The Pennsylvania State University

The Graduate School

Department of Engineering Science and Mechanics

GALLIUM NITRIDE-BASED ELECTRONIC AND OPTOELECTRONIC

DEVICES

A Dissertation in

Engineering Science and Mechanics

by

Li Wang

 2015 Li Wang

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

May 2015

The dissertation of Li Wang was reviewed and approved* by the following:

Jian Xu Associate Professor of Engineering Science and Mechanics Dissertation Advisor Chair of Committee

Michael T. Lanagan Professor of Engineering Science and Mechanics

Samia A. Suliman Assistant Professor of Engineering Science and Mechanics

Jerzy Ruzyllo Distinguished Professor of Electrical Engineering

Judith A. Todd P.B. Breneman Department Head Chair Head of the Department of Engineering Science and Mechanics

*Signatures are on file in the Graduate School

iii

ABSTRACT

For the past decade, Gallium nitride (GaN) material system has earned a significant place in modern power electronic and optoelectronic devices due to its outstanding electric and optical properties. GaN-based device technologies have improved substantially, and are still investigated intensely for advanced performance.

The GaN-based devices studied in this dissertation involve Schottky barrier diodes

(SBDs) and high electron mobility transistors (HEMTs) on the electronic side and light emitting diodes (LEDs) on the optoelectronic side.

In the SBDs part, GaN SBDs with high voltage blocking capability and low on- state voltage on inductively coupled plasma (ICP) etched commercial LED epi-wafers are studied. Their applications in alternating current (AC) LEDs are demonstrated. It is revealed that the potassium hydroxide (KOH) pretreatment with optimized concentration could eliminate the leakage current due to the reduction of the ICP induced surface defects. Moreover, the numerical values of the surface defect density are extracted by analyzing the leakage current mechanism.

In the HEMTs part, the transfer saturation feature of GaN-based HEMTs is investigated firstly. It is observed that the drain current in HEMTs with short gate length becomes saturation as gate bias approaches zero. The theoretical analysis based on a simple series resistance model reveals this saturation feature results from the fact that the total source-drain resistance is independent on gate bias in a short gate length HMET.

This conclusion is further verified by device simulation study. Secondly, novel GaN double-gate (DG) HEMTs featuring enhanced back gate-control of the two dimensional

iv electron gas (2DEG) in AlGaN/GaN heterostructures is designed and modeled. The results indicate that the DG GaN-HEMTs can provide a higher maixmum transconductance gain and better immunity of the short channel effects than traditional single-gate HEMTs. At last, the temperature-dependent electrical characteristics of GaN- based HEMTs from room temperature down to 50K are studied. It is observed that the drain saturation current and transconductance increase with the decrease of the temperature.

In the LEDs part, quantum dots (QDs) coupled non-resonant microcavity light emitting diodes (LEDs) with micro-holes is designed and demonstrated to enhance non- radiative energy transfer between InGaN/GaN quantum wells (QWs) and QDs for the first time by tailoring the radiative relaxation lifetime of excitations in QWs. The blue emission from the InGaN/GaN QWs is detuned from the resonant modes of the microcavity to extend the radiative recombination lifetime in QWs. The direct contact of

QDs and the QWs active layer is achieved by depositing QDs into the micro-holes on the

LEDs. This non-resonant microcavity structure leads to a 3.2 times enhancement of the effective quantum efficiency of QDs in microcavity LEDs than the LEDs without microcavity structure.

v

TABLE OF CONTENTS

LIST OF FIGURES ...... vii

LIST OF TABLES ...... xi

ACKNOWLEDGEMENTS ...... xii

Chapter 1 Introduction ...... 1

1.1 Gallium nitride material properties ...... 1 1.2 Epitaxial GaN film growth ...... 7 1.3 Gallium nitride-based devices ...... 9 1.3.1 Power Electronic devices ...... 10 1.3.2 Optoelectronic devices ...... 15 1.3.3 Monolithic integrated devices ...... 17 1.4 Fundamentals of selective Gallium nitride-based devices ...... 19 1.4.1 Schottky barrier diodes ...... 19 1.4.3 High electron mobility transistros ...... 23 1.4.3 Light emitting diodes ...... 28 1.5 Organization of the Dissertation ...... 33 Reference ...... 34

Chapter 2 High performance GaN Schottky Barrier Diodes (SBDs) on plasma deep-etched surface ...... 45

2.1 Introduction ...... 45 2.2 Experimental Methods ...... 47 2.3 Results and Discussion ...... 49 2.4 Surface defect density extraction ...... 53 2.5 High performance of Schottky barrier diodes ...... 60 2.6 Conclusion ...... 63 Reference ...... 64

Chapter 3 A study of the conductive interface region between gallium nitride and patterned sapphire substrate ...... 68

3.1 Introduction ...... 68 3.2 Experimental ...... 70 3.3 Results and Discussion ...... 71 3.4 Conclusion ...... 76 Reference ...... 77

vi

Chapter 4 Investigation of drain current saturation feature in AlGaN/GaN high electron mobility transistors ...... 80

4.1 Introduction ...... 80 4.2 Experimental methods ...... 81 4.3 Results and Discussion ...... 83 4.4 Conclusion ...... 92 Reference ...... 93

Chapter 5 Modeling the Back Gate Effects of AlGaN/GaN HEMTs ...... 95

5.1 Introduction ...... 95 5.2 Device Structure ...... 96 5.3 Simulation Methodology ...... 97 4.4 Results and Discussion ...... 98 5.5 Conclusion ...... 107 Reference ...... 108

Chapter 6 Temperature dependence of AlInN/GaN HEMTs electrical characteristics ...... 111

6.1 Introduction ...... 111 6.2 Aluminium Indium Nitride/Gallium Nitride heterostructure ...... 112 6.3 Experimental Methods ...... 114 6.4 Results and Discussion ...... 115 5.5 Conclusion ...... 125 Reference ...... 126

Chapter 7 Energy transfer in quantum dots coupled microcavity nitride LEDs ...... 129

7.1 Introduction ...... 129 7.2 Theory ...... 133 7.2.1 Quantum dots and quantum confinement effect ...... 133 7.2.2 Nonradiative energy transfer between quantum well and quantum dot ...... 136 7.3 Device structure and fabrication ...... 138 7.4 Experimental methods ...... 140 7.5 Results and analysis ...... 142 7.6 Conclusion ...... 147 Reference ...... 149

Chapter 8 Future Work ...... 153

8.1 Chip-level integration of gallium nitride light emitting diodes and vertical metal semiconductor field effect transistors ...... 153 8.2 Monolithic integration of LEDs and AlInN/GaN HEMTs...... 154

vii

LIST OF FIGURES

Figure 1-1. Wurtzite structures of GaN...... 2

Figure 1-2. Bandgap energy versus lattice constants of common materials at room temperature...... 4

Figure 1-3. JFM and BFM for several semiconductors normalized to Si...... 11

Figure 1-4. A diagram of a GaN HEMT with a field plate structure...... 14

Figure 1-5. An illustration for white solid state lighting...... 17

Figure 1-6. An illustration of a metal and a n-type GaN (a) prior to contact. (b) after contact (adapted from reference [28])...... 20

Figure 1-7. The struture of the AlGaN/GaN HEMTs...... 24

Figure 1-8. An AlGaN/GaN heterostructure...... 24

Figure 1-9. (a) Separated (b) Thick epitaxial layer (c) Thin epitaxial layer...... 26

Figure 1-10. 2DEG formation in AlGaN/GaN interface...... 26

Figure 1-11. A schematic illustration for (a) non-radiative recombination and (b) radiative recombination...... 29

Figure 1-12. Cross section of a typical MQW GaN LED...... 32

Figure 2-1. The process flow for the SBDs fabricated on LED epi-wafers...... 48

Figure 2-2. J-V curves under reverse bias of GaN SBDs treated by KOH with different concentration...... 50

Figure 2-3. Reverse voltages as the function of KOH concentration at the leakage current J=10 A/cm2...... 50

Figure 2-4. AFM images of (a) the sample without KOH treatment. (b) the sample with 0.5mole/L KOH treatment. (c) the sample with molten KOH treatment. .... 52

Figure 2-5. Forward J-V characteristics of the sample treated by 0.5 mole/L KOH solution at various temperatures between 300K and 420K...... 54

viii

Figure 2-6. Richardson plot of the sample treated by 0.5 mole/L KOH solution...... 54

Figure 2-7. Thin surface barrier (TSB) model...... 55

Figure 2-8. The band diagram of a metal-GaN contact under reverse bias, where Ef is the semiconductor quasi-Fermi level, Efm is the metal Fermi level, Ec is the conduction band bottom, η is the electron energy from the barrier top, E is the electron energy from the conduction band bottom, Vr is applied reverse

voltage, qb0 is the barrier height, and q is the image force barrier lowering...... 58

Figure 2-9. J-V curves for the GaN SBDs (a) treated 0.5 mole/L KOH solution (b) without KOH treatment. The solid lines represent the calculated results...... 59

Figure 2-10. Typical (a) forward (b) reverse J-V curves of the fabricated SBDs...... 61

Figure 2-11. Microscope pictures of (a) top emission AC LED with SBDs (adapted from reference [22]). (b) flip chip AC LED with SBDs (adapted from reference [23])...... 62

Figure 3-1. Temperature-dependent measurements for PSS sample and FSS sample...... 72

Figure 3-2. (a) Schematic of series resistances between two ohmic contacts on a GaN film. (b) Typical measured I-V characteristics between two ohmic contacts with the distance of 400um on the GaN samples with PSS and FSS respectively...... 73

Figure 3-3. Simulated forward I-V characteristics of GaN schottky diodes with three different conductivities in the interface region. The inset is the reverse I-V curves...... 75

Figure 3-4. Measured I-V curves of the SBDs fabricated on GaN LED wafers with PSS. The inset is the structure of the SBDs...... 76

Figure 4-1. Fabricated AlGaN/GaN HEMTs structures...... 82

Figure 4-2. Photo of TLM structure...... 82

Figure 4-3. Measured and simulated ID-VGS curves of AlGaN/GaN HEMTs...... 83

Figure 4-5. Gate-source capacitance as the function of gate voltage...... 85

Figure 4-6. The electron mobility versus 2DEG density for the 0.25 um gate- length sample...... 86

ix

Figure 4-8. The drain current IDS as a function of gate voltage VGS at VDS=0.1 V with different gate length...... 91

Figure 4-9. The channel voltage drop Vch as a function of gate voltage VGS at VDS=0.1 V with different gate length...... 91

Figure 5-1. Structures of (a) single-gate HEMTs. (b) double-gate HEMTs...... 98

Figure 5-2. The simulation result compared with the measurement result of Khan et al. [7] in (a) linear scale. (b) log scale...... 99

Figure 5-5. Electron distributions for both single-gate HEMTs and double-gate HEMTs...... 103

Figure 5-7. Subthreshold swing, SS, versus device gate length at Vds=2.5V...... 105

Figure 5-8. The channel potential variation along the channel of the SG and DG HEMT with the gate length of 50nm...... 107

Figure 6-1. Bandgap energy versus lattice constants of AlN, GaN, and InN...... 113

Figure 6-2. 2DEG formation in AlGaN/GaN interface...... 113

Figure 6-3. Pictures of (a) AlInN/GaN HEMTs, (b) gate structure, and (c) TLM patterns...... 115

Figure 6-4. Results of TLM measurements...... 116

Figure 6-5. Contact resistances extracted from transfer length measurement versus temperature...... 116

Figure 6-6. Sheet resistances extracted from transfer length measurement versus temperature...... 117

Figure 6-7. DC and plused output characteristics of AlInN/GaN HEMTs at (a) 295K. (b) 250K. (c) 150K. (d) 50K...... 120

Figure 6-8. Transfer characteristics of AlInN/GaN HEMTs at 295K, 250K, 200K, 150K, 100K, and 50K...... 120

Figure 6-9. Channel electron mobility versus gate voltage and 2DEG density at various temperatures...... 122

Figure 6-10. Gate capacitance Cgs and 2DEG density ns dependences gate voltage Vgs...... 123

x

Figure 6-11. Extracted 2DEG mobility as a function of the depth from the AlInN/GaN interface ...... 124

Figure 7-1. (a) The particle with velocity V in a one-dimensional potential box. (b) The first three solution of wave function for the particle in a box...... 135

Figure 7-2. Illustration of energy transfer in a QD coupled LED...... 137

Figure 7-3. Schematic diagrams of the microcavity InGaN LEDs with micro- holes...... 140

Figure 7-4. Schematic top view of the microcavity LEDs...... 140

Figure 7-5. An illustion for collecting output emission...... 141

Figure 7-6. Reflection spectrum and emission spectrum at injected current of 10 mA for the microcavity LED without QD coupling...... 142

Figure 7-7. The measured PL lifetime versus wavelength for the microcavity LED and the control sample...... 143

Figure 7-8. EL spectra of (a) the microcavity LED sample with and without QD deposition. (b) the control LEDs with and without QD deposition...... 145

Figure 7-9. PL decay dynamics measured at 450 nm for (a) the microcavity LED and (b) the control sample...... 147

Figure 8-1. The monolithic integration of a GaN LED and a vertical MESFET...... 154

Figure 8-2. The monolithic integration of a GaN LED and a AlInN/GaN HEMT...... 155

xi

LIST OF TABLES

Table 1-1. Material properties of common semiconductors at room temperature [1, 9-15]...... 6

Table 1-2. Some characteristics of the three foreign substrates and the native GaN substrate [19, 34, 48]...... 9

Table 2-1.Comparative results with other references [1, 21]...... 60

xii

ACKNOWLEDGEMENTS

First and foremost, I would like to give the deepest appreciation and gratitude to my advisor Prof. Jian Xu for his invaluable guidance and unceasing support. His vast knowledge and insigntful suggestions are of great benefit not only to this rearch work but also to my future life.

Next, I would like to thank Prof. Michael T. Lanagan, Prof. Samia A. Suliman, and Prof. Jerzy Ruzyllo for their advice, time, and effort as members of my committee.

I would also like to thank all my former and current labmates Dr. Guanjun You,

Dr. Yu Zhang, Dr. Xuefeng Zhang, Dr. Lai Wei, Dr. Jie Liu, Dr. Myo Thein, Dr. Shawn

Pickering, Dr. Zhenyu Jiang, Dr. Xiaoyun Li, Dr. Yan Liu, Mahmoud R. M. Atalla, Asim

M. Elahi, and Shengshi Liu for their generous help.

Finally, I would express my especial thanks to my family for their unconditional love.

Chapter 1

Introduction

The focus of this dissertation is gallium nitride (GaN)-based devices. As one of the wide-bandgap III-V compound semiconductors, GaN has became the center of attention in both academic and industry areas since the breakthroughs achieved in the

GaN film growth technology in the early 1990s[1]. Thanks to the great effort taken by researchers over more than twenty years, both GaN-based power electronic and optoelectronic devices have appeared in the commercial market. Companies carrying

GaN technology include Cree, Toshiba, Avago, Triquint and so on. According to the report published by MarketsandMarkets, the value of GaN market is estimated to be

$15.6 billion by 2022 [2].

This chapter will start with introducing the material properties of GaN. Then, applications of GaN in optical and electronic devices will be discussed and the principles of selective GaN-based devices will be introduced. At last, the organization of this dissertation will be demonstrated.

1.1 Gallium nitride material properties

GaN is one of III-nitrides compound semiconductors, consisting of the group III element Ga and the group V element N with covalent bonds. Although GaN has three typical crystal structures which are wurtzite, zinc-blende, and rocksalt, wurtzite structure

2 is the most common structure under equilibrium conditions because its thermodynamically stability [3]. The basic unit cell of the wurtzite structure is shown in

Fig. 1-1. It is a hexagonal unit with two lattice constants, which are the hexagon edge length a and the unit cell height c. For wurtzite GaN, a=3.189 Å and c =5.185 Å [4].

Figure 1-1. Wurtzite structures of GaN.

The synthesized GaN was first obtained in 1932 [5]. The reaction product between the ammonia (NH3) gas and the metallic gallium at high temperature turns out a dark grey powder. The GaN powder was turned to be impressive stable under ambient conditions. The decomposition temperature of was suggested in the range of 750 oC and

1000 oC [6]. It was reported that GaN could not dissolve in general acids or bases at room temperature but showed slow dissolution rate in hot concentrated H2SO4 and NaOH solutions [4]. Due to the advanced thermal stability, GaN can withstand high

3 temperatures in device fabrication processes. However, the chemical stability of GaN makes conventional wet etching technologies not suitable as etching method in GaN material system [7]. Thus, dry etching technologies are commonly used in GaN device fabrication processes.

In 1969, the first single GaN crystal was grown successfully by Maruska and

Tietjen. The GaN film was on sapphire using the hydride vapor phase epitaxy (HVPE) technique [6]. The single crystalline GaN layers were obtained through reacting gallium

o chloride (GaCl) with NH3 at 850 C. The unintentionally doped GaN films contained high inherent electron concentrations, which made the GaN films were n-type material. The research of GaN material was almost stagnated due to the difficulties of restraining the large intrinsic electron concentration and obtaining high quality conductive p-type GaN for forming p-n junctions [1]. Almost ten years later, Amano et al. [9, 11] and Nakamura et al. [10, 12] sucessefully got high quality GaN layers as well as conductive p-type GaN by metal organic chemical vapor deposition (MOCVD) technique. As like breaking through the bottlenecks, these improvements of GaN technology greatly boosted the research on GaN-based devices since 1990s and this prosperity endures to nowadays.

GaN is a direct wide-bandgap (WBG) material. Figure 1-2 shows the corresponding relation between the bandgap energy and the lattice constant for several semiconductors [32, 35]. This figure shows that compared with Si and GaAs, both GaN and SiC have a much larger bandgap, which are 3.44 eV and 3.26 eV respectively. GaN and its alloys have a wide rang of bandgap energies occupying a continuous value from

1.9eV to 6.2eV, corresponding to a continuous wavelength from 635nm to 200nm. This wavelength range covers the whole visible spectrum and extends to deep ultraviolet

4 wavelengths. This wide spectrum band makes AlGaInN material system superior than

InGaAlP or AlGaAs material systems in optoelectronic applications which wavelength is limited below 500nm [31]. Additionally, the probability of carriers occurring radiative recombination in direct bandgap materials is higher than indirect bandgap materials, because a phonon is needed to change the momentum of electrons to complete the recombination in indirect bandgap materials. The participation of phonons greatly reduces the probability of electrons and holes recombination, leading to the decrease of the quantum efficiency of a device [17]. Thus, as a direct bandgap semiconductor, GaN is popular in optoelectronics applications.

8 Direct Bandgap Indirect Bandgap 6 AlN

4

GaP SiC GaN AlAs 2 InN

Bandgap energy (eV) GaAs Si InP 0 3.0 3.2 3.4 3.6 5.5 6.0 Lattice constant (Å)

Figure 1-2. Bandgap energy versus lattice constants of common materials at room temperature.

Because of the wide bandgap energy, GaN can be used in high operation temperature conditions. There is a temperature point at which the intrinsic carrier

5 concentration in a material will be high enough leading to the failure of devices, generating a large amount of leakage current and losing their ability to control the carrier density [8]. Since such temperature value is in direct proportion to the bandgap of a material, GaN has a much lower intrinsic carrier density than those conventional narrow bandgap materials such as Si and GaAs in a wide temperature range [47]. Therefore,

GaN-based devices could withstand the high temperature operation conditions.

Also, GaN has a relatively higher electric breakdown field than conventional semiconductors due to its large bandgap. The electric breakdown field has a positive correlation with the bandgap energy, according to the study conducted by Jerry et al. [9].

The critical electric field of GaN is estimated as ~5 MV/cm [10]. The large electric breakdown field indicates that GaN material can withstand high supply voltage, which is demanded in power electronic devices.

Moreover, GaN owns excellent carrier transport properties which relate directly to the performance of electronic and optoelectronic devices. Excellent carrier transport properties such as high electron mobility and large electron saturation velocity are required for high frequency and high power applications. The measured electron mobility for GaN is lower than 1000 cm2/V-s [10, 15]. Nevertherless, AlGaN/GaN heterostructures have electron mobility of 2000 cm2/v-s and the two dimensional electron gas (2DEG) with a sheet density of 1×13 cm-2 [16]. The availability of AlGaN/GaN heterostructures combined with the 2DEG, which has a high sheet density and reasonable electron mobility enables GaN a promising material for fast switching power electronics.

Another attractive property of GaN is its high thermal conductivity. Good thermal conductivity guarantees an excellent performance of the device at high operation

6 temperature. The thermal conductivity value of GaN is 1.3 W/cm-K [11], which is similar as the most common conventional material Si, but much higher than GaAs [15]. SiC has a better thermal conductivity [15], than GaN, so GaN on SiC substrate is a good choice for high power applications.

The fundamental material properties at room temperature associated the performance of optoelectronic and electric devices for several semiconductors are listed in Table 1-1.

Table 1-1. Properties of common materials at room temperature [1, 9-15]. GaN GaAs Properties SiC Si (AlGaN/GaN) (AlGaAs/InGaAs) Bandgap energy, 3.45 3.26 1.12 1.43 Eg (eV) Electric breakdown field, 3.3 2.0 0.3 0.4 Ec (MV/cm) Saturated electron velocity, 2.5 2 1 1 7 ʋsat (×10 cm/s) Electron 900 8500 mobility, μ 700 1500 n (>2000) (10000) (cm2/V-s) 2DEG density, 13 -2 1 NA NA <0.2 ns (×10 cm ) Thermal conductivity, 1.3 4.5 1.5 0.5 κ (W/cm-K)

7 1.2 Epitaxial GaN film growth

Three main techniques are used for GaN epitaxial growth, which are hydride vapor phase epitaxy (HVPE), metal organic chemical vapor deposition (MOCVD), and molecular beam epitaxy (MBE). HVPE is the first technology to grow epitaxial GaN films by Maruska and Tietjen in 1969 [18], and was the most popular epitaxial GaN growth technique in the 1970s [33]. This technique can grow high quality epitaxial GaN layers at high growth rates above 100 um/h [34]. Furthermore, the low pressure and low temperature conditions in the HVPE growth process make it a favorable technique for

GaN epitaxial growth [19]. Currently, HVPE is thought to be a highly promising technique for growing thick GaN native substrate [18, 35]. MOCVD has been considered to be the most popular and efficient method in epitaxial GaN growth since the high quality GaN layers were successfully obtained by using this method in 1986 [36].

MOCVD is a growth technique with a low growth rate and can be used to grow heterostructures due to the good control ability [1]. Moreover, the Ga/N flux ratio is very low in MOCVD process, which could greatly reduce the nitrogen vacancies in the GaN layers. MBE was first introduced for growing GaN by Gotoh et al. in 1981 by reacting

Ga with NH3 [37]. The process of MBE is under ultra-high vacuum (UHV) growth conditions and can be controlled as precise as one atomic layer [37]. The high cleanliness of the UHV chamber makes it possible to grow high purity GaN lattices. However, it is difficult to find a nitrogen source for MBE system because the temperature for MBE growth is too low to decompose NH3. Therefore, electron cyclotron resonance or radio

8 frequency plasmas are generally integrated in MBE systems to provide the nitrogen source [1].

Generally, GaN films are grown on foreign substrates because of the absence of the native GaN substrate. Lattice mismatch, thermal expansion coefficient (TEC) mismatch, thermal conductivity, electrical conductivity, and price are on the list for choosing a suitable substrate. Large lattice mismatch and TEC mismatch will lead a high defect density. Since sapphire is an insulating substrate and has a low cost, it is the traditional most common substrate for growing GaN [1]. But its high lattice mismatch, high TEC mismatch, and bad thermal conductivity make it mainly used in GaN LEDs

[19]. SiC owns a very good thermal conductivity and relative low lattice mismatch and

TEC mismatch, which makes it a premium substrate for GaN-based high power applications such as HEMTs. The disadvantage of SiC is the relatively high cost.

Although Si has large lattice mismatch and TEC mismatch as well as low thermal conductivity, it is still on the choice list for GaN epitaxial growth because of the possibility to integrate with other conventional Si devices [1].

Meanwhile, although heteroepitaxial GaN growth on foreign substrate works well for most of GaN devices, the growth of bulk GaN substrate is also under intensive research for improving the GaN device performance [34, 19]. GaN films grown on GaN substrate are expected to have reduced defect density due to the elimination of the lattice and TEC mismatching [34]. The recent achievements in growing native GaN substrate techniques includes 3 inch freestanding GaN wafer with the lowest unintentional doping of 1×1016 cm-3 obtained by HVPE technique, 2 inch wafer with the lowest unintentional doping of 2×1018 cm-3obtained by Ammono-thermal technique, and 1×1016 cm-3obtained

9 by Na-solution growth technique [19]. Despite the big progress in bulk GaN growth, the quality and the size of GaN substrate still could not meet the market requirements and the price is extremely expensive [34, 48]. Therefore, most of GaN-based devices in nowadays are based on foreign substrates. Table 1-2 compares the four substrates.

Table 1-2. Characteristics of the three foreign substrates and the native GaN substrate [19, 34, 48]. Substrate Sapphire SiC Si Bulk GaN

Lattice mismatch 13% 3.5% 17% none

TEC mismatch 34% 25% 56% none

Thermal conductivity, κ (W/cm-K) 0.47 4.5 1.5 1.3

Maximum Available wafer size 8 inch 6 inch 3 inch any

Cost, €/cm2 1 10 0.1 100

1.3 Gallium nitride-based devices

GaN material system has earned an important place in modern electronic and optoelectronic devices due to their numerous advantages including direct wide bandgap energy, excellent transport properties, a wavelength range covering the near infrared band to the near ultraviolet spectral band, and the availability of heterostructures such as

AlGaN/GaN.

1.3.1 Power Electronic devices

Currently, energy consumed in the world wide is mainly in the form of electrical energy and most of the dissipated energy in a power electronic system is associated with the power semiconductor devices [45]. Thus, there is a great interest in increasing the efficiency of power electronic devices to minimize the consumed electrical energy.

Nowadays, the power electronic system largely depends on the Si-based semiconductor devices. However, the maximum blocking voltage capability of current commercial Si devices is lower than 7kV, and many Si devices can only operate under 200 oC. Si-based devices also suffer an inadequate capacity of current conductance and a low switching frequency [38]. These limitations of Si devices promote the development of wide bandgap semiconductor-based electronic devices. Compared with the traditional Si-based devices, wide bandgap material based electronic devices own enormous potential to achieve higher energy conversion efficiency resulting in the reduction of device size, cost, and energy consuming [46].

Among the wide bandgap semiconductor materials, GaN and SiC with mature and well established process technologies are in the focus of attention and the most attractive to the future development of power electronic devices [46]. GaN and SiC have very similar material properties that provide them the capability of withstanding high reverse voltage, high operation frequencies, and high temperature operation. However, GaN has superior theoretical characteristics than SiC and is expected to demonstrate better performance in high-power microwave applications [38, 46]. Figure 1-3 is a summary of

11 three figures of merit (FOMs) for GaN, SiC, GaAs, and Si [49]. FOMs are proposed for comparing the possible performance of the materials in power electronic application [11].

Johnson’s figure of merit (JFM) is an indicator for a semiconductor by taking into

account the electric breakdown field c and the electron drift saturation velocity s ,

which can be given by JFM cs/2 [87]. It can be seen that GaN features a nearly 2 times better JFM than SiC, 790 times than Si, and 77 times than GaAs. which is an indicator of high frequency, and three times better BFM, which is a measurement of on state resistance. Baliga’s figure of merit (BFM) shows the power capability of a certain semiconductor by considering the comprehensive effects of the dielectric constant  ,

3 electric breakdown field c , and electron mobility μ, given as BFM  c [1]. The value of BFM for GaN is 3 times better than SiC, 30 times better than GaAs, and 910 times better than Si.

910 1000 790

410 500 290 1 11 1 28 0 JFM BFM Si GaN SiC GaAs

Figure 1-3. JFM and BFM for several semiconductors normalized to Si.

12 GaN-based optoelectronic devices have already been widely commercialized in the market, while GaN-based power electronic devices are still at the beginning stage in the commercial market [48]. As wide bandgap materials, GaN has a large electric breakdown field (~5 MV/cm) and the ability to withstand high supply voltages and high operation temperatures, which are required in power electronic devices [47]. Another interesting property of GaN for power electronics is the availability of heterostructures and the two-dimensional electron gas (2DEG) formed due to the polarization effects in nitrides. High 2DEG sheet density and not too low electron mobility combined with high electron saturation velocity (~2×107 cm/s) enable GaN based devices to provide high output currents and fast switching frequency, which are necessary elements for high- power microwave devices [1]. These material properties evidently show that GaN is the most promising candidate in power electronic applications.

There is a steady increasing interest to GaN-based power electronic devices in industry nowadays and GaN has huge potential from wireless devices, power supplies, and aerospace in the low voltage area to rail transport, smart power grid, and wind mills in the high voltage area [50]. In a high voltage power electronic system, three-terminal switches and two-terminal rectifiers are two essential components from the power generation applications to custom power devices. For GaN-based power devices,

Schottky barrier diodes fulfill the two-terminal rectifier requirement and AlGaN/GaN

HEMTs perform as switches.

GaN SBDs with breakdown voltages higher than 600V were first reported and commercialized by Liu et al. in 2009 [70]. The devices were grown on sapphire substrate and had the leakage current as low as 200uA at the reverse voltage lower than 600V.

13 Meanwhile, forward current achieves 6A at 1.7V at room temperature. Breakdown voltage of GaN SBDs as high as 1.5 kV have been demonstrated on Si substrate in 2013 by Boles et al. [71]. Although most of the reported GaN SBDs nowadays are lateral or quasi-vertical structures, SBDs fabricated on bulk GaN substrates have been reported owning several advantages over other foreign substrates [19, 46]. High breakdown voltage SBDs on bulk GaN substrates have been reported and a low specific on-state resistance of 1.1 mΩcm2 was achieved [72]. The quality of schottky contacts on GaN is still a main concern in improving the performance of GaN SBDs. Defects and contaminants induced in device fabrication processes as well as the native oxide will cause the lowering of Schottky barrier height and deteriorate the ideal characteristics of

SBDs. Surface cleaning pretreatments including HNO3/HCl, HF/H2O, HCl/H2O, and

NH4OH solutions have been verified as effective ways to dissolve the oxide layer on the

GaN surface [73-75]. It also has been revealed by X-ray photoemission spectroscopy analysis that KOH could effectively increase the band bending and improve the surface stoichiometry of GaN Schottky contacts by removing the gallium atoms and the chemisorbed oxygen atoms [76].

Currently, the most common nitride power electronic devices serving as switches are GaN-based HEMTs. Thay have several advantages including the availability of

AlGaN/GaN heterostructures and the polarization effects induced 2DEG with high charge density and acceptable electron mobility. Thus, GaN-based HEMTs well suited for high- power microwave devcies and attract lots of entention both in academic and industry in recent years [46]. Significant advancements have been achieved since the first

AlGaN/GaN HEMTs were proposed in 1994 [77]. A great deal of effect was made to

14 suppress the drain current degradation problem observed in GaN-based HEMTs. It is thought that this current collapse is caused by the surface donor-like traps which have low mobility and act like a “virtual gate”. This leads to the increase of the channel resistance and consequently the reduction of the drain current [45, 78]. One main method to prevent the current collapse is to add a field plate structure to relax the electric field strength, as shown in Fig. 1-4 [79]. Other structures including a thin GaN cap structure, the recessed gate structure, and the passivation film are also often used in AlGaN/GaN for eliminating the current collapse [46].

Figure 1-4. A diagram of a GaN HEMT with a field plate structure.

Another research effort involved in the development of GaN-based HEMT technologies is the development of normally-off GaN power switching transistors. It is known that AlGaN/GaN HEMTs are normally-on devices due to the polarization induced

2DEG, allowing GaN HEMTs conducting current without gate bias. For safety reason, normally-off switches are preferred in power applications, so it is desired to obtain normally-off GaN HEMTs. One of the most used methods to achieve the normally-off operation is to extend the gate into the barrier layer [82]. In this way, the AlGaN barrier

15 layer becomes very thin (2~5 nm) [45]. The thin AlGaN layer causes the elimination of

2DEG and a positive threshold voltage. The second method is to induce immobile negative charges by injecting plasma treatment underneath the gate region to reduce the

2DEG. Thus, the threshold voltage would increase to a positive value [83]. Another approach involves the use of p-n junction gate structure to form a depletion region in the

AlGaN layer [84].

1.3.2 Optoelectronic devices

III-nitride material system has tunable direct bandgap energies, which cover the rang of from ~2 eV to ~6.2eV. This bandgap energy range corresponds to a wavelength range from ~650 nm to ~300 nm which spans the whole spectral bands of near infrared, visible, and deep ultraviolet [47]. The excellent material properties of GaN, such as chemical stability, reasonable thermal conductivity, and high electron saturation drift velocity, provide GaN great potential in power electronic applications also guarantee it to be a promising material in optoelectronic devices.

The common materials used for LEDs include AlGaAs, InAlGaP, GaAsP, and

GaN [28]. GaN material system covers the widest visible spectrum range from violet to red. Moreover, GaN and its alloys are the unique materials to provide emission with the wavelength shorter than 500nm.

The first application of GaN in optoelectronics is LEDs, which also are the first

GaN-based devices. The first GaN LED emitting blue light with the peak wavelength of

475 nm was fabricated using metal-insolated-semiconductor structure in 1971 by

16 Pankove et al. [51]. With the achievements of GaN growth techniques and the improvements of GaN epixycial layers, GaN LEDs based on pn junction structures and high brightness blue double heterostructrues were proposed in 1989 and in 1993 respectively [43, 44]. The appearance of GaN blue LEDs opens a door to the white solid state lighting, which was expected to be advanced over current incandescent and fluorescent lighting techniques in energy saving, longer lifetime, and smaller size [52].

White solid-state lighting can be produced either by integrating LEDs, producing red color, green color, and blue color separately, into a same package to obtain a mixing white light or by using a phosphor in the GaN-based LED package to produce a white emission by converting and mixing the blue light [14]. Figure 1-5 shows the two ways in creating white light. The color-mixing method has a higher cost than the phosphor-based white LED method and the color rendering is not good [14]. Meanwhile, due to the low quality of green LEDs, the white LEDs based on color-mixing method has a lower performance than the expectation [53]. On the other hand, the phosphor method earns a lot of interest due to the low cost and compact package size. One of the popular phosphor-based white LED techniques involves converting the blue emission from

InGaN/GaN multiple quantum-well (MQW) LEDs into yellow light to generate white light [85]. Another way is to use an ultraviolet LED to pump a wide spectrum phosphor for white light production [47]. Recently, quantum dots (QDs) have been used as phosphor in the solid state white light technique due to their great features of sharp absorption spectrum, excellent photoluminescence efficiency, and the wide emission wavelength range[86]. Also, the discovery that the existence of nonradiative energy transfer between InGaN/GaN QWs and QDs, which is believed a high efficiency color

17 conversion through skipping several color conversion steps, boosts the research of QD phosphors in white LEDs [89].

Figure 1-5. An illustration for white solid state lighting.

1.3.3 Monolithic integrated devices

Recently, the monolithic integration of nitride optoelectronic and power electronic devices is commanding lots of attention for the promise of cost savings, circuit simplification, and system reliability improvements [90].The monolithic integrated GaN devices under investigation involve the monolithic integration of LEDs with SBDs [91] and LEDs with HEMTs [90, 92]. LEDs integrated monolithically with power electronic devices have great potential in developing alternating current (AC) LEDs, free-space visible light communications, smart solid state lighting, and single-chip sensors [90-93].

GaN alternating current (AC) LEDs based on the integration of LEDs and SBDs on a single chip were demonstrated by Wen-Yung Yeh et al. [91]. The integration was

18 expected to be achieved by the re-growth of SBD epi-layers on the LED epi-wafers or the

LED epi-layers on the SBD wafers. The monolithic integration of GaN-based LEDs and

GaN-based HEMTs was reported by Li et al. [92]. The integrated devices were fabricated by growing LED epi-layers on customized AlGaN/GaN HEMT layers firstly and then etching off certain regions of the LED epi-layers to expose the HEMTs areas. Lately, Liu et al. [90] presented and compared two ways in fabricating monolithic integration of

LEDs and HEMTs in GaN. The LED-HEMT integration devices were realized through the selective epitaxial removal (SER) method, which refers to grow the HEMT structure on the LED structure followed by etching off selective areas to expose the LED region, and the selective epitaxial growth (SEG) method, which refers to grow the HEMT structure on selective region of the LED structure. The comparison results showed that

LEDs had better performance in the SEG method due to the avoidance of etching processes [90]. Despite these achievements obtained in the GaN-based monolithic integration devices, plenty of efforts are still needed to improve the integration technologies due to the difficulty and complexity of integration fabrication processes

[90].

19 1.4 Fundamentals of selective Gallium nitride-based devices

1.4.1 Schottky barrier diodes

Schottky barrier height

A Schottky barrier diode is a semiconductor diode which consists of rectifying metal-semiconductor junctions [24]. A metal-semiconductor junction is made by a metal contacting with a semiconductor. Figure 1-6 shows the energy band diagram of a metal and an n-type GaN prior to and after contact. Metals with higher work function than the work function of GaN are required for n-type semiconductor to form rectifying contacts

[28]. Before contact, the smaller work function of GaN makes the Fermi level in GaN side is higher than the one in the metal side. After contact, the Fermi levels should be constant at thermal equilibrium condition, leading to a barrer formed at the metal-GaN interface. Ideally, the Schottky barrier height could simply be calculated by subtracting the electron affinity of the semiconductor χ from the work function of the metal and, as [25]

= ) (1-1)

. The energy band of the semiconductor near the interface appears distortion due to the continuity of the vacuum level. The magnitude of this band bending is the built-in potential [28].

20

Figure 1-6. An illustration of a metal and a n-type GaN (a) prior to contact. (b) after contact (adapted from reference [28]).

The work function for n-type GaN is ~ 4.1eV [26], so the most commonly used metals in GaN SBDs are Ni (5.15 eV), Au (5.10 eV), Pd (5.12 eV), and Pt (5.65 eV) [25,

27]. The barrier height can be estimated using Eq. 1-7, but this equation only valid in an ideal condition. For a given metal, the reported Schottky barrier height does not equal to the calculation value and varies in the literatures due to the fabrication process factors and multiple transport mechanisms [27, 38].

There are two main effects in altering the experimental Schottky barrier height from the ideal theoretical value. The first one is the image-force lowering effect. This effect is caused by the image charges in the metal side, which are induced by the electrons near the semiconductor surface. The image-force lowering effect  can be calculated by [28]

q   m (1-2) 4 s

21 , where [61]

2qN m  (Vbi V  kT/ q) (1-3)  S

is the maximum electric field at the interface,  s is the dielectric constant of GaN, k the

Boltzmann constant, T the Kelvin temperature, q the electron charge, Vbi the built-in potential, V the applied voltage, and N the doping concentration near the GaN surface.

Because of the built-in potential, the image-force lowering happens even at zero bias.

The second effect is the interface states effect. The existence of interface states in the narrow interfacial layer with a thickness of a few angstroms and transparent to electrons could alter the barrier height by pinning the semiconductor Fermi level at a fixed position [28]. The relation between the barrier height and the interface states density is found to be [25]

1  i (Eg  q0  qb0 )  2q s N(b0 n )  [m  ( b0 )] (1-4) qDit qDit

, where Eg is the bandgap of the semiconductor, q0 is the surface potential below

which all states are donor states and above which are acceptor states,  i is the

permittivity of vacuum,  is the thickness of the interfacial layer, and Dit is the interface states density.

1 In the case of D   , the barrier height   (E  q ) is a fixed value. On it b0 q g 0

the other hand, in the case of Dit  0, the barrier height b0  (m  ) is the same as the ideal barrier height calculated by Eq. 1-1 [25].

22 An index is proposed to evaluate the degree to which the barrier height is tailored by interface states. This index is given by [62]

d S  b0 (1-5) d(m  )

. The value of S is between 0 and 1. S = 0 corresponds to the situation that the barrier height is completely pinned by interface states, while S =1 corresponds to the situation that the interface states do not exist. S is ~0.385 in GaN material, indicating a partial pinning by interface states [62].

Carrier transport mechanism

Different from p-n junction diodes, under which the current mechanism is the diffusion of minority carriers, the current in SBD is controlled by the drift of majority carriers [25]. The current in GaN SBDs is produced by the transition of electrons between the metal and GaN. The observed total current I transporting through the interface is the algebraic sum of is the current density transfer from GaN to metal and the current density transfer from metal to GaN [61].

In a moderately doped GaN, thermionic emission process is the dominant current transport mechanism. Assuming kT and under the thermal equilibrium condition, the total thermionic emission current at the interface is [63]

qV I  I [exp( ) 1] (1-6) total s nkT

, where I s is the reverse saturation current and is given by [25]

23

 2  q q I  AA T exp( b0 )exp( ) (1-7) s kT kT

, where A is the effective Schottky contact area, n the ideality factor, and A* the

Richardson constant. If other transport mechanisms except thermionic process exist in a

Schottky diode, n is larger than unity [28]. The effective Richardson constant A* is a material parameter and can be determined by [25]

4qmk 2 A  n (1-8) h3

 , where mn is the effective electron mass and h is the Planck’s constant. For GaN, the theoretical value of A* is 26.4 Acm-2K-2 [64]. q is the image forcing lowering and can be calculated by Eq. 1-2 . Since the image forcing lowering will increase with the reverse voltage, the reverse current will become large at the high reverse bias.

1.4.3 High electron mobility transistros

Basic concepts of GaN HEMTs

AlGaN/GaN heterostructures are the most popular structure for GaN-based

HEMTs. Figure 1-7 is the structure of an AlGaN/GaN HEMT. Commonly, the heterojunction, as an abrupt junction, is formed by a GaN buffer layer and an AlGaN barrier layer with a sudden change of bandgap from AlGaN to GaN, as shown in Fig. 1-8.

Both GaN layer and AlGaN layer are unintentionally doped. Since AlN has a bandgap energy of 6.2 eV and GaN has a bandgap energy of 3.4 eV [6], the bandgap energy of

24 AlGaN is tunable from 3.4 eV to 6.2 eV. A potential well could formed at the heterointerface because of the large band discontinuity. 2DEG are electrons accumulating below the interface of heterojunctions and locating in the quantum well formed by the discontinuity of conductance bands of two different semiconductor materials [56]. The term two-dimensional in the name refers to the fact that they can move freely in the other two directions [57]. In GaN-based HEMTs, the electrons derive from polarization effects.

Source Gate Drain

AlxGa1-xN

Conductive Channel GaN 2DEG

Substrate

Figure 1-7. The struture of the AlGaN/GaN HEMTs.

2DEG

Ec2 Ec1 E F1 EF2

Ev2

Ev1 Al Ga N x 1-x GaN

Figure 1-8. An AlGaN/GaN heterostructure.

25 Two dimensional electron gas in AlGaN/GaN HEMTs

There are two types of polarization effects in III-nitrides like GaN, AlN, and InN due to the existence of nitrogen atoms. First effect is spontaneous polarization which induced by the fact that no electron occupies the outer orbital of a nitrogen atom leading to the strong Coulomb attraction force between the atomic nucleus and the surrounding electrons [58]. Thus, in III-nitride covalent bonds, all electrons will be attracted to the nitrogen atom side. This causes the appearance of strong iconicity in the III-nitride covalent bonds [38]. At last, the nitrogen atom demonstrates negative charge and the gallium (Ga) atom demonstrates positive charge in a gallium-nitrogen covalent bond due to this iconicity. For the whole asymmetry wurtzite GaN crystal, a macroscopic polarization will be produced, named as spontaneous polarization [59].

Another polarization effect is piezoelectric polarization effect. This type of polarization is induced by the external stress from the mismatch of the lattice constants between two materials [58]. An important issue involves in the formation of heterostructures is the matching of the lattice constants in two different materials. An epitaxial growth film will be strained if the lattice constant of the substrate is differeent

[28]. Figure 1-9 (a) shows the two materials in isolation. a1 and a2 denote the lattice constant of the two layers respectively, where a1< a2. Dislocations will appear without the external stress as shown in Fig. 1-9 (b). The top layer can be strained to make its lattice constant coincide with the below layer and eliminate the dislocations, as shown in

Fig. 1-9 (c). The crystal structure will changes due to the applied strain and induce another polarization which is called piezoelectric polarization [59].

26

a 1 strain

a2

(a) (b) (c)

Figure 1-9. (a) Separated (b) Thick epitaxial layer (c) Thin epitaxial layer.

For AlGaN/GaN heterostructures, the smaller AlGaN lattice constant results in the production of a piezoelectric polarization effect in the AlGaN barrier layer [58].

Figure 1-10 gives all of the three polarization effects in an AlGaN/GaN heterostructure.

An interface sheet charge, the 2DEG, will be produced by these polarization effects due to the change of polarization field in space and can be calculated by [60]

 sheet  (PSP  PPE ) AlGaN (PSP)GaN (1-9)

, where PSP is the spontaneous polarization determined by the crystal structure parameters and PPE is the piezoelectric polarization calculated by both the structure parameters and piezoelectric coefficients [59].

Net polarization P SP PPE AlGaN charge

PSP GaN 2DEG

Figure 1-10. 2DEG formation in AlGaN/GaN interface.

27 Principles of operation

GaN-based HEMTs are normally-on devices. The 2DEG formed by polarization effects in the potential well serves as a conductive current channel in a GaN HEMT, under the control of the applied gate voltage. By using complete depletion approximation, the relationship of the 2DEG density ns below the heterojunction and the gate voltage VGS can be demonstrated by [65-67]

 E n() V  V  F (1-10) sqd GS th q

, where  is the dielectric constant of AlGaN, d is the thickness of the AlGaN layer, EF the Fermi energy, and Vth threshold voltage.

To calculate the drain current of HEMT devices, one needs to consider the modulation on drain current by both drain voltage and gate voltage. Using the gradual channel approximation, the charge control model can be modified to [68]

 n( z ) ( V  V  V ( z )) (1-11) sqd GS th

, where V(z) is the channel potential.

In the non-saturation region, the electron velocity is dependent on the distance apart from the source z:

dV() z vz()  (1-12) n dz

, where µn is the electron mobility. Assuming a constant electron mobility µn, the drain current is described by [67]

28

2 ngW VDS IVVVDS[( GS  th ) DS  ] (1-13) dLg 2

, where Lg is the gate length, Wg the gate width, and VDS the bias.

Under the hgih drain voltage VDS condition, the 2DEG density near the drain terminal is close to zero. The channel becomes “pinch-off”. Meanwhile, the drain current tends to saturation and is given by [28]

uWng 2 IVVDsat() GS th (1-14) dLg

1.4.3 Light emitting diodes

Principles of LEDs

Conventionally, incandescent lamps and fluorescent lamps are used for luminescence. In incandescent lamps, the visible light comes from the heated filament

[29], while fluorescent lamps emit visible light by the absorption of phosphors to the UV light [30]. Unlike those traditionally luminescence technologies, LEDs emit the light directly through the radiative recombination process of electrones and holes.

The operation principle of LEDs is spontaneous emission. Electrons and holes can experience either radiative recombination or non radiative recombination in the energy gap of a semiconductor, as illustrated in Fig. 1-11 [40]. The energy is released as electromagnetic wave in the radiative recombination process, while transferred to other

29 energy in the nonradiative recombination process [64]. Since the light emission in LEDs relies in the radiative recombination of electrons and holds, increasing the possibility of radiative recombination and decreasing the possibility of non radiative recombination is desirable in LEDs. To evaluate the radiative recombination in a semiconductor, the radiative recombination rate is defined as [39]

Rr  Bnp (1-15)

, where n and p are the electron concentration and the hole concentration respectively, and B is the bimolecular recombination coefficient which is material dependence.

Figure 1-11. A schematic illustration for (a) non-radiative recombination and (b) radiative recombination.

There are two types of efficiency associated with LEDs. One refers to the internal quantum efficiency. It is the probability of transferring device current to luminance.

Given the injection efficiency equals to unit, internal quantum efficiency can be expressed as [40]:

30

 nr i  (1-16)  r  nr

, where  r is the radiative lifetime and  nr is the nonradiative lifetime. should be kept

longer than  r to get a high internal quantum efficiency. Reducing the defect and impurity densities in the semiconductor is an effective way to prolong the nonradiative lifetime [40].

The other efficiency is the external quantum efficiency ex , which defined as [28]

ex inop (1-17)

, where op is the optical efficiency. relies on the device optic characteristics and does not depends on the electrical characteristics [28]. The external quantum efficiency valuates how much light has been extracted out from devices.

Similar as a normal p-n junction diode, electrons in the n-type semiconductor side will diffuse into the p-type side. A depletion region will be formed in an equilibrium condition. Under the forward bias condition, carriers are injected into the depletion region. They can recombine to each other either in radiative pathway or nonradiative pathway [39]. A radiative recombination process is characterized by releasing the energy as photons. The bandgap of the semiconductor determins the wavelength of the generated light λ, which is given by [40]

1.24   (um) (1-18) Egap

, where Egap is the bandgap energy of a semiconductor material. Thus, a specific semiconductor material should be selected to get lamination with a certain color.

31 Multiple quantum well InGaN/GaN LEDs

The first GaN-based LED was formed by insert insulator between a metal and a

GaN layer due to the unavailability of p-type GaN in early 1970s [41, 42]. Later, in 1989

Amano et al. [43] proposed the first p-n junction GaN LED. High brightness double heterostructure GaN LEDs were firstly reported in 1993 by Nakamura et al. [44]. In the double heterostructure GaN LEDs, electrons and holes are confined in a narrow potential well. Increasing the number of electrons and holes can short the radiative recombination time and result in an increase of efficiency of LEDs [28]. However, double heterostructure LEDs do not have a wide wavelength range and the InGaN layer need to be Zn doped [38]. Also, the thick recombination region in the double heterostructue

LEDs will cause unnecessarily energy losses due to the long traveling path of the light

[69]. To overcome these disadvantages of double heterostructure LEDs, the width of the

InGaN layer was reduced to the order of the de Broglie wavelength, typically smaller than 10nm [28]. This narrow potential well is referred as a quantum well in which carriers can only move freely in two directions that are perpendicular to the crystal growth direction. Since the thickness of the quantum well is extremely small, the epitaxial layers can tolerate large lattice mismatch and consequently have less defects

[28]. On the other hand, electron and holes are easily to be saturated in the limited volume of a single quantum well, leading to the overflow of extra carriers. Therefore, the single quantum well is repeated in several periods and comprises the multiple quantum wells (MQWs) to hold the extra carriers overflowing from the adjacent quantum well

[80].

32 Figure 1-12 shows a typical MQW GaN LED structure. The MQW structure is formed by InGaN quantum wells and GaN barrier layers. The InGaN layers with a smaller bandgap energy are sandwiched by GaN layers with a wider bandgap energy on both sides. Appling a forward voltage to this LED, carriers are injected into the quantum wells. Carriers can move freely inside the quantum wells but little of carriers have the sufficient energy to tunnel through the potential wells, indicating that carriers are confined within the quantum wells [81]. Therefore, the high carrier concentration in the potential wells can lead to the increase of radiative recombination rate and ultimately increase the internal quantum efficiency of LEDs.

Figure 1-12. Cross section of a typical MQW GaN LED.

33 1.5 Organization of the Dissertation

The rest part of this dissertation is organized as below.

Chapter 2 demonstrates the successful fabrication of GaN SBDs with high voltage blocking capability and low on-state voltage on commercial LED epi-wafers.

Chapter 3 investigates the transfer saturation feature of GaN-based HEMTs.

Chapter 4 focuses on the design and modeling of DG HEMTs featuring enhanced back gate-control of the 2DEG in AlGaN/GaN heterostructures.

Chapter 5 studied the temperature-dependent electrical properties of GaN HEMTs over a cryogenic temperature range.

Chapter 6 is concerned with the design of QDs coupled non-resonant microcavity

LEDs with micro-holes to enhance non-radiative energy transfer between QDs and

InGaN/GaN QWs for the first time by tailoring the radiative relaxiation lifetime of electron-hole pairs in QW.

Chapter 7 discusses the future work on the study of the GaN/patterned sapphire substrate interface region and the on-chip integration of LEDs and MESFETs in GaN.

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Chapter 2

High performance GaN Schottky Barrier Diodes (SBDs) on plasma deep- etched surface

In this chapter, GaN SBDs with high voltage blocking capability and low on-state voltage are successfully fabricated on commercial LED epi-wafers. Potassium hydroxide

(KOH) with optimized concentration is used to eliminate the inductively coupled plasma

(ICP) etching-induced surface defects for reducing the leakage current in SBDs.

Furthermore, the numerical values of the surface defect density for samples with and without KOH treatment are extracted by analyzing the leakage current mechanism. At last, GaN alternating current (AC) LEDs with monolithic integration of micro-LEDs and

SBDs are demonstrated.

2.1 Introduction

For decades the technology of III-nitride based devices has been under intense investigation for the improved performance in various optical and electronic applications.

Recently, there have been growing interests in monolithic integration of III-nitride optoelectronics and power electronics, and monolithically integrated light emitting diodes

(LEDs) with Schottky barrier diodes (SBDs) or high electron mobility transistors

(HEMTs) have been demonstrated and proposed for AC-LEDs, smart solid state lighting, free-space visible light communication, and on-chip sensing applications [1-4]. The monolithic LEDs could potentially reduce the cost and shrink the system size, but the

46 performance of the monolithic devices is often compromised by the processing complexity and fabrication imperfection that are associated with the integration scheme

[3]. Among them, inductively coupled plasma (ICP) etching is indispensible removing certain epitaxial layers of the LED epi-wafer to integrate electronic devices. However, the physical ion bombardment involved in the ICP etching generates a damaged surface region with the signature amounts of nitrogen vacancies at the GaN surface, since the lighter nitrogen atoms are more readily to be removed than gallium atoms [5]. These nitrogen vacancies combine with oxygen impurities which are absorbed by dangling gallium bonds and act as donors with the activation energy of 30-40 meV [6, 7]. As a result, the barrier height decreases due to the reduction of the band bending. Also, the surface region becomes a degenerated n-type GaN layer which could facilitate the electrons tunneling through the depletion region significantly. Both the band bending reduction and the tunneling probability enhancement are responsible for the degeneration of the devices. X-ray photoemission spectroscopy analysis has revealed that KOH could effectively eliminate the induced donors on ICP damaged GaN surface to increase the band bending and improve the surface stoichiometry through the removal of the gallium atoms and the chemisorbed oxygen atoms, as well as the passivation of the dangling gallium bonds by forming Ga-H bonds [8]. Although many previous reports have claimed that the performance of GaN SBDs in terms of leakage current, ideal factor, and barrier homogeneity is greatly improved after the KOH treatment [9-11], few GaN SBDs under investigation is fabricated from a LED epi-wafer and undergoes a deep etching process with the etched depth larger than 3 um. The research for the effect of KOH treatment on deep ICP-etched GaN SBDs is still under demand.

47 In the following sections, the fabrication process of our GaN SBDs is described.

The effect of KOH with different solution concentration on the recovery of deep ICP- damaged surface is compared and investigated. Furthermore, leakage current mechanism is analyzed and the value of the surface defect density is extracted. In the end, GaN SBDs fabricated on LED epi-wafers with breakdown voltage higher than 200V and their applications in the AC-LEDs are demonstrated.

2.2 Experimental Methods

Figure 2-1 shows the SBDs fabricated on LED epi-wafers process flow. The

SBDs are fabricated on 2-inch commercial LED epi-wafers, which epitaxial structure

18 -3 comprises, from top down, by a ~200 nm p-type (Na> 1×10 cm ) GaN capping layer, a

17 -3 ~200 nm p-type (Na≈ 5×10 cm ) GaN layer, a multiple quantum well (MQW) emissive layer consisted by fifteen pairs of 2.5nm-In0.1Ga0.9N/12 nm-GaN, a ~2.4 um n-type (Nd ≈

1×1018cm-3) GaN layer, and a ~2.5 um GaN buffer layer on a patterned sapphire substrate. The fabrication areas of SBDs are exposed by a Cl2/Ar ICP cyclic etching process. After removing the up layers with a total thickness of ~3um, KOH treatment is performed before the contact metal deposition. To optimize the performance of the SBDs integrated on LED epi-wafers, the ICP-etched samples are treated by KOH with different concentration. Samples are soaked into 0.1mole/L, 0.5mole/L, 1mole/L aqueous KOH solution for 10min, and molten KOH for 90s respectively.

48

Figure 2-1. The process flow for the SBDs fabricated on LED epi-wafers.

A Ti/Al/Ti/Au (10nm/40nm/40nm/100nm) metal layer was deposited on n GaN by e-beam evaporation serving as ohmic contacts followed by 500 oC annealing for 1min

-5 -2 in N2 environment. Schottky contacts with the contact area of 2.5×10 cm are formed

49 by depositing Ni/Al/Ti/Au (50nm/500nm/100nm/200nm) on u-GaN layer. IV characteristics of the SBDs are measured using a Keithley 2612 current source meter.

2.3 Results and Discussion

Figure 2-2 shows the dependences of the current density on voltage in the SBDs treated by KOH with different concentrations under reverse bias. The samples treated by

KOH are obviously have reduced leakage current in compared with the control samples which do not experience KOH treatment after ICP etching. This verifies that KOH treatment is an effective way in eliminating the ICP-induced damage. However, the degree of the effective does not in direct proportional to the KOH concentration. The reverse voltages at the leakage current density of 10A/cm2 are 17V for the control sample,

48V for the 0.1 mole/L KOH treated sample, 90V for the 0.5 mole/L KOH treated- sample, 59V for the 1 mole/L KOH treated sample, and 39V for the molten KOH treated- sample, as shown in Fig. 2-3. It can be seen that the 0.5 mole/L KOH treated sample has the highest reverse voltage value at a certain leakage current value, indicating that 0.5 mole/L KOH solution can best improve the ICP-damaged GaN surface.

50

105 1 mole KOH molten KOH

) 3 0.1 mole KOH 2 10 0.5 mole KOH control sample

101

A/cm ( 10-1

10-3

10-5

10-7

-9 Current Density 10 -100 -80 -60 -40 -20 0 Voltage (V)

Figure 2-2. J-V curves under reverse bias of GaN SBDs treated by KOH with different concentration.

100

80

60

40

20 Reverse voltage (V)

0 0.0 0.2 0.4 0.6 0.8 1.0 molten KOH concentration (mole/L)

Figure 2-3. Reverse voltages as the function of KOH concentration at the leakage current J=10 A/cm2.

AFM images were taken to inspect the KOH concentration dependent results.

Figure 2-4 (a) shows the AFM image for the control sample as well as the value of the

51 root-mean-square (rms) surface roughness measured over 3um×3um scan areas. A rough surface with numerous etch pits is observed after ICP etching process, implying a large number of nitrogen deficiencies on the GaN surface [12]. Meanwhile, the sample with

0.5mole/L KOH treatment has a much more smooth surface, which surface roughness decreases to 2.03nm as shown in Fig. 2-4 (b). The surface morphology smoothening is resulted from the removal of the ICP damaged region by the KOH etchant. However, the surface roughness increases to 7.42nm in the sample with the molten KOH treatment as shown in Fig. 2-4 (c), which is in agreement with the conclusion that KOH with the high concentration tends to produce a rough surface [13]. One possible explanation in the larger leakage current of samples treated by KOH with high concentration including molten KOH is that intensive etchant is readily to produce more facets on the sample surface. The large number of the surface facets will alter the crystalline orientation and induce interface states locally, resulting in the increase of leakage current [14]. Another explanation might be the difficulty to remove the extra gallium atoms for smoothing the surface in KOH with high concentration due to the reduced composition and dissolution rate of gallium hydroxides in saturation solutions [8]. Thus, the KOH concentration should be specified for getting a smooth surface to recover the ICP etching damage.

52

(a)

(b)

(c)

Figure 2-4. AFM images of (a) the sample without KOH treatment. (b) the sample with 0.5mole/L KOH treatment. (c) the sample with molten KOH treatment.

53 2.4 Surface defect density extraction

In this section, the surface defects induced by the ICP etching process will be quantitatively analyzed. The obtained values of the surface defects will allow us to better understand and evaluate the improvements of KOH treatment to the performance of the

SBDs integrated in the LED epi-wafers.

Firstly, temperature-dependent measurements are performed on the SBDs to

determine the schottky barrier height. In a SBD for the forward voltageV f  kT/ q , the forward current density J can be expressed as [15]

J q V ln( )  ln(A* )  (  f ) (2-1) T 2 kT b n

, where k is the Boltzmann constant, T is the Kelvin temperature, q is the electron charge

* , b is the actual barrier height, A is the Richardson constant, n is the ideality factor.

Figure 2-5 is the forward current-voltage curves of the sample with the 0.5 mole/L KOH treatment over the temperature range of 300K to 420K in the step of 20K. A plot of log

(J/T2) versus 1/T, which is referred as Richardson plot [15], can be obtained by extracting the current density values at Vf =0.3V from Fig. 2-5 at every measurement temperature, as

shown in Fig. 2-6. According to Eq. (2-1), the barrier height b can be given by [15]

2 V f 2.3k d[log( J /T )]    (2-2) b n q d(1/T)

. The ideality factor n can be determined by fitting the J-V curves linearly at various temperatures. n=1.08 is obtained based on Fig. 2-5. The slop for the Richardson plot in

54

Fig. 2-6 is found to be -2.2. Combining with Vf =0.3 V, the Schottky barrier height b

=0.71eV is extracted from the J-V characteristics.

) 2 2 10 420K

1 A/cm

( 10

100

10-1 300K

-2

Current Density 10 0.0 0.2 0.4 0.6 0.8 1.0 Voltage (V)

Figure 2-5. Forward J-V characteristics of the sample treated by 0.5 mole/L KOH solution at various temperatures between 300K and 420K.

1E-9

)

2 K

2 1E-10

A/cm

(

2

J/T 1E-11 V=0.3V

2.4 2.6 2.8 3.0 3.2 3.4 -1 1000/T (K )

Figure 2-6. Richardson plot of the sample treated by 0.5 mole/L KOH solution.

55 Next, the numerical value of the surface defect density will be extracted from the leakage current model based on the tunneling mechanism. It has been proposed that the donor-like surface defects could sharpen the potential profile near the semiconductor surface, leading to the formation of thin surface barrier (TSB) region, as shown in Fig. 2-

7 [16]. The increase probability of electron tunneling leads to that tunneling current is considered as the dominant mechanism under the leakage current of the SBDs [17].

Figure 2-7. Thin surface barrier (TSB) model.

56

Figure 2-8 is a energy band diagram for a SBD with a reverse bias Vr. Given

Vr  kT/ q and most of the reverse current is the tunneling current, the reverse current density Jr can be approximated as [18]

A*T q(Vb ) J  F T()(1 F (V ))d (2-3) r  m s r k 0

, where q is the image force barrier lowering, Vb is the built-in potential at the

reverse bias Vr , Fm and Fs are the electron Fermi-Dirac distribution functions for the metal and the semiconductor respectively, η is the electron energy below the effective barrier, and T() is the electron tunneling probability. Based on the Wentzel-Kramers-

Brillouin approximation, can be expressed as [17]

2m* x2 T()  exp[2  q(x)  Edx] (2-4)  x1

, where (x) is the surface potential profile, E is the electron energy, and x1 and x2 are the classical turning points.

Assuming a triangular Schottky potential barrier profile which is [19]

q(x)  qx  qb0 (2-5)

, where qb0 is the barrier height in the ideal condition. The electron tunneling probability can be given by

3 4 2m* (q  E) 2 T(E)  exp[ b0 ] (2-6) 3  q

, where  is the constant electric field near the top of the effective barrier, and E is electron energy.

57 Since almost all electrons tunnel through the top of the Schottky barrier, we can assume Fs≈0. Meanwhile, Fm can be given by [19]

 E F  exp( ) (2-7) m kT

Thus, the reverse current density, from semiconductor to metal, can be calculated by

Equations (3), (6), and (7) to be [19]

3 A*T q(b0 ) 4 2m* (q    E) 2 E J  exp[ b0  ]dE (2-8) r k  3  q kT (Vr Vn )

It has been reported that the surface defects should have an exponential decaying distribution, which is [20]

Nds  N0 exp(x/) (2-9)

, where Nds is the defect density along depth into the semiconductor, N0 is the defect density at the surface, x is the depth from the surface, and λ is the decay constant.

The surface defects will influence the leakage current through changing the electric field near the top of the effective barrier  and the image force barrier lowering due to

q   [N0 (1 exp(D/ ))  Nd (W  D)] (2-10)  s

q   (2-11) 4 s

, where D is the defect region thickness as shown in Fig. 2-7, Nd is the doping

concentration of the bulk GaN layer, and  s is the permittivity of GaN. The change of the

surface defect density will render the variation of both the barrier height qb0 and the

58 constant electric field  , leading to the increase or reduction of leakage currents ultimately.

Figure 2-8. The band diagram of a metal-GaN contact under reverse bias, where Ef is the semiconductor quasi-Fermi level, Efm is the metal Fermi level, Ec is the conduction band bottom, η is the electron energy from the barrier top, E is the electron energy from the conduction band bottom, Vr is applied reverse voltage, qb0 is the barrier height, and q is the image force barrier lowering.

Figure 2-9 (a) shows the measured and fitted J-V curves of the GaN SBDs with

0.5mole/L KOH treatment. A*=26 Acm-2K-2 is used in the calculation. The good fitting is

17 -3 16 -3 achieved by N0 =8×10 cm , λ=15nm, and Nd =2×10 cm . Following the same procedure, the defect density of the control sample is also determined. As shown in Fig.

18 -3 16 -3 2-9 (b), N0 =2×10 cm , λ=15nm, and Nd =2×10 cm are used in the fitting. The extracted values of the surface defect density indicate that 0.5mole/L KOH treatment can eliminate 60% of the surface defects produced by ICP etching process and thus reduce the leakage current of of the deep ICP-etched GaN SBDs.

59

(a)

(b)

Figure 2-9. J-V curves for the GaN SBDs (a) treated 0.5 mole/L KOH solution (b) without KOH treatment. The solid lines represent the calculated results.

60 2.5 High performance of Schottky barrier diodes

Through the optimized KOH treatment process, the breakdown voltages of the

SBDs fabricated on LED epi-wafers are in the range between 170V and 200V, larger than 200V (the limit of our test equipment) for some devices. Figure 2-10 shows typical

J-V characteristics of the fabricated SBDs. The average reverse voltage Vr for the current of -100uA in our devices is -170V, around 14 times the ones from the SBDs also fabricated on LED epi-wafers but without KOH treatment for AC-LED rectifiers [1] and for electrostatic discharge protection [21]. Meanwhile, the average forward on-state voltage Vf for the current of 20mA in our devices is 3.1 V, comparable to the references, which are 1.8 V and 2.8 V respectively. The comparison results are shown in Table 2-1.

Table 2-1.Compared results with other references [1, 21].

Vr (V) Vf (V) Reference -11 1.8 1 -12 2.8 21 -170 3.1 This work

Moreover, the leakage current of our devices keeps below 1 uA at the reverse bias of 100V, indicating high voltage blocking capability. The high breakdown voltages can reduce the number of SBDs integrated in the system, minimizing the size of integration chips and the power loss.

61

103

) 2

101 A/cm ( 10-1

10-3

10-5

10-7

-9 Current Density 10 0 1 2 3 4 Voltage (V)

(a)

103

) 2

101

A/cm (

10-1

10-3 Current Density 10-5 -200 -150 -100 -50 0 Voltage (V)

(b)

Figure 2-10. Typical (a) forward (b) reverse J-V curves of the fabricated SBDs.

62 SBDs are successfully integrated with LEDs on the same LED epi-wafers and monolithic GaN SBD-based alternating current (AC) LEDs are fabricated to operate under AC 110V. Figure 2-11 shows the optical microscope images of AC LEDs monolithically integrated SBDs on commercial LED epi-wafers.

300 μm (a)

300 μm (b)

Figure 2-11. Microscope pictures of (a) top emission AC LED with SBDs (adapted from reference [22]). (b) flip chip AC LED with SBDs (adapted from reference [23]).

63 2.6 Conclusion

In summary, GaN SBDs with high breakdown voltage (>200V) and small on-state voltage (~3.1 V at 100A/cm2) are successfully fabricated on GaN LED epi-wafers. The effect of KOH with different solution concentration in remving the surface defects that are induced by ICP etching process has been investigated. The results indicate that the leakage current of SBDs is suppressed by four orders of magnitude after employing the

KOH solution with optimized concentration. Moreover, the defect density values by leakage current mechanism analysis have been extracted. Lastly, the integration of GaN

LEDs and SBDs in the AC LED applications has been demonstrated.

64 Reference

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high-electron-mobility transistors for light-emitting power integrated circuits in GaN

on sapphire substrate, Applied Physics Letters, 102 (2013).

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AlGaN/GaN driving transistors,Appl. Phys. Lett. 104, 091103 (2014).

4. F.G. Kalaitzakis, E. Iliopoulos, c, G. Konstantinidis, N.T. Pelekanosa, Monolithic

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applications, Microelectronic Engineering Volume 90, February 2012, Pages 33–36.

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GaN epitaxial structures after reactive ion etching”, Opt. Eng. 42(10), 2918-2922

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coupled-plasma-induced damage on n-type GaN using deep-level transient

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GaN and AlGaN Schottky interfaces," Appl. Phys. Lett. 84, 4884 (2004).

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213–218.

21. S. J. Chang, C. H. Chen, Y. K. Su, J. K. Sheu, W. C. Lai, J. M. Tsai, C. H. Liu, and S.

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MAR. 2003.

22. Jie Liu, COLLOIDAL QUANTUM DOT-ENABLING AND ALTERNATING

CURRENT DRIVEN GALLIUM NITRIDE BASED LIGHT EMITTING DIODE

TECHNOLOGY, the Pennsylvania State University, 2013.

67 23. Zhenyu Jiang, LIGHT EMITTING DIODES AND PHOTODETECTORS BASED

ON III-NITRIDE AND COLLOIDAL QUANTUM DOT MATERIALS, the

Pennsylvania State University, 2014.

68 Chapter 3

A study of the conductive interface region between gallium nitride and patterned sapphire substrate

In Chapter 2, GaN SBDs with high voltage blocking capability and low on-state voltage which are fabricated on commercial LED epi-wafers have been demonstrated.

2 The average forward voltage Vf at the current of 100 A/cm is 1.6 V, and the leakage current is lower than 10 mA/cm2 at 100V reverse voltage. The study in Chapter 2 have revealed that the leakage current is reduced by applying the mixed etching process to decrease the damaged surface region, while this study aims to understand the mechanism under the low on-state voltage in SBDs.

3.1 Introduction

The technology of Gallium nitride (GaN)-based devices system has earned a significant place in optical and electronic applications for decades. Recently, the interests in monolithic integration of GaN optical devices and power electronic devices is growing, and monolithically integrated GaN-based light emitting diodes (LEDs) with schottky barrier diodes (SBDs) or high electron mobility transistors (HEMTs) have been demonstrated and proposed for AC-LEDs, smart light communication system, sensing, and other applications [1-4]. In a monolithic LED system, the dissipated power of power electronic devices should be minimized to improve the efficiency of the whole LED system, hence low-voltage bias is highly desirable when running the power electronic devices that are integrated on LED epi-wafers.

69 Traditionally, single polished flat sapphire substrate (FSS) is the most commonly used substrate for GaN heteroepitaxial growth [5]. However, GaN LEDs on FSS have a low light extraction efficiency [6]. Recently, patterned sapphire substrates (PSS) have been applied to the epi-layer growth GaN LEDs to increase the light extraction efficiency

[7]. It is well known that a significant number of threading dislocations can be observed in the interfacial region due to the lattice mismatches between GaN and sapphire [8].

These dislocations decorated by impurities can form a highly conductive current path and potentially affect the lateral current transport in the fabricated devices [9]. Compared with the GaN and FSS interface, a higher dislocation density has been observed in the

GaN and PSS interface due to the involvement of separated GaN islands at the initial epitaxial growth stage [10]. Thus, it is envisioned that the interface region between GaN epilayer and PSS is expected to have much higher conductance than flat sapphire substrate due to the rough patterned interface.

In this work, we show by comparing the current-voltage characteristics of the

GaN films grown on PSS and FSS that the conductance of the interface region in PSS samples is higher than that in FSS, and suggest that the carriers in the GaN and sapphire interface region are oxygen or nitrogen vacancies by calculating the dopant ionization energies. A two-layer model is used to quantify the donor concentration. Furthermore, the performance of GaN schottky diodes with different donor concentration in the interface region is investigated by Sentaurus TCAD tools. Finally, schottky diodes with low on- state voltages, low leakage currents, and high breakdown voltages are successfully fabricated on commercial GaN LED epi-wafers with PSS.

70 3.2 Experimental

17 -3 Unintentionally doped GaN films (Nd < 1×10 cm ) with the thickness around

2um were grown by one run on PSS and on FSS in a metal organic chemical vapor deposition (MOCVD) system. The sapphire patterns have the dome shape with height of

1.5 um and bottom diameter of 2.5 um. Rectangular transfer length measurement (TLM) patterns, consisting of six 80um ×250um contacts with spacing of 4, 8, 12, 16, 20um, were fabricated.

SBDs are fabricated on 2-inch commercial GaN LED PSS epi-wafers. The epitaxial structure comprises by a 200 nm p-type GaN layer, a 50nm multiple quantum well (MQW) layer, a 2.4 um n-type GaN layer and a 2.5 um unintentionally doped GaN layer. The fabricated SBDs have a mesa structure formed by Cl2/Ar (20sccm/5sccm) inductively coupled plasma cyclic etching (650W source power, 50W rf chuck power).

After the etching process, the samples were immersed in KOH solution to remove ICP induced surface defects. Schottky contacts with the contact area of 2.5×10-5 cm-2 are formed by the deposition of Ni/Al/Ti/Au (50nm/500nm/100nm/200nm) and ohmic contacts were formed by Ti/Al/Ti/Au (10nm/40nm/40nm/200nm). The contact metals

o were deposited by liftoff e-beam evaporation and annealed at 700 C for 2min under N2 ambient.

I-V characteristics were measured by Keithley 2612 source meter. A low- temperature probe station system (including a DE-204 probes from Advanced Research

Systems, an ARS-4HW water-cooled helium compressor from Advanced Research

Systems, and a 336 temperature controller from Lake Shore) were used for the

71 temperature-dependent measurement. The numerical analysis of GaN SBDs with the existence of a parallel current path at the bottom of the GaN in the present study was performed with Synopsys TCAD tools.

3.3 Results and Discussion

Figure 3-1 shows the resistance measurement results in varying temperatures.

Four-point resistance between two ohmic contacts with the distance of 400um on both

PSS and FSS samples are measured from 290k to 70k. The contact resistances were removed from the total resistances by transmission line method (TLM) measurements. It can be concluded from the ln(R)-T curves that the conductance of PSS samples is more than 5times than the one of FSS sample at room temperature, implying higher on-state current can be obtained on PSS samples.

In this study, based on the relationship between the resistance R and the carrier activation energy ΔE at low temperature [11],

E RTR( ) exp( ) (1) 0 2kT

, we obtained the donor activation energies, which were 30meV and 29meV for PSS and

FSS samples respectively, from the slope of the ln(R)-T curve as shown in Fig. 3-1. Since the ionization energies of both oxygen and nitrogen vacancies are reported as 29meV

[12], 29.3meV [11], and 30meV [13], our results suggest that oxygen and/or nitrogen vacancies are the possible physical origin of the GaN and sapphire interface carriers.

72

Figure 3-1. Temperature-dependent measurements for PSS sample and FSS sample.

A two-layer model [14] as shown in Figure 3-2 (a) was used for quantitatively analyze the interface region. A heavy doped interface region is added between the substrate and the GaN epilayer. The total resistance consists of the contact resistances Rc and the channel resistance Rch. The channel resistance can be calculated by the resistance of the GaN region paralleling with the resistance of the interface region. By using [15]

1 Lch Rchi  (3-2) qni i Wchti

, where i=1,2 for GaN region and interface region respectively, Lch is the length of the conductive channel, Wch is the width of the conductive channel, ni is the average carrier concentration, ui is the average mobility, and ti is the thickness of the region.

73

Wch Lch t1 Rc Rc Rch1 GaN t2 Interface Rch2

Sapphire

(a)

(b)

Figure 3-2. (a) Schematic of series resistances between two ohmic contacts on a GaN film. (b) Typical measured I-V characteristics between two ohmic contacts with the distance of 400um on the GaN samples with PSS and FSS respectively.

Figure 3-2(b) shows the typical I-V characteristics between two ohmic contacts with the distance of 400um on the GaN samples with PSS and FSS respectively. After

74 subtracting the contact resistances, the channel resistance Rch=75 ohm for PSS-based samples and Rch=410 ohm for FSS-based samples were obtained. In both PSS and FSS samples, Lch =400 um, Wch =230 um, other parameters can be extracted from the curve fitting by incorporating the condition of t1+t2=2 um and 0 um ≤ t2 ≤ 0.7 um [11] and using the GaN doping dependence model [16]

945   55  (3-3) i n 1 i 21017

, good fittings for IV curves of ohmic contacts with different spaces on PSS samples were

16 -3 20 -3 achieved using following parameters t1=1.5um, t2=0.5um, n1=3×10 cm , n2=1×10 cm .

16 -3 18 -3 The parameters for FSS samples are t1=1.5um, t2=0.5um, n1=3×10 cm , n2=8×10 cm .

The increase of the carrier concentration in PSS samples could be explained by the rough interface which has a larger contact area for GaN epilayers and sapphire, increasing the number and the possibility of the oxygen atoms diffusing from the substrate.

Simulation models are built to investigate the influence of the carrier concentration in the interface region on the behavior of GaN schottky diodes. The device model consists by an 1.5 um-thick GaN film with the conductivity of 4Ω-1cm-1 and a 0.5 um-thick interface layer with alterable conductivity. The schottky barrier height is set to be 1 eV for Ni/GaN metal contact [17]. The doping density in ohmic contact is 1×1020 cm-3. Figure 3-3 shows the simulated I-V characteristics of GaN schottky diodes with three different conductivities in the interface region. The results confirm that the conductive interface region could benefit the on-state characteristics of schottky rectifiers without deteriorating their performance under reverse biases.

75

Figure 3-3. Simulated forward I-V characteristics of GaN schottky diodes with three different conductivities in the interface region. The inset is the reverse I-V curves.

Figure 3-4 shows typical three I-V curves of the SBDs fabricated on GaN LED epi-wafers with PSS. The average forward voltage Vf at the current of 20 mA is 1.2 V, corresponding to a low dissipated power integrated in a system. Meanwhile, the leakage current keeps below 1uA at the reverse bias of 100V and does not exceed -100uA at -

150V. With the absence of interface region, the calculated on-state forward current (the dash line) is much lower than our experimental results, suggesting that the high forward currents in our fabricated SBDs should attributed to the highly conductive GaN and sapphire interface region.

76

Figure 3-4. Measured I-V curves of the SBDs fabricated on GaN LED wafers with PSS. The inset is the structure of the SBDs.

3.4 Conclusion

We confirm that the conductance of the interface region in PSS sample is higher than in FSS. A two-layer model is used to quantify the interface conductivity.

Furthermore, the performance of GaN schottky diodes with different conductivity in the interface region is investigated by Sentaurus TCAD tools. Finally, schottky diodes with low on-state voltage, low leakage current, and high breakdown voltage are demonstrated by fabricating on commercial GaN LED wafers with PSS.

77 Reference

1. W.-Y. Yeh, H.-H. Yen, Y.-J. Chan, The development of monolithic alternating

current light-emitting diode, (2011) 793910-793910.

2. Z. Li, J. Waldron, T. Detchprohm, C. Wetzel, R.F. Karlicek, T.P. Chow, Monolithic

integration of light-emitting diodes and power metal-oxide-semiconductor channel

high-electron-mobility transistors for light-emitting power integrated circuits in GaN

on sapphire substrate, Applied Physics Letters, 102 (2013).

3. Zhaojun Liu1, Jun Ma1, Tongde Huang1, Chao Liu1 and Kei May Lau, Selective

epitaxial growth of monolithically integrated GaN-based light emitting diodes with

AlGaN/GaN driving transistors,Appl. Phys. Lett. 104, 091103 (2014).

4. F.G. Kalaitzakis, E. Iliopoulos, c, G. Konstantinidis, N.T. Pelekanosa, Monolithic

integration of nitride-based transistor with Light Emitting Diode for sensing

applications, Microelectronic Engineering Volume 90, February 2012, Pages 33–36.

5. C.I.H. Ashby, C.C. Mitchell, J. Han, N.A. Missert, P.P. Provencio, D.M.

Follstaedt,G.M. Peake, L. Griego, Appl. Phys. Lett. 77 (2000) 3233.

6. Xiao-Hui Huang, Jian-Ping Liu, Ya-Ying Fan, Jun-Jie Kong, Hui Yang, and Huai-

Bing Wang,Effect of Patterned Sapphire Substrate Shape on Light Output Power of

GaN-Based LEDs, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 23, NO.

14, JULY 15, 2011.

7. Hyoungwon Park, Kyeong-Jae Byeon, Jong-Jin Jang, Okhyun Nam, Heon Lee,

Enhancement of photo- and electro-luminescence of GaN-based LED structure grown

78 on a nanometer-scaled patterned sapphire substrate, Volume 88, Issue 11, November

2011, Pages 3207–3213.

8. Hui-Youn Shin, Y. I. Chang, S. K. Kwon, K. T. Lee, M. J. Cho and K. H. Park The

Growth Characteristics of a GaN Layer on a Cone-Shaped Patterned Sapphire

Substrate by TEM Observation, Journal of the Korean Physical Society, Vol. 50, No.

4, April 2007, pp. 1147∼1151.

9. Sun, X.L., Goss, S.H., Brillson, L.J., Look, D.C., Molnar, R.J., Depth-dependent

investigation of defects and impurity doping in GaN/sapphire using scanning electron

microscopy and cathodoluminescence spectroscopy, JOURNAL OF APPLIED

PHYSICS VOLUME 91, NUMBER 10 15 MAY 2002.

10. Tae Su Oh, Hyun Jeong, Yong Seok Lee, Tae Hoon Seo, Ah Hyun Park, Hun Kim,

Kang Jea Lee, Mun Seok Jeong c, Eun-Kyung Suh, Defect structure originating from

threading dislocations within the GaN film grown on a convex patterned sapphire

substrate , Thin Solid Films 519 (2011) 2398–2401.

11. Abhishek Motayed,Albert V. Davydov, S. N. Mohammad, and John Melngailis,

Experimental investigation of electron transport properties of gallium nitride

nanowires, JOURNAL OF APPLIED PHYSICS 104, 024302 2008.

12. J. Zolper, R. G. Wilson, S. J. Pearton, and R. A. Stall, Appl. Phys. Lett.68, 1945,

1996.

13. M. E. Levinshtein, S. L. Rumyantsev, and M. S. Shur, Properties of Advanced

Semiconductor Materials: GaN, AlN, InN, BN, SiC, SiGe Wiley, New York, USA,

2001, p. 8.

79 14. B.J. Anscll, I,. Harrison, C.T. Foxon, .I.J. Harris and T.S. Cheng, Direct evidence for

defect conduction at interface between gallium nitride and sapphire, ELECTRONICS

LETTERS 6th July2000 Vol. 36 No. 14.

15. Dieter K. Schroder, Semiconductor Material and Device Characterization, John Wiley

& Sons, Feb 10, 2006.

16. Tigran T. Mnatsakanov, Michael E. Levinshtein, Lubov I. Pomortseva, Sergey N.

Yurkov, Grigory S. Simin, M. Asif Khan, Carrier mobility model for GaN, Solid-

State Electronics 47 (2003) 111–115.

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Chapter 4

Investigation of drain current saturation feature in AlGaN/GaN high electron mobility transistors

In this chapter, gate bias dependence of drain current saturation in AlGaN/GaN

HEMTs was studied. A series resistance model was used to explain this saturation and device simulations were carried out to confirm the explaination. As gate bias increases, the drain current of AlGaN/GaN HEMTs tends to be saturated. In addition, the drain current of the devices with shorter gate length saturates more quickly than the devices with longer gate length. The influence of this saturation feature on the transconductance and the extracted value of electron mobility was also analyzed. Moreover, we evaluated the possibility of using AlGaN/GaN HEMTs in logic inverter applications based on the saturation feature.

4.1 Introduction

Recently, GaN-based HEMTs have been studied substantially as promising candidates in high-power microwave applications [1-3]. The static behavior of a transistor could be evaluated by its DC output and transfer characteristics. Some crucial parameters including transconductance and low field electron mobility could be extracted from transfer characteristics to evaluate the device performance. It is shown in our own measurement results of GaN-based HEMTs and also reported in other studies [3, 4] that the drain current has a tendency of saturation in the case of gate bias larger than threshold

81 voltage. Additionally, this saturation characteristic depends on the gate length of the device. The drain current ID of samples with shorter gates achieve saturation more quickly with the increase of the gate-source voltage VGS than the samples with longer gates have been observed. It is also shown that the value of the extracted electron mobility based on the ID-VGS curves decreases quickly when the gate-source voltage approaching zero. This value is much lower than the theoretical calculations and other experimental results [9-11]. Therefore, it is necessarily to comprehensively investigate the transfer characteristics (ID-VGS) of GaN-based HEMTs.

In this work, the reason for the saturation of drain current related to the gate voltage was analyzed based on a simple resistance model. The influence of the transfer saturation feature on the mobility value extraction in nitride HEMTs was examined.

Moreover, this series resistance model was validated by device simulations by using

SENTAURUS [5] TCAD tools. At last, the possibility of AlGaN/GaN HEMTs with short gate length as a logic inverter is discussed.

4.2 Experimental methods

The HEMT structures, consisting of an unintentionally doped GaN buffer layer, a

5 nm-thick AlN spacer layer, and a 20 nm-thick unintentionally doped AlGaN barrier layer, were grown on SiC substrates based on metal organic chemical vapor deposition

(MOCVD). The fabricated devices have the gate length of 0.25 um or 1 um and the gate width of 150 um. The source to drain spacing is 2 um. The e-beam evaporated

Ti/Al/Ti/Au (30 nm/100 nm /30 nm /30 nm) ohmic contact metallization was deposited

82 as source and drain, and Ni/Au (100 nm/100 nm) was chosen as the gate metal. Figure 3-

1 shows the fabricated devices. The transmission line model (TLM) structures were also patterned, as shown in Fig. 3-2.

150 μm

Figure 4-1. Fabricated AlGaN/GaN HEMTs structures.

150 μm

Figure 4-2. Photo of TLM structure.

The measurements of the electrical characteristics for the devices and TLM patterns were carried out in a light and electric field screening chamber by using Keithley

2612 source meter. The capacitance measurements were performed by using HP4284A precision LCR meter.

83 4.3 Results and Discussion

Figure 3-3 shows the measured results as well as simulation results of transfer (ID-

VGS) characteristics for both devices with the gate length of 0.25 μm and 1 μm under the drain-source voltage of 0.1 V. The mobility of 1000-1100 cm2/Vs was used in the fitting and by adjusting the 2DEG density the correct threshold voltages for the devices with the gate length of 0.25 μm and 0.1 μm.

0.005 ID for Lg=0.25 m

ID for Lg=1.0 m

0.004 simulation

(A) D

I 0.003

0.002

0.001 Drain current 0.000

-5 -4 -3 -2 -1 0 1

Gate votage VGS (V)

Figure 4-3. Measured and simulated ID-VGS curves of AlGaN/GaN HEMTs.

The drain current of the device the gate length of 0.25 um tends to saturate at the gate bias higher than -3.4 V, which is the threshold voltage. However, the 1 um gate-length device does not have the obvious saturation feature. The gate leakage currents for both

0.25 um gate-length and 1 um gate-length devices are shown in Fig. 3-4.

84

0.005

IG for Lg=0.25 m

(A) 0.004 IG for Lg=1.0 m G

0.003

0.002

0.001

0.000 Gate leakage current I -5 -4 -3 -2 -1 0 1

Gate votage VGS (V)

Figure 4-4. Measured IG-VGS curves of AlGaN/GaN HEMTs.

On one hand, by comparing the gate leakage current and the drain current, it can be conclude that the gate leakage current is not resposible for this saturation characteristic due to its negligible value in this gate voltage range. On the other hand, the fixed interface charge is considered to be one of the possible reasons that cause the 2DEG density varies from device to device, resulting in the discrepancy in the threshold voltages of the samples with 0.25 um and 1.0 um gate length. Nevertheless, this variation of the 2DEG density is not responsible for the observed drain current saturation. It also can be seen that the reduction of the drain current at the gate voltage larger than 0.5V

(VGS>0.5 V). The rapid increase of gate leakage under positive gate voltage should be the reason that causes the drop in drain current [11, 12].

To investigate this saturation feature, the gate-source capacitance CGS is measured versus gate voltage VGS. The C-V curves for both 0.25 um gate-length and 1 um gate- length devices are shown in Fig. 3-5. It is reported that in the range of the gate-source

85 voltage VGS within which the gate-source capacitance is almost independent of the gate-

source voltage, the channel 2DEG density nch can be described as [6]

C n  GS (V V ) (3-1) ch q GS th where Vth is the threshold voltage, q is the electron charge, and CGS is the gate-source

capacitance. Then, the channel electron mobility ch could be extracted from

Gch Lg ch  (3-2) qnchW

, where Lg is the gate length, W is the gate width, Gch is the channel conductance. The electron mobility versus the 2DEG sheet density for the 0.25 um gate-length sample is shown in Fig. 3-6. This mobility value is significant lower than the values obtained from

Hall measurements and gated TLM measurements. This inconsistency is not in agreement with theoretical predictions and other measurement results [8-10].

Lg=1 m 4.0x10-7 Lg=0.25 m

3.0x10-7

) 2

2.0x10-7

F/cm

(

GS

1.0x10-7 C

0.0

-4 -3 -2 -1 0

VGS (V)

Figure 4-5. Gate-source capacitance as the function of gate voltage.

86

1000

/Vs)

2

500 Mobility (cm

5.00E+012 1.00E+013 -2 2DEG density (cm )

Figure 4-6. The electron mobility versus 2DEG density for the 0.25 um gate-length sample.

The saturation transfer characteristics are investigated using a resistance model to explain the saturation feature in this section.

The channel current (Ich) under the gate can be given by

Ich  qnchvch (3-3)

, where vch is the electron velocity. Under VDS = 0.1 V, the lateral electric filed (E) in channel under the gate region is far below the value that induce the electron velocity saturation, which is ~200 kV/cm [13]. Therefore, in this linear region the electron velocity should be proportional to the lateral electric field E in channel, given by

vch=μchE (3-4)

. Then, the drain current Ids is

I DS  qnch ch (3-5)

87

S Lsg G Ldg D L g AlGaN E

RC RS Rch RD RC

Vch GaN

Figure 4-7. Illustration of the series resistance in a GaN HEMT.

In the Fig. 3-7, Vch repsents the channel voltage and E repsents the channel lateral electric field. Lg, Lsg, and Ldg represent the gate length, source to gate space and drain to gate space respectively. Rc, Rs, Rd and Rch are the contact resistance, source to gate resistance, drain to gate resistance, and channel resistance respectively.

The voltage drop Vch along the channel can be expressed as

Rch VVch DS (3-6) 2RRRRc s  d  ch

, where Rc is the contact resistance, Rch is the channel resistance, Rs is the source-gate resistance, and Rd is the drain-gate resistance. Therefore, the lateral electric field E can be calculated by the channel voltage drop Vch and gate length Lg as

V   ch (3-7) Lg

88

The source-gate resistance Rs, drain-gate resistance Rd, and channel resistance Rch can be given respectively by

Lsg Rs  (3-8) qn00

Ldg Rd  (3-9) qn00

and

Lg Rch  (3-10) qnch ch

, where Lsg is the source-gate space, Ldg is the drain-gate space, n0 is the 2DEG density at

zero gate voltage, and 0 is the electron mobility at zero gate voltage. Usually, the partially depleted region under the gate caused by the schottky contact of gate makes the channel 2DEG density nch a little smaller than the 2DEG density at zero gate voltage n0.

Using the equations (3-5) – (3-10), the drain current can be expressed as

VDS 1 IDS  (3-11) RRsd 2RnLg  c 1   00 Rs R d L sg L gd n ch ch

n00 Given that the mobility is not influence by gate bias, the ratio of in equation (7) nch ch will reduce to unity when the gate voltage VGS increases. Therefore, the term of

L n  L g  00could be neglected in the case of g <<1. This confirms that the Lsg L gd n ch ch LLsg gd variation of 2DEG density cannot influence the saturation characteristics of drain current, even though this variation can lead to the difference in the threshold voltages in

89 individual device. Based on above analysis, it can be concluded that the total source to drain resistance Rds is almost determined by the contact resistance Rc, the source to gate resistance Rs, and the drain to gate resistance Rd. The fact that the drain current Ids is almost independent on gate bias indicates that the change of 2DEG density nch with gate voltage under the condition of VGS>Vth does not have large influence on the drain. This is the reason that the drain current approaches saturation at the high gate bias, as shown in

Figure 3-3. This also explains that the device with shorter gate length will be saturate

L n  more readily since the influence of g  00on the drain current is insignificant. Lsg L gd n ch ch

The channel conductance Gch extracted from the IDS-VGS curve for a HEMT with a short gate length will be an almost constant value due to the current saturation. Since

based on the CGS-VGS measurement or the relationship of nch C GS() V GS V th , the extracted channel 2DEG density nch raises with the increase of VGS, the extracted mobility will present an unreal reduction to keep the constant conductance. So, it is better to use long gate HEMTs to extract more accurate electron mobility because of the satisfaction of the condition of Lg ≈(Lsg+Ldg).

Two-dimensional device simulations on AlGaN/GaN HEMTs are carried out using

SENTAURUS device simulation tool to verify above analysis. The simulation structure of the AlGaN/GaN HEMTs includes an Al0.3Ga0.7N barrier layer with the thickness of 25 nm and a GaN layer with the thickness of 0.2 um. As shown in Fig. 3-3, by using the geometric structure parameters of the real devices, which are 2 um source-drain space,

0.25 um gate length, and 1 um gate length, the simulated ID-VGS curves coincide with the experimental data.

90 A 4-um source-drain distance, which is larger than the real devices, is used in the simulations for presenting the tendency of drain current saturation feature as a function of gate length. The gate length of the devices changes from 3 um to 0.1 um. The work function of gate contact is set as 5.23 eV [14]. The simulations include the piezoelectric polarization effect and the spontaneous polarization effect. According to the polarization calculations and scaling parameters, the 2DEG sheet density and electron mobility are adjusted to 11013 cm-2 and 1200 cm2/Vs in our simulations. Other material parameters used in our simulation are adopted from the reference [5].

Figure 3-8 shows the simulated transfer characteristics ( IDS-VGS) and Fig. 3-9 shows the relevant channel voltage drop Vch as a function of the gate voltage VGS. It is shown

Fig. 3-5 that the drain currents are more easily to be saturated when the gate length Lg decreases from 3 um to 0.1 um. HEMTs with short gate length under the high gate voltage have a more obvious saturation feature than devices with longer gate lengthes in the range of VGS = -2V~0V. The simulation results are in agreement with the resistance model analysis. On the other hand, the channel voltage drop Vch in the devices with shorter gate length begin to sharp decrease at a lower gate voltage than the devices with longer gate, as shown in Fig. 3-9.

91

0.05

Gate length Lg 0.1m 0.04 0.4m 0.7m 1.0m 0.03 2.0m

3.0m

(A/mm)

0.02

D I

0.01

0.00 -4 -3 -2 -1 0

Gate voltage VGS (V)

Figure 4-8. The drain current IDS as a function of gate voltage VGS at VDS=0.1 V with different gate length.

0.10

0.08

0.06

(V) Gate length Lg

ch 0.1um V 0.04 0.4um 0.7um 1.0um 0.02 2.0um 3.0um

0.00 -4 -3 -2 -1 0

Gate voltage VGS (V)

Figure 4-9. The channel voltage drop Vch as a function of gate voltage VGS at VDS=0.1 V with different gate length.

92 HEMTs with short gate length can be used for logic inverters due to the saturation characteristic of IDS-VGS. It is shown in Fig. 3-9 that the channel voltage drop Vch is in inverse relation with the gate voltage VGS. Therefore, inverters based on HEMTs can be realized by using the gate voltage VGS as input signal and the channel voltage drop Vch as output signal. Additional ohmic contacts can be used as the output signal electrodes and they can be placed closely on each side of the gate contact to form a single HEMT invert.

HEMT-based inverters are especially valuable to fabricate logic inverters on InAs substrate, since it is difficult to form pMOSFETs on InAs substrate. Notwithstanding, in the future it is still necessary and very important to study carefully the transient character and its influence on the proposed HEMT-based inverters.

4.4 Conclusion

In conclusion, this chapter investigated the gate length dependence of transfer characteristics for GaN HEMTs. The drain currents become saturation when the gate bias accesses to zero. A simple series resistance model was proposed to explain this saturation feature. The analysis concludes that the contact resistance Rc, the source to gate resistance

Rs, and the drain to gate resistance Rd are the primary contributions to the total source to drain resistance Rds when the gate bias accesses to zero. Therefore, the drain current in

GaN HEMTs with short gate length do not depend on the gate bias. Large error will take place when electron mobility is extracted from the saturation region of IDS-VGS characteristics. At last, logic inverters based GaN HEMTs with ultra short gate length can be realized by utilizing the saturation feature.

93 Reference

1. U. K. Mishra, L. Shen, T. E.Kazior, and Y.-F.Wu, “GaN-based RF power devices and

amplifiers”, Proc. IEEE 96, 287 (2008).

2. W. Saito, T. Nitta, Y. Kakiuchi, Y. Saito, K. Tsuda, I. Omura, and M. Yamaguchi, “A

120-W boost converter operation using a high-voltage GaN-HEMT,” IEEE Electron

Device Lett. 29, 8 (2008).

3. J. Yang, X. Hu, J. Deng, R. Gaska, M. Shur, and G. Simin, in International

Semiconductor Device Research Symposium (2009), pp. 1–2.

4. S. L. Rumyantsev, N. Pala, M. S. Shur, R. Gaska, M. E. Levinshtein, M. A. Khan, G.

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AlGaN/GaN field effect transistors, Appl. Phys. 88, 6726 (2000).

5. Sentaurus User’s Guide, Synopsys, Inc (2009)

6. M. E. Levinshtein, P. A. Ivanov, M. Asif Khan, G. Simin, J. Zhang, X. Hu, and J.

Yang, Mobility enhancement in AlGaN/GaN metal-oxide-semiconductor

heterostructure field effect transistors, Semicond. Sci. Technol. 18, 666 (2003).

7. M. Marso, J. Bernat, P. Javorka, and P. Kordos, Influence of a carrier supply layer on

carrier density and drift mobility of AlGaN/GaN/SiC high-electron-mobility

transistors, Appl. Phys. Lett. 84, 2928 (2004).

8. B. K. Ridley, B. E. Foutz, and L. F. Eastman, Mobility of electrons in bulk GaN and

AlxGa1-xN/GaN heterostructures, Phys. Rev. B 61, 16862 (2000).

9. R. T€ulek, A. Ilgaz, S. G€okden, A. Teke, M. K. €Ozt€urk, M. Kasap, S.Ozc¸elik, E.

Arslan, and E. Ozbay, Comparison of the transport properties of high quality

94 AlGaN/AlN/GaN and AlInN/AlN/GaN two-dimensional electron gas

heterostructuresJ. Appl. Phys. 105, 013707 (2009).

10. X. Z. Dang, P. M. Asbeck, E. T. Yu, G. J. Sullivan, M. Y. Chen, B. T. McDermott, K.

S. Boutros, and J. M. Redwing, Measurement of drift mobility in AlGaN/GaN

heterostructure field-effect transistor, Appl. Phys. Lett. 74, 3890 (1999).

11. Y. Yin, J. A. Cooper, Jr., P. G. Neudeck, M. L. Balzan, and A. E. Geissberger, Appl.

Phys. Lett. 54, 1884 (1989).

12. B. C. Lai and J. Y.-M. Lee, The Observation of Negative Transconductance Effect

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Effect Transistors Fabricated with Ta2O5 Gate DielectricIEEE Electron Device Lett.

22, 142 (2001).

13. B. Benbakhti, A. Soltani, K. Kalna, M. Rousseau, and J.-C. De Jaeger, Effects of

Self-Heating on Performance Degradation in AlGaN/GaN-Based DevicesIEEE Trans.

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14. S. Karmalkar and U. K. Mishra, IEEE Electron Device Lett. 48, 1515 (2001).

Chapter 5

Modeling the Back Gate Effects of AlGaN/GaN HEMTs

This letter reports the design and simulation of novel AlGaN/GaN double-gate high electron mobility transistors (DG HEMTs) featuring enhanced back gate-control of the two dimensional electron gas (2DEG) in AlGaN/GaN heterostructures. A comparison study was carried out to investigate the performance for both the AlGaN/GaN DG

HEMTs and single-gate devices. The results show that the DG GaN-HEMTs can potentially offer a higher transconductance gain and better immunity of the short channel effects of drain induced barrier lowering (DIBL) and subthreshold swing (SS) than traditional single-gate HEMTs.

5.1 Introduction

In recent years, the technology of nitride high electron mobility transistors

(HEMTs) has advanced substantially, and is still under intense investigation for improved performance in power and frequency. The gate length (Lg) of the devices has been reduced to the nanometer dimension for enhanced operation frequency [1-3]. However, the scaled devices have to confront the increasingly significant short-channel effects, one of the primary reasons that prevent devices from achieving higher frequency operation[4-

5]. In addition, the strong polarization fields in GaN-based material system result in the formation of the virtual gate, weakening the gate control to the channel current [6].

96 Moreover, while metal-oxide-semiconductor HEMTs (MOSHEMTs) have been proposed and widely studied to reduce the leakage current of the HEMTs, it has to be achieved at the expense of reduced transconductance [7].

To address the above challenges in nitride HEMTs, we propose the design of

AlGaN/GaN double-gate HEMTs (DG HEMTs) by placing a back Schottky gate underneath the two-dimensional-electron-gas (2DEG) channel of the AlGaN/GaN heterostructure. While field effect transistors (FETs) employing back gate design have been demonstrated with silicon MOSFETs [8-9], AlInAs/InGaAs modulation-doped

FETs [10], and nitride MESFETs [11] to reduce drain potential variation, there is yet any report on back gate-configured AlGaN/GaN double-gate HEMTs to date. In the present work, three-dimensional device simulations were used to perform numerical analysis of the AlGaN/GaN DG HEMTs as well as the conventional single-gate HEMTs (SG

HEMTs). The DC characteristics of both DG and SG devices were investigated.

Furthermore, the short channel effects of DG and SG devices were studied by altering the gate length from 2um down to 50nm. Compared to the SG devices, the DG device output is characterized with a higher transconductance gain and exhibits enhanced suppression of the short channel effects.

5.2 Device Structure

Figure 5-1 (a) and (b) shows, respectively, the structures of the AlGaN/GaN DG and SG HEMTs under study. Device heterostructure consists of 30nm-thick of 20%

AlGaN barrier layer with 50nm-thick GaN layer on top of 1um-thick GaN buffer layer.

97 For electrode configuration, both nitride DG and SG HEMTs have a nickel front gate on the top surface of the AlGaN barrier layer, whereas the GaN buffer layer of the DG device has a trench opening for the access to a nickel back gate on the bottom surface of the GaN channel layer. This trench opening could be fabricated by thinning the substrate through conventional grinding and polishing technologies, followed by dry etching technology. The default gate length for both DG and SG devices is 2um, and the source- drain spacing of 1.5um was used for the simulation study.

5.3 Simulation Methodology

The numerical analysis of the nitride HEMTs in the present study was performed with Synopsys TCAD tools, including Sentaurus Structure Editor, Sentaurus Device, and

Sentaurus Visual. The schottky barrier for the gate is set to be 1.27 eV for Ni/Al0.2Ga0.8N front gate metal contact [12], and 0.99 eV for Ni/GaN back gate metal contact [13]. The doping density in Ohmic contact is 1×1020 cm-3. The Shockley-Read-Hall (SRH) carrier recombination model, Stain and Stress polarization model, Canali high filed saturation mobility model, and Arora doping dependence mobility model were used in the simulation study [14].

(a)

98

(b)

Figure 5-1. Structures of (a) single-gate HEMTs. (b) double-gate HEMTs.

4.4 Results and Discussion

The simulation parameters had been varified by comparing the simulation results with the reported measurement results [7], and the calibration curves in linear and log scale are presented in Fig. 5-2 (a) and (b).

500 measurement 400

simulation (mA/mm)

300

ds

200

100 Drain current I -6 -5 -4 -3 -2 -1 0 Gate voltage Vgs (V)

(a)

99

102 (mA/mm) ds 100

-2 10 measurement simulation

Drain Current I 10-4 -5 -4 -3 -2 -1 0

Gate Voltage Vgs (V)

(b)

Figure 5-2. The simulation result compared with the measurement result of Khan et al. [7] in (a) linear scale. (b) log scale.

400 SG DG

Vds = 6V

(mA/mm) 300

Lg = 2m ds

200

100

Drain Current I 0 -5 -4 -3 -2 -1 0 Gate Voltage Vgs (V)

(a)

100

140 SG 120 DG

(mS/mm) 100 Vds = 6V m 80 Lg = 2m 60 40 20 0 -20 -5 -4 -3 -2 -1 0

Transcondunctance g Gate Voltage Vgs (V)

(b)

Figure 5-3. (a)Drain current Ids versus gate voltage Vgs and (b) transconductance gm versus gate voltage Vgs curves of the SG and DG HEMTs at Vds=6V.

Upon the completion of the calibration procedure, the DC characteristics of both

DG and SG HEMTs were simulated and analyzed by the comparison study. Figure 5-3

(a) and (b) shows the transfer characteristics and transconductance (gm) of each device when the gate bias was swept from 0V to -5V and the drain bias fixed at 6V.

The AlGaN/GaN DG HEMT is characterized with a higher peak transconductance of 138mS/mm than that of the SG HEMT (117mS/mm) at zero gate bias. In addition, our study uncovers a significant reduction of gate pinch-off voltage, by more than 40%, in the nitride DG HEMT as compared with the SG device (2.72V versus 4.55V), suggesting that enhanced gate control of 2DEG is achieved in AlGaN/GaN DG HEMTs for switching applications. The simulation also calculates the channel current of the HEMTs

101 under study, yielding a maximum drain current density of 272mA/mm for the DG device.

While this current density is ~30% lower than that of the SG devices, it is expected that the multi-finger design of the HEMT electrodes will enable high current applications of nitride DG HEMTs while retaining the advantages of high transconductance and low pinch off gate voltage.

To obtain a better insight into the observed distinction in the DC characteristics of the DG and SG HEMTs, the dynamic change of the 2DEG density underneath the gate region was calculated with and without bias. The results were plotted for the SG and DG devices, respectively, along with the respective energy band diagram, as shown in Fig. 5-

4. The conduction band in the n-GaN region of the DG device shows more upward bending than the SG device due to the additional carrier depletion induced by the back gate.

6 Vds=0V_Vgs=0V 4 Vds=0V_Vgs=-2V

2

0

-2

Energy Level (eV) -4

0.00 0.02 0.04 0.06 0.08 Depth x (m), (y=0m)

(a)

102

6 Vds=0V_Vgs=0V 4 Vds=0V_Vgs=-2V

2

0

-2

Energy Level (eV) -4

0.00 0.02 0.04 0.06 0.08 Depth x (m), (y=0m)

(b)

Figure 5-4. Energy band diagrams of (a) single-gate HEMTs. (b) double-gate HEMTs.

The resultant narrower 2DEG well width in the DG device further leads to a lower sheet electron density (7.18×1012 cm-2) than the SG device (7.46×1012 cm-2), as shown in

Fig. 5-5. Under negative gate bias, the energy levels of both devices are lifted up, resulting in the reduction of 2DEG density. While the 2DEG density of the SG HEMT decreases by 46% (from 7.46×1012 cm-2 to 4.01 ×1012 cm-2) after biasing the gate at -2V , the electron density of the DG HEMT decreases by 81% (from 7.18×1012 cm-2 to 1.33

×1012 cm-2). It is postulated that the dual gate control of the conductive channel in the DG device accounts for the faster depletion. This, in turn, gives rise to the high transconductance and low pinch off gate voltage in AlGaN/GaN DG HEMTs.

103

)

-3 8 SG

cm DG

6

19

10 ( 4

2

0

0.020 0.025 0.030 0.035 0.040 Electron Density Depth x (m), (y=0m)

Figure 5-5. Electron distributions for both single-gate HEMTs and double-gate HEMTs.

Next, device models with various gate lengths were established to investigate the short channel effects of both DG and SG devices. Two of the most common short channel effects are drain induced barrier lowing (DIBL) and subthreshold swing (SS). The minimum gate length determined by these two effects limits the further increase of the gain and output power, which are important parameters for nitride power HEMTs [15].

DIBL is conventionally described as the ratio of the threshold voltage difference to the drain bias voltage difference (ΔVth/ΔVds), which remains effective in nitride HEMTs

[16]. In this study, the DIBL was calculated at a drain current of 1mA/mm threshold voltages for Vds = 0.5 V and Vds = 3.5 V. The variation of the DIBL with gate length for both SG and DG devices are plotted in Fig. 5-6 (a).

The DIBL is less than 50mV/V in both SG and DG devices with the gate length above 200nm. Nevertherless, the DIBL of the SG device begins to increase rapidly and

104 exceeds 180mV/V at the gate length of 50nm, whereas the DIBL of the DG nitride

HEMT stays below 100mV/V as the gate length decreases below 200nm. Figure 5-6 (b) shows the representative transfer characteristics of the SG and DG devices (Lg= 50nm) at

Vds=0.5V (dash) and Vds=3.5V (solid), respectively. It can be seen that the shift of the drain current is significantly reduced in the DG device.

The SS [17] was also found to become relieved in the DG device. Figure 5-7 shows SS, calculated at the drain bias of 2.5V, as the function of the gate length for both

SG and DG devices. It is evident that the SS magnitude of the DG device remains ~40% lower than that of the SG device under study. This effectively reduces the leakage current of the nitride DG HEMTs in the off-state.

250

200 SG DG 150

100

DIBL (mV/V) 50

0

-1 0 Drain induced barrier lowing 10 10 Gate length Lg (m)

(a)

105

102 Lg=50nm

0

10 DG (mA/mm) ds SG 10-2

10-4 Vds=3.5V Vds=0.5V -6

Drain Current I 10 -8 -6 -4 -2 0

Gate Voltage Vgs (V)

(b)

Figure 5-6. (a) Drain induced barrier lowering, DIBL, versus device gate length. (b) Ids- Vgs characteristics at Vds=0.5V and 3.5V of the SG and DG HEMT with the gate length of 50nm.

250 SG 200 DG

150 SS (mV/dec)

100 Subthreshold swing

50 10-1 100 Gate length Lg (m)

Figure 5-7. Subthreshold swing, SS, versus device gate length at Vds=2.5V.

106 The observed reduction of short channel effects in the DG device is consistent with our aforementioned model analysis. Since the channel potential is controlled by both gate bias and drain bias in a HEMT structure, the occurrence of DIBL and SS can be attributed to the reinforced drain influence in the short channel devices and subsequently the diminished gate control over the channel potential. The enhanced gate control capability of nitride DG HEMTs, resulting from the effective depletion of channel electrons via dual gate architecture, weakens the drain influence in the DG device, and thus suppresses the short channel effects. The suppressed drain bias effect on the channel potential can be observed directly from Fig. 5-8, which shows the channel potential of each device under two different drain biases. The gate bias is set to be -5.2V for the SG device and -3.5V for the DG device to pinch off both devices. It is evident that the channel potential in the DG device does not increase as much as the one in the SG device.

The lower channel potential, in turn, offers the DG device more immunity to the short channel effects than the SG device.

107

3.0 SG_Vgs= -3.5V DG_Vgs= -5.2V 2.5 Lg=50nm V = 2.5V 2.0 ds

1.5 Channel potential (V) Vds = 0V 1.0 -0.02 -0.01 0.00 0.01 0.02 Distance, y (m) (x=0.031m)

Figure 5-8. The channel potential variation along the channel of the SG and DG HEMT with the gate length of 50nm.

5.5 Conclusion

In this work, a simulation study of DG AlGaN/GaN HEMTs by comparing their

DC characteristics and short-channel effects with those of SG AlGaN/GaN HEMTs have been conducted. It is shown that the DG devices exhibit a higher transconductance and, at the same time, effectively suppress the short channel effects in terms of DIBL and SS due to the enhanced channel control with the dual gate design. Our results indicate that DG

AlGaN/GaN HEMTs are promising candidates for applications in highly scaled circuits.

108 Reference

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Chapter 6

Temperature dependence of AlInN/GaN HEMTs electrical characteristics

In this chapter, the electrical properties of AlInN/GaN HEMTs from room temperature down to 50K have been investigated. The results show that the reduction of the device operating temperature leads to the enhancement of the drain saturation current and transconductance. Over the entire temperature range, a self-heating effect when the power consumption is high has been observed. Moreover, the relationship between electron density and channel electron mobility is studied in detail. It is concluded that in the presence of high negative gate bias, electrons at the AlInN/GaN interface will be pushed into the bulk GaN buffer layer, and these electrons combined with Coulomb scattering causes the electron mobility reduction at high reverse gate voltage.

6.1 Introduction

AlInN/GaN high electron mobility transistors (HEMTs) have been a new favorite for high-power radio frequency (RF) applications recently [1-5]. The sheet charge density of two-dimensional electron gas (2DEG) induced is much higher in AlInN/GaN HEMTs than AlGaN/GaN HEMTs [6]. Meanwhile, the lattice-mached AlInN/GaN heterostructure leads to the elimination of stress and piezopolarization effect and have enormous pontential in the improvement of heterostructure stability and the reduction or disappearance of the current collapse related to the surface states [7-9]. Considerable

112 achievements have been obtained as the improvements of material quality and device fabrication techniques [1, 3].

GaN-based HEMTs are expected to have advanced performance under cryogenic operation temperatures due to the reduction of self-heating and scattering in 2DEG.

Cryogenic electrical characteristics of AlGaN/GaN HEMTs are investigated by several research works, revealing that the more remarkable “current-kink” effect in lower temperatures and the enhancement of cutoff frequency and transconductance under cryogenic temperatures [10, 11]. However, to date most reports involve the investigation of GaN-based HEMTs in the cryogenic temperature range are based on the studies of

AlGaN/GaN HEMTs, few reports about the performance of AlInN/GaN HEMTs is carried out.

In this study, the detail cryogenic properties of AlInN/GaN HEMTs are investigated by both DC and pulse measurements over the temperature range of 50K ~

295K. The C-V measurement results of gate capacitance and gate voltage combined with the channel conductance values give the temperature dependent characteristics between the channel electron mobility and the 2DEG density.

6.2 Aluminium Indium Nitride/Gallium Nitride heterostructure

The lattice constant of AlInN with indium composition around 17% is the same as the one of GaN [32]. As shown in Fig. 5-1, the GaN layer and the Al0.83In0.17N layer are lattice matched at 3.2. Since there is no mismatch between the two layers, piezoelectric

113 polarization disappears in the AlInN layer. As a result, only spontaneous polarization exits in both AlInN layer and GaN layer, as show in Fig. 5-2.

7 AlN 6

Al0.83In0.17N 5

4

3 GaN

Bandgap Energy (eV) InN 2

3.1 3.2 3.3 3.4 3.5 3.6 Lattice Constant (10-10 m)

Figure 6-1. Bandgap energy versus lattice constants of AlN, GaN, and InN.

Net polarization PSP Al0.83In0.17N charge

PSP GaN 2DEG

Figure 6-2. 2DEG formation in AlGaN/GaN interface.

114 6.3 Experimental Methods

AlInN/GaN heterostructures on sapphire substrates in this study were grown based on Migration Enhanced Metalorganic Chemical Vapor Deposition (MEMOCVD®), a new growth technique by Sensor Electronic Technology, Inc [12]. The samples in our experiments have the AlInN layer with the indium composition close to 17%. Transfer length measurement (TLM) patterns and AlInN/GaN HEMTs with 2 um gate length and various gate widths were fabricated. Figure 5-3 shows the fabricated devices and the

TLM patterns. Keithley 2612 source meter was used in the measurements of electrical characteristics of the HEMTs.

150 μm

(a)

150 μm

(b)

115

80 μm

(c)

Figure 6-3. Pictures of (a) AlInN/GaN HEMTs, (b) gate structure, and (c) TLM patterns.

6.4 Results and Discussion

Figure 5-4 pressent the measurement results of TLM pattern. Both sheet resistance

(Rsheet) of the channel 2DEG and contact resistance (Rc) in AlInN/GaN HEMTs were extracted from the TLM pattern measurements. Their values vary with operation temperatures were shown in Fig. 5-5 and Fig. 5-6, respectively. The drop of Rsheet with the decrease of operation temperature could be attributed to the fact that less phonon and alloy scattering occurs at the lower temperature [13]. On the contrary, little fluctuation with temperature was found in the contact resistance values, which is an evidence of good quality of ohmic contacts [14].

116

20 295K 18 250K 200K 150K 16 100K 50K

14 Linear Fit

12 R (Ohm) 10

8

6 0 2 4 6 8 10 12 14 16

Lj (um)

Figure 6-4. Results of TLM measurements.

2.90

2.85

2.80

(Ohm) c

R 2.75

2.70

50 100 150 200 250 300 Temperature T (K)

Figure 6-5. Contact resistances extracted from transfer length measurement versus temperature.

117

250

200

150

(Ohm/sq.)

sheet R 100

50 100 150 200 250 300 Temperature T (K)

Figure 6-6. Sheet resistances extracted from transfer length measurement versus temperature.

The typical output characteristics of the AlInN/GaN HEMTs with the 6 um source- drain space and the gate geometry of 2×150 μm2 ×2 at four different temperatures (50K,

150K, 250K and 295K) were demonstrated in Figure 5-7. It shows that the devices were pinched-off completely under all operation temperatures. As temperature decreases, the drain current (Ids) increases in the linear region due to the reduction of the parasitic access resistances. At the same time, as the operation temperature increases from 50K to 295K the saturation current (Idss) decreases from 2.06 A/mm to 1.13 A/mm. Since Idss is directly proportional to the saturation velocity Vsat , the reason for the decrease of saturation current is the decrease of saturation velocity at high temperature. Furthermore, the discrepancy between the DC and pulse measurements indicates the obvious self-heating effect at any operation temperature. The pulse measurements were carried out with 250-

118 us pulse width and 1%-duty cycle. The fact that the drain current versus drain voltage

(Ids-Vds) curves in DC measurements overlaps with the results of pulse measurements at large reverse gate voltages while differs from the pulse measurement results as gate voltage (Vgs) approaching to zero or at the positive gate bias indicates the presence of self-heating effect. At large negative gate voltage, the drain current is low and consequently the power consumption is small, so the DC measurement and pulse measurement results coincide with each other. On the other hand, self-heating effect becomes prominent due to the high drain current. The drain current from DC measurements is larger than the one from pulse measurements because pulse measurements produce less heat than DC measurements. The reduction of self-heating effect is obvious even though our equipment Keithley 2612 limits the pulse width in the measurements longer than 200ns [14]. The low thermal conductance of sapphire substrates is responsible for the observed self-heating effect in our experiments [13, 15].

2.5 Lines: DC Scatters: pulse 2.0 295K Gate bias from 0 to -12V with -2V step

1.5 0V

(A/mm)

1.0

ds I

0.5 -12V 0.0 0 2 4 6 8 10 Vds (V)

(a)

119

2.5

50K 0V 2.0

1.5

(A/mm) 1.0

ds I

0.5

-12V 0.0 0 2 4 6 8 10 Vds (V)

(b)

2.5

150K 2.0 0V

1.5

(A/mm)

ds 1.0 I

0.5 -12V 0.0 0 2 4 6 8 10 Vds (V)

(c)

120

2.5

2.0 250K

0V

1.5 (A/mm)

1.0

ds I

0.5 -12V

0.00 2 4 6 8 10 Vds (V)

(d)

Figure 6-7. DC and plused output characteristics of AlInN/GaN HEMTs at (a) 295K. (b) 250K. (c) 150K. (d) 50K.

0.7 50K 0.6 100K 150K 0.5 200K 250K 0.4 295K

Vds=1V (A/mm)

0.3

ds I 0.2

0.1 -16 -14 -12 -10 -8 -6 -4 -2 0 Vgs (V)

Figure 6-8. Transfer characteristics of AlInN/GaN HEMTs at 295K, 250K, 200K, 150K, 100K, and 50K.

121 Figure 5-8 shows the transfer characteristics of our AlInN/GaN HEMTs at various temperatures. The values of transconductance can be extracted from the transfer characteristics. As the temperature decrease from 295K to 50K, the maximum transconductance at Vds=1V increases from 0.094 S/mm to 0.168 S/mm. It has been proposed by Pozzovivo et al. [16] that a strong dependent relationship between

transconductance (Gch) and the channel electron mobility ( ch ) for GaN-based HEMTs including AlInN/GaN and AlGaN/GaN HEMTs. The fact that the maximum transconductance increases as the temperature decreases is consistent with the inverse proportional relationship between the channel electron mobility and the operation temperature, as shown in Fig. 5-9. Here, the extraction of the channel electron mobility

was based on the conductance calculation equation which is given by

Gch()/ V gs qn s ch W L g (5-1) and the total source-drain resistance RDS under the gate region

RVRRRGVDS( gs ) 2 c  GS  DG  1/ ch ( gs ) (5-2)

, where RGS is the resistance between the gate and source terminals, RDG is the resistance

between the gate and drain terminals, ns is the 2DEG density, W and Lg are the gate width and gate length of the device, Rc is the contact resistance for both source and drain ohmic contacts [17]. The TLM pattern measurements can give the values of Rc, RGS and RDG.

The gate voltage (Vgs) dependent of the drain-to-source total resistance (RDS) can be extracted from the transfer characteristics. Consequently, the transconductance can be calculated based on Equation (2). Next, the 2DEG density ns can be obtained by integrating gate-source capacitance (Cgs) over Vgs, as shown in Fig. 5-10. It has been

122 known that the 2DEG density in AlInN/GaN heterojunctions is almost independent on temperature confirmed theoretically and experimentally due to the significant degeneracy

[18]. Therefore, the electron mobility can be extracted using the 2DEG density value at room temperature based on Eq. 5-1. The drop in channel electron mobility with channel electron density decreasing was generally attributed to the Coulomb scattering in the

2DEG channel [17, 19].

Vgs (V) -10 -8 -6 -4 -2 0 6x103

) 295K 250K /Vs 3 2 5x10 200K

cm 150K ( 3 100K ch 4x10  50K

3

3x10

2x103

1x103 Electron mobility

5.0x1012 1.0x1013 1.5x1013 2.0x1013 2.5x1013 2 Electron density ns(/cm )

Figure 6-9. Channel electron mobility versus gate voltage and 2DEG density at various temperatures.

123

0.4

13 )

2 2.0x10

n

s

0.2 (cm

F/cm 

-2 (

) gs

C 0.0 0.0 -16 -12 -8 -4 0 Vgs (V)

Figure 6-10. Gate capacitance Cgs and 2DEG density ns dependences gate voltage Vgs.

Nevertheless, the Coulomb scattering only is the partial reason for the decrease of channel electron mobility. The 2DEG density in channel decreases with the increase of the gate bias. Meanwhile, electrons in the heterostructure interface are driven into the

GaN buffer layer. Since the bulk GaN electron mobility is smaller than the mobility of

2DEG, the obtained mobility value is small under deep negative gate bias. Figure 5-11 shows the extracted 2DEG mobility as a function of the depth from the AlInN/GaN interface, which verifies the reduction of channel electron mobility.

124

10000 295K

) 250K

200K /Vs

2 150K

100K cm ( 50K

1000 Electron mobility 100 20 40 60 80 100 120 140 160 180 200 Depth in channel (nm)

Figure 6-11. Extracted 2DEG mobility as a function of the depth from the AlInN/GaN interface

The value of channel mobility ch quickly reduces to the value of electron mobility in bulk GaN, which is 100-400 cm2/Vs at room temperature [12], when electrons are driven into the GaN buffer layer. Moreover, it also can be observed that the channel mobility decrease from its maximum value at each temperature. Alloy scattering, phonon scattering, and interface roughness are not all the factors responsible for this reduction because the decrease also occurs at temperature as low as 50K under which alloy scattering and phonon scattering could be neglected [20]. Also, other proposed mechanism that electrons from the channel diffuse into the AlInN layer forming a parasitic parallel conduction channel with low electron mobility cannot explain the decrease of electron mobility at high 2DEG density in our case [19]. From Fig. 5-9, it can be seen that electron mobility begins to reduce at a low gate bias. The parasitic parallel

125 conduction channel should not be activated at such bias condition. This is also confirmed by the fact that the measured value of gate capacitance keeps constant while the electron mobility decreases, as shown in Fig. 5-10. Thus, intersub-band scattering in the

AlInN/GaN heterostructure potential well should be the reason for the electron mobility drop under low reverse gate bias [21].

5.5 Conclusion

In summary, the DC and pulse electrical properties of AlInN/GaN HEMTs were investigated in low temperature. The experimental results show that the DC saturation current improves from 1.13 A/mm to 2.06 A/mm. Meanwhile, the maximum transconductance at Vds=1V increases from 0.094 S/mm to 0.168 S/mm when the operation temperature was cooled down from room temperature to 50K. The electron mobility decreases at the low 2DEG density region because of strong Coulomb scattering and at the high 2DEG density region because of strong intersubband scattering. Because of low thermal conductance of sapphire substrates, self-heating effect can be detected at all operation temperature. Therefore, it is necessary to use substrates with high thermal conductivity to further reduce the self-heating effect. Furthermore, it is concluded that the drop in electron mobility under high negative gate voltage is partially due to the electrons are drifted to the GaN buffer layer by the high electric field.

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129 Chapter 7

Energy transfer in quantum dots coupled microcavity nitride LEDs

In this chapter, quantum dots (QDs) coupled non-resonant microcavity light emitting diodes (LEDs) are designed and an enhanced non-radiative energy transfer between colloidal QD phosphors and InGaN/GaN quantum wells (QWs) is demonstrated.

It is the first time to improve the non-radiative energy transfer efficiency through tailoring the radiative recombination lifetime of electron-hole pairs in QWs. The microcavity structure is formed by a top metal reflector and a bottom distributed Bragg reflector (DBR). The blue emission from the InGaN/GaN QWs is detuned from the resonant modes of the microcavity to extend the radiative recombination lifetime in QWs.

The direct contact of QDs and the QW active layer is achieved by fabricating micro-holes on the LEDs. The occurrence of non-radiative energy transfer between QW and QD is confirmed by time-resolved photoluminescence (PL) measurement. A longer radiative combination lifetime electron-hole pairs in the microcavity LEDs have been obtained in the wavelength range from 440nm to 460nm in compared with the LEDs without microcavity structure. The non-resonant microcavity structure, combining with the micro-holes, leads to a 3.2 times enhancement of the effective quantum efficiency in compare with the LEDs without microcavity structure.

7.1 Introduction

During last several decades, solid state lighting is replacing incandescent lamps and fluorescent lamps in electrical lighting, and has been used in residential general

130 lighting, devices indicators, computer displays, and so on [1]. The advantages of solid state lighting over the conventional lamps include higher convertible efficiency between electrical energy and lighting, one hundred times longer lifetime than incandescent bulbs and ten times longer lifetime than fluorescent lamps, and the high power intensity [2-4].

One of the core techniques involving in the solid state lighting is the white LED technology. Currently, two main methods, multiple LEDs and phosphor based LEDs, are employed in producing white emission in LEDs. Multiple LEDs method refers to combine the LEDs which can emit the primary colors separately into a single lamp to produce white emission. White emission will be formed when those three prime colors are mixed. This method is a direct way but the cost is high due to the complication of manufacture and the lamp size is large [5]. Another method is using phosphors to convert a single color into multiple colors to obtain white lighting. Generally, nitride LEDs are used as the blue light emitter and yellow phosphors is combined to convert the blue light into the yellow light. A white light is yielded by mixing the blue light with the yellow light. This method encounters the disadvantages of less than 100% phosphor convertible efficiency and a poor color rendering property [29].

Recent years, colloidal compound QDs technique is becoming a new favorite phosphor in the field of white LEDs. The lighting color emitted by this new phosphor could be tuned in a large spectrum range. Comparing to other conventional phosphors, the emission wavelength of quantum dots is sharper and the quantum efficiency is higher

[6, 7]. Without changing the chemical composition, QDs only need to adjust their sizes to luminous a variety of colors. Besides the size-tunable emission wavelength, the disclosure of a brand new way for electron-hole pairs between a bulk semiconductor and

131 QDs is another advantage that motivates the application of QDs in the white-LED technique. Given QDs locating near the immediate vicinity of an InGaN QW, the free carriers inside the QW could transfer the energy to the QDs in a special non-contact, non- radiative way. This energy transfer method based on the dipole-dipole interactions combining with the quantum well to QDs coupling effect [8]. This method skips several color conversion steps, and thus is expected to have a higher luminous efficiency by reduce the energy loss. Based on this method, core/shell CdSe/(Zn, Cd)S QDs have been employed in white LED technique as phosphors combining with InGaN MQW LEDs [9-

11].

The configuration of the nitride LEDs plays an important role in improving the non-radiative energy transfer rate between QWs and QDs. Since the non-radiative energy transfer rate is in reversely proportional to the distance between QWs and QDs and the non-radiative energy efficiency increases with the increase of the radiative recombination lifetime of excitations in the QWs and with the increase of the nonradiative energy transfer rate, prolonging the QW radiative combination lifetime and minimizing the QW-

QD distance are two approaches to enhance the non-radiative energy transfer efficiency.

Several studies have been carried out to increase the nonradiative energy transfer between

QW and QD. By using a very thin n-type capping layer (~5nm), a 13% measured color conversion efficiency is achieved by Achermann et al. from an InGaN/GaN quantum well to a single layer of CdSe/ZnS QDs [11]. However, the capping layer in this configuration is too thin to be a good current spreading layer, leading to the decrease of the efficiency of the LED itself. Moreover, the emission from the QD phosphor is still low compared to the blue emission from the quantum well, resulting in an imbalanced white light emission

132 [10]. Chanyawadee et al.[12] proposed surface-patterned nitride LEDs with elliptical holes reaching to the emissive quantum well layers. An average color conversion efficiency of 20% was obtained by injecting QDs into the holes to make the short distance between the QWs and the QDs [12]. In another work, Fan. et al. deposited

CdSe/ZnS QDs into the spaces in the nanopillar GaN LEDs to enhance the nonradiative energy coupling between the quantum well and the QDs. A 263% increase in internal quantum efficiency from that of planar GaN LEDs [10].

In this work a novel approach for improving non-radiative energy transfer efficiency between QWs and QDs by depositing QDs into GaN-based LEDs with a non- resonant microcavity structure is demonstrated. The non-resonant microcavity, consisting of a narrow-band dielectric distributed Bragg reflector (DBR) lower layer and a broadband metal reflector upper layer, is detuned from the blue emission from the

InGaN/GaN QWs to remarkably prolong the radiative recombination lifetime of electron- hole pairs in the QWs which leads to the enhancement of the QW-QD non-radiative energy transfer rate. The microcavity InGaN/GaN LEDs is patterned and dry-etched to fabricate micro-holes that penetrate through the InGaN/GaN QWs. Colloidal QDs have been deposited into these micro-holes to minimize the distance between QWs and QDs for boosting the non-radiative energy transfer. The measured radiative combination lifetime of QWs increases to 1.44 ns in microcavity LEDs from 1.24 ns in LEDs without microcavity structure at the wavelength of 450 nm. Electroluminescence results show that the effective quantum efficiency of the QDs coupled in microcavity LEDs is 3.2 times than the one in control LEDs. At last, time-resolved photoluminescence study confirms

133 the occurrence of non-radiative energy transfer and an energy transfer rate which is comparable to the radiative recombination rate is obtained.

7.2 Theory

7.2.1 Quantum dots and quantum confinement effect

QDs, also called “nanocrystals”, are crystals with very small dimensions which are in the nanometer scale. The size of a QD is in the range of one to ten nanometers [13].

QDs are made from semiconductors, typically II-VI and III-V semiconductors [19]. They are classified by their fabrication techniques. Colloidal QDs refers to a class of QDs that are synthesized by wet chemical methods, which was first proposed by Murray et al. in

1993 [18]. In this method, QDs are grown from a solution containing the atomic species that could build the nanocrystal blocks [17]. QDs used in this dissertation are CdSe/ZnS

QDs, which are core/cell semiconductor QDs. The bandgap energy of the shell material

(3.61 for ZnS) is much larger than the one of the core material (1.74 for CdSe). Thus, the generated excitons are well restricted in the core. The shell material also can provide good passivation for the surface defects of the core material to increase the fluorescence quantum yield [19, 20].

Although QDs are semiconductors, they have certain unique optical and electronic properties which cannot be obtained from bulk semiconductors. The particularities of QDs derive from the existing quantum confinement effect in the QDs.

134 This quantum confinement effect endows QDs the tunable band gap energy which influences the properties of QDs.

Quantum confinement effect is a concept of quantum mechanics, which varies the band gap energy of QDs. According to the particle-wave duality which is a basic concept of quantum mechanics, every particle associates a matter wave [14]. The rules of quantum mechanics should be applied to the particles with the dimensions comparable to the wavelength. In a bulk semiconductor material, carriers can move around the crystal freely, which motion can be explained by integrating plane waves. However, in a semiconductor material with the size comparable to its associated wavelength, the motion of carriers could be described as a “particle in a box” [15]. Figure 6-1 (a) shows a simple model for a particle in a one-dimensional potential box. The particle bounces back and forth between two impenetrable walls with a constant speed [28]. The behavior of this free particle can be described by the solutions of its Schrodinger equation with mass m

[16],

d 2 (x) 2mE   (x)  0 (6-1) dx 2 2

Considering the boundary condition

 (x  0)  (x  a)  0 (6-2)

, the time-independent wave solution can be obtained as [16]

2 nx  (x)  sin( ) (6-3) a a

, where n is a positive integer number. Figure 6-1 (b) gives the first three solutions.

135

(a)

(b)

Figure 7-1. (a) The particle with velocity V in a one-dimensional potential box. (b) The first three solution of wave function for the particle in a box.

The particle energy can be obtained by substituting the wave solution back to the schrodinger equation [16],

n22 2 E  E  (6-4) n 2ma2

136 It can be seen that the energy levels of the particle are discrete and Wide wells could lead to small energy level spaces.

Different from the bandgap energy of a bulk semiconductor Eg(bulk), the bandgap energy of a QD Eg(qd) is size dependence. Given a sphere QD, the energy gap of the QD can be calculated by [17]

h2 1 1 e2 Ewell  Eg  2 (  ) 1.8 (6-5) 2d me mh 2 s d

, where d is the diameter of the QD, me is the electron effective mass, and mh is the effective hole mass.

7.2.2 Nonradiative energy transfer between quantum well and quantum dot

QW carriers in the excited states have three ways to decay in a coupled QDs and

QWs system. They can relax by radiative process with the lifetime T , non radiative

process with the lifetime  NT , or nonradiative energy transfer into a QD with the lifetime

 ET . The nonradiative energy transfer between QDs and QWs is a type of resonance energy transfer, which also can be referred as Forster resonance energy transfer

(FRET)[24]. In this type of energy transfer process, a donor in the excited state transfers energy to an acceptor by the interactions between donor dipole moment and acceptor dipole moment [25].The non-radiative energy transfer rate RET can be calculated as [12],

6 RdET  (6-6)

, where d is the donor to acceptor spacing.

137 Therefore, the donor and the acceptor should be placed as close as possible to increase the energy transfer rate. Typically only when the spacing is reduced to a few nanometers, such non-radiative energy transfer can be observed [25].

In a QD-pholospher nitride LED, the donors are InGaN/GaN QWs, and QDs serve as acceptors. In the InGaN/GaN QWs, carriers can transfer energy to the very nearby QDs through the FRET process, and then carriers experience a fast radiative relaxation process in the QDs to emit the light with a wavelength that depends on the size of the QDs, see Fig. 6-2 [8].

Figure 7-2. Illustration of energy transfer in a QD coupled LED.

The efficiency of the non-radiative energy transfer between QWs and QDs can be defined by [8],

T ET  (6-7) T ET

138

, where T is the relaxation lifetime of excitations in the QWs including both radiative

and nonradiative decaying, and  TE is the relaxation lifetime due to the non-radiative

energy transfer from QWs to QDs. From Eq. 6-7, it can be concluded that a low  T and a high are desirable for increasing the non-radiative energy transfer efficiency.

While a prolonged could be achieved by minimizing the distance between the

QDs and the QWs separation distance,  T is mainly determined by the radiative relaxation lifetime in a spontaneous emission process for a direct semiconductor and can be expressed as based on Fermi’s golden rule [26]:

2 1 T  mg (6-8)

, where m is the optical transition matrix element and g is the density of photon states which is a function of frequency and direction of the emission light. The optical transition element is determined by the structure of QWs. By integrating the emitter source into a resonant microcavity, g could be adjusted and hence can be tailored. The dependence of the relaxation lifetime to the presence of a cavity is called Purcell effect [27].

7.3 Device structure and fabrication

InGaN LEDs with non-resonant microcavity structure were designed and fabricated in order to extend the radiative recombination lifetime of InGaN QWs and enhance the direct energy transfer efficiency between QWs and semiconductor QDs.

Figure 6-3 shows the structure of the microcavity InGaN/GaN LED with micro- holes filled with CdSe/ZnS QDs. The microcavity LED epi-wafer is configured with a

139 sapphire substrate, a 3 um of buffer GaN layer, a 4.1 um of DBR mirror comprising 41 pairs of 49 nm GaN and 51 nm lattice matched AlInN, then followed with a layer of 650 nm n-type GaN, 5 pairs of InGaN/GaN quantum well, and a p-type GaN layer. The p- type metal is Ni/Al/Ni, which is also used as the mirror on the top of the microcavity. The n-type metal is Ti/Al/Ti/Au. The microcavity structure is formed by inserting the

InGaN/GaN QWs-QDs active layer between the upper metal reflector with a GaN capping layer and the bottom DBR mirror. The mcirocavity configuration is designed as a non-resonance cavity to the blue emission from the InGaN/GaN QWs. The confinement of the blue emission in the cavity leads to the decrease of the density of photon states which could prolong the relaxation lifetime of excitations in the QWs significantly.

Consequently, the energy flux transferred from InGaN/GaN QWs to the QDs could be enhanced accordingly.

For device fabrication, Ni/Al/Ni (3nm/50nm/300nm) was patterned and deposited on the p-type GaN. The first layer of 3 nm Ni is for p-type contact, 50 nm Al acts as the top mirror of the cavity, 300 nm Ni is employed as the etching mask for the next step.

Then the device was etched to n-type GaN by inductively-coupled-plasma (ICP) etching, and the mesa of the LED and the micro-hole structure was obtained. For the formation of the n-type electrode, Ti/Al/Ti/Au (10nm/40nm/40nm/100nm) was deposited and patterned on the n-type GaN. Figure 6-4 shows the top view of the microcavity LEDs with micro-holes. The holes are shown as the red circles, with a diameter of 10 um, and a period of 20 um. InGaN LEDs wafer without the layer of DBR mirror was prepared as control samples.

140

Figure 7-3. Schematic diagrams of the microcavity InGaN LEDs with micro-holes.

300 μm

Figure 7-4. Schematic top view of the microcavity LEDs.

7.4 Experimental methods

In this study, two groups of samples have been prepared. One group of samples are LEDs with microcavity structure which are referred as microcavity LEDs, and another group of samples are LEDs without microcavity structure which are referred as control LEDs. For both groups of samples, half of them are soaked into the colloidal

141 CdSe/CdS QDs for depositing a thin QDs layer over the surface of LEDs. The samples soaked into QDs are referred as hybrid QDs-MQWs LEDs and the samples as fabricated without QDs are referred as bare LEDs. Samples were polished in advance and put on a glass plate, and the output emission of the samples is collected by an Ocean ptics

HR2000 high-resolution spectrometer via an optical fiber, as shown in Fig. 6-5.

For electroluminescence (EL) spectra measurements, a Keithley 2612 Dual- channel System Source Meter is used to electrically pump the samples by implementing forward bias to the LEDs. 10mA of current was applied to all LED samples. For photoluminescence (PL) spectra measurements, optical pumping of the samples is implemented by a 405nm laser. The time resolved PL study is carried out by using 80 femtosecond pulse excitation with 1000 Hz repetition rate. A femtosecond fluorescence up-conversion technique is used to measure the radiative lifetime decay [10].

Figure 7-5. An illustion for collecting output emission.

142 7.5 Results and analysis

The reflection spectrum of the microcavity LED structure is shown in Fig. 6-6.

The DBR mirror has a high-reflection band centered at 450 nm with a bandwidth of ~30 nm. The resonant mode of this microcavity structure is not strong and the resonance wavelength locates at ~450 nm, as indicated by a small dip of reflectance at 450 nm. The blue EL emission of this microcavity LED was measured at a detection angle of 0° under current injection of 10mA. It can be seen that the short-wavelength component (<470 nm) of the QW’s emission is suppressed in the non-resonant microcavity.

4000 Reflectivity 100 EL spectra 3000 80

2000

60 EL intensity (a.u.)

Reflectivity (%) 1000 40 0 400 450 500 550 Wavelength (nm)

Figure 7-6. Reflection spectrum and emission spectrum at injected current of 10 mA for the microcavity LED without QD coupling.

Femtosecond fluorescence up-conversion technique is utilized to characterize the changes of radiative lifetime of QWs in the microcavity. The PL lifetime of blue

143 emission from the microcavity LED was measured over the whole emission band. The radiative lifetime was calculated by [2]

 PL  R  (6-9) QW

, where is the internal quantum efficiency of QW and can be determined by temperature-dependent PL measurement. The calculated versuses detection wavelength is demonstrated in Fig. 6-7. The result of the control sample is also plotted for the sake of comparison. The increase of radiative lifetime is readily seen for the non- resonant emission components, which results from the suppressed density of photo states at non-resonant wavelength in the microcavity LED as described by Fermi’s golden rule.

microcavity LED

1.6 control LED

1.2 Radiative lifetime (ns)

0.8 420 440 460 480 500 Wavelength (nm)

Figure 7-7. The measured PL lifetime versus wavelength for the microcavity LED and the control sample.

Fig. 6-8(a) shows the EL spectra of the microcavity LEDs with and without QD.

A continuouse 10 mA current was used to pump the LEDs. The blue shadow denotes the

144 blue emission from the QW that absorbed by the QDs, and the red shadow denotes the red emission from the QDs.

The effective internal quantum efficiency of QD emission is defined as [10]

QW QD NQD QD  QW QW QD (6-11) NN QW

, where is the part of the blue emission that does not absorbed by the QDs,

is the QD red emission and is the emission of the LEDs without QD deposition. For the QD coated microcavity structure, was calculated to be ~47.3%.

Fig. 6-8(b) shows the EL spectra of the control LEDs with and without QD. The effective internal quantum efficiency of the control sample was calculated to be

~14.7%. The effective quantum efficiency of the QD emission coupled in LEDs with microcavity structure is 3.2 times higher than the one in LEDs without microcavity structure. The increase of the effective quantum efficiency should be attributed to the energy transfer between QWs and QDs, because the two type of samples have the same

QW structures.

145

4000 (a) Without QDs With QDs 3000

2000

EL intensity (a.u.) 1000

0 400 500 600 700 Wavelength (nm)

(a)

2000 (b) Without QDs With QDs 1500

1000

EL intensity (a.u.) 500

0 400 500 600 700 Wavelength (nm)

(b)

Figure 7-8. EL spectra of (a) the microcavity LED sample with and without QD deposition. (b) the control LEDs with and without QD deposition.

The presence of efficient nonradiative energy transfer was verified by time- resolved PL study. Figure 6-9 (a) shows the time-resolved PL data measured at 450 nm for the microcavity LED with and without QDs. Prior to QD deposition, the decay

146 lifetime in the mcirocavity LEDs was ~810 ps, while it reduced to ~490 ps after the QD deposition. This drop shows that the existence of QDs accelerates the carriers decaying process, implying a new decay path in the microcavity LEDs. The time-resolved PL results of the control sample, shown in Fig. 6-9 (b), indicate the decay lifetimes are the same in the control samples with and without QDs. Therefore, the additional relaxation channel in QDs coated microcavity LED should be the efficient non-radiative energy transfer from QWs to QDs. The energy transfer rate can be determined directly as 1/490-

1/810 = 0.81 ns-1. This value is comparable to the radiative recombination rate of

InGaN/GaN QWs as shown in Fig. 6-7, which is 0.70 ns-1. The results indicate that non- radiative energy transfer has an important place in QD-coupled microcavity regarding the

QW-QD color conversion process.

1.2

1.0  = 810 ps 0.8

0.6

0.4  = 490 ps

0.2 Normalized intensity Without QDs 0.0 With QDs -500 0 500 1000 1500 2000 Time (ps)

(a)

147

1.2

1.0 Without QDs With QDs 0.8

0.6

0.4

0.2 Normalized intensity

0.0 -500 0 500 1000 1500 2000 Time (ps)

(b)

Figure 7-9. PL decay dynamics measured at 450 nm for (a) the microcavity LED and (b) the control sample.

7.6 Conclusion

In conclusion, in this chapter our design and investigation of a novel way to improve the nonradiative energy transfer between QWs and QDs by interpreting QDs into InGaN/GaN QW LEDs with non-resonant microcavity structure and micro-holes have been demonstrated. The radiative combination lifetime of electron-hole pairs in

QWs is extended to 1.44 ns at the wavelength of 450 nm in mcirocavity LEDs in compared with 1.24 ns in LEDs without microcavity structure by predesigning the microcavity detuning from the blue emission out of QWs. Furthermore, the existence of nonradiative enengy transfer between QWs and QDs is confirmed by time-resolved

148 photoluminescence study and an enhancement in the effective quantum efficiency is observed in the LEDs with microcavity structure.

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153 Chapter 8

Future Work

8.1 Chip-level integration of gallium nitride light emitting diodes and vertical metal semiconductor field effect transistors

Based on our study of the GaN SBDs fabricated on LED epi-wafers, a monolithic integration of GaN LEDs and vertical metal-semiconductor field effect transistors

(MESFETs) can be achieved, as shown in Fig. 8-1. Selective regions of n-type doped

GaN surface or unintentionally doped GaN surface will be exposed by etching, and a sufficient thin vertical conductive channel which also is an n-type doped GaN mesa will be formed. Top ohmic contact on n-type doped GaN layer will be the drain terminal, and bottom ohmic contact on unintentionally doped GaN layer will be the source terminal.

The gate Schottky metal will be deposite on the uninterntionally doped GaN layer and surround the vertical channel. A negative gate bias should be applied to pinch off the current flowing. The MESFET-integrated GaN LEDs are expected to extend the potential intelligent lighting applications of GaN LEDs in the future.

154

Figure 8-1. The monolithic integration of a GaN LED and a vertical MESFET.

8.2 Monolithic integration of LEDs and AlInN/GaN HEMTs

In this work, we will use re-growth methord to realize the monolithic integration of GaN LEDs and AlInN/GaN HEMTs. The re-growth methord can remove the plasma damage in the integration processes. Moreover, the lattice-matched AlInN/GaN HEMTs can potentially provide higher output current and have improved stability and reliability compared to AlGaN/GaN HEMTs.

155

Figure 8-2. The monolithic integration of a GaN LED and a AlInN/GaN HEMT.

VITA

Li Wang

EDUCATION Ph.D. Department of Engineering Science and Mechanics 08/2010~5/2015 The Pennsylvania State University, University Park, PA, USA

M.S. Department of Electronic Engineering 09/2007~07/2010 Tsinghua University, Beijing, P.R. China

B.S. Department of Communications Engineering 09/2003~07/2007 Communication University of China, Beijing, P.R. China

SELECTED PUBLICATIONS

[1] Li Wang, Xuefeng Zhang, Guanjun You, Feng Xiong, Lixin Liang, Yong Hu, Aping Chen, Jie Liu and Jian Xu, "Modeling the back gate effects of AlGaN/GaN HEMTs," Journal of Computational Electronics, August 7, 2014.

[2] Xue Feng Zhang, Lai Wei, Li Wang, Jie Liu, and Jian Xu, "Gate length related transfer characteristics of GaN-based high electron mobility transistors," Applied Physics Letters, 102, 113501, 2013.

[3] Zhang Xue-Feng, Wang Li, Liu Jie, Wei Lai, Xu Jian, "Electrical characteristics of AlInN/GaN HEMTs under cryogenic operation," Chin. Phys. B 2013, Vol. 22 Issue (1): 017202.

[4] Li Wang, Wenhua Chen, Peng Wang, Xin Xue, Jiaxing Dong and Zhenghe Feng, "Design of asymmetrical spurline filter for a high power sic MESFET class-E power amplifier," Microwave and Optical Technology Letters, v 52, n 7, p 1650-2, July 2010.

[5] Li Wang, Wenhua Chen, Peng Wang, Xin Xue, Jiaxing Dong and Zhenghe Feng, “A High Power SiC MESFET Class-E Power Amplifier With An Asymmetrical Spurline Resonator,” Asia-Pacific Microwave Conference, 2009, p 1120-3, 2009.