DEGREE PROJECT IN INFORMATION AND COMMUNICATION TECHNOLOGY, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2019

Study of ohmic contact formation on AlGaN/GaN heterostructures

KAI-HSIN WEN

KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE DF KTH Royal Institute of Technology

Study of ohmic contact formation on Al- GaN/GaN heterostructures

Master’s thesis in Nanotechnology

Kai-hsin Wen

Information and Communication Technology KTHROYAL INSTITUTEOF TECHNOLOGY Stockholm, Sweden 2019

Master’s thesis 2019

Study of ohmic contact formation on AlGaN/GaN heterostructures

Kai-hsin Wen

Information and Communication Technology KTH Royal Institute of Technology Stockholm, Sweden 2019 Study of ohmic contact formation on AlGaN/GaN heterostructures Kai-hsin Wen

© Kai-hsin Wen, 2019.

Supervisors: Niklas Rorsman, Chalmers University of Technology Ding-yuan Chen, Chalmers University of Technology Examiner: Mattias Hammar, KTH Royal Institute of Technology

Master’s Thesis 2019 Information and Communication Technology KTH Royal Institute of Technology SE-100 44 Stockholm Telephone +46 8 790 60 00

Cover: The contour plot of the obtained Rc from the laser focus/dose matrix.

Typeset in LATEX, template by David Frisk Printed by KTH Stockholm, Sweden 2019 iv Study of ohmic contacts formation on AlGaN/GaN heterostructures Kai-hsin Wen Information and Communication Technology KTH Royal Institute of Technology SE-100 44 Stockholm

Abstract

It is challenging to achieve low-resistive ohmic contacts to III-nitride semiconductors due to their wide bandgap. A common way to reduce the contact resistance is to recess the ohmic area prior to metallization. In the minimization of the contact resistance, parameters like the recess depth, anneal temperature and design of the metal stack are commonly optimized. In this work, three other approaches have been evaluated. All experiments were performed on AlGaN/GaN heterostructures. The fabricated ohmic contacts were recess etched, metallized with a Ta/Al/Ta stack, and annealed at 550-575◦C.

Firstly, it is shown that the laser writer intensity, transmittance and focus offset during optical lithography affect the contact resistance. The reason is believed to be the variation in the resist profile, which has an impact on the metal coverage. At the optimum intensity/transmittance/focus condition, which generates a relatively medium undercut, a contact resistance of 0.23 Ωmm was obtained.

In the second approach, the metal layer of annealed contacts was removed by wet etching, followed by the re-deposition of a metal stack and annealing. The purpose was to increase the amount of N vacancies in the AlGaN, which are responsible for the contact formation. A minimum contact resistance of 0.41 Ωmm was achieved with this method, compared to 0.28 Ωmm with the regular method (without re- metallization).

In the last approach, the bottom Ta layer was sputtered, whereas evaporation was used in all other cases. The minimum contact resistance was found to be 0.6 Ωmm, which was higher than for the evaporated contacts. The reason was assumed that the thickness of sputtered Ta should be thinner than the evaporated Ta due to its higher density. Moreover, the obtained lower sheet resistance is assumed to caused by the atomic scale damage due to the high energy ions during sputtering.

Keywords: ohmic contacts, wide bandgap, Ta-based, recess etch, N-vacancies

v Sammanfattning

En utmaning med III-nitrid-halvledare är att uppnå låg-resistivitetskontakter, på grund av deras breda bandgap. Ett konventionellt tillvägagångsätt för att reducera kontaktresistansen är att fördjupa ohmska ytan före metallisering. I strävandet av att minska den ohmska resistansen sker vanligtvis en optimering av följande parame- trar, recessddjup, anlöpningstemperatur och metallagersdesign. I detta arbete så har samtliga tre parametrar evaluerats. Alla experiment utfördes på AlGaN/GaN- heterostrukturer. De tillverkade ohmska kontakterna var recesssetsade, metalliser- ade med ett Ta/Al/Ta lager och anlöpt vid 550-575◦C.

Den primära undersökningen, visar att laserritar-intensitet, -transmission och - fokusförskjutning under optisk litografi inverkar på kontaktresistansen. Anledningen antas vara variation i resistprofilen, vilket påverkar metallbeläggningen. Vid opti- mal intensitet/transmission/fokus-förhållanden, (som genererar en underskärning), blev den resulterande kontaktresistansen 0.23 Ωmm uppmätt.

I en sekundär undersökning, avlägsnas ohmska kontaktens metallager genom våtet- sning, följt av en återdeponering av ett nytt metallager, samt anlöpning. Syftet var att öka mängden N-vakanser i AlGaN-lagret, som formar ohmska kontakten. Min- sta kontaktresistansen uppmätt var 0.41Wmm, att jämföras med 0.28 Ωmm, som uppnåddes genom den konventionella metoden (utan återmetallisering). Den sista undersökningen jämförde sputtrade med evaporerade bottenlager av Ta, (evaporation användes som standardmetod i de tidigare undersökningarna). Med sputtrning blev den minsta kontakresistansen 0.6 Ωmm, (högre än de evaporerade kontakterna). En hypotetisk förklarning kan vara att det sputtrade Ta-lagret är tunnare än det evaporerade Ta-lagret, på grund av en dess högre densitet. Därutöver, den uppmätta lägre skiktresistansen antas bero på den skada i atomskala som sker vid de höga energi-kollisioner som joner skapar vid sputtrning.

vi

Acknowledgements

This thesis work is pursued at the Department of Microtechnology and Nanoscience - MC2, Microwave Electronics Laboratory. It is a great experience for me to work here and during this period I have gained so much knowledge.

I would like to sincerely thank my supervisor Niklas Rorsman for giving me the chance to come to Chalmers and work with him. Despite his tight schedule, his door is always open for discussion whenever I have any question. Furthermore I would like to express my deepest gratitude to my daily supervisor Ding-yuan Chen for his patience and guidance. Thank you for teaching me the processing techniques in the cleanroom and all the measurements used in this thesis. I would also like to thank Hans Hjelmgren for helping me build a simulation model and teaching me to use TCAD simulation. Also, I would like to appreciate all the people in Microwave Electronics Laboratory for creating such a fantastic working environments.

Finally, I am deeply grateful to my family and all my friends for always being supportive encouraging me to push on.

Kai-hsin Wen, Gothenburg, June 2019

viii x Contents

List of Figures xiii

List of Tables xv

1 Introduction1 1.1 Thesis objectives and summarized results...... 2 1.2 Thesis outline...... 3

2 Ohmic contact technology5 2.1 Metal-semiconductor contact...... 5 2.1.1 Current transport mechanisms...... 6 2.2 Ohmic contact mechanism...... 8 2.3 Ohmic contact types...... 9 2.3.1 Planar contacts...... 9 2.3.2 Recessed contacts...... 10 2.3.3 n+-GaN regrowth contacts...... 12 2.4 Physical modelling...... 13

3 Fabrication Process 17 3.1 Standard Cleaning...... 17 3.2 LPCVD...... 18 3.3 Mesa...... 18 3.3.1 Photolithography...... 19 3.3.2 Plasma Ashing...... 20 3.3.3 Plasma Etching...... 21 3.4 Ohmic Contact...... 21 3.4.1 Metal Deposition...... 21 3.4.2 Re-metalization...... 22 3.4.3 Sputtered Ta...... 23

4 Characterization 25 4.1 Scanning electron microscopy...... 25 4.2 Transmission line method...... 26 4.2.1 TLM structure...... 26 4.2.2 Epi-layer sheet resistance Rsh ...... 27 4.2.3 Contact resistance Rc and specific contact resistivity ρc .... 27

xi Contents

5 Results 31 5.1 Laser writer focus/intensity test...... 31 5.2 Ohmic contact re-metallization...... 33 5.3 Sputtered Ta...... 36

6 Conclusion and future work 39

Bibliography 41

xii List of Figures

1.1 Schematic of GaN HEMT structure...... 1

2.1 Band diagram of metal-semiconductor in equilibrium [18]...... 5 2.2 Schematic of three different carrier transport mechanism for different Nd.[24]...... 7 2.3 E00 plotted as a function of doping concentration for GaN at T= 300K.8 2.4 Illustration of three different recess depth cases. (a)the barrier is still present (b)the barrier is still present but it is too thin to retain 2DEG under it (c)the barrier is completely removed in etching process.... 11 2.5 The simulation model structure(a)model from software (b)schematic of the model...... 13 2.6 Schematic of the simulation model structure and the obtained results of various doping depth, concentration and the temperature...... 15

3.1 Schematic of the ohmic structure in this work...... 17 3.2 The schematic of the position of photoresist and the laser beam.... 19 3.3 The etchant versus etching target material (X: The target material can not be etched by the etchant, : The target material can be etched by the etchant, –: Not found from the literature)...... 23

4.1 (a)Schematic TLM strcture (b)Microscope image of the TLM structure 26 4.2 (a)A schematics shows the different components of Rtot. (b) Total resistance Rtot plotted as a function of isolation distance dx ...... 27 4.3 Current flow through contact for high and low ρc ...... 29 5.1 Illustration of the laser focus/transmittance/intensity matrix with the exact energy received on the substrate...... 31 5.2 The contour plot of the obtained Rc from the laser focus/dose matrix. 32 5.3 The schematic of (a) larger sidewall angle (b) smaller sidewall angle and (c) the GaN depletion region due to too small sidewall angle... 32 5.4 (a)Rc comparison of original and re-deposited ohmic metal stack. (b)The plot of Rc versus annealing time and the comparison of orig- inal and re-deposited ohmic metal stack...... 33 5.5 The plot of Rc versus annealing time while first Ta layer thickness differs...... 34 5.6 The schematic shows the enlargement of the photoresist and the cross section of the ohmic contact...... 34

xiii List of Figures

5.7 Cross sectional SEM image of the sample with the measured Ta thick- ness...... 35 5.8 The obtained (a)Rc and (b)Rshversus annealing time with different metal thickness...... 36

xiv List of Tables

2.1 Comparison of Rc values and different metal schemes on different heterostructure...... 10 2.2 Literature values of Rc for recess contacts on different heterostructure 12 2.3 Rc of regrowth n-GaN contacts on different heterostructure...... 13 2.4 The obtained ρc of different doping concentration...... 15 2.5 The obtained ρc of different doping depth...... 15 2.6 The obtained resistance under different temperature...... 15

xv List of Tables

xvi 1 Introduction

The gallium nitride (GaN) based high electron mobility transistor (HEMT) has attracted considerable attention during the past two decades and has become an attractive candidate for high frequency and high power applications [1,2]. GaN is a semiconductor material which possesses a wide bandgap, a high breakdown field and a high saturation electron drift velocity. Furthermore, the GaN HEMT is based on a heterojunction, commonly AlGaN/GaN, which generates a two-dimensional electron gas at the interface with enhanced electron mobility compared to doped GaN. Grown on silicon carbide (SiC) substrate, high thermal conductivity is also possible. Fig. 1.1 shows a cross-section of a SiNx-passivated AlGaN/GaN HEMT with the three terminals gate, source, and drain marked out. As shown in the figure, the source- and drain terminals are ohmic contacts, while the gate is a Schottky contact.

Figure 1.1: Schematic of GaN HEMT structure.

Since the ohmic contacts constitute parasitic elements in the GaN HEMT, it is essential to minimize the contact resistance (Rc) in order to promote a good per- formance. However, the wide band gap of AlGaN between 3.4 (GaN) and 6.2 eV (AlN) makes this a challenge. The standard ohmic metal stack for GaN HEMTs is Ti/Al/Ni/Au [3,4,5,6]. However, with the Ti-based metal scheme, a high anneal- ing temperature of 800 to 900◦ C is required. Such high anneal temperature may lead unwanted effects on the heterostructure, such as increasing of sheet resistance Rsh as well as rough surface morphology and poor edge acuity due to the formation of highly resistive Al- and Au containing compounds. Previously, the Mo-based ohmic contacts were developed to obtain low-resistance contacts for low annealing temperature needed. Roccaforte et al [7] demonstrated that with the metal scheme

1 1. Introduction

of Mo/Al/Mo/Au and a pre-treatment with SiCl4 plasma, a very low Rc of 0.15 Ωmm was obtained at an annealing temperature of 650◦ C.

Ta- based metal schemes have also been investigated in several studies. The metal stacks of Ta/Ti/Al [8,9, 10] require annealing temperature of 700 to 950◦ C. An- other Ta- based metal scheme,Ta/Al/Ta, was developed demonstrating a lower an- nealing temperature. The lowest Rc of 0.06 and 0.28 were achieved with different Al thickness and annealing temperature of 550 and 575◦, respectively [11]. Lin et al demonstrated recessed Ta/Al/Ta ohmic contacts, where the ohmic contact is formed ◦ on the recess sidewall. The lowest Rc obtained is 0.24 Ωmm with a tilt angle of 10 during evaporation of the first Ta layer and an annealing temperature of 575◦C[12].

To improve the ohmic contacts, several studies have focused on recessed ohmic contact. Zhang et al performed recess etching by inductive coupled plasma etching (ICP) before metal deposition and obtained a Rc of 0.3 Ωmm [13]. Wang et al reported that with 2DEG totally removed, a very low Rc of 0.26 Ωmm was obtained due to direct contact between ohmic electrode and 2DEG [14]. Although the recess etching yields a low Rc, the ohmic recess process needs to have excellent depth control and be low-damage since ion damage on the epitaxial layer may lead to worse Rc. Regrown ohmic contact can produce excellent ohmic contact with an Rc of 0.16 Ωmm [15, 16]. However, the processing is complicated and the associated costs much higher compared to recess etching, which may make it less suitable for large scale processes. Consequently, recess ohmic contacts are preferable and an ohmic recess below 2DEG is performed in order to avoid the requirement of precise control of recess depth.

1.1 Thesis objectives and summarized results

The main purpose of this thesis is to investigate and evaluate three different meth- ods intended for decreasing Rc. In the first method, the parameters of the optical lithography are optimized in order to investigate the impact of the resist profile on recessed contacts. The second method is the removal of the annealed metal, followed by re-metallization to increase the formation of TaN. The third method is to sputter the first Ta layer to improve the metal coverage of the sidewall and the in-situ Ar cleaning is also performed to improve the Rc.

Electrical characterization was performed by the transfer length method (TLM). Scanning electron microscopy is also used to check the cross section of ohmic con- tact. Additionally, technology computer-aided design (TCAD) simulations were per- formed to study the impact of doping concentration, doping depth and temperature on the formation of ohmic contact.

Characterization results show that the lowest Rc of 0.23 Ωmm can be achieved with the optimal laser writer focus offset and intensity. With the optimal laser writer focus/intensity, photoresist profile can be better controlled. Therefore, a medium undercut can be generated and affect the metal coverage. However, with the second approach to re-metalize the ohmic metal, the obtained Rc is 0.41 Ωmm which shows

2 1. Introduction that the idea of enhancing N atoms extraction mechanism is not achieved in this work. The third approach, sputtered first Ta layer, shows the highest Rc of 0.6 Ωmm in this work. Assumptions are made according to the observed Rc, Rsh and the SEM characterization, and these assumptions may be the factors that lead to the failure of lowering the Rc. Further study on the adequate first Ta thickness as well as the in-situ Ar cleaning are required to decrease the Rc.

1.2 Thesis outline

This thesis is organized in six chapters: • The topic and a brief background are introduced in Chapter 1 followed by the aim and outline of the thesis. • Theory concerning the mechanism of ohmic contact formation, current trans- port mechanism and different types of ohmic contacts are presented in Chapter 2. • The fabrication process is described in Chapter 3. • The main characterization methods are introduced in Chapter 4. • The results are presented and discussed in Chapter 5. • A brief conclusion with a future outlook are presented in Chapter 6.

3 1. Introduction

4 2 Ohmic contact technology

2.1 Metal-semiconductor contact

Contacts between metal and semiconductor is an essential part of all the electronic devices. A low contact resistance Rc is an important factor in ohmic contacts. However, ohmic contact formation in III-nitride semiconductors, eg. (Al)GaN, has been an issue due to its wide bandgap. In this project, we investigate various aspects, which potentially could improve the formation of ohmic contact, including electron beam evaporation and sputtering and re-deposition of ohmic metal stack.

Metal-semiconductor contacts can be divided into two types. One is Schottky con- tact acting as a diode with the rectifying property. The other is a low resistive ohmic contact linear I-V characteristic. Generally, the Schottky barrier is formed by a metal contacting with an undoped or low-doped semiconductor. In contrast, when metal contacts with a highly-doped semiconductor and an additional thermal treatment can form ohmic contacts.

Figure 2.1: Band diagram of metal-semiconductor in equilibrium [18]

5 2. Ohmic contact technology

The band diagram of metal contacting an n-type semiconductor is shown in Fig. 2.1. The Fermi level, EF , should be flat when there is no bias applied across the junction [19]. This results in band bending and causes a depletion region with the width W. The Schottky barrier height, φB, is a barrier to electrons. φB depends on the metal work function, φM , and electron affinity, χ, of the semiconductor. The metal work function φM is the the work that is required to remove the electron from the metal surface to the vacuum level, Evacuum. The semiconductor electron affinity, χ, is the energy difference between Evacuum and the conduction band of the semiconductor.The Schottky barrier height is given in Equation.2.1.

φB = φM − χ (2.1)

In order to reach the full potential of semiconductor device technologies, it is nec- essary to minimize parasitic losses. Therefore, ohmic contacts with low resistance and linear I-V characteristic are needed.

In III-nitride devices it is not enough to only deposit metals on the semiconductor to form an ohmic contact. Thermal annealing is required to form a metal nitride at the interface [22, 23].

Specific contact resistivity ρc is commonly used to characterize ohmic contacts. ρc is defined in Equation 2.2, where J is the current density and the voltage is at zero bias. In this project, contact resistance per width, Rc, is calculated with the unit of Ωmm, and is used to evaluate the performance of the ohmic contacts.

∂J !−1 ρc = (2.2) ∂V v=0

2.1.1 Current transport mechanisms There are three different mechanisms for carrier transport which is dependent on the n-type doping concentration, Nd. The depletion width is proportional to Nd (Equation 2.3).

1 W ∝ √ (2.3) Nd 17 −3 For low-doped semiconductor, Nd < 10 cm , thermal energy for the carriers is required to overcome the Schottky barrier, and therefore this mechanism is named thermionic emission (TE, Fig. 2.2a). If the semiconductor is highly-doped, the Schottky barrier height is constant, but with a much narrower depletion width (Equation 2.3). Due to the small W, it is possible for carriers to tunnel through the barrier. This tunnel mechanism is called field emission (FE, Fig. 2.2b). Field 19 −3 emission occurs when the doping concentration Nd is higher than 10 cm . For the 17 −3 19 −3 moderately-doped semiconductors, Nd between 10 cm and 10 cm , the cur- rent transport is a combination of TE and FE. This mechanism is called thermionic

6 2. Ohmic contact technology

field emission (TFE, Fig. 2.2c). The depletion width is too large for carriers to tun- nel through, but with the extra thermal energy, carriers can be thermally excited to an energy above EF where W is thin enough to tunnel through the barrier.

(a) Thermionic emission (b) Field emission (c) Thermionic field emission

Figure 2.2: Schematic of three different carrier transport mechanism for different Nd.[24]

The dominant mechanism can be predicted by the calculated value of kT (Equation qE00 2.4). This calculation gives ratio between TE and the other two mechanisms.

h Nd 1 E = ( ) 2 (2.4) 00 4π mε

where h, m and ε is Plank’s constant, effective mass and the dielectric constant respectively. The calculated value of kT gives the ratio between TE and the other qE00 two mechanisms. When kT/qE00  1 , field emission is the dominating mechanism for carrier transport. Thermionic emission dominates when kT/qE00  1 while kT/qE00 = 1 indicates that thermionic field emission is dominant. This behavior described in Fig. 2.3, in which E00 is plotted as a function of doping concentration [?].

7 2. Ohmic contact technology

Figure 2.3: E00 plotted as a function of doping concentration for GaN at T= 300K.

2.2 Ohmic contact mechanism

The mechanism of forming ohmic contact at AlGaN/GaN heterostructures is com- plicated and has not been fully understood. Many studies were performed to inves- tigate the mechanisms of obtaining the low Rc in ohmic contacts with Ti/Al based contacts. The most frequent and acceptable explanation is that nitrogen atoms are extracted from AlGaN layer and thus leaving N-vacancies as n-dopants [25, 26, 27]. Another explanation for the mechanism of ohmic contact formation is that with the low work function compound/alloy formed at the metal-semiconductor interface, a low barrier height is formed which enhance the thermionic or thermionic field emis- sion [28]. Luther et al proposed that the diffusion of native oxide on GaN and Al through Ti will be reduced, and therefore the ohmic contact with low work function Al-Ti intermetallic phase is formed [28].

The formation of TiN is the most common mechanism adopted to explain the low Rc. Nitrogen atoms are extracted from AlGaN to form TiN, which results N-vacancies in the barrier layer acting as n-dopants. Therefore, the barrier layer becomes heavily doped, which increases the tunneling probability and lowers the Rc [27]. Moreover, it was proposed by Luther et al that during the process, not only TiN but AlN was also formed [28]. Formation of AlN also creates N-vacancies, leaving heavily doped interface that narrows the depletion region. The barrier height might also be de- creased because of the potential across AlN [29]. Chaturvedi et al developed a model to study the mechanism of Ti/Al/Mo/Au metal stacks forming ohmic contacts on AlGaN/GaN heterostructure and the model disclosed that Ga diffused throughout the metal and reacted with Mo [30]. The Ga-vacancies lead to a charge imbalance and therefore, N atoms nearby replace the vacant Ga site in the lattice causing N-vacancies acting as n type dopants as described above.

It is believed that the formation of TaN has the same mechanism as TiN [27]. In this work, we assume that the formation of a thick TaN layer between Ta and AlGaN possibly terminates the mechanism of extracting N from AlGaN. Therefore,

8 2. Ohmic contact technology removing TaN by wet chemical etching and the re-depositing the metal stack were anticipated to increase the extraction of N from AlGaN.

2.3 Ohmic contact types

2.3.1 Planar contacts

Approaches to fabricate ohmic contact on AlGaN/GaN heterostructure includes planar, MBE regrowth and recess etched contacts. Planar contact is the simplest and the standard method to fabricate ohmic contacts on GaN HEMTs. A planar ohmic contact is formed by metallizing the contact area followed by annealing. Values of Rc from the literature are shown in Table. 2.1.

It is clearly found that typical annealing temperature are over 800◦C with an excep- tion of an Mo/Al/Mo/Au metal stack that the required annealing temperature of 650 ◦C. In addition, for Ti/Al based planar ohmic contacts, higher Al concentration in AlGaN barrier leads to higher Rc, which might due to the larger energy barrier. Ruvimov et al suggested that Al might form a barrier for the diffusion of N atoms and therefore higher Al composition in AlGaN layer might lead to higher Rc [31].

9 2. Ohmic contact technology

Metal Stack Barrier layer Annealing T Rc Ref (◦C) (Ωmm)

Ti/Al/Ni/Au GaN/Al0.28Ga0.72N/AlN 820 0.45 [4]

Ti/Al/Ni/Au Al0.24Ga0.76N 830 0.2 [5]

Mo/Al/Mo/Au In0.17Al0.83N/AlN 650 0.15 [7]

Ti/Al/Ni/Au In0.18Al0.82N/AlN 900 0.15 [32]

Ta/Si/Ti/Al/Ni/Au In0.18Al0.82N/AlN 825 0.36 [33]

Ti/TiN GaN/Al0.2Ga0.8N/AlN 850 0.13 [34]

Ti/TiN GaN/Al0.35Ga0.65N/AlN 850 0.6 [34]

Table 2.1: Comparison of Rc values and different metal schemes on different het- erostructure

2.3.2 Recessed contacts For planar contacts, the distance between ohmic metal stack and 2DEG is large and makes it difficult to obtain low Rc. In order to reduce the distance between 2DEG and ohmic metal, and avoid the required high anneal temperature of planar contacts and the complex process of regrown n+-doped contacts, recessed contacts has been extensively studied. Prior to the ohmic metallization, an recess etching step is performed, which is able to reduce the annealing temperature.

Čičo et al reported the Rc of 0.39 Ωmm obtained by recess etching before metal de- position and annealing with temperature of 700◦C while the conventional annealing temperature was above 800◦C[39].

However, the concept of recessed contacts adds more process parameters, such as recess depth and the sidewall slope that have to be optimized. In general, the recess depth can be divided into three different cases. The first case is that part of the barrier is still present and the contact mechanism of this case is similar to planar contacts as in Fig. 2.4(a), but with smaller distance between ohmic metal and 2DEG. However, a thinner barrier results in a smaller 2DEG density, which

10 2. Ohmic contact technology aggravates the formation of ohmic contact. Fig. 2.4(b) and (c) show that the barrier is still presented but it is so thin that there is no 2DEG formed under it and the barrier is completely removed during the etching process, respectively. In this work, to avoid the difficulty of controlling the etching depth precisely, the recess is etched below 2DEG. Therefore, the deposited ohmic metal can have direct contact to the AlGaN and GaN layer at the sidewall.

Figure 2.4: Illustration of three different recess depth cases. (a)the barrier is still present (b)the barrier is still present but it is too thin to retain 2DEG under it (c)the barrier is completely removed in etching process.

Bergsten demonstrated a very low Rc of 0.14 Ωmm with the almost removed InAlN barrier [40]. Lin et al reported a low Rc of 0.24 Ωmm with the completely removed AlGaN barrier layer[12]. Buttari et al obtained the Rc of 0.27 Ωmm with the barrier only slightly etched [41]. Generally, there is no clear conclusions of the recess depth since etching depth is not uniform. Table. 2.2 shows some literature values of Rc obtained from recessed contacts. It can be seen that the annealing temperature of recess contacts is lower than planar contacts.

11 2. Ohmic contact technology

◦ Metal Stack Barrier layer Annealing T ( C) Rc (Ωmm) Ref

Ti/Al/Ni/Au In0.18Al0.82N 700 0.39 [39]

Ta/Al/Ta In0.17Al0.83/AlN 550 0.14 [40]

Ta/Al/Ta Al0.25Ga0.75N 575 0.24 [12]

Ti/AlMo/Au GaN/Al0.3Ga0.7N/AlN 850 0.26 [42]

Table 2.2: Literature values of Rc for recess contacts on different heterostructure

2.3.3 n+-GaN regrowth contacts

Regrowth contacts process starts from etching past the barrier layer, and a lattice matched, highly doped n-GaN is then grown in the recess. Saunier et al utilized the regrowth n-doped GaN contacts to achieve extremely low Rc of 0.1 Ωmm [35], which is similar in Tang’s study [36]. Regrowth n-GaN in the recess improves the Rc due to the direct contact to 2DEG. This fabrication method can offer extremely low Rc value, but the process is complicated and the cost is high, which possibly makes this method unsuitable for mass production. Moreover, to obtain low Rc, a large density of 2DEG is required while the standard electron sheet concentration 13 −2 ns of AlGaN/GaN heterostructure is about 10 cm which makes it more difficult to achieve. Literature values of the contact resistance on different heterostructure are shown in Table. 2.3. From those papers, higher ns in the barrier decreases the contact resistance while the obtained value of Rc from barriers with moderate ns are similar to those metal-based contacts.

12 2. Ohmic contact technology

Berrier Rc ns Ref (Ωmm) (cm−2)

13 In0.17Al0.83N/AlN 0.16 1.92 × 10 [15] (2.5/1.5 nm)

In0.18Al0.82N/AlN 0.10 — [35] (8/1 nm)

13 In0.17Al0.83N/AlN 0.16 2 × 10 [37] (5.6/1 nm)

13 GaN/In0.17Al0.83N/AlN 0.22 1.6 × 10 [38] (2/3.5/1 nm)

Table 2.3: Rc of regrowth n-GaN contacts on different heterostructure

2.4 Physical modelling

TCAD simulation was made to promote further understanding of the ohmic con- tact formation. The simulation model was based on the thermionic field emission mechanism. In the simulation the contact on the top of the structure is modelled as a Schottky contact while the bottom contact is a perfect ohmic contact (Fig. 2.5b). The GaN layer and the lower part of the AlGaN layer are assumed to have a background doping of 1015cm−3.

The n-doping, representing the N-vacancies, is distributed in the upper part of AlGaN layer where the dopants are. The simulations study the impact of n-doping (N-vacancies), doping depth and working temperature.

(a) (b)

Figure 2.5: The simulation model structure(a)model from software (b)schematic of the model

13 2. Ohmic contact technology

As expected, the contact resistance is inversely proportional to the doping concen- tration (Fig. 2.6a and b). The specific contact resistivity is obtained from the slope in Fig. 2.6 and is listed in Table. 2.4. For doping concentration higher than > 1019cm−3, field emission (FE) is dominant. The depletion width becomes thinner while the Schottky barrier keeps the same height. Therefore, it becomes easier for carrier to tunnel through the barrier and hence, the ρc is decreased. For thermionic field emission (TFE), the doping concentration is simulated from 1018 to 1019cm−3. The carriers with enough thermal energy can tunnel through the midsection of the barrier, which implies a direct temperature dependence. The relation between Rc and doping concentration of FE and TFE is shown below [43].

 φB  Rc ∝ exp √ (FE) (2.5) ND

" φ # R ∝ exp √ B (TFE) (2.6) c E00 ND cosh kT

From the equations, it is clearly shown that for FE and TFE, Rc is proportional to √ 1 . The simulation results is consistent to the experimental results reported by ND Yu et al [43]. The investigation of re-metallization is motivated by the assumption that the creation of N-vacancies is limited by the availability of Ta near the metal and semiconductor interface. By removing the metal nitride, more N-vacancies may be created in the barrier layer, and Rc is anticipated to be decreased.

The depth of the region where N-vacancies are created is unknown. From the sim- ulations, it is clear that a larger doping depth promotes a low contact resistivity (Fig. 2.6c and Table 2.5). The doping depth starts at the surface and with larger doping depth, the dopants spread deeper in the AlGaN layer, which increases the effective area. Due to the current crowding effect, which is introduced in Chapter 4, the resistance has an inversely proportional relation to the effective area [46]. Consequently, with a larger doping depth, lower resistance is obtained.

The temperature dependence of the contact resistivity is simulated with a constant doping concentration of 1019cm−3 while the temperature is varied from 300K to 450K. From the Fermi-Dirac distribution function, more carrier are excited above the Fermi level at higher temperature. Therefore, current density increases and leads to smaller Rc. Chang et al reported that for moderate doping concentration, Rc has strong dependence on temperature [44]. The reported results are consistent to the simulation results (Fig. 2.6d and Table 2.6).

14 2. Ohmic contact technology

(a) (b)

(c) (d)

Figure 2.6: Schematic of the simulation model structure and the obtained results of various doping depth, concentration and the temperature.

Doping concentration (cm−3) 1 × 1018 5 × 1018 9 × 1018 1 × 1019 1 × 1020 K 1 × 1021 2 −4 −6 −6 −9 −9 ρc (Ωcm ) 6.98 1.16 × 10 1.18 × 10 1.04 × 10 8.11 × 10 4.47 × 10

Table 2.4: The obtained ρc of different doping concentration

Doping depth (µm) 0.01 0.025 2 −6 −7 ρc (Ωcm ) 1.79 × 10 5.57 × 10

Table 2.5: The obtained ρc of different doping depth

Temperature (K) 300 350 400 450 2 −6 −6 −7 −7 ρc (Ωcm ) 1.79 × 10 1.07 × 10 6.18 × 10 3.53 × 10 Table 2.6: The obtained resistance under different temperature

15 2. Ohmic contact technology

16 3 Fabrication Process

The fabrication process for ohmic contacts on AlGaN/GaN heterostructures is in- troduced in this chapter. The process starts from standard cleaning. Then a silicon nitride passivation layer with a thickness of 55 nm is deposited by low-pressure chemical vapor deposition (LPCVD). Device isolation is achieved with mesa etching by inductive coupled reactive ion etching (ICP-RIE) plasma after photolithography definition of the mesa structure. Then the ohmic structure is patterned with laser writer where an image reversal photoresist AZ5214 is utilized for lift off. After the ohmic recess etching with ICP-RIE, the ohmic metal stack Ta/Al/Ta is deposited by electron beam evaporator with the same resist, making the ohmic metal self-aligned to the recess. In the final step, lift-off is performed to obtain the TLM structure (Fig. 3.1).

Figure 3.1: Schematic of the ohmic structure in this work.

3.1 Standard Cleaning

The 15 × 15mm2 samples used in our project is diced from the same epitaxial wafer, which the AlGaN/GaN epitaxial heterostructures are grown on SiC by metal-organic chemical vapor deposition (MOCVD). Before the high temperature LPCVD SiNx deposition, samples must be cleaned by standard RCA cleaning process to remove all particles and metal contamination on the sample surface to prevent pollution of the LPCVD chamber. Samples are first immersed in remover 400, isopropanol and deionized water for five minutes respectively to remove the photoresist used for protection during dicing. Remover 400 is usually used for photoresist removal and is composed by 1-methyl-2-pyrrolidone. This is followed by cleaning in SC1 solution ◦ NH4OH(25%):H2O2:H2O=1 : 1 : 5 at 80 C for 10 minutes to remove organics; then ◦ immersed in SC2 solution HCl : H2O2 : H2O=1 : 1 : 6 at 75 C for 2 minutes to strip metallic particles. Subsequently immersed in NH4OH(25%):H2O=1 : 20 for two minutes to remove oxide residues on the sample surface.

17 3. Fabrication Process

3.2 LPCVD

Low-pressure chemical vapor deposition is one of the most common manufacturing process in thin film deposition. LPCVD reactors can be divided into hot wall and cold wall system [47]. Temperature distribution in hot wall system is more uniform than cold wall systems and the convection effects can be reduced in hot wall systems [48]. With the advantages of better uniformity, reduced convection effect and good conformal step coverage [49], hot wall LPCVD is used in this study for depositing SiNx passivation layer.

Silicon nitride is a dielectric material that has high electrical resistivity, high chem- ical resistance to acids, bases, salts and molten metals, as well as the ability to withstand elevated temperature exposure. Therefore, silicon nitride thin films have been widely used in device processing such as passivation layer, mechanical protec- tive mask, diffusion barrier and gate dielectrics [50, 51, 52]. For instance, they could be used as passivation layer because they are good barriers to water and sodium, and they can serve as masks during selective oxidation process since they oxidize very slowly.

After cleaning, the wafers were placed on quartz boat in the furnace. Dummy silicon wafers were also placed on the quartz boat in order to stimulate a fully loaded boat and present a consistent thermal mass [48, 53]. A 55nm SiNx passivation layer was grown under temperature of 820◦C, a pressure of 250 mTorr with dichlorosilane (DSC) and ammonia (NH3) flows of 224 sccm and 23 sccm, respectively to supply the silicon and nitrogen. The reaction is described by the formula:

3SiCl2H2 + 4NH3 → Si3N4 + 6HCl + 6H2

3.3 Mesa

Device isolation is commonly achieved by ion implantation or mesa etching. Mesa etching is often performed with Cl2 based plasma dry etching to define the active region of a device [54]. However, 2-D electron gas (2DEG) channel will be exposed on the side wall which may cause excessive gate leakage when contacted with the gate metalization [55]. Another approach to achieve device isolation is high energy ion implantation, which can creates point defects or simple defects that produce deep-level electron and hole traps [56]. Through these defects, high-resistivity is achieved. Ion implantation has the advantage of maintaining the surface flat which simplifies further processing and having lower gate leakage compared to mesa etching [55]. However, the thermal stability of ion implanted material affects the reliabil- ity since implant-damaged material might degrade at high temperature [57, 58]. Consequently, mesa etching is preferred than ion implantation in this work.

18 3. Fabrication Process

3.3.1 Photolithography Photolithography is a technique that transfer a desired pattern to the photoresist on the wafer surface and is extensively used in IC fabrication. In this work, photolithog- raphy was performed using a non-contact laser writer (Heidelberg Instruments DWL 2000). The lithography process includes spin coating, exposure and development.

A laser writer is used instead of stepper or mask aligner since maskless lithography process could increase the precision and flexibility, and direct writing system could obtain better control of the photoresist profile through the control of the exposure dose and focus settings compared to the mask lithography, where mask contact introduces uncertainty in dimension control and resist profiles. The irradiance of laser beam can be described by Gaussian distribution, and the focus offset is the deviation of best exposure focus from default lens position. Focus off set is optimized to ensure that the photoresist is positioned at the best focal plane (Fig. 3.2).

Figure 3.2: The schematic of the position of photoresist and the laser beam.

Prior to the photoresist coating, hexamethyldisilazane (HMDS) was coated on sam- ple, which acts as an adhesion promoter. After applying the HMDS on sample surface, the photoresist is spin coated. The sample is then mounted on a vacuum chuck of a spinner. Spin coating is commonly used to coat thin films in order to obtain good uniformity of the films. The thicknesses are usually of the order of micrometers and the thickness of the resist is determined by spinning acceleration, spinning speed, spinning time and the viscosity of the photoresist.

19 3. Fabrication Process

Mesa isolation and recess ohmic contact are both achieved by ICP-RIE. However, the photoresists used in both process are different. Positive photoresist is used in mesa etching, where the chemical structure of exposed area on the photoresist becomes more soluble and can be removed easily in developer. An image reversal photoresist is applied on the wafer during ohmic contact lithography process. Since ohmic contact is formed at the sidewall between the 2DEG channel and ohmic metals by e-beam evaporation followed by lift-off process [12], the applied photoresist becomes an important factor.

A positive photoresist (S1813) was applied for the definition of the mesa, while a negative, image reversal resist (AZ5214) is used for the ohmic contact definition. The AZ5214 is exposed by laser writer and the exposed area becomes soluble as normal positive photoresist. However, a special crosslinking agent in AZ5214 be- comes active after reversal baking at 125◦C for 1 minutes. The originally exposed area becomes almost insoluble and no longer light sensitive while the unexposed area still remains as normal positive resist. After flood exposure, the originally unexposed area becomes soluble and is developed in spin developer.

In this work, the resist is deposited with a rotation speed of 4000 rpm for 30 s. After spin coating, samples are softbaked at 110◦C for 2 minutes and 1 minute for the S1813 and AZ5214 resist, respectively. Softbake is performed to drive off most of the solvent in the resist since the dissolution rate in the developer can be affected by the solvent concentration in the photoresist. Then the resist coated sample is exposed by laser writer developed in developer MFCD26 for 90 seconds and 351B for 25 s for the S1813 and AZ5214 resists, respectively. Finally, the developed areas are subjected to plasma ashing.

3.3.2 Plasma Ashing Plasma ashing process and plasma descum process are carried out after samples are exposed by laser writer and developed. This step is in order to remove the residual resist, which might affect the uniformity of the etching rate across the whole sample. It is a common used cleaning method, which is highly effective and environmentally safe method. Oxygen is the common gas used in plasma ashing. The samples are put in a vacuum chamber. Oxygen gas fills the chamber and a plasma is generated by RF-power. The oxygen ions break the bonds of the resist. Water and carbon dioxide are generated during the process which is pumped from the chamber. In this work, the sample is etched by oxygen plasma with the power of 40 W for 30 seconds.

20 3. Fabrication Process

3.3.3 Plasma Etching The mesa etching is performed by an inductive coupled plasma reactive ion etching (ICP-RIE) system, which is a common type of high density plasma etching system. ICP-RIE is an anisotropic etching process that can be used to etch a great variety of materials; for instance, semiconductors, metals, dielectrics and polymers. Two separate energy sources independently control the ion density and energy in the system. One is applied to generate bias power to the lower electrode to control the ion energy, while the other applies power to ICP coil and hence can control the ion flux. An oscillating magnetic field is generated by the RF current through the coil. By controlling the induced magnetic field, the plasma density can increase significantly [47]. Different reactive gases are used when etching different materials; for instance, NF3 is commonly used for SiN and Si etching, argon plasma is used as physical etching gas, and Cl2 and SiCl4 gases are often used in III-V etching.

In our work, NF3 gas was used to etch silicon nitride layer with 50 sccm flow rate. The heterostructure was etched with Cl2 gas with flow rate of 20 sccm and Ar gas with 10 sccm flow rate. After mesa etching, an oxygen plasma striping of 100W for 2 minutes was carried out in order to remove etching residues. Then, the samples were cleaned in remover 400, isopropanol and deionized water for five minutes respectively in order to fully remove the photoresist S1813.

3.4 Ohmic Contact

Similar to mesa etching, ohmic recess is also etched by ICP-RIE. Silicon nitride is etched by NF3 with the flow rate of 50 sccm while the heterostructure is etched by Cl2, SiCl4, and Ar gases with the flow rate of 39 sccm, 1 sccm, and 10 sccm, respectively. An oxygen plasma ashing is also carried out with the RF power of 40W for 30 seconds to remove etching residues. Subsequently, ohmic metal deposition and lift-off are performed.

3.4.1 Metal Deposition E-beam evaporation is a form of physical vapor deposition in which the electron beam from charged tungsten filament bombards the target material which is then evaporated to the gaseous state and deposited on the wafer surface. When heating metals in the evaporator, the electron beam commonly sweeps over the target in order to heat the target material with a even heat distribution.

Prior to metal deposition, the wafer is immersed in HF : H2O=1:10 solution for 4 minutes and HCl : H2O=1:10 solution for 60 seconds to strip oxides and other etching residues. In this work, the ohmic metal stack Ta/Al/Ta is deposited by electron beam evaporator in a ultra high vacuum chamber. However, Ta has a very high melting point of 3017◦ C which makes it difficult to evaporate with the sweeping mode. Consequently, the electron beam focuses at a fixed position. The thickness of first Ta layer is investigated in this work since it is considered to affect the formation of the ohmic contact. The Al layer of 280 nm is evaporated in the

21 3. Fabrication Process sweep mode since the melting point of Al is 660◦ C which is lower than Ta and easier to evaporate. The second 20 nm Ta layer is deposited to prevent the oxidation of Al. After the metalization completed, lift-off was performed in remover400 at 65◦ C and ultrasonic bath for 5 minutes.

3.4.2 Re-metalization

The re-metalization step starts from the removal of the original ohmic metal stack and the TaN formed during annealing. Tantalum’s chemically inert and is resistant to most chemicals. The most common method used to etch Ta is dry etching with CF4/O2 gas [68]. Although dry etching is also high selective, coating photoresist as a mask needs to ensure the alignment during the exposure which makes it more complicated compared to wet etching. Therefore, wet chemical etching is chosen in this work. Wet chemical etching is a process that immerse the sample in a bath of liquid chemicals in order to remove a certain type of material. Wet etching is a simple and low cost etching technique. However, etching with liquid chemicals is usually isotropic and less controllable and hence the reproducibility is low compared to dry etching. Since the aim is to etch Ta/TaN, while other layers such as the herterostructure AlGaN/AlN, the buffer layer GaN and the substrate SiC should be remain unaffected.

As listed in table (Fig. 3.3), the heated solution of KOH/NaOH with water can etch most of the material including the AlGaN/GaN heterostructure, making it an unsuitable etchant in this investigation. In this work, we choose HF (5%) : HNO3 : H2O(20%) =1 : 2 : 1 to selectively etch the ohmic contact at room temperature. This solution is suggested in [59] without stating the etching rate. We repeated the Ta etching for several times and obtained two methods. The first way is immersing the sample in Ta etchant for over 50 minutes. However, it is too time-consuming to perform. Since the ohmic metal stack formation was Ta/Al/Ta, another way is using standard Al etchant composed of H3PO4 : CH3COOH : HNO3= 60% : 3.5% : 2.5%. The etching rate is 30 Å/sec.Subsequently, the sample was immersed in Ta etchant, Al etchant and Ta etchant again for 6, 9, 20 minutes respectively. In both methods, agitation during etching secures a scalable etch rate, which otherwise is limited by the formation of bubbles of etching products generated during the process.

22 3. Fabrication Process

Figure 3.3: The etchant versus etching target material (X: The target material can not be etched by the etchant, : The target material can be etched by the etchant, –: Not found from the literature).

3.4.3 Sputtered Ta Sputtering is a type of physical vapor deposition techniques which is based on ion beam bombardment. Sputtering is a commonly used method of thin film deposition, which can be divided into several types such as DC sputtering, magnetron sputtering, and RF sputtering etc. In the DC sputtering system, the target material is placed on the cathode while the substrate is placed on anode. Usually, the sputtering gas filled in the chamber is inert gas (typically Ar). The applied DC voltage between electrodes maintains the glow charge in which the Ar+ is generated. The Ar+ ions are accelerated at the cathode and thus sputter the target material on the cathode resulting the thin film deposition of the target material on the substrate.

In this project, sputtered Ta layer was investigated in order to improve the sidewall coverage. Furthermore, sputtering also offers in-situ cleaning with Ar-plasma. The first Ta layer was changed to be deposited with DC sputtering while the following Al/Ta was still deposited by evaporation since there is no Al in the sputtering tool at Chalmers.

23 3. Fabrication Process

24 4 Characterization

In this work, two main characterization techniques are used. The first one is trans- mission line method (TLM), which is extensively used to examine the ohmic contact behavior by characterizing the contact resistance and sheet resistance. The other is the scanning electron microscopy (SEM), which is used to check the cross section of ohmic contact. Both methods are introduced in this chapter.

4.1 Scanning electron microscopy

Scanning electron microscope (SEM) is a widely used instrument to examine surface characteristics. Instead of light as the illumination source, a focused electron beam is emitted from the electron gun and focused on the sample surface. A SEM image is formed by collecting the scattered electrons. The electro-optical path of the SEM contains several electromagnetic lenses and deflection coils to control the diameter and astigmatism of the electron beam. Due to the interaction between the elec- tron beam and the sample surface, secondary electrons (SE) can be generated from the inelastic scattering, while backscattered electrons (BSE) can be produced from elastic scattering which often retain 60 to 80% of the incident electrons’ energy.

SEs usually have the energy of several electron volts, and SEs can only escape from the volume near the sample surface of the interaction zone. However, BSEs have higher energy and can escape from deeper level of the interaction zone compared to SEs. Consequently, images generated by SEs will have higher resolution of surface topography while images produced by BSEs demonstrate clear compositional con- trast if the sample contains more than one chemical elements. The compositional contrast depends on the backscatter coefficient, η, which increases with the atomic numbers of the chemical elements in the sample. n η = BSE (4.1) ni

η represents the ratio of the number of the BSEs escaping from the sample nBSE to the number of incident electrons ni [69]. Thus, the sample that contains higher atomic number elements would generates more BSEs. The different number of the collected BSEs appear different gray color levels in a black and white image. Hence, the higher atomic number elements the area contains, the brighter it appears in the SEM image. Despite that the SE signal contributes to form compositional contrast image, the contrast is not clear since the SE signal does not have as strong depen- dence on the atomic number as BSE [69]. In this work we use SEM to characterize

25 4. Characterization the cross-sections of the samples in backscattered electrons mode to identify the epitaxial layers and metal stacks.

4.2 Transmission line method

Numerous of techniques can be used to obtain contact resistance, the transmission line method (TLM) is the most common and popular method. It was originally pro- posed by Shockley [70] and was further studied and extended by Berger [71], which provided a convenient and simple method to determine resistive performance of the ohmic contacts, including the contact resistance (Rc), specific contact resistance, (ρc), sheet resistance (Rsh) and transfer length.

4.2.1 TLM structure The ladder structure consisting of more than three rectangular metal contacts, which are identical in the area, are used as the test structure for TLM (Fig. 4.1). These metal pads are separated with adjacent contact pads with incrementing spacing dis- tances (ranging from 5 to 30 µm (d1 to d5) in this work). The resistance between every contact pads are measured with four probes. The outer two probes give the current and the inner two probes measure the voltage difference, and the resistance values are given. Four-probe measurement is utilized instead of two-probe mea- surements since the current and voltage are separated which eliminate the lead and contact resistance from the measurement [72].

(a) (b)

Figure 4.1: (a)Schematic TLM strcture (b)Microscope image of the TLM structure

26 4. Characterization

4.2.2 Epi-layer sheet resistance Rsh The resistance of a homogeneous semiconductor can be expressed by the sheet re- sistance if the semiconductor is homogeneous rectangular. The resistance and sheet resistance is expressed as: L R = ρ (4.2) tW ρ R = (4.3) sh t where ρ, L, t, and W represents the resistivity, length, thickness, and width of the semiconductor, respectively. Introducing the Rsh enables to consider resistors as numbers of squares since the resistance can be simply obtained by multiplying by Rsh. However, it is hard to consider semiconductor as a homogeneous material since the semiconductor is usually a diffusion or epitaxially grown layer. Hence, an average resistivity ρ¯ can be calculated as a first approximation by considering the doping profile of the layers.

4.2.3 Contact resistance Rc and specific contact resistivity ρc

The plot of Rtot as a function of dx is shown in Fig. 4.2b. Rtot is the sum of several components, in which Rm is the resistance of contact metal, Rsemi is resis- tance due to the semiconductor, and Rc is the contact resistance between metal and semiconductor interface.

(a) (b)

Figure 4.2: (a)A schematics shows the different components of Rtot. (b) Total resistance Rtot plotted as a function of isolation distance dx

From the Fig. 4.2b, Rc and Rtot can be obtained. An extrapolation at the point of zero distance d=0 represents 2Rc. In this work, Rc is scaled with the width of the contact, and hence has the unit of Ωmm. Moreover, Rtot can be expressed by Equation 4.4.

27 4. Characterization

Rtot = 2Rm + 2Rc + Rsemi (4.4)

In general, the contact metal resistance Rm is usually much smaller than contact resistance Rc, and hence can be ignored. The slope of the fitting line (Fig. 4.2b) represents the sheet resistance Rsh. Consequently, Rtot can be expressed by Equation 4.7.

Rc >> Rm (4.5)

R R = sh d (4.6) semi W R R = 2R + sh d (4.7) tot c W The transfer length is defined as where the voltage, caused by current transferring between semiconductor and metal contacts, drops to 1/e of its maximum value, which is usually at the contact edge. The transfer length is dependent on specific contact resistance and sheet resistance, and the relation can be expressed as:

s ρc LT = (4.8) Rsh

If L ≥ 1.5LT , the effective contact area, Aeff , can be treated as LT W . Due to the current crowding effect, LT can be much smaller than L, and consequently, Aeff can also be much smaller than the contact metal area. Hence, this means that in the close proximity of the contacts, the current density would be higher than expected and therefore needs to be considered.

2 Specific contact resistivity,ρc, is usually measured in Ωmm which independent of contact area or geometry. It can be defined as in Equation 4.9:

∂J !−1 ρc = (4.9) ∂V v=0 The specific contact resistance contains the contact resistivity of both the interface and the regions above and below it. ρc can also be determined directly by the contact resistance Rc, transfer length LT and the width of contact pads. It can be expressed as:

ρc = RcLT W = RcAeff (4.10)

Aeff in the equation is the effective contact area. The contact area is always different from the Aeff due to the current crowding effect which is presented by Kennedy et al [73]. The current going through the semiconductor is uniform; however, the current flow is not uniform through the metal contacts. Since the current flows through the metal contacts with low resistance, and therefore, the current density at the edge of the contacts are higher and will drops to zero at the far edge. For small ρc, only the edge is used for current conduction and the used area of the contacts would be

28 4. Characterization

expanded for high ρc because of the high transition resistance which is shown in Fig. 4.3.

] (a) High ρ c (b) Low ρc

Figure 4.3: Current flow through contact for high and low ρc

29 4. Characterization

30 5 Results

The main results of this project will be presented in this chapter and can be divided into three sections, including laser writer intensity and focus offset optimization, re-metallization ohmic contact, and initial results of ohmic contacts fabricated by sputter deposition.

5.1 Laser writer focus/intensity test

The optimal laser writer focus offset and dose enable to obtain better control of the photoresist, laser writer focus offset was from 5% to 35%, dose intensity from 30% to 60% and the transmittance keeps constant at 25%. The focus/transmittance/in- tensity matrix settings and the equivalent energy are shown in Fig. 5.1.

Figure 5.1: Illustration of the laser focus/transmittance/intensity matrix with the exact energy received on the substrate.

After evaporation and lift-off, the sample was annealed at 575◦C for 40 minutes, and the contact resistance obtained through TLM is presented as a contour plot in Fig. 5.2.

31 5. Results

Figure 5.2: The contour plot of the obtained Rc from the laser focus/dose matrix.

Lin et al reported that the sidewall angle can be controlled by reversal baking temperature and laser writer exposure intensity [12]. For steeper sidewall angle, Rc increases due to the smaller contact area. However, Rc also increases with smaller sidewall angle since the increase of depleted GaN leads to decreasing AlGaN and hence, hinder the formation of 2DEG [40] (Fig. 5.3). Moreover, photoresist profile is an important factor to determine the sidewall angle. Consequently, the optimal exposure intensity is studied in this work to control the photoresist profile through varying the laser writer focus offset and dose while the transmittance is constant. It can be seen from the plot that the lowest Rc obtained is around 0.23 Ωmm and from Fig. 5.1, the corresponding laser energy received on the substrate is 9 mJ/cm2. This optimal laser writer focus/transmittance/intensity is applied in the subsequent parameter testing.

Figure 5.3: The schematic of (a) larger sidewall angle (b) smaller sidewall angle and (c) the GaN depletion region due to too small sidewall angle.

32 5. Results

5.2 Ohmic contact re-metallization

The re-metallization process includes etching away the ohmic metal stack and re- depositing the metal again. The re-deposition metal stack annealed at 600◦C was characterized by TLM. Rc and Rsh before and after re-metallization process are plotted in Fig. 5.4.

(a) Rc versus annealing time (b) Rsh versus annealing time

Figure 5.4: (a)Rc comparison of original and re-deposited ohmic metal stack. (b)The plot of Rc versus annealing time and the comparison of original and re- deposited ohmic metal stack.

The lowest Rc of the re-deposited metal contacts obtained in Fig. 5.4a is 0.5 Ωmm while the lowest Rc of original metal contacts is 0.28 Ωmm. It is clearly shown that the Rc increases after re-metallization process. The Rsh versus annealing time (Fig. 5.4b) shows that the sheet resistance keeps similar values, which is around 267 Ω/, after the etching followed by metal re-deposition process. The similar Rsh indicates that the epi-layer is not damaged during the wet etching step and hence, the increase in Rc is not caused by epi-layer damage.

The sample was originally deposited with the metal stack Ta/Al/Ta with thickness ◦ of 15/280/20 nm respectively, and annealed at 600 C. Due to the increase in Rc and unaffected Rsh, we assumed that the cause of the increasing Rc might be the annealing temperature and the thickness of the first Ta layer according to the results from Lin et al [12]. Consequently, we reduced the annealing temperature to 575◦C and the thickness of the first tantalum layer was changed to 5 and 10 nm.

33 5. Results

(a) 5 nm (b) 10 nm

Figure 5.5: The plot of Rc versus annealing time while first Ta layer thickness differs.

Fig. 5.5 is the plot of Rc and Rsh versus annealing time for the Ta thickness of 5 and 10 nm respectively. Compared to the results of 15 nm Ta in Fig. 5.4a, value of the lowest Rc for 10 nm Ta decreases from 0.5 Ωmm to 0.41 Ωmm while the lowest Rc for 5 nm Ta increases from 0.5 Ωmm to 0.67 Ωmm. This might be related to the larger mask we applied during the re-metallization process. During the first metallization step, the applied mask was smaller, and hence the samples had to be evaporated with a tilt angle. However, the mask was enlarged in the re-metallization step to ensure the correct alignment. Fig. 5.6a illustrates the original mask in the first metallization process and the larger mask in re-metallization process and the cross section after re-metallization is shown in Fig. 5.6b.

(a) The schematic of the larger pho- (b) The cross section of first and toresist . re-metallization.

Figure 5.6: The schematic shows the enlargement of the photoresist and the cross section of the ohmic contact.

34 5. Results

Because of the smaller mask and the tilt angle during the first metal evaporation, the sidewall Ta thickness is thinner than the bottom Ta thickness which is expected to be 15 nm (Fig. 5.7). The exact sidewall thickness measured with SEM is shown to be 13.95 nm. However, with the larger mask used in the re-metallization step, the re-metallization sidewall Ta thickness is thicker than the sidewall thickness in the first metallization step. Consequently, the expected re-deposited sidewall Ta thickness for 10 nm is close to the original deposited sidewall Ta thickness while 5 nm Ta is too thin for the contacts. This is the possible explanation for the decreased Rc for 10 nm and the increased Rc for 5 nm.

Figure 5.7: Cross sectional SEM image of the sample with the measured Ta thick- ness.

35 5. Results

5.3 Sputtered Ta

To improve the sidewall metal coverage, the first Ta layer were deposited by sput- tering for three different thickness (10, 15, 20 nm) with the following Al/Ta layer (280/20 nm) deposited by e-beam evaporation. The contact resistance was also been characterized and is shown in Fig. 5.8a.

(a) Rc (b) Rsh

Figure 5.8: The obtained (a)Rc and (b)Rshversus annealing time with different metal thickness.

Compared to the lowest Rc of evaporated metal stack (0.23 Ωmm), the lowest Rc of sputtered Ta was 0.6 Ωmm with the Ta thickness of 10 nm and annealing at 575◦C for 64 minutes. However, the sample with thickness of 15 nm did not perform as ohmic contacts even after annealing for 32 minutes while the lowest Rc obtained at the annealing time of 16 minutes was 6.65 Ωmm. Similarly, the lowest Rc of the sample with Ta thickness of 20 nm was obtained at the annealing time of 16 minutes, which is 1.72 Ωmm. The Rc started to rise after 16 minutes annealing for the samples with 15 and 20 nm Ta. The sheet resistance of 10 nm Ta sample is similar to the evaporated sample which is around 280 Ω/. However, Rsh of 15 nm sample is around 215 Ω/ which slightly lower than the evaporated sample while Rsh of 15 nm is much lower than other samples, seen in Fig. 5.8b.

There are two assumptions that we made for the observed high Rc of all sputtered samples and the slightly lower Rsh of 20 nm Ta. The first assumption is that since the sputtered thin film is denser than evaporated thin film due to the incident momentum [74], the sputtered thin film should be thinner than the evaporated thin film. Consequently, the sample with 10 nm sputtered Ta obtained the lowest Rc obtained the lowest Rc while the optimized thickness of evaporation is 15 nm. Another assumption was made due to the lower Rsh of 20 nm Ta. The high energy ion in sputtering process might lead to atomic scale damage on/below the sample surface [75, 76], and thus the damaged epi-layer leads to the lower Rsh.

36 5. Results

Moreover, the Al and second Ta layer are deposited by e-beam evaporation after the first Ta is deposited by sputtering. However, once the sample contact with air, there may be oxide layer formed on the first Ta layer. Therefore, future investigation of depositing Au on Ta while sputtering to prevent from oxidation is also required.

37 5. Results

38 6 Conclusion and future work

The aim of this thesis is to optimize different aspects of ohmic contact formation. There are three types of ohmic contacts, planar contacts, regrowth n+-regrowth contacts and recess etched contacts. In this thesis, recess contacts are used and in order to avoid the depth control problem, the recess was etched beyond the 2DEG. The ohmic contact optimization in this work includes laser writer focus/- transmittance/intensity test, re-metallization as well as electron beam evaporation and sputtering were also investigated.

The optimal laser writer focus/transmittance/intensity test was tested on a sample, which composed of a focus/transmittance/intensity matrix. The lowest Rc obtained in this work is 0.23 Ωmm and the corresponding optimal laser focus/intensity is 25%/35% while transmittance keeps constant at 25% i.e. the laser power received on the substrate is 9 mJ/cm2.

One of the widely accepted explanations of ohmic contact mechanism is that the formation of TiN extracts the nitrogen atoms from AlGaN layer, and hence the N- vacancies are created and act as n-dopants, which makes the barrier layer heavily doped. In this work, we assumed that the formation of TaN between Ta and GaN would block the extraction mechanism of nitrogen atoms. Therefore, wet chemical etching was performed to remove the original ohmic metal stack and the formed TaN. Then the re-deposition of ohmic metal stack was carried out in order to extract more nitrogen atoms from GaN i.e. creating more N-vacancies in barrier layer.

The Rsh obtained from the characterization of re-metallized samples indicates that the epi-layer was not damaged during the wet etching process. However, com- paring the lowest achieved Rc between original metallization (0.28 Ωmm) and re- metallization (0.41 Ωmm), it can be concluded that a lowering of the contact resis- tance was not achieved by re-metallization described in this report.

Different sputtered Ta thickness (10/15/20 nm) was investigated in this project. The lowest contact resistance of the sputtered Ta was 0.6 Ωmm when the Ta thickness was 10 nm, annealed for 64 minutes at 575◦C. Although the samples with 20 nm Ta showed ohmic behavior, the Rsh was about 65 Ω/ lower than the evaporated samples. These are results from the initial tests of sputtered Ta and thus further investigation about the possible damage caused by high energy ions and the thickness of the sputtered Ta is needed.

39 6. Conclusion and future work

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