A Novel Gaas MESFET with Surface Oxygen Implantation

Effect of Cl2/Ar dry etching on p-GaN with Ni/Au metallization characterization

Kuang-Po Hsueh, Hung-Tsao Hsu, Che-Ming Wang, Shou-Chian Huang, Jinn-Kong Sheu+ and

Yue-Ming Hsin*

Department of Electrical Engineering, National Central University, Chung-Li 32054, Taiwan

+Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan

70101, Taiwan

Abstract

The surface root mean square roughness and the depth display monitor (the

Bearing analysis) of etched 500 nm p-GaN after Cl2/Ar reactive ions etching as well as

the I-V characteristics of Ni(20 nm)/Au(20 nm) metallization have been investigated in

order to determine the effect of dry etching on Ni/Au contacts. By varying Cl 2/Ar flow

rate and RF power, there is no significant increase in etching rate by only increasing Cl2

flow rate while keeping the power and chamber pressure constant. The increased RF

power is more efficient in generating concentration of Cl radicals in the plasma to etch

p-GaN. From experimental results, the surface roughness RMS is not directly related to

the I-V characteristics of Ni/Au contacts, but Bearing ratio is. We concluded the I-V

characteristics (and thus Schottky barrier height) is more consisted with the Bearing

analysis of existing nanorods than that with surface roughness in the Cl2/Ar etch.

*Corresponding author: Department of Electrical Engineering, National Central

1 University, Chung-Li 32054, Taiwan, E-mail: [email protected], Tel: +886-3-

4227151 ext 34468, Fax: +886-3-4255830

2 From the success in the gallium nitride (GaN) based devices such as blue light emitting diodes (LEDs) and laser diodes (LDs), metal-semiconductor field effect transistors (MESFETs) and high electron mobility transistors (HEMTs) have been developed comprehensively. 1-5 However, GaN based heterojunction bipolar transistors

(HBTs) is still under developing due to the difficulties of high doping p-GaN base and good base ohmic contacts. 6-12 It is well known that the roughness and the contamination resulted from the dry etching process of p-GaN surface will increase Schottky barrier height (SBH) at the metal/semiconductor interface. 13-16 Because the dry etching process is required for GaN based npn or pnp HBTs, it is the key research to obtain the good ohmic contact on p-GaN after dry etching. This paper presents both the etched surface root mean square (RMS) roughness and the depth display monitor (the Bearing

analysis) of the doped p-GaN after Cl2/Ar reactive ions etching (RIE) as well as the study of Ni(20nm)/Au(20nm) metallization.

All GaN materials used in this study were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. An undoped GaN layer with thickness of

2 µm was grown first, followed by the growth of 1 µm thick p-type GaN doped with

Mg. The activation annealing was carried out at 750℃ for 20 min in the furnace. A bulk carrier concentration of ~ 31017 cm-3 and a mobility of 10 cm2V-1s-1 were obtained from

room temperature Hall measurements. Prior to Cl2/Ar reactive ions etching, all samples

3 were ultrasonically degreased with acetone and isopropyl alcohol for 10 min and then rinsed with de-ionized (DI) water. Dry etching effect of p-GaN using RIE mode of high density plasmas system (Unaxis Nextral 860L) has been investigated by systematically

varying RF plasma power and Cl2/Ar mixture gas composition. Under the different etching conditions, an etched thickness of 500 nm is set to the same for all samples.

After dry etching, samples were analyzed by atomic force microscopy (AFM) to evaluate the surface properties including AFM images, RMS roughness and the depth display monitor (the Bearing analysis). In addition, the relationship between the surface properties and the corresponding I-V characteristics of Ni/Au metallization is studied.

After 500 nm GaN etching, all samples were then patterned by the standard photolithographic technique for contact measurement. Prior to metal deposition, hydrofluoric acid was used to remove the native oxide layer on p-GaN. The Ni(20 nm)/Au(20 nm) contact patterns were deposited on p-GaN by electron beam

evaporation and lift-off. The Ni/Au contact is then annealed in O2-containing ambiance at 500 C. Current-voltage (I-V) data were measured on those Ni/Au contact patterns with spacing of 10 µm using parameter analyzer (HP4156C).

First of all, etching effects of Cl2/Ar gas flow rate at room temperature on the etching rate and surface morphology for p-GaN are investigated. Fig. 1 shows the

etching rates and the RMS roughness for p-GaN as a function of Cl2/Ar gas flow rate at

4 constant RF power of 200W and chamber pressure of 20 mTorr. The Ar flow rate was maintained constant at 10 sccm during the etching process. From Fig. 1 it can be observed that the surface roughness reaches the lowest value of 74 nm at 150 sccm of

Cl2 flow rate. However, the etching rate increases slightly with increasing the Cl2 flow rate from 202 to 217 nm/min because the concentration of Cl radicals is not

17 proportional to the Cl2 flow rate, and thus the surface roughness is not strong function

of the Cl2 flow rate either. The corresponding AFM measurement results for various Cl2 gas flow rate showing nanorods existed on the surface of all p-GaN samples after dry etching as observed in other publications. 16 The exact mechanism of the formation of nano-structure is not fully understood yet. However, the creations of nanorods seem to be related to the crystalline quality of epitaxially grown GaN material and the ability of the dry etching process to dissociate GaN bonding. Fig. 2 shows the Bearing analysis

results as a function of Cl2/Ar gas flow rate. Bearing analysis reveals how much percentage of a surface lies above or below a given height. This measurement provides additional information beyond standard roughness measurements. Surface roughness is generally represented in terms of statistical deviation from average height; however, this gives little indication of height distribution over the surface. By using bearing analysis, it is possible to determine what percentage of the surface (the “bearing ratio”) lies above or below any arbitrarily chosen height. In Fig. 2, the Bearing ratio of the nanorods

5 means that the percentage of the nanorods of the total surface area. And it can be observed that the Bearing ratio of the nanorods has the lowest average value of 13.3% at

150 sccm of Cl2. At this Cl2/Ar gas flow rate combined with constant RF power of 200

W and chamber pressure of 20 mTorr, the etched surface of etched 500 nm p-GaN shows the lowest surface roughness and Bearing ratio of the nanorods. However, the difference is marginal. Fig. 3 shows the I-V characteristics for the Ni/Au contacts on p-

GaN as the function of Cl2/Ar gas flow rate. After 500nm etching on p-GaN, all samples show the Schottky barrier effect from original Ohmic contact. In order to determine the effective SBH of the Ni/Au contacts, the I-V method was employed. The I-V relation is given by 18-19

2 b   qV   J  A**T exp( )exp  1 (1) kT   nkT  

where A**, the effective Richardson constant, is 96.1 Acm-2K-2 for p-GaN ; 19 J is the current density; T is the measurement temperature in Kelvin; Φb is the barrier height; n is the ideality factor and k is the Boltzmann’s constant. SBH calculations showed that the SBH is 0.47 eV for the contact without dry etching, which is similar to

the published data. In addition, SBHs for adding Cl2/Ar etching are increased to around

0.6 eV and summarized in Table I. There is no significant difference in the results of

SBH in the functions of Cl2/Ar gas flow rate, because the RMS and the Bearing analysis

are not significantly different from these samples. Therefore, the Cl2/Ar gas flow rates

6 were fixed at 150/10 sccm respectively for the following experiment in varying RF power.

Fig. 4 shows the etching rates and the RMS roughness for etched 500nm p-GaN as

a function of RF power at constant Cl2/Ar gas flow rates of 150/10 sccm and chamber pressure of 20 mTorr, respectively. It can be observed that the surface roughness increased rapidly with the RF power increased from 50 to 200 W. The same trend for the etching rate is increased with increasing the RF power because of higher effective concentration of Cl radicals in the plasma. The corresponding AFM measurement results of etched p-GaN surface under various RF power showing the nanorods disappear on the etched surface at RF power of 50 W. The Bearing ratio of the nanorods as a function of RF power is again plotted in Fig. 2. The Bearing ratio of the nanorods decreased significantly with decreasing the RF power and reached almost 0% at RIE power of 50 W. Fig. 5 shows the I-V characteristics for the Ni/Au contacts on etched p-

GaN as a function of RF power. Using equation (1), calculations show that the SBHs are

0.50 eV, 0.52 eV and 0.60 eV for RF power of 50 W, 100 W and 200 W, respectively.

The corresponding data are summarized in Table I. From Table I, we can determined the

I-V characteristics (and thus SBH) is more consisted with the Bearing analysis of existing nanorods than that with surface roughness RMS. Less Bearing ratio, lower

SBH and better I-V characteristics has be observed.

7 We have investigated the effect of the I-V characteristics of Ni/Au contacts

(Schottky barrier height) on p-GaN from dry etching of Cl2/Ar mixture gas by varying

the Cl2 flow rate and RF power. There is no significant increase in etching rate by

increasing Cl2 flow rate while keeping the power and chamber pressure constant. The increased RF power is more efficient in generating effective concentration of Cl radicals in the plasma to etch p-GaN. From AFM observation, the surface roughness RMS is not directly related to the I-V characteristics of Ni/Au contacts, but Bearing ratio is. By optimizing the etching conditions to obtain the low Bearing ratio and resulted good I-V characteristics of Ni/Au contacts is the conclusion from this experiment.

8 ACKNOWLEDGEMENT

The authors would like to thank the National Science Council of the Republic of

China for financially supporting this research under contract no. NSC 93-2215-E-008-

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11 12 Figure Caption

FIG.1 RIE physical properties (RMS and etching rate) as a function of Cl2/Ar mixture gas flow rate.

FIG. 2 The Bearing ratio of the nanorods as a function of Cl2/Ar gas flow rate and RIE

RF power.

FIG. 3 I-V characteristics from Ni/Au contacts with 10 µm spacing as a function of

Cl2/Ar gas flow rate.

FIG. 4 RIE physical properties (RMS and etching rate) as a function of RF power.

FIG. 5 I-V characteristics from Ni/Au contacts with 10 µm spacing as a function of RIE

RF power.

Table Caption

Table I Summary of the RIE etching conditions and the corresponding surface physical/electrical characteristics.

120 250 Ar = 10 sccm

RF power = 200 W ) n Chamber pressure = 20 mTorr i 225 m / ) 100 m m n n ( (

t a M r

80 g n 175 i h c t e

60 150 0 50 100 150 200 250 Cl flow rate (sccm) 2

FIG.1 RIE physical properties (RMS and etching rate) as a function of Cl2/Ar mixture gas flow rate.

14 Cl flow rate (sccm) 2 ) 0 50 100 150 200 250 300

% 30 (

Ar = 10 sccm

s Chamber pressure = 20 mTorr

d 25 2 o

r Scanned area = 100 m

o 20 n a n

15 e h

f 10 o

o vary Cl flow rate i 5 2 t

a @ RIE RF power at 200W r vary RIE RF power

n @ Cl flow rate at 150 sccm i 2 r

e 0 50 100 150 200 250 300 350 400 B RIE RF Power (W)

FIG. 2 The Bearing ratio of the nanorods as a function of Cl2/Ar gas flow rate and RIE

RF power.

20 p-GaN Ar = 10 sccm RF power = 200 W 10 Chamber pressure

) = 20 mTorr A  (

e Cl = 50 sccm

r 2 r Cl = 100 sccm u 2 -10 C Cl = 150 sccm 2

Cl2 = 200 sccm without etch

-15 -10 -5 0 5 10 15 Voltage (V)

FIG. 3 I-V characteristics from Ni/Au contacts with 10 µm spacing as a function of

Cl2/Ar gas flow rate.

80 300 Ar = 10 sccm

Cl = 150 sccm ) 2 250 n Chamber pressure i

= 20 mTorr / )

200 m m n n (

40 150 t S

R 100 g n i

20 h c 50 t e

0 0 0 50 100 150 200 250 RIE RF Power (W)

FIG. 4 RIE physical properties (RMS and etching rate) as a function of RF power

20 p-GaN Ar = 10 sccm Cl = 150 sccm 10 2 Chamber pressure

) = 20 mTorr A  (

n e r r

u power = 50 W -10 C power = 100 W power = 200 W without etch

-10 -5 0 5 10 Voltage (V)

FIG. 5 I-V characteristics from Ni/Au contacts with 10 µm spacing as a function of RIE

RF power.

18 Table I. Summary of the RIE etching conditions and the corresponding surface physical/electrical characteristics.

Sample RF power Cl2 flow rate etching rate RMS Bearing ratio of SBH the nanorods W sccm nm/min nm % eV 200 50 202.8 84.5 22.21 0.61

a function of Cl2/Ar 200 100 202.2 82.0 16.86 0.60 gas flow rate 200 150 208.8 74.0 13.34 0.60 200 200 217.8 89.8 17.30 0.61 50 150 11.0 11.1 0.06 0.50 a function of RIE 100 150 81.0 24.3 5.14 0.52 RF power 200 150 208.8 74.0 13.34 0.60