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 Cl2/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.
1
*Corresponding author: Department of Electrical Engineering, National Central
University,
Chung-Li
32054,
Taiwan,
E-mail:
+886-3-4227151 ext 34468, Fax: +886-3-4255830
2
yhsin@ee.ncu.edu.tw,
Tel:
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
3
from room temperature Hall measurements. Prior to Cl2/Ar reactive ions etching, all
samples 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
4
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
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
proportional to the Cl2 flow rate, 17 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
5
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
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
J  A * *T 2 exp( 
b   qV  
) exp 
 1
kT   nkT  
(1)
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
6
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
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
7
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.
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
93-2215-E-008-019.
9
research
under
contract
no.
NSC
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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.
13
250
RMS (nm)
Ar = 10 sccm
RF power = 200 W
Chamber pressure = 20 mTorr
225
100
200
80
175
60
0
50
100
150
200
etching rate (nm/min)
120
150
250
Cl2 flow rate (sccm)
FIG.1 RIE physical properties (RMS and etching rate) as a function of Cl2/Ar mixture
gas flow rate.
14
Bearing ratio of the nanorods (%)
0
Cl2 flow rate (sccm)
50
100
30
150
200
250
300
Ar = 10 sccm
Chamber pressure = 20 mTorr
25
Scanned area = 100 m
2
20
15
10
vary Cl2 flow rate
@ RIE RF power at 200W
vary RIE RF power
@ Cl2 flow rate at 150 sccm
5
0
-5
0
50
100
150
200
250
300
350
400
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.
15
20
Current (A)
p -GaN
Ar = 10 sccm
RF power = 200 W
10 Chamber pressure
= 20 mTorr
0
Cl2 = 50 sccm
Cl2 = 100 sccm
Cl2 = 150 sccm
Cl2 = 200 sccm
without etch
-10
-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.
16
60
RMS (nm)
300
Ar = 10 sccm
Cl2 = 150 sccm
Chamber pressure
= 20 mTorr
250
200
150
40
100
20
50
0
0
50
150
100
200
etching rate (nm/min)
80
0
250
RIE RF Power (W)
FIG. 4 RIE physical properties (RMS and etching rate) as a function of RF power
17
20
Current (A)
p -GaN
Ar = 10 sccm
Cl2 = 150 sccm
10
Chamber pressure
= 20 mTorr
0
power = 50 W
power = 100 W
power = 200 W
without etch
-10
-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
a function of Cl2/Ar
gas flow rate
a function of RIE
RF power
RF power
Cl2 flow rate
etching rate
RMS
W
200
200
200
200
50
100
200
sccm
50
100
150
200
150
150
150
nm/min
202.8
202.2
208.8
217.8
11.0
81.0
208.8
nm
84.5
82.0
74.0
89.8
11.1
24.3
74.0
19
Bearing ratio of
the nanorods
%
22.21
16.86
13.34
17.30
0.06
5.14
13.34
SBH
eV
0.61
0.60
0.60
0.61
0.50
0.52
0.60