Vol. 36, No. 7
Journal of Semiconductors
July 2015
Chemical and electrical properties of (NH4 /2 S passivated GaSb surface
Tao Dongyan(陶东言)Ž , Cheng Yu(程雨), Liu Jingming(刘京明), Su Jie(苏杰), Liu Tong(刘彤),
Yang Fengyun(杨凤云), Wang Fenghua(王凤华), Cao Kewei(曹可慰), Dong Zhiyuan(董志远),
and Zhao Youwen(赵有文)
Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences,
Beijing 100083, China
Abstract: The surface chemical properties of gallium antimonide (GaSb) after ammonium sulfide ((NH4 /2 S) solution passivation have been studied by X-ray photoelectron spectroscopy (XPS), time of flight secondary ion
mass spectroscopy (TOF-SIMS) and I –V measurement. An advantage of neutral (NH4 /2 S C S solution over pure
(NH4 /2 S solution and alkaline (NH4 /2 S C S solution has been found in the ability to passivate the GaSb surface
by contrast and comparison. It has been found that alkaline (NH4 /2 S C S solution passivation effectively removes
oxides of the GaSb surface and forms sulfide products to improve device performance. TOF-SIMS complementally
demonstrates that pure (NH4 /2 S passivation did form sulfide products, which are too soluble to really exist. The
lowest roughness determined using a 3D optical profilometer and the highest improved SBD quality proved that
neutral (NH4 /2 S C S solution passivation worked much better in improving the surface properties of GaSb.
Key words: surface passivation; GaSb; XPS; TOF-SIMS
DOI: 10.1088/1674-4926/36/7/073006
EEACC: 2520
1. Introduction
GaSb, which has a direct energy gap of 0.72 V, is the most
suitable substrate in the epitaxial growth of mixed semiconductors of GaSb system, because of its small lattice mismatchŒ1 3 .
GaSb-based devices are promising candidates for a variety
of applications in infrared regimes, including infrared lasers,
emitters and detectorsŒ4 6 . Unfortunately, the high level of reverse current and surface instabilities deteriorates the performance and reliability of GaSb-based devices. Previous studies revealed that GaSb surface can be easily oxidized by atmospheric oxygen with the formation of native surface oxides several nanometers thickŒ7; 8 . Moreover, chemical processing of the GaSb surface is particularly difficult because
of its high activity. Thurmond et al.Œ9 showed that relevant
reactions on GaSb yield interfacial deposits of Sb via 2GaSb
C Sb2 O3 !Ga2 O3 C 4Sb. The excess Sb always acts as nonradiative center, leading to high surface recombination velocity
and large leakage currents that hinder the enhancement of the
device’s performance. It is important to fabricate surfaces and
interfaces with a low level of electronic states which means native surface oxides need to be prevented and processed as well
as possible.
Much effort has been focused on GaSb crystal growth and
device fabricationsŒ10; 11 . The procedure of chemical preparation of the GaSb surface is still far from satisfactory. To our
knowledge, sulfide passivation, using (NH4 /2 S solutions, has
been performed extensively to reduce density of surface states
and correspondingly lower dark current of mesa diodesŒ12 .
However, the sulfide layer is slightly soluble, thus formation
of sulfides is always unsatisfactory. To overcome this problem, excess sulfur (S) blended (NH4 /2 S solution and neutral
(NH4 /2 S solution were studied to optimize the passivation effect. Generally, surface properties, including chemical components, roughness and electrical parameters, are important aspects to characterize the passivation effect. Moreover, changing relationship between these factors should also be considered. Thus, X-ray photoelectron spectroscopy (XPS) was applied to characterize GaSb surface information on chemical
composition before and after passivation. Time of flight secondary ion mass spectroscopy (TOF-SIMS) and 3D surface topography were employed to detect chemical component variation and surface roughness. In particular, the impact of sulfide passivation on Au/n-GaSb Schottky junction behavior was
studied. Excess S blended (NH4 /2 S solution improves the rectifying behavior and reduces the reverse current more obviously. The purpose of this study is to evaluate surface properties of GaSb surface passivated by different (NH4 /2 S solutions. Additionally, the effect of hydrogen in (NH4 /2 S solution
on the passivation reaction is also discussed.
2. Experimental details
Te-doped n-type GaSb (n D 1.3 1017 cm 3 , D
2446 cm2 /(Vs), T D 300 K) two side polished substrate with
500 m thick was used in this study. Prior to further treatment,
all samples were cleaned by standard surface cleaning steps
consisting of degreasing in acetone, alcohol, and DI water in
order to remove organic contamination. We labeled the treated
samples as samples (a)–(d).
(a) Dipped in concentrated HCl solution for 5 min;
(b) Dipped in concentrated HCl solution for 5 min, and
then sulfured by soaking in pure (NH4 /2 S solution for 5 min;
* Project supported by the National Natural Science Foundation of China (No. 61474104).
† Corresponding author. Email: [email protected]
Received 28 September 2014, revised manuscript received 2 February 2015
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J. Semicond. 2015, 36(7)
Tao Dongyan et al.
Figure 1. XPS (a) Ga 3d and (b) Sb 3d spectra and simulation results obtained from HCl-etched (sample (a)), pure (NH4 /2 S solution passivation
(sample (b)), alkaline (NH4 /2 S C S solution passivation (sample (c)) and neutral (NH4 /2 S C S solution passivation (sample (d)).
(c) Dipped in concentrated HCl solution for 5 min, and
then sulfured by soaking in alkaline (NH4 /2 S C S solution for
5 min at 60 ıC. The alkaline (NH4 /2 S C S solution was prepared by dissolving 2 g sulfur in 100 mL pure (NH4 /2 S solution;
(d) Dipped in concentrated HCl solution for 5 min, and
then sulfured by soaking in neutral (NH4 /2 S C S solution for
5 min at 60 ıC. The pH value of alkaline (NH4 /2 S C S solution
turned to 7 by dipping HCl, referred to as neutral (NH4 /2 S C S
solution.
Finally, all treated samples were dried by nitrogen gas
flow, and then sealed in sample boxes protected by nitrogen prior to XPS and TOF-SIMS measurements. XPS was
carried out in a Kratos Axis-165 system using a monochromated Al source under ultra-high vacuum (base pressure 5
10 7 mTorr). All XPS scans were done at room temperature with 0.1 eV step size, 30 ms dwell time, and no additional
in situ processing. TOF-SIMS analysis was carried out with
a TOF-SIMS V instrument from ION-TOF. Surface roughness
was characterized using a 3D optical profilometer (ContourGTK1, Bruker, Germany). Images were recorded at three different locations on each sample using the 3D profilometer to obtain the average value of surface roughness. The semiconductor parameter analyzer (Agilent B1500) was observed to evaluate sulphurization effects on electrical transport properties of
Au/n-GaSb Schottky diode. Circular Au Schottky contact, ˚
D 0.50 mm, 100 nm thick, was resistively evaporated through
a metal shadow mask. Rear-side Ohmic contact was made by
evaporating AuGeNi and then alloyed at 280 ıC for 2 min.
3. Results and discussion
Figure 1 shows typical room temperature Ga 3d and Sb 3d
doublet spectra obtained from the GaSb surface. Background
subtraction and iterative lineshape decomposition based on the
Gaussian function were performed to analyze the core level
spectra. Sb 3d3=2 was analyzed instead of Sb 3d5=2 , because
the signal peaks of Sb 3d5=2 in XPS spectra almost overlap
with O 1s.
As shown in Figure 1(a), peaks due to oxides of Ga were
detected from samples (a) and (b) at a binding energy of
20.6 eV. The position and intensity of Ga–O signal changed
little after pure (NH4 /2 S solution passivation. However, it was
drastically reduced after alkaline (NH4 /2 S C S solution passivation. A new component at a binding energy of 19.8 eV became visible due to formation of Ga–S bonds. This was verified
by the argument that chemical shifts are mainly due to charge
transferŒ13 . Indeed, the Pauling electro-negativities for Ga, O
and S are 1.8, 3.5 and 2.5, respectively. The binding energy of
Ga–S compound is thus expected to be between that of Ga0 and
Ga–O bond. Although determination of the exact composition
of this component is difficult, judging from Pauling electronegativities difference, it is definitely Ga–S bonds. The intensity and relative area ratio of Ga–S bonds has been increased
after the GaSb surface was subjected to neutral (NH4 /2 S C S
solution passivation. It was clearly shown in Figure 1(b), Sb
3d3=2 spectra were deconvoluted into Sb–Ga and Sb–O bonds
from samples (a) and (b). The relative area ratio of Sb–O to Sb–
Ga bonds from sample (b) decreased compared to that from
sample (a). After alkaline (NH4 /2 S C S solution passivation,
the core level line of Sb–O disappeared and a new peak at
binding energy of 538.8 eV can be readily identified to be Sb–
S bonds. Just like the case in Ga–S bonds, the formation and
intensity of Sb–S bonds became more apparent after neutral
(NH4 /2 S C S solution passivation. The result of passivated surfaces suggests that sulfur agent dissolves native oxide present
on the semiconductor surface and forms S–S, S–Ga, and S–
Sb bonds in the surface layer. The formation of these bonds
causes a reduction in the density of surface states. Considering the fact that sulfide products should exist on the GaSb surface after pure (NH4 /2 S solution passivation, the absence of
Ga–S/Sb–S bonds from sample (b) demonstrates that sulfide
products are unstable, or soluble, in pure (NH4 /2 S solution.
Alkaline (NH4 /2 S C S solution goes through the following reactionsŒ14 :
S C OH ! SC2 SO23 C H2 O .60 ıC/;
(1)
S2 C H2 O ! HS C OH ;
(2)
NHC
4 C HS ! NH4 SH;
(3)
Gax Oy C NH4 SH C H2 O ! Gax 0 Sy 0 C NH4 OH C H2 ;
(4)
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Tao Dongyan et al.
Figure 2. Comparison of TOF-SIMS positive (-pos) and negative (-neg) mass spectrum obtained from (a) HCl-etched (sample (a)) and (b) pure
(NH4 /2 S solution passivation (sample (b)). The unmarked peak positions are caused by the isotope of Ga and/or Sb, which are not considered
during the analysis.
Sbx Oy C NH4 SH C H2 O ! Sbx 0 Sy 0 C NH4 OH C H2 : (5)
Thus, excess sulfur in alkaline (NH4 /2 S C S solution can
increase the amount of sulfide products. The achieved thicker
sulfide layer in neutral (NH4 /2 S C S solution clearly confirms
that neutral (NH4 /2 S C S solution passivation is more effective to remove oxides and form thicker sulfides layer on the
GaSb surface. The additional hydrogen ions condition shifts
the equilibrium of reactions (4) and (5) to the right. The preferential replacement of sulfides with their corresponding oxides
can be explained by Pearson acid-base conceptŒ15 .
Spectra in Figure 2 obtained by TOF-SIMS confirm that
oxide-related peaks such as Ga2 O3 and Sb2 O3 are detected
in sample (a), which are attributed to the nature of the GaSb
surface. Besides, the appearance of Sb signals in Figure 2(a)-
pos, which are not detected in XPS measurement due to the
detection limit, partially clarified the origin of the reverse current. Due to pure (NH4 /2 S solution passivation (sample (b)),
the amount of oxides and excess Sb decreased, promoting the
enhancement of the device’s performance. Meanwhile, signals
of GaS and SbS in Figure 2(b) revealed that there were some
sulfide products on the GaSb surface after pure (NH4 /2 S solution passivation, which is in good agreement with our XPS
assumption and analysis.
Figure 3 depicts surface topography for four respective
treated GaSb samples. The surface of sample (a) is rather irregular and contains a multitude of, what appears to be, etching
related corrosion areas. Samples (b), (c) and (d), however, appear smoother with smaller roughness value, suggesting more
or less uniform passivation of the surface. Table 1 lists the
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Figure 3. Surface topography for HCl-etched (sample (a)), pure (NH4 /2 S solution passivation (sample (b)), alkaline (NH4 /2 S C S solution
passivation (sample (c)) and neutral (NH4 /2 S C S solution passivation (sample (d)).
Table 1. The results of surface roughness
locations on each sample.
Samples
Point 1
Point 2
Rq (nm)
Rq (nm)
(a)
0.545
0.585
(b)
0.499
0.523
(c)
0.562
0.573
(d)
0.502
0.493
recorded at three different
Point 3
Rq (nm)
0.591
0541
0.575
0.462
Average
Rq (nm)
0.574
0.521
0.570
0.486
results of surface roughness recorded at three different locations on each sample. The corrosion areas appear to be relatively shallow with roughness of 0.521 nm after pure (NH4 /2 S
solution passivation. However, the smooth corrosion area is
destabilized during alkaline (NH4 /2 S C S solution exposure to
60 ıC. Passivation reactions continue at a rapid speed, resulting
in increased surface roughness, 0.570 nm. In the case of neutral (NH4 /2 S C S solution, thicker sulfides can be formed due
to the repression of sulfide solubility. The corrosion area can
cease to passivate beyond a certain thickness, resulting in a flat
surface with the lowest roughness, 0.486 nm.
To further understand the detailed information of (NH4 /2 S
sulfide passivation on the GaSb surface, representative I –V
characteristics under forward and reverse biases are shown in
Figure 4. The Schottky diode made on neutral (NH4 /2 S C S solution passivation exhibited the highest improved SBD quality,
as evidenced by strongly reduced reverse currents and relative
high rectification ratio. These observations combined with the
above XPS and TOF-SIMS analysis suggested that electronic
states density, as well as excess Sb on the GaSb surface which
contributes to reverse current, decreases efficiently after neu-
Figure 4. I –V characteristics of Au/n-GaSb Schottky contacts under
forward and reverse bias.
tral (NH4 /2 S C S solution passivation.
4. Conclusion
In this paper, we have investigated surface chemical properties of (NH4 /2 S passivation of the GaSb surface by XPS. Although pure (NH4 /2 S solution passivation did form sulfides,
it has less obvious effect on the reduction of surface states
due to the solubility of sulfide products. On the other hand,
attributed to excess sulfur, alkaline (NH4 /2 S C S solution passivation proved to be better as justified from the appearance
of Ga–S and Sb–S bonds from XPS analysis. Moreover, both
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Tao Dongyan et al.
TOF-SIMS and surface topography measurement confirm that
neutral (NH4 /2 S C S solution passivation is more effective in
forming a thicker sulfide layer with higher stability than the alkaline one. I –V characteristic upon it exhibited the most significant reduction in surface states and improvement in device
performance. In conclusion, neutral (NH4 /2 S C S solution passivation is proved to be more effective in improving surface
properties of GaSb compared to pure (NH4 /2 S solution and alkaline (NH4 /2 S C S solution. We attribute this to the promoted
effect of additional hydrogen ions in neutral (NH4 /2 S C S solution which shifts the equilibrium of passivation reactions to
the right.
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