Thin Solid Films 483 (2005) 407 – 410
www.elsevier.com/locate/tsf
Ammonium sulfide treatment of HgCdTe substrate and its effects
on electrical properties of ZnS/HgCdTe heterostructure
Yong-Chul Junga,b, Se-Young Ana, Sang-Hee Suha, Duck–Kyun Choib, Jin-Sang Kima,*
a
Thin Film Materials Research Center, Korea Institute of Science and Technology P.O. Box 131, Cheongryang, Seoul 130-650, South Korea
b
Department of Ceramic Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-ku, Seoul 133-791, South Korea
Received 3 May 2004; accepted in revised form 22 December 2004
Available online 27 January 2005
Abstract
The semiconductor–passivating layer interface, as well as the dielectric properties of the passivants, plays an important role in HgCdTe
based photodiodes. ZnS is a commonly used surface passivant for HgCdTe. This study examined the effects of sulfidation on the HgCdTe
surface and interfacial characteristics of metal/ZnS/HgCdTe structures. The ZnS layer was deposited by thermal evaporation after sulfidation.
The interfacial properties of the metal insulator semiconductor (MIS) structures were determined. A comparison of an untreated capacitor and
a sulfide treated MIS capacitor showed that the fixed charge density (untreated 6.371011, treated 3.21011 cm 2) and slow state density
(untreated 5.51011, treated 7.51010 cm 2) were 2 and 7 times lower in the treated than in the untreated specimens. Sulfidation results in a
decrease in the concentration of contaminants originating from the native oxide-covered HgCdTe substrates. This reduction may be due to the
formation of S–S or II–S bonds at the surface layer. These bonds might act as barriers against native oxide formation when (NH4)2Sx-treated
HgCdTe substrates are exposed to air.
D 2004 Published by Elsevier B.V.
PACS: 73.40.Qv; 73.20.-r
Keywords: ZnS; Metal–insulator–semiconductor; Surface and interface states; Interface
1. Introduction
The semiconductor–passivating layer interface, as well as
the dielectric properties of the passivants, plays an important
role in HgCdTe based photodiodes. In second-generation
infrared focal plane arrays based on HgCdTe photodiodes
coupled to silicon signal processors, surface passivation is a
decisive factor which determines the device performance
[1,2]. HgCdTe surface passivation is complex because of the
compound nature of the semiconductor, the difference in
chemical properties of the constituents, and the tendency of
electrically active defects to form in the interface region
during the passivating process [3]. The chemical, structural
and electronic defects induce a high density of fixed, fast
and slow interfacial traps, which are usually responsible for
* Corresponding author. Tel.: +82 2958 5693; fax: +82 2958 5692.
E-mail address: [email protected] (J.-S. Kim).
0040-6090/$ - see front matter D 2004 Published by Elsevier B.V.
doi:10.1016/j.tsf.2004.12.057
the excess dark currents and high noise level of the
photodiode [4,5].
The commonly used surface passivant for HgCdTe is
ZnS, which is evaporated thermally with an ordinary
evaporation system. Growth of HgCdTe and deposition of
the ZnS layer can not be carried out in series without
exposure to air, because several other steps, such as etching
of mesas and ion implantation are always needed before the
deposition of ZnS. Thus, appropriate surface preparation is
required and is very important for good ZnS passivation of
HgCdTe. The usual surface preparation method for HgCdTe
is chemical etching with bromine etchants in various
solvents including methanol, ethylene glycol and other
solvents. After surface pretreatment, it may be possible to
obtain clean and stoichiometric HgCdTe surfaces. However,
a clean HgCdTe surface should be inevitably exposed to air
for the deposition of ZnS. Therefore, the possibility of
native oxide formation is unavoidable.
Y.-C. Jung et al. / Thin Solid Films 483 (2005) 407–410
It is well known that the formation of native oxides on
HgCdTe surfaces results in a high density of surface
pinning. In GaAs substrates, sulfur treatment with (NH4)2Sx
solution has been reported to be one of the most efficient
techniques for preventing native oxide formation on the
GaAs surface [6]. In this experiment, the HgCdTe surface
was treated with (NH4)2Sx solution before ZnS layer
deposition. To understand clearly the effect of (NH4)2Sx
treatment, X-ray photoelectron spectroscopy (XPS) was
carried out on the (NH4)2Sx-treated HgCdTe and ZnS
interface. The capacitance–voltage characteristics of the
MIS structures were determined.
2. Experimental details
n-type Hg1 xCdxTe wafers (xc0.26) grown on (001)
GaAs substrate by metal organic vapor phase epitaxy [7],
were used in this study. The evaporation system consisted of
a high vacuum chamber and a Knudsen-effusion-cell, whose
temperature was accurately controlled. Bulk ZnS was used
as the source material for the deposition of ZnS. HgCdTe
wafers were first cleaned in trichloroethylene, acetone and
methanol, and then chemically etched with 0.3% brominein-methanol solution for 1 min. After chemical etching,
wafers were rinsed in methanol several times and then
dipped into an (NH4)2Sx–methanol solution for 5 min at
room temperature. The wafers were dried with high purity N
gas and then transferred immediately into the evaporation
chamber.
After surface treatment, a 300 nm thick ZnS passivation
layer was deposited on the HgCdTe surface, at a deposition
rate of about 0.1 nm/s by a thermal evaporator at room
temperature. The deposition rate was monitored by quartz
crystal thickness monitor and the temperature of Knudseneffusion-cell was automatically adjusted to maintain the
deposition rate of 0.1 nm/s.
Test structures of MIS were processed to characterize the
electrical properties of the interface. The MIS devices
consisted of top Au gate electrode and thermally evaporated
ZnS onto HgCdTe substrates. The gate electrode of 0.5 mm
diameter was formed onto the ZnS layer by thermal
evaporation of gold (Au) through a metal shadow mask. A
part of ZnS layer was chemically etched to make ohmic
contact to HgCdTe substrate. The ohmic contact to HgCdTe
was made by 5 wt/o AuCl2–water solution. A droplet of
AuCl2–water solution was dropped onto HgCdTe surface.
Chlorine spontaneously evaporated at room temperature and
remaining metallic gold onto HgCdTe surface was used as
contact material. The capacitance–voltage (C–V) characteristics of the MIS structure were measured with a frequency
of 1 MHz at 77 K.
The XPS spectra were obtained using a PHI-ESCA 5800
spectrometer. The monochromatized Al Ka radiation
(1488.6 eV) was used as the photon source. To obtain the
interfacial nature of spectra, the accumulation of XPS
spectra was alternated with sputter etching by means of
Ar+ ions. The binding energies of all peaks were calculated
assuming a value for C 1s electron binding energy of 285.0
eV. The analysis of XPS spectra have been carried out using
the XPS peak fitting program for WIN95/98 XPSPEAK
Version 4.1 that have been created by Raymond W. M.
Kwok (http://www.phy.cuhk.edu.hk/~surface).
3. Results and discussion
Photoelectron signals detected from the Oxygen 1s,
Carbon 1s, Cadmium 3d, and Tellurium 3d XPS peaks
were used to compare the surface states of HgCdTe
substrates after Bromine etching with polysulfide (NH4)2Sx
solution treatment. The relative intensities of these signals
are summarized in Fig. 1. Fig. 1 shows that there is an
evident decrease in the concentration of contaminants
(oxygen and carbon) originating from the native oxidecovered HgCdTe substrates after polysulfide (NH4)2Sx
solution treatment. In addition, the intensity of the XPS
core level peaks of the Cd 3d and Te 3d on (NH4)2Sx-treated
HgCdTe surface were higher than those on Br–methanol
etched surface. This can be attributed to the high probability
of native oxide formation in the Br–methanol etched
HgCdTe surface. It is thought that the native oxide layer
is mainly formed when the etched HgCdTe surface is
exposed to air. To a lesser extent, the etchant itself is also
thought to be an oxidation source because the etchant was
not flooded with inert gas and therefore the etchant
contained an unknown amount of dissolved oxygen. These
results suggest that the formation of native oxide layer on
the HgCdTe substrate surface was suppressed by the sulfur
treatment.
The above experimental results suggest that sulfur
treatment may improve the electrical properties of the
passivation layer on HgCdTe because of the lower
possibility of native oxides formation. In order to investigate
Relative Intensity (arb.units)
408
2.87
Br-MeOH etched
(NH4)2Sx-treated
2.0
1.37
1.0
0.74
0.76
0.61
0.23
C1s
O1s
Cd3d
Te3d
XPS core level peaks of HgCdTe surface
Fig. 1. The relative intensity of C 1s, O 1s, Cd 3d, and Te 3d XPS core level
peaks of the HgCdTe surface after Br–menthol etching and (NH4)2Sx
treatment.
Y.-C. Jung et al. / Thin Solid Films 483 (2005) 407–410
Normalized Capacitance
1.00
a
VFB=4.2V
b
VFB =2.1V
0.96
0.92
1.00
0.96
0.92
-10
-5
0
5
10
Voltage (V)
Fig. 2. C–V curves of MIS capacitors fabricated on n-type HgCdTe
substrates. The HgCdTe substrates were (a) etched with 0.3% Bromine in
methanol solution, and (b) treated with 20% (NH4)2Sx–methanol solution
for 5 min.
c
1.00
VFB= 2.1V
0.96
0.92
Normalized Capacitance
the effectiveness of (NH4)2Sx treatment on the electrical
properties of the ZnS passivant, MIS capacitors were
fabricated and characterized. One HgCdTe substrate was
etched in 0.3% Br–methanol solution and another was
treated with (NH4)2Sx solution. After etching and surface
treatment, 300 nm thick ZnS layers were simultaneously
deposited by mounting two substrates close to each other on
the same substrate holder.
Fig. 2 shows the capacitance–voltage (C–V) curves of
the MIS devices for each sample. The width of the
hysteresis loop was greatly decreased in the (NH4)2Sxtreated sample. The hysteresis in characteristics C–V curves
is directly related to slow states which are responsible for
the high noise level of photodiodes. The hysteresis of C–V
curves is thought to result from the native oxide layer on the
HgCdTe surface [8]. The slow states densities determined
from the width of the hysteresis are 5.5x1011 cm 2 for Br–
methanol etched sample and 7.51010 cm 2 for the
(NH4)2Sx-treated sample. An additional feature of the
measured capacitance–voltage characteristics of this experiment is that the shift of flat band voltage (VFB) toward zero
voltage after (NH4)2Sx treatment. From these results, it is
inferred that the (NH4)2Sx treatment is very effective in
improving the electrical properties of ZnS layer on HgCdTe
by reducing native oxide formation in the Br-etched
HgCdTe surface. This result is in agreement with the
decrease in the photoelectron signal of the oxygen 1s peak
in the (NH4)2Sx-treated HgCdTe substrates as shown in Fig.
1. By employing (NH4)2Sx treatment the slow interface state
density and the fixed charge density were decreased by a
factor of seven and two, respectively.
In this experiment, (NH4)2Sx–methanol solution was
used for the surface treatment of HgCdTe. The effect of
(NH4)2Sx concentration is shown in Fig. 3. There is
409
20 (NH 4 )2S x
b
1.00
VFB=1.4V
0.96
0.92
10 (NH 4 )2 Sx
a
1.00
VFB= 1.8V
0.96
5 (NH4 )2 S x
0.92
-2
-1
0
1
2
3
4
Voltage (V)
Fig. 3. The effect of (NH4)2Sx concentration on C–V characteristics of Au/
ZnS/HgCdTe devices. HgCdTe substrates were rinsed into (a) 5%, (b) 10%,
and (c) 20% of (NH4)2Sx–methanol solution for 5 min, respectively.
negligible change in the width of hysteresis of C–V
characteristics. However, the flat band voltage changed
slightly with changes in the volume fraction of (NH4)2Sx in
methanol. All samples have a positive flat band voltage
which corresponds to the negative fixed interface charges.
VFB started to shift toward zero voltage and has a minimum
value at 10% of (NH4)2Sx solution (Fig. 3b). After this
point, VFB shifted toward the positive gate voltage (Fig. 3c)
due to the increased negative fixed charges. One of the
candidates of negative charged species is thought to be S2
ions, which might be incorporated during the sulfidation
step especially in high concentration of (NH4)2Sx solution.
To understand more clearly the effect of (NH4)2Sx
treatment on bonding nature at the ZnS/HgCdTe Interface,
10 nm thick ZnS layers were deposited on HgCdTe
substrates and the XPS spectra at the surface of ZnS and
ZnS/HgCdTe interfaces were measured. XPS spectrum from
ZnS and HgCdTe interface was determined by the appearance of the XPS signals of Hg 4f and Te 3d.
Fig. 4 shows the XPS narrow scan spectra for S 2p peaks
at ZnS and HgCdTe interface in Br-etched HgCdTe (a) and
(NH4)2Sx-treated HgCdTe substrates (b). The shape of S 2p
peak from Br-etched HgCdTe was exactly the same as the
shape of S 2p peaks at the surface of ZnS in both samples
(not shown in Fig. 4). Peak fitting analysis of the S 2p
spectrum of (a) have shown that there are two peaks at the
binding energy of E b1=161.8 eV and E b2=163.0 eV due to
spin-orbit splitting. The peaks at binding energies of 161.8
and 163.0 eV could be assigned to S 2p3/2 and S 2p1/2 of
Zn–S bond, respectively [9]. The ratio of areas for these two
peaks is 1.8 and slightly less than would be expected in
410
Y.-C. Jung et al. / Thin Solid Films 483 (2005) 407–410
time. The ratio of the areas of peaks at E b1 and E b3 gives an
estimate that about 6.8% of S atoms have lower binding
energy value. From these experimental results, it is thought
that sulfidation process on HgCdTe surface results in an II–
S (CdS or HgS) or S–S bond at the surface. These newly
passivated surfaces might act as barriers against native oxide
formation when (NH4)2Sx-treated HgCdTe substrates are
exposed to air.
4. Conclusion
The results of this study show that a sulfur agent
originating from the sulfidation process forms S–S or II–S
bonds at the surface of HgCdTe substrates. These bonds are
very effective in improving the electrical properties of the
ZnS layer on HgCdTe by reducing the possibility of native
oxide formation. After the sulfidation, the electrical properties of HgCdTe MIS capacitors were greatly improved due
to the lower density fixed charge density and reduced
hysteresis width.
Fig. 4. XPS narrow scan spectra for S 2p peaks at the interface of ZnS and
HgCdTe. (a) HgCdTe substrate was etched with 0.3% Bromine in methanol
solution, and (b) treated with 20% (NH4)2Sx–methanol solution for 5 min.
accordance with the ratio of the degree of degeneracy of
these levels (2.0) [9]. The peak half widths are same value
of 1.3 eV, respectively. XPS peak fit analysis of the S 2p
spectrum of the (NH4)2Sx-treated sample (b) showed that
there is another pair of peaks in addition to the peaks from
the Zn–S bond. In peak fit analysis, the doublet separation
value for peaks of E b1 and E b2 (1.2 eV) was employed in the
separation of the peaks of E b3 and E b4. The binding energies
of the newly formed two peaks were E b3=161.4 and
E b4=162.6 eV, respectively. For peaks at E b1 and E b2 the
ratio of areas (1.8) equals to the ratio of areas of the peaks at
E b3 and E b4 (1.8). The peaks half widths are about 1.2 eV.
The sulfidation of HgCdTe substrate lead to the
appearance of two additional peaks shifted to a lower
binding energy at 0.4 eV. It has been reported that the peaks
of S 2p3/2 of CdS were detected at a binding energy of 0.3
eV lower than where the peaks were detected in ZnS [10].
Therefore, the newly formed peaks can be attributed to the
formation of CdS during the sulfidation process. Also, the
possibility of HgS formation can not be excluded at this
Acknowledgements
This research has been supported by the development of
dual infrared detector, Korea, through the thin film
materials research center at Korea Institute of Science
and Technology.
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