Effect of deposition
parameters
on the properties
of ih$&/llnP
junctions
V. Korobov and Yoram Shapira
Department of Electrical Engineering-Physical
Electronics, Tel Aviv University, Ramat Aviv 69978, Israel
B. Ber, K. Faleev, and D. Zushinskiy
Ioffe Physical-Technical
Institute, 19402i St. Petersburg, Russia
(Received 23 August 1993; accepted for publication 2 November 1993)
Transparent conducting indium oxide films have been prepared by means of reactive evaporation
of In onto p-type InP substrates at various deposition temperatures [in the range of 25 “C!
(RT)-330 “C] and under different oxygen pressures (in the range of 8 X 1O-5 Torr up to
9 x 10m4 Torr). The chemical composition and structural properties of the films have been
investigated using such analytical tools as Auger electron spectroscopy (AES), x-ray diffraction,
and scanning electron microscopy (SEM) . The combination of AES and SEM has proved to be
extremely useful for interface analysis. The concentrations of oxidized and unoxidized
(elemental) In in the tested samples have been investigated by deconvolution of the appropriate
Auger MNN transitions using reference spectra of InaOj and InP. The films were found to be
polycrystalline at all deposition temperatures above RT under all the tested range of oxygen
pressures. Nearly stoichiometric In,03 films have been observed on all the investigated samples.
Elemental In at the interfaces of films grown at low deposition temperatures has been noted. The
effect of the oxygen pressure and deposition temperature on the films properties is discussed.
I. INTRODUCTION
Indium oxide ( Inz03) as well as other transparent conducting oxides (TCO) has a wide range of applications as
a top contact of various optoelectronic devices due to their
special properties: a wide band gap (sufficient to be transparent in the visible range) and high carrier concentrations
which make them good conductors.‘-’ High quality In,O,
films have been fabricated by various deposition methods,
such as sputtering, spraying, chemical vapor deposition,
ion plating, and vacuum evaporation. We have selected
reactive evaporation because this technique has several advantages: a minimum of critical parameters; elimination of
sputtering damage (present at the sputtering method)6 and
relative ease of operation. In order to simplify the chemistry of the system, indium oxide (without tin doping) was
used. In,O,/n-type
GaAs junctions prepared by this
method have already been studied.‘-’
This work presents a further step in a systematic study
of indium oxide films on InP substrates and focuses on the
structural and compositional analysis of the grown films
and their interfaces under various deposition conditions of
the reactive evaporation process. It is the continuation of a
recent work
on the electrical
characteristics
of
In,Os/III-V
compound semiconductors.” It aims to clarify the results obtained by Auger electron spectroscopy
(AES), optical and electrical measurements of previous
studies by using a combination of such measurements with
morphological and structural data.
up to 350 “C during 10 min. After this degassing process
the substrate temperature was decreased to the deposition
temperature ( Tdep) . The different depositions were carried
out at substrate temperatures from RT up to 330 “C. Indium (99.99%) was resistively evaporated onto the InP
substrates under pressure of 0, (Po) in the range of
8 X 10U5 Torr up to 9 X 10m4 Tort-, which reacted with the
deposited In at the substrate surface.
Control of the rate of evaporation and the thickness
was done by means of a quartz crystal oscillator during
deposition. The final film thickness was in the order of 100
nm and the deposition rate was 0.1 run/s.
Scanning electron micrographs were obtained using a
CamScan scanning electron microscope (SEM) operated
at 20 kV with a magnification up to 60 000 X . Both the
surface structure micrographs and the cross-section images
were used for exact thickness determination.
The crystallinity of the film was analyzed by x-ray
diffraction using a Cu target, at 50 keV electron energy and
a scanning speed of l”/min.
AES measurements were carried out using a RIBER
“MP-2000” Auger electron microprobe with a 3 keV, 200
-5 pm spot size electron beam, and a cylindrical
d,
mirror analyzer with an energy resolution better than
0.19%. Depth profiling was obtained using a 1.5 keV, 20
nA Ar f ion beam scanning a 700 X 700 pm2 area. The
AES spectra were synthesized using reference spectra from
In,O,, In, and InP by means of nonlinear least square
fitting.
II. EXPERIMENTAL
The
fers (Zn
in 0.3%
loaded
3 X lo-’
samples used were (100) oriented p-type InP wadoped 5 X 1016cm-‘). The top surface was etched
BrMe solution for about 1 min, just before being
into the deposition chamber (base pressure
Torr) . The semiconductor substrates were heated
2264
J. Appl. Phys. 75 (4), 15 February 1994
111.RESULTS
Figures 1 shows a sequence of scanning electron micrographs of indium oxide tllms grown at five different
deposition temperatures and under PO= 5 X lo-” Torr.
The grain size increases with growing Tdep. Films pro-
0021-8979/94/75(4)/2264/6/$6.00
@ 1994 American Institute of Physics
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FIG. 2. Scanning electron micrographs of film surfaces (a)-(e) and their
cross sections (a’)-(e’)
grown under four different deposition oxygen
pressures: (a) 1~10~" Torr; (b) 2~10~~ Torr; (c) 5X10m4 Torr; (d)
9 x low4 Torr.
FIG. 1. Scanning electron micrographs of film surfaces (a)-(e) and their
cross sections (a’)-(e’)
grown at five different deposition temperatures:
(a) RT; (b) 100°C; (c) 170 ‘C; (d) 250°C; (e) 330°C.
duced at RT seem to be quasiamorphous [Fig. 1 (a)]. Figure 1 (b) ( Tdep= 100 “C) indicates a crystalline structure
with grains having a linear dimension--So of 40-80 nm.
The crystallinity is improved at higher deposition temperatures. At Tdep= 170 “C! Fig. l(c)] So-80-100
nm and
reaches saturation (up to 200 nm) at Tdep=250 "C! [Figs.
1 (d) and 1 (e)]. However, it is easy to see a difference
between the films
grown
at Tdep= 250 “C and
T,,,--330 “C. At Tdep= 250 “C the growth is columnar
[Figs. 1 (d) and 1 (d’ j] while at Tdep=330 "C separate homogenous grains are observed on quasiplanar film [Figs.
l(e) and l(e’)].
Figure 2 exhibits a sequence of micrographs for indium
oxide films obtained under various PO at Tdep=250 “C.
Films produced at PO=1 X lo-” Torr show separate islands, comprising some homogenous grains combined toJ. Appl. Phys., Vol. 75, No. 4, 15 February 1994
gether, with sizes up to approximately 400 nm [Figs. 2(a)
and 2(a’)]. The cross-section micrographs [Fig. 2(a’)]
show the islands standing on the substrate. At
Po=2X lo-” Torr [Figs. 2(b) and 2(b’)], SF decreases to
approximately 300 nm but the grains are settled on a 50
nm thick planar film. Further increase in Po leads to coalescence of the islands into a polycrystalline continuous
film. The case of growth under PO=5 X 10e4 Torr has already been described above and under Po=9X 10m4 Torr
[Figs. 2(d) and 2 (d’ j] the roughness of the film decreases.
The samples were also investigated using AES depth
profiling. The In MNN Auger line shape at various depths
is given in Fig. 3. The spectrum from the InP substrate
[Fig. 3 (a)] comprises two peaks with energy positions at
402 eV (Ini) and 408 eV (Inzj. The ratio of the peak
intensities is In@, - 1.1. The same result is obtained
from metallic (elemental) In. The In line of oxidized indium has two peaks which are shifted to In1 = 399 eV and
In,==405 eV, and their intensity ratio is Inl/Ina--,0.9 [see
Fig. 3(c)]. It is-noted that the absolute value of the shift
depends on the analyzer resolution. These changes in the
In MNN line correspond to differences in the In chemical
bonding. *’ The shape of the In line is quite different at the
Korobov et al.
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2265
.--\
ai
: :
.’ -*
=‘- ...-‘y”” ‘;
‘,I
;
b)
60
s :
;:
I ’
-; .‘* -*
*, ‘,, .-’
:
:
*x./
~z~~7~~
..‘.
= ‘*,
c)
-1’ ,
,/.--“”
.-.
.--.-.-A_
- - .--,t”
I8
-‘1
.*
*
;
.,
;*”
‘. ’
-.. /*‘....
385
390
395
400
405
50
loo
150
200
0
50
loo
150
200
60
“--
410
415
Electron energy (eV)
FIG. 3. Experimental (dashed curves) and calculated (solid curve) In
MNN Auger line shapes: (a) in the InP bulk (elemental In); (b) at the
IqO,/InP
interface; (c) in the 1~0~ film (oxidized In).
indium oxide/p-type InP interface [see Fig. 3(b)]. In this
case, the In line, comprising three peaks, seems to be a
superposition of the lines of elemental and oxidized In, i.e.,
the In is observed in both chemical states. This can be
presented as Intot= AIn,+(l-A)&,,
whereAand 1-A
are the fractions of elemental (In,,) and oxidized (IQ
-iridium line shapes, respectively. The calculated total peak
for A=0.58 and is shown in Fig. 3(b) (solid curve) and
fits the experimental spectrum (dashed curve).
Figures 4 and 5 show depth profiles obtained by this
procedure. The ratio of In to 0 concentrations is constant
in the oxide bulk for all tested films obtained at various
temperatures under PO= 5 X lop4 Torr (Fig. 4). The
atomic concentration of 0 is about 60%, which corresponds to stoichiometric In,O, layers. The layer stoichiometry is independent of Tdep, in, accordance with previously obtained results on GaAs and InP.‘*” The apparent
width of the interface increases with increasing Tdepin the
tested range. Elemental In is observed at the interfaces of
the films grown at low deposition temperatures [up to
Tdep= 100 “C!, see Figs. 4(a) and 4(b)], which in a good
agreement with recently reported results.7’8~10
Depth profiles of films grown under different oxygen
pressures at Tdep=250 “C! are shown in Fig. 5. The In,,/0
concentration ratio is 2/3 in the film bulk grown under
Po=5X10-4TorrandPo=2X10-4Torr[Figs.
5(b) and
5(c)] and slightly higher under PO=9 X 10M4 Torr [Fig.
5(a)]. A further decrease of the oxygen pressure leads to
deviation from stoichiometric In,O, [Figs. 5 (d) and 5 (e)].
Figure 5 (d) shows that stoichiometric In,03 film is present
only at the subsurface region and the concentrations of 0
and oxidized In strongly decrease as a function of depth
into the film bulk. Under PO= 8 X low5 Torr [Fig. 5 (e)] P
2266
0
J. Appl. Phys., Vol. 75, No. 4, 15 February 1994
c.
fi
60
-z=
2
=I!
40
0K
20
.-Yii
u
a
Tdep=2500c
60 b
I
150
200
250
I
300
350
Tdep=3300c
60 b=+%
. .,
I
40
20
0
loo
150
200
250
300
Calculated depth (nm)
FIG. 4. AES depth profiles of films grown at five different deposition
temperatures: 0 (open circles); In(ox) (open triangles); In (full triangles); P (full circles).
and elemental In already exist at the surface and the concentrations of the analyzed elements change very little with
depth. There is no distinct border between the film and the
substrate.
Korobov ef al.
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Figure 6 shows a set of x-ray diffraction patterns of
samples grown at three different Tdep's.Oxidation at RT
(not shown) reveals a quasiamorphous film growth with
microcrystalline structure. At Tdep=170 “C, the film has a
50
100
150
200
3
(cl)
1
250
40
20
0.75
0
150
h
200
250
300
350
60
0.25
40
20
2 0 (deg)
F
0
50
0
100
150
200
40
20
0
60 e>
1
Po=8x10” Torr
40
20
0
Calculated depth (nm)
FIG. 5. AES depth profiles of films grown under five different deposition
oxygen pressures: 0 (open circles); In(ox) (open triangles); In (full
triangles); P (full circles).
J. Appl. Phys., Vol. 75, No. 4, 15 February 1994
FIG. 6. X-ray diffraction patterns of films grown at three different deposition temperatures.
bee structure with a= 10.116 A, according to Fig. 6(a),
which is very close to the ASTM Powder Diffraction Data
Card No. ASTM-6-0416. Figure 6(b) presents a diffraction pattern of a film grown at Tdep=250 “C!. The intensity
of the (004) reflection peak is higher than in the ASTM
card. The (002), (006), and (008) peaks also appear. At
Tdep=330“C! the pattern is more similar to the one at
Tdep=170 “C. However, the intensity ratio of the (222)/
(004) peaks is lower [Fig. 6(c)].
Figure. 7 shows a set of x-ray diffraction patterns of
samples grown under 3 different PO'sat Tdep= 250 "C. Figure 7(a) (PO=9 X 10e4 Torr) shows a small reflection
peak, presumably in the ( 111) direction, in addition to the
peaks corresponding to card No. ASTM-6-0416. This additional peak is probably due to a deviation from the cubic
symmetry, discussed below. Diffraction from the film prepared under PO=5 x 10V4 Torr is shown in Fig. 7(b) and
was described in the preceding paragraph. Further decreasing the oxygen pressure (PO= 1 X 10m4 Torr) results in
Fig. 7(c), which is similar to Fig. 7(b) . However, new
reflection peaks with interplanar distances of d=2.71, 2.47,
Korobov et al.
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2267
Fb=Sxl&orr
(222)
(a)
Oxygen
pressure
(~10~
Tom)
8
,>-
(4
(211)
(111)(200)
d&!-L
(b)
5280
Temperature( “C)
>I-
FIG. 8. O/In(ox) peak ratio in the film as a function of deposition
temperature (squares, top scale) and oxygen pressure (circles, bottom
scale ) .
J?
I-
z
c
20
30
40
20 idegl
FIG. 7. X-ray diffraction patterns of films grown under three different
deposition oxygen pressures.
2.30 A appear, which do not correspond to card No.
ASTM-6-0416.
IV. DISCUSSION
Figures 4 and 5 seem to point to an apparent increase
of the interface width with increasing Tdepand decreasing
PO. This trend may be explained either by a possible measurement error due to the primary roughness of the film
surface or by enhanced interdiffusion of 0 and/or P. SEM
has been found to be an extremely useful complementary
tool in conjunction with AES depth protiling for interpreting these results. The SEM micrographs show that the
roughness of the film increases with increasing Tdep(see
Fig. 1) or decreasing PO (see Fig. 2). It is well known that
the initial roughness of the surface to be investigated drastically affects the depth resolution and may lead to misinterpretation of the interface width.12 Thus, the set of results
shown in Fig. 4 cannot be interpreted based on a uniform
layer on a flat substrate, that is sputtered layer by layer.
Rather, the rough island formation causes the substrate
signal to appear at an earlier stage (and the film signal to
2268
J. Appl. Phys., Vol. 75, No. 4, 15 February 1994
linger on), as rdep increases. A similar situation occurs in
the set shown in Figs. 5(a)-5(c) and 2(b)-2(d).
Under
low 0, pressures [Figs. 5(d), 5(e) and 2(a)] the AES
substrate signal is observed at the initial surface since the
coverage is not continuous.
In order to determine the composition of the films
grown under the different deposition parameters, we calculated the exact oxygen to indium (in oxidized form)
ratio--O/In( ox). Figure 8 shows the O/In( ox) peak ratio
in the film as a function of Tdep (squares) and PO (circles).
This ratio is very close to stoichiometry of In.@, (3/2) for
all deposition temperatures and O2 pressures except for a
small deviation at low oxygen pressures ( 1 x 10u4 and
8X 10e5 Torr). This means that the oxide grows as In203
even if it is in the form of islands or clusters.
Our analysis of the AES and SEM data can be examined in view of the x-ray diffraction patterns. They show
that at RT the film is quasiamorphous. At all temperatures
above RT, a polycrystalline film of bee Inz03 is obtained.
At Tdep= 170 “C and Tdep=330 “C the pattern is very similar, with a slight difference in the intensities of the (222)
and (004) peaks [see Figs. 6(a) and 6(c)], indicating a
random polycrystalline orientation. However, the (222)/
(004) peak ratio is quite different for films grown at
Tdep=250 “C! and new peaks appear in the (001) directions. That may be explained by the preferred orientation
of the film, due to the effect of the InP substrate, as evidenced in Fig. 1 (d) .
A similar random growth is observed under the highest
PO (9 X low4 Torr) in Fig. 7(a), which agrees with Fig.
2(d) . There is some sign of a ( 111) peak, in addition to the
peaks of the ASTM-6-0416 card. This is a forbidden line
according to the cubic symmetry rules. Its appearance indicates an excess 0 concentration in the lattice [see also
Figs. 8 and 5(a)], contributing to a deviation from the
perfect bee structure. Under lower pressures [Figs. 7(b)
and 7(c)], preferred (001) orientation is observed (again at
Korobov et
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al.
i
Td,,=250 “C). Under PO= 1 x IO-” Torr peaks corresponding to elemental In appear, in agreement with Fig.
5(d).
V. CONCLUSIONS
Based on the data and their correlations
above, the following conclusions are reached.
discussed
77te effect of Tdepon In203 films and their InP junctions. At T,,,=RT amorphous indium oxide grows on the
InP substrate. At low Tdep)ssome unoxidized (elemental)
In is detected at the interface, which is in agreement with
previous results on GaAs* and Si.12
Above Tdep= 100 “C polycrystalline
stoichiometric
InzO, films are formed. As the temperature is increased,
the grain size increases, the film morphology becomes
smoother, and the desired optical and electrical properties
become established due to the increasing oxidation reaction
rate at the substrate surface. Above T,,,=250 “C! preferred
(001) orientation is obtained.
The effect of PO. Nearly stoichiometric In203 films
have been obtained under all oxygen pressures PO in the
studied range. PO mainly affected the film morphology.
This can be understood in view of the chemistry of the In
oxidation reaction. A shortage of oxygen in the low pressure range should bring about oxygen-poor compounds,
such as In20 and InO. However, the latter do not form
J. Appl. Phys., Vol. 75, No. 4, 15 February 1994
since they are unstable but rather In203 grows in scattered,
separate islands, in between which some unoxidized indium is present. Under higher oxygen pressures In reacts
more strongly with oxygen and forms a more continuous
and smoother film. Excess oxygen in the In203 lattice is
found at the highest pressure measured.
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