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Sputtered indium-tin oxide substrates for CdSCdTe solar cells
So~ ~
ELSEVIER
k~aat~
Solar Energy Materials and Solar Cells 37 (1995) 357-365
Sputtered indium-tin oxide substrates for CdS-CdTe solar cells J. Tou~kovfi
a,
j. K o v a n d a
a,
L. Dobifi~ovfi
a,
V. Pa~izek b, p. K i e l a r b
a Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Physical Institute of Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic
Received 25 September 1992;revised 19 July 1993; accepted 23 December 1994
Abstract
Simple d.c. sputtering was used for the deposition of ITO films on glass substrates. Their structural, electrical, optical, and chemical properties depend on the substrate temperature. A temperature slightly above 400°C is optimal and leads to an increase of the grain size, mobility, carrier concentration, and conductivity of the layers. The lowest resistivity is 1.88 × 10 -4 l-lcm and relevant transmittivity is 83.3%. Moreover, these layers are noted for a good chemical stability, which enables us to use them as transparent conductive electrodes for the CdS-CdTe solar cells prepared by chemical and electrochemical methods. ITO films on unheated substrates do not attain the required properties not even after annealings in various atmospheres.
1. I n t r o d u c t i o n
Metallic oxide films as tin oxide, indium oxide and indium-tin oxide (ITO) are well known as semiconductors with low electrical resistivity, high optical transparency of visible light and infrared reflectance. These properties have been employed in many applications in optoelectronics (liquid crystal displays, solar cells, electroluminescent devices, charge-coupled devices), as thin film resistors, gas sensors, heat mirror coating, etc. We have dealt with I T O (In203:Sn) films which should serve as transparent conductive electrodes for C d S - C d T e photovoltaic cells. Because of a corrosive medium at CdS and CdTe preparation by electrodeposition or chemical methods an additional requirement of good chemical stability of the I T O layers is arrived. In this paper we report on the I T O films prepared by simple d.c. sputtering method from an oxide target. This method has no claim on high vacuum and 0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927 -0248(95 )00029 - 1
358
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
usually yields films with good stoichiometry. Low deposition rate, especially without use of magnetron sputtering arrangement, is a disadvantage. Some structural, electrical, optical and chemical properties of the layers are presented and it is shown that the decisive parameter for the fulfilment of the specified demands is the substrate temperature.
2. Experiment ITO films were prepared by d.c. sputtering from an oxide target with composition: 91 wt% In20 3 - 9 wt% SnO 2. Typical conditions for the growth of ITO layers were as follows: the total pressure of the sputter gas (argon-air mixture) was 10 Pa, the oxygen pressure 2 x 10 -2 Pa, DC voltage 3.4-3.7 kV and discharge current 20 mA for a cathode 5 cm in diameter. The deposition rate was from 0.6 to 1.0 txm/hour. Sample thicknesses (0.5-1.0 txm) were measured mechanically with Surfometer SF 200. Borosilicate glass substrates were mechanically and then chemically cleaned prior to the deposition. The substrates were heated or unheated. Let us note that the temperature of the substrates not intentionally heated might increase up to 200°C during the sputtering. ITO layers were investigated by X-ray diffractometry (XRD) to determine their phase composition and preferred orientation. XRD measurements were carried out on 0 - 2 0 scanning HZG-4 powder goniometer with Ni filtered C u K a radiation. The surface morphology was examinated in a Jeol scanning electron microscope. Measurements of optical transmission of the layers and of the glass substrates were performed with SPM-2 spectrometer in the wavelength range from 0.27 to 2.2 ixm. Hamamatsu R 928 photomultiplier and PbS cell were used as detectors. The temperature dependence of resistivity and Hall coefficient have been measured with standard 4-point van der Pauw method. Ohmic contacts were gold wires fastened by silver paste. The electric current passing the samples was held at values 5 or 10 mA, the electric voltage was measured with an accuracy of + 0.1 ~V, the magnetic field was 0 and +0.5 T or 0 and +0.7 T. Computer controlled measurements were performed in the temperature range from liquid nitrogen to 300 K in 2 K intervals.
3. Results and discussion 3.1. Crystallographic properties
The composition and crystallinity of ITO layers sputtered on unheated and heated substrates were examined by X-ray diffractometry. All layers were polycrystalline, cubic modification of In20 3, and preferentially oriented. No other phase was identified in the films by this method. Lattice parameter was calculated using
J. Tou~kovd et a l . / Solar Energy Materials and Solar Cells 37 (1995) 357-365
359
100
[3
C
-
I
0
I
b 100
o
-4
0
c 100
--
o 20
o
30
co
40
50
20(deg)
60
Fig. 1. Diffraction patterns of ITO films with the texture (222): (a) unheated substrate, (b) the substrate heated to 325 ° C, (c) the substrate heated to 400 ° C.
cos O cotg O extrapolation function [1]. There are no systematic changes in lattice parameters a (hkl) and the average value is 1.0143 + 0.0010 nm. The {222} and the {440} preferential orientations of crystal planes parallel to the substrate surface revealed (Figs. 1 and 2). The texture appeared more expressive at the layers deposited on the unheated substrates. We have not found a relation between the type of the texture and the substrate temperature. The surface topography of the ITO films is shown on the scanning electron micrographs (Figs. 3 and 4). Small grains are typical for the samples on the unheated substrates (Fig. 3) and the feature of the topography was not changed by a post-annealing in argon atmosphere at 425 ° C. The grain size became larger as the substrate temperature Ts increased, the longest dimension reaching approximately 0.2 I~m at T~ = 400 ° C (Fig. 4).
3.2. Electrical properties and chemical stability Transport parameter measurements show that films deposited on heated and unheated substrates are n-type semiconductors. Carrier concentration, electrical resistivity and mobility of all samples varied only weakly in the temperature range
360
J.
Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
loo
--x
c
2 c
@
I
100
V~
0 20
30
z,0
5O 60 2@(deg)
Fig. 2. Diffraction patterns of ITO films with the texture (440): (a) unheated substrate, (b) the substrate heated to 450 ° C.
Fig. 3. Scanning electron micrm, taphs of ITO films on unheated substrates (magnification 30,000 × ).
Fig. 4. Scanning electron micrographs of ITO films on substrates heated to 400°C (magnification 30,000 x ).
J. Tougkovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
361
Table 1
Heat treatment conditions and change in resistance of the samples on unheated substrates Sample
Atmosphere
Temperature (o C)
Duration (min.)
J R / R o (%)
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11
air air H2 Ar-H 2 Ar-H 2 Ar Ar Ar Ar Ar Ar
510 410 400 350 400 280 350 400 425 450 500
30 60 20 60 60 60 60 60 60 60 60
+ 133 + 23 - 13 -70 -65 -55 -52 -55 -42 -15 +7
Ro is the
sheet resistance before heat treatment, AR is change of the sheet resistance after the
treatment. from LN 2 to 300 K. As the values of electrical parameters and chemical stability of these two groups of layers are quite different, we will report on the individual groups separately.
ITO films sputtered onto unheated substrates Typical resistivity of these I T O films was about 60 × 10 4 O c m (sheet resistance 90 12/1~), Hall mobility 4 c m e / V s and electron concentration 2 × 102° cm -3. The layers were then annealed in various atmospheres which is a procedure usually changing electrical and optical properties of I T O or SnO 2 layers ([2], [3] for example). H e a t treatment conditions and resulting changes in resistance are given in Table 1. As evident, the heat treatment in air led to higher resistance, probably by filling O 2 vacancies, a decrease of resistance was found by annealing in hydrogen but transmission of the films lowered already after several minutes of the processing because of a reductive reaction. From the point of view of transport parameters and transmittance of the layers the most convenient is annealing in a mixture of 90% Ar and 10% H 2 or in pure Ar. Maximal reduction in resistance was 70% and 55%, respectively. As the electron concentration was not practically changed and mobility increased (by 50%), the annealing did not enhance the amount of oxygen vacancies in the grains and a desorption of oxygen from the grain boundaries probably occurred as it had been found at sprayed polycrystalline layers [2], [4]. Let us r e m e m b e r that no change in the grain size was observed after this heat treatment. As the temperatures above 400°C did not change the structure of the layers a decrease of resistance change for these temperatures may be attributed a penetration of alkaline ions from glass into the films. With regard to intended employment of the I T O films as substrates for electrochemically deposited CdS and CdTe layers a requirement of their chemical resistance was imposed on them in addition. I T O films were submitted to a simple test by immersion into hydrochloric acid (35%, 60 ° C). The films (1 ~xm) sputtered
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
362
35 -
4x
o
/
E
°I
\o
%
\
o 0
x Z
E o
x x
30-
3--
/o
10
\/ ,
x
/~x
99
~08 25-
z
/x /" /' •
20 - - 1 30O
O -''~
I I I
--07
-06
o
i
I
I
350
400
450
----05 Ts (*C I
Fig. 5. Variation of resistivity ~ (x), mobility Iz(o), and concentration N(o) in ITO films with substrate temperature TS.
on unheated glass substrates disappeared immediately. In water solutions for electrodeposition of CdS (0.2M CdSO4, 0.05M N a 2 S 2 0 3 p H ~ 3 at 80°C) and CdTe (1M CdSO 4 and 0.01M TeO2, p H ~ 2 at 85 ° C) [5] the layers were dissolved in several tens of seconds. Chemical resistance did not improve by the heat treatments neither in argon, nor in hydrogen. After annealing in air the films were dissolved in the solutions within 20 minutes.
ITO films sputtered onto heated substrates The films were deposited on glass substrates intentionally heated to 300-450 ° C. The variation of resistivity ~', Hall mobility /x and carrier concentration N as a function of substrate t e m p e r a t u r e TS is shown in Fig. 5. The measurements were performed at room temperature. There is a minimum in ~" (Ts) dependence at 410°C corresponding to a maximum of mobility. The lowest resistivity is 1.88 x 10 -4 f~cm (2.35 12/[] ). The value of mobility is about 30 c m Z / g s which is usual for polycrystalline I T O layers. Similar results have been received on vacuum-deposited I T O layers [6] or on sprayed F T O [7] or SnO 2 [8] layers. As the t e m p e r a t u r e of the substrate increases, the grain size increases and crystallinity is more perfect, which results in the enhanced mobility and the decrease of resistivity. T h e r e is a contamination of I T O layers by incorporation of impurities from the substrates at higher temperatures. These impurities being
J. Tou~koud et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
8o
363
o
60
4O o
oJ
2O
J
o/ K
I
L
L
20
40
60
I
80 t(s)
Fig. 6. Time d e p e n d e n c e of the thickness change of 0.85 txm ITO layer etched with HCI (35%, 60 ° C, without stirring).
ionised can develop another scattering of charge carriers and reduce mobility, which can lead to a consequential increase in resistivity. Thus, the substrate temperature slightly above 400°C is optimal. The increasing N(T s) dependence is usually related to an increase of amount of oxygen vacancies with the substrate temperature. The chemical resistivity was examined by the HCl-test again. By a successive etching (without stirring the bath) and measurement of thickness it was established that the rate of the etching is not constant as shown in Fig. 6 and that in the first stages the layer is etched more slowly. There was not any problem with the chemical stability of the ITO films immersed into the solutions for electroplating of CdS or CdTe layers.
~001 o
to
oo ,i o "o oo
/
o •
|/ ~ - IoO
~o° " ~o
".. \
O!
~O
oS
°-o.°•
•
0
0
~
60 ~'- o~ •,
•
~0
,~
"o
,.
lio°'
"°. °
.o,
\
L
I
L
I
04
06
08
10
°,o N 12
L
1
14
16
]_
] .
18 20 X(#Jm)
Fig. 7. Spectral d e p e n d e n c e of the optical transmission curve a, ITO film on heated (400 ° C) substrate curve b, ITO film on unheated substrate.
364
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
Subsequent heat treatment of this type of samples in H 2 at 400 and 420°C reduced the electric resistivity only by 5% without a decrease of transmission. As also the chemical stability of the as-deposited layers was high enough, this process was not further carried out. 3.3. Optical transmission
High optical transparency in the visible range is typical for these ITO films without a notable difference between both groups of samples. Fig. 7 shows transmittance spectrum modified by interference effects for 1 Ixm thick ITO films. The large decrease at wavelengths below 0.35 ~m is due to absorption edge which shifts towards the UV with increasing electron concentration. The influence of the increasing free carrier concentration on the spectral dependence of transmittance in the near IR region manifests in a decrease of the transmittance due to the free carrier absorption. This fact becomes very expressive comparing the samples deposited on unheated and on heated substrates as it is shown on this figure (the ratio of the carrier concentration is approximately 4). For the applications of the ITO films as transparent electrodes a knowledge of the sheet resistance and the average visible transmittance is required. Especially, in the case of electrodes for C d S / C d T e heterojunction cells we have appreciated an average transmittance T e of the ITO layers in the region of 500-700 nm according to r700
Tp=(1/2OO) Jo
500
TdA.
(1)
A thickness-independent factor describing a "quality" of the layers has been defined as [7] F = --Rsh lnTp = a / o - ,
(2)
where ~r is the conductivity of the film and a is the absorption coefficient. Dependence of the factor F on the substrate temperature is shown in Fig. 8. The minimum value of F is again for 410 °C, which corresponds to the sheet resistance 2.35 f ~ / [ ] and to the average transmittance 83.3%.
4. Conclusions
Thin ITO films were prepared by d.c. sputtering on glass substrates. No other phase besides that of I n 2 0 3 was found by XRD. Some of the electrical, optical and structural properties of the layers depend predominantly on the substrate temperature. The samples on unheated substrates have small grains, the mobility of order 1 cm2/Vs, the resistivity ~ 10 -3 f~ cm, the carrier concentration about 2 × 102o cm -3. These values were not sufficiently changed by a subsequent annealing in various atmospheres and nor the grain size increased. On the other side, the samples on heated substrates show higher
J. Tougkoud et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
365
/ £ 14-
12
10-
0.8-
04 300
1 350
L ~ 400
Ts (°C'
I 450
Fig. 8. Dependence of factor F on the substrate temperature T~.
mobility ~ 101 cm2/Vs, carrier concentration (5-10) x 1020 cm -3 and lower resistivity 10 -4 ~ cm. It was found that optimal deposition temperature was 410 ° C. These results can be explained by improvement of crystallinity, namely by increase of the grain size with substrate temperature. Higher carrier concentration is probably connected with a higher amount of oxygen vacancies which are created in the layers during the deposition process. Subsequent heat treatments of the samples are not able to replace the influence of temperature during the deposition. Optical transmission of most layers exceeds 80%. In the case of the layers on heated substrates a free carrier absorption decreases transmittance in IR region of spectrum. Quality of the layers was appreciated by a factor F including both the sheet resistance and the transmittance. The optimal value of F obtained for substrate temperature 410°C corresponds to the sheet resistance 2.35 f ~ / D and to an average transmittance 83.3%. Good chemical stability of these layers is evidently related to a better crystalline structure with larger grains. Hence, such ITO films attained electrical, optical and chemical properties available for their use as substrates for chemical or electrochemical deposition of CdS and CdTe layers. References [1] H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials (New York, 1970). [2] E. Shanthi, A. Banerjee, V. Dutta and K.L. Chopra, Thin Solid Films 71 (1980) 237. [3] H. Hoffmann, A. Dietrich, J. Pickl and D.Krause, Appl. Phys. 16 (1978) 381. [4] J. Tou~kovfi, D. Kindl and J. Kovanda, Thin Solid Films 214 (1992) 92. [5] J. Tou~kovfi, D. Kindl and J. Tou~ek, Sol. Energy Mater. 18 (1989) 377. [6] M. Mizuhashi, Thin Solid Films 70 (1980) 91. [7] K.L. Chopra, S. Major and D.K. Pandya, Thin Solid Films 102 (1983) 1. [8] V. Vasu and A. Subrahmamyam, Thin Solid Films 202 (1991) 283.
ELSEVIER
k~aat~
Solar Energy Materials and Solar Cells 37 (1995) 357-365
Sputtered indium-tin oxide substrates for CdS-CdTe solar cells J. Tou~kovfi
a,
j. K o v a n d a
a,
L. Dobifi~ovfi
a,
V. Pa~izek b, p. K i e l a r b
a Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b Physical Institute of Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic
Received 25 September 1992;revised 19 July 1993; accepted 23 December 1994
Abstract
Simple d.c. sputtering was used for the deposition of ITO films on glass substrates. Their structural, electrical, optical, and chemical properties depend on the substrate temperature. A temperature slightly above 400°C is optimal and leads to an increase of the grain size, mobility, carrier concentration, and conductivity of the layers. The lowest resistivity is 1.88 × 10 -4 l-lcm and relevant transmittivity is 83.3%. Moreover, these layers are noted for a good chemical stability, which enables us to use them as transparent conductive electrodes for the CdS-CdTe solar cells prepared by chemical and electrochemical methods. ITO films on unheated substrates do not attain the required properties not even after annealings in various atmospheres.
1. I n t r o d u c t i o n
Metallic oxide films as tin oxide, indium oxide and indium-tin oxide (ITO) are well known as semiconductors with low electrical resistivity, high optical transparency of visible light and infrared reflectance. These properties have been employed in many applications in optoelectronics (liquid crystal displays, solar cells, electroluminescent devices, charge-coupled devices), as thin film resistors, gas sensors, heat mirror coating, etc. We have dealt with I T O (In203:Sn) films which should serve as transparent conductive electrodes for C d S - C d T e photovoltaic cells. Because of a corrosive medium at CdS and CdTe preparation by electrodeposition or chemical methods an additional requirement of good chemical stability of the I T O layers is arrived. In this paper we report on the I T O films prepared by simple d.c. sputtering method from an oxide target. This method has no claim on high vacuum and 0927-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0927 -0248(95 )00029 - 1
358
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
usually yields films with good stoichiometry. Low deposition rate, especially without use of magnetron sputtering arrangement, is a disadvantage. Some structural, electrical, optical and chemical properties of the layers are presented and it is shown that the decisive parameter for the fulfilment of the specified demands is the substrate temperature.
2. Experiment ITO films were prepared by d.c. sputtering from an oxide target with composition: 91 wt% In20 3 - 9 wt% SnO 2. Typical conditions for the growth of ITO layers were as follows: the total pressure of the sputter gas (argon-air mixture) was 10 Pa, the oxygen pressure 2 x 10 -2 Pa, DC voltage 3.4-3.7 kV and discharge current 20 mA for a cathode 5 cm in diameter. The deposition rate was from 0.6 to 1.0 txm/hour. Sample thicknesses (0.5-1.0 txm) were measured mechanically with Surfometer SF 200. Borosilicate glass substrates were mechanically and then chemically cleaned prior to the deposition. The substrates were heated or unheated. Let us note that the temperature of the substrates not intentionally heated might increase up to 200°C during the sputtering. ITO layers were investigated by X-ray diffractometry (XRD) to determine their phase composition and preferred orientation. XRD measurements were carried out on 0 - 2 0 scanning HZG-4 powder goniometer with Ni filtered C u K a radiation. The surface morphology was examinated in a Jeol scanning electron microscope. Measurements of optical transmission of the layers and of the glass substrates were performed with SPM-2 spectrometer in the wavelength range from 0.27 to 2.2 ixm. Hamamatsu R 928 photomultiplier and PbS cell were used as detectors. The temperature dependence of resistivity and Hall coefficient have been measured with standard 4-point van der Pauw method. Ohmic contacts were gold wires fastened by silver paste. The electric current passing the samples was held at values 5 or 10 mA, the electric voltage was measured with an accuracy of + 0.1 ~V, the magnetic field was 0 and +0.5 T or 0 and +0.7 T. Computer controlled measurements were performed in the temperature range from liquid nitrogen to 300 K in 2 K intervals.
3. Results and discussion 3.1. Crystallographic properties
The composition and crystallinity of ITO layers sputtered on unheated and heated substrates were examined by X-ray diffractometry. All layers were polycrystalline, cubic modification of In20 3, and preferentially oriented. No other phase was identified in the films by this method. Lattice parameter was calculated using
J. Tou~kovd et a l . / Solar Energy Materials and Solar Cells 37 (1995) 357-365
359
100
[3
C
-
I
0
I
b 100
o
-4
0
c 100
--
o 20
o
30
co
40
50
20(deg)
60
Fig. 1. Diffraction patterns of ITO films with the texture (222): (a) unheated substrate, (b) the substrate heated to 325 ° C, (c) the substrate heated to 400 ° C.
cos O cotg O extrapolation function [1]. There are no systematic changes in lattice parameters a (hkl) and the average value is 1.0143 + 0.0010 nm. The {222} and the {440} preferential orientations of crystal planes parallel to the substrate surface revealed (Figs. 1 and 2). The texture appeared more expressive at the layers deposited on the unheated substrates. We have not found a relation between the type of the texture and the substrate temperature. The surface topography of the ITO films is shown on the scanning electron micrographs (Figs. 3 and 4). Small grains are typical for the samples on the unheated substrates (Fig. 3) and the feature of the topography was not changed by a post-annealing in argon atmosphere at 425 ° C. The grain size became larger as the substrate temperature Ts increased, the longest dimension reaching approximately 0.2 I~m at T~ = 400 ° C (Fig. 4).
3.2. Electrical properties and chemical stability Transport parameter measurements show that films deposited on heated and unheated substrates are n-type semiconductors. Carrier concentration, electrical resistivity and mobility of all samples varied only weakly in the temperature range
360
J.
Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
loo
--x
c
2 c
@
I
100
V~
0 20
30
z,0
5O 60 2@(deg)
Fig. 2. Diffraction patterns of ITO films with the texture (440): (a) unheated substrate, (b) the substrate heated to 450 ° C.
Fig. 3. Scanning electron micrm, taphs of ITO films on unheated substrates (magnification 30,000 × ).
Fig. 4. Scanning electron micrographs of ITO films on substrates heated to 400°C (magnification 30,000 x ).
J. Tougkovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
361
Table 1
Heat treatment conditions and change in resistance of the samples on unheated substrates Sample
Atmosphere
Temperature (o C)
Duration (min.)
J R / R o (%)
A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11
air air H2 Ar-H 2 Ar-H 2 Ar Ar Ar Ar Ar Ar
510 410 400 350 400 280 350 400 425 450 500
30 60 20 60 60 60 60 60 60 60 60
+ 133 + 23 - 13 -70 -65 -55 -52 -55 -42 -15 +7
Ro is the
sheet resistance before heat treatment, AR is change of the sheet resistance after the
treatment. from LN 2 to 300 K. As the values of electrical parameters and chemical stability of these two groups of layers are quite different, we will report on the individual groups separately.
ITO films sputtered onto unheated substrates Typical resistivity of these I T O films was about 60 × 10 4 O c m (sheet resistance 90 12/1~), Hall mobility 4 c m e / V s and electron concentration 2 × 102° cm -3. The layers were then annealed in various atmospheres which is a procedure usually changing electrical and optical properties of I T O or SnO 2 layers ([2], [3] for example). H e a t treatment conditions and resulting changes in resistance are given in Table 1. As evident, the heat treatment in air led to higher resistance, probably by filling O 2 vacancies, a decrease of resistance was found by annealing in hydrogen but transmission of the films lowered already after several minutes of the processing because of a reductive reaction. From the point of view of transport parameters and transmittance of the layers the most convenient is annealing in a mixture of 90% Ar and 10% H 2 or in pure Ar. Maximal reduction in resistance was 70% and 55%, respectively. As the electron concentration was not practically changed and mobility increased (by 50%), the annealing did not enhance the amount of oxygen vacancies in the grains and a desorption of oxygen from the grain boundaries probably occurred as it had been found at sprayed polycrystalline layers [2], [4]. Let us r e m e m b e r that no change in the grain size was observed after this heat treatment. As the temperatures above 400°C did not change the structure of the layers a decrease of resistance change for these temperatures may be attributed a penetration of alkaline ions from glass into the films. With regard to intended employment of the I T O films as substrates for electrochemically deposited CdS and CdTe layers a requirement of their chemical resistance was imposed on them in addition. I T O films were submitted to a simple test by immersion into hydrochloric acid (35%, 60 ° C). The films (1 ~xm) sputtered
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
362
35 -
4x
o
/
E
°I
\o
%
\
o 0
x Z
E o
x x
30-
3--
/o
10
\/ ,
x
/~x
99
~08 25-
z
/x /" /' •
20 - - 1 30O
O -''~
I I I
--07
-06
o
i
I
I
350
400
450
----05 Ts (*C I
Fig. 5. Variation of resistivity ~ (x), mobility Iz(o), and concentration N(o) in ITO films with substrate temperature TS.
on unheated glass substrates disappeared immediately. In water solutions for electrodeposition of CdS (0.2M CdSO4, 0.05M N a 2 S 2 0 3 p H ~ 3 at 80°C) and CdTe (1M CdSO 4 and 0.01M TeO2, p H ~ 2 at 85 ° C) [5] the layers were dissolved in several tens of seconds. Chemical resistance did not improve by the heat treatments neither in argon, nor in hydrogen. After annealing in air the films were dissolved in the solutions within 20 minutes.
ITO films sputtered onto heated substrates The films were deposited on glass substrates intentionally heated to 300-450 ° C. The variation of resistivity ~', Hall mobility /x and carrier concentration N as a function of substrate t e m p e r a t u r e TS is shown in Fig. 5. The measurements were performed at room temperature. There is a minimum in ~" (Ts) dependence at 410°C corresponding to a maximum of mobility. The lowest resistivity is 1.88 x 10 -4 f~cm (2.35 12/[] ). The value of mobility is about 30 c m Z / g s which is usual for polycrystalline I T O layers. Similar results have been received on vacuum-deposited I T O layers [6] or on sprayed F T O [7] or SnO 2 [8] layers. As the t e m p e r a t u r e of the substrate increases, the grain size increases and crystallinity is more perfect, which results in the enhanced mobility and the decrease of resistivity. T h e r e is a contamination of I T O layers by incorporation of impurities from the substrates at higher temperatures. These impurities being
J. Tou~koud et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
8o
363
o
60
4O o
oJ
2O
J
o/ K
I
L
L
20
40
60
I
80 t(s)
Fig. 6. Time d e p e n d e n c e of the thickness change of 0.85 txm ITO layer etched with HCI (35%, 60 ° C, without stirring).
ionised can develop another scattering of charge carriers and reduce mobility, which can lead to a consequential increase in resistivity. Thus, the substrate temperature slightly above 400°C is optimal. The increasing N(T s) dependence is usually related to an increase of amount of oxygen vacancies with the substrate temperature. The chemical resistivity was examined by the HCl-test again. By a successive etching (without stirring the bath) and measurement of thickness it was established that the rate of the etching is not constant as shown in Fig. 6 and that in the first stages the layer is etched more slowly. There was not any problem with the chemical stability of the ITO films immersed into the solutions for electroplating of CdS or CdTe layers.
~001 o
to
oo ,i o "o oo
/
o •
|/ ~ - IoO
~o° " ~o
".. \
O!
~O
oS
°-o.°•
•
0
0
~
60 ~'- o~ •,
•
~0
,~
"o
,.
lio°'
"°. °
.o,
\
L
I
L
I
04
06
08
10
°,o N 12
L
1
14
16
]_
] .
18 20 X(#Jm)
Fig. 7. Spectral d e p e n d e n c e of the optical transmission curve a, ITO film on heated (400 ° C) substrate curve b, ITO film on unheated substrate.
364
J. Tou~kovd et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
Subsequent heat treatment of this type of samples in H 2 at 400 and 420°C reduced the electric resistivity only by 5% without a decrease of transmission. As also the chemical stability of the as-deposited layers was high enough, this process was not further carried out. 3.3. Optical transmission
High optical transparency in the visible range is typical for these ITO films without a notable difference between both groups of samples. Fig. 7 shows transmittance spectrum modified by interference effects for 1 Ixm thick ITO films. The large decrease at wavelengths below 0.35 ~m is due to absorption edge which shifts towards the UV with increasing electron concentration. The influence of the increasing free carrier concentration on the spectral dependence of transmittance in the near IR region manifests in a decrease of the transmittance due to the free carrier absorption. This fact becomes very expressive comparing the samples deposited on unheated and on heated substrates as it is shown on this figure (the ratio of the carrier concentration is approximately 4). For the applications of the ITO films as transparent electrodes a knowledge of the sheet resistance and the average visible transmittance is required. Especially, in the case of electrodes for C d S / C d T e heterojunction cells we have appreciated an average transmittance T e of the ITO layers in the region of 500-700 nm according to r700
Tp=(1/2OO) Jo
500
TdA.
(1)
A thickness-independent factor describing a "quality" of the layers has been defined as [7] F = --Rsh lnTp = a / o - ,
(2)
where ~r is the conductivity of the film and a is the absorption coefficient. Dependence of the factor F on the substrate temperature is shown in Fig. 8. The minimum value of F is again for 410 °C, which corresponds to the sheet resistance 2.35 f ~ / [ ] and to the average transmittance 83.3%.
4. Conclusions
Thin ITO films were prepared by d.c. sputtering on glass substrates. No other phase besides that of I n 2 0 3 was found by XRD. Some of the electrical, optical and structural properties of the layers depend predominantly on the substrate temperature. The samples on unheated substrates have small grains, the mobility of order 1 cm2/Vs, the resistivity ~ 10 -3 f~ cm, the carrier concentration about 2 × 102o cm -3. These values were not sufficiently changed by a subsequent annealing in various atmospheres and nor the grain size increased. On the other side, the samples on heated substrates show higher
J. Tougkoud et al. / Solar Energy Materials and Solar Cells 37 (1995) 357-365
365
/ £ 14-
12
10-
0.8-
04 300
1 350
L ~ 400
Ts (°C'
I 450
Fig. 8. Dependence of factor F on the substrate temperature T~.
mobility ~ 101 cm2/Vs, carrier concentration (5-10) x 1020 cm -3 and lower resistivity 10 -4 ~ cm. It was found that optimal deposition temperature was 410 ° C. These results can be explained by improvement of crystallinity, namely by increase of the grain size with substrate temperature. Higher carrier concentration is probably connected with a higher amount of oxygen vacancies which are created in the layers during the deposition process. Subsequent heat treatments of the samples are not able to replace the influence of temperature during the deposition. Optical transmission of most layers exceeds 80%. In the case of the layers on heated substrates a free carrier absorption decreases transmittance in IR region of spectrum. Quality of the layers was appreciated by a factor F including both the sheet resistance and the transmittance. The optimal value of F obtained for substrate temperature 410°C corresponds to the sheet resistance 2.35 f ~ / D and to an average transmittance 83.3%. Good chemical stability of these layers is evidently related to a better crystalline structure with larger grains. Hence, such ITO films attained electrical, optical and chemical properties available for their use as substrates for chemical or electrochemical deposition of CdS and CdTe layers. References [1] H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials (New York, 1970). [2] E. Shanthi, A. Banerjee, V. Dutta and K.L. Chopra, Thin Solid Films 71 (1980) 237. [3] H. Hoffmann, A. Dietrich, J. Pickl and D.Krause, Appl. Phys. 16 (1978) 381. [4] J. Tou~kovfi, D. Kindl and J. Kovanda, Thin Solid Films 214 (1992) 92. [5] J. Tou~kovfi, D. Kindl and J. Tou~ek, Sol. Energy Mater. 18 (1989) 377. [6] M. Mizuhashi, Thin Solid Films 70 (1980) 91. [7] K.L. Chopra, S. Major and D.K. Pandya, Thin Solid Films 102 (1983) 1. [8] V. Vasu and A. Subrahmamyam, Thin Solid Films 202 (1991) 283.