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A study of the composition of In2O3(Te) films prepared by the spraying method
Thin Solid Films, 202 ( 1991 ) 243-247
243
PREPARATION AND CHARACTERIZATION
A S T U D Y O F T H E C O M P O S I T I O N OF In203(Te) F I L M S P R E P A R E D BY T H E S P R A Y I N G M E T H O D T. RATCHEVA AND M. NANOVA
Higher Institute qf Chemical Technology, Sofia (Bulgaria) L. KINOVA AND I. PENEV
Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy qf Science, Sqfia (Bulgaria) (Received March 19, 1990; revised October 26, 1990; accepted December 6, 1990)
In this paper we report investigations on the composition of I n 2 0 3 films doped with tellurium, prepared by the spraying method. The aim of this investigation was to establish the optimum conditions for the preparation of highly transparent and conductive films. The film composition was studied by X-ray photoelectron spectroscopy and by instrumental neutron activation analysis. The analysis data allow us to deduce a transition coefficient (1.79 _+0.18) relating the In:Te ratio of the solution to that of the film.
1. INTRODUCTION
Transparent conductive I n 2 0 3 films have found numerous applications in optoelectronic devices I 3. Because of the wide band gap of In/O3 (Eg > 3.6 e V ) 4 the films are transparent in the visible spectral range. Deviation from the stoichiometric composition (the presence of oxygen vacancies) as well as any suitable doping lead to a low resistivity (about 10- 3 f2 cm) of the films. In this paper we present and discuss investigations on the composition o f I n 2 0 3 films doped with tellurium prepared by the spraying method. 2. EXPERIMENTAL DETAILS
In our previous paper 5 we have discussed the preparation ofIn203(Te ) films by the spraying method using an alcoholic solution of InCl3 and TeCl 4 (as the source of the tellurium dopant), n-Si plates 10mm × 10mm in size, with p ~ 6 - 9 ~ cm and (111) orientation, and borosilicate glass plates 30 m m × 30 m m in size were used as substrates. The films were obtained at T~ = 480-500 °C. The InCl 3 concentration in the solution was 1 tool I 1, the spraying rate was 0.08 cm 3 s - 1, the angle of spraying was 30°-45 ° and the distance between the nozzle and the substrate was 50 cm. Three spray solutions with different In:Te ratios were used: (1) 17.1; (2) 35.3; (3) 83.3. The film thickness was measured with a Talystep instrument. The film resistivity was determined by the four-probe method. The film transmission T w a s measured with a Shimadzu double-beam spectrophotometer (UV- 190). 0040-6090/91/$3.50
(~i' Elsevier Sequoia/Printed in The Netherlands
244
T. RATCHEVAet al.
The film composition was studied by X-ray photoelectron spectroscopy and by instrumental neutron activation analysis. Electron spectroscopy for chemical analysis (ESCA) spectra were obtained by irradiation of the specimens with an A1 K s X-ray energy of 1486.6 eV. The films were sputtered with argon ions of 4 keV energy to investigate the bulk. The In203(Te) films and the solutions with three different ratios of In:Te sealed in a quartz vial were irradiated for 12h in a reactor neutron flux of 5×1012 neutrons cm 2 S 1. After the samples had been cooled for 36 h, they were measured on a Ge(Li) detector with an MCA Canberra-40 instrument. Indium was detected by the 7 line at 189.9 keV of the isotope 114In m with a half-life of 49.5 days, and tellurium by the 7 line at 364.5 keV of the daughter isotope1311 with a half-life of 8 days. The weight ratio In:Te in the films was calculated from the measured activity ratio on the basis of the known values of this ratio in the solutions. 3. RESULTS AND DISCUSSION
The ESCA spectra for In203(Te) films obtained by spraying an alcoholic solution with an In:Te ratio of 35.3 are shown in Fig. 1. The observed binding energy of the In 3d5/2 level (Fig. l(b)) is 445.1 eV, which is characteristic of In2036' 7. Figure l(c) shows the Te 3d5/2 ESCA peak with a binding energy of 577.3 eV at the surface, which corresponds to TeO20, while under the surface the binding energy of the Te 3d5/2 level is shifted to 573.3 eV, which can be attributed to free tellurium atoms. Figure l(d) shows the O ls ESCA peak observed in the InzO3(Te) films. As can be seen, on the surface the O ls peak is shifted to 532.6 eV (under the surface it is at 530.6eV) owing to contamination of the surface with carbon compounds (CO, hydrocarbons etc.). The calculated In:Te ratio in the bulk of the films is presented in Table I. The calculated bulk In:Te ratio in the solution and the ratio in the film show that the obtained films are depleted in tellurium. In Table II the values of the In:Te ratios of the solutions are compared with those of the films which were obtained from the neutron activation analysis. The values in the tables are averaged and processed using the least-squares procedure. The film transmission in the visible spectral range is presented in Fig. 2 for the same three samples with different In:Te ratios in the film. The experimental data given in Fig. 2 and Table II show that the films with the highest transmission coefficient in the visible spectral range and the lowest resistivity are obtained from a solution with an In:Te ratio of 35.3. The ESCA and neutron activation analysis data gave us the possibility of deducing a transition coefficient relating the In:Te ratio of the solution to that of the film. The value of this coefficient is 1.79 +__0.18 for In203(Te). As can be seen, the films are depleted in tellurium compared with the starting solution owing to the different lnCl3 and TeCI,~ vapour pressures at the temperatures of film preparation. For example, at T~ = 480 °C the InC13 vapour pressure is 6.7 x 104 Pa and Plec~4 = 3.9 x 105 Pa s. The greater value of the TeC14 vapour pressure at the deposition temperature would favour the lower doping level of the films.
COMPOSITION OF SPRAYED
In203(Te )
245
In 3dsl2
In 3d 3~2
In 3p3~2
01s
0.0
l In 3p,~2
Binding energy
1000.0
(a)
l,
-
-
2
~ dsn
. fT:L
,~.:
,0';.'. ,,:..-,~.~, /~0.0
BINDIN6 ENERGYleVI
/+50,0 565.0
~d3/2
S.E {eV]
-.~,,,~. 590.0 5270
B.E [eV]
537.0
(b) (c) (d) Fig. 1. (a) ESCA spectrum for an InzO3(Te) film obtained by spraying an alcoholic solution with an In:Te ratio of 35.3. (b) In 3ds/z ESCA peaks, (c) Te3ds/2 ESCA peaks and (d) O ls ESCA peaks for different distances from the surface: spectra 1, 0/am (on the surface); spectra 2, 0.008/am; spectra 3, 0.0361am; spectra 4, 0.063/am.
TABLE 1 P E R C E N T A G E C O N T E N T OF T H E ELEMENTS I N D I U M , T E L L U R I U M A N D O X Y G E N A N D THE R A T I O
In:Te IN
Distance from the sutJ'ace (pm)
In (%)
Te (%)
0 (%)
/n.. Te
0 0,008 0,036 0,063
46,8 52.0 48.0 55.8
0.9 1.0 0.8 0.9
47.0" 47.0 52.2 43.3
52.0 52.0 60.0 62.0
T H E FILM
The film thickness was 0.15/am and the resistivity p = 2.9 × 10 -a f~ cm. The transmission is presented in Fig. 2, curve 1. a The carbon content on the surface is 5.3%.
246
T.
TABLE II In:Te R A T I O S
RATCHEVA et al.
OF T H E S O L U T I O N A N D OF T H E F I L M S O B T A I N E D F R O M T H E N E U T R O N A C T I V A T I O N A N A L Y S I S
Sample
ln: Te
1 2 3
Resistivity
In the solution
In the film
35.3 83.3 17.1
63.28 149.20 30.64
cm)
2.9 x 10 4 5.5x10 3 4.0× 10 -3
Thickness (pm)
0.150 0.157 0.162
lOO 9o~ 80 _
7o 6o
]
I
50
o
3C
2C -
~
!
~
+
/
:i
, /
&O0
500
600 700 Wevetengfh (nm)
]
800
900
Fig. 2. Transmission of thin In20 3 films doped with tellurium for various In:Te ratios in the solution: spectrum 1, In:Te = 35.3 (measured film thickness, 0.150 I.tm); spectrum 2, In:Te = 83.3 (measured film thickness, 0.157 pm); spectrum 3, ln:Te = 17.1 (measured film thickness, 0.162/am).
4. CONCLUSION
The study of doped In203 films deposited by the spraying method shows that films with high transmission (the average transmission T > 85% at 2 = 400-850 nm) and low resistivity (p ~ 10 -4 f2 cm) are obtained from solutions with an In:Te ratio of 35.3. A transition coefficient (1.79 _ 0.18) relating the In:Te ratio in the solution to that of the film was deduced. At the deposition temperature used the obtained I n 2 0 3 films are depleted in tellurium compared with the starting solution owing to the greater value of the TeCI 4 vapour pressure which is reflected in the deduced transition coefficient. REFERENCES 1 K.L. 2 Z.M. 3 A.L. 4 R.L. 5 T.M. 189.
Chopra, S. Major and D. K. Pandya, Thin Solid Films, 102 (1983) 1. Jarzebski, Phys. Status Solidi A, 71 (1982) 13. Dawar and J. C. Joshi, J. Mater. Sci., 19 (1984) 1. Weiher and R. P. Ley, J. Appl. Phys., 37 (1966) 299. Ratcheva, M. D. Nanova, L. V. Vassilev and M. G. Mikhailov, Thin Solid Films, 139 (1986)
COMPOSITION OF SPRAYED
In203(Te)
247
V. I. Nefedov, Rentgenoelectronnaja Spectroscopia Khimicheskikh Soedinenii, Khimia, Moscow, 1984. 7 J.C. Fan and J. B, Goodenough, J. Appl. Phys., 48 (1977) 3524. 8 Gmelin Handbuch der Anorganischen Chemie, Springer, Berlin, 1977.
6
243
PREPARATION AND CHARACTERIZATION
A S T U D Y O F T H E C O M P O S I T I O N OF In203(Te) F I L M S P R E P A R E D BY T H E S P R A Y I N G M E T H O D T. RATCHEVA AND M. NANOVA
Higher Institute qf Chemical Technology, Sofia (Bulgaria) L. KINOVA AND I. PENEV
Institute of Nuclear Research and Nuclear Energy, Bulgarian Academy qf Science, Sqfia (Bulgaria) (Received March 19, 1990; revised October 26, 1990; accepted December 6, 1990)
In this paper we report investigations on the composition of I n 2 0 3 films doped with tellurium, prepared by the spraying method. The aim of this investigation was to establish the optimum conditions for the preparation of highly transparent and conductive films. The film composition was studied by X-ray photoelectron spectroscopy and by instrumental neutron activation analysis. The analysis data allow us to deduce a transition coefficient (1.79 _+0.18) relating the In:Te ratio of the solution to that of the film.
1. INTRODUCTION
Transparent conductive I n 2 0 3 films have found numerous applications in optoelectronic devices I 3. Because of the wide band gap of In/O3 (Eg > 3.6 e V ) 4 the films are transparent in the visible spectral range. Deviation from the stoichiometric composition (the presence of oxygen vacancies) as well as any suitable doping lead to a low resistivity (about 10- 3 f2 cm) of the films. In this paper we present and discuss investigations on the composition o f I n 2 0 3 films doped with tellurium prepared by the spraying method. 2. EXPERIMENTAL DETAILS
In our previous paper 5 we have discussed the preparation ofIn203(Te ) films by the spraying method using an alcoholic solution of InCl3 and TeCl 4 (as the source of the tellurium dopant), n-Si plates 10mm × 10mm in size, with p ~ 6 - 9 ~ cm and (111) orientation, and borosilicate glass plates 30 m m × 30 m m in size were used as substrates. The films were obtained at T~ = 480-500 °C. The InCl 3 concentration in the solution was 1 tool I 1, the spraying rate was 0.08 cm 3 s - 1, the angle of spraying was 30°-45 ° and the distance between the nozzle and the substrate was 50 cm. Three spray solutions with different In:Te ratios were used: (1) 17.1; (2) 35.3; (3) 83.3. The film thickness was measured with a Talystep instrument. The film resistivity was determined by the four-probe method. The film transmission T w a s measured with a Shimadzu double-beam spectrophotometer (UV- 190). 0040-6090/91/$3.50
(~i' Elsevier Sequoia/Printed in The Netherlands
244
T. RATCHEVAet al.
The film composition was studied by X-ray photoelectron spectroscopy and by instrumental neutron activation analysis. Electron spectroscopy for chemical analysis (ESCA) spectra were obtained by irradiation of the specimens with an A1 K s X-ray energy of 1486.6 eV. The films were sputtered with argon ions of 4 keV energy to investigate the bulk. The In203(Te) films and the solutions with three different ratios of In:Te sealed in a quartz vial were irradiated for 12h in a reactor neutron flux of 5×1012 neutrons cm 2 S 1. After the samples had been cooled for 36 h, they were measured on a Ge(Li) detector with an MCA Canberra-40 instrument. Indium was detected by the 7 line at 189.9 keV of the isotope 114In m with a half-life of 49.5 days, and tellurium by the 7 line at 364.5 keV of the daughter isotope1311 with a half-life of 8 days. The weight ratio In:Te in the films was calculated from the measured activity ratio on the basis of the known values of this ratio in the solutions. 3. RESULTS AND DISCUSSION
The ESCA spectra for In203(Te) films obtained by spraying an alcoholic solution with an In:Te ratio of 35.3 are shown in Fig. 1. The observed binding energy of the In 3d5/2 level (Fig. l(b)) is 445.1 eV, which is characteristic of In2036' 7. Figure l(c) shows the Te 3d5/2 ESCA peak with a binding energy of 577.3 eV at the surface, which corresponds to TeO20, while under the surface the binding energy of the Te 3d5/2 level is shifted to 573.3 eV, which can be attributed to free tellurium atoms. Figure l(d) shows the O ls ESCA peak observed in the InzO3(Te) films. As can be seen, on the surface the O ls peak is shifted to 532.6 eV (under the surface it is at 530.6eV) owing to contamination of the surface with carbon compounds (CO, hydrocarbons etc.). The calculated In:Te ratio in the bulk of the films is presented in Table I. The calculated bulk In:Te ratio in the solution and the ratio in the film show that the obtained films are depleted in tellurium. In Table II the values of the In:Te ratios of the solutions are compared with those of the films which were obtained from the neutron activation analysis. The values in the tables are averaged and processed using the least-squares procedure. The film transmission in the visible spectral range is presented in Fig. 2 for the same three samples with different In:Te ratios in the film. The experimental data given in Fig. 2 and Table II show that the films with the highest transmission coefficient in the visible spectral range and the lowest resistivity are obtained from a solution with an In:Te ratio of 35.3. The ESCA and neutron activation analysis data gave us the possibility of deducing a transition coefficient relating the In:Te ratio of the solution to that of the film. The value of this coefficient is 1.79 +__0.18 for In203(Te). As can be seen, the films are depleted in tellurium compared with the starting solution owing to the different lnCl3 and TeCI,~ vapour pressures at the temperatures of film preparation. For example, at T~ = 480 °C the InC13 vapour pressure is 6.7 x 104 Pa and Plec~4 = 3.9 x 105 Pa s. The greater value of the TeC14 vapour pressure at the deposition temperature would favour the lower doping level of the films.
COMPOSITION OF SPRAYED
In203(Te )
245
In 3dsl2
In 3d 3~2
In 3p3~2
01s
0.0
l In 3p,~2
Binding energy
1000.0
(a)
l,
-
-
2
~ dsn
. fT:L
,~.:
,0';.'. ,,:..-,~.~, /~0.0
BINDIN6 ENERGYleVI
/+50,0 565.0
~d3/2
S.E {eV]
-.~,,,~. 590.0 5270
B.E [eV]
537.0
(b) (c) (d) Fig. 1. (a) ESCA spectrum for an InzO3(Te) film obtained by spraying an alcoholic solution with an In:Te ratio of 35.3. (b) In 3ds/z ESCA peaks, (c) Te3ds/2 ESCA peaks and (d) O ls ESCA peaks for different distances from the surface: spectra 1, 0/am (on the surface); spectra 2, 0.008/am; spectra 3, 0.0361am; spectra 4, 0.063/am.
TABLE 1 P E R C E N T A G E C O N T E N T OF T H E ELEMENTS I N D I U M , T E L L U R I U M A N D O X Y G E N A N D THE R A T I O
In:Te IN
Distance from the sutJ'ace (pm)
In (%)
Te (%)
0 (%)
/n.. Te
0 0,008 0,036 0,063
46,8 52.0 48.0 55.8
0.9 1.0 0.8 0.9
47.0" 47.0 52.2 43.3
52.0 52.0 60.0 62.0
T H E FILM
The film thickness was 0.15/am and the resistivity p = 2.9 × 10 -a f~ cm. The transmission is presented in Fig. 2, curve 1. a The carbon content on the surface is 5.3%.
246
T.
TABLE II In:Te R A T I O S
RATCHEVA et al.
OF T H E S O L U T I O N A N D OF T H E F I L M S O B T A I N E D F R O M T H E N E U T R O N A C T I V A T I O N A N A L Y S I S
Sample
ln: Te
1 2 3
Resistivity
In the solution
In the film
35.3 83.3 17.1
63.28 149.20 30.64
cm)
2.9 x 10 4 5.5x10 3 4.0× 10 -3
Thickness (pm)
0.150 0.157 0.162
lOO 9o~ 80 _
7o 6o
]
I
50
o
3C
2C -
~
!
~
+
/
:i
, /
&O0
500
600 700 Wevetengfh (nm)
]
800
900
Fig. 2. Transmission of thin In20 3 films doped with tellurium for various In:Te ratios in the solution: spectrum 1, In:Te = 35.3 (measured film thickness, 0.150 I.tm); spectrum 2, In:Te = 83.3 (measured film thickness, 0.157 pm); spectrum 3, ln:Te = 17.1 (measured film thickness, 0.162/am).
4. CONCLUSION
The study of doped In203 films deposited by the spraying method shows that films with high transmission (the average transmission T > 85% at 2 = 400-850 nm) and low resistivity (p ~ 10 -4 f2 cm) are obtained from solutions with an In:Te ratio of 35.3. A transition coefficient (1.79 _ 0.18) relating the In:Te ratio in the solution to that of the film was deduced. At the deposition temperature used the obtained I n 2 0 3 films are depleted in tellurium compared with the starting solution owing to the greater value of the TeCI 4 vapour pressure which is reflected in the deduced transition coefficient. REFERENCES 1 K.L. 2 Z.M. 3 A.L. 4 R.L. 5 T.M. 189.
Chopra, S. Major and D. K. Pandya, Thin Solid Films, 102 (1983) 1. Jarzebski, Phys. Status Solidi A, 71 (1982) 13. Dawar and J. C. Joshi, J. Mater. Sci., 19 (1984) 1. Weiher and R. P. Ley, J. Appl. Phys., 37 (1966) 299. Ratcheva, M. D. Nanova, L. V. Vassilev and M. G. Mikhailov, Thin Solid Films, 139 (1986)
COMPOSITION OF SPRAYED
In203(Te)
247
V. I. Nefedov, Rentgenoelectronnaja Spectroscopia Khimicheskikh Soedinenii, Khimia, Moscow, 1984. 7 J.C. Fan and J. B, Goodenough, J. Appl. Phys., 48 (1977) 3524. 8 Gmelin Handbuch der Anorganischen Chemie, Springer, Berlin, 1977.
6