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Characterization of laser evaporated cadmium telluride films
Volume 10. number
MATERIALS
4,5
Characterization 0. Mahammad Department
Received
of laser evaporated
Hussain and P. Jayarama
of Physrcs, Sri
I Bnkateswara
Unrverslty.
26 June 1990: in final form 8 August
November
LETTERS
cadmium
telluride
I990
films
Reddy
Erupati
5 I7 502, India
1990
CdTe films were deposited using the laser assisted vacuum evaporation technique onto Corning 7059 glass substrates. The effect of deposition parameters on the composition and structure and the influence of dopants (antimony and oxygen) on the electrical properties of these films are reported. The oxygen incorporated CdTe films appear to be better with regard to resistivity and carrier concentration when compared to antimony-doped films.
oxygen) on the electrical are reported.
1. Introduction
Thin films of II-VI compound semiconductors have drawn considerable interest in recent years due to their applications in the fabrication of cost effective thin film solar cells. Among these, CdTe is one of the most attractive candidates because it has an optimum band gap ( 1.5 eV) for solar energy conversion [ 1,2 1. Various deposition techniques such as vacuum evaporation [ 3 1, hot wall epitaxy [ 41, close space vapour transport [ 5 1, electrochemical deposition [6] and screen printing [7] were employed for the preparation of polycrystalline CdTe films. The laser evaporation technique is a relative new deposition technique for the deposition of CdTe films. The main advantages of the laser evaporation technique are that (i) power is delivered outside the vacuum system, (ii) the response time is nearly instantaneous and (iii ) simplicity in simultaneous multi-source evaporation of the material to obtain large area films. In laser evaporation the kinetic energy of the particles in the vapour flux is found to be larger than the expected thermal energy distribution [S] which enhances the ad-atom mobility and improves the crystallinity of the films [9]. In this paper, preparation of CdTe films using the laser assisted vacuum evaporation technique, the effect of deposition parameters on the composition and the structure and the influence of dopants (antimony and 0167-577x/90/$
03.50 0 1990 - Elsevier Science
Publishers
properties
of CdTe films
2. Experimental Thin films of CdTe were prepared by the laser assisted vacuum evaporation technique onto ultrasonically cleaned Corning glass substrates maintained at temperatures (T,) in the range lOO-35O’C. The laser beam from a high-power CO2 laser enters into the vacuum chamber through a ZnSe window and was reflected onto the source material kept in a conical shape tantalum boat. The films were deposited in a vacuum better than 3 x 10e6 Torr with a growth rate of about 25 A/s. The thickness of the films was maintained about 1 ym using a quartz crystal thickness monitor. Pure CdTe films as well as films doped with antimony or oxygen were prepared in the present investigation. A Philips Auger Microprobe model 545C computerized system was used for compositional analysis of the films. The surface of the film was scanned at three different spots and from the average values. the composition was determined. .A Philips thin film X-ray diffractometer having scanning angle 28 from 0 to 180” was employed to determine the structure, lattice constants and grain size of the films. Cu Ka target with I = I .54 18 A was used as an X-ray source. Electrical conductivity and Hall measurements were
B.V. (North-Holland
)
165
Volume
10, number
carried out technique.
4,5
using
3. Composition
MATERIALS
the
standard
van
der
Pauw
and structure
The films were smooth, shiny and dark brown in color, uniformly thick and fairly adherent to the substrate surface. The Auger electron spectra of CdTe films deposited at different substrate temperatures are shown in fig. 1. The spectra showed Auger peaks due to cadmium (375 and 382 eV) and tellurium
Ts - 15O.C
I
-
w
-T z D
LETTERS
November
1990
(473 and 493 eV). The composition data (table 1) indicated that while tellurium was found to be in excess in the films formed at T, < 200’ C, no detectable changes in the composition could be seen in the films formed at T,= 2 lo-350°C. Due to the limitations of the AES technique (AES sensitivity is 0.1 at%) we failed to detect the small deviations ( < 0.1 at%) from stoichiometry. The reproducibility of the composition was observed throughout the investigations for the films deposited by maintaining the above deposition conditions. The observation is slightly different from XRD data. Fig. 2 shows the XRD pattern of CdTe films deposited at different temperatures. The films deposited at T,< 250°C showed small ( 100) and (300) peaks corresponding to the hexagonal phase of tellurium (figs. 2a and 2b). These peaks are absent in the films formed at T,=250-330°C (figs. 2c and 2d). A (002) hexagonal peak appeared just before the ( 111) cubic phase in the XRD spectrum (fig. 2e) of CdTe films formed at TS> 330°C. The presence of the hexagonal phase may be due to excess of cadmium in the films [ 10, 11 ] . The hexagonal phase had ac4.56 a and c= 7.46 A which are in good agreement with the results obtained by El-Akmi and Suryanarayanan [ 12 1. The grain size was found to increase with increasing substrate temperature and was about the maximum (0.7 urn) in the films formed at T,= 3 15-320°C. These films were also found to be stoichiometric and highly textured with a preferred (111) orientation which is the close packing direction of the zinc blende structure. The evaluated lattice parameter was 6.48 A.
Ts- 320’ C
!I! @l--T Table 1 AES composition
, ,
200
,
3cQ
ELECTRON
400
ENERGY
5’00
Fig. 1. Auger electron spectra of laser evaporated formed at different substrate temperatures.
166
600
(~‘4)
CdTe films
data of laser evaporated
Sample no.
Substrate temperature
1
150 180 210 260 320 350
2 3 4 5 6
( “C)
CdTe films
Cd (at%)
Te (at%)
48.8 49.4 49.9 50.0 50.0 50.1
51.2 50.6 50.1 50.0 50.0 49.9
Volume
10. number
MATERIALS
4,5
I
5
@
IN
DEGREES
profiles of CdTe films formed at different
4. Electrical properties The undoped CdTe films were found to be highly resistive. The resistivity of the films deposited at T,z 320°C was about 1O6 Q cm. The resistivity of the films was reduced to 104- 1O5 fi cm by annealing in H2 atmosphere at 425 “C for 30 min. This may be due to reduction in the grain boundary state density by heat treating in Hz ambient, with preferential passivation of states near the band edges which leads to a decrease in the recombination velocities as reported by Thorpe et al. [ 131. The films deposited at T, < 250’ C were found to be p-type, whereas the films deposited at T,> 33O’C were n-type. In the intermediate temperature range from 250 to 330°C the films were found fo be intrinsic. 4.1. Antimony-doped
I990
Ts 21O'C
2 Q
Fig. 2. X-ray diffraction
November
LETTERS
CdTe $lms
Attempts to obtain p-type doping of CdTe films by co-evaporation of dopant were not successful. So in
substrate
temperatures
the present study we prepared CdTe material with different antimony concentrations ranging from 10” to 102’/cm3 and used them for the preparation of the films. The source material was made into 8 mm diameter pellets and placed in the boat for evaporation. The substrate temperature was fixed at 320°C. All the films deposited under these conditions exhibited p-type conductivity. The electrical properties of CdTe films are summarized in table 2. The resistivity of these films was found to be strongly dependent on the dopant concentration. A minimum resistivity of about 580 Q cm with a hole density of 8 X lO”/cm was observed in the films prepared by using a source material with antimony concentration of 8 x 10’8/cm3. The resistivity of the films increased with further increase of antimony concentration probably due to self-compensation or defect interactions. The adhesion was also poor with increase of antimony concentration. The effect of heat treatment in different ambients 167
Volume
IO, number
Table 2 Electrical
properties
Sample no.
4,5
MATERIALS
of CdTe films: resistivityp
N,“’
LETTERS
(a cm) and carrier concentration
As-deposited
Annealed
P
NA
P
2.3x IO4 2.0x lo3 5.8x 102
1.3x 10’5 5.4x lOI 8.0x lOI
1.50x 1.70x 5.56x
1 2 3
4x 10” 2x 10’8 8x 10’8
a) N,=dopant
density of CdTe source material
580 BINDING
168
1990
NA ( cme3)
in vacuum
lo4 10’ IO2
Annealed
in hydrogen
NA
P
NA
1.8x 10’5 5.8x lOI5 8.2x lOI
5.5x 103 1.0x 103 3.2x lo2
3.0x 10’5 6.4x lOI5 1.0x 10’5
(cm-3).
on the electrical properties of p-CdTe films are also included in table 2. The hydrogen heat-treated films showed lower resistivity compared to the films heat treated in vacuum.
Fig. 3. X-ray photoelectron
November
4.2. Oxygen-doped CdTe films CdTe films were prepared in oxygen atmosphere with different partial pressures. The AES data taken on the films formed in oxygen partial pressure greater than lop4 Torr showed an intense oxygen peak in-
410
420 ENERGY
spectra of CdTe films formed at different
41
(eV)
partial pressures
of oxygen.
Volume 10, number 4.5
MATERIALS LETTERS
dicating more than 2 at% of oxygen in the films. The phases corresponding to the formation of TeO, or CdTe03 were also observed in XPS scans (fig. 3a). For films formed at lower oxygen partial pressure (<4x lop5Torr), no detectable oxide phases were seen in XPS spectra (fig. 3b), confirming that the presence of oxygen in these CdTe films is very much less. The films deposited in the oxygen partial pressure range from 4x 10e5 to 8 x lop6 Torr and at T,= 320’ C were found to be p-type. From thermally stimulated current measurements two trap levels at 0.11 eV and 0.5 eV were observed for oxygen-doped films [ 141. Agrinskaya et al. [ 15 ] found that the replacement of tellurium with oxygen in CdTe films give rise to a localized level behaving as an electron trap. Wavelength modulation reflection spectroscopy studies of Hsu et al. [ 161 showed that the oxygen atoms exhibited a local level in the band gap with an ionization energy of about 0.1 eV. Chu [ 17 ] also reported that the incorporated oxygen at lower concentrations (co.5 at%) would behave as an acceptor impurity. The films formed at T,=z 320°C with oxygen partial pressure of 1 x 1O- 5 Torr were single phase having a resistivity of = lo2 fi cm with a hole concentration of (2-3) x 10’6/cm’.
5. Conclusions CdTe films were deposited by the laser assisted vacuum evaporation technique onto glass substrates maintained at different substrate temperatures. The films deposited at T,z 320°C were found to be single phase, stoichiometric and highly crystalline. The films deposited using antimony-doped CdTe as source material showed a resistivity of 580 n cm with a carrier concentration 8X 10”/cm3, which is further reduced to 320 Q cm by annealing in hydrogen atmosphere. The films formed in oxygen atmosphere were found to be p-type with carrier concentration and resistivity of (2-3) x 10’6/cm3 and = lo2 Q cm
November 1990
respectively. The oxygen-doped films appear to be better with regard to resistivity and carrier concentration compared to antimony-doped films.
Acknowledgement The authors are thankful to the department of NonConventional Energy Sources, Government of India, for providing the financial support.
References [ 1] S.S. Chern, H.R. Vydyanath and F.A. Kroger. J. Solid State Chem. 14 (1975) 33. [2] K.W. Mitchell, A.L. Fahrenbruch and R.H. Bube. J. Appl. Phys. 48 (1977) 829. [3] J. Aranda, J.L. Moranza, J. Estava and J.M. Codina. Thin Solid Films 120 ( 1984) 23. [4] A. Lopez-Otero, Thin Solid Films 49 ( 1978) 3. [ 51T.C. Anthony, A.L. Fahrenbruch and R.H. Bube, J. Vacuum Sci. Technol. A 2 (1984) 1296. [6] B.M. Basal, J. Appl. Phys. 55 (1984) 601. [7] H. Matsumoto, A. Nakano, H. Uda. S. Ikegami and T. Miyarawa, Japan. J. Appl. Phys. 21 (1982) 800. [ 81 J.F. Frichnicht, Rev. Sci. Instrum. 45 ( 1974) 5 1. [9] T. Ishida, S. Wavo and U. Ishivo, Thin Solid Films 39 (1976) 227. [ lo] R. Suryanarayanan and C. Paporoditis. Compt. Rend. Acad. Sci. (Paris) B 265 ( 1967) 498. [I I ] K.V. Shirnilova, O.S. Bulotore, E.N. Voronkor and V.A. Dinitrier, Soviet Phys. Cryst. 1I (1966) 431. [ 121 A. El-Akmi and R. Suryanarayanan, International Symposium on Trends and New Applications in Thin Films, [ 13 ] T.P. Thorpe Jr., A.L. Fahrenbruch and R.H. Bube, J. Appl. Phys. 60 ( 1986) 3622. [ 14 ] 0. Mahammad Hussain and P. Jayarama Reddy. Solid State Phys. (India) 31C (1988) 343. [ 15 ] N.V. Agrinskaya, G.I. Aleskadrova, E.N. .4rkad’eva, B.A. Alabekov, O.M. Matveer, G.B. Perapelova, S.V. Prokofev and G. Ishmarenkova, Soviet Phys. Semicond. 8 ( 1974) 202. [ 161 M. Hsu, R.-J. Jih, S.-F. Fan, P.-C. Lin, H.-S. Veng and H.L. Hwang, in: Proceedings of the 17th IEEE Photovoltaic Specialists Conference, Las Vegas ( 1984) p. 846. [ 171 T.L. Chu, Solar Cells 23 (1988) 31.
169
MATERIALS
4,5
Characterization 0. Mahammad Department
Received
of laser evaporated
Hussain and P. Jayarama
of Physrcs, Sri
I Bnkateswara
Unrverslty.
26 June 1990: in final form 8 August
November
LETTERS
cadmium
telluride
I990
films
Reddy
Erupati
5 I7 502, India
1990
CdTe films were deposited using the laser assisted vacuum evaporation technique onto Corning 7059 glass substrates. The effect of deposition parameters on the composition and structure and the influence of dopants (antimony and oxygen) on the electrical properties of these films are reported. The oxygen incorporated CdTe films appear to be better with regard to resistivity and carrier concentration when compared to antimony-doped films.
oxygen) on the electrical are reported.
1. Introduction
Thin films of II-VI compound semiconductors have drawn considerable interest in recent years due to their applications in the fabrication of cost effective thin film solar cells. Among these, CdTe is one of the most attractive candidates because it has an optimum band gap ( 1.5 eV) for solar energy conversion [ 1,2 1. Various deposition techniques such as vacuum evaporation [ 3 1, hot wall epitaxy [ 41, close space vapour transport [ 5 1, electrochemical deposition [6] and screen printing [7] were employed for the preparation of polycrystalline CdTe films. The laser evaporation technique is a relative new deposition technique for the deposition of CdTe films. The main advantages of the laser evaporation technique are that (i) power is delivered outside the vacuum system, (ii) the response time is nearly instantaneous and (iii ) simplicity in simultaneous multi-source evaporation of the material to obtain large area films. In laser evaporation the kinetic energy of the particles in the vapour flux is found to be larger than the expected thermal energy distribution [S] which enhances the ad-atom mobility and improves the crystallinity of the films [9]. In this paper, preparation of CdTe films using the laser assisted vacuum evaporation technique, the effect of deposition parameters on the composition and the structure and the influence of dopants (antimony and 0167-577x/90/$
03.50 0 1990 - Elsevier Science
Publishers
properties
of CdTe films
2. Experimental Thin films of CdTe were prepared by the laser assisted vacuum evaporation technique onto ultrasonically cleaned Corning glass substrates maintained at temperatures (T,) in the range lOO-35O’C. The laser beam from a high-power CO2 laser enters into the vacuum chamber through a ZnSe window and was reflected onto the source material kept in a conical shape tantalum boat. The films were deposited in a vacuum better than 3 x 10e6 Torr with a growth rate of about 25 A/s. The thickness of the films was maintained about 1 ym using a quartz crystal thickness monitor. Pure CdTe films as well as films doped with antimony or oxygen were prepared in the present investigation. A Philips Auger Microprobe model 545C computerized system was used for compositional analysis of the films. The surface of the film was scanned at three different spots and from the average values. the composition was determined. .A Philips thin film X-ray diffractometer having scanning angle 28 from 0 to 180” was employed to determine the structure, lattice constants and grain size of the films. Cu Ka target with I = I .54 18 A was used as an X-ray source. Electrical conductivity and Hall measurements were
B.V. (North-Holland
)
165
Volume
10, number
carried out technique.
4,5
using
3. Composition
MATERIALS
the
standard
van
der
Pauw
and structure
The films were smooth, shiny and dark brown in color, uniformly thick and fairly adherent to the substrate surface. The Auger electron spectra of CdTe films deposited at different substrate temperatures are shown in fig. 1. The spectra showed Auger peaks due to cadmium (375 and 382 eV) and tellurium
Ts - 15O.C
I
-
w
-T z D
LETTERS
November
1990
(473 and 493 eV). The composition data (table 1) indicated that while tellurium was found to be in excess in the films formed at T, < 200’ C, no detectable changes in the composition could be seen in the films formed at T,= 2 lo-350°C. Due to the limitations of the AES technique (AES sensitivity is 0.1 at%) we failed to detect the small deviations ( < 0.1 at%) from stoichiometry. The reproducibility of the composition was observed throughout the investigations for the films deposited by maintaining the above deposition conditions. The observation is slightly different from XRD data. Fig. 2 shows the XRD pattern of CdTe films deposited at different temperatures. The films deposited at T,< 250°C showed small ( 100) and (300) peaks corresponding to the hexagonal phase of tellurium (figs. 2a and 2b). These peaks are absent in the films formed at T,=250-330°C (figs. 2c and 2d). A (002) hexagonal peak appeared just before the ( 111) cubic phase in the XRD spectrum (fig. 2e) of CdTe films formed at TS> 330°C. The presence of the hexagonal phase may be due to excess of cadmium in the films [ 10, 11 ] . The hexagonal phase had ac4.56 a and c= 7.46 A which are in good agreement with the results obtained by El-Akmi and Suryanarayanan [ 12 1. The grain size was found to increase with increasing substrate temperature and was about the maximum (0.7 urn) in the films formed at T,= 3 15-320°C. These films were also found to be stoichiometric and highly textured with a preferred (111) orientation which is the close packing direction of the zinc blende structure. The evaluated lattice parameter was 6.48 A.
Ts- 320’ C
!I! @l--T Table 1 AES composition
, ,
200
,
3cQ
ELECTRON
400
ENERGY
5’00
Fig. 1. Auger electron spectra of laser evaporated formed at different substrate temperatures.
166
600
(~‘4)
CdTe films
data of laser evaporated
Sample no.
Substrate temperature
1
150 180 210 260 320 350
2 3 4 5 6
( “C)
CdTe films
Cd (at%)
Te (at%)
48.8 49.4 49.9 50.0 50.0 50.1
51.2 50.6 50.1 50.0 50.0 49.9
Volume
10. number
MATERIALS
4,5
I
5
@
IN
DEGREES
profiles of CdTe films formed at different
4. Electrical properties The undoped CdTe films were found to be highly resistive. The resistivity of the films deposited at T,z 320°C was about 1O6 Q cm. The resistivity of the films was reduced to 104- 1O5 fi cm by annealing in H2 atmosphere at 425 “C for 30 min. This may be due to reduction in the grain boundary state density by heat treating in Hz ambient, with preferential passivation of states near the band edges which leads to a decrease in the recombination velocities as reported by Thorpe et al. [ 131. The films deposited at T, < 250’ C were found to be p-type, whereas the films deposited at T,> 33O’C were n-type. In the intermediate temperature range from 250 to 330°C the films were found fo be intrinsic. 4.1. Antimony-doped
I990
Ts 21O'C
2 Q
Fig. 2. X-ray diffraction
November
LETTERS
CdTe $lms
Attempts to obtain p-type doping of CdTe films by co-evaporation of dopant were not successful. So in
substrate
temperatures
the present study we prepared CdTe material with different antimony concentrations ranging from 10” to 102’/cm3 and used them for the preparation of the films. The source material was made into 8 mm diameter pellets and placed in the boat for evaporation. The substrate temperature was fixed at 320°C. All the films deposited under these conditions exhibited p-type conductivity. The electrical properties of CdTe films are summarized in table 2. The resistivity of these films was found to be strongly dependent on the dopant concentration. A minimum resistivity of about 580 Q cm with a hole density of 8 X lO”/cm was observed in the films prepared by using a source material with antimony concentration of 8 x 10’8/cm3. The resistivity of the films increased with further increase of antimony concentration probably due to self-compensation or defect interactions. The adhesion was also poor with increase of antimony concentration. The effect of heat treatment in different ambients 167
Volume
IO, number
Table 2 Electrical
properties
Sample no.
4,5
MATERIALS
of CdTe films: resistivityp
N,“’
LETTERS
(a cm) and carrier concentration
As-deposited
Annealed
P
NA
P
2.3x IO4 2.0x lo3 5.8x 102
1.3x 10’5 5.4x lOI 8.0x lOI
1.50x 1.70x 5.56x
1 2 3
4x 10” 2x 10’8 8x 10’8
a) N,=dopant
density of CdTe source material
580 BINDING
168
1990
NA ( cme3)
in vacuum
lo4 10’ IO2
Annealed
in hydrogen
NA
P
NA
1.8x 10’5 5.8x lOI5 8.2x lOI
5.5x 103 1.0x 103 3.2x lo2
3.0x 10’5 6.4x lOI5 1.0x 10’5
(cm-3).
on the electrical properties of p-CdTe films are also included in table 2. The hydrogen heat-treated films showed lower resistivity compared to the films heat treated in vacuum.
Fig. 3. X-ray photoelectron
November
4.2. Oxygen-doped CdTe films CdTe films were prepared in oxygen atmosphere with different partial pressures. The AES data taken on the films formed in oxygen partial pressure greater than lop4 Torr showed an intense oxygen peak in-
410
420 ENERGY
spectra of CdTe films formed at different
41
(eV)
partial pressures
of oxygen.
Volume 10, number 4.5
MATERIALS LETTERS
dicating more than 2 at% of oxygen in the films. The phases corresponding to the formation of TeO, or CdTe03 were also observed in XPS scans (fig. 3a). For films formed at lower oxygen partial pressure (<4x lop5Torr), no detectable oxide phases were seen in XPS spectra (fig. 3b), confirming that the presence of oxygen in these CdTe films is very much less. The films deposited in the oxygen partial pressure range from 4x 10e5 to 8 x lop6 Torr and at T,= 320’ C were found to be p-type. From thermally stimulated current measurements two trap levels at 0.11 eV and 0.5 eV were observed for oxygen-doped films [ 141. Agrinskaya et al. [ 15 ] found that the replacement of tellurium with oxygen in CdTe films give rise to a localized level behaving as an electron trap. Wavelength modulation reflection spectroscopy studies of Hsu et al. [ 161 showed that the oxygen atoms exhibited a local level in the band gap with an ionization energy of about 0.1 eV. Chu [ 17 ] also reported that the incorporated oxygen at lower concentrations (co.5 at%) would behave as an acceptor impurity. The films formed at T,=z 320°C with oxygen partial pressure of 1 x 1O- 5 Torr were single phase having a resistivity of = lo2 fi cm with a hole concentration of (2-3) x 10’6/cm’.
5. Conclusions CdTe films were deposited by the laser assisted vacuum evaporation technique onto glass substrates maintained at different substrate temperatures. The films deposited at T,z 320°C were found to be single phase, stoichiometric and highly crystalline. The films deposited using antimony-doped CdTe as source material showed a resistivity of 580 n cm with a carrier concentration 8X 10”/cm3, which is further reduced to 320 Q cm by annealing in hydrogen atmosphere. The films formed in oxygen atmosphere were found to be p-type with carrier concentration and resistivity of (2-3) x 10’6/cm3 and = lo2 Q cm
November 1990
respectively. The oxygen-doped films appear to be better with regard to resistivity and carrier concentration compared to antimony-doped films.
Acknowledgement The authors are thankful to the department of NonConventional Energy Sources, Government of India, for providing the financial support.
References [ 1] S.S. Chern, H.R. Vydyanath and F.A. Kroger. J. Solid State Chem. 14 (1975) 33. [2] K.W. Mitchell, A.L. Fahrenbruch and R.H. Bube. J. Appl. Phys. 48 (1977) 829. [3] J. Aranda, J.L. Moranza, J. Estava and J.M. Codina. Thin Solid Films 120 ( 1984) 23. [4] A. Lopez-Otero, Thin Solid Films 49 ( 1978) 3. [ 51T.C. Anthony, A.L. Fahrenbruch and R.H. Bube, J. Vacuum Sci. Technol. A 2 (1984) 1296. [6] B.M. Basal, J. Appl. Phys. 55 (1984) 601. [7] H. Matsumoto, A. Nakano, H. Uda. S. Ikegami and T. Miyarawa, Japan. J. Appl. Phys. 21 (1982) 800. [ 81 J.F. Frichnicht, Rev. Sci. Instrum. 45 ( 1974) 5 1. [9] T. Ishida, S. Wavo and U. Ishivo, Thin Solid Films 39 (1976) 227. [ lo] R. Suryanarayanan and C. Paporoditis. Compt. Rend. Acad. Sci. (Paris) B 265 ( 1967) 498. [I I ] K.V. Shirnilova, O.S. Bulotore, E.N. Voronkor and V.A. Dinitrier, Soviet Phys. Cryst. 1I (1966) 431. [ 121 A. El-Akmi and R. Suryanarayanan, International Symposium on Trends and New Applications in Thin Films, [ 13 ] T.P. Thorpe Jr., A.L. Fahrenbruch and R.H. Bube, J. Appl. Phys. 60 ( 1986) 3622. [ 14 ] 0. Mahammad Hussain and P. Jayarama Reddy. Solid State Phys. (India) 31C (1988) 343. [ 15 ] N.V. Agrinskaya, G.I. Aleskadrova, E.N. .4rkad’eva, B.A. Alabekov, O.M. Matveer, G.B. Perapelova, S.V. Prokofev and G. Ishmarenkova, Soviet Phys. Semicond. 8 ( 1974) 202. [ 161 M. Hsu, R.-J. Jih, S.-F. Fan, P.-C. Lin, H.-S. Veng and H.L. Hwang, in: Proceedings of the 17th IEEE Photovoltaic Specialists Conference, Las Vegas ( 1984) p. 846. [ 171 T.L. Chu, Solar Cells 23 (1988) 31.
169