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Electrical and photovoltaic properties of a heterojunction between AsTeGe film and crystalline silicon
Thin Solid Films, 70 (1980) 85-90 © ElsevierSequoia S.A., Lausanne--Printedin the Netherlands
85
E L E C T R I C A L AND PHOTOVOLTAIC PROPERTIES OF A H E T E R O J U N C T I O N BETWEEN As-Te-Ge F I L M AND CRYSTALLINE SILICON M. PER~IN AND VED MITRA "Ruder Bogkovik'" Institute, P.O. Box 1016 Bijenicka 54, Zagreb 41001 (Yugoslavia) (Received December 17, 1979;accepted January 25, 1980)
A heterojunction between an As-Te-Ge film and an n-type silicon crystal was fabricated by evaporating the film onto the silicon substrate. The junction shows rectifying behaviour. Its I - V characteristics were measured in the dark and under illumination with sunlight. The photovoltaic spectral response shows a band peaking at 840 nm. The electrical and photovoltaic properties of the junction were studied and the results are reported. The fill factor value is 43~o. The efficiency is very low.
1. INTRODUCTION In the last decade, much effort has been devoted to the study of various properties of As-Te-Ge chalcogenides 1-6, in particular their application to memory and switching phenomena 7-9 in bulk and thin film form. So far, no attempt has been made to study their interracial properties. Heterojunctions are of reasonable research interest for their various applications. Recently, the electrical and photovoltaic properties of heterojunctions between a Ge-Te-Se film and crystalline silicon have been reportedX o. In the present paper a similar attempt is made to form a heterojunction between an As-Te-Ge film and crystalline silicon (n-Si) using the evaporation technique. Their electrical and photovoltaic properties are studied and results reported. An attempt to propose an energy band diagram of the junction is also made. 2. EXPERIMENTAL A heterojunction was fabricated between an As-Te-Ge film and an n-Si crystal using as the substrate an n/n + silicon wafer of diameter 52.4 mm (Semimetals, Inc., U.S.A.). An n-type epitaxial layer 20-30 ~tm thick and of resistivity 20 ~ cm -+ 20~o was grown on an n + base of resistivity of 0.002-0.005 ~ cm. Glass of composition As16TesaGe 1 was prepared by direct melting and quenching of the constituent elements in fused silica ampoules. It was used as a source of thermal evaporation onto the silicon wafer, using a mask of diameter 40 mm. The deposited film was 0.85 I~m thick and was p type. A gold film was deposited onto a chalcogenide As-Te-Ge film, for an ohmic contact, using a mask of 5 mm in diameter. Silver paste was used for contacts to the gold films as well as to the n + silicon surface.
86
M. PERglN, V. MITRA
The refractive index and film thickness were determined by means ofa Gaertner ellipsometer L 117 and a T E N C O R alpha step, respectively. Absorption coefficient measurements were made on a Carry 17 spectrophotometer. A Carl Zeiss m o n o c h r o m a t o r was used for spectral photovoltaic measurements. Electrical characteristics were measured in the dark as well as under direct sun illumination using Keithley electrometers 616 and 602. 3. RESULTS AND DISCUSSION 3.1. Current-voltage characteristics in the dark Figure 1 shows the I - V characteristics in the dark of a p n junction fabricated between an A s - T e - G e film and an n-Si crystal. The junction shows good rectifying behaviour and the forward current flows when the film is biased negatively with respect to the n-Si. The semilogarithmic plot of the forward characteristics drawn in the inset to Fig. 1 shows more clearly the dependence of current on the bias voltage. At low forward bias, the current is given by I = I o { e x p ( e V / n k T ) - 1}
where I o is the saturation current, e the electronic charge, V the applied bias, n an experimentally determined factor, k the Boltzmann constant and T the absolute temperature. The value o f I o deduced from Fig. 1 is 3 x 10 9 A c m - 2 and the factor n was calculated to be 2. A log-log plot in the higher bias region shows a steep increase obeying the power law 1 oz V p, with p varying from 1.1 to 4.4. This type of I - V characteristic in the forward bias region is interpreted in terms of a space-chargelimited current influenced by traps distributed exponentially in the forbidden gap of the films 1°-14. The reverse current tends to saturate with increasing reverse bias voltage.
3.0
~E •~
16 ~
2.0
16B 1.0 I 0~2 0.4 VOLTAGE (V)
-0.6 i
-0,4
0.6
-0.2
i
02
0.4
0.6
VOLTAGE { V )
-1.0
Fig. 1. Current voltage characteristics of the heterojunction between A s - T e Ge film/n-Si in the dark. The inset shows the semilogarithmic plot in the forward bias region.
HETEROJUNCTION BETWEEN A s - T e - G e
FILM AND CRYSTALLINE Si
87
3.2. Current-voltage characteristics under illumination Figure 2 shows the characteristics of an As-Te-Ge film/n-Si heterojunction when the film surface was illuminated with direct sunlight (flux ~ 0.6 cal cm-2 min -1) and when it was in the dark. There is an increase in the current on illumination with sunlight. Similar behaviour has been observed in an InESe3/Si heterojunction 15. Under illumination, the polarities of Voc and Isc (negative for the As-Te-Ge film side and positive for the silicon side) follow the direction of energy band bending (see Fig. 5). The band bending direction of the heterojunction energy band is dependent on the electron affinity of the semiconductors and on the Fermi level Eyin the material. For example, as Ef - E v increases, a change in polarity of Voc and Is~ is expected in a p-type substrate. In this case the open circuit voltage Voc increases with decreasing resistivity value. This has been observed for an In203/pGe heterojunction 16. For such heterojunctions a change in polarity of Vo¢ and Isc occurs between two p-type Ge substrates having resistivities of 37 and 20 t) cm. The resistivities for a substrate of n-type silicon for Ge-Te-Se film/n-Si lo and As-TeGe/n-Si are l0 and 20 f~ cm, respectively. In this way it is possible to explain why the polarities of Vo~ and Is¢ are in opposite directions in the cases of Ge-Te-Se film/nSi lo and As-Te-Ge film/n-Si heterojunctions.
5.0 E t,.0
0.3
/ /
UNDER SUN-LIGH ~ / /
0.2
/
/
/
3.0
0.1
i/ 2.0 ~ OUT PUT VOLIAGE ( V 1
K 1.0
-0.6
-0.&
/
/
-0.2 ss ~
02
O.t,
VOLTAGE ( V ) /J
0.6 ,~
02
/s / /s s s S s1 A. S ~s
-0/.
s
-0.6
s I
.0.8 1.0
Fig. 2. Current-voltage characteristics for the A s - T e - G e film/n-Si heterojunction under illumination. Note the change in the scale for the reverse current. The inset shows the output characteristics of the heterojunction under direct illumination.
The inset of Fig. 2 shows the output current-voltage characteristics under approximately similar sunlight illumination. A fill factor of 4 3 ~ was calculated from this figure.
88
M. P E R g l N , V. MITRA
3.3. Spectral response of I~ Figure 3 shows the photocurrent as a function of wavelength for an A s - T ~ G e film/n-Si heterojunction, having a bell-shaped band peaking at 840 nm and a negative photocurrent on the lower wavelength side. The negative photocurrent behaviour suggests that there is a reverse field at the surface (reverse flow for carriers generated by non-penetrating light) ~5. The optimal thickness of an absorbing A s T e - G e film for best absorption is about 3.5 pm, as determined using the absorption coefficient ~. In the present case, the film thickness of 0.85 pm is much lower than this value. Therefore, a large amount of incident photons were not contributing to a photocurrent.
500 400
"~
t
300
209
"TE 1 0 0 u
100 ea
m O e,-
-100
7x
-200
7~o
8~o
960
,o'oo ,'oo
WAVELENGTH
o 1.o
1.5
(nm)
21o h y ( eV ) - - ~
Fig. 3. The spectral response of the short circuit current of the As Te-Ge film/n-Si heterojunction. The optical band gap E[ pt of the filmis shown by an arrow. Fig. 4. The dependence of(hw) v2 on photon energy for an As-Te Ge film. It is evident from Fig. 3 that the photoresponse lies in the region of photon energies above the band gap of the silicon crystal (1.12 eV). This result leads to the conclusion that a depletion layer is formed in the chalcogenide film as well as in the silicon crystal. The optical band gap Eg °pt of the film is indicated in Fig. 3 with a vertical arrow. The value ofEg °pt is determined by extrapolating the linear portion of a plot of (hvo~) 1/2 v e r s u s hv to the photon energy axis, as shown in Fig. 4. It is noticeable that the heterojunction responds to photon energies higher than the Eg°pt value of the film. Similar results were observed for a Ge-Te-Se/n-Si heterojunction 1° and were interpreted by the fact that the fundamental absorption edge of a m o r p h o u s semiconductors is gradual and thus these semiconductors transmit photon energies which are somewhat higher than Eg°pt
3.4. Energy band profile of an As- Te-Ge film/n-silicon heterojunction The energy band profile of a heterojunction can be discussed on the basis of the above results. Figure 5(a) shows the energy band diagrams for an isolated n-Si
HETEROJUNCTION BETWEEN
As-Te-Ge FILM AND CRYSTALLINE Si
89
crystal and an isolated As-Te-Ge film, while Fig. 5(b) shows a diagram for a heterojunction between them at equilibrium. These diagrams were drawn assuming that Xl > Z2, q51 > q~2,and Z1 < Z2 + Es2, where X, ~band Eg are the electron affinity, work function and band gap, respectively, for a given semiconductor ~7. Here, the subscripts 1 and 2 refer to the n-Si crystal and the As-Te-Ge film, respectively. The extent of band bending for each semiconductor and the position of Fermi levels are arbitrary because the present results are insufficient for a more exact analysis of the energy band structure.
~E I
ECl
-- -I . . . . . .
_. . . . l -
Evl
E~,
Evz
(a)
E~,
EW (b)
C2
Ef
I
"~
Evz
i
Fig. 5. Energy band diagrams proposed for (a) an isolated As-Te-Ge film and n-Si, and (b) the heterojunction between them at equilibrium.
4. CONCLUSIONS
The physical parameters for the junction are summarized in Table I. From the present study of electrical and photovoltaic properties of a heterojunction between an As-Te-Ge film and an n-Si crystal the following conclusion can be drawn. TABLE
I
PHYSICAL PARAMETERS FOR THE
As-We-Ge/n-Si HETEROJUNCT1ON
Short circuit current (~tAcm- 2) Open circuit voltage (V) Reverse saturation current (A cm- 2) Refractive index for As-Te-Ge film Optical band gap for As--Te-Ge film (eV) Fill factor (%) Series resistance (Q) Efficiency
0.318 (light flux ~ 0.6 cal cm- 2 min 1) 0.115 about 10- 7 1.827 (at 6328 A)
1.34
43 5.38 x 104 very poor
(1) Physical parameters are poor for solar cell application, although an appropriate As-Te-Ge film thickness will show a better performance. (2) As-Te-Ge films exhibit rectifying contacts with n-Si, the polarity of which is forward when the film is negatively biased. Under illumination, the polarities of Voc and Isc are negative for the film side and positive for the silicon side. This is in accordance with the direction of energy band bending. ACKNOWLEDGMENTS
The authors are thankful fo their colleagues Mr. H. Zorc and Mrs. V. Butkovi6 for their help during the measurements, and to Mrs. B. Fanton for reading the manuscript and for useful suggestions.
90
M. PERglN, V. MITRA
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
S. lizima, M. Sugi and M. Kikuchi, Solid State Commun., 8 (1970) 153. C.H. Seager, D. Emin and R. K. Quinn, Phys. Rev. B, 8 (1973) 4746. A . A . A m m a r a n d M . M. Hafiz, ThinSolidFilms, 34(1976) 64. N. Tohge, T. Minami and M. Tanaka, Jpn. J. Appl. Phys., 16 (1977) 977. K . K . Srivastava, D. R. Goyal, A. Kumar, K. N. Lakshminarayan, O. S. Panwar and 1. Krishan, Phys. Status' Solidi A, 41 (1977) 323. O.S. Panwar, A, Kumar, D. R. Goyal, K. K. Srivastava and K. N. Lakshminarayan, J. Non-Cryst. Solids', 30 (1978) 37. R. Utrecht, H. Stevenson, C. H. Sie, J. D. Griener and K. S. Raghavan, J. Non-Crvst. Solids, 2 (1970) 358. M. Pergin, D. Kunstelj, A. Pergin and H. Zorc, Thb~ Solid Films, 36 (1976) 475. D.R. Goyal, O. S. Panwar, K. K. Srivastava, K. N. Lakshminarayan, A. K umar and D. R. Ravikar, Indian J. Pure Appl. Phys., 15 (1977) 10. N. Tohge, T. Minami and M. Tanaka, Thin SolidFilrns, 56 (1979) 377. B. Dunn and J. D. MacKenzie, J. Appl. Phys., 47 (1976) 1010. H.P.D. Lanyon, Phys. Rev., 130 (1963) 134. T. Shiraishi, T. Kurosu and M. lida, Oya Butsuri, 45 (1976) 640. V. Mitra, J. 1. Pankove, N. B. Urli and B. Etlinger, Proc. Int. ('ol¢]~ on Radi~ltion Ph)sics ~[ Semiconductors and Related Materials, Tbilisi, U.S.S.R., 1979. B. Etlinger and V. Mitra, Comples. 18th Int. Cot~ on Solar Energy, New Prospects, 1979, Milan, Italy. E.Y. Wang and L. Hsu, J. Eleetroehem. Soe., 125 (1978) 1328; Proe. 13th IEEE Photovoltaic Speeialists" Conj', Washington, D.C., 1978, IEEE, New York, 1978, p. 536. B.L. Sharma and R. K. Purohit, Semieonduetor Heterojunctions, Pergamon, New York, 1974.
85
E L E C T R I C A L AND PHOTOVOLTAIC PROPERTIES OF A H E T E R O J U N C T I O N BETWEEN As-Te-Ge F I L M AND CRYSTALLINE SILICON M. PER~IN AND VED MITRA "Ruder Bogkovik'" Institute, P.O. Box 1016 Bijenicka 54, Zagreb 41001 (Yugoslavia) (Received December 17, 1979;accepted January 25, 1980)
A heterojunction between an As-Te-Ge film and an n-type silicon crystal was fabricated by evaporating the film onto the silicon substrate. The junction shows rectifying behaviour. Its I - V characteristics were measured in the dark and under illumination with sunlight. The photovoltaic spectral response shows a band peaking at 840 nm. The electrical and photovoltaic properties of the junction were studied and the results are reported. The fill factor value is 43~o. The efficiency is very low.
1. INTRODUCTION In the last decade, much effort has been devoted to the study of various properties of As-Te-Ge chalcogenides 1-6, in particular their application to memory and switching phenomena 7-9 in bulk and thin film form. So far, no attempt has been made to study their interracial properties. Heterojunctions are of reasonable research interest for their various applications. Recently, the electrical and photovoltaic properties of heterojunctions between a Ge-Te-Se film and crystalline silicon have been reportedX o. In the present paper a similar attempt is made to form a heterojunction between an As-Te-Ge film and crystalline silicon (n-Si) using the evaporation technique. Their electrical and photovoltaic properties are studied and results reported. An attempt to propose an energy band diagram of the junction is also made. 2. EXPERIMENTAL A heterojunction was fabricated between an As-Te-Ge film and an n-Si crystal using as the substrate an n/n + silicon wafer of diameter 52.4 mm (Semimetals, Inc., U.S.A.). An n-type epitaxial layer 20-30 ~tm thick and of resistivity 20 ~ cm -+ 20~o was grown on an n + base of resistivity of 0.002-0.005 ~ cm. Glass of composition As16TesaGe 1 was prepared by direct melting and quenching of the constituent elements in fused silica ampoules. It was used as a source of thermal evaporation onto the silicon wafer, using a mask of diameter 40 mm. The deposited film was 0.85 I~m thick and was p type. A gold film was deposited onto a chalcogenide As-Te-Ge film, for an ohmic contact, using a mask of 5 mm in diameter. Silver paste was used for contacts to the gold films as well as to the n + silicon surface.
86
M. PERglN, V. MITRA
The refractive index and film thickness were determined by means ofa Gaertner ellipsometer L 117 and a T E N C O R alpha step, respectively. Absorption coefficient measurements were made on a Carry 17 spectrophotometer. A Carl Zeiss m o n o c h r o m a t o r was used for spectral photovoltaic measurements. Electrical characteristics were measured in the dark as well as under direct sun illumination using Keithley electrometers 616 and 602. 3. RESULTS AND DISCUSSION 3.1. Current-voltage characteristics in the dark Figure 1 shows the I - V characteristics in the dark of a p n junction fabricated between an A s - T e - G e film and an n-Si crystal. The junction shows good rectifying behaviour and the forward current flows when the film is biased negatively with respect to the n-Si. The semilogarithmic plot of the forward characteristics drawn in the inset to Fig. 1 shows more clearly the dependence of current on the bias voltage. At low forward bias, the current is given by I = I o { e x p ( e V / n k T ) - 1}
where I o is the saturation current, e the electronic charge, V the applied bias, n an experimentally determined factor, k the Boltzmann constant and T the absolute temperature. The value o f I o deduced from Fig. 1 is 3 x 10 9 A c m - 2 and the factor n was calculated to be 2. A log-log plot in the higher bias region shows a steep increase obeying the power law 1 oz V p, with p varying from 1.1 to 4.4. This type of I - V characteristic in the forward bias region is interpreted in terms of a space-chargelimited current influenced by traps distributed exponentially in the forbidden gap of the films 1°-14. The reverse current tends to saturate with increasing reverse bias voltage.
3.0
~E •~
16 ~
2.0
16B 1.0 I 0~2 0.4 VOLTAGE (V)
-0.6 i
-0,4
0.6
-0.2
i
02
0.4
0.6
VOLTAGE { V )
-1.0
Fig. 1. Current voltage characteristics of the heterojunction between A s - T e Ge film/n-Si in the dark. The inset shows the semilogarithmic plot in the forward bias region.
HETEROJUNCTION BETWEEN A s - T e - G e
FILM AND CRYSTALLINE Si
87
3.2. Current-voltage characteristics under illumination Figure 2 shows the characteristics of an As-Te-Ge film/n-Si heterojunction when the film surface was illuminated with direct sunlight (flux ~ 0.6 cal cm-2 min -1) and when it was in the dark. There is an increase in the current on illumination with sunlight. Similar behaviour has been observed in an InESe3/Si heterojunction 15. Under illumination, the polarities of Voc and Isc (negative for the As-Te-Ge film side and positive for the silicon side) follow the direction of energy band bending (see Fig. 5). The band bending direction of the heterojunction energy band is dependent on the electron affinity of the semiconductors and on the Fermi level Eyin the material. For example, as Ef - E v increases, a change in polarity of Voc and Is~ is expected in a p-type substrate. In this case the open circuit voltage Voc increases with decreasing resistivity value. This has been observed for an In203/pGe heterojunction 16. For such heterojunctions a change in polarity of Vo¢ and Isc occurs between two p-type Ge substrates having resistivities of 37 and 20 t) cm. The resistivities for a substrate of n-type silicon for Ge-Te-Se film/n-Si lo and As-TeGe/n-Si are l0 and 20 f~ cm, respectively. In this way it is possible to explain why the polarities of Vo~ and Is¢ are in opposite directions in the cases of Ge-Te-Se film/nSi lo and As-Te-Ge film/n-Si heterojunctions.
5.0 E t,.0
0.3
/ /
UNDER SUN-LIGH ~ / /
0.2
/
/
/
3.0
0.1
i/ 2.0 ~ OUT PUT VOLIAGE ( V 1
K 1.0
-0.6
-0.&
/
/
-0.2 ss ~
02
O.t,
VOLTAGE ( V ) /J
0.6 ,~
02
/s / /s s s S s1 A. S ~s
-0/.
s
-0.6
s I
.0.8 1.0
Fig. 2. Current-voltage characteristics for the A s - T e - G e film/n-Si heterojunction under illumination. Note the change in the scale for the reverse current. The inset shows the output characteristics of the heterojunction under direct illumination.
The inset of Fig. 2 shows the output current-voltage characteristics under approximately similar sunlight illumination. A fill factor of 4 3 ~ was calculated from this figure.
88
M. P E R g l N , V. MITRA
3.3. Spectral response of I~ Figure 3 shows the photocurrent as a function of wavelength for an A s - T ~ G e film/n-Si heterojunction, having a bell-shaped band peaking at 840 nm and a negative photocurrent on the lower wavelength side. The negative photocurrent behaviour suggests that there is a reverse field at the surface (reverse flow for carriers generated by non-penetrating light) ~5. The optimal thickness of an absorbing A s T e - G e film for best absorption is about 3.5 pm, as determined using the absorption coefficient ~. In the present case, the film thickness of 0.85 pm is much lower than this value. Therefore, a large amount of incident photons were not contributing to a photocurrent.
500 400
"~
t
300
209
"TE 1 0 0 u
100 ea
m O e,-
-100
7x
-200
7~o
8~o
960
,o'oo ,'oo
WAVELENGTH
o 1.o
1.5
(nm)
21o h y ( eV ) - - ~
Fig. 3. The spectral response of the short circuit current of the As Te-Ge film/n-Si heterojunction. The optical band gap E[ pt of the filmis shown by an arrow. Fig. 4. The dependence of(hw) v2 on photon energy for an As-Te Ge film. It is evident from Fig. 3 that the photoresponse lies in the region of photon energies above the band gap of the silicon crystal (1.12 eV). This result leads to the conclusion that a depletion layer is formed in the chalcogenide film as well as in the silicon crystal. The optical band gap Eg °pt of the film is indicated in Fig. 3 with a vertical arrow. The value ofEg °pt is determined by extrapolating the linear portion of a plot of (hvo~) 1/2 v e r s u s hv to the photon energy axis, as shown in Fig. 4. It is noticeable that the heterojunction responds to photon energies higher than the Eg°pt value of the film. Similar results were observed for a Ge-Te-Se/n-Si heterojunction 1° and were interpreted by the fact that the fundamental absorption edge of a m o r p h o u s semiconductors is gradual and thus these semiconductors transmit photon energies which are somewhat higher than Eg°pt
3.4. Energy band profile of an As- Te-Ge film/n-silicon heterojunction The energy band profile of a heterojunction can be discussed on the basis of the above results. Figure 5(a) shows the energy band diagrams for an isolated n-Si
HETEROJUNCTION BETWEEN
As-Te-Ge FILM AND CRYSTALLINE Si
89
crystal and an isolated As-Te-Ge film, while Fig. 5(b) shows a diagram for a heterojunction between them at equilibrium. These diagrams were drawn assuming that Xl > Z2, q51 > q~2,and Z1 < Z2 + Es2, where X, ~band Eg are the electron affinity, work function and band gap, respectively, for a given semiconductor ~7. Here, the subscripts 1 and 2 refer to the n-Si crystal and the As-Te-Ge film, respectively. The extent of band bending for each semiconductor and the position of Fermi levels are arbitrary because the present results are insufficient for a more exact analysis of the energy band structure.
~E I
ECl
-- -I . . . . . .
_. . . . l -
Evl
E~,
Evz
(a)
E~,
EW (b)
C2
Ef
I
"~
Evz
i
Fig. 5. Energy band diagrams proposed for (a) an isolated As-Te-Ge film and n-Si, and (b) the heterojunction between them at equilibrium.
4. CONCLUSIONS
The physical parameters for the junction are summarized in Table I. From the present study of electrical and photovoltaic properties of a heterojunction between an As-Te-Ge film and an n-Si crystal the following conclusion can be drawn. TABLE
I
PHYSICAL PARAMETERS FOR THE
As-We-Ge/n-Si HETEROJUNCT1ON
Short circuit current (~tAcm- 2) Open circuit voltage (V) Reverse saturation current (A cm- 2) Refractive index for As-Te-Ge film Optical band gap for As--Te-Ge film (eV) Fill factor (%) Series resistance (Q) Efficiency
0.318 (light flux ~ 0.6 cal cm- 2 min 1) 0.115 about 10- 7 1.827 (at 6328 A)
1.34
43 5.38 x 104 very poor
(1) Physical parameters are poor for solar cell application, although an appropriate As-Te-Ge film thickness will show a better performance. (2) As-Te-Ge films exhibit rectifying contacts with n-Si, the polarity of which is forward when the film is negatively biased. Under illumination, the polarities of Voc and Isc are negative for the film side and positive for the silicon side. This is in accordance with the direction of energy band bending. ACKNOWLEDGMENTS
The authors are thankful fo their colleagues Mr. H. Zorc and Mrs. V. Butkovi6 for their help during the measurements, and to Mrs. B. Fanton for reading the manuscript and for useful suggestions.
90
M. PERglN, V. MITRA
REFERENCES
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
S. lizima, M. Sugi and M. Kikuchi, Solid State Commun., 8 (1970) 153. C.H. Seager, D. Emin and R. K. Quinn, Phys. Rev. B, 8 (1973) 4746. A . A . A m m a r a n d M . M. Hafiz, ThinSolidFilms, 34(1976) 64. N. Tohge, T. Minami and M. Tanaka, Jpn. J. Appl. Phys., 16 (1977) 977. K . K . Srivastava, D. R. Goyal, A. Kumar, K. N. Lakshminarayan, O. S. Panwar and 1. Krishan, Phys. Status' Solidi A, 41 (1977) 323. O.S. Panwar, A, Kumar, D. R. Goyal, K. K. Srivastava and K. N. Lakshminarayan, J. Non-Cryst. Solids', 30 (1978) 37. R. Utrecht, H. Stevenson, C. H. Sie, J. D. Griener and K. S. Raghavan, J. Non-Crvst. Solids, 2 (1970) 358. M. Pergin, D. Kunstelj, A. Pergin and H. Zorc, Thb~ Solid Films, 36 (1976) 475. D.R. Goyal, O. S. Panwar, K. K. Srivastava, K. N. Lakshminarayan, A. K umar and D. R. Ravikar, Indian J. Pure Appl. Phys., 15 (1977) 10. N. Tohge, T. Minami and M. Tanaka, Thin SolidFilrns, 56 (1979) 377. B. Dunn and J. D. MacKenzie, J. Appl. Phys., 47 (1976) 1010. H.P.D. Lanyon, Phys. Rev., 130 (1963) 134. T. Shiraishi, T. Kurosu and M. lida, Oya Butsuri, 45 (1976) 640. V. Mitra, J. 1. Pankove, N. B. Urli and B. Etlinger, Proc. Int. ('ol¢]~ on Radi~ltion Ph)sics ~[ Semiconductors and Related Materials, Tbilisi, U.S.S.R., 1979. B. Etlinger and V. Mitra, Comples. 18th Int. Cot~ on Solar Energy, New Prospects, 1979, Milan, Italy. E.Y. Wang and L. Hsu, J. Eleetroehem. Soe., 125 (1978) 1328; Proe. 13th IEEE Photovoltaic Speeialists" Conj', Washington, D.C., 1978, IEEE, New York, 1978, p. 536. B.L. Sharma and R. K. Purohit, Semieonduetor Heterojunctions, Pergamon, New York, 1974.