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Influence of longitudinal electric field on vapour deposited thin films of cadmium sulphide—a report
Micro,onics ELSEVIER
Microelectronics Journal 29 (1998) 403-407
Influence of longitudinal electric field on vapour deposited thin films of cadmium sulphide a report Pyare Lal, S.K. Srivastava Centre for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221005 (U.P.), India
Abstract In this report we give a brief account of the various processes of the formation of thin films. This is followed by a description of the influence of various deposition parameters on the characteristic properties of thin films. The electrical and structural characterization of vapour deposited cadmium sulphide films grown under the influence of electric fields is discussed. This article is a compilation of the previous reported results. Various observations (viz. conductivity measurement, thermoelectric power measurement X-ray observation and crossover voltage measurement) suggest that there is an increase in disorderliness (as a result of freezing in action) due to application of a dc electric field at the time of growth of the film. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Thin film has a pivotal role in the development of challenging areas of microelectronics, optical coatings, superconductivity and metallurgical coatings. Ultra-thin films are two dimensional and may be obtained by any of the following processes: physical vapour deposition (PVD) [1-4]; chemical vapour deposition (CVD) [5-8]; electro chemical deposition (ECD) [9-12]; electroless or solution growth deposition. Generally, thin films ~xe prepared by depositing the films' material atom by atom on a suitable substrate. Such a process of film formation involves phase transformation. Because of the phase transformation involved in the film formation, the ambient conditions influence the parametric properties of the film. The ambient conditions that are likely to change the morphology, microstructure and adhesion of deposits in the case of PVD and CVD are pressure and nature of residual gas ill the deposition chamber, temperature of evaporating source, rate of deposition of condensing atom, temperature of substrate, nature of substrate, surface mobility of deposits on substrate, presence of electric/magnetic field on the surface of substrate, chemical reaction between substrate and deposits and the activation process. The factors influencing electro deposition process are pH of electrolyte, current density, temperature of bath composition, electrode shape and agitation. The influence of deposition parameters on film growth may be evaluated in terms of their effect on nucleation density, sticking coefficient and surface mobility of 0026-2692/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved Pll S0026-2692(97)00023-2
adatoms. This covers a very wide spectrum of the subject and is beyond the scope of this report. One of the important aspects of the subject is the study of the effect of the application of an electric field applied to the substrate at the time of growth of metallic and semiconducting film. Chopra [ 14] has studied the effect of a lateral field deposited onto sodium chloride and noted that the field appears to flatten the discrete islands and increase their surface area. Other workers [15-17] have also reported on the change in electrical resistance of continuous film prepared with an applied electric field and they attributed this to the decrease in frozen-in structural defects during the induced coalescence. However, the influence of the application of a longitudinal electric field at the time of deposition has not been reported to our knowledge. In this report we have compiled our work on the study of the effect of application of longitudinal electric field on vacuum deposited cadmium sulphide film.
2. Device preparation Thin films of cadmium sulphide sandwiched between aluminium electrodes were prepared by a suitable masking arrangement described elsewhere [18]. Before deposition, the electric field system was incorporated as shown in Fig. 1. Special care was taken when the substrate was clipped to the positive terminal of the supply as the whole system under vacuum acts as a diode, resulting in flow of an additional electron current (if the temperature of the evaporation source is high).
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998)403-407
404
Cooling arrangement Evaporation source L Type rubber gasket
Base plate Power supply
Baffle valve
Diffusion
Fig. 1. Electric field arrangement before deposition of CdS film. (a) 104 --
Vacuum sealed electrically insulated plug for applying longitudinal electric field of either polarity
(b) 104 _ 8 6
106 v/cm /65
4
v/cm
2 v/cm 104
104 8 6
8 6
4
2
~102 .98 6
<~.102 .N 8
6
~4
~
L)
4
2 101
1°'68f
8
6 4
+35 v/cm
2 i
Iv 0.1
i 0.2
i i i [ ! 0.4 0.6 0.8 1.0
cl i i i i i 2.0 3.0 4.0 6.0 8.010.0
o o.1
Voltage Fig. 2. Log l versus log V characteristics of the film grown under different field values.
0'2
...... I i Vc , ....... I 0.4 0.6 0.8 1.0 2.0 3.0 4.0 6.0 8.010.0 Voltage
405
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 4 0 3 - 4 0 7
(a) 0,8 '2.4
~ o.6! \ \ \
\
2.2 \
\
O
.,a 0.4
~
\
'2.0
"~ 0.2 [-
'1.8
300
I 320
,
+70 V/cm
, 340
,
, , 380 , , , , 360 400 Temperature (°K)
4120 4130
"1.6
.1.4 ! \
(b)
/ / ' D " - -- ~ " ~ /
/
0, / o
0.6
1I
• 1.2
0.s - A
/ O
C 0.4
I
I
I
-106
-65
-35
I
N.F. 35 Field (v/cm)
I
I
65
106
~
-
1
4
-210 V/cm V/cm
0
N70 V/cm
0.2
Fig. 3. Variation of crossover voltage with electric field. i
300
The thickness of the film was evaluated by the weight difference method and in all the cases it was between approximately 0.4 and 0.5/xm. For thermoelectric power measurement the slides were kept in an electronically controlled oven. The temperature of the hot junction was raised using a small heater in such a way that the temperature difference between the hot and cold junctions was approximately constant (approximately 10°C to avoid non-linearity in thermoelectric power measurement). This was achieved by raising the temperature of the oven accordingly. The resistance of the film was also measured with increasing temperature while the thermoelectric power was being measured. Special precautions were taken in sample preparation for X-ray structural study. Deposited cadmium sulphide (with and without field) was scrapped and coated onto the surface of the fine glass fibre using a small amount of glue. Once the specimen rod was prep~xed it was mounted in its holder in the Debye-Scherrer camera. A copper target was used to produce the beam and ez,ch sample was exposed for 5 h. The asymmetrical method of film loading was used in the present study and'd' values for di:fferent specimens including the mother material (i.e. CdS which was used in this present study) were recorded and compared with the standard ASTM chart.
i
320
I
340
I
I
I
I
I
I
I
360 380 400 Temperature (°K)
I
420
Fig. 4. Variation of thermoelectric power with temperature.
increases with increasing positive or negative field as shown in Fig. 2. There are two sets of lines in each case. For low applied voltage Ohm' s law of conduction is followed where, as for high voltage, space charge limited conduction (SCL conduction) is observed [19,20]. Variation of crossover voltage (i.e. the voltage at which change from Ohm's law to SCL conduction occurs) as a function of field [20] is shown in Fig. 3. It is obvious from this figure that the crossover voltage increases with either polarity of the field value and is always more than the no-field value. However, there is slight departure from this observation at _+ 106 V/cm field value, which may possibly be due to certain errors in thickness evaluation. The variation with temperature in the thermoelectric power [21] for polycrystalline CdS film grown with different field strength of either polarity is shown in Fig. 4. It is found that thermoelectric power initially decreases with an increase in temperature and then starts increasing. It is also observed that the thermoelectric power increases with an increase in the field for either polarity.
3. Results 4. Description of results The current voltage characteristics of different samples were studied. It was found that current at the same voltage
It is obvious from the conductivity measurement that the
406
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 403-407
(b)
+70 V/cm +140 V/cm +210 V/cm
/
lO9 8
"0.65
6 "0.60
/
"~ 4 "
2
lxl08 2.46
i
2, 2
(a) 109 8
i
i
i
I
I
0
.
5
5
i
2.58 2.64 2.70 2.76 2.82 2.88 2.94 1 x 103 °K-1 T 2-70 V/cm 10 V/cm -140 V/cm
•0.45
N_F
•0.40
6 .0.35
e~ 2 i -270 lxl08 2.46
I
I
-140
-70
0.30 N.F.
I
70
I
140
I
270
Field (V/cm) 2.52
2.'58
2164 21.70 2176 21.82 2188 21.94 1 x 103 °K-I T
Fig. 5. Variation of resistance with temperature during thermoelectric power measurement.
conductivity increases with either polarity of the field value. The increase in conductivity may be due to a decrease in number of scattering sites. A decrease in number of scattering sites may be due to flattening or stretching of grains [13,14,22]. However, flattening or stretching of the grains is only possible in the plane of the film. With the field applied along the direction of evaporation, the vapour of CdS, which is partially ionic and partially neutral, freezes immediately as it strikes the substrate. Therefore, with the application of a positive field the concentration of Cd ÷ ions should decrease and hence the conductivity should increase with increasing positive field value. The reverse should be true in the case of a negative field; but such an expected increase or decrease in conductivity is not observed and the conductivity always increases. This may be due to a freezing-in action, which makes the film more disordered. We term this freezing in action as electrical quenching. The initial decrease and then increase in thermoelectric power with temperature may be explained in terms of change in the effective carrier density [18]. The slope of the resistance versus temperature curve Fig. 5 in thermoelectric power measurement gives the activation energy (El). A plot of activation energy (Ef) with electric field E applied at the time of growth of the film is shown in Fig. 6. The slope (d/dE)(zS~f) is positive for either polarity of the field value. This is indicative of the fact that the location of
Fig. 6. Variation of activation energy (Ef) with electric field E.
dominant impurity states changes with field, possibly because of an increase in disorder with increasing field of either polarity. X-ray observations [23] show that the lines of higher angle of reflections are either missing or diffused with increasing electric field of either polarity. However, the effect is more dominant in the case of the negative field than the positive field. This is indicative of the fact that the system is more disordered in the case of a negative field than in the case of a positive field. This is supported by the results reported earlier of the variation of crossover voltage with field. The disappearance or the diffusion of diffracted lines for higher angles of reflection may be due to electrical quenching at the time of growth of the films. It may be possible that the diffused or missing diffracted lines are due to disorderliness or the orientation of lattice planes as a result of the freezing-in action at the time of growth of the film under the influence of an electric field of either polarity. The disorder or the orientation of the reflecting planes increases with increasing field, which results in missing lines in the X-ray diffraction pattern. Finally, we may conclude that the applied longitudinal electric field at the time of growth of CdS film changes its electrical properties by creating disorder in the structure of the film which is due to electrical quenching of the partially ionic and partially neutral ions of the cadmium sulphide. The effect is more dominant in the case of a negative field than a positive field.
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 403-407
References [1] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [2] L.I. Maissel and R. Ghmg, Hand Book of Thin Film Technology, McGraw-Hill, New York, 1970. [3] L. Holland, Vacuum Deposition of Thin Film, Chapman and Hall, London, 1956. [4] J.W. Mathews, Epitaxial Growth, Academic Press, New York, 1975. [5] W. Kern, V.S. Ban, Chemical Vapour Deposition of Inorganic Thin Films, Academic Press, New York, 1978, p. 258. [6] W. Kern, G.L. Schnable, IEEE Electron Devices ED-26 (1979) 647. [7] E.C. Douglas, Solid State Technol. 24 (1981) 65. [8] J.L. Vossen, W. Kern, Phys. Today 33 (1980) 26. [9] G. Milazzo, Electrochemistry, Elsevier, Amsterdam, 1963. [10] F.A. Lowenheim, Modern Electroplating, John Wiley, New York, 1963.
407
[11] A. Brenner, Electrodeposition of Alloys, Vols 1,2. Academic Press, New York, 1963. [12] J.M. West, Electrodeposition and Corrosion Processes, D. Van Nostrand, Company, Princeton, NJ, 1965. [13] K.L. Chopra, Appl. Phys. Lett. 7 (1965) 140. [14] K.L. Chopra, J. Appl. Phys. 37 (1966) 2249. [15] H. Koeing, G. Helwig, Optik 6 (1950) 111. [16] L. Holland, J. Opt. Soc. Am. 43 (1953) 376. [17] K. Mihana, M. Tanaka, J. Cryst. Growth 2 (1968) 51. [18] S.K. Srivastava, P. Lal J. Inst Eng. (India) 53 (1973) 168. [19] P. Lal, P.N. Dixit, S.K. Srivastava, Thin Solid Films 28 (1975) LI7L20. [20] P. Lal, P.N. Dixit, S.K. Srivastava, Solid State Electron. 19 (1976) 545. [21] P. Lal, Thin Solid Films 143 (1986) 217-223. [22] E. Ahilea, A.A. Hirsch, J. Appl. Phys. 42 (1971) 5601. [23] P. Lal, J. Mater. Sci. Lett. 6 (1987) 546.
Microelectronics Journal 29 (1998) 403-407
Influence of longitudinal electric field on vapour deposited thin films of cadmium sulphide a report Pyare Lal, S.K. Srivastava Centre for Research in Microelectronics, Department of Electronics Engineering, Institute of Technology, Banaras Hindu University, Varanasi-221005 (U.P.), India
Abstract In this report we give a brief account of the various processes of the formation of thin films. This is followed by a description of the influence of various deposition parameters on the characteristic properties of thin films. The electrical and structural characterization of vapour deposited cadmium sulphide films grown under the influence of electric fields is discussed. This article is a compilation of the previous reported results. Various observations (viz. conductivity measurement, thermoelectric power measurement X-ray observation and crossover voltage measurement) suggest that there is an increase in disorderliness (as a result of freezing in action) due to application of a dc electric field at the time of growth of the film. © 1998 Elsevier Science Ltd. All rights reserved.
1. Introduction Thin film has a pivotal role in the development of challenging areas of microelectronics, optical coatings, superconductivity and metallurgical coatings. Ultra-thin films are two dimensional and may be obtained by any of the following processes: physical vapour deposition (PVD) [1-4]; chemical vapour deposition (CVD) [5-8]; electro chemical deposition (ECD) [9-12]; electroless or solution growth deposition. Generally, thin films ~xe prepared by depositing the films' material atom by atom on a suitable substrate. Such a process of film formation involves phase transformation. Because of the phase transformation involved in the film formation, the ambient conditions influence the parametric properties of the film. The ambient conditions that are likely to change the morphology, microstructure and adhesion of deposits in the case of PVD and CVD are pressure and nature of residual gas ill the deposition chamber, temperature of evaporating source, rate of deposition of condensing atom, temperature of substrate, nature of substrate, surface mobility of deposits on substrate, presence of electric/magnetic field on the surface of substrate, chemical reaction between substrate and deposits and the activation process. The factors influencing electro deposition process are pH of electrolyte, current density, temperature of bath composition, electrode shape and agitation. The influence of deposition parameters on film growth may be evaluated in terms of their effect on nucleation density, sticking coefficient and surface mobility of 0026-2692/98/$19.00 © 1998 Elsevier Science Ltd. All rights reserved Pll S0026-2692(97)00023-2
adatoms. This covers a very wide spectrum of the subject and is beyond the scope of this report. One of the important aspects of the subject is the study of the effect of the application of an electric field applied to the substrate at the time of growth of metallic and semiconducting film. Chopra [ 14] has studied the effect of a lateral field deposited onto sodium chloride and noted that the field appears to flatten the discrete islands and increase their surface area. Other workers [15-17] have also reported on the change in electrical resistance of continuous film prepared with an applied electric field and they attributed this to the decrease in frozen-in structural defects during the induced coalescence. However, the influence of the application of a longitudinal electric field at the time of deposition has not been reported to our knowledge. In this report we have compiled our work on the study of the effect of application of longitudinal electric field on vacuum deposited cadmium sulphide film.
2. Device preparation Thin films of cadmium sulphide sandwiched between aluminium electrodes were prepared by a suitable masking arrangement described elsewhere [18]. Before deposition, the electric field system was incorporated as shown in Fig. 1. Special care was taken when the substrate was clipped to the positive terminal of the supply as the whole system under vacuum acts as a diode, resulting in flow of an additional electron current (if the temperature of the evaporation source is high).
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998)403-407
404
Cooling arrangement Evaporation source L Type rubber gasket
Base plate Power supply
Baffle valve
Diffusion
Fig. 1. Electric field arrangement before deposition of CdS film. (a) 104 --
Vacuum sealed electrically insulated plug for applying longitudinal electric field of either polarity
(b) 104 _ 8 6
106 v/cm /65
4
v/cm
2 v/cm 104
104 8 6
8 6
4
2
~102 .98 6
<~.102 .N 8
6
~4
~
L)
4
2 101
1°'68f
8
6 4
+35 v/cm
2 i
Iv 0.1
i 0.2
i i i [ ! 0.4 0.6 0.8 1.0
cl i i i i i 2.0 3.0 4.0 6.0 8.010.0
o o.1
Voltage Fig. 2. Log l versus log V characteristics of the film grown under different field values.
0'2
...... I i Vc , ....... I 0.4 0.6 0.8 1.0 2.0 3.0 4.0 6.0 8.010.0 Voltage
405
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 4 0 3 - 4 0 7
(a) 0,8 '2.4
~ o.6! \ \ \
\
2.2 \
\
O
.,a 0.4
~
\
'2.0
"~ 0.2 [-
'1.8
300
I 320
,
+70 V/cm
, 340
,
, , 380 , , , , 360 400 Temperature (°K)
4120 4130
"1.6
.1.4 ! \
(b)
/ / ' D " - -- ~ " ~ /
/
0, / o
0.6
1I
• 1.2
0.s - A
/ O
C 0.4
I
I
I
-106
-65
-35
I
N.F. 35 Field (v/cm)
I
I
65
106
~
-
1
4
-210 V/cm V/cm
0
N70 V/cm
0.2
Fig. 3. Variation of crossover voltage with electric field. i
300
The thickness of the film was evaluated by the weight difference method and in all the cases it was between approximately 0.4 and 0.5/xm. For thermoelectric power measurement the slides were kept in an electronically controlled oven. The temperature of the hot junction was raised using a small heater in such a way that the temperature difference between the hot and cold junctions was approximately constant (approximately 10°C to avoid non-linearity in thermoelectric power measurement). This was achieved by raising the temperature of the oven accordingly. The resistance of the film was also measured with increasing temperature while the thermoelectric power was being measured. Special precautions were taken in sample preparation for X-ray structural study. Deposited cadmium sulphide (with and without field) was scrapped and coated onto the surface of the fine glass fibre using a small amount of glue. Once the specimen rod was prep~xed it was mounted in its holder in the Debye-Scherrer camera. A copper target was used to produce the beam and ez,ch sample was exposed for 5 h. The asymmetrical method of film loading was used in the present study and'd' values for di:fferent specimens including the mother material (i.e. CdS which was used in this present study) were recorded and compared with the standard ASTM chart.
i
320
I
340
I
I
I
I
I
I
I
360 380 400 Temperature (°K)
I
420
Fig. 4. Variation of thermoelectric power with temperature.
increases with increasing positive or negative field as shown in Fig. 2. There are two sets of lines in each case. For low applied voltage Ohm' s law of conduction is followed where, as for high voltage, space charge limited conduction (SCL conduction) is observed [19,20]. Variation of crossover voltage (i.e. the voltage at which change from Ohm's law to SCL conduction occurs) as a function of field [20] is shown in Fig. 3. It is obvious from this figure that the crossover voltage increases with either polarity of the field value and is always more than the no-field value. However, there is slight departure from this observation at _+ 106 V/cm field value, which may possibly be due to certain errors in thickness evaluation. The variation with temperature in the thermoelectric power [21] for polycrystalline CdS film grown with different field strength of either polarity is shown in Fig. 4. It is found that thermoelectric power initially decreases with an increase in temperature and then starts increasing. It is also observed that the thermoelectric power increases with an increase in the field for either polarity.
3. Results 4. Description of results The current voltage characteristics of different samples were studied. It was found that current at the same voltage
It is obvious from the conductivity measurement that the
406
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 403-407
(b)
+70 V/cm +140 V/cm +210 V/cm
/
lO9 8
"0.65
6 "0.60
/
"~ 4 "
2
lxl08 2.46
i
2, 2
(a) 109 8
i
i
i
I
I
0
.
5
5
i
2.58 2.64 2.70 2.76 2.82 2.88 2.94 1 x 103 °K-1 T 2-70 V/cm 10 V/cm -140 V/cm
•0.45
N_F
•0.40
6 .0.35
e~ 2 i -270 lxl08 2.46
I
I
-140
-70
0.30 N.F.
I
70
I
140
I
270
Field (V/cm) 2.52
2.'58
2164 21.70 2176 21.82 2188 21.94 1 x 103 °K-I T
Fig. 5. Variation of resistance with temperature during thermoelectric power measurement.
conductivity increases with either polarity of the field value. The increase in conductivity may be due to a decrease in number of scattering sites. A decrease in number of scattering sites may be due to flattening or stretching of grains [13,14,22]. However, flattening or stretching of the grains is only possible in the plane of the film. With the field applied along the direction of evaporation, the vapour of CdS, which is partially ionic and partially neutral, freezes immediately as it strikes the substrate. Therefore, with the application of a positive field the concentration of Cd ÷ ions should decrease and hence the conductivity should increase with increasing positive field value. The reverse should be true in the case of a negative field; but such an expected increase or decrease in conductivity is not observed and the conductivity always increases. This may be due to a freezing-in action, which makes the film more disordered. We term this freezing in action as electrical quenching. The initial decrease and then increase in thermoelectric power with temperature may be explained in terms of change in the effective carrier density [18]. The slope of the resistance versus temperature curve Fig. 5 in thermoelectric power measurement gives the activation energy (El). A plot of activation energy (Ef) with electric field E applied at the time of growth of the film is shown in Fig. 6. The slope (d/dE)(zS~f) is positive for either polarity of the field value. This is indicative of the fact that the location of
Fig. 6. Variation of activation energy (Ef) with electric field E.
dominant impurity states changes with field, possibly because of an increase in disorder with increasing field of either polarity. X-ray observations [23] show that the lines of higher angle of reflections are either missing or diffused with increasing electric field of either polarity. However, the effect is more dominant in the case of the negative field than the positive field. This is indicative of the fact that the system is more disordered in the case of a negative field than in the case of a positive field. This is supported by the results reported earlier of the variation of crossover voltage with field. The disappearance or the diffusion of diffracted lines for higher angles of reflection may be due to electrical quenching at the time of growth of the films. It may be possible that the diffused or missing diffracted lines are due to disorderliness or the orientation of lattice planes as a result of the freezing-in action at the time of growth of the film under the influence of an electric field of either polarity. The disorder or the orientation of the reflecting planes increases with increasing field, which results in missing lines in the X-ray diffraction pattern. Finally, we may conclude that the applied longitudinal electric field at the time of growth of CdS film changes its electrical properties by creating disorder in the structure of the film which is due to electrical quenching of the partially ionic and partially neutral ions of the cadmium sulphide. The effect is more dominant in the case of a negative field than a positive field.
P. Lal, S.K. Srivastava/Microelectronics Journal 29 (1998) 403-407
References [1] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. [2] L.I. Maissel and R. Ghmg, Hand Book of Thin Film Technology, McGraw-Hill, New York, 1970. [3] L. Holland, Vacuum Deposition of Thin Film, Chapman and Hall, London, 1956. [4] J.W. Mathews, Epitaxial Growth, Academic Press, New York, 1975. [5] W. Kern, V.S. Ban, Chemical Vapour Deposition of Inorganic Thin Films, Academic Press, New York, 1978, p. 258. [6] W. Kern, G.L. Schnable, IEEE Electron Devices ED-26 (1979) 647. [7] E.C. Douglas, Solid State Technol. 24 (1981) 65. [8] J.L. Vossen, W. Kern, Phys. Today 33 (1980) 26. [9] G. Milazzo, Electrochemistry, Elsevier, Amsterdam, 1963. [10] F.A. Lowenheim, Modern Electroplating, John Wiley, New York, 1963.
407
[11] A. Brenner, Electrodeposition of Alloys, Vols 1,2. Academic Press, New York, 1963. [12] J.M. West, Electrodeposition and Corrosion Processes, D. Van Nostrand, Company, Princeton, NJ, 1965. [13] K.L. Chopra, Appl. Phys. Lett. 7 (1965) 140. [14] K.L. Chopra, J. Appl. Phys. 37 (1966) 2249. [15] H. Koeing, G. Helwig, Optik 6 (1950) 111. [16] L. Holland, J. Opt. Soc. Am. 43 (1953) 376. [17] K. Mihana, M. Tanaka, J. Cryst. Growth 2 (1968) 51. [18] S.K. Srivastava, P. Lal J. Inst Eng. (India) 53 (1973) 168. [19] P. Lal, P.N. Dixit, S.K. Srivastava, Thin Solid Films 28 (1975) LI7L20. [20] P. Lal, P.N. Dixit, S.K. Srivastava, Solid State Electron. 19 (1976) 545. [21] P. Lal, Thin Solid Films 143 (1986) 217-223. [22] E. Ahilea, A.A. Hirsch, J. Appl. Phys. 42 (1971) 5601. [23] P. Lal, J. Mater. Sci. Lett. 6 (1987) 546.