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A low technology, low cost silicon solar cell production system
Solar Cells, 11 (1984) 13 - 18
13
A LOW TECHNOLOGY, LOW COST SILICON S O L A R CELL P R O D U C T I O N SYSTEM P. H. FANG, Z. HUAN*, J. H. KINNIER and C. C. SCHUBERT
Department o f Physics, Boston College, Chestnut Hill, MA 02167 (U.S.A.) (Received June 17, 1983; accepted June 20, 1983)
Summary A system has been designed for the mass production of solar cells using simple processes based on evaporation in a low vacuum with a controlled gas. The system involves a minimum a m o u n t of high technology equipment and can be readily constructed.
1. Introduction Solar cell research, development and production are of great current technological interest. Two areas which have received attention are the cost of materials for solar cell production and the efficiency of the solar cell performance [1]. The complexity, the cost and the operation of the materials and equipment involved do n o t seem to be the foremost in these considerations. This practice can be traced to microelectronics technology. The " b i r t h " of transistors and solar cells occurred n o t t o o many years ago and in fact they were essentially the same device. N o w the transistor has entered into maturity through miniaturization, contracting from millimeters to micrometers in linear dimensions, i.e. a reduction by a factor of 10 3 . In contrast, because of the very nature of light dependence and a limited possibility of augmenting the conversion efficiency, at most by a factor of 3 or 4, the size of the solar cell has no alternative b u t to increase. Current technology has expanded solar cells from centimeters by 10 times and arrays of solar cells also by a b o u t 10 times. An enlargement by a factor of 10 3 , while a desirable goal, is not y e t in sight. One possible reason, we believe, is the "transistor mentality". With this mentality, expensive equipment and meticulous processes are naturally accepted production procedures.
*Visiting scholar from Inner Mongolia University, Hohehaote, Inner Mongolia, China. 0379-6787/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
14
In this paper we give an approach which does not involve the high technology of sophisticated equipment, and the operational process is simplified but still meets the requirements of basic physics. The idea is a modification of a m e t h o d for producing vacuum-evaporated thin film solar cells [2 - 4]. This modification will be described in several interdependent constructions.
2. Thin film silicon solar cells Several materials are used to make solar cells, e.g. silicon, cadmium sulfide and gallium arsenide. Among them, silicon is the most abundant, least expensive and also least toxic. From silicon, solar cells have been made by growing first single crystals or polycrystals, by cutting or shaping them into slices of a b o u t 0.5 mm thickness and b y performing thermal diffusion or ion implantation to form p - n junctions of the solar cell. A quite different approach, which has been rapidly advanced in recent years, produces amorphous silicon solar cells [1]. Amorphous silicon has a larger optical absorption coefficient in the dominant region of the solar spectrum than crystalline silicon does, and therefore a thickness o f 0.5 pm, instead of 0.5 mm for the crystal case, is adequate to absorb the incident solar energy, i.e. a reduction of silicon material by a factor of 103 is realized. Thus the cost of the silicon, which is a b o u t one-third of the solar cell cost, is vastly reduced. Of course, we n o w have to add the cost of the substrate material on which the amorphous silicon is deposited. The current popular m e t h o d to produce amorphous silicon solar cells is the glow discharge of silane gas, and with this method solar cells with a conversion efficiency of 10% have been produced [1]. Three drawbacks of the m e t h o d are that (i) the deposition rate is slow, requiring typically a b o u t 1 h to deposit a solar cell, (ii) silane gas is expensive (in terms of silicon weight, the cost is a b o u t U.S. $4000 per kilogram in comparison with the cost of solid polycrystalline silicon which is U.S. $60 per kilogram) and (iii) silane is toxic, inconvenient to transport and not readily available in developing countries and remote regions of the world. An alternative approach is to produce the solar cell in a vacuum by electron beam heating of the silicon source. This approach is unique in three aspects: (i) it gives a fast deposition rate with a high percentage of material utilization; (ii) solid polycrystalline silicon is used (a convenience in transportation and a cost saving); (iii) dopant materials are solids with a low vapor pressure and thus interference and contamination due to the dopants are minimized. Functional amorphous silicon solar cells have been made [3] with this arrangement. More recently, by a combination of submicron-sized polycrystalline silicon [4] as a window on an amorphous silicon film, solar cells with an efficiency exceeding 1% have been achieved [5]. There are many improvements and optimizations y e t to be undertaken, but with these production conditions and the cost effectiveness, even at this state of low conversion efficiency, the approach already has a practical utility.
15
3. Low vacuum solar cell production system In the above vacuum evaporation, the ambient vacuum is in the 10 -° Torr range. This vacuum can be readily obtained b y a diffusion pump action with cryogenic trapping, typically by liquid nitrogen or liquid helium. Under this condition, electron beam heating for vaporizing the silicon is quite convenient. However, there are several deficiencies in this practice. (1) The cryogenic source is impractically expensive, especially for largevolume consumption to produce low cost solar cells. The availability of the cryogenic source is also limited. (2) The construction of the electron beam system is quite complex. The cost of the system represents a large portion of the cost of the deposition system. (3) Because of the high voltage operation involved there is radiation damage on the device from the electrons and X-rays generated by the electron beam. The present approach aims to eliminate these deficiencies through a modification of the processing m e t h o d and will be described according to three principal sources: semiconductor, gaseous ion and dopant.
3.1. S i l i c o n s o u r c e
If the evaporation is to be carried o u t in a low vacuum to eliminate the cryogenic facility, the electron beam will arc and spark; also, the filament for electron emission will deteriorate rapidly. Therefore, a low vacuum and electron beam evaporation are incompatible. The present modification is to replace the electron beam system by other designs. One design is to use the resistive heating of graphite or a conductive refractive material such as boron nitride or b o r o n carbide. Another way is to use indirect heating by means of a tungsten basket in contact with the silicon contained in a crucible of, for example, b o r o n nitride or beryllium oxide. A third way is to heat inductively, e.g. by introducing an r.f. coil inside the vacuum system. It is important in all these installations to have a heat removal facility inside the vacuum system. A convenient m e t h o d is to use a closed water-cooling shield and watercooled power-carrying conduits. The vacuum system is to be operated in the low (10 -4 Tort) to high (10 -s Torr) range, a condition that can readily be obtained by a mechanical pump plus an oil diffusion pump. The low vacuum will also minimize the pumping time requirement. This low vacuum is adequate for evaporation from the point of view of the silicon mean free path. The relation between the mean free path L in centimeters and the vacuum pressure p in torrs is given approximately b y L = 5 × 103/p
Thus, when p = 10 -4 Torr, L is 50 cm, a practical range to exceed the sourcesubstrate separation.
16 3.2. I o n s o u r c e
The low vacuum represents the presence of various gaseous species in the system and they could react with the evaporation material to cause poor solar cell performance. We recall that the thin film silicon solar cell actually contains, besides silicon, a small gaseous component such as hydrogen, oxygen or nitrogen incorporated in an atomic or ionized state rather than a molecular state [6]. The introduction of this atomic or ionic gas into the vacuum system performs two functions simultaneously: the replacement of undesirable residues in the low vacuum and the provision of a small a m o u n t of the ingredient to achieve an annealing of the defects in the silicon film. The proper a m o u n t of this gas to introduce will be decided by the a m o u n t required in the silicon composition, taking into consideration the concentration of the gas available on the deposited surface and the sticking coefficient. A source o f the gaseous ion can be derived from the following constructions. 3.2.1. M e c h a n i c a l c o n s t r u c t i o n The molecular gas at about 0.1 Torr pressure is introduced into the vacuum system through a leak valve. Inside the vacuum system, the outlet of the leak valve is connected to one end of tungsten or tantalum tubing of 1 cm inner diameter and 10 cm length. Inside the tubing is a tungsten filam e n t coil, 5 mm in diameter and 2 cm long, placed about 3 cm from the inlet end o f the tungsten tubing. The other end is constricted to an orifice 0.1 mm in diameter. 3.2.2. Electrical c o n n e c t i o n
There are three electrical connections: (i) those t h a t heat the filament to incandescence to cause electron emission; {ii) a voltage, positive with respect to the filament, connected to the tungsten tubing, or to a grid inside the tubing, to remove the electrons stripped from the hydrogen and thereby to p r o m o t e the production of hydrogen ions; {iii) a negative potential plate with a 2 mm hole in the center aligned with the center of the exit orifice of the end plate, with a 5 mm separation between the plates {this negative potential is introduced to extract the ions and to orient the ions to the location of the substrate where the deposition is being performed). A simpler structure is a gas discharge tube excited by a high voltage of a few kilovolts of a.c., d.c. or r.f. power. The tube could be made of high temperature glass with geometrical dimensions similar to those of the tungsten tube described in Section 3.2.1. 3.3. D o p a n t source
In in solid heating dopant,
order to achieve e c o n o m y and convenience, the d o p a n t sources are form. For an n-type dopant, arsenic or a n t i m o n y metal in a resistive boat made of tungsten or tantalum is satisfactory. For a p-type better results have been obtained with boron.
17 A very high temperature of 1800 °C is required to evaporate boron. A practical construction for a small production system is to have a graphite rod of 5 mm diameter with a constricted central portion of a b o u t 2 - 3 mm in diameter, with an indentation to hold a small boron piece. The t w o ends of the rod are tightly wrapped with tantalum foil electrodes connected to a heavy copper electrical lead. To lower the excessively high temperature required, the source of boron can be boron oxide, boron nitride or boron silicide. In industrial production, the scale described above should be proportionally enlarged. With the sources described, the principal installation of the solar cell production system is complete. A description of the process of production is contained in refs. 2 - 6 except that the deposition should proceed in situ in a gaseous plasma environment instead of in a high vacuum as a first step and incorporation of the ionized gas through a post-annealing.
4. An operational example In the following, an example will be given of h o w to produce a heterostructure solar cell made by the superposition of a microcrystalline silicon window on an amorphous silicon film. (i) A steel substrate is placed in a rotary dome inside the vacuum system for deposition. After a vacuum in the high 10 -s Tort range has been reached, the substrate is heated to 400 °C and gaseous ions are introduced into the system. The following layers of materials are consecutively evaporated while a vacuum in the low 10 -4 Tort range is maintained: (ii) a titanium layer of 500 A; (iii) an n-doped silicon layer of 300 A; (iv) an intrinsic silicon layer of 3000 A; (v) a p-doped silicon layer of 300 A. (vi) Because of the low substrate temperature, these silicon layers will be in amorphous form. At the end of (v), the substrate temperature is raised to 600 °C rapidly in a time of a b o u t 10 min; this time is sufficiently long to establish thermal equilibrium b u t n o t t o o long to induce a crystallization of the amorphous layer. After this temperature has been established, a p layer of silicon 600 A thick is deposited. This layer, because of the new substrate temperature, will be microcrystalline. (vii) At the end of (vi), the temperature is lowered to and maintained at 450 °C in a hydrogen plasma ambient for 30 rain. After this process, the heater is turned off and the specimen removed. (viii) The complete solar cell is made by forming the t o p grid electrode and the t o p surface antireflection coating. 5. Conclusion In conclusion, while the methods employed to produce the individual components are not new, a unique synthesis to fulfilthe objective of low
18
technology requirements and good cost effectiveness is the heart of the present work. With regard to the combination of'a silicon and hydrogen source to produce a plasma, it should be pointed out that this is different from ion plating deposition. Ion plating deposition involves ionizing silicon and accelerating the ionized silicon to attain a high energy state. As pointed out above, the objective of the present work was to maintain the evaporating materi~.ls in a low energy state to minimize the radiation damage during processing.
Acknowledgments This work is an outgrowth of an earlier program supported by the U.S. Department of Energy, Office of Basic Sciences. We wish to record our indebtedness to the program manager, Dr. Ryszard Gajewski, for his understanding and his guidance.
References 1 Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, September 1982, IEEE, New York, 1982. 2 P. H. Fang, L. Ephrath and W. B. Norwak, Appl. Phys. Lett., 25 (1974) 583. 3 P. H. Fang, C. C. Schubert, J. H. Kinnier and D. Pang, Appl. Phys. Lett., 39 (1981) 256. 4 C. C. Schubert, P. H. Fang and J. H. Kinnier, Jpn. J. Appl. Phys., 20 (1981) 437. 5 P. H. Fang, C. C. Schubert, P. Bai and J. H. Kinnier, Appl. Phys. Lett., 41 (1981) 365. 6 P. H. Fang, Z. Huan, Y. Gao, C. C. Schubert and J. H. Kinnier, Jpn. J. Appl. Phys., 22 (1983) 147.
13
A LOW TECHNOLOGY, LOW COST SILICON S O L A R CELL P R O D U C T I O N SYSTEM P. H. FANG, Z. HUAN*, J. H. KINNIER and C. C. SCHUBERT
Department o f Physics, Boston College, Chestnut Hill, MA 02167 (U.S.A.) (Received June 17, 1983; accepted June 20, 1983)
Summary A system has been designed for the mass production of solar cells using simple processes based on evaporation in a low vacuum with a controlled gas. The system involves a minimum a m o u n t of high technology equipment and can be readily constructed.
1. Introduction Solar cell research, development and production are of great current technological interest. Two areas which have received attention are the cost of materials for solar cell production and the efficiency of the solar cell performance [1]. The complexity, the cost and the operation of the materials and equipment involved do n o t seem to be the foremost in these considerations. This practice can be traced to microelectronics technology. The " b i r t h " of transistors and solar cells occurred n o t t o o many years ago and in fact they were essentially the same device. N o w the transistor has entered into maturity through miniaturization, contracting from millimeters to micrometers in linear dimensions, i.e. a reduction by a factor of 10 3 . In contrast, because of the very nature of light dependence and a limited possibility of augmenting the conversion efficiency, at most by a factor of 3 or 4, the size of the solar cell has no alternative b u t to increase. Current technology has expanded solar cells from centimeters by 10 times and arrays of solar cells also by a b o u t 10 times. An enlargement by a factor of 10 3 , while a desirable goal, is not y e t in sight. One possible reason, we believe, is the "transistor mentality". With this mentality, expensive equipment and meticulous processes are naturally accepted production procedures.
*Visiting scholar from Inner Mongolia University, Hohehaote, Inner Mongolia, China. 0379-6787/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
14
In this paper we give an approach which does not involve the high technology of sophisticated equipment, and the operational process is simplified but still meets the requirements of basic physics. The idea is a modification of a m e t h o d for producing vacuum-evaporated thin film solar cells [2 - 4]. This modification will be described in several interdependent constructions.
2. Thin film silicon solar cells Several materials are used to make solar cells, e.g. silicon, cadmium sulfide and gallium arsenide. Among them, silicon is the most abundant, least expensive and also least toxic. From silicon, solar cells have been made by growing first single crystals or polycrystals, by cutting or shaping them into slices of a b o u t 0.5 mm thickness and b y performing thermal diffusion or ion implantation to form p - n junctions of the solar cell. A quite different approach, which has been rapidly advanced in recent years, produces amorphous silicon solar cells [1]. Amorphous silicon has a larger optical absorption coefficient in the dominant region of the solar spectrum than crystalline silicon does, and therefore a thickness o f 0.5 pm, instead of 0.5 mm for the crystal case, is adequate to absorb the incident solar energy, i.e. a reduction of silicon material by a factor of 103 is realized. Thus the cost of the silicon, which is a b o u t one-third of the solar cell cost, is vastly reduced. Of course, we n o w have to add the cost of the substrate material on which the amorphous silicon is deposited. The current popular m e t h o d to produce amorphous silicon solar cells is the glow discharge of silane gas, and with this method solar cells with a conversion efficiency of 10% have been produced [1]. Three drawbacks of the m e t h o d are that (i) the deposition rate is slow, requiring typically a b o u t 1 h to deposit a solar cell, (ii) silane gas is expensive (in terms of silicon weight, the cost is a b o u t U.S. $4000 per kilogram in comparison with the cost of solid polycrystalline silicon which is U.S. $60 per kilogram) and (iii) silane is toxic, inconvenient to transport and not readily available in developing countries and remote regions of the world. An alternative approach is to produce the solar cell in a vacuum by electron beam heating of the silicon source. This approach is unique in three aspects: (i) it gives a fast deposition rate with a high percentage of material utilization; (ii) solid polycrystalline silicon is used (a convenience in transportation and a cost saving); (iii) dopant materials are solids with a low vapor pressure and thus interference and contamination due to the dopants are minimized. Functional amorphous silicon solar cells have been made [3] with this arrangement. More recently, by a combination of submicron-sized polycrystalline silicon [4] as a window on an amorphous silicon film, solar cells with an efficiency exceeding 1% have been achieved [5]. There are many improvements and optimizations y e t to be undertaken, but with these production conditions and the cost effectiveness, even at this state of low conversion efficiency, the approach already has a practical utility.
15
3. Low vacuum solar cell production system In the above vacuum evaporation, the ambient vacuum is in the 10 -° Torr range. This vacuum can be readily obtained b y a diffusion pump action with cryogenic trapping, typically by liquid nitrogen or liquid helium. Under this condition, electron beam heating for vaporizing the silicon is quite convenient. However, there are several deficiencies in this practice. (1) The cryogenic source is impractically expensive, especially for largevolume consumption to produce low cost solar cells. The availability of the cryogenic source is also limited. (2) The construction of the electron beam system is quite complex. The cost of the system represents a large portion of the cost of the deposition system. (3) Because of the high voltage operation involved there is radiation damage on the device from the electrons and X-rays generated by the electron beam. The present approach aims to eliminate these deficiencies through a modification of the processing m e t h o d and will be described according to three principal sources: semiconductor, gaseous ion and dopant.
3.1. S i l i c o n s o u r c e
If the evaporation is to be carried o u t in a low vacuum to eliminate the cryogenic facility, the electron beam will arc and spark; also, the filament for electron emission will deteriorate rapidly. Therefore, a low vacuum and electron beam evaporation are incompatible. The present modification is to replace the electron beam system by other designs. One design is to use the resistive heating of graphite or a conductive refractive material such as boron nitride or b o r o n carbide. Another way is to use indirect heating by means of a tungsten basket in contact with the silicon contained in a crucible of, for example, b o r o n nitride or beryllium oxide. A third way is to heat inductively, e.g. by introducing an r.f. coil inside the vacuum system. It is important in all these installations to have a heat removal facility inside the vacuum system. A convenient m e t h o d is to use a closed water-cooling shield and watercooled power-carrying conduits. The vacuum system is to be operated in the low (10 -4 Tort) to high (10 -s Torr) range, a condition that can readily be obtained by a mechanical pump plus an oil diffusion pump. The low vacuum will also minimize the pumping time requirement. This low vacuum is adequate for evaporation from the point of view of the silicon mean free path. The relation between the mean free path L in centimeters and the vacuum pressure p in torrs is given approximately b y L = 5 × 103/p
Thus, when p = 10 -4 Torr, L is 50 cm, a practical range to exceed the sourcesubstrate separation.
16 3.2. I o n s o u r c e
The low vacuum represents the presence of various gaseous species in the system and they could react with the evaporation material to cause poor solar cell performance. We recall that the thin film silicon solar cell actually contains, besides silicon, a small gaseous component such as hydrogen, oxygen or nitrogen incorporated in an atomic or ionized state rather than a molecular state [6]. The introduction of this atomic or ionic gas into the vacuum system performs two functions simultaneously: the replacement of undesirable residues in the low vacuum and the provision of a small a m o u n t of the ingredient to achieve an annealing of the defects in the silicon film. The proper a m o u n t of this gas to introduce will be decided by the a m o u n t required in the silicon composition, taking into consideration the concentration of the gas available on the deposited surface and the sticking coefficient. A source o f the gaseous ion can be derived from the following constructions. 3.2.1. M e c h a n i c a l c o n s t r u c t i o n The molecular gas at about 0.1 Torr pressure is introduced into the vacuum system through a leak valve. Inside the vacuum system, the outlet of the leak valve is connected to one end of tungsten or tantalum tubing of 1 cm inner diameter and 10 cm length. Inside the tubing is a tungsten filam e n t coil, 5 mm in diameter and 2 cm long, placed about 3 cm from the inlet end o f the tungsten tubing. The other end is constricted to an orifice 0.1 mm in diameter. 3.2.2. Electrical c o n n e c t i o n
There are three electrical connections: (i) those t h a t heat the filament to incandescence to cause electron emission; {ii) a voltage, positive with respect to the filament, connected to the tungsten tubing, or to a grid inside the tubing, to remove the electrons stripped from the hydrogen and thereby to p r o m o t e the production of hydrogen ions; {iii) a negative potential plate with a 2 mm hole in the center aligned with the center of the exit orifice of the end plate, with a 5 mm separation between the plates {this negative potential is introduced to extract the ions and to orient the ions to the location of the substrate where the deposition is being performed). A simpler structure is a gas discharge tube excited by a high voltage of a few kilovolts of a.c., d.c. or r.f. power. The tube could be made of high temperature glass with geometrical dimensions similar to those of the tungsten tube described in Section 3.2.1. 3.3. D o p a n t source
In in solid heating dopant,
order to achieve e c o n o m y and convenience, the d o p a n t sources are form. For an n-type dopant, arsenic or a n t i m o n y metal in a resistive boat made of tungsten or tantalum is satisfactory. For a p-type better results have been obtained with boron.
17 A very high temperature of 1800 °C is required to evaporate boron. A practical construction for a small production system is to have a graphite rod of 5 mm diameter with a constricted central portion of a b o u t 2 - 3 mm in diameter, with an indentation to hold a small boron piece. The t w o ends of the rod are tightly wrapped with tantalum foil electrodes connected to a heavy copper electrical lead. To lower the excessively high temperature required, the source of boron can be boron oxide, boron nitride or boron silicide. In industrial production, the scale described above should be proportionally enlarged. With the sources described, the principal installation of the solar cell production system is complete. A description of the process of production is contained in refs. 2 - 6 except that the deposition should proceed in situ in a gaseous plasma environment instead of in a high vacuum as a first step and incorporation of the ionized gas through a post-annealing.
4. An operational example In the following, an example will be given of h o w to produce a heterostructure solar cell made by the superposition of a microcrystalline silicon window on an amorphous silicon film. (i) A steel substrate is placed in a rotary dome inside the vacuum system for deposition. After a vacuum in the high 10 -s Tort range has been reached, the substrate is heated to 400 °C and gaseous ions are introduced into the system. The following layers of materials are consecutively evaporated while a vacuum in the low 10 -4 Tort range is maintained: (ii) a titanium layer of 500 A; (iii) an n-doped silicon layer of 300 A; (iv) an intrinsic silicon layer of 3000 A; (v) a p-doped silicon layer of 300 A. (vi) Because of the low substrate temperature, these silicon layers will be in amorphous form. At the end of (v), the substrate temperature is raised to 600 °C rapidly in a time of a b o u t 10 min; this time is sufficiently long to establish thermal equilibrium b u t n o t t o o long to induce a crystallization of the amorphous layer. After this temperature has been established, a p layer of silicon 600 A thick is deposited. This layer, because of the new substrate temperature, will be microcrystalline. (vii) At the end of (vi), the temperature is lowered to and maintained at 450 °C in a hydrogen plasma ambient for 30 rain. After this process, the heater is turned off and the specimen removed. (viii) The complete solar cell is made by forming the t o p grid electrode and the t o p surface antireflection coating. 5. Conclusion In conclusion, while the methods employed to produce the individual components are not new, a unique synthesis to fulfilthe objective of low
18
technology requirements and good cost effectiveness is the heart of the present work. With regard to the combination of'a silicon and hydrogen source to produce a plasma, it should be pointed out that this is different from ion plating deposition. Ion plating deposition involves ionizing silicon and accelerating the ionized silicon to attain a high energy state. As pointed out above, the objective of the present work was to maintain the evaporating materi~.ls in a low energy state to minimize the radiation damage during processing.
Acknowledgments This work is an outgrowth of an earlier program supported by the U.S. Department of Energy, Office of Basic Sciences. We wish to record our indebtedness to the program manager, Dr. Ryszard Gajewski, for his understanding and his guidance.
References 1 Proc. 16th Photovoltaic Specialists' Conf., San Diego, CA, September 1982, IEEE, New York, 1982. 2 P. H. Fang, L. Ephrath and W. B. Norwak, Appl. Phys. Lett., 25 (1974) 583. 3 P. H. Fang, C. C. Schubert, J. H. Kinnier and D. Pang, Appl. Phys. Lett., 39 (1981) 256. 4 C. C. Schubert, P. H. Fang and J. H. Kinnier, Jpn. J. Appl. Phys., 20 (1981) 437. 5 P. H. Fang, C. C. Schubert, P. Bai and J. H. Kinnier, Appl. Phys. Lett., 41 (1981) 365. 6 P. H. Fang, Z. Huan, Y. Gao, C. C. Schubert and J. H. Kinnier, Jpn. J. Appl. Phys., 22 (1983) 147.