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Excimer laser induced silicon amorphous-microcrystalline transformation and its solar cell applications
Solar Cells, 25 (1988) 27 - 29
27
EXCIMER LASER INDUCED SILICON A M O R P H O U S MICROCRYSTALLINE T R A N S F O R M A T I O N AND ITS SOLAR CELL APPLICATIONS P. H. F A N G Department o f Physics, Boston College, Chestnut Hill, MA 02167 (U.S.A.) (Received July 10, 1986; accepted in revised f()rm February 29, 1988)
Summary We report an amorphous-microcrystalline transformation in an amorphous silicon surface induced by krypton-fluorine excimer laser radiation. An application of this result to produce a microcrystalline window on single-crystal and on amorphous silicon solar cells is discussed.
There is a general interest in the interaction of lasers with solids, particularly with the extremely short wavelength laser radiation provided by the excimer laser. Our object is to transform a thin amorphous silicon layer into a crystalline layer, thus achieving a combined crystalline-amorphous silicon solar cell structure. We have reported such a solar cell previously, albeit made using a different process [1]. In order to realize this application, there are two pertinent points which must be studied: (1) the optical absorption coefficient in the excimer wavelength region which determines the laser penetration depth; (2) the nature of morphological change due to laser irradiation and the effect on the solar cell properties. Optical absorption with a short pulse YAG laser has been studied by Kanemitsu e t al. [2]. The laser wavelengths were 1064 and 532 nm with a picosecond pulse. They reported the following optical absorption coefficients ~: Laser wavelength 532 nrn
1064 nrn
olc (crn -I)
1 X 10 4
2 X 10 2
~ a ( c m -1)
1Xl0 s
6X10 a
where c denotes crystalline and a amorphous. These values are t o o small for our purpose for an effective attenuation in a region of about 10 nm. There0379-6787/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
28 fore, we have investigated a shorter wavelength KrF excimer laser which has a wavelength of 248 nm. At this wavelength, based on e2 data of Jackson e t al. [3] and taking an index of refraction of 1.6, a value of 1.7 × 106 cm -a for 0~ is obtained. This value is quite close to the value of ac of 1.81 × 106 cm -1 given by Aspnes and Studna [4], i.e. there is no appreciable difference in during the transformation from an a m or pho us to a crystalline state. The all-amorphous silicon film in our study had a p - i - n configuration with the top p layer 15 nm thick. The thickness of the i layer was 460 nm and the n layer was 25 nm thick. The specimen was made by electron beam evaporation in vacuum o n t o a steel substrate [5]. Prior to laser treatment, the specimen was hydr ogenat ed in order t hat the laser effect could be evaluated immediately through observation of the photovoltaic effect. Since the laser pulse was 20 ns, we believe that h ydrogen out-diffusion during the laser radiation is negligible. The laser energy in our experiment was 1 J with a beam cross-section of approximately 3 mm × 7 mm. The e xpe r i m ent was carried out in air and the result was obtained with one single pulse. A first sign t hat the a m o r p h o u s crystalline transformation has taken place is a reduct i on of electrical resistivity by 2 to 4 orders of magnitude, as obtained from a direct probe. In the all-amorphous p - i - n cell, an electrode has to be evaporated o n t o the surface before the photovoltaic voltage and current can be measured. However, when the surface is sufficiently conductive, as in the present case, the electrode is n o t required at least for qualitative evaluation. Next, we took X-ray diffraction patterns, and characteristic crystalline silicon lines were observed. In order to make sure t hat this crystalline state was specifically microcrystalline [6], we devised the following experiment. Instead of a steel substrate, we used a microscope glass slide as the substrate and deposited an am or phous p-Si layer 15 nm thick. After laser t reat m ent , the optical absorption was measured in order to evaluate the optical band gap. The value of 1.7 eV, characteristic of microcrystalline silicon, was confirmed. This value is distinctly different from t hat of ordinary crystalline silicon of 1.1 eV. In am or phous films a value of 1.7 eV can also be obtained, but in this case the value is variable and can be influenced by growth conditions, including the hydrogen content. In the case of microcrystalline silicon, this is a fixed value and is not affected by hydrogenation [6]. Finally, the determination of the thickness of the transformed microcrystalline layer was based on the following observations. When the thickness of the microcrystalline layer is much less than the p-layer thickness, the surface resistivity would be t oo high to measure the photovoltaic effect directly as we have been able to. Secondly, if the transformed region includes an appreciable i region, the short wavelength spectral response would be greatly reduced. Based on these observations, we conclude that the transformed microcrystalline layer has a thickness of a bo ut 15 nm. This dept h is considerably larger than the optical dept h (a-a ~ 6 nm) based on a = 1.7 × 106 cm -1 , and apparently there is appreciable propagation of the crystallization. In this case, the region affected by a r uby laser (wavelength 693 nm) with a pulse of 20 ns is over 200 nm [7], and would n o t be suitable for our purpose.
29 Solar cells m a d e f r o m this e x c i m e r laser t r e a t e d silicon give an o p e n circuit v o l t a g e o f 560 m V and a short-circuit c u r r e n t d e n s i t y of 4.6 m A c m -2 u n d e r air mass 1 s i m u l a t e d light. This result is n e a r l y t h e same as t h a t o b t a i n e d in earlier w o r k o n a c o m b i n e d m i c r o c r y s t a l l i n e - a m o r p h o u s silicon solar cell [ 1 ] . In t h e earlier w o r k , the m i c r o c r y s t a l l i n e layer was m a d e b y e l e c t r o n b e a m e v a p o r a t i o n at a high s u b s t r a t e t e m p e r a t u r e , and a steel substrate w a s used. T h e p r o c e s s p r e c l u d e s using glass or plastic as s u b s t r a t e materials. In the p r e s e n t w o r k , t h e solar cell is first m a d e c o m p l e t e l y in an a m o r p h o u s s t r u c t u r e b y e l e c t r o n b e a m e v a p o r a t i o n o n s u b s t r a t e s at a t e m p e r a t u r e o f 300 °C; t h u s a c o m b i n e d m i c r o c r y s t a l l i n e - a m o r p h o u s solar cell m a d e o n a w i d e r v a r i e t y o f s u b s t r a t e m a t e r i a l s b e c o m e s possible. A d i f f e r e n t p r o c e s s t o p r o d u c e m i c r o c r y s t a l l i n e silicon is the glow discharge m e t h o d [ 8 ] . In this case, t h e high r.f. p o w e r r e q u i r e d i n t r o d u c e s excessive r a d i a t i o n d a m a g e and d e g r a d e s t h e q u a l i t y o f t h e m a t e r i a l . By using t h e e x c i m e r laser, o n e c o u l d first m a k e a c o m p l e t e a m o r p h o u s s t r u c t u r e at low r.f. p o w e r and f o l l o w this b y e x c i m e r laser m i c r o c r y s t a l l i z a t i o n . In c o n c l u s i o n , we have p r e s e n t e d an o b s e r v a t i o n o f the m i c r o c r y s t a l l i z a t i o n o f a m o r p h o u s silicon p r e p a r e d b y an e l e c t r o n b e a m e v a p o r a t i o n . An a p p l i c a t i o n o f this o b s e r v a t i o n t o m a k e a c o m b i n e d m i c r o c r y s t a l l i n e a m o r p h o u s silicon thin film solar cell is r e p o r t e d . I t h a n k Mr. A. K i r k p a t r i c k o f t h e E p i o n C o r p o r a t i o n f o r his g e n e r o u s e x c i m e r laser facility, t e c h n i c a l help in p e r f o r m i n g the e x p e r i m e n t , and v e r y useful discussions.
References 1 P. H. Fang, C. C. Schubert, P. Bai and J. H. Kinnier, Appl. Phys. Lett., 41 (1982) 356. 2 Y. Kanemitsu, H. Kuroda and S. Shionoya, Jpn. J. AppI. Phys., 24 (1984) 618; 24 (1984) 940. 3 W. B. Jackson, S. M. Kelso, C. C. Tsai, J. W. Allen and S.-J. Oh, Phys. Rev., 31 (1985) 5187. 4 D. E. Aspnes and A. A. Studna, Phys. Rev. B, 27 (1983)985. 5 C. C. Schubert, P. H. Fang and J. H. Kinnier, Jpn. J. Appl. Phys., 20 (1981) L437. 6 P. H. Fang, P. Bai, C. C. Schubert and J. H. Kinnier, Jpn. J. Appl. Phys. Lett., 22 (1983) 149. P. H. Fang, P. Bai, C. C. Schubert and J. H. Kinnier, J. Non-Cryst. Solids, 59 - 60 (1983)819. 7 W. Sinke and F. W. Saris, Phys. Rev. Lett., 53 (1984) 2121. J. Narayan and C. W. White, Appl. Phys. Lett., 44 (1984) 35. 8 Y. Uchida, T. Ichimura, M. Veno and M. Ohsawa, Jpn. J. Appl. Phys. Lett., 21 (1962) 586.
27
EXCIMER LASER INDUCED SILICON A M O R P H O U S MICROCRYSTALLINE T R A N S F O R M A T I O N AND ITS SOLAR CELL APPLICATIONS P. H. F A N G Department o f Physics, Boston College, Chestnut Hill, MA 02167 (U.S.A.) (Received July 10, 1986; accepted in revised f()rm February 29, 1988)
Summary We report an amorphous-microcrystalline transformation in an amorphous silicon surface induced by krypton-fluorine excimer laser radiation. An application of this result to produce a microcrystalline window on single-crystal and on amorphous silicon solar cells is discussed.
There is a general interest in the interaction of lasers with solids, particularly with the extremely short wavelength laser radiation provided by the excimer laser. Our object is to transform a thin amorphous silicon layer into a crystalline layer, thus achieving a combined crystalline-amorphous silicon solar cell structure. We have reported such a solar cell previously, albeit made using a different process [1]. In order to realize this application, there are two pertinent points which must be studied: (1) the optical absorption coefficient in the excimer wavelength region which determines the laser penetration depth; (2) the nature of morphological change due to laser irradiation and the effect on the solar cell properties. Optical absorption with a short pulse YAG laser has been studied by Kanemitsu e t al. [2]. The laser wavelengths were 1064 and 532 nm with a picosecond pulse. They reported the following optical absorption coefficients ~: Laser wavelength 532 nrn
1064 nrn
olc (crn -I)
1 X 10 4
2 X 10 2
~ a ( c m -1)
1Xl0 s
6X10 a
where c denotes crystalline and a amorphous. These values are t o o small for our purpose for an effective attenuation in a region of about 10 nm. There0379-6787/88/$3.50
© Elsevier Sequoia/Printed in The Netherlands
28 fore, we have investigated a shorter wavelength KrF excimer laser which has a wavelength of 248 nm. At this wavelength, based on e2 data of Jackson e t al. [3] and taking an index of refraction of 1.6, a value of 1.7 × 106 cm -a for 0~ is obtained. This value is quite close to the value of ac of 1.81 × 106 cm -1 given by Aspnes and Studna [4], i.e. there is no appreciable difference in during the transformation from an a m or pho us to a crystalline state. The all-amorphous silicon film in our study had a p - i - n configuration with the top p layer 15 nm thick. The thickness of the i layer was 460 nm and the n layer was 25 nm thick. The specimen was made by electron beam evaporation in vacuum o n t o a steel substrate [5]. Prior to laser treatment, the specimen was hydr ogenat ed in order t hat the laser effect could be evaluated immediately through observation of the photovoltaic effect. Since the laser pulse was 20 ns, we believe that h ydrogen out-diffusion during the laser radiation is negligible. The laser energy in our experiment was 1 J with a beam cross-section of approximately 3 mm × 7 mm. The e xpe r i m ent was carried out in air and the result was obtained with one single pulse. A first sign t hat the a m o r p h o u s crystalline transformation has taken place is a reduct i on of electrical resistivity by 2 to 4 orders of magnitude, as obtained from a direct probe. In the all-amorphous p - i - n cell, an electrode has to be evaporated o n t o the surface before the photovoltaic voltage and current can be measured. However, when the surface is sufficiently conductive, as in the present case, the electrode is n o t required at least for qualitative evaluation. Next, we took X-ray diffraction patterns, and characteristic crystalline silicon lines were observed. In order to make sure t hat this crystalline state was specifically microcrystalline [6], we devised the following experiment. Instead of a steel substrate, we used a microscope glass slide as the substrate and deposited an am or phous p-Si layer 15 nm thick. After laser t reat m ent , the optical absorption was measured in order to evaluate the optical band gap. The value of 1.7 eV, characteristic of microcrystalline silicon, was confirmed. This value is distinctly different from t hat of ordinary crystalline silicon of 1.1 eV. In am or phous films a value of 1.7 eV can also be obtained, but in this case the value is variable and can be influenced by growth conditions, including the hydrogen content. In the case of microcrystalline silicon, this is a fixed value and is not affected by hydrogenation [6]. Finally, the determination of the thickness of the transformed microcrystalline layer was based on the following observations. When the thickness of the microcrystalline layer is much less than the p-layer thickness, the surface resistivity would be t oo high to measure the photovoltaic effect directly as we have been able to. Secondly, if the transformed region includes an appreciable i region, the short wavelength spectral response would be greatly reduced. Based on these observations, we conclude that the transformed microcrystalline layer has a thickness of a bo ut 15 nm. This dept h is considerably larger than the optical dept h (a-a ~ 6 nm) based on a = 1.7 × 106 cm -1 , and apparently there is appreciable propagation of the crystallization. In this case, the region affected by a r uby laser (wavelength 693 nm) with a pulse of 20 ns is over 200 nm [7], and would n o t be suitable for our purpose.
29 Solar cells m a d e f r o m this e x c i m e r laser t r e a t e d silicon give an o p e n circuit v o l t a g e o f 560 m V and a short-circuit c u r r e n t d e n s i t y of 4.6 m A c m -2 u n d e r air mass 1 s i m u l a t e d light. This result is n e a r l y t h e same as t h a t o b t a i n e d in earlier w o r k o n a c o m b i n e d m i c r o c r y s t a l l i n e - a m o r p h o u s silicon solar cell [ 1 ] . In t h e earlier w o r k , the m i c r o c r y s t a l l i n e layer was m a d e b y e l e c t r o n b e a m e v a p o r a t i o n at a high s u b s t r a t e t e m p e r a t u r e , and a steel substrate w a s used. T h e p r o c e s s p r e c l u d e s using glass or plastic as s u b s t r a t e materials. In the p r e s e n t w o r k , t h e solar cell is first m a d e c o m p l e t e l y in an a m o r p h o u s s t r u c t u r e b y e l e c t r o n b e a m e v a p o r a t i o n o n s u b s t r a t e s at a t e m p e r a t u r e o f 300 °C; t h u s a c o m b i n e d m i c r o c r y s t a l l i n e - a m o r p h o u s solar cell m a d e o n a w i d e r v a r i e t y o f s u b s t r a t e m a t e r i a l s b e c o m e s possible. A d i f f e r e n t p r o c e s s t o p r o d u c e m i c r o c r y s t a l l i n e silicon is the glow discharge m e t h o d [ 8 ] . In this case, t h e high r.f. p o w e r r e q u i r e d i n t r o d u c e s excessive r a d i a t i o n d a m a g e and d e g r a d e s t h e q u a l i t y o f t h e m a t e r i a l . By using t h e e x c i m e r laser, o n e c o u l d first m a k e a c o m p l e t e a m o r p h o u s s t r u c t u r e at low r.f. p o w e r and f o l l o w this b y e x c i m e r laser m i c r o c r y s t a l l i z a t i o n . In c o n c l u s i o n , we have p r e s e n t e d an o b s e r v a t i o n o f the m i c r o c r y s t a l l i z a t i o n o f a m o r p h o u s silicon p r e p a r e d b y an e l e c t r o n b e a m e v a p o r a t i o n . An a p p l i c a t i o n o f this o b s e r v a t i o n t o m a k e a c o m b i n e d m i c r o c r y s t a l l i n e a m o r p h o u s silicon thin film solar cell is r e p o r t e d . I t h a n k Mr. A. K i r k p a t r i c k o f t h e E p i o n C o r p o r a t i o n f o r his g e n e r o u s e x c i m e r laser facility, t e c h n i c a l help in p e r f o r m i n g the e x p e r i m e n t , and v e r y useful discussions.
References 1 P. H. Fang, C. C. Schubert, P. Bai and J. H. Kinnier, Appl. Phys. Lett., 41 (1982) 356. 2 Y. Kanemitsu, H. Kuroda and S. Shionoya, Jpn. J. AppI. Phys., 24 (1984) 618; 24 (1984) 940. 3 W. B. Jackson, S. M. Kelso, C. C. Tsai, J. W. Allen and S.-J. Oh, Phys. Rev., 31 (1985) 5187. 4 D. E. Aspnes and A. A. Studna, Phys. Rev. B, 27 (1983)985. 5 C. C. Schubert, P. H. Fang and J. H. Kinnier, Jpn. J. Appl. Phys., 20 (1981) L437. 6 P. H. Fang, P. Bai, C. C. Schubert and J. H. Kinnier, Jpn. J. Appl. Phys. Lett., 22 (1983) 149. P. H. Fang, P. Bai, C. C. Schubert and J. H. Kinnier, J. Non-Cryst. Solids, 59 - 60 (1983)819. 7 W. Sinke and F. W. Saris, Phys. Rev. Lett., 53 (1984) 2121. J. Narayan and C. W. White, Appl. Phys. Lett., 44 (1984) 35. 8 Y. Uchida, T. Ichimura, M. Veno and M. Ohsawa, Jpn. J. Appl. Phys. Lett., 21 (1962) 586.