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Amorphous silicon with selenium films: Optical absorption
Journal of Non-Crystalline Solids 53 (1982) 11-17 North-Holland Publishing Company
11
A M O R P H O U S SILICON W I T H S E L E N I U M FILMS: O P T I C A L ABSORPTION F . G . W A K I M a n d A. A I - J A S S A R Physics Department, Kuwait University, Kuwait
S.A. A b o - N A M O U S Kuwait Institute for Scientific Research, Kuwait
Received 30 November 1981 Revised manuscript received 3 August 1982
Amorphous silicon (a-Si) and amorphous silicon with selenium (a-SiSe~,)were prepared in an ultrahigh vacuum system using an electron gun evaporation source. Films of similar thickness showed that the optical absorption coefficient for a-Si:Se:, is shifted to higher energies than that for a-Si. The optical gap for a-Si was found to be 1.25 eV and independent of the thickness whereas that for a-Si:Sex was a function of thickness up to 0.3 lam and reached a steady value for thicker films. The usefulness of selenium is assumed to be a result of its ability to satisfy one or more dangling bonds.
1. Introduction A m o r p h o u s silicon (a-Si) films p r e p a r e d b y glow discharge of silane [1] were f o u n d to have a low d e n s i t y of u n d e s i r e d defect states in the m o b i l i t y gap [2,3]. T h e absence of defects in these films m a k e s it possible to d o p e a-Si a n d m a k e p - n j u n c t i o n s , which c o u l d be used in p h o t o v o l t a i c applications. T h e favorable p r o p e r t i e s of glow discharge a-Si was f o u n d to be due to the presence of 8 - 3 5 at% of h y d r o g e n [3]. T h e presence of h y d r o g e n not only saturates d a n g l i n g b o n d s but also m o d i f i e s the whole structure, which m a k e s it of s e m i c o n d u c t o r quality. M o r e o v e r , - t h e presence of h y d r o g e n shifts the optical a b s o r p t i o n coefficient a to higher energies a n d changes its slope [4]. It has been o b s e r v e d that these films are n o t always u n i f o r m b u t show some structural i n h o m o g e n e i t y , with grains having a small a m o u n t of h y d r o g e n , while the intergrain b o u n d a r i e s c o n t a i n a m u c h larger h y d r o g e n c o n c e n t r a t i o n [3,5]. In a d d i t i o n , it has been suggested b y O v s h i n s k y [6] that the a d d i t i o n of o t h e r elements such as fluorine to a-Si : H can i m p r o v e the p h o t o v o l t a i c device p e r f o r m a n c e of such materials. I n t e r c o n n e c t e d m i c r o v o i d s have b e e n d e t e c t e d t h r o u g h o u t e v a p o r a t e d or s p u t t e r e d a m o r p h o u s silicon films [3] which were a s s u m e d to develop d u r i n g the growth of the material. These m i c r o v o i d s were o b s e r v e d b y X - r a y scatter0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
F.G. Wakim et aL / Amorphous silicon with seleniurn films
12
ing [7], electron microscopy [8] and porosity measurements [9]. Even the highest-quality glow discharge a - S i : H films have been assumed to have two phases, a phase dilute in hydrogen (grains) having only monohydride and interconnecting material which has a different hydrogen concentration [5]. The grains have only 2 - 4 at% hydrogen irrespective of the hydrogen content of the material; the rest of the hydrogen is found in the intergrain region which could be clustered (Sill n). The presence of voids reduces the normal tetrahedral coordination of silicon to three or less coordination on the surfaces of the voids. This generates many unpaired electrons or dangling bonds which increase the density of states in the gap and reduce the usefulness of these films for device applications. In films prepared by glow discharge, the hydrogen covers the surfaces and substantially reduces the number of unpaired electrons. To reduce the influence of the voids, it was decided to incorporate selenium in the films. Selenium can bond covalently with one, two or three dangling bonds, as reported by Kastner et al. [10] which will also reduce the density of defect states in the gap.
2. Experimental Samples of SiSex were prepared by grinding silicon (99.999%) to a fine powder, which was then mixed with 66 at% of selenium powder. The mixture was placed in a quartz ampoule which was evacuated to a pressure of 10 -5 Torr and sealed. The ampoule was heated at 1000°C for about 20 h and then quenched in air. The resulting material was sintered SiSex. A section from the middle of this material was evaporated in an ultrahigh vacuum system by thermal means using an electron gun evaporation source and in graphite boats. The starting pressure before evaporation was about 10-9 Torr which reached a stable pressure of the order of 10 - 7 Torr during evaporation. Film thicknesses ranging from 0.02 to 2 /~m were prepared by placing glass substrates at different locations in the vacuum chamber. The thickness of the films was measured by a film thickness monitor and a Varian microscope. Other films were prepared by evaporating the mixture in a diffusion pump vacuum system at a pressure of 10-5 Torr. Tungsten boats were used for heating the specimen. SiSex was found to be unstable in the presence of humid air. When SiSex is exposed to air it hydrolyzes [11] slowly to SiO 2 and H2Se and produces an unpleasant odour. This reaction can be written as SiSe2 + 2 H 2 0 ~ SiO 2 + 2H2Se. To study the uniformity of the thickness of the films on the glass slides, crystallinity, and composition, a scanning electron microscope and a transmission electron microscope were used. It was found that the SEM signal varied in strength with location, which indicates a variation in the thickness of the film. It was also observed that the films contain voids and possibly adsorbed gases.
13
F.G. Wakim et al. / Amorphous silicon with selenium films
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F.G. Wakim et al. / Amorphous silicon with selenium films
14
The scanning electron microscope ruled out the presence of single crystals or large grains of 30/~m in diameter. The method used is based on the channelling patterns where negative evidence of crystallinity was observed in all the samples. The transmission electron microscope was used to rule out the presence of small grain sizes. No diffraction pattern was observed for powder scraped from a slide which ruled out the presence of grain sizes greater than 100 ,~. X-ray fluorescence showed that the concentration of selenium was not uniform.
3. Optical absorption spectrum and optical edge The optical absorption spectra of some thin films containing pure Si or SiSex were studied at room temperature through the range 400-2000 nm. A Cary-17 spectrophotometer was used in all these measurements. Fig. 1 shows the absorption coefficient ct of two films prepared under identical conditions, for pure silicon and for silicon containing selenium. The absorption coefficient of the film containing selenium is shifted towards higher energy than that of the pure silicon film. This shift is very similar to that for a-Sill x reported by Cody et al. [4] and that of evaporated a-Si films found by Pierce and Spicer [12]. The lack of agreement between our a-Si film and that of Pierce and Spicer may be due to preparation or measuring conditions. In any case, there is a shift between our a-Si and a-SiSex which were prepared and measured under identical conditions. The optical absorption coefficient t~ was used to find the optical gap using the following relation given by Tauc [13]:
Fig. 2 gives a straight line for ( a h u ) 1/2 against the photon energy; the intercept I~
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Fig. 3. Variation of the optical gap Eg with thickness. Most of the measurements were done on films where the starting material was SiSe 2.
F.G. Wakim et al. / Amorphous silicon with selenium films
15
gives the optical gap Eg for pure a-Si and a-SiSex. From fig. 2, we conclude that the optical gap for a-Si is 1.25 eV, and for a-SiSex is 1.80 eV for films of about 1 /~m thick. The value of 1.25 eV for a-Si is in good agreement with the value obtained by Brodsky et al. [14] for unannealed a-Si films prepared by RF sputtering. Studying the optical gap of SiSex films as a function of film thickness showed that the gap increases initially with thickness for the same composition. Beyond a certain film thickness, it reaches a saturation value, as is shown in fig. 3. Since selenium close to the surface hydrolyzes and leaves the surface, it seems that the selenium concentration in the film is a function of thickness. The loss of selenium from a thin layer close to the surface has no influence on films thicker than 0.3 ~m. We could conclude that HzSe in the gaseous state either has less chance to form in the deeper layers of the SiSex films or, if it forms, it has less chance to escape from the bulk. On the other hand, the optical gap of pure silicon films was found not to be a function of thickness but was about 1.25 eV for films prepared in an ultrahigh vacuum, and about 1.40 eV for films prepared in the diffusion p u m p system where the pressure was 10 -5 Torr. The higher value of the optical gap is thought to be due to the partial oxidation of the silicon film at higher pressure.
4. Discussion The function that hydrogen takes in glow discharge a - S i : H films can be assumed by selenium, a chalcogen, which is a group-VI element. Selenium can assume a great variety of bonding configurations with adjacent silicon or other selenium atoms. The normal bonding configuration of a selenium atom is a twofold neutral configuration which has been represented by Kastner et al. [10] and by Adler [ 15] as C °, and which is in the configuration of lowest energy. An atom of selenium can have such coordination when it is found in a double silicon vacancy in a silicon tetrahedral structure, (see fig. 4). In this position,
Fig. 4. One position out of several possibilities is a selenium atom replacing two silicon atoms (A and B) in a silicon tetrahedral structure. All the unpaired electrons due to the double vacancy are satisfactorily bonded with the neighboring silicon atoms, or with the selenium atom, which has a twofold bond configuration.
16
F.G. Wakim et al. / Amorphous silicon with selenium films
where it replaces two silicon atoms, no unpaired electrons are found, which reduces the density of states. In addition, selenium can have single C~- and triple C~- coordination as has been concluded for selenium in chalcogenide glasses [10]. The formation of C~- and C~- (known as a valence alteration pair) from two neutral normally coordinated selenium atoms 2C ° requires a relatively small amount of energy. These can satisfy one and three bonds, where C~- can be coordinated with a triply bonded T ° silicon atom, transforming it to a tetrahedrally coordinated T ° silicon atom with lower energy, and C~- can be bonded to three triply-coordinated silicon atoms 3T3°, transforming them to 3T ° with lower energy. C~- has the same function as a hydrogen atom in glow discharge a - S i : H that satisfies a single dangling bond. In addition, selenium can be bonded with other selenium atoms as C~- to terminate a selenium chain of C~- to bond to three other selenium atoms [16]. The concentration of selenium relative to that of silicon in SiSex should determine the optical gap. This has been found to be the case for GeSex [17] where the shift in the optical gap was directly related to the value of x. In addition (h~,a) 1/2 versus hp gave a straight line for the different compositions, GeSe x. It could be assumed that the variation of the optical gap with thickness is the result of the variation of the concentration of selenium, which varies as a result of the hydrolysis of SiSex mentioned earlier. The knowledge that hydrolyis was taking place can be assumed from the strong smell that is observed when SiSe~ is left in air. A similar reaction can take place between a-Si : H and atmospheric oxygen, where water and SiO 2 are formed. The oxidation of Sill by atmospheric oxygen is more favorable energetically [18] * than the hydrolysis of SiSe~, but cannot be observed as easily since the resulting product is water and partially oxidized silicon surface. The oxidation of the surface is likely to happen even with the absence of hydrogen as a result of oxygen adsorption or chemisorption on the surface of pure silicon.
5. Conclusion When selenium atoms are introduced into a-Si films, the optical gap shifts to higher energies in a way similar to that caused by the presence of hydrogen in a-Si. In addition, this study suggests that the presence of selenium may reduce the number of unpaired electrons or dangling bonds and the density of states in the gap. The two functions of increasing the gap and satisfying dangling bonds are necessary to make SiSex a potential material for photovoltaic application.
* The energy of S i - H is 70.4 kC m o l - l, that of O - O is 33.2 kC m o l - ', while that of S i - O is 88.2 kC m o l - i and that of H - O is 110.6 kC m o l - i . This shows that breaking S i - H and O - O and forming H - O and Si-O is a favorable reaction.
F.G. Wakim et al. / Amorphous silicon with selenium films
17
We appreciate the help of Dr. K.Z. Botros in doing the electron microscope measurements. The support of the Research Council of Kuwait University and of Kuwait Institute for Scientific Research is greatly appreciated.
References [1] R.C. Chittick, J.H. Alexander and H.F. Sterling, J. Electrochem. Soc. 116 (1969) 77. [2] N.F. Mort and E.A. Davis, Electronic Properties in Non-Crystalline Materials (Clarendon Press, Oxford, 1979). [3] H. Fritzsche, Solar Enery Mater. 3 (1980) 447. Many references are listed in this review paper. [4] G.D. Cody, C.R. Wronski, B. Abeles, R.B. Stephens and B. Brooks, Solar Cells 2 (1980) 227. [5] J.A. Reimer, R.W. Vaughan and J. Knights, Phys. Rev. Lett. 44 (1980) 193. [6] S.R. Ovshinsky, New Scientist 80 (1978) 647. [7] S.C. Moss and J.F. Graczyk, Phys. Rev. Lett. 23 (1969) 1167. [8] A. Barna, P.B. Barna, G. Rathnoczi, L. Toth and P. Thomas, Phys. Stat Sol (a) 41 (1977) 81. [9] H. Fritzsche and C.C. Tsai, Solar Energy Mater. 1 (1979) 471. [10] M. Kastner, D. Adler and H. Fritzsche, Phys. Rev. Lett. 37 (1976) 1504. [11] E.G. Rochow, in: Comprehensive Inorganic Chemistry, Eds. J.C. Bailor, H.J. Emeleus, Sir R. Nyholm and A.F. Trotmann-Dickenson (Pergamon, Oxford, 1973), p. 1414. [12] D.T. Pierce and W.E. Spicer, Phys. Rev. B5 (1972) 3017. [13] J. Tauc, Optical Properties of Solids, Ed. F. Abeles (North-Holland, Amsterdam, 1970). [14] M.H. Brodsky, R.S. Tittle, K. Weiser and G.D. Pettit, Phys. Rev. BI (1970) 2632. [15] D. Adler, J. Non-Crystalline Solids 35/36 (1980) 819. [16] R.S. Street, Proc. 7th Int. Conf. on Amorphous and Liquid Semiconductors, Ed. W.E. Spear (University of Edinburgh, 1977) p. 509. [17] T. Shimizu, M. Kumeda and M. Ishikawa, J. Non-Crystalline Solids 33 (1979) 1. [18] L. Pauling, The Nature of the Chemical Bond, 3rd edn. (Cornell University Press, Ithaca, NY, 1960).
11
A M O R P H O U S SILICON W I T H S E L E N I U M FILMS: O P T I C A L ABSORPTION F . G . W A K I M a n d A. A I - J A S S A R Physics Department, Kuwait University, Kuwait
S.A. A b o - N A M O U S Kuwait Institute for Scientific Research, Kuwait
Received 30 November 1981 Revised manuscript received 3 August 1982
Amorphous silicon (a-Si) and amorphous silicon with selenium (a-SiSe~,)were prepared in an ultrahigh vacuum system using an electron gun evaporation source. Films of similar thickness showed that the optical absorption coefficient for a-Si:Se:, is shifted to higher energies than that for a-Si. The optical gap for a-Si was found to be 1.25 eV and independent of the thickness whereas that for a-Si:Sex was a function of thickness up to 0.3 lam and reached a steady value for thicker films. The usefulness of selenium is assumed to be a result of its ability to satisfy one or more dangling bonds.
1. Introduction A m o r p h o u s silicon (a-Si) films p r e p a r e d b y glow discharge of silane [1] were f o u n d to have a low d e n s i t y of u n d e s i r e d defect states in the m o b i l i t y gap [2,3]. T h e absence of defects in these films m a k e s it possible to d o p e a-Si a n d m a k e p - n j u n c t i o n s , which c o u l d be used in p h o t o v o l t a i c applications. T h e favorable p r o p e r t i e s of glow discharge a-Si was f o u n d to be due to the presence of 8 - 3 5 at% of h y d r o g e n [3]. T h e presence of h y d r o g e n not only saturates d a n g l i n g b o n d s but also m o d i f i e s the whole structure, which m a k e s it of s e m i c o n d u c t o r quality. M o r e o v e r , - t h e presence of h y d r o g e n shifts the optical a b s o r p t i o n coefficient a to higher energies a n d changes its slope [4]. It has been o b s e r v e d that these films are n o t always u n i f o r m b u t show some structural i n h o m o g e n e i t y , with grains having a small a m o u n t of h y d r o g e n , while the intergrain b o u n d a r i e s c o n t a i n a m u c h larger h y d r o g e n c o n c e n t r a t i o n [3,5]. In a d d i t i o n , it has been suggested b y O v s h i n s k y [6] that the a d d i t i o n of o t h e r elements such as fluorine to a-Si : H can i m p r o v e the p h o t o v o l t a i c device p e r f o r m a n c e of such materials. I n t e r c o n n e c t e d m i c r o v o i d s have b e e n d e t e c t e d t h r o u g h o u t e v a p o r a t e d or s p u t t e r e d a m o r p h o u s silicon films [3] which were a s s u m e d to develop d u r i n g the growth of the material. These m i c r o v o i d s were o b s e r v e d b y X - r a y scatter0022-3093/82/0000-0000/$02.75
© 1982 N o r t h - H o l l a n d
F.G. Wakim et aL / Amorphous silicon with seleniurn films
12
ing [7], electron microscopy [8] and porosity measurements [9]. Even the highest-quality glow discharge a - S i : H films have been assumed to have two phases, a phase dilute in hydrogen (grains) having only monohydride and interconnecting material which has a different hydrogen concentration [5]. The grains have only 2 - 4 at% hydrogen irrespective of the hydrogen content of the material; the rest of the hydrogen is found in the intergrain region which could be clustered (Sill n). The presence of voids reduces the normal tetrahedral coordination of silicon to three or less coordination on the surfaces of the voids. This generates many unpaired electrons or dangling bonds which increase the density of states in the gap and reduce the usefulness of these films for device applications. In films prepared by glow discharge, the hydrogen covers the surfaces and substantially reduces the number of unpaired electrons. To reduce the influence of the voids, it was decided to incorporate selenium in the films. Selenium can bond covalently with one, two or three dangling bonds, as reported by Kastner et al. [10] which will also reduce the density of defect states in the gap.
2. Experimental Samples of SiSex were prepared by grinding silicon (99.999%) to a fine powder, which was then mixed with 66 at% of selenium powder. The mixture was placed in a quartz ampoule which was evacuated to a pressure of 10 -5 Torr and sealed. The ampoule was heated at 1000°C for about 20 h and then quenched in air. The resulting material was sintered SiSex. A section from the middle of this material was evaporated in an ultrahigh vacuum system by thermal means using an electron gun evaporation source and in graphite boats. The starting pressure before evaporation was about 10-9 Torr which reached a stable pressure of the order of 10 - 7 Torr during evaporation. Film thicknesses ranging from 0.02 to 2 /~m were prepared by placing glass substrates at different locations in the vacuum chamber. The thickness of the films was measured by a film thickness monitor and a Varian microscope. Other films were prepared by evaporating the mixture in a diffusion pump vacuum system at a pressure of 10-5 Torr. Tungsten boats were used for heating the specimen. SiSex was found to be unstable in the presence of humid air. When SiSex is exposed to air it hydrolyzes [11] slowly to SiO 2 and H2Se and produces an unpleasant odour. This reaction can be written as SiSe2 + 2 H 2 0 ~ SiO 2 + 2H2Se. To study the uniformity of the thickness of the films on the glass slides, crystallinity, and composition, a scanning electron microscope and a transmission electron microscope were used. It was found that the SEM signal varied in strength with location, which indicates a variation in the thickness of the film. It was also observed that the films contain voids and possibly adsorbed gases.
13
F.G. Wakim et al. / Amorphous silicon with selenium films
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F.G. Wakim et al. / Amorphous silicon with selenium films
14
The scanning electron microscope ruled out the presence of single crystals or large grains of 30/~m in diameter. The method used is based on the channelling patterns where negative evidence of crystallinity was observed in all the samples. The transmission electron microscope was used to rule out the presence of small grain sizes. No diffraction pattern was observed for powder scraped from a slide which ruled out the presence of grain sizes greater than 100 ,~. X-ray fluorescence showed that the concentration of selenium was not uniform.
3. Optical absorption spectrum and optical edge The optical absorption spectra of some thin films containing pure Si or SiSex were studied at room temperature through the range 400-2000 nm. A Cary-17 spectrophotometer was used in all these measurements. Fig. 1 shows the absorption coefficient ct of two films prepared under identical conditions, for pure silicon and for silicon containing selenium. The absorption coefficient of the film containing selenium is shifted towards higher energy than that of the pure silicon film. This shift is very similar to that for a-Sill x reported by Cody et al. [4] and that of evaporated a-Si films found by Pierce and Spicer [12]. The lack of agreement between our a-Si film and that of Pierce and Spicer may be due to preparation or measuring conditions. In any case, there is a shift between our a-Si and a-SiSex which were prepared and measured under identical conditions. The optical absorption coefficient t~ was used to find the optical gap using the following relation given by Tauc [13]:
Fig. 2 gives a straight line for ( a h u ) 1/2 against the photon energy; the intercept I~
1.6
It
0
mJ
~
0
0
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.
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F.G. Wakim et al. / Amorphous silicon with selenium films
15
gives the optical gap Eg for pure a-Si and a-SiSex. From fig. 2, we conclude that the optical gap for a-Si is 1.25 eV, and for a-SiSex is 1.80 eV for films of about 1 /~m thick. The value of 1.25 eV for a-Si is in good agreement with the value obtained by Brodsky et al. [14] for unannealed a-Si films prepared by RF sputtering. Studying the optical gap of SiSex films as a function of film thickness showed that the gap increases initially with thickness for the same composition. Beyond a certain film thickness, it reaches a saturation value, as is shown in fig. 3. Since selenium close to the surface hydrolyzes and leaves the surface, it seems that the selenium concentration in the film is a function of thickness. The loss of selenium from a thin layer close to the surface has no influence on films thicker than 0.3 ~m. We could conclude that HzSe in the gaseous state either has less chance to form in the deeper layers of the SiSex films or, if it forms, it has less chance to escape from the bulk. On the other hand, the optical gap of pure silicon films was found not to be a function of thickness but was about 1.25 eV for films prepared in an ultrahigh vacuum, and about 1.40 eV for films prepared in the diffusion p u m p system where the pressure was 10 -5 Torr. The higher value of the optical gap is thought to be due to the partial oxidation of the silicon film at higher pressure.
4. Discussion The function that hydrogen takes in glow discharge a - S i : H films can be assumed by selenium, a chalcogen, which is a group-VI element. Selenium can assume a great variety of bonding configurations with adjacent silicon or other selenium atoms. The normal bonding configuration of a selenium atom is a twofold neutral configuration which has been represented by Kastner et al. [10] and by Adler [ 15] as C °, and which is in the configuration of lowest energy. An atom of selenium can have such coordination when it is found in a double silicon vacancy in a silicon tetrahedral structure, (see fig. 4). In this position,
Fig. 4. One position out of several possibilities is a selenium atom replacing two silicon atoms (A and B) in a silicon tetrahedral structure. All the unpaired electrons due to the double vacancy are satisfactorily bonded with the neighboring silicon atoms, or with the selenium atom, which has a twofold bond configuration.
16
F.G. Wakim et al. / Amorphous silicon with selenium films
where it replaces two silicon atoms, no unpaired electrons are found, which reduces the density of states. In addition, selenium can have single C~- and triple C~- coordination as has been concluded for selenium in chalcogenide glasses [10]. The formation of C~- and C~- (known as a valence alteration pair) from two neutral normally coordinated selenium atoms 2C ° requires a relatively small amount of energy. These can satisfy one and three bonds, where C~- can be coordinated with a triply bonded T ° silicon atom, transforming it to a tetrahedrally coordinated T ° silicon atom with lower energy, and C~- can be bonded to three triply-coordinated silicon atoms 3T3°, transforming them to 3T ° with lower energy. C~- has the same function as a hydrogen atom in glow discharge a - S i : H that satisfies a single dangling bond. In addition, selenium can be bonded with other selenium atoms as C~- to terminate a selenium chain of C~- to bond to three other selenium atoms [16]. The concentration of selenium relative to that of silicon in SiSex should determine the optical gap. This has been found to be the case for GeSex [17] where the shift in the optical gap was directly related to the value of x. In addition (h~,a) 1/2 versus hp gave a straight line for the different compositions, GeSe x. It could be assumed that the variation of the optical gap with thickness is the result of the variation of the concentration of selenium, which varies as a result of the hydrolysis of SiSex mentioned earlier. The knowledge that hydrolyis was taking place can be assumed from the strong smell that is observed when SiSe~ is left in air. A similar reaction can take place between a-Si : H and atmospheric oxygen, where water and SiO 2 are formed. The oxidation of Sill by atmospheric oxygen is more favorable energetically [18] * than the hydrolysis of SiSe~, but cannot be observed as easily since the resulting product is water and partially oxidized silicon surface. The oxidation of the surface is likely to happen even with the absence of hydrogen as a result of oxygen adsorption or chemisorption on the surface of pure silicon.
5. Conclusion When selenium atoms are introduced into a-Si films, the optical gap shifts to higher energies in a way similar to that caused by the presence of hydrogen in a-Si. In addition, this study suggests that the presence of selenium may reduce the number of unpaired electrons or dangling bonds and the density of states in the gap. The two functions of increasing the gap and satisfying dangling bonds are necessary to make SiSex a potential material for photovoltaic application.
* The energy of S i - H is 70.4 kC m o l - l, that of O - O is 33.2 kC m o l - ', while that of S i - O is 88.2 kC m o l - i and that of H - O is 110.6 kC m o l - i . This shows that breaking S i - H and O - O and forming H - O and Si-O is a favorable reaction.
F.G. Wakim et al. / Amorphous silicon with selenium films
17
We appreciate the help of Dr. K.Z. Botros in doing the electron microscope measurements. The support of the Research Council of Kuwait University and of Kuwait Institute for Scientific Research is greatly appreciated.
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