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Growth and properties of glow-discharge hydrogenated amorphous silicon-carbon alloys from silane-propane mixtures
Thin Solid Films, 164 (1988) 221 226
221 GROWTH AND PROPERTIES OF GLOW-DISCHARGE H Y D R O G E N A T E D A M O R P H O U S S I L I C O N - C A R B O N ALLOYS F R O M S I L A N E - P R O P A N E MIXTURES* A. QAYYUM, J. I. B. WILSON, K. IBRAHIM, S. K. AL-SABBAGH AND U. EICKER
Department of Physics, Heriot- Watt University, Edinburgh EH14 4AS ( U.K. )
We have investigated films with the required wider bandgap and a low Fermi level density of states for use in solar cells, even with large concentrations of propane in the gas mixture. The hydrogen is predominantly bonded to silicon atoms despite the production of higher hydrocarbons and mixed species by the plasma. Borondoped films incorporated into tandem p - i - n cells do improve their blue spectral response but have yet to be optimized.
1. INTRODUCTION
Hydrogenated amorphous Si-C alloys (a-SixCy:H) have attracted much attention because they are not only promising materials for making solar cells but are also an interesting system for the study of amorphous semiconductors 1. Films of a-SixCy:H can be fabricated by the plasma decomposition of silane and hydrocarbon gas mixtures 2'3 or alkylsilanes4; however, their physical and chemical characteristics depend upon the carbon source and the deposition conditions 2. Despite many papers describing the properties of these alloy films, there are few that describe either the preparative conditions and plasma chemistry, or the nature of the defects and their relation to the hydrogen, silicon and carbon bonding arrangements. From the many possible carbon sources, we have selected propane (C3Hs) as a result of an early comparison of alloys made from various alkanes 5. Weider e t al. 6 have made some detailed studies of IR spectra of a-SixCy:H films grown with ethylene, and Fujimoto e t al. 7 have studied the IR spectra of a-SixCy:H grown with methane or ethylene. The present paper describes the IR absorption (180-4000 cm-1 range) of a-SixCy:H films prepared from Sill4 and C3H 8 mixtures on crystalline silicon, as well as other optical and electrical properties of films grown simultaneously on Corning 7059 glass. The longer-lived chemical species produced by the plasma were detected by mass spectroscopy. Studies on films from silane and methane indicate that the density of states (DOS) at mid-gap depends quadratically
*Paper presentedat the 7th International Conferenceon Thin Films, New Delhi, India, December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
222
A. QAYYUMet al.
on the carbon concentration 8. In this paper we show the gas mixture dependence of D O S near the Fermi level. Finally, we have also studied the boron-doped films as wide bandgap window layers in p - i - n / p - i - n tandem photovoltaic cells. 2. EXPERIMENTAL DETAILS The a-SixCy:H films were prepared in the plasma deposition system described in our previous paper 9. The substrate temperature was always in the range 240-300 °C and the r.f. power applied to 100 m m diameter electrodes was either 10 or 36W. The SiHa:C3H8 flow rates were between 20:1.0 and 5.0:5.0 standard cm 3 m i n - 1 I-V characteristics were measured by a Hewlett-Packard 4140B picoammeter/d.c, voltage source controlled by an H P 85 computer. The density of states near the Fermi level, g(Ef), was calculated from the space charge limited conduction through the films according to the method of den Boer z°. The optical band gap Eg was obtained from a Tauc plot. Spectral response measurements on tandem cells used simultaneous monochromatic light with a red or blue bias light to separate the responses of upper and lower cells. 3. RESULTS AND DISCUSSION The optical band gap Eg increases monotonically with increasing propane gas fraction and decreases with an increase in substrate temperature, as is usual for these alloys. This is shown in Fig. 1. 2.4 2.3
•
LOW T E M P .
o HIGH TEMP.
2.2 2.1 d~
2.0 !.9
,?, 1.8 1.7 16 1.5 1.4
I
I
I
I
1
2
3
4
I
I
I
I
5 6 SiH4/C3H8
I
7
8
9
10
Fig. 1. Optical bandgap dependence on gas composition and substrate temperature.
Figure 2 shows the infrared transmission of a set of films where absorption is due to S i - - H bond stretching (about 2090 c m - 1 ) and wagging (about 650 cm-x). The remainder of the spectra show Si-C absorption near 750 c m - t and a small absorption at about 2900cm-~ due to the C - - H stretching mode. Thus most hydrogen is bonded in these films to Si, with very little Sill2. The Si-C and Si-H
GLOW-DISCHARGE HYDROGENATED a-Si-C ALLOYS FROM S i H 4 - C 3 H
s MIXTURES 223
absorptions are shifted slightly to higher frequency with increasing carbon incorporation, due to back-bonding of carbon. (The same effect is observed in a-SixNy:H films). The C - - H absorption bands are less obvious than in other hydrocarbon grown films, possibly due in part to a stronger substrate temperature influence on the depositing species. 100% a = 754 b = 755 c = 756 d = 757 e = 758 f = 759 Z
0
< rr I--
~
a
-----,--------/--
b
~
d
~
e
~
f
0% 3000
i 2800
a ~ b
i
C
e t"
e
f
d I
I
2200 WAVENUMBER
2000 (cm
i J i 700 650 600
400
-1)
Fig. 2. 1R transmission spectra ofthree different compositionsofSiH4:C3Hs (a,b, 15:5;c,d, 10:5;e,f, 5:5) at various substrate temperatures (a,d, 270 °C; b,c,e, 300 °C; f, 250 °C).
Mass spectroscopy 9 shows that the two gases appear to break down independently in the plasma, giving a series of hydrocarbons and silanes which either react or are incorporated into the growing film. There are also significant ion-molecule reactions, producing species like SiCaH 7 and SiCaH 9. A large propane concentration led to the formation of higher molecular weight hydrocarbons, indicating that C - - C bonds were readily formed; this was similar to the appearance of higher silanes when there was a large silane concentration. Obviously Si--Si bonds are favoured. Although our device quality films were always grown with l0 W r.f. power, an increase to 36 W resulted in a significant decrease of both silane and hydrocarbon radicals, which might have been due to the depleted gas stock as the deposition rate increased, but was more likely to be due to a change in plasma dissociation reactions. From the above studies, propane is quite stable in r.f. plasma relative to its elements whereas silane is unstable relative to its elements. Figure 3 shows the temperature dependence of dark conductivity with respect to the gas composition for films deposited at about 240 °C. Note the decrease with increase in propane content. Fermi level DOS shows a dependence not only on gas composition but also on the substrate temperature: in general, a higher substrate temperature gives a higher DOS, as can be seen from Fig. 4. All samples were photosensitive, with the best response from a deposition temperature of 240-250 °C and an SiH4:CaH s ratio of 20:4. Films of p type grown with SiH4:CsHs:I~oPHa/Ar of 20:5:25 standard
GLOW-DISCHARGE HYDROGENATED
a-Si-C ALLOYS FROM SiH4-C3H 8 MIXTURES 2 2 5 LIGHT BIASING BOTTOM TOP
I
4
ISUBSTRATE I
1
0 350
440
530 620 WAVELENGTH (nm)
710
800
Fig. 5. Top cell spectral response (red bias).
4
.
-
3 o
2
1
0 350
440
I 530 620 WAVELENGTH (nm)
I 710
800
Fig. 6. Bottom cell spectral response (blue bias).
matched.) Comparing top and bottom cell responses, we observe that the overall current is limited by the thin top cell, which is due to several factors, particularly recombination due to electrons diffusing against the field and insufficient absorption in the thin entrance cell. 4. CONCLUSION The structure and hence the bandgap of a-SixCy:H depends on carbon concentration and substrate temperature. Most of the hydrogen is bonded to silicon, as Si--H. Mass spectroscopy of the plasma species shows the formation of higher molecular weight hydrocarbons in propane-rich mixtures, similar to the formation
A. QAYYUM etal.
226
of higher silanes, but also the production of mixed species from the two gases by ion-molecule reactions. Films prepared using propane as the carbon source instead of methane or other hydrocarbons are highly photosensitive and have a low Fermi level density of states. Their use in tandem cells requires further study of charge transport through thin ptype layers. ACKNOWLEDGMENT
The award of research studentships is gratefully acknowledged by A.Q. (Government of Pakistan), K.I. (Government of Malaysia), S.A. (Government of Iraq) and U.E. (Heriot-Watt University).
REFERENCES 1 2 3 4 5 6 7 8 9 10
Y. Tawada, H. Okamato and Y. Hamakawa, Appl. Phys. Lett., 39 (1981) 237. D.A. Anderson and W. E. Spear, Philos. Mag. 35 (1977) 1. Y. Catherine, G. Turban and B. Grolleau, Thin Solid Films, 76 (1981) 23. H. Munekata, S. Murasato, H. Kukimoto and Y. Hamakawa, J. Appl. Phys., 53 (1982) 536. S. Nitta, A. Hatano, M. Yamada, M. Watanabe and M. Kuwai, J. Non-Cryst. Solids, 59 60 (1983) 557. H. Weider, M. Cardona and C. R. Guarmierl, Phys. Status Solidi, 92 (1979) 99. F. Fujimota, A. Oatuka, K. Komaki, Y. Inata, I. Yamaha, H. Yamashita, Y. Hashimoto, Y. Tawada, K. Nishmura, H. Okomoto and T. Hamakawa, Jpn. J. Appl. Phys., 23 (1984) 810. M.P. Schmidt, J. Bullot, M. Gauthier, P. Cordier, I. Solomon and H. Tran-Quoc, Philos. Mag. B, 51 (1985) 581. A. Qayyum, J. I. B. Wilson, K. Ibrahim and S. K. Al-Sabbagh, E-MRS Symp. C, Strasbourg, 1987. W. Den Boer, J. Phys. Colloq.,42(4), Suppl. 10(1981)451.
221 GROWTH AND PROPERTIES OF GLOW-DISCHARGE H Y D R O G E N A T E D A M O R P H O U S S I L I C O N - C A R B O N ALLOYS F R O M S I L A N E - P R O P A N E MIXTURES* A. QAYYUM, J. I. B. WILSON, K. IBRAHIM, S. K. AL-SABBAGH AND U. EICKER
Department of Physics, Heriot- Watt University, Edinburgh EH14 4AS ( U.K. )
We have investigated films with the required wider bandgap and a low Fermi level density of states for use in solar cells, even with large concentrations of propane in the gas mixture. The hydrogen is predominantly bonded to silicon atoms despite the production of higher hydrocarbons and mixed species by the plasma. Borondoped films incorporated into tandem p - i - n cells do improve their blue spectral response but have yet to be optimized.
1. INTRODUCTION
Hydrogenated amorphous Si-C alloys (a-SixCy:H) have attracted much attention because they are not only promising materials for making solar cells but are also an interesting system for the study of amorphous semiconductors 1. Films of a-SixCy:H can be fabricated by the plasma decomposition of silane and hydrocarbon gas mixtures 2'3 or alkylsilanes4; however, their physical and chemical characteristics depend upon the carbon source and the deposition conditions 2. Despite many papers describing the properties of these alloy films, there are few that describe either the preparative conditions and plasma chemistry, or the nature of the defects and their relation to the hydrogen, silicon and carbon bonding arrangements. From the many possible carbon sources, we have selected propane (C3Hs) as a result of an early comparison of alloys made from various alkanes 5. Weider e t al. 6 have made some detailed studies of IR spectra of a-SixCy:H films grown with ethylene, and Fujimoto e t al. 7 have studied the IR spectra of a-SixCy:H grown with methane or ethylene. The present paper describes the IR absorption (180-4000 cm-1 range) of a-SixCy:H films prepared from Sill4 and C3H 8 mixtures on crystalline silicon, as well as other optical and electrical properties of films grown simultaneously on Corning 7059 glass. The longer-lived chemical species produced by the plasma were detected by mass spectroscopy. Studies on films from silane and methane indicate that the density of states (DOS) at mid-gap depends quadratically
*Paper presentedat the 7th International Conferenceon Thin Films, New Delhi, India, December7-11, 1987. 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
222
A. QAYYUMet al.
on the carbon concentration 8. In this paper we show the gas mixture dependence of D O S near the Fermi level. Finally, we have also studied the boron-doped films as wide bandgap window layers in p - i - n / p - i - n tandem photovoltaic cells. 2. EXPERIMENTAL DETAILS The a-SixCy:H films were prepared in the plasma deposition system described in our previous paper 9. The substrate temperature was always in the range 240-300 °C and the r.f. power applied to 100 m m diameter electrodes was either 10 or 36W. The SiHa:C3H8 flow rates were between 20:1.0 and 5.0:5.0 standard cm 3 m i n - 1 I-V characteristics were measured by a Hewlett-Packard 4140B picoammeter/d.c, voltage source controlled by an H P 85 computer. The density of states near the Fermi level, g(Ef), was calculated from the space charge limited conduction through the films according to the method of den Boer z°. The optical band gap Eg was obtained from a Tauc plot. Spectral response measurements on tandem cells used simultaneous monochromatic light with a red or blue bias light to separate the responses of upper and lower cells. 3. RESULTS AND DISCUSSION The optical band gap Eg increases monotonically with increasing propane gas fraction and decreases with an increase in substrate temperature, as is usual for these alloys. This is shown in Fig. 1. 2.4 2.3
•
LOW T E M P .
o HIGH TEMP.
2.2 2.1 d~
2.0 !.9
,?, 1.8 1.7 16 1.5 1.4
I
I
I
I
1
2
3
4
I
I
I
I
5 6 SiH4/C3H8
I
7
8
9
10
Fig. 1. Optical bandgap dependence on gas composition and substrate temperature.
Figure 2 shows the infrared transmission of a set of films where absorption is due to S i - - H bond stretching (about 2090 c m - 1 ) and wagging (about 650 cm-x). The remainder of the spectra show Si-C absorption near 750 c m - t and a small absorption at about 2900cm-~ due to the C - - H stretching mode. Thus most hydrogen is bonded in these films to Si, with very little Sill2. The Si-C and Si-H
GLOW-DISCHARGE HYDROGENATED a-Si-C ALLOYS FROM S i H 4 - C 3 H
s MIXTURES 223
absorptions are shifted slightly to higher frequency with increasing carbon incorporation, due to back-bonding of carbon. (The same effect is observed in a-SixNy:H films). The C - - H absorption bands are less obvious than in other hydrocarbon grown films, possibly due in part to a stronger substrate temperature influence on the depositing species. 100% a = 754 b = 755 c = 756 d = 757 e = 758 f = 759 Z
0
< rr I--
~
a
-----,--------/--
b
~
d
~
e
~
f
0% 3000
i 2800
a ~ b
i
C
e t"
e
f
d I
I
2200 WAVENUMBER
2000 (cm
i J i 700 650 600
400
-1)
Fig. 2. 1R transmission spectra ofthree different compositionsofSiH4:C3Hs (a,b, 15:5;c,d, 10:5;e,f, 5:5) at various substrate temperatures (a,d, 270 °C; b,c,e, 300 °C; f, 250 °C).
Mass spectroscopy 9 shows that the two gases appear to break down independently in the plasma, giving a series of hydrocarbons and silanes which either react or are incorporated into the growing film. There are also significant ion-molecule reactions, producing species like SiCaH 7 and SiCaH 9. A large propane concentration led to the formation of higher molecular weight hydrocarbons, indicating that C - - C bonds were readily formed; this was similar to the appearance of higher silanes when there was a large silane concentration. Obviously Si--Si bonds are favoured. Although our device quality films were always grown with l0 W r.f. power, an increase to 36 W resulted in a significant decrease of both silane and hydrocarbon radicals, which might have been due to the depleted gas stock as the deposition rate increased, but was more likely to be due to a change in plasma dissociation reactions. From the above studies, propane is quite stable in r.f. plasma relative to its elements whereas silane is unstable relative to its elements. Figure 3 shows the temperature dependence of dark conductivity with respect to the gas composition for films deposited at about 240 °C. Note the decrease with increase in propane content. Fermi level DOS shows a dependence not only on gas composition but also on the substrate temperature: in general, a higher substrate temperature gives a higher DOS, as can be seen from Fig. 4. All samples were photosensitive, with the best response from a deposition temperature of 240-250 °C and an SiH4:CaH s ratio of 20:4. Films of p type grown with SiH4:CsHs:I~oPHa/Ar of 20:5:25 standard
GLOW-DISCHARGE HYDROGENATED
a-Si-C ALLOYS FROM SiH4-C3H 8 MIXTURES 2 2 5 LIGHT BIASING BOTTOM TOP
I
4
ISUBSTRATE I
1
0 350
440
530 620 WAVELENGTH (nm)
710
800
Fig. 5. Top cell spectral response (red bias).
4
.
-
3 o
2
1
0 350
440
I 530 620 WAVELENGTH (nm)
I 710
800
Fig. 6. Bottom cell spectral response (blue bias).
matched.) Comparing top and bottom cell responses, we observe that the overall current is limited by the thin top cell, which is due to several factors, particularly recombination due to electrons diffusing against the field and insufficient absorption in the thin entrance cell. 4. CONCLUSION The structure and hence the bandgap of a-SixCy:H depends on carbon concentration and substrate temperature. Most of the hydrogen is bonded to silicon, as Si--H. Mass spectroscopy of the plasma species shows the formation of higher molecular weight hydrocarbons in propane-rich mixtures, similar to the formation
A. QAYYUM etal.
226
of higher silanes, but also the production of mixed species from the two gases by ion-molecule reactions. Films prepared using propane as the carbon source instead of methane or other hydrocarbons are highly photosensitive and have a low Fermi level density of states. Their use in tandem cells requires further study of charge transport through thin ptype layers. ACKNOWLEDGMENT
The award of research studentships is gratefully acknowledged by A.Q. (Government of Pakistan), K.I. (Government of Malaysia), S.A. (Government of Iraq) and U.E. (Heriot-Watt University).
REFERENCES 1 2 3 4 5 6 7 8 9 10
Y. Tawada, H. Okamato and Y. Hamakawa, Appl. Phys. Lett., 39 (1981) 237. D.A. Anderson and W. E. Spear, Philos. Mag. 35 (1977) 1. Y. Catherine, G. Turban and B. Grolleau, Thin Solid Films, 76 (1981) 23. H. Munekata, S. Murasato, H. Kukimoto and Y. Hamakawa, J. Appl. Phys., 53 (1982) 536. S. Nitta, A. Hatano, M. Yamada, M. Watanabe and M. Kuwai, J. Non-Cryst. Solids, 59 60 (1983) 557. H. Weider, M. Cardona and C. R. Guarmierl, Phys. Status Solidi, 92 (1979) 99. F. Fujimota, A. Oatuka, K. Komaki, Y. Inata, I. Yamaha, H. Yamashita, Y. Hashimoto, Y. Tawada, K. Nishmura, H. Okomoto and T. Hamakawa, Jpn. J. Appl. Phys., 23 (1984) 810. M.P. Schmidt, J. Bullot, M. Gauthier, P. Cordier, I. Solomon and H. Tran-Quoc, Philos. Mag. B, 51 (1985) 581. A. Qayyum, J. I. B. Wilson, K. Ibrahim and S. K. Al-Sabbagh, E-MRS Symp. C, Strasbourg, 1987. W. Den Boer, J. Phys. Colloq.,42(4), Suppl. 10(1981)451.