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Electrical and optical properties of RF-SP a-Ge:O:(H) deposited from GeO2
Journal of Non-Crystalline Solids 57 (1983) 119-127 North-Holland Publishing Company
119
ELECTRICAL AND OPTICAL PROPERTIES OF RF-SP a-Ge:O:(H) DEPOSITED FROM GeO 2 Y. T A K A N O a n d H. O Z A K I Department of Electrical Engineering. Waseda University, Ohkuho 3- 4-1, Shinjuku, To,~\vo 160, Japan Received 18 October 1982 Revised manuscript received 19 January 1983
The electrical and optical properties of a-Ge:O:(H) prepared from sintered GeO2 are investigated. Unlike SiO2, GeO 2 can be reduced with H 2 gas. The properties of a-Ge:O:(H) thus prepared can be changed widely with the H 2 gas pressure, approaching those of a-Ge. It is found that, in addition to these changes, a configurational change from Ge-O-Ge to oxygen-vacancy complex occurs with increasing the H 2 gas pressure.
1. Introduction
H y d r o g e n a t e d a m o r p h o u s silicon ( a - S i : H ) is a p r o m i s i n g m a t e r i a l for solar cells, etc. [1,2]. However, the i n c o r p o r a t e d h y d r o g e n tends to be effused away from a - S i : H film at elevated temperatures, d a m a g i n g its properties. So, 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 - o x y g e n alloys ( a - S i : O : H ) are a t t r a c t i n g m u c h a t t e n t i o n [3-6]. a - S i : O : H is not m a d e b y the s p u t t e r i n g of SiO 2 target in H 2 gas a t m o s p h e r e . O n the other hand, p o l y c r y s t a l l i n e g e r m a n i u m ( p o l y - G e ) is m a d e b y deoxidizing G e O 2 with H z gas at high temperatures. In this s t u d y G e O 2 was s p u t t e r e d with a H a and A r gas m i x t u r e to m a k e a m o r p h o u s g e r m a n i u m - o x y g e n alloy ( a - G e : O : ( H ) ) . To the a u t h o r s ' knowledge, this is the first r e p o r t of a - G e : O : ( H ) p r e p a r e d from G e O 2.
2. E x p e r i m e n t a l A l l films were d e p o s i t e d b y R F s p u t t e r i n g (13.56 MHz). P o l y - G e a n d sintered G e O 2 were chosen as sputtering targets. A m o r p h o u s g e r m a n i u m ( a - G e ) m a d e from the f o r m e r served as a reference material. T h e system was first p u m p e d to a base pressure of - 1.0 x 10 _6 Torr, then H 2 a n d A r gas m i x t u r e was passed. The total gas pressure was c o n s t a n t at 5 m T o r r a n d the H 2 gas p a r t i a l pressure ratio ( P H 2 / ( P A r + P H 2 ) ) was varied. The films were d e p o s i t e d on K B r disk for I R m e a s u r e m e n t and on u l t r a s o n i c a l l y cleaned C r o w n glass s u b s t r a t e s for o t h e r m e a s u r e m e n t s . The R F p o w e r was 100 W. T h e 0022-3093/83/0000-0000/$03.00
© 1983 N o r t h - H o l l a n d
Y. Takano, H. Ozaki / R F - S P a-Ge.'O.'(H) deposited from GeO 2
120
deposition rate of a-Ge:O:(H) was varied with H 2 gas pressure from about 0.6 A / s for n o H 2 gas to about 0.4 ,~/s for H 2 gas partial pressure ratio of 0.15. The deposition rate of a-Ge was about 1.0 A/s. The substrate temperature during deposition was nominally room temperature. The films were annealed at 200°C for 30 rain in a N 2 gas atmosphere.
3. Results and discussion
The dc electrical conductivities (o) are shown in figs. 1 and 2. In the high-temperature region ( T > 250 K) a's are of activated type as shown in fig. 1, and in the low-temperature region ( T < 220 K ) o ' s fit well Mott's T 1/4 law [7] as shown in fig. 2. It is also seen in these figures that the annealing reduces o by about one order for a-Ge but scarcely changes o for a-Ge:O(H). Separate films prepared simultaneously were used to measure o's before and after annealing, in order to avoid the diffusion of the electrode material (Sn) during deposition. So the small change in o for a-Ge:O:(H) by annealing seems to be a sample dependence. The plots of (ahu) I/2 versus hp for two representative films of a-Ge and a-Ge:O:(H) are shown in figs. 3 (a) and (b), respectively. The optical energy gap, (Eg)opt, is obtained as the point of intersection of the extrapolated line with the abscissa. Annealing shifts (Eg)o m by about 0.1 eV towards higher energies for a-Ge but no shift is seen for a-Ge:O:(H). This shows that the oxygen atoms incorporated make a-Ge:O:(H) stable against the heat treatment up to 200°C. Figs. 4, 5 and 6 show the dependence on the H 2 gas partial pressure of the activation energy E,, (Eg)opt and the room temperature dc electrical conductivity ORT, respectively. Fig. 7 shows the relation between (Eg)opt and E. obtained
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Fig. 2. dc electrical conductivities versus T - i / 4 for a-Ge films (a) and a-Ge:O:(H) (0.10) films (b). T h e m e a n i n g of the decimals is the same as in fig. 1.
from figs. 4 and 5. It is seen that the electrical and optical properties of a-Ge:O:(H) approach those of a-Ge with increasing the H 2 gas partial pressure. a-Ge:O sputtered only with Ar gas is transparent and its (Eg)op, cannot be measured with our monochromator. Fig. 8 shows the variation of the density of states at the Fermi level (N(EF)) derived from Mott's T 1/4 law, as a function of H 2 gas partial pressure ratio, assuming the decay constant of the wavefunctions as 10 ,X,. While the N(EF) of a-Ge is about 10 Is eV 'cm -L that of a-Ge:O:(H) is less by more than one order. This change can be attributed to the reduction of the dangling bonds due to the softening of the lattice induced by oxygen which takes up a G e - O - G e configuration. The enhancement of the
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122
Y. Takano, H. Ozaki / RF-SP a-Ge:O:(H) deposited from GeO2
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N ( E F ) at h i g h e r H 2 gas p a r t i a l p r e s s u r e s is a t t r i b u t e d to the s t r u c t u r a l c h a n g e from a Ge-O-Ge c o n f i g u r a t i o n to the o x y g e n - v a c a n c y c o m p l e x c o n f i g u r a tion, w h i c h i n c r e a s e s the n o n b o n d i n g state o f g e r m a n i u m . T h e I R s p e c t r a for r e p r e s e n t a t i v e s a m p l e s are s h o w n in figs. 9 ( a ) - ( d ) .
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Fig. 5. The dependence on the H 2 gas partial pressure of the activation energy E a for a-Ge:O:(H). The broken line denotes E. for a-Ge prepared without H 2 gas.
Y. Takano, H. Ozaki / R F - S P a-Ge.'O.'(H) deposttedJ}'om GeO,
123
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Fig. 6. The dependence on the H 2 gas partial pressure of O'RT for a-Oe:O:(H). The broken line denotes oRw for a-Ge prepared without H 2 gas. Fig. 7. Relation between E,, without H 2 gas.
and
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Y. Takano, H. Ozaki / RF-SP a-Ge:O:(H) deposited from GeO2
124
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Y. Takano, H. Ozaki / R F - S P a-Ge.'O:(H) deposited from GeO,
125
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Fig. 9. 1R spectra for a-Ge films (a), a-Ge:O films (b), a-Ge:O:(H) (0.10) films (c) and a-Ge:O:(H) (0.15) films (d). The meaning of the decimals is the same as in fig. 1.
Firstly, we will discuss the spectra before annealing. Fig. 9(a) shows the I R spectra for a-Ge, which has no absorption peak. Fig. 9(b) shows the 1R spectra for a-Ge:O prepared by sputtering GeO 2 only with Ar gas, which shows an absorption peak near 820 c m - ] . This peak is assigned to the antisymmetric stretching vibrational mode of the G e - O - G e configuration [8]. With the increase of hydrogenation this peak near 820 c m - ] is gradually quenched and a new peak appears near 760 cm-1. This behavior is shown in figs. 9(c) and (d). It is assumed that the broad peak around 800 cm J in fig. 9(c) consists of two peaks of figs. 9(b) and (d). There is no absorption corresponding to the G e - H bond (near 1855 cm-1) [9,10] nor the O - H bond (near 3500 c m - I ) [11] in figs. 9(c) and (d). This indicates that the role of the hydrogen in the plasma is mostly to deoxidize GeO 2 rather than to hydrogenate the sample, although the possibility of the presence of hydrogen in the films is not completely excluded. To distinguish our samples from a-Ge:O which is prepared by sputtering Ge in O 2 gas atmosphere, we use the notation a-Ge:O:(H). The peak at 760 cm-~ in fig. 9(d) is attributed to an oxygen-vacancy complex. Namely, the oxygen in the vacancy becomes threefold coordinated and bonds to the surrounding germanium atoms. This type of complex is reported to occur in the irradiated oxygen-doped crystalline germanium (c-Ge) [8]. It would be possible for such a complex to occur also in the amorphous state. The I R spectra for annealed samples shown in figs. 9(b)-(d) indicate that the annealing shifts the peaks to larger wavenumbers. The shift in figs. 9(b) and (d) is about 20 cm-~, while that in fig. 9(c) is about 60 cm ~. The difference is explained as follows. In a-Ge:O films with high oxygen concentration, most of the oxygen atoms take G e - O - G e configuration and it becomes rigid on annealing. On the other hand, in a-Ge:O:(H) films with lower oxygen
126
Y. Takano, H. Ozaki / RF-SP a-Ge:O.'(H) depositedfrom GeO,
concentration which correspond to higher H 2 gas partial pressure ratio (0.15), most of the oxygen atoms form oxygen-vacancy complexes, which also becomes rigid on annealing. The two types of configuration coexist in a-Ge:O:(H) films prepared with the H 2 gas partial pressure ratio of 0.10. The concentration of oxygen in the oxygen-vacancy complex is higher than that in the G e - O - G e configuration before annealing. The structural change from oxygen-vacancy to Ge O - G e configuration occurs due to the decrease of the numbers of vacancies on annealing. As the oxygen concentration is not so high in this case, the environment of the G e - O - G e configuration resembles that of the G e - O - G e in c-Ge. This would make the peak near 860 c m - ~, as observed in oxygen-doped c-Ge.
4. Conclusions (1) The absence of the IR absorption peak due to G e - H and O - H bonds indicates that the role of the hydrogen in the plasma is mainly to deoxidize GeO 2 rather than to hydrogenate the sample. (2) a-Ge:O:(H) is heat resistant up to 200°C. The shift of the 1R absorption peak on annealing indicates a structural change, though it is not strong enough to cause changes in the electrical and optical properties in the visible region. (3) Although (Eg)op~ of a-Ge:O:(H) prepared in gas with relatively high H~ partial pressure is near to that of a-Ge, N ( E v) of a-Ge:O:(H), derived through Mott's T W4 law, is lower than that of a-Ge by more than one order of magnitude. (4) As the proportion of H 2 gas increases, the structural change from the G e - O - G e configuration to the oxygen-vacancy complex configuration takes place around the H 2 gas partial pressure ratio of 0.10.
Acknowledgements The authors would like to thank N. Akiyama for the preliminary experiments, and N. Kitaoka, R. Sugie, K. Usami, K. Ueda and S. Tanaka for their kind assistances both in sample preparation and measurements.
References [1] W.E. Spear, P.G. LeComber, S. Kinmond and M.H. Brodsky,Appl. Phys. Lett. 28 (1976) 105. [2] D.E. Carlson and C.R. Wronski, Appl. Phys. Lett. 28 (1976) 671. [3] M.A. Paesler, D.A. Anderson, E.C. Freeman, G. Moddel and W. Paul, Phys. Rev. Lett. 41 (1978) 1492. [4] J.C. Knights, R.A. Street and G. Lucovsky,J. Non-CrystallineSolids 35&36 (1980) 279. [5] B.G. Yacobi, R.W. Collins, G. Moddel, P. Viktorovitch and W. Paul, Phys. Rev. B24 (1981) 5907.
Y. Takano, H. Ozaki / R F - S P a-Ge:O:(H) deposited from GeO,
127
[6] T. Shimizu, M. Kumeda, I. Watanabe and K. Kamono, Jpn. J. Appl. Phys. 18 (1979) 1923. [7] N.F. Mort and E.A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1979). [8] R.W. Whan and H.J. Stein, Appl. Phys. Lett. 3 (1963) 187. [9] D. Bermejo and M. Cardona, J. Non-Crystalline Solids 32 (1979) 421. [10] G. Lucovsky, R.J. Nemanich and J.C. Knights, Phys. Rev. B19 (1979) 2064. [11] J.F. Cordaro, J.E. Kelly III and M. Tomozawa, Phys. Chem. Glasses 22 (1981) 90.
119
ELECTRICAL AND OPTICAL PROPERTIES OF RF-SP a-Ge:O:(H) DEPOSITED FROM GeO 2 Y. T A K A N O a n d H. O Z A K I Department of Electrical Engineering. Waseda University, Ohkuho 3- 4-1, Shinjuku, To,~\vo 160, Japan Received 18 October 1982 Revised manuscript received 19 January 1983
The electrical and optical properties of a-Ge:O:(H) prepared from sintered GeO2 are investigated. Unlike SiO2, GeO 2 can be reduced with H 2 gas. The properties of a-Ge:O:(H) thus prepared can be changed widely with the H 2 gas pressure, approaching those of a-Ge. It is found that, in addition to these changes, a configurational change from Ge-O-Ge to oxygen-vacancy complex occurs with increasing the H 2 gas pressure.
1. Introduction
H y d r o g e n a t e d a m o r p h o u s silicon ( a - S i : H ) is a p r o m i s i n g m a t e r i a l for solar cells, etc. [1,2]. However, the i n c o r p o r a t e d h y d r o g e n tends to be effused away from a - S i : H film at elevated temperatures, d a m a g i n g its properties. So, 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 - o x y g e n alloys ( a - S i : O : H ) are a t t r a c t i n g m u c h a t t e n t i o n [3-6]. a - S i : O : H is not m a d e b y the s p u t t e r i n g of SiO 2 target in H 2 gas a t m o s p h e r e . O n the other hand, p o l y c r y s t a l l i n e g e r m a n i u m ( p o l y - G e ) is m a d e b y deoxidizing G e O 2 with H z gas at high temperatures. In this s t u d y G e O 2 was s p u t t e r e d with a H a and A r gas m i x t u r e to m a k e a m o r p h o u s g e r m a n i u m - o x y g e n alloy ( a - G e : O : ( H ) ) . To the a u t h o r s ' knowledge, this is the first r e p o r t of a - G e : O : ( H ) p r e p a r e d from G e O 2.
2. E x p e r i m e n t a l A l l films were d e p o s i t e d b y R F s p u t t e r i n g (13.56 MHz). P o l y - G e a n d sintered G e O 2 were chosen as sputtering targets. A m o r p h o u s g e r m a n i u m ( a - G e ) m a d e from the f o r m e r served as a reference material. T h e system was first p u m p e d to a base pressure of - 1.0 x 10 _6 Torr, then H 2 a n d A r gas m i x t u r e was passed. The total gas pressure was c o n s t a n t at 5 m T o r r a n d the H 2 gas p a r t i a l pressure ratio ( P H 2 / ( P A r + P H 2 ) ) was varied. The films were d e p o s i t e d on K B r disk for I R m e a s u r e m e n t and on u l t r a s o n i c a l l y cleaned C r o w n glass s u b s t r a t e s for o t h e r m e a s u r e m e n t s . The R F p o w e r was 100 W. T h e 0022-3093/83/0000-0000/$03.00
© 1983 N o r t h - H o l l a n d
Y. Takano, H. Ozaki / R F - S P a-Ge.'O.'(H) deposited from GeO 2
120
deposition rate of a-Ge:O:(H) was varied with H 2 gas pressure from about 0.6 A / s for n o H 2 gas to about 0.4 ,~/s for H 2 gas partial pressure ratio of 0.15. The deposition rate of a-Ge was about 1.0 A/s. The substrate temperature during deposition was nominally room temperature. The films were annealed at 200°C for 30 rain in a N 2 gas atmosphere.
3. Results and discussion
The dc electrical conductivities (o) are shown in figs. 1 and 2. In the high-temperature region ( T > 250 K) a's are of activated type as shown in fig. 1, and in the low-temperature region ( T < 220 K ) o ' s fit well Mott's T 1/4 law [7] as shown in fig. 2. It is also seen in these figures that the annealing reduces o by about one order for a-Ge but scarcely changes o for a-Ge:O(H). Separate films prepared simultaneously were used to measure o's before and after annealing, in order to avoid the diffusion of the electrode material (Sn) during deposition. So the small change in o for a-Ge:O:(H) by annealing seems to be a sample dependence. The plots of (ahu) I/2 versus hp for two representative films of a-Ge and a-Ge:O:(H) are shown in figs. 3 (a) and (b), respectively. The optical energy gap, (Eg)opt, is obtained as the point of intersection of the extrapolated line with the abscissa. Annealing shifts (Eg)o m by about 0.1 eV towards higher energies for a-Ge but no shift is seen for a-Ge:O:(H). This shows that the oxygen atoms incorporated make a-Ge:O:(H) stable against the heat treatment up to 200°C. Figs. 4, 5 and 6 show the dependence on the H 2 gas partial pressure of the activation energy E,, (Eg)opt and the room temperature dc electrical conductivity ORT, respectively. Fig. 7 shows the relation between (Eg)opt and E. obtained
-2
-2
-3
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;
; (K-')
Fig. I. dc electrical conductivities versus IO00/T for a-Oe films (a) and a-Oe:O:(H) (0.10) films (b). Decimals in parentheses denote the H 2 gas partial pressure ratio (Prt2/( PAr + PH2))"
Y. Takano, H, Ozaki / RF-SP a-Ge.'O.'(H) deposited from GeO2 -2
-2
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X
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-9
(annealed)
I
I
I
I
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0.24
0.26
0.28
T-~/4
(K-1/4)
0.30
( K -1/4 )
Fig. 2. dc electrical conductivities versus T - i / 4 for a-Ge films (a) and a-Ge:O:(H) (0.10) films (b). T h e m e a n i n g of the decimals is the same as in fig. 1.
from figs. 4 and 5. It is seen that the electrical and optical properties of a-Ge:O:(H) approach those of a-Ge with increasing the H 2 gas partial pressure. a-Ge:O sputtered only with Ar gas is transparent and its (Eg)op, cannot be measured with our monochromator. Fig. 8 shows the variation of the density of states at the Fermi level (N(EF)) derived from Mott's T 1/4 law, as a function of H 2 gas partial pressure ratio, assuming the decay constant of the wavefunctions as 10 ,X,. While the N(EF) of a-Ge is about 10 Is eV 'cm -L that of a-Ge:O:(H) is less by more than one order. This change can be attributed to the reduction of the dangling bonds due to the softening of the lattice induced by oxygen which takes up a G e - O - G e configuration. The enhancement of the
X 102
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4
4 • a-Ge a-Ge
3
/"
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,//
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/
//
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> c~
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/,/,
1.0 1.5 h~ (eV)
2.0
0 I
0.5
!
7/ •
1.0 hv ( e V )
I
1.5
2.0
Fig. 3. Typical (ahv) 1/2 versus hu plots for a - G e films (a) and a-Ge:O:(H) (0.10) films (b). The m e a n i n g of the d e c i m a l s is the s a m e as in fig. 1.
122
Y. Takano, H. Ozaki / RF-SP a-Ge:O:(H) deposited from GeO2
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>~ UJ
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PHa //( PAr "t" eHz ) Fig. 4. The dependence on the H 2 gas partial pressure of the optical energy gap (Eg)op t for a-Ge:O:(H). The broken line denotes (Eg)opt for a-Ge prepared without H 2 gas.
N ( E F ) at h i g h e r H 2 gas p a r t i a l p r e s s u r e s is a t t r i b u t e d to the s t r u c t u r a l c h a n g e from a Ge-O-Ge c o n f i g u r a t i o n to the o x y g e n - v a c a n c y c o m p l e x c o n f i g u r a tion, w h i c h i n c r e a s e s the n o n b o n d i n g state o f g e r m a n i u m . T h e I R s p e c t r a for r e p r e s e n t a t i v e s a m p l e s are s h o w n in figs. 9 ( a ) - ( d ) .
2.0
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0
0.05
i
0.10
t
i
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0.20
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Fig. 5. The dependence on the H 2 gas partial pressure of the activation energy E a for a-Ge:O:(H). The broken line denotes E. for a-Ge prepared without H 2 gas.
Y. Takano, H. Ozaki / R F - S P a-Ge.'O.'(H) deposttedJ}'om GeO,
123
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Fig. 6. The dependence on the H 2 gas partial pressure of O'RT for a-Oe:O:(H). The broken line denotes oRw for a-Ge prepared without H 2 gas. Fig. 7. Relation between E,, without H 2 gas.
and
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Y. Takano, H. Ozaki / RF-SP a-Ge:O:(H) deposited from GeO2
124
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Y. Takano, H. Ozaki / R F - S P a-Ge.'O:(H) deposited from GeO,
125
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Fig. 9. 1R spectra for a-Ge films (a), a-Ge:O films (b), a-Ge:O:(H) (0.10) films (c) and a-Ge:O:(H) (0.15) films (d). The meaning of the decimals is the same as in fig. 1.
Firstly, we will discuss the spectra before annealing. Fig. 9(a) shows the I R spectra for a-Ge, which has no absorption peak. Fig. 9(b) shows the 1R spectra for a-Ge:O prepared by sputtering GeO 2 only with Ar gas, which shows an absorption peak near 820 c m - ] . This peak is assigned to the antisymmetric stretching vibrational mode of the G e - O - G e configuration [8]. With the increase of hydrogenation this peak near 820 c m - ] is gradually quenched and a new peak appears near 760 cm-1. This behavior is shown in figs. 9(c) and (d). It is assumed that the broad peak around 800 cm J in fig. 9(c) consists of two peaks of figs. 9(b) and (d). There is no absorption corresponding to the G e - H bond (near 1855 cm-1) [9,10] nor the O - H bond (near 3500 c m - I ) [11] in figs. 9(c) and (d). This indicates that the role of the hydrogen in the plasma is mostly to deoxidize GeO 2 rather than to hydrogenate the sample, although the possibility of the presence of hydrogen in the films is not completely excluded. To distinguish our samples from a-Ge:O which is prepared by sputtering Ge in O 2 gas atmosphere, we use the notation a-Ge:O:(H). The peak at 760 cm-~ in fig. 9(d) is attributed to an oxygen-vacancy complex. Namely, the oxygen in the vacancy becomes threefold coordinated and bonds to the surrounding germanium atoms. This type of complex is reported to occur in the irradiated oxygen-doped crystalline germanium (c-Ge) [8]. It would be possible for such a complex to occur also in the amorphous state. The I R spectra for annealed samples shown in figs. 9(b)-(d) indicate that the annealing shifts the peaks to larger wavenumbers. The shift in figs. 9(b) and (d) is about 20 cm-~, while that in fig. 9(c) is about 60 cm ~. The difference is explained as follows. In a-Ge:O films with high oxygen concentration, most of the oxygen atoms take G e - O - G e configuration and it becomes rigid on annealing. On the other hand, in a-Ge:O:(H) films with lower oxygen
126
Y. Takano, H. Ozaki / RF-SP a-Ge:O.'(H) depositedfrom GeO,
concentration which correspond to higher H 2 gas partial pressure ratio (0.15), most of the oxygen atoms form oxygen-vacancy complexes, which also becomes rigid on annealing. The two types of configuration coexist in a-Ge:O:(H) films prepared with the H 2 gas partial pressure ratio of 0.10. The concentration of oxygen in the oxygen-vacancy complex is higher than that in the G e - O - G e configuration before annealing. The structural change from oxygen-vacancy to Ge O - G e configuration occurs due to the decrease of the numbers of vacancies on annealing. As the oxygen concentration is not so high in this case, the environment of the G e - O - G e configuration resembles that of the G e - O - G e in c-Ge. This would make the peak near 860 c m - ~, as observed in oxygen-doped c-Ge.
4. Conclusions (1) The absence of the IR absorption peak due to G e - H and O - H bonds indicates that the role of the hydrogen in the plasma is mainly to deoxidize GeO 2 rather than to hydrogenate the sample. (2) a-Ge:O:(H) is heat resistant up to 200°C. The shift of the 1R absorption peak on annealing indicates a structural change, though it is not strong enough to cause changes in the electrical and optical properties in the visible region. (3) Although (Eg)op~ of a-Ge:O:(H) prepared in gas with relatively high H~ partial pressure is near to that of a-Ge, N ( E v) of a-Ge:O:(H), derived through Mott's T W4 law, is lower than that of a-Ge by more than one order of magnitude. (4) As the proportion of H 2 gas increases, the structural change from the G e - O - G e configuration to the oxygen-vacancy complex configuration takes place around the H 2 gas partial pressure ratio of 0.10.
Acknowledgements The authors would like to thank N. Akiyama for the preliminary experiments, and N. Kitaoka, R. Sugie, K. Usami, K. Ueda and S. Tanaka for their kind assistances both in sample preparation and measurements.
References [1] W.E. Spear, P.G. LeComber, S. Kinmond and M.H. Brodsky,Appl. Phys. Lett. 28 (1976) 105. [2] D.E. Carlson and C.R. Wronski, Appl. Phys. Lett. 28 (1976) 671. [3] M.A. Paesler, D.A. Anderson, E.C. Freeman, G. Moddel and W. Paul, Phys. Rev. Lett. 41 (1978) 1492. [4] J.C. Knights, R.A. Street and G. Lucovsky,J. Non-CrystallineSolids 35&36 (1980) 279. [5] B.G. Yacobi, R.W. Collins, G. Moddel, P. Viktorovitch and W. Paul, Phys. Rev. B24 (1981) 5907.
Y. Takano, H. Ozaki / R F - S P a-Ge:O:(H) deposited from GeO,
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[6] T. Shimizu, M. Kumeda, I. Watanabe and K. Kamono, Jpn. J. Appl. Phys. 18 (1979) 1923. [7] N.F. Mort and E.A. Davis, Electronic Processes in Non-Crystalline Materials (Clarendon Press, Oxford, 1979). [8] R.W. Whan and H.J. Stein, Appl. Phys. Lett. 3 (1963) 187. [9] D. Bermejo and M. Cardona, J. Non-Crystalline Solids 32 (1979) 421. [10] G. Lucovsky, R.J. Nemanich and J.C. Knights, Phys. Rev. B19 (1979) 2064. [11] J.F. Cordaro, J.E. Kelly III and M. Tomozawa, Phys. Chem. Glasses 22 (1981) 90.