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The stretching modes of the Si-H and Ge-H bonds in amorphous and crystalline Ge and Si
Journal of Non-Crystalline Solids 59 & 60 (1983) 181-184 North-Holland Publishing Company
181
THE STRETCHING MODES OF THE Si-H AND Ge-H BONDS IN AMORPHOUSAND CRYSTALLINE Ge AND Si H. RICHTER+ , J. TRODAHL++, and M. CARDONA Max-Planck-lnstitut fur Festk~rperforschung, Heisenbergstrasse I , 7000 S t u t t g a r t 80, Federal Republic of Germany The infrared absorption spectra of hydrogen containing pc-Si and pc-Ge prepared by plasma transport are reported and discussed in terms of v i b r a t i o n s of Si-H bonds at grain boundaries and internal surfaces. 1. INTRODUCTION The stretching vibrations of hydrogenated amorphous Si(a-Si:H) are s p l i t into two components, at ~2000 cm- I and 2100 cm-l. 1'2 The lower frequency is often associated with single Si-H bonds while the higher one is a t t r i b u t e d to Si-H 2 r a d i c a l s .
•
,2,3
I t has, however, been recognlzea
that the 2100 cm- I v i b r a -
tions can also be related to single Si-H bonds, as suggested by the absence of bending modes (~x~O0 cm- I ) c h a r a c t e r i s t i c of Si-H 2.
Measurements on single cry-
stal Si surfaces also place the Si-H stretching mode at around 2100 cm- I .
Si-
m i l a r results have been obtained f o r hydrogen in m i c r o c r y s t a l l i n e Si (~c-Si) presumably bonded to Si at the grain boundaries 6'7. I t has been recently suggested 2 that the 2000 cm- 1 S i - H stretching peak of a-Si corresponds to H atoms "dissolved" in the bulk of the material ( s o l u b i l i t y -I The 2100 cm
~5%) occupying a space which roughly corresponds to a Si-vacancy.
mode corresponds indeed in some samples to the stretching modes of Si-H 2.
De-
pending on preparation conditions, however, i t is possible to obtain a peak at 2100 cm- I without i t s bond bending counterpart (~900 cm-1).
This peak can be
a t t r i b u t e d to single Si-H bonds at internal surfaces ( e . g . , voids, multivacancies).
The amount of these Si-H bonds is not l i m i t e d by s o l u b i l i t y but by the
area of free surface a v a i l a b l e .
In t h i s manner, i t is possible to reconcile the
results f o r a-Si with those f o r pc-Si and c-Si surfaces.
The stretching modes
of H in single crystal Si, however, are considerably more complicated and, as yet, not f u l l y understood 8. In t h i s paper, we present data f o r the i r absorption spectrum of pc-Si and pc-Ge prepared by the plasma transport method 9.
For pc-Si we observe bond
stretching peaks at 2085 and at 2100 cm- I while f o r pc-Ge structures are ob+Present address: Xerox Research Corporation, 800 P h i l l i p s Road, W114, Webster, N.Y. 14607, U.S.A. ++ . . . . V i c t o r i a University of Wellington, Private Bag, Wellington, NEW ZEALAND 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
182
H. Richter et al.
/
The stretching modes o f the Si.'H and Ge:H bonds
served a t 1975, 1996, and 2018 cm- I . 800 cm- I f o r Ge) are absent.
Bond bending peaks (~900 cm- I f o r S i ,
By comparison with the work of Refs. 4 and 5
these v i b r a t i o n s are assigned to Si-H bonds at (111), ( i i 0 ) ,
and (I00) i n t e r n a l
surfaces. 2. SAMPLE PREPARATION The samples were prepared in the plasma t r a n s p o r t r e a c t o r described elsewhereI0 on c - S i ( ~ c - S i ) and e i t h e r c-Si or c-Ge substrates (~c-Ge).
The H-
pressure was kept at ~ 0 . i t o r r while the discharge voltage was held at ~500 V. The source temperature was ~IOOC. The substrate temperature lay between 150 and 450°C f o r uc-Si and between 120 and 325°C f o r pc-Ge. the f i l m s were between 0.5 and I ~m.
The thicknesses of
They were shown to be m i c r o c r y s t a l l i n e by
means of Raman s c a t t e r i n g and x - r a y d i f f r a c t i o n . 3. RESULTS AND DISCUSSION Figure I shows the absorption c o e f f i cients (in a r b i t r a r y u n i t s ) of several ~c-Si samples prepared at d i f f e r e n t subs t r a t e temperatures T s.
From the i n t e -
grated strength of these bands, and using the o s c i l l a t o r 2100 mode of
2085cm -1
IJ, c-Si
I f 2100crn "1
strength f o r the Si-H
a-Sigiven in Ref. 2, we
f i n d a hydrogen concentration CH = 6% f o r Ts = 150°C and CH = 0.5% f o r T s = 450°C.
Figure i indicates v i b r a t i o n a l
frequencies at 2085 and 2100 cm- I ,
the
~ 250°C
higher one being dominant f o r high T
s" The peaks in Fig. I are s i m i l a r to those
~,~50°C
reported in Ref. 6 f o r r e a c t i v e l y sputtered ~c-Si.
In t h i s reference, however,
~,,450"C I
bond bending bands were seen between 850 and 900 cm" I .
No such bands were
found in our samples.
2000
2100
WAVENUMBER
2200
( cm -1)
The bond s t r e t c h i n g bands of pc-Ge, prepared at d i f f e r e n t Ts, are shown in Fig. 2.
The hydrogen concentrations
determined from the strengths of the wagging modes at 640 cm- I are 2 CH = 1.5% f o r
FIGURE 1 I n f r a r e d absorption spectra of the bond s t r e t c h i n g v i b r a t i o n s of pc-Si prepared with d i f f e r e n t substrate temperatures
Ts = 120°C, CH = 1.0% f o r Ts = 160°C and Ts = 180°C. In Fig. 2 structures are seen at 1975, 1996, and 2018 cm-1. We were not able to pin down the systematics
H. Richter et al. / The stretching modes o f the Si:H and Ge:H bonds
183
of the v a r i a t i o n of these structures with preparation c o n d i t i o n s .
I t seems,
however, t h a t the lower frequency structures ( e s p e c i a l l y t h a t at 1996 cm- I )
1996crn-1
are enhanced f o r low T
in a manner s s i m i l a r to t h a t of Fig. I . I t is thus
tc-Ge 1975cm-1 ~
reasonable to associate the 2085 and
0oC
the 2110 cm- I frequencies of Fig. i with the 1996 and 2018 cm- I of Fig. 2, respectively.
/
The 1975 cm- I frequency
\. 16ooc
/2018cn~"I
of Fig. 2 has no partner in Fig, I . I t may, however, correspond to s t r u c ture at ~2065 cm- I reported in Ref. 8. The corresponding frequencies f o r hydro-
, ./,
gen "dissolved" in the amorphous semiconductors 2 are 1895 cm- I f o r a-Ge and
1900 2000 2100 WAVENUMBER ( crn-1 )
2000 cm- I f o r a - S i , in both cases about I00 cm- I lower than the ones reported above. Because of the s i m i l a r i t y of the frequencies of Fig. I with those of Ref. 4 and the lack of bond bending bands, we t e n t a t i v e l y i d e n t i f y
FIGURE 2 Infrared absorption spectra of the bond s t r e t c h i n g bands of uc-Ge prepared with d i f f e r e n t substrate temperature. The strengths are normalized so as to y i e l d equal a~eas f o r the wagging peaks at 640 cm
the 2100 cm- I mode with the s t r e t c h ing of Si-H bonds at (lO0)Si surfaces.
In Ref. 4 a mode at 2087 cm-1 is a t t r i -
buted to the asymmetric stretching of SiH 2. 2085 cm- I mode of Fig. 2.
I t is tempting to r e l a t e i t to the
This would mean, however, t h a t we have f a i l e d to ob-
serve the corresponding bending modes due, f o r instance, to the f a c t t h a t the d i p o l e moment of these modes happens to be perpendicular to the f i l m surface.
We
then r e l a t e the Si-H s t r e t c h i n g mode a t 2077 cm" I , a t t r i b u t e d in Ref. 5 to Si-H at (111) surfaces, to the 1975 cm- I mode of Ge-H (Fig. 2).
This v i b r a t i o n would -I The 2085 cm
thus be due to Ge-H bonds at (111) i n t e r n a l surfaces in the f i l m . mode of Fig. i could also be due to Si-H at (110) surfaces.
We should mention t h a t i n f r a r e d e f f e c t i v e charges e~ between O.Ole and O.02e were found from the attenuated t o t a l r e f l e c t i o n data of Refs. 4 and 5 w h i l e the r e s u l t s reported here suggest much l a r g e r values of e~ (~O.3e).
An independent
determination of the hydrogen concentration of our samples, e i t h e r by effusion or by the nuclear reactions technique, should be performed in order to c l a r i f y the matter.
1t. Richter et al. / The stretching modes o f the Si:H and Ge:H bonds
184
We thank L. Ley for many discussions and A. Breitschwerdt for performing the infrared measurements, and L. Viczian and K. Peters for the x-ray analysis. REFERENCES i) M. Brodsky, M. Cardona, and J.J. Cuomo, Phys. Rev. B 13 (1976) 787. 2) M. Cardona, Phys. Stat. Sol. (b), in press, and references therein. 3) W. Paul, Solid State Commun. 34 (1980) 283. 4) Y.J. Chabal, E.E. Chaban, and S.B. Christman, J. Electron Spectr. 29 (1983) 35. 5) Y.J. Chabal, Phys. Rev. Letters 50 (1983) 1850. 6) A. Hiraki, T. Imura, K. Mogi, and M. Tashiro, J. Physique C4 (1981) 277. 7) H. Richter, Ph.D. Thesis, University of Stuttgart, 1982. 8) T.S. Shi, S.N. Sahu, G.S. Oehrlein, A. Hiraki, and J.W. Corbett, Phys. Star Sol. a 74
(1982) 329.
9) S. Veprek, Z. lqbal, H.R. Oswald, and A.P. Webb, J. Phys. C14 (1981) 295. I0) H. Richter and L. Ley, J. Appl. Phys. 52 (1981) 7281.
181
THE STRETCHING MODES OF THE Si-H AND Ge-H BONDS IN AMORPHOUSAND CRYSTALLINE Ge AND Si H. RICHTER+ , J. TRODAHL++, and M. CARDONA Max-Planck-lnstitut fur Festk~rperforschung, Heisenbergstrasse I , 7000 S t u t t g a r t 80, Federal Republic of Germany The infrared absorption spectra of hydrogen containing pc-Si and pc-Ge prepared by plasma transport are reported and discussed in terms of v i b r a t i o n s of Si-H bonds at grain boundaries and internal surfaces. 1. INTRODUCTION The stretching vibrations of hydrogenated amorphous Si(a-Si:H) are s p l i t into two components, at ~2000 cm- I and 2100 cm-l. 1'2 The lower frequency is often associated with single Si-H bonds while the higher one is a t t r i b u t e d to Si-H 2 r a d i c a l s .
•
,2,3
I t has, however, been recognlzea
that the 2100 cm- I v i b r a -
tions can also be related to single Si-H bonds, as suggested by the absence of bending modes (~x~O0 cm- I ) c h a r a c t e r i s t i c of Si-H 2.
Measurements on single cry-
stal Si surfaces also place the Si-H stretching mode at around 2100 cm- I .
Si-
m i l a r results have been obtained f o r hydrogen in m i c r o c r y s t a l l i n e Si (~c-Si) presumably bonded to Si at the grain boundaries 6'7. I t has been recently suggested 2 that the 2000 cm- 1 S i - H stretching peak of a-Si corresponds to H atoms "dissolved" in the bulk of the material ( s o l u b i l i t y -I The 2100 cm
~5%) occupying a space which roughly corresponds to a Si-vacancy.
mode corresponds indeed in some samples to the stretching modes of Si-H 2.
De-
pending on preparation conditions, however, i t is possible to obtain a peak at 2100 cm- I without i t s bond bending counterpart (~900 cm-1).
This peak can be
a t t r i b u t e d to single Si-H bonds at internal surfaces ( e . g . , voids, multivacancies).
The amount of these Si-H bonds is not l i m i t e d by s o l u b i l i t y but by the
area of free surface a v a i l a b l e .
In t h i s manner, i t is possible to reconcile the
results f o r a-Si with those f o r pc-Si and c-Si surfaces.
The stretching modes
of H in single crystal Si, however, are considerably more complicated and, as yet, not f u l l y understood 8. In t h i s paper, we present data f o r the i r absorption spectrum of pc-Si and pc-Ge prepared by the plasma transport method 9.
For pc-Si we observe bond
stretching peaks at 2085 and at 2100 cm- I while f o r pc-Ge structures are ob+Present address: Xerox Research Corporation, 800 P h i l l i p s Road, W114, Webster, N.Y. 14607, U.S.A. ++ . . . . V i c t o r i a University of Wellington, Private Bag, Wellington, NEW ZEALAND 0022-3093/83/0000-0000/$03.00 © 1983 North-Holland/Physical Society of Japan
182
H. Richter et al.
/
The stretching modes o f the Si.'H and Ge:H bonds
served a t 1975, 1996, and 2018 cm- I . 800 cm- I f o r Ge) are absent.
Bond bending peaks (~900 cm- I f o r S i ,
By comparison with the work of Refs. 4 and 5
these v i b r a t i o n s are assigned to Si-H bonds at (111), ( i i 0 ) ,
and (I00) i n t e r n a l
surfaces. 2. SAMPLE PREPARATION The samples were prepared in the plasma t r a n s p o r t r e a c t o r described elsewhereI0 on c - S i ( ~ c - S i ) and e i t h e r c-Si or c-Ge substrates (~c-Ge).
The H-
pressure was kept at ~ 0 . i t o r r while the discharge voltage was held at ~500 V. The source temperature was ~IOOC. The substrate temperature lay between 150 and 450°C f o r uc-Si and between 120 and 325°C f o r pc-Ge. the f i l m s were between 0.5 and I ~m.
The thicknesses of
They were shown to be m i c r o c r y s t a l l i n e by
means of Raman s c a t t e r i n g and x - r a y d i f f r a c t i o n . 3. RESULTS AND DISCUSSION Figure I shows the absorption c o e f f i cients (in a r b i t r a r y u n i t s ) of several ~c-Si samples prepared at d i f f e r e n t subs t r a t e temperatures T s.
From the i n t e -
grated strength of these bands, and using the o s c i l l a t o r 2100 mode of
2085cm -1
IJ, c-Si
I f 2100crn "1
strength f o r the Si-H
a-Sigiven in Ref. 2, we
f i n d a hydrogen concentration CH = 6% f o r Ts = 150°C and CH = 0.5% f o r T s = 450°C.
Figure i indicates v i b r a t i o n a l
frequencies at 2085 and 2100 cm- I ,
the
~ 250°C
higher one being dominant f o r high T
s" The peaks in Fig. I are s i m i l a r to those
~,~50°C
reported in Ref. 6 f o r r e a c t i v e l y sputtered ~c-Si.
In t h i s reference, however,
~,,450"C I
bond bending bands were seen between 850 and 900 cm" I .
No such bands were
found in our samples.
2000
2100
WAVENUMBER
2200
( cm -1)
The bond s t r e t c h i n g bands of pc-Ge, prepared at d i f f e r e n t Ts, are shown in Fig. 2.
The hydrogen concentrations
determined from the strengths of the wagging modes at 640 cm- I are 2 CH = 1.5% f o r
FIGURE 1 I n f r a r e d absorption spectra of the bond s t r e t c h i n g v i b r a t i o n s of pc-Si prepared with d i f f e r e n t substrate temperatures
Ts = 120°C, CH = 1.0% f o r Ts = 160°C and Ts = 180°C. In Fig. 2 structures are seen at 1975, 1996, and 2018 cm-1. We were not able to pin down the systematics
H. Richter et al. / The stretching modes o f the Si:H and Ge:H bonds
183
of the v a r i a t i o n of these structures with preparation c o n d i t i o n s .
I t seems,
however, t h a t the lower frequency structures ( e s p e c i a l l y t h a t at 1996 cm- I )
1996crn-1
are enhanced f o r low T
in a manner s s i m i l a r to t h a t of Fig. I . I t is thus
tc-Ge 1975cm-1 ~
reasonable to associate the 2085 and
0oC
the 2110 cm- I frequencies of Fig. i with the 1996 and 2018 cm- I of Fig. 2, respectively.
/
The 1975 cm- I frequency
\. 16ooc
/2018cn~"I
of Fig. 2 has no partner in Fig, I . I t may, however, correspond to s t r u c ture at ~2065 cm- I reported in Ref. 8. The corresponding frequencies f o r hydro-
, ./,
gen "dissolved" in the amorphous semiconductors 2 are 1895 cm- I f o r a-Ge and
1900 2000 2100 WAVENUMBER ( crn-1 )
2000 cm- I f o r a - S i , in both cases about I00 cm- I lower than the ones reported above. Because of the s i m i l a r i t y of the frequencies of Fig. I with those of Ref. 4 and the lack of bond bending bands, we t e n t a t i v e l y i d e n t i f y
FIGURE 2 Infrared absorption spectra of the bond s t r e t c h i n g bands of uc-Ge prepared with d i f f e r e n t substrate temperature. The strengths are normalized so as to y i e l d equal a~eas f o r the wagging peaks at 640 cm
the 2100 cm- I mode with the s t r e t c h ing of Si-H bonds at (lO0)Si surfaces.
In Ref. 4 a mode at 2087 cm-1 is a t t r i -
buted to the asymmetric stretching of SiH 2. 2085 cm- I mode of Fig. 2.
I t is tempting to r e l a t e i t to the
This would mean, however, t h a t we have f a i l e d to ob-
serve the corresponding bending modes due, f o r instance, to the f a c t t h a t the d i p o l e moment of these modes happens to be perpendicular to the f i l m surface.
We
then r e l a t e the Si-H s t r e t c h i n g mode a t 2077 cm" I , a t t r i b u t e d in Ref. 5 to Si-H at (111) surfaces, to the 1975 cm- I mode of Ge-H (Fig. 2).
This v i b r a t i o n would -I The 2085 cm
thus be due to Ge-H bonds at (111) i n t e r n a l surfaces in the f i l m . mode of Fig. i could also be due to Si-H at (110) surfaces.
We should mention t h a t i n f r a r e d e f f e c t i v e charges e~ between O.Ole and O.02e were found from the attenuated t o t a l r e f l e c t i o n data of Refs. 4 and 5 w h i l e the r e s u l t s reported here suggest much l a r g e r values of e~ (~O.3e).
An independent
determination of the hydrogen concentration of our samples, e i t h e r by effusion or by the nuclear reactions technique, should be performed in order to c l a r i f y the matter.
1t. Richter et al. / The stretching modes o f the Si:H and Ge:H bonds
184
We thank L. Ley for many discussions and A. Breitschwerdt for performing the infrared measurements, and L. Viczian and K. Peters for the x-ray analysis. REFERENCES i) M. Brodsky, M. Cardona, and J.J. Cuomo, Phys. Rev. B 13 (1976) 787. 2) M. Cardona, Phys. Stat. Sol. (b), in press, and references therein. 3) W. Paul, Solid State Commun. 34 (1980) 283. 4) Y.J. Chabal, E.E. Chaban, and S.B. Christman, J. Electron Spectr. 29 (1983) 35. 5) Y.J. Chabal, Phys. Rev. Letters 50 (1983) 1850. 6) A. Hiraki, T. Imura, K. Mogi, and M. Tashiro, J. Physique C4 (1981) 277. 7) H. Richter, Ph.D. Thesis, University of Stuttgart, 1982. 8) T.S. Shi, S.N. Sahu, G.S. Oehrlein, A. Hiraki, and J.W. Corbett, Phys. Star Sol. a 74
(1982) 329.
9) S. Veprek, Z. lqbal, H.R. Oswald, and A.P. Webb, J. Phys. C14 (1981) 295. I0) H. Richter and L. Ley, J. Appl. Phys. 52 (1981) 7281.