- Email: [email protected]
Incorporation scheme of H reducing defects in a-Si studied by NMR and ESR
Physica 117B & 118B (1983) 926-928 North-Holland Publishing Company
926
INCORPORATION SCHEME OF H REDUCING DEFECTS IN a-Si STUDIED BY NMR AND ESR
Tatsuo Shimizu, Kenji Nakazawa, Minoru Kumeda and Shoichi Ueda Department of Electronics, Faculty of Technology, Kanazawa 920, Japan
Kanazawa University,
By comparing the results of NMR, ESR and photoconductivity measurements, it is found that dispersed H atoms bonded to Si mainly play a role in reducing the density of dangling bonds Ns. Gathered H and F atoms appear to decrease the mobility. It is also concluded that the decrease in N s with an increase in substrate temperature T s does not result from a direct thermal relaxation but from a different H incorporation scheme at a higher T s.
i. INTRODUCTION In order to investigate the role of H and F in a-Si, we have carried out systematic studies of H and F NMR for a-Si:H, a-Si:(F,H) and a-Si:F films prepared by various methods and the results are compared with the results of ESR, photoconductivity and infrared (IR) absorption measurements. The NMR spectra of H and F are known to have a broad and a narrow component [1-5]. The former originates from closely gathered H or F in the form of SiH (SiF), SiH 2 (SiF2) etc. and the latter originates from dispersed H or F in the form of SiH or SiF. 2. EXPERIMENTAL
Sample films were prepared under various conditions as follows, a-Si:H films were prepared by glow discharge decomposition of SiH 4 or SiH 4 + Ar mixture or by reactive sputtering of Si in Ar + H 2 mixture, a-Si:(F,H) films were prepared by glow discharge decomposition of SiH 4 + H2 mixture or SiF 4 + SiH4 + Ar mixture, a-Si:F films were prepared by reactive sputtering of Si in Ar + SiF 4 mixture. Preparation conditions of most of these films are briefly summarized in Ref. 5. Sample films were deposited on aluminum foil substrates for NMR and ESR measurements and on crystalline Si substrates for IR absorption measurements. The aluminum foil substrates were dissolved in HCI solution. NMR measurements were carried out by a pulse method at 16 MHz.
(a) 1020
(b)
1020 li
•
1019
a - S i :H a - S i : (F,H) Carlos-Taylor
•
! I I
I:
eOO
a-Si:H a - S i : (F,H) Carlos-Taylor
1019
o
o
o I
°
•o
IE O 1018
1018
l
o%
O
Z o
•
o
o
o
Z •
0
1017
1017 •
•
oo
o o o
o °o
oo
o •
•
•
•
1016
0
•
i
I
I
I
5
i0
15
20
[H] t Fig.
1.
1016
25
0
I
I
I
I
1
2
3
4
[H~
(at. %)
(at.%)
(a) N s v e r s u s [H] t a n d (b) a l s o s h o w n a f t e r Ref. 2.
N s versus
0 378-4363/83/0000-0000/$03.00 © 1983 North-Holland
[H] n.
Data
by
Carlos
and
Taylor
are
T. Shimizu et al. / Incorporation scheme o f H reducing defects in a-Si
927
A part of the present result has already been published[6].
ranges from 2 to 5 kHz and that of the broad component ranges from 15 to 34 kHz,
3. RESULTS AND DISCUSSION
From the measurement of the IR absorption due to Si-H vibrations, SiH, SiH2, (SiH2) n and SiH 3 can be characterized by the position of the absorption peak. The peak near 2000 cm -I is identified as SiH and that near 2100 cm -I is identified as SiH 2 etc. Knights et al. reported that the intensity of the IR peak around 2000 cm -I is well correlated with N s for a-Si:H prepared by glow discharge decomposition of SiH 4 with various substrate temperature[7]. The correlation, however, was found to be not good for a-Si:H films prepared under a wide variety of preparation methods. A close correlation between N s and [H]n shows that only dispersed SiH plays a role in reducing N s. Gathered SiH's contributing to the 2000 cm -I IR absorption peak exhibit the broad NMR line.
Figures i (a) and (b) show the center density of ESR due to dangling bonds, Ns, versus the total H content, [H]t , deduced from NMR and the content of H contributing to the narrow component of NMR spectra, [H]n, respectively. There appears a tendency that N s decreases with an increase in [H] t up to I0 ~ 15 at.% and then increases, but the data are very scattered and the correlation between N s and [H]t is not good. On the other hand, it is found that [H]n has a close correlation with N s. N s decreases monotonically with an increase in [H]n. Some samples shown in these figures contain F in addition to H, but it appears that N s is determined only by [H] n and F in the film has nothing to do with N s. In a-Si:(F,H) films used in the present work, the broad component in F NMR spectra is dominant. As for a-Si:F films used in the present work, the narrow component of F NMR spectra is present ~ 4 at.%), and is supposed to play a role in reducing Ns because N s is 6 x 1017 cm-3 which is more than one order of magnitude smaller than N s for a-Si with neither H nor F. The ability of reducing N s by F, however, appears to be weaker than that by H. In some cases, the linewidth narrows at higher temperature (even at liquid nitrogen temperature), showing the motional narrowing. Therefore, it should be ascertained that the narrow component is not the motionally narrowed one. The magnitude of the linewidth of the narrow component
Changes in Ns, [H]t , [H]n and [H]b (the content of H contributing to the broad component of NMR spectra) with substrate temperature T s are shown in Fig. 2 (a) and (b) for a-Si:H prepared by glow discharge decomposition of SiH 4 and for a-Si:H prepared by reactive sputtering of Si in Ar + H 2 mixture, respectively. In the latter case, H 2 partial pressure was fixed at 40 %. In both glow discharge decomposed a-Si:H and reactively sputtered a-Si:H, [H]t , [H]b and N s decrease and [H] n increases with an increase in T s. It should be noted that [H]n/[H]b, however, is larger for the former than for the latter at the same T s. Previously, we reported that N s decreases with an increase in the H
(a) GD a-Si:H
(b) •
N
O
SP a-Si:H
•
N
s
O
[H]t
o
i
1018 l
[H~
[ [Hi
o
l o 17
25
o
lo i7
25 z
Z
]_o16 ~20
v 20 4~
4~
© 15 o o
0 o m 10
10
"eL
5 \
.4P'" ~ . . . . .
O0 Fig.
2.
I
I
100 200 Temperature
~"
I
300 (°C)
•
00
40
N s , [H] t , [ H ~ and [H~ versus T s for decomposition a n d (b) r f s p u t t e r i n g .
a-Si:H
l~0 2d0 Temperature (°C) prepared
by
(a)
glow
300
discharge
928
T. Shimizu et al. / Incorporation scheme o f H reducing defects in a-Si
content for a-Si:H films prepared by rf sputtering in Ar + H 2 mixture with both a water-cooled substrate and a substrate heated at 250°C[8]. The decrease in N s is far more remarkable for films w i t h a substrate heated at 250°C than those with a water-cooled substrate. We attributed such difference to thermal relaxation[g]. Figure 2 (b), however, shows that [H] n is larger for films with a higher substrate temperature. Therefore, we may conclude that no direct thermal relaxation is working in reducing N , but the different incorporation scheme of HSat a higher temperature makes N s smaller. The fact that Ns depends only weakly on T s for a-Si without H[8] supports this conclusion. The results shown in Fig. 2 (a) for glow discharge decomposed a-Si:H is also consistent with this conclusion. Closely gathered SiH (SiF), SiH 2 (SiF 2) etc., are expected to make the band tail larger, working as a scatterer or a trap for photo-excited carriers. Accordingly, the mobility ~ should be reduced with an increase in [H]b. We have several such indications. First, one sample of a-Si:(F,H) films were prepared in a chamber equipped with a cathode screen in order for the growing film not to touch the plasma. Both N s and [H]n in this sample are similar to those in a-Si:(F,H) films prepared by glow discharge decomposition without using a cathode screen. a-Si:(F,H) prepared by using a cathode screen, however, has smaller contents of H and F contributing to the broad component of NMR spectra and larger photoconductivity than a-Si:(F,H) prepared without a cathode screen. Second, for a series of a-Si:H prepared by glow discharge decomposition of SiH 4 diluted with Ar, [H] n have close values but [H] b decreases from 15 to 5 at. % with increasing substrate temperature from i00 to 350°C, accompanying an increase in photoconductivity by about three orders of magnitude and a decrease in N s by about one order of magnitude. The photoconductivity is proportional to the mobility u and the lifetime ~ of photo-excited carriers. If we assume that the photo-excited carriers recombine through dangling bonds, • is expected to be inversely proportional to N s. Therefore, the above two cases show that the decrease in ~ causes the decrease in the photoconductivity. In conclusion, the role of H in a-Si can be summarized as shown in Fig. 3. It is also concluded that the decrease in N s with an increase in T s does not result from a direct thermal relaxation but from a different H incorporation scheme at a higher Ts. Acknowledgements The authors wish to thank Mr. A. Morimoto, Mr. K. Yamada and Mr. Y. Yonezawa for their technical assistance. They also wish to thank Dr. H. Matsumura of Tokyo Institute of Technology, Dr. Y. Uchida of Fuji Electric Corporate Research and Development, Ltd. and Dr. S. Usui of Sony Corporation Research Center for supplying
It o t a l
IR
SiH
NMR
Inarrow
H I
SiH 2 etc.
I "-..
I
1
1
line
II b r o a d
line
the density of dangling bonds
Ithe m o b i l i t y icarriers
Fig.
of H
3.
The
role
in
]
of
] |
a-Si.
several samples in the present work. This work was partly supported by the Sunshine Project of the Ministry of International Trade and Industry of Japan. References [i] J. A. Reimer, 9th Int. Conf. Amorphous and Liquid Semiconductors, Grenoble, 1981, J. de Phys. 42 (1981) C4-715. [2] W. E. Carlos and P. C. Taylor, ibid., C4725. [3] S. Ueda, M. Kumeda and T. Shimizu, ibid., C4-729. [4] M. Lowry, F. R. Jeffrey, R. G. Barnes and D. R. Torgeson, Solid State Commun. 38 (1981) 113. [5] S. Ueda, K. Nakazawa, M. Kumeda and T. Shimizu, Solid State Commun. 42 (1982) 261. [6] T. Shimizu, K. Nakazawa, M. Kumeda and S. Ueda, Jpn. J. Appl. Phys. 21 (1982) No. 6 (part2). [7] J. C. Knights, G. Lucovsky and R. J. Nemanich, J. Non-Cryst. Solids 32 (1979) 393. [8] M. Kumeda and T. Shimizu, Jpn. J. Appl. Phys. 19 (1980) L197.
926
INCORPORATION SCHEME OF H REDUCING DEFECTS IN a-Si STUDIED BY NMR AND ESR
Tatsuo Shimizu, Kenji Nakazawa, Minoru Kumeda and Shoichi Ueda Department of Electronics, Faculty of Technology, Kanazawa 920, Japan
Kanazawa University,
By comparing the results of NMR, ESR and photoconductivity measurements, it is found that dispersed H atoms bonded to Si mainly play a role in reducing the density of dangling bonds Ns. Gathered H and F atoms appear to decrease the mobility. It is also concluded that the decrease in N s with an increase in substrate temperature T s does not result from a direct thermal relaxation but from a different H incorporation scheme at a higher T s.
i. INTRODUCTION In order to investigate the role of H and F in a-Si, we have carried out systematic studies of H and F NMR for a-Si:H, a-Si:(F,H) and a-Si:F films prepared by various methods and the results are compared with the results of ESR, photoconductivity and infrared (IR) absorption measurements. The NMR spectra of H and F are known to have a broad and a narrow component [1-5]. The former originates from closely gathered H or F in the form of SiH (SiF), SiH 2 (SiF2) etc. and the latter originates from dispersed H or F in the form of SiH or SiF. 2. EXPERIMENTAL
Sample films were prepared under various conditions as follows, a-Si:H films were prepared by glow discharge decomposition of SiH 4 or SiH 4 + Ar mixture or by reactive sputtering of Si in Ar + H 2 mixture, a-Si:(F,H) films were prepared by glow discharge decomposition of SiH 4 + H2 mixture or SiF 4 + SiH4 + Ar mixture, a-Si:F films were prepared by reactive sputtering of Si in Ar + SiF 4 mixture. Preparation conditions of most of these films are briefly summarized in Ref. 5. Sample films were deposited on aluminum foil substrates for NMR and ESR measurements and on crystalline Si substrates for IR absorption measurements. The aluminum foil substrates were dissolved in HCI solution. NMR measurements were carried out by a pulse method at 16 MHz.
(a) 1020
(b)
1020 li
•
1019
a - S i :H a - S i : (F,H) Carlos-Taylor
•
! I I
I:
eOO
a-Si:H a - S i : (F,H) Carlos-Taylor
1019
o
o
o I
°
•o
IE O 1018
1018
l
o%
O
Z o
•
o
o
o
Z •
0
1017
1017 •
•
oo
o o o
o °o
oo
o •
•
•
•
1016
0
•
i
I
I
I
5
i0
15
20
[H] t Fig.
1.
1016
25
0
I
I
I
I
1
2
3
4
[H~
(at. %)
(at.%)
(a) N s v e r s u s [H] t a n d (b) a l s o s h o w n a f t e r Ref. 2.
N s versus
0 378-4363/83/0000-0000/$03.00 © 1983 North-Holland
[H] n.
Data
by
Carlos
and
Taylor
are
T. Shimizu et al. / Incorporation scheme o f H reducing defects in a-Si
927
A part of the present result has already been published[6].
ranges from 2 to 5 kHz and that of the broad component ranges from 15 to 34 kHz,
3. RESULTS AND DISCUSSION
From the measurement of the IR absorption due to Si-H vibrations, SiH, SiH2, (SiH2) n and SiH 3 can be characterized by the position of the absorption peak. The peak near 2000 cm -I is identified as SiH and that near 2100 cm -I is identified as SiH 2 etc. Knights et al. reported that the intensity of the IR peak around 2000 cm -I is well correlated with N s for a-Si:H prepared by glow discharge decomposition of SiH 4 with various substrate temperature[7]. The correlation, however, was found to be not good for a-Si:H films prepared under a wide variety of preparation methods. A close correlation between N s and [H]n shows that only dispersed SiH plays a role in reducing N s. Gathered SiH's contributing to the 2000 cm -I IR absorption peak exhibit the broad NMR line.
Figures i (a) and (b) show the center density of ESR due to dangling bonds, Ns, versus the total H content, [H]t , deduced from NMR and the content of H contributing to the narrow component of NMR spectra, [H]n, respectively. There appears a tendency that N s decreases with an increase in [H] t up to I0 ~ 15 at.% and then increases, but the data are very scattered and the correlation between N s and [H]t is not good. On the other hand, it is found that [H]n has a close correlation with N s. N s decreases monotonically with an increase in [H]n. Some samples shown in these figures contain F in addition to H, but it appears that N s is determined only by [H] n and F in the film has nothing to do with N s. In a-Si:(F,H) films used in the present work, the broad component in F NMR spectra is dominant. As for a-Si:F films used in the present work, the narrow component of F NMR spectra is present ~ 4 at.%), and is supposed to play a role in reducing Ns because N s is 6 x 1017 cm-3 which is more than one order of magnitude smaller than N s for a-Si with neither H nor F. The ability of reducing N s by F, however, appears to be weaker than that by H. In some cases, the linewidth narrows at higher temperature (even at liquid nitrogen temperature), showing the motional narrowing. Therefore, it should be ascertained that the narrow component is not the motionally narrowed one. The magnitude of the linewidth of the narrow component
Changes in Ns, [H]t , [H]n and [H]b (the content of H contributing to the broad component of NMR spectra) with substrate temperature T s are shown in Fig. 2 (a) and (b) for a-Si:H prepared by glow discharge decomposition of SiH 4 and for a-Si:H prepared by reactive sputtering of Si in Ar + H 2 mixture, respectively. In the latter case, H 2 partial pressure was fixed at 40 %. In both glow discharge decomposed a-Si:H and reactively sputtered a-Si:H, [H]t , [H]b and N s decrease and [H] n increases with an increase in T s. It should be noted that [H]n/[H]b, however, is larger for the former than for the latter at the same T s. Previously, we reported that N s decreases with an increase in the H
(a) GD a-Si:H
(b) •
N
O
SP a-Si:H
•
N
s
O
[H]t
o
i
1018 l
[H~
[ [Hi
o
l o 17
25
o
lo i7
25 z
Z
]_o16 ~20
v 20 4~
4~
© 15 o o
0 o m 10
10
"eL
5 \
.4P'" ~ . . . . .
O0 Fig.
2.
I
I
100 200 Temperature
~"
I
300 (°C)
•
00
40
N s , [H] t , [ H ~ and [H~ versus T s for decomposition a n d (b) r f s p u t t e r i n g .
a-Si:H
l~0 2d0 Temperature (°C) prepared
by
(a)
glow
300
discharge
928
T. Shimizu et al. / Incorporation scheme o f H reducing defects in a-Si
content for a-Si:H films prepared by rf sputtering in Ar + H 2 mixture with both a water-cooled substrate and a substrate heated at 250°C[8]. The decrease in N s is far more remarkable for films w i t h a substrate heated at 250°C than those with a water-cooled substrate. We attributed such difference to thermal relaxation[g]. Figure 2 (b), however, shows that [H] n is larger for films with a higher substrate temperature. Therefore, we may conclude that no direct thermal relaxation is working in reducing N , but the different incorporation scheme of HSat a higher temperature makes N s smaller. The fact that Ns depends only weakly on T s for a-Si without H[8] supports this conclusion. The results shown in Fig. 2 (a) for glow discharge decomposed a-Si:H is also consistent with this conclusion. Closely gathered SiH (SiF), SiH 2 (SiF 2) etc., are expected to make the band tail larger, working as a scatterer or a trap for photo-excited carriers. Accordingly, the mobility ~ should be reduced with an increase in [H]b. We have several such indications. First, one sample of a-Si:(F,H) films were prepared in a chamber equipped with a cathode screen in order for the growing film not to touch the plasma. Both N s and [H]n in this sample are similar to those in a-Si:(F,H) films prepared by glow discharge decomposition without using a cathode screen. a-Si:(F,H) prepared by using a cathode screen, however, has smaller contents of H and F contributing to the broad component of NMR spectra and larger photoconductivity than a-Si:(F,H) prepared without a cathode screen. Second, for a series of a-Si:H prepared by glow discharge decomposition of SiH 4 diluted with Ar, [H] n have close values but [H] b decreases from 15 to 5 at. % with increasing substrate temperature from i00 to 350°C, accompanying an increase in photoconductivity by about three orders of magnitude and a decrease in N s by about one order of magnitude. The photoconductivity is proportional to the mobility u and the lifetime ~ of photo-excited carriers. If we assume that the photo-excited carriers recombine through dangling bonds, • is expected to be inversely proportional to N s. Therefore, the above two cases show that the decrease in ~ causes the decrease in the photoconductivity. In conclusion, the role of H in a-Si can be summarized as shown in Fig. 3. It is also concluded that the decrease in N s with an increase in T s does not result from a direct thermal relaxation but from a different H incorporation scheme at a higher Ts. Acknowledgements The authors wish to thank Mr. A. Morimoto, Mr. K. Yamada and Mr. Y. Yonezawa for their technical assistance. They also wish to thank Dr. H. Matsumura of Tokyo Institute of Technology, Dr. Y. Uchida of Fuji Electric Corporate Research and Development, Ltd. and Dr. S. Usui of Sony Corporation Research Center for supplying
It o t a l
IR
SiH
NMR
Inarrow
H I
SiH 2 etc.
I "-..
I
1
1
line
II b r o a d
line
the density of dangling bonds
Ithe m o b i l i t y icarriers
Fig.
of H
3.
The
role
in
]
of
] |
a-Si.
several samples in the present work. This work was partly supported by the Sunshine Project of the Ministry of International Trade and Industry of Japan. References [i] J. A. Reimer, 9th Int. Conf. Amorphous and Liquid Semiconductors, Grenoble, 1981, J. de Phys. 42 (1981) C4-715. [2] W. E. Carlos and P. C. Taylor, ibid., C4725. [3] S. Ueda, M. Kumeda and T. Shimizu, ibid., C4-729. [4] M. Lowry, F. R. Jeffrey, R. G. Barnes and D. R. Torgeson, Solid State Commun. 38 (1981) 113. [5] S. Ueda, K. Nakazawa, M. Kumeda and T. Shimizu, Solid State Commun. 42 (1982) 261. [6] T. Shimizu, K. Nakazawa, M. Kumeda and S. Ueda, Jpn. J. Appl. Phys. 21 (1982) No. 6 (part2). [7] J. C. Knights, G. Lucovsky and R. J. Nemanich, J. Non-Cryst. Solids 32 (1979) 393. [8] M. Kumeda and T. Shimizu, Jpn. J. Appl. Phys. 19 (1980) L197.