- Email: [email protected]
Relationship between film quality and deposition rate for a-Si:H by ECR plasma CVD
Journal of Non-Crystalline Solids 164-166 (1993) 63-66 North-Holland
JOURNAL or
]~~|,]J]~
U
Relationship between film quality and deposition rate for a-Si:H by ECR plasma CVD M. Zhang, Y. Nakayama, S. Nonoyama and K. Wakita Department of Physics and Electronics, University of Osaka Prefecture, Sakai, Osaka 593, Japan
The relationship between the film quality and the deposition rate (RQD) for a-Si:H using an ECR plasma CVD reactor has been studied. Whereas the RQD is generally negative in glow discharge plasma CVD, a positive RQD is observed for many cases in ECR plasma CVD. We explain this difference with the necessity of adequate ion impingements on the growing surface in order to achieve a better film quality in ECR plasma CVD because of larger sticking coefficients of the dominant precursors.
1. INTRODUCTION Plasma-enhanced chemical vapor deposition (CVD) is widely used in the electronics industry for preparing thin films of semiconductors and insulators. Currently, an electron cyclotron resonance (ECR) plasma is attracting attention for use in various aspects of thin film processing, because of a remote plasma source providing charged particles with energy adjusted to reduce damage to achieve high quality films. The ECR plasma is operated at low gas pressures and forms a stream of high density of charged species achieving anisotropic processing, which is quite different from a glow discharge plasma [1]. One expects a difference in the mechanism of film growth between the two plasmas, In plasma CVD of hydrogenated amorphous silicon (a-Si:H) films using a glow discharge, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at the filmgrowing surface [2]. Reducing the film deposition rate results in high quality films. The relationship of the film quality to the deposition rate (RQD) is so-called negative. This relationship seems to be universal. However, a positive RQD has been reported for a-Si:H prepared at different microwave powers by ECR plasma CVD [3], and this phenomenon has scarcely been studied in detail. In this paper we report a systematic study on the RQD of a-Si:H in ECR plasma CVD and discuss the microscopic picture of high rate deposition of high quality films in terms of the reaction on the growing surface.
2. EXPERIMENT The ECR plasma CVD system was the divergent magnetic type and consisted of two chambers [4]. One was an ECR chamber in which a 1-I2 plasma was produced under the ECR condition established by a microwave frequency of 2.45GHz and a magnetic field of 875Gauss. The other was a CVD chamber in which the source gas of Sill 4 which was fed with or without a dilution gas toward a substrate from several orifices of lmm in diameter on a ring was decomposed by the plasma downstream. The deposition conditions of a-Si:H are summarized in table 1. The diameter of the aperture which separates the ECR and CVD chambers was set at 4era except for experiments of the aperture-size dependence. The Sill4 flow rate was set to be 10seem except for experiments of the dependence on the Sill 4 flow rate. The gas pressure of 5.5mTorr was used when diluting Sill4 by H2 or He. The deposited films were characterized by the photoconduetivity, the hydrogen bonding states and
Table 1. Deposition conditions of a-Si:H Microwave power Aperture diameter ECR gas (1-12) flow rate Sill4 flow rate Dilution gas (H2 or He) flow rate Gas pressure Substrate temperature
0022-3093D3/$06.00 © 1993 - Elsevier Science Publishers B.V. All fights reserved.
400W 2-10ern 10seem 1-10scorn 0-60seem 3 or 55mTolr 250'~
64
M. Zhang et al. / Relationship between film quality and deposition rate
the optical absorption in the subgap spectral region. Photocunductivity measurements were made for the samples with interdigital Al electrodes under illumination with monochromatic light of 50~tW/cm2 in intensity. The subgap absorption was measured by the constant photocurrent method. Plasma properties were measured near the substrate using the Langmuir probe method and optical emission spectroscopy.
-5
, , , , _ . . . - - o . . -6 ~ "bE ~ -7 -fl ~
-9 -10
3. R E S U L T S A N D DISCUSSION
16
I
Figure 1 shows the photoconductivity o plotted ph as a function o f t h e deposition rate RD for the samples prepared with different deposition parameters. Closed circles denote the dependence on the Sill 4 flow rate where He was fed at 60seem as a dilution gas. As the Sill4 flow rate increases from 1 to 7sccrn the photoconductivity and the deposition rate both monotonically increase. However, at Sill4 flow rates beyond 7seem the photoconductivity becomes small although the deposition rate does not change. The deposition mode in the region up to SiI-I4 flow rate 7sccin is the source-gas limited mode and the other is the electron-flux limited mode [5]. The RQD in the source-gas limited mode is positive, Experiments of the dependence on the dilution of Sill 4 and on the aperture size have been made
14
<----°/~'//zx/''D'/'lf"'"
10-5
,
'
,
--E .90 10 -6 ~,~ i~
O, O O
[3
"u roO 10 -7 .t: 13.
10 - 8
•
n
0
I 5
n Aperture size
zx He dilution o H2 dilution
i 10
I 15
D e p o s i t i o n rate (/z m/h)
20
Figure 1. Deposition rate dependence of the photoconductivity for a-Si:H prepared with different deposition conditions,
I
I
I
~-]4 .~/
d
t~ 12 10 q" 0
i J 2 4 [He]/[SiH4]
i -i3
6
Figure 2. He dilution dependence of the deposition rate RD, dark and photoconductivities o d and %h of films and emission intensity IsiH of Sill in the plasma. in the electron-flux limited mode. A positive RQD is obtained when changing the dilution ratio of Sill 4 with He [He]/[SiH4] (triangles) and the aperture size (squares). On the other hand, the RQD is rather negative in the dependence on the H2 dilution (open circles). Figure 2 shows the deposition rate RD, the emission intensity of Sill species in the plasma IsiH, the dark conductivity 0 d and the photoconductivity Oph plotted as a function of the He dilution ratio [He]/[SiH4]. Increasing the He dilution ratio increases not only the deposition rate but also the photoconductivity. It is noted that the deposition rate reaches an extremely high value of-15~tm/h and the photoconductivity simultaneously reaches the device grade value more of than 10-6S/cm (rlptX>10-5em2,cV) when the He dilution ratio is around 5. As shown in the figure the variation of the deposition rate corresponds to that of [Sill, indicating that the increase in the deposition rate is caused by promoting the dissociation of Sill 4 molecules with increasing He dilution ratio. He atoms play a role in reducing the probability of inelastic collisions of electrons by which electrons get more energy from the electric and magnetic fields. This idea has been supported by Langmuir probe measurements which
65
M. Zhang et al. / Relationship between film quality and deposition rate
-5
,
-6
,
,
10 4
0
,
,
,
-
o~
-7
o
,
1oa ,
-9
E
10 2
.~ q
,% -1(1
•
12
I
e, I
~,
I
m
8
-3 5 zx'--~.....-~.'2 ~--.
6'6
:' 4 [H2]/[SiH4]
[He]
-4
lO n-"
101
100
~ ¢n 1
--
6
Figure 3. H 2 dilution dependence of the deposition rate RD, dark and photoconductivities o d and o I1 of films and emission intensity Isin of Sill in t ~ plasma. show an increase in the density of electrons with sufficient energies to dissociate Sill 4 molecules, The aperture-size dependence where the undiluted Sill 4 gas was fed indicates that the aperture has an optimum size of 3cm in diameter giving the highest photoconductivity as well as the highest deposition rate. This variation is also strongly related to the plasma properties in a similar manner as the case of the He-dilution dependence, Figure 3 shows the H2-dilution dependence of the deposition rate, the emission intensity of Sill and the dark and photoconductivities. With increasing dilution ratio [Hz]/[SiH4] the photoconductivity increases a little but the deposition rate obviously decreases. The variations in the deposition rate and IsiH are quite similar and in addition correlate to the plasma properties. In this case the fluctuation of the partial pressure of Sill 4 seems to be the other origin, Thus the dilution of Sill 4 has a strong influence on the photoconductivity, i.e., on the material quality, The improvement in photoconductivity is explained from the standpoint of the structure and defect density in films. For the He dilution the hydrogen content in films is lower than the case without dilution. Monohydride bonds increase and polyhy-
10-1
4 ~
= 0.8
-
=
i
,
1.2 1.6 2.0 Photon energy (eV)
Figure 4. Subgap absorption spectra measured by the constant photocurrent method for a-Si:H prepared by the He dilution process. dride bonds decrease as the dilution rate increases. The lower hydrogen content and dominance of monohydride bonds rather than polyhydride bonds correspond to more rigid and homogeneous network structures. Figure 4 shows the subgap absorption spectra for the films deposited with the He dilution. The increase of the dilution ratio leads to a decrease of defect densities in the midgap. The He dilution is very effective in reducing the defect density. In the case of H2 dilution the defect density is also decreased with increasing dilution ratio but it is not effective as compared to the He case. It is obvious that the improvement in photoconductivity is due to the reduction of defects as recombination centers. The RQD seems to depend on the film-growth process which includes the elimination of hydrogen atoms coveting the growing surface, chemisorption of precursors, change in hydrogen-bond configuration and formation of the Si network. It is generally believed in glow-discharge plasma CVD that the surface diffusion length of precursors on the growing surface should be longer in order to form a better network structure with less strain and fewer voids. In this case precursors can complete their energy relaxation and can find energetically favorable sites to form the bonds on the growing surface. Therefore,
66
M. Zhang et al. / Relationsh~ between film quality and deposition rate
10_ 5
, , • Sill4 flow rate O H2 dilution & He dilution
for the H2 dilution dependence come from the necessity of more ion impingements to get better quality films.
' A
A fluctuation in the photoconductivity at a certain
A •'~' 10-6
O 10 -7 n
10-4
015
1
115
2
RD/Fi (a.u.)
Figure 5. Photoconductivity of a-Si:H shown in Fig.1 plotted as a function of the ratio of the deposition rate to the ion flux. the RQD is negative for a wide range of deposition conditions in glow-discharge plasma CVD. In the glow-discharge case the dominant precursors are Sill 3 which has a small sticking coefficient and the ion bombardment on the growing surface is extremely small. However, in ECR plasma CVD the dominant precursors are rather Sill 2 [6] which has a larger sticking coefficient than Sill3 [7] and there are numbers of energetic ions which directly impinge on the growing surface. Those ions play an important role in determining the film quality in ECR plasma CVD [8]. Some data in Fig.1 are replotted in Fig.5 with RD/Fi as abscissa instead of RD. Here Fi is the ion flux reaching the growing surface and estimated by the multiplication of the electron density and the sheath potential. The data for samples prepared in the source-gas limited mode have a positive slope and the data for the electron-flux limited mode have a negative slope. This indicates that an optimum value exists in ion impingements on the growing surface in order to obtain high quality films. It is clear that the positive ROD of the dependence on the Sill 4 flow rate is caused by a reduction of the defect creation due to strong ion impingements by increasing the deposition rate. The positive ROD for the He dilution dependence and the negative ROD
value of RD/Fi occurs in different series of experiments. This is because the photoconductivity depends not only on RD/Fi but also on other factors such as a composition of different kinds of precursors and of ions and the contribution of metastable (high energy) atoms. We discuss the positive RQD [3] obtained when changing the microwave power. The deposition rate is roughly proportional to the electron density in the electron-flux limited mode. On the other hand, the ion flux is proportional to the multiplication of the electron density and the sheath potential Vsth. Then we have RD/Fi•I/Vsth. The value of Vsth becomes large as the microwave power increases, because the decrease in the fraction of dissociative molecules with increasing the microwave power leads to a decrease in the probability of inelastic collisions of electrons and hence to an increase in the electron temperature. Therefore, the microwave power dependence is believed to have a negative slope in the plot of Oph vS. RD/Fi and the positive RQD due to providing of the ion flux more than the increase in the deposition rate. The RQD in ECR plasma CVD strongly depends on the role of ions. This important role of ions is associated with the precursors with high sticking coefficients. It is believed that these precursors need adequate ion impingements in order to form a rigid Si network.
REFERENCES 1. V.A.Godyak, R.V.Tiejak, and V.M.Alexandrovich, Rev. Sei. Instmm. 61 (1990) 2401. 2. Y.Hishikawa, S.Tsuda, K.Wakisaka, and Y.Kuwano, J. Appl. Phys. 73 (1993) 4227. 3. K.Kobayashi, M.Hayama, S.Kawamoto, and H.Miki, J- J" Appl. Phys. 26 (1987) 202. 4. Y.Nakayama, M.Kondoh, K.I-Iitsuishi, M.Zhang, and T.Kawamura, Appl. Phys. Lett. 57 (1990) 2297. 5. M.Zhang, Y.Nakayama, K.Hitsuishi, H.Imamura and T.Kawamura, Prec. of Int. Conf. on Image Science and ttardeopy, Shanghai, China, 1992, p.66. 6. Unpublished 7. A.Matsuda, K.Nomoto, Y.Takeuchi, A.Suzuki, A.Yuuki, and J.Perrin, Surf. Sci. 227 (1990) 50. 8. Y.Nakayama, K.Hitsuishi, M.Zhang, H.Imamura, and T.Kawamura,J. Non-Cryst. Solids 137&138 (1991) 669.
JOURNAL or
]~~|,]J]~
U
Relationship between film quality and deposition rate for a-Si:H by ECR plasma CVD M. Zhang, Y. Nakayama, S. Nonoyama and K. Wakita Department of Physics and Electronics, University of Osaka Prefecture, Sakai, Osaka 593, Japan
The relationship between the film quality and the deposition rate (RQD) for a-Si:H using an ECR plasma CVD reactor has been studied. Whereas the RQD is generally negative in glow discharge plasma CVD, a positive RQD is observed for many cases in ECR plasma CVD. We explain this difference with the necessity of adequate ion impingements on the growing surface in order to achieve a better film quality in ECR plasma CVD because of larger sticking coefficients of the dominant precursors.
1. INTRODUCTION Plasma-enhanced chemical vapor deposition (CVD) is widely used in the electronics industry for preparing thin films of semiconductors and insulators. Currently, an electron cyclotron resonance (ECR) plasma is attracting attention for use in various aspects of thin film processing, because of a remote plasma source providing charged particles with energy adjusted to reduce damage to achieve high quality films. The ECR plasma is operated at low gas pressures and forms a stream of high density of charged species achieving anisotropic processing, which is quite different from a glow discharge plasma [1]. One expects a difference in the mechanism of film growth between the two plasmas, In plasma CVD of hydrogenated amorphous silicon (a-Si:H) films using a glow discharge, the film properties are governed by a competition between the rate of film growth and the rate of thermally activated surface reactions at the filmgrowing surface [2]. Reducing the film deposition rate results in high quality films. The relationship of the film quality to the deposition rate (RQD) is so-called negative. This relationship seems to be universal. However, a positive RQD has been reported for a-Si:H prepared at different microwave powers by ECR plasma CVD [3], and this phenomenon has scarcely been studied in detail. In this paper we report a systematic study on the RQD of a-Si:H in ECR plasma CVD and discuss the microscopic picture of high rate deposition of high quality films in terms of the reaction on the growing surface.
2. EXPERIMENT The ECR plasma CVD system was the divergent magnetic type and consisted of two chambers [4]. One was an ECR chamber in which a 1-I2 plasma was produced under the ECR condition established by a microwave frequency of 2.45GHz and a magnetic field of 875Gauss. The other was a CVD chamber in which the source gas of Sill 4 which was fed with or without a dilution gas toward a substrate from several orifices of lmm in diameter on a ring was decomposed by the plasma downstream. The deposition conditions of a-Si:H are summarized in table 1. The diameter of the aperture which separates the ECR and CVD chambers was set at 4era except for experiments of the aperture-size dependence. The Sill4 flow rate was set to be 10seem except for experiments of the dependence on the Sill 4 flow rate. The gas pressure of 5.5mTorr was used when diluting Sill4 by H2 or He. The deposited films were characterized by the photoconduetivity, the hydrogen bonding states and
Table 1. Deposition conditions of a-Si:H Microwave power Aperture diameter ECR gas (1-12) flow rate Sill4 flow rate Dilution gas (H2 or He) flow rate Gas pressure Substrate temperature
0022-3093D3/$06.00 © 1993 - Elsevier Science Publishers B.V. All fights reserved.
400W 2-10ern 10seem 1-10scorn 0-60seem 3 or 55mTolr 250'~
64
M. Zhang et al. / Relationship between film quality and deposition rate
the optical absorption in the subgap spectral region. Photocunductivity measurements were made for the samples with interdigital Al electrodes under illumination with monochromatic light of 50~tW/cm2 in intensity. The subgap absorption was measured by the constant photocurrent method. Plasma properties were measured near the substrate using the Langmuir probe method and optical emission spectroscopy.
-5
, , , , _ . . . - - o . . -6 ~ "bE ~ -7 -fl ~
-9 -10
3. R E S U L T S A N D DISCUSSION
16
I
Figure 1 shows the photoconductivity o plotted ph as a function o f t h e deposition rate RD for the samples prepared with different deposition parameters. Closed circles denote the dependence on the Sill 4 flow rate where He was fed at 60seem as a dilution gas. As the Sill4 flow rate increases from 1 to 7sccrn the photoconductivity and the deposition rate both monotonically increase. However, at Sill4 flow rates beyond 7seem the photoconductivity becomes small although the deposition rate does not change. The deposition mode in the region up to SiI-I4 flow rate 7sccin is the source-gas limited mode and the other is the electron-flux limited mode [5]. The RQD in the source-gas limited mode is positive, Experiments of the dependence on the dilution of Sill 4 and on the aperture size have been made
14
<----°/~'//zx/''D'/'lf"'"
10-5
,
'
,
--E .90 10 -6 ~,~ i~
O, O O
[3
"u roO 10 -7 .t: 13.
10 - 8
•
n
0
I 5
n Aperture size
zx He dilution o H2 dilution
i 10
I 15
D e p o s i t i o n rate (/z m/h)
20
Figure 1. Deposition rate dependence of the photoconductivity for a-Si:H prepared with different deposition conditions,
I
I
I
~-]4 .~/
d
t~ 12 10 q" 0
i J 2 4 [He]/[SiH4]
i -i3
6
Figure 2. He dilution dependence of the deposition rate RD, dark and photoconductivities o d and %h of films and emission intensity IsiH of Sill in the plasma. in the electron-flux limited mode. A positive RQD is obtained when changing the dilution ratio of Sill 4 with He [He]/[SiH4] (triangles) and the aperture size (squares). On the other hand, the RQD is rather negative in the dependence on the H2 dilution (open circles). Figure 2 shows the deposition rate RD, the emission intensity of Sill species in the plasma IsiH, the dark conductivity 0 d and the photoconductivity Oph plotted as a function of the He dilution ratio [He]/[SiH4]. Increasing the He dilution ratio increases not only the deposition rate but also the photoconductivity. It is noted that the deposition rate reaches an extremely high value of-15~tm/h and the photoconductivity simultaneously reaches the device grade value more of than 10-6S/cm (rlptX>10-5em2,cV) when the He dilution ratio is around 5. As shown in the figure the variation of the deposition rate corresponds to that of [Sill, indicating that the increase in the deposition rate is caused by promoting the dissociation of Sill 4 molecules with increasing He dilution ratio. He atoms play a role in reducing the probability of inelastic collisions of electrons by which electrons get more energy from the electric and magnetic fields. This idea has been supported by Langmuir probe measurements which
65
M. Zhang et al. / Relationship between film quality and deposition rate
-5
,
-6
,
,
10 4
0
,
,
,
-
o~
-7
o
,
1oa ,
-9
E
10 2
.~ q
,% -1(1
•
12
I
e, I
~,
I
m
8
-3 5 zx'--~.....-~.'2 ~--.
6'6
:' 4 [H2]/[SiH4]
[He]
-4
lO n-"
101
100
~ ¢n 1
--
6
Figure 3. H 2 dilution dependence of the deposition rate RD, dark and photoconductivities o d and o I1 of films and emission intensity Isin of Sill in t ~ plasma. show an increase in the density of electrons with sufficient energies to dissociate Sill 4 molecules, The aperture-size dependence where the undiluted Sill 4 gas was fed indicates that the aperture has an optimum size of 3cm in diameter giving the highest photoconductivity as well as the highest deposition rate. This variation is also strongly related to the plasma properties in a similar manner as the case of the He-dilution dependence, Figure 3 shows the H2-dilution dependence of the deposition rate, the emission intensity of Sill and the dark and photoconductivities. With increasing dilution ratio [Hz]/[SiH4] the photoconductivity increases a little but the deposition rate obviously decreases. The variations in the deposition rate and IsiH are quite similar and in addition correlate to the plasma properties. In this case the fluctuation of the partial pressure of Sill 4 seems to be the other origin, Thus the dilution of Sill 4 has a strong influence on the photoconductivity, i.e., on the material quality, The improvement in photoconductivity is explained from the standpoint of the structure and defect density in films. For the He dilution the hydrogen content in films is lower than the case without dilution. Monohydride bonds increase and polyhy-
10-1
4 ~
= 0.8
-
=
i
,
1.2 1.6 2.0 Photon energy (eV)
Figure 4. Subgap absorption spectra measured by the constant photocurrent method for a-Si:H prepared by the He dilution process. dride bonds decrease as the dilution rate increases. The lower hydrogen content and dominance of monohydride bonds rather than polyhydride bonds correspond to more rigid and homogeneous network structures. Figure 4 shows the subgap absorption spectra for the films deposited with the He dilution. The increase of the dilution ratio leads to a decrease of defect densities in the midgap. The He dilution is very effective in reducing the defect density. In the case of H2 dilution the defect density is also decreased with increasing dilution ratio but it is not effective as compared to the He case. It is obvious that the improvement in photoconductivity is due to the reduction of defects as recombination centers. The RQD seems to depend on the film-growth process which includes the elimination of hydrogen atoms coveting the growing surface, chemisorption of precursors, change in hydrogen-bond configuration and formation of the Si network. It is generally believed in glow-discharge plasma CVD that the surface diffusion length of precursors on the growing surface should be longer in order to form a better network structure with less strain and fewer voids. In this case precursors can complete their energy relaxation and can find energetically favorable sites to form the bonds on the growing surface. Therefore,
66
M. Zhang et al. / Relationsh~ between film quality and deposition rate
10_ 5
, , • Sill4 flow rate O H2 dilution & He dilution
for the H2 dilution dependence come from the necessity of more ion impingements to get better quality films.
' A
A fluctuation in the photoconductivity at a certain
A •'~' 10-6
O 10 -7 n
10-4
015
1
115
2
RD/Fi (a.u.)
Figure 5. Photoconductivity of a-Si:H shown in Fig.1 plotted as a function of the ratio of the deposition rate to the ion flux. the RQD is negative for a wide range of deposition conditions in glow-discharge plasma CVD. In the glow-discharge case the dominant precursors are Sill 3 which has a small sticking coefficient and the ion bombardment on the growing surface is extremely small. However, in ECR plasma CVD the dominant precursors are rather Sill 2 [6] which has a larger sticking coefficient than Sill3 [7] and there are numbers of energetic ions which directly impinge on the growing surface. Those ions play an important role in determining the film quality in ECR plasma CVD [8]. Some data in Fig.1 are replotted in Fig.5 with RD/Fi as abscissa instead of RD. Here Fi is the ion flux reaching the growing surface and estimated by the multiplication of the electron density and the sheath potential. The data for samples prepared in the source-gas limited mode have a positive slope and the data for the electron-flux limited mode have a negative slope. This indicates that an optimum value exists in ion impingements on the growing surface in order to obtain high quality films. It is clear that the positive ROD of the dependence on the Sill 4 flow rate is caused by a reduction of the defect creation due to strong ion impingements by increasing the deposition rate. The positive ROD for the He dilution dependence and the negative ROD
value of RD/Fi occurs in different series of experiments. This is because the photoconductivity depends not only on RD/Fi but also on other factors such as a composition of different kinds of precursors and of ions and the contribution of metastable (high energy) atoms. We discuss the positive RQD [3] obtained when changing the microwave power. The deposition rate is roughly proportional to the electron density in the electron-flux limited mode. On the other hand, the ion flux is proportional to the multiplication of the electron density and the sheath potential Vsth. Then we have RD/Fi•I/Vsth. The value of Vsth becomes large as the microwave power increases, because the decrease in the fraction of dissociative molecules with increasing the microwave power leads to a decrease in the probability of inelastic collisions of electrons and hence to an increase in the electron temperature. Therefore, the microwave power dependence is believed to have a negative slope in the plot of Oph vS. RD/Fi and the positive RQD due to providing of the ion flux more than the increase in the deposition rate. The RQD in ECR plasma CVD strongly depends on the role of ions. This important role of ions is associated with the precursors with high sticking coefficients. It is believed that these precursors need adequate ion impingements in order to form a rigid Si network.
REFERENCES 1. V.A.Godyak, R.V.Tiejak, and V.M.Alexandrovich, Rev. Sei. Instmm. 61 (1990) 2401. 2. Y.Hishikawa, S.Tsuda, K.Wakisaka, and Y.Kuwano, J. Appl. Phys. 73 (1993) 4227. 3. K.Kobayashi, M.Hayama, S.Kawamoto, and H.Miki, J- J" Appl. Phys. 26 (1987) 202. 4. Y.Nakayama, M.Kondoh, K.I-Iitsuishi, M.Zhang, and T.Kawamura, Appl. Phys. Lett. 57 (1990) 2297. 5. M.Zhang, Y.Nakayama, K.Hitsuishi, H.Imamura and T.Kawamura, Prec. of Int. Conf. on Image Science and ttardeopy, Shanghai, China, 1992, p.66. 6. Unpublished 7. A.Matsuda, K.Nomoto, Y.Takeuchi, A.Suzuki, A.Yuuki, and J.Perrin, Surf. Sci. 227 (1990) 50. 8. Y.Nakayama, K.Hitsuishi, M.Zhang, H.Imamura, and T.Kawamura,J. Non-Cryst. Solids 137&138 (1991) 669.