- Email: [email protected]
Work functions of thin LaB6 films
iiiiiiiiiiiiiiiiiii iii iiiiiiii i i!!iiii i iiiiiiiiiiiiiiiiiiiiii
Applied Surface Science 70/71 (1993) 737-741 North-Holland
applied surface science
Work functions of thin LaB 6 films Akie Yutani, Akihiko Kobayashi and Akira Kinbara Department of Applied Physics, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received 28 August 1992; accepted for publication 20 November 1992
LaB 6 thin films were prepared at elevated temperatures by electron beam deposition in vacuum and were stabilized by annealing at 1100 K for 60 min. Their electron emission characteristics were investigated by the thermionic emission method. The work functions of the films fabricated at 300, 750 and 1000 K had values of 2.7-2.8 eV. The work functions of the films formed on tilted substrates (45 ° and 80 ° to the normal direction) at 750 K were also measured, and they had values of ~ 2.7 eV. X-ray diffraction peak positions showed that stress in the films changes from tensile to compressive with increasing substrate tilt angle, and at 45 ° it is nearly zero. It is found that substrate temperature and film strain have little influence on the work function.
I. Introduction
Plasma display devices are expected as largescale displaying equipments. But their life is relatively short due to the cathode sputtering. When the operating voltage is lowered, the incident ion energy decreases and their durability will be improved. Therefore, materials with low work function are preferable for the cathodes. Lanthanum hexaboride (LaB 6) is a material with low work function, low resistivity, high melting point, high chemical stability and low sputtering yield. It has been used as bright and long-life electron emitters. If thin LaB 6 films also possess these properties, they should be suitable for plasma display cathodes. It is well-known, however, that physical properties of thin films often depend on deposition conditions and differ from bulk ones. Many papers have been published on the work functions of LaB 6 bulk [1-3], but there are not many reports on the films. C o n c e r n i n g to s p u t t e r e d L a B 6 films, Mroczkowski has reported physical properties of them [4]. She formed LaB 6 films by R F sputtering, measured their work function by the thermionic emission method and obtained values
of about 2.4 eV. Nakano et al. also fabricated LaB 6 films by RF sputtering and discussed the relationships between internal stress and the orientation of film crystallites [5]. The two papers reported that sputtered LaB 6 films tend to have bad adhesion due to their internal stress. About the evaporated LaB 6 films, Oshima et al. measured their work function and obtained values of 2.6 eV [6]. Okamoto et al. fabricated plasma display devices with LaB6-coated Ni cathodes, which had the work functions of 2.4-2.5 eV [7]. However, fundamental properties of the films such as electric resistivity, carrier density and internal strain, have not been reported yet. Relationships between physical properties and deposition parameter have not also been clarified systematically. It is well-known that sputtered films contain large amounts of impurities such as Ar atoms and defects. These can raise the value of the internal stress, which reduces the stability and the reliability of the films in practical use. The electron beam deposition method is thought to be superior to the sputtering method from this point of view. The work function was principally measured by the thermionic emission method in previous
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
738
A. Yutani et al. / Work functions o f thin LaB 6 films
work [4,6]. We measured it by means of the Schottky barrier method and Kelvin probe method. But it was found that boron diffusion prevented the Schottky barrier formation in the former case. In the latter case the obtained values were about 4 eV; higher than the bulk value ( < 3 eV). Surface contamination is suspected to raise the values. By these two methods the work function is calculated only relatively (the former, to the semiconductor substrate; the latter, to the standard electrode such as Au). The work function of the standard can be influenced by the interface or surface states, and they contribute to the error. In thermionic emission method, f i l m substrate interaction does not affect the values much. The specimen heating decreases surface contamination. The absolute value of the work function is determined. At these points thermionic emission method is preferable. It is well-known that sputtered and evaporated films tend to have compressive and tensile internal stress, respectively, and the stress affects the growth of the crystallites. We have found that sputtered LaB 6 films always have compressive stress which depends on the working gas (Ar) pressure [5]. The work function can be affected by the stress. However, stress in the evaporated films and its effects on the work function have not been clarified yet. In the present p a p e r the work function of electron b e a m deposited LaB 6 films at several condition is shown. The strain in the films and its effects on the work function are also presented.
2. Experimental details The films were formed by electron b e a m deposition. The source LaB 6, manufactured by Mitsui Mining and Smelting Corp., had a purity of more than 99.5%. During the film formation, the electron acceleration voltage was kept in the range of 4.0-4.1 kV and the evaporation power was set at about 180 W. The pressure was in the range of 1 0 - 4 - 1 0 - 5 Pa (10 6-10-7 Torr). The substrates used were 5 m m wide tantalum ribbons for workfunction m e a s u r e m e n t s and Si(100) single crystals for X-ray analysis. LaB 6 was deposited through 3
m m diameter round masks. The substrate temperature was set at 300, 750 and 1000 K. Films were also fabricated on tilted substrates (45 ° and 80 ° to the incident direction of the evaporated particles) at 750 K. Lattice constants of the films were evaluated by a C u K a X-ray diffractometer to investigate the film strain. The work function was measured by means of thermionic emission in vacuum. The chamber was p u m p e d down to ~ 10 -6 Pa (10 - s Torr) by a sputter ion p u m p and a turbomolecular pump. The specimens were resistively heated to 9001400 K. An electron collector was places at a distance of 1 cm from the specimen and biased at 180 V to perform the m e a s u r e m e n t in the temperature-limited region. In this region, emission current density J is determined by R i c h a r d s o n Dushman's law: J=A**T
2 exp - ~
,
(1)
where A* *, 4~ and T are the mean effective Richardson constant (120 A cm -2 K-2), work function and temperature, respectively. According to eq. (1), a In J T -2 versus T - l plot gives a straight line and the work function 4~ is evaluated from its slope.
3. Results and discussion 3.1. Film deposition
All the films deposited were adherent, free from cracks and did not peel off. Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS) analyses showed that the atomic ratio of La : 13 is about 1 : 5. On the other hand, sputtered films tend to peel off and to be lanthanum-rich (typically, La : B = 1 : 4) [4]. Electron beam deposited films have better adhesion and are stoichiometric rather than the sputtered ones. 3.2. Film strain
Fig. 1 shows the X-ray diffraction patterns of LaB 6 films fabricated on tilted substrates (0 °, 45 °
739
A. Yutani et al. / Work functions of thin LaB 6 films
and 80 ° to the incident direction) at 750 K. All the films deposited on Si, glass and stainless steel showed similar patterns. Without substrate-heating, no peaks were observed. With 600-750 K heating, several peaks appeared. No peaks other than LaB 6 were observed in all samples. Taking the above result on the ratio of La to B into account, we consider that the films consist of LaB 6 crystallites and La-rich amorphous or amorphous-like phases such as small LaB 4 clusters. In this figure, (100) and (200) peaks are strong and ( l i d peaks are not observed, which is different from the source material. According to Nakano et al., LaB 6 films with (100) orientation have relatively weak stress ( < 3 × 10 s Pa) [5]. The diffraction patterns of the films deposited on tilted substrates at 750 K are different from the normal one. Peaks except (100) and (200) become weaker; (100) and (200) peaks become broader with the increasing incident angle. Crystallites in the films orienting to (100) direction become small with increasing angle. Fig. 2 indicates the lattice constants of the films formed with the substrate tilts of 0 °, 45 ° and 80 ° calculated from the X-ray diffraction peak positions. When the substrate is normal to the incident direction, the lattice constant is about
4.17
,--, 4.16
Bulk value
_~ _ ~ _ _ .
o 4.15 O
4.14
4.13
I
0
I
I
40
I
I
80
Substrate tilt [deg.] Fig. 2. Lattice constants of crystallites in the films fabricated with incident angles of 0°, 45° and 80°.
0.4% smaller than the bulk value. It becomes almost equal to the bulk value with the tilt of 45°; about 0.2% larger with the tilt of 80 °. Hence it is concluded that the film stress changes from tensile to compressive with increasing angle, and it is nearly free from stress at 45 °. This is in contrast with the sputtered films, which always have strong compressive stress [5].
3.3. Work function
800
A,
~'~-.-..A
45° 0o
A.
A
~.
J
(100) A
i
[
20
i
source
(110) i
i
I
I
,
(111) (200) (210)(211) ,
,
,
I
,
,
,
,
I
,
,
40
Diffraction angle [deg.] Fig. 1. X-ray diffraction patterns of LaB 6 films.
,
,
60
After exposing to the air, the emission current from all the films was small, unstable and not reproducible. Annealing at 1100 K for 60 min in 10 -8 Torr vacuum increased the emission current by several orders of magnitude and stabilized it. The emission current at 1300 K ( ~ 1000°C) was typically 10 A / m 2. Fig. 3 shows electron emission characteristics of the films fabricated at three different temperatures of 300, 750 and 1000 K. All the specimen show good linearity in the figure. The emission current values are almost the same at 300 and 750 K; a little larger at 1000 K. The values of the work function obtained are 2.8, 2.8 and 2.7 eV, respectively. These values are about 0.4 eV higher than bulk (100) value [8] and 0.1-0.4 eV higher
740
A. Yutani et al. -I0
....
I
....
t
....
/ Work functions o f thin LaB 6 films
~ ....
rent from the films deposited on tilted substrates is a little larger than the normal one. The values of the work function are 2.7-2.8 eV and they do not show remarkable dependence on the incident angle. With the results in the preceding subsection, it is concluded that the film strain does not affect the work function. This is in contrast with the sputtered films; their work function can be affected by the stress. Summarizing the above results, electron beam deposited films have work functions of 2.7-2.8 eV, even when two deposition parameters (substrate temperature and incident angle of the evaporated particles) are varied.
"\o
i:300K (2.8eV)
.~" %
" ~ . . ~
O:750K (2.8eV)
-14 ~ ' * ' ~~ , ~ ~
,
"•
<~:1000K(2.7eV)
O\
~..~-] 8
-
, , , , [ , , , , I .... 10 11
-229
i', 12
,~'.~.
13
1/kT [eV -1 ]
4. Summary
Fig. 3. Emission characteristics of LaB 6 films deposited at different temperatures (300, 750 and 1000 K).
than the previous works on evaporated films [6,7]. It is found that the substrate temperature during the deposition has little influence on the work function. Fig. 4 shows the emission characteristics of the films formed on tilted substrates (0 °, 45 ° and 80°). They also have good linearity. The emission cur-10
~
'
'
'
I
. . . .
I
_'N. "o~.~,
. . . .
I
. . . .
0 : 0 ° (2.8eV)
\og , "%~ q"~", , t ~:~
1:45 o (2.7eV)
~"~-14 k~
~ : 8 0 ° (2.7eV)
~-lS
LaB 6 films were formed by electron beam deposition at several temperatures and on tilted substrates. The crystallites in the films are mainly oriented to (100). Film strain changes from tensile to compressive with the increasing incident angle, and at 45 ° it becomes nearly free from strain. According to the thermionic emission method, the work function of the films was found to have values of 2.7-2.8 eV. It does not show remarkable dependence on substrate temperature and film strain.
Acknowledgements
The authors would thank to Dr. T. Kajiwara in Mitsubishi Electric Corp., who provided the LaB 6 source, and Mr. S. Nikaido of Nikon Corp., who determined the film stoichiometries by AES and XPS.
u o-~..<>.m."
References
-229 ' ~
10
11
I , , ~ , ~ 13 12
1/kT [eV -1] Fig. 4. Emission characteristics of LaB 6 films deposited on tilted substrates (0°, 45° and 80°).
[1] P.H. Schmidt, D.C. Joy, L.D. Longinotti and H.J. Leamy, Appl. Phys. Lett. 29 (1976) 400. [2] C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 48 (1977) 3925. [3] L.W. Swanson, M.A. Gesley and P.R. Davis, Surf. Sci. 107 (1981) 263. [4] S.J. Mroczkowski, J. Vac. Sci. Technol. A 9 (1991) 586.
A. Yutani et al. / Work functions of thin LaB 6 films [5] T. Nakano, S. Baba, A. Kobayashi, A. Kinbara, T. Kajiwara and K. Watanabe, J. Vac. Sci. Technol. A 9 (1991) 547. [6] C. Oshima, S. Horiuchi and S. Kawai, Jpn. J. Appl. Phys. Suppl. 2 Part 1 (1974) 281.
741
[7] Y. Okamoto, T. Aida and S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [8] M. Aono, E. Bannai, T. Tanaka and S. Kawai, Appl. Phys. Lett. 31 (1977) 323.
Applied Surface Science 70/71 (1993) 737-741 North-Holland
applied surface science
Work functions of thin LaB 6 films Akie Yutani, Akihiko Kobayashi and Akira Kinbara Department of Applied Physics, the University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan Received 28 August 1992; accepted for publication 20 November 1992
LaB 6 thin films were prepared at elevated temperatures by electron beam deposition in vacuum and were stabilized by annealing at 1100 K for 60 min. Their electron emission characteristics were investigated by the thermionic emission method. The work functions of the films fabricated at 300, 750 and 1000 K had values of 2.7-2.8 eV. The work functions of the films formed on tilted substrates (45 ° and 80 ° to the normal direction) at 750 K were also measured, and they had values of ~ 2.7 eV. X-ray diffraction peak positions showed that stress in the films changes from tensile to compressive with increasing substrate tilt angle, and at 45 ° it is nearly zero. It is found that substrate temperature and film strain have little influence on the work function.
I. Introduction
Plasma display devices are expected as largescale displaying equipments. But their life is relatively short due to the cathode sputtering. When the operating voltage is lowered, the incident ion energy decreases and their durability will be improved. Therefore, materials with low work function are preferable for the cathodes. Lanthanum hexaboride (LaB 6) is a material with low work function, low resistivity, high melting point, high chemical stability and low sputtering yield. It has been used as bright and long-life electron emitters. If thin LaB 6 films also possess these properties, they should be suitable for plasma display cathodes. It is well-known, however, that physical properties of thin films often depend on deposition conditions and differ from bulk ones. Many papers have been published on the work functions of LaB 6 bulk [1-3], but there are not many reports on the films. C o n c e r n i n g to s p u t t e r e d L a B 6 films, Mroczkowski has reported physical properties of them [4]. She formed LaB 6 films by R F sputtering, measured their work function by the thermionic emission method and obtained values
of about 2.4 eV. Nakano et al. also fabricated LaB 6 films by RF sputtering and discussed the relationships between internal stress and the orientation of film crystallites [5]. The two papers reported that sputtered LaB 6 films tend to have bad adhesion due to their internal stress. About the evaporated LaB 6 films, Oshima et al. measured their work function and obtained values of 2.6 eV [6]. Okamoto et al. fabricated plasma display devices with LaB6-coated Ni cathodes, which had the work functions of 2.4-2.5 eV [7]. However, fundamental properties of the films such as electric resistivity, carrier density and internal strain, have not been reported yet. Relationships between physical properties and deposition parameter have not also been clarified systematically. It is well-known that sputtered films contain large amounts of impurities such as Ar atoms and defects. These can raise the value of the internal stress, which reduces the stability and the reliability of the films in practical use. The electron beam deposition method is thought to be superior to the sputtering method from this point of view. The work function was principally measured by the thermionic emission method in previous
0169-4332/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
738
A. Yutani et al. / Work functions o f thin LaB 6 films
work [4,6]. We measured it by means of the Schottky barrier method and Kelvin probe method. But it was found that boron diffusion prevented the Schottky barrier formation in the former case. In the latter case the obtained values were about 4 eV; higher than the bulk value ( < 3 eV). Surface contamination is suspected to raise the values. By these two methods the work function is calculated only relatively (the former, to the semiconductor substrate; the latter, to the standard electrode such as Au). The work function of the standard can be influenced by the interface or surface states, and they contribute to the error. In thermionic emission method, f i l m substrate interaction does not affect the values much. The specimen heating decreases surface contamination. The absolute value of the work function is determined. At these points thermionic emission method is preferable. It is well-known that sputtered and evaporated films tend to have compressive and tensile internal stress, respectively, and the stress affects the growth of the crystallites. We have found that sputtered LaB 6 films always have compressive stress which depends on the working gas (Ar) pressure [5]. The work function can be affected by the stress. However, stress in the evaporated films and its effects on the work function have not been clarified yet. In the present p a p e r the work function of electron b e a m deposited LaB 6 films at several condition is shown. The strain in the films and its effects on the work function are also presented.
2. Experimental details The films were formed by electron b e a m deposition. The source LaB 6, manufactured by Mitsui Mining and Smelting Corp., had a purity of more than 99.5%. During the film formation, the electron acceleration voltage was kept in the range of 4.0-4.1 kV and the evaporation power was set at about 180 W. The pressure was in the range of 1 0 - 4 - 1 0 - 5 Pa (10 6-10-7 Torr). The substrates used were 5 m m wide tantalum ribbons for workfunction m e a s u r e m e n t s and Si(100) single crystals for X-ray analysis. LaB 6 was deposited through 3
m m diameter round masks. The substrate temperature was set at 300, 750 and 1000 K. Films were also fabricated on tilted substrates (45 ° and 80 ° to the incident direction of the evaporated particles) at 750 K. Lattice constants of the films were evaluated by a C u K a X-ray diffractometer to investigate the film strain. The work function was measured by means of thermionic emission in vacuum. The chamber was p u m p e d down to ~ 10 -6 Pa (10 - s Torr) by a sputter ion p u m p and a turbomolecular pump. The specimens were resistively heated to 9001400 K. An electron collector was places at a distance of 1 cm from the specimen and biased at 180 V to perform the m e a s u r e m e n t in the temperature-limited region. In this region, emission current density J is determined by R i c h a r d s o n Dushman's law: J=A**T
2 exp - ~
,
(1)
where A* *, 4~ and T are the mean effective Richardson constant (120 A cm -2 K-2), work function and temperature, respectively. According to eq. (1), a In J T -2 versus T - l plot gives a straight line and the work function 4~ is evaluated from its slope.
3. Results and discussion 3.1. Film deposition
All the films deposited were adherent, free from cracks and did not peel off. Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS) analyses showed that the atomic ratio of La : 13 is about 1 : 5. On the other hand, sputtered films tend to peel off and to be lanthanum-rich (typically, La : B = 1 : 4) [4]. Electron beam deposited films have better adhesion and are stoichiometric rather than the sputtered ones. 3.2. Film strain
Fig. 1 shows the X-ray diffraction patterns of LaB 6 films fabricated on tilted substrates (0 °, 45 °
739
A. Yutani et al. / Work functions of thin LaB 6 films
and 80 ° to the incident direction) at 750 K. All the films deposited on Si, glass and stainless steel showed similar patterns. Without substrate-heating, no peaks were observed. With 600-750 K heating, several peaks appeared. No peaks other than LaB 6 were observed in all samples. Taking the above result on the ratio of La to B into account, we consider that the films consist of LaB 6 crystallites and La-rich amorphous or amorphous-like phases such as small LaB 4 clusters. In this figure, (100) and (200) peaks are strong and ( l i d peaks are not observed, which is different from the source material. According to Nakano et al., LaB 6 films with (100) orientation have relatively weak stress ( < 3 × 10 s Pa) [5]. The diffraction patterns of the films deposited on tilted substrates at 750 K are different from the normal one. Peaks except (100) and (200) become weaker; (100) and (200) peaks become broader with the increasing incident angle. Crystallites in the films orienting to (100) direction become small with increasing angle. Fig. 2 indicates the lattice constants of the films formed with the substrate tilts of 0 °, 45 ° and 80 ° calculated from the X-ray diffraction peak positions. When the substrate is normal to the incident direction, the lattice constant is about
4.17
,--, 4.16
Bulk value
_~ _ ~ _ _ .
o 4.15 O
4.14
4.13
I
0
I
I
40
I
I
80
Substrate tilt [deg.] Fig. 2. Lattice constants of crystallites in the films fabricated with incident angles of 0°, 45° and 80°.
0.4% smaller than the bulk value. It becomes almost equal to the bulk value with the tilt of 45°; about 0.2% larger with the tilt of 80 °. Hence it is concluded that the film stress changes from tensile to compressive with increasing angle, and it is nearly free from stress at 45 °. This is in contrast with the sputtered films, which always have strong compressive stress [5].
3.3. Work function
800
A,
~'~-.-..A
45° 0o
A.
A
~.
J
(100) A
i
[
20
i
source
(110) i
i
I
I
,
(111) (200) (210)(211) ,
,
,
I
,
,
,
,
I
,
,
40
Diffraction angle [deg.] Fig. 1. X-ray diffraction patterns of LaB 6 films.
,
,
60
After exposing to the air, the emission current from all the films was small, unstable and not reproducible. Annealing at 1100 K for 60 min in 10 -8 Torr vacuum increased the emission current by several orders of magnitude and stabilized it. The emission current at 1300 K ( ~ 1000°C) was typically 10 A / m 2. Fig. 3 shows electron emission characteristics of the films fabricated at three different temperatures of 300, 750 and 1000 K. All the specimen show good linearity in the figure. The emission current values are almost the same at 300 and 750 K; a little larger at 1000 K. The values of the work function obtained are 2.8, 2.8 and 2.7 eV, respectively. These values are about 0.4 eV higher than bulk (100) value [8] and 0.1-0.4 eV higher
740
A. Yutani et al. -I0
....
I
....
t
....
/ Work functions o f thin LaB 6 films
~ ....
rent from the films deposited on tilted substrates is a little larger than the normal one. The values of the work function are 2.7-2.8 eV and they do not show remarkable dependence on the incident angle. With the results in the preceding subsection, it is concluded that the film strain does not affect the work function. This is in contrast with the sputtered films; their work function can be affected by the stress. Summarizing the above results, electron beam deposited films have work functions of 2.7-2.8 eV, even when two deposition parameters (substrate temperature and incident angle of the evaporated particles) are varied.
"\o
i:300K (2.8eV)
.~" %
" ~ . . ~
O:750K (2.8eV)
-14 ~ ' * ' ~~ , ~ ~
,
"•
<~:1000K(2.7eV)
O\
~..~-] 8
-
, , , , [ , , , , I .... 10 11
-229
i', 12
,~'.~.
13
1/kT [eV -1 ]
4. Summary
Fig. 3. Emission characteristics of LaB 6 films deposited at different temperatures (300, 750 and 1000 K).
than the previous works on evaporated films [6,7]. It is found that the substrate temperature during the deposition has little influence on the work function. Fig. 4 shows the emission characteristics of the films formed on tilted substrates (0 °, 45 ° and 80°). They also have good linearity. The emission cur-10
~
'
'
'
I
. . . .
I
_'N. "o~.~,
. . . .
I
. . . .
0 : 0 ° (2.8eV)
\og , "%~ q"~", , t ~:~
1:45 o (2.7eV)
~"~-14 k~
~ : 8 0 ° (2.7eV)
~-lS
LaB 6 films were formed by electron beam deposition at several temperatures and on tilted substrates. The crystallites in the films are mainly oriented to (100). Film strain changes from tensile to compressive with the increasing incident angle, and at 45 ° it becomes nearly free from strain. According to the thermionic emission method, the work function of the films was found to have values of 2.7-2.8 eV. It does not show remarkable dependence on substrate temperature and film strain.
Acknowledgements
The authors would thank to Dr. T. Kajiwara in Mitsubishi Electric Corp., who provided the LaB 6 source, and Mr. S. Nikaido of Nikon Corp., who determined the film stoichiometries by AES and XPS.
u o-~..<>.m."
References
-229 ' ~
10
11
I , , ~ , ~ 13 12
1/kT [eV -1] Fig. 4. Emission characteristics of LaB 6 films deposited on tilted substrates (0°, 45° and 80°).
[1] P.H. Schmidt, D.C. Joy, L.D. Longinotti and H.J. Leamy, Appl. Phys. Lett. 29 (1976) 400. [2] C. Oshima, T. Tanaka, E. Bannai and S. Kawai, J. Appl. Phys. 48 (1977) 3925. [3] L.W. Swanson, M.A. Gesley and P.R. Davis, Surf. Sci. 107 (1981) 263. [4] S.J. Mroczkowski, J. Vac. Sci. Technol. A 9 (1991) 586.
A. Yutani et al. / Work functions of thin LaB 6 films [5] T. Nakano, S. Baba, A. Kobayashi, A. Kinbara, T. Kajiwara and K. Watanabe, J. Vac. Sci. Technol. A 9 (1991) 547. [6] C. Oshima, S. Horiuchi and S. Kawai, Jpn. J. Appl. Phys. Suppl. 2 Part 1 (1974) 281.
741
[7] Y. Okamoto, T. Aida and S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [8] M. Aono, E. Bannai, T. Tanaka and S. Kawai, Appl. Phys. Lett. 31 (1977) 323.