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Microwave effect in RF magnetron sputtering of PbTiO3
Applied Surface Science 33/34 (1988) 625-631 North-Holland, Amsterdam
625
MICROWAVE E F F E C T IN RF M A G N E T R O N S P U T Y E R I N G O F PbTiO 3 M. OKUYAMA, Y. T O G A M I and Y. H A M A K A W A Department of Electrical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Received 23 August 1987; accepted for publication 15 October 1987
PbTiO 3 thin films have been prepared by RF magnetron sputtering in which microwave power at 2.45 GHz is added to activate the plasma and improve the electronic properties of the films. The plasma has been excited through electron heating induced by the microwave application. This excitation was characterized by I - V characteristics using a Langmuir probe, optical emission and mass spectroscopy. PbTiO 3 films prepared by this sputtering show better crystalline and dielectric properties than those prepared by conventional sputtering.
1. Introduction
PbTiO 3 is a good ferroelectric material showing large D - E hysteresis, high Curie-point, good pyroelectric, good piezoelectric and good electro-optic properties [1,2]. PbTiO3 ceramics are used for some applications such as an infrared sensor and a piezoelectric oscillator, however many usable devices have not yet been developed. Its application would have been widespread in various fields if its film could easily be prepared on any substrate, because of its low operating voltage, easy electrical connection and large area. Various preparation methods for these kinds of ferroelectric materials have been proposed so far, including electron beam evaporation, RF sputtering, magnetron sputtering [3-11], ion beam sputtering [12] and CVD [13]. PbTiO 3 thin films have been prepared mainly by RF sputtering for many applications such as for a memory device, infrared sensor, ultrasonic sensor and optical modulator [14]. However, there are some problems such as an inferior quality of the film on Si wafers and a high deposition temperature (500-600 o C). So we have tried to improve the quality of the PbTiO 3 thin film by adding a bias voltage in the RF magnetron sputtering [15] or selecting substrate materials [16]. In the sputtering, the sputtered particles pass through the plasma which is not very hot (about a few hundred centigrades), and are cooled down. Therefore, the sputtered particles need to be warmed up again by heating the substrate for chemical reaction. If the plasma can be heated up easily, the sputtered particles will not be cooled down so much as they approach the substrate and 0169-4332/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
626
M. OkLo,ama et al. / Microwave ef]ect in R F magnetron sputtering of PbTiO~
will react with each other to form P b T i Q . In this paper, we have tried to raise the plasma temperature by adding microwave power and so improve the growth method.
2. Preparation of PbTiO 3 thin films by microwave-assisted sputtering The deposition was carried out with a conventional R F magnetron sputtering instrument ( A N E L V A FP-21b). The target, which was PbTiO 3 powder in which 10 wt% of excess Pb304 had been added to compensate for Pb deficiency in the deposited film, was spread on a quartz plate and was put on the lower electrode. Magnets were laid on the lower RF electrode underneath the quartz plate. The sputtering gas was a mixture of Ar (90%) and 02 (10%) and the pressure was 0.1 Tort. The output power of the RF generator was 150 W and the reflected RF power was about 20 W. The 2.45 G H z microwave generator output of 0 100 W was introduced through a coaxial cable into the deposition chamber. The end of the cable was put by the side of the illuminating plasma. The other end of the cable in the sputtering chamber was also covered by ceramics to suppress any contamination of the microwave sputtering, and also was painted with PbTiO3 powder. The reflection of the microwaves in this circuit was small in comparison with the case of connecting a conventional cavity for a resonance lamp, and was changed monotonically from 0.14 to 0.35 as the output power of the microwave generator was increased from 20 to 100 W. The introduced microwaves move electrons by a Lorentz force as ions and molecules are much heavier than electrons. This electron energy is transferred to ions and molecules by collisions. The minimum gas pressure sustaining the glow discharge was reduced one sixth by the application of microwaves of input power 65 W (generator output: 100 W) as the microwaves assist ionization of molecules in the sputtering chamber. Incident and reflected RF powers were not affected by the microwave application.
3. Plasma diagnostics during the sputtering A Langmuir probe was inserted to measure I V characteristics of the plasma. The probe had a small Pt ball as its tip and was connected to a fine wire covered coaxially by ceramic and an outer metallic tube. Fig. 1 shows I - V characteristics in the forward direction. The backward current was very small and has been omitted in fig. 1. The characteristics show typical current flow limited by rectification of ions and electrons in the plasma. In the abrupt region of the current curve, the probe c u r r e n t Ip is expressed by the electron
M. Okuyama et al. / Microwave effect in R F magnetron sputtering of PbTiO 3
627
MICROWAVE ' /A 60/~AWIN65PUTPOWER ///~/
~
40
o
0W
20
0
0
50 100 VOLTAGE (V) Fig. 1. I p - Vs characteristics of a Langmuir probe in the plasma.
temperature Te, electron density he, the probe potential V~ and the plasma potential Vp as follows:
Ip=eneS(kTJ2rrm) '/2
e x p [ e ( Vs -
Vp)/kT~],
(1)
where e and m are the electronic charge and mass, S is the area of the probe and k is Boltzmann's constant. The electron temperature and density of the plasma can be determined by the slope of the In Ip versus ~ curve and the saturated current, and are shown in fig. 2. The electron temperature and density in the plasma increase monotonically as the microwave power increases. Both the temperature and density in the plasma irradiated by an input microwave power 65 W (generator output: 100 W) were almost double the values obtained without the microwaves. An increase of RF power had little effect on the temperature and density, and the electron temperature increase was only 30% when the R F generator power was changed from 50 to 200 W. A large number of the excited electrons also excite the other ions and molecules by impact. A tiny thermocouple was set close to the substrate, and its temperature increased from about 290 to 3 4 0 ° C as the input microwave power was changed from 0 to 65 W. The floating potential determined from the probe voltage of zero current decreased from 4.4 to 2 V as the microwave input power increased from 0 to 65 W. On the other hand, the plasma
628
T3
M. Okuvama et al. / Microwat,e effect in R F magnetron sputtering of PbTiO~
3
~
~
T
/ .O /
%
,
r x
O
'E
i
4
/ W n"
%
/,
2
/ /-
©
/
/
x v
P
p/ W :K iii
Z UJ a
/, : 0/
Z 0 n-"
Z 0 i
U W ~J ~J
i
GAS PRESSURE
13 Pa
I
RF
150W
POWER
!
! 0
L
W
i i ' 0
~ 20
MICROWAVE
' 40
• 60
~ 0 80
I N P U T POWER(W)
F i g . 2. Electron temperature and density of the plasma as a function of the m i c r o w a v e input
power.
potential determined from the saturated current increased from 47 to 58 V as the microwave input power was increased from 0 to 65 W. So the energy of the positive ions and molecules incident on the substrate was increased by the microwave application. Optical emission spectra from the plasma were measured by an optical multichannel analyzer (PAR Model 1215) and an optical fiber cable. The end of the optical cable was set in front of a view port of the sputtering chamber and the emission of the plasma near the substrate was focussed onto the end surface of the fiber with lenses of short focal length. Fig. 3 shows the measured spectra of the plasma with and without the microwaves. The dotted line shows the spectra of the plasma during the sputterings under 130 W RF input power (generator output: 150 W) and the solid line shows the spectra under 130 W RF input power + 31 W microwave input power (generator output: 40 W). Many emission lines appear in the spectra and almost all are assigned to the emission from Ar. The overall emission intensity increases also as the microwaves are added and the excitation of the plasma was confirmed, but there is no special increase of the lines. Mass spectroscopic analysis was carried out with a quadrupole mass spectrometer (ULVAC Model MSQ-400). The particles from the plasma
M. Okuyama et al. / Microwave effect in R F magnetron sputtering of PbTiO+
- - '
WITH
..............
WITHOUT
629
MICROWAVE MICROWAVE
Ar
Pb
: !.. " L S"
.
:.: .....~
+.
400
450
500
550
600
WAVELENGTH (nm)
Fig. 3. Optical emission spectra of the plasma with (solid line) and without (broken line) the microwave. RF and microwave input power are 130 and 31 W, respectively.
passed through a pinhole of diameter 100 ~m set in the sputtering chamber and were taken into the analyzer tube. There are many lines such as mass-charge ratio m / e = 16, 20, 32 and 40 which are identified with O +, Ar 2÷, 02+ and Ar +, respectively. Many lines of the m / e range larger than 50 were very small and were less than 10 -4 of that of m / e = 40 corresponding to Ar +, although m / e of TiO, TiO 2, Pb, PbO and PbO 2 are 64, 80, 207, 223 and 239. The signal of m / e = 16 corresponding to O + is increased by the microwave application.
4. Basic properties of the deposited PbTiO 3 thin films
The crystalline properties of the deposited films were inspected by X-ray diffraction analysis. Fig. 4 shows the X-ray diffraction pattern of the film deposited on Pt at 5 8 0 ° C and 0.08 Tort. The (100) and (001) lines are increased by the microwave application and the film is oriented, although small (110), (100), (111) and (001) do exist in the film deposited without the microwaves. The films deposited on MgO are also oriented more to the (001) and (100) by the microwave application. The film on Si was oriented from (110) to (101). This orientation is preferable because dielectric poling exists in c-axis direction.
630
M. Okuyama et al. / Microwave efJect in R F magnetron sputtering of PhTiO~
PbTiO 3 ON Pt (a)WITHOUT MICROWAVE
t
L
~,~(b)WITH
•
i
J
MICROWAVE
O0
20
30
40
50
60
28 (deg)
Fig. 4. X-ray diffraction pattern of the PbTiO~ film deposited on Pt under the application of the microwave. The dielectric constant measured at 1 MHz is increased also from 59 to 140 by the microwave application. The remanent polarization obtained from the D - E hysteresis increases about 7 times in comparison with that without the microwaves.
5. Summary. The effect of microwaves on the sputtering of PbTiO 3 has been studied. The plasma was excited by the application of the microwaves, as both the electron temperature and density increased almost twice and optical emission from the plasma was intensified in the visible wavelength region in comparison with the conventional magnetron sputtering. The dielectric constant increases almost twice and the remanent polarization was increased about 7 times by the microwave application.
Acknowledgements The authors would like to thank Messrs. I. Hirota and C. Sada of Osaka University for their technical assistance. The authors also wish to thank
M. Okuyama et al. / Microwave effect in RF magnetron sputtering of PbTiO~
631
Messrs. K. Wakino and T. Tanaka for supplying the target material. Part of this work is supported by Nakatani Electronic Measuring Technology Association of Japan.
References [1] V.G. Gabrilyachenko, R.I. Spinko, M.A. Martynenko and E.G. Fesenko, Soviet Phys.-Solid State 12 (1970) 1203. [2] H. Beerman, Infrared Phys. 15 (1975) 225. [3] W.J. Takei, N.P. Formigini and M.H. Francombe, Appl. Phys. Letters 15 (1969) 256. [4] D.W. Chapman, J. Appl. Phys. 40 (1969) 2381. [5] M. lshida, H. Matsunami and T. Tanaka, J. Appl. Phys. 48 (1977) 951. [6] A. Okada, J. Appl. Phys. 48 (1977) 2905. [7] K. Tanaka, Y. Higuma, K. Yokoyama, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. 15 (1976) 1381. [8] M. Okuyama, Y. Matsui, H. Nakano, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. 18 (1979) 1633. [9] M. Okuyama, T. Usuki, Y. Hamakawa and T. Nakagawa, Appl. Phys. 21 (1980) 339. [10] K. Iijima, S. Kawabata and I. Ueda, in: Proc. 1984 IR & MM Waves, Takarazuka, p. 39. [11] T. Shiosaki, S. Mochizuki and A. Kawabata, Ferroelectrics 63 (1985) 227. [12] R.N. Castellano and L.G. Feinstein, J. Appl. Phys. 50 (1979) 4406. [13] M. Kojima, M. Okuyama, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. Suppl. 22-2 (1983) 14. [14] M. Okuyama and Y. Hamakawa, Ferroelectrics 63 (1985) 243, [15] M. Okuyama, T. Ueda and Y. Hamakawa, Japan. J. Appl. Phys. 24-3 (1985) 3. [16] M. Okuyama, T. Ueda and Y. Hamakawa, Japan. J. Appl. Phys. 24-2 (1985) 619.
625
MICROWAVE E F F E C T IN RF M A G N E T R O N S P U T Y E R I N G O F PbTiO 3 M. OKUYAMA, Y. T O G A M I and Y. H A M A K A W A Department of Electrical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan Received 23 August 1987; accepted for publication 15 October 1987
PbTiO 3 thin films have been prepared by RF magnetron sputtering in which microwave power at 2.45 GHz is added to activate the plasma and improve the electronic properties of the films. The plasma has been excited through electron heating induced by the microwave application. This excitation was characterized by I - V characteristics using a Langmuir probe, optical emission and mass spectroscopy. PbTiO 3 films prepared by this sputtering show better crystalline and dielectric properties than those prepared by conventional sputtering.
1. Introduction
PbTiO 3 is a good ferroelectric material showing large D - E hysteresis, high Curie-point, good pyroelectric, good piezoelectric and good electro-optic properties [1,2]. PbTiO3 ceramics are used for some applications such as an infrared sensor and a piezoelectric oscillator, however many usable devices have not yet been developed. Its application would have been widespread in various fields if its film could easily be prepared on any substrate, because of its low operating voltage, easy electrical connection and large area. Various preparation methods for these kinds of ferroelectric materials have been proposed so far, including electron beam evaporation, RF sputtering, magnetron sputtering [3-11], ion beam sputtering [12] and CVD [13]. PbTiO 3 thin films have been prepared mainly by RF sputtering for many applications such as for a memory device, infrared sensor, ultrasonic sensor and optical modulator [14]. However, there are some problems such as an inferior quality of the film on Si wafers and a high deposition temperature (500-600 o C). So we have tried to improve the quality of the PbTiO 3 thin film by adding a bias voltage in the RF magnetron sputtering [15] or selecting substrate materials [16]. In the sputtering, the sputtered particles pass through the plasma which is not very hot (about a few hundred centigrades), and are cooled down. Therefore, the sputtered particles need to be warmed up again by heating the substrate for chemical reaction. If the plasma can be heated up easily, the sputtered particles will not be cooled down so much as they approach the substrate and 0169-4332/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
626
M. OkLo,ama et al. / Microwave ef]ect in R F magnetron sputtering of PbTiO~
will react with each other to form P b T i Q . In this paper, we have tried to raise the plasma temperature by adding microwave power and so improve the growth method.
2. Preparation of PbTiO 3 thin films by microwave-assisted sputtering The deposition was carried out with a conventional R F magnetron sputtering instrument ( A N E L V A FP-21b). The target, which was PbTiO 3 powder in which 10 wt% of excess Pb304 had been added to compensate for Pb deficiency in the deposited film, was spread on a quartz plate and was put on the lower electrode. Magnets were laid on the lower RF electrode underneath the quartz plate. The sputtering gas was a mixture of Ar (90%) and 02 (10%) and the pressure was 0.1 Tort. The output power of the RF generator was 150 W and the reflected RF power was about 20 W. The 2.45 G H z microwave generator output of 0 100 W was introduced through a coaxial cable into the deposition chamber. The end of the cable was put by the side of the illuminating plasma. The other end of the cable in the sputtering chamber was also covered by ceramics to suppress any contamination of the microwave sputtering, and also was painted with PbTiO3 powder. The reflection of the microwaves in this circuit was small in comparison with the case of connecting a conventional cavity for a resonance lamp, and was changed monotonically from 0.14 to 0.35 as the output power of the microwave generator was increased from 20 to 100 W. The introduced microwaves move electrons by a Lorentz force as ions and molecules are much heavier than electrons. This electron energy is transferred to ions and molecules by collisions. The minimum gas pressure sustaining the glow discharge was reduced one sixth by the application of microwaves of input power 65 W (generator output: 100 W) as the microwaves assist ionization of molecules in the sputtering chamber. Incident and reflected RF powers were not affected by the microwave application.
3. Plasma diagnostics during the sputtering A Langmuir probe was inserted to measure I V characteristics of the plasma. The probe had a small Pt ball as its tip and was connected to a fine wire covered coaxially by ceramic and an outer metallic tube. Fig. 1 shows I - V characteristics in the forward direction. The backward current was very small and has been omitted in fig. 1. The characteristics show typical current flow limited by rectification of ions and electrons in the plasma. In the abrupt region of the current curve, the probe c u r r e n t Ip is expressed by the electron
M. Okuyama et al. / Microwave effect in R F magnetron sputtering of PbTiO 3
627
MICROWAVE ' /A 60/~AWIN65PUTPOWER ///~/
~
40
o
0W
20
0
0
50 100 VOLTAGE (V) Fig. 1. I p - Vs characteristics of a Langmuir probe in the plasma.
temperature Te, electron density he, the probe potential V~ and the plasma potential Vp as follows:
Ip=eneS(kTJ2rrm) '/2
e x p [ e ( Vs -
Vp)/kT~],
(1)
where e and m are the electronic charge and mass, S is the area of the probe and k is Boltzmann's constant. The electron temperature and density of the plasma can be determined by the slope of the In Ip versus ~ curve and the saturated current, and are shown in fig. 2. The electron temperature and density in the plasma increase monotonically as the microwave power increases. Both the temperature and density in the plasma irradiated by an input microwave power 65 W (generator output: 100 W) were almost double the values obtained without the microwaves. An increase of RF power had little effect on the temperature and density, and the electron temperature increase was only 30% when the R F generator power was changed from 50 to 200 W. A large number of the excited electrons also excite the other ions and molecules by impact. A tiny thermocouple was set close to the substrate, and its temperature increased from about 290 to 3 4 0 ° C as the input microwave power was changed from 0 to 65 W. The floating potential determined from the probe voltage of zero current decreased from 4.4 to 2 V as the microwave input power increased from 0 to 65 W. On the other hand, the plasma
628
T3
M. Okuvama et al. / Microwat,e effect in R F magnetron sputtering of PbTiO~
3
~
~
T
/ .O /
%
,
r x
O
'E
i
4
/ W n"
%
/,
2
/ /-
©
/
/
x v
P
p/ W :K iii
Z UJ a
/, : 0/
Z 0 n-"
Z 0 i
U W ~J ~J
i
GAS PRESSURE
13 Pa
I
RF
150W
POWER
!
! 0
L
W
i i ' 0
~ 20
MICROWAVE
' 40
• 60
~ 0 80
I N P U T POWER(W)
F i g . 2. Electron temperature and density of the plasma as a function of the m i c r o w a v e input
power.
potential determined from the saturated current increased from 47 to 58 V as the microwave input power was increased from 0 to 65 W. So the energy of the positive ions and molecules incident on the substrate was increased by the microwave application. Optical emission spectra from the plasma were measured by an optical multichannel analyzer (PAR Model 1215) and an optical fiber cable. The end of the optical cable was set in front of a view port of the sputtering chamber and the emission of the plasma near the substrate was focussed onto the end surface of the fiber with lenses of short focal length. Fig. 3 shows the measured spectra of the plasma with and without the microwaves. The dotted line shows the spectra of the plasma during the sputterings under 130 W RF input power (generator output: 150 W) and the solid line shows the spectra under 130 W RF input power + 31 W microwave input power (generator output: 40 W). Many emission lines appear in the spectra and almost all are assigned to the emission from Ar. The overall emission intensity increases also as the microwaves are added and the excitation of the plasma was confirmed, but there is no special increase of the lines. Mass spectroscopic analysis was carried out with a quadrupole mass spectrometer (ULVAC Model MSQ-400). The particles from the plasma
M. Okuyama et al. / Microwave effect in R F magnetron sputtering of PbTiO+
- - '
WITH
..............
WITHOUT
629
MICROWAVE MICROWAVE
Ar
Pb
: !.. " L S"
.
:.: .....~
+.
400
450
500
550
600
WAVELENGTH (nm)
Fig. 3. Optical emission spectra of the plasma with (solid line) and without (broken line) the microwave. RF and microwave input power are 130 and 31 W, respectively.
passed through a pinhole of diameter 100 ~m set in the sputtering chamber and were taken into the analyzer tube. There are many lines such as mass-charge ratio m / e = 16, 20, 32 and 40 which are identified with O +, Ar 2÷, 02+ and Ar +, respectively. Many lines of the m / e range larger than 50 were very small and were less than 10 -4 of that of m / e = 40 corresponding to Ar +, although m / e of TiO, TiO 2, Pb, PbO and PbO 2 are 64, 80, 207, 223 and 239. The signal of m / e = 16 corresponding to O + is increased by the microwave application.
4. Basic properties of the deposited PbTiO 3 thin films
The crystalline properties of the deposited films were inspected by X-ray diffraction analysis. Fig. 4 shows the X-ray diffraction pattern of the film deposited on Pt at 5 8 0 ° C and 0.08 Tort. The (100) and (001) lines are increased by the microwave application and the film is oriented, although small (110), (100), (111) and (001) do exist in the film deposited without the microwaves. The films deposited on MgO are also oriented more to the (001) and (100) by the microwave application. The film on Si was oriented from (110) to (101). This orientation is preferable because dielectric poling exists in c-axis direction.
630
M. Okuyama et al. / Microwave efJect in R F magnetron sputtering of PhTiO~
PbTiO 3 ON Pt (a)WITHOUT MICROWAVE
t
L
~,~(b)WITH
•
i
J
MICROWAVE
O0
20
30
40
50
60
28 (deg)
Fig. 4. X-ray diffraction pattern of the PbTiO~ film deposited on Pt under the application of the microwave. The dielectric constant measured at 1 MHz is increased also from 59 to 140 by the microwave application. The remanent polarization obtained from the D - E hysteresis increases about 7 times in comparison with that without the microwaves.
5. Summary. The effect of microwaves on the sputtering of PbTiO 3 has been studied. The plasma was excited by the application of the microwaves, as both the electron temperature and density increased almost twice and optical emission from the plasma was intensified in the visible wavelength region in comparison with the conventional magnetron sputtering. The dielectric constant increases almost twice and the remanent polarization was increased about 7 times by the microwave application.
Acknowledgements The authors would like to thank Messrs. I. Hirota and C. Sada of Osaka University for their technical assistance. The authors also wish to thank
M. Okuyama et al. / Microwave effect in RF magnetron sputtering of PbTiO~
631
Messrs. K. Wakino and T. Tanaka for supplying the target material. Part of this work is supported by Nakatani Electronic Measuring Technology Association of Japan.
References [1] V.G. Gabrilyachenko, R.I. Spinko, M.A. Martynenko and E.G. Fesenko, Soviet Phys.-Solid State 12 (1970) 1203. [2] H. Beerman, Infrared Phys. 15 (1975) 225. [3] W.J. Takei, N.P. Formigini and M.H. Francombe, Appl. Phys. Letters 15 (1969) 256. [4] D.W. Chapman, J. Appl. Phys. 40 (1969) 2381. [5] M. lshida, H. Matsunami and T. Tanaka, J. Appl. Phys. 48 (1977) 951. [6] A. Okada, J. Appl. Phys. 48 (1977) 2905. [7] K. Tanaka, Y. Higuma, K. Yokoyama, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. 15 (1976) 1381. [8] M. Okuyama, Y. Matsui, H. Nakano, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. 18 (1979) 1633. [9] M. Okuyama, T. Usuki, Y. Hamakawa and T. Nakagawa, Appl. Phys. 21 (1980) 339. [10] K. Iijima, S. Kawabata and I. Ueda, in: Proc. 1984 IR & MM Waves, Takarazuka, p. 39. [11] T. Shiosaki, S. Mochizuki and A. Kawabata, Ferroelectrics 63 (1985) 227. [12] R.N. Castellano and L.G. Feinstein, J. Appl. Phys. 50 (1979) 4406. [13] M. Kojima, M. Okuyama, T. Nakagawa and Y. Hamakawa, Japan. J. Appl. Phys. Suppl. 22-2 (1983) 14. [14] M. Okuyama and Y. Hamakawa, Ferroelectrics 63 (1985) 243, [15] M. Okuyama, T. Ueda and Y. Hamakawa, Japan. J. Appl. Phys. 24-3 (1985) 3. [16] M. Okuyama, T. Ueda and Y. Hamakawa, Japan. J. Appl. Phys. 24-2 (1985) 619.