- Email: [email protected]
Low temperature deposition of Ba0.4Sr0.6TiO3 thin films on LaNiO3-buffered electrode by rf magnetron sputtering
November
1997
Materials Letters 33 (1997) 97-100
ELSEVIER
Low temperature deposition of Ba,,Sr,,TiO, thin films on LaNiO,-buffered electrode by rf magnetron sputtering Chii-Ming Wu, Tai-Bor Wu
*
Deparhnent of Materials Science and Engineering, National Tsing Hua Uniuersity, Hsinchu, Taiwan, ROC Received
12 March 1997; accepted
17 March 1997
Abstract Sputter-deposited LaNiO, (LNO) was used as a conductive buffer layer for the deposition of 80 nm thick (Ba,,,Sr,,,)TiO, (BST) thin films on Pt/Ti/SiO,/Si substrates by rf magnetron sputtering. Smooth and highly (lOO)-oriented perovskite films of BST were grown on the (lOO)-textured LNO by deposition at temperatures 2 200°C. However, a relatively rough and weakly crystallized BST film was obtained by deposition directly on Pt/Ti/SiO,/Si substrates at 500°C. A stoichiometric composition of BST films was closely reached by using the target having 30-40 mol% enrichment of Ba + Sr. The dielectric constant of the BST films notably depended on the film composition and the electrode used for deposition. The 80 nm thick films deposited on LNO containing a slight excess of Ba + Sr exhibited the largest dielectric constant of k = 150 to 310 for deposition at temperatures from 350 to 550°C. All the films showed a low leakage of current < lop9 A/cm2 under an applied voltage within l-2 V. Moreover, the films deposited on LNO would have a higher resistance against the transition of conduction to the high leakage mode than that of the film deposited on Pt. 0 1997 Elsevier Science B.V. PACS: 77.55. + f; 77.84:-s; 77.84.D~ Keywords: Sputtering;
Magnetron;
LaNiO,;
Ba,,,Sr,,TiO,;
Films; Electrode;
1. Introduction Barium
strontium
titanate
(BST)
thin
films
are
high dielectric materials in fabricating cell capacitors for gigabit dynamic random access memories (DRAMS) [l-3]. Noble metal electrodes, such as Pt or Pt/Ti, are generally used for the deposition of BST. However, a high deposition temperature, e.g., 2 6OO”C, was often used to achieve a large
promising
* Corresponding
author. Fax: + 886-3-5722366.
Leakage
current: Large dielectric
constant
dielectric constant due to the low crystallization ability of BST. It, unfortunately, is detrimental to the process integration of the Si devices, and, moreover, may also deteriorate the electrical leakage characteristics of the BST films [4-61. In order to reduce the process temperature for BST thin films, deposition on substrates buffered with a conductive layer of LaNiO, (LNO) by rf magnetron sputtering was studied in this work, which has proved to be quite advantageous for the deposition of perovskite-type ferroelectric thin films other than BST [7-101, due to its good crystallographic compatibility.
00167-577X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-577X(97)00083-9
98
C.-M. Wu, T.-B. Wu/Materials
Letters 33 (1997) 97-100
2. Experimental
=‘
Highly (loo)-oriented LNO buffer layers, 250 nm thick, were prepared on Pt(150 nm)/Ti(SO nM)/SiO,/Si substrates by rf magnetron sputtering deposition at 300°C. Details of the LNO deposition was described elsewhere [l 11. The BST thin films were then deposited on the LNO-buffered substrate by t-f magnetron sputtering at temperatures ranging from 150°C to 500°C with the use of BST targets containing excess Ba and Sr of lo-60 mol%. The substrate was placed on top and parallel to the target at a distance of 5.0 cm. An OJAr mixture gas of lo/90 ratio under a working pressure of 10 mTorr was used for the deposition with an rf power density of 3.0 W/cm*.
3. Results and discussion The influence of target composition on the film composition was first examined by inductively cou-
I
-1 (a)
0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Y
jit; g
--.._A~
, ,o 0.9 ‘? m
m
08
::
I _/
0.7 -
/
0.6 0.5
” 0.0
” 0.1
a 0.2
f 0.3
” 0.4
a 0.5
a 0.6
0.7
Y
Fig. 1. The measured atomic ratios of: (a) Sr/(Ba + Sr), and (b) (Ba+ Sr)/Ti of the BST films deposited on LNO at 450°C from targets having different excess content of Ba + Sr (y-value).
20
30
40
60
60
2e Fig. 2. XRD patterns of the 80 nm thick BST thin films deposited at 500°C on LNO-buffered electrode from targets of different y-value and on Pt electrode from target of y = 0.4.
pled plasma-mass (ICP) spectrometer with films deposited on LNO-buffered Si substrates without the insertion of Pt/Ti layers to avoid the interference from Ti layer. Fig. 1 shows the measured atomic ratios of Sr/(Ba + Sr) and (Ba + Sr)/Ti, respectively, of the 450”C-deposited BST films obtained from targets having different excess content of Ba + Sr (y-value). It was found that the relative content of Ba and Sr in the films was nearly the same as that in the targets, but the Ti had a quite higher deposition rate than those of Ba and Sr. A stoichiometric composition of BST films was closely reached by using the target having 30-40 mol% enrichment of Ba + Sr. A change of deposition temperature only insignificantly affected the resulted film composition. Fig. 2 shows the typical XRD patterns of the BST thin films of 80 nm thick deposited on LNO/Pt/Ti/SiO,/Si substrates at 500°C using targets of different excess content of Ba + Sr. Although the films had quite different content of excess Ti or Ba + Sr, all of them showed intense and highly (lOO)-oriented patterns of the perovskite phase of BST. For a comparison, the XRD pattern of a BST film deposited directly on the Pt/Ti electrode from the target of y = 0.4 under the same sputtering condition is also shown in Fig. 2. It is interesting to see that the film was only weakly crystallized and without (100) preferred orientation. The above results clearly indicate that the (lOO)-textured LNO layer not only controls the orientation but also greatly
C.-M. Wu, T.-B. Wu/Materials
Letters 33 (1997) 97-100
3w
_
0.1
0.7
4OcPc
.
-A-450%
250 -
--V500°C
501
0.2
1
--o-35ooc -o-
J--m
99
I 0.1
0.2
0.3
Y Fig. 3. Crystallization of perovskite phase in 80 nm thick BST films deposited on LNO from targets of different y-values at various
temperatures.
promotes the crystallization of the BST films. Fig. 3 summarizes the XRID result of perovskite phase formation in the BST -Films deposited on LNO at various temperatures prepared from targets having different content of excess Ba + Sr. It is worth to point out that the perovskite phase was quite easy to form in the BST films containing excess Ba + Sr at deposition temperatures as low as 2OO”C, comparing to that in the films containing excess Ti, for which the perovskite phase was formed at temperatures 2 350°C. A similar effect of excess Ba + Sr on the crystallization abiliry of BST films was also observed by other researchers [12]. For electrical property measurements, BST capacitors in a mesa configuration of 100 mm in diameter were made by photo lithography with Pt top electrodes coated by rf magnetron sputtering at room temperature. The dielectric constant at 10 kHz of the above 80 run thick :BST films as a function of target composition is shown in Fig. 4. In consistence with the previous observation of the effect of LNO buffer layer and excess Ba + Sr content on promoting the crystallization of BST films, the films deposited on LNO showed a highler dielectric constant than that of the film deposited on Pt of the same composition, and the films prepared from targets of y 2 0.4 also had a higher dielectric constant than that from targets of y I 0.3 prepared at the same temperature. Among them, the films containing the least excess Ba + Sr
0.4
0.5
0.6
0.7
Y Fig. 4. Dielectric constant (k) of the 80 nm thick BST thin films deposited on LNO at various temperatures and on Pt at 5OO”C, measured at 10 kHz.
(i.e., the one from y = 0.4) exhibited the largest dielectric constant. Satisfactory dielectric constant of 150-3 10 cloud be achieved for deposition at temperatures ranging from 350 to 550°C. The characteristic of current density (J) versus applied voltage (V) under different polarity of bias was then investigated for the above capacitors. The measurements were conducted with a HP 4140B using a duration time of 30 s before taking the current data at each voltage step. Fig. 5 shows the
y=O.3 /LNO + bias y=O.3 /LNO - bias -~.~~~~~~~ y=O.4 /LNO + bias
------
y=O.4 /l-NO - bias y=O.4 /Pt + bias y=O.4 IPt - bias
0.1
10
1 Voltage
Ll
100
(V )
Fig. 5. The current-voltage characteristics of the Pt/BST/LNO capacitors with BST films prepared from targets of y = 0.3 and 0.4 and the Pt/BST/Pt capacitors of y = 0.4. The BST films were deposited at 500°C.
loo
C.-M. Wu, T.-B. Wu /Materials
typical results of the two on LNO from targets of y = 0.3 and 0.4, as well as the one on Pt deposited at 500°C. All the capacitors had a low leakage current, in the low voltage regime up to a < 10m9 A/cm’, transition voltage around l-2 V. Among them, the one made with BST film containing a slight excess content of Ti (i.e., the one form y = 0.3) exhibited a slightly lower leakage than the other two containing slightly excess Ba + Sr, indicating the lower bulk conductivity of the former BST film which is in accordance with the result observed by other researchers [13]. Also shown in Fig. 5, beyond the transition voltage, the leakage of the three capacitors rapidly increased with the applied voltage. However, it is important to notice that the two of BST films deposited on LNO were more resistive against the transition of conduction to the high leakage mode than that of the film deposited on Pt. Similar results were observed for other BST films deposited on LNO. Such an improvement of leakage characteristic most likely resulted from a reduction of inhomogeneous distribution of applied field in the BST films deposited on LNO because a quite smooth surface was found to form for the films on LNO compared to that of films deposited on Pt, as observed from the atomic force microscopy, that the former had a root mean square surface roughness of 1.6 nm but the latter 2.6 nm.
4. Conclusion In conclusion, it is shown in this work that the use of (100) textured LaNiO, as a conductive buffer layer for the deposition of (Ba,,Sr,.,)TiO, is quite advantageous in improving the crystallization ability of the BST thin films deposited by rf magnetron sputtering, especially for the thin films containing a slight excess of Ba and Sr. Satisfactory dielectric constant of 150 to 310 were achieved for the films of 80 nm thick deposited at temperatures ranging from 350 to 550°C. In contrast, the film of the same composition but deposited on Pt at 500°C was only weakly crystallized and had a low dielectric constant
Letters 33 (1997) 97-100
of 120. The films in a Pt/BST/LNO capacitor configuration also exhibited a low leakage current < 1O-9 A/cm* under an applied voltage within l-2 V. Moreover, the films were highly (100)oriented and had a quite smooth surface morphology which led to a higher resistance against the transition of conduction to the high leakage mode, as compared to that of the film deposited on Pt.
Acknowledgements The authors would like to acknowledge the National Science Council of the Republic of China for financial support under contract number NSC852216-EOO7-036.
References [II A.I. Kingon, SK. Streiffer, C. Basceri, S.R. Summerfelt, MRS Bull. 20 (1996) 46. 121T. Kaga, M. Ohkura, F. Murai, N. Yokoyama, E. Takeda, J. Vat. Sci. Technol. B 13 (1995) 2329. [31 H. Achard, H. Mace, in: 0. Auciello and R. Waser (Eds.), Science and Technology of Electroceramic Thin Films, Kluwer Academic Publishers, Dordrecht, 1994, p. 353. [41 S.O. Park, C.S. Hwang, H.J. Cho, C.S. Kang, H.K. Kang, S.I. Lee, M.Y. Lee, Jpn. J. Appl. Phys. 35 (1996) 1548. N. Mikami, T. Makita, J. Tanimura, M. El T. Horikawa, Kataoka, K. Sato, M. Nunoshita, Jpn. J. Appl. Phys. 32 (1993) 4126. t61T. Kuroiwa, Y. Tsunemine, T. Horikawa, T. Makita, J. Tanimura, N. Mikami, K. Sato, Jpn. J. Appl. Phys. 33 (1994) 5187. 171 M.J. Shyu, T.J. Hong, T.B. Wu, Mater. Lett. 23 (1995) 221. 181MS. Chen, J.M. Wu, T.B. Wu, Jpn. J. Appl. Phys. 34 (1995) 4870. [91 T.F. Tseng, K.S. Liu, T.B. Wu, I.N. Lin, Appl. Phys. Lett. 68 (1996) 2505. [lOI M.J. Shyu, T.J. Hong, T.J. Yang, T.B. Wu, Jpn. J. Appl. Phys. 34 (1995) 3647. 1111 CC. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. [121 S. Yamamichi, H. Yabuta, T. Sakuma, Y. Miyasaka, Appl. Phys. Lett. 64 (1994) 1644. [I31 P.C.V. Buskirk, S.M. Bilodeau, J.F. Roeder, P.S. Kirlin, Jpn. J. Appl. Phys. 35 (1996) 2520.
1997
Materials Letters 33 (1997) 97-100
ELSEVIER
Low temperature deposition of Ba,,Sr,,TiO, thin films on LaNiO,-buffered electrode by rf magnetron sputtering Chii-Ming Wu, Tai-Bor Wu
*
Deparhnent of Materials Science and Engineering, National Tsing Hua Uniuersity, Hsinchu, Taiwan, ROC Received
12 March 1997; accepted
17 March 1997
Abstract Sputter-deposited LaNiO, (LNO) was used as a conductive buffer layer for the deposition of 80 nm thick (Ba,,,Sr,,,)TiO, (BST) thin films on Pt/Ti/SiO,/Si substrates by rf magnetron sputtering. Smooth and highly (lOO)-oriented perovskite films of BST were grown on the (lOO)-textured LNO by deposition at temperatures 2 200°C. However, a relatively rough and weakly crystallized BST film was obtained by deposition directly on Pt/Ti/SiO,/Si substrates at 500°C. A stoichiometric composition of BST films was closely reached by using the target having 30-40 mol% enrichment of Ba + Sr. The dielectric constant of the BST films notably depended on the film composition and the electrode used for deposition. The 80 nm thick films deposited on LNO containing a slight excess of Ba + Sr exhibited the largest dielectric constant of k = 150 to 310 for deposition at temperatures from 350 to 550°C. All the films showed a low leakage of current < lop9 A/cm2 under an applied voltage within l-2 V. Moreover, the films deposited on LNO would have a higher resistance against the transition of conduction to the high leakage mode than that of the film deposited on Pt. 0 1997 Elsevier Science B.V. PACS: 77.55. + f; 77.84:-s; 77.84.D~ Keywords: Sputtering;
Magnetron;
LaNiO,;
Ba,,,Sr,,TiO,;
Films; Electrode;
1. Introduction Barium
strontium
titanate
(BST)
thin
films
are
high dielectric materials in fabricating cell capacitors for gigabit dynamic random access memories (DRAMS) [l-3]. Noble metal electrodes, such as Pt or Pt/Ti, are generally used for the deposition of BST. However, a high deposition temperature, e.g., 2 6OO”C, was often used to achieve a large
promising
* Corresponding
author. Fax: + 886-3-5722366.
Leakage
current: Large dielectric
constant
dielectric constant due to the low crystallization ability of BST. It, unfortunately, is detrimental to the process integration of the Si devices, and, moreover, may also deteriorate the electrical leakage characteristics of the BST films [4-61. In order to reduce the process temperature for BST thin films, deposition on substrates buffered with a conductive layer of LaNiO, (LNO) by rf magnetron sputtering was studied in this work, which has proved to be quite advantageous for the deposition of perovskite-type ferroelectric thin films other than BST [7-101, due to its good crystallographic compatibility.
00167-577X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO167-577X(97)00083-9
98
C.-M. Wu, T.-B. Wu/Materials
Letters 33 (1997) 97-100
2. Experimental
=‘
Highly (loo)-oriented LNO buffer layers, 250 nm thick, were prepared on Pt(150 nm)/Ti(SO nM)/SiO,/Si substrates by rf magnetron sputtering deposition at 300°C. Details of the LNO deposition was described elsewhere [l 11. The BST thin films were then deposited on the LNO-buffered substrate by t-f magnetron sputtering at temperatures ranging from 150°C to 500°C with the use of BST targets containing excess Ba and Sr of lo-60 mol%. The substrate was placed on top and parallel to the target at a distance of 5.0 cm. An OJAr mixture gas of lo/90 ratio under a working pressure of 10 mTorr was used for the deposition with an rf power density of 3.0 W/cm*.
3. Results and discussion The influence of target composition on the film composition was first examined by inductively cou-
I
-1 (a)
0.8
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Y
jit; g
--.._A~
, ,o 0.9 ‘? m
m
08
::
I _/
0.7 -
/
0.6 0.5
” 0.0
” 0.1
a 0.2
f 0.3
” 0.4
a 0.5
a 0.6
0.7
Y
Fig. 1. The measured atomic ratios of: (a) Sr/(Ba + Sr), and (b) (Ba+ Sr)/Ti of the BST films deposited on LNO at 450°C from targets having different excess content of Ba + Sr (y-value).
20
30
40
60
60
2e Fig. 2. XRD patterns of the 80 nm thick BST thin films deposited at 500°C on LNO-buffered electrode from targets of different y-value and on Pt electrode from target of y = 0.4.
pled plasma-mass (ICP) spectrometer with films deposited on LNO-buffered Si substrates without the insertion of Pt/Ti layers to avoid the interference from Ti layer. Fig. 1 shows the measured atomic ratios of Sr/(Ba + Sr) and (Ba + Sr)/Ti, respectively, of the 450”C-deposited BST films obtained from targets having different excess content of Ba + Sr (y-value). It was found that the relative content of Ba and Sr in the films was nearly the same as that in the targets, but the Ti had a quite higher deposition rate than those of Ba and Sr. A stoichiometric composition of BST films was closely reached by using the target having 30-40 mol% enrichment of Ba + Sr. A change of deposition temperature only insignificantly affected the resulted film composition. Fig. 2 shows the typical XRD patterns of the BST thin films of 80 nm thick deposited on LNO/Pt/Ti/SiO,/Si substrates at 500°C using targets of different excess content of Ba + Sr. Although the films had quite different content of excess Ti or Ba + Sr, all of them showed intense and highly (lOO)-oriented patterns of the perovskite phase of BST. For a comparison, the XRD pattern of a BST film deposited directly on the Pt/Ti electrode from the target of y = 0.4 under the same sputtering condition is also shown in Fig. 2. It is interesting to see that the film was only weakly crystallized and without (100) preferred orientation. The above results clearly indicate that the (lOO)-textured LNO layer not only controls the orientation but also greatly
C.-M. Wu, T.-B. Wu/Materials
Letters 33 (1997) 97-100
3w
_
0.1
0.7
4OcPc
.
-A-450%
250 -
--V500°C
501
0.2
1
--o-35ooc -o-
J--m
99
I 0.1
0.2
0.3
Y Fig. 3. Crystallization of perovskite phase in 80 nm thick BST films deposited on LNO from targets of different y-values at various
temperatures.
promotes the crystallization of the BST films. Fig. 3 summarizes the XRID result of perovskite phase formation in the BST -Films deposited on LNO at various temperatures prepared from targets having different content of excess Ba + Sr. It is worth to point out that the perovskite phase was quite easy to form in the BST films containing excess Ba + Sr at deposition temperatures as low as 2OO”C, comparing to that in the films containing excess Ti, for which the perovskite phase was formed at temperatures 2 350°C. A similar effect of excess Ba + Sr on the crystallization abiliry of BST films was also observed by other researchers [12]. For electrical property measurements, BST capacitors in a mesa configuration of 100 mm in diameter were made by photo lithography with Pt top electrodes coated by rf magnetron sputtering at room temperature. The dielectric constant at 10 kHz of the above 80 run thick :BST films as a function of target composition is shown in Fig. 4. In consistence with the previous observation of the effect of LNO buffer layer and excess Ba + Sr content on promoting the crystallization of BST films, the films deposited on LNO showed a highler dielectric constant than that of the film deposited on Pt of the same composition, and the films prepared from targets of y 2 0.4 also had a higher dielectric constant than that from targets of y I 0.3 prepared at the same temperature. Among them, the films containing the least excess Ba + Sr
0.4
0.5
0.6
0.7
Y Fig. 4. Dielectric constant (k) of the 80 nm thick BST thin films deposited on LNO at various temperatures and on Pt at 5OO”C, measured at 10 kHz.
(i.e., the one from y = 0.4) exhibited the largest dielectric constant. Satisfactory dielectric constant of 150-3 10 cloud be achieved for deposition at temperatures ranging from 350 to 550°C. The characteristic of current density (J) versus applied voltage (V) under different polarity of bias was then investigated for the above capacitors. The measurements were conducted with a HP 4140B using a duration time of 30 s before taking the current data at each voltage step. Fig. 5 shows the
y=O.3 /LNO + bias y=O.3 /LNO - bias -~.~~~~~~~ y=O.4 /LNO + bias
------
y=O.4 /l-NO - bias y=O.4 /Pt + bias y=O.4 IPt - bias
0.1
10
1 Voltage
Ll
100
(V )
Fig. 5. The current-voltage characteristics of the Pt/BST/LNO capacitors with BST films prepared from targets of y = 0.3 and 0.4 and the Pt/BST/Pt capacitors of y = 0.4. The BST films were deposited at 500°C.
loo
C.-M. Wu, T.-B. Wu /Materials
typical results of the two on LNO from targets of y = 0.3 and 0.4, as well as the one on Pt deposited at 500°C. All the capacitors had a low leakage current, in the low voltage regime up to a < 10m9 A/cm’, transition voltage around l-2 V. Among them, the one made with BST film containing a slight excess content of Ti (i.e., the one form y = 0.3) exhibited a slightly lower leakage than the other two containing slightly excess Ba + Sr, indicating the lower bulk conductivity of the former BST film which is in accordance with the result observed by other researchers [13]. Also shown in Fig. 5, beyond the transition voltage, the leakage of the three capacitors rapidly increased with the applied voltage. However, it is important to notice that the two of BST films deposited on LNO were more resistive against the transition of conduction to the high leakage mode than that of the film deposited on Pt. Similar results were observed for other BST films deposited on LNO. Such an improvement of leakage characteristic most likely resulted from a reduction of inhomogeneous distribution of applied field in the BST films deposited on LNO because a quite smooth surface was found to form for the films on LNO compared to that of films deposited on Pt, as observed from the atomic force microscopy, that the former had a root mean square surface roughness of 1.6 nm but the latter 2.6 nm.
4. Conclusion In conclusion, it is shown in this work that the use of (100) textured LaNiO, as a conductive buffer layer for the deposition of (Ba,,Sr,.,)TiO, is quite advantageous in improving the crystallization ability of the BST thin films deposited by rf magnetron sputtering, especially for the thin films containing a slight excess of Ba and Sr. Satisfactory dielectric constant of 150 to 310 were achieved for the films of 80 nm thick deposited at temperatures ranging from 350 to 550°C. In contrast, the film of the same composition but deposited on Pt at 500°C was only weakly crystallized and had a low dielectric constant
Letters 33 (1997) 97-100
of 120. The films in a Pt/BST/LNO capacitor configuration also exhibited a low leakage current < 1O-9 A/cm* under an applied voltage within l-2 V. Moreover, the films were highly (100)oriented and had a quite smooth surface morphology which led to a higher resistance against the transition of conduction to the high leakage mode, as compared to that of the film deposited on Pt.
Acknowledgements The authors would like to acknowledge the National Science Council of the Republic of China for financial support under contract number NSC852216-EOO7-036.
References [II A.I. Kingon, SK. Streiffer, C. Basceri, S.R. Summerfelt, MRS Bull. 20 (1996) 46. 121T. Kaga, M. Ohkura, F. Murai, N. Yokoyama, E. Takeda, J. Vat. Sci. Technol. B 13 (1995) 2329. [31 H. Achard, H. Mace, in: 0. Auciello and R. Waser (Eds.), Science and Technology of Electroceramic Thin Films, Kluwer Academic Publishers, Dordrecht, 1994, p. 353. [41 S.O. Park, C.S. Hwang, H.J. Cho, C.S. Kang, H.K. Kang, S.I. Lee, M.Y. Lee, Jpn. J. Appl. Phys. 35 (1996) 1548. N. Mikami, T. Makita, J. Tanimura, M. El T. Horikawa, Kataoka, K. Sato, M. Nunoshita, Jpn. J. Appl. Phys. 32 (1993) 4126. t61T. Kuroiwa, Y. Tsunemine, T. Horikawa, T. Makita, J. Tanimura, N. Mikami, K. Sato, Jpn. J. Appl. Phys. 33 (1994) 5187. 171 M.J. Shyu, T.J. Hong, T.B. Wu, Mater. Lett. 23 (1995) 221. 181MS. Chen, J.M. Wu, T.B. Wu, Jpn. J. Appl. Phys. 34 (1995) 4870. [91 T.F. Tseng, K.S. Liu, T.B. Wu, I.N. Lin, Appl. Phys. Lett. 68 (1996) 2505. [lOI M.J. Shyu, T.J. Hong, T.J. Yang, T.B. Wu, Jpn. J. Appl. Phys. 34 (1995) 3647. 1111 CC. Yang, M.S. Chen, T.J. Hong, C.M. Wu, J.M. Wu, T.B. Wu, Appl. Phys. Lett. 66 (1995) 2643. [121 S. Yamamichi, H. Yabuta, T. Sakuma, Y. Miyasaka, Appl. Phys. Lett. 64 (1994) 1644. [I31 P.C.V. Buskirk, S.M. Bilodeau, J.F. Roeder, P.S. Kirlin, Jpn. J. Appl. Phys. 35 (1996) 2520.