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Pulsed laser deposition of barium titanate films on silicon
Solid State Communications, Vol. 71, No. 1, PP. 9-12, 1989. Printedin Great Britain.
RllsBDIAsJlR~ITIaeoF~-
FILM
0038-1098/89$3.00+.00 MaxwellPergsmonMacmillanplc
Ix
SILIam
RashmiNawathey,R.D.Vispute, S.M.Chaudhari, S.M.Kanetkarand S.B.Ogale Departmentof Ehysics,Universityof Foona,Pune - 411 007, INDIA (ReceivedApril 11, 1989 by C.N.R.Rao) Barium titanate thin films have been deposited on single crystal siliconby using pulsed ruby laser inducedvaporizationfrom sintered bulk material.The dependenceof the film proportieson laser energy density,oxygen partialpressureduring depositionand substratebias has been investigated snd optimum conditionshave been broughtout for obtainingsinglephase stoichiometric thin films. The films have been characterized by Small angleX-ray Diffraction(XRD),ScanningElectron Microscopy(SEM)and Spectroscopic ellipsometry.
Amongst ferroelectric substancesBaTiO3is perhaps the most widely investigated material becauseof its extremelyinteresting structurespecific proporties which are of great importance to device technologyl.Also, this materialbeing chemicallyand mechanically very stable it can be used in a variety of application environments. Most of the applications of this materialexploredtilltodate use its sinteredpolycrystallineceramic form suitablyprocessedand doped to generate the desiredferroelectricresponse.Recently, with the growingemphasisof modern technology on miniaturization, a need is felt for development of processes for obtaining good qualitythin films of such materials.Moreover, it is further desirable if such processes are dry-processingtype and are compatible with similar other processes. pulsed laser deposition is one such process which is receiving considerable attention recently, since its successfulapplicationto deposition of hot superconductor films2-5. In this communication, we have appliedthe pulsedlaser evaporationprocess to depositbarium titanate thin films on silicon.In additionto being an importantmaterialin its own right,BaTi has also been used as one of the barrierlayersfor depositionof superconductor films on Siliconb. Hence laserdevositionof BaTi on siliconcan be regarded as an importan< step towards development of an all-laser process for deposition of oxide superconductorfilms on silicon for realization of superconductorelectronics. BaTi pellet used as a target for laser vaporization was synthesizedby using sol-gel routey. One such pellet was mounted in a chamber pumped by a varian diffstack system capable of ‘renderingan ultimate vacuum of 5 x10-7 Torr.TheRuby laser beam was derived from J.K. laser (system 2000). capable of yieldinga maximum energyof 10 Joulesjpulse at a pulse repetition rate of 6 pulseslmin. The laser pulse duration was 30-ns. The target orientation was at 450 with respect to the direction of laser beam.The distance between
the target and the substratesurfacewas 3 cm. The substrateholder was mounted on a heater assembly capable of raising the substrate temperature to about 500°C. To control the oxygen partialpressurea precisionleak valve was used.The depositionswere carried out on (111) oriented single crystal silicon substrates, at a substratetemperatureof 45oOC by varying&e other depositionparameterssuch as energy density, oxygen partial pressure, substratebias etc. Typicaldepositionrate was found to be lo-15 AO/pulse depending on the depositionparameters. The depositedfilms were characterized by small angleX-ray diffraction (performed on Rigaku, Japan machine, Model no. Rotaflex RU For small angle 200 B ), using -radiation. XRD measurements Seeman-Bohlingeometry was usedg. The morphology of these films was studied using scanning electron microscope (Cambridge-Strioscan 150 machine). The dielectricpropertiesof the film were obtained using spectroscopic ellipsometer (SOPRA, France). The qualityof the laser evaporatedoxide films is known to dependupon the laser energy density as well as ambient oxygen partial pressure9, hence, in this experiment,these two parameterswere variedsystematically to obtain the desiredstoichiometryof the ferroelectric material.The small angle XRD patternof BaTi pellettakenat a glancingangle of lo is shown in fig.l(a).All the difxrasion lines match closely with those reported10 for polycrystalline barium titanate.Initially,the deposition was carried out at lower energy density of 6.0 J/cm2 and oxygen partial pressure of 5 x 10-b Torr. The corresponding XRD pattern is shown in fig.l(b).There is nz indication of the formation of BaTi chase. When the laser energydensitywas incrgasedto 10.0J/cm2 and oxygenpartialpressurewas kept constant,presenceof a small amount of BaTiO3 could be discerned from the XRD pattern [fig.l(c)]. Alongwith the required tetragonal BaTiO3phase however, differentstoichiometric phases were also seen. Though the structural
I0
PULSED LASER DEPOSITION OF BARII/M TITANATE FILMS ON SILICON
Vol. 71, No. I
(a)
(d)
(b)
(e)
(¢1
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A ~n
&.
<[ >z LO P Z
200 300 400 500
7
2 8 (Degrees)
L
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=
FIG. I - Small angle X-ray diffraction patterns of (a) BaTiO 3 pellet and thin films deposited at substrate temperature of 450°C and (b) Energy density = 6 J/cm2,02P.P.=5x10-4 Torr. (c) Energy density = 10 J/cm 2,O2P.P.=5x10-4 Torr. (d) Energy density = 13 J/cm2,02P.P.=Ix10"J Torr. (e) Energydensity = 13 J/cm2, O2P.p. =IxI0-I Torr. (f) Energy density = 15 J/cm 2, 02P,P. = Ixi0 -I Tort. quality of this film was poor, it became clear that increase in energy density did influence the formation of BaTiO 3. Hence the energy density was further increased to 13.0 J/cm 2 and oxygen partial pressure was also increased to 10- Tort. The corresponding XRD pattern is given in fig.1(d) and it clearly shows improvement in the film quality as compared to the case of fig.1(c). In order to still improve the quality of film, oxygen partial pressure was increased to 10 -I Torr keeping the energy density at 13.0 J/cm 2. This condition led to the formation of a stoichiometric BaTiO 3 thin film [fig.1(e)]. When this XRD pattern is compared with that of BaTiO 3 pellet, it is seen that the laser evaporated thin film exhibits some texture effects possibly due to partial preferred grain orientation. It was found that these texture effects and slight broadening of the diffraction lines could be avoided by the laser evaporation of the BaTi03 pellet at the energy density of 15.0 J/cm z with oxygen partial pressure of 10-I Torr. Xhe small angle XRD pattern for this case is given in fig.1(f) which closely matches with that of the original BaTiO 3 pellet. Thus good quality stoichiometric thin ~ilms of BaTiO 3 can be obtained by laser
evaporation technique via control of process parameters. Recently, in the studies on deposition of high T c Y-Ba-Cu-O superconducting thin films it has been reported that the overall quality of thin film can be drastically improved by application of a bias field between the target and the substrate11. It has been argued that control of excited and reactive ionic radicals present in the laser generated plume could be responsible for these effects. In order to examine the use of this effect for the case of BaTi03, depositions were performed at an energy density of 15.0 J/eraz and pressure of 10-6 Tort with and without applied electric field. The result for deposition without field is shown in fig.2(a). The 'd' values do not match with those reported for BaTiO 3. When an electric field of 330 V/cm was applied between the substrate (+ve) and the target (ground), (pressure was kept at 10 -6 Torr and the laser energy density at 15.0 J/cm2) good quality stoichiometric thin film of BaTiO 3 could be deposited [fig.2(b)]. It may be noted that in the field assisted case good film is obtained at much lower oxygen partial pressure than the case of deposition without field. It appears
Vol. 71, No. I
PULSED LASER DEPOSITION OF BARIUM TITANATE FILMS ON SILICON
II
(o)
j5
(b)
>-
z uJ
20,0
30.0
40.0
50.0
60.0
70-0
80.0
90.0
2 e (Degrees)
FIG.2 - Small angle X-ray diffraction patterns of thin films deposited at substrate temper~ ature of 450°C, Energy density = 1~ J/cm and oxygen partial pressure of 10-v Torr (a) Without electric field (b) With electric field of 330 V/cm.
that the substrate bias promotes growth of BaTiO 3 phase by enhanced incorporation of the ionic Torm of oxygen. It is n o w important to comment on the morphology of the deposited films. The SEM micrographs for the film deposited at substrate t e m p e r a t u r e of 4 5 0 o c and o x y g e n p a r t i a l pressure of 10 -I Torr is shown in fig.3(a) while that for the film deposited at 450oc at an oxygen partial pressure of 5 x10 -6 Torr with a bias of IkV/cm is shown in fig.3(b). In both the cases the film shows granularity; though the film deposited with field assistance shows considerable grain compactionandgrain size uniformity. Clearly, the bombardment of f i l m s u r f a c e by f i e l d - e n e r g i z e d charged particles in laser g e n e r a t e d plume is responsible for this effect. Finally, it is interesting to bring out the dielectric properties of the film. The spectroscopic ellipsometry result on the film is s h o w n in fig.4. It is s e e n that the dielectric constant of the film in the visible region is nearly 2.3. This is lower than the dielectric constant of crystalline BaTiO 3 by 5 %. Such lowering can be attributed to the presence of free spaces in the film both in t e r m s of g r a i n b o u n d a r i e s and voids or vacancies. The latter type of inhomogenities can arise in rapidly synthesized materials such as in the present case and post annealing treatments are essential to eliminate them. It is also important to note that the imaginary
FIG.3 - Scanning Electron Micrographics of thin films deposited under the conditions of (a)IEnergy density = 15 J/cm , 09 P.P. = 10-" Torr, T - = 450°C ~ sub. " 2 (b)gEnergy denslty = 15 J/cm , 07 P.P. = 10-v Torr, T ~ = 450°C and Electric field = 330 V/cm. SUP
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,
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....
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FIG.4 - Spectroscopic ellipsometry pattern of the thin fi~m deposited at ~nergy density = 15 J/cm-, 02 P.P. = 10-U Torr, T u b = 450°C and Electric field = 330 V/cm. s
12
PULSED LASER DEPOSITION OF BARIUM TITANATE FILMS ON SILICON
index (K) which represents ~i@l@ctric 10ss is less than 0.05 over almost the entire range of spectral width studied. In conclusion, good quality stoichiometric films of BaTiO 3 on crystalline silicon can be obtained by controlling oxygen partial pressure, laser energy density and substrate bias during laser deposition process. AcknowledgmentThe authors gratefully acknowledge the financial support for this work
Vol. 71, No. I
by Department of Atomic Energy (DAE) and Department of Science and Technology (DST). Tne authors would like to thank Dr. Basu and Dr. Deshpande (SANI~IE) for their help in SEM work and S.V. Rajarshee (Pune University) for his assistance in spectroscopic ellipsometry. The authors are also grateful to Dr. S.K.Date (NCL) for fruitful discussions.
REFERENCES |. " P r i n c i p l e s and a p p l i c a t i o n s of ferroelectrics and related materials" by M.E.lines and A.M.Glass Clarendon Press, 6. Oxford (I 979). 7. 2. D.Dij kkamp, T.Venkatesan, X.D.Wu, S~.Shaheen, N. Jisrawi, Y.H. Min-iee, W.L. Maclean and M. Croft, Applied Physics 8. letter, 51, 619 (1987). 3. T.Venkatesan, E.W.Chase, X.D.Wu, A.Inam, 9. C.C.Chang and F.K.Shokoohi, Applied Physics letter, 53, 243 (1988). 4. C.Richard Guarnieri, R,A.Roy, K.l.Saenger, S~.Shivashankar, D.S.Yee and J.J.Cuomo, 10. Applied Physics letter, 53, 532 (1988). 5. B.F.Kim, J.Bohandy, T.E.Phillips, 11. W.J.Green, E.Agostinelli, F.J.Adrian, K.Moorjani, l.J.Swartzendruber, R.D.S~ulI,
L.H.Bennett and J.S.Wallace, Applied Fnysics letter, 53, 321 (1988). T.Venkatesan, Private Communication. P.D.Godbole, S.B.Deshpande, S.K.Date, Rashmi Nawathey and S.B.Ogale, Ferroelectrics (in press). K.N.Tu and B.S.Berry, Journal of Applied Fnysics, 43, 3283 (1972). Rashmi Nawathey, R.D.Vispute, S.M.Chaudhari, S.M.Kanetkar, A.Mitra, S.K.Date and S.B.Ogale, Journal of Applied Fnysics (in press). Powder diffraction file, Ed. W.F.McClune (JCPDS, Pa, U.S.A.), 5, 654 (1984). S.W.Witanachchi, H.s.Kwok, X.W.Wang and D.T.Shaw, Applied Physics Letter, 53, 234
(1988),
RllsBDIAsJlR~ITIaeoF~-
FILM
0038-1098/89$3.00+.00 MaxwellPergsmonMacmillanplc
Ix
SILIam
RashmiNawathey,R.D.Vispute, S.M.Chaudhari, S.M.Kanetkarand S.B.Ogale Departmentof Ehysics,Universityof Foona,Pune - 411 007, INDIA (ReceivedApril 11, 1989 by C.N.R.Rao) Barium titanate thin films have been deposited on single crystal siliconby using pulsed ruby laser inducedvaporizationfrom sintered bulk material.The dependenceof the film proportieson laser energy density,oxygen partialpressureduring depositionand substratebias has been investigated snd optimum conditionshave been broughtout for obtainingsinglephase stoichiometric thin films. The films have been characterized by Small angleX-ray Diffraction(XRD),ScanningElectron Microscopy(SEM)and Spectroscopic ellipsometry.
Amongst ferroelectric substancesBaTiO3is perhaps the most widely investigated material becauseof its extremelyinteresting structurespecific proporties which are of great importance to device technologyl.Also, this materialbeing chemicallyand mechanically very stable it can be used in a variety of application environments. Most of the applications of this materialexploredtilltodate use its sinteredpolycrystallineceramic form suitablyprocessedand doped to generate the desiredferroelectricresponse.Recently, with the growingemphasisof modern technology on miniaturization, a need is felt for development of processes for obtaining good qualitythin films of such materials.Moreover, it is further desirable if such processes are dry-processingtype and are compatible with similar other processes. pulsed laser deposition is one such process which is receiving considerable attention recently, since its successfulapplicationto deposition of hot superconductor films2-5. In this communication, we have appliedthe pulsedlaser evaporationprocess to depositbarium titanate thin films on silicon.In additionto being an importantmaterialin its own right,BaTi has also been used as one of the barrierlayersfor depositionof superconductor films on Siliconb. Hence laserdevositionof BaTi on siliconcan be regarded as an importan< step towards development of an all-laser process for deposition of oxide superconductorfilms on silicon for realization of superconductorelectronics. BaTi pellet used as a target for laser vaporization was synthesizedby using sol-gel routey. One such pellet was mounted in a chamber pumped by a varian diffstack system capable of ‘renderingan ultimate vacuum of 5 x10-7 Torr.TheRuby laser beam was derived from J.K. laser (system 2000). capable of yieldinga maximum energyof 10 Joulesjpulse at a pulse repetition rate of 6 pulseslmin. The laser pulse duration was 30-ns. The target orientation was at 450 with respect to the direction of laser beam.The distance between
the target and the substratesurfacewas 3 cm. The substrateholder was mounted on a heater assembly capable of raising the substrate temperature to about 500°C. To control the oxygen partialpressurea precisionleak valve was used.The depositionswere carried out on (111) oriented single crystal silicon substrates, at a substratetemperatureof 45oOC by varying&e other depositionparameterssuch as energy density, oxygen partial pressure, substratebias etc. Typicaldepositionrate was found to be lo-15 AO/pulse depending on the depositionparameters. The depositedfilms were characterized by small angleX-ray diffraction (performed on Rigaku, Japan machine, Model no. Rotaflex RU For small angle 200 B ), using -radiation. XRD measurements Seeman-Bohlingeometry was usedg. The morphology of these films was studied using scanning electron microscope (Cambridge-Strioscan 150 machine). The dielectricpropertiesof the film were obtained using spectroscopic ellipsometer (SOPRA, France). The qualityof the laser evaporatedoxide films is known to dependupon the laser energy density as well as ambient oxygen partial pressure9, hence, in this experiment,these two parameterswere variedsystematically to obtain the desiredstoichiometryof the ferroelectric material.The small angle XRD patternof BaTi pellettakenat a glancingangle of lo is shown in fig.l(a).All the difxrasion lines match closely with those reported10 for polycrystalline barium titanate.Initially,the deposition was carried out at lower energy density of 6.0 J/cm2 and oxygen partial pressure of 5 x 10-b Torr. The corresponding XRD pattern is shown in fig.l(b).There is nz indication of the formation of BaTi chase. When the laser energydensitywas incrgasedto 10.0J/cm2 and oxygenpartialpressurewas kept constant,presenceof a small amount of BaTiO3 could be discerned from the XRD pattern [fig.l(c)]. Alongwith the required tetragonal BaTiO3phase however, differentstoichiometric phases were also seen. Though the structural
I0
PULSED LASER DEPOSITION OF BARII/M TITANATE FILMS ON SILICON
Vol. 71, No. I
(a)
(d)
(b)
(e)
(¢1
(I)
A ~n
&.
<[ >z LO P Z
200 300 400 500
7
2 8 (Degrees)
L
LA_A,,,A,
^, ,...
043 30.0 40.0 50.0 60.0 70.0 BO.O 90.0 2 8 (Degrees)
=
FIG. I - Small angle X-ray diffraction patterns of (a) BaTiO 3 pellet and thin films deposited at substrate temperature of 450°C and (b) Energy density = 6 J/cm2,02P.P.=5x10-4 Torr. (c) Energy density = 10 J/cm 2,O2P.P.=5x10-4 Torr. (d) Energy density = 13 J/cm2,02P.P.=Ix10"J Torr. (e) Energydensity = 13 J/cm2, O2P.p. =IxI0-I Torr. (f) Energy density = 15 J/cm 2, 02P,P. = Ixi0 -I Tort. quality of this film was poor, it became clear that increase in energy density did influence the formation of BaTiO 3. Hence the energy density was further increased to 13.0 J/cm 2 and oxygen partial pressure was also increased to 10- Tort. The corresponding XRD pattern is given in fig.1(d) and it clearly shows improvement in the film quality as compared to the case of fig.1(c). In order to still improve the quality of film, oxygen partial pressure was increased to 10 -I Torr keeping the energy density at 13.0 J/cm 2. This condition led to the formation of a stoichiometric BaTiO 3 thin film [fig.1(e)]. When this XRD pattern is compared with that of BaTiO 3 pellet, it is seen that the laser evaporated thin film exhibits some texture effects possibly due to partial preferred grain orientation. It was found that these texture effects and slight broadening of the diffraction lines could be avoided by the laser evaporation of the BaTi03 pellet at the energy density of 15.0 J/cm z with oxygen partial pressure of 10-I Torr. Xhe small angle XRD pattern for this case is given in fig.1(f) which closely matches with that of the original BaTiO 3 pellet. Thus good quality stoichiometric thin ~ilms of BaTiO 3 can be obtained by laser
evaporation technique via control of process parameters. Recently, in the studies on deposition of high T c Y-Ba-Cu-O superconducting thin films it has been reported that the overall quality of thin film can be drastically improved by application of a bias field between the target and the substrate11. It has been argued that control of excited and reactive ionic radicals present in the laser generated plume could be responsible for these effects. In order to examine the use of this effect for the case of BaTi03, depositions were performed at an energy density of 15.0 J/eraz and pressure of 10-6 Tort with and without applied electric field. The result for deposition without field is shown in fig.2(a). The 'd' values do not match with those reported for BaTiO 3. When an electric field of 330 V/cm was applied between the substrate (+ve) and the target (ground), (pressure was kept at 10 -6 Torr and the laser energy density at 15.0 J/cm2) good quality stoichiometric thin film of BaTiO 3 could be deposited [fig.2(b)]. It may be noted that in the field assisted case good film is obtained at much lower oxygen partial pressure than the case of deposition without field. It appears
Vol. 71, No. I
PULSED LASER DEPOSITION OF BARIUM TITANATE FILMS ON SILICON
II
(o)
j5
(b)
>-
z uJ
20,0
30.0
40.0
50.0
60.0
70-0
80.0
90.0
2 e (Degrees)
FIG.2 - Small angle X-ray diffraction patterns of thin films deposited at substrate temper~ ature of 450°C, Energy density = 1~ J/cm and oxygen partial pressure of 10-v Torr (a) Without electric field (b) With electric field of 330 V/cm.
that the substrate bias promotes growth of BaTiO 3 phase by enhanced incorporation of the ionic Torm of oxygen. It is n o w important to comment on the morphology of the deposited films. The SEM micrographs for the film deposited at substrate t e m p e r a t u r e of 4 5 0 o c and o x y g e n p a r t i a l pressure of 10 -I Torr is shown in fig.3(a) while that for the film deposited at 450oc at an oxygen partial pressure of 5 x10 -6 Torr with a bias of IkV/cm is shown in fig.3(b). In both the cases the film shows granularity; though the film deposited with field assistance shows considerable grain compactionandgrain size uniformity. Clearly, the bombardment of f i l m s u r f a c e by f i e l d - e n e r g i z e d charged particles in laser g e n e r a t e d plume is responsible for this effect. Finally, it is interesting to bring out the dielectric properties of the film. The spectroscopic ellipsometry result on the film is s h o w n in fig.4. It is s e e n that the dielectric constant of the film in the visible region is nearly 2.3. This is lower than the dielectric constant of crystalline BaTiO 3 by 5 %. Such lowering can be attributed to the presence of free spaces in the film both in t e r m s of g r a i n b o u n d a r i e s and voids or vacancies. The latter type of inhomogenities can arise in rapidly synthesized materials such as in the present case and post annealing treatments are essential to eliminate them. It is also important to note that the imaginary
FIG.3 - Scanning Electron Micrographics of thin films deposited under the conditions of (a)IEnergy density = 15 J/cm , 09 P.P. = 10-" Torr, T - = 450°C ~ sub. " 2 (b)gEnergy denslty = 15 J/cm , 07 P.P. = 10-v Torr, T ~ = 450°C and Electric field = 330 V/cm. SUP
3.0
2.6
~ 2.4 ¢'y
I.. . . . . . . .
2.0
,
. . . .
2-2
I ....
I ....
I * iii
2-4 Energy
I , i l l l l l l
2.6
iI
....
2,8
I,
,
3.0
(in e V )
o.so[ o.2o t ~ o.~0L
!
=~
0 [ . . ~•- ~ i ~ •. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.0
2-2
2.4
Energy
2.6
(in
2.8
3.0
eV)
FIG.4 - Spectroscopic ellipsometry pattern of the thin fi~m deposited at ~nergy density = 15 J/cm-, 02 P.P. = 10-U Torr, T u b = 450°C and Electric field = 330 V/cm. s
12
PULSED LASER DEPOSITION OF BARIUM TITANATE FILMS ON SILICON
index (K) which represents ~i@l@ctric 10ss is less than 0.05 over almost the entire range of spectral width studied. In conclusion, good quality stoichiometric films of BaTiO 3 on crystalline silicon can be obtained by controlling oxygen partial pressure, laser energy density and substrate bias during laser deposition process. AcknowledgmentThe authors gratefully acknowledge the financial support for this work
Vol. 71, No. I
by Department of Atomic Energy (DAE) and Department of Science and Technology (DST). Tne authors would like to thank Dr. Basu and Dr. Deshpande (SANI~IE) for their help in SEM work and S.V. Rajarshee (Pune University) for his assistance in spectroscopic ellipsometry. The authors are also grateful to Dr. S.K.Date (NCL) for fruitful discussions.
REFERENCES |. " P r i n c i p l e s and a p p l i c a t i o n s of ferroelectrics and related materials" by M.E.lines and A.M.Glass Clarendon Press, 6. Oxford (I 979). 7. 2. D.Dij kkamp, T.Venkatesan, X.D.Wu, S~.Shaheen, N. Jisrawi, Y.H. Min-iee, W.L. Maclean and M. Croft, Applied Physics 8. letter, 51, 619 (1987). 3. T.Venkatesan, E.W.Chase, X.D.Wu, A.Inam, 9. C.C.Chang and F.K.Shokoohi, Applied Physics letter, 53, 243 (1988). 4. C.Richard Guarnieri, R,A.Roy, K.l.Saenger, S~.Shivashankar, D.S.Yee and J.J.Cuomo, 10. Applied Physics letter, 53, 532 (1988). 5. B.F.Kim, J.Bohandy, T.E.Phillips, 11. W.J.Green, E.Agostinelli, F.J.Adrian, K.Moorjani, l.J.Swartzendruber, R.D.S~ulI,
L.H.Bennett and J.S.Wallace, Applied Fnysics letter, 53, 321 (1988). T.Venkatesan, Private Communication. P.D.Godbole, S.B.Deshpande, S.K.Date, Rashmi Nawathey and S.B.Ogale, Ferroelectrics (in press). K.N.Tu and B.S.Berry, Journal of Applied Fnysics, 43, 3283 (1972). Rashmi Nawathey, R.D.Vispute, S.M.Chaudhari, S.M.Kanetkar, A.Mitra, S.K.Date and S.B.Ogale, Journal of Applied Fnysics (in press). Powder diffraction file, Ed. W.F.McClune (JCPDS, Pa, U.S.A.), 5, 654 (1984). S.W.Witanachchi, H.s.Kwok, X.W.Wang and D.T.Shaw, Applied Physics Letter, 53, 234
(1988),