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Thin epitaxial CeO2 films prepared by aerosol MOCVD
December 1994
Materials Letters 21 (1994) 377-380
Thin epitaxial CeO, films prepared by aerosol MOCVD K. Friihlich a, D. Machajdik a, F. Weiss b, B. Bochu b aInstitute ofElectrical Engineering, SAS, Dicbravskh cesta 9, 842 39 Bratislava, Slovak Republic b LMGP-ENSPG, CNRS URA 1109, BP 46, 38 402 Saint-Martin d’Heres, France Received 14 August 1994; accepted I6 August 1994
Abstract Thin epitaxial CeOt films were prepared on (00 I ) MgO and ( 1i02 ) A1,Ox single crystal substrates by the aerosol MOCVD method. X-ray diffraction analysis using a four-circle texture goniometer revealed small amount of in-plane misoriented Ce02 crystallites for the MgO substrate and high epitaxial degree of Ce02 films deposited on ( 1102) A&O,.
1. Introduction
Thin films of CeOz became very attractive in connection with the preparation of high-T, superconductor films. Because of chemical and structural compatibility, CeO, films are suitable as a buffer material. Lattice mismatch of CeOz and YBa$&O, rotated by 45” in plane is less than 1%. Such a small mismatch strongly favours the growth of epitaxial YBa$&O, films. However, for the growth of YBa2Cu307 films with high critical current density the buffer layer should be epitaxial. There are several recent reports on the preparation of high quality YBa2Cu30, thin films using CeO, buffer layer [ l-3 1. The films were grown on LaAlO, and R-plane cut ( 1i02) sapphire and exhibit high critical temperature and current densities together with excellent microwave properties [ 2,3]. Furthermore, YBa$&O, films prepared on multiple buffer layer, including CeOz film, were shown to be promising for integrated superconductor and semiconductor applications [ 4,5]. Up to now, epitaxial CeO, films were prepared by pulsed laser deposition, coevaporation, sputtering and metalorganic chemical vapour deposition. In the case of MOCVD, difficulties connected with a transport
of metalorganic precursors to a substrate were encountered, resulting in low deposition rates at temperatures appropriate to epitaxial growth [6]. Recently, a novel single-source MOCVD process has been successfully used for the growth of epitaxial CeOz thin films [ 71. The typical growth rate of this method was 0.1 pm/h. We have developed a new aerosol MOCVD process which enables reproducible growth of oxide thin films with growth rates up to 10 l.trn/h [ 8 1. Using this method we have succeeded in the preparation of thin epitaxial Ce02 films. In this paper we describe the growth and structural properties of thin CeO, films on single crystal MgO and sapphire substrates.
2. Experimental Thin CeOz films were prepared in a low-pressure aerosol MOCVD apparatus constructed for the growth of thin oxide films [ 8 1. The solution of heptafluorodimethyl octanodionate of cerium Ce (fod), in diethyleneglycol dimethyl ether (diglyme) in a concentration of 0.02 mol/Q was sprayed by an ultrasonic transducer. The aerosol was then transported
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10167-577x(94)00203-7
K. Friihlich et al. /Materials Letters 21 (I 994) 377-380
318
c
'
40
60
80
120
100 4
140
[degl
Fig. 1. X-ray diffraction patterns for CeOz film on MgO substrate: (a) $279 scan showing (001) preferred orientation, (b) w-scan of (002) diffractions of CeOz and MgO, (c) @scan of ( 111) CeOz diffraction indicating epitaxial growth. Inset shows small amount of inplane misoriented crystallites.
by At-, evaporated in the preheating zone and mixed with oxygen. Deposition then took place on substrates mounted on stainless steel sample holder. The sample holder was heated by high-frequency induction to temperatures between 800 and 920°C. The total pressure during the growth was p = 10 Torr. After the deposition, the oxygen pressure was increased and samples were cooled down to room temperature in atmospheric pressure. The deposition time was 15 min and typical thickness of the films was up to 100 nm. Slides of (001) single crystalline MgO and ( 1i02), R-plane cut sapphire were used as substrates. The substrates were chemically cleaned by trichlorethylene and aceton before the deposition. Grown films were characterized by X-ray diffraction using the standard 79-279technique and CuKa radiation. The quality of epitaxial growth and orientational relationships of the Ce02 film and the sub-
strate were determined using w and o-scans on a Siemens D 5000 four-circle texture diffractometer.
3. Results and discussion 3.1. CeO, films on MgO substrate Both MgO and CeOz crystals have cubic structure and space group Fm3m. However, the lattice parameters of MgO and Ce02 are rather different, resulting in considerable lattice mismatch of 28.5%. X-ray diffraction pattern IY-219of CeOl films prepared on (00 1) MgO substrate in the whole deposition temperature range revealed strong (001) preferred orientation with small amount of Ce02 grains oriented with the [ 1111 direction perpendicular to the substrate surface, Fig. la. The amount of [ 111 ]-
K. FrChlich et al. /Materials Letters 21(1994) 377-380
oriented CeOz grains was estimated to be less than 1% for all films deposited between 800 and 920°C. The values of the lattice parameter, calculated from the position of the (006) diffraction peak varied between 0.5411 and 0.5415 nm. These values are very close to the value for bulk material given by PDF-2 database, a=0541 1 nm [9]. Preferred orientation in the [00 111 direction was investigated by w-scans of the (002) peaks of CeO, and MgO, Fig. lb. Full width at half maximum (fwhm) of the rocking curve yields for MgO substrate the value fwhm=0.3”. For the CeO, films we have obtained the values of the rocking curves ranging between fwhm = 1.34” for the deposition temperature T,, = 800°C and fwhm = 0.64” for T, = 920°C. @-scan of the ( 111) CeO, diffraction showed four strong maxima modulated by 90”, Fig. 1c. The fwhm of the peaks was 1.2”. This means, that the CeOz film
c
-0
90
319
is epitaxially grown on MgO substrate with orientational relationship Ce02 (001) [ 1001 ])MgO (001) [ 1001. However, a more detailed inspection revealed the presence of some small amount of CeO, grains oriented with a-axis parallel to the [ 1 lo] direction of MgO substrate and another set of CeO, grains rotated by 23.5” from the [ 1001 MgO direction, see inset in Fig. lc. The quantity of the CeOz grains with such misorientation estimated from the ratio of the peaks in @-scan is less than 0.5%.
3.2. CeO,films on AlJO substrate
Sapphire has a rhombohedral structure. Its ( 1i02) plane (R-plane) has a pseudocubic surface structure. The atomic spacings in the R-plane along the [ 1Oi 1 ] and [ 12101 directions are 0.512 and 0.4759 nm, re-
180
270 0
360
[degl
Fig. 2. X-ray diffraction patterns for CeOz film on A1203 substrate: (a) 8-219 scan showing (001) preferred orientation, (004) diffraction of Ce02 together with (2204) diffraction of A1203, (c) @-scan of ( 111) Ce02 diffraction indicating epitaxial growth.
(b) w-scan of high degree of
380
K. Friihlich et al. /Materials Letters 2lfi994)
spectively. The lattice mismatch for CeOz film in these directions is 5.7 and 13.7%. The films grown on ( 1iO2) A1203substrate showed strong 1001 ] preferential orientation, as it is seen from X-ray diffraction pattern in Fig. 2a. Only a small portion (less than 1%) of [ 1111 oriented CeOz grains was present in the films deposited in the whole deposition range. The lattice parameters calculated from the position of (006) diffraction were slightly lower than the value for bulk material (a=0.5411 nm), with values ranging from 0.5400 to 0.5403 nm. This suggests, that the strains generated due to different thermal expansion coefficient of the film and the substrate during cooling down were released by lattice distortion. The preferential orientation in the [ 0011 direction, inferred from the fwhm values of o-scan of (004) ~ffraction, Fig. 2b, was found to be very good. The values of fwhm were around fwhm= 0.5”; the lowest value fwhm = 0.3 lo was observed for the highest deposition temperature T, = 920”. Rocking curve of the (2204) diffraction of Alz03 in Fig. 2b implies that the substrate consists of several slightly misoriented blocks. For better quality of the substrate, rocking curves of CeOz are expected to be even more narrow. Epitaxial character of CeOz films was revealed by @-scan of ( 111) diffraction, which exhibited four narrow peaks with fwhm = 1.14”, Fig. 2c. High degree of in-plane epitaxy is also evidenced from the high ratio for the signal of the peaks to the signal between them (peak to valley ratio).
377-380
4. Conclusions We have succeeded in the preparation of thin epitaxial CeOz films by the aerosol MOCVD technique. The films prepared on (00 1) MgO and ( 1i02) A1203 single crystal substrates exhibited strong (00 1) preferential orientation. While the films on MgO substrates showed small amount of in-plane misoriented CeO grains, the high degree of epitaxy was observed for the films on A1203 substrates. The CeO, films on sapphire are suitable as a buffer layer for the growth of YBa$&O, thin films. References [ I] X.D. Wu, R.C. Dye, R.E. Muenchausen, S.R. Foltyn, M. Maley, A.D. Rollett, A.R. Garcia and N.S. Nogar, Appl. Phys. Letters58 (1991) 2165. [2] P. Merchant, R.D. Jacowitz, K. Tibbs, R.C. Taber and S.S. Laderman, Appl. Phys. Letters 60 ( 1992) 763. [3] B.F. Cole, G.-C. Liang, N. Newman, K. Char, G. Zacharchuk and J.S. Martens, Appl. Phys. Letters 6 1 ( 1992) 1727. [4] C.A. Copetti, H. Soltner, J. Schubert, W. Zander, 0. Hollricher, Ch. Buchal, H. Schulz, N. Tellmann and N. Klein, Appl. Phys. Letters 63 (1993) 1429. [ 5 ] M.J. Burns, P.R. de la Houssaye, SD. Russel, G.A. Garcia, S.R. Clayton, W.S. Ruby and L.P. Lee, Appl. Phys. Letters 63 (1993) 1282. [ 61 K.-H. Dahmen, M. Brecht and T. GerBn, in: Proc. High-T, Superconductor Thin Films, ed. L. Correra (Elsevier, Amsterdam, 1992) p. 715. [ 7 ] Z. Lu, R.S. Feigelson, R.K. Route, S.A. DiCarolis, R. Hiskes and R.D. Jacowitz, J. Crystal Growth 128 ( 1993) 788. [ 81 F. Weiss, K. Friihlich, R. Haase, M. Labeau, D. Selbmann, J.P. Senateur and 0. Thomas, J. Physi. IV C3 3 ( 1993) 32 1. [ 91 Powder diffraction file database PDF-2, file No. 34-394.
Materials Letters 21 (1994) 377-380
Thin epitaxial CeO, films prepared by aerosol MOCVD K. Friihlich a, D. Machajdik a, F. Weiss b, B. Bochu b aInstitute ofElectrical Engineering, SAS, Dicbravskh cesta 9, 842 39 Bratislava, Slovak Republic b LMGP-ENSPG, CNRS URA 1109, BP 46, 38 402 Saint-Martin d’Heres, France Received 14 August 1994; accepted I6 August 1994
Abstract Thin epitaxial CeOt films were prepared on (00 I ) MgO and ( 1i02 ) A1,Ox single crystal substrates by the aerosol MOCVD method. X-ray diffraction analysis using a four-circle texture goniometer revealed small amount of in-plane misoriented Ce02 crystallites for the MgO substrate and high epitaxial degree of Ce02 films deposited on ( 1102) A&O,.
1. Introduction
Thin films of CeOz became very attractive in connection with the preparation of high-T, superconductor films. Because of chemical and structural compatibility, CeO, films are suitable as a buffer material. Lattice mismatch of CeOz and YBa$&O, rotated by 45” in plane is less than 1%. Such a small mismatch strongly favours the growth of epitaxial YBa$&O, films. However, for the growth of YBa2Cu307 films with high critical current density the buffer layer should be epitaxial. There are several recent reports on the preparation of high quality YBa2Cu30, thin films using CeO, buffer layer [ l-3 1. The films were grown on LaAlO, and R-plane cut ( 1i02) sapphire and exhibit high critical temperature and current densities together with excellent microwave properties [ 2,3]. Furthermore, YBa$&O, films prepared on multiple buffer layer, including CeOz film, were shown to be promising for integrated superconductor and semiconductor applications [ 4,5]. Up to now, epitaxial CeO, films were prepared by pulsed laser deposition, coevaporation, sputtering and metalorganic chemical vapour deposition. In the case of MOCVD, difficulties connected with a transport
of metalorganic precursors to a substrate were encountered, resulting in low deposition rates at temperatures appropriate to epitaxial growth [6]. Recently, a novel single-source MOCVD process has been successfully used for the growth of epitaxial CeOz thin films [ 71. The typical growth rate of this method was 0.1 pm/h. We have developed a new aerosol MOCVD process which enables reproducible growth of oxide thin films with growth rates up to 10 l.trn/h [ 8 1. Using this method we have succeeded in the preparation of thin epitaxial Ce02 films. In this paper we describe the growth and structural properties of thin CeO, films on single crystal MgO and sapphire substrates.
2. Experimental Thin CeOz films were prepared in a low-pressure aerosol MOCVD apparatus constructed for the growth of thin oxide films [ 8 1. The solution of heptafluorodimethyl octanodionate of cerium Ce (fod), in diethyleneglycol dimethyl ether (diglyme) in a concentration of 0.02 mol/Q was sprayed by an ultrasonic transducer. The aerosol was then transported
0167-577x/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSD10167-577x(94)00203-7
K. Friihlich et al. /Materials Letters 21 (I 994) 377-380
318
c
'
40
60
80
120
100 4
140
[degl
Fig. 1. X-ray diffraction patterns for CeOz film on MgO substrate: (a) $279 scan showing (001) preferred orientation, (b) w-scan of (002) diffractions of CeOz and MgO, (c) @scan of ( 111) CeOz diffraction indicating epitaxial growth. Inset shows small amount of inplane misoriented crystallites.
by At-, evaporated in the preheating zone and mixed with oxygen. Deposition then took place on substrates mounted on stainless steel sample holder. The sample holder was heated by high-frequency induction to temperatures between 800 and 920°C. The total pressure during the growth was p = 10 Torr. After the deposition, the oxygen pressure was increased and samples were cooled down to room temperature in atmospheric pressure. The deposition time was 15 min and typical thickness of the films was up to 100 nm. Slides of (001) single crystalline MgO and ( 1i02), R-plane cut sapphire were used as substrates. The substrates were chemically cleaned by trichlorethylene and aceton before the deposition. Grown films were characterized by X-ray diffraction using the standard 79-279technique and CuKa radiation. The quality of epitaxial growth and orientational relationships of the Ce02 film and the sub-
strate were determined using w and o-scans on a Siemens D 5000 four-circle texture diffractometer.
3. Results and discussion 3.1. CeO, films on MgO substrate Both MgO and CeOz crystals have cubic structure and space group Fm3m. However, the lattice parameters of MgO and Ce02 are rather different, resulting in considerable lattice mismatch of 28.5%. X-ray diffraction pattern IY-219of CeOl films prepared on (00 1) MgO substrate in the whole deposition temperature range revealed strong (001) preferred orientation with small amount of Ce02 grains oriented with the [ 1111 direction perpendicular to the substrate surface, Fig. la. The amount of [ 111 ]-
K. FrChlich et al. /Materials Letters 21(1994) 377-380
oriented CeOz grains was estimated to be less than 1% for all films deposited between 800 and 920°C. The values of the lattice parameter, calculated from the position of the (006) diffraction peak varied between 0.5411 and 0.5415 nm. These values are very close to the value for bulk material given by PDF-2 database, a=0541 1 nm [9]. Preferred orientation in the [00 111 direction was investigated by w-scans of the (002) peaks of CeO, and MgO, Fig. lb. Full width at half maximum (fwhm) of the rocking curve yields for MgO substrate the value fwhm=0.3”. For the CeO, films we have obtained the values of the rocking curves ranging between fwhm = 1.34” for the deposition temperature T,, = 800°C and fwhm = 0.64” for T, = 920°C. @-scan of the ( 111) CeO, diffraction showed four strong maxima modulated by 90”, Fig. 1c. The fwhm of the peaks was 1.2”. This means, that the CeOz film
c
-0
90
319
is epitaxially grown on MgO substrate with orientational relationship Ce02 (001) [ 1001 ])MgO (001) [ 1001. However, a more detailed inspection revealed the presence of some small amount of CeO, grains oriented with a-axis parallel to the [ 1 lo] direction of MgO substrate and another set of CeO, grains rotated by 23.5” from the [ 1001 MgO direction, see inset in Fig. lc. The quantity of the CeOz grains with such misorientation estimated from the ratio of the peaks in @-scan is less than 0.5%.
3.2. CeO,films on AlJO substrate
Sapphire has a rhombohedral structure. Its ( 1i02) plane (R-plane) has a pseudocubic surface structure. The atomic spacings in the R-plane along the [ 1Oi 1 ] and [ 12101 directions are 0.512 and 0.4759 nm, re-
180
270 0
360
[degl
Fig. 2. X-ray diffraction patterns for CeOz film on A1203 substrate: (a) 8-219 scan showing (001) preferred orientation, (004) diffraction of Ce02 together with (2204) diffraction of A1203, (c) @-scan of ( 111) Ce02 diffraction indicating epitaxial growth.
(b) w-scan of high degree of
380
K. Friihlich et al. /Materials Letters 2lfi994)
spectively. The lattice mismatch for CeOz film in these directions is 5.7 and 13.7%. The films grown on ( 1iO2) A1203substrate showed strong 1001 ] preferential orientation, as it is seen from X-ray diffraction pattern in Fig. 2a. Only a small portion (less than 1%) of [ 1111 oriented CeOz grains was present in the films deposited in the whole deposition range. The lattice parameters calculated from the position of (006) diffraction were slightly lower than the value for bulk material (a=0.5411 nm), with values ranging from 0.5400 to 0.5403 nm. This suggests, that the strains generated due to different thermal expansion coefficient of the film and the substrate during cooling down were released by lattice distortion. The preferential orientation in the [ 0011 direction, inferred from the fwhm values of o-scan of (004) ~ffraction, Fig. 2b, was found to be very good. The values of fwhm were around fwhm= 0.5”; the lowest value fwhm = 0.3 lo was observed for the highest deposition temperature T, = 920”. Rocking curve of the (2204) diffraction of Alz03 in Fig. 2b implies that the substrate consists of several slightly misoriented blocks. For better quality of the substrate, rocking curves of CeOz are expected to be even more narrow. Epitaxial character of CeOz films was revealed by @-scan of ( 111) diffraction, which exhibited four narrow peaks with fwhm = 1.14”, Fig. 2c. High degree of in-plane epitaxy is also evidenced from the high ratio for the signal of the peaks to the signal between them (peak to valley ratio).
377-380
4. Conclusions We have succeeded in the preparation of thin epitaxial CeOz films by the aerosol MOCVD technique. The films prepared on (00 1) MgO and ( 1i02) A1203 single crystal substrates exhibited strong (00 1) preferential orientation. While the films on MgO substrates showed small amount of in-plane misoriented CeO grains, the high degree of epitaxy was observed for the films on A1203 substrates. The CeO, films on sapphire are suitable as a buffer layer for the growth of YBa$&O, thin films. References [ I] X.D. Wu, R.C. Dye, R.E. Muenchausen, S.R. Foltyn, M. Maley, A.D. Rollett, A.R. Garcia and N.S. Nogar, Appl. Phys. Letters58 (1991) 2165. [2] P. Merchant, R.D. Jacowitz, K. Tibbs, R.C. Taber and S.S. Laderman, Appl. Phys. Letters 60 ( 1992) 763. [3] B.F. Cole, G.-C. Liang, N. Newman, K. Char, G. Zacharchuk and J.S. Martens, Appl. Phys. Letters 6 1 ( 1992) 1727. [4] C.A. Copetti, H. Soltner, J. Schubert, W. Zander, 0. Hollricher, Ch. Buchal, H. Schulz, N. Tellmann and N. Klein, Appl. Phys. Letters 63 (1993) 1429. [ 5 ] M.J. Burns, P.R. de la Houssaye, SD. Russel, G.A. Garcia, S.R. Clayton, W.S. Ruby and L.P. Lee, Appl. Phys. Letters 63 (1993) 1282. [ 61 K.-H. Dahmen, M. Brecht and T. GerBn, in: Proc. High-T, Superconductor Thin Films, ed. L. Correra (Elsevier, Amsterdam, 1992) p. 715. [ 7 ] Z. Lu, R.S. Feigelson, R.K. Route, S.A. DiCarolis, R. Hiskes and R.D. Jacowitz, J. Crystal Growth 128 ( 1993) 788. [ 81 F. Weiss, K. Friihlich, R. Haase, M. Labeau, D. Selbmann, J.P. Senateur and 0. Thomas, J. Physi. IV C3 3 ( 1993) 32 1. [ 91 Powder diffraction file database PDF-2, file No. 34-394.