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High-quality langasite films grown by liquid phase epitaxy
Journal of Crystal Growth 237–239 (2002) 714–719
High-quality langasite films grown by liquid phase epitaxy C. Klemenz* Institute of Micro- and Optoelectronics, Swiss Federal Institute of Technology, Lausanne EPFL, CH-1015 Lausanne, Switzerland
Abstract Single-crystalline La3Ga5SiO14 (LGS) films could be grown by liquid phase epitaxy (LPE) for the first time. These films were obtained on X - and Y -oriented LGS substrates (homoepitaxy) from a PbO-based flux, and were characterized by Nomarski microscopy, AFM, SEM/EDX and XRD. Best results were obtained for Y -oriented films, with macrosteps propagating over macrosopic dimensions (>1 1 cm2). Very flat areas with height variations within 1.4 nm over 8 mm lateral distance were measured by AFM. These films have a thickness of a few micrometer, are crackfree due to perfect lattice match, and no solvent inclusions or secondary phases were found. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.a; 77.65.Dq; 81.15.Lm Keywords: A1. Etching; A1. Surface sturcture; A3. Liquid phase epitaxy; B1. Oxides; B1. Rare earth compounds; B2. Piezoelectric materials
1. Introduction Major efforts have recently been made to develop langasite-type crystals La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0.5O14 (LGT) for bulk (BAW) and surface acoustic wave (SAW) applications. These compounds have piezoelectric properties [1] which are intermediate between those of quartz and LiNbO3. Compared to quartz, they have higher electromechanical coupling coefficient (k2 ), and reduced acoustic velocities, which allows fabrication of miniaturized intermediate frequency band-pass filters for new mobile communication systems. Additionally, they show no structural phase
*Corresponding author. Tel.: +41-21-6935483; fax: +41-216934525. E-mail address: christine.klemenz@epfl.ch (C. Klemenz).
transition between room and melt temperature (about 14701C for LGS), which makes them suitable for high-temperature applications. Langasites belong to the structure type of Ca3Ga2Ge4O14, with space group P321 and the same trigonal crystal class 32 as quartz. The lattice ( constants of LGS are about a ¼ 8:161 A, ( [2]. The relations between the crystalc ¼ 5:087 A lographic and crystal-physical (Cartesian) coordinates [3] in the trigonal–hexagonal system are shown in Fig. 1. These conventions are used in this paper, and X - and Y -cut substrate surfaces refer to surfaces normal to the X and Y directions, with YZ; XZ in-plane axis, respectively. Langasite crystals can be grown from the melt by Czochralski [1,4–8]. However, these crystals show a defect structure (precipitates, inclusions, impurities, striations (transverse growth banding), dislocations, twins, cracks) which limit their performance in high-quality devices.
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 1 2 - 7
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
Fig. 1. Conventions used in the trigonal–hexagonal system, in relation to the crystal-physical axis X ; Y ; and Z:
Among other inhomogeneities, striations are very common in Czochralski-grown langasite crystals. These striations are likely to be due to non-congruent starting melt composition [9], to variations of melt composition during the growth process, to solid-solubility of Si:Ga in LGS, Nb:Ga in LGN, and Ta:Ga in LGT, associated with temperature fluctuations at the growth interface. Depending on the material and specific growth conditions, the 3D pattern and periodicity of striations [5,10] can vary. At a microscopic scale, not only the concentration, but also the distribution (or ordering) of Si, Ta, Nb in the crystals is affected. Additionally, at a macroscopic scale, the composition of crystals varies along radial and growth direction. Hence, no crystal is alike and even the same crystal shows variations of structural (lattice parameters, density, etc.) and physical properties. As a consequence, parameters of devices even fabricated from same crystal shifts, and cannot meet specific demand [9]. For SAW devices, the quality of the substrate surface and the
715
structural perfection within a thickness of about one acoustic wavelength of the surface are further important aspects [11,12]. Films and crystals grown from solutions close to thermodynamic equilibrium usually show lowest dislocation and defect densities and very flat equilibrium surfaces (facets) may develop. Furthermore, dopants (and impurities) will be incorporated in a more homogeneous way than in melt growth, resulting in a better ordering in the crystal. High-quality langasite films grown by liquid phase epitaxy (LPE) are therefore also important for investigation of the temperature dependance of the piezoelectric properties of these materials. The development of LPE growth of LGS-type films can be quite demanding, and requires the study and control of all solution, substrate, and growth parameters, which may influence the growth mode of the films. For extremely perfect and crack-free LPE films, the substrate misorientation and misfit have to be practically zero, and the supersaturation has to be very low [13]. Furthermore, the substrate surface has to be free of residual strain or defects, which influence nucleation and subsequent film growth. In this study, a suitable solvent was first searched through available or related phase diagrams in the La–Ga–Si–O system and complemented by flux and top-seeded solution growth (TSSG) experiments. Substrate preparation/ etching was investigated. Search of the liquidus was performed by phase stability considerations upon cooling of the solution and X-ray analysis of crystallized phases. X - and Y -oriented LGS films could be grown by LPE on LGS substrates (homoepitaxy), and were characterized by Nomarski microscopy, atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and X-ray diffraction (XRD).
2. Solvent Finding a suitable solvent/solvent system is one of the most important aspects. Alkali vandates, molybdates and tungstates are often used for flux
716
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
growth of complex oxides like silicates and germanates. Austerman [14] used an Li2MoO3– MoO3 flux for the growth of BeO. He observed that the crystal habit was influenced by impurities, and also changed from pyramids, prisms, to platelets with decreasing MoO3 content in the solution. Platelets usually show a high structural perfection, and this was an additional reason to investigate this flux for LGS. Li2MoO4:MoO3 fluxes with ratios of 1:1 to 1:4 were used for flux, TSSG, and LPE experiments of LGT and LGS, in a temperature range of 1150–8501C. The langasite phase did not crystallize in the investigated region, and in the lower temperature range, La,Li(MoO4)2 grow epitaxially on the substrates [15]. Strong interface reactions occurs, even for solute concentrations up to 50 wt%. Bi2O3-based fluxes have similarities with PbO fluxes, and can lead to a lowering of the liquidus temperature when added to BaO–B2O3 fluxes. However, when large rareearth (RE) ions are present, substitutions may ( occur. In LGS, Bi3+ with radii of rVIII =1.17 A Bi3þ 3+ VIII ( would easily substitute La with rBi3þ =1.16 A (ionic radii taken from Ref. [16]). PbO–B2O3 fluxes as well as BaO–BaF2–B2O3 ternary solvent systems were successfully used in LPE of garnets for magnetic bubble applications [17,18]. PbO-based fluxes were also used for the growth of perovskites ABO3 (A=La,Nd,Pr;B=Ga) [19]. The advantage of borate-containing fluxes is a lower volatility, but at the expense of a higher viscosity. B2O3 is in most cases used as an additive (about 1–2%). PbF2 and PbO2 are also frequently used as additives, in order to reduce the nucleation and to preserve the Pt crucibles, respectively. Preliminary LGS flux experiments were done using PbO with different additives. The addition of B2O3 and/or PbF2 leads to the crystallization of LaBO3 and LaF3, respectively, and these additives were therefore avoided. LaGaO3 (LGO) was the major phase, which was obtained from these flux experiments, but no LGS. However, LPE films could be grown from PbO fluxes containing 15–20 wt% stoichiometric LGS and 1% PbO2. With a solute concentration higher than 20 wt%, the soaking temperature and time had to be increased, the solution became very viscous, and LGO crystallized. There exist several possible explanations for the fact that the LGS
phase could not be obtained from the same flux, e.g. without LGS substrate. First, LGO is the stable phase in the higher temperature range. Once formed, LGO continues to grow which prevents the formation of the LGS phase upon cooling. It is also known that crystals from another phase may grow in the stability field of another material, when a seed (or substrate) of the required phase is present. Thus, when the LGS substrate is present, epitaxy of LGS is energetically favored, similar to earlier observations on the growth of metastable phases [20,21].
3. Growth system and growth procedure The LPE experiments were performed in a vertical tube furnace schematically shown in Fig. 2. The starting oxides of 3–4 N purity are well mixed by hand and pressed into a cylindrical
Fig. 2. Schema of the LPE growth furnace system.
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
Pt/Au 95/5% crucible of about 45 cm2. The crucible is then placed in the furnace, at a position that can be adjusted for optimal temperature gradients. The polished X - and Y -cut LGS substrates were etched in hot orthophosphoric acid at 1301C for 2–3 h [22], rinsed with distilled water and ethanol, and then mounted with Ptwires on an alumina rod in a vertical position. During LPE growth, alterned rotation of 15– 35 rpm was applied. A Pt lid is placed on top of the crucible to prevent solvent evaporation. The furnace is heated to 8001C in 3 h, then to 11501C in 4 h. After 5 h soaking, the temperature is lowered to about 9501C and equilibrated there for about 24 h. The Pt lid is then removed, and a search of the liquidus temperature is carried out by dipping LGS crystal pieces and substrates in the solution and X-ray analysis of the phases which crystallized on them or in the solution. LPE films of LGS could be successfully grown in a temperature range of 950–8701C, depending on the initial solute concentration. After the growth, the films are slowly withdrawn from the growth solution and cooled down while being taken out from the furnace. The films are then cleaned in diluted nitric acid at room temperature, and rinsed with distilled water and ethanol.
4. Film characterization The Y -oriented LGS LPE films grown on Y -cut LGS substrates present a tendency to facet formation, as can be recognized in the microphotographies of Figs. 3 and 4. A faceting effect is also observed on the side of the macrosteps, which indicates that there may also exist other surface orientations which could potentially develop to facets. This aspect is very important, since the acoustic properties which are interesting for specific applications do not necessarily correspond to major crystal orientations. The film of Figs. 3 and 4 was grown at 8901C from the PbO flux containing 1 wt% PbO2, and La2O3, Ga2O3, SiO2 starting oxides corresponding to 15 wt% stoichiometric LGS. The substrate rotation rate was 15 rpm, and the growth time 25 min. Macrosteps propagate over the whole area of the film of about
717
Fig. 3. Nomarski microphotography of a Y -oriented LGS LPE film: step-flow mode and step bunching with formation of macrosteps.
Fig. 4. Magnified view of a growth terrace of about 100 mm diameter, on the same film.
1 1.5 cm2, but with relatively large variations in step heights and interstep distances. Step bunching has to be prevented when high-quality films have to be grown, since they are a source of solvent inclusions and striations. However, this phenomenon is not well understood and needs to be investigated from case to case. Besides growth parameters, impurities, hydrodynamics, and substrate misorientation are important factors. The substrates used herein had typical misorientations of about o0.51. Sometimes, flatter hillocks and terraces develop. Fig. 5 shows an AFM profile on a very flat region, which shows height variations within about 1.4 nm (roughly 2 lattice constants) over 8 mm of lateral distance. Due to perfect lattice match, these films are free of cracks. No secondary
718
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
growth parameters, to further improve the film quality.
Acknowledgements
Fig. 5. AFM profile of a flat terrace on a Y -oriented LGS LPE film.
phases and no solvent ions incorporation could be found by XRD and EDS (detection limit of about 1%). They have a thickness of 3–4 mm, which gives ( a mean growth rate of about 23 A/s, and an FWHM of 0.21 is obtained from the (0 0 4) rocking curve [22]. Rarely, residual flux droplets remain while removing the films from the growth solution. Continued layer growth was observed in such areas, once the residual solvent dissolved. LGS film was also grown on X -oriented LGS substrates. In contrast to Y -oriented films, they show a rather wavy surface, no (macro)step formation, no clear growth habit.
5. Conclusions Basic conditions were established, which allow homoepitaxial growth of LGS films on X - and Y oriented LGS substrates by LPE for the first time. Step-flow mode occurs over macroscopic distances of 1 1.5 cm2 so far on Y -oriented films, and no inherent limitations are expected for upscaling, which could be developed similar to LPE of garnets. These results also show the high potential of LPE to obtain high-quality single-crystalline films of complex oxides, which is specially important not only for acoustic, but also for optical (laser, etc.) applications. Further work is needed to understand the phase relations in this solute–solvent system, and to establish the solubility curve. This will allow a better control of
The author expresses her thanks to S. Tidrow, C. Fazi and J. Zavada for support and discussions, M. Erwin and I. Utke for EDS/SEM, and K. Kirchner for XRD analysis. Financial support by the European Research Office of the US Army (Contract No. N68171-99-M-6663) is gratefully acknowledged.
References . [1] J. Bohm, E. Chilla, C. Flannery, H.-J. Frohlich, T. Hauke, R.B. Heimann, M. Hengst, U. Straube, J. Crystal Growth 216 (2000) 293. [2] A.A. Kaminskii, B.V. Mill, G.G. Khodzhabagyan, A.F. Konstantinova, A.I. Okorochkov, I.M. Silvestrova, Phys. Stat. Sol. 80 (1983) 387. [3] IEEE Standard on Piezoelectricity, ANSI/IEEE Standards 176-1987, New York, 1988, pp. 1–53. [4] A.N. Gotalskaya, D.I. Drezin, V.V. Bezdelkin, V.N. Stassevich, IEEE Frequency Control Symposium, 1993, p. 339. [5] S. Asakharov, Y. Pisarevsky, A.V. Medvedev, P.A. Senushencov, V. Lider, IEEE Frequency Control Symposium, 1995, p. 642. [6] K. Shimamura, H. Takeda, T. Kohno, T. Fukuda, J. Crystal Growth 163 (1996) 388. [7] B. Chai, J.L. Lefaucheur, Y.Y. Ji, H. Qiu, IEEE Frequency Control Symposium, 1998, p. 748. [8] J. Bohm, R.B. Heimann, M. Hengst, R. Roewer, J. Schindler, J. Crystal Growth 204 (1999) 128. [9] S.-Q. Wang, J. Harada, S. Uda, J. Crystal Growth 219 (2000) 263. [10] S. Uda, A. Bungo, C. Jian, Jpn. J. Appl. Phys. 38 (1999) 5516. [11] H. Matthews, Surface Wave Filters, Wiley, New York, 1977. [12] S. Kelling, et al., J. Chem. Phys. 107 (1997) 5609. [13] C. Klemenz, I. Utke, H.J. Scheel, J. Crystal Growth 107 (1999) 62; C. Klemenz, I. Utke, H.J. Scheel, J. Crystal Growth 204 (1999) 62. [14] S.B. Austerman, J. Nucl. Mater. 14 (1964) 225. [15] C. Klemenz, unpublished. [16] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751. [17] M.H. Randles, in: C.J.M. Rooijmans (Ed.), Crystals: Growth, Properties, and Applications, Crystals for
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719 Magnetic Applications, Vol. 1, Springer, Berlin, Heidelberg, New York, 1978, pp. 71–96. [18] S.L. Blank, Crystal Growth: Magnetic Garnets by Liquid Phase Epitaxy, JEMMSE, The Pennsylvania State University, 1979. [19] B.E. Watts, H. Dabkowska, B.M. Wanklyn, J. Crystal Growth 94 (1989) 125.
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[20] L.G. Schulz, Acta Crystallogr. 4 (1951) 487. [21] S.L. Blank, J.W. Nielsen, J. Crystal Growth 17 (1972) 302. [22] C. Klemenz, in: Proceedings of the 15th European Frequency and Time Forum, March 6–8, 2001, Neuch#atel, Switzerland.
High-quality langasite films grown by liquid phase epitaxy C. Klemenz* Institute of Micro- and Optoelectronics, Swiss Federal Institute of Technology, Lausanne EPFL, CH-1015 Lausanne, Switzerland
Abstract Single-crystalline La3Ga5SiO14 (LGS) films could be grown by liquid phase epitaxy (LPE) for the first time. These films were obtained on X - and Y -oriented LGS substrates (homoepitaxy) from a PbO-based flux, and were characterized by Nomarski microscopy, AFM, SEM/EDX and XRD. Best results were obtained for Y -oriented films, with macrosteps propagating over macrosopic dimensions (>1 1 cm2). Very flat areas with height variations within 1.4 nm over 8 mm lateral distance were measured by AFM. These films have a thickness of a few micrometer, are crackfree due to perfect lattice match, and no solvent inclusions or secondary phases were found. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.a; 77.65.Dq; 81.15.Lm Keywords: A1. Etching; A1. Surface sturcture; A3. Liquid phase epitaxy; B1. Oxides; B1. Rare earth compounds; B2. Piezoelectric materials
1. Introduction Major efforts have recently been made to develop langasite-type crystals La3Ga5SiO14 (LGS), La3Ga5.5Nb0.5O14 (LGN), and La3Ga5.5Ta0.5O14 (LGT) for bulk (BAW) and surface acoustic wave (SAW) applications. These compounds have piezoelectric properties [1] which are intermediate between those of quartz and LiNbO3. Compared to quartz, they have higher electromechanical coupling coefficient (k2 ), and reduced acoustic velocities, which allows fabrication of miniaturized intermediate frequency band-pass filters for new mobile communication systems. Additionally, they show no structural phase
*Corresponding author. Tel.: +41-21-6935483; fax: +41-216934525. E-mail address: christine.klemenz@epfl.ch (C. Klemenz).
transition between room and melt temperature (about 14701C for LGS), which makes them suitable for high-temperature applications. Langasites belong to the structure type of Ca3Ga2Ge4O14, with space group P321 and the same trigonal crystal class 32 as quartz. The lattice ( constants of LGS are about a ¼ 8:161 A, ( [2]. The relations between the crystalc ¼ 5:087 A lographic and crystal-physical (Cartesian) coordinates [3] in the trigonal–hexagonal system are shown in Fig. 1. These conventions are used in this paper, and X - and Y -cut substrate surfaces refer to surfaces normal to the X and Y directions, with YZ; XZ in-plane axis, respectively. Langasite crystals can be grown from the melt by Czochralski [1,4–8]. However, these crystals show a defect structure (precipitates, inclusions, impurities, striations (transverse growth banding), dislocations, twins, cracks) which limit their performance in high-quality devices.
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 1 2 - 7
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
Fig. 1. Conventions used in the trigonal–hexagonal system, in relation to the crystal-physical axis X ; Y ; and Z:
Among other inhomogeneities, striations are very common in Czochralski-grown langasite crystals. These striations are likely to be due to non-congruent starting melt composition [9], to variations of melt composition during the growth process, to solid-solubility of Si:Ga in LGS, Nb:Ga in LGN, and Ta:Ga in LGT, associated with temperature fluctuations at the growth interface. Depending on the material and specific growth conditions, the 3D pattern and periodicity of striations [5,10] can vary. At a microscopic scale, not only the concentration, but also the distribution (or ordering) of Si, Ta, Nb in the crystals is affected. Additionally, at a macroscopic scale, the composition of crystals varies along radial and growth direction. Hence, no crystal is alike and even the same crystal shows variations of structural (lattice parameters, density, etc.) and physical properties. As a consequence, parameters of devices even fabricated from same crystal shifts, and cannot meet specific demand [9]. For SAW devices, the quality of the substrate surface and the
715
structural perfection within a thickness of about one acoustic wavelength of the surface are further important aspects [11,12]. Films and crystals grown from solutions close to thermodynamic equilibrium usually show lowest dislocation and defect densities and very flat equilibrium surfaces (facets) may develop. Furthermore, dopants (and impurities) will be incorporated in a more homogeneous way than in melt growth, resulting in a better ordering in the crystal. High-quality langasite films grown by liquid phase epitaxy (LPE) are therefore also important for investigation of the temperature dependance of the piezoelectric properties of these materials. The development of LPE growth of LGS-type films can be quite demanding, and requires the study and control of all solution, substrate, and growth parameters, which may influence the growth mode of the films. For extremely perfect and crack-free LPE films, the substrate misorientation and misfit have to be practically zero, and the supersaturation has to be very low [13]. Furthermore, the substrate surface has to be free of residual strain or defects, which influence nucleation and subsequent film growth. In this study, a suitable solvent was first searched through available or related phase diagrams in the La–Ga–Si–O system and complemented by flux and top-seeded solution growth (TSSG) experiments. Substrate preparation/ etching was investigated. Search of the liquidus was performed by phase stability considerations upon cooling of the solution and X-ray analysis of crystallized phases. X - and Y -oriented LGS films could be grown by LPE on LGS substrates (homoepitaxy), and were characterized by Nomarski microscopy, atomic force microscopy (AFM), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and X-ray diffraction (XRD).
2. Solvent Finding a suitable solvent/solvent system is one of the most important aspects. Alkali vandates, molybdates and tungstates are often used for flux
716
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
growth of complex oxides like silicates and germanates. Austerman [14] used an Li2MoO3– MoO3 flux for the growth of BeO. He observed that the crystal habit was influenced by impurities, and also changed from pyramids, prisms, to platelets with decreasing MoO3 content in the solution. Platelets usually show a high structural perfection, and this was an additional reason to investigate this flux for LGS. Li2MoO4:MoO3 fluxes with ratios of 1:1 to 1:4 were used for flux, TSSG, and LPE experiments of LGT and LGS, in a temperature range of 1150–8501C. The langasite phase did not crystallize in the investigated region, and in the lower temperature range, La,Li(MoO4)2 grow epitaxially on the substrates [15]. Strong interface reactions occurs, even for solute concentrations up to 50 wt%. Bi2O3-based fluxes have similarities with PbO fluxes, and can lead to a lowering of the liquidus temperature when added to BaO–B2O3 fluxes. However, when large rareearth (RE) ions are present, substitutions may ( occur. In LGS, Bi3+ with radii of rVIII =1.17 A Bi3þ 3+ VIII ( would easily substitute La with rBi3þ =1.16 A (ionic radii taken from Ref. [16]). PbO–B2O3 fluxes as well as BaO–BaF2–B2O3 ternary solvent systems were successfully used in LPE of garnets for magnetic bubble applications [17,18]. PbO-based fluxes were also used for the growth of perovskites ABO3 (A=La,Nd,Pr;B=Ga) [19]. The advantage of borate-containing fluxes is a lower volatility, but at the expense of a higher viscosity. B2O3 is in most cases used as an additive (about 1–2%). PbF2 and PbO2 are also frequently used as additives, in order to reduce the nucleation and to preserve the Pt crucibles, respectively. Preliminary LGS flux experiments were done using PbO with different additives. The addition of B2O3 and/or PbF2 leads to the crystallization of LaBO3 and LaF3, respectively, and these additives were therefore avoided. LaGaO3 (LGO) was the major phase, which was obtained from these flux experiments, but no LGS. However, LPE films could be grown from PbO fluxes containing 15–20 wt% stoichiometric LGS and 1% PbO2. With a solute concentration higher than 20 wt%, the soaking temperature and time had to be increased, the solution became very viscous, and LGO crystallized. There exist several possible explanations for the fact that the LGS
phase could not be obtained from the same flux, e.g. without LGS substrate. First, LGO is the stable phase in the higher temperature range. Once formed, LGO continues to grow which prevents the formation of the LGS phase upon cooling. It is also known that crystals from another phase may grow in the stability field of another material, when a seed (or substrate) of the required phase is present. Thus, when the LGS substrate is present, epitaxy of LGS is energetically favored, similar to earlier observations on the growth of metastable phases [20,21].
3. Growth system and growth procedure The LPE experiments were performed in a vertical tube furnace schematically shown in Fig. 2. The starting oxides of 3–4 N purity are well mixed by hand and pressed into a cylindrical
Fig. 2. Schema of the LPE growth furnace system.
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
Pt/Au 95/5% crucible of about 45 cm2. The crucible is then placed in the furnace, at a position that can be adjusted for optimal temperature gradients. The polished X - and Y -cut LGS substrates were etched in hot orthophosphoric acid at 1301C for 2–3 h [22], rinsed with distilled water and ethanol, and then mounted with Ptwires on an alumina rod in a vertical position. During LPE growth, alterned rotation of 15– 35 rpm was applied. A Pt lid is placed on top of the crucible to prevent solvent evaporation. The furnace is heated to 8001C in 3 h, then to 11501C in 4 h. After 5 h soaking, the temperature is lowered to about 9501C and equilibrated there for about 24 h. The Pt lid is then removed, and a search of the liquidus temperature is carried out by dipping LGS crystal pieces and substrates in the solution and X-ray analysis of the phases which crystallized on them or in the solution. LPE films of LGS could be successfully grown in a temperature range of 950–8701C, depending on the initial solute concentration. After the growth, the films are slowly withdrawn from the growth solution and cooled down while being taken out from the furnace. The films are then cleaned in diluted nitric acid at room temperature, and rinsed with distilled water and ethanol.
4. Film characterization The Y -oriented LGS LPE films grown on Y -cut LGS substrates present a tendency to facet formation, as can be recognized in the microphotographies of Figs. 3 and 4. A faceting effect is also observed on the side of the macrosteps, which indicates that there may also exist other surface orientations which could potentially develop to facets. This aspect is very important, since the acoustic properties which are interesting for specific applications do not necessarily correspond to major crystal orientations. The film of Figs. 3 and 4 was grown at 8901C from the PbO flux containing 1 wt% PbO2, and La2O3, Ga2O3, SiO2 starting oxides corresponding to 15 wt% stoichiometric LGS. The substrate rotation rate was 15 rpm, and the growth time 25 min. Macrosteps propagate over the whole area of the film of about
717
Fig. 3. Nomarski microphotography of a Y -oriented LGS LPE film: step-flow mode and step bunching with formation of macrosteps.
Fig. 4. Magnified view of a growth terrace of about 100 mm diameter, on the same film.
1 1.5 cm2, but with relatively large variations in step heights and interstep distances. Step bunching has to be prevented when high-quality films have to be grown, since they are a source of solvent inclusions and striations. However, this phenomenon is not well understood and needs to be investigated from case to case. Besides growth parameters, impurities, hydrodynamics, and substrate misorientation are important factors. The substrates used herein had typical misorientations of about o0.51. Sometimes, flatter hillocks and terraces develop. Fig. 5 shows an AFM profile on a very flat region, which shows height variations within about 1.4 nm (roughly 2 lattice constants) over 8 mm of lateral distance. Due to perfect lattice match, these films are free of cracks. No secondary
718
C. Klemenz / Journal of Crystal Growth 237–239 (2002) 714–719
growth parameters, to further improve the film quality.
Acknowledgements
Fig. 5. AFM profile of a flat terrace on a Y -oriented LGS LPE film.
phases and no solvent ions incorporation could be found by XRD and EDS (detection limit of about 1%). They have a thickness of 3–4 mm, which gives ( a mean growth rate of about 23 A/s, and an FWHM of 0.21 is obtained from the (0 0 4) rocking curve [22]. Rarely, residual flux droplets remain while removing the films from the growth solution. Continued layer growth was observed in such areas, once the residual solvent dissolved. LGS film was also grown on X -oriented LGS substrates. In contrast to Y -oriented films, they show a rather wavy surface, no (macro)step formation, no clear growth habit.
5. Conclusions Basic conditions were established, which allow homoepitaxial growth of LGS films on X - and Y oriented LGS substrates by LPE for the first time. Step-flow mode occurs over macroscopic distances of 1 1.5 cm2 so far on Y -oriented films, and no inherent limitations are expected for upscaling, which could be developed similar to LPE of garnets. These results also show the high potential of LPE to obtain high-quality single-crystalline films of complex oxides, which is specially important not only for acoustic, but also for optical (laser, etc.) applications. Further work is needed to understand the phase relations in this solute–solvent system, and to establish the solubility curve. This will allow a better control of
The author expresses her thanks to S. Tidrow, C. Fazi and J. Zavada for support and discussions, M. Erwin and I. Utke for EDS/SEM, and K. Kirchner for XRD analysis. Financial support by the European Research Office of the US Army (Contract No. N68171-99-M-6663) is gratefully acknowledged.
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