Single crystal growth of PrGaO3

Journal of Crystal Growth 123 (1992) 126—132 North-Holland

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Single crystal growth of PrGaO3 Masahiro Sasaura and Shintaro Miyazawa NTT LSI Laboratories, 3-1, Morinosato Wakamiya, Atsugi-Shi, Kanagawa 243-01, Japan Received 6 March 1992

Czochralski growth of PrGaO1 single crystal is investigated. A multitude of twins and fine cracks are still included in the crystal boules, but crystals less than 5 mm in diameter are almost entirely crack- and twin-free. To clear up the occurrence of twins, high temperature measurements (high temperature X-ray diffractometry and thermal analysis) were performed. The results suggest the existence of a phase transition, presumably orthorhombic—rhombohedral transition, above 14OO~C.This high temperature transition causes twins.

1. Introduction In order to realize high quality thin films of high-To superconducting cuprates, namely YBa2 Cu3O~ (abbreviated here after to YBCO) for electronic device applications, substrate materials must be well lattice-matched with the YBCO. At the present time, MgO and SrTiO3 are widely used as substrates for YBCO thin film, but their crystallinities are not good because of the existence of sub-boundaries. Moreover, the respective lattice-mismatch of MgO and SrT1O3 is 8.72% and1”2) 1.06% (for adeposition YBCO lattice constant ofaround (a~+ at film temperatures b~) 700°C in general. To achieve epitaxial thin film growth, lattice-matching at film deposition temperatures is very important, as is the matching of the thermal expansivity. Among oxides, the lanthanide aluminate (REAl03 RE lanthanoide series element) and gallate (REGaO1) families have a better latticematch to YBCO. However, the unit cells of lanthanide aluminates are smaller than those of lanthanide gallates, and the lattice-mismatch of aluminates to YBCO is larger. Especially, LaGaO1, NdGaO3 and PrGaO1 appear to offer the best match because the orthorhombicity (a0 b0) of other types of gallates becomes larger as the atomic number of the lanthanide ions increases =

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[1], but their lattice constants at elevated temperatures are not well known. Among these three gallates, LaGaO3 is inadequate for use as a substrate because of complex twinning associated with a phase formation at 140—150°C[2]. On the other hand, single crystal growth of NdGaO3 has been reported [31,and twin-free NdGaO3 can he successfully grown by a conventional Czochralski method [4]. Moreover, its potential as a substrate for YBCO thin films has been proved [51. There have, however, been few reports on the growth of PrGaO3 single crystal, in spite of the fact Here, that it we has report the bestthe lattice-match YBCO [6]. Czochralskiwith growth of PrGaO 3 crystals.

2. Crystal growth The raw materials of Pr6011 and Ga2O3 (both 99.99% purity) were mixed in a stoichiometric ratio of PrGaO1. The mixture was calcined at 1200°C for 15 h in air. X-ray powder diffraction patterns of the calcined specimen showed it to be a distorted perovskite with a GdFeO1 structure in the orthorhombic system, as shown in fig. la. When Pr203 was used as a raw material, the X-ray powder diffraction pattern showed an unidentified peak at 20 32° for the calcineci

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M. Sasaura, S. Miyasawa

/ Single crystal growth of PrGaO3

127

ceramics, as indicated by the arrow in fig. lb. Mizuno and Yamada [7] reported that PrGaO3 could be made from Pr203 and Ga203 after the following two calcination processes: 1500°Cfor 10 h and 1600°Cfor 5 h. This exhibited no peak at I ____________ ‘7/—

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Fig. 1. X-ray powder diffraction patterns of PrGaO3 calcined from two different praseodymium oxides, (a) Pr6011 and (b) Pr203, with Ga203 as the raw material. An unidentified peak caused by the existence of an unreacted intermediate compound is indicated by the arrow in (b). (c) X-ray powder diffraction patterns of a pulled PrGaO3 crystal.

32°.Therefore, the origin of the unidentified peak may be an unreacted intermediate compound

formed during low temperature calcination. Fig. lc shows the X-ray powder diffraction pattern of a pulled PrGaO3 crystal described below. No unidentifiable peaks were observed and all dominant peaks could be indexed with the orthorhombic system. Fig. 2 shows a cross-section of a typical Czochralski growth furnace. A cone shaped after-heater made of iridium was placed above the Jr crucible (50 mm diameter x 50 mm in height). The temperature was optimized experimentally by adjustinggradient the distance between the crucible and the after-heater. The axial temperature gradient in the after-heater was 18°C/cm,as found from temperature measurements using a thermocouple. At 50°Cabove the melting temperature, relatively clear spokes, due to a convection in the melt, were observed on the melt surface, which disappeared at the melt-

M. Sasaura, S. Miyasawa

128

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crystal growth of PrGaO

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Fig. 3. As-grown PrGaO~crystals pulled along (a) [1001 and (b) [0011axes.

ing temperature. Therefore, the axial temperature gradient just above the melt surface would be extremely low on pulling. The growth chamber was evacuated after charging the calcined material and then backfilling with N2 + 3%O2 gas before melting the starting material. During the first attempt to pull crystals, Pt—Rh wire was used as a seed. The pulling rate was 2—3 mm/h and the crystal rotation rate was 10—15 rpm. Crystallographically onented seed crystals for the subsequent growth runs were prepared from this boule. Seed crystals oriented [001], [1001, and [110] in orthorhombic symmetry were used in this study. Fig. 3 shows examples of as-grown PrGaO3 crystals pulled along (a) the [100] and (b) the [0011 axes. The crystals appeared green in colour when the melt was fresh, while the colour of crystals further pulled from the same melt turned greenish red. Other rare earth (La, Ce, Nd, Sm, Er, Th) and iron ions were not detected (<50 ppm wt%) by an inductively coupled plasma atomic emission spectrometry measurement (ICP—AES). The colours of Pr6011, Pr2O1 were

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brown, green, respectively. Calcined PrGaO3 powders from two different praseodymium oxides .

were green. This difference in colour is thought

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Fig. 5. {001)-oriented polished wafer sliced from a small [0011-pulled crystal. Cracks were not observed, but a few twins existed.

M. Sasaura, S. Miyasawa

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to be due to praseodymium ions whose valency must be dependent on the oxygen partial pressure of the growth atmosphere. That is to say, a repetition of growth from the same might 37Pr2~ in themelt melt. Eichange of Pr ther thatthe or ratio a composition change due to Ga 203 evaporation [8] from the melt must contribute to changing colour. Fig. 4 shows a (001)-oriented polished wafer sliced from the [001] axis boule under transmitted polarized light. Small cracks and numerous twins were observed. When the sample was rotated under a polarization microscope, the contrast around the twin boundaries was altered at intervals of about 90°. Some of the twin boundaries were perpendicular to the wafer surface, while others were inclined 45°. Cracks occurred mainly along the (112) plane, and the dominant twin plane was (110), which is similar to the

crystal growth of PrGaO

3

129

findings in LaGaO3 [9] and NdGaO3 [3]. Crystals with a diameter of less than 5 mm were almost entirely crack-free. Fig. 5 shows an example of a crack-free wafer, inwhere a few may twinsbestill Crack formation the crystals due exist. to a large amount of residual thermal and/or transformation strain. Crystal growth runs with different pulling axes were performed to examine the formation of cracks and twins. Figs. 6a and 6b show vertically cut and polished slices of [1001- and [1101-pulled crystals. Observation with a polarized light clearly shows (112) cracks and (110) twins. Fig. 7 shows schematic drawings of twin formations in the crystals grown along three different axes: [100], [1101 and [001]. In the [1001-pulled crystal, (112) cracks inclined 45°from the pulling direction, and crossed (112) cracks lay along the pulling axis on other crystals. The cracks mainly oco~irredfrom

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M. Sasaura, S. Miyasawa

130

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temperature were observed. The gradual fluctuation below 1000°Cwas due to furnace characteristics. These peaks may be proof that the crystal transforms from the orthorhombic to the primitive rhombohedral system. Geller [101 reported

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crystal growth of PrGaO

that three PrAlO 1 and rare-earth NdAlO1 aluminates belong to spaceLaAlO~, group D~d-R3m,while most of the Ill—Ill perovskitc (ABO3) aluminates, gallates and vanadates with —

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Fig. 7. Schematic drawings of crack and twin formations in crystals grown along three different axes: (a) [tool,(b) [1101 and (c) [001].

the top of the crystal or propagated from the crystal surface. Their origin may be associated with strain. The (110) twins appeared mainly at the end of the boule on the [110]- and [0011-pulled crystal. The growth direction changed from [1101 to [001] in the [1101-pulled crystal. A twinned sample was examined under an optical microscope equipped with a small furnace on the stage. A heating and cooling cycle with a high temperature limit below 1000°Cdid not resuit in remarkable change in twinned features, so twinning may have its origin at temperatures above 1000°C. Differential thermal analysis (DTA) was conducted using Mac Science TGDTA2200 to determine the origin of twinning, Fig. 8 shows a typical DTA curve, where two sharp endothermic peaks close to the melting

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GdFeO3 structure belong to space group D~Pbnm. According to Geller’s description, PrGaO3 may belong to R3m. Therefore, it can be reasonably asserted that the orthorhombic structure at room temperature may in some cases (for example, LaGaO3 or PrGaO3) transform to the rhombohedral structure around 1400°C and that the rhombohedral transforms to the ideal perovskite structure at temperatures ranging from above 1400°Cup to the melting temperature. The DTA shown in fig. 8 should show these transformations. During crystal growth, the grown part goes through these transitions, probably resulting in the occurrence of transformation twins associated with internal thermal stress/strain. Therefore. doping with other rare earth ions may shift these transition temperatures above the melting ternperature. Doping experiments are now being conducted. The smallest quantity of twins was obtained in a [001] axis boule, and a twin-free area as large as 30% was sometimes obtained. However, under a transmitted polarized optical microscope, several misoriented subgrains/sub-boundaries were observed in even twin- and crack-free areas. These hazy subgrains resulted in 220 arc sec of full width at half maximum (FWHM) measured by X-ray double-crystal diffractometry (fl-scan).

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3. Lattice constants and thermal expansivity of PrGaO3 Lattice byconstants of thediffractometry grown crystalwith were measured X-ray powder Si powder as a reference and were determined to be a1) b11 5.493 C0 7.740lattice A at room5.462 temperature. TableA 1and summarizes =

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/ Single crystal growth of PrGaO3

M. Sasaura, S. Miyasawa

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Table 1 Lattice constants of PrGaO3 reported so far along with our own results

tablished 6°C’talong to be 8.36the x 106, a 7.31 x 106 and 6.99 x 10 0, b0 and c0 axes, respec-

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tively. Up to 1100°C,there was no evidence of a distinct phase transition. When a0 and b0 are extrapolated to higher temperatures, they cross each other around 1400°C,which is very close to the endothermic peaks of the DTA curve. Therefore, it can be assumed that the phase transition

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causing twins lies above 1400°C. To evaluate the suitability of PrGaO3 as a substrate, superconducting YBCO thin films were deposited using a pulsed laser ablation technique. The X-ray diffraction pattern of as-deposited 500 A thick film on PrGaO3 (001) substrate showed a strong preference pendicular to the for substrate. the c0 axis The orientation FWHM of perthe (005) peak (20 38.52°) measured by a 0/20 scan method was 0.07°.This means that the irregularity in the cell length of the c0 axis in the film is small. However, fl-scan curves show the FWHM of the (005) peak to be 0.24°. Generally, as lattice mismatch becomes smaller, the crystallinity of the film reflects the crystallinity of the substrate; that is, the c0 axis orientation of the film was determined by the substrate subgrains in =

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suits [1,9—12].They are quite consistent with each other. Fig. 9 shows the temperature dependence of the lattice constants, measured with a hightemperature X-ray diffractometer. From these results, the thermal expansion coefficients were es-

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this experiment. This indicates that the film deposited epitaxially. The temperature dependence of electrical resistivity for a film with a thickness of 500 A exhibited good superconductivity with a superconducting transition at 90 K and a transition width of about 1 K. Although it is well known that praseodymium ions degrade the su-

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132

M. Sasaura, S. Miyasawa

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perconductivity of the YBCO superconductor [13], Auger electron spectroscopy (AES) revealed the inter-diffusion layer to be thinner than 88 A. This means that the thin film of 500 A thickness was free from the inter-diffusion that breaks down superconductivity. A detailed study of YBCO film quality has been published in a separate paper [61.

4. Dielectric properties The frequency dependence and the temperature dependence of the dielectric properties were measured using square samples (5 mm x 5 mm) from twin-free portions. Fig. 10 shows the frequency dependence and the temperature dependence of the dielectric properties. The dielectric constant e of 24 is independent of both frequency (1 kHz to 1 MHz) and temperature (16— 300°C).The loss tangent tan ~ changes between 3.6 x iO~and 5.4 X iO~ and is roughly linear to frequency and temperature. These values are comparable to those of LaGaO3 and NdGaO1 [14].

crystal growth of PrGaO~

High temperature X-ray diffraction and differential thermal analysis reveal that the origin of twinning can most probably be traced to a phase transition above 1400°Cclose to the melting temperature.

Acknowledgments The authors thank Professor H. Kojima of Yamanashi University for measuring dielectric properties.

References [1] M. Marezio, J.P. Remeika and PD. Dernier, lnorg. Chem. 7 (1968) 1339. [2] S. Miyazawa, AppI. Phys. Letters 55 (1989) 2230. [3] 5. Miyazawa, M. Mukaida, M. Sasaura and Y. Tazo, J. Electrochem. Soc. 137 (1990) 227C. [4] G.F. Ruse and S. Geller, J. Crystal Growth 29 (1975) 305. [51M. Mukaida, S. Miyazawa, M. Sasaura and K. Kuroda, Japan. J. AppI. Phys. 29 (1990) L938. [6] M. Sasaura, M. Mukaida and S. Miyazawa, Appl. Phys. Letters 57 (1990) 2728. [7] M. Mizuno and T. Yamada, Yogyo Kikai Shi 93 (1985) 686. [8] C.D. Brandle, D.C. Miller and J.W. Nielsen, J. Crystal

5. Summary We have investigated the growth of PrGaO3 by the Czochralski method. Pr6O~1was used instead of Pr203 as the raw material. The crystals pulled along the [001], [110], and [1001axes contained twins and cracks, but crystals less than 5 mm in diameter were almost completely devoid of any cracks and twins. The twin plane was (110) and crack plane was (112).

Growth 12 (1972) 195. [9] G.W. Berkstresser, A.J. Valentino and C.D. Brandle, J. Crystal Growth 109 (1991) 457. [10] 5. Geller, Acta Cryst. 10 (1957) 243. [11] S. Geller, P.J. Curlander and G.F. Ruse, Mater. Res. Bull. 9 (1974) 637. [12]H.M. O’Bryan, P.K. Gallagher, G.W. Berkstresser and CD. Brandle, J. Mater. Res. 5 (1990) 183. [13] A. Matsuda, K. Kinoshita, T. Ishii. H. Shibata, T. Watanabe and T. Yamada, Phys Rev. B 38 (1988) 2910. [14] T. Konaka, M. Sato, H. Asano and S. Kubo, J. Superconductivity 4(1991)283.