Thermal analysis of rare earth gallates and aluminates
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H. M. O'Bryan, P. K. Gallagher, G.W. Berkstresser, and C. D. Brandle AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 28 June 1989; accepted 30 August 1989) Dilatometry, high-temperature x-ray diffraction, differential thermal analysis, and differential scanning calorimetry have been performed on LaGaO3, NdGaO3, PrGaO3, SmAlO3, and LaAlO3 single crystals grown by the Czochralski technique. First order phase transitions have been located at 145 °C for LaGaO3 and 785 °C for SmAlO3, and Ai/ has been measured for the LaGaO3 transition. Second order transitions have been identified for LaGaO3, PrGaO3, NdGaO3, and LaAlO3. The usefulness of these compounds as substrates for high temperature superconducting films is discussed in terms of thermal expansion matching.
I. INTRODUCTION
The current interest in thin films of high temperature superconductors has made the compatibility of film and substrate an important concern. For an oriented film a single-crystal substrate is required. The ideal substrate should have minimal reaction with the film during processing, possess a low dielectric constant and low loss tangent, and be readily available in large sections. In addition, epitaxial growth requires an excellent lattice match, similar thermal expansion coefficients between the film processing temperature and Tc, and no disruptive phase transitions. Critical current densities of >106 A/cm2 have been measured for c-axis perpendicular Ba2YCu3O7 films grown on SrTiOs.1 These densities were obtained for the superconducting film and SrTiO3 which have a lattice mismatch of (0.3905 - 0.3896)/0.3896 = 0.0023 at 25 °C and have average coefficients of thermal expansion (CTE) between 25 and 600 °C of -11.0 x 10-6/°C for the cubic SrTiO32 and 13 x HT6/°C (average of 22 and 4 x 10"6/°C for the a and b axes) for the orthorhombic Ba2YCu3O7.3 However, its large dielectric constant (—300) makes SrTiO3 unsuitable as a substrate for electronic applications. A recent report has described the successful deposition of epitaxial Ba2YCu3O7 films on single-crystal LaGaO3.4 There is a lattice mismatch of 0.0005 between thefr-axisof Ba2YCu3O7 and the Ga-O-Ga distance in the ab plane of LaGaO3. It was also reported that LaGaO3 undergoes a large, abrupt length change (-0.13%) associated with a transition at 150 °C and exhibits extensive twinning. The surface of films deposited on these substrates shows undesirable discontinuities. LaAlO35 and NdGaO36 single crystal substrates have also been used for depositions. No phase transition data have been reported, but it was noted that the NdGaO3 crystals were almost twin-free. The present study uses dilatometry, thermal analysis, and high temperature x-ray diffraction to locate structural transiJ. Mater. Res., Vol. 5, No. 1, Jan 1990
tions, measure the associated lattice changes, and to obtain values of the thermal expansion and CTE for several rare earth gallates and aluminates. II. EXPERIMENTAL PROCEDURE A. Czochralski crystal growth
Single crystals of lanthanide gallates were grown from a
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