Heteroepitaxial growth of lanthanum aluminate films derived from mixed metal nitrates
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Heteroepitaxial growth of lanthanum aluminate films derived from mixed metal nitrates Man Fai Ng and Michael J. Cima Ceramics Processing Research Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 30 March 1995; accepted 14 October 1996)
Epitaxial lanthanum aluminate (LaAlO3 ) thin films were deposited on single-crystal substrates by pyrolysis of spin-on mixed nitrate precursors. The films are epitaxial without any second phase. TEM micrographs show that all of these films have pores with sizes ranging from 5 to 30 nm. Grain boundaries are not observed. Selected area diffraction shows that the films are single-crystal-like, despite the porosity. All the films are smooth and crack-free. The precursors first decompose into an amorphous mixture. Heterogeneous nucleation occurs on the lattice-matched, single-crystal substrate surface. The epitaxial films grow upward and consume the amorphous regions. The crystallization temperature of LaAlO3 is lower for thin films than for bulk samples due to nucleation on the substrate. The crystallization of LaAlO3 does not exhibit linear growth kinetics. The Johnson–Mehl–Avrami exponent of growth is between 1.4 and 1.5. This deviation from the linear growth model (n 1) can be attributed to continuous nucleation on the substrate/film interface.
I. INTRODUCTION
A Ba2 YCu3 O7–x (BYC) superconductor multilayer device requires both high quality superconductor and dielectric films. Only epitaxial BYC films have demonstrated the capability to carry high current densities.1–4 This limitation on the BYC film microstructure requires that the underlying dielectric films also be deposited epitaxially in order for the subsequent superconductor films to grow epitaxially. This constrains the dielectric films to be lattice-matched. Practical devices also require that these dielectric layers to have low dielectric constant and loss, low electrical conductivity, high dielectric strength, good mechanical strength, and chemical compatibility with BYC. The deposited films must be smooth, uniformly thick, and crack- and pinhole-free. The temperatures used in the dielectric deposition process must be low to prevent potential chemical reactions between BYC and the dielectric materials. Single-crystal (100) lanthanum aluminate has become one of the favorite substrate materials for BYC films deposition because of its lattice matching properties (only 2% mismatch with BYC), relatively low dielectric constant, and chemical stability with respect to BYC. It is also a suitable dielectric material. Lanthanum aluminate has two crystal structures: a rhombohedral structure at low temperatures, and a cubic perovskite structure at high temperatures. Lanthanum aluminate is generally referred to as pseudo-cubic even at room temperature because the difference between the rhombohedral and the cubic cell is very small. The cubic notation 1306
http://journals.cambridge.org
J. Mater. Res., Vol. 12, No. 5, May 1997
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