Heteroepitaxial Growth of Strontium Titanate on Lanthanum Aluminate Using the Metallo-Organic Decomposition Technique
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structure and unit cell size. SrTiO3 exhibits a perovskite structure with a lattice constant of 3.905 A while LaA103 has a slightly 'distorted perovskite structure at room temperature, with a lattice constant of 3.792 A, and becomes cubic above 435 °C. In addition, there is a difference of only about 6% in the thermal expansion coefficients of these two materials. In this report we demonstrate the heteroepitaxial growth of SrTiO3 on LaA103 using the MOD method of thin-film deposition. Although previous authors have shown evidence for the heteroepitaxial alignment of films prepared by MOD [3,4], the technique used for the determination epitaxy was x-ray diffraction and, to our knowledge, none has reported the more stringent ion-channeling measurements utilized in the present work. EXPERIMENTAL The sample preparation has been reported in detail previously.[5] The metallo-organic precursors used to prepare SrTiO3 were strontium carboxylate and titanium alkoxy-carboxylate mixed to a Sr: Ti, 1 : 1 ratio. Abietic acid (rosin) was added to the mixture to adjust the rheological properties for spin coating. Typically, the thermogravimetric analysis of the precursor solution showed a solid residue (at 800 °C) of 12 - 13%. When needed, toluene was added to adjust the viscosity and the concentration of solids in the solution as well as to reduce the thickness of the films. The precursor solution was spin coated on the LaA103 substrates [6] and decomposed by heating to about 550 °C on a hot plate. To induce epitaxy, the samples were further heat treated at higher temperatures (1100 - 1200 °C) in a quartz tube furnace in flowing air. The epitaxial alignment of the MOD films was studied by Rutherford backscattering spectrometry (RBS) combined with ion channeling, using 2 MeV He++ ions. The thickness, index of refraction, and extinction coefficient of one of the films were determined using variable angle spectroscopic ellipsometry. RESULTS AND DISCUSSION Figure 1 shows the random and aligned RBS spectra of a set of four samples of MOD SrTiO3 deposited onto single-crystal, LaA103 substrates (MOD SrTiO 3 / LaA10 3 ) and annealed at 1100 'C for 1 h. For each sample the regions of the spectra corresponding to the Sr and Ti sublattices of SrTiO3 have been indicated to reveal the thickness of the corresponding MOD film. The sample whose spectra appear in Fig. la was prepared using two coatings of the original precursor solution and has the 566
thickest SrTiO3 film. The precursor solution was then diluted by adding toluene in a weight ratio of approximately 2 : 1, solution : toluene, in order to reduce the thickness of the MOD films. Two samples were prepared at this concentration using two and one coatings and their spectra are shown in Fig. lb and 1c, respectively. The precursor solution was further diluted by adding a few drops of toluene (unrecorded dilution) to get an even thinner film. A new sample was prepared whose spectra appear in Fig. 1d. A comparison of aligned (along the direction) and random (off ) spectra for each of the
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