Epitaxial Alignment and Microstructural Development in Lithium Niobate Thin Films Prepared by Metallo-Organic Decomposit
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In the present study we demonstrate the heteroepitaxial alignment of thin films of LiNbO 3 deposited by MOD on A120 3. We also discuss the microstructural evolution of the films. EXPERIMENTAL The LiNbO3 metallo-organic precursor was prepared from niobium alkoxy-carboxylate and Lithium-2-ethylhexanoate using 2-ethylhexanoic acid as solvent. Films were deposited by spin coating onto single-crystal, oriented, A12 0 3 substrates, followed by heating to 500 - 550 'C on a hot plate. Further heat treatment was carried out in a quartz tube furnace. The morphology of the films was studied by secondary electron microscopy (SEM). Structural characterization was obtained from x-ray diffraction analysis and the epitaxial alignment was determined by 2 MeV He++ ion channeling. RESULTS The films were deposited by spin coating, pyrolyzed at 500 - 550 'C and subsequently heat treated in the quartz tube furnace, in flowing air, at temperatures ranging from 500 'C to 1000 'C for 30 min to 1 h. X-ray diffraction (XRD) analysis of films treated at 600, 900, and 1000 'C revealed only the (0006) reflection of LiNbO 3 indicating that the films had grown epitaxially oriented with respect to the A120 3 substrates. Further characterization of the epitaxial alignment was carried out using Rutherford backscattering spectrometry (RBS) combined with ionchanneling. Figure 1 shows the aligned and random spectra of a sample of MOD LiNbO 3 deposited on A120 3 (MOD LiNbO 3/AI 2 0 3 ) and heattreated at 900 °C for 30 min. The Nb peak (corresponding to the Nb Energy (MeV) 1.0
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Figure 1. Aligned and random RBS-channeling spectra of a sample of MOD LiNbO 3 /A1 2 0 3 heat-treated at 900 'C for 30
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sublattice of the LiNb0 3 film) in the aligned spectrum is significantly lower than the corresponding Nb peak in the random spectrum, again indicating that the MOD LiNb03 film is epitaxially aligned with respect to the A1203 substrate. The Al marker in Fig. 1 indicates the channel or energy corresponding to He++ particles backscattered at the top surface of the sample by Al atoms. Since the Al signal extends all the way to the marker, the RBS-channeling spectra shown in Fig. 1 reveal the presence of Al directly exposed to the beam without LiNb03 overlayer. This could be attributed to the out diffusion of Al to the surface of the LiNb03 or to the formation of pinholes in the MOD film leaving the substrate exposed to the probing beam. In order to check these possibilities as well as to characterize the morphology of the films, they were examined using SEM. Fig. 2 shows SEM micrographs of two samples, one only pyrolyzed in the hot plate at 500 'C for approximately 10 min and the other pyrolyzed and subsequently heat-treated at 900 'C for lh. It is evident that the films are not uniform; instead, they are formed by a network of grains that grow in size with increasing heat-treatment temperature but never cover
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