BaTiO 3 Waveguides on MgO Buffered-Sapphire Single Crystals
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were achieved in air using the tapping mode. The scans were taken over an area of 3x3 pm . For the optical analysis, a He-Ne laser (,k=632.8 nm) was coupled into the film by means of a rutile prism [2]. More details about the optical setup are given in Ref. 2. TEM investigations were performed at 400 KV. For this purpose the samples were prepared in the conventional manner as it has been described elsewhere [3]. RESULTS (a) MgO layers The parameters under study were the substrate temperature and the oxygen pressure. The temperature ranged from 450 'C to 900 'C, in intervals of 30-40 'C. The pressure was 2-6x10-3 mbar and 7x] 0-2 mbar. The results obtained by XRD and RBS were not sensitive to the oxygen pressure at the low pressure regime (2-6x10-3 mbar). However, at 7x10-2 mbar the MgO films were amorphous. It is suggested at very high oxygen pressure an oxygen rich phase of Mg and O is formed, which is not stoichiometric and not crystalline. The results described in the following correspond to those samples prepared at 2x10-3 mbar. At temperatures below 630 'C only reflections from the substrate could be detected by XRD. That implies that the layers were amorphous or polycrystalline with a crystallite size of a few tens of nanometers. Above 630 'C, only the 200 MgO reflection was observed. This growth orientation is in agreement with that observed by other authors [5-8]. XRD rocking curves investigations showed that the MgO(200) plane was tilted with respect to (1102) substrate plane by an angle ranged from 1P to 3'. This effect has also been observed by other authors [5,7,8]. This tilt did not show any correlation with the substrate temperature and the oxygen pressure used during depositions. In Table I we show the full width half maximum (FWHM or Aw) of the XRD rocking curves of the MgO(200) plane as a function of the deposition temperature. Note that Ao(MgO(200)) decreases as the temperature increases to 750 'C where the best FWHM value of 0.53' is observed for a film of 200 nm. This value is approximately seven times the resolution of our diffractometer (0.08' for MgO(100)). At higher temperatures, the FWHM increases slowly up to 860 'C. The sample prepared at 890 'C was polycrystalline. It is important to mention that the FWHM decreases for thicker films. This suggests, that the influence of defects at the interface, which give rise to some misaligned crystallites, is reduced for the thicker films. Table I: Full width at half maximum (AG)) of MgO(200) rocking curves obtained by XRD. For comparison, (*) shows a normalized Ao for a nominal film thickness of 200 nm. The calculations were performed using linear interpolation. (**) measured value. T
(CC) 630 670 710 750
t(MgO) (nm) 240 200 200 200
Ao (0) 2.10 0.64 0.69 0.53
T (°C) 750 790 830 860
AW (*) (0) 2.52 ----
t(MgO) (nm) 280 240 280 280
A(O (0) 0.40 1.00 0.46 0.50
AO (*) (0) 0.53(**) 1.20 0.64 0.70
The in- and out-of-plane crystallographic orientations of MgO films prepared at 750 'C, were investigated by TEM diffraction pattern (Fig.I. The fig
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