Integration of PLZT and BST Family Oxides with GaN
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barium, strontium and lanthanum carboxylates, and titanium isopropoxide in watermethanol mixture was used to grow PLZT and BST films. The temperature of the ferroelectric films’ growth was kept between 650-700oC to avoid decomposition of GaN and to reduce the chemical interaction between GaN and PLZT in the interface layer. Thickness of the ferroelectric films was in 0.3-5 µm range. Deposition was performed using standard dip-coating technique. Details of the deposition procedure are described elsewhere [2]. XRD spectra were recorded on “Rigaku” diffractometer using CuKα radiation. Films thickness was measured using profilometry, i.e. “Dektak”. Ferroelectric measurements were taken using standard Sawyer-Tower scheme. A HP4140A picoampermeter was used for I-V characterization of the grown films. RESULTS AND DISCUSSION. Ferroelectric properties of PLZT films on GaN. Oxide films deposited on GaN with or without ITO layer showed good adhesion to the substrate, and passed the “scotch tape” test. X-ray diffraction (XRD) measurement indicated that both PLZT and BST films were single phase and polycrystalline without predominant texture. We deposited 20-40 nm thick layer of ITO on GaN in order to obtain a highly (110)-textured ferroelectric oxide films (Figure 1). Details of this approach are given in [3]. Orientation of the ferroelectric films appears to be one of the most important factor affecting their electro-optical properties. In PLZT phase with composition 8/65/35 (“rhombohedral” phase), polar axis is parallel to [111] direction. In (110)-oriented PLZT films the [111] direction lies in the substrate plane and is, therefore, perpendicular to the incident light beam. In this arrangement, maximum phase retardation between ordinary and extraordinary components of the light beam can be reached, if the material is used in transmission mode. Therefore, the control of the ferroelectric film orientation is very critical if electro-optic applications are implied. 500
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Figure 1. X-ray diffraction patterns of the 0.3 µm thick BST film on GaN/c-sapphire (left) and 3 µm thick PLZT film grown on ITO/GaN/c-sapphire. (right) As it was shown by AFM study, PLZT films on GaN had a submicron grain size. This is smaller than the wavelengths used in optical communication systems, which are
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1300 and 1550 nm. Hence, small optical loss related to the scattering on grain boundaries can be expected. For characterization of electric properties of PLZT/n+-GaN and PLZT/ITO/ n+-GaN structures, vertical test capacitors were fabricated. Rectangular Au/Ti contact pads with 0.3x0.5 mm2 area were deposited by e-beam evaporation on the top of the oxide films. Highly conductive n+-GaN epilayer served as a bottom electrode to the f
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