Microstructure and properties of sol-gel derived Pb(Zr0.3Ti0.7)O3 thin films on GaN/sapphire for nanoelectromechanical systems

Highly (111) oriented, phase-pure perovskite Pb(Zr0.3Ti0.7)O-3 (or PZT 30/70) thin films were deposited on single-crystal, (0001) wurtzite GaN/sapphire substrates using the sol-gel process and rapid thermal annealing. The phase, crystallinity, and stoichiometry of annealed PZT films were evaluated by X-ray diffraction and Rutherford backscattering spectroscopy. The atomic force microscopy revealed a smooth PZT surface (rms roughness similar to 1.5 nm) with striations and undulations possibly influenced by the nature of the underlying GaN surface. The cross-sectional field-emission scanning electron microscopic images indicated a sharper PZT/GaN interface compared to that of sol-gel derived PZT on (111) Pt/TiO2/SiO2/(100) Si substrates. The capacitance-voltage (C-V) characteristics for PZT in the Pt/PZT/GaN (metal-ferroelectric-semiconductor or MFS) configuration were evaluated as a function of annealing temperature and applied voltage. The observed C-V hysteresis stemmed from trapped charge at defect sites within PZT. Also, the lower capacitance density (C/A = 0.35 mu F/cm(2), where A is the area of an electrode) and remnant polarization (P-r similar to 4 mu C/cm(2)) for PZT in the MFS configuration, compared to the values for PZT in the MFM configuration (Pt/PZT/Pt), were attributed to the high depolarization field within PZT.