We report the structural evolution and optical properties of lanthanum doped lead zirconatetitanate(PLZT) thin filmsprepared on Pt/TiO 2 /SiO 2 /Si substrates by chemical solution deposition. X-ray diffraction demonstratesthe post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T a) range of 550-750 ºC. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E g) values determined from UV-visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5-3.8 eV. E g decreases with increasing T a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46-2.50(λ=632.8 nm) with increase in T a. The increase in refractive index at higher T a is attributed to the improved packing density and crystallinity with the temperature.
We report the effect of post deposition annealing temperature (T a = 550 and 750 o C) on the surface morphology, chemical bonding and structural development of Lanthanum doped lead zirconate titanate (Pb 0.95 La 0.05 Zr 0.54 Ti 0.46 O 3 ; referred to PLZT) thin films prepared using chemical solution deposition method. Atomic force microscopy demonstrates formation of nanocrystallites in the film annealed at T a =750 o C. X-ray photoelectron spectroscopy (XPS) analyses indicate that the binding energies (BE) of the Pb 4f , Zr 3d, and Ti 2p doublet experience a positive energy shift at T a = 750 o C, whereas the BE of O 1s and La 3d doublet show negative shift with respect to the BE of the films annealed at T a =550 o C. Thermal induced crystallization and chemical modification is evident from XPS results. The Ar+ sputtering of the films reveal change in oxidation state and chemical bonding between the constituent atoms, with respect to T a. Raman spectroscopy used to study phonon-light interactions show the shift in longitudinal and transverse optical modes with the change in T a , confirming the change in phase and crystallinity of these films. The results suggest annealing at T a = 750 o C yield crystalline perovskite PLZT films, which is essential to obtain photovoltaic response from devices based on such films.
We investigated the influence of space charge region (SCR) on the electrical properties of Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin film based capacitor structures. The metal/PLZT/Pt capacitors were prepared by using metal electrodes (Pt, Au, Al) of varying work function as top electrodes, and Pt as a bottom electrode. The SCR formed at the top interface between the PLZT film and the metal differs from each other in three capacitor structures. The results obtained from the hysteresis, dielectric, and leakage current measurements were analyzed to understand the variation in the electrical properties of the capacitors arising from the different SCRs formed at the interfaces. Among the three, the Pt/PLZT/Pt capacitor showed highest polarization and permittivity, and lowest FCC and coercive field. The Al/PLZT/Pt capacitor showed lowest polarization and permittivity, but higher coercive field and FCC. The leakage current increased with a decrease in work function of the top electrode. The Schottky conduction mechanism was found to be dominant in all the three capacitors. Both the photovoltaic parameters (short circuit photocurrent denisty and open circuit photovoltage) increased with the increase in work function of the top electrodes. These studies suggest the existence of wider SCR and higher space charge field at the top interface in the case of capacitor designed with an Al (or Au) top electrode as compared to the capacitor designed with a Pt top electrode.
The effect of Pb content and solution concentration of lead titanate (PbxTiO3) seed layer on the texture and electric properties of Pb1.1(Zr0.52,Ti0.48)O3 (PZT) thin films was investigated. A variety of seed layers (y PbxTiO3) with varying solution concentration (y = 0.02, 0.05, 0.1, and 0.2 M) and Pb content (x = 1.0, 1.05, 1.1, and 1.2) was deposited on Pt/TiO2/SiO2/Si substrates using chemical-solution deposition method. PZT films were then deposited on these seed layers using the same process. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy investigations of the seed layers confirm change in crystal structure with variation in the solution properties. XRD studies of PZT films deposited on seed layers demonstrate that the seed layer helps in enhancing {100}-texture and suppressing {111}-texture. It was observed that PZT films prepared on seed layers with lower solution concentrations results in highly {100}-textured films, which further helps to improve the electric properties. The polarization and dielectric constant of the PZT films were seen to increase while the coercive field decreased with increase in {100}-texture. Irrespective of the seed layer solution concentration, higher Pb content in the seed layer deteriorates the PZT film properties. Ninety-five percent to ninety-six percent {100}-texture was obtained from thin PZT films deposited on seed layers of 0.02 M solution concentration with 1.05 and 1.10 Pb contents, which is higher than the values reported for thick PZT films. Optimization of both Pb content and solution concentration of the seed layer is a promising route to achieve highly {100}-textured PZT films with improved electric properties.
We report the effects of illumination on the ferroelectric and photovoltaic properties of the Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin film based asymmetric metal/ferroelectric/metal capacitor structure, using Au as a top electrode and Pt as a bottom electrode. Conductive-AFM (atomic force microscopy) measurements demonstrate the evolution of charge carriers in PLZT films on illumination. The capacitance-voltage, the polarization-electric field, and the leakage current-voltage characteristics of the asymmetric Au/PLZT/Pt capacitor are discussed under dark and illuminated conditions. The light generates charge carriers in the film, which increase the coercive field and net remnant polarization and decrease the capacitance. The leakage current of the capacitor increases by an order of magnitude upon illumination. The leakage current data analyzed to study the conduction mechanism shows that the capacitor structure follows the Schottky emission “1/4” law. The illuminated current density-voltage curve of the capacitor shows non-zero photovoltaic parameters. An open circuit voltage (Voc) of −0.19 V and a short circuit current density (Jsc) of 1.48 μA/cm2 were obtained in an unpoled film. However, after positive poling, the illuminated curve shifts towards a higher voltage value resulting in a Voc of −0.93 V. After negative poling, the curve shows no change in the Voc value. For both poling directions, the Jsc values decrease. The photocurrent in the capacitor shows a linear variation with the incident illumination intensity.
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