Structural, optical and ac conduction properties of Bi₂V_1-xNb_xO_5.5 (0≤x≤0.4) thin films fabricated by pulsed laser deposition technique

Abstract

Bi₂V_1-xNb_xO_5.5 {(x = 0, 0.1, 0.2, 0.3, 0.4), (BVN)} thin films were grown by pulsed laser deposition on (1 1 1) Pt/TiO₂/SiO₂/Si and Corning glass substrates and investigated systematically for their microstructural, optical and ac conduction properties. The undoped bismuth vanadate, Bi₂VO_5.5 (BVO) thin films were highly textured and have been associated with the c-axis oriented grains of the layered perovskite structure, while the Nb doped films consisted of randomly oriented crystallites. The scanning electron microscopy of the films indicates that the grain size increases with increase in Nb content. The optical transmission studies carried out on the samples deposited on Corning glass substrates revealed that these films were nearly 80% transparent in the 400-900 nm range and the band gap of Nb doped BVO thin films was slightly higher (3.13 eV for x = 0.4) than that of the undoped (2.91 eV) films. The dielectric constant of the Nb doped films increased with increase in Nb content, while the dielectric loss decreased especially in the 3-100 kHz frequency range. At a particular frequency, the conductivity decreased with increase in Nb content in the BVN thin films. In the higher temperature range, the activation energy varied from 0.61 eV (x = 0.1) to 0.76 eV (x = 0.4) measured at 100 Hz. The frequency analysis of the dielectric and ac conduction properties of these films suggests the conduction process in these films to be via oxygen ion vacancy motion through various defect sites.