Influence of annealing on microstructure, nonlinear optical and electrical properties of spray pyrolyzed Sn0.97La0.03O2 films

Abstract

Investigation on the effect of annealing on structure, morphology, photoluminescence, electrical, linear and nonlinear optical properties of spray pyrolyzed Sn0.97La0.03O2 films is presented. The annealing enhanced the relative intensities of the major diffraction peaks and the crystallite size due to the reduction in strain and dislocation density. Aggregation of crystallites was observed due to annealing at higher temperature. Enhancement of the transmittance and energy band gap of the films with annealing. Photoluminescence spectra showed the existence of various defects that resulted in emissions in the UV, violet, blue and yellow regions. Annealing decreased the intensity of the photoluminescent peaks in the case of 400 and 450 °C. The Z-scan experiment revealed the third-order nonlinear behaviour of the as-prepared and annealed samples. Annealing changed the reverse saturable absorption (RSA) nature of the as-prepared films to saturable absorption (SA). It was observed that annealing decreased the nonlinear absorption coefficient and refractive index. The third-order nonlinear susceptibility was in 10−3-10−4 esu range. The existence of defect sites as revealed by PL spectra can act as centers that trap excited electron which dominates the photo-induced electron-hole pair recombination and led to modification of optical properties of the samples. The variation in the density of defect states influenced the nonlinear optical performance of the films wherein thermal treatment caused the decrease of the defect states depending on the annealing induced structural modification in the film and leaded to broadening of energy band gap. Electrical characterization using the Hall effect measurement showed the n-type conductivity nature of both the as-prepared and annealed samples. The film annealed at 400 °C possessed least resistivity and maximum charge carrier density of 2.18 Ω cm and 1.04 × 1018 cm−3, respectively. The present work sheds light on the role of annealing in modulating the defect induced changes in the nonlinear properties of Sn0.97La0.03O2 films.