Optical properties of crystalline and non-crystalline iron oxide thin films deposited by spray pyrolysis

Abstract

Crystalline and non-crystalline iron oxide (α-Fe 2O 3) thin films were obtained by spray pyrolysis onto glass substrate at different temperatures. The results of X-ray diffraction showed that with increasing the deposition time, the film structure changed from non-crystalline to crystalline at the same substrate temperature. At different substrate temperatures and low deposition times (5 min), iron oxide appears almost in non-crystalline form. With rising the substrate temperature and deposition time, the crystallinity was improved. The effect of substrate temperature as well as deposition time on the optical features (absorption coefficient and bandgap) and optical constants of these films has been investigated. Optical constants of the films were determined from spectrophotometric measurement of reflectance and transmittance. Analysis of the results showed that, for non-crystalline iron oxide films of different thicknesses (76–118 nm) deposited at deposition time 5 min at a substrate temperature of 350, 400 and 450 °C, direct and indirect transitions occur with energies 1.94±0.02 and 1.52±0.01 eV, respectively. The influence of film thickness and substrate temperature on the determined energy gaps has been studied. The crystalline films of different thicknesses (472–634 nm) displayed a bandgap from 1.95±0.02 to 2.125±0.02 eV for direct transition and from 1.71±0.02 to 1.88±0.02 eV for indirect transition. The optical constants (refractive index, n and the extinction coefficient, k) were dependent on the film thickness and independent of the growth temperature, T sub. The carrier concentration by using Drude’s theory is of the order 10 26 m −3 and the dielectric constant, ε ∞ is found to be 5.02.