Optical properties of nanocrystallite films of ?-Fe2O3 and ?-Fe2-xCrxO3 (0.0 ? x ? 0.9) deposited on glass substrates

Abstract

?-Fe2O3 films are deposited on fluorine-doped tin oxide (FTO) and indium-doped tin oxide (ITO) substrates for 1, 4 and 6 min using a spray pyrolysis technique. We also deposited ?-Fe2-xCrxO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.7 and 0.9) films on the FTO substrate for a deposition time of 35 s. The structural and optical properties of these films were then studied. The x-ray diffraction (XRD) patterns show that all the films are crystalline in nature with a hexagonal crystal structure. The average grain size and unit cell volume were calculated using XRD data. It is found that the average grain size and unit cell volume increase with an increasing film thickness and Cr-doping concentration. The value of strain decreases with an increasing film thickness and Cr-doping content. It is also found that films with the same deposition time on the ITO substrate are more crystalline than on the FTO substrate. Furthermore, the average grain size is obtained from field emission scanning electron microscopy (FESEM) images. FESEM analysis confirms that the average grain size increases with the film thickness and Cr-doping concentration. The optical absorption spectra of the films show that the absorbance increases with an increasing deposition time and Cr concentration. The energy band gap (Eg) of all the films has been calculated using Tauc's relation. A narrowing of the band gap was observed with an increase in film thickness and Cr-doping content. The reduction of the band gap with the increase in film thickness of the films deposited on the ITO substrate is larger than for the film deposited on the FTO substrate. The refractive index is also obtained from the absorption spectra of the films using the Moss relation: n = 4 (k/Eg), where k =108 eV. The refractive index decreases with an increase in the optical band gap. The band gaps of the films are also calculated from the FTIR spectra. This is in good agreement with the UV data. The correlation between the structural and optical properties of the deposited films has been discussed.