Investigation of structural and photoluminescence properties of nanocrystalline tin oxide thin films grown by ultrasonic spray pyrolysis method

Nanocrystalline SnO₂ thin film was, successfully, synthetized by using USP method. The structural and optical properties of SnO₂ nanocrystallites were studied by using XRD, AFM, UV-Vis-NIR and PL spectroscopies. The XRD pattern confirmed the tetragonal rutile structure of SnO₂ nanocrystallites with average crystalline grain size of 41 nm. AFM images show a dense surface of columnar grains with RMS surface roughness of 8.87 nm. UV-Vis-NIR measurements exposed a direct band gap of energy 3.58 eV. The PL spectrum recorded at 7 K reveals the presence of PL peaks centered in IR and Vis regions, attributed to radiative transitions via oxygen vacancies, Sn interstitials and dangling bonds. A schematic bands diagram is proposed with the approximate positions of intrinsic point defect levels in nanocrystalline SnO₂ thin films. The integrated PL measurements demonstrate the good thermal stability of our sample. The unusual behavior of PL peaks and their FWHM evolution as a function of temperature indicates the thermal sensitivity of the intrinsic point defects energy levels present in band gap. Indeed, the shallower energy levels due to dangling bonds and/or oxygen vacancies are more sensitive to the temperature. However, the volume defects like Sn interstitials are thermally stable and constitute deep and stable energy levels for photoexcited electrons. Small redshifing of PL peaks is observed with the increasing of temperature. This is attributed to the reduction of oxygen vacancies. (author)