The effects of carrier gas and substrate temperature on ZnO films prepared by ultrasonic spray pyrolysis

ZnO films were deposited on glass substrates in the temperature range of 350-470 degrees C under an atmosphere of compressed air or nitrogen (N-2) by using ultrasonic spray pyrolysis technique. Structural, electrical and optical properties of the ZnO films were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical two-probe and optical transmittance measurements. The ZnO films deposited in the range of 350-430 degrees C were polycrystalline with the wurtzite hexagonal structure having preferred orientation depending on the substrate temperature. The ZnO films deposited below 400 degrees C had a preferred (100) orientation while those deposited above 400 degrees C mostly had a preferred (002) orientation. The resistivity values of ZnO films depended on the types of carrier gas. The ZnO thin films deposited under N-2 atmosphere in the range of 370-410 degrees C showed dense surface morphologies and resistivity values of 0.6-1.1 Omega-cm, a few orders of magnitude lower than those deposited under compressed air. Hydrogen substition in ZnO possibly contributed to decreasing resistivity in ZnO thin films deposited under N-2 gas. The Hall measurements showed that the behavior of ZnO films deposited at 410 degrees C under the N-2 atmosphere was n-type with a carrier density of 8.9-9.2 x 10(16) cm(-3) and mobility of similar to 70 cm(2)/Vs. ZnO thin films showed transmission values at 550 nm wavelength in a range of 70-80%. The values of band gaps extrapolated from the transmission results showed bandgap shrinkage in an order of milli electron volts in ZnO films deposited under N-2 compared to those deposited under compressed air. The calculation showed that the bandgap reduction was possibly a result of carrier-carrier interactions. (c) 2012 Elsevier Ltd. All rights reserved.