Raman spectroscopy of ZnO thin films and nanorods deposited by Pulsed Laser Deposition (PLD) on Sapphire

Abstract

ZnO is a promising material for optoelectronic applications. ZnO thin films and nanostructures are under active investigation for such applications as solid state lasers, light emitting diodes and photovoltaic devices as examples. In this study, ZnO thin films and nanorods grown by Pulsed Laser Deposition (PLD) onto sapphire substrates have been characterized by Raman spectroscopy, and the results have been correlated with prior X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Ellipsometry measurements. The main objective has been to evaluate the dependence of material quality on growth conditions to determine optimized PLD growth parameters for ZnO thin films and nanorods on sapphire.The thin films and nanorods were deposited under different growth conditions, with the substrate temperature, chamber pressure, pulsed laser fluence, and gas content as main variables. Three symmetry allowed ZnO modes, identified as E1 at around 587 cm-1, E2 at around 438 cm-1 and the difference mode E2 (high) E2 (low) at around 333 cm-1, have been identified for all the thin film and nanorods samples but with varying line widths and intensities. The E2 Raman mode is indicative of the wurtzite structure, and is reported to be positively correlated with crystalline quality. The E1 Raman mode is negatively correlated to crystalline quality. A strong E1 mode is also an indication of oxygen deficiency. The E2/E1 Raman mode ratio have provided interesting information about the samples, and through combined analysis with the FWHM of the ZnO (002) crystal plane x-ray diffraction, suggestions have been made on crystalline quality and oxygen deficiency.A high quality thin film with low oxygen deficiency was obtained with substrate temperature of 700°C and oxygen pressure of 0.05 mbar. Such thin film was also obtained with substrate temperature of 600°C and oxygen pressure of 0.005 mbar.ZnO nanorods have, to some degree, been successfully deposited with high crystalline quality and low oxygen deficiency on a few samples, including the sample deposited with a gas combination of 50 % O2 and 50 % Ar. Very dense nanorods, almost thin films, was the result for most of the intended nanorods depositions.