Structural, optical, spectroscopic and electrical properties of Mo-doped ZnO thin films grown by radio frequency magnetron sputtering

Undoped and Mo-doped ZnO (2% Mo) films about 1 mu m thick were deposited by radio-frequency magnetron sputtering on Si(100) and glass substrates at 30 and 300 degrees C. X-ray diffraction patterns show that all films exhibit the hexagonal wurtzite crystal structure with a preferred orientation of the crystallites along the [002] direction. Plane view and cross-section transmission electron microscopy observations showed that the films present a columnar growth. Rutherford backscattering spectrometry indicates that Mo is homogeneously distributed inside the films. Scanning electron microscopy and atomic force microscopy show that Mo doping leads to a reduction of the grain size and surface roughness. According to X-ray photoelectron spectroscopy measurements, the valence of the Mo ions in the ZnO matrix is +5 and +6. Optical measurements in the UV-Visible range show a transmittance increasing from about 60 to 80% when increasing the wavelength from 400 to 800 nm. A sharp absorption onset is observed at about 375 nm corresponding to the fundamental absorption edge of ZnO at 3.26 eV. This gap value remains unchanged upon Mo doping. The Hall effect measurements carried out at room temperature show that both undoped and Mo-doped ZnO films present an n-type conduction. The 2% Mo doping increases the carrier concentration and decreases the resistivity measured in pure ZnO by about three orders of magnitude. A comparison with 2% Al-doped ZnO films grown in the same conditions underlines the important role of the preparation conditions on the transport properties of ZnO based transparent conductive oxides. (C) 2014 Elsevier B.V. All rights reserved.