PLD deposited ZnO films on different substrates and oxygen pressure: A study of surface morphology and optical properties

Abstract

Thin films of ZnO were prepared by pulsed laser ablation on single crystal strontium titanate (STO, 001) and yttrium stabilized zirconium oxide (YSZ, 001) substrates at temperature ~600 °C under varying oxygen pressure of ~50, 100 and 200 mTorr. These as deposited ZnO thin films have been subjected to structural, microstructural and optical characterization by employing X-ray diffraction (XRD), scanning electron microscopy (SEM), and PL-UV spectroscopy, respectively. XRD reveals that the films are epitaxial having single phase wurtzite structure with preferred orientation along (002) direction. The film quality increases with increase in oxygen pressure as evident by enhanced (00.2) peak intensity at higher oxygen pressure (~200 mTorr) on both substrates. SEM results show that the density and morphology of film improves at higher oxygen pressure because of increased particle size. Photoluminescence (PL) measurements indicate that all the thin films show intense near bandedge PL emissions around ~380 nm without the presence of any deep level emission at higher wavelength. All the UV peaks on YSZ substrate are located close to ~380 nm whereas on STO substrates the UV peaks shifts towards lower wavelength as oxygen pressure increases from 50 mTorr to 200 mTorr. The optical band gap on YSZ was found to be ~3.28 eV whereas on STO it varies in the range of ~3.28 to 3.30 eV with increasing oxygen pressure. The variation in band gap on STO has been attributed to the compressive strain on ZnO films due to lattice mismatch between ZnO and STO. The improved optical properties at higher oxygen pressure and on different substrate have been discussed in relation to the microstructural variation because of oxygen incorporation and lattice strain arising due to lattice mismatch of substrate (STO and YSZ) with ZnO. © 2012 by American Scientific Publishers.