Effect of thermal annealing on the structural and optical properties of nanostructured zinc oxide thin films prepared by pulsed laser ablation R. Vinodkumar a , K.J. Lethy a , D. Beena a , M. Satyanarayana a , R.S. Jayasree b , V. Ganesan c , V.U. Nayar a , V.P. Mahadevan Pillai a,Ã a Department of Optoelectronics, University of Kerala, Kariavattom 695581, Thiruvananthapuram, India b Department of Radiology, Sree Chithra Tirunaal Institute of Medical Science and Technology, Thiruvananthapuram 695011, India c Inter University Consortium for DAE Facilities, Khandwa Road, Indore 452 017, India article info Article history: Received 6 September 2007 Received in revised form 17 April 2008 Accepted 18 April 2008 Available online 6 June 2008 Keywords: Zinc oxide nanostructured films Transparent conducting oxides (TCO) Pulsed laser ablation Solar cell materials Photoluminescence spectra Photoluminescent materials abstract Zinc oxide (ZnO) films are prepared by pulsed laser ablation, on an optically flat quartz substrate for different deposition time. The influence of annealing temperature, on the structural and optical properties of ZnO films is investigated systematically using X-ray diffraction (XRD), energy dispersive X- ray analysis (EDX), scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectra, UV–vis spectroscopy and photoluminescence spectroscopy (PL). The XRD pattern shows that the as- deposited films are amorphous and the annealed films are polycrystalline. The average size of the crystalline grains varies from 9 to 26 nm in the films. The SEM and AFM images reveal uniform distribution of grains in the films and the grains are in the nanoscale dimension. Raman spectra suggest the hexagonal wurtzite phase for the ZnO films. The UV–visible spectra show an increase in transmittance with annealing temperature. The observation of very intense PL emission from the films annealed at 773 K, suggest the suitability of these films for applications as light emitters in the visible region. The ability to produce the stochiometric ZnO thin films with reproducible structural, morphological and optical characteristics should be useful as a suitable window material for practical industrial solar cell and display devices. & 2008 Elsevier B.V. All rights reserved. 1. Introduction ZnO is an n-type semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV [1]. Transparent conducting oxide films with a large band gap energy exhibit high-electrical conductivity, high-optical transmittance in the visible region and high reflectance in the IR region. ZnO is one of the most promising materials for the fabrication of next generation optoelectronic devices [2,3]. In recent years great interests are focused on nanostructured ZnO because of its wide direct band gap, strong electronic binding energy and promising application for UV laser with low threshold, transistor and biosensor in nano scale [4]. Pulsed laser ablation technique has emerged as a suitable method for the growth of ZnO thin films due to its advantages like ability to reproduce the stochiometry of the target materials under optimized deposition conditions and a large number of parameters can be changed during the deposition time. In this paper morphological and structural evolutions of ZnO films prepared by pulsed laser deposition under different deposi- tion time and their optical properties as a function of annealing temperature are discussed. Films are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), atomic force microscopy (AFM), UV–visible spectra, Raman spectra and photoluminescence (PL) spectra. 2. Experiment The deposition was carried out using a Q switched Nd:YAG laser (Quanta Ray- INDI Series- Spectra Physics) with a frequency tripled radiation at 355 nm (pulse width—7 ns, repetition rate—10 Hz). ZnO films were grown by pulsed laser deposition on an optically flat quartz substrate kept at an on-axis distance of 4 cm from the ZnO target. The chamber vacuum was kept at 10 6 mbar. The ZnO pellet was ablated for 10, 15 and 20min to get single layered thin films (named as A1, A2 and A3, respectively). All the films were post-annealed at different temperatures viz. 573, 673 and 773K for 1 h. The crystalline structure of the films was characterized by XRD measurements (XPERT PRO diffract- ometer) using CuK a radiation with a wavelength of 0.15405nm. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells 0927-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2008.04.014 Ã Corresponding author. Tel.: +914712412167. E-mail address: vpmpillai9@rediffmail.com (V.P. Mahadevan Pillai). Solar Energy Materials & Solar Cells 93 (2009) 74– 78