Journal of Crystal Growth 305 (2007) 36–39 Microstructures, electrical and optical characteristics of ZnO thin films by oxygen plasma-assisted pulsed laser deposition Yanfei Gu a,b , Xiaomin Li a,Ã , Weidong Yu a , Xiangdong Gao a , Junliang Zhao a,b , Chang Yang a,b a State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b Graduate School of Chinese Academy of Sciences, Beijing 100039, PR China Received 12 March 2007; accepted 30 March 2007 Communicated by D.P. Norton Available online 10 April 2007 Abstract In order to decrease the free-electron concentration and increase the crystalline quality, zinc oxide (ZnO) thin films were deposited on sapphire (0 0 0 1) substrates by oxygen plasma-assisted pulsed laser deposition (PLD). ZnO films showed higher oxygen composition, stronger diffraction intensity of the (0 0 0 2) direction, and larger grain size with regular hexagonal grain shape. The free-electron concentration was decreased greatly from 10 19 to 10 14 cm 3 and the Hall mobility was increased from 6.8 to 37 cm 2 V 1 s 1 . Furthermore, the intensity of the resonant Raman scattering and ultraviolet photoluminescence emission was increased. This enhancement of the crystalline, electrical and optical quality would be attributed to the increase of high activity oxygen density introduced by the plasma oxygen source. r 2007 Published by Elsevier B.V. PACS: 81.15.Fg; 81.05.Dz; 68.55.a; 72.20.i; 87.64.Je; 78.55.m Keywords: A1. Crystal structure; A1. Electrical properties; A1. Photoluminescence; A1. Raman scattering; A3. Pulsed laser deposition; B1. Zinc oxide 1. Introduction In view of the demand for fabrication of high-quality optoelectronic devices such as light-emitting diodes [1–3] or ultraviolet detectors [4], zinc oxide (ZnO) has recently attracted widespread research interest because of its exceptional optical and electronic properties [5,6], includ- ing direct wide band gap of 3.37 eV and a high exciton bonding energy of 60 meV at room temperature. A variety of methods were employed to grow ZnO thin films, such as RF sputtering [7], chemical-vapor deposition [8], molecu- lar-beam epitaxy [9], and pulsed laser deposition (PLD) [10]. Among these techniques, PLD is widely used for its simplicity and experimental flexibility. ZnO films grown by these low oxygen pressure thin film deposition methods show low resistivity and large free- electron concentration. Reducing the background carrier (free electron) concentration in ZnO films is one of the major challenges ahead of realizing high-performance ZnO-based optoelectronic devices [10]. Furthermore, the compensation effect by a large background electron concentration makes the p-type doping of ZnO too difficult [11], which is the bottleneck for the application of ZnO. It is commonly accepted that free electrons in ZnO are generated due to intrinsic defects such as oxygen vacancies (V O ) or zinc interstitials (Zn i ) [12]. Thus, free-electron concentration in ZnO can be deduced by removing the V O or Zn i defects at higher oxygen pressure. Unfortunately, in higher oxygen pressure, the ejected species by laser ablation from the target undergo much more collisions with the high density of oxygen molecules. The kinetic energy of the species is decreased greatly, resulting in the ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.jcrysgro.2007.03.050 Ã Corresponding author. Tel.: +86 21 52412554; fax: +86 21 52413122. E-mail address: lixm@mail.sic.ac.cn (X. Li).