Journal of Crystal Growth 306 (2007) 316–320 Lowered stimulated emission threshold of zinc oxide by hydrogen doping with pulsed argon–hydrogen plasma Naoki Ohashi à , Yu-Guang Wang, Takamasa Ishigaki, Yoshiki Wada, Hiroyuki Taguchi, Isao Sakaguchi, Takeshi Ohgaki, Yutaka Adachi, Hajime Haneda National Institute for Materials Science (NIMS), 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan Received 6 May 2007; accepted 8 May 2007 Communicated by D.P. Norton Available online 18 May 2007 Abstract The effects of hydrogen doping on the optical and electrical properties of zinc oxide (ZnO) films were investigated. We prepared about 50-nm thick ZnO films on a sapphire substrate by using a pulsed laser deposition method and used a pulse-modulated inductively coupled plasma technique (PM-ICP) to intentionally insert hydrogen into the films. An increase in the electron concentration by hydrogen doping indicated that hydrogen is actually a cause of shallow donors. Moreover, the excitation intensity threshold for optically pumped stimulated emissions was significantly reduced after hydrogen doping. r 2007 Elsevier B.V. All rights reserved. PACS: 81.05.Dz; 68.55.Ln; 78.45.h; 78.55.Et Keywords: A1. Doping; A1. Defects; B1. Oxide; B2. Semiconducting II–VI materials 1. Introduction Zinc oxide (ZnO) is one of the most promising materials in the field of wide-band-gap semiconductors. In fact, Tsukazaki et al. [1] have recently succeeded in fabricating a light-emitting diodes (LEDs) made from ZnO. To utilize ZnO-based LEDs, we have to improve the conductivity of the p-type ZnO and enhance the luminescence efficiency of the active layer in LED structures. For these reasons, understanding and controlling the defects in ZnO is of great importance. Prior research has indicated that charge compensation mechanisms and the origin of the native donors in ZnO are still unsolved problems [2]. Oxygen vacancies and/or zinc interstitials were thought to be the origin of native shallow donors in ZnO [3]. However, theoretical studies using the density functional theory [4] indicated that they may not be the real origin of the shallow donors in ‘nominally’ undoped ZnO. To explain electron injection in ‘nominally’ undoped ZnO, unintentional hydrogen contamination has been investigated [5]. A study on muonium implantation into ZnO [6] and an electron paramagnetic study on hydrogenated ZnO crystals [7] have revealed that hydrogen is in fact the cause of shallow donors in ZnO. Although Ip et al. [8] has suggested that most hydrogen can easily diffuse out of ZnO by using thermal treatment at relatively low temperatures, understanding the effects of hydrogen doping in ZnO is one of the important issues needed to clarify charge compensation and carrier injection mechan- isms in ZnO. Since synthesis of nano-structured ZnO from aqueous solution is one of the recent technological trends, insertion of hydrogen and/or OH groups into ZnO lattice during wet chemical process is also an important issue. In fact, several reports, e.g., Refs. [9,10], have indicated that desorption of hydrogen/water from ZnO deposited from aqueous solution caused change in optical properties. Exposing ZnO to an atomic hydrogen-containing plasma is a conventional way to obtain hydrogen doped ZnO [11–14]. We have found that hydrogen impurities in ZnO ARTICLE IN PRESS www.elsevier.com/locate/jcrysgro 0022-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2007.05.024 à Corresponding author. Tel.: +81 29 859 2000; fax: +81 29 855 1196. E-mail address: ohashi.naoki@nims.go.jp (N. Ohashi).