Non-equilibrium defects in aluminum-doped zinc oxide thin films grown with a pulsed laser deposition method
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Isao Sakaguchi, Naoki Ohashi,a) Takashi Sekiguchi, and Shunichi Hishita National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan
Hajime Haneda National Institute for Materials Science, Tsukuba, Ibaraki 305-0044 Japan; and Department of Applied Science for Electronics & Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan (Received 10 April 2005; accepted 26 July 2005)
Zinc oxide (ZnO) films doped with aluminum (Al) were deposited with a pulsed laser deposition technique to characterize the charge compensation phenomena in ZnO. In particular, oxygen radical (O*) irradiation during film deposition was used to modify the oxygen stoichiometry. Irradiation with O* decreased electron concentration in Al-doped ZnO. The lattice parameter of the resultant films also varied with the growth conditions. However, no obvious correlation between electron concentration and lattice parameter was found. The self-diffusion coefficients indicated the presence of non-equilibrium defects. The properties of the films are discussed from the viewpoint of non-equilibrium compensated defects detected in the diffusion measurements.
I. INTRODUCTION
A lot of attention has been paid to zinc oxide (ZnO) and its related materials because ZnO is regarded as a potential material for optoelectronic applications. For example, light emission from a diode structure using a nZnO/p-SrCuO2 junction has been demonstrated,1 and ZnO-based transparent field effect transistors (FETs)2 have been fabricated. Very recently, Tsukazaki et al.3 obtained p-type ZnO after sufficient nitrogen doping by using a growth temperature modulation technique, and they fabricated a light-emitting diode (LED) based on a p-n junction of ZnO. These achievements encouraged us to investigate the structure–composition–property relationship of ZnO to enable precise control of its structure and properties. The most important issue regarding the application of ZnO to optoelectronics is the charge compensation phenomenon. It is known that doping with group III elements, i.e., Al, Ga, and In, introduces shallow donor states in ZnO.4 Heavily doped ZnO has been extensively studied in an effort to increase electron concentration to develop transparent electrode materials for displays and
a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0353 2866
J. Mater. Res., Vol. 20, No. 10, Oct 2005
solar cells. As reviewed by Minami,5 ZnO thin film exhibits very low electric resistivity on the order of 10−4 ⍀cm. Such low resistivity is due to a high electron concentration of up to 1020 cm−3. To realize such a high electric conductivity, the oxygen partial pressure in the growth environment is of great importance. For example, the electron concentration in indium (In)-doped ZnO films grown with a sputtering technique was found to vary with the oxygen partial pressure in the growth ambient.6 Regarding the application of ZnO to transparent electronics, the elimination of deep centers is also an important issue. Particularly
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