Nitrogen function of aluminum-nitride codoped ZnO films deposited using cosputter system
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Chia-Hsun Chen Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan, Republic of China
Li-Ren Lou and Ching-Ting Leea) Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University, 701 Tainan, Taiwan, Republic of China (Received 16 January 2009; accepted 31 March 2009)
AlN codoped ZnO films were deposited on sapphire substrates at low temperature using a cosputter system under various N2/(N2 + Ar) flow ratios. To investigate the nitrogen function, the ratio of nitrogen ambient was varied during cosputtering. AlN codoped ZnO films with various crystallographic structures and bonding configurations were measured. With an adequate nitrogen atmosphere deposition condition and postannealing temperature at 450 C, the p-type conductive behaviors of AlN codoped ZnO films were achieved due to the formation of Zn–N bonds. According to the low-temperature photoluminescence spectra, the binding energy (EA) of 0.16 eV for N acceptors can be calculated. Using time-resolved photoluminescence measurement, the carrier lifetime in AlN codoped ZnO films increases due to the reduction of oxygen vacancies caused by the occupation of adequate nitrogen atoms. I. INTRODUCTION
Group V elements, such as N, P, As, and Sb, are evidently the most suitable sort of dopants for forming shallow acceptors in ZnO films.1–4 Among them, nitrogen, because of its ionic radius similar to oxygen atoms, is thought of as a promising candidate for substituting oxygen atoms in ZnO. Many researchers have made efforts in doping N into ZnO using different nitrogen sources and various deposition methods. However, because of the higher chemical activity of oxygen atoms,5 Zn atoms prefer to bond with O atoms than N atoms, which results in a low solubility of N in ZnO. Moreover, there exist various unintentional donors in ZnO. Hence, N-doped p-type ZnO is difficult to produce. To overcome the difficulty, the N-III codoping method, such as N–In, N–Ga, and N–Al, has been reported and demonstrated to be a promising technique for the formation of p-type ZnO.6–8 In this work, ZnO–AlN films were cosputtered on sapphire substrates using ZnO and AlN targets in a mixed Ar and N2 atmosphere, in which AlN was used as the source of N acceptors and Al reactive codopants. The films deposited with different N2/(N2 + Ar) flow ratios and posta)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0265
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 7, Jul 2009 Downloaded: 08 Jul 2014
annealed at different temperatures were characterized. The electrical and optical properties of the resulted films were analyzed, from which we deduced an adequate deposition condition and postannealing temperature for getting stable p-type ZnO film. II. EXPERIMENTAL PROCEDURE
The radio frequency (rf) magnetron cosputtering system equipped with a dual rf power supply was used to deposit Al, N codoped ZnO films on sapphire substrates. Pure AlN (99.99%) and pure Zn
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