P -type conduction in room-temperature high-energy electron-irradiated ZnO thin films
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Jin Woo Jung, Chae Il Cheon, and Jeong Seog Kim Department of Semiconductor and Display Engineering, Hoseo University, Asan, Chungnam, 336-795, Republic of Korea
Young Hwan Han, Min-Wan Kim, and Byung Cheol Lee Laboratory for Quantum Optics, Korea Atomic Energy Research Institute, Daejeon, 305-353, Republic of Korea (Received 9 October 2008; accepted 21 January 2009)
We report the realization of the p-type conductivity and the enhancement of the photoluminescence (PL) intensity in undoped ZnO films treated with high-energy (1 MeV) electron-beam irradiation (HEEBI), suggesting that the HEEBI process is compatible with a low-temperature requirement for the fabrication of transparent thin film transistors with good efficiency on a plastic substrate. The p-type conductivity of the films was revealed by the Hall, x-ray photoelectron spectroscopy, and PL measurements after being electronirradiated in air at room temperature. The major acceptor-like defects were determined to be oxygen interstitial and zinc vacancy. A model was proposed in terms of O as well as Zn diffusion to explain the observed results. It was also observed that HEEBI treatment has little influence on the optical transmittance of ZnO films, whereas HEEBI treatment shifts the optical band gap toward the lower energy region from 3.29 to 3.28 eV.
I. INTRODUCTION
Zinc oxide (ZnO) has attracted great attention in recent years for its potential optoelectronic applications in the ultraviolet (UV) region because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV. In particular, ZnO thin film has recently emerged as a promising material for fabrication of thin film transistors with good efficiency on a glass or a plastic substrate at low temperatures,1–3 which is important for transparent flexible electronics, for instance, drivers for the next generation information display panels. However, ZnO has largely failed to live up to its potential since the realization of p-type ZnO has proven difficult due to low solubility of the acceptor dopants, deep acceptor level, and its self-compensation between the acceptor dopants and the intrinsic donors.1–12 It is known that the postannealing in oxygen (O2) ambient13–15 or in vacuum16 at high temperature around 600 C or higher is required to realize the p-type conductivity and improve the optical properties of ZnO thin films. Unfortunately, this high-temperature process is often incompatible with a low-temperature requirement a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0186 J. Mater. Res., Vol. 24, No. 5, May 2009
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for the fabrication of transparent thin film transistors with good efficiency on a plastic substrate. Several research groups17–19 have previously demonstrated for Mg-doped GaN films that the low-energy electron beam irradiation (LEEBI) annealing at room temperature is effective to activate the p-type conductivity and enhance the luminescence intensity. More recently, Lee et al.
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