Electrical and Optical Properties of Zinc Oxide Thin Films and Heavily Aluminum-doped Zinc Oxide Thin Films Prepared by
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Electrical and Optical Properties of Zinc Oxide Thin Films and Heavily Aluminum-doped Zinc Oxide Thin Films Prepared by Molecular Beam Epitaxy Takeshi Ohgaki1, Yuji Kawamura2, Naoki Ohashi1, Hirofumi Kakemoto2, Satoshi Wada2, Yutaka Adachi1, Hajime Haneda1 and Takaaki Tsurumi2 1 Advanced Materials Laboratory National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0047, Japan. 2 Graduate School of Science and Engineering, Tokyo Institute of Technology 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan. ABSTRACT ZnO (ZO) and Aluminum (Al) doped ZnO (AZO) films were grown on sapphire substrate via oxygen radical assisted molecular beam epitaxy (MBE) technique. The results of XRD measurement and temperature dependent Hall measurement confirmed that the AZO films were typically highly degenerate semiconductor with good crystallinity. The resistivity of these films were closed to the theoretical lower limit. The optical properties of these films were investigated by photoluminescence (PL) spectra and absorption spectra. Strong band-edge emission was observed in the AZO films with good crystallinity in spite of high carrier concentration more than 1020 cm-3. A sift of absorption edge to higher energy side and a gradual increase of the absorption was observed for the AZO film.
INTRODUCTION ZnO is a semiconductor with a wide direct band-gap (Eg =3.3 eV at RT) and high exciton binding energy, EB =60 meV. Because of these features, ZnO is a promising candidate for light emitting devices in short wave length from blue to ultraviolet (UV) [1,2]. The light emission properties of ZnO have been studied extensively, and recent years have seen many achievements regarding crystal growth of ZnO [3,4]. However, there is as yet not clear understanding of impurity-related ultraviolet emission in ZnO [5]. On the use of ZnO as optoelectronics device, the controlling emission properties in UV region are also an important issue. There were many reports about the donor doping in ZnO films, such as aluminum, gallium and indium, which yield the shallow-donor level in ZnO, as an application of transparent conductive coating films, so far. However, in most cases, AZO films were poly-crystalline films grown on grass substrates, and resistivity and transmittance spectra of these films were investigated. Therefore, no study as yet examined the electrical properties and optical properties of heavily shallow-donor doped ZnO films with good crystallinity. In this study, AZO epitaxial films on sapphire substrates were grown by applying the MBE method with irradiation of oxygen radical to elucidate the electrical and optical properties of the heavily shallow-donor doped ZnO films with good crystallinity.
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EXPERIMENTAL DETAILS We used sapphire single crystals with a well-polished (11 2 0) face for the substrates. They were rinsed in acetone followed by ethanol with an ultra sonic bath to remove dusts and surface contaminations. To obtain a clean and highly crystallized surface the substrates were heated in a vacuum at 60
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