Effect of growth conditions on structural and electrical properties of Ga-doped ZnO films grown by plasma-assisted MBE
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1201-H05-20
Effect of growth conditions on structural and electrical properties of Ga-doped ZnO films grown by plasma-assisted MBE V. Avrutin1∗, H.Y. Liu1, N. Izyumskaya1, M.A. Reshchikov2, Ü. Özgür1, A.V. Kvit3, P.M. Voyles3, and H. Morkoç1,2 1
Department of Electrical and Computer Engineering, Virginia Commonwealth University, Richmond, VA 23284 2 Physics Department, Virginia Commonwealth University, Richmond, VA 23284 3 Deparment of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706
Abstract: ZnO has recently attracted a great deal of attention as a material for transparent contacts in light emitters and adsorbers. ZnO films heavily doped with Ga (carrier concentration in the range of 1020 - 1021 cm-3) were grown on a-plane sapphire substrates by RF plasma-assisted molecular beam epitaxy. Oxygen pressure during growth (i.e. metal (Zn+Ga)–to–oxygen ratio) was found to have a crucial effect on structural, electrical, and optical properties of the ZnO:Ga films. As-grown layers prepared under metal-rich conditions exhibited resistivities below 3×10-4 Ω-cm and an optical transparency exceeding 90% in the visible spectral range. In contrast, the films grown under the oxygen-rich conditions required thermal activation and showed inferior structural, electrical, and optical characteristics even after annealing. INTRODUCTION Due to the well-established fabrication technology, indium-tin oxide (ITO) is the predominant material for transparent conducting electrodes [1]. However, the scarcity of indium in nature (and associated cost) limits the application of ITO. Zinc oxide heavily doped with aluminum (AZO) or gallium (GZO) is receiving increasing attention as a material with potential to replace ITO for transparent electrode applications in solar cells, light-emitting devices, and transparent thin film transistors [2,3,4,5]. AZO and GZO were demonstrated to have low resistivity and high transparency in the visible spectral range and, in many cases, outperform ITO. For instance, Agura et al. [4] reported a very low resistivity of ~8.5x10-5 Ω-cm for AZO, and Park et al. [5] reported even lower resistivity of ~8.1x10-5 Ω-cm for GZO, both values are nearly the same as the lowest reported resistivity of ~7.7x10-5 Ω-cm for ITO [6]. Ga is an excellent n-type dopant in ZnO with a more compatible covalent bond length (1.92 Å for Ga–O and 1.97 Å for Zn–O) than that of Al (2.7 Å for Al–O) [7]. GZO thin films can be grown by various methods, including ion plating [8], sol-gel [9], magnetron sputtering [3], metal organic chemical vapor deposition [2], pulsed laser deposition [5], and molecular beam epitaxy (MBE) [10,11,12]. The MBE technique, with its precise control over the process parameters, allows one to gain insight into the nature of physical phenomena governing the optical and electrical properties of a material and ∗
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thus provide valuable information for further advancements. However, the influence of MBE growth conditions on the electrical and optical properties of GZO
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