Structural and optical properties of delafossite-type CuAlO 2 thin films prepared by RF reactive sputtering
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Structural and optical properties of delafossite-type CuAlO2 thin films prepared by RF reactive sputtering B. Yang, Y. M. Lu, C. Neumann, A. Polity, C. Z. Wang, B. K. Meyer I. Physikalisches Institut, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany ABSTRACT Delafossite-type CuAlO2 thin films have been deposited by radio frequency (RF) reactive sputtering on sapphire using a CuAlO2 ceramic target. A study of structural and optical properties was performed on films of varying deposition parameters such as substrate temperature and oxygen partial pressure and also post annealing. The crystalline phase in the films was identified to be the delafossite structure by x-ray diffraction. The optical properties, such as the wavelength dependence of the transmittance and the band gap, were determined. The average transmittance is 80% in the wavelength range of 400-1500 nm and the band gap is 3.81 eV. INTRODUCTION Transparent conducting oxide (TCO) compounds are widely used as transparent electrodes because almost all of the compounds are n-type semiconductors such as ZnO:Al, In1-xSnxO3, SnO2:F, Cd2SnO4 etc [1-5]. Transparent p-type semiconductors are required in preparing novel transparent p-n junction for optic-electronic device applications. CuAlO2 is a known p-type semiconductor with a delafossite structure and attracted great attention for the applications of a novel transparent device[6,7]. Kawazoe et al. reported the preparation of a p-type CuAlO2 transparent semiconductor film with delafossite structure [6]. For the preparation of CuAlO2 thin films, most of the research groups used pulsed laser deposition (PLD) with a bulk CuAlO2 target. Unfortunately, PLD is not a technique employed by industries. There are some limitations of this method such as the difficulty in scaling-up of the technology and the low quality films due to droplet formation. Hence it is of utmost importance to develop alternative fabrication route for the full exploitation of its technological potential. Reactive sputtering, a commonly used thin film deposition technique in industrial wafer preparation, would be more suitable for mass production because large-area deposition is possible by this method. In this work, delafossite-type CuAlO2 thin films have been deposited on sapphire substrates by RF sputtering using a CuAlO2 ceramics target. We preferred the ceramic target over the Cu-Al alloy target because it is much more convenient to get the Cu+ ion using CuAlO2 ceramic. We report the measured results on the structural, surface, and optical properties of CuAlO2 thin films in this letter.
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Figure 1. XRD pattern of CuAlO2 sintered target. All the diffraction peaks observed could be indexed by the delafossite structure.
EXPERIMENTAL CuAlO2 targets used in RF sputtering were prepared by a conventional solid-state reaction technique with start
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