Pole Figure Analysis of Epitaxial Films of ZnO:2wt%Al Grown on Sapphire Substrates by RF Magnetron Sputtering
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Pole Figure Analysis of Epitaxial Films of ZnO:2wt%Al Grown on Sapphire Substrates by RF Magnetron Sputtering P. Kuppusami1, S. Fiechter, K.Ellmer Hahn-Meitner-Institut, dept. Solar Energetics, D-14109 Berlin, Glienicker Str. 100, Germany 1 Indira Gandhi Centre for Atomic Research, 603102 Kalpakkam, India ABSTRACT Aluminium-doped zinc oxide (ZnO:Al) films have been grown on c-plane (001) and a-plane (110) sapphire by RF magnetron sputtering from a ceramic target. The films grew epitaxially, even at room temperature. However, the crystalline quality depends both on the substrate temperature as well as on the sapphire orientation. The best films, proved by X-ray diffraction (rocking curves and pole figure measurements) were grown on (110)-oriented sapphire in the temperature range 473 to 773 K. The minimum rocking curve half width was about 0.75 °. By Rutherford backscattering analysis it could be shown, that the films exhibit a significant variation of the defect density over the film thickness. The highest density, as expected, is observed at the interface sapphire/ZnO:Al. Films grown on (001)-oriented sapphire have higher rocking curve half widths (about 1.3 °) and exhibit sometimes two types of domains in the same film twisted by 30 °. INTRODUCTION Zinc oxide (ZnO ) with a band gap of 3.37 eV, similar to that of GaN, has recently attracted attention as a material for optoelectronic applications such as short wavelength light emitting devices [1]. In addition, this material finds applications in semiconductor piezo electric devices, surface acoustic wave devices, gas sensors, solar cells and flat panel displays [2, 3]. However, the latter applications use polycrystalline ZnO, while the optoelectronic devices must be based on epitaxial films grown on single crystalline substrates with p- as well as n-type conductivities. Various deposition techniques, including chemical vapour deposition [4], pulsed laser ablation [5], magnetron sputtering [6-8], molecular beam epitaxy [9, 10] or ion-beam assisted deposition [11] have been used for the growth of ZnO thin films. Substrates such as sapphire, ZnS, KCl, LiNbO3, Si, InP, GaAs and metallic substrates were used to grow epitaxial films. Among several single crystalline substrates reported so far, sapphire (Al2O3) is relatively inexpensive and a widely used material in the semiconductor industry. However, the large mismatch between the lattice constants of sapphire (a= 0.47588 nm, c= 1.2992 nm) and zinc oxide (a= 0.3249 nm, c = 0.5206 nm) could produce films with a significant crystal mosaicity. There has been a considerable effort to circumvent this difficulty by optimizing the deposition parameters [6, 8, 9]. Here, we report a comparative study on structure and pole figure analysis of ZnO:2wt% Al thin films grown on (110) or a-plane and (001) or c-plane sapphire substrates. In the future this project is aimed to investigate the influence of the crystallographic quality on the charge carrier transport in highly degenerately doped zinc oxide films by comparing electrical tra
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