Growth of a-plane ZnO Thin Films on r-plane Sapphire by Plasma-assisted MBE
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Growth of a-plane ZnO Thin Films on r-plane Sapphire by Plasma-assisted MBE J. Q. Xie,1 J. W. Dong,1 A. Osinsky,1 P. P. Chow,1 Y. W. Heo,2 D. P. Norton,2 S. J. Pearton,2 X. Y. Dong,3 C. Adelmann,3 and C. J. Palmstrøm3 1 SVT Associates Inc., Eden Prairie, MN 55344 2 Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 ABSTRACT ZnO thin films have been epitaxially grown on r-plane sapphire by RF-plasma-assisted molecular beam epitaxy. X-ray diffraction (XRD) and transmission electron microscopy (TEM) studies indicate that the epitaxial relationship between ZnO and r-plane sapphire is ( 1120 )ZnO // ( 1102 )sapphire and [0001]ZnO // [ 1101 ]sapphire. Atomic force microscopy measurements reveal islands extended along the sapphire [ 1101 ] direction. XRD omega rocking curves for the ZnO ( 1120 ) reflection measured either parallel or perpendicular to the island direction suggest the defect density anisotropy along these directions. Due to the small lattice mismatch along the ZnO [0001] direction, few misfit dislocations were observed at the ZnO/Al2O3 interface in the highresolution cross-sectional TEM image with the zone axis along the ZnO [ 1100 ] direction. INTRODUCTION There has been growing interest in ZnO, a wurtzite II-VI semiconductor with a direct bandgap of 3.37 eV at room temperature [1]. With an exciton binding energy of 60 meV [2], as compared to 28 meV for GaN, ZnO is a promising candidate for optoelectronic applications, such as UV/blue light emitting diodes and UV laser diodes [3,4]. Compared to other widebandgap materials like GaN and SiC, ZnO possesses significant advantages including higher quantum efficiency, greater resistance to high-energy radiation, and greater compatibility with wet chemical etching [5]. Moreover, by alloying with the dielectric MgO, which has a bandgap of 7.9 eV at room temperature, MgxZn1-xO compounds can be formed with bandgaps ranging from 3.4 to 7.9 eV [6]. This opens up opportunities for bandgap engineering, leading to optimized device performance [7,8]. So far, epitaxial ZnO thin films have been grown on c-plane sapphire by metalorganic chemical vapor deposition (MOCVD) [9,10], pulsed laser deposition (PLD) [11], and plasmaassisted molecular beam epitaxy (MBE) [12-15]. When grown on c-plane sapphire, analogous to III-nitride, the total polarization of ZnO is aligned along the [0001] growth direction and the polarization-induced fields may cause reduction in oscillator strength and a redshift of optical transitions in ZnO-based quantum wells. One way to eliminate the out-of-plane polarization effects is to grow the structures along non-polar directions, e.g., grow a-plane ZnO on r-plane sapphire. Compared to the growth on c-plane sapphire, the epitaxial growth of a-plane ZnO on rplane sapphire results in in-plane strain and optical anisotropy, which could be used to design polarization sensitive acousto-optic and
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