In-plane and polar orientations of ZnO thin films grown on atomically flatsapphire

Epitaxial ZnO thin films were prepared on atomically flat sapphire (0001) s ubstrates at various temperatures by laser molecular beam epitaxy. On the a s-polished substrates, the in-plane orientation of ZnO thin films was found to be ZnO [10 (1) over bar 0]parallel to sapphire [11 (2) over bar 0] rega rdless of the deposition conditions. However, films on atomically flat subs trates showed two in-plane orientations, ZnO [10 (1) over bar 0]parallel to sapphire [10 (1) over bar 0] and ZnO [10 (1) over bar 0]parallel to sapphi re [11 (2) over bar 0], rotated by 30 degrees for films grown at low (400-4 50 degrees C) and high (800-835 degrees C) temperatures respectively. Ion s cattering spectroscopy revealed that the former films were (0001) oriented with the Zn-face forming the topmost surface, whereas the latter films had the (000 (1) over bar) orientation with the O-face at the film surface. Thi s orientation change is discussed by taking thermodynamic stability and gro wth kinetics into account. The films grown at very high temperature (835 de grees C) showed superior crystallinity even in comparison with bulk single crystals. (C) 1999 Elsevier Science B.V. All rights reserved.