Uniaxial locked growth of high-quality epitaxial ZnO films on (11(2)over-bar-0)α-Al2O3

ZnO epitaxial films have been grown on sapphire substrates using molecular beam epitaxy (MBE). Elemental sources of Zn and O were used with a RF radical source being employed to increase the reactivity of the oxygen source gas. High-sensitivity pole figure measurements indicated that the films were uniquely (0001) oriented with no trace of secondary orientations. The unique orientation is a consequence of the coincidental near zero lattice mismatch of the ZnO a lattice constant of 0.3250 nm and the sapphire c lattice constant over four or 0.3248 nm leading to the term uniaxial locked epitaxy. Atomic force microscopy of as-grown samples indicated that the films were flat with a RMS roughness of less than 0.4nm. Two dimensional X-ray reciprocal space mapping of the ZnO(1 0 (1) over bar 4) asymmetric reflection using a triple axis configuration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0001)ZnO grown on sapphire (0001) substrates to about 0.5 mu m for growth on (1 (1) over bar 20) sapphire substrates. This is interpreted as being a consequence of less in-plane twisting of domains due to stress from lattice mismatch. Preliminary photoluminescence measurements indicate a dramatic increase in intensity with bound exciton features less than 0.7 meV. (C) 2000 Elsevier Science B.V. All rights reserved.