Uniaxial locked growth of high-quality epitaxial ZnO films on [formula omitted]-Al 2O 3

Abstract

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 (0 0 0 1) 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 0.4 nm. Two dimensional X-ray reciprocal space mapping of the ZnO (1 0 1 ̄ 4) asymmetric reflection using a triple axis configuration indicated that the lateral correlation lengths increased from several hundred nanometers for the case of (0 0 0 1)ZnO grown on sapphire(0 0 0 1) substrates to about 0.5 μm for growth on (1 1 ̄ 2 0) 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.