Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C-2v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34x10(9)Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship (1 0 (1) over bar 0) ZnO//(0 0 0 (2) over bar) GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch. (C) 2010 Elsevier B. V. All rights reserved.