Thin film epitaxy and magnetic properties of STO/TiN buffered ZnO on Si(001) substrates

We report integration of epitaxial ZnO films with Si(0 0 1) substrates using STO/TiN buffer layers. It has been demonstrated that the preferential orientation of the a-plane (1 1 (2) over bar 0) and c-plane (0 0 0 2) of ZnO can be controlled via deposition temperature and oxygen partial pressure. At lower substrate temperatures ZnO grows solely in the (0 0 0 2) orientation, while the (1 1 (2) over bar 0) orientation was dominant at high substrate temperatures and low oxygen pressures. At higher pressures, the (0 0 0 2) orientation is preferred, while (1 1 (2) over bar 0) becomes weaker and a (1 0 (1) over bar 2) ZnO appears. Epitaxial relationships have been determined from X-ray diffraction phi-scans and it was found that both c- and a-ZnO had two types of orientations due to the cubic symmetry of the STO buffer layer. The orientation relationship of c-ZnO on STO(0 0 1) was ZnO(0 0 0 1) parallel to STO(1 0 0); ZnO[1 1 (2) over bar 0]parallel to STO[1 1 0] and ZnO[(1) over bar 2 (1) over bar 0] parallel to STO[(1) over bar 1 0], while that of a-ZnO on STO(0 0 1) was ZnO(1 1 (2) over bar 0) parallel to STO(1 0 0); ZnO[1 1 (2) over bar 0] parallel to STO[1 1 0] and ZnO[0 0 0 2] parallel to STO[(1) over bar 1 0]. High-resolution transmission electron microscopy studies revealed atomically sharp interfaces with no reaction at the interface. Reversible d(0) ferromagnetism was found to be present in both ZnO and STO layers. Our electron-energy-loss spectroscopy studies conclusively rule out the presence of any external ferromagnetic ions or impurities. Taken together, our data indicate that the ferromagnetic order in these undoped oxides might be defect mediated. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.