Thin film epitaxy and magnetic properties of STO/TiN buffered ZnO on Si(001) substrates
Abstract 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 ¯ 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 ¯ 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 ¯ 0 ) becomes weaker and a ( 1 0 1 ¯ 2 ) ZnO appears. Epitaxial relationships have been determined from X-ray diffraction φ-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) ∥ STO(1 0 0); ZnO[ 1 1 2 ¯ 0 ] ∥ STO[1 1 0] and ZnO[ 1 ¯ 2 1 ¯ 0 ] ∥ STO[ 1 ¯ 1 0 ], while that of a-ZnO on STO(0 0 1) was ZnO( 1 1 2 ¯ 0 ) ∥ STO(1 0 0); ZnO[ 1 1 2 ¯ 0 ] ∥ STO[1 1 0] and ZnO[0 0 0 2] ∥ STO[ 1 ¯ 1 0 ]. High-resolution transmission electron microscopy studies revealed atomically sharp interfaces with no reaction at the interface. Reversible d0 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.