Heteroepitaxial growth of ZnO on perovskite surfaces

The microstructural properties of heteroepitaxial ZnO thin films prepared by laser molecular beam epitaxy (L-MBE) were investigated on SrTiO3 substrates and BaTiO3/SrTiO3 pseudo substrates with different orientations. The interface characteristics were in situ monitored by reflection high-energy electron diffraction (RHEED), and the epitaxial orientation relations were reconfirmed by ex situ x-ray diffraction (XRD) measurements. ZnO films grown on SrTiO3(0 0 1) and BaTiO3/SrTiO3(0 0 1) contained a poly-domain structure. For the former, the lattice mismatch was about - 1.7% by four types of domain growth with the epitaxial relation of ZnO(1 1 0)parallel to SrTiO3(0 0 1) and ZnO[ - 1 1 1]parallel to SrTiO3 < 100 >. For the latter, twin domains would result in a smaller mismatch of - 0.8% by the epitaxial relation of ZnO(0 0 1)parallel to BaTiO3(0 0 1) and ZnO[1 1 0]parallel to BaTiO3 < 1 1 0 >. On SrTiO3(1 1 1) and BaTiO3/SrTiO3(1 1 1), single-domain films following the c-axial direction were observed with in-plane orientation ZnO[1 1 0]parallel to SrTiO3[1 1 0] and ZnO[1 0 0]parallel to BaTiO3[1 1 0], respectively. This 30 degrees rotation in the in-plane direction of the ZnO epilayer with respect to the perovskite surfaces increased the lattice mismatch from about - 2% to - 14.5% after inserting BaTiO3 layers. The orientation of ZnO films could be attributed to the characteristic difference of the interface energy. It is determined entirely by interface stress and crystallographic symmetry for the growth on nonpolar (0 0 1)-orientated perovskite surfaces while the competition between elastic energy and chemical energy plays an important role for that on polar (1 1 1)-surfaces.