Defects and interfaces in an epitaxial ZnO/LiTaO₃ heterostructure

ZnO films were grown on a (0112) r-plane LiTaO₃ substrate by electron cyclotron resonance-assisted molecular-beam epitaxy. The detailed structures of the interface and defects are investigated by high-resolution electron microscopy and image simulation. The epitaxial relationship was found to be [0001]_ZnO||[0111]_LiTaO3 and (1120)_ZnO||(0112)_LiTaO3. This epitaxial relationship corresponds to the c axis of ZnO parallel to the piezoelectric r-plane LiTaO₃, which results in the enhanced electromechanical coupling factor. The interfaces were very smooth and structurally semicoherent with a comparative regular array of misfit dislocations at the interface accommodating a lattice mismatch of 9.49% when the incident electron beam is parallel to the [0001]_ZnO||[0111]_LiTaO3 direction. A high efficiency of transfer of acoustic energy across the interface is expected for surface acoustic wave devices with such an interface. The dominant defects commonly observed in ZnO films were found to be the type-I₁ intrinsic stacking fault. The formation of stacking faults was shown to be growth kinetics on particular crystallographic planes during the initial stage of film growth. The effects of these defects and interfaces on electrical and optical properties for device applications are discussed.