Tailoring the optical properties of epitaxially grown biaxial ZnO/Ge, and coaxial ZnO/GeZnO and Ge/ZnOGe heterostructures

Heterostructures of epitaxially grown biaxial ZnO/Ge, and coaxial ZnO/Ge/ZnO and Ge/ZnO/Ge heterostructured nanowires with ideal epitaxial interfaces between the semiconductor ZnO sublayer and the Ge sublayer have been fabricated via a twostage chemical vapor-solid process. Structural characterization by high-resolution transmission electron microscopy and electron diffraction indicates that both the ZnO and Ge sublayers in the heterostructures are single crystalline. A good epitaxial relationship of (100)(ZnO)||(220)(Ge) exists at the interface between ZnO and Ge in the ZnO/Ge biaxial heterostructure. There is also an epitaxial relationship of (010)(ZnO)||(020)(Ge) at the interface between the ZnO and Ge substructures in the coaxial ZnO/Ge/ZnO heterostructures, and a good epitaxial relationship of (010)(ZnO)||(020)(Ge) at the interface between ZnO and Ge in the Ge/ZnO/Ge coaxial heterostructure. Structural models for the crystallographic relationship between the wurtzite-ZnO and diamond-like cubic-Ge subcomponents in the heterostructures are given. The optical properties for the synthesized heterostructures are studied by spatially resolved cathodoluminescence spectra at low temperature (20 K). Excitingly, the unique biaxial and coaxial heterostructures display unique new luminescence properties. It is concluded that the ideal epitaxial interface between ZnO and Ge in the prepared heterostructures induces new optical properties. The group II-VI Ge-based nanometer-scale heterostructures and their interesting optical properties may inspire great interest in exploring related epitaxial heterostructures and their potential applications in lasers, gas sensors, solar energy conversion, and nanodevices in the future.