Recently we showed lateral growth of ZnO nanowires and nanowalls on single crystal GaN and formation of bi- and tri-directional assembly of nanowires and nanowalls using a surface-directed vapor-liquid-solid process (SVLS). Taking advantage of this growth technique, planar arrays of electrically addressable n-p heterojunctions were fabricated. Each n-p heterojunction is formed at the interface of a nanowire or nanowall with the underlying substrate. In the present work we further investigated the coherency of the crystal structure of the heterojunctions both along their width and length using focused-ion beam microscopy and high resolution transmission electron microscopy (HRTEM). We explain the anisotropic growth of ZnO in six directions using a SVLS mechanism, while conventional crystal growth techniques only anticipate an isotropic island formation of ZnO on GaN. During the SVLS process, a ZnO nanocrystal forms inside a Au nanodroplet, and elongates via a sequential ZnO island formation during which ZnO remains single crystal as evident by HRTEM analysis alongside nanowires and nanowalls. We also directly observe the trace of motion of a Au nanodroplet on the surface of a nanowire suggestive of a move and pause mode for a Au nanodroplet during the ZnO nanocrystal growth. The examined segments of the interface along the growth direction show abrupt heterojunctions with high coherency and a small number of misfit dislocations likely due to an un-relaxed interface. To this end, we also observe residual strain stored in both sides of the heterojunctions as well as few stacking faults in the ZnO section parallel to the interface. The site selective growth, structural quality and scalability of the formed heterojunctions can make this technique useful in light generation and detection applications.
Citation: Journal of Physical Chemistry
Pub Type: Journals
Surface-directed growth, directed assembly, nanowires, nanowalls, zinc oxide, p-n heterojunction