Formation of planar arrays of one-dimensional p-n heterojunctions using surface-directed growth of nanowires and nanowalls.

We report a surface-directed vapor-liquid-solid process for planar growth of one-dimensional heterojunctions of zinc oxide on single crystal gallium nitride (GaN) that enables their hierarchical assembly to light emitting diodes. An individual heterojunction is about 10 μm in length and 80 nm in width and is formed by planar growth of an n-type ZnO nanowire or nanowall on p-type GaN surface using Au catalyst. Our results show that a ZnO nanocrystal at its nucleation site has six possible growth directions that can be engineered and controlled using an intentional blockade of the nanocrystal growth in certain directions owing to similarities in crystal structures of ZnO and GaN. The ZnO nanowalls are formed when nanowires during their planar growth slowly grow in direction normal to the substrate via a self-catalytic process. The crystal structure of these heterojunctions is examined from two different crystallographic perspectives using high resolution transmission electron microscopy. Results indicate abrupt and epitaxial formation of n-p heterojunctions, which are difficult to achieve in thin film growth of these heterojunctions. The collective light emission of micrometer- to millimeter-size arrays of the heterojunctions is demonstrated via a simple design that is scalable to literally any platform size. This technique allows in situ growth and combinations of II-VI and III-V semiconductors and offers their easier integration to photonic and lab-on-chip platforms with applications in energy generation and light detection.