Near field imaging of charge transport in gallium nitride and zinc oxide nanostructures

Abstract

A novel technique for imaging minority carrier diffusion in semiconductor nanostructures has been applied to the characterization of GaN and ZnO nanowires and nanobelts. Near field scanning optical microscopy (NSOM) is performed within a scanning electron microscope (SEM) to image carrier recombination with a spatial resolution exceeding the diffraction limit. The electron beam provides a high resolution, highly controlled source of carrier generation at a point. Diffusion lengths can be extracted directly from the resulting distribution of the recombination luminescence. A Nanonics Multi View 2000 provides a unique open architecture to allow the electron beam to be incident on a fixed point on the nanowire with independent motion of a collecting fiber to map the luminescence distribution. Probe tips are cantilevered optical fiber tips with diameters from 100 to 500 nm. Simultaneous NSOM, AFM and SEM imaging provides topographic, optical emission, and carrier transport information. This characterization technique has been used to measure minority carrier diffusion lengths in GaN nanowires, ZnO nanowires, and ZnO nanobelts, with diffusion lengths extracted from carrier recombination profiles. Evidence of waveguiding in some nanowires and nanobelts was also observed. The first measure of ZnO nanowires using this technique resulted in a measured diffusion length of approximately 150 nm for nanowires grown by the hydrothermal method and approximately 640 nm for those grown by physical vapor deposition. Additional results comparing diffusion lengths in n-type, p-type and unintentionally doped GaN nanowires, ZnO nanowires, and ZnO nanobelts are presented. While measuring the diffusion lengths of these structures, it was also observed that diffusion length measurements were sometimes impacted by combined effects associated with surface topography and optical waveguiding and interference.