Morphology of catalytically grown 1-D nanostructures is often defined by the crystallographic direction describing their growth axis. GaN nanowires have been observed to grow in a variety of crystallographic directions. Moreover it has been demonstrated that their growth axis can be controlled by the choice of the catalyst, substrate or growth conditions. The resulting nanowires exhibit different surface terminations that radically affect their properties including reactivity, which allows the selective deposition of optically active layers on desired nanowire surfaces to create light emitting heterostructures. Optical characterization at length scales below the optical diffraction limit is usually accomplished by near-field scanning optical microscopy, where a local probe acts as a sub-wavelength light source, detector or scattering center. We utilize a different technique, cathodoluminescence (CL), where the focused electron beam of a scanning or transmission electron microscope (SEM or STEM) acts as a local probe, depositing energy in the sample material, and creating electron-hole pairs in semiconductors and insulators. When these carriers recombine, light is emitted, providing information on local band-gaps, impurities and point-defect concentrations. By using low incident electron-beam energies or thin, electron-transparent samples, spatial resolution better than 20 nm can be achieved. Correlating cathodoluminescence data from individual GaN nanowires with their structure as defined by transmission electron microscopy allow us to deduce information about the optical and electronic structure of the various GaN surfaces and heterostructures.
Molecular Foundry, Lawrence Berkeley National Laboratory