(c)

1. (a) (b) (c) Surface effects on minority carrier transport in ZnO nanowiresYi Gu, Washington State University, DMR 0845007 (a) Near-field photocurrent maps of a single n-type ZnO nanowire Schottky diode under forward- and reverse-biased conditions indicated on the current-voltage curve of the Schottky diode. (b) Measured minority carrier diffusion length (LD, solid circles) as a function of nanowire diameter; literature values (black dashed lines) for bulk n-type ZnO with different electron concentrations (n) are plotted for comparison; the blue dotted line represents calculated LD using a surface-mediated carrier recombination model. (c) The extracted mid-bandgap surface state density as a function of nanowire diameter, showing a strong diameter dependence. Carrier transport in semiconductors is of both fundamental and technological significance. Minority carrier transport is particularly important as it controls the majority carrier transport in p-n junction based devices. In semiconductor nanostructures, particularly semiconductor nanowires, surface effects can be significant in limiting carrier transport. We are investigating how surface effects affect minority carrier transport in ZnO nanowires. We have discovered that surface effects are negligible (significant) for nanowires with diameters larger (smaller) than 40 nm, and that, surface effects, when present, decrease the minority carrier diffusion length. Moreover, our results suggest a diameter-dependent surface electronic structure; particularly, the mid-bandgap surface state density increases with decreasing nanowire diameter. These results suggest that, besides the surface-to-volume ratio, possible changes in the surface electronic structures need to be considered in understanding the effects of surfaces on various material properties.

2. Undergraduate research: variable-temperature Raman scattering spectroscopy as a probe of anharmonic phonon coupling in semiconductor nanowires Yi Gu, Washington State University, DMR 0845007 Rodolfo Lopez (right), a undergraduate student, and the PI, connect a N2 gas line to a liquid nitrogen dewar for cryogenic Raman measurements. Rodolfo aligns the optics for the cryogenic Raman scattering spectroscopy measurements on ZnO nanowires in PI’s lab.