Student Achievements Yi Gu, Washington State University, DMR 0845007

1. Diameter-dependent Surface State Density in Semiconductor NanowiresYi Gu, Washington State University, DMR 0845007 Figure 1 (a) Atomic force microscopy topography image; surface potential maps of a ZnO nanowire in (b) the dark and (c) under an optical illumination; simulated cross-sectional (d) potential and (e) electron density distributions; (f) surface potential barrier height (solid circles) and surface state density (solid squares); (g) calculated surface lifetime (τsur, solid circles) and total effective lifetime (τeff, solid squares), with the inset showing a plot of τeff at various diameters. Dashed lines in (f) and (g) are spline interpolations. Surface effects are important in limiting carrier transport in low-dimensional semiconductors. The significance of the surface effects is commonly understood to scale with the surface-to-volume ratio. However, recent studies, including our own, suggest that the surface-to-volume ratio alone significantly underestimates the significance of surface effects at small diameters. Through surface photovoltage measurements on single nanowires, shown in Figs. 1 (a) – (c), and finite-element electrostatic simulations, shown in Figs. 1 (d) and (e), we obtained surface state density [solid squares in Fig. 1 (f)], which increases as the diameter decreases. We suggest that this diameter dependence of the surface state density is an important origin of the strong surface effects, and accounts for the transition from bulk-limited to surface-limited carrier transport in the rather narrow 30 – 40 nm diameter regime observed in our previous studies.

2. Student Achievements Yi Gu, Washington State University, DMR 0845007 Afsoon Soudi (left), a Ph.D student in PI’s group, was presented a Materials Research Society (MRS) Graduate Student Silver Award at the 2012 Spring MRS Meeting in San Francisco, in recognition of her achievement in materials research. She is expected to defend her PhD thesis in fall 2012.