Surface Photovoltage Study of Indium Phosphide Nanowire Networks
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Surface Photovoltage Study of Indium Phosphide Nanowire Networks Andrew J. Lohn1,2, Jin-Woo Han3 and Nobuhiko P. Kobayashi1,2 1
Baskin School of Engineering, University of California Santa Cruz, Santa Cruz, CA 95064 USA Nanostructured Energy Conversion Technology and Research (NECTAR), Advanced Studies Laboratories, Univ. of California Santa Cruz – NASA Ames Research Center, Moffett Field, CA 94035 USA 3 Center for Nanotechnology, NASA Ames Research Center, Moffett Field, CA 94035, USA 2
ABSTRACT Surface photovoltage of three-dimensional networks composed of fused indium phosphide (InP) nanowires is discussed. Particular emphasis is given to the dependence of surface photovoltage on the chopping frequency of light that excites the nanowire network as observed in regions which are laterally separated from the excitation. The nanowire network is modeled as a thin film to simplify numerical solutions to transport equations which aids in the interpretation of diffusion and drift of photo-generated carriers within the nanowire network. INTRODUCTION Nanowire-based materials systems offer a variety of advantages1 over traditional thin film technologies and as such their applications are being rapidly developed. Typically scientific as well as application-based studies focus on single nanowires in isolation. Our system of randomly-oriented and interconnected InP nanowires creates a three-dimensional nanowire network with added functionality including long-range electrical transport. Understanding this type of system will lead to better design of nanowire-based large area devices such as those needed in energy, lighting, and display applications. Surface photovoltage (SPV) is a non-contact characterization technique that has been developed in part by the community dedicated for silicon microelectronics 2 for non-contact defect quantification. Band bending in semiconductors due to surface charges results in a voltage that can be measured at the surface. Optically excited free carriers act to screen3 charges thereby reducing band bending and the surface potential. SPV has been used to determine minority carrier lifetime4 and minority carrier diffusion length5 in thin film and bulk samples. More recently the technique has been used to evaluate surface passivation in GaN nanowires6, identify doping concentrations in InAs/InP heterostructure nanowires7, and to characterize band tailing in GaAs nanowires8. In this work, we investigate the SPV of InP nanowire networks and support the measurements with numerical simulations of carrier transport in thin-films EXPERIMENT Fused silica substrates with 300 nm thick films of micro-crystalline silicon (µ-c:Si) served as the growth platform for InP nanowire networks. The nanowires were approximately 50 nm in diameter with lengths in excess of 10 µm as shown in Fig 1. The random nature of µ-
c:Si results in randomly oriented nanowires that fuse together during growth creating conducting pathways which lead to transport over long-range beyond nominal length of individual nanowires.
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