Electrical transport in semiconductor nanowires is commonly measured in a field effect transistor configuration, with lithographically defined source, drain, and in some cases, top gate electrodes. This approach is labor intensive, requires high-end fabrication equipment, exposes the nanowires to extensive processing chemistry, and places practical limitations on minimum nanowire length. Here we describe a simple method for characterizing electrical transport in nanowires directly on the growth substrate, without any need for post growth processing. Our technique is based on contacting nanowires using a nano-manipulator probe retrofitted inside of a scanning electron microscope. Using this approach we characterize electrical transport in GaN nanowires grown by catalyst-free selective epitaxy, as well as InAs and Ge nanowires grown by Au-catalyzed vapor solid liquid technique. We find that in situations where contacts are not limiting carrier injection (GaN and InAs nanowires), electrical transport transitions from Ohmic conduction at low bias, to space-charge limited conduction at higher bias. Using this transition and a theory of space charge limited transport which accounts for the high aspect ratio nanowires, we extract the mobility and the free carrier concentration. For Ge nanowires, we find that the Au catalyst forms a Schottky contact resulting in rectifying current-voltage characteristics, and which are strongly dependent on the nanowire diameter. This dependence arises due to increase in depletion width at decreased nanowire diameter and carrier recombination at the nanowire surface.

We demonstrate selective emission from a heterogeneous metasurface that can survive repeated temperature cycling at 1300 K. Simulations, fabrication, and characterization were performed for a cross-over-a-backplane metasurface consisting of platinum and alumina layers on a sapphire substrate. The structure was stabilized for high temperature operation by an encapsulating alumina layer. The geometry was optimized for integration into a thermophotovoltaic (TPV) system, and was designed to have its emissivity matched to the external quantum efficiency spectrum of 0.6 eV InGaAs TPV material. We present spectral measurements of the metasurface that result in a predicted 22% optical-to-electrical power conversion efficiency in a simplified model at 1300 K. Furthermore, this broadly adaptable selective emitter design can be easily integrated into full-scale TPV systems.

Abstract-Recent theory and experiment have suggested that space-charge-limited transport should be prevalent in high aspect-ratio semiconducting nanowires. We report on InAs nanowires exhibiting this mode of transport and utilize the underlying theory to determine the mobility and effective carrier concentration of individual nanowires, both of which are found to be diameter-dependent. Intentionally induced failure by Joule heating supports the notion of space-charge limited transport and proposes reduced thermal conductivity due to the nanowires' polymorphism.

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