Applied Physics Letters

A simple technique is explored to determine the temporal photo-response, s, of individual semiconducting SiC and Si nanowires (NWs), with a high time resolution. Laser-assisted field emission (LAFE) from the NWs is first shown to be highly sensitive to continuous laser illumination. Pulsed illumination is then combined with measurements of the total energy distributions to determine s which were rather large, 4–200 ls. The time response scaled roughly with the square of the NWs length and could be attributed to laser-induced heating. LAFE is thus a new tool for quantifying rapid thermo-optical effects in such nano-objects. Photo-field emission or laser-assisted field emission (LAFE) has been studied for both fundamental physics1,2 and within the perspective of developing new photocathodes.3–5 The potential for optically controlled rapid field emission (FE) sources4 is particularly attractive for applications such as microwave tubes6 and time-resolved electron microscopy.7 In the interesting case of properly prepared semiconducting (SC) emitters, FE can be highly sensitive to light for both p-type and high resistivity n-type samples.8,9 This high sensitivity is associated with a current saturation in the FE I/ V curves and voltage drops within the SC. Though the emission at low current follows the FowlerNordheim (FN) behavior for metallic emitters and is insensitive to light, the supply of mobile carriers at higher current can be insufficient for full screening and the electric field then penetrates into the SC creating a band bending and a high resistance depletion region to considerable depths associated with strong voltage drops. The dependence of FE with light (and temperature T) is because the depletion zone is highly sensitive to the generation of free carriers.8,9 These effects have been demonstrated for manually-fabricated tips2,9 and for Si tip arrays fabricated from doped Si wafers by photolithography.3,4 It is timely to explore these effects in SC nanowires (NWs) because they are attracting enormous attention these last years. Understanding the optical response of such NWs is thus important for a wider community interested in other photonic devices such as photodetectors, solar cells, etc.10 Our previous studies on FE from nominally undoped SiC NWs,11 and more recent studies on Si NWs,12 revealed strong nonlinearities in the FN plots with T-dependent saturation currents as predicted by theory.8 The drops were determined by the total energy distributions (TEDs) of the emitted electrons which, when combined with the measured total current, becomes a type of two point transport measurement.13 In the saturation region, measured voltage drops along the NWs reached up to several hundreds of volts11,12 and were T-dependent. These measurements allowed us to identify the carrier transport mechanisms14 in our SiC NWs, which is difficult using only FN plots as measured by most authors (e.g. Refs. 3 and 4). An important point in our work is that we showed a clear SC FE effects for mass-produced NWs. ha l-0 07 94 03 2, v er si on 1 2 M ar 2 01 3 Author manuscript, published in "Applied Physics Letters 99 (2011) 072115" DOI : 10.1063/1.3627168