Studies of photoconductivity and field effect transistor behavior in examining drift mobility, surface depletion, and transient effects in Si-doped GaN nanowires in vacuum and air

Abstract

Variable intensity photoconductivity (PC) performed under vacuum at 325 nm was used to estimate drift mobility (μ) and density (σs) of negative surface charge for c-axis oriented Si-doped GaN nanowires (NWs). In this approach, we assumed that σs was responsible for the equilibrium surface band bending (∅) and surface depletion in the absence of illumination. The NWs were grown by molecular beam epitaxy to a length of approximately 10 μm and exhibited negligible taper. The free carrier concentration (N) was separately measured using Raman scattering which yielded N = (2.5 ± 0.3) × 1017 cm−3 for the growth batch studied under 325 nm excitation. Saturation of the PC was interpreted as a flatband condition whereby ∅ was eliminated via the injection of photogenerated holes. Measurements of dark and saturated photocurrents, N, NW dimensions, and dimensional uncertainties, were used as input to a temperature-dependent cylindrical Poisson equation based model, yielding σs in the range of (3.5 to 7.5) × 1011 cm−2 and μ in the range of (850 to 2100) cm2/(V s) across the (75 to 194) nm span of individual NW diameters examined. Data illustrating the spectral dependence and polarization dependence of the PC are also presented. Back-gating these devices, and devices from other growth batches, as field effect transistors (FETs) was found to not be a reliable means to estimate transport parameters (e.g., μ and σs) due to long-term current drift. The current drift was ascribed to screening of the FET back gate by injected positive charge. We describe how these gate charging effects can be exploited as a means to hasten the otherwise long recovery time of NW devices used as photoconductive detectors. Additionally, we present data illustrating comparative drift effects under vacuum, room air, and dry air for both back-gated NW FETs and top-gated NW MESFETs.