Variable intensity photoconductivity (PC) performed under vacuum at 325 nm was used to estimate drift mobility and negative surface charge density sigma for c-axis oriented Si-doped GaN nanowires (NWs). In this approach we assumed that sigma was responsible for the equilibrium surface band bending(SBB)and depletion in the dark. The NWs were grown by molecular beam epitaxy (MBE) to a length of roughly 10 micrometers and exhibited negligible taper. The free carrier concentration N was separately measured using Raman scattering which yielded N = (2.5 ± 0.3)E17 cm-3 for the growth batch studied. Saturation of the PC was interpreted as a flatband condition whereby the SBB was eliminated via the injection of photogenerated holes. Measurements of dark and saturated photocurrents in combination with N, and NW dimensional uncertainties, were used as input to a temperature-dependent cylindrical Poisson solver yielding sigma in the range of (3.57.5)E11 cm-2 and mobility in the range of 8502100 cm2/(V-s) across the 78350 nm span of individual NW diameters examined. Back-gating these devices as field effect transistors (FETs) was not a reliable means to estimate transport properties since, under sustained depletion bias, the devices drifted in a manner commensurate with injected positive charge screening the gate. 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.
Citation: Journal of Applied Physics
Pub Type: Journals
nanowires, nanotechnology, GaN, gallium nitride, photoconductivity, nanowire FET, nanowire transport, GaN mobility