Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy

Abstract

Analysis of steady-state and transient photoconductivity measurements at room temperature performed on c-axis oriented GaN nanowires yielded estimates of free carrier concentration, drift mobility, surface band bending, and surface capture coefficient for electrons. Samples grown (unintentionally n-type) by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of (3–6)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(V s) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of ≈3–5 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner (≈100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1–1 μA. Under the same conditions, thicker (151–320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 μA. With the light blocked, the dark current was restored in a few minutes for the thinner samples and an hour or more for the thicker ones. The samples with lower carrier concentration showed similar trends. Excitation in the 360–550 nm range produced substantially weaker photocurrent with comparable decay rates. Nanowire photoconductivity arises from a reduction in the depletion layer via photogenerated holes drifting to the surface and compensating ionized surface acceptors. Simulations yielded (dark) surface band bending in the vicinity of 0.2–0.3 V and capture coefficient in the range of 10−23–10−19 cm2. Atomic layer deposition (ALD) was used to conformally deposit ≈10 nm of Al2O3 on several devices. Photoconductivity, persistent photoconductivity, and subgap photoconductivity of the coated nanowires were increased in all cases. TaN ALD coatings showed a reduced effect compared to the Al2O3 coated samples.