We report steady-state and time-resolved photoluminescence ͑TRPL͒ measurements on individual GaN nanowires ͑6-20 m in length, 30-940 nm in diameter͒ grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si͑111͒ and dispersed onto fused quartz substrates. Induced tensile strain for nanowires bonded to fused silica and compressive strain for nanowires coated with atomic-layer-deposition alumina led to redshifts and blueshifts of the dominant steady-state PL emission peak, respectively. Unperturbed nanowires exhibited spectra associated with high-quality, strain-free material. The TRPL lifetimes, which were similar for both relaxed and strained nanowires of similar size, ranged from 200 ps to over 2 ns, compared well with those of low-defect bulk GaN, and depended linearly on nanowire diameter. The diameter-dependent lifetimes yielded a room-temperature surface recombination velocity S of 9 ϫ 10 3 cm/ s for our silicon-doped GaN nanowires.

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-type͒ by nitrogen-plasma-assisted molecular beam epitaxy primarily from two separate growth runs were examined. The results revealed carrier concentration in the range of ͑3-6͒ ϫ 10 16 cm −3 for one growth run, roughly 5 ϫ 10 14 -1ϫ 10 15 cm −3 for the second, and drift mobility in the range of 500-700 cm 2 / ͑V s͒ for both. Nanowires were dispersed onto insulating substrates and contacted forming single-wire, two-terminal structures with typical electrode gaps of Ϸ3-5 m. When biased at 1 V bias and illuminated at 360 nm ͑3.6 mW/ cm 2 ͒ the thinner ͑Ϸ100 nm diameter͒ nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA ͑noise level͒ to 0.1-1 A. Under the same conditions, thicker ͑151-320 nm͒ nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 A. 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 ͑dark͒ surface band bending in the vicinity of 0.2-0.3 V and capture coefficient in the range of 10 −23 -10 −19 cm 2 . Atomic layer deposition ͑ALD͒ was used to conformally deposit Ϸ10 nm of Al 2 O 3 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 Al 2 O 3 coated samples.

We have grown GaN and AlGaN nanowires on Si (111) substrates with gassource molecular beam epitaxy (MBE). No metal catalysts were used. The nanowires displayed a number of interesting materials properties, including room-temperature luminescence intensity greater than that of free-standing HVPE-grown GaN, relaxed lattice parameters, and the tendency of nanowires dispersed in solvents to align in response to electric fields. The wires were well separated, 50-250 nm in diameter, and grew to lengths ranging from 2 mm to 7 mm. Transmission electron microscopy indicated that the wires were free of defects, unlike the surrounding matrix layer.

The location of GaN nanowires is controlled with essentially perfect selectivity using patterned SiNx prior to molecular beam epitaxy growth. Nanowire growth is uniform within mask openings and absent on the mask surface for over 95% of the usable area of a 76 mm diameter substrate. The diameters of the resulting nanowires are controlled by the size of the mask openings. Openings of approximately 500 nm or less produce single nanowires with symmetrically faceted tips.

Polarization- and temperature-dependent photoluminescence (PL) measurements were performed on individual GaN nanowires. These were grown by catalyst-free molecular beam epitaxy on Si(111) substrates, ultrasonically removed, and subsequently dispersed on sapphire substrates. The wires were typically 5–10μm in length, c-axis oriented, and 30–100nm in diameter. Single wires produced sufficient emission intensity to enable high signal-to-noise PL data. Polarized PL spectra differed for the σ and π polarization cases, illustrating the polarization anisotropy of the exciton emission associated with high-quality wurtzite GaN. This anisotropy in PL emission persisted even up to room temperature (4–296K). Additionally, the nanowire PL varied with excitation intensity and with (325nm) pump exposure time.