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-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.

Statistical analysis of excitonic transitions in single, free-standing GaN nanowires: Probing impurity incorporation in the poissonian limit

“…Figure 2(b) shows that these latter NWs would already contain 10 donors per NW on average. However, recent experimental results from Sanford et al [17] suggest that the donor concentration in GaN NWs that have not been intentionally doped may be even lower than the value estimated above (down to 5 × 10 14 cm −3 ).…”

Imaging the p-n junction in a gallium nitride nanowire with a scanning microwave microscope

“…On the p-type side the dopant was Mg with N A ¼ 1 Â 10 18 cm À3 , E A ¼ 170 meV, which together yielded n p ¼ 1 Â 10 17 cm À3 ; the downward band bending on the p-type side was 1 eV. 24,33 The geometry of the device structure resting on a metal-coated and oxidized Si substrate (as described earlier) is illustrated as insets in Figs. 4(a) and 4(b).…”

Electrically driven polarized single-photon emission from an InGaN quantum dot in a GaN nanowire

“…The surface recombination velocity in GaN nanowires is B10 3 cm s À 1 , two orders of magnitude smaller than that on a planar surface of GaAs (ref. 35). It is possible to grow one or multiple In(Ga)N disks in Al(Ga)N nanowires to provide quantum confinement, such as in a quantum well [36][37][38] .…”