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

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Author(s): Norman A. Sanford; Paul T. Blanchard; Kristine A. Bertness; Lorelle Mansfield; John B. Schlager; Aric W. Sanders; Alexana Roshko; Beau Burton; Steven George;
Title: Steady-state and transient photoconductivity in c-axis GaN nanowires grown by nitrogen-plasma-assisted molecular beam epitaxy
Published: February 12, 2010
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)X1016 cm-3 for one growth run, roughly 5 X 1014-1 X 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 {approximately equal} 3-5 υm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) the thinner ({approximately equal} 100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1-1 {mu)A. Under the same conditions, thicker (151-320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 υA. 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 {approximately equal} 10 nm of A1203 on several devices. Photoconductivity, persistent photoconductivi
Citation: Journal of Applied Physics
Volume: 107
Pages: pp. 034318-1 - 034318-14
Keywords: Nanowires;wide-bandgap semiconductors;GaN;nanotechnology;photoconductivity;depletion layers;atomic layer deposition
Research Areas: Nanophotonics, Nanowires, Semiconductors, Nanotechnology, Optoelectronics, Characterization, Nanometrology, and Nanoscale Measurements
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