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

• Published in: Journal of applied physics 2010-02, Vol.107 (3), p.034318-034318-14
• Main Authors: Sanford, N. A., Blanchard, P. T., Bertness, K. A., Mansfield, L., Schlager, J. B., Sanders, A. W., Roshko, A., Burton, B. B., George, S. M.
• Format: Article
• Language: English
• Subjects: ALUMINIUM COMPOUNDS; ALUMINIUM OXIDES; CARRIER DENSITY; CARRIER MOBILITY; CHALCOGENIDES; COATINGS; CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; CRYSTAL GROWTH METHODS; DEPLETION LAYER; DEPOSITION; ELECTRIC CONDUCTIVITY; ELECTRICAL PROPERTIES; ELECTRONS; ELEMENTARY PARTICLES; EPITAXY; FERMIONS; GALLIUM COMPOUNDS; GALLIUM NITRIDES; HOLES; LAYERS; LEPTONS; MATERIALS; MATERIALS SCIENCE; MOBILITY; MOLECULAR BEAM EPITAXY; NANOSCIENCE AND NANOTECHNOLOGY; NANOSTRUCTURES; NITRIDES; NITROGEN COMPOUNDS; OXIDES; OXYGEN COMPOUNDS; PHOTOCONDUCTIVITY; PHYSICAL PROPERTIES; PLASMA; PNICTIDES; QUANTUM WIRES; REFRACTORY METAL COMPOUNDS; SEMICONDUCTOR MATERIALS; SUBSTRATES; TANTALUM COMPOUNDS; TANTALUM NITRIDES; TRANSITION ELEMENT COMPOUNDS
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.3275888

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 ) × 10 16   cm − 3 for one growth run, roughly 5 × 10 14 - 1 × 10 15   cm − 3 for the second, and drift mobility in the range of 500 - 700   cm 2 / ( 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   μ m . When biased at 1 V bias and illuminated at 360 nm ( 3.6   mW / cm 2 ) 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   μ 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   μ 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 (dark) 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 (ALD) was used to conformally deposit ≈10 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.