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) x 10(16) cm(-3) for one growth run, roughly 5 x 10(14) - 1 x 10(15) cm(-3) for the second, and drift mobility in the range of 500 - 700 cm(2)/(Vs) for both. Nanowires were dispersed onto insulating substrates and contacted forming single-sire, two-terminal structures with typical electrode gaps of approx. 3-5 microns. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm(2)) the thinner (approx. 100 nm diameter) nanowires with the higher background doping showed an abrupt increase in photocurrent from 5 pA (noise level) to 0.1 - 1 microA. Under the same conditions, thicker (151-320 nm) nanowires showed roughly ten times more photocurrent, with dark currents ranging from 2 nA to 1 microA. 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 260-550 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 approx. 10 nm of Al(2)O(3) on several devices.
Document Details
- Document Type
- Technical Report
- Publication Date
- Feb 01, 2010
- Accession Number
- ADA553593
Entities
People
- A. Roshko
- A. W. Sanders
- B. B. Burton
- J. B. Schlager
- K. A. Bertness
- L. Mansfield
- N. A. Sanford
- P. T. Blanchard
- S. M. George
Organizations
- National Institute of Standards and Technology