Gallium nitride (GaN)-based semiconductors have successfully been incorporated into commercial light emitting diodes and commercial laser diodes that operate from ultraviolet (UV) to green wavelengths. AlxGa1−xN and InxGa1−xN alloys offer the potential to extend the wavelength range of GaN-based photonic devices to the deep UV and into the near infrared. These attributes make GaN a very attractive material for optical sources and optical sensors, but material quality remains an important issue. Conventional growth methods on lattice mismatched substrates produce material with a relatively high density of defects that decrease device yield and increase device cost. Researchers in the EEEL Optoelectronics Division are growing GaN in the form of nanowires with catalyst-free molecular-beam epitaxy (MBE), however, can produce strain-free crystalline material with very low-defect density and in a morphology useful for optoelectronic, electronic, and even nanomechanical applications. Optical characterization of these nanowires yields information about the intrinsic material properties, the sensitivity of the material to environmental changes, and the nanowire structure itself.
EEEL researcher John Schlager and collaborators report steady-state and time-resolved photoluminescence (TRPL) measurements on individual GaN nanowires (6–20 μm in length, 30–940 nm in diameter) grown by a nitrogen-plasma-assisted, catalyst-free molecular-beam epitaxy on Si(111) 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 photoluminescence (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 9X103 cm/ s for our silicon-doped GaN nanowires, over five times smaller than that reported for undoped GaN epilayers and over 100 times smaller than that reported for GaAs. The low surface recombination velocity and long carrier decay times indicate this nanowire material is relatively insensitive to surface states and make it a good candidate for nanoscale optoelectronic device applications where the surface-to-volume ratios for the active materials will necessarily be large.
J.B. Schlager, K.A. Bertness, P.T. Blanchard, L.H. Robins, A. Roshko, and N.A. Sanford, "Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy," J. Appl. Phys 103, 124309 (2008).