Nanowire laser
Demonstrated optically pumped laser action in an as-grown GaN nanowire, in collaboration with the Semiconductor Growth and Devices Project. Using a q-switched pump source at 266 nm, the spectral width of initial superluminescence narrowed and shifted to longer wavelengths with increased pump intensity, and the output vs. pump intensity was superlinear. In addition, the lasing spectrum showed distinct longitudinal modes corresponding to the approximately 25 micrometer long, MBE-grown wires.
Contact: Dr. John Schlager

X-ray characterization of wide bandgap films and nanowires
Compiled 'a' and 'c' lattice constants of GaN/InGaN and GaN/AlGaN films, GaN nanowires, and ZnO nanowires by collecting and fitting the data from symmetric and asymmetric x-ray diffractions. The samples were supplied in-house (MBE-grown nanowires from the Semiconductor Growth and Devices Project) or by government and industry collaborators. It was demonstrated through x-ray measurements that the in-house MBE-grown GaN nanowires are fully relaxed, aligned to the silicon substrate, and have a high degree of crystalline perfection.
Contact: Dr. Norman Sanford

High luminescence efficiency demonstrated in nanowires
Demonstrated that the GaN nanowire ensembles grown by MBE have high photoluminescence efficiency comparable to that of bulk GaN, without correction for the non-unity fill fraction of the nanowires. This further confirms the low defect density of the nanowires.
Contact: Dr. John Schlager

Nanowire LED demonstrated
Co-developed and demonstrated, with members of the Semiconductor Growth and Devices Project, the operation of an ultraviolet light-emitting diode based on gallium nitride nanowires. The LED demonstration required growth of isolated, low-defect-density GaN nanowires on silicon, structural characterization as feedback to growth parameters, removal of the wires from the growth substrate and dispersal on an n-type GaN substrate, optimization of electrical contacts, and spectroscopy of the LED output to verify emission from the nanowires. The LED emission center wavelength near 385 nm was believed to be the shortest wavelength demonstrated for this type of nanowire device at the time.
Contact: Dr. Norman Sanford

Photoluminescence anisotropy of gallium nitride nanowires characterized Made polarization-resolved photoluminescence measurements, from 4 K to room temperature, of individual gallium nitride nanowires grown in-house by MBE. The wires were typically 5-10 um in length and 30-100 nm in diameter. Details of the spectra varied from wire to wire but, in general, spectral peaks associated with free and bound exciton peaks of strain-free GaN were readily identified. The nanowires enabled the first unambiguous room-temperature measurement of photoluminescence anisotropy from unstrained wurtzite gallium nitride that arises from the symmetry, bandstructure, and selection rules for radiative electronic transitions in the material. These measurements are difficult to perform in typical thin-film gallium nitride or quasi-bulk free-standing platelets of the material but are readily enabled by the natural geometry and strain-free material offered by the nanowires. Contact: Dr. John Schlager

Nanowire strain measurements using photoluminescence
Demonstrated the utility of photoluminescence (PL) measurements in understanding post-processing-induced strain in otherwise strain-free GaN nanowires (NWs), in collaboration with the Semiconductor Growth and Devices Project. For example, NWs incorporated in metal bridge structures (for photoconductivity and other electrical tests) exhibited tensile strain. NWs coated with atomic-layer-deposited alumina exhibited compressive strain. Controlled surface passivation and functionalization of NWs is vital for the development of electronic and optoelectronic devices. The measured shifts in peak PL wavelength were consistent with theoretical calculations.
Contact: Dr. John Schlager

Spectral photoconductivity of nanowires
Characterized the spectral and time dependences of photoconductivity in GaN nanowires. High differential UV photoconductivity of up to four orders of magnitude in photocurrent was observed. The observations are consistent with the optical gating of the depletion layer in the wires. Relatively fast persistent photoconductivity recovery is consistent with thin fully depleted nanowires. The effects of surface coatings on photoconductivity depend on the coating material. This study is important for the understanding of nanowire surfaces and interfaces, especially relevant to sensors and other electronic and optoelectronic applications of nanowires.
Contact: Dr. Norman Sanford

Nanowire field-effect transistors Fabricated and characterized high-quality metal-semiconductor field effect transistors (MESFETs) based on individual gallium nitride nanowires (NWs). The Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior and ideality factors significantly lower than those previously reported for individual GaN NWs. In addition, the Schottky-gated MESFETs showed low threshold gate voltages, high transconductances, near-ideal subthreshold swings, and large on/off drain-source current ratios. The MESFET design shows promise for significant improvement in the performance of GaN NW field-effect devices. Contact: Dr. Norman Sanford