Matthew D. Brubaker

Homoepitaxial n-core: p-shell gallium nitride nanowires: HVPE overgrowth on MBE nanowires

We present the homoepitaxial growth of p-type, magnesium doped gallium nitride shells by use of halide vapor phase epitaxy (HVPE) on n-type gallium nitride nanowires grown by plasma-assisted molecular beam epitaxy (MBE). Scanning electron microscopy shows clear dopant contrast between the core and shell of the nanowire. The growth of magnesium doped nanowire shells shows little or no effect on the lattice parameters of the underlying nanowires, as measured by x-ray diffraction (XRD). Photoluminescence measurements of the nanowires show the appearance of sub-bandgap features in the blue and the ultraviolet, indicating the presence of acceptors. Finally, electrical measurements confirm the presence of electrically active holes in the nanowires.

Microstructure evolution and development of annealed Ni/Au contacts to GaN nanowires.

The development of Ni/Au contacts to Mg-doped GaN nanowires (NWs) is examined. Unlike Ni/Au contacts to planar GaN, current-voltage (I-V) measurements of Mg-doped nanowire devices frequently exhibit a strong degradation after annealing in N(2)/O(2). This degradation originates from the poor wetting behavior of Ni and Au on SiO(2) and the excessive void formation that occurs at the metal/NW and metal/oxide interfaces. The void formation can cause cracking and delamination of the metal film as well as reduce the contact area at the metal/NW interface, which increases the resistance. The morphology and composition of the annealed Ni/Au contacts on SiO(2) and the p-GaN films were investigated by scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) and x-ray diffraction (XRD) measurements. Adhesion experiments were performed in order to determine the degree of adhesion of the Ni/Au films to the SiO(2) as well as observe and analyze the morphology of the films underside by SEM. Device degradation from annealing was prevented through the use of a specific adhesion layer of Ti/Al/Ni deposited prior to the nanowire dispersal and Ni/Au deposition. I-V measurements of NW devices fabricated using this adhesion layer showed a decrease in resistance after annealing, whereas all others showed an increase in resistance. Transmission electron microscopy (TEM) on a cross-section of a NW with Ni/Au contacts and a Ti/Al/Ni adhesion layer showed a lack of void formation at the contact/NW interface. Results of the XRD and TEM analysis of the NW contact structure using a Ti/Al/Ni adhesion layer suggests Al alloying of the Ni/Au contact increases the adhesion and stability of the metal film as well as prevents excessive void formation at the contact/NW interface.

Near-field control and imaging of free charge carrier variations in GaN nanowires

Despite their uniform crystallinity, the shape and faceting of semiconducting nanowires (NWs) can give rise to variations in structure and associated electronic properties. Here, we develop a hybrid scanning probe-based methodology to investigate local variations in electronic structure across individual n-doped GaN NWs integrated into a transistor device. We perform scanning microwave microscopy (SMM), which we combine with scanning gate microscopy to determine the free-carrier SMM signal contribution and image local charge carrier density variations. In particular, we find significant variations in free carriers across NWs, with a higher carrier density at the wire facets. By increasing the local carrier density through tip-gating, we find that the tip injects current into the NW with strongly localized current when positioned over the wire vertices. These results suggest that the strong variations in electronic properties observed within NWs have significant implications for device design and may lead ...

An optical Bragg scattering readout for nano-mechanical resonances of GaN nanowire arrays

We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in periodic selected-area growth arrays. Hexagonal 2D arrays of GaN NWs with pitch spacings of 350–1100 nm were designed and prepared to allow optical Bragg scattering of 632.8 nm laser light. The NWs studied have diameters ranging from 100 to 300 nm, lengths from 3 to 10 μm, resonance frequencies between 1 and 10 MHz, Q-values near 10 000 at 300 K, and Youngs modulus of 310 ± 45 GPa. The optical system can detect the thermally induced Brownian mechanical motion of the NWs and driven NW motion and allows the simultaneous monitoring of hundreds of mechanical resonators with a single laser beam. The read-out system allows large arrays of NWs to be characterized and, upon mapping the resonance frequencies to individual array elements, to be applied as, e.g., spatially resolved temperature and mass sensors.We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in periodic selected-area growth arrays. Hexagonal 2D arrays of GaN NWs with pitch spacings of 350–1100 nm were designed and prepared to allow optical Bragg scattering of 632.8 nm laser light. The NWs studied have diameters ranging from 100 to 300 nm, lengths from 3 to 10 μm, resonance frequencies between 1 and 10 MHz, Q-values near 10 000 at 300 K, and Youngs modulus of 310 ± 45 GPa. The optical system can detect the thermally induced Brownian mechanical motion of the NWs and driven NW motion and allows the simultaneous monitoring of hundreds of mechanical resonators with a single laser beam. The read-out system allows large arrays of NWs to be characterized and, upon mapping the resonance frequencies to individual array elements, to be applied as, e.g., spatially resolved te...We report the use of optical Bragg scattering and homodyne interferometry to simultaneously measure all the first order cantilever-mode mechanical resonance frequencies and quality factors (Q) of gallium nitride nanowires (GaN NWs) in 100 NW periodic selected-area growth arrays. Hexagonal 2D arrays of 100 GaN NWs with pitch spacings of 350-1100 nm were designed and prepared to allow optical Bragg scattering. The NWs studied have diameters ranging from 100-300 nm, lengths from 3-10 μm, resonance frequencies between 1-10 MHz, and Q-values near 10,000 at 300 K. The system can passively detect the thermally induced Brownian mechanical motion of the NWs and can study driven NW motion, enabling the simultaneous monitoring of hundreds of mechanical resonators in a 10-100 μm2 area with a single optical beam. The read-out system allows large arrays of NWs to be characterized and applied as e.g. spatially resolved temperature and mass sensors.

Influence of Morphology on Current–Voltage Behavior of GaN Nanowires

We demonstrate the effect that the different morphologies of molecular beam epitaxy-grown GaN nanowires (NWs) can have upon current-voltage (I-V) behavior. Two aspects of NW morphology were investigated. The first aspect was the NW diameter, dNW. For single-crystal Si-doped GaN NW devices with dNW <; 120 nm, I-V curves were nonlinear. In contrast, single-crystal Si-doped NWs from the same growth run with dNW > 120 nm consistently showed ohmic I-V behavior. This discrepancy is likely the result of the comparatively larger surface depletion in thin NWs, which contributes to 1) an increased contact barrier, and 2) a barrier resulting from an axial band offset between the portion of the NW directly beneath the contact and the portion extending from the contact. The second aspect of NW morphology that we investigated was NW coalescence, which occurs when neighboring NWs fuse together during growth. I-V measurements of undoped coalesced NWs showed that these structures can have a free carrier concentration that is significantly higher than the background carrier concentration that is present in single-crystal (noncoalesced), undoped NWs.

CATALYST-FREE GAN NANOWIRES AS NANOSCALE LIGHT EMITTERS

Catalyst-free growth of GaN nanowires with molecular beam epitaxy produces material of exceptionally high quality with long minority-carrier lifetimes and low surface recombination velocity. The nanowires grow by thermodynamic driving forces that enhance the sticking coefficient of incoming reagents to the end facets of the nanowire while inhibiting growth on the m-plane sidewalls. Photoluminescence (PL) studies confirm that the material is essentially free of detrimental chemical impurities and crystalline defects. The nanowires are readily excited to lasing with modest optical pump power. Recent progress in methods for selective epitaxy has made it possible to control both the diameter and placement of the nanowires. Despite the high material quality, the energy-conversion efficiency of single nanowire LEDs remains low. The primary limitation appears to be optimizing the p-type doping with Mg, which is both a growth and a measurement problem.