Paul T. Blanchard

Steady-state and time-resolved photoluminescence from relaxed and strained GaN nanowires grown by catalyst-free molecular-beam epitaxy

We 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 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 9×103 cm/s for our silicon-doped GaN nanowires.

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

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)×1016 cm−3 for one growth run, roughly 5×1014–1×1015 cm−3 for the second, and drift mobility in the range of 500–700 cm2/(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 μm. When biased at 1 V bias and illuminated at 360 nm (3.6 mW/cm2) 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 μA. Under the same conditions, thicker (151...

Polarization-resolved photoluminescence study of individual GaN nanowires grown by catalyst-free molecular beam epitaxy

Polarization- and temperature-dependent photoluminescence (PL) measurements were performed on individual GaN nanowires. These were grown by catalyst-free molecular beam epitaxy on Si(111) substrates, ultrasonically removed, and subsequently dispersed on sapphire substrates. The wires were typically 5–10μm in length, c-axis oriented, and 30–100nm in diameter. Single wires produced sufficient emission intensity to enable high signal-to-noise PL data. Polarized PL spectra differed for the σ and π polarization cases, illustrating the polarization anisotropy of the exciton emission associated with high-quality wurtzite GaN. This anisotropy in PL emission persisted even up to room temperature (4–296K). Additionally, the nanowire PL varied with excitation intensity and with (325nm) pump exposure time.

On-chip optical interconnects made with gallium nitride nanowires.

In this Letter we report on the fabrication, device characteristics, and optical coupling of a two-nanowire device comprising GaN nanowires with light-emitting and photoconductive capabilities. Axial p-n junction GaN nanowires were grown by molecular beam epitaxy, transferred to a non-native substrate, and selectively contacted to form discrete optical source or detector nanowire components. The optical coupling demonstrated for this device may provide new opportunities for integration of optical interconnects between on-chip electrical subsystems.

MESFETs Made From Individual GaN Nanowires

In this paper, we demonstrate novel MESFETs based on individual GaN nanowires. The Pt/Au Schottky gates exhibited excellent two-terminal Schottky diode rectification behavior. The average effective Schottky barrier height was 0.87 eV, with an average ideality factor of 1.6. In addition, the Schottky gates efficiently modulated the conduction of the nanowires. The threshold gate voltages required for complete pinch off were as small as -2.6 V, and transconductances exceeded 1.4 muS. Subthreshold swings approaching 60 mV/decade and on/off current ratios of up to 5times108 were achieved. These results show that the Schottky gate has the potential to significantly improve the performance of GaN nanowire field-effect devices.

High frequency characterization of a Schottky contact to a GaN nanowire bundle

A two-port GaN nanowire (NW) device with one Schottky contact and one Ohmic contact was characterized up to 10 GHz using on-wafer microwave measurements. In addition to the measurement of the broadband response, two additional applications of microwave measurements are introduced: (1) the capability to distinguish a Schottky-type contact from an Ohmic contact based on the reflected broadband signals (S11 and S22) and (2) the measurement of a capacitance voltage (CV) curve for a Schottky contact to a bundle of a few NWs. The junction capacitance of the Schottky contact is determined at various bias voltages by fitting the broadband response with a microwave circuit model. The carrier concentration is estimated from the resulting CV curve to be 5.3×1018/cm3 and the Schottky barrier height is estimated to be 0.89 eV.

MOSFETs Made From GaN Nanowires With Fully Conformal Cylindrical Gates

We report novel metal-oxide-semiconductor field effect transistors (MOSFETs) based on individual gallium nitride (GaN) nanowires with fully conformal cylindrical gates. The W/Al2O3 gates were deposited by atomic layer deposition. Reverse-bias breakdown voltages exceeded the largest gate voltage tested (-35 V). The nanowire MOSFETs showed complete pinchoff, with threshold voltages between -4 and -12 V. Maximum transconductances exceeded 10 μS, and ON/OFF current ratios higher than 10 8 were measured. Significant gating hysteresis and memory effects were also present, indicative of charge traps. Although further optimization is needed, these results represent a promising step forward in the development of efficient GaN nanowire-based FETs.

Electrical Characterization of Photoconductive GaN Nanowires from 50 MHz to 33 GHz

The electrical response of two-port photoconductive GaN nanowire devices was measured from 50 MHz to 33 GHz. The admittance of the nanowire devices showed an increase on the order of 10% throughout the measured frequency range after exposure to steady ultraviolet illumination. Two different two-port microwave network models were used to extract microwave circuit parameters in the photoconductive and dark states. After illumination, the GaN nanowire devices showed a measurable increase in shunt capacitance and decreases in both the contact and nanowire resistances.

Gallium nitride nanowire probe for near-field scanning microwave microscopy

We report on the fabrication of a GaN nanowire probe for near-field scanning microwave microscopy. A single nanowire was Pt-bonded to a commercial Si cantilever prior to evaporation of a Ti/Al coating to provide a microwave signal pathway. Testing over a microcapacitor calibration sample shows the probe to have capacitance resolution of at least 0.7 fF with improved sensitivity and reduced uncertainty compared with a commercial microwave probe. High wear resistance of the defect-free nanowire enabled it to maintain a tip radius of 150 nm after multiple contact-mode scans while demonstrating nanometer-scale topographical resolution.

Measurement of the electrostatic edge effect in wurtzite GaN nanowires

The electrostatic effect of the hexagonal corner on the electronic structure in wurtzite GaN nanowires (NWs) was directly measured using Kelvin probe force microscopy (KPFM). By correlating electrostatic simulations with the measured potential difference between the nanowire face and the hexagonal vertices, the surface state concentration and band bending of GaN NWs were estimated. The surface band bending is important for an efficient design of high electron mobility transistors and for opto-electronic devices based on GaN NWs. This methodology provides a way to extract NW parameters without making assumptions concerning the electron affinity. We are taking advantage of electrostatic modeling and the high precision that KPFM offers to circumvent a major source of uncertainty in determining the surface band bending.