David P. R. Aplin

Direct observation of ballistic and drift carrier transport regimes in InAs nanowires

Conductive atomic force microscopy has been used to characterize distance-dependent electron transport behavior in InAs nanowires grown by metal-organic chemical vapor deposition. Using a conducting diamond-coated tip as a local electrical probe in an atomic force microscope, the resistance of the InAs nanowire has been measured as a function of electron transport distance within the nanowire. Two regimes of transport behavior are observed: for distances of ∼200nm or less, resistance independent of electron transport distance, indicative of ballistic electron transport, is observed; for greater distances, the resistance is observed to increase linearly with distance, as expected for conventional drift transport. These observations are in very good qualitative accord with the Landauer formalism for mesoscopic carrier transport, and the resistance values derived from these measurements are in good quantitative agreement with carrier concentrations and mobilities determined in separate experiments. These res...

Heteroepitaxial Growth of Vertical GaAs Nanowires on Si (111) Substrates by Metal−Organic Chemical Vapor Deposition

Epitaxial growth of vertical GaAs nanowires on Si(111) substrates is demonstrated by metal-organic chemical vapor deposition via a vapor-liquid-solid growth mechanism. Systematic experiments indicate that substrate pretreatment, pregrowth alloying temperature, and growth temperature are all crucial to vertical epitaxial growth. Nanowire growth rate and morphology can be well controlled by the growth temperature, the metal-organic precursor molar fraction, and the molar V/III ratio. The as-grown GaAs nanowires have a predominantly zinc-blende crystal structure along a <111> direction. Crystallographic {111} stacking faults found perpendicular to the growth axis could be almost eliminated via growth at high V/III ratio and low temperature. Single nanowire field effect transistors based on unintentionally doped GaAs nanowires were fabricated and found to display a strong effect of surface states on their transport properties.

InGaN/GaN Schottky Diodes With Enhanced Voltage Handling Capability for Varactor Applications

In this letter, we demonstrate a design of GaN-based Schottky diodes for microwave varactor applications. Using suitable InGaN surface layer structures, the Schottky diodes exhibit an enhanced high-voltage handling capability. The breakdown voltage is increased by ~40 V, and the reverse leakage current is significantly suppressed under high reverse voltages. The breakdown-voltage enhancement and leakage-current suppression are attributed to a reduction in the surface electric field and an increase in the electron tunneling distance due to polarization-induced charges at the InGaN/GaN interface.

Scanned electrical probe characterization of carrier transport behavior in InAs nanowires

Conductive atomic force microscopy combined with detailed calculations of electronic subband structure has been used to analyze electron transport behavior across the transition between ballistic and drift/diffusive motion in InAs nanowires grown by metal organic chemical vapor deposition. Using a conducting tip in an atomic force microscope as a local, positionable electrical probe, InAs nanowire resistance as a function of electron transport distance within the nanowire has been measured. For distance of ∼200nm or less, this resistance is observed to be nearly independent of distance, while for larger distances the resistance increases linearly with distance. Analytical calculations indicate that a resistance only weakly dependent on distance should be observed for distances up to a few times the electron mean free path, and calculations of the mean free path using a variety of approaches yield values in the range of ∼50nm, very consistent with the observation of distance-independent resistance for tran...

Analysis of Reverse Leakage Current and Breakdown Voltage in GaN and InGaN/GaN Schottky Barriers

A study of the reverse-leakage-current mechanisms in metal-organic-chemical-vapor-deposition (MOCVD)-grown GaN Schottky-barrier diodes is presented. An analysis is carried out of the characteristics of GaN Schottky diodes as well as of diodes with an InGaN surface layer to suppress the reverse leakage current and increase the breakdown voltage. The experimental results of the diodes with InGaN surface layers showed a ~ 40-V breakdown voltage increase and a significant leakage-current reduction under high reverse bias, in comparison with the design with GaN only. Such improvements are attributed to the reduced surface electric field and the increased electron tunneling distance induced by the polarization charges at the InGaN/GaN interface. We also report the effect of a high-pressure (near atmospheric pressure) MOCVD growth technique of the GaN buffer layer to further improve the leakage current and breakdown voltage.

Effects of the gas ambient in thermal activation of Mg-doped p-GaN on Hall effect and photoluminescence

The effects of thermal annealing in N2, O2, Ar or mixed gas ambient on the electrical and optical properties of metal-organic-chemical-vapor-deposition-grown Mg-doped p-type GaN were investigated by Hall effect and photoluminescence. A systematical study on optimizing annealing gas combination, time, and temperature for achieving high activation efficiency of Mg acceptors was conducted simultaneously. High hole concentration of 9.07 × 1017 cm−3 and low resistivity of 0.622 Ω-cm using the optimized annealing condition were achieved. In agreement with some previous studies, annealing in the mixed gas ambient of N2 and O2 provided significant improvements in activation efficiency of Mg acceptors compared with annealing in pure O2, N2, or Ar. The room- and low-temperature photoluminescence spectra measured from the samples annealed in N2-rich ambient showed significantly higher photoluminescence intensity at both 2.8- and 3.2-eV band transitions. Consistent with the study of others, the experimental results o...

InGaN/GaN microwave varactors with high Q, high-breakdown voltage and high linearity

InGaN-GaN Schottky-diode microwave varactors with Q >; 100 at 1 GHz, breakdown voltage >; 120 V, and OIP3 >; 71 dBm are reported. The combination of Q, voltage handling capability, and OIP3 represents advancement from any other reported varactors, and is important for applications in adaptive and tunable base-station radio front-ends.