N. Mark Williams

Temperature-dependent electrical characteristics of bulk GaN Schottky rectifier

The temperature-dependent electrical characteristics of Schottky rectifiers fabricated with a SiO2 field plate on a freestanding n− gallium nitride (GaN) substrate were reported in the temperature range of 298–473K. The Schottky barrier heights evaluated from forward current-voltage measurement revealed an increase of Schottky barrier height and series resistance but a decrease of ideality factor (n) with increasing temperature. However, the Schottky barrier heights evaluated from capacitance-voltage measurement remained almost the same throughout the temperature range measured. The Richardson constant extrapolated from ln(J0∕T2) vs 1∕T plot was found to be 0.029Acm−2K−2. A modified Richardson plot with ln(J0∕T2) vs 1∕nT showed better linearity, and the corresponding effective Richardson constant was 35Acm−2K−2. The device showed a high reverse breakdown voltage of 560V at room temperature. The negative temperature coefficients were found for reverse breakdown voltage, which is indicative of a defect-assi...

Electrical characteristics of bulk GaN-based Schottky rectifiers with ultrafast reverse recovery

A vertical Schottky diode rectifier was fabricated using a bulk n−GaN wafer. Pt Schottky contacts were prepared on the Ga face and full backside ohmic contact was prepared on the N face by using Ti∕Al. The root mean square surface roughnesses of the Ga and N faces are 0.61 and 4.7nm, respectively. A relatively high breakdown field of 5.46kV∕cm was achieved with no additional edge termination. The breakdown field decreases as the size of the device increases. The background electron concentration of the bulk GaN wafer was low (5×1015cm−3), which may lead to a relatively high breakdown field even with no special edge termination. The forward turn-on voltage was as low as 2.4V at the current density of 100A∕cm2. The device exhibited an ultrafast reverse recovery characteristics (reverse recovery time <20ns).

Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors

The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk n-GaN substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of 0.56pA at −10V reverse bias. A responsivity of ∼0.09A∕W at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured (50mW∕m2–2.2kW∕m2). The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of 0.3V was also obtained under a broadband UV illumination.

Characterization of Non-Polar Surfaces in HVPE Grown Gallium Nitride

Non-polar surfaces of HVPE grown GaN were characterized by cathodoluminescence (CL), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). Both of a- and m-plane GaN were prepared by growing thick GaN along the c-axis, and cutting in transverse orientations. The exposed non-polar surfaces were prepared by mechanical polishing (MP) and chemically mechanical polishing (CMP). Non-uniform luminescent characteristics on a- and m-plane GaN were observed in CL images, indicating a higher concentration of impurities in the area of more luminescence. CL spectra from the bulk samples revealed two peaks: 364 nm and 510 nm, related to band edge and impurity defects respectively. The detection by SIMS confirmed that oxygen was inhomogeneously incorporated during the growth of thick GaN layers. Surface qualities of a- and m-plane GaN were also investigated. The lower optical intensities from a-plane GaN at low acceleration voltages indicated more surface damages were introduced during polish. The optical intensity difference from the two samples was reduced at higher acceleration voltages. Similar CL intensities at low acceleration voltages from a- and mplane GaN substrates prepared by CMP indicated improved surface qualities.