Tomihito Miyazaki

Extremely Low On-Resistance and High Breakdown Voltage Observed in Vertical GaN Schottky Barrier Diodes with High-Mobility Drift Layers on Low-Dislocation-Density GaN Substrates

Vertical GaN Schottky barrier diodes (SBDs) were fabricated on freestanding GaN substrates with low dislocation density. High quality n-GaN drift-layer with an electron mobility of 930 cm2 V-1 s-1 was obtained by optimizing the growth conditions by reducing the intensity of yellow luminescence using conventional photoluminescence measurements. The specific on-resistance (RonA) and the breakdown voltage (VB) of the SBDs were 0.71 mΩ cm2 and over 1100 V, respectively. The figure of merit (VB2/RonA) was 1.7 GW/cm2, which is the highest value among previously reported SBDs for both GaN and SiC.

High-Breakdown-Voltage GaN Vertical Schottky Barrier Diodes with Field Plate Structure

Gallium nitride (GaN) vertical Schottky barrier diodes (SBDs) with a SiNx field plate (FP) structure on low-dislocation-density GaN substrates have been designed and fabricated. We have successfully achieved the SBD breakdown voltage (Vb) of 680V with the FP structure, in contrast to that of 400V without the FP structure. There was no difference in the forward current-voltage characteristics with a specific on-resistance (Ron) of 1.1mcm2. The figure of merit V2b/Ron of the SBD with the FP structure was 420MWcm-2. The FP structure and the high quality drift layers grown on the GaN substrates with low dislocation densities have greatly contributed to the obtained results.

Fabrication of High-Density Diamond Nanotips by Electron Beam Lithography

For application to fabricating of diamond field emitters, nanosize hard masks for forming high-density diamond nanotips were produced on polycrystalline diamond wafers by electron beam lithography. Fabricated hard masks are of two kinds: one is a TiN–Al stack mask, which forms a uniquely shaped diamond tip after etching because of the difference in etching characteristics between TiN and Al, and the other is an amorphous Si mask, which replaces the conventional Al mask in order to form much smaller diamond tips. These hard masks were arrayed on a diamond wafer at a pitch of 200 or 300 nm. The diameters of a TiN–Al stack mask and of an amorphous Si mask are 100–110 nm and 70–80 nm, respectively. The density of diamond nanotips fabricated using hard masks is 25 pieces per µm2 (200 nm pitch), and the dispersion in diamond tip height is 5% or less.