Publications

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We present progress on the synthesis of semimetal Cd3As2 by metal–organic chemical-vapor deposition (MOCVD). Specifically, we have optimized the growth conditions needed to obtain technologically useful growth rates and acceptable thin-film microstructures, with our studies evaluating the effects of varying the temperature, pressure, and carrier-gas type for MOCVD of Cd3As2 when performed using dimethylcadmium and tertiarybutylarsine precursors. In the course of the optimization studies, exploratory Cd3As2 growths are attempted on GaSb substrates, strain-relaxed InAs buffer layers grown on GaSb substrates, and InAs substrates. Notably, only the InAs-terminated substrate surfaces yield desirable results. Extensive microstructural studies of Cd3As2 thin films on InAs are performed by using multiple advanced imaging microscopies and x-ray diffraction modalities. The studied films are 5–75 nm in thickness and consist of oriented, coalesced polycrystals with lateral domain widths of 30–80 nm. The most optimized films are smooth and specular, exhibiting a surface roughness as low as 1.0 nm rms. Under cross-sectional imaging, the Cd3As2-InAs heterointerface appears smooth and abrupt at a lower film thickness, ~30 nm, but becomes quite irregular as the average thickness increases to ~55 nm. The films are strain-relaxed with a residual biaxial tensile strain (εxx = +0.0010) that opposes the initially compressive lattice-mismatch strain of Cd3As2 coherent on InAs (εxx = −0.042). Importantly, phase-identification studies find a thin-film crystal structure consistent with the P42/nbc space group, placing MOCVD-grown Cd3As2 among the Dirac semimetals of substantial interest for topological quantum materials studies.

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This work provides the first demonstration of vertical GaN Junction Barrier Schottky (JBS) rectifiers fabricated by etch and regrowth of p-GaN. A reverse blocking voltage near 1500 V was achieved at 1 mA reverse leakage, with a sub 1 V turn-on and a specific on-resistance of 10 mΩ-cm2. This result is compared to other reported JBS devices in the literature and our device demonstrates the lowest leakage slope at high reverse bias. A large initial leakage current is present near zero-bias which is attributed to a combination of inadequate etch-damage removal and passivation induced leakage current.

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Edge termination for vertical power devices presents a significant challenge, as improper termination can result in devices with a breakdown voltage significantly less than the ideal infinite-planar case. Edge termination for vertical GaN devices is particularly challenging due to limitations in ion implantation for GaN, and as such this work investigates a bevel edge termination technique that does not require implantation and has proven to be effective for Si and SiC power devices. However, due to key differences between GaN versus Si and SiC p-n junctions (specifically, a grown versus an implanted junction), this technology needs to be reevaluated for GaN. Simulation results suggest that by leveraging the effective bevel angle relationship, a 10-15° physical bevel angle can yield devices with 85-90% of the ideal breakdown voltage. Results are presented for a negative bevel edge termination on an ideally 2 kV vertical GaN p-n diode.

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