Vertical heterojunction NiO/β n-Ga2O/n+ Ga2O3 rectifiers employing NiO layer extension beyond the rectifying contact for edge termination exhibit breakdown voltages (VB) up to 4.7 kV with a power figure-of-merits, VB2/RON of 2 GW·cm−2, where RON is the on-state resistance (11.3 mΩ cm2). Conventional rectifiers fabricated on the same wafers without NiO showed VB values of 840 V and a power figure-of-merit of 0.11 GW cm−2. Optimization of the design of the two-layer NiO doping and thickness and also the extension beyond the rectifying contact by TCAD showed that the peak electric field at the edge of the rectifying contact could be significantly reduced. The leakage current density before breakdown was 144 mA/cm2, the forward current density was 0.8 kA/cm2 at 12 V, and the turn-on voltage was in the range of 2.2–2.4 V compared to 0.8 V without NiO. Transmission electron microscopy showed sharp interfaces between NiO and epitaxial Ga2O3 and a small amount of disorder from the sputtering process.

The band alignment of sputtered NiO on β-Ga2O3 was measured by x-ray photoelectron spectroscopy for post-deposition annealing temperatures up to 600 °C. The band alignment is type II, staggered gap in all cases, with the magnitude of the conduction and valence band offsets increasing monotonically with annealing temperature. For the as-deposited heterojunction, ΔE V = −0.9 eV and ΔE C = 0.2 eV, while after 600 °C annealing the corresponding values are ΔE V = −3.0 eV and ΔE C = 2.12 eV. The bandgap of the NiO was reduced from 3.90 eV as-deposited to 3.72 eV after 600 °C annealing, which accounts for most of the absolute change in ΔE V−ΔE C. Differences in thermal budget may be at least partially responsible for the large spread in band offsets reported in the literature for this heterojunction. Other reasons could include interfacial disorder and contamination. Differential charging, which could shift peaks by different amounts and could potentially be a large source of error, was not observed in our samples.

NiO/β-Ga2O3 vertical rectifiers exhibit near-temperature-independent breakdown voltages (VB) of >8 kV to 600K. For 100 µm diameter devices, the power figure of merit (VB)2/ RON, where RON is the on-state...

Optimized vertical heterojunction rectifiers with a diameter of 100 µm, consisting of sputter-deposited p-type NiO forming a p–n junction with thick (10 µm) Ga2O3 drift layers grown by halide vapor phase epitaxy (HVPE) on (001) Sn-doped (1019 cm−3) β-Ga2O3 substrates, exhibited breakdown voltages >8 kV over large areas (>1 cm2). The key requirements were low drift layer doping concentrations (<1016 cm3), low power during the NiO deposition to avoid interfacial damage at the heterointerface and formation of a guard ring using extension of the NiO beyond the cathode metal contact. Breakdown still occurred at the contact periphery, suggesting that further optimization of the edge termination could produce even larger breakdown voltages. On-state resistances without substrate thinning were <10 mΩ.cm−2, leading to power figure-of-merits >9 GW.cm−2. The devices showed an almost temperature-independent breakdown to 600 K. These results show the remarkable potential of NiO/Ga2O3 rectifiers for performance beyond the limits of both SiC and GaN. The important points to achieve the excellent performance were: (1) low drift doping concentration, (2) low power during the NiO deposition and (3) formation of a guard ring.

P3HT, as one of the most important conjugated polymers, has been reported to possess low mobility and poor stretchability. In this study, two polythiophenes are prepared by attaching ester-substituted, biaxially extended conjugated side chains with backbones of 3-(thiophen-2-yl)-6-(thiophen-3-yl)thieno-[3,2-b]thiophene-bithiophene (PDCTT2T) and 3-(thiophen-2-yl)-6-(thiophen-3-yl)thieno[3,2-b]thiophene-difluorobithiophene (PDCTT2T-F) and compared with their respective counterparts without the biaxially extended side chains (thieno[3,2b]thiophene-bithiophene and thieno[3,2-b]thiophene-difluorobithiophene backbonesfor PDCTT and PDCTT-F). Through investigating mobility−stretchability properties of these four polymers, the synergetic effect of ester-substituted, biaxially extended conjugation and backbone fluorination is demonstrated to improve mobility−stretchability properties of polythiophenes, especially in a highly stretched state. On the one hand, the biaxially extended side chains implant a more amorphous structure and reinforce the intramolecular charge transfer between polymer backbones. On the other hand, the backbone fluorination confers to a rigidified polymer backbone, facilitating effective intrachain charge transport. As a result, PDCTT2T-F delivers superior mobility (0.20 cm 2 V −1 s −1 ) to PDCTT-F (0.054 cm 2 V −1 s −1 ) alongside 11 times higher mobility retention at a 100% strain. In addition, both PDCTT2T and PDCTT2T-F encouragingly retain decent mobility throughout 800 stretching−releasing cycles (at a 60% strain), outperforming their respective parent polymers without side-chain conjugation that show a one-order decrease in mobility. The results shown in this work signify an effective design strategy to fine-tune mobility−stretchability properties of polythiophenes.