AbstractAlthough the first lasers invented operated in the visible, the first on-chip devices were optimized for near-infrared (IR) performance driven by demand in telecommunications. However, as the applications of integrated photonics has broadened, the wavelength demand has as well, and we are now returning to the visible (Vis) and pushing into the ultraviolet (UV). This shift has required innovations in device design and in materials as well as leveraging nonlinear behavior to reach these wavelengths. This review discusses the key nonlinear phenomena that can be used as well as presents several emerging material systems and devices that have reached the UV–Vis wavelength range.

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Vertical GaN-on-GaN Schottky barrier diodes based on as-grown and regrown samples were fabricated to investigate the effects of the etch-then-regrow process on device performance. The surface roughness increased slightly after dry etching and decreased after regrowth. According to X-ray diffraction results, the etch-then-regrow process caused a slight increase of defect density due to increased edge dislocations. Schottky parameters extracted from forward current-voltage curves, such as turn-on voltages of 0.74 V and 0.72 V, ideality factors of 1.07 and 1.10, and barrier heights of 1.07 eV and 1.05 eV, were obtained for diodes based on the regrown and as-grown samples, respectively. The breakdown voltage of the regrown sample was much lower than the as-grown sample. The regrowth interface can be regarded as a n-doping GaN layer due to the high interface charge density after the etch-then-regrown process. This equivalent n-doping GaN layer reduced the effective thickness of the UID-GaN under the Schottky contact thus causing lower breakdown voltage for the regrown sample. Poole-Frenkel emission and trap-assisted tunneling processes were responsible for the leakage of both as-grown and regrown samples according to the temperature dependence of the reverse currents.INDEX TERMS Schottky barrier diodes, GaN-on-GaN, regrow, leakage, interface.

This work demonstrates the construction of p-n heterojunctions between mechanically exfoliated beta-phase gallium oxide (β-Ga2O3) and p-GaN. The detailed mechanical exfoliation process was developed and can be used for further device applications. The atomic force microscopy study showed that the exfoliated β-Ga2O3 flakes had a very smooth surface with a roughness of 0.65 nm. Transmission electron microscopy revealed a clearly defined interface between the exfoliated β-Ga2O3 and p-GaN. The p-n heterojunction exhibited a turn-on voltage of 3.6 V and a rectification ratio of ∼105. The heterojunction also showed good thermal performance up to 200 °C. Ideality factors and turn-on voltages decrease with temperature, tending toward the ideal threshold voltage of 3.2 V as determined by Silvaco simulations. This work provides valuable information on a mechanically exfoliated β-Ga2O3/GaN p-n heterojunction, which opens up the opportunities for a variety of photonic and electronic applications.

The reverse bias current-voltage (I–V) characteristics of GaN-based light-emitting diodes (LEDs) were investigated. The leakage current exhibits exponential dependence on the bias voltage with different exponents for various voltage ranges. The leakage current is closely related to the density of dislocations. The number of dislocations in GaN was determined by atomic force microscopy combined with hot H3PO4 etching. Dislocations with a screw component in the GaN films were found to have a strong influence on the reverse leakage current of LEDs. The dislocation electrical activity in GaN grown on c-plane sapphire is different from that in GaN grown on a-plane sapphire.