Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar $c$-plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar $(20\overline{2}1)$ and $(20\overline{2}\overline{1})$ LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.

This paper reports a comprehensive study on the anisotropic electrical properties of vertical (201) and (010) β -Ga₂O₃ Schottky barrier diodes (SBDs). The devices were fabricated on single-crystal substrates grown by an edge-defined film-fed growth method. The temperature-dependent current-voltage (I-V) and capacitance-voltage (C-V) characteristics were systematically measured, analyzed, and compared. The (201) and (010) SBDs exhibited on-resistances (RON) of 0.56 and 0.77 mΩ · cm², turn-ON voltages (VON) of 1.0 and 1.3 V, Schottky barrier heights (SBHs) of 1.05 and 1.20 eV, electron mobilities of 125 and 65 cm²/(V· s), respectively, with an on-current of ~ 1.3 kA/cm² and on/off ratio of ~ 10⁹. The (010) SBD had a larger VON and SBH due to anisotropic surface properties (i.e., surface Fermi level pinning and band bending), as supported by X-ray photoelectron spectroscopy measurements. Temperature-dependent I-V also revealed the inhomogeneous nature of the SBH in both devices, where the (201) SBD showed a more uniform SBH distribution. The homogeneous SBH was also extracted: 1.33 eV for the (201) SBD and 1.53 eV for the (010) SBD. The reverse leakage current of the devices was well described by the two-step trap-assisted tunneling model and the 1-D variable range hopping conduction model. The (201) SBD showed a larger leakage current due to its lower SBH and/or smaller activation energy, and thus a smaller breakdown voltage. These results indicate that the crystalline anisotropy of β -Ga₂O₃ can affect the electrical properties of vertical SBDs and should be taken into consideration when designing β -Ga₂O₃ electronics.

Ultrawide bandgap (UWBG) materials such as diamond, Ga₂O₃, hexagonal boron nitride (h-BN) and AlN, are a new class of semiconductors that possess a wide range of attractive properties, including very large bandgap, high critical electric field, high carrier mobility and chemical inertness. Due to these outstanding characteristics, UWBG materials are promising candidates to enable high-performance devices for power electronics, ultraviolet photonics, quantum sensing and quantum computing applications. Despite their great potential, the research of UWBG semiconductors is still at a nascent stage and represents a challenging interdisciplinary research area of physics, materials science and devices engineering. In this review, the material properties, synthesis methods and device applications of UWBG semiconductors diamond, Ga₂O₃, h-BN and AlN will be presented and their recent progress, challenges and research opportunities will be discussed.

We perform comprehensive studies on the fundamental loss mechanisms in III-nitride waveguides in the visible spectral region. Theoretical analysis shows that free carrier loss dominates for GaN under low photon power injection. When optical power increases, the two photon absorption loss becomes important and eventually dominates when photon energy above half-bandgap of GaN. When the dimensions of the waveguides reduce, the sidewall scattering loss will start to dominate. To verify the theoretical results, a high performance GaN-on-sapphire waveguide was fabricated and characterized. Experimental results are consistent with the theoretical findings, showing that under high power injection the optical loss changed significantly for GaN waveguides. A low optical loss ~2 dB/cm was achieved on the GaN waveguide, which is the lowest value ever reported for the visible spectral range. The results and fabrication processes developed in this work pave the way for the development of III-nitride integrated photonics in the visible and potentially ultraviolet spectral range for nonlinear optics and quantum photonics applications.

Beta-phase gallium oxide (β-Ga₂O₃) is a cutting-edge ultrawide bandgap (UWBG) semiconductor, featuring a bandgap energy of around 4.8 eV and a highly critical electric field strength of about 8 MV/cm. These properties make it highly suitable for next-generation power electronics and deep ultraviolet optoelectronics. Key advantages of β-Ga₂O₃ include the availability of large-size single-crystal bulk native substrates produced from melt and the precise control of n-type doping during both bulk growth and thin-film epitaxy. A comprehensive understanding of the fundamental growth processes, control parameters, and underlying mechanisms is essential to enable scalable manufacturing of high-performance epitaxial structures. This review highlights recent advancements in the epitaxial growth of β-Ga₂O₃ through various techniques, including Molecular Beam Epitaxy (MBE), Metal-Organic Chemical Vapor Deposition (MOCVD), Hydride Vapor Phase Epitaxy (HVPE), Mist Chemical Vapor Deposition (Mist CVD), Pulsed Laser Deposition (PLD), and Low-Pressure Chemical Vapor Deposition (LPCVD). This review concentrates on the progress of Ga₂O₃ growth in achieving high growth rates, low defect densities, excellent crystalline quality, and high carrier mobilities through different approaches. It aims to advance the development of device-grade epitaxial Ga₂O₃ thin films and serves as a crucial resource for researchers and engineers focused on UWBG semiconductors and the future of power electronics.