Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

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The 2018 GaN power electronics roadmap

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

This Review discusses recent advances towards electric-field control of magnetism in ferromagnetic semiconductors and metals, and in multiferroics. The electrical manipulation of magnetism and magnetic properties has been achieved across a number of different material systems. For example, applying an electric field to a ferromagnetic material through an insulator alters its charge-carrier population. In the case of thin films of ferromagnetic semiconductors, this change in carrier density in turn affects the magnetic exchange interaction and magnetic anisotropy; in ferromagnetic metals, it instead changes the Fermi level position at the interface that governs the magnetic anisotropy of the metal. In multiferroics, an applied electric field couples with the magnetization through electrical polarization. This Review summarizes the experimental progress made in the electrical manipulation of magnetization in such materials, discusses our current understanding of the mechanisms, and finally presents the future prospects of the field.

Control of magnetism by electric fields

The excitation of surface-phonon-polariton (SPhP) modes in polar dielectric crystals and the associ- ated new developments in the field of SPhPs are reviewed. The emphasis of this work is on providing an understand- ing of the general phenomenon, including the origin of the Reststrahlen band, the role that optical phonons in polar dielectric lattices play in supporting sub-diffrac- tion-limited modes and how the relatively long opti- cal phonon lifetimes can lead to the low optical losses observed within these materials. Based on this overview, the achievements attained to date and the potential tech- nological advantages of these materials are discussed for localized modes in nanostructures, propagating modes on surfaces and in waveguides and novel metamaterial designs, with the goal of realizing low-loss nanophoton- ics and metamaterials in the mid-infrared to terahertz spectral ranges.

Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons

Deep-ultraviolet photoluminescence spectroscopy has been employed to study the temperature and compositional dependence of the band gap of AlxGa1−xN alloys in the temperature range between 10 and 800 K. Band-edge emission peaks in AlxGa1−xN alloys were fitted by the Varshni equation to obtain Varshni coefficients, which increase nonlinearly with x. The values of Varshni coefficients obtained for GaN and AlN binary compounds in the present study are in good agreement with the previously reported values. Based on the experimental data, the compositional and temperature dependence of the band gap of AlxGa1−xN alloy has been empirically determined for the entire alloy range.

Temperature and compositional dependence of the energy band gap of AlGaN alloys

Deep ultraviolet photoluminescence spectroscopy was employed to study the impurity transitions in Mg-doped AlGaN alloys. A group of deep level impurity transitions was observed in Mg-doped AlxGa1−xN alloys, which was identified to have the same origin as the previously reported blue line at 2.8eV in Mg-doped GaN and was assigned to the recombination of electrons bound to the nitrogen vacancy with three positive charges (VN3+) and neutral Mg acceptors. Based on the measured activation energies of the Mg acceptors in AlGaN and the observed impurity emission peaks, the VN3+ energy levels in AlxGa1−xN have been deduced for the entire alloy range. It is demonstrated that the presence of high density of VN3+ deep donors translates to the reduced p-type conductivity in AlGaN alloys due to their ability for capturing free holes.

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Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys

AlN epilayers were grown by metal organic chemical vapor deposition on sapphire substrates. X-ray diffraction measurements revealed that the threading dislocation (TD) density, in particular, the edge TD density, decreases considerably with increasing the epilayer thickness. Photoluminescence results showed that the intensity ratio of the band edge emission to the defect related emission increases linearly with increasing the epilayer thickness. Moreover, the dark current of the fabricated AlN metal-semiconductor-metal deep ultraviolet (DUV) photodetectors decreases drastically with the AlN epilayer thickness. The results suggested that one effective way for attaining DUV optoelectronic devices with improved performance is to increase the thickness of the AlN epilayer template, which results in the reduction of the TD density.

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Correlation between optoelectronic and structural properties and epilayer thickness of AlN

https://iopscience.iop.org/article/10.1149/2.0341907jss/pdf

Editors' Choice—Review—Theory and Characterization of Doping and Defects in β-Ga2O3

Deep ultraviolet photoluminescence (PL) spectroscopy has been employed to investigate impurity transitions in Si doped Al-rich AlGaN alloys. In addition to the previously reported donor compensating centers—isolated cation vacancy with three negative charges (VIII)3− and cation vacancy complex with two-negative charges (VIIIcomplex)2−—a group of impurity transitions with higher emission energies has been observed in AlGaN alloys grown under different conditions, which has been assigned to the recombination between shallow donors and cation vacancy complexes with one-negative charge (VIIIcomplex)−1. Similar to (VIII)3− and (VIIIcomplex)2−, the energy levels of (VIIIcomplex)1− deep acceptors in AlxGa1−xN (0⩽x⩽1) alloys are pinned to a common energy level in vacuum. A strong correlation between the resistivity and PL emission intensities of the impurity transitions associated with cation vacancies (and complexes) was found.

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Neeraj Nepal

Papers

Temperature and compositional dependence of the energy band gap of AlGaN alloys

Photoluminescence studies of impurity transitions in Mg-doped AlGaN alloys

Correlation between optoelectronic and structural properties and epilayer thickness of AlN

Editors' Choice—Review—Theory and Characterization of Doping and Defects in β-Ga2O3

Photoluminescence studies of impurity transitions in AlGaN alloys