The status of research on both wurtzite and zinc-bien de GaN, AlN, and InN and their alloys is reviewed including exciting recent results. Attention is paid to the crystal growth techniques, structural, optical, and electrical properties of GaN, AIN, InN, and their alloys. The various theoretical results for each material are summarized. We also describe the performance of several device structures which have been demonstrated in these materials. Near-term goals and critical areas in need of further research in the III-V nitride material system are identified.

In the past several years, research in each of the wide-band-gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high-temperature electronics and short-wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high-power operation. The present advanced stage of development of SiC substrates and metal-oxide-semiconductor technology makes SiC the leading contender for high-temperature and high-power applications if ohmic contacts and interface-state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high-temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra. Blue SiC light-emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN-based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short-wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN-based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe-based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide-band-gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

We review the family of optoelectronic devices whose performance is enhanced by placing the active device structure inside a Fabry-Perot resonant microcavity. Such resonant cavity enhanced (RCE) devices benefit from the wavelength selectivity and the large increase of the resonant optical field introduced by the cavity. The increased optical field allows RCE photodetector structures to be thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. Off-resonance wavelengths are rejected by the cavity making RCE photodetectors promising for low crosstalk wavelength division multiplexing (WDM) applications. RCE optical modulators require fewer quantum wells so are capable of reduced voltage operation. The spontaneous emission spectrum of RCE light emitting diodes (LED) is drastically altered, improving the spectral purity and directivity. RCE devices are also highly suitable for integrated detectors and emitters with applications as in optical logic and in communication networks. This review attempts an encyclopedic overview of RCE photonic devices and systems. Considerable attention is devoted to the theoretical formulation and calculation of important RCE device parameters. Materials criteria are outlined and the suitability of common heteroepitaxial systems for RCE devices is examined. Arguments for the improved bandwidth in RCE detectors are presented intuitively, and results from advanced numerical simulations confirming the simple model are provided. An overview of experimental results on discrete RCE photodiodes, phototransistors, modulators, and LEDs is given. Work aimed at integrated RCE devices, optical logic and WDM systems is also covered. We conclude by speculating what remains to be accomplished to implement a practical RCE WDM system. 8

The valence-band discontinuity at a wurtzite GaN/AlN(0001) heterojunction is measured by x-ray photoemission spectroscopy. The method first measures the core level binding energies with respect to the valence-band maximum in both GaN and AlN bulk films. The precise location of the valence-band maximum is determined by aligning prominent features in the valence-band spectrum with calculated densities of states. Tables of core level binding energies relative to the valence-band maximum are reported for both GaN and AlN. Subsequent measurements of separations between Ga and Al core levels for thin overlayers of GaN film grown on AlN and vice versa yield a valence-band discontinuity of ΔEV=0.8±0.3 eV in the standard type I heterojunction alignment.

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We report epitaxial GaN layers grown on 6H-SiC (0001) substrates. A two stage substrate preparation procedure is described which effectively removes oxygen from the SiC substrate surface without the need of elaborate high temperature processing. In the first step, dangling Si bonds on the substrate surface are hydrogen passivated using a HF dip before introduction into vacuum. Second, the substrate is treated with a hydrogen plasma reducing the amount of oxygen-carbon bonding to below the x-ray photoemission detection limit. Upon heating in the molecular beam epitaxy (MBE) growth chamber, the SiC substrates are observed to have a sharp (1×1) reconstruction with Kikuchi lines readily visible. GaN epilayers deposited on AlN buffer layers by plasma enhanced MBE show sharp x-ray diffraction and photoluminescence peaks.

Erratum: Use of Raman spectroscopy to characterize strain III-V epilayers: Application to InAs on GaAs(001) grown by molecularbeam epitaxy [J.