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.

Large‐band‐gap SiC, III‐V nitride, and II‐VI ZnSe‐based semiconductor device technologies

During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calcu...

III–nitrides: Growth, characterization, and properties

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

The formation of the Schottky barrier height (SBH) is a complex problem because of the dependence of the SBH on the atomic structure of the metal-semiconductor (MS) interface. Existing models of the SBH are too simple to realistically treat the chemistry exhibited at MS interfaces. This article points out, through examination of available experimental and theoretical results, that a comprehensive, quantum-mechanics-based picture of SBH formation can already be constructed, although no simple equations can emerge, which are applicable for all MS interfaces. Important concepts and principles in physics and chemistry that govern the formation of the SBH are described in detail, from which the experimental and theoretical results for individual MS interfaces can be understood. Strategies used and results obtained from recent investigations to systematically modify the SBH are also examined from the perspective of the physical and chemical principles of the MS interface.

The physics and chemistry of the Schottky barrier height

Heterojunction band offset engineering

Optical characterization and band offsets in ZnSe-ZnSxSe

We present low‐temperature photoluminescence and transmission electron microscopy data to show that two transitions I0V at ∼2.774 eV and Y0 at ∼2.60 eV, frequently observed in unintentionally doped zinc selenide epitaxial layers, are directly related to structural defects. It is shown that these transitions are strong in those samples which have very low background impurities and high density of structural defects and weak in those cases that have either high background impurities or low density of structural defects.

Correlation between radiative transitions and structural defects in zinc selenide epitaxial layers

The elastic strains in regions near the top surface of strained-layer structures can be quite different from those in deeper portions of the samples. This effect has been established in epitaxial layers of ZnSe grown on GaAs and ZnSe-$\mathrm{Zn}{\mathrm{S}}_{x}{\mathrm{Se}}_{1\ensuremath{-}x}$ strained-layer superlattices. The depth dependence of the strains was determined with Raman scattering measurements performed under laser excitation below and above the band gap. The strain values near the top surface are driven by a stronger relaxation of the in-plane lattice constants towards equilibrium. This relaxation appears to be independent of the generation of misfit dislocations.

Depth profiling of elastic strains in lattice-mismatched semiconductor heterostructures and strained-layer superlattices.

The structural properties of ZnSe doped with N, in the concentration range of 1×1018–2×1019 cm−3, were characterized by transmission electron microscopy, x‐ray diffraction, and Raman spectroscopy techniques. The relaxation of the lattice mismatch induced compressive strain between ZnSe and GaAs is less for N doped layers for a given ZnSe thickness. The smaller amount of strain relaxation with N doping results in layers that contain residual compressive strain up to thicknesses of at least 1.7 μm. In addition, the misfit dislocation array becomes a regular rectangular grid when N is incorporated in ZnSe layers. The ZnSe lattice constant, as measured by x‐ray diffraction, decreases as the N concentration increases. The reduction in lattice constant, however, is greater than can be explained by the shorter Zn‐N bond distance of model predictions. We attribute the excess lattice contraction to the generation of point defects accompanying N doping. The Raman spectra display a broadening of the linewidth as the N concentration increases, which supports the notion of point defect creation with N doping.

Effect of N doping on the structural properties of ZnSe epitaxial layers grown by molecular beam epitaxy

Chemical composition, core-level line shape, and electron affinity measurements are performed on the Se-terminated (2\ifmmode\times\else\texttimes\fi{}1) and Zn-terminated c(2\ifmmode\times\else\texttimes\fi{}2) ZnSe(100) surfaces. The (2\ifmmode\times\else\texttimes\fi{}1) reconstruction corresponds to a complete monolayer of Se dimers with one filled dangling bond per surface atom. The c(2\ifmmode\times\else\texttimes\fi{}2) reconstruction corresponds to a half-monolayer vacancy structure which leaves twofold-coordinated Zn atoms in the top layer. The 150-meV decrease in surface electron affinity at the (2\ifmmode\times\else\texttimes\fi{}1)\ensuremath{\rightarrow}c(2\ifmmode\times\else\texttimes\fi{}2) transition is consistent with this model and results from a dipole induced by the charge transfer required to empty and fill all cation and anion dangling bonds on the c(2\ifmmode\times\else\texttimes\fi{}2) surface, respectively.

D. J. Olego

Papers

Optical characterization and band offsets in ZnSe-ZnSxSe

Correlation between radiative transitions and structural defects in zinc selenide epitaxial layers

Depth profiling of elastic strains in lattice-mismatched semiconductor heterostructures and strained-layer superlattices.

Effect of N doping on the structural properties of ZnSe epitaxial layers grown by molecular beam epitaxy

ZnSe(100) surface: Atomic configurations, composition, and surface dipole