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

The role of extended and point defects, and key impurities such as C, O, and H, on the electrical and optical properties of GaN is reviewed. Recent progress in the development of high reliability contacts, thermal processing, dry and wet etching techniques, implantation doping and isolation, and gate insulator technology is detailed. Finally, the performance of GaN-based electronic and photonic devices such as field effect transistors, UV detectors, laser diodes, and light-emitting diodes is covered, along with the influence of process-induced or grown-in defects and impurities on the device physics.

Gan : processing, defects, and devices

Calculations of ground-state and excited-state properties of materials have been one of the major goals of condensed matter physics. Ground-state properties of solids have been extensively investigated for several decades within the standard density functional theory. Excited-state properties, on the other hand, were relatively unexplored in ab initio calculations until a decade ago. The most suitable approach up to now for studying excited-state properties of extended systems is the Green function method. To calculate the Green function one requires the self-energy operator which is non-local and energy dependent. In this article we describe the GW approximation which has turned out to be a fruitful approximation to the self-energy. The Green function theory, numerical methods for carrying out the self-energy calculations, simplified schemes, and applications to various systems are described. Self-consistency issue and new developments beyond the GW approximation are also discussed as well as the success and shortcomings of the GW approximation.

https://iopscience.iop.org/article/10.1088/0034-4885/61/3/002/pdf

The GW method

We present $GW$ calculations for small and large gap systems comprising typical semiconductors (Si, SiC, GaAs, GaN, ZnO, ZnS, CdS, and AlP), small gap semiconductors (PbS, PbSe, and PbTe), insulators (C, BN, MgO, and LiF), and noble gas solids (Ar and Ne). It is shown that the ${G}_{0}{W}_{0}$ approximation always yields too small band gaps. To improve agreement with experiment, the eigenvalues in the Green's function $G$ $(G{W}_{0})$ and in the Green's function and the dielectric matrix $(GW)$ are updated until self-consistency is reached. The first approximation leads to excellent agreement with experiment, whereas an update of the eigenvalues in $G$ and $W$ gives too large band gaps for virtually all materials. From a pragmatic point of view, the $G{W}_{0}$ approximation thus seems to be an accurate and still reasonably fast method for predicting quasiparticle energies in simple $sp$-bonded systems. We furthermore observe that the band gaps in materials with shallow $d$ states (GaAs, GaN, and ZnO) are systematically underestimated. We propose that an inaccurate description of the static dielectric properties of these materials is responsible for the underestimation of the band gaps in $G{W}_{0}$, which is itself a result of the incomplete cancellation of the Hartree self-energy within the $d$ shell by local or gradient corrected density functionals.

Self-consistent G W calculations for semiconductors and insulators

The results of first-principles full-potential linear muffin-tin orbital calculations of the elastic constants and related structural and electronic properties of BN, AlN, GaN, and InN in both the zinc-blende and wurtzite structures are presented. The results include all of the equilibrium lattice constants, the bulk moduli, the TO-phonon frequencies at \ensuremath{\Gamma}, their mode Gr\"uneisen parameters, the full set of cubic elastic constants, and deformation potentials. The elastic constants for the wurtzite crystals are first obtained from those calculated for zinc blende by Martin's transformation method. The components related to strains along the c axis (${\mathit{C}}_{13}$ and ${\mathit{C}}_{33}$) are found to be less accurate than the others. An elaboration of Martin's approach utilizing first-principles calculation for distortions which maintains hexagonal symmetry but allows for a nonideal c/a ratio is implemented. As a byproduct of the relaxation calculations of the wurtzite internal parameter u we also obtain the ${\mathit{A}}_{1}$ and an estimate of the ${\mathit{E}}_{1}$ TO-phonon frequencies in the hexagonal materials. Good agreement is obtained with recent experimental results for the elastic constants of wurtzite AlN and GaN and zinc-blende BN as well as for the other properties mentioned above for all materials. Our results provide predictions for the remaining crystal structure materials combinations for which no direct experimental data are presently available. From these results and experimental LO-TO splittings, we determine the bond-stretching and bond-bending parameters \ensuremath{\alpha} and \ensuremath{\beta} of Keating's semiempirical valence-force-field model. We use this model to rationalize some of the observed trends in the behavior with the cation. The shift and splittings of the energy bands due to strains are used to obtain a complete set of deformation potentials for the zinc-blende crystals at symmetry points for several of the important eigenvalues. We also define deformation potentials for the valence-band maximum of the wurtzite structure and relate them to the corresponding [111] strain deformation and optical mode deformation potentials in zinc blende. \textcopyright{} 1996 The American Physical Society.

Elastic constants and related properties of tetrahedrally bonded BN, AlN, GaN, and InN

GaN films have been epitaxially grown onto (001) Si by electron cyclotron resonance microwave‐plasma‐assisted molecular‐beam epitaxy, using a two‐step growth process, in which a GaN buffer is grown at relatively low temperatures and the rest of the film is grown at higher temperatures. This method of film growth was shown to lead to good single‐crystalline β‐GaN and to promote lateral growth resulting in smooth surface morphology. The full width at half‐maximum of the x‐ray rocking curve in the best case was found to be 60 min. Optical‐absorption measurements indicate that the band gap of β‐GaN is 3.2 eV and the index of the refraction below the absorption edge is 2.5. Conductivity measurements indicate that the films may have a carrier concentration below 1017 cm−3.

Epitaxial growth and characterization of zinc‐blende gallium nitride on (001) silicon

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Epitaxial growth and characterization of zinc‐blende gallium nitride on (001) silicon