A.-B. Chen

Theory of AlN, GaN, InN and their alloys

Abstract This review focuses on the fundamental properties of III–V compound semiconductors from a theoretical or computational standpoint. Its purpose is to summarize the contributions of electronic structure theory to the present context and to provide some foundations for the modern techniques. This will enable one to assess the limitations of the techniques employed previously.

Structural properties of bismuth-bearing semiconductor alloys

Materials currently used for detection in the infrared spectral region have notoriously poor structural properties. In search of a better narrow‐gap material, we have addressed the structural properties of bismuth‐bearing III‐V semiconductor alloys theoretically. Because the Bi compounds are not known to form zinc‐blende structures, only the anion‐substituted alloys InPBi, InAsBi, and InSbBi are considered candidates as narrow‐gap semiconductors. We calculate the bond energies and lengths for the zinc‐blende Bi compounds and their diluted and concentrated alloys. Strain coefficients for the compounds are calculated, and predictions for the mixing enthalpies, miscibility gaps, and critical temperatures are made. Miscibility calculations indicate that InSbBi will be the most miscible, and because of the large lattice mismatch of the constituents, InPBi will be the most difficult to mix. Tendencies toward cluster formation and deviations from randomness in the alloys are considered. Calculations of the hardn...

Bandstructure effect on high-field transport in GaN and GaAlN

The velocity-field characteristics in zinc-blende GaN are calculated from the Boltzmann equation, using realistic energy bands taken from ab initio theory. The drift velocity and the high-field negative differential resistance are shown to be largely determined by the inflection point in the bands centered around the Γ valley, instead of the usual intervalley scattering. We analyze the relative importance of these competing mechanisms for GaN and Al0.5Ga0.5N. The importance of this anomaly to device properties is also discussed.

Near band edge absorption spectra of narrow‐gap III–V semiconductor alloys

Near band edge absorption spectra of the narrow‐gap semiconductor alloys InxTl1−xP, InxTl1−xAs, and InxTl1−xSb were calculated and compared with those of HgxCd1−xTe. To test accuracy, we compared the calculated absorption spectra in GaAs with experimental results and found good agreement. Within 50 meV from the absorption edge, the absorption coeffi cient of InxTl1−xP is found to have about the same magnitude as that in HgxCd1−xTe and GaAs, whereas that in InxTl1−xAs and InxTl1−xSb is much smaller. This result and other merits found from previous studies indicate that InxTl1−xP has a potential to compete favorably with HgxCd1−xTe for long‐wavelength infrared applications.

Accurate calculation of Auger rates in infrared materials

The Auger recombination rates in small-gap semiconductor alloys are calculated using full band structures with electron–electron interactions in Coulomb and phonon fields. We find that the results are sensitive to band structure details and the calculated minority carrier lifetimes can differ by two orders of magnitude depending on the approximations used to describe the energy bands and wave functions. The full band structure results agree well with experiments in Hg0.78Cd0.22Te. Similar calculations were carried out for lifetimes in In0.67Tl0.33P, In0.85Tl0.15As, and In0.92Tl0.08Sb as a function of temperature. The minority carrier lifetimes in In0.67Tl0.33P and In0.92Tl0.08Sb are shorter than that in Hg0.78Cd0.22Te at all temperatures. However, the low-temperature minority carrier lifetime in In0.85Tl0.15As is an order of magnitude longer than that in Hg0.78Cd0.22Te. Our calculations further suggest a possibility of increasing the lifetimes of minority carriers by decreasing the density of states insid...

Temperature dependence of band gaps in HgCdTe and other semiconductors

Band-edge shifts induced by the electron-phonon interaction are calculated for HgCdTe alloys and various semiconductor compounds starting from accurate zero-temperature band structures. The calculated temperature variation of gaps agrees with experiments to better than 10% in all materials except InAs and InSb where the deviation is about 50%. While the simple picture that the intra (inter)-band transitions reduce (increase) the gap still holds, we show that both the conduction band edge Ec and valence band edge Ev move down in energy. These shifts in Ev affect the valence band offsets in heterojunctions at finite temperature. The temperature variations of valence band offset and the electron effective mass are also reported.

Ballistic transport in semiconductor alloys

The electronic structure of semiconductor compounds GaAs, InAs, and InP and alloys Ga0.5In0.5As, Ga0.7Al0.3As, and InP0.5As0.5, obtained in the coherent potential approximation, is used to calculate the group velocity and velocity relaxation time limited by longitudinal optical phonons, alloy disorder, and ionized impurities as a function of electron energy at 300 K. The nonparabolic nature of the band structure is found to severely limit the electron mean free path. With the types of interactions considered to date, the presence of L valleys does not limit the mean free path of electrons moving in the 〈100〉 direction. At 1018‐cm−3 doping, electron‐electron interactions reduce the mean free path by only 15% to 20%. InAs and GaInAs alloys offer advantages over all the other materials for devices with base widths greater than 500 A; however, for thinner devices, ∼100 A, no material is appreciably better than GaAs, the III‐V compound currently under best control. The ballistic device‐related properties of se...

Cleavage energies in semiconductors

We present a method for the calculation of the surface and cleavage energies, Eγ, for semiconductors, based on a tight‐binding Green’s function approach and a difference‐equation solution for the layered structure. Energies are calculated for a representative group of semiconductors, and cleavage energies are found to agree well with the available experimental data. We find ESiγ(111)=1360 ergs/cm2, and Eγ(110)=1000, 180, and 120 ergs/cm2 for GaAs, CdTe, and HgTe, respectively.

Velocity‐field characteristics of III‐V semiconductor alloys: Band structure influences

We have calculated the velocity‐field characteristics of semiconductor alloys based on realistic band structures and have obtained the band structures and alloy‐scattering rates from a generalization of the coherent potential approximation method. Although we use proper band structures, we still consider a single electron‐temperature model. The results agree surprisingly well with experiments, and suggest that InP‐based alloys are good candidates for high‐speed devices.

Below band-gap optical absorption in semiconductor alloys

We have used accurate Hamiltonians and resulting wave functions to calculate the two-photon absorption coefficient and the free-carrier absorption coefficient in InAs and a HgCdTe alloy with the same band gap. Detailed results are obtained for the dependencies of the absorption on photon energy and incident intensity. Optical matrix elements are calculated from the wave number dependent wave functions. We have further solved the appropriate steady-state differential equation, with the calculated values of absorption coefficients, for depth dependence of the intensity in the material. We find that the nonlinear absorption in a HgCdTe alloy is about 100% larger than that in InAs of the same band gap and window thickness.