Fundamental reflection spectrum and the electronic band structure of chlorine-doped CdTe

The effect of chlorine impurity on the fundamental reflection spectrum and the electronic band structure of cadmium telluride crystals has been studied. At the impurity concentration N _Cl>5.0×10¹⁹ cm^?3, a peak appears in the reflectance spectra. This peak is due to electron transitions at the X point of the Brillouin zone from the upper split valence band to Cl levels lying 0.05 eV above the Γ minimum of the conduction band. The other features in the reflectance spectra and band structure are explained as being due to the effect of spin-orbit splitting at the X point and to indirect electronic transitions from the Cl levels to the Γ minimum.     

Many-body effects on the width of the band gap in Bi2Te2X (X = Te, Se, S) topological insulators

The spectrum of quasiparticles of Bi₂Te₂X (X = Te, Se, S) three-dimensional topological insulators has been theoretically studied in the GW approximation with the inclusion of the spin-orbit interaction in the construction of the Green’s function and self-energy. It has been shown that many-body corrections to the Kohn-Sham states in Bi₂Te₂X increase the fundamental band gap similar to conventional semiconductors. However, the band gap at the Γ point decreases in this case. Gaps in the quasiparticle spectrum obtained in agreement with the experimental data correspond to the difference between the minimum of the conduction band, which is located on the Γ-Z line, and the maximum of the valence band, which lies beyond the symmetric directions in the mirror plane.     

Nitrogen trap bound states in In1−xGaxP

Photoluminescence and absorption data are reported for nitrogen-doped In_1?xGa_xP (0.64 ? x ? 1.0) demonstrating the existence of a shallow N-trap bound state, N_Γ, in the direct-gap composition region. The N_Γ state, which is derived from the Γ conduction band minimum, exists along with a deeper bound state, N_X, derived from the X minima. These states, which are not observed in N-doped GaP, exhibit non-crossing behavior in the composition region of strong coupling between the N_Γ and N_X states. This observation is in accord with recent experimental and theoretical work indicating that the N-trap bound states in the related alloy GaAs_1-xP_x are produced by a combination of long and short range potentials which should also exist in In_1-xGa_xP.     

Camel's back structure of the conduction band in GaP

A simple k · p theory based on Si and the ionic X gap is applied to the conduction band of GaP. It is found that recent evidence may indicate a location of the absolute minima away from the zone boundary. In the directions parallel to 〈100〉, the band structure is highly nonparabolic for carrier energies in the range 1–50 meV.     

On the concentration dependence of the direct and indirect band gaps of substitutional GaPxAs1−x alloys

The variation of the direct and indirect band gaps of the substitutional GaP_xAs_1?x alloy with concentration has been studied by the KKRZ-ATA method. It is found that the gaps plotted against concentrations have little bowing on the low concentration side of GaP. This bowing has been discussed in the light of existing theoretical and experimental results.     

Energy band structure and refractive properties of LiRbSO4 crystals

The energy band structure of mechanically free and compressed LiRbSO₄ single crystals is investigated. It is established that the top of the valence band is located at the D point of the Brillouin zone [k = (0.5, 0.5, 0)], the bottom of the conduction band lies at the Γ point, and the minimum direct band gap E _g is equal to 5.20 eV. The bottom of the conduction band is predominantly formed by the Li s, Li p, Rb s, and Rb p states hybridized with the S p and O p antibonding states. The pressure coefficients corresponding to the energies of the valence and conduction band states and the band gap E _g are determined, and the pressure dependences of the refractive indices n _i are analyzed.     

First principles study on electronic structure of β-Ga2O3

We have studied the electronic structure of β-Ga₂O₃ using the first principles full-potential linearized augmented plane wave method. It is found that β-Ga₂O₃ has an indirect band gap with a conduction band minimum (CBM) at Γ point and a valence band maximum on the E line. The anisotropic optical properties are explained by the selection rule of the band-to-band transitions. On the other hand, the shape of the CBM is almost isotropic and, therefore, the observed electronic anisotropy in the n-type semiconducting state should not be attributed to the properties of a perfect lattice. The Burstein-Moss shift is discussed using the effect of several allowed transitions between the levels of the valence band and the CBM.     

Bandgap evolution of GaN1−x As x in the whole composition range

The bandgap evolution of GaN_1?x As _x in the whole composition range is investigated and a model describing its bandgap energy is developed. It is found that the bandgap evolution is due to two factors. One is the interaction between the impurity band and the Γ conduction band or the Γ valence band of the host materials. The other one is the intraband coupling within the conduction band and separately within the valence band. The former is dominant in the As-rich GaNAs and the N-rich GaNAs. The latter plays an important part in the N-rich range and the moderate composition range.     

Electrical and optical properties of GexFexSe100−2x chalcogenide thin films

Optical absorption at room temperature and electrical conductivity at temperatures between 283 and 333 K of vacuum evaporated Ge_xFe_xSe_100−2x (0≤x≤15) amorphous thin films have been studied as a function of composition and film thickness. It was found that the optical absorption is due to indirect transition and the energy gap increases with increasing both Ge and Fe content; on the other hand, the width of the band tail exhibits the opposite behavior. The optical band gap E_opt was found to be almost thickness independent. The electrical conductivity show two types of conduction, at higher temperature the conduction is due to extended states, while the conduction at low temperature is due to variable range hopping in the localized states near Fermi level. Increasing Ge and Fe contents were found to decrease the localized state density N(E_F), electrical conductivity and increase the activation energy for conduction, which is nearly thickness independent. Variation of the atomic densities ρ, molar volume V, glass transition temperature T_g cohesive energy C.E and number of constraints N_Co with average coordination number Z was investigated. The relationship between the optical gap and chemical composition is discussed in terms of the cohesive energy C.E, average heat of atomization and coordination numbers.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

Electronic structures of N- and C-doped NiO from first-principles calculations

The large intrinsic band gap of NiO has hindered severely its potential application under visible-light irradiation. In this Letter, we have performed first-principles calculations on the electronic properties of N- and C-doped NiO to ascertain if its band gap may be narrowed theoretically. It was found that impurity bands driven by N 2p or C 2p states appear in the band gap of NiO and that some of these locate at the conduction band minimum, which leads to a significant band gap narrowing. Our results show that N-doped NiO may serve as a potential photocatalyst relative to C-doped NiO, due to the presence of some recombination centres in C-doped NiO.     

Exploring the electronic structure of Pb2+ ions containing material Pb16(OH)16(NO3)16

A theoretical band structure calculation for lead nitrate hydroxide Pb₁₆(OH)₁₆(NO₃)₁₆ single crystal was performed based on the experimental crystallographic data obtained by Chang et al. Calculations exhibit that the conduction band minimum (CBM) is situated at Γ the center of the Brillouin zone (BZ) while the valence band maximum (VBM) is located between Γ and Y points of the BZ, resulting in an indirect energy band gap of about 3.70 eV in close agreement to the measured one (3.78 eV). The angular momentum resolved projected density of states reveals the existence of the strong hybridization between the orbitals and the VBM is originated from Pb-6s/6p and O-2p orbitals while the CBM from N-2p and Pb-6p orbitals. The calculated valence electronic charge density distribution explore the bond characters and the dominancy of the covalent bonding between Pb–O of PbO_n ployhedra and N–O of [NO₃]⁻ triangle. The calculated bond lengths and angles show good agreement with the experimental data.     

Electronic structure and optical gain of InAsPN/GaP(N) quantum dots

The electronic structure and optical gain of InAsPN/GaP(N) quantum dots (QDs) are investigated in the framework of the effective-mass envelope function theory. The strain distribution is calculated using the valence force field (VFF) method. With GaP barrier, for smaller InAsPN QDs, the minimum transition energy may occur at a lower phosphorous (P) composition, but for larger QDs, the transition energy increases as P composition increases due to the increased bandgap of alloy QDs. When the nitrogen (N) composition increases, the transition energy decreases due to the stronger repulsion between the conduction band (CB) and the N resonant band, and the transition matrix element (TME) is more affected by the transition energy rather than N–CB mixing. To obtain laser materials with a lattice constant comparable to Si, we incorporated 2% of N into the GaP barrier. With this GaP_0.98N_0.02 barrier, the conduction band offset is reduced, so the quantum confinement is lower, resulting in a smaller transition energy and longer wavelength. At the same time, the TME is reduced and the optical gain is less than those without N in the barrier at a low carrier density, but the peak gain increases faster when the carrier density increases. Finally it can surpass and reach a greater saturation optical gain than those without N in the barrier. This shows that incorporating N into GaP barriers is an effective way to achieve desirable wavelength and optical gain.     

Electronic band structure of partially inverse spinel MgIn2S4 as calculated by the semiempirical pseudopotential method

The electronic band structure of the partially inverse spinel MgIn₂S₄ has been calculated on the symmetry lines ΓΛL, ΓΔX and ΓΣK by the semiempirical pseudopotential method. The general features of the band structure of MgIn₂S₄ are quite similar to those of the normal spinel CdIn₂S₄. The conduction band minimum is located at Γ and the valence band maximum is along the Σ line. The indirect energy gap (Γ_1cΣ_4v) is 2.50 eV. The effects of magnesium vacancy and variations in the cation distribution and in the parameter u are examined and shown to be small.     

Second order resonant Raman scattering at the E1 and E1 + Δ1 edges of germanium

The resonance of the 2TO phonon second order Raman band of germanium was investigated in the vicinity of the E₁, E₁ + Δ₁ gaps. The peak reported earlier for 2TO phonons at Γ is interpreted as due to a resonant process of the iterative type, involving two first order electron-phonon vertices. The rest of the 2TO band is interpreted as a process involving a second order electron-two phonon interaction vertex. From these measurements three phonon coupling constants for the electron-two phonon interaction are obtained.     

Band-gap bowing and p-type doping of (Zn,Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N_O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.     

Coexistence of type-I and type-II band alignment in Ga(Sb,P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb_{1 ? x} P _x /GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the X _XY valley of GaSb_{1 ? x} P _x conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.     

Properties of Hg1−xZnxSe (0.09 ⩽ x ⩽ 1.0) thin films prepared by flash evaporation technique

Flash evaporated mercury zinc selenide films are observed to grow as single phase ternary alloys of the type Hg_1−xZn_xSe in the composition range 0.09 ⩽ x ⩽ 1.0, with a f.c.c./sphalerite structure on substrates maintained at T_s between 30°C and 175°C. The grain size is observed to increase with increase in T_s for all compositions. The films are observed to have a direct optical band gap which increases from 0.01 to 2.60 eV as the Zn concentration x is varied between 0.09 and 1.0. The band gap V_scomposition shows a bowing, typical of pseudobinary solid solutions. Zn rich films were observed to be p-type whereas Hg-rich films were n-type. Room temperature resistivity was observed to increase with Zn concentration x, which can be attributed to the increase in the band gap of the semiconductors. Higher resistivity in films deposited at high substrate temperatures is due to the decrease in contribution to conduction from the highly conducting grain boundaries.     

A first-principles study of the electronic structure of the sulvanite compounds

We have investigated by means of first-principles total energy calculations the electronic structure of the sulvanite compounds: Cu₃VS₄, Cu₃NbS₄ and Cu₃TaS₄; the later is a possible candidate as a p-type transparent conductor with potential applications in solar cells and electrochromic devices. The calculated electronic structure shows that these compounds are indirect band gap semiconductors, with the valence band maximum located at the R-point and the conduction band minimum located at the X-point. The character of the valence band maximum is dominated by Cu d-states and the character of the conduction band minimum is due to the d-states of the group five elements. From the calculated charge density and electron localisation function we can conclude that the sulvanite compounds are polar covalent semiconductors.