Screened-exchange Local Density Approximation Research Articles

First-principle calculation of the band structure of Ge1−xSnx alloy by screened-exchange local-density approximation theory

The electronic properties of Ge1−xSnx were studied by the sX-LDA method. It is found that the bowing coefficient of the direct bandgap energy depends on composition obviously while the bowing coefficients of the Γ–L and Γ–X bandgap energies depend on composition weakly. As the atom size mismatch (ASM) between Ge and Sn atoms is large and the electronegativity difference (ED) between them is relatively small, the ASM should play a more important role than the ED in determining the bowing coefficient. The decrease of the direct bandgap bowing coefficient is owing to that the direct bandgap energy goes through from the impurity-like region to the band-like region in the Ge-rich range. According to the fitting results, the transition from the indirect to direct bandgap occurs when about 6% of Ge atoms are replaced by Sn atoms. The larger band bowing of Ge1−xSnx than Si1−xGex can be attributed to the ASM. In addition, the larger bowing of the CBM than the VBM is relative to the components of the two bands and the band offsets between Ge and Sn.

Temperature and carrier concentration dependence of thermoelectric properties of GeBi4Te7

We investigate the temperature and carrier concentration dependence of thermoelectric properties including the Seebeck coefficient and thermoelectric power factor of GeBi4Te7 compound by the ab initio electronic structure calculations within density functional theory (DFT). The DFT Kohn-Sham equation is solved by employing the local density approximation (LDA) as well as the screened-exchange local density approximation (sX-LDA). We obtain a band gap of 0.13 eV in sX-LDA for GeBi4Te7 while the LDA calculation predicts a metallic electronic structure due to its well-known bandgap underestimation problem. The optimal concentration to maximize the thermoelectric power factor is found to be 1.8 × 1020 cm −3 for p-type doping and 3.0 × 1020 cm−3 for n-type doping at room temperature, yielding the power factors of 10.2 and 8.2 μWcm −1K −2, respectively. Anisotropy of transport coefficients also depends on the doping type indicating that the in-plane and the perpendicular direction is preferred in p-type and n-type doping, respectively.

Effect of strain on the electronic structure and optical properties of germanium

The effects of biaxial strain parallel to the (001) plane on the electronic structures and optical properties of Ge are calculated using the first-principles plane-wave pseudopotential method based on density functional theory. The screened-exchange local-density approximation function was used to obtain more reliable band structures, while strain was changed from −4% to [Formula: see text]4%. The results show that the bandgap of Ge decreases with the increase of strain. Ge becomes a direct-bandgap semiconductor when the tensile strain reaches to 2%, which is in good agreement with the experimental results. The density of electron states of strained Ge becomes more localized. The tensile strain can increase the static dielectric constant distinctly, whereas the compressive strain can decrease the static dielectric constant slightly. The strain makes the absorption band edge move toward low energy. Both the tensile strain and compressive strain can significantly increase the reflectivity in the range from 7 eV to 14 eV. The tensile strain can decrease the optical conductivity, but the compressive strain can increase the optical conductivity significantly.

New insights into the structure, chemistry, and properties of Cu4SnS4

The ambient temperature structure of Cu4SnS4 has been revisited and the recently reported low temperature structure has been confirmed from single-crystal X-ray diffraction data. A structural phase transition from a large monoclinic unit cell at low temperature to a smaller orthorhombic unit cell at high temperature has been observed. The room temperature phase exhibited disorder in the two copper sites, which is a different finding from earlier reports. The low temperature monoclinic form crystallizes in P21/c space group, which is isostructural with Cu4GeS4. The phase transition has also been studied with variable temperature powder X-ray diffraction and 119Sn Mössbauer spectroscopy. The Seebeck coefficients and electrical resistivity of polycrystalline Cu4SnS4 are reported from 16 to 400K on hot pressed pellets. Thermal conductivity measurements at high temperatures, 350 – 750K exhibited very low thermal conductivities in the range 0.28 – 0.35WK–1m–1. In all the transport measurements the phase transition has been observed at around 232K. Resistivity decreases, while Seebeck coefficient increases after the phase transition during warming up from low to high temperatures. This change in resistivity has been correlated with the results of first-principles electronic band structure calculations using highly-accurate screened-exchange local density approximation. It was found that both the low hole effective mass of 0.63 me for the Γ→Y crystallographic direction and small band gap, 0.49eV, are likely to contribute to the observed higher conductivity of the orthorhombic phase. Cu4SnS4 is also electrochemically active and shows reversible reaction with lithium between 1.7 and 3.5 volts.

Density Functional Theory Calculations of Atomic Configurations and Bandgaps of C-, Ge-, and Sn-Doped Si Crystals for Solar Cells

Poly-Si crystals are mainly used in solar cells because of their low cost. Here, the zones of sensitivity to wavelengths in sunlight should be expanded to increase the engineering efficiency of solar cells. Group IV compound semiconductors films, e.g., Si (Ge) films doped with C, Ge (C, Si), and/or Sn atoms with contents of several %, on a Si or Ge substrate have been identified as potential solutions to this technical problem. In this study, we calculated the formation energy of each atomic configuration of C, Ge, and Sn atoms in Si by using density functional theory. The “Hakoniwa” method proposed by Kamiyama et al. [Materials Science in Semiconductor Processing, 43, 209 (2016)] was applied to a 64-atom supercell of Si including up to three atoms of C, Ge, and/or Sn (up to 4.56%) in order to obtain the ratio of each atomic configuration and the average value of the Si bandgaps. Not only the conventional generalized gradient approximation (GGA) but also the screened-exchange local-density approximation (sX-LDA) functional was used to obtain more reliable Si bandgaps. The results of the analysis are fourfold. First, two C (Sn) atoms are energetically stable when they are 3rd, 4th, 6th, 7th, and 9th neighbors of each other, while the stability of two Ge atoms is independent of the atomic configuration. Second, C and Ge (Sn) atoms are stable when they are 2nd, 5th, and 8th (1st and 8th) neighbors, while the stability of Sn and Ge atoms is independent of the atomic configuration. Third, the Si bandgap depends (does not depend) on the atomic configuration when Si includes C and/or Sn atoms (Ge atoms). Uniformly mono-doping C by up to 4.68% and Ge (Sn) by up to 3.12% decreased the average value of the Si bandgaps. C doping decreased the Si bandgap the most, while Ge doping decreased it the least. Fourth, uniformly co-doping C and Sn in a 1:1 ratio (C and Ge 1:1, Ge and Sn 1:1) at 1.56% also decreased the Si bandgap. The results shown here will be useful for predicting the bandgap for a given content of Si crystals, which is important for the solar cell application.

Electronic structures and thermoelectric properties of layered chalcogenide PbBi4Te7 from first principles

The electronic structures and the thermoelectric (TE) properties of the ternary chalcogenide PbBi4Te7 are investigated by using first-principles calculations within the density functional theory and the solutions of semi-classical Boltzmann equation. Employing the screened-exchange local density approximation, we found that PbBi4Te7 to be a narrow-gap semiconductor with an indirect band gap of 0.11 eV. The combination of light and heavy valence bands near the band edge gives rise to large Seebeck coefficients, S, for p-type doping, which is found to be improved by 11% from that of Bi2Te3 at room temperature (RT). Moreover, in contrast to conventional Bi2Te3 where the value of S decreases rapidly with temperatures higher than RT, the values of S increases with temperature reaching up to 350 μVK−1 at 500 K indicating that PbBi4Te7 is a promising TE material with operating temperatures above RT. Our result also reveals that the intrinsic layered structure results in a prominent anisotropy in the TE coefficients, implying that the TE performance can be optimized by using the transport direction, as well as the type and the level of doping.

Existence of the β-tin structure in Sr: First evidence from computational approach

Molecular Dynamics (MD) calculation is one of the most powerful theoretical methods widely used to predict and to confirm structural phase transitions. In this work, the MD method has been used to verify phase transition from body-centered cubic (bcc) to β-tin structure, then, to the Cmcm and hexagonal close-packed (hcp) structure, respectively. The transition sequence from previous theoretical works has been confirmed. In this study, Density Functional Theory (DFT), has been used to calculate phonon dispersion to confirm the stability of β-tin and hcp phases. The long time discrepancies in transition sequence between the calculation and the experimental works has been explained by conventional DFT calculation using screened exchange local density approximation (sX-LDA). More importantly, the existence of β-tin structure is finally predicted and the transition nature of Sr has also been revealed.

Accurate screened exchange band structures for the transition metal monoxides MnO, FeO, CoO and NiO

We report calculations of the band structures and density of states of the four transition metal monoxides MnO, FeO, CoO and NiO using the hybrid density functional sX-LDA (‘screened exchange local density approximation’). Late transition metal oxides are prototypical examples of strongly correlated materials, which pose challenges for electronic structure methods. We compare our results with available experimental data and show that our calculations generally yield accurate predictions for the fundamental band gaps and valence bands, in favourable agreement with previously reported theoretical studies. For MnO, the band gaps are still underestimated, suggesting additional many-body effects that are not captured by our screened hybrid functional approach.

Screened-exchange electronic structures and thermodynamic properties of the cubic perovskite BiAlO3

The screened exchange local density approximation (sX-LDA) hybrid functional within the pseudopotential plane wave method is performed to improve the electronic structures of cubic BiAlO3 and its thermodynamic properties are investigated using the density functional perturbation theory (DFPT). The equilibrium lattice constant of cubic BiAlO3 is 3.67Å which is applied to calculate its electronic structures. The calculated sX-LDA electronic structures are compared to standard generalized gradient approximation (GGA) approach. It is found that BiAlO3 is an indirect band gap material at M(1/2,1/2,0)-X(0,0,1/2) point with the gap value of 1.84, and 3.43eV for GGA, and sX-LDA calculations, respectively. The linear response method is employed in order to calculate the phonon dispersion relations and phonon density of states. The predicted thermodynamic quantities, specific heat and Debye temperature, of cubic perovskite BiAlO3 are also reported.

First-Principles Study of Electronic, Elastic, and Lattice Vibrational Properties of Pbnm Orthorhombic SrHfO3

Through first-principles pseudopotential calculations based on density functional theory, the electronic structure and lattice vibrational properties of Pbnm orthorhombic SrHfO3 were investigated in the framework of standard functional approximation and density functional perturbation theory, respectively. The calculated equilibrium lattice constants of Pbnm orthorhombic SrHfO3 are in good agreement with available experimental and theoretical results. The results show that Pbnm orthorhombic SrHfO3 is an insulator with a direct band gap of 3.9 eV and 4.0 eV within the calculations using local density approximation (LDA) and generalized gradient approximation (GGA), respectively. Use of the screened exchange local density approximation (sX-LDA) as a functional in a successive band calculation has also been performed. The band gap is predicted to be 6.7 eV within sX-LDA, somewhat higher than the gap values of 6.1 ± 0.1 eV and 6.5 eV obtained from recent x-ray photoelectron spectroscopy. The phonon dispersion curves of Pbnm orthorhombic SrHfO3 were also calculated. All-positive phonon frequencies were observed in the whole Brillouin zone, indicating stability of the Pbnm orthorhombic SrHfO3 structure. In addition, the infrared-active and Raman-active vibrational modes of SrHfO3 were calculated and compared with available theoretical and experimental investigations.

Electronic and lattice vibrational properties of cubic BaHfO3 from first principles calculations

The electronic structure and lattice vibrational properties of cubic SrHfO3 were investigated by first-principles calculations based on density functional theory in the framework of the local density approximation (LDA), generalized gradient approximation (GGA), and density functional perturbation theory (DFPT), respectively. The calculated equilibrium lattice constant of cubic SrHfO3 is in good agreement with available experimental and theoretical results. The results show that cubic SrHfO3 is an insulator with an indirect LDA (GGA) band gap of 3.6 (3.7) eV. Use of the screened exchange local density approximation (sX-LDA) as a functional in successive band calculation has also been performed. The band gap is predicted to be 6.27 eV within sX-LDA, in excellent agreement with the gap value of 6.1 ± 0.1 eV obtained from x-ray photoelectron spectroscopy. The phonon dispersion curves and LO–TO splitting of cubic SrHfO3 were also calculated. Negative phonon frequencies were observed along the M–C–R–M line in the Brillouin zone, indicating instability of the SrHfO3 structure, consistent with previous theoretical investigation.

Temperature and carrier-concentration dependences of the thermoelectric properties of bismuth selenide dioxide compounds

We performed an ab-initio electronic structure calculation for the bismuth selenide compound with oxygen substitution by using the all-electron full-potential linearized augmented plane-wave (FLAPW) method. The spin-orbit coupling in the second variational scheme and the screenedexchange local density approximation (sX-LDA) were adapted, and a large enhancement of the indirect band gap from 0.13 to 0.78 eV over the conventional LDA result was found. Exploiting the FLAPW wave-functions, we employed Boltzmann’s equation in a constant relaxation-time approach to estimate the thermoelectric (TE) transport properties, including the Seebeck coefficient, the electrical conductivity and the power factor. The temperature and the carrier-concentration dependences of the TE properties were investigated in detail to find the effect of oxygen substitution on and to search for the optimal doping and temperature range for better thermoelectric performance.

Structure and electronic properties of lead-free KTa1−xNbxO3: Accurate Wu–Cohen and screened-exchange study

We present first-principles calculations of structure and electronic properties of lead-free potassium tantalate niobate KTa1−xNbxO3 (KTN) using both Wu–Cohen functional (WC) and screened-exchange local density approximation (sX-LDA). WC predicts more accurate the lattice constants and atomic structure of KTN over standard LDA and generalized gradient approximation (GGA-PBE). sX-LDA successfully corrects the band-gap problem of LDA and PBE. The correctness of sX-LDA is further supported by excellent accordance between calculated absorption spectra and experiment. We show that in paraelectric state Ta and Nb atoms have obvious off-center displacements, which agree with experimental results and suggest that KTN are not displacive but order–disorder materials. We found that Ta and Nb have little contribution to valance band-top states, and therefore optical properties of KTN are not sensitive to Ta/Nb disorder.

Electronic properties of layered multicomponent wide-band-gap oxides: A combinatorial approach

The structural, electronic, and optical properties of 12 multicomponent oxides with layered structure $RAM$O${}_{4}$, where $R$${}^{3+}$ = In or Sc, $A$${}^{3+}$ = Al or Ga, and $M$${}^{2+}$ = Ca, Cd, Mg, or Zn, are investigated using first-principles density functional approach. The compositional complexity of $RAM$O${}_{4}$ leads to a wide range of band-gap values varying from 2.45 eV for InGaCdO${}_{4}$ to 6.29 eV for ScAlMgO${}_{4}$ as obtained from our self-consistent screened-exchange local density approximation calculations. Strikingly, despite the different band gaps in the oxide constituents, namely, 2--4 eV in CdO, In${}_{2}$O${}_{3}$, or ZnO, 5--6 eV for Ga${}_{2}$O${}_{3}$ or Sc${}_{2}$O${}_{3}$, and 7--9 eV in CaO, MgO, or Al${}_{2}$O${}_{3}$, the bottom of the conduction band in the multicomponent oxides is formed from the $s$ states of all cations and their neighboring oxygen $p$ states. We show that the hybrid nature of the conduction band in multicomponent oxides originates from the unusual fivefold atomic coordination of $A$${}^{3+}$ and $M$${}^{2+}$ cations, which enables the interaction between the spatially spread $s$ orbitals of adjacent cations via shared oxygen atoms. The effect of the local atomic coordination on the band gap, the electron effective mass, the orbital composition of the conduction band, and the expected (an)isotropic character of the electron transport in layered $RAM$O${}_{4}$ is thoroughly discussed.

Density functional theory calculations for two-dimensional silicene with halogen functionalization

The electronic structures and band gaps of silicene (the Si analogue of graphene) adsorbed with halogen elements are studied using the density functional theory based screened exchange local density approximation method. It is found that the band gaps of silicene adsorbed with F, Cl, Br and I have a nonmonotonic change as the periodic number of the halogen elements increases. This is attributed to the transfer of contributions to band gaps from Si-Si bonding to Si-halogen bonding.

Electronic and Lattice Vibrational Properties of Cubic SrHfO3 from First-Principles Calculations

The electronic structure and lattice vibrational properties of cubic SrHfO3 were investigated by first-principles calculations based on density functional theory in the framework of the local density approximation (LDA), generalized gradient approximation (GGA), and density functional perturbation theory (DFPT), respectively. The calculated equilibrium lattice constant of cubic SrHfO3 is in good agreement with available experimental and theoretical results. The results show that cubic SrHfO3 is an insulator with an indirect LDA (GGA) band gap of 3.6 (3.7) eV. Use of the screened exchange local density approximation (sX-LDA) as a functional in successive band calculation has also been performed. The band gap is predicted to be 6.27 eV within sX-LDA, in excellent agreement with the gap value of 6.1 ± 0.1 eV obtained from x-ray photoelectron spectroscopy. The phonon dispersion curves and LO–TO splitting of cubic SrHfO3 were also calculated. Negative phonon frequencies were observed along the M–Γ–R–M line in the Brillouin zone, indicating instability of the SrHfO3 structure, consistent with previous theoretical investigation.

Tl2Hg3Q4 (Q = S, Se, and Te): High-Density, Wide-Band-Gap Semiconductors

We present the synthesis, crystal structures, and physical properties of Tl2Hg3Q4 (Q = S, Se, and Te). The incongruently melting Tl2Hg3Q4 crystals were grown in a TlxQ flux. These compounds are isostructural and crystallize in a monoclinic cell with a layered structure, adopting the space group C2/c with a = 11.493(2) A, b = 6.6953(13) A, c = 12.937(3) A, β = 114.98(3)° for Tl2Hg3S4, a = 11.977(2) A, b = 6.9264(14) A, c = 13.203(3) A, β = 116.36(3)° for Tl2Hg3Se4 and a = 12.648(3) A, b = 7.3574(15) A, c = 13.701(3) A, β = 117.48(3)° for Tl2Hg3Te4. The structures feature infinite chains of [Hg3Q4]2–, which are linked into layers by charge balancing Tl atoms. The compounds have very high densities (>8.3 g/cm3) with experimentally determined band gaps of 2.05, 1.57, and 0.90 eV for Q = S, Se, and Te, respectively. Using the refined crystal structures, we performed detailed band structure calculations at the density functional theory (DFT) level, using the screened-exchange local density approximation (sx-LDA...

Implementation of screened hybrid functionals based on the Yukawa potential within the LAPW basis set

The implementation of screened hybrid functionals into the WIEN2k code, which is based on the LAPW basis set, is reported. The Hartree-Fock exchange energy and potential are screened by means of the Yukawa potential as proposed by Bylander and Kleinman [Phys. Rev. B 41, 7868 (1990)] for the calculation of the electronic structure of solids with the screened-exchange local density approximation. Details of the formalism, which is based on the method of Massidda, Posternak, and Baldereschi [Phys. Rev. B 48, 5058 (1993)] for the unscreened Hartree-Fock exchange are given. The results for the transition-energy and structural properties of several test cases are presented. Results of calculations of the Cu electric-field gradient in Cu2O are also presented, and it is shown that the hybrid functionals are much more accurate than the standard local-density or generalized gradient approximations.

Thallium Chalcogenide-Based Wide-Band-Gap Semiconductors: TlGaSe2 for Radiation Detectors

The wide-band-gap semiconductor thallium gallium selenide (TlGaSe2) is promising for X-ray and γ-ray detection. In this study, the synthesis and crystal growth of semiconducting TlGaSe2 was accomplished using a stoichiometric combination of TlSe, Ga, and Se and a modified Bridgman method. These large detector-grade crystals can be synthesized and cut to dimensions appropriate for a detector. The crystals have mirror-like cleaved surfaces and are transparent red, in agreement with a band gap of 1.95 eV observed in absorption measurements. Single-crystal X-ray diffraction refinements confirm that TlGaSe2 crystallizes in the monoclinic C2/c space group with a layered crystal structure consisting of planes of GaSe4 corner-sharing tetrahedra connected by weak Tl–Se bonds. Electronic band structure calculations made using the full-potential linearized augmented plane wave method with the screened-exchange local density approximation, including spin orbit coupling, indicate the unusual characteristic of the hole...

Electronic Structure and Characteristics of Chemical Bonds in CuInSe2, CuGaSe2, and CuAlSe2

Electronic structural calculations of chalcopyrite-type CuInSe2 and its related compounds, CuGaSe2 and CuAlSe2, were performed by a recently proposed screened-exchange local density approximation (sX-LDA) method. The theoretical band gaps of CuInSe2, CuGaSe2, and CuAlSe2, obtained by the conventional calculation method using a generalized gradient approximation (GGA) functional, were 0.04, 0.14, and 1.11 eV, respectively. These values were considerably underestimated in comparison with their experimental values of 1.04, 1.68, and 2.67 eV because the exchange–correlation energy was not precisely calculated. Therefore, electronic structural calculations of chalcopyrite-type CuInSe2 and related compounds were performed with an sX-LDA functional to obtain accurate electronic structure. The present sX-LDA calculation successfully reproduced the band gaps of CuInSe2 (0.96 eV), CuGaSe2 (1.36 eV), and CuAlSe2 (2.22 eV). The obtained electronic structures and band gap energies of CuInSe2, CuGaSe2, and CuAlSe2 are discussed on the basis of schematic molecular orbital diagrams of tetrahedral CuSe47-, InSe45-, GaSe45-, and AlSe45- clusters.