Effect of interband scattering on the thermoelectric properties of semiconductors and semimetals

This paper reports on the study of the effect of interband scattering on the thermoelectric figure-of-merit of semiconductors and semimetals. It has been shown that the inclusion of interband scattering in the case of two types of carriers of the same sign leads to a decrease in the thermoelectric figure-of-merit. By contrast, if the material contains carriers of two types of opposite signs and the conduction and valence bands overlap, as is the case with semimetals, the interband scattering favorably affects on the magnitude of the thermoelectric figure-of-merit.     

量子线/铁基超导隧道结中隧道谱的研究

考虑铁基超导中能带间的相互作用和界面对每一个能带的散射作用, 利用推广的Blonder-Tinkham-Klapwijk模型, 并通过求解Bogoliubov-de Gennes 方程研究了具有不同类型双能隙系统的量子线/铁基超导隧道结中准粒子的输运系数和隧道谱. 研究表明: 1)在弹道极限时, 随着带间相互作用的增大, s_± 波隧道谱中零偏压附近的平台演变成电导峰; s₊₊ 波的平台演变成凹陷; p波的零偏压电导峰被压低. 2)界面对两个能带的散射作用不为零时, 随着带间相互作用的增大, s_± 波和s₊₊ 波两能隙处的峰值将降低, 而两峰间的凹陷值将变大; p波的零偏压电导峰被压低, 非零偏压电导增大. 3)界面对每个能带的散射, 可使其产生的电导峰变得更加尖锐, 但可压低和抹平另一个带产生的电导峰值. 这些结果对于澄清铁基超导体的能隙结构和区别不同类型铁基超导体有所帮助.     

Polarization dependence of angle-resolved photoemission spectroscopy of graphite

We have used variable polarization synchrotron radiation to map the valence band electronic structure of graphite by angle-resolved photoemission spectroscopy (ARPES). The experimental results with two orthogonal linear polarization of light signifies the contribution of either even or odd symmetry with respect to the crystal mirror plane towards the photoemission intensity. The σ₁ and σ₂ valence bands show odd reflection symmetry while the π valence band shows even symmetry with respect to the mirror plane. The measured ARPES spectrum using left and right circular polarized lights shows asymmetry in intensity around M point of the Brillouin zone, which ultimately mimicking different partial wave character of σ₁ and σ₃ bands.     

Electron and hole conductivity in CuInS2

Single crystals of CuInS₂ have been grown from the melt and annealed in In or S to produce good n- or p-type conductivity, respectively. Two donor levels, one shallow and one deep (0.35 eV), and one acceptor level at 0.15 eV are identified. The hole-mobility data are best fitted with an effective mass $\text{m}{}_{\mathrm{<mtext>p</mtext>}}\text{1?1.3m}{}_{\mathrm{e}}$, which can be explained by simple, two band $\text{k}$. $\text{p}$ theory if the valence band has appreciable d character. Above 300°K, the hole mobility falls rapidly, evidently due to multiband conduction and/or interband scattering between the nondegenerate and degenerate valence bands. The conduction band mobility appears to be dominated, in many samples, by large concentrations ( >10¹⁸cm^?3) of native donors and acceptors, which are closely compensated.     

Valence band density of states of amorphous and trigonal selenium determined by X-ray and U.V. photoemission

The densities of states of the entire valence bands of amoprhous and trigonal selenium have been measured by X-ray (hv = 1486.6 eV) and u.v. (hv = 21.2 eV) photoemission. It is found that the primary differences in the density of states of the two forms occurs in the p-like valence bands, in particular the lower bonding set. The density of states calculated by Kramer et al. for the amorphous form is shown to be in better agreement with the experimental density of states of the trigonal form than with that of the amorphous form.     

Electrical resistivity anomaly in the vicinity of structural phase transition of p-SnTe

Electrical resistivity of p-SnTe is calculated taking into account the second-order scattering processes through the inter-band electron-phonon interaction based on the Kristoffel-Konsin model. The experimental results can be reasonably explained by our numerical results.     

Circular photogalvanic effect at inter-band excitation in semiconductor quantum wells

We observed a circular photogalvanic effect (CPGE) in GaAs quantum wells at inter-band excitation. The spectral dependence of the CPGE is measured together with that of the polarization degree of the time-resolved photoluminescence. A theoretical model takes into account spin splitting of conduction and valence bands.     

Spin-dependent intravalley and intervalley electron-phonon scatterings in germanium

The spin-dependent electron-phonon scattering in the L and Γ valleys of germanium crystals has been investigated theoretically. For this purpose, the 16 × 16 k · p Hamiltonian correctly describing the electron dispersion in the vicinity of the L point of the Brillouin zone in germanium in the lowest conduction bands and the highest valence bands has been constructed. This Hamiltonian facilitates the analysis of the spin-dependent properties of conduction electrons. Then, the electron scatterings by phonons in the L and Γ valleys, i.e., intra-L valley, intra-Γ valley, inter-L-Γ valley, and inter-L-L valley scatterings, have been considered successively. The scattering matrix expanded in powers of the electron wave vectors counted from the centers of the valleys has been constructed using the invariant method for each type of processes. The numerical coefficients in these matrices have been found by the pseudopotential method. The partial contributions of the Elliott and Yafet mechanisms to the spin-dependent electron scattering have been analyzed. The obtained results can be used in studying the optical orientation and relaxation of hot electrons in germanium.     

The effects of strain and surface roughness scattering on the quasi-ballistic characteristics of a Ge nanowire p-channel field-effect transistor

The effects of strain and surface roughness scattering on the quasi-ballistic hole transport in a strained gate-all-around germanium nanowire p-channel field-effect transistor （pFET） are investigated in this work. The valence subbands are shifted up and warped more parabolically by the influence of HfO2 due to the lattice mismatch. However, the boundary force only shifts the subbands downwards and has little effect on the reshaping of bands. Strain induced by HfO2 increases both the hole mobility and ON-current （/ON）, but has little effect on the hole mobility. The/ON is degraded by the surface roughness scattering in both strained and unstrained devices.     

Electronic structure and optical properties of ordered compounds potassium tantalate and potassium niobate and their disordered alloys

The electronic energy band structure, site and angular momentum decomposed density of states (DOS) of cubic perovskite oxides KNbO₃ and KTaO₃ have been obtained from a first principles density functional based full potential linearized augmented plane wave (FLAPW) method within a generalized gradient approximation (GGA). The total DOS in valence region is compared with the experimental photo-emission spectra (PES). The calculated DOS is in good agreement with the experimental energy spectra and the features in the spectra are interpreted by comparison with the projected density of states (PDOS). The valence band PES is mainly composed of Nb-4d/Ta-5d and O 2p states in KNbO₃ and KTaO₃, respectively. Using the PDOS and the band structure we have analyzed the inter-band contribution to the optical properties of these materials. The real and imaginary parts of the dielectric function have been calculated and compared with experimental data. They are found to be in a reasonable agreement. The role of band structure on the optical properties have been discussed.     

Detection of Yb impurities in the VUV spectral range of NdGaO3 crystals

Substantial influence of the 1 at.% Yb doping of NdGaO₃ single crystals on the optical functions ε₁(E) and ε₂(E) in the spectral range of electronic excitations is established. The corresponding differences of the optical functions for pure and doped NdGaO₃ have been monitored using spectroscopic ellipsometry method and synchrotron radiation light source. This opens an opportunity for the spectroscopic diagnostics of the rare earth dopants in crystals using the spectroscopic information concerning the electronic inter-band optical transitions. To understand better the experimental results obtained, the ab-initio calculations of band structure and optical spectra of the centrosymmetric single crystal NdGaO₃ have been performed using the VASP code (Vienna Ab-initio Simulation Package). The calculated dielectric functions ε₁(E) and ε₂(E) agree satisfactorily with our experimental results obtained with using the spectroscopic ellipsometry method and synchrotron radiation. It was demonstrated that differences of the optical functions for pure and doped crystals using stable synchrotron source may serve as a powerful tool for spectroscopic diagnostic of localised rare earth ions with respect to the strong inter-band transitions.     

Electronic Raman scattering from inter-valence band transition in photo-excited GaP and GaAs1−xPx crystals

Raman scattering from photo-created free carriers in undoped GaP and GaAs_1?xP_x (x = 0.85, 0.73 and 0.66) under high excitation intensity has been studied. Two new Raman bands have been observed and assigned to electronic transitions from the split-off hole band to the heavy hole band and from the light hole band to the heavy hole band. The spin-orbit splitting energies in these crystals have been determined from the analysis of observed Raman bands, and compared with other experimental values.     

Bulk band gaps in divalent hexaborides

Complementary angle-resolved photoemission and bulk-sensitive k-resolved resonant inelastic x-ray scattering of divalent hexaborides reveal a >1 eV X-point gap between the valence and conduction bands, in contradiction to the band overlap assumed in several models of their novel ferromagnetism. This semiconducting gap implies that carriers detected in transport measurements arise from defects, and the measured location of the bulk Fermi level at the bottom of the conduction band implicates boron vacancies as the origin of the excess electrons. The measured band structure and X-point gap in CaB6 additionally provide a stringent test case for many-body quasiparticle band calculations.     

Resonant Mandelstam-Brillouin light scattering and Raman scattering in semiconductors with intermediate exciton states belonging to discrete exciton bands and the continuous spectrum

The results of a theoretical and experimental investigation of resonant Mandelstam-Brillouin light scattering by thermal acoustic phonons with k=0 near the direct absorption edge (in the case of ZnSe crystals) are analyzed. The appearance of a new type of resonant increase in the intensity of Raman scattering by optical phonons with k≠0, which corresponds to resonance with the scattered light in the output channel, near the indirect absorption edge (in the case of semi-insulating GaP:N crystals) is also reported. The resonant gain reaches ∼4×10³ at frequencies corresponding to overtone scattering assisted by LO(X) and LO(L) phonons. Exciton states belonging to both discrete exciton bands and to the continuous spectrum are considered as the intermediate states involved in the scattering processes in calculations of the resonant scattering tensors. In addition, all the intraband transitions, as well as the interband transitions between the conduction band, the valence bands, and the spin-orbit split-off band are taken into account, and good agreement with the experimental results is obtained. Fiz. Tverd. Tela (St. Petersburg) 40, 938–940 (May 1998)     

Temperature dependent energy bands and the metal-nonmetal transition in NiS

The LCMTO (linear combination of muffin-tin orbitals) technique has been used to calculate the first temperature dependent energy bands for a transtion metal compound. The particular compound studied was hexagonal NiS which undergoes a first-order metal-to-nonmetal transition as the temperature is lowered below 264K. We find the bands to agree well with XPS data and that the Sp bands overlap the bottom of the d bands as predicted by White and Mott. This p-d overlap, which reduces the correlation energy, is found to increase with temperature as a result of lattice vibrations whereas the lattice distortion accompanying the transition has very little effect on the energy bands. This suggests that the Mott-Hubbard transition in NiS is driven by the Debye-Waller modification of the potential and not the lattice distortion as suggested by White and Mott.     

Semimetal behavior of bilayer stanene

Stanene is a two-dimensional (2D) buckled honeycomb structure which has been studied recently owing to its promising electronic properties for potential electronic and spintronic applications in nanodevices. In this article we present a first-principles study of electronic properties of fluorinated bilayer stanene. The effect of tensile strain, intrinsic spin-orbit and van der Waals interactions are considered within the framework of density functional theory. The electronic band structure shows a very small overlap between valence and conduction bands at the Γ point which is a characteristic of semimetal in fluorinated bilayer stanene. A relatively high value of tensile strain is needed to open an energy band gap in the electronic band structure and the parity analysis reveals that the strained nanostructure is a trivial insulator. According to our results, despite the monolayer fluorinated stanene, the bilayer one is not an appropriate candidate for topological insulator.     

The effect of non-orthogonality in the study of adsorption: Hydrogen on graphite

The effect of non-orthogonality between the localized orbitals of an adsorbed atom (or impurity atom) and those of the surrounding atoms of the substrate solid has been studied. The overlap matrix S takes in this case a simple form which enables us to calculate local densities of states and other local effects such as charges on the atoms by the continued fraction expansion. The method has been applied to a simplified model for the adsorption of hydrogen on graphite, and the results show an improvement with respect to calculations where the overlap is neglected.     

Electron spectroscopic studies of the adsorption of methanol on zinc oxide powder

X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and Auger electron spectroscopy (AES) have been used to study the adsorption of methanol on ZnO powder. The He(II) spectrum of electron-beam-eleaned ZnO was similar to that reported by Rubloff .¹ for the ZnO (11&#x0304; 0 0) surface. The spectrum of methanol condensed at ?150°C matched that of gas-phase methanol. At low methanol coverages, difference spectra showed no increase in the methanol O(2p) lone pair binding energy reported to be indicative of chemisorption. A residual carbon and oxygen surface component on the ZnO powder may have prevented chemisorption. XPS measurements were found to be more useful in observing this layer than was AES. A decrease in the ZnO valence band intensity near the valence band maximum indicated that a strong interaction with the substrate had occurred.In this system the valence bands of the adsorbate and substrate overlap, and the true position of the methanol O(2p) lone pair orbital is obscured by the effects of the substrate band modification. Also, for insulators or semiconductors, a change in the measured position of the valence band maximum relative to the Fermi level may occur upon adsorption. Since the Zn(3d) core level should not be affected, it was used as an energy reference for the present work. The problems of energy referencing in UPS studies of adsorption on semiconductors are discussed.     

Effective Heisenberg exchange integrals of diluted magnetic semiconductors determined within realistic multi-band tight-binding models

Diluted magnetic semiconductors (DMS) like Ga_1?x Mn _x As are described by a realistic tight-binding model (TBM) for the (valence) bands of GaAs, by a Zener (J-)term modeling the coupling of the localized Mn-spins to the spins of the valence band electrons, and by an additional potential scattering (V-) term due to the Mn-impurities. We calculate the effective (Heisenberg) exchange interaction between two Mn-moments mediated by the valence electrons. The influence of the number of bands taken into account (6-band or 8-band TBM) and of the potential (impurity) scattering V-term is investigated. We find that for realistic values of the parameters the indirect exchange integrals show a long-range, oscillating (RKKY-like) behavior, if the V-term is neglected, probably leading to spin-glass behavior rather than magnetic order. But by including a V-term of a realistic magnitude the exchange couplings become short ranged and mainly positive allowing for the possibility of ferromagnetic order. Our results are in good agreement with available results of ab initio treatments.