Effective mass of heavy holes in diamond-like semiconductors

Nonparabolicity of the heavy hole band in diamond-like semiconductors, which occurs within the framework of the three band model with the perturbation from the other bands taken into account to the Löwdin prucedure, is studied. A direct dependence of nonparabolicity on the band anisotropy (caused by the different effect of _15c and _12c bands) and the inverse dependence on the magnitude of the spin-orbit splitting is established. A connection between the effective mass of heavy holes and their energy is obtained, which is valid for the majority of diamond-like semicondactors, except for materials with very strong nonparabolicity of the band of silicon type.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 66–70, February, 1987.     

sp3sTight-binding parameters for transport simulations in compound semiconductors

A genetic algorithm approach is used to fit orbital interaction energies of sp3s* tight-binding models for the nine binary compound semiconductors consistent of Ga, Al, In and As, P, Sb at room temperature. The new parameters are optimized to reproduce the bandstructure relevant to carrier transport in the lowest conduction band and the highest three valence bands. The accuracy of the other bands is sacrificed for the better reproduction of the effective masses in the bands of interest. Relevant band edges are reproduced to within a few meV and the effective masses deviate from the experimental values typically by less than 10%.     

Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si(1-x)Sn(x) solid solutions

Mg(2)Si and Mg(2)Sn are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition x, Mg(2)Si(1-x)Sn(x) solid solutions must display a convergence in energy of the two conduction bands. Since Mg(2)Si(1-x)Sn(x) solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of Mg(2)Si(1-x)Sn(x) solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at x=0.7. At this concentration the thermoelectric figure of merit ZT reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content x≈0.7 the two conduction bands coincide in energy. We explain the high Seebeck coefficient and ZT values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.     

Thermal destruction of spin-polaron bands in the narrow-gap correlated semiconductors FeGa3 and FeSb2

We report muon spin rotation spectra in the narrow-gap semiconductors FeGa(3) and FeSb(2) consistent with a narrow band of small spin polarons (SPs). The characteristic sizes obtained for these SPs are R(FeGa(3)) ≈ 0.3-0.6 nm and R(FeSb (2)) ≈ 0.3 nm, respectively. Such SP states are expected to originate from the exchange correlations between localized and itinerant electrons. Our data suggest that SP bands are formed at low temperature, but are destroyed by thermal fluctuations above 10 K in FeGa(3) and above 7 K in FeSb(2). Formation of such SP band states can explain many of the low-temperature properties of these materials.     

Magnetic circular dichroism from the impurity band in III-V diluted magnetic semiconductors

The magnetic circular dichroism of III-V diluted magnetic semiconductors, calculated within a theoretical framework suitable for highly disordered materials, is shown to be dominated by optical transitions between the bulk bands and an impurity band formed from magnetic dopant states. The real-space Green's functions incorporate spatial correlations in the disordered conduction band and valence-band electronic structure, and include extended and localized states on an equal basis. Our findings reconcile unusual trends in the experimental magnetic circular dichroism in III-V diluted magnetic semiconductors with the antiferromagnetic p-d exchange interaction between a magnetic dopant spin and its host.     

Effects of finite temperatures,valence bands and energy gaps on the indirect exchange interaction in intrinsic semiconductors

The indirect exchange interaction between localized magnetic moments via virtual excitations from the valence band in intrinsic semiconductors is calculated taking into account the temperature, energy gaps, finite valence bands and effective masses. The inclusion of finite temperature effects changes significantly the phase and magnitude of the oscillations. For small gap semiconductors the oscillatory character and the possibility of ferro-magnetic as well as anti-ferromagnetic coupling are obtained. The exchange interaction oscillates with temperature suggesting interesting applications of this model.     

Hydrides as materials for semiconductor electronics

Systematic studies using density functional theory have shown that some hydrides possess the features of semiconductors. These features include larger fundamental band gap, well dispersed bottom-most conduction band and/or top-most valence band, small electron/hole effective masses and small intrinsic carrier concentration. It is demonstrated that depending upon the composition, hydrides possess a wide range of band gap values and hence they can be regarded as materials for narrow to wide band gap semiconducting applications. The possibility of designing hydride-based p–n junctions, and also their advantages as well as deficiencies compared to existing oxide semiconductors, are discussed. Replacing oxide-based semiconductors by hydrides can help to avoid problems such as formation of an oxide layer, band offsets, large concentration of defect states at the interface between the oxide and semiconductor, etc. Moreover, hydrides can be regarded as an alternative to conventional semiconductors and hence can be used in future-generation electronic devices called “hydride electronics”.     

Anisotropy of the Sommerfeld coefficient in magnesium diboride single crystals

The anisotropic field dependence of the Sommerfeld coefficient gamma has been measured down to B-->0 by combining specific heat and Hall probe magnetization measurements in MgB2 single crystals. We find that gamma(B,theta) is the sum of two contributions arising from the sigma and pi band, respectively. We show that gammasigma(B,theta)=B/Bc2(theta) where Bc2(theta)=Bc2ab/sqrt[sin2theta+Gamma2cos2theta] with Gamma approximately 5.4 (theta being the angle between the applied field and the c axis) and gammapi(B,theta)=gammapi(B)=B/Bpi(B). The "critical field" of the pi band Bpi is fully isotropic but field dependent increasing from approximately 0.25 T for B< or =0.1 T up to 3 T approximately Bc2c for B-->3 T. Because of the coupling of the two bands, superconductivity survives in the pi band up to 3 T but is totally destroyed above for any orientation of the field.     

Critical temperature shift in weakly interacting Bose gas

With a high-performance Monte Carlo algorithm we study the interaction-induced shift of the critical point in weakly interacting three-dimensional /psi/(4) theory (which includes quantum Bose gas). In terms of critical density, n(c), mass, m, interaction, U, and temperature, T, this shift is universal: Deltan(c)(T) = -Cm(3)T(2)U, the constant C found to be equal to 0.0140+/-0.0005. For quantum Bose gas with the scattering length a this implies DeltaT(c)/T(c) = C(0)an(1/3), with C(0) = 1.29+/-0.05.     

BEC transition temperature of a dilute homogeneous imperfect Bose gas

The leading-order effect of interactions on a homogeneous Bose gas is theoretically predicted to shift the critical temperature by an amount DeltaT(c) approximately equal to ca(sc)n(1/3)T(0) from the ideal gas result T(0), where a(sc) is the scattering length, n is the density, and c is a pure number. There have been several different theoretical estimates for c. We claim to settle the issue by measuring the numerical coefficient in a lattice simulation of O(2) straight phi(4) field theory in three dimensions-an effective theory which, as observed previously in the literature, can be systematically matched to the dilute Bose gas problem to reproduce nonuniversal quantities such as the critical temperature. We find c = 1.32+/-0.02.     

The spatial unmasking of speech: evidence for within-channel processing of interaural time delay

Across-frequency processing by common interaural time delay (ITD) in spatial unmasking was investigated by measuring speech reception thresholds (SRTs) for high- and low-frequency bands of target speech presented against concurrent speech or a noise masker. Experiment 1 indicated that presenting one of these target bands with an ITD of +500 micros and the other with zero ITD (like the masker) provided some release from masking, but full binaural advantage was only measured when both target bands were given an ITD of + 500 micros. Experiment 2 showed that full binaural advantage could also be achieved when the high- and low-frequency bands were presented with ITDs of equal but opposite magnitude (+/- 500 micros). In experiment 3, the masker was also split into high- and low-frequency bands with ITDs of equal but opposite magnitude (+/-500 micros). The ITD of the low-frequency target band matched that of the high-frequency masking band and vice versa. SRTs indicated that, as long as the target and masker differed in ITD within each frequency band, full binaural advantage could be achieved. These results suggest that the mechanism underlying spatial unmasking exploits differences in ITD independently within each frequency channel.     

First-principles Band Structures Calculation of Tin-phthalocyanine

SnPc(Tin-phthalocyanine)因在无机/有机二极管等光电结构器件中表现出了很多有趣的特性而备受关注.为了更深地理解载流子的传输特性,利用密度泛函理论,采用广义梯度近似(DFT-GGA),关联函数选择BLYP计算了SnPc的能带结构.从点波函数、能带带宽以及带隙分析了载流子的传输行为. 从前线轨道的带宽以及电子和空穴的有效质量,可以看到电子的传输要比空穴的传输容易两倍左右.而且,当研究费米能级附近的能带时,发现未占有带的带隙总体上要小于占有带的带隙,这表明在考虑声子参与的情况下,电子在带间的跳跃要比空穴容易得多.以上的事实说明SnPc是一种电子传输占主导的材料.     

Group velocity matters for accurate prediction of phonon-limited carrier mobility

First-principles approaches have recently been developed to replace the phenomenological modeling approaches with adjustable parameters for calculating carrier mobilities in semiconductors. However, in addition to the high computational cost, it is still a challenge to obtain accurate mobility for carriers with a complex band structure, e.g., hole mobility in common semiconductors. Here, we present a computationally efficient approach using isotropic and parabolic bands to approximate the anisotropy valence bands for evaluating group velocities in the first-principles calculations. This treatment greatly reduces the computational cost in two ways: relieves the requirement of an extremely dense κ mesh to obtain a smooth change in group velocity, and reduces the 5-dimensional integral to 3-dimensional integral. Taking Si and SiC as two examples, we find that this simplified approach reproduces the full first-principles calculation for mobility. If we use experimental effective masses to evaluate the group velocity, we can obtain hole mobility in excellent agreement with experimental data over a wide temperature range. These findings shed light on how to improve the first-principles calculations towards predictive carrier mobility in high accuracy.     

Ab initio study of II-(VI)2 dichalcogenides

The structural stabilities of the (Zn,Cd)(S,Se,Te)(2) dichalcogenides have been determined ab initio. These compounds are shown to be stable in the pyrite phase, in agreement with available experiments. Structural parameters for the ZnTe(2) pyrite semiconductor compound proposed here are presented. The opto-electronic properties of these dichalcogenide compounds have been calculated using quasiparticle GW theory. Bandgaps, band structures and effective masses are proposed as well as absorption coefficients and refraction indices. The compounds are all indirect semiconductors with very flat conduction band dispersion and high absorption coefficients. The work functions and surface properties are predicted. The Te and Se based compounds could be of interest as absorber materials in photovoltaic applications.     

Simple theory of the density-of-states function in heavily doped non-linear optical and optoelectronic materials

We study theoretically the dispersion relation of the conduction electrons and the corresponding density-of-state (DOS) function in heavily doped non-linear optical, tetragonal, III–V, ternary and quaternary materials forming band tails. It has been found, taking CdGeAs₂, Cd₃As₂, InAs, In_1−xGa_xAs_yP_1−y lattice matched to InP and Hg_1−xCd_xTe as examples of the aforementioned materials that the complex nature of the energy spectrum, the oscillatory DOS for the negative values of the energy and the creation of a new forbidden zone is due to the presence of the crystal field splitting constant, the anisotropic effective electron mass and the anisotropic spin–orbit splitting for the tetragonal and non-linear optical materials and because of the interaction of the impurity atoms in the tails with the spin–orbit splitting of the valence bands for the other compounds. Analytically, the presence of non-removable poles in the dispersion relation of the undoped material creates the complex energy spectrum for the corresponding heavily doped sample and the solution of the Boltzmann transport equation will introduce new physical ideas and new experimental findings under different external conditions. The results of the perturbed III–V semiconductors whose unperturbed energy band structures are defined by the two band model of Kane and that of parabolic energy bands are special cases of our generalized analysis and no oscillations in the DOS are found in the aforementioned cases. The well-known results of the DOS for all materials in the absence of doping have also been obtained from the theoretical analysis under certain limiting conditions. This compatibility is the indirect test of our generalized formalism.     

de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs

We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T(c) ≈ 18 K and LiFeP with T(c) ≈ 5 K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ~3 for LiFeP and ~5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively.     

Growth of Mn5Ge3 ultrathin film on Ge（111）

The growth of Mn5Ge3 ultrathin films with different thicknesses, prepared by solid phase epitaxy, is studied. The results of scanning tunnelling microscopy and low energy electron diffraction studies show that the film can be formed and it is terminated with a （√3 × √3） R30° surface reconstruction when the thickness of Mn exceeds 3 monolayers. The magnetic properties show that the Curie temperature is about 300 K and the T^2-dependent behaviour is observed to remain up to 220 K.