Average Effective Mass Research Articles

Natural sulvanite Cu3MX4 (M = Nb, Ta; X = S, Se): Promising visible-light photocatalysts for water splitting

The identification of natural mineral photocatalysts with excellent visible-light absorption, suitable band edge positions, and large carrier mobility is significant for water-splitting applications. Herein, by applying density functional theory simulation, the natural sulvanite Cu3MX4 (M = Nb, Ta; X = S, Se) compounds are proposed as efficient photocatalysts for visible-light water splitting. Calculation of the phonon dispersions and elastic constants of the Cu3MX4 (M = Nb, Ta; X = S, Se) compounds reveal that they are dynamically stable. In addition, their electronic structures indicate that these compounds are indirect semiconductors with band gaps in the desired range of 2–3 eV, and the position of the band-edge energy perfectly straddles the water oxidation and reduction potentials. Partial density charge analysis reveals that the electrons are mainly located near the Nb/TaS/Se bond, whereas the holes occupy the CuS/Se bonds. Furthermore, the effective mass, deformation potential parameters, and carrier mobility were quantitatively investigated to evaluate the photocatalytic dynamics. A small average effective mass and effective separation of photogenerated carriers in space was observed. Our calculations provide guidance for experimental synthesis and will expand the scope of these natural minerals for visible-light-driven water splitting with excellent performance.

Optical, charge transport and magnetic properties of palladium retrieved from photometric measurements: approaching the quantum mechanics background

A parametric Drude–Lorentz (DL) model is used to describe the spectral variation of the dielectric functions of bulk palladium samples at low and room temperature. In addition to the contribution of conduction electrons, the contribution of holes is also explicitly accounted for in the model. A simulated annealing method is applied to obtain the optimized values of the parameters involved in the model: volume plasma frequency of conduction electrons, high frequency dielectric constant, collision frequency of holes and corresponding relaxation time, and two additional parameters from which the effective mass of holes and collision frequency of conduction electrons are evaluated. Oscillatior strengths, resonance frequencies, and widths entering in the Lorentz contribution to the dielectric function are also optimized. Renormalization of the oscillator strengths requires the introduction of a new parameter in the context of the DL model: the ratio between number density of conduction electrons and number density of metal atoms, whose optimized value fits very well with its evaluation from band structure calculations and from independent measurements. Inclusion of this parameter in the framework allows us to evaluate additional quantities related to the charge-carrier transport: average effective masses, Fermi energies and electronic densities of states at the corresponding Fermi energies, intrinsic electrical resistivity, intrinsic mean free paths, heat capacities, mobilities, as well as paramagnetic and diamagnetic susceptibilities, for both electrons and holes. The optimized resonance frequencies are compared with energy differences between plausible interband transitions, in accordance with reported band structure diagrams and with our own band structure obtained from density functional theory calculations.

Ensemble properties of charge carriers injected by an ultrashort laser pulse

The average effective mass of charge carriers produced by an intense ultrashort laser pulse in a transparent solid increases significantly as the excitation mechanism changes from multiphoton transitions to interband tunneling. We theoretically investigate this phenomenon for several dielectrics and semiconductors. For diamond as a representative dielectric, we present a detailed analysis of the laser-induced change of optical properties. When the concentration of free carriers is high, we find that the average effective mass controls not only the intraband charge-carrier transport but also the interband contributions to the optical response. We observe that the excitation-induced birefringence is particularly large for parameters where the plasma response compensates for the linear response of an unperturbed solid.

Electron effective mass in Sn-doped monoclinic single crystal β-gallium oxide determined by mid-infrared optical Hall effect

The isotropic average conduction band minimum electron effective mass in Sn-doped monoclinic single crystal β-Ga2O3 is experimentally determined by the mid-infrared optical Hall effect to be (0.284 ± 0.013)m0 combining investigations on (010) and (2¯01) surface cuts. This result falls within the broad range of values predicted by theoretical calculations for undoped β-Ga2O3. The result is also comparable to recent density functional calculations using the Gaussian-attenuation-Perdew-Burke-Ernzerhof hybrid density functional, which predict an average effective mass of 0.267m0. Within our uncertainty limits, we detect no anisotropy for the electron effective mass, which is consistent with most previous theoretical calculations. We discuss upper limits for possible anisotropy of the electron effective mass parameter from our experimental uncertainty limits, and we compare our findings with recent theoretical results.

Optical, magnetic, and charge-carriers transport properties of a transition metal: bulk palladium.

Three parametric models of Drude-Lorentz (DL) are used to describe the spectral variation of the dielectric function of bulk palladium. An improved version of the acceptance-probability-controlled simulated annealing method is applied to optimize the values of the parameters involved in the models: high-frequency dielectric constant, free-electron collision frequency and corresponding relaxation time, oscillation strengths, nominal resonance frequencies, and Lorentzian widths. Normalization of the oscillation strengths allows the introduction of a new parameter in the context of the original DL model: the number density ratio, which is the ratio between the number density of conduction electrons and the number density of metal atoms. Inclusion of this parameter in the optimization procedure allows us to evaluate additional parameters related to the charge-carriers transport: the number density of conduction electrons, average effective mass of conduction electrons and holes, Fermi energy and electronic density of states at the Fermi energy, electrical resistivity, intrinsic mean-free path of conduction electrons, electronic heat capacity, Hall coefficient, as well as the mobilities of conduction electrons and holes. The paramagnetic and diamagnetic susceptibilities are also included as derived parameters. A parametric form of the bulk Pd dielectric function, with incorporation of the average local electric field effect in the Lorentz contribution, is reported.

Tuning the tunneling probability by mechanical stress in Schottky barrier based reconfigurable nanowire transistors

Mechanical stress is an established and important tool of the semiconductor industry to improve the performance of modern transistors. It is well understood for the enhancement of carrier mobility but rather unexplored for the control of the tunneling probability for injection dominated research devices based on tunneling phenomena, such as tunnel FETs, resonant tunnel FETs and reconfigurable Schottky FETs. In this work, the effect of stress on the tunneling probability and overall transistor characteristics is studied by three-dimensional device simulations in the example of reconfigurable silicon nanowire Schottky barrier transistors using two independently gated Schottky junctions. To this end, four different stress sources are investigated. The effects of mechanical stress on the average effective tunneling mass and on the multi-valley band structure applying the deformation potential theory are being considered. The transfer characteristics of strained transistors in n- and p-configuration and corresponding charge carrier tunneling are analyzed with respect to the current ratio between electron and hole conduction. For the implementation of these devices into complementary circuits, the mandatory current ratio of unity can be achieved by appropriate mechanical stress either by nanowire oxidation or the application of a stressed top layer.

Easily doped p-type, low hole effective mass, transparent oxides.

Fulfillment of the promise of transparent electronics has been hindered until now largely by the lack of semiconductors that can be doped p-type in a stable way, and that at the same time present high hole mobility and are highly transparent in the visible spectrum. Here, a high-throughput study based on first-principles methods reveals four oxides, namely X2SeO2, with X = La, Pr, Nd, and Gd, which are unique in that they exhibit excellent characteristics for transparent electronic device applications – i.e., a direct band gap larger than 3.1 eV, an average hole effective mass below the electron rest mass, and good p-type dopability. Furthermore, for La2SeO2 it is explicitly shown that Na impurities substituting La are shallow acceptors in moderate to strong anion-rich growth conditions, with low formation energy, and that they will not be compensated by anion vacancies VO or VSe.

Band Structure Effects in Extremely Scaled Silicon Nanowire MOSFETs With Different Cross Section Shapes

A multiscale simulation package based on ab initio calculation is used to study the band structure effects in extremely scaled gate-all-around silicon nanowire (SNW) MOSFETs with different cross-sectional shapes. All the interactions are computed directly from ab initio method without semiempirical parameters, and the effects of crystal atom relaxations and boundary atom dangling bond saturations are included intrinsically. The tight-binding Hamiltonian is obtained from ab initio results with maximally localized Wannier functions. The device performances are obtained with nonequilibrium Green’s function method. With this simulation flow, the band structure and its impacts on the device performance of SNWs with different cross-sectional shapes are studied. Simulation results show that the crystal relaxation has notable effects on the band structure with different cross-sectional shapes and sizes. The SNWs with triangular cross-sectional shape are very competitive due to a smaller average conductivity effective mass ascribed to the valley splitting at extremely scaled size.

Nearly free electrons in a 5d delafossite oxide metal.

Understanding the role of electron correlations in strong spin-orbit transition-metal oxides is key to the realization of numerous exotic phases including spin-orbit-assisted Mott insulators, correlated topological solids, and prospective new high-temperature superconductors. To date, most attention has been focused on the 5d iridium-based oxides. We instead consider the Pt-based delafossite oxide PtCoO2. Our transport measurements, performed on single-crystal samples etched to well-defined geometries using focused ion beam techniques, yield a room temperature resistivity of only 2.1 microhm·cm (μΩ-cm), establishing PtCoO2 as the most conductive oxide known. From angle-resolved photoemission and density functional theory, we show that the underlying Fermi surface is a single cylinder of nearly hexagonal cross-section, with very weak dispersion along k z . Despite being predominantly composed of d-orbital character, the conduction band is remarkably steep, with an average effective mass of only 1.14m e. Moreover, the sharp spectral features observed in photoemission remain well defined with little additional broadening for more than 500 meV below E F, pointing to suppressed electron-electron scattering. Together, our findings establish PtCoO2 as a model nearly-free-electron system in a 5d delafossite transition-metal oxide.

Polarization characteristics of 480–680 nm (1 1 [formula omitted] 2) InGaN/GaN quantum well structures with strain relaxation effects

The polarization characteristics as a function of the In composition for semipolar (112¯2) InGaN/GaN quantum well (QW) structures with strain relaxation effects are investigated by using the multiband effective-mass theory. We assumed that the strain relaxation ratio ∊y′y′0-∊y′y′/∊y′y′0 is linearly proportional to the difference in the lattice constants between the well and the substrate. The energy spacing between the first two subbands increases with increasing In composition, and the average hole effective mass of the first subband is reduced for a QW structure with a larger In composition. The [1¯1¯23] x′-polarized matrix elements are shown to be much larger than the [11¯00] y′-polarized matrix elements. As a result, the x′-polarized light emission is shown to be dominant and a weak function of the In composition. On the other hand, the light emission for y′-polarizations rapidly decreases with increasing In composition, mainly due to a decrease in the matrix elements. Thus, the absolute value of the polarization ratio rapidly increases with increasing transition wavelength.

The Effects of Direct Source-to-Drain Tunneling and Variation in the Body Thickness on (100) and (110) Sub-10-nm Si Double-Gate Transistors

Using an atomistic quantum simulation based on sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> d <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> s* tight-binding bandstructure and ballistic nonequilibrium Green's function, we have investigated the effects of sidewall orientation and the channel direction on transport characteristics of n-and p-type Si double-gate (DG) MOSFETs in sub-10-nm regime. Considering quantum confinement effects on average conductivity effective mass, ION-IOFF ratio of (100)/(100) devices turns out better under iso-IOFF compared with (110)/(110) devices for both n-and p-type DG MOSFETs. This is due to the effective mass tradeoff between direct source-to-drain tunneling and carrier mobility, leading to different net effects on n-and p-type DG MOSFETs. It is also shown that (100)/(100) devices have an advantage over (110)/(110) devices with respect to body thickness variation since their effective mass is less sensitive to quantum confinement effect.

Strain induced change of band structure and electron effective mass in wurtzite ZnO: A first-principles study

The first-principles hybrid density functional theory is applied to the contrastive investigation on the structural and electronic properties of wurtzite ZnO under hydrostatic strain and uniaxial strain along c axial, and the effect of strain on electron effective mass. It is observed that the structural transformation of the wurtzite ZnO occurs under tensile hydrostatic strain and compressive uniaxial strain. The transformation of ZnO from wurtzite phase to graphitelike phase is accompanied by the transition from a direct to indirect band gap. The deformation potentials that quantify the changes of band gap with strain are also reported. When the hydrostatic strain changes from compression to tension, the electron effective masses at Γ point in Σ, T and Δ directions, and average electron effective mass decrease linearly. For the uniaxial strain, the changes of electron effective mass are dissimilar to the hydrostatic results.

Optical Gain Characteristics in GaAsPN/GaPN Quantum Well Lasers for Silicon Integration

Optical gain characteristics of strain-compensated GaAsPN/GaPN quantum wells (QWs) with the GaPN barrier under 1.0 % tensile strain were investigated using the multiband effective-mass theory and the non-Markovian gain model. The transition energy linearly increases from 1.12 to 1.22 eV when P content ratio changes from 0.1 to 0.4. The theoretical transition energy reasonably agrees with the experiment. The Coulomb enhancement ratio, g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">many</sub> /g <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">free</sub> , decreases with increasing compressive strain because the average hole effective mass decreases with increasing strain. As a result, the QW structure with a smaller compressive strain has larger optical gain than that with a larger compressive strain for higher carrier densities. On the other hand, in the case of smaller carrier densities, the QW structure with a larger compressive strain shows larger optical gain than that with a smaller compressive strain because the former has larger optical matrix element producted by Kane's parameter and quasi-Fermi level separation than the latter. The absolute value of the bandgap renormalization increases with increasing carrier density and is about 99 meV at 5×10 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , which is slightly larger than that of GaAs-based or InP-based QW structures.

Geometrical and band-structure effects on phonon-limited hole mobility in rectangular cross-sectional germanium nanowires

We calculated the phonon-limited hole mobility in rectangular cross-sectional [001], [110], [111], and [112]-oriented germanium nanowires, and the hole transport characteristics were investigated. A tight-binding approximation was used for holes, and phonons were described by a valence force field model. Then, scattering probability of holes by phonons was calculated taking account of hole-phonon interaction atomistically, and the linearized Boltzmann's transport equation was solved to calculate the hole mobility at low longitudinal field. The dependence of the hole mobility on nanowire geometry was analyzed in terms of the valence band structure of germanium nanowires, and it was found that the dependence was qualitatively reproduced by considering an average effective mass and the density of states of holes. The calculation revealed that [110] germanium nanowires with large height along the [001] direction show high hole mobility. Germanium nanowires with this geometry are also expected to exhibit high electron mobility in our previous work, and thus they are promising for complementary metal-oxide-semiconductor (CMOS) applications.

Effects of depolarization on the electronic and the optical properties of InGaN/MgZnO quantum-well structures

The effects of depolarization on the electronic and the optical properties of InGaN/MgZnO quantum-well (QW) structures are investigated by using the multiband effective-mass theory. The average hole effective mass of the InGaN/MgZnO QW structure with a negligible internal field is shown to be similar to that of the conventional QW structure. On the other hand, the matrix element for the InGaN/MgZnO QW structure is improved greatly compared to that of the conventional InGaN/GaN QW structure because the internal field in the well is reduced. As a result, the light emission and the radiative recombination coefficient of the InGaN/MgZnO QW structure are found to be much larger than those of the conventional InGaN/GaN QW structure.

Crystal orientation effects on electronic and optical properties of wurtzite ZnO/CdZnO quantum well lasers

The study of electronic and optical properties of ZnO/CdZnO quantum well (QW) structures, considering the crystal orientation dependence and Gaussian line shape function for optical gain are explored including many body effects and spontaneous-piezoelectric built-in field. Results are confronted with those for GaN-based QW structures. The effect of internal field in the c-plane oriented ZnO/CdZnO QW structure is relatively small compared to that of GaN/InGaN QW structure and thus, a larger optical gain is shown while it disappears in the a- and m-planes for both QW structures where the optical gain as a result is match larger. Energy dispersion, transition strength and the average hole effective masses are anisotropic in non-polar structures. The bandgap transition wavelength of the QW structure with a-plane orientation is smaller than that of m-plane orientation by 2nm in ZnO/CdZnO QW while it is 10nm in the case of GaN/InGaN QW structure.

Theoretical study of the effect of W-doping on the conductivity of β-Ga2O3

The conductivity of W-doped β-Ga2O3 is investigated by using the ultra-soft pseudopotential (USP) approach of the plane-wave based upon density functional theory. The optimized structural parameters, total electron density of states, and energy band structures of β-Ga2(1-x)W2xO3 (x=0, 0.0625, 0.125) are calculated. It is found that the volumes are slightly increased and the total energies are going up in the Ga2(1-x)W2xO3 system with increasing W-doping concentration, which causes the system instability. When the W concentration is smaller, the calculated conductivity and electronic mobility are higher, but when the W concentration is increased, the average electron effective mass becomes bigger and the energy gap becomes narrower. The results are consistent with experimental data.

Optimization of the carrier concentration in phase-separated half-Heusler compounds

Phase separated half-Heusler compounds and the application of an average effective mass model to optimize the carrier concentration.

New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO2.

In general, anatase TiO2 exhibits higher photocatalytic activities than rutile TiO2. However, the reasons for the differences in photocatalytic activity between anatase and rutile are still being debated. In this work, the band structure, density of states, and effective mass of photogenerated charge carriers for anatase, rutile and brookite TiO2 are investigated by the first-principle density functional theory calculation. The results indicate that anatase appears to be an indirect band gap semiconductor, while rutile and brookite belong to the direct band gap semiconductor category. Indirect band gap anatase exhibits a longer lifetime of photoexcited electrons and holes than direct band gap rutile and brookite because the direct transitions of photogenerated electrons from the conduction band (CB) to valence band (VB) of anatase TiO2 is impossible. Furthermore, anatase has the lightest average effective mass of photogenerated electrons and holes as compared to rutile and brookite. The lightest effective mass suggests the fastest migration of photogenerated electrons and holes from the interior to surface of anatase TiO2 particle, thus resulting in the lowest recombination rate of photogenerated charge carriers within anatase TiO2. Therefore, it is not surprising that anatase usually shows a higher photocatalytic activity than rutile and brookite. This investigation will provide some new insight into understanding the difference of photocatalytic activity among anatase, rutile and brookite.

Electron effective mass in Al0.72Ga0.28N alloys determined by mid-infrared optical Hall effect

The effective electron mass parameter in Si-doped Al0.72Ga0.28N is determined to be m∗=(0.336±0.020) m0 from mid-infrared optical Hall effect measurements. No significant anisotropy of the effective electron mass parameter is found supporting theoretical predictions. Assuming a linear change of the effective electron mass with the Al content in AlGaN alloys and m∗=0.232 m0 for GaN, an average effective electron mass of m∗=0.376 m0 can be extrapolated for AlN. The analysis of mid-infrared spectroscopic ellipsometry measurements further confirms the two phonon mode behavior of the E1(TO) and one phonon mode behavior of the A1(LO) phonon mode in high-Al-content AlGaN alloys as seen in previous Raman scattering studies.