Light-hole Bands Research Articles

Low-temperature electrical conductivity and the superconductor-insulator transition induced by indium impurity states in (Pb0.5Sn0.5)1 − x In x Te solid solutions

A study is reported of the low-temperature electrophysical (including superconducting) characteristics of the (Pb0.5Sn0.5)1 − xInxTe semiconducting solid solutions with an indium content variable within x = 0.05–0.20. A decrease in the impurity content x in the material has been found to bring about a decrease in the superconducting transition temperature Tc and the onset of an “insulating” state of the material. These effects manifest themselves in an increase in the low-temperature (T = 4.2 K) resistivity of (Pb0.5Sn0.5)0.95In0.05Te by more than three orders of magnitude as compared to that of (Pb0.5Sn0.5)0.8In0.2Te. A decrease in the In content in the solid solution also gives rise to a radical change in the shape of the temperature dependence of the electrical resistivity from a metallic behavior in the material with x = 0.20 (decrease in the electrical resistivity with decreasing temperature in the range 300–4.2 K) to a semiconducting behavior in a sample with x = 0.05 (exponential increase in the resistivity at T < 25 K). This transition to the insulating state with decreasing content of the impurity should be assigned to the displacement of the impurity band of quasi-local indium states toward the top of the light-hole valence band of the material and its emergence into the band gap of the solid solution.

Light-hole and heavy-hole transitions for high-temperature long-wavelength infrared detection

Hole transitions from the heavy-hole (hh) to the light-hole (lh) band contributing to the 4–10 μm response range are reported on p-GaAs/AlGaAs detectors. The detectors show a spectral response up to 16.5 μm, operating up to a temperature of 330 K where the lh-hh response is superimposed on the free-carrier response. Two characteristic peaks observed between 5–7 μm are in good agreement with corresponding energy separations of the lh and hh bands and thus originated from lh-hh transitions. Results will be useful for designing multi-spectral detection which could be realized on a single p-GaAs structure.

Effects of symmetry-breaking perturbations on excitonic states bound to systems of reduced symmetry

Using an invariant expansion, we build an Hamiltonian describing the influence of the crystal-field, the electron-hole exchange interaction, and any symmetry-breaking perturbations on the fine structure of excitons bound to systems of reduced symmetry: D2d, C3v, and C2v. Several perturbations are considered, including, but not limited to, an electric field, a magnetic field, a strain field, and their combinations. For each symmetry system considered, symmetrized excitonic wave functions, build from heavy- and light-holes states, are used to expand the Hamiltonian in the form of matrices, whose eigenvalues directly provide the energy of the excitonic states and whose eigenstates can be used to determine oscillator strengths of optical transitions. Using this model, we satisfactorily reproduce the excitonic emission observed from nitrogen dyads in GaAs and tellurium dyads in strained ZnSe. We also present Hamiltonians for independent heavy- and light-hole subsystems. Comparing the two models, we demonstrate that the splitting observed in strained quantum dots of C2v symmetry does not necessarily imply a significant anisotropic exchange interaction. This splitting can be produced by a weak coupling between heavy- and light-hole bands.

Optical study of GaAs 1− xSb x layers grown on GaAs substrates by gas-source molecular beam epitaxy

The optical properties of GaAs 1− x Sb x (5.9% ≤ x ≤ 9.7%) layers grown on GaAs substrates by gas-source molecular beam epitaxy were characterized by photoreflectance (PR) and piezoreflectance (PzR). Identification of conduction to heavy-hole (HH) band and conduction to light-hole (LH) band transitions originated from the strained induced valence band splitting have been achieved by comparing the relative intensities of PR and PzR spectra. The results indicate increases of the valence band splitting with increasing of Sb content. The temperature dependences of near band edge transition energies were analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 to 300 K. The parameters that describe the temperature variations of the near band edge transition energies and broadening function were evaluated and discussed.

Dispersive resonance bands within the space-charge layer of a metal-semiconductor junction

Based on measurements of angle resolved photoemission, we report that in the Pb/Ge(111)- \sqrt{3}x\sqrt{3} R30^\circ structure, in addition to three bands resembling Ge heavy hole (HH), light hole (LH), and split off (SO) bulk band edges, a fourth dispersive band resembling the non split off (NSO) band is found near the surface zone center. While three Ge bulk-like bands get distorted due to strong coupling between Pb and Ge, the NSO-like band gets weaker and disappears for larger thickness of Pb, which, when combined with ab initio calculations, indicates its localized nature within space charge layer. Our results are clearly important for designing electronics involved with metal-semiconductor contacts.

Self-consistent solution of the Schrödinger-Poisson equations for hydrogen-terminated diamond

A self-consistent one-dimensional solution of the Poisson and Schr\odinger equations is developed to describe the electronic properties of the hole gas confined below the hydrogen-terminated surface of surface conducting diamond. The energy eigenstates of the confinement potential and the position of the Fermi level are examined as a function of carrier sheet density and temperature. A comparison is made with experimental studies of the hole sheet density for both atmosphere-induced and synthetic surface acceptors. For hole sheet densities below $9\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$ the Fermi level is within the band gap while for hole sheet densities greater than $9\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }{\text{cm}}^{\ensuremath{-}2}$ the Fermi level resides below the valence-band maximum in the valence band. The dimensionality of the heavy-hole, light-hole, and split-off bands was examined for varying Fermi energy and temperature.

Strain of GaAs/GaAsP Superlattices Used as Spin-Polarized Electron Photocathodes, Determined by X-Ray Diffraction

The strains in GaAs/GaAsP superlattices used in spin-polarized photocathodes grown on GaAs and GaP (001) substrates were determined by X-ray diffraction. The thicknesses of the GaAs wells and GaAsP barrier layers were also determined. The band structures of the superlattices were calculated on the basis of these experimentally determined strains and layer thicknesses. The thicknesses and band structures were in good agreement with those observed by transmission electron microscopy and photoluminescence, respectively. The strains induced in the GaAs well layers were approximately linearly dependent upon the phosphorous fraction in the GaAsP layer, and the splitting between the heavy hole band and the light hole band of the superlattices grown on GaP substrates was larger than that of superlattices grown on GaAs substrates. In photocathodes grown on GaP substrates, low polarizations were observed, not due to a lack of band splitting, but to depolarization scattering caused by crystal defects, which were different from that induced in superlattices grown on GaAs substrates. [DOI: 10.1380/ejssnt.2010.125]

Ferromagnetic properties of selectively Mn-doped (Ga,Mn)As quantum wells

We investigate temperature dependence of spin-resolved subband structure and spontaneous magnetization for different doping profiles of magnetic impurities in diluted (Ga,Mn)As quantum well structures. The self-consistent hole subband energies and wave functions are determined by solving combined Schr\"odinger and Poisson equations numerically. We included the coupling effects among heavy-hole, light-hole, and split-off bands (six-band model) in our calculation and compared the result with the case of two-band model. We show that selective doping of magnetic ions in GaAs quantum well enhances the Curie temperature ${T}_{c}$ significantly, but holes occupying light-hole subbands result to reduce spontaneous magnetization and hence the Curie temperature of the system. This reduction effect reveals more clearly near the $T\ensuremath{\sim}{T}_{c}$ hindering an increase in the Curie temperature in a selectively Mn-doped system.

The characterization of the hole transport in Sb based strained quantum wells

Compressively strained InSb structures aimed for p-channel heterostructure field effect transistors (HFETs) are analysed in order to maximise their hole mobility. We optimise the heterostructure by the change of the material composition, thickness of the layers and position of δ-doping. The splitting of light and heavy hole bands in compressively strained channels are calculated by nextnano3 software as a function of material mole fraction and quantum well thickness. The 8 × 8 k.p method is used for the determination of corresponding hole eigenvalues. The strained layer bandstructure calculation is carried out with Quantumwise ATK for the Kohn-Sham DFT using double-zeta polarised basis set orbitals as well.

Infrared dielectric function ofp-typeGe0.98Sn0.02alloys

The dielectric function of heavily doped $p$-type ${\text{Ge}}_{0.98}{\text{Sn}}_{0.02}$ alloys was determined using infrared spectroscopic ellipsometry. The free holes contribute a Drude-like term to the dielectric response, from which the resistivity and carrier relaxation time can be determined. The transport parameters extracted from such optical experiments are similar to those found for pure Ge with comparable doping levels. Optical transitions between the split-off, light-hole, and heavy-hole bands contribute a second term to the dielectric function. The overall line shape of the dielectric function corresponding to these transitions shows differences between ${\text{Ge}}_{0.98}{\text{Sn}}_{0.02}$ and pure Ge. These differences may provide important clues to the valence-band electronic structure of ${\text{Ge}}_{1\ensuremath{-}y}{\text{Sn}}_{y}$ alloys.

Hole mobility enhancement by double-gate mode in ultrathin-body silicon-on-insulator p-type metal-oxide-semiconductor field-effect transistors

Hole mobility enhancement in double-gate (DG) ultrathin-body (UTB) silicon-on-insulator (SOI) p-type metal-oxide-semiconductor field-effect transistors (MOSFETs) is investigated systematically in comparison with single-gate (SG) UTB MOSFETs for various SOI thicknesses (TSOI) ranging from 2 to 30 nm. It is found that mobility in DG mode (μDG) is higher than that in SG mode (μSG) in all the measured ranges of TSOI and the surface carrier concentrations (Ns). In particular, enhancement of mobility of the sub-10-nm-thick TSOI devices is greater at higher Ns. As a result, it is demonstrated that μDG of the 8.1-nm-thick TSOI device is 12.9% greater than the universal mobility when Ns is 6×1012 cm−2. Higher μDG is attributed to the average effective mass reduction in DG mode because of the increased population in the light hole (LH) bands. Careful investigation also clarifies that mobility in the sub-10-nm-thick TSOI devices is enhanced greatly at higher Ns, since the DG mode-induced increase in the hole population in LH bands is enhanced at higher Ns; the reduction in the hole population in LH bands due to the high-Ns-enhanced carrier confinement in the normal direction is larger in SG mode than in DG mode.

Optical characterization of thin epitaxial GaAs films on Ge substrates

Two thin epitaxial GaAs films on Ge substrates were characterized by photoluminescence (PL), contactless electroreflectance (CER), and piezoreflectance (PzR) techniques. The GaAs films containing antiphase domains (APDs) and those APD free were grown at 650 °C by metal organic vapor phase epitaxy. The PL intensity of the APD-containing film is four orders of magnitude lower than that of the APD-free sample. The reduction in intensity is due to the electrically active Ga–Ga and As–As bonds at the boundaries between the different APDs. The electric fields deduced from Franz–Keldysh oscillations of the CER spectra were used to determine the net carrier concentration of the GaAs films. In addition, the conduction to light-hole band transitions and the conduction to heavy-hole band transitions that originated from strain induced valence band splitting in GaAs thin films were observed and identified through a comparison of the relative intensities of light- and heavy-hole related features in the PzR and CER spectra. The results demonstrate that PL, CER, and PzR are useful nondestructive tools to characterize thin epitaxial GaAs films on Ge substrates.

Accurate band structures and effective masses for InP, InAs, and InSb using hybrid functionals

The band structures of $\text{In}\text{ }X$ ($X=\text{P}$, As, and Sb) are calculated using the hybrid HSE06 functional and $GW$ with spin-orbit coupling effects included up to second order. Conventional local or semilocal density functionals predict an incorrect band ordering for InAs and InSb when spin-orbit coupling is included. We show that inclusion of one quarter of the exact exchange allows us to predict very accurate band gaps for InP, InAs, and InSb, i.e., 1.48, 0.42, and 0.28 eV, respectively, in good agreement with recent experiments. Furthermore, calculated effective masses for the conduction band (electron) and the valence band (heavy-hole, light-hole, and split-off band) are in fairly good agreement, whereas the values obtained using semilocal functionals deviate significantly from experiment even for InP. The calculated Luttinger parameters are also in reasonable agreement with experiment, although a tendency toward underestimation is observed with increasing anion mass. This underestimation is shown to be partially related to a tendency to overestimate the band gaps for the heavier elements using the particular hybrid functional used in this study. By adjusting the screening parameter in the hybrid functional, agreement with the experimental band gap can be achieved, but even then the Luttinger parameters are 15% smaller than the experimental values for InAs and InSb.

Packaging-Induced Strain Measurement Based on the Degree of Polarization in GaAsP–GaInP High-Power Diode Laser Bars

The strain caused by device packaging was studied in high-power semiconductor laser bars by measuring the degree of polarization. Polarization measurement with intentionally GaAsP-GaInP strained laser bar packaged on a Cu heat sink for 799-nm emission revealed the variation of band edges between the conduction band and heavy-hole or light-hole bands in the active region. This served as a method for evaluating the strain. In the packaging process, a maximum of 1800-ppm strain was transmitted to the active region. It was found that the defect density of 14.3% was induced.

Finite Cylinders of Si1 − x Ge x Alloy Under the Double-Punch Test and Effect on Three Valence-Bands

An analytical solution for the stress and strain fields within a finite circular solid cylinder of Si1 − xGex alloy under the double-punch test is presented. The method of solution uses the displacement function approach, together with proposing a new expression for the displacement function so that all of the boundary conditions are satisfied exactly. The present solution covers the solution by Wei and Chau [1] for isotropic cylinders under the double-punch test when the isotropic limit case is considered. Numerical results show that strain and stress distributions within the cylinder are not uniform, and strain or stress concentrations are usually developed near the end surfaces, but the strain and stress distributions within the central part are relatively uniform. Moreover, according to quantum mechanics and the energy band theory, the effects of strain on three valence bands, namely the heavy hole (HH) band, the light hole (LH) band and the split-off (SO) band of Si1 − xGex alloy and the corresponding conductivity masses are analyzed. It was found that the quantum behavior of the three valence bands, such as the energy distribution, the shape of constant energy surfaces, the magnitude and the distribution pattern of the conductivity masses can be modified considerably by the external force applied in the double-punch test, depending on the proportion between silicon and Germanium in Si1 − xGex alloy.

High-Brightness Spin-Polarized Electron Source Using Semiconductor Photocathodes

We developed AlGaAs photocathodes with low electron affinity for long negative electron affinity (NEA) lifetime. AlGaAs photocathodes achieved 10 times longer NEA lifetime than conventional NEA-GaAs photocathodes. We estimated the appropriate superlattice structure for small conduction mini band width, high density of state in the conduction band and high splitting energy between the heavy- and light-hole bands by theoretical energy band calculation. We conclude that the AlGaAs–GaAs superlattice semiconductor is a suitable NEA-GaAs photocathode that not only has acceptable NEA lifetime but also fulfills the requirements of small energy spread of photoelectrons, high quantum yield and highly spin-polarized electrons.

Optical Switching Based on the Conditional Faraday Effect With Electron Spins in Quantum Dots

We present the theoretical foundation of a novel concept for optical switching based on the conditional Faraday effect with electron spins in quantum dots (QDs). Calculations for light of wavelengths lambda = 1.3 mum and lambda = 1.55 mum show devices with active regions of a few hundred microns are possible. Case studies for both spherical and nonspherical QDs are considered, with calculations for the nonspherical case showing a 20% reduction in the required device length due to the lifting of the light and heavy hole band degeneracy.

Piezoreflectance and photoreflectance study of annealing effects on GaAs0.916Sb0.084 and GaAs0.906Sb0.075N0.019 films on GaAs grown by gas‐source molecular beam epitaxy

AbstractThermal annealing effects of GaAs0.916Sb0.084 and GaAs0.906Sb0.075N0.019 films grown on GaAs substrates by gas‐source molecular beam epitaxy have been characterized by piezoreflectance (PzR) and photoreflectance (PR). By a comparison of relative intensity of PzR and PR spectra, the identification of conduction to heavy‐hole (HH) band and conduction to light‐hole (LH) band transitions originated from the strained induced valence band splitting have been achieved. The near band edge transition energies are blue‐shifted, and the splitting of HH and LH bands is reduced after thermal annealing treatment. The annealing effects of GaAs0.906Sb0.075N0.019 are found to be more pronounced than that of GaAs0.916Sb0.084. The temperature dependences of near band edge transition energies are analyzed using Varshni and Bose–Einstein expressions in the temperature range from 15 K to 300 K. The parameters that describe the temperature variations of the near band edge transition energies are evaluated and discussed. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Superconducting gap structure of heavy-Fermion compound URu2Si2 determined by angle-resolved thermal conductivity

In heavy-Fermion compound URu2Si2, unusual superconductivity is embedded in an enigmatic ‘hidden order’ phase. Recently, it has been shown that URu2Si2 is essentially a multiband superconductor associated with the semimetallic compensated electronic structure. Here, to pin down the detailed superconducting gap structure, we have performed thermal transport measurements on ultraclean URu2Si2 single crystals in magnetic fields rotating various directions relative to the crystal axes. By changing the amplitude of magnetic fields, we determined the nodal topology of electron and hole band separately. The results indicate a new type of unconventional superconductivity with two distinct gaps, in which horizontal line nodes lie within the basal ab plane of the light-hole band with small gap and point nodes along the c-axis in the heavy electron band with large gap. This gap structure is consistent with ‘chiral’ d-wave symmetry with a form .

Rashba spin-orbital splitting in kane type quantum anti-wires

In the present study, using three-band Kane's model including the conduction band, light and spin-orbital hole bands, the electronic states of a semiconductor quantum anti-wire is studied in presence of Rashba spin-orbital interaction and external magnetic field and compared with those of a quantum anti-wire of the same size. It's calculated the radii, height, coupling strength and magnetic field dependence of Rashba splitting for carriers respectively. It has been found for the InSb quantum anti-wire that Rashba splitting of electrons are decreased with the increasing of radius.