Showing papers on "Effective mass (solid-state physics) published in 1986"

Resonant tunneling through double barriers, perpendicular quantum transport phenomena in superlattices, and their device applications

Abstract: New results on the physics of tunneling in quantum well heterostructures and its device applications are discussed. Following a general review of the field in the Introduction, in the second section resonant tunneling through double barriers is investigated. Recent conflicting interpretations of this effect in terms of a Fabry-Perot mechanism or sequential tunneling are reconciled via an analysis of scattering. It is shown that the ratio of the intrinsic resonance width to the total scattering width (collision broadening) determines which of the two mechanisms controls resonant tunneling. The role of symmetry is quantitatively analyzed and two recently proposed resonant tunneling transistor structures are discussed. The third section deals with perpendicular transport in superlattices. A simple expression for the low field mobility in the miniband conduction regime is derived; localization effects, hopping conduction, and effective mass filtering are discussed. In the following section, experimental results on tunneling superlattice photoconductors based on effective mass filtering are presented. In the fifth section, negative differential resistance resulting from localization in a high electric field is discussed. In the last section, the observation of sequential resonant tunneling in superlattices is reported. We point out a remarkable analogy between this phenomenon and paramagnetic spin resonance. New tunable infrared semiconductor lasers and wavelength selective detectors based on this effect are discussed.

Correction to "Reduction of lasing threshold current density by the lowering of valence band effective mass"

Abstract: In present day semiconductor lasers, there is a serious asymmetry between the very light conduction band mass and the very heavy valence band mass. Under laser threshold conditions, the hole occupation remains classical even while the electrons are degenerate. This results in a significant penalty in terms of threshold current density, carrier injection level, and excess Auger and other nonradiative recombination. We propose a combination of strain and quantum confinement to reduce the valence band effective mass and to lessen the laser threshold requirements.

Zero-dimensional "excitons" in semiconductor clusters

Abstract: Semiconductor crystallites of characteristic dimension \sim20-100 A can be made by precipitation in liquids and dielectrics. These crystallites have bulk-like internal lattices. The optical spectra show partially resolved discrete features that result from carrier spatial confinement in three dimensions. Spectral shifts of more than 1 eV are observed. The electronic wavefunctions of small crystallites are discussed within the effective mass approximation. The pattern of discrete hole states is qualitatively different in small crystallites than in slab superlattices. In small CdS crystallites, luminescence is observed from trapped carriers that may be in localized surface states. The recombination emission is strongly coupled to crystallite phonons.

Superconductivity and Magnetic Order in a Strongly Interacting Fermi-System: URu2Si2

Abstract: Measurements are reported on the electrical resistivity, susceptibility, dc-Meissner effect and specific heat of the tetragonal U-compound URu2Si2. At T N ≃17 K, an antiferromagnetic phase transition is observed. This magnetically ordered state appears to coexist with superconductivity, which is found below T c ≃1.5 K. An analysis of the upper critical field data reveals rather unusual properties of both the normal-and the superconducting state. The effective mass of the quasiparticles at low temperatures is substantially enhanced (m*≃50 m 0), though the measured low-temperature specific heat coefficient γ does not exceed 65 mJ/K2mol.

Density and magnetic field dependences of the conductivity of two-dimensional electron systems

Abstract: Based on a model in which a localised state lies below a conduction band, the density and magnetic field dependences of the magnetoconductance of two-dimensional electron systems are determined for low/intermediate magnetic fields. When plotted against electron density, the magnetoconductance shows a broad maximum which is shifted towards a larger electron density and lowered when the magnetic field is increased. Formulae for the resistivity, density of states, effective mass and relaxation time are also derived.

Enhanced photoredox chemistry in quantized semiconductor colloids

Abstract: This, in turn, leads to enhanced redox potentials for photoexcited electrons and holes in very small semiconductor particles In this Letter, we present both photochemical and photophysical evidence for large quantization effects in small-sized HgSe, PbSe, and CdSe colloids To a first approximation, the energy of the quantized levels is inversely proportional to the effective mass and the square of the particle diameter In our study, HgSe and PbSe colloids were used because they have very small electron effective masses (-005) Since the band gaps of HgSe and PbSe are both 03 eV, colloids consisting of large particle sizes (>500 A) are black and opaque; colloids with small particle sizes exhibit different colors depending on the size CdSe, with an effective mass of 01 and a band gap of 17 eV, was studied for COz reduction since it has a greater stability against photocorrosion compared to HgSe and PbSe Colloids with extremely small particle sizes were made by controlled precipitation of metal selenide in water or acetonitrile in the presence of stabilizers, such as (NaP03)6, SO2, Nafion, styrene/maleic anhydride copolymer, and polyethylene glycol All these colloids show large blue shifts in their absorption edge and in their emission spectra All the stabilizers were checked for their resistance against reduction processes For this purpose, T1° colloids were used as the reducing agent; its standard redox potential is -19 V vs NHE We found that only Si02 and Nafion are very stable in the presence of TlO, and therefore only these stabilizers were used to study photoredox chemistry SiO, colloids9 with a particle diameter of 50 A were used for the preparation of selenide colloids in water The colloids were prepared in water-alcohol mixtures at -20 OC, and the sols were then filtered through membrane filters with a pore size of 100 A For HgSe, 20 mL of solution containing 2 X M HgC12, 6 X M CH3COONH4 (buffer) were M SOz, and 2 X

Resonant magnetotunneling in GaAlAs-GaAs-GaAlAs heterostructures

Abstract: We report the observation of resonant tunneling of electrons through Landau levels in double-barrier GaAlAs-GaAs-GaAlAs heterostructures, in the presence of a strong magnetic field perpendicular to the interfaces. This is a two-dimensional magnetotunneling effect which manifests itself as periodic structures in the current-voltage characteristics, with a period proportional to the electron cyclotron energy in the GaAs quantum well, from which the electron effective mass is determined.

Photocurrent spectroscopy in GaAs/AlGaAs multiple quantum wells under a high electric field perpendicular to the heterointerface

Abstract: Photocurrent (PC) spectroscopy under high electric fields perpendicular to the heterointerface has been utilized to characterize subband structures in GaAs/AlGaAs multiple quantum wells (MQW’s). The PC spectra from MQW’s under high electric fields show clear exciton peaks corresponding to forbidden transitions between electron and hole subbands. From PC spectra, we have precisely determined the splitting energies of the two lowest electron and four highest hole subbands in 55‐, 82‐, and 105‐A‐thick wells. The experimental results indicate that the conduction‐band offset is 60±3% of the band‐gap discontinuity. Our data are also in excellent agreement with finite square well calculations using a heavy‐hole effective mass of 0.34 and a light‐hole effective mass of 0.12.

HgTe-CdTe superlattice subband dispersion

Abstract: The results of band-structure calculations for (001) and (111) HgTe-CdTe superlattices using a multiband tight-binding model are presented. The band structures of superlattices in the semiconducting and semimetallic regimes are found for directions both parallel and perpendicular to the growth direction. The tight-binding model automatically incorporates the correct space-group symmetry of the superlattice. Band mixing, band crossings or anticrossings, degeneracies, and spin splittings are therefore correctly produced. For semiconducting superlattices the strain-induced reversal of light- and heavy-hole subbands depends on growth orientation as well as layer thicknesses. The light-hole subband, as defined by the effective mass in the growth direction, is found to be higher than the heavy-hole subband for the (001) 50-A\r{}--40-A\r{} superlattice by 9.2 meV, but lower for the (111) superlattice with similar layer thicknesses. This feature is somewhat sensitive to the precise deformation potentials and bulk valence-band parameters assumed in the calculation. The semi- metallic superlattice which had been the subject of magnetoabsorption experiments is studied in detail. In agreement with the calculation of Wu and McGill, it is found that the inclusion of strain significantly distorts the valence-band-edge band structure, implying that a reappraisal of the 40-meV HgTe-CdTe valence-band-offset determination is needed. It is found that the stress opens up a gap at the Brillouin-zone center, but that the superlattice is still semimetallic due to a stress-induced indirect valence-band maximum with energy above that of the conduction-band minimum. The conduction-band-minimum wave function has a large interfacial component. The relative magnitude of the amplitude of this component increases approximately with the inverse square root of the energy of the state.

Effect of conduction‐band nonparabolicity on quantized energy levels of a quantum well

Abstract: The effects of conduction‐band nonparabolicity on quantized energy levels of an In0.5Ga0.5As /In0.5Al0.5As quantum well have been analyzed using the original Luttinger–Kohn ‘‘effective mass’’ equation, which is, in principle, valid as long as the perturbation to the periodic lattice potential can be regarded as slowly varying. The results differ substantially from those previously reported which employed the energy‐dependent effective mass approach.

Magneto-optical studies of n-GaAs under high hydrostatic pressure

Abstract: Far-infrared laser spectroscopy and Fourier spectroscopy are used in order to study the influence of hydrostatic pressure on shallow donor and cyclotron resonance spectra in high-purity epitaxial films of n-GaAs. Accurate pressure variations for the effective mass and dielectric constant are obtained at a temperature of 4.2K and pressures of up to 17.3kbar. These parameters are subsequently used to interpret the pressure variation of the chemical shifts of the 1s and 2s levels. It is shown that the dependence of these chemical shifts on magnetic field at fixed pressure can be described well by lowest-order perturbation theory. The ratio of the chemical shifts of the 1s level to the corresponding chemical shifts of the 2s level at zero magnetic field is found to be pressure independent and equal to 7.2+or-0.3 for all donors. A much stronger increase of chemical shifts than that predicted by lowest-order perturbation theory is observed on increasing pressure. In addition, a resonant interaction of the 1s level of the deepest shallow donor with a strongly localised level of the same impurity centre is observed on increasing pressure. A qualitative interpretation of all these phenomena is given.

Current–voltage characteristics through GaAs/AlGaAs/GaAs heterobarriers grown by metalorganic chemical vapor deposition

Abstract: Current–voltage characteristics of GaAs/AlxGa1−xAs /GaAs heterobarriers grown by metalorganic chemical vapor deposition were investigated for x from 0.32 to 0.46. Calculation of the current, which included just two components—a tunneling component and a thermionic component—agreed well with experimental results. The tunneling effective mass used is the same as the Γ point effective mass when x is 0.32; this value allows the calculated results to fit the experimental data. The tunneling effective mass becomes much larger than the Γ point mass as x increases. This fact suggests that the other band edge X participates in the tunneling process, making the effective mass larger as it approaches the Γ point edge. The barrier heights deduced from I–V and I–T relations are in good agreement. The results support a ratio of 60%–65% of AlGaAs/GaAs conduction‐band discontinuity to the total band‐gap difference.

Thermionic emission in heterosystems with different effective electronic masses.

Abstract: It is shown that the usual way of calculating thermionic emission takes into account incorrectly the conservation law of the quasimomentum of the electrons on the boundary separating two regions with different effective masses of electrons. Therefore, at a thermodynamic equilibrium the opposite flows of the electrons crossing this boundary are different (because the Richardson constants are different). This paradox disappears if one takes into account that not all electrons with larger effective mass can cross the boundary even if the part of their kinetic energy perpendicular to the interface is larger than the barrier height. In fact this condition is contained in the quantum-mechanical coefficient of electron transmission. It is shown that in the spherical and parabolic effective-mass approximation the Richardson constant is defined by the smallest effective mass. This fact is important in the calculations of the thermionic currents in semiconductor contact phenomena especially in heterostructures. It makes clear what effective mass must be used in the definition of the pre-exponent factor of the thermionic current.

Electrical conduction and effective mass of a hole in single-crystal NiO

Abstract: The electrical conductivity and Seebeck coefficient of single-crystal NiO were measured at 1100 to 1400° C. The activation energies for conductivity and thermoelectric power were 84.1 and 76.5 kJ mol−1, respectively, which was interpreted as showing band-like conduction occurring at high temperatures. Doubly ionized cation vacancies were believed to be the dominant point defects, and the impurity effect gave rise to a $$P_{{\text{O}}_{\text{2}} }^{1/5} $$ dependence of conductivity. The effective mass of a hole, m *, was calculated based on a band-conduction scheme using the literature value of the deviation from stoichiometry, δ, and the hole mobility μ. The value of m * was in the range 0.8 to 1.0m 0, and slightly decreased with increasing temperature.

Perimeter corrections to the Landau diamagnetism

Abstract: If the free electron gas is enclosed in a box of finite volume the Dirichlet boundary condition imposed on the wavefunction modifies the density of states (it increases the energy levels). This is also true in presence of a uniform magnetic field and gives rise to the perimeter corrections chi ' to the Landau diamagnetic susceptibility chi 0. The author analyses the effect for the zero-field susceptibility by a Green function approach rather than by enumerating the energy levels. The perimeter contribution chi ' to the susceptibility is always positive (paramagnetic). The relative correction chi '/ chi 0 is given by (apart from a constant of order unity) l*surface area/volume, where l is a characteristic length and is equal to the thermal de Broglie wavelength at high temperatures and to the Fermi wavelength in another extreme of complete degeneracy. Thus the effect may be observable in small metallic particles of size 10-100 AA, in particular if the electron effective mass is small such as, e.g., in bismuth.

Theoretical study of hole transport in ZnSe

Abstract: Hole mobilities in ZnSe were calculated by solving the Boltzmann transport equation by the variational method, with all major scattering mechanisms included. Valence‐band state symmetries were accounted for, both intra‐ and interband scattering terms for light and heavy holes were included, and generalized Fermi–Dirac statistics were used throughout. Screening of polar optical phonon and ionized impurity scatterings was also included. The contributions of all component scattering mechanisms to the total hole mobility as a function of hole concentration at 300 and 77 K, have been calculated for the first time. The influence of the compensation ratio, and the concentration of ionized impurities on both the hole mobility and resistivity has also been calculated for the first time; these results are shown to facilitate rapid and direct evaluation of electronic materials quality. In addition, the theoretical model was used to calculate the temperature dependence of the hole mobility in ZnSe, which proved to be...

Electrical transport of holes in GaAs/InGaAs/GaAs single strained quantum wells

Abstract: We report electrical transport data for holes in single strained quantum well structures of the type GaAs/InxGa1−xAs/GaAs with x≊0.2. With modulation doping, 4 K mobilities of ∼3×104 cm2/V s have been achieved. This value is near that attained for electrons in comparable structures, illustrating the enhanced transport possible due to the strain‐induced light‐hole planar mass.

Calculation of phonon‐assisted tunneling between two quantum wells

Abstract: We have calculated the longitudinal optical phonon scattering between two subbands of a system of two coupled quantum wells. Our model takes the delocalization of the wave function over the whole structure into account, thus avoiding the shortcomings of earlier computations for thin barriers. The dependence on the various parameters (width of the wells and of the barrier, band offset, voltage drop between the wells, temperature, and effective mass of carriers) is discussed and illustrated.

London penetration depth of heavy-fermion superconductors.

Abstract: The dynamic effective mass in heavy-fermion systems is different from the bare mass, unlike in one-component Fermi liquids. This is used to explain the appearance of a heavy mass in the London penetration depth at all temperatures. ${F}_{0}^{s}\ensuremath{\sim}{10}^{2}$ and ${F}_{1}^{s}\ensuremath{\sim}1$ are found, reflecting the strong frequency dependence but weak momentum dependence of the conduction-electron self-energy.

Refractive index of hexagonal II–VI compounds CdSe, CdS, and CdSeXS 1– X

Abstract: The theoretical refractive index of the direct-gap II–VI compounds CdSe, CdS, and CdSexS1–x with the wurtzite lattice structure is calculated for frequencies below the fundamental absorption edge. Use is made of the band structure of the Kane theory, modified to include the dependence of the refractive index on the effective hole masses of the three valence bands, the lattice constants a(A) and C(A) of the hexagonal structure, and the direction of polarization of the incident wave. An extension of the previous result for the refractive index of a cubic zinc blende crystal, in which birefringence is absent, to the hexagonal wurtzite structure, where birefringence occurs, is given. Results are obtained for the refractive indices of the ordinary and extraordinary rays in terms of experimentally available quantities, which include the direct-band-gap energy G, the effective electron mass mn, the three effective hole masses mp1,mp2, and mp3 of the three valence bands at k = 0, and lattice constants a(A) and C(A). Numerical values of the refractive index for values of x in the range 0 ≤ x ≤ 1 are given and compared with experimental results.

‘‘Ballistic’’ injection devices in semiconductors

Abstract: ‘‘Ballistic’’ electron transistors are of considerable interest for high‐frequency operation. Regardless of the mechanism of electron injection or collection it is anticipated that device performance will be dominated by base transit dynamics. We address this issue by calculating the scattering rate for hot electrons in selected semiconductor materials holding some common band structure and transport properties. It is shown that the scattering rate is critically dependent on the carrier concentration and that GaAs is not suitable for fabrication of traditional ‘‘ballistic’’ electron transistors. We suggest that semiconductors with small effective electron mass or a two‐dimensional system would be more suitable.

Effective mass of neutrons diffracting in crystals.

Abstract: Neutrons propagating in a crystal under diffraction conditions exhibit an effective inertial mass which is lower by 5--6 orders of magnitude than their vacuum rest mass and of both positive and negative sign. This is verified experimentally by measurement of the enormously enhanced deflection of neutrons subjected to a magnetic force while passing through a silicon crystal.

Temperature Dependence of the Effective Masses in III–V Semiconductors

Abstract: Effective masses in GaAs, InSb and InP are determined in the temperature range from 77 to 300 K from the magnetophonon resonance experiments. An expression for the conduction band effective mass valid for zinc blende crystals is derived by the \(\bm{k.p}\) perturbation theory and by taking into account the higher lying bands, where two momentum matrix elements P 0 and P 1 appear. The momentum matrix elements are estimated from the parameters determined from the energy band calculations. Temperature dependence of the effective mass is calculated using the expression derived in this paper and dilational change of the energy gaps which are estimated from the pseudopotential method and the experimental data of the linear thermal expansion coefficient. Reasonable agreement is obtained for GaAs but agreement is not so good in InSb and InP.