Short-period InAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb superlattices may allow strong optical transitions in the long-wavelength infrared (>10 μm) spectral region. Absorption calculations can be difficult, however, because of the strongly type-II interface and because of the large lattice mismatch. We present a comparative study of band-structure calculations for strained-layer type-II InAs/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$Sb superlattices grown on GaSb. The energy of superlattice band gaps (${\mathit{E}}_{\mathit{g}\mathit{s}}$) and the cutoff wavelengths (${\lambda }_{\mathit{c}}$) are computed in the empirical tight-binding, effective-bond-orbital, and 8×8 k⋅p models. In the empirical tight-binding model (ETBM) the strain is included by scaling the matrix elements according to Harrison’s universal 1/${\mathit{d}}^2$ rule and by appropriately modifying the angular dependence. The bond-orbital model (EBOM) and k⋅p calculations include the strain via the deformation-potential theory. We find in all cases that the superlattice band gap decreases rapidly with increasing x and that the proper inclusion of strain is critical in the ETBM. Our results compare favorably with existing experiments. In addition, we compare directly the results of the EBOM and k⋅p models. Contrary to expectations, the two models give quite different results for InAs/InSb superlattices.

We present a study of the emission of electrons from a well into the conduction band of the associated barrier. The process involved is not the usual thermionic one but a defect-assisted tunneling process, occurring from the quantum states of the well to the energy levels of the conduction band of the associated barrier, and via the energy level associated with a defect located in the barrier. This tunneling process occurs because there is band bending of the conduction band associated with the charge located in the well. This mechanism results in an apparent lowering of the band offset, which is a function of the ionization energy ${\mathrm{\Delta }}_0$ associated with the defect. Evidence of this effect is obtained on a ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As/${\mathrm{Ga}}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{-}\mathit{y}}$As/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{In}}_{1\mathrm{-}\mathit{x}}$As quantum well. The apparent offset lowering is measured using the admittance spectroscopy technique and is consistent with theory.

We present a supercell model calculation of the vibrational properties and Raman response of (113)-oriented GaAs/AlAs superlattices. Phonon-dispersion relations are calculated both along the growth axis and in the normal planes. The angular dispersion of the zone-center optical phonons is calculated as well, and some unusual features are discussed. We also calculate the Raman response in the backscattering geometry: to this aim, the bond polarizability model has been suitably extended to the case of arbitrarily oriented superlattices. The effects of the interface cationic diffusion on the Raman spectra are discussed. A comparison with experimental data is finally presented and discussed.

We investigate theoretically the intrasubband collective plasmon modes and the optical transmission in a specially constructed semiconductor superlattice consisting of n- and p-type doped semiconductors separated by an undoped intrinsic (i) semiconductor. The thicknesses of the constituent layers are randomly distributed in accordance with a designed probability, and all of them are assumed to be sufficiently large so that the quantum-size effect can be ignored. The materials of the layers are characterized by frequency-dependent dielectric functions. The discontinuity at the interfaces is dealt with by using the transfer-matrix method. The calculation of dispersion relation and optical transmission shows that in varying the degree of thickness randomness, the frequencies of the plasmon modes only exhibit small shifts, but the transmission coefficients are changed. A particular random structure is found in which some modes of electromagnetic waves with special frequencies are completely unscattered by the randomness, whereas the other modes rapidly decay. This provides a possibility of building a high-quality optical filter.

The paper is devoted to calculation of the ‘‘classical’’ (thermal and/or shot) intrinsic noise of the single-electron transistor (SET) caused by the stochastic character of electron tunneling. Exact solution of the master equation describing the dynamics of the SET is obtained in the frequency representation. The low-frequency limit for the spectral calculations is considered in detail.

The Landau levels pertaining to a symmetric band-inverted junction and quantum well are calculated in the presence of a magnetic field parallel to the interface. It is shown that the effective band gap is widened at the interfaces, the Landau eigenenergies vary nonmonotonically with distance of the orbit’s center from the structure interfaces and that these levels have a noninteger number of harmonic-oscillator energy quanta. The Landau levels in the presence of a magnetic field perpendicular to the interfaces are briefly discussed.

The effects of tensile strain on the energy-band structures of semiconductor quantum wells and superlattices (SL’s) are studied theoretically, with emphasis on structures with unique valence-subband configurations achievable only through the use of tensile strain. Quantum wells are treated using finite-element envelope-function calculations which fully treat interactions between the light-hole, heavy-hole, and split-off valence bands, whereas strained SL’s are modeled using a superlattice K⋅p approach modified to treat strain effects. The two models are described in detail, tested for appropriate cases where both models should be applicable, and applied to prototype structures based on the III-V ${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As and ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/InP heterostructure systems. Single quantum wells are considered first. Transition energies are calculated and conveniently plotted as functions of composition (or strain) and layer thickness for both systems, and valence-subband band structures and k-dependent optical matrix elements are examined in detail for both systems in regimes where crossing of the uppermost light- and heavy-hole bands is induced by composition or well-width changes. Superlattices in both material systems are then considered, with emphasis on structures in which crossing of the uppermost valence subbands is induced by variation of barrier width. Band structures and optical matrix elements are calculated for wave vectors along directions parallel and perpendicular to the layer planes both in free-standing ${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$/${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{y}}$As SL’s with strain shared between the well and barrier layers and ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/InP SL’s strained to lattice match to InP substrates. Finally, general features of band structures and optical matrix elements in tensile-strained structures inferred from these studies are summarized. Implications for tensile-strained quantum-well lasers are discussed briefly.

The energy-loss rate of hot carriers in several modulation-doped Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ heterostructures has been studied. The Ohmic properties of the Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ samples, which were grown by ultrahigh-vacuum chemical-vapor deposition, were studied by Hall effect, conductivity, Shubnikov–de Haas, and quantum Hall effect measurements. For the samples with mobilities ranging from 1.3×${10}^4$ to 1.3×${10}^5$ ${\mathrm{cm}}^2$/Vs at T≊2 K the ratio of the transport time to the single-particle scattering time increases from 2.4 to 7.7. This result clearly indicates the change from dominant short-range to rather long-range scattering mechanisms in the higher quality Si/${\mathrm{Si}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ge}}_{\mathit{x}}$ heterostructures. The dependence of the energy-loss rate (${\mathit{P}}_{\mathit{E}}$) on electron temperature (${\mathit{T}}_{\mathit{e}}$) was obtained from the damping of the Shubnikov–de Haas oscillations with applied electric field up to 5 V/cm. In the electron temperature range from 1.6 to 7 K, the functional dependence of ${\mathit{P}}_{\mathit{E}}$ does not change when the mobility of the samples is varied by a factor of 10, and thus ${\mathit{P}}_{\mathit{E}}$(${\mathit{T}}_{\mathit{e}}$) is unaffected by the nature of the elastic-scattering mechanisms within these limits. In this electron temperature range the dominant energy-loss mechanism is due to acoustic-phonon scattering via deformation-potential coupling. For a deformation-potential coupling constant of 9 eV, taking static screening into account, a quantitative agreement between experimental and calculated values of the energy-loss rate is obtained without any fit parameter.

Double quantum wells are grown on Au(111) and Cu(111) substrates using Ag as the well material and either Cu or Au as a barrier material. The barrier thickness is varied to change the coupling between the two wells, and the resulting double-quantum-well states are probed with photoemission. In the limit of a very thick barrier, the coupling is eliminated, and the allowed states become those of two independent wells. When the barrier thickness is reduced to zero, the two quantum wells combine to form a single well. Between these two limits, the observed evolution of the binding energies and intensities is compared with the predictions of a simple theoretical model based upon Bloch wave functions. The effects of smoothing the barrier potential are studied by annealing a Au barrier sample, allowing atomic interdiffusion between the Au barrier and the Ag wells.

We consider the image potential and its effect in calculating the binding energy of the ground bound state of an exciton in a finite quantum well subjected to an external electric field by means of the variational technique. The influence of the image potential is obtained as a function of the well width, the electric-field strength, and the ratio of the dielectric constants of different media. It is found that the image-potential effect will give significant corrections to the exciton binding energy and its Stark shift. The self-image potential and mutual-image potential have opposite effects on the Stark shift.

The magnetoconductivity of parabolic quantum wires in strong tilted magnetic fields, associated with the magnetophonon resonance effect, is obtained analytically for optical-phonon scattering, by taking the linear-response limit of the results of the nonlinear-response theory developed previously. Neglecting the coupling Hamiltonian term ∼${\mathit{B}}_{\mathit{x}}$${\mathit{B}}_{\mathit{z}}$xz, since its contribution to the total electron energy is minor, the results for the magnetophonon resonance effect display two different periods of oscillation as a function of the strength and direction of the magnetic field and the confinement frequencies. In particular, it is shown that the direction of the magnetic field plays a significant role in determining the relaxation rates and the magnetoconductivity related to the magnetophonon resonances.

We have determined the pressure dependences of the band offsets in the GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterojunctions by measuring the pressure dependences of the harmonic-oscillator-like electron-energy-level spacings ħ${\mathrm{\omega }}_{\mathit{e}}$ of two ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As parabolic quantum wells at hydrostatic pressures up to ∼26 kbar at 2 K using photoluminescence-excitation spectroscopy. We found that the conduction-band offset for x≊0.3 increases with pressure at a rate of 0.73±0.25 meV/kbar. Using this result and the finding that the pressure coefficient of the direct-band-gap energy of ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As is independent of x for x=0-0.4, we found that the fractional conduction- (valence-) band offset ${\mathit{Q}}_{\mathit{c}}$ (${\mathit{Q}}_{\mathit{v}}$) of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As increases (decreases) with pressure at a rate of 0.0020±0.0007 ${\mathrm{kbar}}^{\mathrm{-}1}$. From this result, we conclude that the previous determinations of the band-offset values of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As by high-pressure measurements overestimated (underestimated) the fractional conduction- (valence-) band offset by 0.06±0.02, and the corrected ${\mathit{Q}}_{\mathit{c}}$ value at atmospheric pressure should be 0.63±0.04.

A variational calculation within the effective-mass approximation of the ground and lowest excited states of a donor impurity in a cylindrical GaAs-(Ga,Al)As quantum-well wire is presented. The corresponding impurity binding energies are calculated for various values of the GaAs-(Ga,Al)As quantum-wire radius and donor positions within the wire. The line strengths of transitions from the donor ground state to excited states of 2s-like and 2${\mathit{p}}_{\mathit{z}}$-like symmetries are calculated as the donor position varies along the radial direction in the wire, for polarizations of the incident radiation perpendicular and parallel to the wire axis, respectively. Although the 1s→2s donor transition is forbidden in bulk materials, this transition is allowed for incident radiation polarized along the y radial direction of the wire with a quite considerable oscillator strength—comparable to the strength of the 1s→2${\mathit{p}}_{\mathit{z}}$ transition—for impurities away from the wire axis.

High-quality molecular-beam-epitaxy-grown decoupled GaAs-(Al,Ga)As multiple quantum wells (MQW) of various well thickness (2.7 ≤${\mathit{L}}_{\mathit{W}}$≤11.9 nm; x≊0.3) and different barrier compositions (0.12≤x<=1; ${\mathit{L}}_{\mathit{W}}$≊11 nm) have been studied by x-ray diffraction (XRD), photoluminescence excitation, and emission (PL). The temperature dependence of the MQW properties has also been studied. The well width and barrier composition of the MQW were obtained by XRD and PL, respectively. 2s-excitonic features and the free electron-hole sublevel transitions can be resolved. We compare the 1s-2s energy difference and the ground-state binding energies of the heavy- and light-hole excitons with a recent accurate theory of exciton binding energies, taking the structure parameters from an independent determination. Experimental and theoretical values of the heavy- and light-hole exciton binding energies are found to agree within 1 meV. The theoretically predicted and experimentally observed excitonic transition energies associated with the lowest (n=1) electron, heavy-hole, and light-hole sublevels agree well, if the theoretical approach includes the split-off valence band. Interpolation formulas for the heavy- and light-hole ground-state exciton binding energies and for the n=1 electron, heavy-, and light-hole sublevel energies are given.

The dynamical response of a double-barrier resonant-tunneling structure (DBRTS) on external ac bias V(t) is studied. The rate equation for the occupation number of the level in the DBRTS well as well as the conditions required for this equation are derived in the case of coherent tunneling. The analysis of the equation in question provides the possibility to obtain easily the analytic results for the current through the structure in all limiting cases of interest.

A transition from semimetallic to semiconducting behavior induced by a magnetic field has been observed in type-II superlattices of InAs/${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$${\mathrm{In}}_{\mathit{x}}$Sb by the use of very high magnetic fields, in excess of 100 T. The carrier densities and effective masses were measured by the study of cyclotron resonance using wavelengths in the region 10.6–3.39 μm. This was used to demonstrate that the zero-point energy associated with the lowest-electron and highest-hole Landau levels was sufficient to uncross the energy bands in long-period, semimetallic structures. For (100)-oriented structures this transition was found to occur in the region of 50–60 T, while for (111)A-oriented samples the uncrossing field was found to move up to the region of 100 T, due to an orientational dependence of the band offset. The effective masses, studied as a function of both photon energy and superlattice period, were found to be in good agreement with the predictions of k⋅p theory.

Andreev reflection point-contact spectroscopy is performed on a bilayer consisting of 50-nm degenerately doped Si backed with Nb. Due to the short mean free path both injection into and transport across the Si layer are diffusive, in contrast to the ballistic conditions prevailing in clean metal layers. Nevertheless a large Andreev signal is observed in the point-contact characteristics, not reduced by elastic scattering in the Si layer or by interface scattering, but only limited by the transmission coefficient of the metal-semiconductor point contact. Two peaks in the Andreev reflection probability are visible, marking the values of the superconducting energy gap at the interface on the Nb and Si sides. This interpretation is supported by a method of solving the Bogolubov equations analytically using a simplified expression for the variation of the order parameter close to the interface. This observation enables a comparison with theoretical predictions of the gap discontinuity in the proximity effect. It is found that the widely used de Gennes model does not agree with the experimental data.

A numerical method, in which the average bond energy is considered as a reference level for determining the valence-band offset (VBO) in strained-layer superlattices (SLS’s), is suggested and tested by performing ab initio electronic-structure calculations for the SLS’s (InP${)}_{\mathit{n}}$/(InAs${)}_{\mathit{n}}$(001), (n=1,3,5), (GaP${)}_1$/(GaAs${)}_1$(001), (AlP${)}_1$/(AlAs${)}_1$(001), and (AlP${)}_1$/(InP${)}_1$(001). Based on this method, the VBO’s of the SLS’s composed of any two of the six compounds InP, InAs, GaP, GaAs, AlAs, and AlP under arbitrary elastic strain are determined systematically, and their strain-induced effects are discussed. Good agreement is found between the present results and the available experimental data.

We investigate theoretically the time evolution of electron and exciton wave packets in semiconductor superlattices in a uniform along-axis electric field. We obtain analytic results showing that electron wave packets undergo Bloch oscillations as predicted by semiclassical theory, but with spatial amplitudes which are strongly dependent on the initial conditions. We examine the nature of exciton wave packets in the experimentally relevant problem of short-pulse photoexcited electron-hole pairs in undoped semiconductor superlattices. We find that, when we neglect the effects of the electron-hole Coulomb interaction on the wave function in the direction of the field, the motion of the electron-hole separation ranges from a ‘‘breathing mode’’ all the way to a semiclassical-type Bloch oscillation, depending on the laser pulse width and frequency. When the electron-hole Coulomb interaction is included, much of the character of the motion is unaffected. There are, however, some significant changes in the nature of the wave packet evolution; in particular a breathing mode motion no longer exists, and there is an enhancement in the total oscillating dipole over a wide range of laser pulse parameters.

Applying the transformation of fermion operators to the fermion quasiparticles introduced by Halperin, Lee, and Read, we estimate the scaling behavior of the ground-state energy and quasiparticle gaps as a function of filling fraction for a ‘‘principal sequence’’ of fractional-quantum-Hall-effect ν=N/(2N±1) states converging towards the gapless state at half filling. The exponent describing the shape of the cusp δE(ν)∼‖δν${\mathrm{\Vert }}^{\mathrm{\eta }}$ is found to be greater than 1 and to depend nontrivially on the interaction potential. The dependence of quasiparticle gaps agrees with the results of recent measurements by R. R. Du et al.