A study of the optical properties of the layered compound ${\mathrm{TlInS}}_2$ is presented. Reflection spectra were measured at room temperature, in the energy region 1.5–6.2 eV. Transmission spectra were measured in the temperature range 10–290 K, in the energy region 1.5–3.5 eV. Two structures, at energy positions 2.58 and 2.87 eV (10 K), are attributed to transitions to ground exciton states, from valence states split because of the spin-orbit interaction, at the point Γ of the Brillouin zone. Two other structures at higher energies are attributed to transitions at ${\mathit{M}}_0$ and ${\mathit{M}}_3$ critical points, respectively. The second-derivative spectra ${\mathit{d}}^2$ɛ̃/d${\mathrm{\omega }}^2$ were fitted, accordingly, to analytic two-dimensional critical-point line shapes. Lorentzian line shapes are used to fit the structures of excitonic origin. The temperature dependence of the peaks and their broadening parameters are also presented.

It is often suggested that the rate of interdiffusion at semiconductor heterostructure interfaces may be strongly dependent on elastic strain in the structure. Experimental and theoretical results show that strain does not, and should not, affect interdiffusion.

The oscillator strength of excitons is investigated systematically in lattice-matched GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As, strained-well ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs, and strained-well GaAs/${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ quantum wells (QW’s) using Fourier-transform reflectance spectroscopy. The oscillator strength in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As QW’s increases as the well width is reduced, in agreement with the existing theory, while those in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs and GaAs/${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ QW’s show a maximum at a certain well width. The well-width dependence of the band nonparabolicity and the overlap of the electron and hole wave functions is responsible for this behavior. The oscillator strengths in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs and GaAs/${\mathrm{GaAs}}_{1\mathrm{-}\mathit{x}}$${\mathrm{P}}_{\mathit{x}}$ QW’s are differently dependent on the alloy composition; this is also explained in terms of the band nonparabolicity and the wave-function overlap. The temperature dependence of the oscillator strength in these structures is well described by a modified Debye-Waller expression with an averaged phonon mode. The exciton-phonon interaction, deduced from the temperature dependence of the oscillator strength of the zero-phonon line, is mainly determined by confined phonons in GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As QW’s, and by quasi-three-dimensional phonons in ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs QW’s.

We have measured the temperature dependence of the Raman frequency and linewidth of the GaP-like LO phonon of random and spontaneously ordered ${\mathrm{GaInP}}_2$ alloys. The width of the asymmetric Raman band is strongly influenced by disorder and the anharmonicity of the lattice. The half-width towards the high-energy side of the Raman band of the GaP-like phonon is independent of temperature, while the half-width on the low-energy side does depend on temperature. Though no obvious differences are observed in the temperature dependence of the Raman spectra of the random and the ordered alloys, the GaP-like Raman band of the ordered samples narrows for incident photon energies near the fundamental gap of the alloy. We attribute the narrowing of the Raman band to the inhomogeneities in the material caused by the existence of a statistical distribution of partially ordered domains with different order parameters η. From the observed width of the dispersion of the Raman linewidth as a function of incident photon energies under resonance conditions, it is concluded that samples grown at higher temperatures possess a narrower distribution of domains.

The influence of localization effects on the radiative recombination of two-dimensional confined electrons and holes in single-side modulation-doped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures is investigated by magneto-optical measurements. The results show a rapid decrease of photoluminescence (PL) intensity with increasing temperature and a striking increase of PL intensity when a weak magnetic field is applied. This is explained as an enhancement of hole localization caused by an inhomogenous distribution of background impurities. The effective thermal-activation energy of the hole localization potential is found to vary from about 0.6 to 1.7 meV with increasing magnetic field from 0 to 4.5 T, respectively.

A direct general method for deriving effective-mass equations for microstructures with atomically abrupt boundaries is presented. The principal assumption is that the envelope functions are slowly varying on the scale of the lattice period. The band-edge Bloch functions are not assumed to be the same on both sides of an interface and it is shown how the differences can be taken into account perturbatively. The particle in a box method is known to work well in many situations. To demonstrate why, a derivation of the effective-mass equation is carried out explicitly for the case of conduction-band states of a type-I microstructure composed of zinc-blende crystals without spin-orbit interaction. The derivation provides much insight into why the effective-mass method works so well. The method is illustrated by applying it to a one-dimensional superlattice problem. For this model problem, the effective-mass approximation to the wave function is seen to be good even for a quantum well one lattice period wide.

We introduce and analyze a class of model systems to study transitions in the integer quantum Hall effect (IQHE). Even without disorder our model exhibits an IQHE transition as a control parameter is varied. We find that the transition is in the two-dimensional Ising universality class and compute all associated exponents and critical transport properties. The fixed point has time-reversal, particle-hole, and parity invariance. We then consider the effect of quenched disorder on the IQHE transition and find the following. (i) Randomness in the control parameter (which breaks all the above symmetries) translates into bond randomness in the Ising model and is hence marginally irrelevant. The transition may equally well be viewed as a quantum percolation of edge states localized on equipotentials. The absence of random-phase factors for the edge states is responsible for the nongeneric (Ising) critical properties. (ii) For a random magnetic field (which preserves particle-hole symmetry in every realization) the model exhibits an exactly solvable fixed line, described in terms of a product of a Luttinger liquid and an SU(n) spin chain. While exponents vary continuously along the fixed line, the longitudinal conductivity is constant due to a general conformal sum rule for Kac-Moody algebras (derived here), and is computed exactly. We also obtain a closed expression for the extended zero-energy wave function for every realization of disorder and compute its exact multifractal spectrum f(α) and the exponents of all participation ratios. One point on the fixed line corresponds to a recently proposed model by Gade and Wegner. (iii) The model in the presence of a random on-site potential scales to a strong disorder regime, which is argued to be described by a symplectic nonlinear-sigma-model fixed point. (iv) We find a plausible global phase diagram in which all forms of disorder are simultaneously considered. In this generic case, the presence of random-phase factors in the edge-state description indicates that the transition is described by a Chalker-Coddington model, with a so far analytically inaccessible fixed point.

A multiband theory of Bloch electron dynamics in a uniform electric field of arbitrary strength is presented. In this formalism, the electric field is described through the use of the vector potential. Multiband coupling is treated through the use of the Wigner-Weisskopf approximation, thus allowing for a Bloch-electron transition out of the initial band due to the power absorbed by the electric field; also, the approximation insures conservation of total transition probability over the complete set of excited bands. The choice of the vector-potential gauge leads to a natural set of extended time-dependent basis functions for describing Bloch-electron dynamics in a homogeneous electric field; an associated basis set of localized, electric-field-dependent Wannier and related envelope functions are utilized in the analysis to demonstrate the inherent localization manifest in Bloch dynamics in the presence of relatively strong electric fields. From the theory, a generalized Zener tunneling time is derived in terms of the applied uniform electric field and the relevant band parameters; specific results are derived from the general theory using a nearest-neighbor tight-binding, multiband model, and are shown to have identical parametric dependence on electric field, but different, more realistic dependence on the appropriate band-structure parameters than those of the well-known Kane and effective-mass two-band model. Further, the analysis shows an electric-field-enhanced broadening of the excited-state probability amplitudes, thus resulting in spatial lattice delocalization and the onset of smearing of discrete, Stark-ladder, and band-to-band transitions due to the presence of the electric field.

Thin films of α-Sn were epitaxially grown on InSb(111)A and InSb(111)B substrates at room temperature. We studied growth modes and phase transitions of the films by using reflection high-energy electron diffraction and Auger electron spectroscopy. The films on InSb(111)A grew in biatomic layer-by-layer modes. Characteristic growth features for Sn/InSb(111)B were due to the segregation of Sb to the α-Sn film surface. Thus, we used the Sn/InSb(111)A system to examine the stability of the films as a function of Sn coverage. The as-grown film surfaces changed irreversibly from the (3×3) structure, to the (2×2), and, subsequently, to the (1×1) structure, and finally to melting while going to elevated temperatures. The transition temperatures for the thinner film were more than 10 °C higher than for the thicker film. Finally, to study the role of an interface in the stable growth of α-Sn on InSb(111)A, we performed discrete variational Xα cluster calculations.

The free-carrier-induced plasma excitation in δ-doped p-type GaAs(001) has been studied by means of high-resolution electron-energy-loss spectroscopy (HREELS). Several samples, with different values of doping and depth of the dopant layer, have been investigated at various primary-beam energies. The HREEL spectra show a strong dependence on the doping level. We were able to reproduce satisfactorily all the measured spectra using a suitable dielectric model of a classically confined free-carrier gas, pointing out the two-dimensional character of the free-carrier gas in the samples having the two lowest dopings. On the contrary, a characteristic three-dimensional behavior of the plasma excitation is exhibited by the most doped sample.

The scattering-theoretic Green’s-function method incorporated with band structures, which takes into account the indirect band gap, the band nonparabolicity, and the multiorbital and external-bias effect, is used to study AlAs/GaAs/AlAs double-barrier quantum-well structures in a [001] direction. The calculated spectral local density of states simultaneously exhibits confined states originating from both the quantum-well Γ valley and the barrier X valley. The calculated energies of confined quantum-well electron states and light-hole states are compared with the reported photoluminescence-excitation experimental data.

A Fourier transform analysis is used to derive a general formula for the determination of the dependence of the electron and hole spatial distribution functions on the carrier-generation function, under a small-signal nonuniform illumination of a photoconductor. The suggested procedure also yields general basic theorems regarding the relation between any carrier-generation function and the resulting electron and hole concentration distributions in the steady state. We do not know of any previous attempt to formulate such general theorems. In particular, we prove that in the most general case, provided the Einstein relation holds, the only carrier-generation function that can yield functionally similar distributions for the two types of carriers is spatially sinusoidal. The present results explain well the uniqueness of the sinusoidal generation function and provide the rigorous basis for the recently suggested photocarrier grating method, which is used for the experimental determination of the ambipolar diffusion length in photoconductors.

Magnetoreflectivity measurements of the 1s and 2s exciton energies in a CdTe/${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Mn}}_{\mathit{x}}$Te superlattice have been made in magnetic fields up to 45 T, showing the resonant polaron coupling of electrons to LO phonons. Strong reflectivity features are seen for both the 1s and 2s excitons, which show a strong field-dependent spin splitting due to the dilute magnetic barriers. At ${\mathit{B}}_{\mathit{z}}$=0, the 2s exciton feature is observed lying 18 meV above the 1s state, and is shifted upward in energy by the magnetic fields. No resonant behavior occurs when the 2s state passes through the LO-phonon energy of 21 meV, but at higher fields of around 20 T, the resonances for both spin states (${\mathrm{\sigma }}^{\pm }$) of the 2s exciton broaden and show a strong anticrossing behavior. These experiments are shown to be in excellent agreement with a theoretical treatment which includes the resonant polaron coupling of the electrons alone. Both experiment and theory demonstrate an extremely strong resonant splitting of the 2s exciton states of approximately 11 meV, which is over 50% of the LO-phonon energy. The dominance of single-particle polaron coupling is attributed to the relative sizes of the polaron (35 Å) and the exciton (50 Å) radius.

X-ray-absorption fine-structure measurements were carried out to probe the local environment surrounding Mn ions in Mn-doped nanocrystals of ZnS with different size distributions ranging from 30–35 Å to 50–55 Å. The interatomic distances between Mn and neighboring atoms, the coordination number, local disorder, and effective valency determined for the nanocrystals are compared with those in bulk Mn-doped ZnS. The Mn ions are found to substitute for the Zn sites in the host ZnS but with significant size-dependent local structural changes. The questions of Mn-cluster formation and the presence of Mn impurities on the surface of the nanocrystals are addressed. Near-edge x-ray-absorption fine structures indicate that the effective valency of Mn ions in the nanocrystals is close to +2 with a weak size dependence. These local structures are believed to be closely related to the novel optical properties observed in this new class of semiconductors.

The breakdown of the integral quantum Hall effect at high current is studied in wide high-quality GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructures, on the i=2 and i=4 Hall plateaus. The striking features of the phenomenon are pronounced steps in longitudinal resistance with rich time-dependent structures, which are analyzed with techniques normally applied to the study of chaotic behavior in dynamical systems. A number of characteristic parameters of the signal are obtained, showing that the underlying dynamics may be of a chaotic kind.

To investigate useful properties caused by various interference effects of electron waves in a mesoscopic system, we carry out a numerical analysis of electron-wave propagation by a combination of several techniques for solving the two-dimensional Schrödinger equation. The techniques provide an accurate solution for a realistic potential profile in a point-contact structure, and are simple to apply even under magnetic fields. By using this calculation method, we investigate the detection of the propagation from a quantum point-contact injector to a point-contact detector under magnetic fields. We calculate electron-wave propagation and transfer conductance through a wedge-shaped detector, which has a smaller scattering cross section for injected electron waves than ordinary line-shaped detectors, and analyze the interference due to the detector as a function of detector parameters. We conclude that a well-designed wedge-shaped point contact could provide good detection of electron-wave propagation.

In this paper, we examine interwell tunneling between a pair of fractional quantum Hall liquids in a double-quantum-well system in a tilted magnetic field. Using a variational Monte Carlo method, we calculate moments of the intra-Landau-level tunneling spectrum as a function of in-plane field component ${\mathit{B}}_{\mathrm{R}}$∥and interwell spacing d. This is done for a variety of incompressible states including a pair of ν=1/3 layers ([330]), a pair of ν=1/5 layers ([550]), and Halperin’s [331] state. The results suggest a technique to extract interwell correlations from the tunneling spectral data.

The time evolution of an ion-sputtered Si(111) surface was investigated between 300 and 650 °C using the high-resolution low-energy electron diffraction technique. Below 450 °C, a (1×1) rough phase shows an anomalous dynamic-scaling behavior on the short-range scale where the measured height-height correlation has a form of ∼ln(t)${\mathit{r}}^{2\mathrm{\alpha }}$, which grows in time with a roughness exponent α=1.15±0.08. Such a behavior is consistent with the prediction by a recent dynamic-scaling theory. Above 530 °C, we found that the (7×7) phase does not undergo roughening evolution. The dramatic morphology change with temperature indicates a dynamic phase transition.

The electrophonon resonance, based on the long-wavelength optical-phonon population detectable by the Raman-scattering technique and on the hot-electron cooling rate, is studied theoretically with the aim of obtaining the separate contributions of the confined and interface modes to the resonance pattern. Two models of optical phonons are applied to a semiconductor ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As-GaAs quantum well with the square shape of the potential profile: the bulk phonons of the GaAs and the dielectric continuum model of the optical phonons in a quantum well. The control of the onset of the resonant intersubband optical-phonon scattering is assumed to be provided by means of either varying the quantum-well width or the applied homogeneous electric field normal to the interfaces. The electrophonon resonance based on the detection of the long-wavelength optical-phonon population is shown to give a strongly modulated resonance pattern for both the confined modes and the interface modes. The intensities of the resonance patterns due to the confined and the interface phonons appear to be comparable with each other for a broad range of well widths.

We present an exact calculation of the persistent current of electrons with Hubbard interactions in disordered one-dimensional rings. Numerical simulations show that the currents exhibit an interesting dependence on the configuration and strength W of disorder, and the value of the Hubbard interaction U. Our numerical simulations show that disorder and interaction both can monotonically suppress the amplitude of the current. The interaction can sometimes enhance the current of a disordered ring depending on its configuration and degree of disorder, even though smaller than the corresponding ideal-ring value ${\mathit{V}}_{\mathit{F}}$e/L. We have also studied the effect of negative U, and shown that it can influence the current as the positive U does. Based on the eigenvalue theory of a matrix, we give a simple explanation of the effects of disorder and interaction on currents. The main results are consistent with the numerical results for the case of long-range Coulomb interactions.