Recent time-domain observations of the dynamics of wavepackets in semiconductor heterostructures are reviewed. The most simple example are coherent tunneling oscillations of a wavepacket in a coupled double quantum well. The composition and the oscillation frequency of the wavepacket are tunable with an electric field. Coherent microwave radiated by the oscillating carriers can be observed. Wavepacket dynamics in biased superlattices are identified as Bloch oscillations of the electrons.

Recently several proposals of quantum interference devices have appeared based on the close proximity of two coupled electron waveguides. The steady-state analysis of these new electron-wave directional couplers indicates that these devices are feasible using current fabrication technology, and could be used as basic switches in high-performance digital circuit at low enough temperature. In order to investigate the potential high-frequency applications of these newly proposed electron directional couplers, we have studied the transfer of wavepackets between parallel quantum wells using a block alternating direction implicit technique to solve the two dimensional time-dependent Schrodinger equation following the scalar ADI scheme of Douglas and Gunn. While injecting a wavepacket in one well, we show that only a partial transfer between wells (varying with incident kinetic energy) is reached even for perfectly symmetric double-quantum wells (in the direction of growth). This contrasts with the 100% probability of transfer predicted using the steady state coupled mode theory. The length and time scales for maximum transfer are found to be around a few thousand angstroms and in the sub-picosecond regime, respectively. In the presence of an external magnetic field B^yields parallel to the interfaces of the double quantum well, the probability of transfer between wells can be partially reduced with a magnetic field of few Teslas. We also illustrate the more efficient possibility of tuning the probability of transfer between wells using an external gate voltage as proposed by Dagli et al. Finally, the presence of a small amount of impurities in the direction coupler is found to have a relatively small influence on the probability of transfer of electron wavepackets between wells as long as the Fermi energy is large compared to the strength of the impurity potential. The probability of transfer between waveguides also varies with the actual impurity configuration.

A Monte Carlo solution to the Boltzmann transport equation is used to simulate hot-carrier relaxation and transport in a unipolar superlattice base transistor. Simulated results show that, due to the reduced density of states and wavefunction overlap, interminiband scattering is suppressed and high-energy transport is maintained in the superlattice base longer than in a bulk base region. However, an increased probability of reverse scattering and a lower magnitude of velocity along the superlattice axis result in a reduced transfer ratio across the superlattice base.

The injection of photoexcited carriers into higher subbands by resonant tunneling in GaAs- AlAs superlattices is directly observed by photoluminescence spectroscopy. For a conduction subband spacing larger than the longitudinal optical phonon energy, the relative occupation of the second subband is much smaller than one. However, if the conduction subband spacing is below the longitudinal optical phonon energy, the relative occupation increases, but so far no intersubband inversion has been detected.

We have designed (using a specially developed simulation package) some novel GaAs/AlAs tunnel structures with highly asymmetric current-voltage (I-V) characteristics for use as microwave detectors. The asymmetry arises from having unequal spacer regions either side of a single AlAs tunnel barrier. Recent designs have a microwave performance at 9.4 GHz which matches a zero-bias Schottky diode in terms of voltage sensitivity and dynamic range, but with a much better (equals weaker) temperature dependence. The new diodes also out-perform existing germanium back diodes in their voltage sensitivity and dynamic range, although the variation of sensitivity with temperature is not quite as small; it is expected however that this gap can be closed. The main competition for the new diodes is expected to come from the recently developed planar-doped-barrier detector (PDB) diodes, which combine the high sensitivity and dynamic range of the Schottky diode with a somewhat weaker variation with temperature. However, our diodes are still expected to have a weaker temperature dependence, and they would seem to be more easily and cheaply manufactured due to the problems associated with control over the p+ doping spike in a PDB diode: we have successfully made structures by both MBE and MOCVD. In this paper, we describe the design of our diodes and demonstrate the above points with detailed graphs of d.c. and microwave performance for one MBE-grown structure and one MOCVD-grown structure.

Space charge build-up in the well is shown to be the cause of the inductive effects in double- barrier diodes. A new impedance model for the diode is presented, built on a static model of coherent tunneling in a selfconsistent electron potential. The corresponding equivalent circuit is made up of two capacitances--related to the charge accumulations in the emitter and in the well--, and two conductances--one for each barrier. The numerical results of this circuit model are in qualitative agreement with experimental data. The success of the earlier quantum inductance model of Brown et al. is explained in terms of the presented model, without the need of introducing such a quantum inductance.

Electric field modulated spectra obtained from heterojunction samples have oscillatory structure that depends not only on the magnitude of the applied field but on confinement near the heterojunctions as well. Experiments indicate two field regions, each with distinct spectral lineshapes. Spectra obtained with relatively small dc electric fields have rapid oscillatory structure whose period does not significantly depend on the magnitude of the applied field. In contrast, the period of the bulk Franz-Keldysh oscillations that emerge with large field does. Theoretical calculations suggest that a crossover from the low to high field behavior occurs when carriers can freely move through the crystal.

Picosecond pulses are used to measure the hot phonon generation rate ((partial)N_q/(partial)t) of Raman active GaAs LO phonons in several GaAs/Al_xGa_1-xAs superlattices (SL's) and quantum wells (QW's). Drastic increase in (partial)N_q/(partial)t is observed as the barrier width (L_b) decreased below a critical value for SL's with x >= 0.4. This is interpreted as due to a phonon transition from confinement to propagation. In contrast, for x b's considered. We have also observed the existence of a critical x equals x₀ below which the LO phonons are no longer confined. Estimate of the LO phonon penetration depth into the barriers are also obtained for the different x values.

Micro-resonator modulators exploiting the quantum confined Stark shift and incorporating the asymmetric Fabry-Perot have been fabricated which exhibit high contrast (15 - 20 dB), and large reflectivity changes (> 50%) at low voltages (3V - 4V). This paper will review progress made in the development of these devices and will consider how the performance can be modified, post growth, by use of intermixing techniques to modify the shape of the quantum wells. The asymmetric Fabry Perot structure is also a convenient configuration to investigate the intermixing process itself. Measurement of the lateral extent of the intermixing process promoted by vacancies in GaAs/AlGaAs structures obtained using this structure are reported.

The use of vacancy enhanced interdiffusion to accurately and reproducibly modify quantum well (QW) shapes from abrupt (the square, as-grown shape) to rounded in a spatially selective manner has now been demonstrated in many binary and ternary lattice matched and strained material systems. The processing increased the effective bandgaps of the QWs significantly. We now report on vacancy enhanced interdiffusion studies in quaternary quantum wells (InGaAsP wells with InP barriers or InGaAs wells with InGaAsP barriers) grown nominally lattice matched on InP substrates using metalorganic vapor phase epitaxy. Essentially the same behavior with processing parameters was observed as in the previous systems with only slight changes due, probably, to modified strain. Thus it should be possible to utilize this simple process to integrate optoelectronic devices with differing functions in these long wavelength systems.

The polarization dependent exciton ground state optical absorption in a diffusion induced AlGaAs/GaAs undoped non-square single quantum well has been calculated. The exciton Hamiltonian is expressed in the effective mass approximation using a separable enveloped function to determine both the electron and hole subbands and the 1S-like hydrogenic wavefunction for the coulombic interaction. The k(DOT)p approach is used, assuming a parabolic band without mixing, to obtain the dipole matrix elements, which are then used to determine the TE and TM absorption coefficients.

A quantum mechanical manipulation of electron- and photon systems was realized in a semiconductor system, and thereby we systematically studied controllable excitonic spontaneous emission through the continuous tuning of the emission wavelength by quantum confined Stark effect in a GaAs single quantum well sandwiched between pairs of AlAs/AlGaAs distributed Bragg reflectors. Device-applications of this phenomenon are also discussed.

Serpentine superlattices (SSL) with a parabolic-crescent cross section defining the wells and barriers, have been grown on vicinal GaAs substrates by molecular beam epitaxy. The SSL structures have been studied by photoluminescence (PL) and photoluminescence excitation (PLE) measurements at 1.4 K, showing a strong polarization anisotropy in both PL and PLE. The carrier confinement has been characterized by measuring the linear polarization dependence of the PL from the surface as well as from the cleaved edges by using a photoelastic modulation technique. Calculations of the conduction band and valence band electronic structure describe the polarization dependence as a function of segregation into lateral wells and barriers. We find that about 30% of the Al intended for the barriers end up in the well giving Al_xGa_1-xAs wells and barriers of x equals 0.12 and 0.21, instead of the nominally intended values of 0.00 and 0.33, corresponding to a lateral conduction band barrier of 70 meV. Linear polarized PLE has been used to reveal the laterally induced heavy-light hole splitting. PL decay time measurements of the serpentine emission, shows a longer decay time than for a reference alloy-well structure, indicating a reduced carrier relaxation in the serpentine structure. The linear polarization of the PL is found to be rather constant over large areas of the wafer indicating uniform quantum wire like states, showing the intended advantage of the serpentine structure over tilted superlattices.

We have measured bound exciton luminescences from PbI₂ microcrystallites in the shape of a platelet embedded into E-MAA copolymer at 4.2 K. An energy shift of the bound exciton from the free exciton increases as the thickness of the crystallite decrease, and amounts to 51 meV in the crystallite with a monolayer, which is about seven times as large as that of the bulk crystal. This fact strongly suggests that the binding energy of the free exciton becomes larger both for the size confinement and dielectric screening effects.

Data from a series of experiments on porous silicon are presented, which provide important information about the luminescence processes in this promising new material. Raman spectra were correlated with PL spectra to clarify the significance of the silicon microcrystallites sizes on the photoluminescence (PL). The temperature dependence of the PL intensity, time constants, and peak PL energies was determined to reveal the role of more highly localized states such as defects and impurities. The dielectric constant was measured using angel resolved ellipsometry to relate quantum size effects to possible excitonic levels in the microcrystallites. The excitation power dependence of the PL was determined to be linear, indicating a one photon-one electron process is responsible for the excitation of the PL. The excitation spectrum of the PL was measured to provide information about the PL excitation process and the critical energy levels.

Various approaches have been used to calculate the quantum-mechanical tunneling time through potential barriers including the phase-delay method first introduced by Bohm and Wigner, Buttiker's analysis of the Larmor clock and its generalization, the dwell time of Smith, and numerical studies of wavepacket propagation through potential barriers among others. Most of those previous estimates have only dealt with the tunneling time through simple obstacles, including delta-potential scatterers and simple rectangular barriers under zero bias condition. Even for these simple cases, the agreement among the various estimates is far from being satisfactory. In this manuscript, the transmission line technique is employed to solve the time-dependent Schrodinger equation and an expression of the quantum-mechanical tunneling time is derived for an arbitrary potential profile under non zero bias condition. An exact analytical expression of the tunneling time through a rectangular barrier is derived and shown to be identical to the one obtained recently by Spiller et. al. using the Bohm's quantum potential approach. The tunneling time through resonant tunneling structures under zero bias is also calculated and is shown to be minimum at the quasi-boundstate energy. Finally, the quantum-mechanical tunneling time through the emitter-base junction of a typical heterojunction bipolar transistor is shown to be larger than its semiclassical counterpart.

Acceptor-related photoluminescence spectra are calculated for both homogeneous and on- center spike-doped distribution of acceptors in a cylindrical GaAs-(Ga,Al)As quantum-well wire. Results are dependent on the temperature, on the choice of the quasi-Fermi energy level of the conduction-subband electron gas and on the distribution of acceptors in the wire. The photoluminescence spectra corresponding to a on-center spike-doped Gaussian distribution show a peak for energies associated with on-center impurity states, as it is expected, whereas for a homogeneous distribution of acceptors in the well we essentially found an edge in the spectra associated to transitions involving on-center acceptors and a peaked structure related to the onset of transitions from the conduction subband to on-edge acceptors.

Based on information obtained from steady-state and time-resolved photoluminescence (PL) experiments done as a function of uniaxial stress, the symmetry of the lowest conduction band state for a series of type II Al_0.15Ga_0.85As/AlAs multiple quantum wells with a nominally fixed alloy thickness of 4.2 nm and AlAs thicknesses from 4.8 nm to 7.8 nm, is reported. Structures with AlAs thickness less than 6.2 nm have the X_z state (heavy mass in the growth direction z) as the lowest lying conduction band level while for thicker AlAs layers the X_x,y states are lowest in energy. For samples with X_x,y ground states, a time-windowing technique is used to simultaneously observe the PL from the X_z and lower-lying X_x,y states, providing a direct measure of the energy separation of excitons associated with the in- and out-of-plane X valleys in the AlAs. By studying the effect of the uniaxial stress on the time-resolved PL spectra, the X₆ shear tetragonal deformation potential of AlAs, E₂, can be estimated without complications due to the stress dependence of the valence band energy. The value of E₂ obtained from two samples with different layer thicknesses is 5.8 +/- 0.1 eV.

The effect of different tunneling modulations on the quantized conductance in a coupled double electron waveguide is theoretically investigated with the use of a model of two coupled chains. The calculated results show that both the accuracy of the quantization in the conductance and the resonance pattern strongly depend on the tunneling modulation between two channels. The resonance structures in the conductance plateaus are smeared when the corresponding tunneling modulation alters smoothly over the obstacle region. We also study the variation of the quantized conductance with the Fermi energy for various doubly-connected structures. A series of new novel features in the conductance curve emerge. We find that this two coupled chain model is shown to describe the essence of quantum ballistic transport of electrons in the two coupled electron waveguide in a simple and transparent way.

Propagation of guided electron waves in two coupled quantum wells is studied and analyzed by decomposing 1-D coupled eigenstates in terms of multiple eigenstates of individual wells. The energy transfer from one mode to the other modes in either channel is characterized. The dominant transfer is to the matched mode in the other channel. But tunneling to other modes nearest in energy to the incident mode is found to be quite large, particularly under high mode injection.

A semi-analytical calculation of the screened exciton binding energies in modulation-doped quantum wells with one occupied subband is presented, whereby the finite thickness of the confined screening charge is included, giving good agreement with recent experimental reports. It is forecasted that in one-side doped quantum well, above a critical sheet carrier density, the exciton binding energy increases, as a consequence of the band-bending effect. Under an applied magnetic field the magneto-exciton binding energy increases very rapidly, which is compared to a much weaker field dependence in the undoped well.

The optical-absorption spectra associated with transitions between the n equals 1 valence subband and the donor-impurity band and between the acceptor-impurity band and the n equals 1 conduction subband have been calculated for GaAs-(Ga,Al)As quantum-wells under constant applied electric field perpendicular to the interfaces. We have described the impurity states within a variational scheme in the effective-mass approximation. The distribution of impurities in the quantum well has been assumed to be homogeneous and interaction between impurities neglected. As a general feature, the impurity-related optical absorption for finite electric fields exhibits three van Hove-like singularities corresponding to the binding energies associated with impurities at the two edges of the quantum well and at the position at which the binding energy has a maximum.

(GaAs)₆(AlAs)₆ sample was grown on [001]-oriented semi-insulating GaAs substrate by MBE. The Raman scattering was measured at room temperature and under off- and in-resonance conditions. The GaAs even and odd modes were observed in the polarized and depolarized spectra, respectively, while the interface modes appear in both configurations. The second-order Raman spectra show that the polarized spectra are composed of the overtones and combinations of the even and interface modes, while the depolarized spectra are composed of the combinations of one odd mode and one even mode or one interface mode.

Tunneling time is used to study localization in a quasi-1D system in the presence of random elastic scattering. Schrodinger and Poisson's equation are solved self-consistently to calculate the developed space charge. The self-consistently calculated tunneling time is used to calculate shot noise in such structures.

In most electronic bandstructure calculations of semiconductor superlattices (SLs), periodically extended boundary conditions are used. However the presence of a terminating layer, which breaks the periodicity of the SL, is expected to modify the electronic bandstructure. In this work, we report a model for calculating the localized surface-like states of a terminated SL which includes, for the first time, the nonparabolic nature of the host bandstructures by way of band mixing, unlike the simple one band Kronig-Penney type model. This model is within the framework of the multiband envelope function approximation, which has proven to be a useful and efficient method for the calculation of SL electronic bandstructure.

Measurements of the in-plane electron effective mass in GaInAs/InP single quantum wells as a function of well thickness using far-infrared optically detected cyclotron resonance (FIR- ODCR) is reported. The FIR-ODCR technique is described, the mechanism of detection is explained, and the experimental results are compared with a theoretical calculation. In the thinnest QW investigated (80 angstroms) the in-plane mass is found to increase by about 50% compared to the bulk GaInAs value.

We report the observation of the excitonic recombination of degenerate quasi-two-dimensional electrons with localized photoexcited holes. Low-temperature photoluminescence spectra exhibit a sharp Fermi surface and a well resolved 'Mahan' exciton resonance which is sensitive to electron density n_s and temperature. We observe a sharp decrease in the exciton linewidth with a concomitant double peak spectrum which is attributed to the formation of biexcitons and a large discontinuity in the exciton groundstate energy at n_s approximately equals 1.9 X 10¹¹ cm^-2. An abrupt transition from excitonic to free electron-hole recombination occurs at n_s approximately equals 2.2 X 10¹¹ cm^-2.

The electroabsorption in ZnSe_0.35Te_0.65--ZnTe type II superlattice has been studied using the tight binding method. The effective blue shift of the absorption edge has been found to be up to 50 meV at the field strength about 300 kV/cm.

The homogeneously broadened, linear gain model of semiconductor lasers does not predict many of the observed dynamic and spectral properties of these devices. We begin by phenomenologically introducing the simplest form of nonlinear gain compression into the single-mode rate equations and review its effect on the laser's dynamic response. A recently developed rate-equation model of quantum-well lasers is then introduced to explain how carrier transport, in conjunction with single-mode nonlinear gain, determines the intrinsic structure-dependent frequency response of quantum-well lasers. The need for a two-mode, nonlinear gain model to describe the low-frequency relative intensity noise (RIN) spectra of semiconductor lasers is briefly reviewed. Finally, recent experimental measurements of the longitudinal mode spectra and the beat spectra between adjacent modes of nearly single-mode lasers are presented. It is shown that understanding of three-mode coupling is required for a proper description of these measurements.

We have studied the effect of strain on the laser threshold current density in the 1.3 and 1.55 micrometers wavelength regions using both GaInAsP/InP and AlGaInAs/InP material systems. Low threshold current densities have been obtained for both compressive- and tensile-strained quantum well lasers. We have also fabricated 20-wavelength distributed-feedback laser arrays using both compressive- and tensile-strained quantum well active layers. A wide optical gain spectrum and a sub-MHz linewidth have been demonstrated.

Previously, strain has shown to reduce the threshold current density for long cavity semiconductor lasers. However, also important is the cavity length for which the threshold current has a minimum. We find that for a given well width, an increase of the indium ratio from the lattice-matched value substantially reduces this optimum cavity length. We attribute this to the combined effects of strain on the valence bands and an increase in the confinement of the conduction band electrons to the well.

We studied the optical properties of a type-II (GaAs)₆/(GaP)₆ strained layer superlattice grown by Atomic Layer Molecular Beam Epitaxy (ALMBE) on GaAs substrate. The evolution of the photoluminescence peaks as a function of the temperature and excitation power as well as the photoluminescence excitation spectra supported the assignment of the transition involved. For (GaAs)₆/(GaP)₆, the lowest conduction band is the X level in GaP layers. The energy distance between the X level in GaP layers and the (Gamma) level in GaAs layers is approximately 44 meV. This sample is spatially indirect (type-II) superlattice. We found that the temperature dependence of the photoluminescence spectra is different from other's results.

Single layers of GaAs strained to InP have been grown by reduced pressure metal-organic vapor phase epitaxy. Raman spectroscopy along with cathodoluminescence show that the layers are fully strained and consisting of GaAs. The photoluminescence is strong, allowing detailed hydrostatic pressure experiments to be performed. These structures, which are type II at atmospheric pressure, have been transformed to type I structures at high hydrostatic pressure, where the GaAs layer has an indirect conduction band. This transformation is seen as a change in the pressure derivative of the transition energy and a rapid disappearance of the luminescence intensity. We find that the pressure derivative of the valence band offset is less than 1 meV/kbar.

The photoluminescence (PL) of CdTe/CdMnTe quantum wells (QWs) with well widths from 20 to 150 angstroms has been investigated as a function of hydrostatic pressure (0 - 37 kbar) at liquid-helium temperature. No band crossovers were observed before the phase transition at approximately 33 kbar. Pressure coefficients of the E^(Gamma )_1h transitions between the quantized ground levels of the (Gamma) conduction band and the heavy-hole valence band are presented for various well widths. The pressure coefficient is found to increase with decreasing well widths, as in InGaAs/GaAs strained quantum wells but unlike that observed in the GaAs/AlGaAs quantum well system.

An error function is used to model the compositional profile in an undoped InGaAs/GaAs single quantum well after disordering. Calculation of the subband structure, interband transition energies, overlapping wavefunction and polarization dependent interband absorption coefficient are presented for various stages of disordering. It is shown that the combined effects of strain and disordering in quantum well structures can be used to tailor the absorption edge.

Recent progress in wide-gap II-VI heterostructures is reviewed, including results on blue/green diode lasers and light emitting diodes in ZnSe-based quantum wells configurations. An important element in these structures are quasi-2 dimensional exciton effects which are likely to be of relevance as practical optoelectronic devices emerge at short visible wavelengths.

In this paper, it is demonstrated that establishing metal-semiconductor interfaces at the heterojunctions of polar semiconductor quantum wells introduces a set of boundary conditions that dramatically reduces or eliminates unwanted carrier energy loss caused by interactions with interface longitudinal-optical (LO) phonon modes.

We report on recent theoretical and experimental results on reverse biased photoconductive detectors and bistable optical switches. Using a special design and taking advantage of the 'giant ambipolar diffusion constant' of n-i-p-i structures, the photogenerated carriers are transferred from the 'absorption area' into a small detection area within very short times. The combination of small contact separations and small RC- and diffusion time constants results in very high gain-bandwidth values with adjustable 3-dB frequencies.

We report here a detailed study of intersubband absorption at 10.7 micrometers between the localized ground state and the global miniband state in an InAlAs/InGaAs multiple quantum well and short-period superlattice (SL) barrier heterostructure. The use of enlarged quantum well and the superlattice reinforced miniband structure has shown a significant enhancement in the net intersubband absorption. An integrated optical absorption strength of I_A equals 19.5 Abs-cm^-1 was obtained under the Brewster's incident angle at T equals 300 K, which is about five times larger than that of the conventional single bound-to-bound transition in the InAlAs/InGaAs quantum well structure.

Intersubband absorption spectra in the 10-micrometers region are measured between 4.2 and 290 K in four n-type GaAs/Al_0.28Ga_0.72As multiple-quantum-well samples. The carriers responsible for the absorption are generated by a cross-gap pump laser operating at 0.75 micrometers . The absorption strength per unit pump power is found to depend strongly on the background electron sheet density (sigma) _B, and is greatest by far in a sample having (sigma) _B approximately equals 4 X 10¹⁰ cm^-2. By measuring the speed of response, the cause of the strong photoabsorption is found to be a long photoelectron lifetime. The sample with the strongest photoabsorption is used to make an efficient CO₂-laser modulator.

We study optical intersubband transitions in quantum well structures as a function of the energy position of the excited state with respect to the continuum edge of the well. The position is varied from a bound state in the well to an extended state above the continuum edge. The experimental results are compared to theoretical calculations. In the case of strongly coupled quantum wells, a significantly larger peak absorption is observed than that for the normal wells with the same well width. We show that this is due to the camel back shaped ground state wave function in such structures.

Experimental results on GaAs-AlAs multiple quantum wells where the confined electron level is initially delocalized due to the mixing between the (Gamma) and X levels are presented. The applied electric field reduces this coupling and reconfines the electron in the GaAs layer. This causes an increase in oscillator strength and a blue shift of the heavy hole to (Gamma) - electron transition. Reduction of the charge transfer from narrow wells to a wide well has also been observed.

We have studied the field dependence of the absorption coefficient of three GaAs-AlAs superlattices using a new modulation technique, the wavelength modulated photocurrent spectroscopy. At small applied electric fields we observe transitions corresponding to the edges of the joint miniband density of states between electrons and heavy holes as well as light holes. At intermediate fields Franz-Keldysh oscillations appear at the lower and upper band edges of the heavy and light hole joint miniband. With further increasing electric field these oscillations transform gradually into Wannier-Stark ladder transitions. The experimentally observed features are well reproduced by numerical calculations.

Electroreflectance (ER) spectroscopy has been used to study the influence of an electric field on the optical properties of a strongly coupled GaAs-AlAs superlattice. At certain electric field strengths, anticrossings of the fundamental transitions for heavy- and light-holes are observed, which can be assigned to resonant coupling of the electron wave functions over one, two, and three superlattice periods. The lineshapes of the ER-spectra are discussed for a wide range of temperatures and field strengths.

Surprisingly, the onset of superlattice (SL)-type electroabsorption (EA) (localization/delocalization and Stark ladder phenomena) as multi-quantum well (MQW) barriers are lowered does not correlate with miniband width. As the barriers of a 100/100 angstroms GaAs/Al_xGa_-1As MQW are lowered to x equals 0.01, similar (quantum confined Stark effect) EA as for x equals 0.3 samples is seen. For x equals 0.02, as the MQW period (d) is shortened to 140 angstroms, SL-type EA is observed but does not correlate with miniband width. This shows that standard models do not apply for shallow systems. For x equals 0.07 and d equals 175 angstroms SL effects consistent with standard models are observed.

We report extremely efficient and fast (approximately 25 pS FWHM) escape times of optically generated carriers in a reverse biased GaAs/AlGaAs graded index separate confined heterostructure single quantum well (GRINSCH-SQW) laser. Room temperature photoconductivity (PC) measurements in a high speed ridge waveguide detector are compared with time resolved photoluminescence (PL) measurements at T equals 20 K, 70 K, and 150 K. By comparing the experimental PL and PC response times and efficiencies as a function of bias voltage and temperature with theory, we show that the results are consistent with a simple model based on electron recombination and escape out of the quantum well. Electron escape occurs by either direct tunneling out of the lower electronic level, by thermally assisted tunneling out of the upper weakly bound state, or by thermionic emission over the barrier, depending on the bias voltage and temperature.

General properties of emission spectra of an interacting two-dimensional (2D) electron-hole (e- h) system in a strong magnetic field are discussed. The procedures are proposed for determining some parameters of magnetorotons, elementary excitations of an incompressible 2D liquid, from emission spectra. A manifestation of fractional charges in optical spectra is also discussed. the violation of a hidden symmetry inherent in the magnetospectroscopy of 2D systems in the framework of the standard model is discussed as applied to different spectroscopic experiments.

We have studied the electroabsorption properties of GaAs/AlAs superlattices with a 1 monolayer (ML) and 2 ML wide AlAs barriers. At large electric fields F parallel to the growth direction, Wannier-Stark localization of the carrier states is observed, even for the case of 1 ML AlAs barriers. In the low-field regime (up to 60 kV/cm), Franz-Keldysh oscillations are found. These oscillations are associated with the miniband dispersion of the superlattice. The oscillation period shows an F^2/3 behavior, as theoretically expected. Numerical calculations on electroabsorption in strongly coupled superlattices are also presented.

To understand the feasibility of using strain to reduce the Auger recombination rate of 1.5 micrometers laser, we have directly measured the carrier lifetime in strained-layer InGaAs/AlGaInAs quantum well systems, using time-resolved photoluminescence measurements. We find that the Auger recombination rate can be reduced by applying either biaxial compressive or tensile strain. A longer radiative carrier lifetime is observed for the tensile-strained materials. The effect of strain and quantum confinement on the carrier lifetime is discussed.

It has been shown that semiconductor superlattices can be engineered to achieve a significant enhancement of the higher order virtual susceptibilities. These studies also reveal that the strength of the optical transitions between minibands originating from different regions of the wave vector space reflect the microscopic character of the superlattice wave function. However, this relationship has not been systematically studied and neither its precise origin nor its impact upon the spectral shapes can be understood in terms of the standard language of band theory. Accordingly, in the present study, we show that the spectral shape of the frequency-dependent third order susceptibility of such a system (e.g. AlSb-InGaSbAs) in a sensitive function of novel cancellation processes that reflect not only the position in energy but also the widths and the shape throughout the momentum space of the superlattice minibands spanning the energy range of the order of the fundamental gap away from the band edges.

The critical layer thickness (CLT), based on the transition from two dimensional to three dimensional (3D) growth, was investigated by the use of photoluminescence in highly strained In_xGa_1-xAs/GaAs (0.36