Recent experimental results on the physics of coherent and incoherent resonant tunneling in superlattices with an electric field perpendicular to the layers are discussed. For the case of weak coupling between the wells, we observe a decrease of the time constant for electron transport if different subbands of adjacent wells are at resonance. In this case) transport is incoherent and sequential. This leads to an efficient population of higher conduction subbands by non-thermal carriers which can be probed by photoluminescence experiments. For the case of strong inter-well coupling, on the other hand, coherence gives rise to resonance-induced delocalization phenomena which have been studied by photocurrent spectroscopy.

A theoretical investigation of the influence of longitudinal magnetic fields on the charge build-up in double barrier tunneling structures is presented. We find that both the peak current and the hysteresis in the I-V curve can be modulated by variation of the strength of the magnetic field.

Photoluminescence (PL), PL excitation (PLE), and time-resolved PL are employed to study the tunneling of photoexcited holes through a GaAs/A1GaAs double-barrier resonant tunneling structure. Lifetime measurements of the n= 1 heavy-hole (hh) exciton transition from the well were obtained as a function of the applied voltage. For voltages biasing the structure in the non-resonant tunneling regime, beyond the region of negative differential resistance (NDR), the exciton decays with two time constants. The fast component, which was observed at all voltages, is attributed to the decay of the exciton population originating from holes photoexcited directly in the well. The slower time constant is associated with excitons that are created from hOles which are photoexcited in the GaAs contact region, and which subsequently tunnel into the well. This picture for hole tunneling is further supported by the observation of the n= 1 hh exciton emission using exciting photon energies lower than the quantum well bandgap but larger than the GaAs bandgap, when the structure is biased beyond the region of NDR.

We study resonant and nonresonant hole tunneling in an asymmetric double quantum well structure by picosecond timeresolved photoluminescence. The tunneling times are directly determined by studying the luminescence decay time in one of the wells. Various hole levels in the two quantum wells are brought in resonance by applying an electric field to the doped layers which clad the inirinsic region containing the quantum well structures. The luminescence decay shows a sharp resonance due to tunneling of carriers when two heavy-hole levels are brought in resonance. The tunneling time at resonance, however, is much longer than expected from a simple theoretical model assuming a coherent tunneling process. We develop a quantitative theory of resonant tunneling under the influence of scattering and relaxation processes. The results predict large increases in the tunneling times in good agreement with the experimental observations.

Pseudomorphic Al 35Ga 65As/In 1Ga 9As resonant tunneling diodes fabricated with asymmetric spacer layers adjaceht to the tunnel barrier were characterized via magneto-transport measurements. Novel tunneling effects (ground vs excited state tunneling) were observed in the current-voltage characteristics of these devices which depend upon the bias direction. Shubnikov-de Haas oscillations obtained at high magnetic fields show a strong asymmetry with bias direction and give evidence of silicon dopant out diffusion during molecular beam epitaxy.

We report on the investigation of the miniband transport regime in GaAs-AlAs superlattices by electrical time-of-flight experiments. The temperature dependence of the low-field drift mobility is used to obtain information about the underlying transport mechanisms. The photocurrent as a function of the applied field can be fitted over a wide temperature range with a modified Kazarinov-Suris model.

High-quality GaAs-GalnP heterojunctions, quantum wells and superlattices have been grown using low-pressure metalorganic chemical vapo deposition. We showed using photoluminescence that the growth rate of the thin layers can not be extrapolated from the thick layer growth rate. In situ reflectance anisotropy (RA) measurements were used to monitor the growth. Correlations were made between the RA signals and film quality. RA signals were also measured for GaAs quantum wells of various thickness, as well as f superlattice growth.

We report the first observation of optically pumped spontaneous emission from intersubband transitions by electrons in an n-type GaAs/A1GaAs coupled-quantum-well structure. Electrons were pumped from the first to fourth conduction subbands with a CW CO2 laser operating at a photon energy of 133 meV. The transition from the fourth to the third subbands occurred at an energy of 17 meV and had a full width at half-maximum of 3 meV.

We have fabricated and characterized several GaAs/AlGaAs multiquantum well infrared detectors at temperatures ranging from 6 K to 77 K. The detectors were designed to have a single bound state in the quantum well and the first excited state in the continuum above the AIGaAs conduction band edge. The difference in energy between the two levels, as determined by the quantum well width and aluminum mole fraction in the barrier, was chosen such that absorption would occur in the 8-14 tm wavelength region. Each detector was characterized by FTIR absorption, dark current, responsivity, spectral noise density, and thermal activation energy measurements. The -maximum observed detectivity is 1 .8 x 1 012 cmIHz/W at ?= 8.3 jim and 6 K.

We demonstrate waveguide devices for optical fiber communications that employ unique semiconductor quantum well electrorefractive phenomena. Field- and carrier-induced effects are applied to electro-optic directional coupler switches and interferometric intensity modulators that are considerably more compact than bulk semiconductor waveguide devices. These quantum well waveguide components are -' 500 microns long, suitable for monolithic integration with semiconductor laser diodes, and limited in speed only by RC parasitics.

We discuss that hybrid concepts are fundamentally more suited to meet simultaneously the goal of low optical and electrical switching energies and high speed operation. Recently, we have developped an optical threshold switch with gain which can be monolitically integrated and which is suitable for frequencies up into the GHz range and has a gain-bandwidth product of up to 30 GHz. By combining this switch with n-i-p-i or hetero n-i-p-i based electro-optical modulators to form "smart pixels" we obtain optical gates which fulfill all the requirements for applications in optical processors such as low optical and electrical switching energy, high speed, high fan-out, and large tolerances in signal energy.

The nonlinear response in semiconductors is normally achieved via a band filling mechanism accompanied by strong energy dissipation. An attractive alternative is a virtual process which is based on multi-photon excitations to higher lying levels. We show that in certain superlattices the position of such energy levels and the relevant transition probabilities can be engineered so as to obtain a significant enhancement of nonlinear response. We predict structural parameters for several lattice matched systems where this enhancement should occur and report the first full scale calculation of the frequency dependence of the third order susceptibility.

We present an experimental and theoretical investigation of coupling between two identical GaAs/AlGaAs quantum wells separated by a narrow AlGaAs barrier. The optical properties of this heterostructure were characterized with photoluminescence (PL), time resolved PL, and PL excitation spectroscopies with and without the application of electric fields and other external perturbations. Experimental results for level splittings and Stark shifts agreed well with calculations derived from a simple model.

Excitonic spectra and certain dynamical features of electron-hole pairs in coupled double-quantum wells under electrical bias are analyzed using numerical data on the evolution of envelope wavefunctions. The excitonic system was described by a two-band effective-mass model. The spectra and their dynamical counterparts were deduced, respectively, from the Fourier transform and the direct time-dependence of the autocorrelation function linking the evolved and initial states. Several measures of quantum dynamics are discussed.

We have studied excitonic effects in resonantly coupled GaAs/A1GaAs multiple quantum wells. We use photocurrent spectroscopy to deduce the resonant field from the field-dependent exciton energies, and find an unexpected variation of -10% in the resonant field between different excitonic transitions involving the same coupled electron levels. We construct a variational model of the coupled excitons which explains the results in terms of Coulomb mixing of the delocalised electron states.

We use an ensemble Monte Carlo simulation of coupled electrons, holes and polar optical phonons in multiple quantum well systems to model the intersubband relaxation of hot carriers measured in ultra-fast optical experiments. Our simulated results are in good agreement with experimental results in modulation doped quantum wells and coupled double well structures where we find that the intersubband relaxation time is controlled by the spatial overlap of the subband envel ope wavefuncti ons.

An alternative mechanism to produce an optical blue shift in a double quantum well heterostructure is suggested, distinct from that recently proposed by means of a static electric field1 . For this purpose we consider the tuning effects of the energy levels of the heterostructure due to the laser-dressing of the quantum well by a pulsed low-energy long-wavelength laser field. Appropriate parameters of the heterostructure are chosen so as to fulfil the necessary tuning conditions. Symmetric and asymmetric double quantum wells are discussed as well as the respective blue shifts attainable by different laser field strengths.

Studies are reported of an MBE-grown, two-quantum-well structure which uses photon-assisted resonant tunneling between the two quasi-confined well states to provide a detection current. Bias applied across the device allows for tuning of the wavelength of the detected light by changing the difference in energy of the two states. Various charactrization measurements of this structure will be described, and their ramifications will be discussed.

The results of magnetoreflection, photoluminescence and photoluminescence excitation experiments are reported in GaAs-AlAs multiple quantum wells of different well widths, demonstrating the influence of the X-band in the AlAs on the electron levels in GaAs. Evidence is presented for the existence of an exciton formed from a delocalized electron, and for the conduction of electrons from narrow wells to wide wells via the X-band of AlAs.

The essential elements of strain effects on the electronic structure of lnGa1As-(AlGa)As on GaAs are elucidated. Attention is focused on the optical properties of quantum wells and superlattices(SL) with In fractions <0.12. Photoluminescence and photoluminescence excitation spectroscopy at '-4K has revealed sharp exciton features and allowed us to measure the el-hhl binding energy versus well width, identify Ln =0 and transitions and follow the development of SL minibands. Comparison with envelope function calculations suggests the band offset ratio remains constant (67:33) up to x=O.12.

Three types of ultrathin single quantum well structure grown by MBE , GaAs/InAs/GaAs (type I), GaAs/GaSb/GaAs (typell , staggered) and GaSb/InAs/GaSb (type II , misaligned) have been studied by photolusinescence (PL) measurements to understand the band alignment and effects of lattice strain at the interface. Observed PL spectra are discussed in conjunction with the interface structure probed by RHEED analysis during the MBE growth. The well width dependence of PL energy is cospared with the theoretical results based on a finite potential well model.

PIN diodes whose intrinsic region s composed of a strained monolayer superlattice (SLS) have been fabricated by molecular beam epitaxy. The optical and geometrical properties of these structures as a function of the substrate orientation have been investigated. Transmission Electron Microscopy (TEM) and X-ray spectroscopy have been used to characterize the crystalline quality of the samples. The absorption spectra have been measured using photocurrent spectroscopy. Optical transitions extending well into the silicon bandgap have been observed. The type of the transition has been analyzed using envelope function approximation and curve fitting procedures.

In order to gain information about the band offset in the strained layer InGa1As/GaAs system we have investigated photorefleCtanCe (PR) from two single quantum wel 1 samples at 300 K and 77 K. Our samples have we 1 1 width L= 110 A (sample 1) and L = 107 A (sample 2) with In Composition x = 0. 11 (sample 1 ) and x = 0. 19 (sample 2 ) . We have observed a number of intersubband transitions in the spectra of both samples. By studying the polarization dependence of the PR at 300 K using an internal reflection mode we have identified spectral features Corresponding to light and heavy hole to conduction subband transitions. Good agreement between our experimental results and an envelope function calculation (including strain) is obtained for conduction band offset Q = 0.45 0.07 (sample 1) and Q = 0.67 0.07 (sample 2). These values comply with the compositional dependence of proposed by Joyce et al [Phys.Rev. B 38, 10978 (1988)1.

We emphasize the importance of fabricating planar superlattice (PSL) structures with the lateral dimension of 100A, in which most carriers are accommodated in sharply defined ground level. Furthermore, we review our recent studies on the molecular beam epitaxial (MBE) growth and electronic properties of novel planar superlattice (PSL) structures, in which an array of monolayer(ML) thick AlAs bars with period of 80-160A is inserted in GaAs layer. These grid inserted heterostructures (GIHSs) are prepared by depositing 0. 5 monolayer of AlAs during the growth of GaAs on misoriented substrates. We have found anisotropic electronic structures reflecting misorientation induced atomic terraces in both optical and electronic properties, which are in good agreement with theory. This demonstrates that the PSL states are formed in our GIHSs.

Far infrared photothermal ionization spectroscopy of well-center doped shallow donors in GaAs/AlCaAs'multiple-quantum-well structures has been carried out to investigate the electron - optical-phonon interaction. The ls-2p(m=+1) and ls-3p(m=-i-1). transitions are tuned through the resonant region with the GaAs optical phonons by magnetic fields up to 23 .5T . Both two -level and three -level resonance measurements show similar anti-level-crossing behavior with extremely large interaction gaps. The lower branches are well below (-50 cm ) the resonant energies involving the bulk zone centr LO phonons. These anomalous features strongly suggest that electrons in the wells interact with phonon modes at several different frequencies, or a phonon band, in contrast to the single frequency LO phonons in the bulk. A partial explanation may be found in terms of electron - interface-phonon and electron - confined-LO-phonon interactions. Other possible mechanisms are also discussed.

We have computed the bound state energies of an electron which is trapped at the intersection of a cross formed by two quantum wires. The widths of the wires forming the cross are assumed to vary independently. When the widths are equal, a bound state exists and the binding energy corresponds to the value obtained recently by Schult, Ravenhall and Wyld. When the ratio of the two widths is varied the binding energy changes. The variation of the energy with the ratio of the widths is obtained and shown graphically. A similar study is also completed for YTT? and "LT' shaped geometries. It is shown that a nondegenerate bound state of the electron exists for a range of values of the ratio of the two widths; outside the range the state becomes degeneiate with the energy continuum.

Optical phonon modes in a semiconductor double heterostructure (DHS) are examined within the continuum model. The interface modes found here can account for the novel phenomena observed in right-angle Raman scattering. The Hamiltonian for the electron interaction with phonon eigenmodes is derived and employed to study polaronic effects in the DHS. The ls2p+transition energy of a magnetopolaron bound to a hydrogenic impurity in the quantum well is calculated, and excellent agreement with experiments is obtained.

The photoluminescence (PL) and photoluminescence excitation (PLE) spectra of a GaAs/AlGaAs doublebarrier resonant tunnelling diode have been studied with sub-meV resolution as a function of the applied bias voltage. For voltages which bias the device in the resonant tunnelling regime, a monotonic blue shift of the PLE peak is observed, concomitant with a monotonic red shift of the corresponding PL peak. Over the same range of voltages, the linewidth (FWHM) increases from 4.8 to 6.3 meV in the case of the PL and from 3.6 to 8.7 meV in the case of the PLE. These results are interpreted as representing the influence of the resonantly accumulated electron population in the well region on the heavy hole exciton resonance.

Theoretical predictions of electronic and optical properties of short period Si/Ge superlattices are presented. Attention is focused on critical point structure and the effects of the superlattice compositional modulation and the buffer on band edge absorption. The band edge absorption is the main distinguishing feature of the different superlattice configurations. The construction conditions for direct-gap Si/Ge superlattices are specified. Theoretical results are discussed in relation to recent experimental work.

Photoconductivity measurements of two-photon magnetoabsorption in GaAs/AlGaAs multiple quantum wells have been performed using a one-beam pumping technique. The splitting of 2P-exciton states in magnetic fields up to 6 T have been studied in Faraday configuration with the linear polarization parallel to the layers of the quantum wells. By applying a theory of linear-Zeeman effect in the hydrogen atom, effective masses for motion parallel to the quantum-well layers can be extracted from the data to be O.1Om for heavy holes and O.18m for light holes.

It is shown that Coulomb attraction between electron and hole in coupled quantum wells leads to localization of states in the direction of growth, i.e. in one of the wells, even when the wells are symmetric and unbiased. As a result, oscillator strength of the lowest excitonic transition can increase by more than 100%, while the oscillator strengths of higher energy transitions can decrease. These results, obtained using novel model, are important for understanding of the Wannier-Stark localization effect in quantum wells and superlattices.

The stimulated emission spectra of photoexcited quantum-well structures were measured at room temperature, 77 and 1.8K as a function of the illuminated stripe length and excitation intensity. Three samples are examined: a multiple quantum well, and two superlattices (one of them is type-Il). Based on these data, the gain spectra and gain saturation behavior were obtained using Agrawal's model for the analysis. Both the spontaneous photoluminescence spectra and the stimulated emission spectra are used in order to analyze the spectral shifts. Kronig-Penney model was used to approximate the energy level diagram of our samples.

The samples of (GaAs) (AlAs) superlattices (SLs) were grown by MIBE method on (001)-oriented semi-thsulathg GaAs substrates. The photoluminescence (PL) was measured at 77 K and under hydrostatic pressure in the range of 0- 30 Kbar. The dependence of the energy separation between conduction band Flike and X- like levels on n values was obtained from the measured results. The transition from type I to type II was observed at atmosphere in a SL of n = 1 1. It is in good agreement with the calculated result based on Kronig-Penney model. It is found that the pressure coefficient of X-like states decreases with decreasing the n values. And it is explained by a combined effects. The pressure dependence of the levels and luminescence intensities demonstrates that F-X mixing is quite weak.

Excitonic effects associated with subband dispersion and subband mixing have been investigated using photoluminescence excitation spectroscopy at liquid helium temperatures. A wide energy range has been studied in high quality AlGa1As/GaAs superlattices (SL's) by tunable cw dye lasers. Series of SL's grown by molecular beam epitaxy (MBE) have been designed with constant well widths and aluminum concentrations while the barrier widths (Lb's) vary. Each SL series has been carefully designed to produce a broad range of subband dispersion. We have observed significant changes in the excitonic spectra of the n=1 heavy hole (1HH), light hole (1LH), and n=2 heavy hole (2HH), as the Lb's vary. In particular, a wealth of excitonic peaks and structures were observed in the vicinity of the 2HH excitons within a certain range of Lb's in our SL samples. These arise from band mixing between 1LH and 2HH in conjunction with subband-dispersion associated excitonic effects. The observed lineshapes were compared with computer generated curves based on a comprehensive exciton theory. We also report observations of pronounced line narrowing effects in a SL. The linewidths of the 1HH and 1LH exciton peaks decrease significantly as the Lb's decrease. This narrowing can be explained with a theory which incorporates combined effects of well-to-well coupling and layer width irregularities.

We have investigated the effects of complex barrier structures in double barrier resonant tunneling diodes (DBRTDs). The largest room temperature peak-to-valley current ratios (PVCR) to date have been observed for AlGaAs/GaAs DBRTD. PVCRs as high as 5. 1 were observed in AlAs/GaAs DBRTD with an AL14Ga86As chair barrier in the cathode. We attribute the improvement in the PVCR to the chair barrier in the cathode which significantly reduces the valley current. The effects of a real, spatially separated Al,14Ga086As barrier in the anode and cathode sides of the DBRTDs were also investigated and a PVCR as high as 4.8 was observed when the A10• 14Ga86As barrier was on the anode side.

The third order nonlinear optical susceptibility, of the GaAs/Gao.7A10.3As superlattice is calculated by determining the enhanced band nonparabolicity in the superlattice growth direction. A 4-band k.p model is used for the band structure of the constituent bulk semiconductors, and the superlattice conduction minibands are obtained using a transfer matrix method. Estimates for ) , averaged over the Fermi distribution of energies, are made at T = 0 and at room temperature for a number of representative values for carrier concentrations and for various layer thicknesses. In-plane nonparabolicity is incorporated, and no approximations are made for the Fermi distribution of carriers over the lowest and the first excited minibands. For the GaAs/Gai_AlAs superlattice, it is found that the in-plane nonparabolicity has a negligible effect on both at T = 0 and at T = 300K. For T = 0 a superlinear behavior of x) with carrier concentration is obtained for Fermi energies near the top of the lowest miniband. Significant contributions to () occur from the first excited miniband in the case of wide wells, and that the magnitude of () is enhanced for wider wells and/or barriers.

The nearly lattice-matched InAs/GaSb/A1Sb system offers tremendous flexibility in designing novel heterostructures due to its wide range of band alignments. We have recently exploited this advantage to demonstrate a new class of negative differential resistance (NDR) devices based on interband tunneling. We have also studied "traditional" double barrier (resonant) and single barrier NDR tunnel structures in the InAs/GaSb/AlSb system. Several of the interband and resonant tunneling structures display excellent peak current densities (as high as 4 x 1O A/cm2 ) and/or peak-to-valley current ratios (as high as 20:1 and 88:1 at 300 K and 77 K, respectively), offering great promise for high frequency and logic applications.