Table of contents

Some of the earliest theoretical speculation, stimulated by the growth of semiconductor superlattices, focused on novel devices based on vertical transport through engineered band structures; Esaki and Tsu promised Bloch oscillators in narrow mini-band systems and Kazarinov and Suris contemplated electrically stimulated intersubband transitions as sources of infrared radiation. Nearly twenty years later these material systems have been perfected, characterized and understood and experiments are emerging that test some of these original concepts for novel submillimetre wave electronics. The authors describe recent experiments on intersubband emission in quantum wells stimulated by resonant tunnelling currents. A critical issue at this time is devising a way to achieve population inversion. Other experiments explore 'saturation' effects in narrow miniband transport. Thermal saturation may be viewed as a precursor to Bloch oscillation if the same effects can be induced with an applied electric field.

The Monte Carlo procedure developed for the 'exact' solution of the Liouville-von-Neumann equation for the density matrix is presented in a way that throws light on its similarities and differences to the corresponding procedure, the ensemble Monte Carlo technique, for the solution of the Boltzmann equation. The Monte Carlo algorithm for the quantum case is strikingly similar to that for semiclassical transport, and the differences are directly related to well known quantum effects. Applications to real cases are reviewed, and related to the relaxation of photoexcited electrons, and overshoot of the drift velocity in silicon and gallium arsenide, the impact ionization rate and the polaron dressing time.

Recently, three-dimensionally laterally confined semiconductor quantum wells ('quantum dots') have been realized. These structures are analogous to semiconductor atoms, with energy level separation of order 25 meV, and tunable by means of the confining potentials. A systematic study reveals a (radius)^-1 dependence on the energy separation. The electronic transport through quantum dots is presented and analysed. The spectra correspond to resonant tunnelling from laterally confined emitter contact subbands through the discrete three-dimensionally confined quantum dot states. The effects of two dots in series, and Fermi level effects, are presented.

When the energy separation of two quantum levels equals the energy of an LO phonon, resonant interaction occurs which leads to an enhancement of the scattering rate and consequently to a suppression of the conductivity. This effect is called magneto-phonon resonance (MPR) when the resonant scattering occurs between two Landau levels, and electro-phonon resonance (EPR) when the scattering occurs between two electrical subbands. The authors discuss both effects in GaAs/Al_xGa_1-xAs heterostructures under hot-electron conditions. The influence of the heating of the electrons on the resonant condition and the amplitude of the effect is studied.

The new technique of ballistic electron luminescence spectroscopy (BELS) is discussed, and applied in several situations. The authors describe the use of BELS to determine the conduction band offset in Al_xGa_1-xAs/GaAs heterostructures and the ballistic range of electrons in undoped GaAs. The range for the emission of longitudinal optical (LO) phonons is found to be approximately 1000 AA for electrons with an initial kinetic energy of about 250 meV.

Continuous wave (CW) hot electron luminescence (HEL) spectra indicate that hot electrons scatter predominantly with cold electrons rather than with hot electrons or hot holes. Two series of experiments are compared. In the first series the sample is GaAs:Be. Increasing the concentration of optically injected carriers results in a widening distribution of hot electrons due to scattering and to an increasing band-gap-related background. In the second series, the authors keep the injection of hot electrons low, but study different samples GaAs:Be,Si which contain a rising background of donor electrons. In this case the hot electron population widens in a comparable way, indicating that hot electrons scatter with electrons low in kinetic energy.

The authors have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (2 DEG) in a GaAs/AlGaAs heterojunction. The 2 DEG was heated by applying short ( approximately=100 ns) electrical pulses to the drain-source contacts of the device. The electrons lost energy by emitting phonons which were detected by a CdS bolometer on the opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at an excitation level of about 5 pW per electron indicated a change in the phonon emission process. The corresponding electron temperature, T_e, at which optic phonon emission is expected to become the dominant energy relaxation process was estimated to be about 60 K. At powers well below the change-over, the authors found that the energy loss rate per electron, P_e, due to acoustic phonon emission is proportional to T_e³. At much higher powers, P_e varies as exp(-h(cross) omega _LO/kT_e), where h(cross) omega _LO is the longitudinal optic phonon energy. They obtained a value of 3.3 ps for the electron-optic phonon scattering time, which is consistent with the range of values found in the literature.

The authors present a gauge-invariant formulation of the eigenfunction representation of the Baym-Kadanoff-Keldysh theory of nonequilibrium quantum statistical mechanics. The eigenfunction representation, which for uniform fields consist of Airy functions, allows a nonperturbative treatment of the accelerating electric field. For the special case of a momentum-independent electron-phonon coupling, treated in the first Born approximation, the authors are able to carry through the complete nonequilibrium calculation, which in this case consists of the following steps: (i) determination of spectral densities, which are consistent with the frequency sum rule, and include collisional broadening and intra-collisional field effect on equal footing; (ii) derivation of a closed equation for a generalized distribution function, which contains information about the nonequilibrium occupation of energy states (in the special case the central 'kinetic' equation is a homogenous integral equation for a function of only one variable, well suited for a numerical solution); (iii) relation of the function determined in step (ii) to experimentally observable quantities, such as number densities or nonlinear conductivitiesl (iv) checking of the consistency of the results with the linearized Boltzmann equation in the appropriate limit; and (v) finding a numerical solution of the derived transport equation.

Quantum effects on the electron-phonon interaction under high electric fields are investigated by a fully quantum mechanical method in the real space representation, in which electric fields are included in the unperturbed Hamiltonian and the electron-phonon interactions are incorporated through the perturbation scheme. The intracollisional field effect and the interference effect associated with the wave character of electron under high electric fields are investigated. It is explicitly shown that the intracollisional field effect becomes significant when the electric field is above tens of kV cm^-1 and that the interference effect is reduced as the electric field becomes greater.

Hot-electron magnetophonon resonances in n-type Ge samples have been investigated in pulsed high magnetic fields up to 40 T for B//I//(110), (111) and (100) directions at a fixed temperature of 20 K. Owing to the high magnetic fields, an observation of almost all resonance series of both intervalley and intravalley processes from the first harmonic number (Landau quantum number N=1) has become possible for the first time. The resonant cooling process of electrons was consistently understood by taking into account an initial energy distribution of electrons, anisotropy of the effective mass at each valley and the mobility change toward the current direction.

Magnetophonon resonance (MPR) recombination of hot carriers has been studied in Hg_1-x-yCd_xMn_yTe crystals, which were grown by LPE method from Hg solution in a wide compositional range (x<or approximately=0.15, y<or approximately=0.04). In crystals in the negative energy gap range (E_g<0), recombination processes of hot carriers excited by high voltages with the emission of two TA phonons were detected as minute increases of magnetoresistance. In particular, MPR recombination due to the transition from the conduction band to the acceptor state originating from Hg-vacancy was observed and the E_g dependence of the acceptor binding energy was directly deduced for the first time in a wide range of E_g=-300 approximately 0 meV.

The authors calculate electron-LO phonon intersubband scattering rates and intrasubband scattering rates in a 70 AA GaAs/Ga_0.7Al_0.3As quantum well considering the phonons to be described by the hydrodynamic model, the slab mode model and the bulk phonon approximation. These rates for the different phonon models are used in a Monte Carlo simulation of the electron drift velocity in the quantum well under the influence of an electric field.

Macroscopic dielectric continuum models of optical-phonon modes predict enhancements in the magnitudes of the surface-optical (SO) modes in double-barrier heterostructures as the heterojunction-to-heterojunction separation is reduced. In this paper, the ratio of electron scattering by the SO-phonon modes to that by the (electrostatic) confined longitudinal-optical- (LO-) phonon modes is calculated for a GaAs/AlAs short-period superlattice based on the assumption that electron-SO-phonon scattering may be described by a scalar potential. The scaling of the ratio of electron-SO-phonon scattering to electron-LO-phonon scattering as a function of the superlattice period provides a sensitive test of the appropriateness of the scalar-potential model. The effect of phonon confinement on electron-optical-phonon scattering rates is presented for rectangular quantum wires as well. A major conclusion of these new results is that it is essential to model phonon confinement properly in predicting carrier transport properties in mesoscale structures.

Theoretical models that treat optical lattice vibrations in heterostructures by viewing the lattice-matched crystalline media as isotropic homogeneous continua separated by smooth planar interfaces, offer conceptual simplicity and are, without doubt, computationally advantageous when calculations involving electron-phonon interactions are carried out. The two (apparently conflicting) continuum models, namely the hydrodynamic (HD) and the electromagnetic (EM) models, are discussed, their calculational frameworks in the context of semiconductor heterostructures examined and some of their predictions displayed. A statement of the HD model in terms of a polariton subset plus an LO subset is given, and this permits a transparent comparison with the EM model to be made. It is then shown that the EM model is a non-dispersive, non-retarded and subsequently mishandled version of the polariton subset of the HD model and it completely disallows proper considerations of longitudinal optical modes. The sources of the current controversies concerning HD versus EM boundary conditions-confined mode identification, existence of localized alloy interface LO modes and interface FK modes, Frohlich-type coupling e Phi together with the e Phi versus-(e/m*c)A^T.p couplings to electrons-are all pointed out and discussed. A close agreement between some very recent predictions of the two models concerning interface mode contributions to energy relaxation is explained and this helps to clarify the nature of the conflict regarding other contributions. Further evidence in support of the HD model from recent theoretical and experimental results is pointed out.

The role of confined phonon modes in determining the energy relaxation of hot electrons in low-dimensional semiconductor microstructures is discussed within a dielectric continuum model for the LO phonon confinement and a long wavelength Frohlich model for the electron-phonon interaction. Numerical results are provided for the hot-electron relaxation rate as a function of electron temperature and density for GaAs quantum wells and quantum wires by taking into account emission of slab phonon modes. Comparison with existing experimental results shows some evidence for slab phonon emission in intersubband electronic relaxation in reasonably narrow quantum wells. It is argued that most experiments can be interpreted in terms of an electron-bulk phonon interaction model (i.e. by taking into account the effect of confinement only on the electrons and assuming the phonons to be the usual bulk three-dimensional phonons) because a number of important physical processes, such as screening, the hot phonon effect, phonon self-energy correction etc, make it difficult to distinguish quantitatively between various models for phonon confinement, except perhaps in the narrowest (<50 AA) wells and wires. Detailed numerical results for the calculated intra-subband relaxation rate in GaAs quantum wires are provided within the slab phonon and the electron temperature model, including the effects of dynamical screening, quantum degeneracy and nonequilibrium hot phonons.

The electron-optical-phonon scattering rates in GaAs/AlAs quantum wells (QW) are calculated on the basis of a fully microscopic description of the phonon spectrum. The results indicate the great importance of confined as well as GaAs-like and AlAs-like interface phonons. The implications of these findings are discussed by using the calculated rates in Monte Carlo simulations of ultrafast carrier relaxation in a 50 AA QW.

Scattering rates of electrons in GaAs/AlAs superlattices are calculated in an envelope-function approximation which almost completely reproduces long-wavelength optical phonons. The results are compared with those obtained in the bulk-phonon model and the dielectric continuum model, which shows that the dielectric continuum model can provide an accurate estimate of the strength of electron-phonon scattering and that even the bulk-phonon model explains the layer thickness dependence reasonably well. The underlying physics leading to this result is discussed.

Photoexcitation of III-V compounds with a 50 fs laser pulse leads to ultrafast longitudinal field changes of up to 100 kV cm^-1. These are generated via screening and transport effects of the photoexcited carriers in the surface space-charge region. The field rise is sufficiently short to impulsively launch coherent LO phonons. A detailed study of the frequency and dephasing behaviour of these phonons as a function of excitation density and excess optical excitation energy is performed in GaAs and AlGaAs.

The description of the interaction of electrons and optical phonons in semiconductor superlattices and quantum well structures requires analytical expressions for atomic displacements and macroscopic electrostatic potentials accompanying the excitation of polar phonons. Recent discussion of this problem has revealed that the conventional macroscopic dielectric continuum theory contains uncertainties giving rise to controversial results even in the long-wavelength limit. The authors demonstrate that the uncertainties can be lifted if the complete branch dispersion of the underlying bulk phonons and boundary conditions derived from microscopic equations of motion are taken into the continuum theory. In the limit of vanishing dispersion the resulting envelopes confirm the predictions of Huang and Zhu as well as of Bechstedt and Gerecke within the unified model.

The tunnelling current through a resonant tunnelling structure is calculated including electron-interface-phonon coupling in the quantum well. The Hamiltonian describing electron-phonon interaction in a double-barrier structure is derived using the dielectric continuum model. The result shows that interface phonons of the barrier play a significant role in the double-barrier structure whose barrier width is greater than the well width.

Based on numerical results, the characteristics of each type of optical phonon mode in one-dimensional (1D) quantum wires are illustrated. The analytical formulae for 1D bulk-like optical displacements and associated electrostatic potentials are presented. Compared with the two-dimensional (2D) case, both the optical displacements and Frohlich potentials for the bulk-like modes vanish at the interfaces, but the finite dispersion of bulk phonons has a more pronounced effect on the 1D phonon modes.

A new theoretical approach to describe the interaction of electrons with Fuchs-Kliewer (FK) interface phonon polaritons is outlined. The quantized vector potential A of the FK modes in GaAs/AlAs quantum wells is derived and employed via the minimal coupling interaction (-e/m^*)A.p to evaluate the intersubband and intrasubband relaxation rates. A novel resonance in the intersubband rate is predicted and shown to be intimately related to the FK mode dispersion. As for the intrasubband rate, it is shown that for small well widths the AlAs-like FK interface mode is crucial for relaxing the carrier energy in GaAs/AlAs systems.

Elastic boundary conditions are shown to emerge naturally from a simple unifying theory describing long-wavelength excitations in bulk semiconductors. Their application to the cases of a freely vibrating layer and to a layer enclosed by an infinitely rigid medium (roughly approximated by a GaAs layer in vacuum and by a GaAs/AlAs quantum well respectively) shows that the disappearance of relevant dilational and shear stresses in the first case and the inhibition of displacement in the second case can only be obtained by s-polarized TO modes without mixing-LO and p-polarized TO modes are forced to mix coherently. It is also necessary for Fuchs-Kliewer interface polaritons to mix with LO modes in order to satisfy elastic boundary conditions. The use of elastic, as distinct from hydrodynamic, boundary conditions brings the continuum model much closer to the predictions of microscopic theory.

Intersubband relaxation of hot carriers in III-V compound quantum well systems depends explicitly on the form of the polar optical coupling. In the GaAs/AlGaAs system, it is well known that the electrical and mechanical boundary conditions imposed on the atomic displacement at the interface lead to the formation of GaAs-like and AlAs-like slab and surface phonon modes. Thus, a variety of possible intersubband relaxation channels exist rather than a single phonon mechanism, as is the case with bulk-like phonons. Here the authors study the effect of such modes on the intersubband scattering rate in asymmetric coupled quantum wells in which an ensemble Monte Carlo simulation of electrons and holes is used to compare with recent time-resolved photoexcitation experiments. Their results show that, when energetically permitted, the dominant contribution to the transfer time between wells is due to AlAs modes originating in the barrier rather than GaAs-like modes in the well.

Anti-Stokes Raman scattering has been used to investigate hot carrier dynamics in GaAs/AlGaAs quantum wells and has revealed new information on the physics of the electron-phonon interaction. Measurements of inter-subband relaxation have for the first time revealed sub-picosecond scattering rates. Also, it has been demonstrated that intra-subband relaxation in narrow quantum wells is dominated by the emission of interface modes. Furthermore, the lifetime of LO phonons in quantum wells is found to be smaller than in bulk GaAs.

In recent years, a number of time-resolved photoexcitation experiments have been reported which directly measure the decay of photoexcited carriers due to inter-subband scattering in semiconductor quantum wells and superlattices. Various techniques such as differential transmission, photoluminescence spectroscopy and Raman spectroscopy have probed different regimes of this non-equilibrium carrier relaxation. Particle simulation using Monte Carlo techniques has proved particularly useful for modeling the non-stationary behaviour of carriers during and after ultrafast laser excitation in such experiments. Here a discussion is given of hot carrier dynamics in heterojunction systems during photoexcitation, and the modelling of the dynamics using ensemble particle simulation. Comparison is made with different experiments which measure the effects of intersubband relaxation in single and multiple quantum wells.

The interaction of electrons with interface phonons is predicted to be of major importance in narrow quantum wells. Time-integrated Raman measurements of non-equilibrium phonons in GaAs/AlAs structures show strong coupling to AlAs interface modes, in good agreement with theoretical predictions based on a microscopic phonon model. Monte Carlo simulations of time-resolved Raman measurements of interface phonons in GaAs/AlGaAs structures provide further confirmation of this result.

The authors report time-resolved anti-Stokes Raman scattering measurements of short-period GaAs/AlAs type II superlattices which reveal directly that q approximately 0 confined and interface optical phonons are emitted by electrons when they transfer from the Gamma valley in the GaAs layers to the X_z valley in the AlAs layers in structures where the Gamma -X_z separation is greater than the optical phonon energy. The transfer time is estimated from the risetime of the non-equilibrium phonon population to be approximately=1 ps.

Time resolved optical techniques have been proven as powerful tools to investigate hot carrier relaxation channels in semiconductors on a subpicosecond time scale. Their improvements in time resolution by the use of femtosecond laser pulses opened the way to quantitative details concerning the initial steps in the chain of relaxation processes. In this paper, the most recent experimental results obtained from time resolved spectroscopy with femtosecond laser pulses are reviewed. Special attention is paid to the internal thermalization of optically generated electron-hole pairs, their ultrafast separation in surface space charge fields and to specific intercarrier interactions during cooling via phonon emission. Ultrafast hot carrier transport in surface space charge fields results in the synchronous excitation of coherent LO-phonons. The nature of the driving force for these coherent vibrations is explored.

The authors demonstrate that a transient-grating experiment is a sensitive optical method to study the dynamics of the spatial separation of electrons and holes in an electric field, i.e. the screening of the electric field by charge rearrangement. It provides high temporal as well as spatial resolution. The technique is applied to study the field-induced carrier dynamics in the plane of an intrinsic GaAs/AlAs quantum well layer.

The authors have studied high-field parallel transport of photoinjected carriers in GaAs quantum wells using femtosecond optical spectroscopy. They have directly observed the transient nonequilibrium distribution functions that occur during the field-induced acceleration of the electrons. At a high field (16 kV cm^-1), a nonthermal high-energy tail in the distribution function is apparent during the first 150 fs, which is due to electrons ballistically accelerated from the band edge. At later times, the authors observe the relaxation of the applied electric field due both to radiation from the accelerating carriers and from the build-up of space charge as the electron and hole gases separate.

The authors have developed a semi-empirical model for the short-range deformation-potential (DP) interaction in bulk (elemental or compound) semiconductors (not semiconductor alloys), which governs the transfer of carriers between different equivalent or non-equivalent conduction band (CB) valleys. Their formalism treats the electron-phonon interaction in the rigid-ion approximation and uses parametrized models for describing electrons (empirical local pseudopotential method) and phonons (shell models). The parameters for electrons and phonons were taken from the literature and no additional parameter was introduced to model the electron-phonon coupling. This model, when applied to the scattering times of electrons between different CB valleys in GaAs, gives reasonable agreement with a number of recent ultrafast optical experiments and resolves apparent contradictions between them. The present model can also be used in Monte-Carlo simulations of electronic transport under high-field conditions. The authors' main conclusion is that the simple formula for intervalley scattering due to Conwell (based on parabolic CB valleys for the electrons, an Einstein model for the phonons, and a single coupling constant describing the interaction) can only qualitatively explain most experiments and leads to differing values of the strength of the DP interaction. The full electronic band structure and all six phonon modes have to be taken into account in order to obtain a consistent picture.

Thermalization of a non-equilibrium distribution of electrons and holes generated by femtosecond photoexcitation is studied in GaAs and InP via spectrally and temporally resolved luminescence. In both materials, a rapid onset of luminescence is observed over a broad spectral range from the bandgap up to 1.7 eV. The data demonstrate the redistribution of both electrons and holes over a wide energy range within 100 fs, even for excitation densities as low as 10¹⁷ cm^-3. Equilibration is dominated by carrier-carrier collisions with scattering rates that are considerably higher than predicted by theoretical simulations using static screening of the interaction potential. The authors' Monte Carlo calculations, applying a molecular dynamics scheme, give enhanced scattering among the carriers with respect to a static screening approach, resulting in smoother distribution functions and luminescence spectra which are in good agreement with the experimental results.

The net energy-loss rate of a hot electron in GaAs due to Coulomb-mediated interactions with a quasi-thermal electron-hole plasma is calculated for a range of plasma densities, temperatures and initial hot-electron energies. The energy-loss rates obtained using a fully dynamic treatment (within the random phase approximation) of the screening are significantly larger than those estimated using the static screening approximation. Furthermore, a substantial portion of the hot-electron's energy can be transferred to the plasma via the hole system when dynamic screening and intervalence band processes are included.

The authors studied the lifetimes and the luminescence spectra of photoexcited carriers in H⁺-bombarded InP for different damage doses by means of femtosecond luminescence spectroscopy. The lifetime decreases down to 95 fs for a dose of 10¹⁶ cm^-2 and the time-integrated luminescence spectrum of this sample is inverted. Taking into account the different thermalization and relaxation conditions for electrons and holes, they find that the time-averaged hole distribution is hot but thermalized, whereas the time-averaged electron distribution is inverted, meaning that the electron distribution function increases with increasing energy within the conduction band.

The use of femtosecond laser pulses to excite electron-hole plasmas in semiconductors has become a major method of studying fast processes in this system in recent years. Transition times from the central Gamma valley in GaAs to the satellite X and L valleys are comparable to the reciprocal of the frequency of the phonon involved in these transitions, bringing into question the use of standard perturbation theory approaches. The authors present an initial investigation of the role of retardation arising from the finite collision duration required for such a transition to occur, through the use of a path-integral form of the quantum kinetic equation. A secondary self-scattering process and a probability of completed collisions are used to cast this equation in a semi-classical form.

Photoconductive and electro-optic sampling of the photocurrent of GaAs metal-semiconductor-metal photodiodes reveals the influence of field screening and optical phonon scattering on the frequency bandwidth of these detectors. The experimental results agree with theoretical predictions of a self-consistent two-dimensional Monte Carlo calculation.

Femtosecond pump-and-probe spectroscopy was adopted to study the hot carrier distribution in a CdSe thin film. In the experiment, a nonthermalized hot carrier distribution was observed when the pump was turned on. The nonthermalized distribution was observed at the 200 meV broad energy range extending from the pump energy to the low energy side. The low energy tail of the nonthermalized distribution indicates that the carrier-LO phonon scattering competes with the carrier-carrier scattering in the thermalization process. In this sense, this observation is a unique example of the nonthermalized hot carrier distribution in semiconductors. From the ratio of the nonthermalized distribution to the total one, the thermalization time was estimated to be 16-40 fs. In the wake of the thermalization, cooling of photogenerated carriers was observed. With an increase of the excitation density up to 10¹⁹ cm^-3, the carrier cooling rate was slower. The observed cooling rate was slower than the theoretical calculation, taking account of the screening of the carrier-phonon interaction. This is ascribed to the hot phonon effects of the LO phonon and the TO phonon.

The gain dynamics of thin GaAs films is measured near the band edge with femtosecond time resolution following photoexcitation of a large carrier density with approximately 100 fs pulses at 2 eV. The authors attribute the subpicosecond onset of gain to very efficient cooling of carriers near the Gamma point of the Brillouin zone, which implies nearly instantaneous equilibration and preferential intervalley scattering of high-energy electrons. Evidence for a heated-hole distribution on a subpicosecond timescale is presented, which points to the importance of electron-hole interactions.

The authors measure the hot phonon population of the GaAs LO phonons using a picosecond Raman scattering technique. Their samples were a series of GaAs/Al_0.4Ga_0.6As superlattices (SLS) with a fixed well width (L_z) and varying barrier width (L_b). An abrupt increase in the phonon generation rate was observed as L_b decreased over a narrow range. This is interpreted as the onset of significant well-to-well coupling of the confined GaAs LO phonons. An estimate of the LO phonon penetration depth into the Al_0.4Ga_0.6As barriers is obtained from this transition.

Exciton trapping time to interface islands in GaAsP/AlGaAs strained quantum wells is found to be of the order of 30 ps.

The time evolution of the electron population in the bottom of the X₆ valley in Al_0.6Ga_0.4As was obtained by a femtosecond visible-pump and infrared (IR)-probe absorption spectroscopy. The L₆ to X₆ intervalley scattering time of approximately 200 fs was determined from the measured kinetic data. The role of the L₆ to X₆ intervalley scattering for the relaxation process of hot carriers was revealed.

Subpicosecond luminescence and transmission spectroscopy for a broad range of doping and optical excitation densities are used for a systematic study of thermalization and energy relaxation of highly photoexcited electrons and holes in bulk intrinsic, n- and p-doped GaAs and InP at room temperature. The measurements are complemented by an ensemble-Monte-Carlo analysis, including non-equilibrium phonons and plasmons as well as electron degeneracy.

The authors present a Monte Carlo study of the Coulomb interaction in polar semiconductors involving both single-particle (screened carrier-carrier interaction) and collective excitations (plasmon scattering). Relaxation of high energy carriers, created by laser excitation, occurs through the interaction with the lattice (mainly via optical phonon emission) and with other carriers. The lattice (mainly via optical phonon emission) and with other carriers. The importance of the latter mechanism has been demonstrated recently by time-resolved and CW hot (e, AA) luminescence experiments which allow the authors to distinguish the separate contributions due to phonon and intercarrier scattering. They study the energy exchange mechanisms in a multi-component plasma using an ensemble Monte Carlo simulation where the intercarrier scattering is treated with two different approaches, the first based on a k-space model that allows the separation of short range (single-particle collisions) and long range (plasmon scattering) Coulomb potential, the second adopting a molecular dynamics scheme. The results of the simulation and the comparison between the two models are discussed for both GaAs and InP. A very good agreement with the experimental results is found.

Time-resolved probe beam deflection and degenerate four-wave mixing (DFWM) have both been used to determine the temporal evolution of the refractive index changes induced in ZnSe and CdS_0.75Se_0.25 single crystals at T=300 K by lambda =620 nm, 120 fs pulses with irradiances (I) up to 500 GW cm^-2. Negative refractive index changes which follow the pump pulse profile are attributed to virtual electronic transitions and two-photon resonances, while two-photon-generated hot carriers and their cooling rate determine the magnitude and temporal evolution of the refractive index change for probe delays up to 4 ps. No further evolution is observed up to 100 ps unless I>30 GW cm^-2, in which case the refractive index appears to recover on a 10 ps timescale following carrier relaxation: the possible role of a threshold dependent recombination process in this recovery is discussed.

Differential transmission measurements have been performed on a 200 nm thick Al_0.25Ga_0.75As layer at 300 K. The transient photoinduced transmission is observed in the vicinity of the fundamental absorption edge using tunable 150 fs pump pulses with 11 meV and 75 meV excess energy and white light continuum probe pulses. Comparison of the experimental data with a theoretical model, including excitonic as well as plasma effects reveals the dominant influence of Coulomb screening on the band-edge-absorption kinetics. Agreement with the experimental data could only be achieved by using dynamical instead of static screening of all possible scattering processes.

Electron-optical phonon interactions in ultrathin GaAs-AlAs multiple quantum well structures have been studied by using picosecond time-resolved Raman spectroscopy. The authors experimental results have shown that the electrons primarily relax through the emission of interface optical phonons and that the electrons interact not only with GaAs-like interface phonons but also with AlAs-like interface phonons. The interaction strength of electrons with AlAs-like interface phonons has been shown to be much stronger than that of electrons with GaAs-like interface phonons for ultrathin GaAs quantum wells. The strength of electron-AlAs-like interface phonon interaction has been demonstrated to decrease very rapidly as the thickness of the GaAs well increases.

Picosecond time- and energy-resolved photoluminescence (PL) spectroscopy is used to investigate hot carrier dynamics in GaAs epilayer structures grown on silicon substrates. The existence of biaxial stress arising from the thermal expansion difference between GaAs and Si substrates alters the valence band of GaAs epilayer structures on Si. Using the radiative transitions from electrons in donor (Si, 10¹⁶ cm^-3) states to separated valence bands in a 2 mu m GaAs epilayer on Si, energy relaxation of thermalized hot holes is studied. Different electron energy relaxation dynamics are observed in thin and wide modulation Be-doped GaAs-AlGaAs multiple quantum wells grown on Si substrates, arising from different valence subband structures.

Improved modelling of photoconductive switching experiments can be obtained by embedding a bipolar ensemble Monte Carlo model of the photoconductive gap into a time domain solution of Maxwell's equations. In addition to making fewer assumptions than previous models, this technique allows for the simulation of a probe pulse used in pump and probe experiments. Thus it is possible to accurately simulate the entire experiment and plot the quantity actually observed in the experiments-the phase shift of the probe beam.

The authors discuss a wide variety of new phenomena caused by high levels of confinement in mesoscopic systems, which affects the dynamics and statistical distribution of the carriers. A simulation of the low-temperature transport in quantum wires shows the onset of velocity oscillations arising from optical phonon emission in moderate electric fields. At high temperatures, transport nonlinearities are manifested by high carrier mobility and substantial carrier cooling. Pronounced resonances between the optical-phonon energy and the subband separation give rise to population inversion between adjacent off-resonance subbands induced by 'phonon pumping'. Under high-field conditions carriers rapidly escape from quantum wires through intervalley scattering and real space transfer.

The electron transport properties in quantum wires are investigated in the ballistic region and the hot-electron region by solving the Boltzmann equations. In the ballistic region, the effect of the scattering due to the surface roughness of a quantum wire on the conductance quantization is studied, and it is made clear how the scattering at the surface roughness destroys the conductance quantization. In the hot-electron region it is shown that the optical-phonon scattering plays an important role in the electron transport, and two types of transport take place as a result. One is electron confinement in a energy region below the optical-phonon energy and the other is electron velocity runaway. With these characteristics the possibility of a velocity modulation field effect transistor is suggested. Finally, a quantum wire resonant tunnelling transistor which works as a kind of multichannel rotational switch is proposed. As an application of this transistor, a switch for a superconducting interconnection network on which signals can propagate without attenuation and distortion is proposed to reduce the number of interconnection lines and also to simplify the circuit configuration.

The conductance, the thermal conductance, the thermopower and the Peltier coefficient of a quantum point contact all exhibit quantum size effects. The authors review and extend the theory of these effects. In addition, they review their experimental work on the quantum oscillations in the thermopower, observed using a current heating technique. New data are presented showing evidence for quantum steps in the thermal conductance, and (less unequivocally) for quantum oscillations in the Peltier coefficient. For these new experiments the authors have used a quantum point contact as a miniature thermometer.

This paper reviews the authors work on the experimental and theoretical analyses of the angular distribution of electrons injected through a single quantum point contact. They observed double peaks in the distribution with the point contact quantized in two modes. The authors calculation of the distribution using a Fraunhofer diffraction approximation through a quantized single slit agreed well with results. In this calculation they use a Green's function in weak magnetic fields and constructed mirror images there. They also investigated the possibility of observing interference in the angular distribution between the first and second modes, and found that the interference terms of the angular distribution were cancelled due to system symmetry. Another possible way to observe interference is due to electron waves through double point contacts. To measure this interference, the authors developed submicron air bridges to make double point contacts with independently controlled widths. The authors measured the controlled additivity of the conductance for four point contacts.

The authors compare the effects of current heating, hot-electron injection and lattice temperature on the attenuation of a collimated ballistic electron beam in a two-dimensional electron gas. In the electron gas heating and injection experiments, they use a quantum point contact as a local thermometer. They find that electron-electron scattering is the dominant factor determining the temperature dependence of the collimation signal.

The authors extend their previous studies of the heating by DC electric fields of submicron-diameter free-standing wires of doped GaAs at low lattice temperatures. For lower fields, equivalent to sub-nanowatt dissipated power in their samples, all thermal processes could be accounted for in terms of electron heating alone. They consider here the regime where the hot electrons deposit some of their excess energy into the host lattice, they extract a characteristic electron-phonon scattering length that is an appreciable fraction of the sample length, and the authors provide evidence for ballistic phonon transport.

The authors have studied lateral exciton and electron-hole pair transport by space- and time-resolved spectroscopy under conditions where enhanced mobilities and diffusivities are observed. A transport/transfer model, including relaxation, lateral diffusion, transfer between the islands and recombination, is used to evaluate the authors measurements of growth-interrupted quantum wells. For 40 K<T<180 K the variation of the diffusivities with temperature can be described by an isothermal diffusion of excitons considering various scattering mechanisms. For growth-interrupted quantum wells they found considerably higher mobilities and diffusivities at low temperatures compared to continuously grown samples. This is attributed to a partial suppression of interface roughness scattering due to island formation on growth-interrupted interfaces. Unexpectedly high diffusivities were observed for 10 K<T<40 K which may be caused by non-equilibrium effects in the carrier and phonon system. The unexpected increase of the diffusivities with rising temperature observed in thin quantum wells for 180 K<T<230 K is attributed to the growing thermal dissociation of excitons into free carrier pairs.

Picosecond pump-probe measurements of the exciton absorption resonance in a GaAs quantum well display a bleaching caused by collisional broadening. The authors have used this effect to measure how the effective diffusivity of photoexcited carriers depends on the excess kinetic energy imparted to them by the absorbed photons.

The authors describe a many-body semiclassical approximation for simulating electronic motion. Semiclassical trajectories in this approach are determined from spin-dependent forces and effective mass corrections, which incorporate effects of Heisenberg uncertainty, and the Pauli repulsion and exchange interactions of fermion statistics. The method has been implemented jointly with an ensemble Monte Carlo treatment of phonon scattering, and numerical simulations performed for GaAs. Numerical results indicate that in the femtosecond-scale relaxation of laser-excited plasmas, quantum corrections to the electronic motion will become significant for excitation densities above 2*10¹⁸ cm^-3.

The authors measured photoluminescence (PL) spectra under an applied electric field along the hetero-interface to study hot electron scattering mechanisms in HEMTs. The electron temperature obtained from the PL spectra, indicates hot electron scattering conditions. An undoped AlGaAs barrier layer is inserted into the HEMT's channel layer. PL intensity is enhanced by introducing an undoped AlGaAs barrier layer on the substrate side, since holes are confined within the quantum well. PL spectra are obtained for three types of AlGaAs/GaAs/AlGaAs single quantum wells (SQWs) whose doping conditions differed. They also measured barrier layer PL spectra to observe the effect of real space transfer (RST). The authors propose an energy cooling model to explain the anomalous cooling of hot electrons observed when RST occurred.

By using hot-electron transistor structures, the authors showed that the hot-electrons in the base suffer little or no energy loss within a distance less than 1500 AA. They utilize this hot-electron property to construct a highly sensitive 10 mu m infrared hot-electron transistor.

The effects of a hot two-dimensional electron gas (2 DEG) on optical properties of modulation-doped GaAs/AlGaAs heterostructures have been investigated by photoluminescence (PL) and optically detected cyclotron resonance (ODCR) techniques. A decrease of the PL intensity from shallow excitons and an enhancement of the interband recombination between the 2 DEG (confined in a notch potential near the heterointerface) and photo-excited holes are observed, when the cyclotron resonance (CR) condition for electrons is fulfilled. The mechanisms responsible for these observations are discussed in terms of a hot 2 DEG heated in the microwave (MW) field. The angular dependence of the ODCR peak proves the 2D character of the corresponding hot electrons. In this way the authors identify these PL emissions as arising from the same GaAs active layer.

Harmonics of hole cyclotron resonance are more pronounced in optically detected cyclotron resonance than in ordinary cyclotron resonance. Thanks to this enhancement, harmonics have been clearly observed to the third order for light holes and to the sixth order for heavy holes Ge. This phenomenon is explained by consideration of the wavenumber distribution of hot holes produced by cyclotron resonance and its harmonics.

The authors analyse hot-electron transient transport in a Bloch-type superlattice miniband, under conditions leading to saturation in the regime of negative differential conductance (NDC), using a balance equation approach with a dynamical force-balance equation for the centre-of-mass acceleration, and a dynamical energy balance. They solve these balance equations numerically for a GaAs-based superlattice, considering Gamma -valley electrons with a tight-binding miniband structure. Both impurity scattering and phonon scattering are taken into account. For a step electric field in the NDC regime, the drift velocity exhibits very pronounced overshoot. In GaAs systems, the maximum drift velocities occur within 0.1-0.3 ps after turning on an electric field of 10 kV cm^-1. About this time, the instantaneous inverse effective CM mass for perpendicular transport falls within the range of negative values. The rise of the electron temperature, however, is much slower, such that the behaviour of the drift velocity response to an impulsive electric field is markedly dependent on the duration of the impulse.

The non-equilibrium distribution function of a two-dimensional electron gas in a static high electric field is calculated within the Lei-Ting approach of high-field electronic transport. The separation of the centre of mass motion from the relative motion of the electrons leads to anisotropic interactions with impurities and phonons which displace the relative electron gas state from thermodynamic equilibrium. The calculated distribution function is highly anisotropic with a 'cold' part in the direction of the drift velocity and a 'hot' part in the opposite direction. An effective temperature is introduced by averaging the electronic distribution function in all the directions of the reciprocal space. This effective temperature is compared with the experimental temperatures measured by photoluminescence, during the steady state high electric field transport experiments. A finite LO-phonon lifetime as low as 1 ps is shown to give good agreement between theory and experiment for the energy relaxation rate of a two-dimensional hot electron gas.

The authors have investigated the temperature dependence and electron concentration dependence of the drift velocity in AlGaAs/GaAs heterostructures for electric fields up to 2 kV cm^-1. The saturation behaviour of the drift velocity at different temperatures could be well described by assuming LO-phonon emission as well as intersubband scattering processes. An increase of the saturation drift velocity with increasing electron concentration was found which can be explained by screening effects. All experimental data were compared with theoretical calculations based on a Boltzmann integral as well as a balance equation method. In addition, the electron distribution function was measured directly using the Smith-Purcell effect. The authors observed a clear cut-off of the distribution function at the LO-phonon energy and a shift to higher velocity values with increasing applied electric field. Distinct deviations of the emission spectra from a drifted hot Fermi distribution were found.

The authors report optical analysis of hot carrier distribution and transport properties in InP/AlInAs type II heterostructures under electric fields applied both normal and parallel to the heterointerface. They have observed photoluminescence which originates from the transition between electrons and holes spatially separated in the InP and AlInAs, respectively. An applied electric field across the heterointerface has significantly modulated the photoluminescence spectrum emitted from the interface. This effect can be interpreted from the field-dependent modulation of the carrier distributions near the interface in the staggered heterostructure. The hot electron velocity of 2D electrons in the heterostructures has been also discussed in experimental and theoretical aspects.

The influence of a non-equilibrium population of LO phonons (hot phonons) on hot-electron energy and momentum relaxation in modulation-doped GaAs quantum wells are studied both experimentally and theoretically. The experimental results on high-field parallel transport indicate strongly that: (i) non-equilibrium phonons in GaAs quantum wells, contrary to the assumptions made in the conventional theories, are non-drifting; (ii) therefore, the production of hot phonons not only reduces the energy relaxation rate but also enhances the momentum relaxation rate; (iii) the enhancement of the momentum relaxation at high fields inhibits negative differential conductivity via real space transfer or intervalley transfer; (iv) the enhancement of the momentum relaxation rate also reduces the drift velocity at high fields, which is detrimental to the speed of the hot electron devices; (v) hot phonon effects increases with increasing 3D carrier concentration. The results are compared with a comprehensive theoretical model involving non-drifting hot phonons and scattering from remote impurities and interface roughness. The agreement between the theory and the experiments is excellent.

The conductance of geometrically asymmetric quantum point contacts formed in 2D electron gas (2 DEGs) in GaAs/(AlGa)As heterojunctions has been studied as a function of the applied bias and magnetic field. The nonlinear conductance is found to be independent of bias direction in zero magnetic field in agreement with other workers but at higher magnetic fields the I(V) curves are highly asymmetric in terms of both the onset and the type of nonlinearity observed. The curves are described using simple theory. The same experimental arrangement is also used to study nonlinearity in the quantum Hall regime when the device acts as an adjustable, narrow channel. The nonlinearity is qualitatively different to the first case and there are discontinuous changes in the dissipation which the authors identify with scattering between magneto-electric subbands.

A semi-classical theory of the non-linear miniband conduction in crossed electric and magnetic fields is presented. The Boltzmann transport equation is solved with realistic scattering mechanisms, i.e. electron-phonon scattering and interface potential fluctuations. The technique consists of a characteristic solution of the electron dynamics along the free trajectories associated with an iterative convergent algorithm. The main effect of the transverse magnetic field is interpreted in the frame of the Esaki-Tsu model by the modification of the free trajectories by the Lorentz force which repels the onset of Bragg scattering to higher electric fields. A direct comparison with experimental current-voltage data is performed in taking into account the electric field inhomogeneity.

The authors report the results of a spectroscopic investigation of the radiative recombination processes occurring in a dense electron-hole plasma confined in quasi-one-dimensional semiconductor quantum wires. Intersubband transitions involving quantum wire states with quantum number as high as n_y=5 are observed in the photoluminescence spectra recorded under stationary conditions of high excitation intensity. The temporal evolution of the luminescence is characterized by a decay time of about 800 ps longer than the one measured in the two-dimensional reference heterostructure.

The quantized thermopower in the ballistic regime was estimated directly from measurements in a single point contact with a specific configuration of ohmic electrodes. If the DC current supplied to heat electrons increased over a critical value, increases of both the electron and lattice temperatures were observed. For the lower indices of the one-dimensional subbands (i=1 and 2), the measured thermopower peak differed by as much as 20% from the theoretically predicted value. At low currents, where only the electron temperature was raised, the agreement with theory was within 10%.

Electron-electron interactions are investigated in the context of a two-quantum dot 'capacitor' in order to determine changes in the energy due to the transfer of single electrons between quantum dots. The calculations are performed by a linear combination of atomic orbitals (LCAO) technique with a spherical square well basis. It is found that the transfer of just one electron results in substantial changes of both the energy quantization in the dots and the total energy stored in the 'capacitor'. The role of direct and exchange energy interactions is discussed.

The authors report 4.2 K magnetoresistance measurements of InAs quantum wires based on AlGaSb/InAs/AlGaSb heterostructures grown by molecular beam epitaxy (MBE). The widths of the channel have been defined by electron-beam lithography and wet chemical etching. In the low magnetic field region, the Landau plots deviated from the linear relations indicating the lateral confinement. In the high magnetic field region, pronounced spin-splitting of Landau levels was observed in quantum wires compared with two-dimensional electron gas.

The Monte Carlo simulation of electron transport in multisubband quasi-one-dimensional GaAs/AlAs quantum wires (QWIs) are presented. The electron intrasubband and intersubband scattering by surface-optical and confined longitudinal-optical phonons in QWIs has been included in the program. It is demonstrated that at room temperature the electron drift velocity in the QWI is suppressed by electron intersubband scattering and does not exceed the bulk material values. The energy dependence of the total scattering rate in ideal QWIs exhibits multiple sharp peaks related to intersubband transitions. The scattering rates in the real QWIs with variable thicknesses are calculated as well. The results show that even small variation in thickness leads to a significant broadening of the very first peaks and complete washing-out of the peak-like structure at higher energies.

The authors report the results of femtosecond optical measurements following injection of a very large density (>10¹⁹ cm^-3) of carriers with 100 fs 2 eV pulses in a-Si:H. They measure the momentum scattering time of the free carriers (<1 fs), the thermalization rate of hot carriers (>or approximately=1 eV ps^-1), the non-radiative carrier recombination lifetime (1-10 ps), and the decay time of optic phonons into acoustic phonons (<or approximately=100 fgs). The lack of momentum conservation in amorphous materials is responsible for the much shorter characteristic times in a-Si:H than in a typical direct-gap semiconductor such as GaAs.

The authors calculate the finite temperature inelastic scattering rate for hot electrons injected into n-doped GaAs for various doping densities and injected electron energies. They use the Born approximation and treat the base region as a three-dimensional coupled electron-phonon system, so that the Coulomb and Frohlich interactions with the injected electron are put on an equal footing. As the temperature increases, the coupled electron-phonon modes broaden considerably, resulting in a rounding of the sharp emission thresholds which occur at T=0. For moderate to high doping densities (n>or approximately=8*10¹⁷ cm^-3) and injection energies of E-E_F>or approximately=100 meV (1) the increase in the scattering rate from T=0 K to T=300 K is typically less than a factor of 2, and (2) the scattering rates decrease with increasing doping density.

A direct numerical method is developed for solving the electron transient coupled Boltzmann equations in the Gamma and L valleys in InP. The complicated shape of the transient distribution function is obtained with a great accuracy. Fine structures of the velocity and the energy are obtained in the Gamma and L valleys when the population of L valleys is very small.

The authors have carried out Monte Carlo simulations of hole transport in a range of InGaAs/GaAs strained quantum well structures. The simulations include phonon, impurity and alloy scattering, with rates obtained using k.p band structure. The simulations give good agreement with the experimentally determined magnitude and carrier density dependence of the low field hole mobility, for samples with carrier densities p_s<or=4.0*10¹¹ cm^-2. The results suggest that alloy scattering is primarily responsible for the low mobilities observed in these structures. At p_s=4.0*10¹¹ cm^-2 strong coupling of optical phonon and plasmon modes leads to enhanced scattering and consequent reduction of the hole drift velocity, but in the current model this effect still does not fully account for the strong saturation of the drift velocity observed experimentally.

High-field dielectric response is investigated using the technique of nonequilibrium Green's functions based on an Airy-coordinate formulation. The field-dependent polarization function of the system is derived. De-screening of the Coulomb interaction due to collisional broadening and high electric field is examined.

Many authors have calculated, and measured, the velocity-field curve in bulk GaAs. However, many new incorporations within a proper ensemble Monte Carlo simulation have improved greatly the ability to understand fully the physics inherent in the velocity-field curves. One such improvement, which has arisen from the treatment of the electron-impurity interaction via real-space molecular-dynamics approach, is the recognition that screening is overestimated in most simple approaches, and the typical electron interacts with many impurities simultaneously. Here, the authors incorporate these more complicated approaches to determine the velocity-field curve at densities above 3*10¹⁷ cm^-3.

The influence of electron-electron scattering on the parameters of a hot electron system in a semiconductor has been analysed. The strongest influence has been observed in the slightly heated electron system (in the case of so-called 'warm electrons') at the lattice temperature T approximately=T_o/5(T_o being the characteristic temperature of the optical phonon).

Recently, hot electron to neutral acceptor recombination luminescence experiments under continuous wave laser excitation have been utilized to investigate ultrafast scattering processes. In this study, a numerical simulation of the hot e-A degrees problem is performed using the energy diffusion equation approach. The effects of elastic scattering processes and optical phonon scattering have been incorporated, as well as a static screening approximation to the electron-electron coulombic interaction, for GaAs irradiated by a CW laser source under low temperature (10 K). By comparing the Gamma -L intervalley population transfer results with experimental data, a value for the intervalley coupling constant and scattering lifetime was deduced.

The authors discuss the modelling of macroscopic balance equations in irreversible thermodynamics as resulting from stationary Monte Carlo bulk simulations for electrons. The Onsager coefficients related to these equations are determined from time integrals of fundamental microscopic correlation functions. Special interest is given to the validity of the relaxation time approximation and the Wiedemann-Franz law. The simulation results for silicon clearly show that a high doping level (>10¹⁸ cm^-3) is a sufficient condition for the validity of both approximations in inhomogeneous systems.

Data from a detailed investigation of the variation of longitudinal and transverse noise temperature with changing electric field strength in compensated n-InSb(Cr) are presented. It is found that longitudinal noise temperature decreases with the diminishing electron concentration, n, and it does not exceed the lattice temperature T_O, when n<2*10¹¹ cm^-3 at electric fields E<100 V cm^-1. It has been explained that the existing electron-gas cooling effect in electric fields is mainly responsible for this decrease in the noise temperature. Investigation of the noise temperature in the transverse magnetic field and at different lattice temperatures supports the presence of the cooling effect. The influence of the fluctuating potential barriers of the conduction band edge on the noise temperature and electric conductivity are also discussed.

The dynamical behaviour of carriers in highly photo-excited Ge and GaAs is investigated by far-infrared magneto-absorption and photoconductivity measurement. The authors observe far-infrared absorption due to hot holes in electron-hole gas in Ge and extension of carrier lifetime by impact-ionization of impurities in GaAs.

An investigation of the hot carrier relaxation in GaAs/(AlGa)As quantum wells and bulk GaAs in the high carrier density limit is presented. Using a time-resolved luminescence up-conversion technique with <or=80 fs temporal resolution, carrier temperatures are measured in the 100 fs to 2 ns range. The results show that hot carrier cooling in quantum wells becomes significantly slower than in the bulk for carrier densities greater than 2*10¹⁸ cm^-3.

The effects of carrier-carrier interactions on the transient valley repopulation and velocity in bulk silicon has been investigated using an ensemble Monte Carlo method. The expressions for electron-electron scattering rates take into account the ellipsoidal nature of the energy surfaces in silicon. The relative effects of both electron-plasmon and short range e-e interactions strongly depend on the orientation and magnitude of the electric fields. The e-e interactions reduce the transfer rates of electrons from the hot valleys to the cold valleys, increase the energy of the electrons in the cold valleys, and reduce the transient velocity overshoot in the hot valleys. These effects are more significant at higher electron concentration.

Results on energy relaxation in low carrier concentration two-dimensional structures over the lattice temperatures, T_l, of 3 K to 150 K are presented. The power loss per carrier as a function of electron temperature, T_e, was experimentally determined using the mobility-field, mobility-temperature thermometric technique. In the acoustic phonon regime, below about T_l=20 K, the power loss per carrier was found to be proportional to T_e-T_l. Theoretical calculations in this temperature regime predict a similar form but do not agree in magnitude. Above T_l=40 K the power loss per carrier was modelled using an optic phonon scattering model, which includes the effects of hot phonons, and which predicts a phonon lifetime of 11 ps+or-1 ps. Between these two regimes, for the multi-quantum well, the experimental power loss was found to be greater than the theoretical power loss. This discrepancy has been explained by invoking scattering due to optic phonon-plasmon coupling. Using the optic phonon-plasmon coupled mode energy and the scattering time as fitting parameters values of 10 meV for the energy of the coupled mode and 400 ps for the scattering time were obtained.

Minority-electron transport properties in p-GaAs have been investigated using time-of-flight (TOF) and time-integrated/time-resolved photoluminescence (PL) measurements. It was found that with an increase in hole concentration the drift velocity decreases, a thermal nonequilibrium energy state exists between electrons and holes, and the energy relaxation time decreases. By combining these experimental data with a Monte Carlo calculation the scattering mechanisms in highly doped semiconductors were clarified. It was also found that energy and momentum transfers during the electron-hole interaction play an important role in the minority-electron transport for p-GaAs. This investigation revealed that the combination of an experimental approach using optical techniques, such at TOF or PL, with a theoretical approach such as a Monte Carlo calculation is indispensable in clarifying hot-carrier transport.

The authors present a numerical method that allows them to solve the transient Boltzmann equation in p-Si with accuracy as high as in previous methods, but up to 200 times faster than is possible by the direct method. It consists in expanding the carrier distribution into a set of displaced Maxwellian functions and using a least-squares-like approximation method.

The results of studying a submicron GaAs ballistic diode, using a new multi-valley ( Gamma ,L and X) nonparabolic hydrodynamic transport model, are presented. Numerical simulations indicate that accurately including the effects of nonparabolicity in the streaming terms and k-space transfer in the velocity and energy equations is very important in correctly determining the conductance of the device. The existence and amount of negative differential conductance was determined to be strongly influenced by both of these physical factors. Furthermore, the sensitivity of device conductance to changes in the thermal conductivity is diminished significantly when nonparabolicity is accurately incorporated.

The authors have constructed a unique Monte Carlo simulation which takes into account (i) the bandstructure of the semiconductor, (ii) the anisotropy of the scattering rates due to both the density-of-states and the electron-phonon matrix elements (iii) realistic phonon dispersion, and (iv) an impact ionization scattering rate including the anisotropy of the rate and the intracollisional field effect. The sole inputs to the simulation are the empirical local pseudopotential form factors of the semiconductor, and an appropriate dynamical matrix to describe the lattice dynamics. The only freedom they have in constructing the transport model is in choosing an interpolation of the pseudopotential between 'known' form factors. This is necessary for the calculation of the rigid pseudo-ion matrix elements. This computational model provides the authors for the first time with a rigorous test of the ability to formulate and calculate semiclassical transport properties based on fundamental physical principles, i.e. they calculate both the bandstructure and the electron-phonon interaction using the same pseudopotentials to describe the periodic crystal potential.

Barrier calculations based upon solutions of the Liouville equation in the coordinate representation reveal a complicated spatial dependence of the quantum distribution function near and within the barriers. Within the framework of classical transport this spatial dependence suggests equilibrium electron temperature values that differ from the ambient. The prospect of quantum heating and cooling under equilibrium conditions is examined and dispelled in favour of an interpretation that includes density-gradient contributions.

Using the results of a Monte Carlo simulation, a set of coupled Langevin equations for the relevant variables (the fraction of free carriers, their velocity and their energy) is constructed and applied to the case of p-type Si at 77 K under hot-carrier conditions. The relaxation and generalized-diffusion matrices, describing dissipation and fluctuations respectively, are evaluated. From these results, the authors derive a model spectral density of current fluctuations which is applied to interpret experimental noise spectra.

Hot-electron transport in n-type GaAs has been investigated in the crossed applied DC electric field and DC magnetic field. The energy band structure of conduction electrons is taken to be a nonparabolic band and the dominant interaction between hot electrons and lattices is assumed to be the deformation-potential coupling. Results show that the dissipative current density increases nonlinearly with the applied electric field owing to the nonparabolic band structure of electrons in semiconductors. In the lower electric field region, the dependence of the current density on temperature does not appear to follow a regular trend. It is also shown that the current density depends strongly on the magnetic field.

The effects of disorder on the electronic conduction properties at subsidiary energy minima (upper valleys) of In_0.53Ga_0.47As alloys are evaluated using a novel Monte Carlo supercell approach with both the bond-length and bond-angle fluctuations considered. It is found that the width of the spectral function obtained in the present approach is much larger than that obtained using the coherent-potential approximation (CPA). This result indicates that the bond-length and the bond-angle fluctuations-which are not sufficiently considered in the CPA framework-induce strong effects on the electronic properties of the upper valleys.

High field electron transport properties were investigated for a pulse-doped GaAs structure which consisted of p^-/n⁺/n^- layers. Enhanced mobility was observed due to the electron transfer from doped layer to undoped layer. Owing to this effect, the drift velocity of a pulse-doped structure is higher than that of a bulk structure.

Hot-carrier transport is analysed by a formalism of irreversible thermodynamics. The entropy production rate in hot-carrier transport is derived to evaluate the validity of the analytical flow expressions, which had been derived using high carrier-carrier scattering. The analytical expressions are used to investigate thermodynamics involved in hot-carrier transport, which includes the relation to thermoelectric effects, the conformation to Onsager's theorem and the effect of scattering mechanisms. The expressions have already been successfully applied to silicon hot-electron transport and silicon hot-hole transport.

A new analytical band structure model for high energy carrier transport is presented, in which not only X valley (the lowest set of minima of conduction band) but also L or Gamma valleys are taken into account. All the physical parameters (effective mass, nonparabolicity and the location of each valley) are adjusted to fit to the calculated local empirical pseudopotential band structure. The main features of the density of states, which influence the scattering rate, are well reproduced with the new band model. It was found that the Monte Carlo simulation using the high energy valley model makes it possible to accurately predict high energy electron transport.

A simulation of charge quantum transport in silicon is presented. The authors discuss how effects such as energy nonconserving transitions (or collisional broadening) and intracollisional field effect influence transient transport and, in particular, drift-velocity overshoot. The analysis is based on an improved version of the quantum Monte Carlo method developed by the authors during the last few years. Results show that, for the case of silicon, the intracollisional field effect plays the dominant role in determining deviations from the semiclassical results.

The frequency dependence of the longitudinal differential mobility of hot electrons is calculated using velocity averaging over the before- and after-scattering ensembles by the single-particle Monte Carlo simulation of the steady state. A dynamic negative differential mobility (NDM), due to the transit-time resonance of hot electrons in the momentum space under the predominant role of spontaneous emission of optical phonons at low lattice temperature, is observed in n-InP. The frequency- and field-behaviour of the NDM and the noise characteristics, as well as the possibilities to use the effect for amplification and generation of the millimetre-wave radiation, are investigated. The techniques for the experimental investigation of the transit-time resonance are discussed. The noise temperature measurements are shown to be the most suitable tool for this. The transit-time resonance characteristics in n-InP are compared with the experimentally realized and theoretically calculated parameters of the cyclotron resonance NEMAG in p-Ge. The conditions for the generation and amplification are found to be better in the former case than in the latter one.

The authors have used a self-consistent Monte Carlo simulation to investigate the effect of high currents on the base-collector region of a heterostructure bipolar transistor. By considering identical device structures in Si and GaAs they are able to show that an accurate description of carrier response to field is necessary in order to obtain the correct interdependence of velocity overshoot, field configuration and self-consistent effects.

The authors discuss tunnelling processes in double quantum well structures in two limiting cases. Firstly, they report the coherent oscillations of a photogenerated electronic wavepacket between the wells. The THz oscillations are traced by an optical pump-probe as well as by four-wave-mixing spectroscopy. A comparison with theory shows that the two experiments give complementary information about the various relaxation processes. Secondly, they discuss hole tunnelling in weakly coupled wells as an example of resonant tunnelling in the limit of strong scattering. The experimental and theoretical results show that the resonant tunnelling process is considerably slowed down by the scattering and relaxation processes.

The use of optical techniques as spectroscopic probes of vertical transport in double-barrier resonant tunnelling structures is reviewed. Particular topics that are emphasized include the determination of the magnitude and temperature of charge build-up in the active quantum well region, deduction of the nature of the tunnelling processes and study of the tunnelling characteristics of both majority and minority carriers.

The occupation of higher subbands in superlattices is investigated with photoluminescence spectroscopy. The carriers are injected into the higher subbands by resonant tunnelling when an electric field is applied perpendicular to the layers of the superlattice. The occupation of the second subband is larger for a subband spacing below the longitudinal optical phonon energy h(cross) omega _LO than for a subband spacing larger then h(cross) omega _LO. However, the intensity of the observed transition is strongly reduced for a subband spacing below h(cross) omega _LO due to the strong mixing of the heavy- and light-hole subbands.

Magnetic fields up to 41 T (B perpendicular to J) are used to investigate hole tunnelling in p-type double-barrier AlAs/GaAs structures and to probe the dispersion curves of the hole states of the quantum well. The high magnetic fields reveal the cubic anisotropy of these states. A detailed comparison is made between the experimental data and the theoretical hole dispersion curves. Hole space charge build-up is observed in the quantum well and is measured using the Landau quantization effect for B//J.

Simultaneous measurements of electron and hole tunnelling times, and the exciton formation rate in asymmetric double well structures are achieved by the nonlinear double-pulse photoluminescence cross-correlation technique.

Time-resolved photoluminescence measurements in the picosecond regime on GaAs/Al_0.35Ga_0.65As asymmetric double quantum well structures with perpendicular electric fields applied in both directions are reported. A large number of electron and hole resonances are detected. A splitting of the ground state resonances reveals the importance of excitonic effects. Longitudinal optical phonon assisted tunnelling plays a minor role for narrow quantum wells in comparison with impurity or interface roughness assisted transfer. Resonant electron tunnelling times depend exponentially on the square root of the effective barrier height and are an order of magnitude shorter than resonant hole tunnelling times.

The decay time constant of photoexcited hot electrons in a double quantum well structure is measured by picosecond luminescence spectroscopy. By applying electric fields perpendicular to the heterointerface, the resonant and off-resonant tunnelling times are investigated as a function of the difference in energy between the quantized levels in the double quantum wells.

Time-resolved photoluminescence (PL) spectra of GaAs/Al_0.3Ga_0.7As double quantum well wires are studied. The decay time constant of photo-excited carriers is reduced in quasi-one-dimensional (1D) samples, indicating the enhancement of tunnelling probability and/or the reduction of the energy relaxation time of electrons. This is attributed to the surface roughness scattering.

Non-equilibrium quantum mechanical distribution of electrons in reservoirs is revealed and its effects on quantum tunnelling and interference are demonstrated for the first time. A new contact model using local chemical potential is proposed, and it is described here how an infinite open system can be reduced to a finite analysis region and the role of maintaining the electric potential differences between contacts is undertaken by energy dissipation due to dielectric relaxation and scattering processes in the reservoirs. Consequently, the effects of reservoirs are characterized by the bias voltage dependence of the local chemical potential, of the ohmic drop and particularly the quantum mechanical accumulation in a cathode region. This bias voltage dependence diminishes the quantum transport phenomena, and may produce transmission probability pictures very unlike the actual system. Designing a flat potential structure in the cathode region would eliminate this degradation in the quantum transport properties.

The time-dependent Liouville equation for the density matrix in the coordinate representation, incorporating scattering effects through a quasi-Fermi level, and Poisson's equation have been solved numerically for a single- and double-barrier structure using algorithms based on 'characteristics', and show significant charge accumulation on the emitter side of the barrier, as well as significant charge screening in multiple barrier structures.

The effect of inelastic scattering on resonant tunnelling through a double-barrier semiconductor heterostructure is studied using the transfer matrix method incorporating a phenomenological damping of the wavefunction propagation in the quantum well. The coherent and sequential contributions to the transmission coefficient are evaluated analytically in a Lorentzian lineshape approximation close to resonance. In contrast to previous results it is found that the integrated total transmission coefficient is in general dependent on the inelastic scattering rate. Numerical calculations show that realistic values of current peak to valley ratios at 77 K are obtained assuming a mean inelastic scattering time approximately 10^-12 s. The tunnelling is then almost totally sequential.

The authors have investigated the I(V) characteristics of a GaAs/(AlGa)As resonant tunnelling diode in which the effective cross-sectional area, A, may be varied using a gate. As A is progressively reduced I(V) becomes asymmetric. In particular the peak to valley ratio in forward bias is decreased from a value approximately=20 to approximately=1, but in reverse bias remains constant at approximately=20. They propose that this arises from a lateral variation of the voltage drop across the tunnel barriers which leads to a smearing of the resonance. As A is reduced they also observe additional maxima and minima in I(V).

Oscillatory structure in the low temperature current-voltage characteristics of Be-doped p-type GaAs/(AlGa)As/GaAs single barrier tunnelling devices is observed. The oscillations have a period Delta V=39 mV, close to h(cross) omega _L/e, where h(cross) omega _L is the LO phonon energy in GaAs. They result from energy relaxation of hot holes injected through the tunnel barrier. The sheet density in the hole accumulation layer adjacent to the tunnel barrier is obtained from the magneto-oscillations in I(V) for magnetic field B//J. This allows one to calculate the hole effective mass for tunnelling.

The authors have calculated the tunnelling current in GaAl-Al_xGa_1-xAs heterostructures using non-equilibrium distribution functions. These functions are obtained by means of a set of rate equations for carrier and phonon dynamics which takes account of the fundamental scattering mechanisms in a semiconductor. They analyse the resonant tunnelling as well as the time evolution of the carrier population generated by a laser pulse in a well. In particular, they pay attention to the escape time and its dependence on the external parameters of excitation.

The authors have studied the effect of final states, the ground states and the first excited state, on electron tunnelling in asymmetric double QW structures. The tunnelling transfer time reached its minimum under resonant conditions when the destination was the first excited state. When the final state is the ground state of the adjacent well, however, it is found that the tunnelling transfer time reaches its minimum not under on-resonant but rather under off-resonant conditions. The energy difference between the initial and the final states at the transfer minimum is estimated to be around 17 meV, which eliminates LO phonon-assisted tunnelling as the mechanism. The actual transfer mechanism is most likely related to the interface roughness when the destination is the ground state.

The authors observe resonant tunnelling of electrons and holes in a GaAs p-i-n diode incorporating two AlAs barriers in the undoped region. Peaks due to the HH1, LH1, E1 and HH2 (E=electron, LH=light hole, HH=heavy hole) resonances are observed, in that order of increasing voltage. The voltage separation of the LH1 and E1 resonance is significantly larger than expected from a simple quantum mechanical model due to an interaction effect involving resonant buildup of electron space charge in the quantum well. High magnetic fields B//J and B perpendicular to J are used to investigate the device and confirm the assignment of the resonances.

The transfer Hamiltonian method is used to study tunnelling between the subbands of barrier-separated 2D electron systems. Application of the theory is made to a double-barrier semiconductor heterostructure in which a 2D accumulation layer is formed under bias. A region of bistability is found in the voltage dependence of the charge stored in the quantum well and in the accumulation layer. Comparison is made with magneto-oscillation studies of asymmetric double-barrier structures.

The conductance of laterally confined double-barrier quantum well resonant tunnelling heterostructures is investigated. The lateral confinement is provided by a Schottky gate and can be varied in a continuous way. Data for dots with nominal diameters in the submicrometre range are reported.

A GaAsSb (8.50 nm)/InGaAs (4.40 nm)/InAlAs (5.27 nm) staggered resonant tunnelling barrier structure, lattice-matched to InP, has been grown by MBE and a strong illumination effect on its current-voltage characteristics has been observed at room temperature. The peak current density increased by 40% and the peak voltage shifted by -75 mV at an excitation power density of 30 W cm^-2. The authors attributed these effects to the holes accumulated in the GaAsSb layer which acts as a deep quantum well trapping the holes.

The knowledge of hot electron noise properties in FETs and the dependence on biasing conditions and frequency is essential in device optimization. The recent development of submicron gate MODFETs with epitaxial III-V semiconductor heterolayers has considerably complicated the problem of producing accurate device models able to predict at the same time the static and microwave characteristics and the noise results as well. Low cost and fast simulators that can be implemented on small computers are in high demand in the device industry. In this paper the authors try to describe the present state of this problem.

Chaotic carrier transport induced by impact ionization of neutral impurities (i.e. impurity avalanche breakdown at low temperature) in semiconductors (e.g. GaAs Ge, InSb) has been summarized. Viewed over the wide spectrum of the observations, a universality class of low-dimensional dissipative chaos is recognized in the related nonlinear carrier transport. Among various types of the current instabilities reviewed, driven chaos and crossover instability in n-GaAs have been used as examples to show typical low-dimensional chaos. High-dimensional complex behaviour of the current instability has also been discussed.

Magnetic-field-induced oscillatory instabilities in semiconductors are explained by a novel dynamic Hall effect of hot electrons. The basic new idea is to consider the nonlinear dynamics of both the drift and the Hall field due to dielectric relaxation coupled with impact ionization of hot carriers. This results in self-generated current or voltage oscillations under DC bias, period-doubling, and intermittency routes to chaos. Numerical simulations are presented for p-Ge, n-InSb, and n-GaAs with field-dependent mobilities and one or two shallow impurity levels at 4.2 K. In particular, the universal scaling behaviour of type-I intermittency as a function of magnetic field is demonstrated.

Chaotic oscillations of the photoluminescence intensity detected via optically detected cyclotron resonance, are studied in Ga_0.47In_0.53As layers grown by the liquid-phase epitaxy technique on semi-insulating InP:Fe. The impact ionization of bound excitons and donors is shown to be the driving force for the chaotic oscillations in the photoluminescence intensity and the corresponding free-carrier density.

The self-generated formation of spontaneous current oscillations developing during low-temperature impact ionization breakdown of slightly doped p-type germanium is explained, taking advantage of a model experiment. Upon applying a relatively small transverse magnetic field, the spatially inhomogeneous current distribution, manifest in the form of individual high conducting current filaments, undergoes distinct travelling dynamics that is oriented perpendicular to the direction of the electric and the magnetic field (i.e. not coincident with the direction of the current flow). The resulting magnetic field induced oscillatory behaviour can be described qualitatively by simple model considerations.

Chaotic conductivity oscillations in n-Si and p-Si have been investigated in the freeze-out temperature range between 15 and 37 K. The oscillations were found in zero magnetic field for n-Si<110>, n-Si<111> and p-Si<111>. For n-Si<100>, on the contrary, chaotic behaviours were found only in the presence of a magnetic field B of about 18 T. The fundamental oscillation frequency f and the attractor dimension D for n-type Si were obtained as f=5.3 kHz and D=1.1-1.54 for <111>, f=870 Hz and D=1.1-2.0 for <110>, and f=12 kHz and D=1.1-4.2 for <100>, respectively. For p-Si<111>, f=1.25 kHz and D=1.6-2.6 were obtained in zero magnetic field. The fractal structure was directly shown by the number density of the current trajectory crossing a special phase in the phase space for n-Si<110> sample. The chaotic oscillation was observed for the doped n-Si<111> with N_D=6*10¹⁶ cm^-3 at T=15 K, whilst only harmonic oscillation was observed for a higher purity sample of N_D=10¹⁴ cm^-3. The fundamental oscillation frequency for n-Si<100> decreased by 20% with increasing magnetic field up to 16 T in an electric field E=100 V cm^-1.

It is shown that GaAs MESFET drain current low-frequency oscillations (LFOs) correlate with side-gating effect S-type negative differential resistances (SNDRs). The LFOs are observed only around the SNDR region, which appears near an extrapolated cross-point for a low-voltage Ohm's law curve and a high-voltage super linear injection current curve. The LFO waveforms are chaotic and they vary with temperature and applied side-gate voltage in a complicated way. Fractal dimensions for the LFO waveform are estimated to be 1.7-3.6, which may support the waveform complexity.

The current correlation function and noise spectrum for a finite quasi-one-dimensional system, biased by an external voltage, are calculated. The time-dependence of the correlation function is analysed in terms of the transit times of the carriers through the system. For a multi-subband system in the degenerate limit, the low-frequency spectral density exhibits a step-like shape. Under the presence of an applied voltage the step height is reduced to one half of its equilibrium value.

The effect of electron space charges in a potential well on the transmission coefficient of resonant tunnelling of electrons through a one-dimensional double barrier is investigated using the standard transfer-matrix method. It is shown that the bistable states in the transmission coefficient appear due to the positive feedback loop to incident electron space charges in the well. The growth rate of the small perturbation in the unstable region between the bistable states is also investigated numerically.

The authors have studied impact ionization with a quantum-mechanical approach beyond the Boltzmann equation. The theoretical background is a two-band density-matrix formalism where, in an electron-hole picture, particle conservation means that only the difference of electrons and holes remains constant. A quantum Monte Carlo procedure has been extended, in the second-quantization formalism, to include variable numbers of electrons and holes. The second-order correction to the number of electrons as a function of time has been evaluated. For delta-like initial distribution functions, quantum-mechanical and semiclassical results are compared. In contrast to a semiclassical treatment, nonconserving energy transitions at short times and the intracollisional field effect influence impact ionization above and below threshold.

The influence of collision broadening due to phonons and the intra-collisional field effect on the inverse Auger process have been examined. Results are presented for silicon and show for the first time the wave-vector dependence of the ionization rate including collision broadening.

The potential of utilizing strain for suppressing impact ionization is evaluated. It is found that if compressive strain is introduced without altering the bandgap (e.g. by using properly tailored InGaAlAs alloys) the threshold energy for electron impact ionization is significantly increased for both GaAs-based and InP-based materials.

Threshold energies and rates for hole-initiated impact ionization are calculated as functions of energy and direction in k-space. The calculations are based on anisotropic band structure and wavefunctions derived from four-band k.p theory with remote bands taken into account by perturbation theory. It is found that in InP the transitions initiated by carriers in the split-off band are dominant, while for In_0.53Ga_0.47As transitions initiated by light and heavy holes are also important. Simple algebraic expressions depending on energy and one direction parameter have been developed to approximate proximate the computed transition rates so that these may be used in further calculations or simulations of transport involving hot carriers.

The authors present a study of the effect of a parallel electric field on the excitonic photoluminescence (PL) from AlGaAs/n-GaAs quantum wells (QWs). They observe sharp thresholds in the quenching of luminescence from free and bound excitons at fields near a few tens of V cm^-1. PL lifetime measurements of the excitons show that the recombination lifetime is unaffected during the quenching. The mechanism responsible for the observed effects is shown to be due to impact ionization of the excitons by the free carriers heated in the electric field.

The charge injection transistor, or CHINT, is a three-terminal semiconductor device based on controlled real-space transfer of hot electrons between two conducting layers separated by a potential barrier. The symmetry of hot-electron injection by real-space transfer with respect to the polarity of the heating field allows the implementation of novel circuit elements. Thus, in the basic CHINT structure, the collector current is an exclusive OR function of voltages applied to the emitter electrodes. Moreover, the authors have proposed and demonstrated a multiterminal device structure with three symmetric logic inputs that performs both the NOR and the AND logic functions, and can be switched between these functions in the course of the circuit operation. Symmetry properties of real-space transfer transistors have been studied theoretically, with the help of continuation modeling and transient device simulation. These studies reveal a variety of instabilities and a striking novelty of multiply connected current-voltage characteristics. The authors have found that the CHINT can support anomalous steady states in which hot-electron injection occurs in the absence of any voltage between the emitter electrodes. In these states, some of which are not only stationary but also stable with respect to small perturbations, the electron heating is due to the fringing field from the collector electrode. Some of the anomalous states break the reflection symmetry in the plane normal to the channel at midpoint. The study elucidates the formation of hot-electron domains.

Hot electron transport in InAlGaAs/InGaAs ballistic collection transistors (BCTs) is simulated using a self-consistent Monte Carlo method. The authors analyse the effects of hot electron injection on the device performance of BCTs with various injection energies. The simulation has shown that the base and collector transit times in BCTs have conspicuous dependences on the energy of hot electrons in the base region.

The authors have measured transport properties in an AlGaAs/Al_xGa_1-xAs triangular quantum well whose energy spectrum has been varied by means of a gate bias. They have observed several nonlinear effects in the lateral conductance arising at positive gate voltages as the increasing Fermi level is moved towards the lowering energy positions of the excited subbands in the quantum well. They interpret the results in terms of electron population of the excited subbands in which electrons possess low mobility. Finally, they find new features at high lateral voltages which are considered to be evidence of previously predicted electro-phonon resonance.

Three-terminal devices based on real-space transfer (RST) of heated electrons across heterointerfaces, are promising for applications at microwave frequencies. Recently, a real-space transfer transistor (RSTT) similar to the charge injection transistor (CHINT), but with a pseudomorphic InGaAs/AlGaAs/GaAs material system and a collector-up structure has been demonstrated, exhibiting a 60 GHz gain-bandwidth product. The authors analyse the RSTT of Hueschen and coworkers using a self-consistent ensemble Monte Carlo technique. In particular, they focus on the location of RST with respect to the source-to-collector distance. They also propose an RSTT structure with an additional 'gate' electrode for response to changes in the heater voltage.

The authors describe a vertical hot electron transistor structure in the GaAs/AlGaAs materials system, in which the base is a two-dimensional electron gas in GaAs. They undertake initial studies of the hot electron spectroscopy at low temperature, and corroborate the high common base current gain measured to be >100 at low temperature, the best result ever achieved for a unipolar transistor. They compare these attractive results with the less satisfactory results achieved earlier with a uniformly doped (3D) GaAs base. A number of devices have been made which allow magnetotransport and Hall measurements to be performed on the base while the transistor is operating.

The authors report a study by Monte-Carlo simulation of an npn HBT with a Si_1-xGe_x base. The injection of ballistic electrons over an abrupt emitter-base heterojunction allowed the base transit time to decrease to 0.2 ps. However, the interest of this ballistic transport was limited by the following two-dimensional phenomena: the defocalization of the emitter current and the Kirk effect.

Impact ionization phenomena accompanied by light emission in the 1.1-3.1 eV energy range take place in GaAs MESFETs and in AlGaAs/GaAs HEMTs at high drain voltages. The dominant mechanism for the emission of photons with hv>E_g is the recombination between impact ionization generated holes and channel electrons.

The physical effects inherent in the operation of ultra-small devices are based on the fact that the critical length (e.g. the gate length or the depletion length) becomes so small that it approaches the coherence length of the electrons that provide the operation, which suggests that such small devices must be treated as quantum-mechanical objects. The accurate simulation of ultra-small devices requires quantum effects such as tunnelling and quantum repulsion (complementary to barrier penetration) to be included. The authors have developed a numerical model based upon a full quantum description-the moments of the Wigner distribution function. Numerical simulation of ultra-small MESFETs has been carried out using this model. Here, they emphasize the hot carrier effects such as ballistic transport and velocity overshoot. They also investigate the change of the velocity due to the quantum effects.

New hot-electron injection electroluminescent (HEI-EL) devices fabricated on p-Si substrate were demonstrated to have a maximum luminescent efficiency of 1.14 ImW^-1. The addition of a guard-ring-like n⁺ layer to control the number of electrons injected into the luminescent layer increases the luminance over 30 times compared with HEI-EL devices without an n⁺ layer. The luminance was easily controlled by the low bias voltage applied to the n⁺ region instead of the high voltage applied to the substrate.

By using the Monte Carlo method, submicron Si n⁺nn⁺ structures are analysed at different bias voltages and lengths of the n region. The autocorrelation function of current fluctuations and the variance of carrier number in different parts of the device is used to investigate the correlations induced by the self-consistent solution of the Boltzmann and Poisson equations and the influence of the modelling of the contacts.

Hot carriers cause charge trapping in the gate oxide of MOSFETs and generate interface traps at Si/SiO₂ interfaces of MOSFETs and bipolar transistors. Models for N-channel and P-channel MOSFETs and for bipolar transistor degradations have been developed and implemented in an IC reliability simulator BERT. Several comparisons between simulation results and measurements are shown. There remain to be answered questions concerning the presence of excess degradation when the stressing signal frequency is high.

The authors present a detailed discussion, critical tests and applications of a new lattice gas simulation technique for high-field transport in semiconductors. This method may be viewed as a variant of the ensemble Monte Carlo technique but can easily handle ensembles with more than 10⁵ particles, can efficiently deal with complex geometries and achieve accelerations on multiprocessor computers that scale linearly with the number of processors. The stationary and transient transport properties of Si and GaAs that have been calculated with this new technique compare very favourably with the standard Monte Carlo results.

The authors measure the spectrum of radiative emission from a high quality HEMT device and relate this to the energy distribution of the channel electrons. The discussion focuses on the intraband and interband contributions to the spectrum. The intraband, bremsstrahlung component gives evidence for very energetic electrons (T_el approximately=6000 K). The radiative recombination of carriers, however, is characterized by a very much lower temperature.

Visible light emission from short-channel silicon MOSFETS biased deep into saturation is studied. A group of peaks is observed in the range of 1.3 to 2.3 eV. Devices from different manufacturers are all found to have similar luminescence features. Neither of the current models for light emission from MOSFETs, electron-hole pair recombination and bremsstrahlung emission, appears to be able to explain the observed spectra. The authors suggest that the luminescence arises from radiative transitions between conduction bands in the silicon.

Studies have been made of Si 2DEGS formed on (001) high resistivity ( rho approximately 1000 Omega cm) p-type substrates in the hot-electron region. Thermal population of an excited subband leads to a decrease in resistance in contrast with the increase seen in 2DEGS formed on lower resistivity substrates, and this suggests that mu (E_ex)> mu (E₀) in these samples, though which of the excited subbands, E'₀ or E₁, is lowest is not known. Tensile stress measurements indicate that mu (E'₀)> mu (E₀) and this is attributed to the greater average distance of the electrons from the interface resulting from their weaker confinement in high-resistivity substrates.

Electron mobility in Si inversion layers has been investigated. The effective-field and temperature dependences of the effective mobility are analysed by the model, which is based on the interactions of a two-dimensional electron gas confined in the inversion layer with acoustic phonons, inter-valley phonons, surface roughness and ionized impurities.

The photon spectrum is analysed for two types of LDD n-MOSFET. It is found that at a special wavelength the photon spectrum has close correlation with the degradation rate of transconductance or V_t shifts, independent of device sizes and stress conditions. Photon spectrum analysis is found to be an effective monitor for the hot carrier degradation.

The hot-carrier degradation mechanism in PMOSFETs (p-channel metal-oxide-semiconductor field-effect transistors) is studied for three transistor types over ten orders of magnitude in time. The authors observe oxide charge and interface states separately by charge pumping. Degradation for various gate lengths at many gate and drain voltages is measured and interpreted consistently. Negative oxide charge and interface states are generated by electrons near the drain. This charge attracts an inversion layer that extends the drain and reduces the effective transistor length logarithmically in time, simultaneously preventing degradation of the transistor operation by the generated interface states. However, holes may generate interface states outside the drain extension, decreasing the transconductance with a power of time.

The effect of the sidewall spacer thickness on the hot-carrier degradation of sidewall-offset single drain PMOS transistors was studied. At the stress bias condition of maximum gate current, a large degradation was observed when there is no overlap between gate and drain. The trapping of a large number of electrons in the sidewall oxide spacer is attributed to this. In the off-state, PMOS transistors were degraded by electrons generated by the band-to-band tunnelling process. Transistors with no gate-to-drain overlap show little degradation at relatively low V_d, because of the small drain leakage current by band-to-band tunnelling. However, as V_d is increased and the drain leakage current reaches a certain level, the degree of degradation drastically increases. As in the case of the DC stress test at the peak gate current, this is probably a result of enhanced trapping of injected electrons in the sidewall spacer. These results suggest that there should be an overlap between gate and drain but that the overlap distance should be kept to a minimum in order to maximize the hot-carrier resistance of PMOSFETs. The single drain PMOS transistor with the optimum sidewall spacer thickness has a lifetime which is much longer than 10 years at an operation voltage of -3.3 V.

Monte Carlo simulations of carrier transport in the channel of metal-oxide field-effect transistors show a tail in the electron distribution with an effective temperature equal to the lattice temperature. Based on the solution of the Boltzmann transport equation in the relaxation approximation, the author gives a physical explanation of this feature. The tail at the lattice effective temperature is due to a fundamental energy cutoff: the finite energy the electric field can supply to the carriers on their way from source to drain.

Mechanical stress related hot-carrier effects are investigated experimentally by imposing mechanical stress on the Si chip and analytically by using a three-dimensional simulator. The main viewpoints are: (i) applied stress direction dependence, (ii) device scaling, and (iii) degradation mechanisms. The generation of surface states by hot carrier is found to decrease under compressive stress, while V_th shift is enhanced. Consequently, mechanical stress might cause the variation of bond length of Si-O, Si-OH, Si-H, resulting in the enhancement of electron trapping efficiency. In terms of scalability, mechanical stress-induced hot-carrier effect becomes less with scaling of the channel length, L which coincides with the simulated mechanical stress sigma _x which decreases as L is reduced. This is probably due to a spreading effect of localized stress at the gate edge for scaled MOS devices, and will be a positive effect for lateral device scaling. Thus, the mechanical stress was found to influence differently on carrier trapping and N_ss generation. From now on, reliability problems such as TDDB, junction leaks and so forth will have to be re-examined as mechanical stress-related phenomena, which will lead to the realization of more reliable ULSIS.

A new microscopic silicon model for hole transport at high electric fields featuring two valence bands in a finite Brillouin zone is presented. The band parameters and the electron-phonon coupling constants were determined by best fitting the density of states and the experimental and theoretical results for transport properties in the low and intermediate field-strength range. Hole impact ionization has been introduced following a new scheme that goes beyond the limitations contained in the Keldysh formula. The present model, coupled to an analogous model already developed for electrons, allows study of bipolar transport silicon devices. Applications to bulk Si and Si p-MOSFETs are presented.

Anisotropy in the substrate current has been observed experimentally in Si MOSFETs on the (100) surface. The impact ionization rate along the (001) axis becomes larger than that along the (011) axis. The ratio of the ionization rate between the two directions has been found to be as high as 5 at 81 K. This anisotropy can be explained by the anisotropic threshold energy for impact ionization. Furthermore, it has been found that the anisotropy ratio increases with a decrease in temperature and the length of the pinch-off region in MOSFETs. These results suggest that the non-stationary transport in the pinch-off region can lead to the enhancement of the anisotropy in the impact ionization.

Stimulated far infrared radiation is observed due to cyclotron emission in the light hole band of p-Ge. For the first time the authors have observed in samples with an acceptor concentration of 8*10¹² cm^-3 an extended lasing range between 1.4 and 3.7 T, corresponding to a frequency range between 28 and 76 cm^-1. For a higher doping of 5*10¹³ cm^-3 the magnetic field range lies between 3 and 4.5 T with frequencies between 60 and 90 cm^-1. For the latter sample they have found a maximum output power of 0.3 W. They have carried out a detailed study of the emission frequency and of cyclotron resonance absorption as a function of the applied magnetic field. A substantially improved calculation of the Landau levels in crossed fields including non-parabolicity effects was performed, to make a quantitative assignment of the transitions involved.

The role of quantum effects determined by interaction and mixing of light and heavy hole states in the scattering processes and population inversion of hot carriers in Ge in crossed E perpendicular to H fields is studied. The results of experimental investigations of stimulated emission for intersubband and cyclotron transitions are presented, the experimental data being in good agreement with quantum model calculations.

Theoretical investigation through quantum-mechanical calculations of far-infrared (FIR) laser oscillation in p-Ge is presented and compared with the experimental results. Analysis on the basis of energy spectra of germanium driven from the Luttinger Hamiltonian under crossed fields is performed for both cyclotron-resonance (CR) and intervalence band (IVB) laser oscillations. The numerical estimations of the population inversion and amplification coefficient for CR-type show that the transition between n=0 and n=1 light-hole (LH) Landau levels yields net amplification in the field region that substantially matches the observations. In these calculations, it is clarified that the valence-band mixture and nonequidistant energy spacing of LH-Landau levels play an essential role. As for the IVB-type, it is suggested that the transitions from n=2-3 LH-Landau levels to the heavy-hole band take part in laser emissions in the shorter-wavelength region. However, the observed gap in the emission wavelength between the shorter- and longer-wavelength regions cannot be explained by the calculations and further investigation based on quantum-mechanical treatments is needed.

Discrete lines with frequencies corresponding to the optical transitions from the excited states to the ground state of an acceptor impurity have been observed in the spectra of a FIR p-Ge laser, which operates on hole intersubband transitions in crossed electric and magnetic fields. Stimulated emission on impurity transitions arises due to the redistribution of holes in the nonlinear mode of laser generation. A gain mechanism on these transitions is discussed. It is also observed that these lines are tuned under the uniaxial stress on an active p-Ge crystal.

The possibility of a negative response of bunched carriers to a small DC electric field is discussed. According to the results of Monte Carlo calculations the response becomes negative when the carrier-optical phonon interaction is appropriately strong, and it shows a pronounced dip in the case of p-type germanium, around (a) particular microwave amplitude(s).

A proposal to achieve population inversion between first and second levels of the superlattice wells due to the intrawell optical phonon emission and 1-3 level interwell tunnelling is discussed. A brief outline of proposed stimulated FIR emission by ballistic electrons in the barriers of a superlattice is presented.

The authors study the response of holes in germanium to intense radiation from far-infrared gas lasers in the range of 28 to 174 cm^-1. Two types of nonlinear response are evaluated: (i) the saturation of inter-valence-band absorption and (ii) the saturation of stimulated inter-valence-band emission. The effect (i) yields critical intensities I_S between 10 and 1000 W cm^-2, depending also on temperature between 20 and 100 K; the saturation behaviour as well as the values of I_S follow a model that treats the holes as inhomogeneously broadened two-level systems. In the presence of electric fields I_S is found to increase drastically. They interpret this as a transit-time effect of rapidly moving carriers. The effect (ii) measures saturation in the presence of strong E perpendicular to B fields in an active p-Ge laser. In this case the transit-time broadening is extreme and imposes a fully homogeneous character to the transition. Conclusions are drawn on the relaxation mechanisms of low-energy holes as well as on the functioning of hot-hole lasers.

A spectral investigation of the cyclotron absorption and amplification of germanium light holes in strong crossed electric and magnetic fields has been made. It is shown that cyclotron generation is possible due to separation of the amplification and absorption frequencies. The cyclotron emission gain has also been measured, and found to be about 0.24 cm^-1.

Hot holes in p-type germanium are investigated via picosecond changes of the inter-valence band absorption in the mid-infrared. The transient absorption from the heavy to the light hole band monitored in spectrally and time resolved pump-probe experiments gives evidence of the subpicosecond inter-valence band scattering of photoexcited holes. This process is followed by the cooling of heavy holes on a time scale of several tens of picoseconds. Nonequilibrium split-off holes created by intense picosecond excitation give rise to a short-lived population inversion between optically coupled states in the split-off and the light hole band. A transient gain of 5 cm^-1 is observed with probe pulses of a photon energy around 260 meV. The amplification is due to the very high generation rate of split-off holes, the different transient temperatures of split-off and light holes and the exceptionally strong radiative transition probability.

The light hole cyclotron stimulated emission in p-Ge in strong E perpendicular to H fields was studied as a function of E and H field orientation and uniaxial stress applied to the crystal. Quantum model calculations of light hole scattering rates and Landau level population are used to interpret the results. The low frequency part of the intersubband emission spectra was investigated, and new emission lines were found. The role of impurity transitions in stimulated emission processes is discussed.

Far-infrared laser oscillation is studied in p-Ge subjected to crossed electric and magnetic fields. Compared with the oscillation in the Faraday configuration, the region of radiation in the optical resonator is propagated perpendicularly to the external magnetic field. Compared to the oscillation in the Faraday configuration, the region of the electric (E) and magnetic (B) fields for oscillation is expanded significantly. Also, longer-wavelength oscillation ( lambda >150 mu m), which is missing in the Faraday configuration, coexists with shorter-wavelength oscillation ( lambda <120 mu m) in the whole region of E and B fields. The laser emission is suggested to be of linear polarization, where the plane polarization is aligned either parallel or normal to the external B-field, depending on E- and B-fields and the oscillation wavelength.

Compressive stress is found to improve the characteristics of all types of hot hole masers and lasers in p-Ge. Effects of the uniaxial stress of p-Ge crystal on the FIR emission in both E//H and E perpendicular to H fields are discussed. Mechanisms of the influence of uniaxial stress on the population inversion and the stimulated emission of hot holes are revealed.

For bulk InSb at the quantum limit, it is shown that the lowering of the Landau levels is associated with excitation of excess electron-hole pairs by the action of the J*H force, when the stimulated emission sets out, and that the value of the gain of the emission becomes extremely high, diverging as omega _c tau to infinity in accordance with the observation.