Table of contents

In the present paper we present a generalized Monte Carlo method recently developed by the authors for the solution of the coupled set of quantum kinetic equations for the distribution functions and the interband polarization. The aim of this method is to combine the advantages of the description within a fully quantum mechanical picture with the power of the Monte Carlo technique for the treatment of stochastic processes. It is based on a decomposition of the kinetic equations into a coherent and an incoherent part. The former is integrated directly while the latter is solved by means of a Monte Carlo simulation. Band renormalization and excitonic effects are included in a self-consistent way. The calculations allow us to treat carrier thermalization and relaxation, as well as the dephasing process, on the same level.

We detect the coherent submillimetre-wave radiation from electron Bloch oscillations in a biased GaAs/AlGaAs semiconductor superlattice by time-resolved terahertz (THz) spectroscopy. The emitted THz radiation is tunable from 0.3 to 4 THz. The emission from Bloch oscillations is observable for temperatures as high as 100 K.

We study Bloch oscillations (BO) in GaAs/AlGaAs superlattices with miniband widths of 19 and 38 meV at various lattice temperatures. In the time domain, we use transient degenerate four-wave mixing (FWM), while simultaneously monitoring the Wannier-Stark ladder (WSL) in the frequency domain by transmission spectroscopy. Spectrally resolved FWM measurements prove unambiguously that the oscillations in the decay of the FWM signal are due to quantum interference as expected for a wavepacket performing the BO.

We investigate the coherent dynamics of excitons and continuum states at the direct gap of bulk germanium using femtosecond four-wave mixing. In particular, the dependence on the polarization of the incident beams is studied. Spectrally resolving the diffracted wave exposes a strikingly different behaviour of excitons and continuum states. We explain the different polarization dependences on the basis of the recently discussed density-induced dephasing. Applying a high magnetic field substantially changes the coherent response of the underlying system. This manifests itself in a significant modulation of the spectrally integrated four-wave mixing signal, which is interpreted as the time-resolved cyclotron motion of carriers.

Optical third-order polarizations close to the bandgap of bulk GaAs are investigated by degenerate four-wave mixing using 100 fs pulses. Spectrally and temporally resolved experiments reveal excitonic and free-carrier components both occurring within the bandwidth of the femtosecond pulses. The excitonic component dominates at excitation densities below 10¹⁶ cm^-3 and shows a time behaviour governed by many-body effects. At higher carrier concentrations, we observe an additional contribution due to non-thermalized free carriers that shows a spectrum resonant to the pulse and a photon-echo-like time behaviour. Theoretical calculations based on the semiconductor Bloch equations and a Monte Carlo simulation of the scattering dynamics are in good agreement with the experimental results.

The polarization dependence of the coherent optical response of GaAs QWS reveals that modelling of 2D excitons as non-interacting two-level systems is inadequate. Extensions of the model taking into account many-body Coulomb effects and disorder-induced coupling of the valence band states results in a fairly good description of all experimental data.

Various single-particle and collective-mode scattering processes that contribute to the dephasing time of photoexcited electron-valence hole pairs in degenerate, modulation-doped quantum wells are discussed. Detailed calculations show that sharp structures in the energy dependence of the electron dephasing time can give information about short-range correlations associated with large-wavevector and high-frequency excitations of the coupled electron-Lo phonon system. To extract dephasing times from coherent optical excitations the effects of the complex valence band structure, i.e. mixing of light and heavy holes in GaAs quantum wells, have to be taken into account. This is discussed in terms of the three-level semiconductor Bloch equations. Calculations are compared with recent experimental results by Kim et al.

As recently observed, the process of optically generating carriers in semiconductors has to be treated with a fully quantum mechanical description. We demonstrate that the description of carrier generation on the basis of the Bloch equations leads to much broader energy distributions than can be expected semiclassically. We also point out that the detection of such distributions by the probe pulse is a non-trivial process due to quantum effects, and that the probe spectrum is not always a direct measure of the carrier distribution function. A quantum mechanical calculations of the probe spectrum shows that it is considerably broader than the distribution function, and that sharp features in the time domain are smoothed out in the probe transmission.

On short time-scales two fundamental assumptions made in the derivation of the semiclassical Boltzmann equation become questionable: first, the concept of instantaneous collisions described by scattering rates due to Fermi's golden rule and, second, the neglect of interference terms between different interactions. We present a full quantum-kinetic description of a photoexcited carrier-phonon system and discuss the various types of approximations leading to the semiclassical case. Numerical results for the relaxation and dephasing process on different levels of approximations are given.

The echo phenomenon is observed in the course of band-to-band transitions produced by two successive pulses of coherent light separated by a time interval tau . It is due to the existence of independent 'two-level systems' represented by mixed electron-hole states with given quasimomenta p. Here we investigate a mechanism responsible for the observed very short times of the echo decay (of the order of a few femtoseconds). It is associated with the loss of phase memory as a result of interaction of the mixed state with the unscreened random Coulomb potential of the photocarriers or with the random static potential of the impurities. We have calculated the phase breaking time tau _psi within the framework of the elkonal approximation using a diagrammatic technique. It is shown that tau _psi is typically much shorter than both the period of plasma oscillations and the time of electron-electron collisions. It is proportional to n^-1d/ (where n is the carrier concentration and d the dimensionality of a system) which is consistent with the experimental results. However, the law of echo decay of the form exp(-( tau / tau _psi )³) does not agree with the existing experimental data.

We theoretically estimate the possibility of detectable coherence effects in nonlinear femtosecond pump-and-probe absorption spectroscopy of semiconductors at high excitation energies and densities by hybridizing the quantum mechanical equations of motion for the intraband and interband density matrices of the photoexcited carriers with an ensemble Monte Carlo simulation of the detailed scattering dynamics. We predict realizable experimental scenarios for Rabi-type oscillations in the transient absorption change at the excitation frequency.

Nearly all femtosecond carrier relaxation studies are dominated by electron dynamics. We have investigated femtosecond hole dynamics by a judicious choice of experimental parameters: n-modulation-doped GaAs/AlGaAs quantum wells at low temperatures excited with small excess energy and low photoexcitation density. We show that holes are non-thermal for approximately the first 800 fs and determine the hole-electron energy loss rates by comparing experimental results with Monte Carlo simulations. These results represent the first definitive study of hole relaxation dynamics in a semiconductor.

The energy relaxation process is successfully measured for various quantum well structures. The measurements show a marked difference from bulk characteristics as the well width becomes narrower. This implies that the two-dimensional nature of carriers plays an important role in the energy relaxation process. Monte Carlo simulation results support the experimental findings.

The ultrafast relaxation of minority electrons in highly doped p-GaAs ( rho =1.0*10¹⁹cm^-3) has been investigated through femtosecond lime-resolved luminescence. For low excitation densities the hole plasma temperature stays at 300 K and the transient luminescence spectra reveal the rapid cooling of the minority electrons within the first picoseconds. The electron-hole energy transfer is much larger (up to 10^-7W) than the known electron-LO phonon scattering rate, which allows the quantitative determination of the electron-hole energy transfer.

Femtosecond bleaching measurements are performed in n-doped GaAs at both 300 K and 14 K using low densities of carriers injected at 2 eV. The dwell time of electrons excited far above the Fermi sea is reduced because of hot-electron/cold-electron interactions. The response at 14 K is much stronger and slower than that at 300 K. This is attributed to: (i) the proximity to the Fermi level of the conduction band states probed via split-off valence band coupling; (ii) slow Landau damping of non-equilibrium plasmons, primarily by the minority holes which then heat the majority electrons.

A novel technique, which is a special embodiment of the three-pulse pump-probe scheme, and conventional subpicosecond transmission spectroscopy are used to investigate carrier-carrier scattering in GaAs and In_0.5Ga_0.5P. Results obtained for a broad range of plasma temperatures and injected carrier densities yield information on how carrier-carrier scattering is affected by these conditions.

The density dependence of the initial scattering rate of free carriers within nearly monoenergetic distributions is studied both experimentally and theoretically in bulk GaAs. The net decay rate of the population at the centre of such athermal distributions due to carrier-carrier scattering within the distribution is found to increase slowly, as the density of carriers to the power 0.3+or-0.1. The calculated result, which is quite insensitive to the approximation used to treat the screening of the Coulomb interaction, indicates that this carrier-carrier scattering rate increases significantly with the mean energy of the athermal distribution.

We present a theoretical analysis of the effects of carrier-carrier interaction in polar semiconductors. Though a Monte Carlo simulation we show that carrier-carrier scattering is the dominant scattering mechanism in the femtosecond time regime after a laser photoexcitation. We compare our theoretical results with both ultrafast absorption saturation studies and up-conversion luminescence experiments. The agreement with all types of experiments is extremely good and shows the accuracy of our simulations. We also verify that pump and probe experiments are the ideal tool for the study of the strength of carrier-carrier interaction.

We calculate the finite-temperature inelastic scattering rate for hot electrons injected parallel to an n-doped GaAs/GaAlAs interface or quantum well for various doping densities n, temperatures T and injection energies. We use the Born approximation and a T-dependent two-pole Pade approximation to the full finite T, frequency omega and wavevector q dependent RPA expression for the dielectric function epsilon (q, omega ,T), which contains the screening of the 2D electron gas and the 3D polar LO phonons treated equally. We calculate the mean free path length and average energy loss for a hot electron scattering from the plasmon-coupled LO phonon and single-particle excitation modes. We find that these quantities are very dependent upon T, n and the energy of the hot carrier. We also find that the 2D plasmon and polar LO phonons mutually screen each other so that the modes must be considered together. We will show that this high-T, low-n approximation, which incorporates Maxwell-Boltzmann statistics, is valid for GaAs systems at n<7*10¹¹cm^-2 at 300 K and n<2*10¹¹cm^-2 at 77 K and should be used at lower n in preference to the commonly used zero-temperature RPA.

Carrier-carrier scattering of a near-bandgap excited electron-hole plasma in an intrinsic GaAs quantum well is studied using two different Monte Carlo (MC) techniques. In the first one, the two-particle collisions are modelled using a screened Coulomb interaction. In the second one, the many-particle interaction is incorporated through classical molecular dynamics (MD). At low excitation densities (2*10¹⁰cm^-2) molecular dynamics gives a much slower thermalization of the plasma than the 'two-particle collision' technique. This difference is ascribed to the breakdown of the two-particle collision model, because the screening of Coulomb interaction at low densities is too weak to limit the range of the two-particle interaction to the mean interparticle distance. The agreement of 'two-particle collision' simulations with experiment seems to be fortuitous. The disagreement of MD with experiment implies that the theory is still incomplete.

Thermalization of near-bandgap excited electron-hole (e-h) plasma in an intrinsic GaAs quantum well is studied by Monte Carlo simulation. Carrier-carrier interaction is treated as two-particle e-e, e-h and h-h collisions due to screened Coulomb interaction. The indistinguishability of two colliding electrons (holes) is included by taking into account the exchange e-e (h-h) scattering between the electrons (holes) of like spin. The model is modified to the case of spin-polarized (SP) e-h plasma. We show that the thermalization of the SP plasma is several limes slower than the thermalization of the spin-randomized plasma, when the density of photoexcited e-h pairs is 3*10¹¹cm^-2. The effect is due to the exchange e-e (h-h) scattering, which significantly weakens the direct Coulomb scattering in the dense SP plasma. This yields a chance to observe the exchange experimentally.

The effects of electron-hole scattering on the transient transport of minority electrons in room-temperature silicon is examined using an ensemble Monte Carte approach. The transport model includes the dynamics of both electrons and holes with coupling between the electron and hole systems through their Coulombic interaction. Calculations are reported for an acceptor concentration of 10¹⁸cm^-3 with applied electric field strengths of 10 and 29 kV cm^-1. The calculations show that electron-hole scattering reduces mobility, velocity overshoot and energy in both hot and cold valleys, and that the valley repopulation rate is reduced for an applied field of 10 kV cm^-1 while at 20 kV cm^-1 it is enhanced at times longer than one picosecond.

The thermalization and energy relaxation of highly photoexcited carriers in p⁺-GaAs is investigated at room temperature using an ensemble Monte Carlo approach. We present an alternative model for treating carrier-carrier scattering in Monte Carlo simulations. The model takes into account the wavevector and frequency dependence of the dielectric function by using a screened Coulomb potential in q-space due to a charge moving at the velocity of the centre of mass. In comparison with the statically screened model, the calculated effective carrier temperatures for an excitation density of 2*10¹⁸cm^-3 and background p-doping of the same value are in good agreement with the experimental results.

Resonant tunnelling spectroscopy is used to investigate the energy level spectrum of a wide potential well in the presence of a large magnetic field oriented at angles theta between 0 degrees and 90 degrees to the normal to the plane of the well. In the tilted field geometry, the current-voltage characteristics exhibit a large number of quasiperiodic resonant peaks, even though the classical motion of electrons in the potential well is chaotic. The voltage range and spacing of the resonances both change dramatically with theta . We give a quantitative explanation for this behaviour by considering the classical period of unstable periodic orbits within the chaotic sea of the potential well.

Resonant tunnelling through a GaAs-AlAs triple-barrier structure with coupled quantum wells of unequal widths has been studied experimentally. Peaks in the current-voltage characteristic due to tunnelling via the lowest subband of each well were observed. Application of a magnetic field parallel to the interfaces broadens the peaks and at a certain critical field an additional resonance peak appears at a bias voltage between the original resonances. The field-induced peak is interpreted as a double-resonance effect, even though the subbands are not energetically aligned. This interpretation is supported by detailed calculations of current-voltage characteristics using a transfer-matrix approach.

A realistic multiband and multichannel scattering theory of the Zener current in semiconductors is developed that fully incorporates the periodic crystal potential and the high electric field in a device. The non-perturbative treatment of the spatial variation of the electric field in a p-i-n structure permits us to calculate the Zener tunnelling current unambiguously and to explain the interplay between Stark ladders and Zener tunnelling. For low and intermediate fields, we can express our results for the Zener generation rate in terms of analytical results that may be used in transport simulations.

Longitudinal uniaxial stress (O<S<36 kbar) at 300 K and 77 K was used to experimentally examine intervalley tunnelling processes in a GaAs/AlAs resonant tunnelling diode (RTD) by lowering quasibound states confined in AlAs X-point quantum wells with respect to the Gamma -point conduction band profile. Intervalley resonances were observed that arose from tunnelling through: (i) large effective mass longitudinal X-states, (ii) a small-mass transverse X-state, and (iii) longitudinal X-states in AlAs aligned with the Gamma -point ground state confined in the adjacent GaAs quantum well, resulting in a resonance with a bias voltage position that increased (decreased) with increasing stress for X-states in the emitter (collector) AlAs layer. At 77 K, intervalley tunnelling can provide resonances of magnitude comparable with the intravalley Gamma resonance. Intervalley resonances remained observable in the differential conductance at 300 K. Self-consistent calculations of device characteristics agreed with the data and established voltage-stress domains for all possible intervalley resonances in the RTD.

The time behaviour of tunnelling phenomena in relaxation processes of various semiconductor heterostructure systems is investigated. We analyse the dynamical changes of scattering mechanisms and their effect on the corresponding carrier distribution which determines the tunnelling current. This is done for the case of resonant escape of a carrier population injected in a mesoscopic well between double barriers. Results are compared with those for non-resonant escape. Dynamical screening and charge build-up effects are also studied with and without an applied electric field. Competition between the various escape mechanisms is discussed.

We have measured current-voltage characteristics at liquid-helium temperature and for magnetic fields up to 7T(parallel to the current flow) for three similar asymmetric GaAs/AlAs double-barrier structures, all of which possess large phonon-assisted tunnelling currents. Confined longitudinal optical (LO) phonons in the GaAs well layer and LO-like symmetric interface phonons treated within a dielectric continuum picture suffice to account for the measured currents. Phonon-assisted tunnelling current levels as well as magnetotunnelling data are found to depend sensitively on well and barrier widths.

Terahertz dynamics in double-barrier resonant-tunnelling devices are studied using a Langevin equation approach taking into account the dynamic motion of electrons in the contacts. Damping constants are determined by the second-order perturbation for the Coulomb interaction between electrons inside each contact. It turns out that the present theory, which does not include empirical parameters, predicts resonance enhancements in the frequency response function and the noise spectrum of the tunnelling current in the terahertz region. It is shown that the 3 dB bandwidth is dependent on temperature T and barrier width d and can be as high as several tens of THz in the devices with higher T and smaller d values because of the existence of the resonance enhancements.

We have studied the problem of coherent tunnelling through a double-barrier structure of GaAs-AlGaAs in the presence of light, considered to be present all over the structure: emitter, well and collector, with a frequency in the infrared range. By means of a canonical transformation, and in the framework of the time-dependent perturbation theory approach, we have calculated the transmission coefficient and the electronic resonant current. Our calculations have been compared with experimental results, turning out to be in good agreement.

We investigated resonant tunnelling of electrons and holes between coupled quantum wells using time-resolved luminescence spectroscopy. Exponential dependence of tunnelling times on barrier width is observed for electrons but not for holes. The tunnelling times of electrons are correctly described by a model which takes into account inhomogeneous broadening.

We demonstrate that transient optical dephasing experiments on electrically biased, shallow multiple quantum wells offer the unique possibility of studying the first step in carrier sweep-out, namely the escape tunnelling out of the individual quantum wells. We perform transient four-wave mixing experiments on two shallow GaAs/Al_xGa_1-xAs multiple quantum well samples with x=0.04 and x=0.06. We find that the electron tunnelling times out of the quasibound n=1 quantum well states into the continuum states decrease with increasing electric field, and lie in the subpicosecond regime at high fields. In addition, for the x=0.06 sample, the experimentally determined escape rate is resonantly enhanced when the quasibound n=1 quantum well state couples to the n=2 state embedded in the continuum above the second-nearest-neighbour well.

We report the use of a time-resolved transient-grating technique to study the in-plane ambipolar diffusion coefficient and interwell tunnelling probability for excitons in a number of multiple quantum well samples. Our specimens were selected with barrier thicknesses ranging from 14 to 141 AA thus giving complete coverage of the transition from isolated quantum well to superlattice behaviour. Two types of MBE growth conditions were used giving rise to samples with low or high surface defect density. The surface defect density had a strong effect on the in-plane mobility but not the tunnelling probability. The samples show surprisingly strong interwell coupling even with the thickest barrier, which we attribute to defect assisted tunnelling.

We have measured the broad-band terahertz response of a state-of-the-art InGaAs/AlAs resonant tunnelling diode from 120 GHz to 3.9 THz using the free-electron lasers at the University of California, Santa Barbara. A tungsten whisker antenna in a conventional probe station is used to couple the far-infrared radiation into the device. By normalizing the rectified response in the resonant tunnelling regime with the off-resonant response we are able to remove the antenna frequency effects and the frequency dependence controlled by the much slower RC time constant and measure the relaxation time due to the quantum inductance.

In this paper we present an electroluminescence study of electron populations in the quasiconfined excited states of two GaAs-AlGaAs double-barrier resonant tunnelling structures. We investigate the dependence of the electron population on both electron collector barrier width, which controls the electron tunnelling-out time, and on the confined level energy spacing, which controls the intersubband scattering time. We show that the observed excited state populations are in good agreement with the predictions of a rate equation analysis.

Resonant tunnelling between different subbands of adjacent wells is investigated with electroluminescence spectroscopy as a function of forward bias voltage. The observed near-bandgap electroluminescence lines indicate a strong inhomogeneity in the electric field as well as in the carrier distribution. This interpretation is supported by calculations. Electroluminescence lines originating from excited conduction and valence subband states demonstrate an efficient occupation of these subbands by resonant tunnelling.

Magnetotransport and electroluminescence measurements are reported for a p-i-n AlAs/GaAs double-barrier resonant tunnelling structure incorporating superlattices in the emitter and collector. When a magnetic field of 15 T is applied parallel to the current a pronounced region of negative differential resistance (NDR) is observed. Associated with the NDR region in the current-voltage characteristic the energy of the electroluminescence peak from the centre quantum well is shifted by more than 10 meV for a change in applied bias of only 1 mV as the device is swept through resonance.

We have used CW photoluminescence and photocurrent spectroscopy to study the vertical transport mechanisms in GaAs/Al_xGa_1-xAs multiple quantum well p-i-n structures. In an x=0.33 asymmetric coupled well sample we have observed resonant electron tunnelling through barriers as thick as 153 AA above the transition field for spatially indirect recombination. In a superlattice with x=0.4 we have observed both electron and hole resonant tunnelling. The temperature and intensity dependence of the photocurrent suggest that non-resonant tunnelling is assisted by charged impurity scattering below 100 K.

Photoluminescence spectroscopy is used to investigate the donor-assisted resonant tunnelling processes in double-barrier structures which incorporate a low-density delta -doped donor layer in the centre of the quantum well. A quantum well luminescence line corresponding to hole-neutral donor recombination is observed, along with two other lines at higher photon energy. The possible origin of these two lines is discussed.

Photoluminescence spectroscopy is used to investigate p-type double-barrier resonant tunnelling structures based on GaAs/AlAs. Strong photoluminescence from the quantum well is observed due to recombination of resonantly tunnelling holes with minority photogenerated electrons, which also tunnel into the quantum well. The luminescence undergoes a red shift with increasing bias and its intensity shows peaks at biases corresponding to the first four hole resonances in the current-voltage characteristics. Two additional strong peaks are also seen in this intensity-bias plot, due to electron resonant tunnelling.

The electroluminescence (EL) spectra of forward biased p-i-n double-barrier heterostructures are investigated. Two EL lines, separated by about 20 meV, due to recombination in the quantum well are observed. Their intensities peak at bias voltages corresponding to electron and hole resonant tunnelling. By investigating a series of structures with different spacer layer thicknesses between the barriers and p-type contact layers and with intentional p-type delta -doping in the central plane of the quantum well, we are able to confirm that the lower-energy line corresponds to electron recombination with neutral accepters in the quantum well.

It is demonstrated theoretically that a stable and tunable semiconductor oscillator can be designed by using a novel method of chaos control. By application of a small time-continuous delayed feedback voltage control signal, different unstable periodic orbits embedded in the chaotic attractor of a semiconductor can be stabilized. Thus different modes of self-generated periodic voltage oscillations can be selected, for example by choosing an appropriate delay time. This is illustrated for two different oscillation mechanisms involving hot-carrier transport: (i) self-generated oscillations under crossed electric and magnetic fields in the regime of low-temperature impurity breakdown (dynamic Hall effect), and (ii) driven real-space transfer oscillations in modulation-doped heterostructures.

A closed hydrodynamic approach based on the system of conservation equations for carrier number, drift velocity and mean energy coupled with the Poisson equation is used to calculate the response functions in the time domain of the local electric field and applied voltage for a near-micrometre n⁺nn⁺ InP diode. By Fourier analysis, the generation bands in the frequency domain are then obtained. The good agreement achieved with available experiments validates the present theoretical approach. A significant increase of the cut-off frequency of the microwave power generation up to 600-700 GHz is predicted for submicrometre GaAs and InP diodes.

Transient drift-velocity response of miniband transport in semiconductor superlattices is analysed using a balance-equation theory. Carrier occupation of transverse subbands and realistic electron-impurity and electron-phonon scatterings are taken into account. We find that thermal excitations of the carrier transverse movement suppress the Bloch oscillation. In a high-electron-density system where carriers are thermalized rapidly in both longitudinal and transverse directions, Bloch-type oscillatory current is unlikely to appear in miniband transport under the influence of a constant electric field.

The photoinduced microwave absorption in undoped bulk GaAs crystals is studied at low temperatures. It is observed that when the microwave power and exciting light intensity exceed certain threshold values, self-oscillations appear in the microwave absorption. The oscillation frequency is dependent on the light intensity and also on the external magnetic field. This phenomenon is qualitatively explained in terms of impact ionization of shallow donors that are neutralized by the photoexcitation.

We report on experimental investigations of the dynamics of formation and evolution of the thermodiffusional autosolitons (AS) in a non-equilibrium electron-hole plasma in Ge samples with different crystallographic orientations. The AS are a new type of dissipative structure, which manifest themselves as static, pulsating or moving narrow strata and which are characterized by a high temperature and a lowered concentration of carriers.

The electric field dependence of the microwave noise temperature and the diffusion coefficient are found to be significantly different for two sets of AlGaAs/GaAs structures with different spacer thickness and aluminium mole fraction. In the samples with a 'wide' quantum well (QW), one maximum of the diffusion coefficient is observed at fields below the threshold for the intervalley diffusion, but two maxima are characteristic of a 'narrow' QW with an intersubband gap wider than the K_BT_O. A microscopic explanation of the observed noise spectra of AlGaAs/GaAs is given and used to determine kinetic parameters of the heterostructures.

We present a microscopic analysis of current and voltage fluctuations in GaAs Schottky barrier diodes under forward-bias conditions in the absence of 1/f contributions. Calculations are performed by coupling self-consistently an ensemble Monte Carlo simulator with a one-dimensional Poisson solver. By using current and voltage operation modes we provide the microscopic origin and the spatial location of the noise sources respectively. The coupling between fluctuations in carrier velocity and self-consistent field is found to be essential in determining the noise spectra. Different types of noise (shot, thermal and excess) are exhibited by the device at different voltages. In particular excess noise due to hot carriers and intervalley transfer is detected for the highest voltages.

We present a theoretical analysis of number and current fluctuations in homogeneous n-Si resistors of submicron dimensions at increasing electric field strengths. To this purpose, we calculate the corresponding correlation functions. In the ballistic regime a simple scaling relationship of the transit time accounting for carrier heating is found to hold over the whole range of fields considered. In the diffusive regime different time scales associated with diffusion, drift and dielectric relaxation are found to characterize the behaviour of number fluctuations.

A physical model is proposed for limitation and suppression of spectral intensity of hot-electron fluctuations in submicrometre semiconductor layers, where the thickness 2d of the sample is comparable to the electron energy relaxation length L_epsilon . We suppose two identical layer surfaces, no space charge and surface charges at zero bias, the electron temperature approach, and the surface velocity approximation. Electric field E and dimension 2d dependences of spectral densities of fluctuations, small-signal conductivities and noise temperatures are investigated. A decrease of the spectral intensity of hot-electron fluctuations and noise temperature takes place when 2d<L_epsilon in the range of fluctuation frequencies omega tau _epsilon <<1, where tau _epsilon is the electron energy relaxation time.

In this work we present a new model describing the electrical transport in the heterostructure hot-electron diode. As in an earlier but less sophisticated approach, we find an interesting spatiotemporal behaviour of current filaments switching on and off (spiking). Comparison with a simple generic model indicates that spiking might be a generic mechanism for many types of devices exhibiting bistability.

An experimental and theoretical study has been made of electron transport over a series of wide Al_xGa_1-xAs barriers with graded interfaces, containing GaAs quantum wells in the central Al_0.25Ga_0.76As region. Extensive measurements of both the voltage and temperature dependence of the current were made, as well as measurements of magnetoresistance. In these structures tunnelling is expected to be negligible and hence drift-diffusion thermionic emission theory was used to interpret the data. Good agreement between theory and experiment was obtained using a low-field model for electric fields up to 10 kV cm^-1. At applied electric fields greater than 10 kV cm^-1 agreement between theory and experiment was obtained to within an order of magnitude when the model was modified to include a saturated drift velocity plus the effect of injected space charge. At high electric fields periodic negative differential resistance (NDR) oscillations were observed. The model usually given to explain these oscillations involves tunnelling and is, therefore, not obviously applicable to these structures. An alternative description of their origin is discussed, based upon the NDR associated with intervalley transfer.

We present a model which describes the onset of current instabilities and space-charge domains in extrinsic semiconductors (especially p-Ge) under DC voltage bias. Just above the onset voltage for current instability we have found numerically small-amplitude fast oscillations due to the periodic motion of solitary waves which decay before reaching the receiving contact. For slightly larger applied voltages there is an abrupt and slightly hysteretic transition to slower large-amplitude solitary waves similar to those in the Gunn effect. An amplitude equation is derived for long samples which reveals a quasicontinuum of oscillatory modes that become linearly unstable at onset.

Current instabilities in a p-i-n Si diode heavily doped with gold impurities are investigated numerically. In order to clarify the explicit effect of the space charge of impurities on the current instabilities, we re-examine numerically the steady-state solutions for this particular diode without the quasineutrality assumption. The linearized stability analysis shows that the spontaneous self-sustained current oscillations can occur in the low-injection regime, where the DC resistance remains positive.

Period-doubling bifurcation and the related noise effect have been investigated experimentally in the periodically driven instability of n-GaAs. An S-shaped I-V curve formed by the impact ionization avalanche of neutral shallow donors is modulated periodically by a DC+AC bias of the form V₀+V_acsin(2 pi f₀t). With f₀ approximately 2 MHz, we observe a smooth period-doubling bifurcation as a function of V₀. We have found a remarkable noise effect which appears as a deterministic noise amplification. The behaviour has been attributed to the sensitive nature of the NDR instability and thus the sensitivity of the filamentary current. In a theoretical frame, we have proposed some novel applications using the chaos system such as a deterministic noise amplifier and a chaos memory device.

Nonlinear charge transport parallel to the layers of modulation-doped GaAs/Al_xGa_1-xAs heterostructures is studied theoretically and experimentally. In the field regime of about 2 kV cm^-1 we find DC-induced current oscillations associated with N-shaped negative differential resistance. We develop a dynamic model based on real space transfer of hot electrons from the undoped high-mobility GaAs layers to the adjacent n-doped low-mobility Al_xGa_1-xAs layers. In particular, we extend previous models to multilayer structures and investigate the dependence of the self-generated oscillations upon circuit conditions and the lattice temperature in the range T_L=77-200 K. In the light of the experimental results the theoretical predictions are analysed and discussed.

Utilizing a high electron-electron scattering rate assumption, a new type of bulk negative differential conductance is predicted from hydrodynamic Boltzmann transport equations. N-type negative differential conductance appears in two-valley semiconductors, in which the upper valley has a higher density of states and the saturation drift velocities have the relation V_SAT(LOWER)>V_SAT(UPPER). In selecting low-field mobility values for both valleys, the V_d-E curves show a sharp peak or even a loop near the peak with triple steady states for an electric field. The peculiar V_d-E curves are attributed to the extraordinarily high drift velocities appearing around the onset of the carrier population inversion from the lower valley to the upper valley. The high velocity emerges because of the efficient heat removal from the upper-valley carriers through electron-electron scattering to the lower-valley carriers.

Two novel techniques-field contrast with a scanning electron microscope and electro-optic probing-have been employed to image the formation, and the resulting transients, of high-field domains in GaAs/Al_xGa_1-xAs quantum wells. The field contrast mode of a scanning electron microscope involves the analysis of the energy of secondary electrons emitted from the surface of the specimen, the energies of which depend on the local electrostatic fields present at or near the surface, whilst the electro-optic probing technique makes use of the electric-field-induced birefringence that occurs in non-centrosymmetric crystals, such as GaAs. Both techniques are therefore sensitive to changes in the local electric fields within the samples. In this work both techniques were used to profile the electric field distribution along the GaAs quantum well samples where negative differential resistance, and the associated current instabilities, have been induced at room temperature. Unlike the case of bulk material, where high-field domains propagate along the samples (Gunn domains), in 2D GaAs the high-field domains are found to be either static or annihilate before reaching the anode. A model based on a lateral dissipative mechanism is proposed and the merits of the two experimental techniques are discussed in terms of time and spatial resolution.

We analyse the effects of a non-equilibrium phonon population on noise and diffusion phenomena in polar semiconductors. We calculate the current and energy autocorrelation functions and the current-energy cross-correlation functions, for both the case with and without hot-phonon effects. Owing to the presence of hot phonons, we find an increase of the noise equivalent temperature and of the longitudinal diffusion coefficient at intermediate field strengths below the threshold for negative differential mobility.

We have performed saturation spectroscopy measurements of the lowest intersubband transition in a single 400 AA GaAs/Al_0.3Ga_0.7As modulation-doped square quantum well. We couple intense tunable far-infrared radiation from the Santa Barbara free electron laser into our sample using an edge-coupling technique and measure absorption as a function of frequency and intensity. Saturation and frequency shifts in the absorption line are clearly observed. We attribute the frequency shifts to reductions in the many-body depolarization shift. From our preliminary measurements, we estimate the intersubband relaxation time to be 600 ps to within a factor of three.

We report the first combination of far-infrared linear and nonlinear (saturation) magnetoabsorption experiments on coupled subband Landau level excitations in a series of modulation-doped multiple coupled double quantum well (MCDQW) structures. Linear spectroscopy was carried out with a Fourier transform spectrometer over a range of temperature, and saturation spectroscopy was carried out with the free electron laser at UC Santa Barbara. Based on theoretical analysis, an electron lifetime of approximately 1.2 ns was obtained.

Large third-order, free-carrier nonlinear susceptibilities, X⁽³⁾ (to approximately 0.2 esu), and subcubic dependence of the third-harmonic power on the incident intensity, have been observed between 19 cm^-1 and 23 cm^-1 for InAs/AlSb quantum wells with electron sheet densities between 2.5*10¹² cm^-2. and 8*10¹² cm^-2. We find that the transmission of the fundamental, and the samples' DC conductivity, decrease with increasing incident intensity, indicating a large rise in the scattering rate. Using the intensity-dependent transmission to account for absorption in the sample is not sufficient to recover a cubic power law for the third-harmonic intensity. In addition, given the increased scattering rate indicated by the conductivity data, the bulk free-carrier X⁽³⁾ due to non-parabolicity should decrease dramatically with increasing fundamental intensity, contrary to our results. Thus, non-parabolicity alone cannot account for the observed third-harmonic response.

We investigate the stimulated emission spectra of p-Ge hot-hole lasers. To clarify the influence of impurity absorption we use for the first time a Tl-doped p-Ge laser. Since we observe a clear difference between the spectrum of this laser and that of normally used Ga-doped lasers with respect to the 'emission gap' and to the low-frequency range, we can give definite evidence that impurity transitions are involved in the lasing process.

The results of experimental investigations of an FIR laser on hot holes in germanium in constant crossed electrical and magnetic fields in the Voight configuration are presented. Similar studies have been carried out before, mainly for the Faraday configuration. The Voight configuration provides a greater intensity, a greater (E,H) region and a wider spectral range of generation than the Faraday configuration. A new type of injectionless FIR laser is proposed-a heavy hot-hole cyclotron resonance laser. Calculation of optical gain, alpha _gain, spectral dependence is carried out by balance equations of power and number of particles. The Monte Carlo method is used to calculate light and heavy hot-hole distribution functions and the value of alpha _gain. Some advantages of such lasers are presented in comparison with an FIR laser in constant crossed electrical and magnetic fields.

We performed a detailed study of the emission spectra of a hot-hole cyclotron p-germanium laser. The laser was operated in crossed electric and magnetic fields, which were parallel to two different (110) axes of the germanium crystal. Using a high-resolution Fabry-Perot spectrometer a broadening of the laser line was found. A band structure analysis made it possible to understand the observed feature. The line broadening is explained by the inhomogeneity of the electric field inside the crystal, which leads to slight deviations of the resonance frequency in different places. We manufactured a non-rectangular shaped laser crystal for enhancing the electric field homogeneity. Our first results show that the new laser configuration leads to a narrowing of the laser line.

The efficiency of p-Ge hot-hole lasers is shown to depend on the shape of the sample. This is due to the inhomogeneity of the electric field caused by the Hall effect. To estimate the effect on gain, Monte Carlo (MC) simulations are performed for the case of intervalence band (IVB) lasing, while for light-hole cyclotron resonance (LHCR) lasing characteristics are taken from the literature. It is concluded that samples with an elongated rectangular cross section perpendicular to B are advantageous because they provide a relatively large region with positive gain.

This paper reviews the most recent experimental results concerning the characterization of hot-electron effects and light emission in GaAs MESFETS, AlGaAs/GaAs and AlGaAs/InGaAs high electron mobility transistors (HEMTS), and AlGaAs/GaAs heterojunction bipolar transistors (HBTS). In MESFETS and HEMTS, light emission has been correlated with impact-ionization-induced gate current, providing insights into possible emission mechanisms. In HBTS, impact ionization can be evaluated by measuring the changes in the base current as a function of base-collector voltage. The measured multiplication factor correlates well with the results of a Monte Carlo simulation of the device, which also provides general microscopic details of the pre-avalanche regime.

We have measured the hot-electron luminescence of GaAs HEMT devices over a broad spectral range in the infrared (0.4-1.4 eV). The emission intensity has spectral features suggesting that k-conserving, interconduction-band transitions dominate the emission process. The interpretation of emission spectra in terms of a hot-carrier distribution requires consideration of band-structure effects. In general this tends to reduce values of electron temperature T_el determined by the exponential fit to the experiment.

A broad-band component of the optical emission by reverse-biased diodes observed in a number of semiconductors is described. The emission properties have been found to be essentially independent of the diode structure type, the doping impurity content or the excitation conditions. The results are interpreted by using the model of intraband hot-carrier radiative transitions.

We present a theoretical study of hot-carrier-induced light emission in III-V semiconductor devices. Carrier heating in the intense electric fields present under high-bias conditions is studied using self-consistent Monte Carlo simulations. The carrier distribution functions obtained from the simulation are then incorporated into a pseudopotential algorithm that describes the direct optical transitions and calculates the corresponding spectra. We show that, in general, no straightforward extrapolation of 'carrier temperatures' from the slope of the measured spectra is possible. Our analysis indicates that light emission due to hot carriers is made up by direct radiative interband transitions within the conduction and valence bands as well as between them.

We present a combined experimental and theoretical study of hot-carrier effects in GaAs PIN diodes. Electroluminescence spectra are measured both below and above bandgap, evidencing features connected to gap states and to hot-carrier distributions. The results of a self-consistent Monte Carte investigation illustrate the microscopic details of carrier transport in the device and confirm that under breakdown conditions average carrier energies as high as several tenths of an eV can be reached.

In this paper a theoretical-evaluation is given of the absolute intensity and polarization of light emission from silicon devices due to conduction-conduction (c-c) and valence-valence (v-v) direct transitions. The matrix elements of the momentum operator between Bloch states have been obtained from a full band-structure calculation performed with the pseudopotential method. Results have been obtained by using both analytical model distribution functions and realistic hot-carrier distributions obtained from Monte Carlo (MC) simulations based on the same band model. They show a polarization degree of a few per cent, which should be observable for these transitions.

Experimental results on hot-electron parallel transport in GaAs quantum wells, situated in the n-side of the depletion region of a Ga_1-xAl_xAs p-n junction, are presented. In our structures hot electrons are thermionically excited from the n-side of the junction into the quantum wells in the depletion region, by the application of pulsed longitudinal electric fields. The accumulation of the electrons in the depletion region modifies the potential profile of the junction, resulting in a reduction of the potential barrier and the depletion width in the p-side. Thus, hot holes are excited into the valence bands of the quantum wells via thermionic emission or tunnelling to recombine radiatively with the electrons. A simple modelling of the device is carried out by solving the Poisson and Schrodinger equations for the p-n junction incorporating a single quantum well. The results of this modelling describe the experimental observations very well. It is also shown that population inversion in the junction can be achieved at high fields. The device is therefore a very efficient light emitter with the main advantages over the conventional laser diodes being: the emitted light is independent of the polarity of the applied voltage; and only two point contacts are required, thus making it a very simple device to fabricate.

An optical method for obtaining the valence subband dispersion of GaAs/AlGaAs quantum wells, including warping, is reviewed. Utilizing the recombination of hot electrons at neutral acceptors, the method gives meV accuracy. The dispersion has been measured for quantum wells with widths ranging from 3.3 nm to 9.8 nm. Measurements have been made for wavevectors from zero to greater than 10% of the Brillouin zone, corresponding to holes with as much as 50 meV of kinetic energy. The measurements are in excellent agreement with k.p calculations.

The heavy-hole dispersion of bulk GaAs along specific high-symmetry directions may be obtained from hot electron to neutral acceptor photoluminescence spectroscopy. The degree of linear polarization eta is analysed in the light of a detailed lineshape calculation to identify the emission from carriers with a particular wavevector direction. The technique is demonstrated for the (100) direction and in principle also for the (111) and (110) directions. It allows a systematic plot of E_hh(k) with sub-meV accuracy over a range of wavevectors near Gamma .

Relaxation phenomena of holes in p-type Ge and GaAs are studied by ultrafast infrared spectroscopy in the wavelength range from 0.8 to 10 mu m. In germanium, carriers are excited to the split-off band by 250 fs pulses and the subsequent scattering dynamics is measured via the transient intervalence band absorption. The excited carriers undergo intervalence band scattering within 100 fs and thermalize with time constants around 700 fs. In contrast, equilibration among unexcited holes proceeds within 100 fs. Inelastic carrier-carrier collisions via the dynamically screened Coulomb interaction are the main mechanism of thermalization. In GaAs, we monitor the picosecond recombination of free holes with shallow accepters. After infrared photoionization of impurity atoms, hole capture occurs within several 10^-11 S^-1. Emission of single longitudinal optical phonons represents the predominant process by which the impurity ground state is repopulated.

Ultrafast (150 fs) time-resolved photoelectron spectroscopy is used to directly observe the energy relaxation of optically excited electrons in silicon. Conduction band electrons fit a thermal distribution by 120 fs pump-probe delay. The initial cooling rate of the excited distribution is found to be extremely fast, followed by an electron-phonon thermalization time of approximately 1 ps, and an overall much slower cooling rate as the electrons lose energy. Here, we also report a new effect in two-photon photoemission unique to ultrashort laser pulses. Our model and results demonstrate this effect to be a sensitive new monitor of electron dynamics.

We have studied carrier-carrier scattering in hot electron-hole plasmas, using both experiments and molecular dynamics calculations. Experiments were performed using photoluminescence via neutral acceptor recombination in GaAs. Moderate plasma densities were generated using femtosecond laser pulses at 1.7 eV. Experiments and calculations indicate that for hot non-equilibrium distributions carrier-carrier scattering can dominate the electron scattering at carrier densities as low as 8*10¹⁵cm^-3. We discuss the nature of the carrier-carrier scattering and the validity of classical molecular dynamics calculations.

We have determined for the first time the inter-Landau level lifetimes in GaAs/GaAlAs heterostructures in the spin polarized region (filling factor <1). Intensity-dependent cyclotron resonance absorption has been measured using a high-intensity optically pumped far-infrared (FIR) laser. A detailed analysis of the lineshape of the cyclotron resonance absorption within a Drude model shows saturation at intensities of 0.1 W cm^-2. The electronic lifetimes deduced using a three-level model depend inversely on the electron concentration in the excited Landau level, indicating electron-electron scattering to be the dominant relaxation mechanism.

We performed a novel experiment to directly observe the influence of non-equilibrium optical phonons on the cooling rate of hot carriers in GaAs grown at low temperature by molecular beam epitaxy. Hot carriers in this material recombine in 1 ps leaving behind only non-equilibrium optical phonons, which reduce strongly the cooling rate of hot carriers generated within the phonon lifetime. Our experiments rule out a noticeable role for plasmon-phonon coupling and related effects of free-carrier screening. Monte Carlo simulations confirm the dominant contribution of the hot-phonon effect for the reduction of carrier cooling rate.

The ultrafast carrier dynamics in the high electric field at an Au-GaAs interface has been studied both experimentally, using a subpicosecond photoluminescence correlation technique, and theoretically by a Monte Carlo simulation. The photoluminescence decay time has been found to increase drastically with input power, ranging from a few picoseconds at low excitation to a considerably higher value (10-20 ps) at high excitation. From the Monte Carlo calculations it has been found that at high excitation the applied field collapses almost instantaneously. Even for a recharging time constant of 1 ps, which corresponds to the estimated device RC time, a drastic slowing down of the carrier sweep-out has been found, in almost quantitative agreement with the experimental findings.

Alloy semiconductor structures are important in bandgap engineered devices because of the tunability of the bandgap and lattice constant. Transport in such alloys is inherently affected by disorder scattering arising from the random placement (in this case) of Al and Ga atoms on the cation sublattice. Intervalley scattering involves large k-vector components which must exist in the disorder-scattering potential in order for these to cause intervalley transitions. Most previous studies of disorder scattering involve averaging of the entire Brillouin zone to obtain a long-range effective scattering potential. Here, a general k-dependent formulation of the disorder-scattering potential, based on the coherent potential approximation, is made. Disorder-induced intervalley scattering rates are computed from the imaginary self-energy and subsequent integration over partial densities of states. We find a scattering potential (= square root (x(1-x))V₀ in the long-range model) of 18 meV at the L point and 38 meV at the X point in the Brillouin zone. Using an ensemble Monte Carlo simulation, the phonon-induced intervalley scattering and disorder-induced intervalley scattering transitions are compared for femtosecond photoexcited Al_0.6Ga_0.4As. We find that L-X scattering is affected slightly by the presence of disorder scattering. The disorder-induced scattering lifetimes for L-X scattering are found to be three to five times longer than the phonon-induced processes.

The combined effect of a short carrier lifetime and of phonon emission on the cooling rate of nearly resonantly excited carriers in bulk GaAs has been investigated using picosecond time-resolved photoluminescence. Intense photoexcitation of carriers results in very short carrier density decay times of about 20 ps, leading to a considerable reduction of the cooling rate. The rapid density decay is attributed to a high stimulated recombination rate due to a high density accompanied by a low effective carrier temperature. Inclusion of the stimulated recombination in a model calculation for the energy relaxation by phonon emission leads to a significant recombination heating, which nicely explains the observed reduction of the carrier cooling rate.

Exciton photoluminescence in ultrapure GaAs (total shallow impurity concentration N_D approximately 10¹² cm^-3) was studied in magnetic fields up to 20 T. The observed magnetic field enhancement of free-exciton luminescence and exciton cooling were found to depend strongly on the excitation energy, giving direct evidence for the role of hot photoexcited electrons. The observed effect in fact reflects cooling of electrons in a magnetic field due to their increasing acoustic phonon energy relaxation rate and an efficient exciton-electron interaction. In a magnetic field parallel to the sample surface a suppression of the exciton-electron interaction was found because of the spatial separation of excitons and electrons.

Time-resolved Raman spectroscopy with subpicosecond resolution has been used to measure L- Gamma intervalley scattering of photoexcited electrons in GaSb. Our experiment measures directly the time-dependent occupancy of non-equilibrium longitudinal optic phonons with wavevector approximately 10⁶ cm^-1 which are generated by intra- Gamma relaxation of electrons near the Gamma minimum. The phonon population shows a two-component decay: at times <or approximately=10 ps, phonons are generated by direct electron relaxation within the Gamma valley, and their decay is due to anharmonicity and reabsorption. At later times the phonon decay rate slows, as L- Gamma intervalley scattering heats electrons at Gamma . At low temperature the intervalley relaxation time is measured to be 54 ps, corresponding to an effective intervalley deformation potential D_{Gamma L}=3.2 eV AA^-1.

The dynamics of photoexcited carriers in GaAs quantum wells and superlattices has been measured with femtosecond resolution using luminescence as well as pump-probe experiments. The electron capture mechanism shows well defined resonances with a time as short as 500 fs at room temperature for a well thickness of 60 AA. This gives strong evidence for the importance of the quantum mechanical, LO phonon-assisted capture mechanism predicted by theoretical calculations. In type-II superlattices, we show that the transfer of electrons from the GaAs layers to the AlAs layer is mediated by zone edge phonons (both LO and TA) in a way similar to intervalley scattering. The observed times range from 140 fs up to 30 ps depending on the thickness of the GaAs layer.

We study experimentally and theoretically both vertical transport in the barriers of GaAs/AlGaAs quantum well (QW) heterostructures and the capture of electron-hole pairs into the QWS. By a numerical simulation we determine ambipolar diffusivities, mobilities, surface-recombination velocities, transmission and capture parameters for 40 K <or=T<or=150 K. Alloy-disorder scattering is found to limit the ambipolar mobilities between 40 and 120 K in the Al_0.5Ga_0.5As barriers.

We calculate the electron capture rates for GaAs-Al_xGa_1-xAs multiple quantum wells by the emission of confined phonons via electron-phonon (Frohlich) interaction. The carrier capture process as detected by optical measurements is discussed and we show the importance of a knowledge of the details of carrier excitation in order to obtain an appropriate interpretation of the experimental results. In particular we show that if the carriers are excited into a confined state with a large kinetic energy intrasubband transitions strongly influence the capture rates. We obtain remarkable agreement with several experimental results.

We have experimentally studied the time evolution of the exciton population in a higher subband of GaAs quantum wells, below the free-carrier continuum. The lifetime of the exciton formed by an electron of the lowest subband and a heavy hole of the second subband in GaAs quantum wells is determined by time-resolved luminescence as 130+or-20 ps. This result is consistent with theoretical estimations of intersubband scattering by acoustic phonon emission. We discuss subband transitions of excitons in quantum wells as a new appealing concept for optically pumped coherent sources in the meV range.

The temperature dependence of intersubband scattering in n-modulation-doped GaAs/Al_xGa_1-xAs quantum well structures is systematically investigated for the first time by an infrared bleaching technique. In a highly doped sample, where the excited well subband is located above the bottom of the potential minimum of the barrier, a ground state recovery time of several picoseconds is found at T=300 K. At temperatures below 200 K an additional non-exponential component appears extending over approximately 100 ps. In samples where the energy of the excited well subband is lower than the energy of the subbands of the barrier potential minimum, the direct intersubband transition from the excited subband to the lowest subband is observed at T=10 K( tau approximately 2 ps). With rising temperature the measured time constants increase. The results are discussed taking into account the band structures of the samples.

We report an experimental realization of a new hot-electron resonant tunnelling transistor, where the resonant tunnelling current between two terminals is modulated by a third terminal. By using this new transistor we find that the intersubband hot-electron energy relaxation time, obtained from magnetotunnelling spectroscopy, is only several tens of femtoseconds.

The purpose of this paper is to give a theoretical interpretation of the experimental results of Goede et al. (Superlatt. Microstruct. 12 363 (1992)) describing the relaxation of heavy-hole excitons in asymmetric double quantum well systems based on the semimagnetic CdTe-CdMnTe system. In order to explain certain spectroscopic data, Goede et al. had to invoke excitonic tunnelling between the wells for systems with narrow (25 AA) inner barriers. In this work it is shown that the experimental results imply tunnelling via a 'crossed' excitonic state with the electron and hole localized in different wells. The experimental observations are interpreted in terms of a model involving phonon scattering and calculations of intersubband relaxation rates via confined phonon modes are shown to be in agreement with experimental observations.

The influence of a quasi-equilibrium electron-hole plasma on the population lifetime of non-equilibrium longitudinal optic (LO) phonons is studied in GaAs quantum wells using a multiple-beam Raman scattering technique to separately control the phonon generation and free-carrier injection processes. The lifetime of non-equilibrium, small-wavevector LO phonons is found to decrease from 4.5 ps to 1.9 ps as the density of the cold plasma is increased to approximately 3*10¹¹cm^-2 by adjusting the intensity of an infrared laser tuned close to the lowest subband gap of the quantum wells.

We outline a continuum model of optical modes in GaAs/AlAs superlattices with particular emphasis on the modes vibrating in the AlAs restrahl band. Results are given for the AlAs modes of (100) and (111) superlattices and are shown to give excellent agreement with a microscopic model. We also demonstrate the effect of using the mass approximation on the dispersion of the superlattice modes.

In GaAs/AlAs quantum well structures both the confined GaAs optic phonon and the AlAs interface polariton (IP) mode contribute to the energy and momentum relaxation of hot electrons. It has been shown that in real multiple quantum well structures elastic scattering of phonons can lead to a non-drifting population of hot phonons, which results in a suppression of the high-field electron drift velocity. The consequences for device application are obvious. The non-drift of the mode is determined by the relative magnitudes of its energy and momentum relaxation rates. We present a calculation of the lifetime and the elastic scattering rates for confined and interface modes in GaAs/AlAs quantum wells. It is found that the decay rates of the two modes exhibit different well-width dependences, and that scattering from interface roughness is the dominant mechanism for momentum relaxation. The implications for high-field transport are discussed.

Optical carrier heating by quasi-CW excitation has been studied in an InGaAs/GaAs single quantum well structure. At lower excitation intensities, until the quantum well is filled with the photoexcited carriers, the carrier temperature in the well is much higher than in the barriers. At higher excitation intensities, additional carrier heating in the barriers by the hot carriers located in the well is observed. The difference between carrier temperatures in the well and in the barriers is discussed comparing the carrier-LO phonon and carrier-carrier scattering times.

We report time-resolved luminescence measurements of MBE-grown ZnTe/GaSb. Above-bandgap excitation is provided by second-harmonic generation using infrared pulses from a Ti:sapphire laser. Cooling of the hot-carrier distribution is observed on a picosecond time-scale, and is seen to be limited by phonon bottle-necking at these carrier densities, allowing an estimate of the LO phonon lifetime in this material.

We report time-resolved optical transmission and photoluminescence measurements of exciton dynamics and recombination in a ZnSe/ZnSe_0.82S_0.18 superlattice which displays low-threshold lasing. The carrier densities in our experiments are below threshold for stimulated emission. We find clear evidence for strong exciton-LO phonon scattering which gives rise to emission at the same wavelength where stimulated emission is reported to occur.

The energy loss rate of two-dimensional hot electrons has been studied in modulation-doped Si/SiGe heterostructures grown by ultrahigh-vacuum chemical vapour deposition using the damping of the amplitudes of Shubnikov-de Haas oscillations with applied electric field up to 2V cm^-1. The samples investigated had a carrier concentration of 1*10¹²cm^-2. The ohmic properties of the samples were studied by photo-Hall effect, conductivity and quantum Hall effect measurements. From comparison of the experimental data with calculations it is shown that the dominant energy loss mechanism is due to acoustic phonon emission via deformation potential coupling for electron temperatures between 1.8 and 7 K. In addition the electric field dependence of the negative magnetoresistance due to weak localization for electric fields up to 2.1 Vcm^-1 was investigated for a lattice temperature of 1.8 K. The data were analysed to yield the dephasing time tau _phi as well as the elastic scattering time tau _e.

Monte Carlo simulation is carried out to investigate the high-field transport properties of the two-dimensional electron gas in strained Si/SiGe heterostructures. In the simulation we take into account the intervalley scattering between twofold and fourfold valleys of an Si well layer split by the tensile strain. Intervalley scatterings within the twofold or fourfold valleys are also incorporated in the simulation as well as the acoustic phonon scattering. We obtained an electron drift velocity at room temperature as high as 1*10⁷ cm s^-1 at 10 kV cm^-1. Calculated results of 4.2 and 77 K show negative differential mobility beyond 10 kV cm^-1. At 77 K the transient response of the drift velocity shows a remarkable overshoot, reaching about 3*10⁷ cms^-1 at 0.2 ps at 10 kV cm ^-1.

The in-plane transport properties of a strained (100) Si layer on a relaxed Si_1-xGe_x substrate are studied for an ungated modulation-doped structure. We use an ensemble Monte Carlo technique. These results are then used to study a gated device structure with a moment equation method. Similar velocity-field characteristics are found for ungated strained Si with any valley splitting energy Delta E>or=0.1 eV. These phonon-limited electron mobilities reach 4000 cm²V^-1s^-1. at 300 K, and 23 000 cm²V^-1s^-1 at 77 K. There is only a slight increase in the saturation velocity at both temperatures. However, a significant overshoot peak transient velocity is found to depend upon Delta E, and for Delta E=0.4 eV it reaches 4.1*10⁷ cm s^-1 at 300 K, and 5.2*10⁷ cm s^-1 at 77 K. Impact ionization increases with Delta E, due to the reduction in the bandgap. For the gated device structure, our numerical simulation of a deep submicrometre modulation-doped device shows velocity overshoot with a peak velocity of 2.6*10⁷ cm s^-1 in the quantum well at 300 K, which is important in achieving a high transconductance of about 300 mS mm^-1.

Electron-optical phonon interaction is studied in longitudinal transport through a series of Al_0.3Ga_0.7As/GaAs superlattices by measuring longitudinal magnetophonon resonance (MPR). In addition to well resolved MPR peaks caused by GaAs phonons, peaks ascribable to AlAs interface phonons are observed when a thin AlAs barrier is inserted at each GaAs/Al_0.3Ga_0.7As heterointerface. A calculation based on the Huang-Zhu model describes the features of experimental spectra. When the well width is reduced in Al_0.3Ga_0.7As/GaAs superlattices with narrow quantum wells, the AlAs-like phonon mode also becomes important in miniband transport.

We demonstrate via hot-electron photoluminescence and high-temperature mobility measurements the importance of the AlAs interface mode in the energy relaxation of electrons in GaAs/AlAs multi-quantum wells. A corresponding investigation of a similar GaAs/Al_0.24Ga_0.76As system illustrates that this is not the case for AlGaAs barrier devices where GaAs modes are the dominant energy relaxation process. The importance of the AlAs interface mode is not simply related to its intrinsic scattering rate but also to its shorter lifetime (compared with GaAs modes). Hot-phonon effects are therefore crucial to a full understanding of the experimental data.

In this paper we report a detailed study of the phonon emission by hot holes in (311)A GaAs/(AlGa)As heterojunctions. The two-dimensional hole gas (2DHG) was heated by short ( approximately 20 ns) electrical pulses and the emitted phonons were detected using several Al bolometers on the opposite face of the GaAs substrate. Analysis of the phonon signals as a function of mode, emission angle and power dissipated in the 2DHG directly yielded information regarding the hot-hole energy relaxation processes: at moderate power inputs (<or=50 pW/hole) acoustic phonon emission was dominant. In this regime the energy relaxation rate was found to be proportional to T_h⁵ and the phonon emission was mostly confined to within 30 degrees of the 2DHG normal. At higher powers the dominant relaxation process changed to optic phonon emission. We were also able to deduce the average separation of the 2DHG from the heterointerface, 3 delta ₀ approximately=6 nm, which is in reasonable agreement with theory.

We investigate the nonlinear conduction mechanisms of electrons in a semiconductor superlattice miniband. Experimental results obtained from time-of-flight measurements in GaAs/AlAs superlattices are compared with analytical semiclassical approaches. It is shown that the peak velocity can be accounted for when electron heating is assumed to take place. The dynamics of electron transport is also investigated by numerically solving the time-dependent Boltzmann equation including polar optical phonon and interface roughness scattering.

We have directly observed the electric-field-driven hot-electron distribution in high-mobility GaAs/AlGaAs heterostructures by detecting the grating-induced far-infrared Smith-Purcell emission. In samples with very low concentration the distribution develops a non-thermal shape at low electric fields due to the dominance of LO phonon emission. For the first time the average mean free path of the hot electrons is determined by studying the emission signal as a function of the grating period length.

Monte Carlo simulation is carried out to investigate the transport parameters of the two-dimensional electron gas in AlGaAs/GaAs heterostructures. In addition to diffusion coefficient and drift velocity at high electric fields, we derive the length coefficient defined by Price for an extended drift-diffusion model, which describes non-equilibrium carrier transport in semiconductors. The length coefficient is found to be almost zero at electric fields less than 3 kV cm^-1 and shows a maximum of about 550 nm at 4 kV cm^-1, decreasing gradually to 100 nm at 10 kV cm^-1. The diffusion coefficient is found to be strongly affected by the degeneracy of the distribution function.

We have made measurements of the phonon intensity emitted by hot carriers in the entry and exit corners of two-dimensional electron and hole gases (2DEGs and 2DHGS) in single GaAs/(AlGa)As heterojunctions in the quantum Hall regime (QHR). The intensity was measured normal to the 2DG. In 2DEGs, for the whole power range studied, the normal intensity from the electron exit corner is appreciably lower and we attribute this to a change in angular distribution due to hot-electron transmission into the 3D contact. The difference in behaviour from Si is attributed to the difference in relaxation rate associated with the effective masses. Dissipation from the entry and exit corners has also been seen for the first time in a magnetically quantized 2DHG.

We study the effect of the interaction of electrons confined in one spatial direction in a parabolic quantum well with longitudinal optical (LO) phonons in a tilted magnetic field. In detail we analyse the polaron cyclotron mass, calculated using second-order perturbation theory, in dependence on the magnetic field strength and the tilt angle of the magnetic field. We find a strong dependence of the cyclotron masses on the tilt angle, corresponding to the two measurable transitions in cyclotron resonance experiments.

We present Monte Carlo (MC) simulation of transmitted acoustic phonon (TAP) drag between barrier-separated two-dimensional (2D) electron gases in the AlGaAs/GaAs system. Non-equilibrium acoustic phonons emitted by the hot 2D electron gas in the biased GaAs channel travel across the sample. These phonons are partially absorbed in an unbiased 2D channel where they induce a drag current. Simulation includes 2D electron-non-equilibrium acoustic phonon interaction for both deformation-potential and piezoelectric coupling. Non-equilibrium phonon distribution is calculated numerically. TAP drag is simulated at 4.2 K in a multiple quantum well containing equivalent high-mobility 2D electron gases. Drift velocities around 1000 m s^-1 are found in the drag channel (2D gas without outer field), when it is driven by TAP drag from a large number (10-50) of 2D electron gases subjected to electric field of 1000 V m^-1 TAP drag is mainly due to the deformation-potential coupling.

Low-field electron transport properties in heavily doped n-GaAs channels at 300 K are investigated by Monte Carlo simulation. The matrix element for electron-impurity scattering is obtained from the Fourier-transformed Coulomb potential accounting for the screening effects of the two-dimensional electron gas. The effects of the screening and the impurity profile are analysed. Electron mobility is calculated for several values of sheet electron density and n-GaAs thickness. For low sheet electron density, the calculated mobility increases with the sheet electron density and is almost independent of n-GaAs thickness. For high sheet electron density, the mobility approaches a constant value and becomes independent of sheet electron density. These findings are explained for the first time as a transition from a two- to three-dimensional nature and are well confirmed by our experiments.

A novel multiband theory of Bloch electron dynamics in homogeneous electric fields of arbitrary strength and time dependence is presented. In this formalism, the electric field is described through the use of the vector potential. Multiband coupling is treated through the use of the Wigner-Weisskopf approximation, thus allowing for a Bloch electron transition out of the initial band due to the power absorbed by the electric field; also, the approximation ensures conservation of the total transition probability over the complete set of excited bands. The choice of the vector potential gauge leads to a natural set of extended time-dependent basis functions for describing Bloch electron dynamics in a homogeneous electric field; an associated basis set of localized, electric-field-dependent Wannier and related envelope functions are developed and utilized in the analysis to demonstrate the inherent localization manifest in Bloch dynamics in the presence of relatively strong electric fields. From the theory, a generalized Zener tunnelling time is derived in terms of the applied uniform electric field and the relevant band parameters. The analysis shows an electric-field-enhanced broadening of the excited state probability amplitudes, thus resulting in spatial lattice delocalization and the onset of smearing of discrete, Stark ladder and band-to-band transitions due to the presence of the electric field. In addition, the velocities of a Bloch oscillation will be observed only for the electron that is initially in a Bloch state before Zener tunnelling. Further, the influence of electric fields on resonant tunnelling structure is examined.

We have studied the transport properties of an Al_xGa_1-xAs/GaAs single heterostructure using a Monte Carlo method, focusing in particular on the effect of the polar interaction between electrons and phonons. A two-valley ( Gamma and L) model for both GaAs and Al_xGa_1-xAs layers has been used, which includes size quantization effects through the numerical self-consistent solution of the coupled Schrodinger-Poisson equations. The optical mode description is given in terms of the dielectric continuum model (DCM); within this model the alloy is described by a two-pole dielectric function, which depends on the Al composition. We have then evaluated the scattering probabilities for the confined electrons interacting with half-space and interface modes. These rates are inserted in our Monte Carlo code to study the electron response to an electric field applied along the heterointerface.

This paper presents a study of electron transport in ultra-high-speed devices with a heterojunction launcher which achieves high-energy electron injection into an active transport region. The potential and limitations for realizing near-ballistic transport in practical device structures under realistic bias conditions are examined. Specific devices considered in this work are charge injection transistors and vertical electron transistors based on lattice-matched AllnAs/GalnAs material systems. Self-consistent Monte Carlo simulations indicate that in order to effectively utilize electron near-ballistic transport for achieving improved performances, the device structure and operating conditions must be properly designed.

We report 4.2 K studies of the dependence of the in-plane, DC conductivity of a quasi 2D electron gas on the amplitude E_omega of applied fields with frequencies from 0.25 THz to 3.5 THz. We analyse the dependence of sigma _DC on E_omega assuming that electron-optical phonon scattering dominates energy relaxation, that the absorbed power has a Drude form and that the electron distribution is thermal. This simple analysis is self-consistent: Arrhenius plots of the estimated energy loss rate have a slope near -h omega _LO/k₈, for all frequencies, as expected for energy loss by optical phonon emission. We find that the effective energy relaxation time tau _epsilon varies with the frequency of the applied field, from tau _epsilon approximately 4 ps at 0.34 THz to tau _epsilon approximately 0.3 ps at 3.45 THz. This may indicate a frequency-dependent form for the hot-phonon distribution.

An investigation has been made of the far-infrared emission (FIR) from two-dimensional electron and hole gases (2DEGs and 2DHGs) in GaAs/(AlGa)As heterostructures as a function of source-drain current and magnetic field. The results on 2DEGs indicate that the proportion of FIR coming from the bulk of the 2DEG, rather than from the diagonally opposite corners near the contacts where a significant amount of the dissipation takes place, varies with both current and magnetic field. The results on PDHGs suggest that the carrier temperatures are very much lower than those in 2DEGs for the same source-drain current.

Investigations of the differential conductivity and time-of-flight spectra of phonons emitted by hot electrons were performed in Si delta -doped GaAs. Together with a Green function balance equation method for numerical simulations, carrier heating in an electrical field is convincingly described. The results emphasize the importance of carrier transfer from the confined to the extended states and the role of phonon emission.

We present a full band Monte Carlo algorithm based on phase-space simplexes which has all of the advantages of analytical band Monte Carlo while preserving the accuracy of a full band structure. An adaptive, contour-aligned grid represents the energy band structure within the irreducible wedge to within an arbitrary tolerance. This discretization allows exact treatment of the equations of motion and final state selection for a wide class of scattering mechanisms. Results using this method show at least an order of magnitude improvement in performance over previous full band codes.

A rigorous quantum treatment of real-space transfer for electrons from a quantum well is developed within the density matrix and Wigner function formalisms. A continuous electron dynamical evolution (not an abrupt collision) corresponding to a phonon absorption is described. From the evaluation of the lowest-order correction to the unperturbed density matrix, the spatial and momentum distributions can be evaluated. The Wigner and Husimi functions are also shown for physical situations of interest.

The transient photocurrent flowing over a wide undoped AlGaAs barrier has been investigated as a function of bias voltage, voltage polarity, laser intensity and sample temperature. Excitation was by pulses of length 100 ps from a Nd:YAG laser. The radiation, which uniformly illuminated the top surface of the mesa, could produce excess carrier densities of the order of 1*10¹⁹ cm^-3. The time constant of the decay for zero bias was 4 ns at 100 K. It was independent of laser intensity, which suggests that the lifetime was determined by radiative recombination in the contact regions. A trap-limited hole mobility of 2 cm² V^-1 s^-1 was deduced from the low-field data.

We have constructed a unique Monte Carlo simulator incorporating the full band structure of the semiconductor, a realistic phonon spectrum and anisotropic electron-phonon scattering rates generated by ab initio electron-phonon matrix elements. Our computational model provides us with a rigorous test of our ability to formulate and calculate semiclassical transport properties based on fundamental physical principles. By treating the relevant scattering mechanisms with a greater degree of sophistication, we have drastically reduced the number of adjustable parameters and thereby hope to gain some measure of confidence in the calculated high-energy tail of the distribution function.

Through the use of examples, various forms of the quantum potential are examined for modelling and interpreting the operation of semiconductor devices.

Algorithms for obtaining quantum distribution functions from the Liouville equation in the coordinate representations, incorporating dissipation, are applied to the current-voltage (IV) and capacitance-voltage (CV) relations of single-barrier structures. IV reveals signature charge distributions and energies in excess of equilibrium values; CV demonstrates a reliable means of delineating the width of the barrier.

The scattering matrix approach (SMA) is a new technique for solving the space-dependent Boltzmann equation in semiconductor devices. In this paper we apply the SMA to examine electron transport in a simple AlGaAs/GaAs heterojunction bipolar transistor. The results demonstrate that the scattering matrix approach resolves off-equilibrium transport with the accuracy of a Monte Carlo simulation while also treating near-equilibrium transport and injection across high-energy barriers. These results establish the viability of the SMA for simulating carrier transport in advanced, compound semiconductor devices.

We report on theoretical investigations bearing on carrier relaxation among electronic states in III-V semiconductors. Electron scattering by longitudinal optical and acoustic phonons is considered for quasi-two-, one- and zero-dimensional (2D, 1D and 0D) structures. In the 0D case, the calculated electron-phonon scattering rates decrease strongly with increasing spatial quantization. Exciton-phonon scattering and electron relaxation by Auger processes in 0D structures are studied. We briefly discuss the impact of the relaxation properties on the prospect of devices and compare the theoretical results with optical experiments.

A Monte Carlo analysis of the carrier relaxation dynamics in a GaAs quantum wire system following laser photoexcitation is presented. Relaxation mechanisms due to electron-electron and electron-polar optical phonon interaction are included within a multisubband picture taking into account both intrasubband and intersubband scattering mechanisms for the case of rectangular quantum wire structures. Degeneracy and hot-phonon effects are also investigated as a function of carrier density and kinetic energy.

We have investigated the temporal evolution of the electron-hole plasma recombination and the carrier density dependence of the bandgap renormalization in GaAs quantum wires.

Quantum wire lasers are expected to require very low threshold currents owing to the nature of the 1D density of states which develops a sharp peak at the band edge and ensures superior laser characteristics. However, carrier relaxation processes in quasi-1D structures may be much slower than in bulk material owing to reduction in the momentum space. For very long relaxation times, these equilibrium processes are expected to limit the maximum modulation frequency of the quantum wire lasers. We perform a Monte Carlo simulation of electron relaxation in quantum wires with the inclusion of the electron-bulklike polar optical and acoustic phonon, electron-electron and electron-hole interactions as well as Thomas-Fermi screening. We find that for a carrier density of 10¹⁸ cm^-3 the electron relaxation time ranges from 120 ps for the 100 AA*100 AA wire to 30 ps for the 200 AA*200 AA wire. Since the threshold current in a quantum wire laser increases with the wire cross section, within the limits of our relaxation model, this indicates possible existence of a trade-off between speed and efficiency in a quantum wire laser. We also analyse the effects of carrier relaxation on gain compression in quantum wire lasers by solving the Boltzmann equation using a novel Monte Carlo technique. A spectral hole forms in the carrier distribution at high injected currents with the resulting decrease in the slope of the light-current characteristic. The effect of a non-fermi-Dirac distribution of electrons is found to result in a suppression of the peak gain as compared with the peak gain calculated using the equilibrium distribution.

Under certain biasing conditions, the intersubband energy separation in a quantum wire can match the optical phonon energy, thereby enhancing the scattering of charge carriers. We provide direct theoretical evidence that an anomalous negative differential transconductance (NDT) recently observed in an array of quantum wires at high temperatures is caused by resonant intersubband optical phonon scattering. Self-consistent 2D solutions of the Schrodinger and Poisson equations with a subsequent Monte Carlo analysis of intersubband scattering reveal a strong resonant polar optic phonon coupling between subbands at the gate bias indicated by the experimental data. The peak-to-valley ratio of the NDT exhibits a maximum near 170 K, thereby confirming the predictions of several theoretical investigations. The observation of resonant scattering is only possible because the quasi-1D form of Gauss's law shows an opposite trend to that for 2D systems in the dependence of the eigenenergy spectrum on gate bias.

Monte Carlo simulation of hot photoexcited electron relaxation in rectangular quantum wires is carried out. Simulation shows that at the initial stage the electron cooling dynamics is defined by electron-optical phonon interaction and exhibits strong dependence on excitation energy. When electrons are excited above the optical phonon energy they cool down in a subpicosecond time-scale to the bottom of the first subband. Electrons may even occur below thermal equilibrium energy and then slowly (during tens of picoseconds) relax to equilibrium due to interaction with acoustic phonons. At certain excitation energies strong intersubband electron scattering by optical phonons leads to carrier redistribution and intersubband population inversion.

We present results of Monte Carlo simulations of electron relaxation dynamics in rectangular GaAs quantum wires (QWIS) embedded in AlAs. Electron interactions with confined LO phonons, interface optical phonons, bulk-like acoustic phonons as well as non-equilibrium (hot) optical phonons have been taken into account. It has been found that hot phonons come into play at electron concentrations exceeding 10⁵ cm^-1. In QWIS electrons having appreciably different initial energies generate non-equilibrium phonons at different q-space regions which do not overlap. In turn, these phonons can be reabsorbed only by the electrons that have generated them. Consequently, hot-phonon effects become weaker as the energy distribution of excited electrons broadens. This result is in complete contrast to the case of bulk materials and quantum wells where the injected electron energy distribution virtually does not affect non-equilibrium phonon build-up and the reabsorption rate.

We report measurements of hot-carrier photoluminescence from GaAs V-groove quantum wires. CW spectra reveal distinct one-dimensional subbands: at low excitation density recombination from laterally confined n_v=1 states is dominant, which shows that carriers relax efficiently to the lowest subband; n_v=2 and 3 recombination becomes pronounced at higher temperature and density, which indicates inhibited relaxation, possibly due to band filling. Time-resolved photoluminescence measurements at 10 K show a rise time of approximately 100 ps, which is due to energy relaxation. At later times the intensity of the n_v=1 line shows a monotonic decay at low carrier density, but there is a n_v pronounced plateau at high density that extends for approximately 200 ps. These results provide clear evidence of carrier relaxation and band filling in quantum wires.

We have investigated thermalization and band renormalization in zero-dimensional electron-hole plasmas in InGaAs/GaAs quantum dots with geometrical widths down to 24 nm by time-resolved photoluminescence spectroscopy. Our experiments indicate a reduced relaxation of carriers in quantum dots with decreasing size, while the carriers in the dots seem to be thermally distributed on the discrete energy levels. Exchange-correlation interaction of the carriers in the dots lead to a renormalization of the bandgap.

The theory of coherent transmission resonances in multimode nanostructures is developed by considering a quasi-level, a decaying electron state localized within the structure, as primary, and deriving steady-state transmission and reflection probabilities versus energy as consequences. General (though not completely general) formulae are obtained for these as Lorentzian peaks and inverted peaks, in terms of the quasi-level decay lifetime and the mode fractions of the decay current. Both positive peaks and inverted peaks of the transmission probabilities equally are shown to be consequences of a decaying quasi-level. The dwell times, which determine the localized space charge for given occupations of the itinerant steady states, are obtained in terms of the same parameters, and it is shown that when all these states are filled through a resonance range of energy then the total localized space charge is equal to an electron charge times two (the spin degeneracy). A 'sum rule' is derived, giving the dwell time of each mode in terms of the phase-delay propagation times.

Current heating is used to measure the thermopower of a quantum dot in the Coulomb blockade regime. We observe sawtooth-like oscillations as a function of gate voltage in the thermovoltage across the dot. These observations are compared with measured Coulomb blockade oscillations in the conductance, and with theory.

Dephasing of ballistic electrons is measured as a function of both temperature and Fermi energy in a high-mobility two-dimensional electron gas. We find a qualitative agreement between the measured phase-breaking length and the theoretical prediction for the electron-electron scattering length using the value of E_F measured with large-area Hall bars. A good quantitative agreement is obtained when a local value of E_F, measured via on-chip magnetic focusing, is used. The good agreement between the measured phase-breaking length and the theoretical electron-electron scattering length strongly suggests that these two quantities are the same in the ballistic regime.

A critique is given of the Bohm-de Broglie pilot-wave representation of quantum mechanics as applied to quantum transport theory. It is argued that particle trajectories can exist only if they are stochastic. An extension of the Bohm-de Broglie theory is outlined which generates the conventional picture from an ensemble-averaged coupled particle-stochastic field. Consequences for tunnel time estimates and possible experimental verification with single-electronic devices are discussed.

We examine statistical fluctuations in the transmission properties of quantum dots with interface roughness and neutral impurities. For this purpose we employ a supercell model of quantum transport capable of simulating potential variations in three dimensions. We find that sample to sample variations in interface roughness in a quantum dot waveguide can lead to substantial fluctuations in the n=1 transmission resonance position, width and maximum. We also find that a strongly attractive impurity near the centre of a quantum dot can reduce these fluctuations. Nevertheless, the presence of more than a single impurity can give rise to a complex resonance structure that varies with impurity configuration.

We report on the observations of S-type negative differential conductance in the current-voltage characteristics of a pinched cavity quantum dot structure. An energy balance approach is used to explain the existence of bistable current-voltage characteristics due to thermal runaway of the hot carriers in the dot structure. This runaway is shown to arise from heating of carriers in the quantum dot by incident electrons injected over the barrier of the input constriction. The observed S-type negative differential conductance is controlled by a third-terminal gate bias, and may be turned on or off depending on the bias voltage. Thus the effect may be utilized to realize a multiterminal switching device.

We derive, with the Keldysh non-equilibrium Green function technique, an expression for the fully nonlinear, time-dependent current through an interacting mesoscopic system. For proportionate coupling to the leads, the time-averaged current is simply the integral between the chemical potentials of the time-averaged density of states, weighted by the coupling to the leads, in close analogy to the time-independent result of Meir and Wingreen (1992 Phys. Rev. Lett. 68, 2512). Explicit results for the exactly solvable non-interacting resonant tunnelling system are presented. Due to the coherence between the leads and the resonant site, the current shows a non-adiabatic behaviour with a ringing period which depends on the amplitude of the AC driving voltage. Finally, we establish a connection to recent linear response calculations.

A previous coupled-mode formalism for quantum transport in semiconductor two-dimensional electron gases is extended to interacting electrons. The monomode case has similar effective potentials to the single-carrier theory arising from the curvature of the actual confinement potential. In the case of the tunnelling of two electrons through a constriction in monomode a new phenomenon of Coulomb-assisted resonant tunnelling on single barriers is proposed and confirmed by numerical solutions to the two-body wave equation.

A generalized Monte Carlo method for the solution of the Wigner transport equation in semiconductor devices is proposed. The theoretical approach is based on the Wigner transport equation describing the time evolution of our electron-phonon system, and the Monte Carlo procedure is based on an iterative expansion of such an equation in powers of the various interaction coupling constants. In addition to a fully coherent description of the electron dynamics, the proposed approach allows us, in principle, to introduce in a quantum framework all the various interaction processes, such as electron-phonon, electron-impurity, and electron-electron scattering. Furthermore, boundary conditions, and therefore open systems, can be considered, as required for the analysis of semiconductor devices. Some numerical results are presented for a biased double-barrier structure with electron-phonon interaction.

The non-Markovian behaviour of confined systems of electrons interacting with both elastic and inelastic scattering is studied by the holomorphic memory-function method. First, a 'ballistic memory function' is identified, which shows that the conductance quantization of waveguides is caused by long-lived correlations between the device contacts. Second, an 'interacting memory function' shows the mutual influence of geometrical confinement and dissipative mechanisms. Lateral quantization is associated with diffusive boundary scattering which causes phase-breaking effects when it interacts with elastic scattering centres. This affects both the electron mass through renormalization, and the transport relaxation time, through a shift.

Transport responses of hot electrons in quantum structures at room temperature are calculated including longitudinal optical phonon scattering by using an iterative method based on the non-equilibrium Green function technique to study how the potential fluctuation affects the transport responses. The average kinetic energy in a quantum wire is found to be influenced only a little, while the drift velocity is considerably affected by the potential fluctuation especially at lower electric fields. The possibility of Bloch oscillation in a coupled quantum box is also discussed.

Photoluminescence spectra and electron transport in p-type doped quasi-one-dimensional (1D) quantum well structures are studied at room temperature. The electric field applied along the wire raises the electron temperature. It is higher than that in two-dimensional (2D) structures. The apparent electron mobility is analysed using the time-of-flight method, and it is found that the apparent electron mobility is larger in 1D than in 2D.

We have investigated low-temperature electron transport in quantum wires which were fabricated on high-mobility GaAs/AlGaAs heterostructures using laser holography and electron beam lithography. A decrease of the electron mobility with decreasing wire width and increasing wire length is found that is attributed to roughness scattering. At high electric fields saturation of the drift velocity is observed similar to the case of a two-dimensional electron gas.

We show that the electron-phonon scattering rates in a semiconductor quantum wire can be significantly affected by an external magnetic field. This suggests the possibility of using a magnetic field to modulate the electron mobility, the saturation velocity, the relaxation rate of photocarriers, etc., in a quantum wire. A magnetic field increases the optical phonon scattering rates but decreases the acoustic phonon scattering rates. The latter has serious implications for the quantum Hall effect. Finally the possibility of a magnetic field promoting negative differential mobility caused by the Riddoch-Ridley mechanism in a quantum wire is also discussed.

A new model for Monte Carlo simulation is proposed for one-dimensional electron gases. In the model, electron energy and momentum are treated as independent variables, and the Wigner distribution function f(k, omega ,t) is simulated instead of the semiclassical distribution function f(k,t). Using this method, the transport responses of one-dimensional electron gases in a quantum wire at room temperature are evaluated in which electrons are interacting with optical phonons only, and the results reveal that the electron temperature increases monotonically with applied electric field without showing anomalous carrier cooling.

We report electron and thermal transport in InAs nanostructure free-standing wires fabricated by electron beam lithography and wet-chemical etching on an InAs/AlGaSb heterostructure grown by molecular beam epitaxy. Conductance measurements of InAs free-standing wires with the thickness of 150-550 AA and a few thousand angstroms wide were conducted by non-dissipative AC measurement over the temperature range between 4.2 and 40 K. The dependence of resistance on heating current revealed that the Wiedemann-Franz law holds when the power dissipation is below approximately=3 nJ. Above the critical power dissipation, the I-V characteristics show deviation from the simulation curve which takes into account the electric contribution, the phonon drag thermopower, the diffusive thermopower, the electron-phonon generation term, blackbody radiation, and thermal diffusion from the surface to the helium atmosphere of the cryostat. The experimental results suggest some other heat conduction mechanism which does not contribute to the electric conduction.

We report the first demonstration of the 1D quantized conductance at around 80 K on InAs/(AlGa)Sb split gate devices fabricated by using electron beam lithography and wet chemical etching. Nonlinear transport properties measured between 4.2 K and 115 K are discussed.