Table of contents

We discuss the role of excitonic polarons on the interband optical properties of single self-assembled semiconductor quantum dots. We show in particular that the strong coupling to optical phonons leads to fundamental modifications in the description of the excitation, relaxation and luminescence processes, as compared to previous schemes, and that the use of excitonic polaron states provides a coherent model for the description of various interband optical processes.

The influence of impurities on the transport properties of a heterostructure is studied by ballistic electron emission microscopy/spectroscopy (BEEM/BEES) on a single AlAs barrier embedded in a GaAs matrix. The formation of a continuous impurity band is observed for doping concentrations of N_D = 1 × 10¹⁷ cm⁻³. A local influence of the impurities on the transmission behaviour is demonstrated and subsequently used to image areas of enhanced transmission associated with the impurities.

We present a model which treats non-perturbatively the effects of both dipolar coupling and reservoir fluctuations on the interband optical response of a single quantum dot. We are able to study the transition to a Rabi-like situation with increasing excitation power. The incorporation of an energy relaxation process (for instance, due to polaron relaxation among the dot polaron levels) in a three-level system allows us to simulate both photoluminescence (PL) and photoluminescence excitation experiments. The PL spectrum displays one or a few lines depending on the excitation intensity, relaxation efficiency and reservoir fluctuation rates.

We present a unified theoretical approach, the finite-U slave-boson mean-field approach developed initially by Kotliar and Ruckenstein (1986 Phys. Rev. Lett.57 1362), for the description of the linear and nonlinear Kondo-type transport through coupled double quantum dots (DQDs) connected in series to leads and through a multi-level lateral quantum dot. We find that, with increasing inter-dot hopping, the DQDs manifest three distinct physical scenarios: the Kondo singlet state of each dot with its adjacent lead, the spin singlet state consisting of local spins on each dot and the doubly occupied bonding orbital of the coupled dots. For a two-level quantum dot, our calculation demonstrates a substantial conductance enhancement near the degeneracy point of the spin singlet and triplet states, a non-monotonic temperature dependence of conductance and a sharp dip and non-zero bias maximum of the differential conductance. These agree well with recent experimental observations.

Monte Carlo simulation of a two-dimensional hole gas in a GaAs/AlGaAs single quantum well is performed to study the effects of the anticrossing behaviour of the in-plane dispersion on the transport characteristics.

We report experimental results on simultaneous measurement of electron as well as hole transient transport in an Al_0.3Ga_0.7As-based p–i–n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy. Electron and hole velocity overshoots are directly observed. These experimental results are discussed and explained.

Transient subpicosecond Raman spectroscopy has been used to measure electron transport properties in an In_xGa_1−xAs-based semiconductor nanostructure under the application of an electric field. The deduced electron drift velocity has been found to be significantly larger than either GaAs or InP-based p–i–n nanostructures under similar experimental conditions. We attribute this finding to both the smaller electron effective mass and the larger Γ to L(X) energy separations in In_xGa_1−xAs. The experimental results are compared with ensemble Monte Carlo calculations.

At temperatures between 2 and 10 K the phonon induced conductivity of δ-doped GaAs:Si was investigated as a function of carrier heating. Two peculiarities were observed at about 1 and 10 V cm⁻¹. The analysis allows us to conclude that they are connected with destruction of weak localization and redistribution of hot carriers between states with different mobilities, respectively.

The scheme of a laser which can operate in the far-infrared range (λ ∼ 150 µm) is suggested. In order to attain the inversion of the subband population, it is suggested that electron transport in three tunnel-coupled quantum wells in a strong electric field, which lies in the plane of quantum wells, be used. An important specific feature of the structure suggested is the presence of a single rough heterointerface. The electron transport was simulated by the Monte Carlo method for the Al_xGa_1−xAs/GaAs (x = 0.2–0.3) heterostructure. The simulation demonstrated that the population inversion in the first and second subbands is realized for fields above 1.2 kV cm⁻¹ at T = 4.2 and 77 K.

Monte Carlo simulations of electron transport in Al_xGa_1−x As/GaAs/In_yGa_1−yAs double quantum well heterostructures in high lateral electric fields are carried out. It is shown that, under the conditions of intervalley Γ–L electron transfer, there exists a population inversion between the first and the second quantum-confinement subbands in the Γ valley. The population inversion appears in fields exceeding 4 and 5.5 kV cm⁻¹ at 77 and 300 K, respectively. The gain in a superlattice composed of such quantum wells is estimated to be on the order of 100 cm⁻¹ for radiation with a wavelength of 12.6 µm.

We analyse the electronic and phononic dynamics in GaAs and GaN quantum dot structures due to their interaction with acoustic phonons. We compare results for two specific quantum dot heterostructures which have been proposed as hardware building blocks for a quantum computer in recent quantum computation/information schemes. In particular, we are interested in the loss of coherence after excitation with an ultrashort laser pulse and in the dynamics of phonons which are created as a consequence of the optical excitation process. Our results are non-perturbative with respect to both carrier–phonon and carrier–light interaction and therefore include multi-phonon processes of arbitrary order. We find that, due to different quantum dot sizes, involved electric fields and material parameters, the decoherence is stronger in the GaN dots. The interplay of these effects also strongly determines the details of the created phonon occupation, which splits into two parts: one remains in the vicinity of the dot and forms a stable polaron and another leaves the dot as a phononic wave packet travelling with the velocity of sound. Due to the dot geometry and the carrier–phonon coupling matrix elements this phonon emission is strongly anisotropic.

Semiconductor superlattices driven into the regime of negative differential conductivity often exhibit complex or even chaotic self-sustained current oscillations due to travelling field domains. For a reliable operation of a superlattice as an electronic oscillator, such unpredictable and irregular conditions should be avoided. In this work we present a theoretical analysis, proposing a novel scheme to stabilize high-frequency domain oscillations in semiconductor superlattices by a time-delayed feedback loop.

Due to the Fermi–Dirac statistics of electrons the temporal correlations of tunnelling events in a double barrier setup are typically negative. Here, we investigate the shot noise behaviour of a system of two capacitively coupled quantum dot states by means of a Master equation model. In an asymmetric setup positive correlations in the tunnelling current can arise due to the bunching of tunnelling events. The underlying mechanism will be discussed in detail in terms of the current–current correlation function and the frequency-dependent Fano factor.

Quantum Hall systems can show a bistability in the hysteresis of the breakdown. We realized a relaxation oscillator based on a quantum-Hall device with Corbino geometry. Investigations of the performance of such an oscillator revealed an increase of the hysteresis of the current–voltage curve of the device in comparison with the width of hysteresis at dc voltages. By direct measurements of the hysteresis of the current–voltage curve at different frequencies, we found a marked increase of the breakdown hysteresis at low frequencies of some Hz. We explain the observed dynamical enhancement of the breakdown hysteresis applying an electron heating model with a background (delocalization-related) component of conductivity, which decreases with increasing frequency, indicating a more effective localization already at low frequencies.

We investigate the electron kinetics in quantum dot (QD) devices due to Coulomb scattering with carriers in a remote, non-local quantum well or bulk contact region. Our numerical calculations show that Auger relaxation times of microseconds are possible even for distances of several 100 nm between the QDs and an electron bulk layer.

We investigate lasing and transport properties of GaAs/Al_xGa_1−xAs quantum cascade laser structures with varying injector doping density n_e. Calculations from a nonequilibrium Green function theory show a linear dependence of the current density on n_e in agreement with the experimental trend. We compare theoretical results for the threshold current density J_th and laser emission energy as a function of n_e with previously obtained experimental results.

A transverse (in-plane) magnetic field (B = 0–7 T) was used to modify the shape of the tunnelling resonance between electronic states in neighbouring quantum wells of a superlattice and to investigate the effect of the modification of the tunnelling resonance profile on electric field domain stability and self-sustained current oscillations effect in GaAs/AlGaAs superlattices.

Wavelength selective charging of a QD sub-ensemble is demonstrated, presenting a basis of future wavelength dependent parallel optical memory. High-resolution saturation spectroscopy in two-colour photocurrent experiments is used to investigate the binding energy of the positively charged trion complex as well as the temperature dependence of the coupling to acoustic phonons.

We studied electrical transport properties of coupled InAs quantum dots (QDs) embedded in GaAs. A resonance peak in the current–voltage characteristics was observed in the low temperature region. When the magnetic field was applied, a linear shift of the resonance voltage was observed. As a result of the g-factor estimation, the resonance is attributed to the current corresponding to the electron transport through coupled InAs QDs.

We have investigated the breakdown of the self-averaging of impurity scattering in ultra-small semiconductor devices which occurs when less than 1000 impurities are present. Non-equilibrium Green function techniques are used to show that in the atomistic regime there are strong interference processes that are not described by self-energies based on self-averaging. The predicted current flow comprises meandering open orbits between the impurities and closed localized vortices that block the flow. Substantial fluctuations are predicted in the transmission function as the micro-configurations are changed at fixed doping concentration. The action of a strong electric field dampens the interference processes.

We present Joule heating measurements carried out over a wide temperature range on two-dimensional electron gases (2DEGs) and quantum wires of varying widths fabricated in an In_0.52Al_0.48As/In_0.53Ga_0.47As/In_0.52Al_0.48As heterostructure system that has a 25 nm wide In_0.53Ga_0.47As quantum well region. The power dissipated per electron is extracted and the electron–phonon coupling processes in these systems are studied. The temperature decay of the power loss at the 2DEG points towards unscreened piezoelectric coupling to the acoustic modes over temperatures of 1–30 K, with boundary scattering in the ohmic contacts gaining importance at very low temperatures. In the wires, we observe different behaviour and the effect of wire width and carrier density on the observed energy-loss rates. Possible phonon confinement and exponential suppression in these structures are also looked at.

The inter- and intra-subband relaxation of electrons emitting an LO-phonon in GaAs/AlGaAs mesoscopic discs were investigated by femtosecond reflectance spectroscopy. It was found that the relaxation time depends on the size as well as on the shape of the disc. The relaxation time in a stadium disc is shorter than that found in circular discs. A possible origin of the enhancement of the LO-phonon emission rate in a stadium disc is discussed in terms of the electron–electron scattering rate modified in the mesoscopic discs.

In this paper we present an extended transport model that has been specifically derived to account for the silicon thickness dependence of low-field electron mobility as observed in ultra-thin silicon-on-insulator MOS transistors, operating both in single- and double-gate modes. The systematic comparison with available experimental data suggests that conventional models for bulk transistors are inadequate to quantitatively describe the peculiarities of electron transport in such devices. The role of bulk phonons, surface roughness and Coulomb scattering is analysed, together with that of surface optical phonons and of the space, low-frequency fluctuations of the silicon thickness of such devices. Simulation results show that the inclusion of the last two scattering mechanisms significantly improves the comparison with available experimental data.

We model resonant terahertz photoconductance recently observed in field-effect transistors with a double-quantum-well (DQW) channel. Comparison of the measured THz resonant photoresponse to the calculated THz absorption spectrum establishes that the resonances are determined by standing plasma waves in the DQW channel under metallic portions of the grating gate. It is found theoretically that the DQW asymmetry mixes the acoustic and optical plasmons resulting in a rather intense ac electric field between the QWs.

We analyse photomixing using a structure akin to a high-electron mobility transistor. We develop a device model which takes into account the resonant excitation of plasma oscillations in the high-electron mobility transistor and near-ballistic transport of the photogenerated carriers. The transport of the photogenerated carriers is considered using an ensemble Monte Carlo particle modelling. It is shown that the velocity overshoot due to near-ballistic propagation of just photogenerated electrons can substantially increase the efficiency of the resonant photomixing and generation of terahertz radiation.

Self-oscillations of the current with frequencies of 0.7–3.6 MHz depending on the dc bias have been detected in weakly coupled GaAs/AlGaAs superlattices at 4.2 K. A study of the static and dynamic characteristics of the structure shows that self-oscillations arise in the local region of the superlattice, restricted by the size of the domain boundary expansion, due to the periodic coupling and decoupling of subbands in adjacent quantum wells forming the expanded domain boundary.

In the present work, we show that a usually unstable transparent state of a superlattice can be stabilized by a high-frequency biharmonic field with frequencies that are close, but not multiples of one another (with one of the components having sufficiently low amplitude). Furthermore, an incident biharmonic field having multiple frequencies gives rise to an absolute negative conductivity and consequent spontaneous generation of a static field in superlattices for specific phase relations of the harmonics.

Low-field measurements of carrier mobility, coupled with a perturbative, analytical calculation of the momentum–relaxation time, provide a method for a direct experimental extraction of the relation between the impurity concentration and the inverse screening length involved in the Coulombic scattering. The method is applied here to the non-degenerate, fully ionized case, showing significant differences with respect to the same quantity as derived from the solution of Poisson's equation.

A comprehensive theoretical study of the operation of the strained hot hole p-Ge RSL is performed. Hot hole kinetics in strained p-Ge under an applied electric field is studied and heavy and light hole distribution functions are found as a solution to the Boltzmann kinetic equation, which includes interactions with acoustic and optical phonons, as well as resonant and nonresonant ionized impurity scattering. Nonequilibrium population of the localized acceptor states is obtained as a solution to the system of rate equations within the two-level model for an impurity level structure. Conditions for the formation of the intracentre population inversion are studied as a function of the electric field and external strain. We have observed that the population inversion is formed in a wide range of electric fields and, therefore, could be easily achieved. Net gain at the THz frequencies is calculated using the nonequilibrium hot hole distribution function and the localized states population. The possibility of achieving net THz gain in semiconductors by injection of hot carriers into impurity resonant states is demonstrated.

We report the generation of widely tunable mid-infrared (5–13 µm) electroluminescence in a heterostructure bipolar transistor-like device. The radiation is associated with transitions between Wannier–Stark ladder states of a periodic superlattice included in the intrinsic region between base and collector. At low fields the spectrum is dominated by diagonal 1–1 transitions between ground levels localized in neighbouring wells. At larger fields, merging with the 2–1 intersubband emission is observed until intervalley Γ–X electronic scattering limits the radiative efficiency above 230 kV cm⁻¹.

We report the observation of electric-field domain formation in an asymmetric double-quantum-well GaAs/AlAs superlattice. Photoluminescence (PL) spectra exhibit several branches with increasing bias voltage. These steeply red-shifted PL branches can be attributed to Stark-ladder transitions between the heavy hole ground state in the narrow quantum well and the electron ground state in the wide quantum well in the high-field domain. According to our calculations, electrons and holes in the high-field domain are separated into different quantum wells, resulting in a large reduction of the radiative recombination energy with increasing electric field. Each PL branch most likely corresponds to the moving of the domain boundary.

We show how inverse scattering theory can be used to design superlattice structures with specified energy versus crystal momentum dispersion relations. We illustrate the method by calculating the superlattice unit cell required to maximize the curvature of the lowest miniband near the centre of the minizone. This type of structure should have a high electron drift velocity and might therefore be useful for high frequency applications.

Visible and infrared light emission bands arising from avalanche breakdown between gate and drain contacts of high-frequency high-power GaAs field effect transistors have been studied. The decay time of the mid-infrared (mid-IR) light (photon energies in the range 0.25–0.5 eV) is considerably less than 25 ms, while light with photon energies below 0.25 eV decays in a few seconds after switching off the current. The mid-IR spectrum of the light emitted by the GaAs transistors at high current consists of almost equidistant peaks with widths of ≈15 meV. The mid-IR emission is attributed to radiative transitions of holes from the split-off band to the heavy-hole band.

Transport properties of GaAs–GaAlAs weak barrier superlattices are studied by computer simulation and observation of current–voltage curves and terahertz emission. Existence of terahertz dynamic negative differential conductivity is demonstrated in the simulation. The observed terahertz emission is considered as due to incoherent Bloch oscillation emission. Prospects for the terahertz Bloch oscillator based on such superlattices are discussed.

It is shown theoretically that a considerable increase in the transit time dynamical negative differential conductivity may be achieved in graded gap heterostructures with a special barrier profile. It occurs due to an increase in the population inversion. The semiconductor devices utilizing this dynamical negative differential conductivity can operate at frequencies up to terahertz frequency band.

Hot electron spectroscopy was used to study transport through two GaAs/AlAs/GaAs heterostructures with undoped AlAs layers of 10 and 13 nm width. We observed transport through the individual bound states of the quantum well formed in the X-valley of the GaAs/AlAs/GaAs system. From the energetic positions of these bound states, we derived X_AlAs − Γ_GaAs= 136 meV for both samples and m*_X,AlAs = (0.8 ± 0.1)m₀ for the 10 nm sample and m*_X,AlAs = (0.7 ± 0.1)m₀ for the 13 nm sample.

Electron transport in GaN/AlGaN and GaAs/AlGaAs terahertz quantum cascade lasers is calculated using a fully self-consistent intersubband scattering model with thermal balancing. Subband populations and carrier transition rates are calculated and all relevant intra- and inter-period electron–electron and electron–LO-phonon scattering mechanisms are included. In particular, the influence of the lattice temperature on relevant scattering rates, carrier lifetimes and the population inversion is investigated in both cases.

A microscopic investigation of hot-carrier vertical transport phenomena in semiconductor-based infrared photodetectors is presented. In order to overcome the intrinsic limitations of the conventional Monte Carlo method in describing the electro-optical response of such quantum devices—i.e. huge photocurrent fluctuations—a novel weighted Monte Carlo scheme is proposed. As shown by our simulated experiments, this new Monte Carlo strategy, particularly suited for the analysis of steady-state conditions, allows for a fully three-dimensional analysis of the basic microscopic processes governing new-generation infrared photodetectors.

Using microprobe photoluminescence we measured the electronic and lattice temperatures in operating quantum cascade lasers having similar chirped-superlattice active regions but different (GaAs/AlAs or GaAs/Al_0.45Ga_0.55As) conduction band discontinuities. Our results demonstrate the establishment of a thermalized hot-electron distribution. Coupling between the electronic ensemble and the lattice increases with the band-offset and influences the optical characteristics of the devices.

We first present a brief review of the basic requirements, proposals and prototype implementations for quantum computing devices. Then we introduce a specific universal set of quantum gates and describe a physical system, based on coupled quantum wires, that should be able to realize the proposed quantum transformations. Finally, the dynamics of entanglement-creation between two ballistic electrons is analysed for a model system.

With the scaling of semiconductor devices to nano-size dimensions, gate lengths reach their physical limits of 8–15 nm. In such devices, a variety of undesirable effects begin to be prominent, including gate leakage, discrete impurity effects and source-to-drain tunnelling. To address the issue of source-to-drain tunnelling we apply to a prototypical 10 nm MOSFET device a novel highly efficient method, termed contact block reduction (CBR) method. It has been reported recently [1] that the CBR method allows us to calculate the ballistic current (transmission function) through an arbitrarily shaped, multi-terminal two- or three-dimensional device. Here we show that in the ballistic case the density matrix of the open device can be computed using the same method. This opens a possibility for an efficient fully quantum-mechanical self-consistent calculation within the ballistic limit.

Self-consistent full-band Monte Carlo simulations are employed to compare the performance of nanoscale strained-Si single-gate (SG) and unstrained-Si double-gate (DG) MOSFETs for a gate length of 25 nm. Almost the same on-current as in the DG-MOSFET can be achieved by strain in a SG-MOSFET for the same gate overdrive. This is due to the compensation of the higher electron sheet density in the two inversion channels of the DG-MOSFET by the higher strain-enhanced velocity in the channel of the SG-MOSFET. The on-current of the strained-Si SG-MOSFET is almost 10% larger for a channel orientation along the crystallographic ⟨100⟩ direction than for the ⟨110⟩ direction. This confirms that the on-current is determined by quasi-ballistic transport, because the maximum enhancement of the in-plane velocity in bulk (001)-strained Si in the ⟨100⟩ direction is 5% at medium electric fields (the low-field mobilities and saturation velocities are the same and the difference in the thermal injection velocities is negligibly small), whereas the transient bulk velocity overshoot peak is 30% larger in the ⟨100⟩ direction.

We present a Monte Carlo analysis of InAlAs/InGaAs-based four-terminal ballistic rectifiers operating at room temperature. The rectifying effect is explained in terms of the vertical asymmetry of electron concentration originating, in the presence of ballistic transport, from the action of the obstacle located at the centre of the structure. An increase in temperature is found to degrade the performance of the structures as transport becomes more diffusive. Different geometries of the rectifier are analysed.

The coherent control of excitonic and biexcitonic polarization in pulse-transmission and four-wave-mixing (FWM) experiments is analysed with real time resolution. As a model system a ZnSe single quantum well is exclusively excited at the heavy-hole exciton and exciton–biexciton resonance. In pulse-transmission experiments the coherent control of excitonic polarization is shown for low excitation intensities. At elevated excitation intensities the dephasing time of the polarization is found to be strongly dependent on the relative phase of two phase-locked laser pulses which perform the coherent manipulation of the polarization. In FWM experiments the separate coherent control of the excitonic and biexcitonic FWM polarization is demonstrated which enables a coherent manipulation of the amplitude and phase of exciton–biexciton beats which are observed on the FWM polarization transients.

We have investigated the impact of quantum-mechanical space-quantization effects on the operation of a narrow-width SOI device structure. The presence of a two-dimensional carrier confinement results not only in a significant increase in the threshold voltage but also in its pronounced channel width dependence. In this work, we have used classical 3D Monte Carlo particle-based simulations. Quantum-mechanical space-quantization effects have been accounted for via an effective potential scheme. In a second effort, we have studied the fluctuations in the device characteristics introduced by discreteness of charge in the channel region of the device.

We have investigated threshold voltage for each gate length in order to find the optimum value for side gate length and side gate voltage. We find clear on–off stages according to the bias conditions of both gates when side gate voltage is above 2 V and side gate length is above 70 nm. We have obtained the minimum short channel effect for a double-gate metal oxide semiconductor field effect transistor (DG MOSFET). We know that the optimum side gate voltage for each side gate length is about 2 V in the case that the main gate length is 70 nm. The cut-off frequency value is 41.4 GHz at L_SG = 70 nm. As a result, the DG MOSFET has excellent electrical characteristics and very high cut-off frequency.

We present a theoretical study of the realization of a quantum logic gate based on a laterally structured high mobility GaAs/AlGaAs 2DEG. The I–V characteristics exhibit a pronounced switching behaviour which demonstrates that quantum logic gates can be fabricated and operated experimentally. The calculations have been performed with the recently developed contact block reduction method. We show that this method can be readily extended to multi-band k ⋅ p-Hamiltonians while retaining its efficiency.

Scaling of conventional MOSFETs to channel lengths of 20 nm and below will require channel doping in the range of 10¹⁹ cm⁻³. At such size and doping there are only around ten dopant atoms within the channel, resulting in significant statistical mobility variations. Simulation of charge transport in this region must reflect the discreteness of the doping, both in position and number, in order to incorporate the mobility variations, from device to device, due to random dopant variations that are known to be important. Here, we discuss the particle–particle particle–mesh method employed within 3D ensemble Monte Carlo simulations in order to include ab initio ionized impurity scattering.

We show that spurious peaks in velocity profiles emerging from hydrodynamic simulators of submicron n⁺nn⁺ Si structures are washed out by using a field-dependent thermal conductivity calculated within a correlation function formalism.

We report on the low-frequency noise of the drain currents (at and above 8 GHz) in nanometre double-gate MOSFETs at room (T = 300 K) and low (T = 77 K) temperatures simulated by ensemble Monte Carlo method. The drain current fluctuations reveal the excessive noise relative to the equilibrium thermal level that is attributed rather to hot-carrier effects than to shot noise.

Measurements of optical Rabi oscillations in the excited states of individual InGaAs are presented. Under pulsed resonant excitation we observe Rabi oscillations with increasing pulse area, which are damped after the first maximum and minimum. We show that the observed damping comes from an additional non-resonant generation of carriers in the quantum dot. The observation of Rabi oscillations provides an efficient way of directly measuring the excitonic transitions' dipole moments. A polarization anisotropy of the dipole moment is resolved in some of the quantum dots.

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in a short-gate MOSFET. For this purpose, a careful description of electron–ion interaction suitable for the case of discrete impurities has been implemented in a 3D (three-dimensional) particle Monte Carlo simulator. This transport model is applied to the investigation of a 50 nm MOSFET operation. The paper shows that a small change in the position of a single discrete impurity may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but rather to variations in transport properties in the inversion layer.

In this paper we focus on the problem of interface roughness scattering which is treated non-perturbatively by incorporating the effects of scattering as a boundary condition for the Boltzmann transport equation. The scattering probability is derived and implemented in an ensemble Monte Carlo simulation of electron transport in relaxed and strained Si n-MOSFETs. We compare the internal device behaviour as a function of the interface quality between short channel relaxed and strained Si n-MOSFETs. We provide further evidence that the strained Si/SiO₂ interface is less rough, requiring a longer correlation length and smaller rms height for the strained Si/SiO₂ interface. The high doping densities encountered necessitate the self-consistent use of degenerate (Fermi–Dirac) statistics.

The solution method for the Boltzmann transport equation based on the spherical-harmonics expansion has been applied to the transport problem in a Si–SiO₂ structure. The solver has been improved in order to fully simulate MOS structures. At the Si–SiO₂ interface, the solution of the Schrödinger equation takes into account the carrier tunnelling across the energy-barrier discontinuity. Results of a 2D simulation are shown for different devices to validate the model.

Shot noise suppression in double barrier resonant tunnelling diodes with a Fano factor well below the value of 0.5 is theoretically predicted. This giant suppression is found to be a signature of the coherent transport regime and can occur near zero temperature as a consequence of the Pauli principle or above about 77 K as a consequence of long range Coulomb interaction. These predictions are validated by experimental data.

We study the resistance fluctuation distribution of a thin film near electrical breakdown. The film is modelled as a stationary resistor network under biased percolation. Depending on the value of the external current, on the system sizes and on the level of internal disorder, the fluctuation distribution can exhibit a non-Gaussian behaviour. We analyse this non-Gaussianity in terms of the generalized Gumbel distribution recently introduced in the context of highly correlated systems near criticality. We find that when the average fraction of defects approaches the random percolation threshold, the resistance fluctuation distribution is well described by the universal behaviour of the Bramwell–Holdsworth–Pinton distribution.

Femtosecond electron dynamics in GaAs is investigated tracing ultrafast modifications of the Franz–Keldysh absorption spectrum. A sophisticated heterostructure design allows for a study of unipolar transport as well as a nanometre scale definition of layers for both carrier injection and detection of the propagating electron ensemble. Transit times through the structure are directly measured for electric fields between 7 kV cm⁻¹ and 180 kV cm⁻¹ comparing room temperature operation to results for T_L = 4 K. For electric fields F ⩾ 400 kV cm⁻¹ a clear signature of impact ionization and avalanche multiplication on a picosecond time scale is found.

We show the existence of an upconversion of low-frequency intergroup noise by means of Monte Carlo simulations in bulk materials and semiconductor structures operating under periodic large-signal regime. The intergroup noise is associated with stochastic transitions in momentum space of carriers between regions characterized by different dynamics originated by the presence of hot-carrier conditions.

Electron transport in 3 µm long InN n⁺nn⁺ structures is theoretically investigated by the Monte Carlo particle technique at low lattice temperatures when optical phonon emission is the dominating scattering mechanism. It is shown that at constant bias a free-carrier grating can be formed inside the n-region. Such a grating is found to be responsible for microwave power generation in the THz frequency range.

We simulate transport and fluctuations in 0.3 µm gate length n-MOSFETs operating in the hot-carrier regime using ISE-TCAD software. Our results are compared with other results given by microscopic simulations. As a first step, drift-diffusion and hydrodynamic models are employed to study the static and dynamic characteristics of these devices in common-source configuration. The velocity overshoot and other hot electron phenomena in the pinch-off region of the channel as well as the admittance parameters and other elements of small-signal equivalent scheme are in good agreement with the results of Monte Carlo simulations. The calculation of the spectral density of current fluctuations is implemented within the two-dimensional direct impedance field method of ISE-TCAD software at RF frequencies, when current fluctuations are dominated by diffusion noise. Among the two macroscopic models, only the hydrodynamic scheme simulates correctly the excess noise due to the presence of hot carriers.

We consider the coherent dynamics of electrons in double quantum wells under ultrafast intersubband photoexcitation. We analyse the effect of the Rabi oscillations both on the population redistribution and on the dipole moment oscillations which gives rise to the THz emission from the excited structure. The crossover from the quantum beating to the Rabi oscillations has been analysed.

We have investigated the nonequilibrium transport of a two-dimensional electron gas (2DEG) in an InAs/GaAs(111)A heterostructure coupled with superconducting Nb electrodes. In the superconductor (S)/normal conductor (N) hybrid system, the transport properties of the N are affected by the superconducting proximity effect. According to the updated theory, this proximity correction to the conductance (PCC) shows the reentrant behaviour to be a function of energy. We observed reentrant behaviour in the voltage and temperature dependences of the conductance of the 2DEG. The conductance peak is observed at T_cmax ∼ 6 K and the PCC reaches ∼20% of the normal-state conductance and almost disappears at low energies. These values are one order of magnitude higher than those obtained in the previous works. These results are most likely achieved by diminishing the length of the N channel L because the PCC as well as the correlation energy (Thouless energy) is proportional to 1/L². The measured conductance showed a very good agreement with that of the theory.

We have investigated photo-generated carrier transport (photocurrent) in the p–n junction experimentally and theoretically. The photocurrent was measured with various laser-pulse intensities under different reverse bias conditions applied at the p–n junction. For a low laser intensity, the measured photocurrent maintains the same shape as the irradiated laser pulse with diminished time delay. Increasing the laser intensity, however, transport delay of the measured photocurrent is observed with broadened pulse shapes, especially under weak bias conditions. This broadened photocurrent shape could not be reproduced by conventional two-dimensional (2D) device simulations, which solve the basic device equations. Under highly generated carrier concentration, it is demonstrated here that the carrier transport is mostly governed by non-equilibrium carrier dynamics, which is not included in the conventional device equations.

Gallium nitride with its high negative differential mobility threshold is an appealing material for high power millimetre-wave oscillators as a Gunn diode. By means of extensive ensemble Monte Carlo simulations, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz is studied in detail. Their operations are checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations cease. The degeneracy effects due to the Pauli exclusion principle are also considered, but their effects are seen to be negligible within the realistic bounds of the Gunn diode operation.

We present a study of the thermal noise locally generated in a Si/SiGe MOSFET. We use a Langevin stochastic noise source model in order to obtain the noise generated locally by random velocity fluctuations. Physical magnitudes governing local noise have been obtained using a two-dimensional device simulator that implements the hydrodynamic transport model for majority carriers. Local calculation of the spectral density of noise current, S_in, pinpoints the importance of an accurate physics-based noise model in order to fully understand the impact of technological parameters on the device noise performance. Our study shows that a reduction in the channel length will increase the local value of the noise sources across the channel. Results also suggest that a reverse doping of the channel could lead to significant reduction of S_in.

Monte Carlo simulations show that electrons do perform coherent, real-space THz oscillations in suitably designed space charge potentials, if the kinetic energy is restricted (by the choice of the potential depth) below threshold for intervalley scattering. Oscillations have been detected by femtosecond pump and probe spectroscopy and electro-optic sampling in V-shaped as well as in parabolic potentials. A complete calculation of the observed optical signal is in good agreement with the experimental findings.

In this work, the RF and microwave noise performance of bulk and fully-depleted silicon-on-insulator (FD SOI) transistors with directly comparable topologies is investigated by means of an ensemble Monte Carlo simulator, and in the case of FD SOI devices, the influence of varying the active layer thickness is also studied. Intrinsic noise sources are calculated and analysed, and important figures of merit such as NF_min or R_n are also determined and evaluated. Moreover, a microscopic understanding of the noise phenomena is achieved, which is essential for the optimization of the transistors. FD SOI devices show a reduced influence of induced gate noise and drain noise as compared to the bulk transistor due to the minor influence of phonon scattering and energetic carriers at the pinch-off region. Whereas reducing the active layer thickness minimizes the effect of hot carriers in the noise parameters, it can lead to a degradation of other important high-frequency figures of merit such as g_m or f_T.

The performance potential of an 80 nm physical gate length MOSFET with GaAs channel and high-k gate insulator is investigated using Monte Carlo simulations. The results show that such a device could deliver a 100–125% increase in the drive current at both low and high drain biases compared to equivalent Si based MOSFETs. Various transport model enhancements including the Fermi–Dirac statistics and the interface roughness scattering have been systematically studied in order to attain a more realistic prediction of the device performance.

We perform ab initio band structure calculations within density functional theory using an exact exchange formalism and a local density approximation for correlations. Ensemble Monte Carlo simulations consider all relevant scattering mechanisms including a realistic impact ionization rate. This full-band ensemble Monte Carlo method is applied to study the high-field electron transport. Results can be given for the total electron–phonon scattering rate, the drift velocity, the mean kinetic energy, the valley occupations and the ionization coefficient. We show here exemplary results for the wide band-gap materials ZnS and GaN.

We analyse the noise temperature versus applied voltage of ballistic n⁺–n–n⁺ nanodiodes. We show that by increasing the doping density of the ballistic channel the minimum of the noise temperature decreases. In particular, noise temperature values can be suppressed below the absolute minimum value predicted for undoped n⁺–i–n⁺ ballistic diodes. This result could be of interest for the development of low-noise ballistic devices.

A gauge-invariant formulation of high-field transport in semiconductors is proposed. We revisit the conventional description of carrier–phonon scattering within the Fermi golden rule scheme by means of a gauge-invariant generalization of the scattering rates. With a density-matrix approach, we show that the so-called intracollisional field effect, as usually accounted for, has always been overestimated due to the neglect of the time variation of the basis states, which in turn leads to a ill-defined Markov limit in the carrier–phonon interaction process. This is confirmed by a fully three-dimensional simulation of charge transport in state-of-the-art semiconductor superlattices.

Two-pair correlations are shown to induce dependences of the nonlinear optical response of semiconductors that can be used for a coherent manipulation of the resulting emission. Influences of biexcitonic transitions as well as transitions to two-pair scattering states are discussed within a microscopic density matrix approach. It is shown that the coherent control of biexciton beats is selective with respect to the propagation direction of the emitted signal, in contrast to the control of beats where all beat partners are generated by single photon processes. Furthermore, it is demonstrated that memory structures induced by exciton–exciton scattering states enable coherent manipulations of the excitonic line shape that allow us to estimate lower bounds for the corresponding memory time from experimentally accessible data.

We present a generalization of the nonequilibrium Green's function formalism to the treatment of dynamically open systems, such as the active region of a modern small semiconductor structure. This approach relies on the partial-trace-free, time-convolutionless approach to the open-system reduced density matrix. We illustrate how one can analyse the short-time evolution of an open system within the present approach, and obtain corrections to the single-particle spectrum and level broadening.

We demonstrate the creation of a transient spin-polarized current grating in bulk GaAs at room temperature. The spin current is injected through quantum interference between two-photon absorption of fundamental (1.55 µm) and one-photon absorption of second harmonic (775 nm) pulses. Because the fundamental and second harmonic pulses do not propagate collinearly, a spin current grating is formed, which decays by electron diffusion.

The Wigner equation was used for the calculation of carrier transport in mesoscopic devices. The carrier transport has a coherent part determined by the Wigner potential V_w and a dissipative part accounting for the interaction with phonons. Models have been developed which solve the equation by using quasi-particles evolving over pieces of classical Newton trajectories. In a backward evolution approach [1], the interaction with the Wigner potential is interpreted as a scattering process. The quantum information is carried by the particle weight. In the ensemble model proposed in [2], the quantum information from V_w is included by the particle affinity. Both weight and affinity are artificial numerical quantities. We propose a model which interprets the Wigner equation with a Boltzmann scattering term as a Boltzmann equation with a generation term. The quantum information is carried by the sign of the quasi-particles. In all other aspects quasi-particles behave as classical particles. The sign has a physical meaning since particles of opposite sign can annihilate. The model ensures a seamless transition between classical and quantum regions. Negative values of the Wigner function are explained in a direct way.

We study the dynamics of an electronic wave packet in a quantum wire with an embedded quantum dot interacting with an LO-phonon bath at room temperature. Two different cases are studied: in the first scenario (escape scenario), we determine the probability for the electrons to leave the dot by absorption of an LO-phonon. In the second scenario (capture and escape scenario), we study the capture process from the delocalized states of the wire into the localized states of the dot accompanied by re-emission into the delocalized states. These scenarios are analysed by using a quantum kinetic theory for the electron LO-phonon interaction. The thermal escape scenario shows a continuous flow of density directed with equal probabilities to both sides from the dot accompanied by small oscillations in the density of bound carriers. For the capture and escape scenario, we find an interplay between thermal escape and a coherent escape process, the latter being present even at zero temperature. In the coherent escape, a part of the captured density re-escapes and forms additional travelling wave packets following the original wave packet.

On the basis of the equivalence of the principle of detailed energy balance when expressed at micro- and macro-level, we propose a quantum generalization of the viscous friction law for a macroscopic Langevin equation describing thermal fluctuations of the damped harmonic oscillator.

A quantum transport formalism based upon the balance equations for momentum and energy will be used to investigate nonequilibrium steady-state transport in mesoscopic systems. We show that our formalism is able to probe the global and local properties of mesosopic systems. We will derive the Landauer–Büttiker (LB) conductance for a quantum point contact (QPC) and show that dissipation is induced in the QPC. As a result the notion of ballistic transport is questioned, because locally dissipation is enhanced in the QPC.

A method is presented for treating particle–particle interactions that can be used within Monte Carlo simulations that include quantum potential corrections to the classical potential profile. We use the effective potential method to treat the quantum confinement effects, along with a new molecular dynamics implementation to treat short-range particle interactions. We demonstrate the need for including electron–electron interactions to properly examine energy relaxation of hot electrons in the drain. We also demonstrate the influence of discrete dopants on the uniformity of current flow in ultra-thin SOI nMOSFETs.

In electron tunnelling from channel to gate in a field effect transistor, and in alpha-particle emission from atomic nuclei, the tunnelling transition is from a quasi-level to a continuum of states, to which conventional tunnelling theory does not apply. Gamow, in the alpha-emission case, proposed to consider particle states with an imaginary component of energy, , where λ is the tunnelling decay rate. It is shown in this paper, in terms of an appropriate real wavefunction ψ(x), that λ is then given by u/λ = ∫^∞₀r² dx, where r(x) = ψ(x)/ψ(0) and where u = ℏk/m is the velocity of the tunnelling particle. The application of this formula is discussed. The connection with a quantum resonance of the electron system is shown.

In this work we present a recently developed transport model, based on the Wigner-function approach and allowing for non-parabolic band profiles. Two scattering mechanisms are included by means of a Boltzmann-like collision operator, describing the collisions between electrons and polar optical and intervalley phonons. The transport equation for the Wigner function is solved by the splitting-scheme algorithm. We have chosen a one-dimensional model band profile, which exhibits satellite valleys, besides the minimum at the centre of the Brillouin zone, similar to the band profile of GaAs.

The effect of exchange-interaction on the two-terminal conductance of fully ballistic samples is studied using a many-particle wave packet formalism. The approach shows that the puzzling nonuniversal conductance quantization for long quantum wires can be explained as a fermion exchange interaction effect. The Landauer conductance step G_o ≡ 2e²/h is obtained when two-electron interaction is considered. Reductions of G_o are due to a wave packet many-particle exchange interaction effect. For ballistic GaAs/AlGaAs quantum wires longer than 1 µm, a conductance step reduction is obtained in agreement with experimental results.

In compound semiconductor crystals with a substantial degree of ionicity Froelich interaction of charge carriers with longitudinal optical (LO) phonons is strong enough to produce detectable polaron effects. In this paper polaron effects are analysed using a quantum theory of electron transport based on the momentum- and frequency-dependent Wigner function f_w(p, ω), defined starting from the G^< Green function, simply related to the electron spectral function A(p, ω). The theoretical approach considers the dynamical evolution of the electron Wigner function in the presence of phonon scattering. An elaboration of the quantum dynamical equation in terms of Wigner paths formed by free flights and scattering events is used. These paths are especially suitable for a Monte Carlo solution of the transport equation for the Wigner function very similar to the semiclassical traditional Monte Carlo solution of the Boltzmann equation. Numerical results for GaAs and CdTe in a variety of physical conditions are presented.

In mesoscopic systems the Wigner function picture is used for studying electron quantum transport. In this work we present theoretical results on the effect of contact proximity in nanometric devices. If the correlation of the electron wavefunction vanishes inside the contact regions, the transport properties inside the device are affected. We have verified this influence analytically. The main effect is the dependence of the tunnel current through potential barriers on the distance between the contacts. Using three different simulation schemes we have observed modifications in the electron density and conductance for simple devices depending on the distance between the contacts.

A density-matrix approach for the description of quantum-transport phenomena in systems with open boundaries is proposed. We shall show that the application of the conventional Wigner-function formalism within a source-term boundary-condition scheme leads to unphysical results, such as injection of coherent electronic states from the contacts. To overcome such basic limitations, we propose a generalization of the standard Weyl–Wigner formulation. The latter can be regarded as a quantum mechanical derivation of the phenomenological injection model commonly employed in the simulation of open quantum devices.

A novel experimental technique, combining near-field optics and femtosecond pump–probe spectroscopy, is demonstrated to analyse the coherent nonlinear optical response of single quantum dots on ultrafast time scales. The technique is used to study the effects of strong non-resonant light fields on the optical spectra of single excitons in interface quantum dots. Transient reflectivity spectra show dispersive line shapes reflecting the light-induced shift of the quantum dot resonance. The nonlinear spectra are governed by the phase shift of the coherent quantum dot polarization acquired during the interaction with the light field. The phase shift is measured and ultrafast control of the quantum dot polarization is demonstrated.

The effect of continuous-wave (cw) laser illumination on THz radiation from semiconductor surfaces such as n- and p-type GaAs, semi-insulating (SI-) GaAs and SI-InP has been studied. The THz radiation power for SI-InP decreases under cw laser illumination, which is explained by the screening of the surface electric field by the photogenerated carriers. On the other hand, an increase of 40% of the THz radiation power is observed for SI-GaAs by cw laser illumination with 167 mW. The origin of this increase is discussed in relation to the metastable defect and temperature rise in SI-GaAs.

An experimental study of extreme nonequilibrium transport of charge carriers in Si and SiGe p–i–n diodes at fields up to 420 kV cm⁻¹ is presented. In Si the carriers are photoinjected up to 400 meV above the conduction band edge by a 10 fs laser pulse. Our measurements allow a direct observation of the average carrier acceleration at room temperature. The observed dynamics in SiGe turns out to be slower than in silicon. In Si, we find indications for coherent feedback processes of impulsively excited phonons onto the carriers for the highest fields applied.

We have detected using superconducting bolometers the phonons generated by ultra-fast pulsed laser excitation of gallium arsenide/aluminium arsenide superlattice structures. In addition to the enhancement of the longitudinal acoustic (LA) phonon previously reported, we observe an enhancement of the signal due to ballistic transverse acoustic (TA) phonons when the condition for coherent phonon generation is satisfied, that is, the pump photon energy is equal to the exciton energy in the quantum wells. TA modes are not Raman active in (001) GaAs, therefore impulsively stimulated Raman scattering (ISRS), believed to be the mechanism for generation of LA modes, is unlikely to be responsible for the TA emission. We consider other processes which might be responsible for generation of the TA phonons.

The dynamics of interband transitions in InGaAs/AlGaAs heterostructures with funnel-shaped quantum wells, when a narrow and deep well is positioned asymmetrically in wide and shallow ones, has been studied experimentally. These structures have been proposed as a dual-colour laser in mid- and near-IR range simultaneously. The lifetime of electrons in the ground and excited states in the quantum wells has been determined by the time of the photoluminescence intensity decline. The obtained lifetime values allow precise identification of peaks in the photoluminescence spectra and a deep insight into the process of high electron states populating. The experimentally found electron lifetime for the interband transitions corresponds on the order of magnitude with the interband transitions lifetime in GaAs. Analysis of the photoluminescence spectra as well as the time dependence of the photoluminescence intensity decline indicates a possibility of population inversion achievement in these structures under high excitation.

We demonstrate that exciton–exciton correlations strongly affect FWM signals. In particular, A exciton–B exciton correlation (bound and unbound AB-biexcitons), which can be generated even in co-circular polarizations, has a significant qualitative impact on the optical response of bulk ZnO.

We investigate the coherent nonlinear response of a 2D electron gas under ultrafast mid-IR excitation. Laser-induced Rabi oscillations result in population redistribution between resonantly excited subband levels. We calculate temporal redistributions using non-Markovian balance equations, taking into account population relaxation and dephasing of the intersubband polarization. Detection schemes, based on terahertz emission of radiation from non-symmetric quantum wells, as well as a two-colour pump probe method, are discussed for signatures of intersubband Rabi oscillations.

We report measurements of the temperature and excitation energy dependence of the photoluminescence (PL) decay time of InAs quantum dots (QDs) embedded in a GaAs quantum well (QW). The investigated structure displays high PL efficiency up to high temperatures (>300 K) due to the effect of the barriers which inhibit carrier escape. It is therefore an ideal structure for device applications, e.g., QD lasers. In the work described here, we have made a detailed study of the recombination dynamics in a QD/QW structure. In particular, we focus on the temperature dependence of the QD PL decay time which in recent years has been a matter of some controversy. In QDs the radiative lifetime is believed to be independent of temperature and so any temperature dependence must be due to other processes, e.g., carrier relaxation and redistribution. We find that, for non-resonant pumping and at low temperatures, the presence of the barriers slows down the PL decay time. However, at high temperatures and/or for resonant pumping of the dots, the barriers have less effect and the PL decay time is governed by carrier redistribution between the dots.

Rabi oscillations of intersubband transitions in n-type modulation-doped GaAs/AlGaAs multiple quantum wells are directly observed by electro-optic sampling of ultrashort THz transients with large electric-field amplitudes transmitted through the sample.

We present time-resolved measurements of photoinduced intraband absorption in undoped GaAs/AlGaAs quantum well and InAs/GaAs self-assembled quantum dot structures after interband excitation. By using ultrabroadband mid-infrared pulses we determine the temporal evolution of the intersubband absorption spectrum of a double quantum well sample (subband spacing < longitudinal optical phonon energy) after excitation. From these measurements we determine an intersubband relaxation time of 14 ps at T = 5 K. By tuning mid-infrared pulses into resonance with intraband transitions between confined quantum dot states and the wetting layer continuum, the electron population of the quantum dot ground and first excited states is determined as a function of time-delay after the pump. We find that the most efficient relaxation pathway into the quantum dot ground state is the step-wise relaxation through the excited states of the dot. The capture time at T = 5 K is 4.7 ps.

The carrier dynamics and the collective properties of a photogenerated quasi-two-dimensional electron gas in an asymmetric undoped GaAs/AlGaAs quantum well (QW) structure are investigated using an all-optical interband pump–intersubband probe absorption experiment. At a carrier density of 1 × 10¹² cm⁻² per QW, an e₁–e₂ absorption band at 126 meV is found. Transient resonance frequency blueshifts with respect to the single-particle resonance as well as a transient spectral band narrowing are observed. At the carrier density 1 × 10¹³ cm⁻² per QW, the e₂–e₃ absorption band at 99 meV with a spectral width of 5 meV is measured. This band is blueshifted by 30 meV through collective effects and broadened at an excitation density of 3 × 10¹³ cm⁻² per QW.

We present picosecond time resolved photoluminescence measurements of GaAs/AlGaAs quantum dot structures—grown by modified droplet epitaxy—where the dimensionality of the barrier states has been varied from 2D to 3D. This is realized by the insertion of a variable thickness quantum well at the quantum dot base. We find a fast carrier relaxation, which is made faster by the increase of the photogenerated carrier injection in all the samples due to Auger-like mechanisms. Hence the dimensionality of the electronic states in the barrier is not relevant in determining the quantum dot capture, in contrast with the conclusions of several models so far presented in the literature. Our findings suggest that the Auger-like processes only involve the zero-dimensional levels of the quantum dots.

We present a study of time-dependent transmission spectra of a modulation-doped Cd_1−xMn_xTe/Cd_1−y−zZn_yMg_zTe quantum well with variable hole gas concentration. The temporal evolution of the neutral and charged exciton absorption intensities was analysed. We found that the dominant effect is an 'oscillator strength stealing' from charged to neutral excitons. Due to the fact that charged exciton intensity is proportional to spin subband population the spin flip of heavy holes was observed.

We show that the interaction between carriers confined in a quantum dot and the surrounding lattice under external driving of carrier dynamics has a dynamical, resonant character. The quality of Rabi oscillations in such a system depends on the relation between nonlinear spectral characteristics of the driven dynamics and the spectral density of effectively coupled lattice modes (phonon frequencies and density of states). For a large number of Rabi oscillations within a fixed time (allowed by, e.g., exciton recombination) the spectrum of the dynamics extends towards high frequencies, coming into resonance with acoustic and optical phonons. Thus, this resonant lattice response strongly restricts the possibility of fully coherent control over the charge state in a quantum dot.

In this study, a full-band particle-based simulator is used to model photo-generated electron–hole pairs in Si to investigate ultra-fast charge transport. This work is motivated by previous simulations of transient transport in III–V material as well as recent experimental measurements of optically excited Si pin diodes. The simulation results show evidence of velocity overshoot at high fields (greater than 100 kV cm⁻¹), and can be attributed to momentum relaxation.

We present a calculation of terahertz (THz) radiation by coherent optical phonons in GaAs p–i–n diode structure under high electric fields. The ultrafast dynamics of photoexcited carriers is simulated by adopting a self-consistent ensemble Monte Carlo method. It is demonstrated that the coherent phonon oscillations emitting the THz radiation are excited resonantly by synchronization between the proper lattice oscillation and ultrafast transient of electronic polarization originating from a drift overshoot within the initial period of lattice vibration.

The heat transport in heavy-doped n-GaAs has been investigated at temperatures T = 300 K and 77 K using the irradiation of the metal–semiconductor contact by modulated CO₂-laser radiation. It is shown that this approach gives an opportunity to determine the thermo-diffusion coefficient χ and Seebeck coefficient S_T without direct measurements of the temperature gradient. It was also found that the ratio of the characteristic heated-zone scale to the excited heat wavelength exceeds by an order of magnitude the assessment that can be obtained from the reference data for GaAs. To elucidate the origin of the observed phenomenon the measurements were conducted with Schottky contacts made on the thin-doped GaAs layer epitaxially grown on the semi-insulating GaAs substrate. In this case, the degenerate electron gas occupies only an insignificant part of the heat-conducting medium. In addition, the injection of hot electrons into the semiconductor by current pulses through the Schottky barrier was used to clear up whether there is a dependence of the effect on the method of electron heating. The nonequilibrium of LO-phonons and the change in the electron–phonon collisional integral due to the nonequilibrium pair correlations of the electrons are suggested as a possible explanation.

Strong coupling of a confined exciton to optical phonons in semiconductor quantum dots (QDs) leading to the formation of a polaron is considered for a model system including two lowest exciton states and several optical phonon modes. Both intra- and inter-level terms are taken into account. The Hamiltonian has been exactly diagonalized including a finite number of phonons allowed for each mode, large enough to guarantee that the result can be considered exact in the physically important region of energies. Based on this polaron spectrum, the Raman scattering probability is obtained, which is compared with the one calculated using the standard perturbation theory approach. It is shown that, when either diagonal or non-diagonal coupling is sufficiently strong, the Raman spectrum line shape and especially its resonant behaviour differ considerably from the perturbation theory predictions. The dependence of the scattering intensity on the excitation wavelength contains features similar to those expected in the optical absorption spectra of QDs.

Polaron relaxation in n-type InAs quantum dots has been studied by picosecond time-resolved pump–probe spectroscopy. Due to inhomogeneous broadening of the absorption features associated with transitions between s-like ground state and p-like first excited state, the energy dependence of the polaron decay time has been measured over a wide spectral region from 40 to 60 meV. The polaron lifetime increases continuously from 20 ps at 40 meV to 65 ps at 54 meV. By analysing the temperature dependence of the polaron lifetime the main polaron decay channel has been identified as the cubic overtone (2LA) decay channel. By fitting the experimental data we extract the LO-phonon lifetime of 10 ± 2 ps (at 33 meV) for InAs quantum dots and electron–phonon coupling strength of 6.2 ± 0.5 meV.

Bound electron–hole pairs—excitons—are Bose particles with small mass. Quantum degeneracy of exciton gases and eventually exciton Bose–Einstein condensation is expected to occur at a few Kelvin—a temperature many orders of magnitude higher than for atoms. Experimentally, an exciton temperature well below 1 K is achieved in coupled quantum well semiconductor nanostructures. In this contribution, we briefly review experiments revealing bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons. Experiments with exciton rings in the photoluminescence patterns of excitons and macroscopically ordered exciton state are also presented.

We review and discuss the first fully three-dimensional study of non-equilibrium carrier dynamics governing semiconductor-based intersubband optoelectronic devices, such as quantum-cascade lasers. First, a multisubband Monte Carlo simulation scheme in a kinetic Boltzmann-like approach is presented. Then, its generalization into a density-matrix quantum-transport formalism is discussed. This allows us to directly access microscopic key features of the electron relaxation dynamics (without resorting to phenomenological parameters) as well as to investigate the nature, coherent versus incoherent, of charge injection/transport processes. Applications to state-of-the-art mid-infrared quantum-cascade lasers and novel far-infrared emitters are reviewed. The extremely good agreement between theoretical results and experimental findings demonstrates that our approach is a valid and predictive tool for the understanding of charge transport in these quantum devices.

We present a microscopic quantum theory of spatially resolved photoluminescence in quantum wells with interface fluctuations. The theory can model low-temperature photoluminescence and photoluminescence excitation experiments performed in illumination, collection, illumination–collection mode or diffusion experiments where the spatial positions of excitation and collection are scanned independently. Numerically calculated two-dimensional images clarify the impact of the microscope setup on the obtained images and resolutions.

Fast optical nonlinearities which arise in dc-biased GaAs crystal have been investigated numerically and experimentally using the time-resolved picosecond four-wave mixing technique. Hot electron transport induced enhancement of optical nonlinearity as well as oscillatory behaviour of light diffraction efficiency in the subnanosecond time domain has been observed in the region of negative differential resistance. The kinetics of the four-wave mixing is compared with the numerical calculation of refractive index modulation taking into account coexisting free-carrier and quadratic electrooptic nonlinearities. The numerical modelling of hot electron dynamics in a spatially modulated GaAs crystal enabled us to explain the experimentally observed features and extract the modulation coefficient of quadratic electrooptic nonlinearity.

We have observed marked oscillations in the time-resolved photoluminescence of a semiconductor microcavity under non-resonant excitation conditions. Hot excitons, created with an ultrashort light pulse, rapidly relax into polaritons in the cavity with a large in-plane momentum K. Shortly after illumination, above a certain excitation power, the polaritons accumulate into an energy trap at the bottom of the dispersion and the light emission, governed by final-state stimulated scattering, starts at K ∼ 0 (0° with respect to the normal of the sample). The angular dependence of the photoluminescence at negative detunings reveals that the emission is rapidly transferred to K ∼ 2 × 10⁴ cm⁻¹ (∼15°), close to the point of inflection of the lower polariton branch, giving rise to an annular emission. The oscillations arise from a macroscopic coherent population in the lower polariton branch.

We investigated the shift in emission wavelength of GaAs-based distributed feedback quantum cascade lasers during pulsed operation. This shift is due to current-induced self-heating. Together with a fast detector this shift in the emission spectra can be used to investigate absorption lines in ammonium isotopes. The linewidth of these single mode devices was determined to be below 0.1 cm⁻¹. These measurements reveal the potential of these devices for chemical sensing.

Time-resolved absorption bleaching and four-wave mixing techniques have been used for studies of spin relaxation and nonequilibrium carrier dynamics in resonantly excited InGaAs/GaAs multiple quantum well structures. Kinetics of absorption bleaching at 1064 nm provided spin relaxation times of 240–280 ps and carrier lifetimes of 750–1000 ps at low excitations. At higher excitations, nearly three-fold increase of spin-relaxation rate has been measured by the FWM technique, while the decrease of carrier lifetime was much less pronounced. The kinetics of nonequilibrium carrier grating with randomized spins allowed determination of the in-plane bipolar diffusion coefficient in the wells D = 11.5 cm² s⁻¹ and hole mobility of 230 cm² V⁻¹ s⁻¹.

A joint theoretical and experimental investigation on the influence of the doping concentration in InP-based quantum cascade lasers on the device performance is presented. The theoretical analysis is based on a Monte Carlo simulation that includes both electron–phonon and electron–electron scattering and explicitly considers non-equilibrium phonon effects. Good agreement is found between the experimental data and the Monte Carlo results, which clarify the role played by doping in the alignment of the electron levels and in the achievement of population inversion.

We report an experimental study of the optical bistability in a semiconductor microcavity (MC), consisting of a wedged λ GaAs spacer containing a single 8.5 nm In_0.04Ga_0.96As quantum well (QW). Increasing the excitation power, a transition between the normal coupling and the weak coupling regime of the MC modes is observed. At intermediate excitation power the reflectivity spectrum shows the simultaneous presence of three resonances, with a third peak spectrally lying between the two normal modes. Monitoring the reflected signal for different laser energy as a function of the excitation power, we found that optical bistability is observed only at the energy of the third peak. The experimental findings do not agree with transfer matrix calculations based on exciton bleaching, suggesting that more complex nonlinear mechanisms are relevant in the observed bistable behaviour.

We have measured the emission intensity and spectra of terahertz quantum-cascade lasers in an external magnetic field applied normal to the epilayers. We have observed a reduction of the threshold current, an enhancement of laser emission intensity and shifts of the emission line. A wider operating range was predicted for the selected waveguide design according to our finite-difference time-domain simulation results. The intensity enhancement and the threshold current reduction are attributed to the suppression of nonradiative Auger-intersubband transitions by Landau quantization of the in-plane electron motion, to the modulation of the injection rate via resonant inter-Landau-level transfer, and to the modulation of waveguide properties.

We report on experimental and theoretical study of GaA/InGaAs quantum systems grown along the [N11] direction. Time-resolved photoluminescence measurements have revealed a strong non-linear optical response from a sidewall quantum wire. The experimental results were reproduced by theoretical calculations taking into account strain, piezo-charge and excitonic effect. Also we theoretically investigated the effect of strain and piezo-charge on the properties of a quantum dot.

Transport properties of a field effect transistor based on pentacene have been investigated by experimental and numerical analysis. Moreover, thin film transistors based on porphyrin have been realized and characterized. In order to derive from basic principles the transport properties of the organic semiconductors, we used a Monte Carlo (MC) simulator to calculate the field-dependent mobility. The overall device simulation is performed by using two-dimensional drift-diffusion simulations taking into account field-dependent mobility obtained from MC simulations, interface/bulk trap states and fixed charge density at the organic/oxide interface. We demonstrate how carrier mobility can be extracted from experimental device characteristics, without using the conventional MOSFET theory that could lead to the wrong result. Finally we demonstrate the realization of simple organic logic circuitry, namely an organic inverter with a different configuration of load transistor.

The Green function for the coupled electron–phonon system is derived from a quantized version of the SSH Hamiltonian and is solved exactly for relevant modes of vibration. This Green function is used to compute the elastic and inelastic current components thought a di-thio-phenylene molecule in between Au contacts.

The optoelectronic behaviour of poly(para)phenylenevinylene is explored through solid-state ab initio approaches, which are appropriate for extended crystalline systems, and allow one to access transfer integrals and excitonic spectra. The microscopic interpretation of our results highlights the impact of interchain coupling on both transport and emissive properties of semiconducting polymer crystals.

We have investigated the polariton recombination and spin dynamics in the nonlinear emission regime as a function of the cavity–exciton detuning (δ). The relaxation to K ∼ 0 states, after non-resonant excitation, is governed by polariton final-state stimulated scattering. The time evolution of the degree of polarization of the photoluminescence (℘) shows a very rich and novel behaviour, never observed in the excitonic emission in bare quantum wells. ℘ attains its maximum at a finite time after the excitation with a light pulse instead of monotonically decaying to zero as in the case of bare excitons. Furthermore, the sign of ℘ is strongly dependent on the detuning: it is positive for δ > 0 and negative for δ < 0, showing an oscillatory behaviour for δ ∼ 0. The change in the sign of ℘ with the detuning is correlated with an energy splitting between the σ⁺- and σ⁻-polarized components of the photoluminescence.

We demonstrate the ballistic transport of electron spin in the absence of net transport of electron charge. Spin-up electrons move in one direction, while an equal number of spin-down electrons move with the same velocities in the opposite direction, yielding no net charge motion. This effect, also referred to as a 'pure' spin current, results from the quantum mechanical interference of the transition amplitudes associated with two-photon absorption of an optical field of frequency ω and one-photon absorption of a field of frequency 2ω. By changing the relative phase of ω and 2ω fields, we demonstrate coherent control of the magnitude and direction of this pure spin current. Results agree with recent theoretical predictions. We use polarization-sensitive pump–probe techniques to monitor the resulting ballistic spin transport with <10 nm spatial resolution.

By using a circularly polarized pulse that is tuned well below resonance, we create a virtual population of spin-polarized coherent excitons in a GaAs/AlGaAs multiple quantum well, and we study its effects on the polarization state of a weak signal pulse using a combination of time-resolved and spectrally resolved ellipsometric techniques. From these data the polarization state contributions to an all-optical polarization switch, which exhibits a ∼415 fs switching time and large contrast ratios, are identified and the performance of the switch is demonstrated.

We propose a mechanism to decrease the time needed to establish nuclear spin polarization by resonant coupling of the nuclear spins to a heat bath composed of nanomechanical degrees of freedom. To achieve this, we suggest that the nuclear spins be attached to the tip of a cantilever oscillating in a nonuniform magnetic field in the presence of another, perpendicular, constant, uniform magnetic field. The gradient of the magnetic field couples the spin and mechanical degrees of freedom, inducing an additional spin relaxation mechanism. The nanomechanical oscillator can also serve as one of the contacts of a tunnel junction, facilitating a voltage control of the nuclear spin relaxation time.

The photoluminescence of intrinsic excitons in high purity Al_xGa_1−xAs layers is studied under interband excitation by circularly polarized light. To determine the nature of excitons, the luminescence depolarization effect in a transverse magnetic field (Hanle effect) is used. The experimental curves represent two contours. For the composition x = 0.15, additional peaks arranged symmetrically with respect to zero magnetic field are observed. It is shown that the luminescence is determined by excitons localized in a potential associated with alloy fluctuations. The appearance of the additional peaks is due to the crossing of J = 1 and J = 2 levels in a magnetic field. The exchange splitting of exciton levels, recombination and spin relaxation times are determined from a comparison between theoretical and experimental dependences.

We report on time-resolved photoluminescence studies of tunnelling in a system of asymmetric double quantum wells (QWs). We employ a structure with a semimagnetic QW to align carrier spins and with excess concentration of electrons to provide a sensitive tool for studying the heavy hole dynamics. We prove the existence of hole tunnelling and demonstrate an efficient spin transfer in the process. A semi-empirical model describes qualitatively the experimental findings.

We analyse the combined effect of high electric field and Coulomb interaction on spin transport in semiconductors. We concentrate on the experimentally relevant regime characterized by T ⪅ T_F (intermediate to degenerate), where T_F = ε_F/k_B is the Fermi temperature. By the careful analysis of the downstream spin diffusion length L_d, we (i) demonstrate that in this regime carrier–carrier interactions become important and (ii) introduce the 'semiconductor degenerate' regime, in which spin-transport properties of a degenerate semiconductor are substantially modified by applying a modest electric field. Finally we discuss how the electric field and Coulomb interaction concur to influence the magnetoresistance of a trilayer system.

We report the magnetic field dependence of the degree of circular polarization on electroluminescence (EL) from InAs Schottky diode with ferro- and non-ferromagnetic electrodes by measuring the σ⁺ and σ⁻ components of the luminescence P_L intensity. These data are compared with reference photoluminescence (PL) data from p-InAs substrate. We found an interesting circular polarization behaviour of the positive slope (0 < B < 5 T) and the negative slope (5 T < B < 10 T) reaching a maximum value of 12–15% at B ≈ 5 T in both non-magnetic Schottky diode (EL data) and p-InAs substrate (PL data). The positive dependence is presumably attributed to the Zeeman effect and the negative dependence to magnetic depopulation of Landau levels of the valence band. We have also found an interesting behaviour of the degree of circular polarization from PL data as a function of external magnetic field containing double peaks. The intensity of the higher field peak is quenched at higher temperatures while the lower field peak stays almost the same. We attribute the higher field and lower field peaks to the radiative recombination of electrons with acceptor trapped holes and free excitons, respectively. The degree of circular polarization of the ferromagnetic metal Schottky diode was found to be negative and alters its sign according to the external magnetic field, B, and its magnitude reaches a maximum value of 12% at B ≈ ±5 T. The clear difference between the ferro- and non-ferromagnetic samples indicates successful spin polarized electron injection from the ferromagnetic metal into the semiconductor. The net degree of circular polarization is estimated to be a much higher value of ≈20% which is translated to be ≈40% of spin injection efficiency assuming selection rules between heavy and light holes.

We investigate spin resonance of two-dimensional (2D) conduction electrons in a modulation-doped SiGe/Si/SiGe quantum well structure. We find spin relaxation times of the order of microseconds and a 2D anisotropy of both the line broadening (dephasing time) and of the longitudinal spin relaxation. The spin relaxation is dominated by the D'yakonov–Perel spin relaxation mechanism originating from the Bychkov–Rashba spin–orbit field. We find evidence that for high mobility samples the cyclotron motion causes an additional modulation of spin–orbit coupling which leads to an effective reduction of the spin relaxation. For 2D electrons the dependence of the cyclotron frequency on the direction of applied field causes the observed anisotropy of spin relaxation.

Potential application of elementary excitation in semiconductor quantum dot to quantum computation is discussed. We propose a scalable hardware and all optical implementation of a logic gate that exploits the discrete nature of electron–hole states and their well-concentrated oscillator strength for ultrafast gate operation. A multiple-bit gate function is based on the nearest neighbour dipole–dipole coupling. Rabi population oscillation and long-lived coherence of an excitonic two-level system in an isolated In_xGa_1−xAs quantum dot guarantees quantum gate functions. Technologies that will realize and facilitate the exciton quantum computing are discussed.

We report independent coherent control of the density and spin of a non-equilibrium electron population in (111)-oriented GaAs. The symmetry and orientation of the GaAs combined with the polarizations and phases of the excitation fields provide a number of control parameters that allow for controlling either the density or spin—or both or neither—of the population. We show that coherent control arises through interference processes involving one- and two-photon absorption.

Gunn domain luminescence has been investigated based on our earlier studies on GaAl_xAs_1−x (0 < x < 0.45) layers, which suggested two regimes of radiative recombination: inside the domain, and outside. With multiple domain transits the build-up of excess carriers introduced long-term effects that made the device essentially a broad-area emitter. In this paper we argue that, in devices with high enough gain, simple Fabry–Perot cavity laser structures (Gunn lasers) emitting in the 630–850 nm wavelength ranges can be fabricated. The device was investigated with the luminescence spectrum of each facet. The related theoretical approach about the lasing action is described in detail.

We propose a single-electron-doped quantum dot in a field-effect structure as an optically triggered turnstile for spin-entangled electrons. A short laser pulse excites a charged exciton, whose quantum properties are transferred through tunnelling and relaxation to the spin entanglement between electrons in the dot and contact.

We demonstrate the presence of a resonant interaction between a pair of coupled quantum wires, which are realized in the ultra-high mobility two-dimensional electron gas of a GaAs/AlGaAs quantum well. Measuring the conductance of one wire, as the width of the other is varied, we observe a resonant peak in its conductance that is correlated with the point at which the swept wire pinches off. We discuss this behaviour in terms of recent theoretical predictions concerning local spin-moment formation in quantum wires.

We have studied the various types of Coulomb interactions in triple quantum dots and estimated their effects on the gate operation of quantum computation. In view of the magnitude of the Coulomb energies, the system requirements for error-free and high-speed operation are discussed.

In this work we present a simulation of the coherent evolution of electrons in two coupled quantum wires assisted by surface acoustic waves. The system under study has recently been proposed as a model for a solid-state realization of quantum gates. A quantum NOT transformation and an electron beam splitter have been numerically simulated. Results show that the surface acoustic waves improve the functionality of the device by preventing the spreading of the electron wavefunction and reducing undesired reflection effects.

The aim of this work is the study of electrostatically confined single and coupled quantum wires realized within a high-mobility two-dimensional electron gas (2DEG) at a GaAs/AlGaAs heterointerface. The one-dimensional channels are formed by the potential created by suitably biased surface electrodes. The shape of the bottom of the conduction band energy at the heterojunction has been obtained by numerically solving the three-dimensional Poisson equation over the whole structure at 20 K. Special attention has been paid to the depletion condition for the 2DEG within the quantum wires. To this purpose, the 1D Schrödinger equation is solved along the growth direction consistently with the Poisson solution, to accurately calculate the local electron distribution at the heterojunction. Finally, a single-electron wavefunction is propagated within the structure by means of a two-dimensional time-dependent Schrödinger solver, and results are shown for a single-qubit gate able to split the wavepacket. This investigation is part of a feasibility study carried out to identify the experimental requirements for the realization of basic quantum-computing gates.

We discuss several aspects of a recently proposed mesoscopic spin-polarizing beam splitter (Ionicioiu R and D'Amico I 2003 Phys. Rev. B 67 041307(R)). The device can be used for both spin preparation and measurement and can be viewed as the mesoscopic analogue of a Stern–Gerlach apparatus. We also discuss how it can be used as an orientable Stern–Gerlach.

A microwave noise technique is applied to study hot phonons in a biased two-dimensional AlGaN/GaN channel. The longitudinal optical (LO)-phonon-conversion lifetime is estimated from the measured dependence of hot-electron temperature on supplied power. At a lattice temperature of 373 K, the mean value of the effective LO-phonon lifetime (350 fs) is close to the value obtained at room temperature.

Ensemble Monte Carlo simulation of two-dimensional electron transport is performed for a biased undoped AlGaN/GaN channel. The effect of hot phonons on electron drift velocity and energy dissipation is investigated, the model is found correct within a factor of ∼2 when LO-phonon conversion lifetime is assumed to be 0.35 ps.

Field-induced electron transport in an In_xGa_1−xN(x ≅ 0.4) sample grown on GaN has been studied by subpicosecond Raman spectroscopy. Non-equilibrium electron distribution and electron drift velocity due to the presence of piezoelectric and spontaneous fields in the In_xGa_1−xN layer have been directly measured. The experimental results are compared with ensemble Monte Carlo calculations and reasonable agreements are obtained.

We study the shuttling instability in an array of three quantum dots the central one of which is movable. We extend the results by Armour and MacKinnon on this problem to a broader parameter regime. The results obtained by an efficient numerical method are interpreted directly using the Wigner distributions. We emphasize that the instability should be viewed as a crossover phenomenon rather than a clear-cut transition.

We calculate the transresistivity ρ₂₁ between two concentric armchair nanotubes in a diffusive multiwall carbon nanotube as a function of temperature T and Fermi level ε_F. We approximate the tight-binding band structure by two crossing bands with a linear dispersion near the Fermi surface. The cylindrical geometry of the nanotubes and the different parities of the Bloch states are accounted for in the evaluation of the effective Coulomb interaction between charges in the concentric nanotubes. We find a broad peak in ρ₂₁ as a function of temperature at roughly T ∼ 0.4T_F. Further, we predict a peak in ρ₂₁ as a function of ε_F, which can be studied experimentally by changing a gate voltage or by doping.

We propose a microwave diode based on a modulation-doped GaAs/Al_0.25Ga_0.75As structure. The principle of the diode operation relies on a non-uniform heating of the two-dimensional electron gas in microwave electric fields arising due to the asymmetric shape of the device. The voltage sensitivity of the diode at room temperature is close to 0.3 V W⁻¹ at 10 GHz, which is comparable to the value obtained using similarly shaped and sized diodes based on bulk n-GaAs. At liquid nitrogen temperature, the voltage sensitivity strongly increases reaching a value of 20 V W⁻¹ due to the high mobility of the two-dimensional electron gas. The detected signal depends linearly on power over 20 dB, until hot-electron real-space-transfer effects begin to predominate. We discuss noise temperature measurements at 10 GHz, consider the frequency dependence of the voltage sensitivity in the microwave range and compare the performance data of the proposed device and the asymmetrically shaped bulk GaAs diode within the 10 GHz–2.5 THz frequency range.

We report measurements of the rate of energy loss per electron, P_e, by a two-dimensional electron gas in a series of AlGaN/GaN heterostructures grown on different substrates by MBE. The measurements cover a range of electron temperatures, T_e, from 4–40 K. A combination of zero and high magnetic field electrical transport measurements was used to determine T_e as a function of the power dissipated in the device. It was found that for the samples grown on the better lattice matched substrates P_e ∝ Tⁿ_e, with n= 5 at the lowest temperatures, T_e≪ 10 K. At higher temperatures, T_e> 10 K, n→ 1 in all samples. The experimental results are compared with numerical calculations of the energy relaxation rate. We find that the emission of piezoelectrically coupled acoustic phonons is the dominant energy loss process in the studied temperature range.

We present a theoretical analysis of both static and small-signal electron transport in Hg_0.8Cd_0.2Te in order to study the high-frequency behaviour of this material usually employed for infrared detection. Firstly we simulate static conditions by using a Monte Carlo simulation in order to extract transport parameters. Then, an analytical method based on hydrodynamic equations is used to perform the small-signal study by modelling the high-frequency differential mobility. This approach allows a full study of the frequency response for arbitrary electric fields starting only from static parameters and to overcome technical problems of direct Monte Carlo simulations.

We present simulations of terahertz photo-response experiments on a double quantum well FET structure, with a periodic metallic grating. When radiation is shed on the device at a certain frequency, measurements show that photo-response peaks appear as the gate voltage is varied, over a range of frequencies and temperatures. To explain the basic physical phenomena at the basis of this experiment, we have developed a non-equilibrium Green's function model for the electron transport problem inside the device structure. Following a semi-empirical approach, we have developed a formulation to express the drag rate inclusive of radiation power absorption and Coulomb drag effects, starting from an existing approach for coupled wells in the absence of radiation. A single calibration factor is evaluated for one experimental condition and left unchanged for all other situations examined. The simulation results follow closely the measured data in the range of frequencies and temperatures covered by the experiments.

It is shown experimentally that surface-field terahertz (THz) emitters can produce well-collimated beams of THz radiation, making them useful devices for time-domain spectroscopy applications. Simulations of the carrier-dynamics are used to explain the mechanism of THz generation in InAs and GaAs, and it is shown that inter-valley scattering of electrons must be considered in order to fully describe THz emission from InAs.

We investigate the performance of n-GaAs surface plasmon type emitters of terahertz radiation having surface layers grown by low-temperature molecular beam epitaxy technology. Emitters with Be-doped low-temperature-grown GaAs layers on the surface radiate transients that are up to five times more powerful than those obtained from reference devices without these layers. Moreover, terahertz output from the modified emitters shows much weaker saturation at large optical intensities. Differences in the shapes of terahertz transients generated by both types of devices are discussed in terms of electron and hole dynamics inside the structures.

Recent investigations showed that the THz photoresponse of quantum Hall systems (QHS) is a complex combination of different mechanisms. In this work, we separate these parts by varying the electron concentration and the source–drain current. For our photoconductivity measurements, we used a pulsed p-Ge cyclotron laser as a monochromatic THz radiation source and changed the magnetic field, the source–drain current and the electron concentration (via a backgate) of the QH sample. The spectral resolution of the QHS was found to improve with increasing values of the source–drain current up to the breakdown of the QH effect.

We have conducted a computational investigation of electron motion in wurtzite GaN crystal under intense mid-infrared irradiation by a free electron laser, and studied effects of polar-optical-phonon scatterings on electron heating processes.

The intrasubband transition rates of bound carriers interacting with piezoelectric and polar optical phonons in a bulk GaN lattice are presented, considering the anisotropy of the uniaxial wurtzite structure. The directional dependence of the excitonic Bohr radius has a measurable effect for both types of piezoelectric and polar optical phonon scattering. In the latter case, the anisotropy of the uniaxial crystal induces exciton scattering involving both strongly mixed transverse-like and longitudinal-like polar optical phonons. By taking into account the angular frequency dispersion of the zone centre polar optical phonons, we can relate the anisotropy of the excitonic scattering rate to a particular orientation of the transverse-like phonon wave vector with respect to the c-axis. The same effect is not as pronounced in the case of the longitudinal-like polar optical phonon emission due to the much lower anisotropy of the longitudinal-like phonon frequency branch.

Ultrafast electron dynamics in the inversion layer of an AlGaN/GaN transistor is studied in pump–probe experiments with 50 fs mid-infrared pulses. Two-colour pump–probe measurements show an instantaneous transmission increase for all spectral positions of the probe, which demonstrates that homogeneous broadening is an important contribution to the total linewidth in this material. We observe the maximum of the induced transmission change around 5 µm. This large Stokes shift might be caused by the extremely large electron–LO-phonon scattering rate.

Optical excitation of shallow donor centres in silicon creates an inverted electron distribution on the Coulomb centres at low lattice temperature. The population inversion is formed due to the peculiarities of the electron–phonon interaction of free and bound charge carriers. At certain conditions stimulated emission in the THz frequency range from group-V donors in silicon can be obtained.

This communication reports the results of experimental studies of the electroluminescence (EL) of forward-biased Schottky barrier (SB) diodes fabricated on heterostructures where an InAs/GaAs quantum dot (QD) layer was placed in the space charge region. In order to reach a higher EL wavelength, the QD layer was overgrown by a thin combined GaAs/InGaAs cladding layer. The heterostructures with Au SBs were found to show pronounced EL originating from several confined exciton states in InAs QDs which is higher at 300 K than at low temperatures. It is shown that the EL wavelength can be tuned within the 1.3–1.57 µm range by changing the thickness and composition of the combined GaAs/InGaAs capping layer. We discuss possible mechanisms of the injection of holes needed for the radiative recombination in the QDs and possibilities of improving the EL quantum yield.

In this paper, we simulate the mixed electronic and mechanical vibrational behaviour of Coulomb-coupled nanodevices subject to electromagnetic chirped pulses. The electronic states of the devices are approximated with a finite number of quantum states coupled to a classical vibration and to a heat bath, as well as subject to optical pumping. We show that mechanical vibrational states of the nuclei can be switched by suitable optical chirps and binary information can be loaded onto nanodevice chains. Our simple model indicates that the loss of coherence in the electronic states can be reconstructed by proper excitation of the nuclear vibration.

A full-band electron transport calculation in wurtzite phase GaN based on an accurate model of electron–phonon interactions using a rigid pseudo-ion model is reported. The calculated deformation potentials are used to estimate the scattering rate in a cellular Monte Carlo (CMC) simulator. Longitudinal optical (LO)-like and transverse optical (TO)-like polar optical phonon scatterings are also employed in this work. The calculated velocity is lower than the previous works which used fixed deformation potentials.

AlGaN/GaN test structures were fabricated with an etched constriction. A nitrogen plasma treatment was used to remove the disordered layer, including natural oxides on the AlGaN surface, before the growth of the silicon nitride passivation film on several of the test structures. A pulsed voltage input, with a 200 ns pulse width, and a four-point measurement were used in a 50 Ω environment to determine the room temperature velocity–field characteristic of the structures. The samples performed similarly over low fields, giving a low-field mobility of 545 cm² V⁻¹ s⁻¹. The surface treated sample performed slightly better at higher fields than the untreated sample. The highest velocity measured was 1.25 × 10⁷ cm s⁻¹ at a field of 26 kV cm⁻¹.

Despite the difficulty in fabrication, resonant tunnelling diodes (RTD) have found a great deal of usage in the analogue, digital and mixed signal realms as a means of increasing the speed of signal processing circuitry, or in reducing the static power dissipation in the circuitry. Nevertheless, RTDs suffer from their non-planar structure. One possible solution is a planar diode, which operates via coupling of injected electron modes from an input waveguide to a corresponding output waveguide in a semiconductor hetrostructure, or a reflective coupling diode (RCD). In this paper, we investigate the role of temperature on the operation of an RCD.

We study few-particle interactions in a coupled quantum dot system by the use of a semi-analytical model we designed. The model allows for very accurate results and a substantial speed up of calculations. We apply our analysis to the design of a single-electron read-out device based on conditional excitonic dynamics driven by trains of multicolour laser pulses.

We report the first combined photoluminescence and pump–probe measurements on single wall carbon nanotubes isolated in micelles. The bandgap photoluminescence is observed at room temperature and is concomittent with a drastic change in the photocreated carrier dynamics as compared to the one observed in usual macroscopic samples. Probing the fundamental bandgap leads to a non-exponential dynamics which is related to a lifetime distribution within the macroscopic nanotube population. Probing the second van Hove transition leads to a strong increase of the excitation lifetime which demonstrates that some non-radiative relaxation channels are quenched for isolated nanotubes. These observations constitute an important step in the understanding of the existence of photoluminescence in isolated nanotubes.

A detailed analysis, both experimental and theoretical, of light–matter interaction in dielectrically coupled semiconductor microcavities is presented. More specifically, we have designed and realized all a-SiN:H coupled planar microcavities, whose optical investigation via transmission/reflection as well as photoluminescence experiments reveals a clear fingerprint of cavity–cavity coupling effects. The latter have been quantitatively modelled via fully three-dimensional calculations based on a scattering-state approach including energy-dependent refractive indices and absorption coefficients.

Magnetic resonance studies allow us to distinguish paramagnetic, ferromagnetic and ferrimagnetic phases in Ga_1−xMn_xAs. The transition from ferromagnet to ferrimagnet is correlated with a metal to insulator transition. The analysis of spin wave resonance spectra, which occur in the ferrimagnetic phase, allows us to estimate the magnitude and the distance dependence of exchange coupling. The experimentally evaluated long range of exchange causes an effective averaging of the fluctuation of exchange interactions. As a consequence, in the semimetallic phase both spin subsystems coherently precess forming the ferrimagnetic structure. In the insulating phase, fluctuations of the local exchange field lead to a fast decoherence of the carrier spins and only the localized Mn spins form the ferromagnetic moment.

The International Conference on Nonequilibrium Carrier Dynamics in Semiconductors (HCIS-13) celebrates 30 years since it first took place in Modena. Nonequilibrium dynamics of charge carriers, pioneered by the hot-electron concept, is an important issue for understanding electro-optic transport properties in semiconductor materials and structures. In these 30 years several topics have matured, and new ones have emerged thus fertilizing the field with a variety of physical problems and new ideas. The history of the conference is summarized in the opening paper `30 years of HCIS'. The future of the conference seems secure considering the continued lively interest of the participants.

The conference addressed eleven major topics which constitute the backbone of the proceedings and are summarized as follows: carrier transport in low dimensional and nanostructure systems, nonequilibrium carriers in superlattices and devices, small devices and related phenomena, carrier dynamics and fluctuations, carrier quantum dynamics, coherent/incoherent carrier dynamics of optical excitations and ultra-fast optical phenomena, nonlinear optical effects, transport in organic matter, semiconductor-based spintronics, coherent dynamics in solid state systems for quantum processing and communication, novel materials and devices.

Nanometric space scale and femtosecond time scale represent the ultimate domains of theoretical, experimental and practical interest. Traditional fields such as bulk properties, quantum transport, fluctuations and chaotic phenomena, etc, have received thorough and continuous attention. Emerging fields from previous conferences, such as quantum processing and communication, have been better assessed. New fields, such as spintronics and electron transport in organic matter, have appeared for the first time. One plenary talk, 11 invited talks, 230 submitted abstracts covering all these topics constituted a single-session conference. Following scientific selection through the Advisory and Program Committees and peer review, 162 papers were selected for publication by the Institute of Physics Publishing in this special issue of Semiconductor Science and Technology.

The financial support that allowed conference organization and helped researchers with budget difficulties to attend came from the following institutions which are gratefully acknowledged: Office of Naval Research (ONR), Defense Advanced Research Project Agency (DARPA), Office of Naval Research International Field Office (ONRIFO), International Union of Pure and Applied Physics (IUPAP), Italian Ministry of Education University and Research (MIUR), National Institute for the Physics of Matter (INFM), University of Modena and Reggio Emilia, Dipartimento di Ingegneria dell' Innovazione of the Lecce University. Finally, sincere thanks are addressed to the technical staff who provided assistance during the conference: G Angelone, M Benassi, F Grossi, M Leuzzi, A Magnani, S Montanto, L Zagni and D Zanfi. The staff of the University Press Office together with F Goggi and N Minto are acknowledged for their excellent job in printing the conference documents.

On the occasion of the 30th anniversary of the HCIS conference, we would like to provide a general, although necessarily not exhaustive, picture of how the conference has changed over the years and of how the natural evolution of research has influenced its various editions. By performing a statistical analysis of specific sets of data, we try to identify some characteristic trends that, we believe, can contribute to a better understanding of how our field of research has been developing in the last three decades. On request of the editor, we added the data relative to HCIS-13.