Nonequilibrium Electron Transport Research Articles

Non-equilibrium electron transport in degenerate nitride heterostructures—dynamic screening effects

We show how dynamic screening effects on non-equilibrium electron transport can be incorporated in the case of electronically dense GaN-based quantum wells. The theory is based on the Boltzmann equation, leading to evaluations of the momentum relaxation time and, hence, the electron mobility in these heterostructures. We find that both screening and anti-screening effects are manifest as the electron density varies. However, anti-screening dominates over a wide range of densities, with screening commencing at densities appropriate for phonon–plasmon coupling.

Studies of field-induced nonequilibrium electron transport in an InxGa1−xN (x≅0.6) epilayer grown on GaN

Field-induced electron transport in an InxGa1−xN (x≅0.6) sample grown on GaN has been studied by subpicosecond Raman spectroscopy. Nonequilibrium electron distribution and electron drift velocity due to the presence of piezoelectric and spontaneous fields in the InxGa1−xN layer have been directly measured. The experimental results are compared with ensemble Monte Carlo calculations and reasonable agreements are obtained.

Nonequilibrium electron and phonon transport and energy conversion in heterostructures

We establish a unified model dealing with the transport of electrons and phonons in double heterojunction structures with the coexistence of three nonequilibrium processes: (1) nonequilibrium among electrons, (2) nonequilibrium among phonons, and (3) nonequilibrium between electrons and phonons. Using this model, we investigate the energy conversion efficiency based on concurrent thermoelectric and thermionic effects on electrons and size effects on electrons and phonons. It is found that heterostructures can have an equivalent figure of merit higher than the corresponding bulk materials.

Effect of the lead dimensionality over transport properties in quantum dots

We theoretically investigate the effect of the lead dimensionality on the non-equilibrium electron transport through quantum dots. More precisely, we study nonlinear transport in a quantum dot coupled to leads of diverse dimensionality. We show that the presence of the latter strongly determines the resulting transport properties. Differently from higher dimensional leads (wide and smooth band limit), van Hove singularities in the density of states of low-dimensional reservoirs determine sharp resonances in the differential conductance at -nite applied voltages as well as in the dot spectral density. It is also shown that, due to the finiteness of the terminal bandwidth, the differential conductance change its sign at higher biases. These results clearly indicate that the environment doesplay an important rôle in determining transport properties in mesoscopic systems.

Nonequilibrium electron transport in wide minibandGaAs/AlxGa1−xAssuperlattices at room temperature

Nonequilibrium electron transport in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{0.3}{\mathrm{Ga}}_{0.7}\mathrm{As}$ superlattice with a miniband width of 100 meV has been investigated by time-domain terahertz spectroscopy. When the superlattice is pumped at the bottom of the miniband, the transient electron velocity is found to exhibit acceleration-deceleration characteristics unique to miniband dispersion. The time taken to reach the steady-state velocity was determined to be of the order of 1--2 ps. When pumped at the top of the miniband, no phase reversal in the transient velocity was observed, indicating the importance of the density of photoexcited states by femtosecond laser pulses.

Density-functional method for nonequilibrium electron transport

We describe an ab initio method for calculating the electronic structure, electronic transport, and forces acting on the atoms, for atomic scale systems connected to semi-infinite electrodes and with an applied voltage bias. Our method is based on the density functional theory (DFT) as implemented in the well tested Siesta approach (which uses non-local norm-conserving pseudopotentials to describe the effect of the core electrons, and linear combination of finite-range numerical atomic orbitals to describe the valence states). We fully deal with the atomistic structure of the whole system, treating both the contact and the electrodes on the same footing. The effect of the finite bias (including selfconsistency and the solution of the electrostatic problem) is taken into account using nonequilibrium Green's functions. We relate the nonequilibrium Green's function expressions to the more transparent scheme involving the scattering states. As an illustration, the method is applied to three systems where we are able to compare our results to earlier ab initio DFT calculations or experiments, and we point out differences between this method and existing schemes. The systems considered are: (1) single atom carbon wires connected to aluminum electrodes with extended or finite cross section, (2) single atom gold wires, and finally (3) large carbon nanotube systems with point defects.

High-field electron transport in Al xGa 1− x as diodes investigated tracing ultrafast absorption changes

The topic of this contribution is the investigation of transient, nonequilibrium high-field electron transport in GaAs. Our two key ingredients are a unique laser system, which provides fs pump and probe pulses of independently tunable wavelengths and an AlGaAs hetero p–i–n diode with sophisticated design. This combination allows us to obtain time- and spatially resolved information about the temporal evolution of a photo-generated electron ensemble from ballistic to side-valley dominated transport, without disturbing hole contributions. Fields ranging from 7 to 180 kV/cm are investigated. Results are compared successfully to Monte Carlo Simulations.

Miniband transport in vertical superlattice field effect transistors

The non-equilibrium transport of electrons confined at an atomically sharp interface and subject to a periodic potential is studied. We find a pronounced negative differential resistance, the magnitude of which increases with increasing modulation strength. The data is qualitatively consistent with the Esaki–Tsu transport model. We emphasize the significance of the two-dimensionality of the electron system and the gate to inhibit domain formation. Additional features in the source–drain current are attributed to Bloch-phonon resonances.

Numerical analyses of the nonequilibrium electron transport through the Kondo impurity beside the Toulouse point

Nonequilibrium electron transport through the Kondo impurity is investigated numerically for the system with 20 conduction-electron levels. The electron current under finite voltage drop is calculated in terms of the ‘conductance viewed as transmission’ picture proposed by Landauer. Here, we take into account the full transmission processes of both the many-body correlation and the hybridization amplitude up to infinite order. Our results demonstrate, for instance, how the exact solution of the differential conductance by Schiller and Hershfield obtained at the Toulouse point becomes deformed by more realistic interactions. The differential-conductance-peak height is suppressed below e 2/ h with the width hardly changed through reducing the Kondo coupling from the Toulouse point, whereas it is kept unchanged by further increase of the coupling. We calculated the nonequilibrium local Green function as well. This clarifies the spectral property of the Kondo impurity driven far from equilibrium.

Nonequilibrium electronic transport and interaction in short metallic nanobridges

We have observed interaction effects in the differential conductance $G$ of short, disordered metal bridges in a well-controlled non-equilibrium situation, where the distribution function has a double Fermi step. A logarithmic scaling law is found both for the temperature and for the voltage dependence of $G$ in all samples. The absence of magnetic field dependence and the low dimensionality of our samples allow us to distinguish between several possible interaction effects, proposed recently in nanoscopic samples. The universal scaling curve is explained quantitatively by the theory of electron-electron interaction in diffusive metals, adapted to the present case, where the sample size is smaller than the thermal diffusion length.

Nonequilibrium electron transport in HBTs

Nonequilibrium electron transport in heterojunction bipolar transistors (HBTs) becomes clearly observable as their vertical dimensions are reduced, This is the reason for the superior tradeoff relationships between the device parameters and the resultant high-speed performance of HBTs fabricated with III-V semiconductor materials. This paper reviews the hot carrier effect in the base and the velocity overshoot effect in the collector and discusses the roles these effects play in reducing carrier traveling times and improving device performance.

Molecular-scale electronics

A review of nonequilibrium electronic transport in molecules and molecular-scale synthetic systems is given. Although the basic concepts and mechanisms for electronic conduction in polymers have been discussed for some time, recently developed experimental techniques provide systems for their validation. New fabrication techniques that create metallic contacts to a small number of conjugated organic molecules allow the study of the basic transport mechanism of these systems and will provide direction for the potential development of molecular-scale electronic systems.

Preparation and ultrafast optical characterization of metal and semiconductor colloidal nano-particles

The ultrafast dynamics of photoinduced electrons in several metal and semiconductor colloidal nano-particle systems are characterized using femtosecond laser spectroscopy. Various preparation methods are used and, in several cases, modified for making particles with long-term stability and narrow and controllable size distributions. The particle size and size distribution are determined using transmission electron microscopy and electronic absorption spectroscopy. For aqueous gold and silver colloids, spatial size confinement is found to cause substantially slower electronic relaxation due to reduction of non-equilibrium electron transport and weaker electron-phonon coupling. In gold colloids, photoejection of electrons into the liquid is observed, which is attributed to a two-photon enhanced ionization process. The effect of surfactant on the electron dynamics in CdS colloids is examined and found to be significant, substantiating the notion that electrons are dominantly trapped at the liquid-solid interface. In Ru3+-doped TiO2 colloids, the electronic decay is found to be as fast as or even faster than in undoped TiO2 and other semiconductor colloids such as CdS, suggesting that ion doping of large bandgap semiconductor colloids is not necessarily effective in lengthening the electron lifetime. In almost all cases studied, the majority of the photoinduced electrons are found to decay within a few tens of picoseconds due to non-radiative relaxation. The results are discussed in the context of the potential applications of metal and semiconductor nano-particles in areas including photocatalysis and photoelectrochemistry.

Hot electron and quasiballistic transport of nonequilibrium electrons in diamond thin films

A molecular dynamics simulation of conduction-band transport in diamond has been carried out that combines classical propagation of electrons with choice of scattering events determined by a Monte Carlo algorithm using quantum mechanical rates. The high-field regime is dominated by hot optical phonon scattering for n⩽1019 cm−3 and by electron–plasmon emission for higher electron concentrations. Electron–electron and electron–hole binary processes do not significantly influence the transport characteristics but extend the high-energy tail and asymmetry of the electron distribution, making it more “Maxwellian.” The results demonstrate the possibility of achieving quasiballistic transport in thin diamond films with a significant portion of the field energy imparted to the electrons at high fields (F≈102 V/μm) and electron concentrations up to n≈1018 cm−3.

Non-equilibrium carrier transport in the base of heterojunction bipolar transistors

The conventional drift-diffusion equation is inadequate to analyze ballistic effects. In most device analysis, thermoelectric effects associated with gradients in the carrier temperature are assumed to be small. The base thickness dependence of current gain as a result of non-equilibrium transport of electrons in the base was experimentally shown by many researchers. We present a simulation to analyze the electron transport in the base region of an HBT based on the carrier density balance, momentum balance, and energy balance equations. Transistors with base thickness of 1000 Å and 2500 Å are investigated. Simulation results of electron transport in the base region indicates that tunneling electrons give rise to unique transport characteristics in the base of HBTs. The density profile is found as a result of transition from ballistic to diffusive transport in the base region. The range of observed non-equilibrium transport and the carrier density and temperature profiles in the base region are functions of average momentum and energy relaxation times. The non-equilibrium effects are shown to exist up to 1000 Å in the base region of HBTs. The simulation results are in good agreement with experimental data.

Numerical studies of two-dimensional photocarrier transport in quantum wells

Ensemble Monte Carlo simulation has been performed to study the initial spatial transport of nonequilibrium photogenerated electrons and holes in a quantum well structure. The spatial distributions have been simulated over a wide range of excitation energies and temperatures, including both the carrier - carrier and carrier - LO phonon scatterings with the help of the local density approximation. It has been found that the energy relaxation and hence the spatial transport are strongly dependent on the excitation energies, reflecting whether the relaxation channel via LO phonon emission is available or not. The spatial transport is quenched when the energy relaxation is accelerated by LO phonon emission. This excitation energy dependence of the spatial transport reduces with the increase of the carrier densities because the relative weight of the carrier - carrier scattering to the LO phonon emission changes.

Fourier Analysis-Based Method for High-Frequency Performance Calculation of Heterojunction Bipolar Transistor

A method to evaluate high-frequency performance of heterojunction bipolar transistors based on Fourier analysis of the non-stationary collector current response is proposed. The nonequilibrium electron transport and velocity overshoot effects are taken into account by ensemble Monte Carlo particle simulation. The influence of the emitter and collector capacitances on the high-frequency performance are also included in our consideration. It is found that the conventional method for cut off frequency calculation underestimates its value. Experimental results reported in the literature are compared with the results of numerical simulation. The comparison shows high accuracy and validity of the developed method.

Direct measurements of the transport of nonequilibrium electrons in gold films with different crystal structures.

The transport of femtosecond-laser-excited nonequilibrium electrons across polycrystalline and single-crystalline gold films has been investigated through time-of-flight measurements. The thicknesses of the films range from 25 to 400 nm. Ballistic electrons as well as electrons interacting with other electrons and/or with the lattice have been observed. The ballistic component dominates the transport in the thinner films, whereas the interactive transport mechanism is dominant at the upper end of the thickness range. A slower effective velocity of the interactive component is observed in the polycrystalline samples, and is assumed to arise from the presence of grain boundaries. The reflection coefficient of excited electrons at the grain boundaries is extracted from the experiment and is estimated to be r\ensuremath{\simeq}0.12. The experiment also suggests that thermal equilibrium among the excited electrons is not fully established in the first \ensuremath{\sim}500 fs after excitation.

Forward transit delay in In/sub 0.53/Ga/sub 0.47/As heterojunction bipolar transistors with nonequilibrium electron transport

The authors have performed a comprehensive study of the high-frequency current gain h/sub 21/ in InP-In/sub 0.53/Ga/sub 0.47/As HBTs (heterojunction bipolar transistors) (with an emitter area A/sub E/=3.5 mu m*3.5 mu m and 500 AA base thickness with p-type doping level of 1*10/sup 20/ cm/sup -3/) over the temperature range 55 K<or=T<or=340 K. Detailed device modeling was used to extract tau /sub F/ from the measured data. Results show that the intrinsic emitter-collector forward delay decreases from a value of tau /sub F/=0.5 ps at a temperature T=340 K and saturates at a value of tau /sub F/ approximately 0.28 ps for T=150 K and below. This marked decrease in measured forward delay with decreasing temperature arises from the temperature dependence of electron scattering in the device. The extrinsic current gain cutoff frequency, f/sub T/, of these devices at T=55 K is greater than 300 GHz. Such a high value of f/sub T/ can only be explained by the existence of extreme nonequilibrium electron transport in the base and collector.

Forward delay in scaled Al0.48In0.52As/In0.53Ga0.47As heterojunction bipolar transistors

We present experimental measurements and numerical simulations of the intrinsic forward delay as a function of base thickness in abrupt junction n-p-n Al0.48In0.52As/In0.53Ga0.47As heterojunction bipolar transistors. For base thicknesses up to 1350 Å and impurity concentration p=1.5×1019 cm−3 we find that nonequilibrium electron transport ensures that base transit delay is less than that in the 3000-Å-thick collector space-charge region. This provides an opportunity to increase base thickness and reduce base resistance without sacrificing the intrinsic forward delay time.