R. Mickevičius

Electron high-field transport in multisubband quantum wire structures

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

HOT-PHONON EFFECTS ON ELECTRON TRANSPORT IN QUANTUM WIRES

Hot (nonequilibrium) phonon effects on electron transport in rectangular GaAs/AlAs quantum wires have been investigated by a self‐consistent Monte Carlo simulation. Confinement and localization of optical phonons have been taken into account. We have demonstrated that at room temperature hot optical phonons lead to a significant increase in electron drift velocity. This hot‐phonon drag effect is due to the strongly asymmetric nonequilibrium phonon distribution. As a result, phonon absorption for forward transitions (electron gains momentum along electric field) is enhanced, whereas absorption for backward transitions (electron gains momentum against electric field) is suppressed. At low temperatures diffusive heating of electrons by hot phonons dominates over hot‐phonon drag and the electron drift velocity decreases.

Superlinear electron transport and noise in quantum wires

We have employed a Monte Carlo technique for the simulation of electron transport and noise (diffusion) in GaAs rectangular quasi‐one‐dimensional quantum wire structures at low temperatures. It is demonstrated that with the heating of electron gas the efficiency of acoustic phonon scattering decreases and the mobility increases. The increase of electron mobility appears as a superlinear region on velocity‐field dependence. It is shown that electron noise increases in the superlinear region. The transition from superlinear transport to the regime close to electron streaming with a further increase of electric fields is reflected on the diffusivity‐frequency dependence by the appearance of a separate peak at the streaming frequency. The electron streaming regime which takes place at higher fields causes the collapse of the diffusion coefficient (noise spectral density) to the streaming frequency.

Electron intersubband scattering in real quantum wires

Abstract The rates of electron intra- and intersubband scattering by surface optical (SO) and confined longitudinal optical (LO) phonons in quasi-one-dimensional GaAs/AlAs quantum wires (QWIs) are calculated. It is shown that electron-SO-phonon intersubband scattering can be resonant, so that the scattering rate tends to infinity when intersubband energy separation approaches SO phonon energy. The energy dependence of the total scattering rate in ideal QWIs exhibits multiple sharp peaks related to intersubband transitions. The scattering rates in the real QWIs with variable thickness are calculated. The results show that even small variation in thickness leads to the significant broadening of the very first peaks and complete washing-out of the peak-like structure at higher energies. Monte Carlo simulations of electron transport in the QWI have been performed. It is demonstrated that electron drift velocity in the QWI is considerably suppressed by electron intersubband scattering and is considerably lower than the bulk material values. The role of SO phonons in electron energy dissipation is discussed.

Hot phonons in quantum wires

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

Radiation of acoustic phonons from quantum wires

We have investigated by the Monte Carlo technique the radiation of ballistic acoustic phonons from quasi‐one‐dimensional electron gas in quantum wires. At low temperatures and over a wide range of electric fields, all excess heat in quantum wires is dissipated by means of acoustic phonons. Due to the uncertainty of momentum conservation during electron–acoustic‐phonon scattering, electrons emit acoustic phonons with large transverse momentum components. Consequently, in this transport regime quantum wires radiate fluxes of nonequilibrium acoustic phonons into surrounding material. Nonequilibrium acoustic phonons can propagate ballistically over macroscopic distances. Ballistic fluxes of nonequilibrium acoustic phonons have been previously detected experimentally in quantum well structures. We have calculated the angular and energy spectrum of nonequilibrium acoustic phonons radiated from quantum wires.

Hot-electron overcooling and intersubband population inversion in quantum wires

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

Hot‐electron relaxation dynamics in quantum wires

Monte Carlo simulations of hot nonequilibrium electron relaxation in rectangular GaAs quantum wires of different cross sections are carried out. The simulations demonstrate that the initial stage of hot‐electron cooling dynamics is determined by cascade emission of optical phonons and exhibits strong dependence on the excitation energy. The second (slow) relaxation stage is controlled by strongly inelastic electron interactions with acoustic phonons as well as by nonequilibrium (hot) optical phonons. The relaxation times obtained in our simulations are in good agreement with the results of recent luminescence experiments. At low electron concentrations where hot phonon effects are negligible the cascade emission of optical phonons may lead to the overcooling of the electron system to temperature below the lattice temperature. These electrons then slowly (during tens of picoseconds) relax to equilibrium due to the interaction with acoustic phonons. At certain excitation energies strong intersubband electro...

Hot-phonon effects on electron runaway from GaAs quantum wires

Nonequilibrium (hot) optical phonon effects on electron runaway from GaAs quantum wires embedded in AlGaAs have been investigated by Monte Carlo technique. We have simulated the carrier runaway kinetics in the 0<E<1000 V/cm electric-field range for a lattice temperature of 30 K. Due to optical phonon mode confinement by GaAs/AlGaAs heterointerfaces, the buildup of generated hot phonons is strongly pronounced in the quantum wires. Even at moderate electron concentrations and electric fields, the accumulation of these phonons may become significant and substantially affect all transport properties in the structure. As a result of reduced hot electron cooling rates in the presence of nonequilibrium optical phonons, the high-energy tail of the carrier distribution function extends above the potential barriers at the quantum wire boundaries. This may eventually lead to significant electron escape from the potential well, even at relatively low electric fields, what significantly affects the performance of such...

Oscillations of photoconductivity and negative absolute conductivity in quantum wires

The phenomenon of oscillating photoconductivity as a function of photoinjection energy is studied by Monte Carlo simulation in quasi-one-dimensional quantum wire structures. It is demonstrated that the amplitude of the oscillations of photoconductivity may be so large that this leads to a negative absolute conductivity at injection energies that are multiples of the optical phonon energy. These oscillations are associated with inelastic optical phonon scattering, leading to an asymmetric electron distribution function established under conditions of intensive electron photoinjection to the subband bottom or close to energies that are multiples of the optical phonon energy. Simulation results suggest that quasi-one-dimensional quantum wires are ideal for the experimental observation of negative absolute conductivity at low lattice temperatures.