L. Rota

Hot carrier relaxation in GaAs V‐groove quantum wires

We have measured hot carrier relaxation in GaAs V‐groove quantum wires using time‐resolved photoluminescence. Relaxation between confined one‐dimensional subbands is clearly observed on the time scale of several hundred picoseconds. A simulation of the experiment using a hybrid multisubband Monte Carlo method which includes hot phonon and degeneracy effects gives good agreement with the measurements.

Monte Carlo calculations of hot‐carrier noise under degenerate conditions

We present a Monte Carlo investigation of noise and velocity fluctuations in Si at 300 K under full degenerate conditions. The presence of the Pauli principle is found to strongly modify the shape of the correlation functions: at low electric fields, the time decay decreases due to the shortening of momentum relaxation time and the variance evidences nonparabolicity effects. At increasing electric fields, the correlation function exhibits a negative part that can be associated with ballistic carriers crossing the velocity space from negative to positive values of the Fermi velocity without scattering. A microscopic analysis in terms of diagonal and off‐diagonal contributions to the velocity correlation confirms this interpretation.

Femtosecond optical absorption measurements of electron‐phonon scattering in GaAs quantum wells

We present a combined experimental and theoretical investigation of intra‐ and intersubband carrier relaxation in GaAs/AlAs quantum wells following femtosecond laser excitation at low density. Time‐resolved optical absorption has been measured using 125 fs pulses from a tunable Ti:sapphire laser which allows carriers to be excited preferentially into different subbands. The experiments have been analyzed using a multi‐subband Monte Carlo simulation which contains all the important scattering mechanisms. Electron‐phonon scattering rates obtained from dielectric continuum theory have been used, and we find excellent agreement with the experiment. The dominance of interface phonons in intrasubband relaxation is confirmed.

The role of vertical quantum wells in carrier trapping in v‐groove quantum wire lasers

We report a theoretical investigation of carrier trapping in GaAs v‐groove quantum wire structures. Our results show that trapping is fast in wires confined by AlGaAs alloy barriers where the growth process creates a Ga‐rich vertical quantum well at the center of the v‐groove: this acts as a highly effective scattering channel into quantum wires states. The results indicate that with suitable growth engineering, high‐efficiency quantum wire structures can be obtained.

Resonant Raman scattering from phonons in GaAs/(GaAs)m(AlAs)n quantum wire structures

Raman spectroscopy has been used to measure phonons in GaAs v‐groove quantum wire structures containing (001) and (111) GaAs/AlAs superlattice barrier regions. Resonance enhancement permits the identification of modes in different regions of the structure, and the measured phonon frequencies provide structural information which shows clear evidence of GaAs migration during growth from (001) surfaces into the grooves. Confined and interface phonons with large in‐(111) plane wavevectors are observed.

Electronic states in GaAs v‐groove quantum wire structures with superlattice barriers

We present a joint theoretical and experimental investigation of GaAs v‐groove quantum wires confined in GaAs/AlAs superlattice barriers. We have computed the electronic states for both the quantum wire and the barriers. The intrinsic bending of the superlattice layers, together with systematic spatial variations of their thickness, create localized states in the barriers that are separated from the wire. This effect has a strong impact on the overall luminescence efficiency of the wires. The results are in excellent agreement with photoluminescence and photoluminescence excitation spectra.

An investigation of carrier dynamics in semiconductor quantum wires following femtosecond laser excitation

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

Influence of hot phonons on electronic noise in GaAs

We present a Monte Carlo investigation of the influence of the nonequilibrium phonon population on second‐order transport properties in GaAs. We calculate the velocity and energy autocorrelation functions and the velocity–energy cross‐correlation functions, both for the case with and without phonon perturbation. By comparing the results, we find significant modifications in the correlation functions and consequently in the equivalent noise temperature due to the presence of the nonequilibrium phonons.

Phonons in quantum wires

We present a detailed study of phonons in rectangular quantum wires within the dielectric continuum model and within a fully microscopic approach. From the latter, we calculate the phonon dispersions and the potentials associated to the individual modes. The classification of such modes is much more complex than in the corresponding two dimensional case, owing to the intrinsic coupling of confined and interface modes associated to both directions perpendicular to the wire. This indicates a failure of the current implementations of the macroscopic dielectric continuum model.

Position broadening effect in hot-electron transport

Abstract We utilize the Kadanoff-Baym-Keldysh methods to develop a formalism appropriate for high, homogeneous fields. The aim is to derive a spectral density model which can account for both the energy dependence of the collision rate and the intracollisional field effect in a relatively simple and rigorous way. We solve the appropriate Dysons equation in the first Born approximation for the case of scattering mechanisms described by a momentum-independent self-energy. We obtain an analytical expression for the spectral density and derive an integral equation for the correlation function G , which is proportional to the density of particles. We show that the field, acting in conjunction with the phonons, leads to an effective quantization of the energy in the direction of the electric field suggesting the presence of a novel quantum effect consisting in a discontinuous trajectory of the electron along this direction.