S. S. Prabhu

Dynamics of the pump-probe reflectivity spectra in GaAs and GaN

The pump-probe reflectivity (PPR) technique is a quick way to characterize the short carrier lifetime in materials which may be potentially good terahertz (THz) emitters or detectors. Here, we study the PPR signal in semiconductors theoretically in the frequency domain (at various energies above and below the band gap) as a function of pump-probe delay. We consider two conditions of carrier relaxation. In one, the carriers are assumed to form a hot, thermalized energy distribution during excitation itself and then to cool via phonon emission, as is expected in the case of high density excitation in GaAs. In the other case, the carriers essentially remain in a nonequilibrium, nonthermal state even as they relax. This can happen when the carrier-longitudinal optical phonon interaction is stronger than carrier–carrier scattering, as is likely in GaN even at moderately high densities. In addition, effects of carrier trapping and recombination determining the carrier lifetime are included. The calculation take...

Quasi-donor-acceptor pair transitions in GaAsSb and AlGaAsSb on InP

We identify quasi-donor-acceptor pair transitions in the photoluminescence spectra of GaAsSb and AlGaAsSb layers, lattice matched to InP, and grown by molecular-beam epitaxy. These alloys show compositional inhomogeneity due to phase separation resulting from miscibility gaps. The presence of Al in the quaternary alloy increases the fluctuation of the electrostatic potential in the epitaxial layer, increasing the variation of recombination energy as a function of intensity excitation in the range of low temperatures.

Measurements of frequency upconversion and picosecond excitation‐correlation luminescence spectra in GaAs quantum wells and determination of time constants describing exciton dynamics

We study the picosecond dynamics of free carriers and excitons in high‐quality GaAs quantum wells at 8 K using frequency upconversion (UC) and nonlinear picosecond excitation‐correlation (PEC) luminescence spectroscopy under nonresonant excitation conditions with carrier densities of a few 1010 cm−2. We analyze the measurements using a theoretical model of the coupled free electron–hole pair and exciton dynamics, incorporating important density‐dependent nonlinear effects, caused by exciton collisions and fermion exclusion. The time‐resolved UC photoluminescence (PL) spectra are used to deduce time constants related to free electron–hole (e–h)‐pair transformation into excitons and exciton transfer from large K to small K (k is the exciton wave vector). It is shown that unambiguous determination of exciton radiative and nonradiative decay times are possible when both UC and PEC PL spectra are considered simultaneously. We carry out a detailed line‐shape analysis of the picosecond UC and PEC PL spectra. We ...

Femtosecond energy relaxation of nonthermal electrons injected in p-doped GaAs base of a heterojunction bipolar transistor

We study femtosecond relaxation of minority carriers (electrons) injected into a heavily p-doped base of a heterojunction bipolar transistor (HBT). Here, we consider the case of p-doped GaAs, to be specific. The electrons are assumed to have a peaked energy distribution at t=0, with kinetic energies a few hundred meV above the conduction band threshold. We solve the time dependent Boltzmann equation governing the dynamics of these electrons. The main feature of this work is a detailed calculation of the time dependent nonthermal, nonequilibrium electron energy distribution, that relaxes due to single particle excitations via electron–hole scattering and interaction with coupled optical phonon-hole plasmon modes in the sub and picosecond time domains. We highlight the significant role that the electron-hole scattering plays in this relaxation. The majority carriers (holes) are assumed to remain in quasiequilibrium with the lattice, taken to be at room temperature (or at 77 K). We present calculations of el...

On the line shape of intensity correlated nonlinear photoluminescence spectra due to delocalized excitons in GaAs quantum wells

Abstract Measurements of the picosecond time-resolved intensity-correlated nonlinear photoluminescence (NLPL) spectra are made at different excitation densities in the range 4 × 10 9 to 2 × 10 11 cm −2 on high quality GaAs Quantum Wells (QWs) at 8K. We consider various possible models to understand the observed NLPL spectral shapes, which show a sharp prominent negative peak at the exciton energy with a positive sidewing on either side. We suggest that the most likely origin of the observed lineshape is collision broadening of the luminescence due to delocalized excitons. The presence of additional nonlinear effects, such as the density dependent exciton radiative lifetime and fermion exclusion, can also be deduced from the analysis.

Comments on the steady state photocarrier grating technique to measure diffusion lengths

The steady state photocarrier grating technique has emerged as an important technique for measurement of the diffusion length in amorphous silicon. In this communication we show that morphological inhomogeneities lead to an overestimation of the magnitude of the diffusion length. The magnitude of this error cannot be easily estimated.