Excitonic recombination dynamics in shallow quantum wells

Abstract

We report a comprehensive study of carrier-recombination dynamics in shallow Al x Ga 1Ϫx As/GaAs quantum wells. At low crystal temperature ͑2 K͒, the excitonic radiative recombination time is shown to be strongly enhanced in shallow quantum wells with xϾ0.01, consistently with a model that takes into account the thermal equilibrium between the three-dimensional exciton gas of the barrier and the two-dimensional exciton gas, which are closer in energy as x decreases. Furthermore, we demonstrate the existence of a thermally activated escape mechanism due to the low effective barrier height in these structures. The nonradiative recombination is shown to dominate the carrier dynamics for temperatures as low as 10 K for xϷ0.01. Our experimental observations are analyzed using three different variational exciton calculations. In particular, we study the crossover from the two-dimensional to the three-dimensional behavior of the exciton, which occurs for x as low as 0.01 and affects mainly the oscillator strength, whereas the transition energies in shallow quantum wells can be calculated, to a large extent, using the same approximations as for conventional quantum wells. The peculiar behavior of the oscillator strength at the crossover to the weak confinement regime is obtained by expansion in a large basis. ͓S0163-1829͑98͒05035-8͔

Theory of laser cooling of semiconductor quantum wells

“…As a result, Auger effects are negligible in the cooling analysis in bulk GaAs below 100 K, whereas they are still appreciable in the determination of b τ at lower temperatures. Available experimental measurements of non-radiative lifetime in GaAs-based QWs [29,30] give values between 1 ns and 10 ns which is two orders of magnitude shorter than b τ shown in Fig. 5.…”

Thermalization and Photoluminescence Dynamics in Quantum Well Heterostructures

“…In our approach, 60 Ph. Roussignol et al [1] are also reported electrons, holes and excitons form ideal and dilute gases described by Boltzmann distribution functions. These ideal gases are at thermal equilibrium with each other and characterized by a single, time dependent, temperature.…”

“…Recent time-resolved photoluminescence experiments, performed on shallow GaAs/ Ga 1± ±x Al x As quantum wells with x as low as 1%, have revealed that good samples display a photoluminescence decay time, t r , which increases with decreasing x [1]. On the contrary, an accurate theory of excitons in these structures reveals that very little variations with x are expected for the oscillator strength of the excitonic transition if x`3% [2].…”

“…This indicates that the interpretation of t r as the exciton radiative lifetime is dubious, if not invalid. We proposed that thermal equilibrium between 1s excitons and barrier excitons could explain the apparent increase of t r upon decreasing x [1]. Here, we wish to include free electrons and free holes in the modelling of the photoluminescence signal and calculate the time dependence of the excitonic recombination in shallow GaAs/Ga 1± ±x Al x As quantum wells (QW) versus x.…”

“…We shall compare the prediction of our model to experimental data obtained by means of time-resolved photoluminescence spectroscopy in a set of single GaAs/Ga 1± ±x Al x As quantum wells with x ranging from 0.5 to 17.8%. The samples consist in nominally 100 # e GaAs single quantum wells clad with two 500 # e Ga 1± ±x Al x As barriers [1].…”