Excitons, biexcitons and trions in self-assembled (In,Ga)As/GaAs quantum dots: Recombination energies, polarization and radiative lifetimes versus dot height

TL;DR

This study models how excitonic complex energies, polarization, and lifetimes in self-assembled (In,Ga)As/GaAs quantum dots vary with dot height, revealing shifts, polarization anisotropy, and temperature-dependent radiative lifetimes.

Contribution

It provides a detailed atomistic pseudopotential and configuration-interaction analysis of excitonic properties as a function of quantum dot height, including polarization and radiative lifetime predictions.

Findings

Recombination energies shift with dot height, showing blue or red shifts depending on the excitonic complex.

Polarization anisotropy switches sign with height for X^0 and XX^0, but remains negligible for X^- and X^+.

Radiative lifetimes are short (~ ns) for charged excitons and biexcitons, and long (~ ms) for neutral excitons at low temperature.

Abstract

We calculate the height dependence of recombination energies, polarization and radiative lifetimes of the optical transitions of various excitonic complexes: neutral excitons (X^0), negatively- (X^-) and positively-charged (X^+) trions, and biexcitons (XX^0) in lens-shaped, self-assembled In_0.6Ga_0.4As/GaAs quantum dots. By using an atomistic pseudopotential method combined with the configuration-interaction method, we predict the following. (i) The recombination energy of the lowest transition of X^- blue-shifts as height increases, whereas that of X^+ red-shifts. Remarkably, the recombination of XX^0 shows a red-shift at small heights, reaches a maximum shift, and then blue-shifts for taller dots. (ii) Changes in dot height lead to a bound-to-unbound crossover for X^-, X^+ and XX^0. (iii) When considering the [110] and [1\bar{1}0] directions, the lowest transitions of X^0 and XX^0 manifest [110] vs [1\bar{1}0] in-plane polarization anisotropy that switches sign as a function of height as well as alloy randomness. $X^-$ and $X^+$ show transitions with negligible polarization anisotropy regardless of height. (iv) The ground state of X^0 is split in a low-energy pair that is forbidden (dark) and a high-energy pair that is allowed; thus, at T=0K the radiative lifetime \tau(X^0) is long (~ ms) due to the dark exciton. On the other hand, at T=10K, \tau(X^0) decreases moderately as height increases and its magnitude ranges from 2-3ns. The ground state of X^- and X^+, and that of XX^0 is allowed (bright); so, \tau(X^-), \tau(X^+) and \tau(XX^0) are fast (~ ns) even at T=0K. These radiative lifetimes depend weakly on height. In addition, \tau(X^-) ~ \tau(X^+) ~1.1ns, while \tau(XX^0)~0.5ns$. We compare our predictions with available spectroscopic data.