Self-assembled semiconductor quantum dots (QDs) are zero-dimensional structures, being solid state systems with an atomic-like density of states. QDs are currently deeply investigated for their potentiality as building blocks for novel optoelectronic devices and for quantum information technologies. Several promising applications were recently developed in these fields, such as single photon sources for quantum cryptography, lasers[2, 3], and optical amplifiers. In all the foreseen applications, good knowledge of recombination kinetics and carrier dynamics at high temperature in the QDs is of the utmost importance.
Semiconductor QDs are known to efficiently capture the carriers generated by optical absorption. The electron hole pairs created in the barrier region rapidly relax to the QD ground state from where they radiatively recombine. The role of non-radiative channels on temperature dependence of the QD recombination efficiency has been analyzed in many details and with different experimental techniques[5–8]. It is nowadays well established that the high-T thermal quenching of the photoluminescence (PL) is due to the escape of carriers from the QD to the wetting layer or barrier material, where they undergo non-radiative recombination. The presence of PL thermal quenching at intermediate temperatures has been attributed to the loss of carriers within the barriers during the relaxation path[9, 10]. Despite the large experimental and theoretical effort devoted to the understanding of the carrier thermodynamics in QDs, a few aspects still need a better understanding.
In particular, it has been long debated, and still not completely clarified, whether the capture and escape of the carriers in QDs occur via single carrier[8, 11, 12] or electron-hole pairs[7, 13–15]. In this discussion, a relevant point is taken by the interpretation of the superlinear dependence of the QD integrated PL (I
) on the excitation power density (Pexc) at high T that is sometimes observed in QD samples (see, e.g., the works of Le Ru et al. and Sanguinetti et al.[8, 14] and references therein). It is claimed that such observation is the fingerprint of a bimolecular recombination inside the QDs, thus inferring that single carrier dynamics dominates in the QDs[8, 12]. On the contrary, on the basis of comparison of the temperature dependence of I
on Pexc under non-resonant and resonant, with QD states, excitation conditions, Sanguinetti et al. have attributed the superlinear I
behavior to the saturation of temperature-activated trap states, which affect the carrier diffusion in the barrier.
Here, we show that it is possible to turn off, in a controlled manner, the superlinear behavior of the QD I
on Pexc at high T through the reduction of the non-radiative recombination centers of the QD material. This further supports the extrinsic origin of the observed PL superlinearity showing that it stems directly from a poor quality of the QD material. As the superlinearity is the outcome of temperature-activated non-radiative recombination centers in the barrier, we propose its suppression as an effective method to test the quality of the QDs and of the surrounding barrier.
For this study, we use QD samples fabricated by droplet epitaxy (DE), which is a technique that allows for the fabrication of quantum nanostructures in lattice matched and mismatched systems with a precise control on the density, size, and shape. These achievements are due to the intrinsic high design flexibility of DE, making accessible a large variety of different structures, ranging from QDs, QD molecules, rings, multiple rings, and more complex shapes[20–22]. However, the DE growth is typically performed at low substrate temperature (between 150°C and 350°C). On one side, this allows to maintain a low thermal budget, making DE perfectly suited for the monolithic integration of GaAs QDs on CMOS devices; on the other side, the crystalline quality of the grown materials at such low temperature is quite poor. Post-growth annealing is then commonly used to recover material quality. Different studies have been proposed to explain the effect of annealing on the QDs grown by DE, demonstrating the modifications induced on both morphological and electronic aspects of the nanostructure material[24–26].