Role of the continuum background for carrier relaxation in InAs quantum dots

Abstract

We study the carrier capture and relaxation in self-assembled InAs/ GaAs quantum dots ͑QDs͒ using bleaching rise time measurements as a function of the excitation density, at 5, 77, and 293 K. We observe that the bleaching rise time and the carrier lifetime of the first excited state are longer than the bleaching rise time of the QD ground state, indicating that the excited state does not act as an intermediate state. For high excitation density, we observe a temperature-dependent plateau in the initial bleaching rise time, contradicting an Augerscattering-based relaxation model. Both these experimental results point toward a relaxation through the continuum background, followed by a single LO-phonon emission toward the QD ground state. The relaxation through the continuum background is governed by Coulomb or acoustic phonon coupling between the continuum and the discrete QD energy levels.

Dichroic reflection of InAs∕GaAs quantum dots

“…8͒ can also be determined by differential reflectivity, i.e., by using in-plane polarization-resolved two-color pump-probe time-resolved differential reflection spectroscopy ͑TRDR͒. [12][13][14] The carriers generated by the pump pulses are captured in the QD energy states which leads to the bleaching of the QD transition. The bleaching results in a decrease of the probe pulse absorption and thus also to a change of the QD reflection due to the carrier-induced change in the QD refractive index.…”

Coherent acoustic phonons in strain engineered InAs∕GaAs quantum dot clusters

“…That is, the QD reflectivity changes with the PZE field modulation in the QD as a linear response to the modulated strain, 18 which is enhanced in the clusters due to a locally higher QD density compared to random QD layers. Hence, the amplitude of the oscillation in the differential reflection signal scales linearly with the periodical change of the refractive index 15,18 at the probe energy. To locate the origin of the coherent acoustic phonons, the amplitude of the oscillations is measured as a function of excitation density, as shown in Fig.…”

“…[11][12][13][14] In this letter, we report on the differential reflectivity of QDs arranged in a small ordered group of four QDs per cluster on average, grown by strain engineering. 7,8 Using pump-probe time-resolved differential reflection spectroscopy ͑TRDR͒, 15 we are able to measure the carrier capture and carrier relaxation process, and the carrier recombination process within the QDs. For structures with a random QD distribution, the decay of the TRDR signal is described by a single exponential function.…”

“…For more experimental details, we refer to Ref. 15. Figure 1 depicts the transient differential reflection signal, ͑⌬R / R 0 ͒͑t͒, measured at 5 K for an excitation density of 1.7 kW/ cm 2 .…”

“…The initial and dominant changes of the QD reflectivity are due to the carrier capture in the QD ground state. 15 However, the refractive index can also be changed by a modulated local strain field. That is, the QD reflectivity changes with the PZE field modulation in the QD as a linear response to the modulated strain, 18 which is enhanced in the clusters due to a locally higher QD density compared to random QD layers.…”