Electron intraband capture and relaxation in self-assembled InAs/GaAsquantum dots

We will present a detailed investigation of intraband electron capture and relaxation processes in InAs quantum dots (QDs) using an energy and temperature dependent, mid-infrared pump-probe (PP) measurement. We find that the electron relaxation time is strongly dependent on the photo-excitation energy and varies from ~4 ps for relaxation from the higher energy excited states inside the QDs up to ~10 ps for relaxation from the wetting layer (WL)/barrier states. Although the ‘phonon bottleneck’ effect predicts the capture time to be much longer, the measured times still exceed significantly the typical ~1ps relaxation time for quantum wells. Also our intraband measurements clearly indicate that the electron capture/relaxation occurs directly to the QD ground state, not step-wise between the QD excited states as it was suggested previously. An advantage of the intraband PP measurement is that the electron population is determined by the doping, varying from 0.5 up to >2 electrons per dot, giving a more precise control of the QD population compared with interband excitation.
Our previous studies have shown that the electron relaxation in QDs from the p-like first excited state to s-like ground state (low energy peak in Fig.1) is typically >30ps [1]. Surprisingly, in the present work, the PP signal corresponding to the relaxation of electrons from higher energy excited states (dashed line in Fig.2) decays with time constant τ ~4ps. Such a short decay time allows us to conclude, that electron relaxation occurs directly to the ground state, avoiding the p-state. When we excite into the WL/barrier states the intraband relaxation time increases to ~10ps for the PP energy of ~210meV. Furthermore, at low temperatures a bi-exponential decay of the PP signal corresponding to electron relaxation from WL/barrier states to QD ground state with τ1=(8 ± 2) ps and τ2=(300 ± 100) ps is observed (solid line in Fig.2 and inset). The short decay time is related to electron capture in the same QD from which it was originally excited, whereas the long decay time is strongly dependent on the temperature and originates from the electron capture and thermal re-emission in adjacent QDs.
Additionally, interband pump/intraband probe measurements were performed from which the electron capture times from the GaAs barrier and WL states into the QDs of ~6ps and ~4ps, respectively, has been determined. Since interband excitation creates photo-generated electron-hole pairs, we explain the shortening of the electron capture time by Auger-type carrier-carrier scattering.
Using a combination of these two techniques we have been able to gain detailed insight into the electron relaxation processes in QDs, which are significant for quantum dot based lasers and infrared photodetectors. Our results show that due to longer WL states lifetime (~10ps) QD infrared photodetectors have the potential to be more efficient compared to quantum well analogues.