Excited states and energy relaxation in stacked InAs/GaAs quantum dots

Abstract

Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap layers grown by migration enhanced epitaxy. Photoluminescence excitation ͑PLE͒ spectra reveal the excited state spectrum as a function of size for self-assembled InAs QD's in multilayered samples with 36-ML spacers. The observed energy shifts and splittings are consistent with those of hole states numerically calculated for pyramidal QD's supporting assignment to the transition between the electron ground ͉000͘ and the ͉001͘ excited hole state. The optical results suggest the island shape uniformity to improve in multilayered samples, which is attributed to the contribution of the buried islands to the surface strain altering the island formation kinetics and energetics that also underlie vertical self-organization. Time-resolved photoluminescence ͑TRPL͒ results yield a lifetime of 40 ps for the first excited ͉001͘ hole state, attributed to multiphonon relaxation processes bridging the approximately 100 meV level separation, and ground-state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, showing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE spectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing recombination processes. ͓S0163-1829͑98͒03115-4͔

Suppression of LO phonon scattering in Landau quantized quantum dots

“…10 In the present work we observe clear phonon suppression in n-type quasi 0D dots ͑i.e., Landau quantized rather than spatially quantized͒ by a time-resolved intraband absorption measurement. This provides unambiguous evidence for the phonon bottleneck effect independently of arguments concerning which processes dominate in the interband photoluminescence measurements in dots [3][4][5][6][9][10][11][12] and quasi dots 13 such as electron-hole scattering. Further, because of the very clean model system ͑much sharper interfaces and no wetting layer, etc.͒, the interpretation is not complicated by detailed questions about different growth techniques and the quality of different dot sample structures.…”

“…[2][3][4][5][6] However, partly as a result of different groups using different growth techniques for interband photoluminescence samples and partly on fundamental grounds, this is controversial and is the subject of much debate. [7][8][9][10][11][12] Indeed several mechanisms have been proposed that may bypass the bottleneck, such as multiphonon scattering, 7 Auger processes, 8 excitonic effects, 9 and defect related processes. 10 In the present work we observe clear phonon suppression in n-type quasi 0D dots ͑i.e., Landau quantized rather than spatially quantized͒ by a time-resolved intraband absorption measurement.…”

Quantum dot materials for terahertz generation applications

“…These timescales are determined by generation and diffusion of carriers from the GaAs barrier layers into the QDs, simultaneous state-filling of QD energy levels, and subsequent energy-dependent exciton recombination. Importantly however, it is observed that an increasing optical pump power leads firstly to saturation of the dot ground state (GS) and subsequently of the higher energy states due to comparatively low degeneracy of the GS [24]. Photoluminescence recombination measurements www.lpr-journal.org indicate a shorter recombination time from the excited states (ES) as compared to the GS.…”

Electronic coupling and thermal relaxation in self-assembled InAs quantum dot superlattices

“…As in Ref. 22 , we suppose uncoupled states arise from QD lateral size variations within the vertical stack. Coupled states, due to their lower ground state, are rst populated.…”