Self-assembled InAs/GaAs quantum dots studied with excitation dependent cathodoluminescence

Abstract

We have examined the optical properties of self-assembled InAs quantum dots (QDs) with polarization sensitive and time-resolved cathodoluminescence (CL) techniques. The InAs QDs were formed via self-assembly during molecular beam epitaxial growth of InAs on unpatterned GaAs(001). CL spectra exhibited a two-component line shape whose linewidth, intensity, and peak positions were found to be temperature and excitation dependent. The two components are found to be consistent with state filling of the QDs, resulting in emission involving ground state and excited state excitonic transitions. The luminescence intensities and lineshapes of the QD and wetting layer (WL) excitonic transitions were analyzed with constant excitation and time-resolved CL for various temperatures and excitation levels to study the thermal activation, re-emission, and recombination kinetics of carriers. Thermal quenching of the QD ground state and excited state components in the 105–175 K range is correlated with a rise in the WL emission, showing that the WL carrier distribution is fed partially by thermal reemission of carriers from the QDs. A more rapid thermal quenching of the QD and WL excitonic emissions in the 185–300 K range is consistent with thermal reemission of carriers into the GaAs matrix. Time delayed CL spectra show that carriers in the excited states rapidly feed the ground state during the decay phase of the luminescence, further confirming that the two components are derived from the same QD. The CL decay time for the QD luminescence was found to be wavelength and temperature dependent, owing to thermal reemission into the WL. The CL decay times ranged from 0.1 to 2.0 ns. A polarization anisotropy was found in CL spectroscopy, revealing the importance of the stress anisotropy in both the formation of QDs and their subsequent optical emission.