Observation of both heavy- and light-hole excitons in (25–60)-nm diameter GaAs-(Al,Ga)As quantum dots

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Abstract
Uniform arrays of free-standing quantum dots (QD’s) with lateral dimensions between 25 and 60 nm have been made from a 5-nm single-quantum-well (SQW) structure using electron-beam lithograpy and low-damage, electron-cyclotron-resonance plasma etching. Low-temperature (5 K) photoluminescence (PL) and photoluminescence-excitation (PLE) spectroscopy have been used to map the samples carefully both before and after processing into QD’s. The PL linewidth increases with decreasing dot diameter but remains in the range 8–4 meV (full width at half maximum) for the diameters we have studied. This should be compared to the 3 meV that we see for the unprocessed SQW material. Although we have made no intentional effort to passivate the exposed surfaces of the etched dots we continue to see strong 5-K PL from the samples when ‘‘in-well’’ excitation is employed. Following correction for the area of QW sample remaining in the dots we see a rapid increase in the PL intensity with decreasing dot diameter. PLE measurements show a decrease in the Stokes shift of the heavy-hole exciton as the dot diameter is reduced. In addition, blueshifts of both the light-hole (lh) and heavy-hole (hh) exciton are observed as the lateral diameter of the dot diminishes. We believe these are the clearest observations of both hh and lh excitons in a range of QD sizes.