Acceptor-induced threshold energy for the optical charging of InAs single quantum dots

We study the photoluminescence of single InAs/GaAs self-assembled quantum dots for a range of excitation powers, excitation energies and sample temperatures $4\mathrm{K}<T<\mathrm{}50\mathrm{}\mathrm{K}.$ Our results demonstrate the existence of a well-defined excitation energy threshold, above which negatively charged excitons could be effectively created in a single quantum dot at helium temperatures. This threshold energy is in the range between the wetting layer ground state energy and the GaAs barrier energy, and is ascribed to the acceptor to the conduction band transition in GaAs. A model is presented, according to which a laser excitation of this energy creates an extra number of free electrons in the GaAs barrier in addition to the equal number of electrons and holes in the wetting layer. The excitation power dependence of these extra electrons has been calculated and found to be in good agreement with the experimentally derived values of the total electron charge, accumulated in the quantum dot. At elevated temperatures $(T>\mathrm{}30\mathrm{}\mathrm{K}),$ this effect vanishes due to the essential decrease of the steady-state free electron concentration in the GaAs barrier as a result of thermally excited free holes appearing in the GaAs barrier valence band which provides an effective recombination channel for the free electrons. These experimental observations could be used as an effective tool to create and study charged excitons in quantum dots.