Enhanced luminescence from InAs/GaAs quantum dots

Single quantum dots (QDs), based on the InAs/GaAs material system have been characterized by micro-photoluminescence (mu PL). The self-organized quantum dots studied are fabricated by the Stransky-Krastanov method, taking advantage of the strain caused by the lattice mismatch between InAs and GaAs. Well-defined narrow excitonic features from individual QDs are monitored in the mu PL spectra, upon single or dual tunable laser excitation. The charge state of the quantum dot is revealed from these excitonic lines in the mu PL spectra. However, by tuning the laser excitation energy, it is demonstrated that the charge state of the dot can be altered: The distribution of neutral and charged excitons is demonstrated to be extremely sensitive on the laser energy. In addition, with an additional infrared laser, striking changes are induced in the mu PL spectra. The results achieved demonstrate the existence of two well-defined excitation energy regions for the main laser, in which the presence of the infrared laser will decrease or increase, respectively, the integrated dot mu PL intensity. For excitation above the critical threshold energy of the main laser, the addition of the infrared laser will induce a considerable increase, by up to a factor 5, in the QD emission intensity. At laser excitation below the threshold energy, on the other hand, the QD emission intensity will decrease. This fact is due to reduced carrier capture efficiency into the dot as determined by the internal electric field driven carrier transport. In order to get further insight into the carrier capture process due to the electric field in the vicinity of the QD, the dots have also been subjected to an external electric field. In most optical experiments with QDs, electrically injected or photoexcited carriers are primarily created somewhere in the sample outside the QDs, e.g. in the barriers or in the wetting layer. Consequently, excited carriers undergo a transport in the wetting layer and/or barriers prior to the capture into the QDs. This circumstance highlights the crucial role of the carrier transport and capture processes into the dot for the performance and operation of the dot based devices such as QD lasers, QD infrared detectors and QD memory devices. This transport effect on the optical response of the quantum dots has been investigated by subjecting the carriers to an external electric field in mu PL measurements. This external field is formed by application of a lateral field between two top contacts. It is demonstrated that the QD PL signal intensity could be increased several times (> 5 times) by optimizing the magnitude of this external field.