InAs quantum dot emission and annealing impact in quantum wells with strain reduced InAlGaAs layers

InAs quantum dots (QDs) embedded in Al_0.30Ga_0.70As/InGaAs/InGaAlAs/Al_0.30Ga_0.70As quantum wells (QWs) have been investigated in the as grown state and after thermal annealing at 640°C or 710 °C for two hours. Two types of QD structures with different InGaAlAs capping layers (In_0.15Ga_0.85As (#1) and In_0.15Al_0.4Ga_0.45As (#2)) are compared and studied by means of photoluminescence (PL), X-ray diffraction (XRD) and high resolution HR-XRD methods. XRD and HR-XRD techniques are used with the aim to control the quality of crystal structures, as well as the variation of material compositions and elastic strains in QW layers at thermal annealing. The higher PL intensity and lower energy of ground state (GS) emission are detected in the structure with the In_0.15Al_0.4Ga_0.45As capping layer. Thermal annealing leads to the shift of PL spectra into the higher energy range and the value of this shift is more relevant in the structure with the In_0.15Ga_0.85As capping layer. The variation of GS emission peak versus temperature has been monitored within the range of 10-500 K for as grown and annealed states and is compared with the temperature shrinkage of energy band gap in the InAs and GaAs bulk crystals. It is revealed that the QD composition in #2 is InAs and the efficiency of Ga/Al/In intermixing at thermal annealing in #2 is less than in #1. Thermal decay of the integrated PL intensities has been investigated in studied structures as well. The PL thermal decay 10-fold is revealed for the range of 10-300 K in #1 with the In_0.15Ga_0.85As capping layer compared with the 5-fold thermal decay in #2 for the In_0.15Al_0.4Ga_0.45As capping layer. Finally the reasons for PL spectrum transformation at annealing, the mechanism of PL thermal decay, and the advantages of QD structures with the In_0.15Al_0.4Ga_0.45As capping layer have been analyzed and discussed.