The microstructural and optical properties of self-assembled quantum dots (QDs) formed by Stranski-Krastanow (S-K) growth mode have been particularly attractive for several years because of their potential applications to optoelectronic devices. Defect-free QDs in highly lattice-mismatched heterostructures can be generated utilizing the S-K growth mode. Even though a significant narrowing and blue-shift of the PL spectrum for self-assembled QDs due to RTA treatment have been reported, the detailed studies underlying the variation of the microstructural properties for self-assembled QDs after RTA treatment have not yet been clearly clarified.
This study reports effects of thermal annealing on the microstructural properties of InAs/GaAs QDs, which were grown by using molecular beam epitaxy. The RTA process was performed on proximity capped samples in a nitrogen atmosphere. Experiments of sample #1, which is without GaAs capping layer, were carried out for 60s at a temperatures of 600, 650, 700, 750, and 800℃. Experiments of sample #2, which is with GaAs capping layer, were performed for 60s at a temperature of 650, 700, 750, 800, and 850℃. InAs QDs were analyzed by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL).
Results of as-grown, 600, and 650℃ annealed samples, which is uncapped GaAs layer, show that incomplete shape of QDs are forming complete shape due to mismatch strain relaxation. But 700, 750, and 800℃ annealed samples display that QDs are destroying by high thermal energy. Peak emission is unchanged because diffusion between QD and barrier materials is not occurred. Only intensity is reduced due to As outdiffusion.
As increasing annealing temperature of GaAs capped layer samples, width of QDs is increased and height of QDs is reduced. At 850℃ annealing, no QDs are observed and only InAs wetting layer remains. Density of QDs is reduced and degree of size uniformity increases in accordance with increasing annealing temperature. PL spectra is blueshifted because of a change in the dot composition by In/Ga interdiffusion. Narrowing of the PL linewidth is observed due to increasing size uniformity. Also, intersublevel energy spacing is narrowed. These results can help improve the understanding of thermal annealing effect on the microstructural and optical properties of InAs/GaAs QDs.
