Anomalous strain profiles and electronic structures of a GaAs-capped InAs/In_0.53Ga_0.47As quantum ring

We investigate the strain profiles and electronic structures of a novel quantum ring capped by a material different from the substrate. By comparing the novel quantum ring with an ordinary quantum dot and quantum ring, which are capped by the same material as the substrate, we find that the novel quantum ring exhibits noticeably different properties, such as an anomalous strain relaxation, band alignments, and blueshift of the emission energy. We investigate the novel properties of our quantum ring by separating the ring evolution process into (i) the step of geometric change from a dot to ring and (ii) the step of GaAs capping. The GaAs embedded in the In_0.53Ga _0.47As matrix provides sufficient space for the relaxation of InAs and, thus, individual strain and biaxial strain of the InAs ring are considerably reduced by the GaAs layer. We show that the blueshift in the emission energies due to ring formation is mainly caused by (i) the geometric change from a dot to ring and (ii) the weakened heavy hole-light hole splitting, and not by the enhanced compressive strain. The relaxation of the ring along the radial direction also considerably enhances the shear strain and piezoelectric potential, and the piezoelectric potential partially compensates for the blueshift resulting from the ring evolution and GaAs capping. We also show that the tensile-strained GaAs selectively acts as a potential well for light holes and as potential barriers for both the electrons and heavy holes. As a consequence, the GaAs layer considerably enhances the light-hole character of the hole states in our quantum ring.