Manipulation of the structural and optical properties of InAs quantum dots by using various InGaAs structures

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ layer below the InAs QD layer with a 6 nm ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ overgrowth layer only. By covering the InAs QDs on a 1 nm ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ layer with an 8 nm ${\mathrm{In}}_x{\mathrm{Ga}}_{\text{1−x}}\mathrm{As}$ layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an ${\mathrm{In}}_{\text{0.15}}{\mathrm{Ga}}_{\text{0.85}}\mathrm{As}$ layer. The longer emission wavelength with relatively larger energy-level spacing was largely related to the change in the QD shape and size, especially the aspect ratio (height/width), which was confirmed by cross-sectional TEM images.