InAs quantum dots capped by GaAs, In0.4Ga0.6As dots, and In0.2Ga0.8As well.

We have fabricated and characterized three types of InAs quantum dots (QDs) with different InxGa1-xAs capping layers. Post-growth atomic force microscopy measurements show that the In0.2Ga0.8As/InAs structure has a smooth surface (dot-in-well structure), whereas the In0.4Ga0.6As/InAs structure revealed large QDs with a density similar to that underneath InAs QDs on GaAs (dot-in-dot). With increasing In mole fraction of the capping layer and increasing In0.4Ga0.6As thickness, the energy position of the room-temperature photoluminescence (PL) peak is red-shifted. The quantum dot-in-dot structure emits stronger room-temperature PL than does the quantum dot-in-well structure. With a spatially distributed strain in the InAs quantum dot, we have solved the three-dimensional Schrödinger equation by the Green's function theory for the eigenvalues and eigen wave functions. It is concluded that the ground state increases its wave function penetration into the low-barrier InxGa1-xAs capping layer so that its energy position is red-shifted. The reduced PL peak intensity of the dot-in-well (compared with GaAs covered dots) is due to the reduced overlapping between the ground state and the extended states above the GaAs barrier. The overlapping reduction in the dot-in-dot is over compensated for by the reduced relaxation energy (full width at half-maximum), indicating the importance of the sample quality in determining the PL intensity.