Controlling the aspect ratio of quantum dots: From columnar dots to quantum rods

We demonstrate the feasibility and flexibility of artificial shape engineering of epitaxial semiconductor nanostructures. Novel nanostructures including InGaAs quantum rods (QRs), nanocandles, and quantum dots (QDs)-in-rods were realized on a GaAs substrate. They were formed by depositing a short-period GaAs/InAs superlattice (SL) on a seed QD layer by molecular beam epitaxy growth. The InAs layer thickness in the SL plays an important role in obtaining the QRs. The growth of the QRs is very sensitive to growth interruption and growth temperature. By properly choosing both growth parameters, QRs with length of 41 nm corresponding to an extremely large aspect ratio of 4.1 were obtained. The evolution from a 0-D to 1-D confinement type is evidenced in the optical properties. The origin of the optical transitions from the QRs was understood by calculations of the electronic states within a fully 3-D approach in the eight-band k$\cdot$ p approximation. The QRs are embedded in a GaAs matrix and are therefore free from surface traps. This feature enables high material quality and consequently their application in real devices. At room temperature, laser diodes based on QR active regions lasing around 1120 (or 1130) nm are demonstrated.