A detailed investigation of the impact of varying number of dot layers in strain-coupled multistacked InAs/GaAs quantum dot heterostructures

Strain-coupled InAs quantum dot (QD) heterostructures has been compared in terms of their optical properties, with varying the number of stacks. Each structure consists of seed layer dots (2.5 monolayer of InAs) with a capping layer of 6.5nm GaAs followed by active layer dots (2.1 monolayer of InAs). The active layer QD with the capping layer is repeated by one, two, four, and six times in bilayer, trilayer, pentalayer, and heptalayer samples, respectively. Thickness of the GaAs spacer layer in between active layer QD stacks is different for each structure. A red shift in photoluminescence (PL) emission was obtained for the strain-coupled multi-stack samples compared to the conventional uncoupled one. This is due to the formation of larger dot size in coupled structures. We also observed a monomodal dot distribution till the pentalayer sample, but after that a bimodal distribution was found, which may be due to the enhancement of strain as we further increase the stacks. Compared to an uncoupled sample, all coupled samples exhibited lower full width at half maximum (FWHM) values (uncoupled-35.89nm, bilayer-32.83nm, trilayer-30.17nm, pentalayer-68.91nm, and heptalayer-67.55nm) which attributes to homogeneous dot size distribution. Higher activation energies were measured in coupled samples compared to the conventional uncoupled one. Trilayer sample claimed the highest PL activation energy of 303.42meV, whereas the uncoupled sample has only 243.89meV. This increased activation energy in the coupled structures will be helpful for lower dark current in the devices.