Growth strategy to achieve mono-modal quantum dot size distribution in InAs/GaAs multi stack coupled heterostructures

Abstract

In multi-stack structures, the strain field existing in the seed layer induces the nucleation of subsequent dots on the preexisting dots, and as the InAs quantum dot (QD) coverage is fixed we eventually get a dissimilar overgrowth percentage between subsequent layers. Therefore, such structures are prompt to defects and dislocations and also produce a multimodal distribution of dots. In this paper, a detailed investigation has been done on the growth strategy of strain-coupled multi-stack InAs/GaAs heterostructures, to achieve mono-modal distribution of InAs QDs. The heterostructures discussed in this paper are grown with fixed seed layer coverage of 2.5 monolayer (ML) InAs in order to maintain the constant overgrowth percentage between the subsequent QD layers. The subsequent QD layer coverage has been varied from 2.5ML to 2.1ML and the GaAs spacer thickness in between them varied from 10nm to 12nm. Power dependent photoluminescence (PL) spectra at 18K revealed the transition from multimodal to monomodal as the growth parameters varied. We have also optimized the spacer thickness between the seed layer and immediate dot layer to 6.5nm, by keeping other parameters constant. It results in a red shift in PL emission peak and lowers the full width half maximum by 12nm, which seems to be improving in dot size and homogeneity. The highest activation energy has been obtained from the optimized structure, which attributes to a better QD confinement and hence lowers dark current value. An enhancement in the optical properties may happen with further optimization.