Tuning of near infrared excitonic emission from InAs quantum dots by controlling the sub-monolayer coverage

In this paper, we present a combined optical and structural study of GaAs-hosted InAs sub-monolayer QD (SML-QD) vertical multi-stacks. The main feature of this paper is to demonstrate the feasibility of sub-monolayer InAs QD with as low coverage as 0.4 ML which shows all the characteristics of QD excitonic emission, emitting in the NIR region (1.496 eV). This emission energy could be precisely tuned successfully by systematically controlling InAs coverage fraction in the range of 0.4-0.8 with corresponding emission in the range of 1.406-1.496 eV. The luminescence efficiency (4 K) exhibited an increasing trend with the decrease in InAs coverage. This paper elaborately discusses the interdependence of structure, strain and emission characteristics through a combined study of high resolution x-ray diffraction, Raman Scattering and Photoluminescence measurement. Strain-induced growth of the dots with different vertical size distribution (height similar to 2.3-1.4 nm) have been explored which were found to depend strongly on the InAs coverage at low temperature. The varying size distribution of the dot ensembles lead to different degree of carrier confinement, capture and localization, as determined from the low temperature (4 K) PL spectra. Significant enhancement of carrier localization inside almost 2D-like exciton was achieved by reducing InAs coverage well below one monolayer. From the relative temperature dependent photoluminescence measurements, it has been shown that the coupling and relaxation pathways of photo-carriers through the SML-QD multi-structure can be controlled by adjusting the InAs coverage.