In-situ tailoring of band alignment between strain-coupled surface and buried InAs/GaAs quantum dots for sensor applications

Abstract

This paper reports a comparative study of InAs/GaAs quantum dots (QDs) heterostructures with vertically aligned strain-coupled uncapped and capped buried dots, epitaxially grown by solid-state MBE. Here an in-situ method is used to optimize the band alignment among the coupled QD heterostructures. In this work, the stable uncapped QDs are grown with reduced surface energy using the self-assembly growth technique called Stranski Krastanov (SK) QDs. During growth we reduced the Indium flux to the top uncapped QDs layer referred to as Surface QDs (SQDs), keeping a constant overgrowth percentage (2.7 ML) for capped QDs known as buried QDs (BQDs). Up to 2 ML SQD, two distinguished energy states for BQD and SQD are observed, showing a gradual blue-shift as the InAs content reduces from 2.2 to 2 ML. As we get into the regime of 1.6ML, the energy states of SQD are in resonating condition with the BQDs. This resonance enhances the electronic interaction between the coupled dot layers. The corresponding photoluminescence response depicts the wave function overlapping surface and buried dots. In addition, AFM images show a homogeneous distribution in size and shape of the SQDs in this regime. Strain analysis of the heterostructure is performed by Raman spectroscopy and HRXRD measurement. The heterostructure with 1.6 ML coverage would promise a sensor based on SK-QDs with high efficiency due to inter-dot carrier communication. Here the underneath capped QD supplies surplus carriers act like a reservoir and the surface QD layers act as a primary receptor.