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The effect of thin In0.15Ga0.85As strain-reducing layer on the structural and optical characteristics of multilayer InAs/In(Ga)As Stranski-Krastanov (SK) quantum dots (QDs) electronically coupled to Sub monolayer (SML) QDs has been investigated. The capping of In0.15Ga0.85As material over the InAs SK QDs reduces the out-diffusion of Indium atoms from the InAs dot resulting in an increased QD size. Moreover, the In0.15Ga0.85As material has a lattice constant between that of InAs and GaAs that aids in undulated transition of strain from the dot to the capping material and the GaAs spacer helping the growth of multilayer QD structure with high crystalline quality. Five different heterostructures are used in this study by varying the number of SK QD layers i.e., single layer (x1), bi layer (x2), penta layer (x5), hepta layer (x7) and deca layer (x10) which are grown on the same six stack SML QDs. A growth strategy has been employed while growing these multilayer SK QDs such that similar size QDs are grown even for deca layer structure with superior dot size homogeneity. The emission full width at half maxima computed at a temperature of 19 K came out to be ~50 nm for the penta layer structure indicating formation of uniform size QDs. The SK and SML QD sizes are chosen such that the ground eigen state of SML QDs coincide with excited states of SK QDs allowing the possibility of carriers to tunnel from the SML to SK QDs. The use of In0.15Ga0.85As capping led to a red shift in the PL peak position as compared to the structure without In0.15Ga0.85As capping. High-Resolution X-Ray Diffraction (HRXRD) measurements are carried out on these structures to understand the structural characteristics of multilayer SK on SML structures. The HRXRD results show that the hepta layer and the deca layer structures have the minimum strain with best crystalline quality. Considering both the optical and the structural characteristics, it has been concluded that the growth strategy helps in growing similar size QDs for the multilayer structures which can be used in various optoelectronic device applications.
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