A comparative study of Stranski-Krastanov (SK), sub-monolayer (SML) and coupled SK on SML InAs quantum dots as active region in InGaAs/GaAs/AlGaAs DDWELL heterostructure was done. Incorporation of additional high band gap confinement enhancing (CE) Al_xGa_1-xAs barrier helps to create new energy levels, increase the absorption coefficient, reduce dark current and improve crystalline quality of the heterostructure. This is because of the CE barrier which reduces In-adatom out-diffusion. Three different DDWELL heterostructure A, B and C with active regions as SK, SML and SK on SML respectively, had been modelled using the Nextnano simulation tool keeping all other parameters same. Photoluminescence (PL) emission wavelength, biaxial strains and hydrostatic strain profiles of heterostructures A, B and C were compared. Hydrostatic strain with less magnitude leads to better carrier confinement within the conduction band, and biaxial strain with high magnitude increases splitting between heavy-hole and light- hole bands, generating a red-shift in PL emission wavelength. It can be observed from the computed result that biaxial and hydrostatic strain in the SK QD are enhanced in structure C compared to A. Likewise, biaxial strain and hydrostatic strain in the SML QD stacks are enhanced in structure C compared to B. PL emission wavelength of structures A, B, and C were observed to be 1116nm, 864nm and 1170nm respectively. Therefore, structure C exhibits minimum strain among the heterostructures and highest PL emission wavelength for SWIR applications.

In the current study, Y-doped vertically aligned ZnO nanowires are grown on p-Si substrates by employing cost-effective double-step chemical bath deposition technique. The doping percentages are varied systematically (Y_xZn_1-xO, x= 0.0, 0.01, 0.02, 0.03, 0.04 M) to investigate the impact of rare earth doping on structural, optical and luminescence properties of ZnO nanowires. The crystalline quality, morphology, optical absorbance and defect states of grown nanowires are studied extensively by employing XRD, FESEM, UV-VIS and room-temperature PL. XRD results reveal that Y-doped ZnO nanowires have single phase hexagonal structure without any extra peaks related to the Y-mixed oxides. FESEM analysis indicates that the dopant with higher radius dose not affected the morphology of ZnO nanowires. The optical energy band gaps of such nanowires are calculated by employing UV-VIS spectroscopy and values are estimated to be 3.11 eV to 2.97 eV. The absorption coefficient, reflective index and extinction coefficient are also extracted and analysed for all of such nanowires. PL results showed that the undoped ZnO nanowires exhibit low UV emission (374 nm) and relatively high green emission (552 nm), whereas, after a low amount of Y doping, UV emission peak enhanced along with an additional blue emission peak at 437 nm. Such blue emission peak is associated with the Zn interstitials related defects, which is generally responsible for the enhancement of donor concentrations in ZnO. Significantly, the oxygen vacancy related green emission peak reduces gradually with increasing Y incorporation. The work provides a detailed study on optical properties of Y-doped ZnO nanowires, which is essential for developing the next-generation heterojunction based optoelectronic devices.

In current study, the variation of sub-capping thickness of InGaAs strain reducing layer (SRL) of InAs quantum dot heterostructure using digital alloy approach is presented. The thickness of 6 nm SRL of conventional structure (sample A) is divided equally with 2 nm thickness (sample B) by using digital alloy approach. Further, using such approach, this thick 6 nm capping is divided in unequal fashion for sample C (1 nm, 2 nm and 3 nm) and sample D (3 nm, 2 nm and 1 nm) from InAs QD towards top GaAs layer. The In-content inside the SRL of the sample A is 15%, whereas, In-content inside the divided-SRL is considered as 45%, 30% and 15% for all other samples. Such composition of SRLs helps in reducing the In-out diffusion, minimizing the lattice mismatch at InAs QD-SRL and SRL-top GaAs layer interfaces, and also reduces the strain inside the overall heterostructures. Two strains, namely hydrostatic and biaxial are calculated by using Nextnano for all the structures and compared simultaneously. The hydrostatic strain inside the QD of sample D is reduced by 4.74%, 1.07% and 2.269% and the biaxial strain inside the QD of sample D is improved by 1.66%, 0.696% and 1.276% as compared to that of samples A, B and C, respectively. The computed PL emission of samples A, B, C and D are observed to be 1305 nm, 1365 nm, 1349 nm and 1375 nm, respectively. Hence, sample D is the optimum choice for fabricating future opto-electronic devices.