Browsing by Author "Chakrabarti, Subhananda"

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.

In this work, the concept of the novel approach called linear alloy capping layer (LACL) has been investigated on the strain-coupled bilayer InAs/GaAs1-ySby QD heterostructures. Here, two analog structures with low (structure BA1) and high (structure BA2) antimony (Sb) contents, and one linear alloyed structure (BL) with varying Sb-content inside the capping layer is considered. The Sb-content inside the CL of structure BA1 and BA2 are 10% and 20%, respectively. Whereas, it is varying linearly from 20% to 10% inside structure BL. The CL and GaAs spacer layer thickness has been taken as 8 nm and 13 nm, respectively. All these three structures have been modeled using Nextnano++ simulation software. Two strain components, hydrostatic and biaxial have been computed and compared. These two strain components help in decreasing the ground state energy gap which leads to a red-shifted PL emission. The structure BL offers improved biaxial strain by 1.11% and 0.56% inside QD compared to structures BA1 and BA2. In addition, the magnitude of hydrostatic strain inside QD of structure BL is reduced by 1.78% and increased by 0.64% compared to structures BA1 and BA2. The strain inside the CL of structure BL is reduced very smoothly in a linear fashion as compared to other analog structures. The computed PL emission of structures BA1, BA2, and BL are 1371 nm, 1665 nm, and 1617 nm, respectively. Also, the proposed structure BL offers a type-II band profile. Hence this proposed approach is useful for future optoelectronic applications.

In this work, the authors introduced a novel approach called digital alloy capping layer (DACL) and investigated its effect on the optical and structural properties of InAs quantum dot-in-a-well (DWELL) heterostructures. In DACL, a conventional thick well layer is digitized equally with different compositions analogous to short-period-superlattice (SPS). The DACL approach's effect has been studied experimentally and theoretically on DWELL heterostructures with InxGa1-xAs as the well material. The photoluminescence (PL) study reveals that DACL observes a red-shift of ∼55 nm as compared to AACL approached heterostructures. High-resolution X-ray diffraction (HRXRD) results reveal higher In-content, controlled In-out diffusion from InAs QD, and improved in-plane strain in DACL samples compared to the analog sample. The study has been extended to QD heterostructures with GaAs1-xNx and GaAs1-ySby as well materials, and comprehensive analysis has been carried out. Hence, the DWELL heterostructures with the DACL approach can be utilized to fabricate infrared photodetector devices.

An experimental study of optical phonon modes, both normal and interface (IF) phonons, in bilayer strain-coupled InAs/GaAs_(1-x)Sb_x quantum dot heterostructures has been presented by means of low-temperature polarized Raman scattering. The effect of Sb-content on the frequency positions of these phonon modes has been very well correlated with the simulated strain. The Raman peaks show different frequency shifts in the heterostructure with varying Sb-content in the capping layer. This shift is attributed to the strain relaxation, bigger size of quantum dots and type-II band alignment.

The role of combinational (ternary and quaternary) capping layers to understand the strain distribution mechanism and optical properties through digital alloy capping layer (DACL) has been presented in this work. GaAsSb as ternary and InGaAsSb as quaternary capping materials have been used, and a combination of ternary-quaternary/quaternary-ternary has been implemented. Digital alloy technique has been employed to cover the quantum dots with the combinational capping materials. The biaxial as well as hydrostatic strain is obtained using Nextnano++ software and compared to analyze the strain distribution inside the heterostructure. Digital alloyed structures offer a red shift in emission wavelength (∼ 2 μm) compared to conventional structures based on the selection of capping material. However, the selection of capping materials (GaAsSb and InGaAsSb) exhibits both type-I as well as type-II band profiles which can be utilized in multiple optoelectronic applications. This detailed theoretical study of the digital and analog alloy approach of the combinational ternary/quaternary and quaternary/ternary capping layer would help to optimize advanced futuristic device heterostructures with reduced strain and better crystal quality.

Strain – induced intermixing in sub – monolayer (SML) quantum dots (QDs) affects primarily the dot size distribution in an erratic way and also the inter – dot coupling efficiency for carrier transport. In this study, we have explored the role of strain and carrier confinement effects in order to control the dot size distribution by varying InAs QDs coverage (X: 0.3 and 0.5 ML) and stacks (Y: 4, 6, and 8) simultaneously. The size variation affects the position of localized levels inside QDs primarily as it is the inter – dot coupling that decides the luminescence efficiency. Next, through multiple stacking of QD layers, strain variation becomes inhomogenous that facilitate a higher degree of In – Ga intermixing which necessitates to estimate the amount of Indium (%) present inside such In–rich islands to progress for a superior optical performance. Hence, we combine and address these above concerns in great detail. The 20 K ground-state (GS) photoluminescence (PL) energy for 0.3 and 0.5 ML samples were centered at 1.24–1.33 eV, and 1.11–1.19 eV respectively. The PL linewidth and activation energy variation with temperature validated that there exists two possible thermal escape pathways for carrier recombination. Besides, PL – excitation (PLE) studies comprehended the influence of phonon and excited states (ES) in the samples. Raman analysis helped us calculate the In% inside QDs by figuring out the shift in InAs QD phonon modes (amount of compressive strain). Furthermore, the structural analysis by high-resolution X-ray diffraction (HRXRD) was done to calculate the strain moderated defect density and also the effect of in – plane compressive strain to the reduction in quantum confinement. The increase in compressive strain inside QDs caused more In–Ga intermixing in 0.3 ML samples and higher defect densities. Altogether we see that sample with 0.5 ML, 4 stacks showed vertically-correlated dot growth (from HR-XTEM) with minimum strain (−0.03383 a.u.), dislocation density (4.887 x 10¹¹ cm⁻²) and higher In content (62.41%). This high degree of excitonic carrier confinement and strain tuning makes SML QDs an attractive candidate for room-temperature (RT) operable photodetector (PD) devices.

This work presents the impact of barrier spacer on the structural and optical properties of strain-coupled Stranski-Krastanov (SK) on Sub Monolayer (SML) quantum dot (QD) heterostructures. Various ternary and quaternary materials have been employed as the barrier layer of SK-SML QD heterostructures. In the coupled SK-SML QDs, the residual strain propagates from the SML seed layer towards the top SK dots, introducing defects and dislocations. After employing the ternary (GaAs1-ySby) and quaternary (InxAl0.21Ga1-.0.21-xAs and In0.18Ga0.82As1-ySby) materials, the residual strain reduces, reducing the defects which thereby helps in increasing the crystalline quality of the heterostructure. Nextnano software has been used to compute the structures' strain, energy band profile, probability density functions, and emission wavelength. Two strain components, viz. hydrostatic as well as biaxial strain, have been computed and compared for all the heterostructures to understand the distribution of the strain profile. The emission wavelength is red-shifted for the SK-SML QD heterostructures with ternary and quaternary materials as a barrier layer as compared to that of the GaAs barrier. Moreover, type-I and II energy band profiles are observed for Sb-based barrier material, appropriate for various optoelectronic applications. This is a comparative study of SK-SML QDs with various barrier materials helps minimize the strain and defects and improve the device performance.