Self-assembled InAs quantum dot heterostructures have been employed as a promising entrant for various optoelectronic device applications over the past few decades. This is because of their properties such as 3D carrier quantum confinement, normal incidence photon absorption, etc. In this work, we have qualitatively studied the effect of growth rate variations of MBE-grown 3ML SK QD in DWELL heterostructures on their optical and strain behaviour. Three samples A, B, and C with a growth rate of 0.1 ML/s, 0.075 ML/s, and 0.05 ML/s with growth time 30s, 40s, and 60s, respectively have been considered. The bottom and top In0.15Ga0.85As well thickness is kept constant at 2 nm and 6 nm respectively. From the optical study, it has been observed that the PL emission wavelength of structures A, B, and C was approximately 1045 nm, 1054 nm, and 1071 nm respectively. Also, the observed PL intensity of sample C is almost three times that of sample A. This is due to the homogeneous dot formation in sample C which has a low growth rate and promotes uniform dot sizes. From the simulation results, it has been observed a significant improvement in hydrostatic and biaxial strain of sample C as compared to that of samples A and B. The simulated PL spectra for samples A, B, and C were also calculated to be 1053 nm, 1052 nm, and 1082 nm respectively. Hence, sample C with high PL intensity can be utilized for SWIR optoelectronic applications.

This research investigates the effects of continuous tungsten inert gas arcing (C-TIGA) and pulsed tungsten inert gas arcing (P-TIGA) processes on the hardness of case-hardenable low-alloy steels, focusing on AISI 8620 steel. The study investigates the influence of arc parameters on mechanical properties and microstructural changes. The modified surface of AISI 8620 steel was investigated analytically and validated through experiments. Using an 18 mm thick AISI 8620 steel plate, C-TIGA and P-TIGA processes are employed in one pass with subsequent hardness measurements in the fusion zone and heat-affected zone (HAZ). The results show a remarkable improvement in the hardness of the modified zone compared to the base metal, with a noteworthy increase of 127 +/- 20% and 77 +/- 25% for C-TIGA and P-TIGA processes, respectively. Notably, at equivalent arc currents, P-TIGA outperforms C-TIGA, achieving a hardness increase of over 20 +/- 10% in the modified zone. In addition, the careful control of process parameters in P-TIGA leads to a better change in the thermal and mechanical properties of rolling bearing steel compared to C-TIGA. The combination of significantly increased surface hardness and an extensively modified zone depth contributes to increased wear resistance and compressive stresses in the matrix. In particular, the P-TIGA process is shown to be superior as it has greater modified zone depth, narrower HAZ width, and higher hardness. This research establishes P-TIGA as a promising method for surface modification and offers significant improvements over the traditional C-TIGA method.

The present work introduces an in-situ technique to tune the inter-dot coupling between vertically aligned InAs surface and buried quantum dots (SQD and BQD). Here, we utilize the self-assembly growth kinetics to get the most stable SQDs with lowest possible surface states. We vary the monolayer (ML) coverage of top SQD layer (from 2.2 ML to 1.6 ML), keeping monolayer coverage of BQD constant (2.7 ML). The reduced Fermi level pinning effect is observed through the monolayer coverage minimization, and the upshifting of energy level in the conduction band is obtained. The degeneracy between BQD and SQD energy levels is maximized in case of the 1.6 ML SQD and an enhanced coupling between the two QD families is observed through the photoluminescence (PL) result analysis. The type-II carrier distribution with effect of the Fermi level pinning is reduced in the 1.6 ML SQD structure, which claims the highest level of Coulomb coupling between the two QD families. Also, AFM images show lower dispersion in size and shape of the surface dot with 1.6 ML coverage. The proposed optimum QD heterostructure would promote a QD based sensor with high efficiency in effect with higher level of inter- dot carrier tunneling.

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 the past decade, surface quantum dots (SQDs) have been thoroughly investigated for sensing applications. The SQDs suffer from the limitations of non-uniformity dot distribution and weak oscillator strength, which affect their response to ambient contaminants. We have achieved uniformity by coupling buried quantum dots (BQDs) with SQDs. Moreover, BQDs provide additional carriers to SQDs for enhancing sensitivity. In this study, we have theoretically investigated the impact of varying the capping material of BQDs on their strain and optical properties. Investigations have been carried out with three samples having different capping materials as GaAs (sample A1), InGaAs (sample A2), and InAlGaAs (sample A3). A decreasing trend in the magnitude of hydrostatic strain and an increasing trend in biaxial strain inside the BQD from samples A1-A3 is observed. With a decrease in hydrostatic strain, the conduction band eigenstate lowers towards the band edge resulting in a lowering bandgap. With an increase in biaxial strain, the bandgap lowers due to the heavy hole (HH) and light hole (LH) band splitting. The lowering of the bandgap enhances the luminescence of BQD in sample A3. The computed photo-luminescence (PL) emission wavelength is found to be 1547 nm, 1558 nm, and 1568 nm for GaAs, InAlGaAs, and InGaAs capping respectively. The lowering in the bandgap of BQD leads to band alignment between SQD and BQDs, which may improve the carrier communication between these layers and become a promising candidate for better carrier reservoirs for SQDs in sensor applications.

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 (GaAs 1-y Sb y ) and quaternary (In x Al 0.21 Ga 1-.0.21-x As and In 0.18 Ga 0.82 As 1-y Sb y ) 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.

Many times the power industry shutdowns due to unplanned maintenance. In such environments, the spallation/cracking is increased in the protective oxide layer, results an explosion in tube joints which threatens the safety of the power industry. In this investigation, microstructurally different regions of TIG joint in T11 boiler steel were examined by the thermogravimetry method under cyclic conditions. The kinetics of oxides in these regions of TIG joint after some initial cycles was parabolic with increased cracks at cyclic stresses. The higher spallation of oxide scale on the HAZ than the weld metal was because more cracking occurred under the cyclic process. The weld metal showed slow oxidation kinetics in comparison to HAZ.

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-periodsuperlattice (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 similar to 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 inplane 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.

The current work represents the influence of Sb composition on strain profiles, band alignment of InAs/GaAsSb Stranski-Krastanov (SK) quantum dots (QD) heterostructure. A single layer InAs/GaAs SK QD heterostructure has been utilised as a reference structure (sample A). Three different structures B, C and D are chosen with the Sb composition of 10%, 14% and 22% in the capping layer on the InAs QDs. The strain distribution of InAs quantum dots with GaAs1-xSbx capping has been investigated in detail by using Nextnano simulation software. The transition from type-I to type-II band alignment occurs after 14% Sb incorporation. The effect of variation in the Sb component on the strain profile is analysed in terms of hydrostatic and biaxial strain. In the strain profiles, the biaxial strain increases significantly from samples A to D, whereas the hydrostatic strain is reducing from samples A to D inside the QD region simultaneously. Further, low temperature photoluminescence (PL) has been performed to validate the simulated results. The PL peak is originated at ~1030 nm for sample A, which shows a good agreement with the simulated value ~1071 nm. With the similar dot size, the simulated PL peaks are found at 1116, 1284, 1675 nm for samples B, C and D, respectively. A slight variation is observed in the experimentally obtained PL peaks and simulated one due to the incorporation of higher Sb content. With the higher Sb content, relatively more defects are generated, which affects the experimental PL peaks. Also, a red shift is found in the PL peaks from ~1030 nm to ~1213 nm at 19K and it gives an insight to develop various optoelectronic devices using such QDs heterostructure.

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) AlxGa1-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 this study, the author proposed a new technique for strain minimization, called linear alloy technique (LAT), for the symmetric dot-in-a-well (DWELL) heterostructure. Here, three different DWELL InAs QDs heterostructures with 6 nm thick InxGa1-xAs as well material have been simulated using 8 band k.p. model-based Nextnano software. Here, the first sample is analog alloyed DWELL heterostructure having In0.15Ga0.85As well (Sample A), the second sample is digital alloyed DWELL heterostructure where the well layer is divided into three sub-layers of 2nm thickness with indium composition varied from 45% to 15% in the step of 15%(Sample D), and the third sample is linear alloyed DWELL heterostructure where indium composition is varied from 45% to 15% (Sample L) in linear fashion have been studied. The Lower the magnitude of hydrostatic strain better will be carrier confinement. The more the biaxial strain, the more the heavy-hole and light-hole band splitting, which reduces the transition energy gap. The computed biaxial strain is increased by 1.52% and 2.21%, and the magnitude of hydrostatic strain is reduced by 3.66% and 1.13% in sample Lcompared with samples A and D, respectively. Strain inside the well layer of sample L reduces more smoothly than samples A and D, respectively. The computed PL emission wavelength for all three samples are 1329, 1418, and 1419 nm for the samples A, D, and L, respectively. Hence, this proposed technique can be the best choice for fabricating future optoelectronic-based devices.

This research examined the SMAW and TIG weldments in T22 steels after exposure in air at 900 degrees C under thermal-cyclic conditions. The kinetics of oxidation was identified by thermogravimetry-technique. The SMAWwelded steel oxidized at faster rate than TIG-welded because the formation of cracks of high extents in surfaceoxide and voids in inner-scale, leads the oxide-scale in-thicker on the weld and the heat-affected zone together. TIG weldment showed slow oxidation kinetics and considerably lower weight-gains than that of SMAW, reason the minor presence of chromium oxide in inner oxide-scale reduced the oxidation rate as detected by EDX.

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 study, we minimize the strain by using the new technique called linear alloy technique (LAT) for the Stranski-Krastanov (SK) quantum dot heterostructure. Here, three different SK InAs QDs heterostructures with 6 nm thick capping layer (CL) having InxGa1-xAs as capping material have been simulated using the 8-band k.p. model-based Nextnano software. Here, the first sample is analog alloyed SK QDs heterostructure having In0.15Ga0.85As capping (Sample A1), the second sample is digital alloyed SK QDs heterostructure where CL is divided into three sub-layers each of 2nm thickness with indium composition varied from 45-30-15% (Sample D1), and the third sample is linear alloyed SK QDs heterostructure where indium composition is varied from 45% to 15% (Sample L1) in a linear fashion, have been studied. The biaxial and hydrostatic strain is computed for all three heterostructures and compared. The biaxial strain is improved by 2.03% and 2.0%, and hydrostatic strain is reduced by 3.49% and 0.071% inside the QD region of sample L1 compared with samples A1 and D1, respectively. Additionally, digital sample D1 offers a step-wise strain reduction inside CL compared to analog sample A1. However, sample L1 offers an even more relaxed strain inside CL than samples A1 and D1, respectively. The PL emission wavelength is observed at 1317, 1372, and 1379 nm for samples A1, D1, and L1, respectively. Hence the linear alloy technique is useful for making future optoelectronic devices where strain reduction is the main factor.

In this study, variation in Sb composition in In0.18Ga0.82AsYSb1-Y as capping layer (CL) over SK QDs and matrix material (MM) in 6 stacks of SML QDs for InAs SML, and strain coupled InAs SK-on-SML QDs heterostructure have been done theoretically. The structural and optical properties have been investigated through Nextnano++ software. Different single-layer SK (A-series), SML (B-series), and coupled SK-on-SML QDs (C- series) structures have been modeled for this study. Two main strain components, hydrostatic and biaxial strain have been computed simultaneously with the solution of 3-D Schrodinger's equation. The hydrostatic strain is compressive in the growth direction and gives information about carrier confinement in the conduction band while a biaxial strain is tensile in the growth direction and gives information about splitting in the valance band. There is a significant improvement of hydrostatic and biaxial strain observed for A-series and B-series structures respectively, which have quaternary capping, which helps in reduced overall strain and reduced InAs QDs desorption. In Addition to that, the coupled SK-on-SML structures (C-series) with InGaAsSb (Sb-15 and 25%) as capping layer and matrix layer possess more biaxial strain and reduced hydrostatic strain as compared to the conventional coupled structures with InGaAs as matrix material of SML and GaAs as capping layer over SK. The computed PL emission wavelength for proposed C-series structures offers a red-shift over conventional coupled structures. These configurations show type-II band alignment which can be used for various optoelectronic applications in the SWIR regime such as solar cells, long-range communication, etc. This study can be advantageous for the optimization of strain-coupled heterostructures.

Welding is the primary joining-process, produced the heat affected zone which is mostly damaged severely because of prolonged period particularly in corrosive environment. Most industrial components are joined by shielded metal arc welding (SMAW) process. The present article, therefore, investigated the performance of corrosion of weld-metal (WM) and heat-affected zones (HAZs) of weldment in ferritic T22 steel exposed in salt (Na2SO4-60 wt% V2O5) environment at 900 degrees C under thermal-cyclic conditions. During the experiments the weight-gain measurements were applied on these regions of weldment to observe the hot corrosion kinetics. The weight gain per unit area increased with the increased number of testing cycles. The parabolic growth rate was described the corrosion kinetics after few initial cycles. HAZs corroded at faster rates than WM due to more spallation and cracking in surface oxide-scale.

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.

A detailed theoretical study of optical and structural properties of heterogeneously coupled Stranski-Krastanov (SK) on submonolayer (SML) QDs heterostructure with In0.18Ga0.82AsYSb1-Y capping on InAs SK QD has been done using Nextnano++ software. Variation of Sb composition was taken as 10, 15, 20 and 25%. By solving 3-D Schrodinger’s equation the biaxial and hydrostatic strain distribution, energy band diagram and PL peak observation is done. It has been noted that increasing Sb composition contributes in transition from type I to type II. This transition can be discovered in positions of probability density function for electron and holes and from energy band illustrations. The biaxial strain is responsible for energy splitting in light hole (LH) and heavy hole (HH) and its distribution in SK QD increases with increasing Sb composition. The hydrostatic strain is compressive strain in nature, which is responsible for carrier confinement in conduction band. With increasing Sb composition the magnitude of hydrostatic strain is observed as diminishing. From the ground state energy levels of electron-hole Eigen state (E1-H1) PL emission wavelength have been observed for all the four structures. It has been noted that higher the composition of Sb, higher the wavelength emission. In overall analysis it has been observed that type-II has lower compressive strain inside the QDs and higher wavelength emission. The composition selection demonstrates both type-I and type-II energy band profile which can be advantageous in many optoelectronic device properties. This theoretical study can be useful in optimization of strain coupled heterostructures for better crystalline quality.

Stranski-Krastanov (SK) Quantum dots (QDs) based lasers and detectors are now used in many fields because of the plentiful advantages they offer, some of which are normal incident absorption and phonon bottleneck. In order to optimize the properties of the devices, growth parameters have to be tuned properly. The effects of growth temperature (GT) and growth rate (GR) variations on the QD devices have been studied here by comparing simulations and experiments. In this study, InAs QDs are grown on GaAs substrates with four different temperatures, from 480°C to 510°C, and with five different GRs, from 0.15 ML/s to 0.025ML/s. To observe the grown heterostructures' structural and optical properties photoluminescence (PL) and PL excitation (PLE) have been performed on the samples. The size, shape, and composition of the QDs ultimately decide the energy levels in the heterostructure. Hence, it determines the optical and electrical properties of the devices. Here we simulated 3-D strain profiles of the QD and compared the results with PL and PLE. The trends in simulated biaxial strain and heavy hole (HH) - light hole (LH) band splitting, observed in the PLE, match pretty well. We observed that the change in GT drastically affects the composition of the dots and the wetting layer, whereas a change in GR only changes the lateral size of the QDs, and do not affect the strain or composition. These studies can be beneficial for p-i-p short-wave infrared (SWIR) detectors since their spectral response is tuned by the HH-LH band splitting.

In this study, we have discussed the effect of strain distribution and optical properties on In0.14Ga0.86As matrix thickness variation (t(mat) ) in self-assembled InAs quantum dot (QD) heterostructure using temperature and power-dependent photoluminescence (PL) measurements. The calculated ground-state transition energies are 1.12, 1.14 and 1.09 eV for t(mat) of 2, 4 and 6 ML (monolayer) In0.14Ga0.86As matrix thickness respectively. We also discern that the full-width at half-maximum (FWHM) broadens gradually as temperatures increases due to electron-phonon scattering. The calculated activation energy (E-alpha) values are 231, 302 and 98 meV for increasing tmat. The partial strain relief due to varying In0.14Ga0.86As layer thickness occurs due to QD size tunability by preventing Indium (In) segregation effect, that sets the possibility to understand about InAs inter-band and inter-subband transitions of PL emission. This has been validated with HRXRD results where strain decreases linearly with increasing tmat. Here In0.14Ga0.86As layer acts as a strain-reducing layer (SRL) in QD heterostructure as well. Thus helps in reducing the hydrostatic strain (epsilon(hyd)) of InAs QDs, while the lower InGaAs layer increases the QD density, leading to a remarkable rise in PL intensity due to state filling of carriers. The effect of strain distribution for varying tmat in the heterostructure was also studied using nextnano++ simulations. The relative percentage change in hydrostatic (biaxial) strain was calculated to be 5.5% (8%) respectively. Thus, the results so obtained can help in tuning matrix thickness on the PL emission properties of QDs and therefore in the realization of several optoelectronic devices.