Satyesh Kumar Yadav

Copper (I) oxide thin films are deposited on quartz substrates by DC magnetron reactive sputtering. This study examines the effect of post-annealing on their optoelectronic properties in detail. The films are grown by sputtering from copper in an atmosphere of argon and oxygen. The substrate temperature is held at 200 °C, while annealing in ambient atmosphere has been carried out between 100 and 600 °C. X-ray diffraction analysis, Raman and UV–Vis spectroscopy, and four-probe measurements were used to characterise the films. XRD indicates that deposited Cu2O has a preferred orientation of (110). Post-annealing did not show any measurable conversion to copper (II) oxide until about 500 °C, and the process was incomplete even at 600 °C. The highest conductivity is observed in the sample post-annealed at 100 °C. These results are of substantial technological importance for using Cu2O for a variety of applications, including transparent solar cell fabrication.

The effect of doping on the thermoelectric properties of Half-Heusler (HH) high-entropy alloy (HEA) Ti2NiCoSnSb was studied. Lower thermal conductivity was observed with increased Sb doping. Mass scattering by heavy (Ta, Zr) and light (Al) dopants was studied to further lower the thermal conductivity. Dopants at the level of up to 50% at the Ti site were studied. A high HH phase content was obtained in the Zr-doped samples, and a low-lattice thermal conductivity of 1.9 W/(m·K) was observed. This value is one of the lowest reported lattice thermal conductivities in HH alloys. The poor solubility of Ta led to undissolved Ta in the samples, which enhanced the electrical properties. In the case of Al doping, the NiAl phase raised the power factor value of Ti1.8Al0.2NiCoSn0.5Sb1.5 to 2.2 × 10-3 W/(m·K2), which is almost twice the corresponding value reported for Ti2NiCoSnSb. Interestingly, a maximum ZT of 0.29 was found in all of the doped systems, although the transport mechanism and microstructure varied widely with the type of dopant. An optimum dopant level of 25% of Zr, 7.5% of Ta, and 10% of Al is necessary to obtain the maximum ZT in these alloys. Compared to HH systems, the HH high-entropy alloy (HEA) systems provide a larger composition field for tuning the transport properties by simultaneous doping of multiple elements to lower the thermal conductivity.

Developing ductile refractory alloys have remained a challenge. Decreasing the valence electron concentration of refractory alloys has been widely suggested for improving their ductility. However, Re has been used to ductilize W, which goes against the low valency suggestion. The thermodynamic stability of refractory alloys has never been considered while suggesting alloying elements to improve ductility. Here we use first-principles density functional theory simulations to unravel the role of enthalpy of formation in improving the intrinsic ductility of refractory alloys. The intrinsic ductility is assessed using the D-parameter, which is the ratio of surface energy and unstable stacking fault energy. We studied 25 equiatomic binary refractory alloys and found that positive enthalpy of formation improves ductility. The small positive enthalpy of formation could be compensated by sufficiently large entropy; hence the alloy is expected to be a single phase. Our present work explains the role of high-valency and low-valency alloying elements in improving the ductility of refractory alloys. These findings provide a path to design thermodynamically stable and intrinsically ductile high-temperature alloys.

A series of (D-π)2-An-A based organic dyes containing a boron dipyrromethene (BODIPY) moiety as an ancillary acceptor (An) derivative were chosen, and the effect of donor moieties (diarylamine, carbazole, azepine, and dibenzazepine) was investigated to understand their photophysical and photoelectrochemical properties by employing density functional theory (DFT) and time-dependent DFT. It is experimentally proved that BODIPY enhances light-harvesting in the red and near IR regions of visible light. Electron density distribution analysis was performed for all the dyes to confirm the intramolecular charge transfer, envisioned from the simulated absorption spectra of the dyes. Carbazole donor-based dyes exhibited the lowest reorganization energy. A dye attached to the TiO2(1 0 1) surface was modeled to estimate the adsorption energy of the dyes. The density of states analysis revealed that the absence of defect states in the bandgap of TiO2 facilitates smooth electron transfer from the excited state of the dye to the conduction band of TiO2. Considering the lowest unoccupied molecular orbital (LUMO) energy level of the dyes and the conduction band energy level of TiO2, it is understood that all the dyes studied in this report are capable of electron injection upon photoexcitation. Considering the driving force for dye regeneration and the magnitude of reorganization energy, a carbazole donor-based dye (D2) would be the best performing dye in DSSCs. Previously, the power conversion efficiencies of the dyes have been reported, and the carbazole donor-based dye (D2) exhibited the highest efficiency among all the dyes. Our computational investigations are in good agreement with the experimental results.

Interface by definition is two-dimensional (2-D) as it separates 2 phases with an abrupt change in structure and chemistry across the interface. The interface between a metal and its nitride is expected to be atomically sharp, as chemical gradation would require the creation of N vacancies in nitrides and N interstitials in metal. Contrary to this belief, using first-principles density functional theory (DFT), we establish that the chemically graded Ti/TiN interface is thermodynamically preferred over the sharp interface. DFT calculated N vacancy formation energy in TiN is 2.4 eV, and N interstitial in Ti is −3.8 eV. Thus, diffusion of N from TiN to Ti by the formation of N vacancy in TiN and N interstitial in Ti would reduce the internal energy of the Ti–TiN heterostructure. Diffusion of N is thermodynamically favorable till ∼23% of N has diffused from TiN to Ti, resulting in an atomically chemically graded interface, which we refer to as a 3-D interface. We show gradual variation in lattice parameters and mechanical properties across the Ti/TiN interface. This opens a new way to control properties of metal/ceramic heterostructures, in line with the already established advantage of gradation at interfaces in micrometer length scale.

In a DC magnetron sputtering system, the deposition rate is significantly affected by the sputtering power and working pressure, and the precise nature of this dependence varies from machine to machine. An understanding of the effect of sputtering parameters on the deposition rate is required before undertaking further studies. To do so, we have carried out a design of experiments study on the optimization of deposition process parameters using a Taguchi design methodology. Taking an example of silver (Ag) deposited on silicon (100) substrates using the DC magnetron sputtering method, we use a Taguchi L9 design to reduce the experimental design space from 27 to 9 combinations. The attributes of the Ag thin films were quantified with respect to surface roughness (Ra), thickness (t), and sheet resistance (R s). The objectives of this study are 3-fold: establishing the effect of sputtering process parameters, namely the power and working pressure, on the deposition rate for the system under consideration; optimizing the sputtering process parameters within the chosen design space to ensure the formation of smooth conductive films using Taguchi analysis and response surface models; and analyzing the effect of annealing on the thin film characteristics. We found that the deposition rate increases with increasing the sputtering power. The highest deposition rate for the maximum power considered (25 W) was achieved at an intermediate working pressure of 6.1 × 10−3 mbar. The lowest deposition rate was obtained for the minimum power (5 W) and the highest working pressure (8.5 × 10−3 mbar) considered. For the given substrate–material system, we also found that the critical thickness below which the deposited films are non-conductive was 16 nm, which agrees with the existing literature.

An effective method for the formation of a Zn-doped Ru liner is demonstrated that realizes a self-forming barrier to achieve low resistivity interconnects for future back-end of line interconnect nodes. The "Ru-Zn"exhibits significantly improved adhesion to the dielectric and better electrochemical nucleation as compared to those of pristine Ru. In addition, time-dependent dielectric breakdown (TDDB) measurements indicate the inhibition of Cu ions drifting into the dielectric that precedes the TDDB failure. Complementary analysis using x-ray absorption spectroscopy, transmission electron microscope, and energy dispersive spectroscope suggests that the "Ru-Zn"forms an interfacial Zn-Si-O compound, and Zn, being more electronegative than Cu, protects the latter from oxidation. Calculation using density function theory also indicates that the Zn-Si-O compound adopts an intercalated structure at the interface of Ru/dielectric in which Zn occupies the interstitial sites within the Si-O lattice. We propose a twofold mechanism for improved TDDB performance: (1) the intercalated Zn atoms effectively block the diffusion of Cu ions through the dielectric and (2) Zn provides the cathodic protection of Cu that prevents the generation of mobile Cu ions that accelerate the TDDB.

Valence electron concentration (VEC), atomic size difference (δ), and Pugh’s ratio (B/G) are a few of the empirical parameters widely used to design ductile refractory alloys. Here, we used the intrinsic ductility parameter (D), which is the ratio of surface energy (γ s) and unstable stacking fault energy (γ u s f e), to design ductile refractory alloy. We found that the D correctly captures the experimentally observed ductility in concentrated refractory alloys. Here, we studied the enthalpy of formation (Δ E f), lattice distortion, and D of 9 refractory metals and 36 equiatomic refractory alloys using density functional theory simulations. We found that the Δ E f strongly influences the D of concentrated refractory alloys. The positive Δ E f and δ lead to large lattice distortion in concentrated refractory alloys. However, we did not find a strong correlation between lattice distortion and D in the presently studied alloys. We found that the success of VEC and Pugh’s ratio in designing ductile refractory alloys has a strong dependence on the underlying Δ E f of the alloy. We have developed a bottom-up method, which drastically reduces the number of alloys to be studied, to design ductile concentrated refractory alloys that can be thermodynamically stable.

Heterostructure interfaces play a major role in defining the performance of thin-film devices. High-k dielectric oxide-semiconductor heterostructures are being extensively investigated as promising candidates for future integrated circuits, thus it becomes important to precisely probe the interfaces at the atomic scale for technological advancements. In this work, a high-k dielectric oxide (Gd2O3)-semiconductor (Ge) interface was characterized at the atomic scale using complex-valued exit wave reconstructed from a set of focal series high-resolution transmission electron microscopy (HRTEM) images acquired without objective lens spherical aberration correction. The complexity of this characterization lies in removing image artefacts produced by amorphous layer deposited on the imaged region during ion milling which was successfully solved using an algorithm to remove amorphous background developed recently. The final result reveals that the interface of the present study is atomically sharp and flat. The thickness of the imaged region along viewing direction was estimated from channelling map. Comparing reconstructed amplitude of experimental data with that of simulated one generated using Density Functional Theory (DFT) optimized interface structure, it was found that the Gd2O3 layers were terminated at the Gd atoms in the interface.

Present work studies 126 quaternary and 126 quinary equiatomic refractory high entropy alloys (RHEAs), made from Group IV (Ti, Zr, Hf), Group V (V, Nb, Ta) and Group VI (Cr, Mo, W) elements. Rule-of-mixtures (ROM) technique is used to calculate liquidus temperature, density (ρ), Young's modulus (E), % atomic size difference (δ), valence electron concentration (VEC) and specific heat at constant pressure and at 1273 K (Cp). CALPHAD technique is used to predict the number of phases formed at 298 K, ρ and liquidus temperature. ROM calculated densities are matching perfectly with CALPHAD values. Densities and E are directly proportional to the VEC and liquidus temperature of the alloys. Ti, Zr, Hf are ductilizing the alloys and making them light; whereas Cr, Mo and W, are reducing the alloys' ductility and making them heavy. For quinary RHEAs, Cp shows six distinct groups with δ, but a similar trend is not observed in quaternary RHEAs. A methodology is developed to screen a large number of alloys based on various properties. Correlations between those properties are also studied.