Materials Science (cond-mat.mtrl-sci)

Abstract: The thermal conductivity (k) of polycrystalline silicon carbide thin films is relevant for thermal management in emerging silicon carbide applications like MEMS and optoelectronic devices. In such films k can be substantially reduced by microstructure features including grain boundaries, thin film surfaces, and porosity, while these microstructural effects can also be manipulated through thermal annealing. Here, we investigate these effects by using microfabricated suspended devices to measure the thermal conductivities of nine LPCVD silicon carbide films of varying thickness (from 120 - 300 nm) and annealing conditions (as-grown and annealed at 950 degrees Celsius and 1100 degrees Celsius for 2 hours, and in one case 17 hours). Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) spectra and density measurements are also used to characterize the effects of the annealing on the microstructure of selected samples. Compared to as-deposited films, annealing at 1100 degrees Celsius typically increases the estimated grain size from 5.5 nm to 6.6 nm while decreasing the porosity from around 6.5% to practically fully dense. This corresponds to a 34% increase in the measured thin film thermal conductivity near room temperature, from 5.8 W/m-K to 7.8 W/m-K. These thermal conductivity measurements show good agreement of better than 3% with fits using a simple theoretical model based on kinetic theory combined with a Maxwell-Garnett porosity correction. Grain boundary scattering plays the dominant role in reducing the thermal conductivity of these films compared to bulk single-crystal values, while both grain size increase and porosity decrease play important roles in the partial k recovery of the films upon annealing. This work demonstrates the effects of modifying the microstructure and thus the thermal conductivity of silicon carbide thin films by thermal annealing.

Abstract: The morphology of a collision cascade is an important aspect in understanding the formation of defects and their distribution. While the number of subcascades is an essential parameter to describe the cascade morphology, the methods to compute this parameter are limited. We present a method to compute the number of subcascades from the primary damage state of the collision cascade. Existing methods analyse peak damage state or the end of ballistic phase to compute the number of subcascades which is not always available in collision cascade databases. We use density based clustering algorithm from unsupervised machine learning domain to identify the subcascades from the primary damage state. To validate the results of our method we first carry out a parameter sensitivity study of the existing algorithms. The study shows that the results are sensitive to input parameters and the choice of the time-frame analyzed. On a database of 100 collision cascades in W, we show that the method we propose, which analyzes primary damage state to predict number of subcascades, is in good agreement with the existing method that works on the peak state. We also show that the number of subcascades found with different parameters can be used to classify and group together the cascades that have similar time-evolution and fragmentation.

Abstract: Semiconducting oxides possess a variety of intriguing electronic, optical, and magnetic properties, and native defects play a crucial role in these systems. In this study, we study the influence of native defects on these properties of $\alpha$-MoO$_{3}$ using the first-principles density functional theory (DFT) calculations. From the formation energy calculations, it is concluded that Mo vacancies are difficult to form in the system, while O and Mo-O co-vacancies are energetically quite favorable. We further find that vacancies give rise to mid-gap states (trap states) that remarkably affect the magneto-optoelectronic properties of the material. Our calculations indicate that a single Mo vacancy leads to half-metallic behavior, and also induces a large magnetic moment of 5.98 $\mu_{B}$. On the other hand, for the single O vacancy case, the band gap disappears completely, but the system remains in a non-magnetic state. For Mo-O co-vacancies of two types considered in this work, a reduced band gap is found, along with an induced magnetic moment of 2.0 $\mu_{B}$. Furthermore, a few finite peaks below the main band edge are observed in the absorption spectra of configurations with Mo and O vacancies, while they are absent in the Mo-O co-vacancies of both types, just like in the pristine state. From the ab-initio molecular dynamics simulations, stability and sustainability of induced magnetic moment at room temperate is verified. Our findings will enable the development of defect strategies that maximize the functionality of the system, and further help in designing highly efficient magneto-optoelectronic and spintronic devices.

Abstract: The interest in nanowire photodetectors stems from their potential to improve the performance of a variety of devices, including solar cells, cameras, sensors, and communication systems. Implementing devices based on nanowire ensembles requires a planarization process which must be conceived to preserve the advantages of the nanowire geometry. This is particularly challenging in the ultraviolet (UV) range, where spin coating with hydrogen silsesquioxane (HSQ) appears as an interesting approach in terms of transmittance and refractive index. Here, we report a comprehensive study on UV photodetectors based on GaN or AlGaN/AlN nanowire ensembles encapsulated in HSQ. We show that this material is efficient for passivating the nanowire surface, it introduces a compressive strain in the nanowires and preserves their radiative efficiency. We discuss the final performance of planarized UV photodetectors based on three kinds of nanowire ensembles: (i) non-intentionally-doped (nid) GaN nanowires, (ii) Ge-doped GaN nanowires, and (iii) nid GaN nanowires terminated with an AlGaN/AlN superlattice. The incorporation of the superlattice allows tuning the spectral response with bias, which can enhance the carrier collection from the AlGaN/AlN superlattice or from the GaN stem. In all the cases, the performance of the planarized devices remains determined by the nanowire nature, since their characteristics in terms of linearity and spectral selectivity are closer to those demonstrated in single nanowires than those of planar devices. Thus, the visible rejection is several orders of magnitude and there is no indication of persistent photocurrent, which makes all the samples suitable for UV-selective photodetection applications.

Abstract: In semiconductor spintronics, the generation of highly spin-polarized carriers and the efficient probe of spin order (due to strong ferromagnetism) -- at or above room temperature -- are crucial because it allows for the design of spin-based semiconductor devices. Usually, such goals were fulfilled in room-temperature ferromagnetic semiconductors, being rare materials in nature. While room-temperature antiferromagnetic semiconductors are plentiful, the possibility for creating highly spin-polarized carriers and strong ferromagnetism in these materials remain to be unraveled. Here, we explore such a possibility by first-principles simulations, working with CaTcO$_3$ and NaOsO$_3$ perovskites -- being room-temperature antiferromagnetic semiconductors. We find that doping them by electrons or holes results in these materials to be highly spin-polarized, carrying enormous ferromagnetic moments. Doping electrons with moderate carrier density can yield strong ferromagnetism in them, with the ferromagnetic moments being comparable to that in typical ferromagnetic semiconductors. Our work thus indicates the merit of perovskite antiferromagnetic semiconductors in spintronics -- for a possible replacement of ferromagnetic semiconductors.

Abstract: We present first principles calculations of the electronic properties of trigonal selenium with emphasis on photovoltaic applications. The band gap and optical absorption spectrum of pristine selenium is calculated from many-body perturbation theory yielding excellent agreement with experiments. We then investigate the role of intrinsic as well as extrinsic defects and estimate the equilibrium concentrations resulting from realistic synthesis conditions. The intrinsic defects are dominated by vacancies, which act as acceptor levels and implies $p$-doping in agreement with previous predictions and measurements, and we show that these do not give rise to significant non-radiative recombination. The charge balance remains dominated by vacancies when extrinsic defects are included, but these may give rise to sizable non-radiative recombination rates, which could severely limit the performance of selenium based solar cells. Our results thus imply that the pollution by external elements is a decisive factor for the photovoltaic efficiency, which will be of crucial importance when considering synthesis conditions for any type of device engineering.

Abstract: Multiferroic particulate composites $(1-x)$ Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$-$(x)$CoFe$_2$O$_4$ with ($x$ = 0.1, 0.2, 0.3, 0.4 and 0.5) have been prepared by mechanical mixing of the calcined and milled individual ferroic phases. X-ray diffraction and Raman spectroscopy analysis confirmed the formation of both perovskite Ba$_{0.95}$Ca$_{0.05}$Ti$_{0.89}$Sn$_{0.11}$O$_3$ (BCTSn) and spinel CoFe$_2$O$_4$ (CFO) phases without the presence of additional phases. The morphological properties of the composites were provided by using Field Emission Scanning Electron Microscopy. The BCTSn-CFO composites exhibit multiferroic behavior at room temperature, as evidenced by ferroelectric and ferromagnetic hysteresis loops. The magnetoelectric (ME) coupling was measured under a magnetic field up to 10 kOe and the maximum ME response found to be 0.1 mV /cm/ Oe for the composition 0.7 BCTSn-0.3 CFO exhibiting a high degree of pseudo-cubicity and large density.

Abstract: We study the effects of the uniaxial tensile strain and shear deformation as well as their combinations on the electronic properties of single-layer black phosphorene. The evolutions of the strain-dependent band gap are obtained using the numerical calculations within the tight-binding (TB) model as well as the first-principles (DFT) simulations and compared with previous findings. The TB-model-based findings show that the band gap of the strain-free phosphorene agrees with the experimental value and linearly depends on both stretching and shearing: increases (decreases) as the stretching increases (decreases), whereas gradually decreases with increasing the shear. A linear dependence is less or more similar as compared to that obtained from the ab initio simulations for shear strain, however disagrees with a non-monotonic behaviour from the DFT-based calculations for tensile strain. Possible reasons for the discrepancy are discussed. In case of a combined deformation, when both strain types (tensile/compression + shear) are loaded simultaneously, their mutual influence extends the realizable band gap range: from zero up to the values respective to the wide-band-gap semiconductors. At a switched-on combined strain, the semiconductor-semimetal phase transition in the phosphorene is reachable at a weaker (strictly non-destructive) strain, which contributes to progress in fundamental and breakthroughs.

Abstract: Liquid metal batteries (LMBs) are a promising alternative for large-scale stationary energy storage for renewable applications. Using high-abundance electrode materials such as Sodium and Zinc is highly desirable due to their low cost and excellent cell potential. LMBs undergo multiple complex mass transport dynamics and as a result, their operation limits and other critical parameters are not fully understood yet. In this work, a multiphase numerical model was developed to resolve electrode and electrolyte components in 1D and simulate the discharge process of a Na-Zn battery including the interfacial displacement of the molten metal electrodes. The variation in electrolyte composition was predicted throughout the process, including the species distribution and its effect on the cell conductivity and capacity. Volume change and species redistribution were found to be important in predicting the maximum theoretical capacity of the cell when neglecting convective phenomena.

Abstract: In this work we study phonon-phonon coupling in bismuth vanadate (BiVO4), known for its second-order transition involving a variety of coupling mechanisms. Using Raman spectroscopy as a probe, we identify two optical coupled phonon modes of the VO4 tetrahedron and study them by varying light polarization and temperature. The coupling manifests in non-Lorentzian line-shapes of Raman peaks and frequency shifts. We use theoretical framework of coupled damped harmonic oscillators to model the coupling and capture the phenomena in the temperature evolution of the coupling parameters. The coupling is negligible at temperatures below 100 K and later increases in magnitude with temperature until 400 K. The sign of the coupling parameter depends on the light polarization direction, causing either phonon attraction or repulsion. After 400 K the phonon-phonon coupling diminishes when approaching phase transition at which the phonon modes change their symmetry and the coupling is no longer allowed.

Abstract: We study the frequency non-reciprocity of the spin waves in symmetric CoFeB/Ru/CoFeB synthetic antiferromagnets stacks set in the scissors state by in-plane applied fields. Using a combination of Brillouin Light Scattering and propagating spin wave spectroscopy experiments, we show that the acoustical spin waves in synthetic antiferromagnets possess a unique feature if their wavevector is parallel to the applied field: the frequency non-reciprocity can be so large that the acoustical spin waves transfer energy in a unidirectional manner for a wide and bipolar interval of wavevectors. Analytical modeling and full micromagnetic calculations are conducted to account for the dispersion relations of the optical and acoustical spin waves for arbitrary field orientations. Our formalism provides a simple and direct method to understand and design devices harnessing propagating spin waves in synthetic antiferromagnets.

Abstract: Hydrogen embrittlement is a prime cause of several degradation effects in metals. Since grain boundaries (GBs) act efficiently as sinks for hydrogen atoms, H is thought to segregate in these regions, affecting the local formation of dislocations. However, it remains unclear at which concentrations H begins to play any role in the mechanical properties of Cu. In the current study, we use density functional theory (DFT) to assess the accuracy of a bond order potential (BOP) in simulating the segregation of H in Cu GB. BOP accurately predicts the most favorable segregation sites of H in Cu GB, along with the induced lattice relaxation effects. H is found to weaken the crystal by reducing the GB separation energy. Classical molecular dynamics (MD) simulations using BOP are performed to evaluate the concentration of H in bicrystalline Cu required to substantially impact the crystal's mechanical strength. For concentrations higher than 10 mass ppm, H significantly reduces the yield strength of bicrystalline Cu samples during uniaxial tensile strain application. This effect was attributed to the fact that H interstitials within the GB promoted the formation of partial dislocations.