SearcharxivSearch

arXiv subjects

Prashun Gorai

Publications and source records attributed to Prashun Gorai.

17 recordsLinked to original sources

Accelerating point defect simulations using data-driven and machine learning approaches

Point defects in solid-state materials are now routinely simulated using large supercell structures, requiring efficient quantum mechanical solutions. Data-driven and machine learning (ML) models trained on computational data can enable rapid defect property predictions and high-throughput screening. In this article, we provide an overview of prominent efforts to accelerate defect simulations using these approaches. We begin by discussing the motivations for data-driven techniques in defect modeling, and describe efforts over the past decade to use descriptor-based models for rapid screening of defect properties -- most notably in oxides. In particular, we discuss case studies where surrogate models and interatomic potentials were trained on density functional theory (DFT) data, leading to predictions with quantum-mechanical accuracies at a fraction of the cost. In addition to geometry relaxation and formation energy predictions, these interatomic potentials are capable of predicting phonon modes and vibrational free energies to yield defect energetics at finite temperatures -- representing a key frontier for computational defect research. We finish with a discussion on how to connect these approaches and their outputs with experimental data, and provide an outlook on this burgeoning sub-field.

cond-mat.mtrl-sci

Ferroelectricity of Wurtzite Al$_{1-x}$Hf$_{x}$N Heterovalent Alloys

Thin films of aluminum hafnium nitride (Al$_{1-x}$Hf$_{x}$N) were synthesized via reactive magnetron sputtering for Hf contents up to $x$ = 0.13. X-ray diffraction showed a single $c$-axis oriented wurtzite phase for all films. Hard X-ray photoelectron spectroscopy demonstrated homogeneous Al:Hf distribution through the thin films and confirmed their insulating character. A collection of complementary tests showed unambiguous polarization inversion, and thus ferroelectricity in multiple samples. Current density vs. electric field hysteresis measurements showed distinct ferroelectric switching current peaks, the piezoelectric coefficient d$_{33,f,meas}$ measured using a double beam laser interferometer (DBLI) showed a reversal in sign with similar magnitude, and anisotropic wet etching confirmed field-induced polarization inversion. This demonstrates the possibility of using tetravalent--and not just trivalent--alloying elements to enable ferroelectricity in AlN-based thin films, highlighting the compositional flexibility of ferroelectricity in wurtzites and greatly expanding the chemistries that can be considered for future devices.

cond-mat.mtrl-sci

From Prediction to Experimental Realization of Ferroelectric Wurtzite Al$_{1-x}$Gd$_{x}$N Alloys

AlN-based alloys find widespread application in high-power microelectronics, optoelectronics, and electromechanics. The realization of ferroelectricity in wurtzite AlN-based heterostructural alloys has opened up the possibility of directly integrating ferroelectrics with conventional microelectronics based on tetrahedral semiconductors such as Si, SiC and III-Vs, enabling compute-in-memory architectures, high-density data storage, and more. The discovery of AlN-based wurtzite ferroelectrics has been driven to date by chemical intuition and empirical explorations. Here, we demonstrate the computationally-guided discovery and experimental demonstration of new ferroelectric wurtzite Al$_{1-x}$Gd$_x$N alloys. First-principles calculations indicate that the minimum energy pathway for switching changes from a collective to an individual switching process with a lower overall energy barrier, at a rare-earth fraction $x$ of $x>$ 0.10$-$0.15. Experimentally, ferroelectric switching is observed at room temperature in Al$_{1-x}$Gd$_x$N films with $x>$ 0.12, which strongly supports the switching mechanisms in wurtzite ferroelectrics proposed previously (Lee et al., $\textit{Science Advances}$ 10, eadl0848, 2024). This is also the first demonstration of ferroelectricity in an AlN-based alloy with a magnetic rare-earth element, which could pave the way for additional functionalities such as multiferroicity and opto-ferroelectricity in this exciting class of AlN-based materials.

cond-mat.mtrl-sci

GdWN$_3$ is a Nitride Perovskite

Nitride perovskites $AB$N$_3$ are an emerging and highly under-explored class of materials that are of interest due to their intriguing calculated ferroelectric, optoelectronic, and other functional properties. Incorporating novel $A$-site cations is one strategy to tune and expand such properties; for example, Gd$^{3+}$ is compelling due to its large magnetic moment, potentially leading to multiferroic behavior. However, the theoretically predicted ground state of GdWN$_3$ is a non-perovskite monoclinic structure. Here, we experimentally show that GdWN$_3$ crystallizes in a perovskite structure. High-throughput combinatorial sputtering with activated nitrogen is employed to synthesize thin films of Gd$_{1-x}$W$_{x}$N$_{3-y}$ with low oxygen content within the bulk of the films. Ex-situ annealing crystallizes a polycrystalline perovskite phase in a narrow composition window near $x=1$. LeBail fits of synchrotron grazing incidence wide angle X-ray scattering data are consistent with a perovskite ground-state structure. New density functional theory calculations that included antiferromagnetic configurations confirm that the ground-state structure of GdWN$_3$ is a distorted $Pnma$ perovskite with antiferromagnetic ordering, in contrast to prior predictions. Initial property measurements find that GdWN$_3$ is paramagnetic down to $T=2$ K with antiferromagnetic correlations and that the absorption onset depends on cation stoichiometry. This work provides an important stepping stone towards the rapid expansion of the emerging family of nitride perovskites and towards our understanding of their potential multiferroic properties.

cond-mat.mtrl-sci

Defects and Oxygen Impurities in Ferroelectric Wurtzite Al$_{1-x}$Sc$_x$N Alloys

III-nitrides and related alloys are widely used for optoelectronics and as acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest because of their potential for direct integration with Si and wide bandgap semiconductors, and unique polarization switching characteristics; such interest has taken off since the first report of ferroelectric Al$_{1-x}$Sc$_x$N alloys. However, the coercive fields needed to switch polarization are on the order of MV/cm, which is 1-2 orders of magnitude larger than oxide perovskite ferroelectrics. Atomic-scale point defects are known to impact the dielectric properties, including breakdown fields and leakage currents, as well as ferroelectric switching. However, very little is known about the native defects and impurities in Al$_{1-x}$Sc$_x$N, and their effect on the dielectric properties. In this study, we use first-principles calculations to determine the formation energetics of native defects and unintentional oxygen incorporation in Al$_{1-x}$Sc$_x$N. We find that nitrogen vacancies are the dominant native defects, and that they introduce multiple mid-gap states that can lead to premature dielectric breakdown in ferroelectrics and carrier recombination in optoelectronics. Growth under N-rich conditions will reduce the concentration of these deep defects. We also investigate unintentional oxygen incorporation on the nitrogen site and find that the substitutional defect is present in high concentrations, which can contribute to increased temperature-activated leakage currents. Our findings provide fundamental understanding of the defect physics in Al$_{1-x}$Sc$_x$N alloys, which is critical for future deployment of ferroelectric devices.

cond-mat.mtrl-sci

Structural and Optoelectronic Properties of Thin Film LaWN$_3$

Nitride perovskites are an emerging class of materials that have been predicted to display a range of interesting physics and functional properties, but they are under-explored due to the difficulty of synthesizing oxygen-free nitrides. LaWN3, recently reported as the first oxygen-free nitride perovskite, exhibited polar symmetry and a large piezoelectric coefficient. However, the predicted ferroelectric switching was hindered by large leakage current, which motivates better understanding of its electronic structure and optical properties. Here, we study the structure and optoelectronic properties of thin film LaWN3 in greater detail, employing combinatorial techniques to correlate these properties with cation stoichiometry. We report a two-step synthesis that utilizes a more common RF substrate bias instead of a nitrogen plasma source, yielding nanocrystalline films that are crystallized by ex-situ annealing. We investigate the structure and composition of these films, finding polycrystalline La-rich and highly textured W-rich films. The optical absorption onset and temperature- and magnetic field-dependent resistivity are consistent with semiconducting behavior and are highly sensitive to cation stoichiometry, which may be related to amorphous impurities: metallic W or WNx in W-rich samples and insulating La2O3 in La-rich samples. The fractional magnetoresistance is linear and small, consistent with defect scattering, and a W-rich sample has n-type carriers with high densities and low mobilities. We demonstrate a photoresponse in LaWN3: the resistivity of a La-rich sample is enhanced by 28% at low temperature, likely due to a defect trapping mechanism. The physical properties of LaWN3 are highly sensitive to cation stoichiometry, like many oxide perovskites, which therefore calls for precise composition control to utilize the interesting properties observed in this nitride perovskite.

cond-mat.mtrl-sci

Ionic Bonds Control Ferroelectric Behavior in Wurtzite Nitrides

Ferroelectricity enables key integrated technologies from non-volatile memory to precision ultrasound. Wurtzite ferroelectric Al1-xScxN has recently attracted attention because of its robust ferroelectricity and Si process compatibility in addition to being the first known ferroelectric wurtzite. However, the origin and control of ferroelectricity in wurtzite materials is not yet fully understood. Here we show that the local bond ionicity, rather than simply the change in tetrahedral distortion, is key to controlling the macroscopic ferroelectric response, according to our coupled experimental and computational results. Across the composition gradient in Sc < 0.35 range and 140-260 nm thickness in combinatorial thin films of Al1-xScxN, the pure wurtzite phase exhibits a similar c/a ratio regardless of the Sc content, due to elastic interaction with neighboring crystals. The coercive field and spontaneous polarization significantly decrease with increasing Sc content despite this invariant c/a ratio, due to the more ionic bonding nature of Sc-N relative to the more covalent Al-N bonds, supported by DFT calculations. Based on these insights, ionicity engineering is introduced as an approach to reduce coercive field of Al1-xScxN for memory and other applications and to control ferroelectric properties in other wurtzites.

cond-mat.mtrl-sci

Ternary Wide Band Gap Oxides for High-Power Electronics Identified Computationally

As electricity grids become more renewable energy-compliant, there will be a need for novel semiconductors that can withstand high power, high voltage, and high temperatures. Wide band gap (WBG) semiconductors tend to exhibit large breakdown field, allowing high operating voltages. Currently explored WBG materials for power electronics are costly (GaN), difficult to synthesize as high-quality single crystals (SiC) and at scale (diamond, BN), have low thermal conductivity ($β$-Ga$_2$O$_3$), or cannot be suitably doped (AlN). We conduct a computational search for novel semiconductors across 1,340 known metal-oxides using first-principles calculations and existing transport models. We calculate the Baliga figure of merit (BFOM) and lattice thermal conductivity ($κ_L$) to identify top candidates for n-type power electronics. We find 40 mostly ternary oxides that have higher $κ_L$ than $β$-Ga$_2$O$_3$ and higher n-type BFOM than SiC and GaN. Among these, several material classes emerge, including 2-2-7 stoichiometry thortveitites and pyrochlores, II-IV spinels, and calcite-type borates. Within these classes, we propose In$_2$Ge$_2$O$_7$, Mg$_2$GeO$_4$, and InBO$_3$ as they are the most favorable for n-type doping based on our preliminary evaluation and could be grown as single crystals or thin film heterostructures. These materials could help advance power electronic devices for the future grid.

cond-mat.mtrl-sci

Search and Structural Featurization of Magnetically Frustrated Kagomé Lattices

We have searched nearly 40,000 inorganic solids in the Inorganic Crystal Structural Database to identify compounds containing a transition metal or rare earth kagomé sublattice, a geometrically magnetically frustrated lattice, ultimately identifying $\sim$500 materials. A broad analysis of the chemical and structural trends of these materials shows three types of kagomé sheet stacking and several classes of magnetic complexity. Following the search and classification, we rapidly screen the magnetic properties of a subset of the materials using density functional theory to eliminate those that are unlikely to exhibit magnetic frustration. From the results of our computational screening, we rediscover six materials that have previously been explored for their low temperature magnetic behavior, albeit showing symmetry breaking distortions, spin glass behavior, or magnetic ordering. However, all are materials with antiferromagnetic behavior, which we correctly predict. Finally, we also report three materials that appear to be unexplored for their magnetic properties.

cond-mat.mtrl-sci

Charting Lattice Thermal Conductivity of Inorganic Crystals

Thermal conductivity is a fundamental material property but challenging to predict, with less than 5% out of about $10^5$ synthesized inorganic materials being documented. In this work, we extract the structural chemistry that governs lattice thermal conductivity, by combining graph neural networks and random forest approaches. We show that both mean and variation of unit-cell configurational properties, such as atomic volume and bond length, are the most important features, followed by mass and elemental electronegativity. We chart the structural chemistry of lattice thermal conductivity into extended van-Arkel triangles, and predict the thermal conductivity of all known inorganic materials in the Inorganic Crystal Structure Database. For the latter, we develop a transfer learning framework extendable for other applications.

cond-mat.mtrl-sci

On the Dopability of Semiconductors and Governing Material Properties

To be practical, semiconductors need to be doped. Sometimes, to nearly degenerate levels, e.g. in applications such as thermoelectric, transparent electronics or power electronics. However, many materials with finite band gaps are not dopable at all, while many others exhibit strong preference toward allowing either p- or n-type doping, but not both. In this work, we develop a model description of semiconductor dopability and formulate design principles in terms of governing materials properties. Our approach, which builds upon the semiconductor defect theory applied to a suitably devised (tight-binding) model system, reveals analytic relationships between intrinsic materials properties and the semiconductor dopability, and elucidates the role and the insufficiency of previously suggested descriptors such as the absolute band edge positions. We validate our model against a number of classic binary semiconductors and discuss its extension to more complex chemistries and the utility in large-scale material searches.

cond-mat.mtrl-sci

Effective $n$-type Doping of Mg$_3$Sb$_2$ with Group-3 Elements

The recent discovery of high thermoelectric performance in Mg$_3$Sb$_2$ has been critically enabled by the success in $n$-type doping of this material, which is achieved under Mg-rich growth conditions, typically with chalcogens (Se, Te) as extrinsic dopants. Using first-principles defect calculations, we previously predicted that higher electron concentrations ($\sim10^{20}$ cm$^{-3}$) can be achieved in Mg$_3$Sb$_2$ by doping with La instead of Se or Te. Subsequent experiments showed that free electron concentration in La-doped Mg$_3$Sb$_{2-x}$Bi$_x$ indeed exceeds those in the Te-doped material. Herein, we further investigate $n$-type doping of Mg$_3$Sb$_2$ and predict that, in addition to La, other group-3 elements (Sc, Y) are also effective as $n$-type dopants; Y is as good as La while Sc slightly less. Overall, we find that doping with any group-3 elements should lead to higher free electron concentrations than doping with chalcogens.

cond-mat.mtrl-sci

Ionic vs. van der Waals Layered Materials: Identification and Comparison of Elastic Anisotropy

In this work, we expand the set of known layered compounds to include ionic layered materials, which are well known for superconducting, thermoelectric, and battery applications. Focusing on known ternary compounds from the ICSD, we screen for ionic layered structures by expanding upon our previously developed algorithm for identification of van der Waals (vdW) layered structures, thus identifying over 1,500 ionic layered compounds. Since vdW layered structures can be chemically mutated to form ionic layered structures, we have developed a methodology to structurally link binary vdW to ternary ionic layered materials. We perform an in-depth analysis of similarities and differences between these two classes of layered systems and assess the interplay between layer geometry and bond strength with a study of the elastic anisotropy. We observe a rich variety of anisotropic behavior in which the layering direction alone is not a simple predictor of elastic anisotropy. Our results enable discovery of new layered materials through intercalation or de- intercalation of vdW or ionic layered systems, respectively, as well as lay the groundwork for studies of anisotropic transport phenomena such as sound propagation or lattice thermal conductivity.

cond-mat.mtrl-sci

Large Piezoelectric Response of van der Waals Layered Solids

The bulk piezoelectric response, as measured by the piezoelectric modulus tensor (\textbf{d}), is determined by a combination of charge redistribution due to strain and the amount of strain produced by the application of stress (stiffness). Motivated by the notion that less stiff materials could exhibit large piezoelectric responses, herein we investigate the piezoelectric modulus of van der Waals-bonded quasi-2D ionic compounds using first-principles calculations. From a pool of 869 known binary and ternary quasi-2D materials, we have identified 135 non-centrosymmetric crystals of which 48 systems are found to have \textbf{d} components larger than the longitudinal piezoelectric modulus of AlN (a common piezoelectric for resonators), and three systems with the response greater than that of PbTiO$_3$, which is among the materials with largest known piezoelectric modulus. None of the identified materials have previously been considered for piezoelectric applications. Furthermore, we find that large \textbf{d} components always couple to the deformations (shearing or axial) of van der Waals "gaps" between the layers and are indeed enabled by the weak intra-layer interactions.

cond-mat.mtrl-sci

Enhanced piezoelectric response of AlN via CrN alloying

Since AlN has emerged as an important piezoelectric material for a wide variety of applications, efforts have been made to increase its piezoelectric response via alloying with transition metals that can substitute for Al in the wurtzite lattice. Herein, we report density functional theory calculations of structure and properties of the Cr-AlN system for Cr concentrations ranging past the wurtzite-rocksalt transition point. By studying the different contributions to the longitudinal piezoelectric coefficient, we propose that the physical origin of the enhanced piezoelectricity in Cr$_x$Al$_{1-x}$N alloys is the increase of the internal parameter $u$ of the wurtzite structure upon substitution of Al with the larger Cr ions. Among a set of wurtzite-structured materials, we have found that Cr-AlN has the most sensitive piezoelectric coefficient with respect to alloying concentration. Based on these results, we propose that Cr-AlN is a viable piezoelectric material whose properties can be tuned via Cr composition; we support this proposal by combinatorial synthesis experiments, which show that Cr can be incorporated in the AlN lattice up to 30\% before a detectable transition to rocksalt occurs. At this Cr content, the piezoelectric modulus $d_{33}$ is approximately four times larger than that of pure AlN. This finding, combined with the relative ease of synthesis, may propel Cr-AlN as the prime piezoelectric material for applications such as resonators and acoustic wave generators.

cond-mat.mtrl-sci

A Computational Framework for Automation of Point Defect Calculations

A complete and rigorously validated open-source Python framework to automate point defect calculations using density functional theory has been developed. The framework provides an effective and efficient method for defect structure generation, and creation of simple yet customizable workflows to analyze defect calculations. The package provides the capability to compute widely-accepted correction schemes to overcome finite-size effects, including (1) potential alignment, (2) image-charge correction, and (3) band filling correction to shallow defects. Using Si, ZnO and In$_2$O$_3$ as test examples, we demonstrate the package capabilities and validate the methodology.

cond-mat.mtrl-sci

Solar energy conversion properties and defect physics of ZnSiP$_2$

Implementation of an optically active material on silicon has been a persistent technological challenge. For tandem photovoltaics using a Si bottom cell, as well as for other optoelectronic applications, there has been a longstanding need for optically active, wide band gap materials that can be integrated with Si. ZnSiP$_2$ is a stable, wide band gap (2.1 eV) material that is lattice matched with silicon and comprised of inexpensive elements. As we show in this paper, it is also a defect-tolerant material. Here, we report the first ZnSiP$_2$ photovoltaic device. We show that ZnSiP$_2$ has excellent photoresponse and high open circuit voltage of 1.3 V, as measured in a photoelectrochemical configuration. The high voltage and low band gap-voltage offset are on par with much more mature wide band gap III-V materials. Photoluminescence data combined with theoretical defect calculations illuminate the defect physics underlying this high voltage, showing that the intrinsic defects in ZnSiP$_2$ are shallow and the minority carrier lifetime is 7 ns. These favorable results encourage the development of ZnSiP$_2$ and related materials as photovoltaic absorber materials.

cond-mat.mtrl-sci