First-principles calculation of temperature-composition phase diagrams of semiconductor alloys

ChecMatE: A workflow package to automatically generate machine learning potentials and phase diagrams for semiconductor alloys

Semiconductor alloy materials are highly versatile due to their adjustable properties; however, exploring their structural space is a challenging task that affects the control of their properties. Traditional methods rely on ad hoc design based on the understanding of known chemistry and crystallography, which have limitations in computational efficiency and search space. In this work, we present ChecMatE (Chemical Material Explorer), a software package that automatically generates machine learning potentials (MLPs) and uses global search algorithms to screen semiconductor alloy materials. Taking advantage of MLPs, ChecMatE enables a more efficient and cost-effective exploration of the structural space of materials and predicts their energy and relative stability with ab initio accuracy. We demonstrate the efficacy of ChecMatE through a case study of the InxGa1-xN system, where it accelerates structural exploration at reduced costs. Our automatic framework offers a promising solution to the challenging task of exploring the structural space of semiconductor alloy materials.

Equilibrium phase diagrams of isostructural and heterostructural two-dimensional alloys from first principles

Alloying is a successful strategy for tuning the phases and properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). To accelerate the synthesis of TMDC alloys, we present a method for generating temperature-composition equilibrium phase diagrams by combining first-principles total-energy calculations with thermodynamic solution models. This method is applied to three representative 2D TMDC alloys: an isostructural alloy, MoS_2(1-x)Te_2x , and two heterostructural alloys, Mo_1-x W _x Te₂ and WS_2(1-x)Te_2x . Using density-functional theory and special quasi-random structures, we show that the mixing enthalpy of these binary alloys can be reliably represented using a sub-regular solution model fitted to the total energies of relatively few compositions. The cubic sub-regular solution model captures 3-body effects that are important in TMDC alloys. By comparing phase diagrams generated with this method to those calculated with previous methods, we demonstrate that this method can be used to rapidly design phase diagrams of TMDC alloys and related 2D materials.

Design of High-Performance Lead-Free Quaternary Antiperovskites for Photovoltaics via Ion Type Inversion and Anion Ordering

The emergence of halide double perovskites significantly increases the compositional space for lead-free and air-stable photovoltaic absorbers compared to halide perovskites. Nevertheless, most halide double perovskites exhibit oversized band gaps (>1.9 eV) or dipole-forbidden optical transition, which are unfavorable for efficient single-junction solar cell applications. The current device performance of halide double perovskite is still inferior to that of lead-based halide perovskites, such as CH₃NH₃PbI₃ (MAPbI₃). Here, by ion type inversion and anion ordering on perovskite lattice sites, two new classes of pnictogen-based quaternary antiperovskites with the formula of X₆B₂AA' and X₆BB'A₂ are designed. Phase stability and tunable band gaps in these quaternary antiperovskites are demonstrated based on first-principles calculations. Further photovoltaic-functionality-directed screening of these materials leads to the discovery of 5 stable compounds (Ca₆N₂AsSb, Ca₆N₂PSb, Sr₆N₂AsSb, Sr₆N₂PSb, and Ca₆NPSb₂) with suitable direct band gaps, small carrier effective masses and low exciton binding energies, and dipole-allowed strong optical absorption, which are favorable properties for a photovoltaic absorber material. The calculated theoretical maximum solar cell efficiencies based on these five compounds are all larger than 29%, comparable to or even higher than that of the MAPbI₃ based solar cell. Our work reveals the huge potential of quaternary antiperovskites in the optoelectronic field and provides a new strategy to design lead-free and air-stable perovskite-based photovoltaic absorber materials.

Decisive effects of atomic vacancies and structural ordering on stable phases and band structures in copper-gallium-chalcogenide compounds

Atomic vacancies usually exist in the Cu-Ga-S ternary system, except for chalcopyrite CuGaS₂ as a promising light-harvesting material for solar cells, and are expected to have decisive effects on the structure stability and electronic structure. We demonstrate that ordered arrangements of the straight lines locally formed by atomic vacancies prefer a stable structure through lowering the formation energy. Accidentally, we confirm that a metastable van der Waals P2₁/c-Cu₂S phase shares better optical properties than newly-found ground-state P4₂-Cu₂S, and possesses the photovoltaic-potentially direct band gap of 1.09 eV. We find anomalous changes in band gap induced by varying chemical composition and applying pressure, according to the variation in p-d coupling between S and Cu atoms. Our Monte Carlo simulations together with the special quasirandom structures further suggest that the band gap of CuGaS₂ can be tuned continuously from 2.51 eV for the chalcopyrite phase to 0.13 eV for the fully disordered configuration by controlling the degree of ordering, which determined by the synthesis temperature and annealing time experimentally.

Ab Initio Discovery of Stable Double Perovskite Oxides Na2BIO6 (B = Bi, In) with Promising Optoelectronic Properties

Recently, oxide perovskites are garnering tremendous attention from the scientific community as possible alternatives to the currently used active materials in photovoltaic (PV) and photoelectrochemical (PEC) devices. Herein, we report the stability and promising optoelectronic properties of a few previously unexplored periodates A₂BIO₆ (A = alkali metal; B = Bi, Sb, In, Tl, Ga). Our compositional phase diagram analysis reveals two compounds Na₂BIO₆ (B = Bi, In) that stabilize in monoclinic phase at thermodynamic equilibrium, showing band gaps (E_g) in the visible region. Band engineering via alloying Bi in Na₂InIO₆ introduces Bi 6s lone pair bands above the valence band maxima (VBM), while alloying Tl in Na₂BiIO₆ introduces an intermediate band (Tl s character) below the conduction band minima. These alloys, Na₂Bi_0.25In_0.75IO₆ and Na₂Tl_0.25Bi_0.75IO₆, acquire E_g's of 1.66 and 1.78 eV, respectively. Apart from band gap, the antibonding VBM, favorable optical absorption, highly dispersive band edges, and well-positioned VBM (for efficient oxygen evolution reaction) make these compounds highly promising for PV and PEC applications.

Quantification of Material Gradients in Core/Shell Nanocrystals Using EXAFS Spectroscopy

Core/shell nanocrystals with a graded interface between core and shell exhibit improved optoelectronic properties compared with particles with an abrupt, sharp interface. Material gradients mitigate interfacial defects and define the shape of the confinement potential. So far, few works exist that allow to quantify the width of the gradient. In this study, ZnSe/CdS nanocrystals with graded shells made at different temperatures are characterized using extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. The average coordination number of the probed element with respect to the two possible counterions is fit to a simple, geometric model. It is shown that at the lower temperature limit for shell growth (260 °C), substantial interfacial alloying can be attributed mainly to cation migration. At higher temperature (290 °C), strain minimization leads to atomic ordering of the metal ions and an anomalously low degree of phase mixing.

Stable Bandgap-Tunable Hybrid Perovskites with Alloyed Pb-Ba Cations for High-Performance Photovoltaic Applications

The intrinsic poor stability of MAPbI₃ hybrid perovskites in the ambient environment remains as the major challenge for photovoltaic applications. In this work, complementary first-principles calculations and experimental characterizations reveal that metal cation alloyed perovskite (MABa _xPb_{1- x}I₃) can be synthesized and exhibit substantially enhanced stability via forming stronger Ba-I bonds. The Ba-Pb alloyed perovskites remain phase-pure in ambient air for more than 15 days. Furthermore, the bandgap of MABa _xPb_{1- x}I₃ is tailored in a wide window of 1.56-4.08 eV. Finally, MABa _xPb_{1- x}I₃ is used as a capping layer on MAPbI₃ in solar cells, resulting in significantly improved power conversion efficiency (18.9%) and long-term stability (>30 days). Overall, our results provide a simple but reliable strategy toward stable less-Pb perovskites with tailored physical properties.

Closing the bandgap for III-V nitrides toward mid-infrared and THz applications

A theoretical study of InNBi alloy by using density functional theory is presented. The results show non-linear dependence of the lattice parameters and bulk modulus on Bi composition. The formation energy and thermodynamic stability analysis indicate that the InNBi alloy possesses a stable phase over a wide range of intermediate compositions at a normal growth temperature. The bandgap of InNBi alloy in Wurtzite (WZ) phase closes for Bi composition higher than 1.5625% while that in zinc-blende (ZB) phase decreases significantly at around 356 meV/%Bi. The Bi centered ZB InNBi alloy presents a change from a direct bandgap to an indirect bandgap up to 1.5625% Bi and then an oscillates between indirect bandgap and semi-metallic for 1.5625% to 25% Bi and finally to metallic for higher Bi compositions. For the same Bi composition, its presence in cluster or uniform distribution has a salient effect on band structures and can convert between direct and indirect bandgap or open the bandgap from the metallic gap. These interesting electronic properties enable III-nitride closing the bandgap and make this material a good candidate for future photonic device applications in the mid-infrared to THz energy regime.

Stability, Electronic Structure, and Phase Diagrams of Novel Inter- Semiconductor Compounds

High-technology electronic devices are based on highly specialized core materials that make their operation pos sible : semiconductors. Unfortunately, the range of mate rial properties that make high-technology devices work is extremely narrow, since the sheer number of useful semiconductors is small. Hence, a major challenge has been to predict and develop new, potentially useful semiconductors. We describe here how the use of state- of-the-art techniques in both quantum and statistical mechanics can lead to predictions of new, stable, and ordered semiconductor alloys. A number of laboratories have already grown experimentally these new materials; efforts to characterize their useful material properties are ongoing in the United States, Japan, and Europe. This work describes the theoretical methodologies of our ap proach, and shows how supercomputers make possible the quantum-mechanical architectural analysis of new materials.

Anomalous Alloy Properties in Mixed Halide Perovskites

Engineering halide perovskite through mixing halogen elements, such as CH3NH3PbI3-xClx and CH3NH3PbI3-xBrx, is a viable way to tune its electronic and optical properties. Despite many emerging experiments on mixed halide perovskites, the basic electronic and structural properties of the alloys have not been understood and some crucial questions remain, for example, how much Cl can be incorporated into CH3NH3PbI3 is still unclear. In this Letter, we chose CsPbX3 (X = I, Br, Cl) as an example and use a first-principle calculation together with cluster-expansion methods to systematically study the structural, electronic, and optical properties of mixed halide perovskites and find that unlike conventional semiconductor alloys, they exhibit many anomalous alloy properties such as small or even negative formation energies at some concentrations and negligible or even negative band gap bowing parameters at high temperature. We further show that mixed-(I,Cl) perovskite is hard to form at temperature below 625 K, whereas forming mixed-(Br,Cl) and (I,Br) alloys are easy at room temperature.

Thermodynamic theory of epitaxial alloys: first-principles mixed-basis cluster expansion of (In, Ga)N alloy film

Epitaxial growth of semiconductor alloys onto a fixed substrate has become the method of choice to make high quality crystals. In the coherent epitaxial growth, the lattice mismatch between the alloy film and the substrate induces a particular form of strain, adding a strain energy term into the free energy of the alloy system. Such epitaxial strain energy can alter the thermodynamics of the alloy, leading to a different phase diagram and different atomic microstructures. In this paper, we present a general-purpose mixed-basis cluster expansion method to describe the thermodynamics of an epitaxial alloy, where the formation energy of a structure is expressed in terms of pair and many-body interactions. With a finite number of first-principles calculation inputs, our method can predict the energies of various atomic structures with an accuracy comparable to that of first-principles calculations themselves. Epitaxial (In, Ga)N zinc-blende alloy grown on GaN(001) substrate is taken as an example to demonstrate the details of the method. Two (210) superlattice structures, (InN)(2)/(GaN)(2) (at x = 0.50) and (InN)(4)/(GaN)(1) (at x = 0.80), are identified as the ground state structures, in contrast to the phase-separation behavior of the bulk alloy.

Indium-indium pair correlation and surface segregation in InGaAs alloys

In-In pair correlations and In surface segregation in In xGa 1-xAs alloys are studied by first-principles total-energy calculations. By calculating the substitution energy of a single In atom, we find that the near-surface energetics explains the observed In segregation on InGaAs(001)-beta2(2x4) surfaces. Indium surface segregation further enhances the In site selectivity, thus the long-range ordering. We find that the [110] and [001] In-In pair correlations are repulsive and nearly isotropic in bulk but are highly anisotropic near the (001) surface. The sign of the [110] In-In interaction energies vs the distance from the surface is oscillatory. These findings explain the recent puzzling cross-sectional x-STM results.