Session F16: Computational Materials Design and Discovery -- Semiconductors

Machine learned defect level predictor for semiconductors

Electronic levels introduced by impurities and defects in the band gap are critically important in semiconductors for optoelectronic and photovoltaic (PV) applications. The energetics and energy levels of point defects can be reliably predicted using density functional theory (DFT) computations. However, the requirement of large supercells and inclusion of charged states make these computations very expensive, and trends and knowledge from previous calculations are not exploited in subsequent ones. In this work, we develop machine learned defect level predictors based on substantial DFT data for two classes of semiconductors: halide perovskites (MAPbX₃, MA = methylammonium, X = Cl/Br/I), and Cd-based chalcogenides (CdX, X=Te/Se/S). DFT data was generated for formation energies and transition levels of hundreds of vacancy, interstitial and substitutional defects, following which correlation analysis and random forest regression were used to map the properties to a set of unique numerical descriptors, resulting in cheap, accurate, quantitative prediction models. Such models can lead to accelerated prediction of defect states and allow efficient materials design of defect-tolerant semiconductors as well as semiconductors with suitably placed defect levels.   

Ternary semiconductors with tunable band gaps from machine-learning and crystal structure prediction

Computational tools are being employed at an increasing rate to discover and design novel materials with tailored properties to tackle global environmental challenges. Besides the two most common approaches based on high-throughput density functional theory (DFT) calculations and crystal structure prediction schemes, novel methods based on materials informatics and machine learning (ML) models have recently emerged to assist the search for materials with improved properties in industrially relevant applications.

Here, we present a computational investigation of a series of ternary X₄Y₂Z compounds with X={Mg, Ca, Sr, Ba}, Y={P, As, Sb, Bi}, and Z={S, Se, Te}, which we identify by a combined search using a machine learning model and the minima hopping crystal structure prediction method. Accroding to our ab initio results, these compounds are thermodynamically stable and semiconducting with band gaps in the range of 0.3 to 1.8 eV, well suited for various energy applications. We show that several candidate compounds exhibit good photo absorption in the visible range, and excellent thermoelectric performance due to high power factors and extremely low lattice thermal conductivities.   

Polarization engineering with novel nitride heterostructures from first principles

Novel heterostructures based on II-IV-nitride semiconductors in combination with the widely used III-nitrides are being explored due to the ability to access a wide range of band offsets and polarization charges at the interface. The polarization charges arise from contributions due to spontaneous and piezoelectric polarization. Exploiting polarization requires a systematic methodology for evaluating the polarization properties and band alignments for a variety of materials and their heterostructures. We study the spontaneous and piezoelectric polarization of the II-IV-nitrides using density functional theory with a hybrid functional. To determine band offsets we use surface calculations as well as alignments based on the electronic level of interstitial hydrogen. We use our results to explore II-IV-nitride/III-nitride heterostructures that give rise to low polarization fields, which is favorable for light emitters.   

First-principles calculations of BAlGaN alloys nearly latticed matched to AlN for deep UV light emission

With the great success of InGaN LEDs visible light production, deep UV LEDs based on AlGaN have also been the subject of intense research activity. However, material issues still hamper the manufacturing of deep UV LEDs with efficiencies matching those of InGaN-based visible LEDs. Of note are the difficulty of traditional hole injection into p-AlGaN and the UV absorption in p-GaN often used in place of p-AlGaN. We used hybrid density functional theory to analyze the electronic, structural, and thermodynamic properties of BAlGaN alloys nearly latticed matched to AlN as potential new materials for deep UV LED design. We show that boron incorporation into AlGaN allows for improved lattice matching to AlN with only a small effect on the band gap. The a lattice parameter is more sensitive to boron incorporation than the c lattice parameter. We predict similar boron incorporation limits (~15% boron) as in BAlN, but with easier incorporation of low boron content than in BAlN due to increased configurational entropy.   

First-principle study of phosphors for white-LED applications : Stokes shift, emission linewidth and thermal quenching.

Despite current impact of white-LED technology, available blue-to-red phosphors exhibit a too wide emission linewidth or some other drawback, such as strong thermal quenching behavior. In view of large-scale high-throughput search for better phosphors, an accurate but fast computational methodology is needed, not restricted to the ground state (formation energy), but covering excitation and emission energies, Stokes shift, emission linewidth, and emission intensity reduction through thermal quenching. We will present results from a constrained density functional theory methodology for about 30 Ce- and Eu- doped materials [1,2] with 4f → 5d transitions. It matches experimental data within 0.3 eV for both absorption and emission energies (ranging between 2.0 eV and 5.0 eV) and provides Stokes shifts usually within 30%. By contrast, the largely used semi-empirical Dorenbos approach [3] does not perform as well, although it provides physical insights to explain trends among the materials, including the different Stokes shifts. This first-principles approach also delivers emission linewidth and assessment of mechanisms for thermal quenching, based on a simple 1-dimensional configuration coordinate approach. For our representative set of Eu-doped materials, we find that the 4f-5d crossover model cannot be the dominant thermal quenching mechanism: the predicted barrier at the 4f-5d crossing is always higher than 1.5 eV. Also, it will be shown how to waive the one-dimensional restriction in the search for the lowest 4f-5d crossing energy barrier. [1] Y. Jia, S. Poncé, A. Miglio, M. Mikami & X. Gonze, Adv. Opt. Mat. 5, 1600997 (2017). [2] Y. Jia, A. Miglio, S. Poncé, M. Mikami & X. Gonze, Phys. Rev. B 96, 125132 (2017). [3] P. Dorenbos, J. Lumin. 91, 155 (2000); Phys. Rev. B 62, 15640 (2000).   

Dielectric Behavior as a Screen in Rational Searches for Overlooked Electronic Materials: Metal Pnictide Sulfosalts

Dielectric screening plays an important role in preventing carrier scattering and trapping by point defects for many semiconductors such as the halide perovskite solar materials. However, it was rarely considered as a screen to find new electronic semiconductors. We performed a material search study using the dielectric properties as a screen to identify potential electronic materials in the class of metal-pnictide ternary sulfosalts, containing Bi or Sb. We find significant cross-gap hybridization between the S p derived valence bands and pnictogen pderived conduction bands in many of the materials. This leads to enhanced Born effective charges, and highly enhanced dielectric constants. We find a chemical rule for high dielectric constant in terms of the connectivity of the structure. Through first principles screening, we find a series of compounds with low effective mass, high dielectric constant and other properties that suggest good performance as electronic materials, and also several potential thermoelectric compounds. The results illustrate the utility of dielectric properties as a screen for identifying complex semiconductors.   

Sn(II)-containing phosphates as promising p-type optoelectronic semiconductors

High-performance and stable p-type optoelectronic semiconductors, such as transparent conductors, have been searched for with decades of efforts. We herein proposed based on first-principles straightforward calculations and structure searches Sn(II)-containing phosphates Sn_nP₂O_5+n (n=2, 3, 4, 5, …) as promising p-type semiconductors for optoelectronic applications. We found that these materials have large band gaps and can have moderate effective masses for both holes and electrons. Calculations of optical properties show that interband transitions in the visible are weak under hole doping. We also find an interesting inverse Burstein-Moss shift, which can be understood in terms of the Sn character of both the states at band edges. By investigating intrinsic defects properties, we identified dominant carrier traps and revealed ideal growth conditions for p-type Sn(II) phosphates. The results indicate that Sn_nP₂O_5+n with large n may be doped to p-type with promising attainable hole density. The unusual combinations of relatively high band gap, low carrier masses and high chemical stability suggest possible optoelectronic applications of Sn(II) phosphates.   

Computational design of new polymorphs of two-dimensional semiconductor InSe with enhanced interlayer interaction

Atomically thin, two-dimensional InSe has attracted considerable attention due to its widely tunable band gap and high electron mobility. The intriguingly high dependence of band gap on layer thickness remains poorly understood, and is generally attributed to quantum confinement effect. We demonstrated via systemic first-principles calculations in our previous work that strong interlayer coupling may be mainly responsible for this phenomenon, especially in the fewer-layer region. The interlayer coupling was also found to be an essential factor influencing other thickness-dependent material properties such as indirect-to-direct band gap transitions, fan-like phonon frequency diagrams, carrier mobilities, etc. In this work, by combining global structure search algorithm and first-principles calculations, we strikingly discovered two new polymorphs of InSe consisting of the monolayer in point group D_3d, distinct from the known one in point group D_3h. The new polymorphs show thermodynamic and lattice dynamical stability, and large transition barrier to the existing phases. One newly discovered polymorph exhibits the enhanced interlayer coupling, manifested by the most tunable band gap and the highest electron mobility among all the InSe phases.   

A quantum-mechanical map for bonding and properties in materials

Materials with rationally controlled properties play important parts in the development of new and advanced technologies. For instance, the properties of thermoelectric, phase-change, or topologically insulating materials can be traced back, to a significant extent, to the nature of bonding in materials. Here, we develop a two-dimensional map based on a quantum-topological description of electron sharing and electron transfer. This map intuitively identifies the fundamental nature of ionic, metallic, and covalent bonding in a range of elements and binary materials [1]. Furthermore, it highlights a distinct region for a mechanism recently termed “metavalent” bonding [2]. Extending this map into the third dimension by including physical properties of application interest, we show that bonding in metavalent compounds differs from the classical textbooks views of bonding. This map could be used to help designing new materials: by searching for desired properties in a 3D space and then mapping this back onto the 2D plane of bonding.

[1]Advanced Materials, accepted.
[2]Advanced Materials, accepted   

Structural, electronic and thermoelectric properties of PbTe-based chalcogenide compounds

Lead telluride (PbTe) and other lead-chalcogenides such as PbSe and PbS have been extensively investigated due to their potential applications in thermoelectric (TE) devices. Researchers previously reported good values of the figure-of-merit (zT) > 1.7 for PbTe – CdTe alloy at 775 K ¹, and about 1.5 for doped PbTe with Thalium at 773 K ². In addition, experiments showed that doping PbTe with Sn increases the figure of merit to 0.8 at 700 K ³. These promising experimental results were the motivation to carry out the electronic structure calculations of Pb_1-xSn_xTe (x = 0.25, 0.5, 0.75) compounds using density functional theory ⁴ in this study. Based on the semi-classical Boltzmann theorem ⁵, the TE properties are computed and the results will be presented.

References:
¹ Y. Pei et al., Adv. En. Mater. 2, 670 (2012).
² J. P. Heremans et al., Science 321, 554 (2008).
³ M. Orihashi et al., J. of Phys. and Chem. of Solids 61, 919 (2000).
⁴ P. Hohenberg et al., Phys. Rev. 136, B864 (1964).
⁵ Georg K. H. Madsen et al., Comp. Phy. Commun. 175, 67 (2006).   

Intermediate-phase method for computing the natural band offset between two materials with dissimilar structures

The band offset between different semiconductors is an important physical quantity determining carrier transport properties near the interface in heterostructure devices. Computation of the natural band offset is a longstanding challenge. We propose an intermediate-phase method to predict the natural band offset between two structures with different symmetry, for which the superlattice model cannot be directly constructed. With this method and the intermediate phases obtained by our searching algorithm, we successfully calculate the natural band offsets for two representative systems, (i) zinc-blende CdTe and wurtzite CdS and (ii) diamond and graphite. The calculation shows that the VBM of zinc-blende CdTe lies 0.71 eV above that of wurtzite CdS, close to the result 0.76 eV obtained by the three-step method. For the natural band offset between diamond and graphite which could not be computed reliably with any superlattice methods, our calculation shows that the Fermi level of graphite lies 1.51 eV above the VBM of diamond. This method, under the assumption that the transitivity rule is valid, can be used to calculate the band offsets between any semiconductors with different symmetry on condition that the intermediate phase is reasonably designed.