Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L63: First Principles Approaches to DefectsFocus
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Sponsoring Units: DMP DCMP FIAP Chair: Joel Varley, Lawrence Livermore Natl Lab Room: Mile High Ballroom 4D |
Wednesday, March 4, 2020 8:00AM - 8:36AM |
L63.00001: First principles study of defect properties of II-VI photovoltaic materials Invited Speaker: Suhuai Wei First-principles study of photovoltaic materials plays an important role in developing solar cell technologies because it can provide useful physical insights, fresh perspective and new design principles for developing innovative solar cell materials with high solar power conversion efficiency and reduced cost. Similar to other semiconductors, one of the most important issues in slolar cell absorber materials are to control the defects, either for introducing charge carrier, improving charge transport, or reducing non-radiative carrier recombination. For example, a good solar cell material should have good defect properties, that is, it can be easily doped such that charge carriers can be created to generate the required electric field and has less defect-induced recombination centers such that it has high carrier life time and minority carrier mobility, so photo-generated charge can be easily collected. In this talk, using II-V thin-film solar cell absorber materials Cd(Te,Se), Cu(In,Ga)Se2 (CIGS) and Cu2ZnSnS2 (CZTS) as examples, I will discuss how theoretical first-principles studies can be used to better understand and optimize the solar cell performance. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L63.00002: First-Principles Study on the Defect Physics of Ternary Chalcopyrite ZnGeP2 through Hybrid Functional Menglin Huang, Shanshan Wang, Shiyou Chen Zinc germanium diphosphide (ZnGeP2) is a chalcopyrite semiconductor that can be used for infrared frequency conversion applications. It is believed that the intrinsic defects are responsible for the defect-related absorptions and emissions observed in experiments, and the performance of the corresponding device is also limited by those defects. Here we study the defect physics in ZnGeP2 using the density functional theory (DFT) calculations with a hybrid functional. We will firstly present the result of phase stability, which differs significantly from the previous result calculated by semi-local DFT. GeZn, ZnGe, GeP, PGe antisites are the dominant point defects among all the Fermi level range and growth conditions. Under Zn-rich condition, the Fermi level is about 0.6 eV above valence band maximum (VBM), which shows the p-type conductivity, while under Ge-poor condition, the Fermi level is located at the middle of the band gap, consistent with the experiment results. We will also discuss the result of calculated optical transition that is related to photoluminescence peak and the possible impurities and defect complexes. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L63.00003: Dimensionality-suppressed chemical doping in 2D semiconductors: the cases of phosphorene, MoS2, and ReS2 from first-principles Ji-Hui Yang In spite of great appeal of two-dimensional (2D) semiconductors for electronics and optoelectronics, to achieve required charge carrier concentrations by means of chemical doping remains a challenge, due to large defect ionization energies (IEs). Here by decomposing the defect IEs into the neutral single-electron defect level, the structural relaxation energy gain, and the electronic relaxation energy cost, we propose a conceptual picture that the large defect IEs are caused by two effects of reduced dimensionality. While the quantum confinement effect (QCE) makes the neutral single-electron point defect levels deep, the reduced screening effect (RE) leads to high energy cost for the electronic relaxation. The first-principles calculations for monolayer, few-layer, and bulk black phosphorus (BP), MoS2, and ReS2 do demonstrate the general trend. Based on the gained insight into defect behaviors, strategies can be envisaged for reducing defect IEs and improving charge carrier doping. Using BP monolayer either embedded into dielectric continuum or encapsulated between two h-BN layers, as practical examples, we demonstrate the feasibility of increasing the screening to reduce the defect IEs and boost carrier concentration. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L63.00004: First-principles study of defect physics in a photovoltaic semiconductor Cu2ZnGeSe4 Lele Cai, Shiyou Chen Cu2ZnGeSe4 is of interest for the development of next-generation thin-film photovoltaic technologies. The additional number of elements in Cu2ZnGeSe4 increases the flexibility of material properties relative to binary and ternary semiconductors. However, a variety of intrinsic lattice defects are formed that have a significant impact on its photovoltaic performance. Here we study the intrinsic point defects in Cu2ZnGeSe4 using the density functional theory calculations with both the generalized gradient approximation and the hybrid functional. Examination of the thermodynamic stability of Cu2ZnGeSe4 shows that the stable chemical potential region for the formation of stoichiometric compound is small. Under Zn-poor condition, the shallow acceptor, CuZn antisite is the dominant defect with the lowest formation energy, accounting for the observed p-type conductivity, while in a Ge-rich condition, the Fermi level is close to the middle of band edge because of the lower formation energy of ZnCu. We will discuss the effects of these defects on the photoelectric properties of Cu2ZnGeSe4. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L63.00005: First principles calculations of phonons and Raman and infrared spectra in ZnGeGa2N4 . Amol Ratnaparkhe, Walter Lambrecht ZnGeN2 is a semiconductor compound closely related to GaN. They have similar band gaps and are closely lattice matched but have a large band-offset, which enables new capabilities in heterostructure and alloy band structure engineering. Recently, a well-defined crystal structure with space group Pmn21 was proposed for the 50% compound ZnGeGa2N4 which satisfies the local octet rule around each N [PHYSICAL REVIEW MATERIALS2, 114602 (2018)]. Here, a first-principles study of the phonons in this material is presented. The calculations are performed using the density functional perturbation theory within the ABINIT software. The phonon frequencies at the Brillouin zone center and their symmetry analysis, as well as various associated parameters, will be presented: Born effective charge tensors, oscillator strength tensors, |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L63.00006: Energy landscape of vacancy-related defects in silicon: a comprehensive picture from DFT and GW Gabriela Herrero-Saboya, Layla Martin-Samos, Anne Hemeryck, Nicolas Richard Vacancy related point-like defects are one of the most common degrading centers in silicon-based technology. They have been the subject of extensive studies both experimental and theoretical. However, a comprehensive theoretical model capable of explaining experimental evidence is often missing. By means of first principles calculations we revisit structural, optical and electronic properties of these common point defects in silicon. Guided by simple theoretical models, we are able to perform accurate simulations, including many-body-perturbation corrections based on the GW approximation, in close quantitative agreement with the experiment. Going beyond the common total energy approach allows us to predict the electronic activity and the low temperature dynamics of such centers. At higher temperatures, vacancies become mobile centers, forming more complex systems and/or contributing to the diffusion of impurities. Starting from experimentally proposed mechanisms, we are able to theoretically characterize activation energies of such technologically relevant processes. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L63.00007: Charged Defects in TiO2 Anatase: A Comparative Study of Hybrid DFT, GW and BSE to Explore Optical Properties Pooja Basera, Saswata Bhattacharya Titanium dioxide (TiO2) anatase is one of the most abundant, functionally versatile oxide materials. However, owing to its large bandgap, the photo-absorption efficiency is limited only in the UV region. Doping-mediated modulation is one of the most pragmatic approaches in the pursuit to improve the photocatalytic and solar energy conversion efficiencies of TiO2. We report here using state-of-the-art hybrid density functional and ab initio atomistic thermodynamics, the thermodynamic (meta-) stability of different non-metal dopants X (X= N, C, S, Se) as a function of charge state at realistic temperature and pressure. Knowing the most stable defects, we aim an accurate theoretical estimation of the optical properties of doped TiO2. To do this, we have used many body perturbation theory viz. one particle Green's function (GW) method and higher order Green's function techniques Bethe-Salpeter Equation (BSE). Our calculations reveal the highly anisotropic nature of the doped TiO2. The n-type doped TiO2 is optically active in both x, z direction, whereas the p-type doped TiO2 is optically inactive along z-direction. The most evident hallmark of the doped system is the appearance of absorption peaks at low energy below 3 eV, to give rise visible-light absorption. |
Wednesday, March 4, 2020 9:48AM - 10:00AM |
L63.00008: The Barrier to the Small Polaron Formation in Metal Peroxides Shuaishuai Yuan, Zi Wang, Maximilian L. F. Baron, Kirk H Bevan Polaron formation is the process by which free electrons (or holes) in a material find a lower energy localized state by distorting their surrounding host lattice. Since polaron formation involves changes in both electronic and atomic coordinates, a deeper understanding of polaron formation promises to open up new avenues towards tailoring the physical properties of materials. Ab initio studies have predominantly focused on studying stabilized small polaron states and their hoping physics, while less investigation has been devoted to the equally important process of polaron formation. In this work, we provide ab initio insights into the polaron formation process and focus on exploring the physical origins of the barrier to polaron formation. We utilize the HSE06 hybrid functional to study small polaron formation in four similar metal peroxides, which exhibit a wide range of polaron formation barriers heights. Our results show that polaron hybridization with the conduction band minimum (CBM) plays a significant role in determining the magnitude of a polaron formation barrier. Moreover, by satisfying the generalized form of Koopmans’ theorem, we are able to show that the degree of hybridization is directly correlated with the electronic relaxation delay during polaron formation. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L63.00009: Machine learned defect and impurity levels in perovskite halides and CdTe Arun Kumar Mannodi Kanakkithodi, Fatih G Sen, Michael Toriyama, Michael J Davis, Maria Chan Electronic levels introduced by impurities and intrinsic defects in the band gap of semiconductors affect optoelectronic performance. Predictions of these defect levels are possible, but expensive, using first principles density functional theory (DFT), and chemical trends are often not easily available. In this talk, we will describe using machine learning (ML) trained on DFT data for defect levels in hybrid perovskite halides [1] and CdTe [2] photovoltaic materials. Relevant descriptors, relative performance of different ML approaches, and insight from resultant ML models will be discussed. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L63.00010: Many-body embedding for excited states of Fe in III-nitrides Cyrus Dreyer, Lukas Muechler, Malte Roesner A quantitative understanding of the electronic excited states of point defects in semiconductors and insulators is crucial for identifying defects detrimental to device performance, as well as those that have attractive properties for quantum applications. In particular, transition metal impurities in semiconductors have been widely studied in both contexts. One of the key limitations for a first-principles description of such systems is that transition mental excited states often involve correlated low-spin multiplets that require a multideterminant treatment. This is beyond the capabilities of density functional theory (DFT), which is the workhorse for computational studies of point defects. Using Fe in GaN and AlN as an example, we address this issue by treating the defect as a correlated subspace embedded in the lattice. The defect structure is obtained from hybrid-functional DFT calculations, and the subspace is isolated via Wannierization. The dielectric screening effects from the surrounding lattice to the local defect Coulomb interactions is accounted for using the constrained RPA approach. Finally, the many-body problem within the subspace is solved exactly. We compare our results to previous calculations using constrained DFT, and experimental measurements. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L63.00011: Monte Carlo Simulations of Non-Linear Evolution of Materials Resistivity Steven Hancock, Yohannes Abate, David P Landau We employed an experimentally motivated Monte Carlo simulation to calculate the conductance as a function of charged dopant concentration to investigate the non-linear evolution of the resistivity with annealing time in oxygen deficient thin films. The model consists of an N x N square lattice with hydrogenic atoms placed at each lattice site. The valence electrons are bound to their respective lattice site via a harmonic potential, and electrons at different sites are allowed to interact via a screened Coulomb potential. At each Monte Carlo step, electrons attempt to change their position slightly, and in addition attempt to transition between conducting and non-conducting states. We found nonlinear dependence of the resistance as a function of dopant concentration. The dopants act as electron traps where they lower the energy of nearby electrons and increase the energy requirement to transition to the conducting state. As more dopants are added, this effect increases, further suppressing the conductivity. This would indicate that the inclusion of the charged dopants advances the percolative evolution of such a phase transition. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L63.00012: Formation energies of charged defects in 2D materials - a new perspective Andrew O'Hara, Blair Tuttle, Xiaoguang Zhang, Sokrates T Pantelides Formation energies of defects in semiconductors play a major role in understanding a wide range of properties. Supercell schemes have been widely used for calculations. For charged defects, a divergence in the potential arises from the periodic Coulomb interactions and, in the “jellium” approach, is removed by setting the average electrostatic potential to zero. A posteriori corrections are typically used to determine the infinite-supercell limit. For 2D materials, where unscreened Coulomb tails are present in the vacuum regions, additional complications arise. In this work, we present an alternative formalism based on statistical mechanics, which dictates that supercells are naturally neutral: “charged defects” are merely ionized, by trading carriers with the energy bands (charge neutrality of the crystal is an essential ingredient of the statistical mechanics of electrons in semiconductors). We show that the jellium approach can be derived from the statistical-mechanics-backed theory by invoking approximations with unknown consequences. We report density-functional-theory calculations showing that the differences between the two methods are especially large in 2D materials, e.g., h-BN, where they can be of order 1 eV. Convergence rates are excellent. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L63.00013: Origin of n-type conductivity of monolayer MoS2 Akash Singh, Abhishek Singh Monolayer MoS2 is a promising two-dimensional material for electronic and optoelectronic devices. As-grown MoS2 is an n-type semiconductor, however, the origin of this unintentional doping is still not clear. Here, using hybrid density functional theory (DFT), we carried out an extensive study of often observed native point defects i.e., VS , VMo , VS2 , VMoS3 , VMoS6 , MoS2 , and S2Mo and found that none of them cause n-type doping. Specifically, S vacancy (VS ), which has been widely attributed to n-type conductivity, turns out to be an electron compensating centre. We report that hydrogen, which is almost always present in the growth environments, is most stable in its interstitial (Hi ) and H-S adatom forms in MoS2 and acts as a shallow donor provided the sample is grown under S-rich condition. Furthermore, they have high migration barrier (in excess of 1 eV), which would ensure their stability even at higher temperature and hence, lead to n-type conductivity. |
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