Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session E11: Defects in Semiconductors -- OxidesFocus
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Sponsoring Units: DMP DCOMP FIAP Chair: Leonard Brillson, Ohio State University Room: BCEC 152 |
Tuesday, March 5, 2019 8:00AM - 8:36AM |
E11.00001: Electronic states in electron-doped rare-earth nickelates from first principles Invited Speaker: Michele Kotiuga Correlation effects in transition metal based materials give rise to many interesting and exotic properties. The rare-earth nickelates, with a rich composition-phase diagram, are no exception. Doping rare-earth nickelates can lead to electron localization, introducing defect states that are unlike typical shallow or deep donor states familiar in conventional semiconductors. We present first-principles density-functional-theory-based calculations of rare-earth nickelates, with a focus on lanthanum nickelate and samarium nickelate, in which we add electrons to the material. Here, we investigate doping concentrations on the order of one electron per formula unit with the goal of changing the orbital occupation and triggering a phase transition, akin to the phase control seen with strain modulation. We carry out calculations where a uniform compensating background charge (‘jellium”) has been added to maintain charge neutrality when electrons are added, as well as supercell configurations with defects that electron dope the system, and superlattices where an electron is transferred at interfacial layers. In particular, we explore the effects of intercalated hydrogen and lithium as well as oxygen vacancies in samarium nickelate as well as lanthanum nickelate/strontium iridate superlattices. In comparing these calculations, we find the jellium-background calculations capture the changes to the electronic structure seen with the explicit inclusion of defects and interfaces. The resulting changes to the electronic structure, intimately linked to structural changes, cannot be understood with a rigid shift of the states: the bands are reorganized and the character of the gap is fundamentally altered. This class of doping effects introduces a new knob to turn in the field of materials design. |
Tuesday, March 5, 2019 8:36AM - 8:48AM |
E11.00002: Energy Band Gap Tuning based on Ag doped LaFeO3 Nano-porous WAFAA AZOUZI, Hicham labrim, Mohammed Benaissa porous lanthanum ferrite LaFeO3 has been elaborated with wet chemical method. We have employed both DFT simulation and Rietveld refinement to understand deeply the complex optical behavior accurate in synthesized Nano-powders [1], with different amount of silver as dopant, and also evaluate the stoichiometry-optical properties relationship in this insulator oxide. It found that all samples, with different gap value depending on the silver concentration, exhibit a p-type semiconductor with a direct gap in the visible light spectrum. The results showed that lanthanum defects present in the synthesized samples improve optical absorption [2] in the visible range comparing with bulk calculated spectrum making it potentially useful in solar photovoltaic application. |
Tuesday, March 5, 2019 8:48AM - 9:00AM |
E11.00003: The role of hydrogen in the persistent photoconductivity of SrTiO3 Zhiqiang Zhang, Anderson Janotti Strontium titanate (STO) is a perovskite oxide with an indirect bandgap of 3.25 eV at room temperature. It has been often used as substrate to grow other materials, and it is the basis of widely reported perovskite heterostructures that show high-density two-dimensional electron gas. Recently, an extremely long persistent photoconductivity (PPC) at room temperature has been observed in annealed strontium titanate single crystals upon illumination with sub-gap light [M. C. Tarun, F. A. Selim, and M. D. McCluskey,Phys. Rev. Letters, 111, 187403 (2013)]. Annealed studies indicate the importance of hydrogen impurities as well as oxygen vacancies to induce PPC. We use electronic structure calculations based on density functional theory (DFT) to investigate possible mechanisms behind the PPC effect. We present results of defect formation energies, defect-related absorption, and defect migration barriers, in an attempt to identify the defect associated with the observed PPC in STO. The results are combined to explain the experimental observations, and we further generalize the model to predict similar effects in other oxide like TiO2 and BaTiO3. |
Tuesday, March 5, 2019 9:00AM - 9:12AM |
E11.00004: Characterization and modulation of oxygen vacancy defects in metal/SrTiO3-δ Schottky heterojunctions Haoming Jin, Xiaochen Zhu, Arthur F Hebard Strontium titanate, SrTiO3-δ (STO), can be made semiconducting and even metallic using high temperature anneals by introducing oxygen vacancy defects (δ > 0). Controlled thermal anneals with various temperatures and durations are employed to create specific defect profiles near the STO surfaces. Thermal sublimation or e-beam evaporation technique is then used to apply gold (Au) contact electrodes to the thermally treated STO substrates and create Schottky junctions. The presence of the oxygen vacancy defects strongly alters the electrical properties of these junctions as measured by transport, capacitance and low frequency noise. Our comparative study of these Schottky Au/STO junctions reveals that (i) the concentration of oxygen vacancy defects increases at the higher annealing temperature (ii) an improved spatial distribution/uniformity of defects is obtained by either lengthening the annealing duration or raising the annealing temperature. We show that the electrical characterizations of Schottky junction discussed here can serve as a useful tool to probe defects near the interfaces of novel materials and unravel information that is not easily accessed by other approaches. |
Tuesday, March 5, 2019 9:12AM - 9:24AM |
E11.00005: Doping in perovskite stannates Santosh KC, Leigh Weston, Chris Van de Walle The perovskite stannates (ASnO3 ; A = Ba, Sr, Ca) are being actively explored for potential |
Tuesday, March 5, 2019 9:24AM - 9:36AM |
E11.00006: The Effects of Excess Charge Carriers in BiVO4 Iflah Laraib, Marciano alves Carneiro, Anderson Janotti Research on photoelectrochemical (PEC) performance of BiVO4 has gained momentum in the past few decades due to the potentially low cost solar-to-hydrogen conversion pathway provided by using this material as a PEC cell anode. While this material exists in four different phases, it is only its monoclinic phase doped with n-type carriers that exhibits significant PEC activity. These carriers typically involve hydrogenation, which supposedly introduces hydrogen interstitial and oxygen vacancy defects in the lattice. In this work, we use density functional theory calculations to explore the effects of excess n-type carriers on the structure and electronic properties of BiVO4, focusing on the most prominent tetragonal and monoclinic phases, and link it to its enhanced photo activity and possible carrier localization through electron small-polaron formation. The obtained results are compared to previous calculations and available experimental findings. |
Tuesday, March 5, 2019 9:36AM - 9:48AM |
E11.00007: Role of Point Defects in Enhancing the Conductivity of BiVO4 Hosung Seo, Yuan Ping, Giulia Galli Bismuth vanadate (BiVO4) is a promising photoanode for solar-to-fuel photocatalytic applications, and it has been extensively studied in recent years. However, the microscopic mechanism underlying the observed changes in electronic conductivity due to oxygen vacancies and nitrogen dopants remains unclear. Here, we combine electronic structure calculations at the hybrid density functional theory (DFT) level with constrained DFT, and we elucidate the role of defects in enhancing the transport properties of the material [1]. We show that at low temperature, oxygen vacancies give rise to deep levels within the fundamental gap of BiVO4; however even as deep levels, oxygen vacancies can act as effective n-dopants and polaronic charge carriers, due to their favorable position in energy relative to polarons in the pristine bulk. In addition, we show that N atoms can be easily introduced in n-doped BiVO4 and that the presence of substitutional nitrogen affects the formation energy of polarons, effectively contributing to an increase of the carrier mobility in the material. Our results reconcile apparently conflicting experiments and they may provide a foundation for polaronic defect engineering for photoanodes oxides. [1] H. Seo et al, Chem. Mater. |
Tuesday, March 5, 2019 9:48AM - 10:00AM |
E11.00008: Computational Study of LiGaO2 Electron Paramagnetic Resonance Spectra of Li and Ga Vacancies Dmitry Skachkov, Walter R L Lambrecht LiGaO2 is an ultra-wide-band-gap material with a wurtzite-like crystal structure and band gap of 5.3 eV. Electron paramagnetic resonance (EPR) experiments on irradiated samples of this material were recently published by Lenyk et al. (J. Appl. Phys. 124, 135702, 2018). In the present work, density functinal theory (DFT) calculations are carried out of the Ga and Li vacancies using the DFT+U approach in the charge states which carry an unpaired spin. In both vacancies the p-hole is located on one oxygen atom adjacent to the vacancy. Apical O and basal plane O are considered. The magnetic resonance parameters of the defects are determined using the Gauge Including Projector Augmented Wave (GIPAW) method. The EPR spectra of VGa2- is characterized by a quasi-isotropic superhyperfine interaction with one Ga nucleus and for the apical O spin gives a g-tensor with maximum oriented along b axis. For VLi0 there is a quasi-isotropic superhyperfine interaction with two Ga nuclei and the g-tensor maximum is along c for the basal plane O spin. Both of these are in agreement with experiment but we predict also the g-tensors for the other possible localization of the spins. The energy ordering and transition levels for the different models will be discussed. |
Tuesday, March 5, 2019 10:00AM - 10:12AM |
E11.00009: Towards bipolar tin monoxide: revealing unexplored dopants Jose Flores Livas, Migle Grauzinyte, Stefan A C Goedecker The advancement of transparent electronics, one of the most anticipated technological developments for the future, |
Tuesday, March 5, 2019 10:12AM - 10:24AM |
E11.00010: Experimental and computational evidence of defect centres in amorphous titanium dioxide and their confluence in resistive switching DIP DAS, Arabinda Barman, Anil Kumar Sinha, Mukul Gupta, D. M. Phase, Rahul Singhal, Sergei Zvyagin, Priya Johari, Aloke Kanjilal This work identifies dominant defect centres in amorphous titanium dioxide (a-TiO2) films and their role in resistive random access memories with the help of combined experimental and theoretical studies. X-ray absorption spectroscopy reveals a significant decrease in peak splitting between t2g and eg sub-bands at the Ti-L edge, which is attributed to lower valence of Ti-ions. Deconvolution of the Ti-2p X-ray photoelectron spectrum also suggests the existence of Ti3+ state along with oxygen vacancies in a-TiO2. Further, the role of oxygen vacancies as intrinsic electron trapping centres in such system is recognised by low-temperature electron paramagnetic resonance measurements. The same has been verified using ab-initio density functional theory based simulations for a-TiO2 phase, generated using ab-initio molecular dynamics simulations. The partial charge density and Bader charge analysis were done, in particular, to manifest the localized character of the excess electrons ejected during the creation of oxygen vacancies. Such electronic localisations facilitate local high conducting regions in the amorphous matrix, which in turn leads to forming free resistive switching behaviour in Pt/a-TiO2/Pt devices. |
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