Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session M63: Defects in Oxides and ChalcogenidesFocus
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Sponsoring Units: DMP DCMP FIAP Chair: Cyrus Dreyer, State Univ of NY - Stony Brook Room: Mile High Ballroom 4D |
Wednesday, March 4, 2020 11:15AM - 11:51AM |
M63.00001: Phase change material programmable visible photonics Invited Speaker: Robert Simpson Chalcogenide phase change materials can exist in multiple structural states at room temperature. There is a large difference in the electrical and optical properties between the structural states of these materials. Importantly the phase transition time can be just picoseconds [1], and this makes these materials ideal for both optical and electrical data storage[2]. Indeed, they have recently been commercialised in Intel’s Optane memory[3]. |
Wednesday, March 4, 2020 11:51AM - 12:03PM |
M63.00002: Electron mobility in crystalline and amorphous metal-oxide semiconductors Igor Evangelista, Anderson Janotti Charge carrier mobility in amorphous semiconductors is significantly reduced when compared to the crystalline phase. For instance, room-temperature electron mobility in crystalline silicon is 1400 cm2/V.s, while that for amorphous silicon is only ~0.1 cm2/V.s. Amorphous metal-oxide semiconductors are unique in this respect, as they display relatively high electron mobilities when compared to their crystalline counterpart. Here we investigate the microscopic origin of this intriguing effect. Using first-principles molecular dynamics to generate amorphous configurations and electronic structure calculations of prototype metal oxides, we investigate why metal-oxide semiconductors display relatively high electron mobilities even in an amorphous phase. We analyze their band structure, the effects of volume, the change in the absolute band edge positions, and provide a detailed comparison between amorphous silicon and Ga2O3, paying special attention to the presence of band tail states and the change in band gaps. |
Wednesday, March 4, 2020 12:03PM - 12:15PM |
M63.00003: Amorphous Oxide Semiconductors: role of disorder and composition in defect formation, carrier generation, and transport properties Julia Medvedeva Unlike Si-based semiconductors, amorphous oxide semiconductors (AOSs) exhibit optical, electrical, thermal, and mechanical properties that are comparable or even superior to those possessed by their crystalline counterparts. Although amorphous materials lack grain boundaries and periodicity, the electron transport is more complex than in the crystalline phases: strong local distortions in the metal-oxygen polyhedra and intricate structural morphology in AOSs affect carrier mobility via composition, defects, thermal vibrations, nano-crystallinity, and lattice strain. Moreover, given many degrees of freedom in amorphous materials, defects in AOSs have the structural, thermal, and electronic characteristics that differ fundamentally from those in the crystalline transparent conducting oxides. |
Wednesday, March 4, 2020 12:15PM - 12:27PM |
M63.00004: Structural Dynamics in Amorphous Oxide Semiconductors and Its Role on Defect Formation and Transport Corey Burris, Julia Medvedeva Amorphous oxide semiconductors (AOS) possess many unique properties, including high carrier mobility (>>a-Si:H) and optical transparency, making them attractive materials for a wide range of optoelectronic applications. Unlike Si-based semiconductors or silica glasses, AOS exhibit strong distortions in local polyhedral structure due to weaker Metal-Oxygen bonds that are ionic in nature. These distortions have been shown to cause strong electron localization near the valence and conduction band edges and deep inside the band gap when oxygen content, known to dope the oxide materials n-type, is varied. Moreover, due to the large number of degrees of freedom in AOS, thermal or photo activation may switch bond configurations out of shallow states into deep bound states or vice versa, making a static characterization inadequate for AOS. In this work, ab-initio molecular dynamics simulations are combined with accurate density-functional calculations to study time- and temperature-dependent characteristics of defects with different formation energies and degree of localization in amorphous InOx . The microscopic understanding of the structural dynamics highlights the complex nature of AOS and helps explain the observed non-equilibrium conductivity in the materials under illumination. |
Wednesday, March 4, 2020 12:27PM - 12:39PM |
M63.00005: Structure and properties of amorphous SrTiO3: a first principles study Ivan Zhuravlev, Julia Medvedeva Recent discoveries of amorphous and quasi-amorphous phases in SrTiO3 (a-STO) open up a wide range of possible applications in electronic devices that prohibit utilization of crystalline STO (c-STO) due to fundamental or technological barriers. While oxygen vacancies are known to play key role in the electronic properties of c-STO, the results may not be applicable to a-STO. Upon amorphization, changes in the local structure and morphology may affect the formation energy of oxygen defects and their diffusion. In addition, a reduction of the material’s density may lead to voids and other structural defects, important for material’s behavior. All the above calls for a comprehensive study of the microscopic origins of the observed electronic and optical properties in a-STO. |
Wednesday, March 4, 2020 12:39PM - 12:51PM |
M63.00006: Hydrogen doping of crystalline/amorphous In2O3 interface: the structure and electronic properties. Kapil Sharma, Julia Medvedeva Hydrogen doped In2O3 has grabbed attention of scientific community due to its high carrier mobility, high carrier concentration, and transparency in near-IR region, with values exceeding those in commercial ITO. While H-doped crystalline In2O3 has been studied, the results may not be applicable to amorphous In-based oxides, employed in state-of-the-art display technologies. The lack of periodicity, the strong local and medium-range distortions in the Metal-Oxygen (M-O) polyhedra, and the increased number of degrees of freedom in amorphous materials are likely to affect the formation of M-OH and M-H bonds as well as their thermal stability. Hence, the resulting properties of hydrogenated amorphous oxide semiconductors may differ from those in the crystalline counterpart. |
Wednesday, March 4, 2020 12:51PM - 1:03PM |
M63.00007: Thermoelectric properties of Pb2-xYxRu2O7-z pyrochlore Sepideh Akhbarifar, Werner Lutze, Ian L Pegg Thermoelectric materials convert heat to electrical energy and are considered for various applications such as refrigeration or in power generations. Thermoelectric properties are characterized by a figure of merit (ZT), ZT=S2σT k-1. ZT is a function of the Seebeck coefficient or Thermopower (S), electrical conductivity (σ), temperature (T) and thermal conductivity (k). This work aims to study the respective properties of lead ruthenate (Pb2Ru2O6.5) doped with yttrium oxide in the temperature range of 25 to 300°C. Lead ruthenate has a defect pyrochlore crystal structure and has metal-like electrical conductivity. Y2Ru2O7 is an insulator. Molar fractions of Pb have been substituted by yttrium, i.e., Pb2-xYxRu2O7-z with x varied between 0 and 1. The compounds were prepared by solid-state synthesis and characterized by XRF, X-ray diffraction, and SEM/EDX for composition, structure, phase content, morphology and crystal size. Micron-sized powders were pressed into pellets to measure the thermoelectric properties. The results will be discussed, particularly the transition from conductive to insulating and its effect of ZT. |
Wednesday, March 4, 2020 1:03PM - 1:15PM |
M63.00008: Role of defects in photocatalytic water splitting: Monodoped vs codoped SrTiO3 Manish Kumar, Saswata Bhattacharya SrTiO3 can be utilized as a photocatalyst for water splitting owing to its suitable band edge positions, thermal stability and low cost. However, it could not harness the visible light due to its wide band gap (3.25 eV). Doping is one of the prominent solutions to tailor the band gap and inducing the visible light response and thus, enhancing the photocatalytic activity. We address the role of monodoping of a metal (Mn) and a non-metal (N), as well as their codoping in SrTiO3 for photocatalytic water splitting using state of the art density functional theory and ab initio atomistic thermodynamics. NO reduces the band gap, but introduces the deep trap states that degrade the photocatalytic efficiency. MnSr could not reduce the band gap of SrTiO3. MnTi could reduce the band gap, but it lowers down the conduction band edge and hence, lowers the reduction power. Our results reveal that the MnSrNO (codoping of Mn at Sr and N at O in SrTiO3) is the promising candidate as a photocatalyst for water splitting as it induces the visible light response as well as passivates the deep level states and its band edge positions straddle the redox potential of water. |
Wednesday, March 4, 2020 1:15PM - 1:27PM |
M63.00009: Rutile GeO2: an ultra-wide-band-gap semiconductor with ambipolar doping Sieun Chae, Kelsey A. Mengle, Hanjong Paik, Jihang Lee, John Heron, Emmanouil Kioupakis Ultra-wide-band-gap (UWBG) semiconductors have tremendous potential to advance electronic devices as device performance improves superlinearly with increasing gap. Ambipolar doping, however, has been a major challenge for UWBG materials as dopant ionization energy and charge compensation generally increase with increasing band gap. Using hybrid density functional theory, we demonstrate rutile germanium oxide (r-GeO2) to be an alternative UWBG (4.68 eV) material that can be ambipolarly doped. We identify SbGe, AsGe, and FO as possible donors with low ionization energies and propose growth conditions to avoid charge compensation by native acceptor-type defects. Acceptors such as AlGe have relatively large ionization energies (0.45 eV) due to the formation of localized hole polarons. Yet, we find that the co-incorporation of AlGe with Hi can increase the solubility limit of Al and enable hole conduction in the impurity band. We also calculate electron (153.6 cm2V-1s-1) and hole mobilities (4.7 cm2V-1s-1) of r-GeO2 at 300 K, suggesting r-GeO2 has outstanding electronic properties that can compete with the state-of-the-art UWBG semiconductors such as β-Ga2O3. We will also discuss on our recent experimental progress on thin-film growth and electrical characterization of r-GeO2. |
Wednesday, March 4, 2020 1:27PM - 1:39PM |
M63.00010: Quasiparticle Properties of Sulfur Doped SrTiO3 Gabe Lopez-Candales, Peihong Zhang Strontium Titinate (SrTiO3) is a potential candidate for photocatalytic water-splitting application due to a favorable conduction band edge position with respect to the hydrogen reduction potential level [1]. Despite having an ideal conduction band placement, the large band gap (3.25 eV (indirect) and 3.75 eV (direct)) [1] limits its photocatalytic and photovoltaic applicability. The distorted chalcogenide-perovskite counterpart (SrTiS3), on the other hand, is predicted to have a significantly lower band gap (0.9 eV) [2]. In this work, we report |
Wednesday, March 4, 2020 1:39PM - 1:51PM |
M63.00011: Defect physics in rare-earth doped wide band-gap materials Khang Hoang Rare-earth (RE) doped wide band-gap materials are of great interest for optoelectronic and spintronic applications. The determination of defect energy levels associated with RE-related defect centers has been challenging, however, both in experimental and theoretical/computational studies. Yet such knowledge is crucial to understanding luminescence or persistent luminescence in the materials. In this talk, we present a hybrid density-functional study of the interaction between RE dopants (Er, Eu, Dy, etc.) and host materials (GaN and SrAl2O4), including intrinsic point defects and other impurities that may be present in the host materials. In light of our results, we identify possible luminescent RE-related centers and elucidate luminescence (or persistent luminescence) mechanisms in the RE-doped materials and develop guidelines for defect engineering to design materials with improved performance. |
Wednesday, March 4, 2020 1:51PM - 2:03PM |
M63.00012: Tuning the Transport Properties of Artificial Synapses: A DFT-Supported Experimental Approach DIP DAS, Arabinda Barman, Pranab Kumar Sarkar, Rene Hubner, Dinakar Kanjilal, Priya Johari, Aloke Kanjilal Defect engineering is an essential aspect of resistive switching (RS) memories. This work illustrates cationic dopant (Ni) controlled RS characteristics in the thin film of anatase titanium dioxide (A-TiO2). A significant increase in ON/OFF ratio to 103 with a gradual change in resistance state is accomplished and such RS characteristics are highly expedient for neuromorphic computing applications. Detailed investigations by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) as well as density functional theory (DFT)-based first-principles calculations establish the role of Ni in memristive operation. In particular, TEM results show an evolution of two different Ni-concentration regions within the films that help to reduce the host Ti-site from its +4 valence state, as obtained from Ti-2p XPS analysis. This claim is further verified by DFT calculations, showing a change in the charge density near the Ni-site form the Bader charge analysis and electron localisation function (ELF) plots. Moreover, the role of those two Ni-concentration regions in regulating transport properties of the Au/ Ni:A-TiO2 /Pt devices similar to the biological synapse is discussed. |
Wednesday, March 4, 2020 2:03PM - 2:15PM |
M63.00013: Hot-electron mediated ion diffusion in semiconductors for ion-beam nanostructuring Cheng-Wei Lee, Andre Schleife Ion-beam-based techniques are widely utilized to synthesize, modify, and characterize semiconductors at the nanoscale. Interactions of the beam with the target are fundamentally interesting, as they trigger multi-length- and time-scale processes that need to be quantitatively understood to achieve nanoscale precision. Here we demonstrate for magnesium oxide1 that, in a beam energy regime in which electronic effects are usually neglected, a proton beam efficiently excites oxygen-vacancy-related electrons and changes the charge state of the F-center. We quantify the beam-energy-dependent electronic excitation and the emerging ion dynamics using first-principles techniques. We further bridge time scales from ultrafast electron dynamics directly after impact to ion diffusion over migration barriers in semiconductors and discover a diffusion mechanism that is mediated by hot electrons. Our quantitative simulations predict that this mechanism strongly depends on the projectile-ion velocity, suggesting the possibility of using it for precise sample manipulation via nanoscale diffusion enhancement in semiconductors with a deep, neutral, intrinsic defect. |
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