Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J53: Thermal and Electronic PropertiesFocus Live
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Sponsoring Units: DMP DCOMP FIAP Chair: Matthew McCluskey, Washington State Univ |
Tuesday, March 16, 2021 3:00PM - 3:36PM Live |
J53.00001: Defect Chemistry and Dopability of Telluride Diamond-Like Semiconductors and Ordered Vacancy Compounds for Thermoelectric Applications Invited Speaker: Elif Ertekin Computation-driven search for candidate thermoelectric materials has recently resulted in several successes, but many of the predicted materials often prove to be difficult to dope in the lab. This presentation will review our recent computational and experimental efforts to tailor and understand defect chemistry and dopabaility of a chemically-diverse set of telluride-based diamond-like semiconductors (DLS) of interest for their potential as thermoelectrics. We consider the IBIIITe2 with IB=(Cu,Ag), III=(In,Ga) and Cu2(Zn,Cd,Hg)(Si,Ge,Sn)Te4 material space, and use first-principles methods and experimental phase boundary mapping to comprehensively assess dopabilities in this search space. These materials are typically observed to be p-type, but a materials descriptor suggests that they would be more effective as thermoelectrics if they could be doped n-type. Therefore, we comprehensively establish the achievable range of carrier concentrations using calculations of phase stability, defect formation energies, and carrier concentrations. Using phase boundary mapping, experimental carrier concentrations are measured and compared to the predicted values, showing a correspondance within a few orders of magnitude. For all compounds, a delicate competition between IIII, IIII, and VI defects governs the achievable range of carrier concentrations -- and enhancing n-type behavior requires suppressing the IIII and VI defects while enhancing the IIII antisites. Using this observation as a design strategy, we identify candidate diamond like semiconductors that may be more amenable to n-type doping. The results of this comprehensive search are used to generate a chemically intuitive framework for predicting dopabilities in this family of materials without the need to carry out full-scale first-principles analysis. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J53.00002: How do defects limit the ultrahigh thermal conductivity of BAs? A first principles study Mauro Fava, Nakib Protik, Chunhua Li, Navaneetha Krishnan Ravichandran, Jesus Carrete, Ambroise van Roekeghem, Georg Madsen, Natalio Mingo, David Broido The promise enabled by BAs high thermal conductivity in power electronics cannot be assessed without taking into account the reduction incurred when doping the material. Using first principles calculations, we determine the thermal conductivity reduction induced by different group IV impurities in BAs as a function of concentration and charge state. We unveil a general trend, where neutral impurities scatter phonons more strongly than the charged ones. CB and GeAs impurities show by far the weakest phonon scattering and retain BAs' κ values of over 1000 W m-1 K-1 even up to high densities making them ideal n-type and p-type dopants. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J53.00003: Effect of Stress-Strain Profile on Bulk Thermal Conductivity Ahmet Gunay, Junichiro Shiomi Thermal conductivity reduction via stress-strain engineering has serious implications in the fabrication of high efficiency thermoelectric generators due to the introduction of additional lattice defects (increased phonon scattering) and decreasing of the speed of sound (lattice softening). Here, we systematically study the effect of stress-strain profile on thermal conductivity through experiments. We fabricate bulk buttons of porous silicon (∼3 cm3) from micropowders (∼50 µm size) under different initial pressure conditions using plasma-assisted sintering. We then obtain the stress-strain and hardness profiles of the substrates using nanoindentation. After obtaining the mechanical properties, we perform laser flash analysis and differential scanning calorimetry to obtain the bulk thermal conductivity of the samples. Our preliminary results show that higher strain (i.e. lower initial pressure) yields lower bulk thermal conductivity due to the better preservation of nanoscale properties. Furthermore, we see a reduction in speed of sound with higher strain, highlighting the effect lattice softening. This study sheds light on the effect of mechanical properties on thermal conductivity and offers insights into new avenues for the design of low thermal conductivity nanomaterials. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J53.00004: Far-infrared studies of shallow thermal donors and dilute impurities in high-purity silicon Vladimir Martinez, David Burnham Tanner, rana adhikari, Koji Arai, Aidan F Brooks, Christopher Wipf Silicon is a candidate material for the test masses of future gravitational-wave detectors; it also has applications in high-resolution infrared spectroscopy in astronomy. We have measured the temperature-dependent infrared transmission of high-purity silicon samples having impurity concentrations of ~10^15 /cm^3. Measurements were made in a frequency range from 10–2000 cm^-1 at temperatures from 10–300K. At 10 K, silicon is transparent in the far infrared (10–600 cm^-1 ) apart from narrow absorption lines caused by residual impurities. In our samples, these absorption lines are mostly due to hydrogen-like transitions in interstitial oxygen occurring in the form of thermal double donors. The oxygen is introduced during the Czochralski growth method and occurs as an uneven distribution of oxygen throughout the boule as determined by how and where the Si melt makes contact with the fused-silica crucible. At higher temperatures, the electrons are ionized by the thermal energy in the crystal and become free electrons, causing a Drude-like response in the far infrared. Using the results of our transmission spectra, we can determine the type of impurities in the sample and their concentrations. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J53.00005: Adiabatic and nonadiabatic contributions to the cross section for carrier capture by defects in semiconductors Guanzhi Li, Yue Yu, Laura R Nichols, Andrew O'Hara, Georgios D Barmparis, Sokrates T Pantelides, Xiaoguang Zhang Recent progress in first-principles calculations of multiphonon processes in solids [1,2] allows direct calculation of nonradiative hot-carrier capture cross sections of defects in semiconductors. Here we report the development of a faster and more accurate time-domain integration method for calculations of capture cross sections that are converged with respect to the number of phonon modes. By applying the improved method, we calculated the Frank-Condon (zeroth-order) term as well as the adiabatic and non-adiabatic first-order terms [1] for electron capture cross section by the Si dangling bond of a triply-hydrogenated vacancy in Si and hole capture cross section by substitutional carbon in GaN. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J53.00006: Piezoresistance in nano-silicon Alistair Rowe, Steve Arscott, Jeffrey McCallum, Brett Johnson, Christopher Lew, Heng Li, Abel Thayil, Marcel Filoche Piezoresistance (PZR) in nano-silicon has long promised to provide a means to sensitively transduce motion in nano-electromechanical systems. Giant or anomalous effects loosely ascribed to mechanically sensitive electronic defects have been reported in lightly doped nano-objects. On the basis of two recent works [1, 2] a quantitative description of the piezoresponse of trap-mediated, space-charge-limited transport will be given. Using silicon nano-membranes containing both native and engineered defects, it will be shown that under steady-state conditions the magnitude of the piezoresistance is always comparable to that of charge-neutral, bulk silicon although a sign change can be induced under bipolar conditions due to stress-induced shifts in the trap activation energies. Under non-steady-state conditions, this same shift in trap activation energies can yield a giant piezoresponse at measurement frequencies close to the characteristic trapping rates. In terms of possible nano-sensing applications, the difficulties likely to be encountered when trying to exploit this giant piezo-impedance will be discussed. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J53.00007: Band Structure Modifications of Ag2Se by Sulfur: A Density Functional Theory Study Maxim Makeev, Ayaskanta Sahu, Nav Nidhi Rajput Silver selenide is known to possess optical response in the range that covers medium to long wavelengths. The methods of finetuning the band gap include its alloying with sulfur resulting in Ag2Se1-xSx structures. In this work, the band-structure analysis of crystalline Ag2Se with and without substitutional sulfur atoms is performed using density functional theory. The band structures of Ag2Se1-xSx are computed using different theory levels, and the results are discussed and compared with experimental data. The atomic structures of bulk crystalline Ag2Se with one to three selenium atoms substituted by sulfur are studied and the effects of substitutions on band structure specifics is discussed in the context of structural and chemical modifications taking place in these systems. The studies of strain-induced evolution of the Ag2Se band structures for samples with and without substitutional sulfur atoms are performed to unravel the combined effects of strain and sulfur on electronic structure of Ag2Se1-xSx. All the calculations have been performed for an Ag2Se unit cell and supercells of varied sizes. The implications of the obtained results for Ag2Se-based photodetectors will be discussed. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J53.00008: Anomalous Hopping Conduction Discovered in N-Type a-Si Brianna Western, Michael Harcrow, Athanasios J Syllaios, Vincent C Lopes, Christopher Littler Historically, n-type amorphous silicon has been shown to obey a conductivity temperature dependence of ln σ ∝ 1/T, determiend by a semi-log Arrhenius plot. This work presents measurements of dark conductivity in amorphous silicon doped with high amounts of phosphine ([PH3]/[SiH4] = 0.01 - 0.03) and grown with argon dilution that have yielded an anomalous hopping conduction mechanism: ln σ ∝ 1/Tp where p ≈ 0.75, determined by Resistance Curve Derivative Analysis. A comparison between these results and historical data is presented. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J53.00009: Anomalous Hopping and Multi-Phonon Hopping Charge Transport in Amorphous Silicon-Germanium Alloy Thin Films Lis Stolik Valor, Mohammad Ali Eslamisaray, James Kakalios Hydrogenated amorphous Si and Ge alloys (a-SixGe1-x:H) are particularly interesting for device applications such as solar cells, due to the addition of Ge to the a-Si:H network allows for the tuning of the optical gap. We describe studies that find that the electronic mobility gap model traditionally employed to describe charge transport in a-Si:H is not the best description for a-SixGe1-x:H alloys. Pure a-Si:H films exhibit anomalous hopping conduction, while the conductivity of a-Ge:H is best fit by a power-law temperature dependence, characteristic of multi-phonon hopping. Corresponding measurements of the thermopower find that conduction is n-type for the purely a-Si:H and a-Ge:H samples but that the Seebeck coefficient exhibits a transition from n-type to p-type as a function of Ge content and temperature. This change in sign may be indicative of the cross-over in conduction mechanism. These findings are interpreted in terms of charge transport either through a density of exponential bandtail states or hopping via dangling bond defects, that does not involve the traditional Mott mobility gap model of highly disordered materials. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J53.00010: Formation and Diffusion of Charged and Neural Defect States in Crystalline GeSe for Synaptic Electronics Luca Bursi, Rajiv K. Chouhan, Alessandra Catellani, Arrigo Calzolari Information and communication technologies have been historically powered by silicon. The current major worldwide drive for big data, machine learning and computing threatens to overwhelm Si-based resources. The search for alternative materials and technologies is thus crucial and represents a unique opportunity to explore and link materials’ properties and performances in unexplored architectures. In this upcoming process, new disrupting solutions referred to as in-memory computing, based on synaptic electronics, are emerging. Ge-based compounds have been proposed as switching materials for nonvolatile memory devices and selectors. However, the interplay between the properties of materials and the devices’ performances has not been completely understood. Here by means of state-of-the-art high-throughput simulations workflows for first principles condensed matter simulations, as implemented in the AiiDA infrastructure for the Quantum ESPRESSO, we study charged and neural defect states in GeSe chalcogenide with particular attention to their formation energies and diffusion barriers. Our results deepen current understanding of the interplay between structural, doping, and electrical properties of complex GeSe compounds in their application as switching materials. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J53.00011: Scanning Tunneling Spectroscopy of Cubic Boron Arsenide Single Crystals Hwijong Lee, Geethal Amila Gamage, John L Lyons, Fei Tian, Brandon Smith, Evan Richard Glaser, Zhifeng Ren, Li Shi Recent theoretical and experimental studies have validated cubic boron arsenide (BAs) as the first known semiconductor with an unusual high lattice thermal conductivity comparable to those of graphite and diamond. Knowledge of the surface electronic structure of BAs is required for realizing its potential as both a heat spreading material and an active layer in future-generation electronic devices. Here, we report scanning tunneling spectroscopy (STS) measurements of the electronic structures of as-grown and in situ cleaved BAs single crystal surfaces. While the onset of tunneling from the conduction band cannot be identified clearly on the as-grown bare surface, the bandgap measured at several interior locations of the cleaved surface is close to 2.1 eV, in agreement with a recent optical measurement and theoretical calculation. However, the measured bandgap decreases to about 1.9 eV near the two edges of the cleaved surface due to tunneling from high-concentration shallow acceptors. The tunneling peaks observed by STS within the bandgap are compared with calculated energy levels for lattice defects and substitutional impurities. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J53.00012: Copper Defects and Charge Collection in Cadmium Telluride Photovoltaic Devices Trumann Walker, Tara Nietzold, Niranjana Mohan-Kumar, Michael Stückelberger, Eric Colegrove, Barry Lai, Mariana Bertoni In CdTe photovoltaic devices, the CdTe layer must be doped with Cu to reduce its resistivity, increase carrier lifetime, and improve hole transport. However, the Cu concentration is typically very low around the interfaces and within the CdTe layer, making it difficult to determine the influence Cu has on charge transport at the nanoscale; Cu concentrations are difficult to detect due to low ionization probability of Cu atoms [1]. Here, we use synchrotron X-ray microscopy to probe the nanoscale distribution of Cu and correlate it to local charge collection in CdTe photovoltaic devices. We demonstrate Cu segregation around grain boundaries, and, using cross-section charge collection measurements and transport modelling, show recombination center concentration in the CdTe layer dictates the interface at which charge collection occurs. The work gives insights on how Cu distributes in CdTe photovoltaic systems, and an understanding of how these distributions affect charge transport. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J53.00013: Electronic and magnetic properties of pristine and lanthanide element doped yttrium iron garnet Santosh KC, Durga Paudyal The rare-earth based iron garnets (RIGs) have attracted significant attraction because of their wide technological applications such as in microwaves, optics, acoustics, magneto-optics, lasers, data storage, spintronics, and potentially in quantum information devices. In order to realize their potential applications, atomic-level details of the electronic, magnetic, intrinsic defect, and doping behavior in those materials need to be understood. Using first-principles electronic structure calculations, we investigate these properties in yttrium iron garnet (YIG). In addition, the role of lanthanide element doping in YIG and its impact on magnetic exchange interactions and defect states is explored. Moreover, incorporation of the lanthanide ions in YIG can potentially provide the defects with long spin coherence and stable optical transition. |
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