Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P63: Defects: Structure and Strain 1Focus
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Sponsoring Units: DMP DCOMP FIAP Chair: Kunal Mukherjee, University of California, Santa Barbara Room: Mile High Ballroom 4D |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P63.00001: Defects in Chalcogenide Compound Semiconductors for Photovoltaics Invited Speaker: Angus Rockett This talk briefly reviews recent progress in solar cell energy generation to set the stage. The most promising include the thin film chalcogenide compound based devices. The talk then reviews the characterization and understanding of defects in the two primary materials classes used in solar cells (photovoltaics), CdTe and CuInSe2 and related compounds. The characterization methods discussed include scanning tunneling microscopy, nuclear magnetic resonance, and capacitance methods including scanning microwave impedance microscopy. The implications of the results for device performance are described. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P63.00002: Does lattice expansion really occur in defected low-temperature GaAs? Mary Clare Escano, Tien Quang Nguyen, Hideaki Kasai, Masahiko Tani A prominent example of lattice expansion caused by point defects is that of GaAs grown at low temperature (LT-GaAs). A linear correlation between the lattice expansion and concentration of As-antisite (AsGa) point defect is obtained experimentally by XRD. However, using full first-principles method (DFT-LDA) on a large 512-atom supercell of GaAs bulk, we found that, in the limit of small concentrations of AsGa (less than 1%), the lattice does not expand but the As-As bond length near the defect increased. DFT-LDA method is tested vis-a-vis the more sophisticated DFT-hybrid functionals and the former yields the same lattice changes as the latter. We propose that the observed expansion maybe due to the increased bonds near the defect. We also found that AsGa point defects do not prefer isolated structure even at low concentration. Using molecular dynamics simulation, we confirm that the above results hold true even at room temperature. Study of electronic properties reveals that, in this low concentration regime, abrupt changes in density of states near the fermi level occur. These findings pose impact in the use of LT-GaAs for optoelectronic/terahertz devices, where fundamental understanding of defect concentration's relation with material structure/functionality is vital. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P63.00003: Off-centered Pb Interstitials in PbTe Sungjin Park, Byungki Ryu The nature of off-stoichiometry of PbTe has not been fully understood yet though many studies existed by investigating its intrinsic defects. We reinvestigated the formation of intrinsic defects of PbTe by DFT study. From this, Te vacancy (VTe) was the major defect under Pb-rich condition. Due to experiments that reported the increase of the lattice parameter as the Pb excess content grew up, a controversy between the DFT and the experimental results existed since the existence of VTe could not explain the volume increase. So we assumed Pb interstitial (PbInt) as the major defect in PbTe instead VTe because it not only exhibited the second-lowest formation energy, but also could be responsible for the lattice parameter increase. From this, all possible PbInt positions in PbTe were first investigated to understand its off-stoichiometry. As a result, we found new stable positions of PbInt other than the conventional one at the subcubic center. Moreover, among the various locations, PbInt toward the nearest Te and Pb dimer with PbInt were the ground and a metastable state. Finally, we believed that the many local minima of PbInt might be responsible for the off-stoichiometry of PbTe during the sintering of thermoelectric PbTe. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P63.00004: Quantifying Cation Disorder in ZnGeP2Thin Films Using Resonant Energy X-ray Diffraction Rekha Schnepf, Ben Levy-Wendt, Brooks Tellekamp, Brenden Ortiz, Celeste Melamed, Laura Schelhas, Kevin Stone, Michael Toney, Eric Toberer, Adele Tamboli II-IV-V2 materials, ternary analogs to III-V’s, are emerging for their potential applications in devices such as LEDs and solar cells. Controlling cation ordering in II-IV-V2’s offers the potential to tune properties at nearly fixed compositions and lattice parameters. Cation disorder is prevalent in many II-IV-V2’s and has a profound effect on properties – however, quantification of disorder remains difficult. In this work, we investigate two different methods to quantify cation ordering in ZnGeP2thin films: a stretching parameter calculated from lattice constants (c/a), and an order parameter determined from the cation site occupancies (S). We use high resolution X-ray diffraction (HRXRD) to determine c/a and resonant energy X-ray diffraction (REXD) to extract S. REXD is critical to distinguish elements with similar Z-number (e.g. Zn and Ge) through X-ray diffraction. We found that samples with a c/a corresponding to the ordered chalcopyrite structure had only partially ordered S values. The optical absorption onset for these films occurred at lower energy than expected for fully ordered ZnGeP2, indicating that S better captures disorder in these samples. This work highlights the importance of nuanced techniques (e.g. S) when analyzing more complex ternary systems. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P63.00005: Phonon Dispersion in Anisotropic Dilute Alloys by Weighted Dynamical-Matrix Approach Mina Aziziha, Saeed Akbarshahi, Sayandeep Ghosh, Prativa Pramanik, James Patrick Lewis, Aldo H Romero, Suresh Pittala, Subhash Thota, Mohindar S Seehra, Matthew Bruce Johnson The calculation of phonon frequencies in dilute anisotropic alloys is not a simple undertaking and the successful methodologies are not well developed. We propose weighted dynamical-matrix (WDM) approach for calculating optical phonon spectra that is applicable to such complex alloys for a large range of alloying elements. Our WDM approach forms the dynamical-matrix for random alloys using the weighted average of parent structures mass and calculated force constants that allows us to calculate the phonon spectra. This WDM approach is computationally efficient and easily implemented. We demonstrate its use for the effect of Fe-doping on the phonon modes in dilute alloy of CuAl1-xFexO2 delafossite powders whose structural and magnetic properties have been reported previously1,2 and the phonon frequencies are measured using Raman and FTIR spectroscopies. The observed phonon modes are in good agreement with our WDM approach calculations. These results are consistent with the lattice expansion accompanying the Fe-doping, weakening the bonds. Also, Fe-doping effect on optical properties of these alloys is explored and explained by the first-principle calculated band structures. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P63.00006: Simulations of Nitrogen Incorporation into GaAsN Alloys: The Role of Lattice Relaxation Emily Oliphant, Rachel Goldman, Liang Qi, Timothy Jen, Albert Tsui, Drimik Chowdhury, Yongqiang Wang Due to the significant band gap narrowing induced by dilute fractions of N in III-V semiconductors, dilute nitride semiconductor alloys are of significant interest for long-wavelength optoelectronics. However, rapid thermal annealing (RTA) is often needed to achieve suitable transport properties and emission efficiencies. Therefore, identification of the local N environments and the influence of RTA on those environments is needed. For GaAsN, it has been suggested that N shares an As site with either As or N, with N-As and N-N pairs (split interstitials) aligned along [010] and [111] directions, respectively. However, the effect of lattice relaxation in the vicinity of incorporated N has not been considered. Here, we utilize density functional theory to compute minimum energy positions of NAs and surrounding neighbors. To identify N incorporation sites, we compare nuclear reaction analysis (NRA) spectra with Monte Carlo-Molecular Dynamics simulations along the [100], [110], and [111] directions. The measured NRA spectra exhibit the highest (lowest) yields in the [111]([100]) directions. Similar trends are observed for simulations of (N-As)As, confirming the presence of interstitial complexes, predominantly (N-As)As, in GaAsN. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P63.00007: Diffusion of Acceptor Dopants in Atomically Precise Devices Jeffrey Ivie, Evan Anderson, Scott W Schmucker, Lisa A Tracy, DeAnna Campbell, David Scrymgeour, Aaron Katzenmeyer, Juan P Mendez, Ping Lu, Xujiao Gao, Dan R. Ward, Ezra Bussmann, Tzu-Ming Lu, Shashank Misra Atomically precise (AP) electrical devices, fabricated using hydrogen depassivation lithography in a scanning tunneling microscope, offer a potential pathway to ultra-efficient transistors. Almost all previous work regarding AP devices has been focused on understanding the properties of phosphorus-based donor devices integrated into intrinsic or donor-implanted substrates. Equivalent knowledge for integration on to substrates having acceptor dopants is critically lacking. Here, we present our work in understanding the diffusion processes of acceptor dopants during CMOS compatible AP processing, which demonstrates highly counterintuitive behavior compared to donor dopants. We speculate on the origin of this diffusion mechanism, which is neither thermally-driven nor concentration gradient-driven. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P63.00008: p-Type Doping of Pyrite FeS2 Bryan Voigt, William Moore, Debmalya Ray, Michael Manno, Jeff D Jeremiason, Laura Gagliardi, Eray Aydil, Chris Leighton Pyrite FeS2 is a potentially ideal absorber for thin film solar cells as it is composed of earth-abundant, inexpensive elements, has a suitable band gap (0.95 eV), and is strongly absorptive. Lack of doping control, however, has precluded p-n homojunctions. Heterojunction solar cells have disappointing efficiencies (≤3 %), likely limited by a leaky surface inversion layer. The ability to controllably n- and p-dope FeS2 would make possible a homojunction solar cell that could, uniquely, avoid this surface inversion entirely. Recently, we established S vacancies as n-dopants in FeS2 and achieved transport control in bulk crystals. Here, we demonstrate p-doping by introducing phosphorus (P) during growth. Increasing P concentrations above ~40 ppm triggers a majority carrier inversion from n- to p-type. Thermal activation energy, room temperature hole density, and mobility in p-type crystals are ~170 meV, ~1018 cm-3, and 1 cm2V-1s-1, respectively. Density functional theory confirms that P substitution for S creates an acceptor level 100’s of meV from the valence band, in agreement with experiment. With p-type control thus achieved, p-n FeS2 homojunctions now become possible. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P63.00009: Insulator-Metal Transition in Co-Doped Pyrite FeS2 Single Crystals Bhaskar Das, Bryan Voigt, Moumita Maiti, William Moore, Michael Manno, Eray Aydil, Chris Leighton Iron pyrite (FeS2) is a low-cost, earth-abundant, non-toxic semiconductor with attractive electronic and optical properties, both for fundamentals, and applications (e.g. photovoltaics). Sulfur vacancies (VS)[1] and Co[2] are the only well-established n-dopants in FeS2. VS are deep donors, however, preventing facile study of phenomena such as the insulator-metal transition (IMT), while surface conduction complicates transport at low Co doping. In this work the problem of surface conduction is circumvented via a contacting scheme that provides access to the bulk, enabling wide-T-range transport studies of Co-doped FeS2 single crystals. An IMT is found at ~4Χ1017cm-3 Hall density, with Efros-Shklovskii variable-range hopping and/or activated transport below this, and electron-electron interaction-corrected metallic conductivity above it. A number of unexpected features also occur, however, including non-monotonic T dependence of the Hall coefficient, resistivity anomalies at intermediate T, and non-saturating non-parabolic positive magnetoresistance. These will be discussed in detail. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P63.00010: Toward Deterministic Doping of Silicon via Dopant Containing Homopolymer Michele Perego, Gabriele Seguini, Elisa Arduca, Andrea Nomellini, Francesco Caruso, Katia Sparnacci, Diego Antonioli, Valentina Gianotti, Riccardo Chiarcos, Michele Laus Manipulation of isolated impurity atoms in semiconductors fostered a vision of novel classical and quantum single atom devices. The main roadblock toward their exploitation is the lack of strategies to control the positioning of dopant impurities within the semiconductor lattice by a methodology compatible with current semiconductor technology. Recently, we developed a doping strategy using self-assembled monolayers of polymers terminated with P-containing moieties, used as dopant-carrying molecules.(1,2) Their self-limiting “grafting-to” reaction from melt determine the areal density of the grafted molecules and consequently the number of P atoms in the dopant source. Subsequent injection and activation of P atoms into the Si substrate is achieved by high temperature annealing.(1) Control of the lateral distribution of P atoms over the substrate could be achieved by integrating these materials with suitable lithographic techniques. In this talk, we will review our results on this topic providing preliminary data on the development of a deterministic doping strategy based on self-assembly materials. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P63.00011: Epitaxial growth, structure, and properties of MgZrN2, an emerging nitride semiconductor Sage Bauers, John S Mangum, John Perkins, Stephan Lany, Andriy Zakutayev Inorganic nitrides are important technological materials, many of which belong to one of two families: hexagonal main-group metal nitride semiconductors and cubic transition-metal nitride superconductors. We break this dichotomy with our discovery of several new semiconducting Mg-TM-N (TM=Ti, Zr, Hf, Nb, Ta) nitrides which adopt rocksalt crystal structures. Ab-initio calculations on this family of mid-gap semiconductors reveal extremely large dielectric constants (up to 80 ε0), and a striking tolerance to structural defects relative to other ternary nitrides. This talk will focus on MgxZr2-xN2. We find that this material forms over a broad metal composition range. At stoichiometric MgZrN2 compositions, this material behaves as a heavily-doped n-type semiconductor exhibiting a negative temperature coefficient of resistivity and thermally-activated carriers. The transport properties can be radically tuned with Mg:Zr ratio from metallic (Zr-rich) to non-degenerately-doped (Mg-rich). X-ray diffraction and electron microscopy reveal that MgZrN2 can be epitaxially grown on both GaN and MgO substrates. The combination of elemental abundance, compelling properties, and structural compatibility highlights the potential of these materials for integration with known nitrides. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P63.00012: Proton radiation-induced enhancement of conductivity of composite amorphous/nanocrystalline silicon thin films Zvie Razieli, Nathan Bosch, Lagy T Baby, Lis Stolik Valor, Rachel Yohay, Roger W Rusack, James Kakalios While most semiconductor devices are susceptible to radiation damage, we report here observations of an enhancement of the conductivity of undoped composite hydrogenated amorphous silicon thin films containing silicon nanocrystalline inclusions (a/nc-Si:H) following irradiation with high-energy protons. A series of films with varying nc content, synthesized in a unique dual-chamber co-deposition system, were irradiated with 16 MeV protons from a superconducting linear accelerator for fluences ranging from 2 x 10^13 cm^-2 to 10^15 cm^-2. For the highest irradiation doses, the dark conductivity is enhanced by over an order of magnitude. Unlike the persistent photoconductivity effect observed in amorphous semiconductors, the radiation-induced enhancement is permanent and is not removed by annealing, remaining unchanged at least eight months after irradiation. Various mechanisms are tested to explain the irradiation-induced conductivity enhancement, but none are found to be able to fully account for our observations. |
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P63.00013: Impurity diffusion induced dynamic electron donors in semiconductors Wen-Hao Liu, Jun-Wei Luo, Shu-Shen Li, Lin-Wang Wang Low-energy impurity diffusion in a host material is often regarded as an adiabatic process. Here, we present that the diffusion process in semiconductors can involve nonadiabatic electron excitations, rending it to be a more complicated process. Impurity diffusion in a device at working temperature can pump one electron up from localized impurity state into the host conduction band and causes the impurity to be a dynamic donor since it temporarily loses its electron to the host. This nonadiabatic process, against a common belief, fundamentally change the diffusion behavior, including its barrier height and diffusion path. Although we mainly demonstrate this process with Au metal impurity in bulk Si through time-dependent density functional theory simulations, we believe this could be a rather common phenomenon as it is shown that the similar phenomena also exist in Zn, Cd impurities diffusion in bulk Si, and Ti diffusion in TiO2. We believe this study can open up a new direction of inquiry for such diffusion behavior in semiconductor. |
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