Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session C01: Applications: Semiconductors |
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Sponsoring Units: FIAP Chair: Weining Man, San Francisco State Univ Room: LACC 150A |
Monday, March 5, 2018 2:30PM - 2:42PM |
C01.00001: Absence of redshift in the direct bandgap of silicon nanocrystals with reduced size Jun-Wei Luo, Shu-Shen Li, Ilya Sychugov, Federico Pevere, Jan Linnros, Alex Zunger The high-energy Γ-Γ direct band gap transition in Si nanocrystals was recently argued to rapidly lower its energy (i.e. be ‘red-shifted’) with decreasing nanocrystal size, in contrast to what is expected from conventional quantum confinement (‘blue-shift’). This ‘anti-confinement’ trend, projected to lead at sufficiently small nano sizes to a truly directgap Si nanocrystal. Here, we combine predictive atomistic pseudopotential theory with single dot spectroscopy to investigate the origin of the red-shifted transition in Si nanocrystals. The measured and calculated absorption and emission bands are in good mutual accord, the direct band gap Γ-Γ transition does not show a redshift, nor does it appear in energy below the Γ-valley direct edge of bulk Si. We find the Γ-valley dominant conduction band state is slightly blue-shifted to higher energy instead of being rapidly red-shifted with reduced nanocrystal size. We conclude that the observed red-shifted band is not the intrinsic direct band edge and that the early theory that supported the previous assignment, based on assigning a negative effective mass to the Si electrons at Γ, is not supported by atomistic calculations. |
Monday, March 5, 2018 2:42PM - 2:54PM |
C01.00002: First-principles study of the electronic and transport properties of van der Waals heterostructures. Anh Khoa Augustin Lu, Michel Houssa, Mathieu Luisier, Geoffrey Pourtois Van der Waals heterostructures are emerging as promising building blocks for nanoelectronics applications. The understanding of the interlayer interactions and their impact on the electronic and transport properties are therefore crucial aspects. In this work, we perform first-principles simulations to show that an electrostatic doping can be achieved in van der Waals heterostructures by applying an external electric field. We demonstrate that the doping concentration depends on both the nature and the relative positions of the layers. Then, we perform electronic transport calculations to study the influence of the relative position of the layers (in translation and in rotation) on the current-voltage characteristics of tunnel field-effect transistors based on MoS_{2}/ZrS_{2} heterostructures. Our results indicate significant variations in the current-voltage curves due to modulations of the orbital overlap and of the effective masses. These results stress out the importance of the interlayer interactions in van der Waals heterostructure and their impact on the device behavior. |
Monday, March 5, 2018 2:54PM - 3:06PM |
C01.00003: Characteristics of proton-induced point-defects for electron energy loss and optical absorption Andrii Iurov, Danhong Huang, Fei Gao, Godfrey Gumbs, David Cardimona We report the results of our investigation on the effects due to point defects on the loss of either energies of incident electron beams or photons on a quantum well. Many body effects and its consequences in the structure are included through defect-induced vertex corrections to the bare polarizability of electrons within the ladder approximation, and the intra- and inter-layer screening in the random-phase approximation. Our principal focus has been directed toward obtaining a wide range of numerical results for effects due to defect on both energy- the loss and optical-absorption spectra. The yielded outcomes from our simulations could provide specific designs of optoelectronic quantum devices that will be operated in space research with radiation-hardening protection. |
Monday, March 5, 2018 3:06PM - 3:18PM |
C01.00004: Electronic Properties of Monolayer and Bulk C_{3}N: Fully converged GW quasiparticle calculations Yabei Wu, Weiyi Xia, Fanhao Jia, Wei Ren, Peihong Zhang Layered carbon nitride compounds (such as g-C_{3}N_{4}, beta-C_{3}N_{4}, C_{2}N and C_{3}N) have long been investigated for their novel electronic and optical properties. C_{3}N is a newly emerged semiconductor material, predicted in 2012 theoretically [1] and has recently been synthesized [2]. The electronic properties, in particular, the evolution of the electronic properties with respect to number of layers, of this material, however, have not been well understood. We have carried out fully converged quasiparticle (QP) calculations [3] for both monolayer and buk C_{3}N with different stackings within the GW approximation. We will also discuss the importance of the convergence issue in GW calculations, especially for complex layered structures. |
Monday, March 5, 2018 3:18PM - 3:30PM |
C01.00005: First-principles study of the nonsymmorphic Dirac insulator Benjamin Wieder, Barry Bradlyn, Zhijun Wang, Jennifer Cano, Youngkuk Kim, Hyeong-Seok Kim, Andrew Rappe, Charles Kane, Andrei Bernevig Recently, we have found a novel topological crystalline insulator phase, referred to as the nonsymmorphic Dirac Insulator [1]. Unlike the time-reversal Z2 topological insulator, the nonsymmorphic Dirac insulator is a characterized by a single four-fold degenerate Dirac point, occurring on the p4g or pgg invariant surface of the otherwise band insulator. Inthis talk, we will present our first-principles study to find the material realization of the nonsymmorphic Dirac insulator. After introducing a general scheme to calculate the Z4XZ2 topological invariants using the symmetry-labeled Wilson loop calculations, we show Sr2Pb3 can host the (2,2) phase, hosting a single four-fold degenerate Dirac on the (001) surface. |
Monday, March 5, 2018 3:30PM - 3:42PM |
C01.00006: Ab-initio Calculations of Electronic Properties of Tin Selenide (SnSe) Yuriy Malozovsky, Lashounda Franklin, Diola Bagayoko We present results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic properties of tin selenide (SnSe) in the orthorhombic B16 crystal structure. We utilized a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. Our calculations minimized the energy down to the ground state, as required by the second DFT theorem. This process ensures the full, physical content of our findings that include electronic energy bands, total and partial densities of states, and electron and hole effective masses. |
Monday, March 5, 2018 3:42PM - 3:54PM |
C01.00007: Ab-initio Calculations of Electronics, Transport and Related Properties of Hexagonal Boron Nitride (h-BN) Cheick Bamba, Anthony Stewart, Yuriy Malozovsky, Diola Bagayoko We present an ab-initio, self - consistent density functional theory (DFT) description of electronic and related properties of hexagonal boron nitride (h-BN). We used a local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO) formalism. Our implementation of the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF), ensures the full physical content of the results of our calculations, as per the derivation of DFT [AIP advances, 4, 127104 (2014)]. Our calculated band gap of 4.37 eV, obtained with a room temperature experimental lattice constants of a = 2.504 Å and c = 6.661 Å, is in good agreement with the measured value. The hybridization of valence s and p states, as per our calculated, partial densities of states, is in agreement with corresponding, experimental findings. |
Monday, March 5, 2018 3:54PM - 4:06PM |
C01.00008: Ab-initio computations of electronic, transport and bulk properties of Magnesium Sulfide (MgS) in rock salt structure Uttam Bhandari, Yuriy Malozovsky, Lashounda Franklin, Diola Bagayoko We report findings from ab-initio, self-consistent density function theory (DFT) calculations of electronic, transport and bulk properties of rock salt magnesium sulfide (MgS). We employed a local density approximation (LDA) potential and the linear combination of atomic orbital (LCAO) formalism. The main difference between our calculations and previous ab-initio DFT ones stems from our utilization of successively larger basis sets in consecutive, self consistent calculations to attain the ground state of the material. For a room temperature lattice constant of 5.200 Å, we predict an indirect (Γ-X) band gap of 3.278 eV. With our predicted equilibrium lattice constant of 5.183Å, we predict an indirect (Γ-X) band gap of 3.512 eV. The calculated bulk modulus of 79.76 GPa is in excellent agreement with experiment result of 78.9 ±3.7 GPa. |
Monday, March 5, 2018 4:06PM - 4:18PM |
C01.00009: Observation of the topological surface state in the nonsymmorphic topological insulator KHgSb Aiji Liang, Juan Jiang, Zhongkai Liu, Yulin Chen Topological insulators represent unusual topological quantum states, typically with gapped bulk band structurebut gapless surface Dirac fermions protected by time-reversal symmetry. Recently, a distinct kind of topologicalinsulator resulting from nonsymmorphic crystalline symmetry was proposed in the KHgX (X = As, Sb, Bi) compounds. Unlike regular topological crystalline insulators, the nonsymmorphic glide-reflection symmetry in KHgX guarantees the appearance of an exotic surface fermionwith hourglass shape dispersion (where two pairs of branches switch their partners) residing on its (010) side surface, contrasting to the usual two-dimensional Dirac fermion form. Here, by using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the electronic structures of KHgSb on both (001) and (010) surfaces and reveal the unique in-gap surface states on the (010) surface with delicate dispersion consistent with the “hourglass Fermion” recently proposed. Our experiment strongly supports that KHgSb is a nonsymmorphic topological crystalline insulator with hourglass fermions, which serves as an important step to the discovery of unique topological quantum materials and exotic fermions protected by nonsymmorphic crystalline symmetry. |
Monday, March 5, 2018 4:18PM - 4:30PM |
C01.00010: Flexible hybrid graphene／Cu_{2}O phototransistors with ultrahigh responsivity QiaoLi Liu, ZongHai Hu, Xia Guo Highly-sensitive and flexible photodetectors are desirable for future optoelectronics. However, lacking of obvious current gain mechanism, the sensitivity of graphene-based photodetectors still can’t compare with traditional PMTs and APDs. Here we report a transfer-free structure of hybrid phototransistor consisting of graphene/Cu_{2}O QDs. It has an ultrahigh responsivity over 10^{10}A/Wand fW light detectivity even at room temperature. It demonstatrates excellent flexibility with responsivity still higher than 4×10^{6} A/W. Our results revealed an important QD-graphene base-collector amplification mechanism which mimics the carrier density amplification in usual transistors, only much larger. This mechanism was strongly supported by our quantum capacitance measurements and the inversion relation between the responsivity and the incident power. This mechanism should serve as a guidance for further efforts seeking ultrahigh gain using 2D hybrid structures. |
Monday, March 5, 2018 4:30PM - 4:42PM |
C01.00011: Tensilely Strained Ge Films on Si Substrates Created by Physical Vapor Deposition Yize Li, John Nguyen, Andrew Kelly Ge, owing to its pseudo-direct band gap property and compatibility with Si-based semiconductor technology, is a promising material for Si-compatible photonic devices. Ge-on-Si materials are typically grown in ultra-high vacuum chemical vapor deposition (UHVCVD) systems using toxic and/or flammable Ge precursor gases, such as GeH_{4}. We demonstrate the creation of Ge films on Si substrates through physical vapor deposition of toxin-free solid Ge sources, where ultra-high purity Ar gas serves as the carrier gas. Structural characterization indicates that a high tensile strain is introduced in the Ge film during the deposition process. A strong peak, resulting from the direct band gap photoluminescence (PL) of Ge, is evident. The growth mechanism, revealed through the evolution of film topography from the early stage of the deposition until a complete film is formed, is currently under investigation and will be reported. |
Monday, March 5, 2018 4:42PM - 4:54PM |
C01.00012: Ab-initio Calculations of Electronics, Transport and Related Properties of Hexagonal Chromium Disiliside (CrSi_{2}) Shaibu Mathias, Yuriy Malozovsky, Diola Bagayoko We report results from ab-initio, self-consistent density functional theory (DFT) calculations of electronic, transport, and bulk properties of chromium disiliside (CrSi_{2}) in hexagonal C40 crystal structure. Our computations utilized the Ceperley and Alder local density approximation (LDA) potential and the linear combination of atomic orbitals (LCAO) formalism. As required the second DFT theorem, our calculation minimized the occupied energies, far beyond the minimization obtained with self-consistency with a single basis set, reach the ground state of the system. Our calculated, indirect band gap is 0.31 eV, at room temperature (using experimental lattice constants of a= 4.4276 Å and c= 6.368 Å). We discuss the energy bands, total and partial densities of states, and electron and hole effective masses. |
Monday, March 5, 2018 4:54PM - 5:06PM |
C01.00013: Fifty Years of an Understandable Misunderstanding of Density Functional Theory (DFT) Diola Bagayoko, Yacouba Diakité, Yuriy Malozovsky The second DFT theorem states that, for a material, the energy reaches its minimum (the ground state), when the charge density is that of the ground state. As this charge density is not known à priori, one necessarily has to minimize the energy to obtain true DFT results. For 50 years, we understandably believed that this minimization is realized upon the attainment of self-consistency, with a single basis set. Such a solution, however, is only one among an infinite number of stationary solutions. Within the linear combination of atomic orbital (LCAO) formalism, we illustrate, for representative semiconductors, that the full minimization of the energy can be realized with successive, self-consistent calculations with a basis set augmented from one calculation to the next. This process leads to the absolute minima of the occupied energies, i.e., the ground state, and results that possess the full physical content of DFT and agree with experiment. We show such excellent results for several semiconductors, including h-BN, zb-GaAs, zb-BAs, w-BeO, and zb-BeSe. |
Monday, March 5, 2018 5:06PM - 5:18PM |
C01.00014: Functional-defect design and optimization on disordered photonic bandgap materials. Bowen Yu, Brandon Gunn, Francisco Baltazar, Shervin Sahba, Weining Man Recently, various disordered photonic band gap materials not limited to any crystalline symmetry or translational order have been proposed. Some have shown inherent advantages associated with the isotropy of the structures, offering unprecedented freedom for functional defect designs impossible to achieve in photonic crystals. Based on our previous study on disordered or quasi-crystalline photonic bandgap materials, we conduct a systematic study on functional-defect design and optimization for various disordered photonic bandgap materials and explore their potential advantages in making highly compact, flexible, and energy-efficient photonic devices. The electric field profiles of the defect modes have been generated and compared for optimizing the geometry design for targeted frequencies, Q factors, and footprint sizes. Particularly, the rich point-defect flavors and flexible line/curve defect arrangements available in disordered systems have been explored. Temperature stability of these functional defects have also been investigated. |
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