Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L44: Focus Session: Defects in Semiconductors: Nano Materials |
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Sponsoring Units: DMP FIAP Chair: Jun-Wei Luo, National Renewable Energy Laboratory Room: Mile High Ballroom 4C |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L44.00001: Dopant binding energies in P-doped Ge[110] nanowires using a real-space pseudopotential approach Alex J. Lee, James R. Chelikowsky, Tzu-Liang Chan We apply a real-space pseudopotential formalism for charged one-dimensional periodic systems to examine the binding energies of P dopants in Ge$[110]$ nanowires with varying periodicities and diameters. Binding energies calculated by density functional quasiparticle energies of the neutral dopant are severely underestimated whereas those calculated by quasiparticle energies of the ionized defect are overestimated. We found the best method for determining binding energies is to adopt a composite approach that evaluates the total energy difference between charged and neutral systems for the ionization energy of the P dopant, but uses the quasiparticle energy for the electron affinity of the pure Ge nanowire. Our formalism offers a simple density functional method for calculating dopant binding energies of small nanowire systems without the use of computationally intensive many-body perturbation theory calculations. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L44.00002: Donor Wave Functions Delocalization in Silicon Nanowires Alberto Debernardi, Guido Petretto, Marco Fanciulli The localization of the donor electron wave function can be of key importance in various silicon application, since it determines the interactions between neighboring donors and influences the charge density close to the donor atom. This is important in light of applications like nuclear spin qubits [1] or for determining the critical density of metal-insulator transitions [2]. In particular the delocalization is a critical feature when dealing with nanostructures, where the confinement induces a squeezing of the donor wave function. Using ab-initio calculations, we have studied the delocalization of the donor electron wave function along the axis of a nanowire with different orientations for P and Se donors[3]. We show that the shape and delocalization is greatly influenced by the orientation of the nanowire and that it considerably larger for [011] oriented nanowires, compared to [001] and [111] orientations. We also demonstrate that its value can be controlled by applying a compressive or tensile uniaxial strain. We also show the effect of the delocalization on the hyperfine parameters. [1] B. E. Kane, Nature 393, 133 (1998) [2] D. Belitz, T. R. Kirkpatrick, Rev. Mod. Phys. 66, 261 (1994) [3] G. Petretto, A. Debernardi, M. Fanciulli, Nano Letters 13, 4963 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L44.00003: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L44.00004: Efficient $n$-type doping of zinc-blende III-V semiconductor nanowires Lucas V. Besteiro, Luis Tortajada, J. Souto, L.J. Gallego, James R. Chelikowsky, M.M.G. Alemany We demonstrate that it is preferable to dope III-V semiconductor nanowires by $n$-type anion substitution as opposed to cation substitution. Specifically, we show the dopability of zinc-blende nanowires is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of \textit{DX}-like defect centers. The comparison with previous work on $n-$type III-V semiconductor nanocrystals also allows to determine the role of dimensionality and quantum confinement on doping characteristics of materials. Our results are based on first-principles calculations of InP nanowires by using the PARSEC code. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L44.00005: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L44.00006: First-principles method to study defect properties in semiconductor nanostructures Bart Partoens, Mozhgan Amini, Bob Schoeters, Rolando Saniz, Dirk Lamoen The standard theoretical approach to examine the deep or shallow nature of defects in bulk crystals is through first-principles calculations of their (neutral and charged) formation energies. The character of a defect in a nanostructure might differ from its character in the bulk material and may vary with its position in the nanostructure. However, the standard method cannot be transferred directly to nanostructures. In calculations for a charged defect, a uniform background charge is considered. While this is well-defined for bulk calculations, the total energy of a charged nanostructure depends on the vacuum width. Therefore, total energies of charged nanostructures cannot be used to calculate defect formation energies. Here we propose a solution to this problem and present a first-principles method to determine formation energies for defects in different charge states in a nanostructure, together with the transition levels. As example, we focus on $V_O$ in ZnO slabs and Si$_{\mbox{Ga}}$ in GaAs slabs. Their preferential position as function of the distance to the surface is determined, together with the evolution of their optical and thermal ionization energies. This new method allows to study the character of a wide range of intrinsic and extrinsic defects in nanostructures. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L44.00007: Effects of dopants on the band structure of quantum dots Robert Meulenberg, Joshua Wright, Stuart Lawson Understanding the role that chemical dopants play in modifying the properties of quantum dots (QDs) has been an active field of research for the last decade. In this presentation, we will discuss our efforts towards investigating the effects of copper doping in CdSe QDs. Extended x-ray absorption fine structure (EXAFS) spectroscopy measurements provide conclusive evidence for substitutional doping of Cu in the CdSe lattice. EXAFS suggests the local coordination environment is reduced, likely due to surface doping. Both x-ray absorption near edge structure spectroscopy (XANES) and theoretical modeling are used to examine effects of hybridization on the conduction band minimum (CBM) in doped CdSe quantum dots (QDs). Experimentally, Cd $M_3$-edge XANES provides evidence for a lowering of the CB minimum for Cu doped CdSe QDs that is dependent on Cu concentration. Theoretical modeling suggests the effects of hybridization between Cu and Cd atoms in the QD can explain our experimental results. The hybridization effect leads to active emissive states below the CBM resulting in tunable near-infrared photoluminescence. Our work shows that a simple chemical model can provide a predictive tool towards probing the effects of hybridization on the CB levels in QDs. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L44.00008: Size-dependent properties of Ga- and Al-doped zinc oxide nanocrystals N. Scott Bobbitt, Minjung Kim, Noa Marom, Na Sai, James R. Chelikowsky The feasibility of introducing n-type dopants into ZnO, a popular semiconductor for many photovoltaic and optoelectronic applications, suggests the possibility of controlling the carrier concentration in ZnO nanocrystal systems. We use a real-space pseudopotential method constructed within density functional theory to examine the role of Ga and Al dopants in ZnO nanocrystals in the size regime of 0.8 - 1.5 nm. Nanocrystals with both wurtzite and zincblende structures are examined. We find that while the dopants affect the width of the gap slightly, the highest occupied dopant states are nearly degenerate with the lowest empty state of the undoped nanocrystal. We find the spatial distribution of the dopant state quite similar to the lowest empty state, i.e., they are both localized on the central atom of the nanocrystal. We find that the defect formation energy decreases with increasing particle size, suggesting a less favorable formation energy for smaller nanocrystals. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L44.00009: Strong green emission from ZnO-MgO nanocomposite and its origin Ramachandra Reddy A, Sowri Babu K, Mallika AN, Venugopal Reddy K ZnO-MgO nanocomposite was prepared through a simple sol-gel method. The effect of high thermal annealing on photoluminescence of ZnO-MgO nanocomposite was studied. PL of ZnO showed only a sharp and intense UV emission positioned at 396 nm when annealed at 600 $^{\circ}$C. ZnO-MgO nanocomposite also exhibited same emission peak with enhanced intensity at the same temperature i.e. at 600 $^{\circ}$C. But, as the temperature increased from 600 $^{\circ}$C to 900 $^{\circ}$C an intense green emission positioned at 503 nm was observed with monotonous increase in its intensity. But further increase in temperature to 1000 $^{\circ}$C decreases the intensity of green emission. XRD results demonstrated that strain increased with increase of temperature till 900 $^{\circ}$C and decreased at 1000 $^{\circ}$C. Moreover, intensity of the diffraction peak corresponding to MgO phase was decreased gradually with temperature. It was also found that intensity of green emission depended on concentration of MgO in the sample. By combining the XRD and PL results, it can be concluded that the huge enhancement in the green PL intensity is due to the increase in oxygen vacancies due to the formation of highly dislocated region at the interface of ZnO and MgO due to the large lattice mismatch between them. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L44.00010: Understanding polarity in semiconductor nanorods with linear-scaling density-functional theory simulations Peter Haynes, Philip Avraam, Nicholas Hine, Paul Tangney Binary polar semiconductors with the wurtzite structure have been observed to exhibit large dipole moments along $[0001]$. To explore the origin of these dipole moments, we use a linear-scaling density-functional theory code [1] to perform first-principles calculations of entire wurtzite GaAs nanorods consisting of several thousand atoms. We find that both the direction and magnitude of the dipole moment of a nanorod, and the electric field, depend sensitively on how its surfaces are terminated and not strongly on the spontaneous polarization of the underlying lattice [2]. We show that our calculations can be explained in terms of a pinning of the Fermi level at the polar surfaces that fixes the potential difference across the nanorod, and that this effect can have a determining influence on the polarity of nanorods, with consequences for the way a nanorod responds to changes in its surface chemistry, the scaling of its dipole moment with its size, and the dependence of polarity on its composition [3]. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly. \\[4pt] [1] J.~Chem.~Phys. 122, 084119 (2005).\\[0pt] [2] Phys.~Rev.~B 83, 241402(R) (2011).\\[0pt] [3] Phys.~Rev.~B 85, 115404 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L44.00011: The electronic and structural properties of SnO$_{2}$ nanoparticles doped with antimony and fluorine Minjung Kim, Noa Marom, Scotty Bobbitt, James R. Chelikowsky Transparent conducting oxide (TCO) materials are important owing to their broad industrial applications such as optoelectronic devices and photovoltaics. The most widely used TCO material is In-doped tin oxide (ITO), but In is not abundant in nature. Sb- and F-doped tin oxide nanoparticles are considered as a good candidate of ITO as they have been successfully synthesized. The electronic properties of these nanoparticles depend on the impurity species, the nanoparticle size and shape. We present electronic structure calculations on Sb- and F-doped tin oxide nanoparticles by employing real-space pseudopotential calculations based on density functional theory. We examine the impurity formation energies and electron binding energies with respect to the size of the nanoparticle and the location of the impurity site. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L44.00012: The Quantum Pinch Effect in Semiconducting Quantum Wires M.S. Kushwaha A two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a {\em radial} confining harmonic potential and an applied magnetic field in the symmetric gauge is investigated. It is demonstrated that such a system as can be realized in semiconducting quantum wires offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a {\em determinable} maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale. We will shed light on the observability of the quantum pinch effect [Appl. Phys. Lett. {\bf 103}, 173116 (2013)]. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L44.00013: Describing Nanomaterials: A Uniform Description System John Rumble, Steve Freiman, Clayton Teague Products involving nanomaterials are growing rapidly and nanoparticles also occur naturally. Materials, scientists, engineers, health officials, and regulators have realized they need a common description system. Led by CODATA and VAMAS, a Uniform Description System (UDS) for nanomaterials is being developed to meet the requirements of a broad range of scientific and technical disciplines and different user communities. The goal of the CODATA/VAMAS effort is the creation of a complete set of descriptors that can be used by all communities, e.g., materials, physics, chemistry, agricultural, medical, etc., interested in nanomaterials. The description system must be relevant to researchers, manufacturers of nanomaterials, materials selectors, and regulators. The purpose of the UDS for materials on the nanoscale is twofold: Uniqueness and Equivalency. The first step in the development of the UDS has been the creation of a Framework that will be used by the different communities to guide in the selection of descriptors relevant to their needs. This talk is a brief description of the draft of such a Framework, and how the framework will be translated into a robust description system with input from many scientific communities including physics. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L44.00014: Optical and structural properties of (III-V)$_{x}$(IV)$_{5-2x}$ alloys Jos\'e Men\'endez, Patrick Sims, Liying Jiang, John Kouvetakis A novel class of (III-V)-IV semiconductor alloys was recently introduced by our group. The alloys are designed to incorporate entire (IV)$_{3}$-V-III tetrahedral building blocks formed in the gas phase by reactions of (V)-(IV-H$_{3}$)$_{3}$ molecules with group-III atomic beams. This structure leads to the highest possible concentration of isolated III-V pairs in a group-IV matrix. Thick, highly crystalline films have been grown on Si and Ge substrates using the group-III elements In and Al, the group-V elements N, P, and As, and the group-IV elements Si and Ge. Results from an array of structural and optical characterization probes will be compared with theoretically proposed structures and predicted optical properties for these new alloys. The existence of III-V compounds with lattice constants very similar to those of elemental Si and Ge implies that the corresponding (III-V)$_{x}$(IV)$_{5-2x}$ alloy will have a tunable electronic structure at a fixed lattice constant, a property that may find applications in areas such as photovoltaics. [Preview Abstract] |
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