Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Z22: Properties of Semiconducting Nanosystems |
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Sponsoring Units: DCMP Chair: Stefan Badescu, Air Force Research Laboratory, Dayton, Ohio Room: D163 |
Friday, March 25, 2011 11:15AM - 11:27AM |
Z22.00001: ABSTRACT WITHDRAWN |
Friday, March 25, 2011 11:27AM - 11:39AM |
Z22.00002: Conductivity of Thin Film Structures Fabricated by E-Beam Lithography from Gold Nanoparticle Resists Stefan Dickert, Myoung-Hwan Park, Colin Jermain, Vincent M. Rotello, Mark T. Tuominen Drop- and spin-coated solutions of ligand coated nanoparticles act as a novel ``direct write'' e-beam resist, which can be prepared with metallic, magnetic and semiconducting nanoparticles (Y.Ofir. et. al, Adv. Mater. 20, 2561-2566 (2008)). We prepared thin films from gold nanoparticles in which we varied the ligand length, ligand type and the film thickness. Small angle X-Ray scattering experiments as well as SEM imaging of the samples were performed to determine structural properties of the nanoparticle films at various stages of the fabrication process, after drop coating, ebeam exposure and annealing. We further performed resistance measurements in the 2-350K temperature range and report different conductivity mechanisms based on the ligand type and film thickness, ranging from insulating to Mott hopping conduction to metallic. We observed different results for Thioalkylated trimethyl ammonium (TMA) and thioalkyl tetra (ethylene glycol)ated trimethyl ammonium (TTMA) ligands. [Preview Abstract] |
Friday, March 25, 2011 11:39AM - 11:51AM |
Z22.00003: Terahertz Ionization of Highly Charged InGaAs Quantum Posts Christopher Morris, Dominik Stehr, Tuan-Anh Truong, Hyochul Kim, Pierre Petroff, Mark Sherwin, Craig Pryor Quantum posts (QPs) are quantum dot based nanostructures grown by MBE. They form short In$_{.4}$Ga$_{.6}$As cylinders embedded in an In$_{.1}$Ga$_{.9}$As quantum well (QW). Terahertz absorption measurements are performed on 30 nm high QPs with $\sim $6 electrons per post and a charge density of $\sim $2.4x10$^{11}$/cm$^{2}$ in the surrounding well. Comparison of spectra from QP and reference QW samples shows an absorption feature due to the QPs absent in the QW sample. Temperature dependent measurements show this absorption is due to electrons in the QPs. 8 band k.p calculations of post and well energies are performed as a function of the number of electrons in the posts, and the absorption is determined to be due to an ``ionizing'' transition from the posts to the well. The highest filled QP state absorbs a terahertz photon, transitioning to a weakly bound unfilled post state $\sim $20 meV higher. From there, the electron quickly scatters into the two dimensional electron gas in the quantum well matrix. Coulomb repulsion from QP electrons locally depopulates the quantum well states, leaving open states for the QP electrons. QPs represent a promising structure for investigation of Coulomb blockade physics and ionizing transitions in artificial atom systems. [Preview Abstract] |
Friday, March 25, 2011 11:51AM - 12:03PM |
Z22.00004: The dipole moments of ZnO nanorods Sefa Dag, Shuzhi Wang, Lin-Wang Wang A self-consistent linear scaling three dimensional fragment (LS3DF) method is used to study the dielectric properties of large ZnO nanorods. Our ab initio calculations show that the ZnO nanorod with unpassivated (10-10) side surface has a large dipole moment which is caused by both surface and interior bulk dipoles. An systematic analysis is carried out, and we found that the biggest contribution to the total dipole moment is from the (10-10) surface. Dielectric screening model is used to illustrate how the dipole moment changes with nanostructure size and geometry. We also show the effect of the dipole moment on the interior electronic structure of the nanorod. [Preview Abstract] |
Friday, March 25, 2011 12:03PM - 12:15PM |
Z22.00005: Natural interface states in coherent and isovalent III-V heterostructures Voicu Popescu, Alex Zunger Interface states occur in semiconductor heterojunctions whenever a significant perturbation is present, caused by interface defects, lattice mismatch, discontinuities in the effective mass or sharp variations in the potential across the interface. We discuss the natural interface states appearing in perfectly coherent and isovalent III-V heterojunctions when a $\Gamma$-well and an $X$-anti-well coexist in the conduction band. We use empirical pseudopotential calculation to illustrate this type of states for a few III-V heterostructures. For InP/GaP the interface localised states lie energetically in the band-gap and possess, because of their mixed $\Gamma-X$ character, a strong optical signature. This allows us to provide a different interpretation of the photoemission data existent in the literature for InP/GaP quantum wells and dots. We further discuss the presence of the interface localised states in other III-V heterojunctions, investigating the conditions under which they might be experimentally observed. [Preview Abstract] |
Friday, March 25, 2011 12:15PM - 12:27PM |
Z22.00006: Electronic Phase Diagram of Single-Element Silicon ``Strain'' Superlattices Zheng Liu, Wenhui Duan, Jian Wu, Max Lagally, Feng Liu The evidence that the band gap of Si changes significantly with strain suggests that by alternating regions of strained and unstrained Si one creates a single-element electronic heterojunction superlattice (SL), with the carrier confinement defined by strain rather than by the chemical differences in conventional compositional SLs. Using first-principles calculations, we map out the electronic phase diagram of a one- dimensional pure-silicon SL. It exhibits a high level of phase tunability, e.g., tuning from type I to type II. Our theory rationalizes a recent observation of a strain SL in a Si nanowire and provides general guidance for the fabrication of single-element strain SLs. [Preview Abstract] |
Friday, March 25, 2011 12:27PM - 12:39PM |
Z22.00007: Quantum Boundary Effect in Nanomaterials: Undo the Quantum Size Effect by Surface Passivation of Silicon Nanofilms Yiyang Sun, Xin Liu, Shengbai Zhang It is well known that when the size of a semiconductor is reduced, its band gap will increase due to the increased kinetic energy of the electrons and holes. However, first-principles calculations reveal that there should also be a quantum boundary effect (QBE), which can drastically change the band gap to the extent that the quantum size effect (QSE) is completely erased. It is found that, for Si(001) nanofilms, surface passivations could show such a strong QBE. While the films are passivated by hydrogen, they show a clear QSE with significant increase in band gap. When some of the hydrogen atoms are replaced by =NH ligands, however, the band gap recovers to that of bulk silicon even for film size as small as two nanometers. The concept of zero confinement state for semiconductors will be introduced. It elucidates why it is possible to remove the seemingly universal QSE. The finding here could be highly desirable for certain applications of nanostructured semiconductors where gap increasing due to QSE is detrimental. [Preview Abstract] |
Friday, March 25, 2011 12:39PM - 12:51PM |
Z22.00008: Excitation energy dependence of the exciton inner ring Yuliya Kuznetsova, Jason Leonard, Leonid Butov, Joe Wilkes, Alex Ivanov, Arthur Gossard We report on the excitation energy dependence of the inner ring in the emission pattern of indirect excitons. The contrast of the inner ring is found to increase with excitation energy until it reaches the direct exciton plus LO phonon energy and saturate at higher excitation energies. The data show that excitation by low-energy laser light tuned to the direct exciton resonance can effectively suppress the laser-induced heating of the exciton gas. The observed dependence is explained in terms of exciton transport and cooling. [Preview Abstract] |
Friday, March 25, 2011 12:51PM - 1:03PM |
Z22.00009: Dynamic coherent exciton condensates in a semiconductor planar microcavity Chih-Wei Lai, Kyaw Zin Latt, Yi-Shan Li, Sheng-Di Lin We observed a coherent exciton state with a long decay time $\sim $1000 ps in a semiconductor planar microcavity structure. The lifetime of cavity polariton condensates previously reported has been limited to $\sim $10 ps. The sample consists of InGaAs quantum wells positioned near anti-nodes of the photon field in a GaAs $\lambda $-cavity sandwiched by GaAs/AlAs-based Bragg mirrors. Under a pulsed excitation above the stop-band of the Bragg mirrors (excess energy $>$150 meV), spatially coherent exciton emissions were observed to last for $\sim $1 ns. Conventional dynamic exciton-polariton condensates with a $\sim $10 ps lifetime were observed under a near-resonant (excess energy $\sim $ 6 meV) ps pulsed excitation. Dynamics of spatial coherence, energy relaxation, and spin polarization were characterized by time-resolved spectroscopies, including double-slit and polarimetry experiments. The fluctuation of excitonic emissions was characterized by a photon-correlation measurement. The existence of such a long-lived coherent exciton state is attributed to formation of dark excitons under an excitation with significant excessive energy. [Preview Abstract] |
Friday, March 25, 2011 1:03PM - 1:15PM |
Z22.00010: Amplitude oscillations in a non-equilibrium polariton condensate Richard Brierley, Peter Littlewood, Paul Eastham Like cold atomic gases, semiconductor nanostructures provide new opportunities for exploring non-equilibrium quantum dynamics. In semiconductor microcavities the strong coupling between trapped photons and excitons produces new quasiparticles, polaritons, which can undergo Bose-Einstein condensation. Quantum quenches can be realised by rapidly creating cold exciton populations with a laser [Eastham and Phillips, PRB 79 165303 (2009)]. The mean field theory of non-equilibrium polariton condensates predicts oscillations in the condensate amplitude due to the excitation of a Higgs mode. These oscillations are the analogs of those predicted in quenched cold atomic gases and may occur in the polariton system after performing a quench or by direct excitation of the amplitude mode. We have studied the stability of these oscillations beyond mean field theory. We show that homogeneous amplitude oscillations are unstable to decay into lower energy phase modes at finite wavevectors, suggesting the onset of chaotic behaviour. The resulting hierarchy of decay processes can be understood by analogy to optical parametric oscillators in microcavities. Polariton systems thus provide an interesting opportunity to study the dynamics of Higgs-like modes in a solid state system. [Preview Abstract] |
Friday, March 25, 2011 1:15PM - 1:27PM |
Z22.00011: Coherence dynamics of a long-lived excitonic condensate in an optical microcavity Kyaw Zin Latt, Yi-Shan Li, Sheng-Di Lin, Chih-Wei Lai We report dynamics of long-range spatial coherence of an excitonic condensate with a $\sim $1000 ps life-time in a planar Fabry-Perot microcavity. The sample consists of three sets of three InGaAs(8nm)/GaAs (14nm) quantum wells positioned near anti-nodes of the photon field in a GaAs $\lambda $-cavity sandwiched by two GaAs/AlAs-based Bragg mirrors. Conventional dynamic exciton-polariton condensates with a $\sim $10 ps lifetime were observed under a near-resonant (excess energy $\sim $ 6 meV) ps pulsed excitation at a 50 degree incident angle. Under an excitation above the stop-band of the Bragg mirrors (excess energy $\sim $170 meV), an excitonic state with 100-$\mu $eV luminescence linewidth was observed to last for $\sim $1 ns. Coherence dynamics were characterized by time-resolved double-slit experiment in a confocal geometry with a ps streak camera system as a function of excitation intensity (fluence) and temperature. The visibility of interference fringes reached above 0.3 within 40 ps and remained above 0.1 up to $\sim $1ns for a double-slit-distance of 12 $\mu $m. [Preview Abstract] |
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