Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A50: Quantum Dots and Nanocrystals: Optical and Electronic Properties |
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Sponsoring Units: DCMP Chair: Sam Carter, U.S. Naval Research Laboratory Room: Mile High Ballroom 1D |
Monday, March 3, 2014 8:00AM - 8:12AM |
A50.00001: Photoluminescence in arrays of doped semiconductor nanocrystals Tianran Chen, Konstantin Reich, Alexander Efros, Boris Shklovskii We study dependence of the quantum yield of photoluminescence of a dense, periodic array of semiconductor nanocrystals (NCs) on the level of doping and NC size. Electrons introduced to NCs via doping quench photoluminescence by Auger process, so that practically only NCs without electrons contribute to the photoluminescence. Computer simulation and analytical theory are used to find a fraction of such empty NCs as a function of the average number of donors per NC and NC size. For an array of small spherical NCs, the quantization gap between 1S and 1P levels leads to transfer of electrons from NCs with large number of donors to those without donors. As a result, empty NCs get extinct and photoluminescence is quenched abruptly at the average number of donors per NC close to 1.8. The relative intensity of photoluminescence is shown to correlate with the type of hopping conductivity of an array of NCs. Ref: http://arxiv.org/abs/1310.0849 [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A50.00002: Millikelvin magneto-photoluminescence of isoelectronic bound excitons in type-II quantum dot superlattices Igor Kuskovsky, Haojie Ji, Siddharth Dhomkar, Jonathan Ludwig, Dmitry Smirnov, Maria Tamargo Photoluminescence (PL) spectrum of Zn-Se-Te multilayer system grown via migration enhanced epitaxy with submonolayer insertion of Te, has been reported to demonstrate coexistence of the isoelectronic centers along with the type-II quantum dots (Gu, \textit{et al.}, Phys. Rev. B \textbf{71}, 045340 (2005)). Spectrally, the band edge emission, originating from the isoelectronic bound excitons (IBE), is observed as characteristic `sharp lines' and their phonon replicas, whereas the low energy side is dominated by spatially indirect, type-II excitons. The latter exhibit robust Aharanov-Bohm oscillations in the intensity of the magneto-PL up to 30 K, while no such effect was expected for the IBEs. Here we report a high resolution spectral analysis of the magneto-PL spectra of various samples with relatively low Te content measured at millikelvin temperatures. The analysis reveals additional features in magneto-PL intensity at specific magnetic fields that appear only in the spectral region dominated by the `sharp lines'. Although the precise origin of these distinctive peaks is still unknown, they are thought to be arising due to 2-dimensional confinement of IBEs. Supported by the National Science Foundation under Award No. DMR-1006050 [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A50.00003: Control over density of submonolayer type-II ZnTe/ZnSe quantum dots grown via migration enhanced epitaxy Siddharth Dhomkar, Haojie Ji, Bidisha Roy, Igor L. Kuskovsky, Alice Wang, Maria C. Tamargo For practical applications of self-assembled semiconductor quantum dots (QDs), it is important to control their density, distribution and size; parameters that remain essentially elusive in case of submonolayer QDs. Such QDs grown via migration enhanced epitaxy (MEE), form without the formation of wetting layers, an advantageous feature for practical applications. Here we present a study of submonolayer type-II ZnTe/ZnSe QDs by combining a series of characterization tools, to obtain precise estimates of the dot densities, and their size. Type-II ZnTe/ZnSe QDs are particularly interesting because of their relatively large valence and conduction band offsets which can be utilized to tune optical and electrical properties in unique ways. Specifically, we have employed low temperature photoluminescence to estimate QD thicknesses while QD radius has been accurately determined via excitonic Aharonov-Bohm effect. Secondary ion mass spectroscopy has been employed to obtain average Te concentration, which was used to calculate QD density. The results demonstrate that the QD density can be controlled relatively precisely by varying Te flux and number of MEE Te cycles. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A50.00004: Magneto-photoluminescence studies of optical Aharonov-Bohm effect in type-II ZnTe/ZnSe semiconductor heterstructure Y.H. Chang, C.H. Hsu, C.C. Huang, W.C. Chou, Y.W. Suen Although the absolute phase of a quantum state is not measurable, the relative phase of a coherent charged particle wave could be manipulated. Recently, the effect of the magnetic flux on the excitonic energy has received much attention. In this talk we'll present our magneto-photoluminescence studies on the optical properties of type-II ZnTe/ZnSe self-assemble QDs system. The ZnTe/ZnSe samples were grown by molecular beam epitaxy with ZnSe layer grown on the GaAs substrate first and then ZnTe was grown on top of ZnSe. The ZnTe layers used in this study has thickness of 2.0, and 2.5 monolayers, respectively. Magneto-photoluminescence experiment was performed at T$=$1.4 K with a 14 T superconducting magnet in conjunction with a 405-nm diode laser and a monochromator. Sharp and clean emission peaks in magneto-PL spectra was observed and oscillation on the peak energy of the photoluminescence spectra as a function of magnetic field were observed for both the 2.0 ML and 2.5 ML samples and are attributed to the optical Aharonov-Bohm effect. The AB period changes from 9T for 2.0ML sample to about 6T for 2.5ML sample, i.e., we observed different AB periods for samples with different quantum dot size of the same system. In addition, the effect of impurity and defect on the AB oscillation will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A50.00005: Optical Anisotropy in Type-II ZnTe/ZnSe Submonolayer Quantum Dots Haojie Ji, Siddharth Dhomkar, Maria Tamargo, Igor Kuskovsky Type-II semiconductor quantum dots (QDs) characterized by spatial separation of charge carriers are good candidates for photovoltaics and photon manipulation applications. Implementation of practical devices requires detail understandings of the QD morphology, the mechanism of strain relief and defect formation. Here we report our study of polarization dependent photoluminescence (PL) in type-II ZnTe/ZnSe submonolayer QD superlattices, grown by migration-enhanced epitaxy. We show that the PL does not depend on the polarization state of excitation and exhibits strong linear polarization, indicating strong anisotropy in this material. We spectrally analyze the degree of linear polarization in samples grown with different Te fluxes, spacer thicknesses and number of periods. Based on our observations, we propose several reasons for the optical anisotropy, focusing on the anisotropic shape of the QDs and the anisotropy at the interfaces in the superlattices. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A50.00006: Statistical Properties of Exciton Fine Structure Splittings and Polarization Angles in Quantum Dot Ensembles Ming Gong, B. Hofer, E. Zallo, R. Trotta, Junwei Luo, Alex Zunger, O.G. Schmidt, Chuanwei Zhang We propose an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle of quantum dot ensembles (QDEs). We derive the distributions of FSS and polarization angle for QDEs and show that their statistical features can be fully characterized using at most three independent measurable parameters. The effective model is confirmed using atomistic pseudopotential calculations as well as experimental measurements for several rather different QDEs. The model naturally addresses three fundamental questions that are frequently encountered in theories and experiments. The answers to these fundamental questions yield a completely new physical picture for understanding optical properties of QDEs. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A50.00007: Sources of optical transition broadening in room temperature CdSe/ZnS nanocrystal quantum dots Michael Wolf, Jesse Berezovsky To understand the origins of optical transition broadening in CdSe/ZnS nanocrystal quantum dots (NCQDs) at room temperature, we study the photoluminescence excitation (PLE) spectra of individual NCQDs. The PLE spectra from single NCQDs reveal broadening of the optical transitions and variations of the transition energies between NCQDs. The observed features in the spectra are identified by comparison to transition energies calculated using an 8-band effective mass model. We attribute the broadening to three effects: phonon broadening, spectral diffusion, and size inhomogeneity. The first two mechanisms contribute to the broadening of transitions in single NCQDs. The third mechanism contributes to ensemble broadening. The broadening caused by spectral diffusion and size inhomogeneity both depend on the sensitivity of each transition to variations in the confining potential, leading to linewidths that depend on the particular electron and hole states involved in the transition. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A50.00008: Controlling Auger Decay Rates of CdSe/CdS Nanocrystals via Core/Shell Interfacial Alloying Young-Shin Park, Wan Ki Bae, Lazaro Padilha, Jeffrey Pietryga, Victor Klimov We report single-dot spectroscopic studies to evaluate the effect of the core/shell interface (i.e., the shape of the confinement potential) on nonradiative Auger decay rates of CdSe/CdS quantum dots (QDs) that have either a sharp or a graded interface. Alloyed QDs with a graded potential are prepared by incorporating a CdSe$_{\mathrm{x}}$S$_{\mathrm{1-x}}$ alloy layer of a controlled composition and thickness between the core and the shell. In second-order intensity correlation (g$^{\mathrm{(2)}})$ measurements, we observed that the interfacial layer has a negligible effect on single-exciton dynamics, but leads to a systematic increase in the biexciton photoluminescence quantum yield ($Q_{\mathrm{BX}})$. We found that $Q_{\mathrm{BX}}$ of alloyed QDs can be up to $\sim$ 10 times higher than that of the reference QDs with a sharp interface. These results are further supported by independent measurements of biexciton dynamics that show a considerable elongation of biexciton lifetimes (to several ns) upon interfacial alloying. Finally, a statistical investigation of over 100 individual QDs shows that the CdS shell thickness has only a minor effect on $Q_{\mathrm{BX}}$. All of these findings point a significant role of the shape of the confinement potential in Auger recombination and should facilitate the development of ``Auger-recombination-free'' QDs. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A50.00009: Entangled Photon Pairs via Two-Photon Transitions in a Quantum Dot Molecule Cameron Jennings, Andrew Jacobs, Michael Scheibner We theoretically investigate the use of tunnel-coupled quantum dots as a source of entangled photon pairs. By preparing the system in a molecular biexciton state [a], with the charges of one exciton in separate dots, the anisotropic electron-hole exchange splitting is greatly reduced for the first transition in the resulting radiative biexciton cascade. While the splitting returns for the second (intradot) transition, polarization-entangled photon pairs can still be recovered [b]. The fidelity of such a process depends crucially on the excitation conditions; we consider various scenarios, from non-resonant incoherent to coherent two-photon excitation of the biexciton state. We simulate two-photon processes in this system and determine optimal parameters for entangled photon generation experimentally accessible by electrical control of the energy level structure in quantum dot molecules. [a] Scheibner et al., Phys. Rev. Lett. 99, 197402 (2007) [b] Scheibner et al., J. Opt. Soc. Am. B 29, A82 (2012) [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A50.00010: 0D- to 2D-transition of electronic states in SK quantum dots Cyprian Czarnocki, Mark Kerfoot, Allan Bracker, Daniel Gammon, Michael Scheibner Level anti-crossing spectroscopy (LACS) has demonstrated the capabilities of mapping the discrete electronic states of a nearby quantum dot by sequentially tunnel coupling another dot's ground state to the energy level ladder of the quantum dot of interest [1]. Here we expand this method and identify the dot to wetting layer transition, i.e., the transition from zero- to two-dimensional states. We find a wealth of states at elevated energies. The identification of these states takes into account the charge and spin configuration of the involved optical transitions. Thereby we identify cross-talk channels between quantum dot molecule states. We anticipate our findings to provide insights for the lateral coupling of neighboring dots and dot molecules through extended 2-D like states. \\[4pt] [1] M. Scheibner, M. Yakes, A. S. Bracker, I. V. Ponomarev, M. F. Doty, C. S. Hellberg, L. J. Whitman, T. L. Reinecke, D. Gammon, \textit{Optically mapping the electronic structure of coupled quantum dots}, Nature Phys. Lett. \textbf{4}, 291-295 (2008). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A50.00011: Group velocity slowdown using phonon-induced transparencies in a quantum dot molecule Andrew Jacobs, Cameron Jennings, Mark Kerfoot, Michael Scheibner In a recent study we have demonstrated coherent, non-dissipative behavior of phonons due to optical excitation, which is revealed via optical transparency [1]. Using a single external driving field, the absorption of the molecule demonstrates a marked reduction as a Fano-type resonance of a spatially indirect exciton and direct polaron form a molecular polaron state. The phonon coherence contrasts the typical role of these particles as a channel for non-radiative state decay or pure state dephasing. The optical response of the system is indicative of a coherent phenomenon, similar to electromagnetically induced transparency. Here we investigate theoretically how this phonon coherence affects the optical response of a 3-level V-type system in a tunnel-coupled quantum dot molecule. From the properties of the molecular polaron, we are able to determine the slowdown factor of the driving field group velocity, as well as the dependence of the slowdown on system parameters such as polaron and exciton lifetimes, tunneling strength, and transition dipole moments. The presence of slow light suggests this system is suitable for use in quantum computational components such as optical storage or qubit logic gates. \\[4pt] [1] M. Kerfoot et. al. ``Optophononics with coupled quantum dots'' (submitted) [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A50.00012: Exchange interaction and the tunneling induced transparency in coupled quantum dots Halyne Borges, Augusto Alcalde, Sergio Ulloa Stacked semiconductor quantum dots coupled by tunneling are unique ``quantum molecule'' where it is possible to create a multilevel structure of excitonic states. This structure allows the investigation of quantum interference processes and their control via electric external fields. In this work, we investigate the optical response of a quantum molecule coherently driven by a polarized laser, considering the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. In our model we consider interdot transitions mediated by the the hole tunneling between states with the same total spin and, between bright and dark exciton states. Using realistic experimental parameters, we demonstrate that the excitonic states coupled by tunneling exhibit an enriched and controllable optical response. Our results show that through the appropriate control of the external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra, whenever states of appropriate symmetry are mixed by the hole tunneling. We explore the relevant parameters space that would allows with the experiments. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A50.00013: Electronic states in InGaAs/GaAs asymmetric quantum rings Vivaldo Lopes-Oliveira, Victor Lopez-Richard, Sergio Ulloa Semiconductor quantum rings (QRs) have attracted a great deal of attention due to the interesting multiply-connected geometry they provide for charge carriers. The observation of Aharonov-Bohm [1] effects on the excitonic response in this geometry and the appearance of localized defects during the growth processes call for theoretical studies of the impact of defects on the optical response in magnetic fields. We present here systematic studies of asymmetry in QRs under magnetic fields within a k.p formalism. Different kinds of perturbations are studied, as the model used allows modulation of confinement and perturbation strength. The angular momentum hybridization is characterized for different field intensity and confinement/defect shape. The coupling of unperturbed and symmetric states defines the potential appearance of crossings and new anticrossings in the electronic structure as function of field and structural parameters. The electronic orbitals are contrasted with unperturbed states and the effects on the optical response for inter-subband transitions are discussed as signatures of symmetry breakings.\\[4pt] [1] M.D.Teodoro et al. PRL 104, 086401 (2010). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A50.00014: Photoconductivity and free-carrier dynamics of silicon quantum dots Matthew Bergren, Peter Palomaki, Nathan Neale, Thomas Furtak, Matthew Beard Silicon quantum dots (QDs) have recently been investigated for use in novel optoelectronic devices such as LEDs and PV. Plasma synthesized SiQDs offer very good control of SiQD size, crystallinity and size distribution. These dots have reported PLQYs \textgreater 60{\%}, making them excellent candidates for LEDs or biomarkers. In PV applications, charges need to be extracted from films, while for LEDs charges are injected into the film, and thus information about their electrical properties is desired. Time-resolved terahertz spectroscopy is uniquely suited to investigate photoinduced charge generation and transport in nanoscale systems in that it can measure the sample's photoconductivity with sub-ps resolution. In this study, we present the complex-frequency dependent photoconductivity of isolated SiQDs for a range of diameters. The ultrafast dynamics show a fast decay followed by a longer lifetime. We attribute the rapid decay to hot-carriers relaxing to bound excitons within the first ps after excitation for the isolated dots. A comparison is made between the isolated dots and thin films composed of the same material, illustrating the importance of QD-QD electronic coupling to achieve charge transport in these films. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A50.00015: Carrier recombination in mid-wave infrared InAs/InAsSb superlattices Yigit Aytac, Benjamin Varberg Olson, Jin K. Kim, Eric A. Shaner, Sam D. Hawkins, John F. Klem, Michael E. Flatt\'e, Thomas F. Boggess Measurements of carrier recombination rates using a temperature-dependent time-resolved differential transmission technique are reported for mid-wave infrared $InAs/InAs_{1-x}Sb_{x}$ type-2 superlattices (T2SLs). By engineering the layer widths and antimony compositions a 16K band-gap of $\sim$ 238 meV was achieved for all five unintentionally doped T2SLs. Carrier recombination rates were determined for all five samples by fitting a rate equation model to the density and temperature dependent data. Minority-carrier lifetimes as long as 22$\mu s$ were measured at 14K, while lifetimes in excess of 2$\mu s$ were measured for all five samples at 200K. The minority-carrier lifetimes were observed to generally increase with increasing antimony content. While minority-carrier lifetimes are much longer than those observed in InAs/Ga(In)Sb T2SLs, Auger recombination processes were found to be more prominent in the Ga-free T2SLs. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. This research was funded by the U.S. Government. [Preview Abstract] |
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