DOCSLIB.ORG

Chapter 6: Light‐Emitting Diodes

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Chapter 6: Light‐Emitting Diodes Chapter 6: Light‐Emitting Diodes Photoluminescence and electroluminescence Basic transitions Luminescence efficiency Light-emitting diodes Internal quantum efficiency External quantum efficiency Device structures LED materials Heterojunction high-intensity LEDs LED characteristics PHYS5320 Chapter Six 1 Photoluminescence and Electroluminescence Photon absorption can create electron‒hole pairs. Excess electrons and holes can recombine and result in the emission of photons in direct bandgap materials. The general property of light emission is referred to as luminescence. When excess electrons and holes are created by photon absorption, then photon emission from the recombination process is called photoluminescence. Electroluminescence is the process of generating photon emission when excess carriers result from an electric current caused by an applied electric field. We will mainly be concerned with injection of carriers across a pn junction. The pn junction light-emitting diode (LED) and laser diode (LD) are examples of this phenomenon. In these devices, electric energy, in the form of a current, is converted directly into photon energy. PHYS5320 Chapter Six 2 Basic Transitions Basic interband transitions: (i) Emission energy is very close to the bandgap energy of the material. (ii) Emission involves energetic electrons. (iii) Emission involves energetic holes. A combination of these emission processes will produce an emission spectrum with a certain bandwidth. 2 1/ 2 h Eg I h Eg exp kT GaAs PHYS5320 Chapter Six 3 Basic Transitions Recombination processes involving impurity or defect states: (i) Conduction band to acceptor transition. (ii) Donor to valence band transition. (iii) Donor to acceptor transition. (iv) Recombination due to a deep trap. This process is usually non-radiative. PHYS5320 Chapter Six 4 Basic Transitions Non-radiative Auger process: (i) Recombination between an electron and hole, accompanied by energy transfer to a free hole. (ii) Recombination between an electron and hole, accompanied by energy transfer to a free electron. The involved third particle will eventually lose its energy to the lattice in the form of heat. The process involving two holes and one electron occurs predominantly in heavily doped p-type materials, and the process involving two electrons and one hole occurs primarily in heavily doped n-type materials. PHYS5320 Chapter Six 5 Luminescence Efficiency Since not all recombination processes are radiative, an efficient luminescent material is one in which radiative transitions predominate. The quantum efficiency is defined as the ratio of the radiative recombination rate to the total recombination rate for all processes. Because the recombination rate is inversely proportional to the lifetime, the quantum efficiency can also be expressed in terms of lifetimes. Rr nr q R nr r The interband recombination rate will be proportional to the number of electrons and holes available. Rr Bnp The values of B for direct bandgap materials are on the order of 106 larger than for indirect bandgap materials. One problem encountered with the emission of photons from a direct bandgap material is the re-absorption of emitted photons. This re-absorption problem must be solved to achieve highly efficient devices. PHYS5320 Chapter Six 6 Light‐Emitting Diodes When a voltage is applied across a pn junction, electrons and holes are injected across the space charge region where they become excess minority carriers. These excess minority carriers diffuse into the neutral semiconductor regions where they recombine with majority carriers. If this recombination process is a direct band-to-band process, photons are emitted. Such devices are then called light-emitting diodes (LEDs). The recombination rate is proportional to the diode diffusion current, so the output photon intensity will also be proportional to the ideal diode diffusion current. The maximum emission wavelength from a direct bandgap semiconductor material is related to (but not exactly equal) the bandgap energy. hc 1.24μm Eg EgeV PHYS5320 Chapter Six 7 Internal Quantum Efficiency The internal quantum efficiency of a LED is the fraction of diode current that will produce luminescence. It is a function of the injection efficiency and a function of the percentage of radiative recombination events compared with the total number of recombination events. internal q The three current components in a forward-biased diode are the minority carrier electron diffusion current, the minority carrier hole diffusion current, and the space charge recombination current. eD n eV J n p0 exp 1 n eWn eV L kT i n JR exp 1 20 2kT eDp pn0 eV Jp exp 1 Lp kT PHYS5320 Chapter Six 8 Internal Quantum Efficiency The recombination of electrons and holes within the space charge region is in general through traps near the middle gap and is a non-radiative process. In gallium arsenide, electroluminescence originates primarily on the p-side of the junction because the efficiency for electron injection is higher than that for hole injection. The injection efficiency is then defined as the ratio of electron current to total current. Jn Jn Jp JR The injection efficiency can be made to approach unity by using an n+p diode so that Jp is a small fraction of the diode current and by forward biasing the diode sufficiently so that JR is a small fraction of the total diode current. The radiative efficiency is the fraction of radiative recombination. Rr 1/r nr q Rr Rnr 1/r 1/nr nr r The non-radiative rate is proportional to the density of trapping sites within the forbidden bandgap. Clearly, the radiative efficiency increases as the density of trapping sites is reduced. PHYS5320 Chapter Six 9 External Quantum Efficiency The external quantum efficiency quantifies the conversion efficiency of electrical energy into an emitted external optical energy. It incorporates the internal quantum efficiency and the subsequent efficiency of photon extraction from the device. P optical out external IV There are three loss mechanisms the generated photons may encounter: photon absorption within the semiconductor, Fresnel loss, and critical angle loss. Since some of the emitted photons have energies above Eg, the photons generated deep inside the semiconductor will be re-absorbed by the semiconductor. PHYS5320 Chapter Six 10 External Quantum Efficiency Fresnel loss: generated photons must transmit across the dielectric interface between the semiconductor and air. The reflectance at normal incidence depends on the refractive indexes of the two dielectric materials. 2 n2 n1 R n2 n1 Example: the refractive index for GaAs is n2 = 3.66 and for air is n1 = 1.0. Calculate the reflectance coefficient at the interface at normal incidence. 2 3.66 1.0 R 0.326 3.66 1.0 PHYS5320 Chapter Six 11 External Quantum Efficiency Critical angle loss: the refractive indexes of semiconductor materials are generally larger than that of air. There will be a critical incidence angle above which generated photons will experience total internal reflection. n sin1 1 c n2 Example: the refractive index for GaAs is n2 = 3.66 and for air is n1 = 1.0. The critical angle for total internal reflection at the GaAs-air interface is n sin1 1 15.9 c n2 PHYS5320 Chapter Six 12 External Quantum Efficiency PHYS5320 Chapter Six 13 LED Structures p-layer grown p-layer is formed by dopant epitaxially on n+-layer diffusion into the epitaxial layer n+p junctions p side (~m) closer to the surface to reduce re-absorption Epitaxial growth to reduce lattice strain-induced defects PHYS5320 Chapter Six 14 LED Structures How do we reduce the A common procedure is the TIR loss? encapsulation of the device within a transparent plastic It is possible to shape the surface of the medium (an epoxy) that has a semiconductor into a dome. The main refractive index matching with drawback is the difficult fabrication the semiconductor and has a process and associated increase in cost. domed surface. PHYS5320 Chapter Six 15 LED Materials Gallium arsenide is an important direct bandgap semiconductor material for optical devices. Compound semiconductor AlxGa1-xAs is of great interest. For 0 < x < 0.35, the bandgap energy is expressed as Eg 1.424 1.247xeV PHYS5320 Chapter Six 16 LED Materials Another compound semiconductor used for optical devices is the GaAs1-xPx system. PHYS5320 Chapter Six 17 LED Materials PHYS5320 Chapter Six 18 Heterojunction High Intensity LEDs A junction between two differently doped semiconductors that are of the same material (same bandgap energy) is called a homojunction. A junction between two different bandgap semiconductors is called a heterojunction. A semiconductor device structure that has a junction between two different bandgap materials is called a heterostructured device. The p-region must be narrow to allow photons to escape without much re- absorption. However, when the p-side is too narrow, some of injected electrons in the p-side will reach the surface by diffusion and recombine non-radiatively through crystal defects near the surface. We have to make a balanced choice to achieve a maximal external efficiency. PHYS5320 Chapter Six 19 Heterojunction High‐Intensity LEDs One approach for increasing the intensity of LED output light makes use of the double heterostructure. The potential barrier, Ec, prevents electrons in the conduction band in p-GaAs passing to the conduction band of p-AlGaAs. PHYS5320 Chapter Six 20 Heterojunction High Intensity LEDs The bandgap energy of AlGaAs is greater than that of GaAs. The emitted photons do not get re-absorbed. Since light is not absorbed by p-AlGaAs either, it can be reflected to increase the light output. There are negligible strain-induced defects due to a small lattice mismatch. PHYS5320 Chapter Six 21 LED Characteristics The energy of an emitted photon from a LED is not simply equal to the bandgap energy because the electrons in the conduction band are distributed in energy and so are the holes in the valence band. The electron concentration as a function of energy is asymmetrical and has a peak at (1/2)kBT above Ec.
Load more

Recommended publications
  • M7 Electroluminescence of Polymers
    Universität Potsdam Institute of Physics and Astronomy Advanced Physics Lab Course March 2020 M7 ELECTROLUMINESCENCE OF POLYMERS I. INTRODUCTION The recombination of holes and electrons in a luminescent material can produce light. This emitted light is referred to as electroluminescence (EL) and was discovered in organic single crystals by Pope, Magnante, and Kallmann in 1963.[1] EL from conjugated polymers was first reported by Burroughes et al.[2] The polymer used was poly(p-phenylenevinylene) (PPV). Since then, a variety of other polymers has been investigated. Organic EL devices have applications in a wide field ranging from multi-color displays and optical information processing to lighting. Polymers have the advantage over inorganic and monomolecular materials in the ease with which thin, structurally robust and large area films can be perpared from solutions. Using printing techniques, patterned structures can be produced easily. Even flexible displays can be produced because of the good mechanical properties of polymers. In this lab course, basic optical and electrical properties of conjugated polymers will be investigated. Advanced Lab Course: Electroluminescence of Polymers 2 EXPERIMENTAL TASKS Measure the absorption spectra of your polymers (thin films spin coated onto glass substrates). Characterize the setup used for luminescence measurements. Identify possible sources of error and collect data necessary for their correction. Measure the photoluminescence emission spectra for the polymer films, using suitable excitation wavelengths. Measure the photoluminescence excitation spectra for the polymer films, using suitable detection wavelengths. Measure the current through the OLEDs and the spectral radiant intensity of electroluminescence as a function of applied voltage (the current-radiance-voltage characteristics).
    [Show full text]
  • Xeu3+ Phosphors: X‑Ray Absorption and Emission Studies
    www.nature.com/scientificreports OPEN Correlation among photoluminescence and the electronic and atomic 3+ structures of ­Sr2SiO4:xEu phosphors: X‑ray absorption and emission studies Shi‑Yan Zheng1,2, Jau‑Wern Chiou3*, Yueh‑Han Li3, Cheng‑Fu Yang4, Sekhar Chandra Ray5*, Kuan‑Hung Chen1, Chun‑Yu Chang1, Abhijeet R. Shelke1, Hsiao‑Tsu Wang1, Ping‑Hung Yeh1, Chun‑Yen Lai6, Shang‑Hsien Hsieh7, Chih‑Wen Pao7, Jeng‑Lung Chen7, Jyh‑Fu Lee7, Huang‑Ming Tsai7, Huang‑Wen Fu7, Chih‑Yu Hua7, Hong‑Ji Lin7, Chien‑Te Chen7 & Way‑Faung Pong1* 3+ 3+ 3+ A series of Eu ‑activated strontium silicate phosphors, Sr2SiO4:xEu (SSO:xEu , x = 1.0, 2.0 and 5.0%), were synthesized by a sol–gel method, and their crystalline structures, photoluminescence (PL) behaviors, electronic/atomic structures and bandgap properties were studied. The correlation among these characteristics was further established. X‑ray powder difraction analysis revealed the formation of mixed orthorhombic α’‑SSO and monoclinic β‑SSO phases of the SSO:xEu3+ phosphors. When SSO:xEu3+ phosphors are excited under ultraviolet (UV) light (λ = 250 nm, ~ 4.96 eV), they emit yellow (~ 590 nm), orange (~ 613 nm) and red (~ 652 and 703 nm) PL bands. These PL emissions 5 7 typically correspond to 4f–4f electronic transitions that involve the multiple excited D0 → FJ levels (J = 1, 2, 3 and 4) of Eu3+ activators in the host matrix. This mechanism of PL in the SSO:xEu3+ phosphors is strongly related to the local electronic/atomic structures of the Eu3+–O2− associations and the bandgap of the host lattice, as verifed by Sr K‑edge and Eu L3‑edge X‑ray absorption near‑edge structure (XANES)/extended X‑ray absorption fne structure, O K‑edge XANES and Kα X‑ray emission 3+ spectroscopy.
    [Show full text]
  • New Lighting—New Leds
    New Lighting—New LEDs Aspects on light‐emitting diodes from social and material science perspectives Editors Mats Bladh & Mikael Syväjärvi Published by Linköping University Electronic Press, 2010 ISBN: 978‐91‐7393‐270‐7 URL: http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva‐60807 © The Authors Contents Foreword ...................................................................................... 5 Authors ........................................................................................ 7 Introduction: A Paradigmatic Shift? Mats Bladh & Mikael Syväjärvi ................................................................................. 9 Materials and Growth Technologies for Efficient LEDs Mikael Syväjärvi, Satoshi Kamiyama, Rositza Yakimova & Isamu Akasaki ............... 16 Light Excitation and Extraction in LEDs Satoshi Kamiyama, Motoaki Iwaya, Isamu Akasaki, Mikael Syväjärvi & Rositza Yakimova ...................................................................................................... 27 ‘No Blue’ White LED Haiyan Ou, Dennis Corell, Carsten Dam‐Hansen, Paul‐Michael Petersen & Dan Friis .................................................................................................................... 35 User Responses to Energy Efficient Light Sources in Home Environments Monica Säter ............................................................................................................. 43 Prospects for LED from a Historical Perspective Mats Bladh ...............................................................................................................
    [Show full text]
  • Lecture ˮincoherent Light Sources“ Summer Term 2008 Tim Pohle Electroluminescence Light Sources – Table of Contents
    Lecture ˮIncoherent Light Sources“ Summer term 2008 Tim Pohle Electroluminescence Light Sources – Table of contents Table of contents . Overview of electroluminescence . LED (Light Emitting Diode) History of development Technical details Applications . OLED (Organic Light Emitting Diode) History of development Technical details Applications . Electroluminescent strings and foils – Light Emitting Capacitor History of development Technical details Applications Incoherent Light Sources 2008 Tim Pohle 2 Electroluminescence Light Sources – Overview of Electroluminescence Overview of Electroluminescence [6] [3] . It depeds on luminescence [1] . It is distinct from fluorescence3/phosphorescence, [2] chemoluminescence2, sonoluminescence5, bioluminescence1, superluminescence6, triboluminescnece4, etc. [4] [5] . It is an optical/electrical phenomenon . Material emits light in response to an electric current passed through it, or to a strong electric field CB . It is result of radiative recombination of electrons and holes in a material VB Incoherent Light Sources 2008 Tim Pohle 3 Electroluminescence Light Sources – LED LED (Light Emitting Diode) History of development 1907 Henry Joseph Round (Marconi Labs) „invents the LED“. He finds out that some inorganic substances glow if a electric voltage is impress on them. He publishted his observation in the journal „Electrical World“ 1921 Ignorant of Round`s invention, Oleg Vladimirovich Losev makes the same observations 1927- Losev investigates this effect 1942 and guesses that it is the inversion of Einstein„s photoelectrical effect Incoherent Light Sources 2008 Tim Pohle 4 Electroluminescence Light Sources – LED 1951 Satisfactory explanation of the light emission due to the semiconductor and transistor development 1961 Bob Biard and Gary Pittman (Texas Instruments) find out that gallium arsenide (GaAs) give off infrared radiation when electric current is applied.
    [Show full text]
  • Properties of Light Library Document
    Light + Air North America Properties of Light Library Document 2 Properties Library Document of Light Properties of Light Light as energy The other way of representing light is as a wave phenomenon. This is somewhat more difficult for most people to understand, but Light is remarkable. It is something we take for granted every day, perhaps an analogy with sound waves will be useful. When you play but it is not something we stop and think about very often or even a high note and a low note on the piano, they both produce sound, try to define. Let us take a few minutes and try to understand but the main thing that is different between the two notes is the some things about light. Simply stated, light is nature’s way frequency of the vibrating string producing the sound waves--the of transferring energy through space. We can complicate it by faster the vibration the higher the pitch of the note. If we now shift talking about interacting electric and magnetic fields, quantum our focus to the sound waves themselves instead of the vibrating mechanics and all of that, but just remember, light is energy. Light string, we would find that the higher pitched notes have shorter travels very rapidly, but it does have a finite velocity. In vacuum, wavelengths, or distances between each successive wave. Likewise the speed of light is 186,282 miles per second (or nearly 300,000 (and restricting ourselves to optical light for the moment), blue kilometers per second), which is really humming along! However, light and red light are both just light, but the blue light has a higher when we start talking about the incredible distances in astronomy, frequency of vibration (or a shorter wavelength) than the red light.
    [Show full text]
  • Decoherence in Optically Excited Semiconductors: a Perspective from Non-Equilibrium Green Functions
    Decoherence in Optically Excited Semiconductors: a perspective from non-equilibrium Green functions by Kuljit Singh Virk A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto Copyright c 2010 by Kuljit Singh Virk Abstract Decoherence in Optically Excited Semiconductors: a perspective from non-equilibrium Green functions Kuljit Singh Virk Doctor of Philosophy Graduate Department of Physics University of Toronto 2010 Decoherence is central to our understanding of the transition from the quantum to the classical world. It is also a way of probing the dynamics of interacting many-body systems. Photoexcited semiconductors are such systems in which the transient dynamics can be studied in considerable detail experimentally. Recent advances in spectroscopy of semiconductors provide powerful tools to explore many-body physics in new regimes. An appropriate theoretical framework is necessary to describe new physical effects now accessible for observation. We present a possible approach in this thesis, and discuss results of its application to an experimentally relevant scenario. The major portion of this thesis is devoted to a formalism for the multi-dimensional Fourier spectroscopy of semiconductors. A perturbative treatment of the electromagnetic field is used to derive a closed set of differential equations for the multi-particle correlation functions, which take into account the many-body effects up to third order in the field. A diagrammatic method is developed, in which we retain all features of the double-sided Feynman diagrams for bookkeeping the excitation scenario, and complement them by allowing for the description of interactions. We apply the formalism to study decoherence between the states of optically excited excitons embedded in an electron gas, and compare it with the decoherence between these states and the ground state.
    [Show full text]
  • Biofluorescence
    Things That Glow In The Dark Classroom Activities That Explore Spectra and Fluorescence Linda Shore [email protected] “Hot Topics: Research Revelations from the Biotech Revolution” Saturday, April 19, 2008 Caltech-Exploratorium Learning Lab (CELL) Workshop Special Guest: Dr. Rusty Lansford, Senior Scientist and Instructor, Caltech Contents Exploring Spectra – Using a spectrascope to examine many different kinds of common continuous, emission, and absorption spectra. Luminescence – A complete description of many different examples of luminescence in the natural and engineered world. Exploratorium Teacher Institute Page 1 © 2008 Exploratorium, all rights reserved Exploring Spectra (by Paul Doherty and Linda Shore) Using a spectrometer The project Star spectrometer can be used to look at the spectra of many different sources. It is available from Learning Technologies, for under $20. Learning Technologies, Inc., 59 Walden St., Cambridge, MA 02140 You can also build your own spectroscope. http://www.exo.net/~pauld/activities/CDspectrometer/cdspectrometer.html Incandescent light An incandescent light has a continuous spectrum with all visible colors present. There are no bright lines and no dark lines in the spectrum. This is one of the most important spectra, a blackbody spectrum emitted by a hot object. The blackbody spectrum is a function of temperature, cooler objects emit redder light, hotter objects white or even bluish light. Fluorescent light The spectrum of a fluorescent light has bright lines and a continuous spectrum. The bright lines come from mercury gas inside the tube while the continuous spectrum comes from the phosphor coating lining the interior of the tube. Exploratorium Teacher Institute Page 2 © 2008 Exploratorium, all rights reserved CLF Light There is a new kind of fluorescent called a CFL (compact fluorescent lamp).
    [Show full text]
  • Top- and Bottom-Emission-Enhanced Electroluminescence of Deep-UV
    OPEN Top- and bottom-emission-enhanced SUBJECT AREAS: electroluminescence of deep-UV INORGANIC LEDS NANOPHOTONICS AND light-emitting diodes induced by PLASMONICS localised surface plasmons Received 26 November 2013 Kai Huang, Na Gao, Chunzi Wang, Xue Chen, Jinchai Li, Shuping Li, Xu Yang & Junyong Kang Accepted 27 February 2014 Department of Physics, Fujian Provincial Key Laboratory of Semiconductor Materials and Application, Xiamen University, Xiamen, 361005, P. R. China. Published 14 March 2014 We report localised-surface-plasmon (LSP) enhanced deep-ultraviolet light-emitting diodes (deep-UV LEDs) using Al nanoparticles for LSP coupling. Polygonal Al nanoparticles were fabricated on the top surfaces of the deep-UV LEDs using the oblique-angle deposition method. Both the top- and Correspondence and bottom-emission electroluminescence of deep-UV LEDs with 279 nm multiple-quantum-well emissions requests for materials can be effectively enhanced by the coupling with the LSP generated in the Al nanoparticles. The primary should be addressed to bottom-emission wavelength is longer than the primary top-emission wavelength. This difference in K.H. (k_huang@xmu. wavelength can be attributed to the substrate-induced Fano resonance effect. For resonance modes with edu.cn) or J.C.L. shorter wavelengths, the radiation fraction directed back into the LEDs is largest in the direction that is nearly parallel to the surface of the device and results in total reflection and re-absorption in the LEDs. ([email protected]) emiconductor deep-ultraviolet (deep-UV) light sources based on III-nitride light-emitting diodes (LEDs) have been intensively investigated because of their potential applications, including air and water purifica- tion, germicidal and biomedical instrumentation systems, and ophthalmic surgery tools1–3.
    [Show full text]
  • Fabrication of Organic Light Emitting Diodes in an Undergraduate Physics Course
    AC 2011-79: FABRICATION OF ORGANIC LIGHT EMITTING DIODES IN AN UNDERGRADUATE PHYSICS COURSE Robert Ross, University of Detroit Mercy Robert A. Ross is a Professor of Physics in the Department of Chemistry & Biochemistry at the University of Detroit Mercy. His research interests include semiconductor devices and physics pedagogy. Ross received his B.S. and Ph.D. degrees in Physics from Wayne State University in Detroit. Meghann Norah Murray, University of Detroit Mercy Meghann Murray has a position in the department of Chemistry & Biochemistry at University of Detroit Mercy. She received her BS and MS degrees in Chemistry from UDM and is certified to teach high school chemistry and physics. She has taught in programs such as the Detroit Area Pre-College Engineering Program. She has been a judge with the Science and Engineering Fair of Metropolitan Detroit and FIRST Lego League. She was also a mentor and judge for FIRST high school robotics. She is currently the chair of the Younger Chemists Committee and Treasurer of the Detroit Local Section of the American Chemical Society and is conducting research at UDM. Page 22.696.1 Page c American Society for Engineering Education, 2011 Fabrication of Organic Light-Emitting Diodes in an Undergraduate Physics Course Abstract Thin film organic light-emitting diodes (OLEDs) represent the state-of-the-art in electronic display technology. Their use ranges from general lighting applications to cellular phone displays. The ability to produce flexible and even transparent displays presents an opportunity for a variety of innovative applications. Science and engineering students are familiar with displays but typically lack understanding of the underlying physical principles and device technologies.
    [Show full text]
  • Determination of Exciton Binding Energy
    Journal of Luminescence 132 (2012) 345–349 Contents lists available at SciVerse ScienceDirect Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin Photoluminescence study of polycrystalline CsSnI3 thin films: Determination of exciton binding energy Zhuo Chen a,b, Chonglong Yu a,b, Kai Shum a,b,n, Jian J. Wang c, William Pfenninger c, Nemanja Vockic c, John Midgley c, John T. Kenney c a Department of Physics, Brooklyn College of CUNY, Brooklyn, NY 11210, United States b The Graduate Center of CUNY, 365 5th Avenue, New York, NY 10016, United States c OmniPV Inc., 1030 Hamilton Court, Menlo Park, CA 94025, United States article info abstract Article history: We report on the determination of exciton binding energy in perovskite semiconductor CsSnI3 through Received 13 May 2011 a series of steady state and time-resolved photoluminescence measurements in a temperature range of Received in revised form 10–300 K. A large binding energy of 18 meV was deduced for this compound having a direct band gap 12 July 2011 of 1.32 eV at room temperature. We argue that the observed large binding energy is attributable to the Accepted 2 September 2011 exciton motion in the natural two-dimensional layers of SnI tetragons in this material. Available online 10 September 2011 4 & 2011 Elsevier B.V. All rights reserved. Keywords: Exciton Radiative emission Perovskite semiconductor Photoluminescence 1. Introduction accurately determined by the low temperature absorption mea- surements in which 1s and 2s excitonic absorption peaks were Wannier
    [Show full text]
  • That You Want to Know About the Light Emitting Diodes Or Leds
    ALL THAT YOU WANT TO KNOW ABOUT THE LIGHT EMITTING DIODES OR LEDS LED=LIGHT EMITTING DIODE IS A SEMICONDUCTOR DEVICE WHICH EMITS LIGHT WHEN PROPERLY BIASED. LIKE THE BASIC SEMICONDUCTOR DIODE IN FORWARD BIAS, THE ELECTRONS AND THE HOLES IN THE LED COMBINE TO RELEASE ENERGY BY A PROCESS CALLED ELECTROLUMINESCENCE. THE COLOUR OF THE LIGHT IS DETERMINED BY THE ENERGY GAP OF THE SEMICONDUCTOR. A LIGHT EMITTING DIODE CONSISTS OF MULTIPLE LAYERS OF SEMI-CONDUCTING MATERIAL. WHEN THE DIODE IS BEING USED WITH DIRECT CURRENT, LIGHT IS PRODUCED IN THE ACTIVE LAYER. THE LIGHT PRODUCED IS DECOUPLED DIRECTLY OR BY REFLECTIONS. IN CONTRAST TO INCANDESCENT REFLECTOR LAMPS, WHICH EMIT A CONTINUOUS SPECTRUM, AN LED EMITS LIGHT IN A PARTICULAR COLOR. THE LIGHT'S COLOR DEPENDS ON THE SEMICONDUCTOR MATERIAL USED. TWO MATERIAL SYSTEMS ARE MAINLY USED, IN ORDER TO PRODUCE LEDS WITH A HIGH DEGREE OF BRIGHTNESS IN ALL COLORS FROM BLUE TO RED AND, BY MEANS OF LUMINESCENCE CONVERSION, ALSO IN WHITE. DIFFERENT VOLTAGES ARE NECESSARY, TO OPERATE THE DIODE IN FORWARD BIAS. LEDS ARE SEMICONDUCTOR CRYSTALS. DEPENDING ON THE COMPOSITION OF THE CRYSTAL COMPOUNDS, THEY EMIT LIGHT IN THE COLORS OF RED, GREEN, YELLOW OR BLUE, WHEN CURRENT FLOWS THROUGH THEM. WITH THE THREE RGB COLORS, ANY NUMBER OF COLOR TONES MAY BE MIXED BY VARYING THE PROPORTIONS OF THE INDIVIDUAL COLORS. IN THIS WAY, THE LED LIGHTING CAN CREATE FASCINATING WORLDS OF EXPERIENCE. HISTORY THE FIRST LED WAS INVENTED BY OLEG LOSE (1927), JAMES R. BIARD (1961), NICK HOLONYAK (1962) SEPARATELY. EARLY LEDS EMITTED LOW INTENSITY RED LIGHT, MODERN ONES ARE AVAILABLE ACROSS THE VISIBLE, ULTRAVIOLET AND INFRARED WAVELENGTHS WITH VERY HIGH BRIGHTNESS.
    [Show full text]
  • Influence of Ultrafast Carrier Dynamics on Semiconductor Absorption Spectra Henni Ouerdane
    Influence of Ultrafast Carrier Dynamics on Semiconductor Absorption Spectra Henni Ouerdane To cite this version: Henni Ouerdane. Influence of Ultrafast Carrier Dynamics on Semiconductor Absorption Spec- tra. Atomic Physics [physics.atom-ph]. Heriot Watt University, Edinburgh, 2002. English. tel- 00001905v2 HAL Id: tel-00001905 https://tel.archives-ouvertes.fr/tel-00001905v2 Submitted on 1 Oct 2003 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. INFLUENCE OF ULTRAFAST CARRIER DYNAMICS ON SEMICONDUCTOR ABSORPTION SPECTRA Henni Ouerdane Submitted for the Degree of Doctor of Philosophy at Heriot-Watt University on completion of research in the Department of Physics November 2001. This copy of the thesis has been supplied on the condition that anyone who consults it is understood to recognise that the copyright rests with its author and that no quo- tation from the thesis and no information derived from it may be published without the prior written consent of the author or the university (as may be appropriate). I hereby declare that the work presented in this the- sis was carried out by myself at Heriot-Watt University, Edinburgh, except where due acknowledgement is made, and has not been submitted for any other degree.
    [Show full text]