Inkohärente Lichtquellen Sommersemester 2010 Uli Engelhardt und Kai Kruse Outline 1. Introduction: Light Emitting Diodes History Principle/ assembly Applications 2. UV - LEDs Different types Applications 3. AlGaN UV - LEDs Technical Details Development/outlook 1. Introduction: Light Emitting Diodes History 1907: H. J. Round of Marconi Labs discovered that some inorganic substances glow if a electric voltage is impress on them. 1927: The russian Oleg Vladimirovich Losev independently reported on the creation of an LED, but no practical use was made of the discovery. 1961: Bob Biardand and Gary Pittman (Texas Instruments) find out that gallium arsenide (GaAs) give off infrared radiation when electric current is applied. They receive a patent for this diode. 1. Introduction: Light Emitting Diodes History 1962: First visible red GaAsP-LEDs was developed by Nick Holonyak ("father of the light-emitting diode”) at General Electric Company. 1971: The first blue LEDs (GaN) were made by Jacques Pankove at RCA Laboratories. Too little light output to be of much practical use. 1993: Shuji Nakamura (Nichia Corporation) demonstrates the first high-brightness blue LED based on InGaN. Blaue LEDs aus InGaN 1. Introduction: Light Emitting Diodes Principle LED consists of a chip of semiconducting material with a p-n junction. As in other diodes, current flows easily from the p-side (anode) to the n-side (cathode), but not in the reverse direction. At the barrier layer electrons and holes recombinat and energy in the form of a photon is emitting. The wavelength of the light, depends on the band gap energy. 1. Introduction: Light Emitting Diodes Principle Color Wavelength (nm) Semiconductor Material Infrared > 760 e.g.: GaAs, AlGaAs Red 610 - 760 e.g.: AlGaAs, AlGaInP Orange 590 - 610 e.g.: GaAsP, GaP Yellow 570 - 590 e.g.: GaAsP, GaP Green 500 - 570 e.g.: InGaN, GaN Blue 450 - 500 e.g.: ZnSe, InGaN Ultraviolet < 400 e.g.: AlN AlGaN AlGaInN 1. Introduction: Light Emitting Diodes Assembly different LED types 1. Introduction: Light Emitting Diodes Applications . Illumination with high brightness . Visual signal application . Automotive lighting . Optical measurement systems . Flashlights . LCD Backlit Screens . Grow lights 1. Introduction: Light Emitting Diodes Advantages/ Disadvantages Efficiency High initial price Direct colour generation Temperature dependence Small size Current-regulated power supplys Fast switch time Colour rendering Frequent on-off cycling Easy dimming Cool light without IR Long lifetime Shock resistance Parallel ray emission No mercury needed 2. UV - LEDs Different Types Blue-LEDs, λ 400 - 500 nm UV-LEDs, λ < 400 nm • Zinc selenide (ZnSe) • Diamond (235 nm) • Indium gallium nitride (InGaN) • Boron nitride (215 nm) Group III- • Indium gallium nitride (InGaN) • AlN (210 nm) nitrides • AlGaN • AlGaInN (210 nm) 2. UV - LEDs Applications Adhesive hardening Lacquer hardening (ink-jet-printer) Quality control scratch-resistant coating of a diffusion disc bank note control Ink-jet-printer 2. UV - LEDs Applications Sterilisation/ Disinfection (water, air, …) Medical technology (dermatology, light therapy) Organic analytics (fluorescence microscopy) Drinking water sterilisation Bottle cap sterilisation 2. UV - LEDs Applications Convert light from a blue or UV LED to broad-spectrum white light 2. UV - LEDs Comparison UV-LED and low pressure vapour Hg-Lamp Efficiency: 5-10 % (LED) 40% (Hg-Lamp) Life-time: > 10.000 hours (LED) 8.000 hours (Hg-Lamp) LEDs are environmentally friendly than Hg-Lamps LEDs need no warm up, they are ideal for use in applications that are subject to frequent on-off cycling 3. AlGaN UV - LEDs Technical Details Output power 0.1 to 10 mW range Efficiency of max 10% Sharp spectrum due to qantum well Cw and pulsed operation 3. AlGaN UV - LEDs Technical Details Semiconductor compound • Increasing Al concentration decreases wavelength • AlGaInN: more degrees of freedom for bandgap and lattice constant, helps growth, increases radiative efficiency but lowers wavelength 3. AlGaN UV - LEDs Technical Details Quantum well • Electron movement confined: 3D -> 2D • Sandwich layers of Well (AlGaN) & Barrier (AlN) • Wavelength tunable by semicond. composition and layer thickness (2-10 nm) „Blue shift“ of emission 3. AlGaN UV - LEDs Development/ outlook Heat sinking: Output and lifetime limit DUV-LED → DUV Laser Diode: challenging Wavelength: Theoretical limit is 205nm Fabrication: Quality + mass production Increase efficiency: layer processing, reduce resistive losses Multiple chip packing: 11mW @ 280nm Thank you for your attention! Sources: www.lightemittingdiodes.org Wikipedia: Light-emitting Diode (english) http://en.wikipedia.org/wiki/Led#Ultraviolet_and_blue_LEDs AlGaN Deep-Ultraviolet Light-Emitting Diodes, Jianping ZHANG et al, Japanese Journal of Applied Physics Vol. 44, No. 10, 2005, pp. 7250–7253 227nm AlGaN Light-Emitting Diode with 0.15mW Output Power Realized Using a Thin Quantum Well and AlN Buffer with reduced Threading Dislocation Density, Hideki Hirayama et al, Applied Physics Express 1 (2008) 051101 Improved local thermal management of AlGaN-based deep-UV light emitting diodes, M. Khizar et al, Semicond. Sci. Technol. 22 (2007) 1081-1085.