NON-COHERENT LIGHT SOURCES II LUMINESCENT RADIATORS AND LIGHT EMITTING DIODES Dr. Bálint Pődör INTRODUCTION Subjects: Electroluminescent light sources Semiconductor light emitting diodes (LEDs) Organic light emitting diodes (OLEDs) ELECTROLUMINESCENT LIGHTING Electroluminescence: excitation by moving electrons driven by electric field. Important mechanism: injection (electro)luminescence, ocurring in a forward biased pn junction. Fundamental mechanism of the operation of light emitting diodes (LED) and laser diodes (LD). The original patent of injection electroluminescent lighting: (Tungsram Rt. Újpest)! SiC electroluminescent light source (the origin of today’s light emitting diodes): György Szigeti and Zoltán Bay US patent No. 2,254,952 (1942) ELECTROLUMINESCENT MATERIALS, ZnS ZnS: One of the most important electroluminescent material, widebandgap semicondcutor. Forms: fine powder or sputtered layers. Control of radiated color: Zn-excess or „activators”, i.e. Ag, Cu, or Mn dopants (0.0001-3% ). Compound Melting p. K Bandgap eV ZnS 2103 3,7 Electron mobility m2/Vs 0,014 Hole mobility m2/Vs CdS 2023 2,4 0,024 0,005 CdSe 1531 1,8 0,06 0,005 HgSe 1073 0,6 1,8 - 0,0005 ZnS: ELECTROLUMINESCENCE SPECTRUM ZnS:Mn orange-yellow ZnS:Cu green Electric field strength: 108 V/m i.e. 100 V across 1 µm thick layer ELECTROLUMINESCENT CELL Simplified band diagram of an electroluminescent crystal. fénypor szigetelõ réteg insulating layer luminescent powder in insulating matrix insulating layer back electrode szigetelõ réteg A.C. leads Cross-section of the ZnS electroluminescent cell (Destriaux-cell). Light can be excited by applying ac voltage to the cell. glass plate transparent electrode emitted light ELECTROLUMINESCENT DISPLAY Al-electrode insulating layer phosphor layer insulating layer glass substrate transparent electrode COLOUR ELECTROLUMINESCENT DISPLAY R, G, B szûrõk RGB filters Row electrodes sor elektródák üveg lemez Glass szigetelõ réteg Insulating layer fénypor réteg Phosphor layer Insulating layer szigetelõ réteg Column electrodes oszlop elektródák Base alaplap CATHODOLUMINESCENCE Excitation by electron beam, vacuum: TV screens and computer monitors (however technological change!) Colour TV picture tube (shadow mask, three e-beams, three phosphor layers): Blue: ZnS:Ag Green: ZnCdS:Cu,Al Red: Y2O3:Eu LUMINESCENCE IN SEMICONDUCTORS Conduction band Impurity/defect centres Valence band 10 DIRECT AND INDIRECT TRANSITION Simplified band diagram of direct and indirect bandgap semiconductors: Energy versus the momentum (wave number) in the reciprocal lattice. RADIATIVE RECOMBIBATION In direct band-structure semiconductors (e.g. GaAs, InP, InGaAs, GaN) momentum conservation is easily satisfied in the transitions between the Γ-point (zone centre) band extrema, therefore the probability of radiation recombination is large. For indirect band-structure (e.g. (pl. Si, SiC, GaP) an additional particle (usually a phonon) is necessary because of momentum conservation, therefore the probability of radiative recombination is much less. The difference in the radiative recombination rates for these two types of semiconductors can amount to 3 to 5 orders of magnitude! 12 INJECTION LUMINESCENCE IN A FORWARD BIASED PN JUNCTION foton emisszió kiürülési tartomány vezetési sáv alja szabad elektron p-tip Fermi nívó n-tip. Fermi nívó szabad lyuk vegyérték kötési sáv teteje az átmenetre helyezett feszültség 13 HETEROJUNCTION LED b. Simplified energy band diagram. EF must be uniform. c. Simplified energy band diagram for forward bias. There is a large overlap of electrons and holes. d. Schematic illustration of photons escaping reabsorption in the AlGaAs (window) layer and being effectively emitted from the device. BANDGAPS AND LED COLOURS 15 BANDGAPS AND POSSIBLE WAVELENGTH RANGES IN SEMICONDUCTORS PROGRESS IN LED-S LED STRUCTURE LED BASICS Large wavelength content Incoherent Limited directionality Electron-hole recombination emits photon Need a location with both electrons and holes Requires a large applied voltage Small emission region Poor optical confinement LED BASICS Spontaneous emission dominates Random photon emission Broad spectrum (∆λ~0.03 µm) Not all photons exit the LED Broad far field emission pattern Quantum efficiency Fraction of optical power emitted out of the LED, caused by material absorption, surface reflection Dominant reduction in quantum efficiency is critical angle Dome used to extract more of the light Dome makes LED more directional LED SPECTRA LED SPECTRA Maximum wavelength corresponds to EG + kT. Spectral width (FWHM) typically 2.5kT - 3kT (on energy scale) Spectral width on wavelength scale if ∆Eph ≈ 3kT then hc λ = —— Eph and dλ hc —— = - —— dEph Eph2 3kT ∆λ ≈ λ2 —— hc λ = 870 nm λ = 1300 nm λ = 1550 nm ∆λ = 47 nm ∆λ = 105 nm ∆λ = 149 nm InGaAsP/InP LED SPECTRA InGaAsP/InP LED series (1100-1700 nm). The half widths of the spectra are ∼65 nm (at 1100 nm), and ∼120 nm (at 1700 nm). COMMERCIAL LED SPECTRA Data from thee LED types (Hewlett-Packard) (from data sheets): Type Material λp nm ∆λ1/2 nm Ep eV Red GaAsP 635 41 1.953 25 Yellow GaAsP 583 30 2.127 22 Green GaP 565 20 2.195 20 ∆λ1/2 nm (calc. with 3kT) Diode voltage (at 10-20 mA) 2.2 – 2.4 V. The last column is the theoretical estimation with 3 kT. Quantum mechanics adequately describes operational parameters! LED STRUCTURES AND OPERATIONAL CHARACTERISTICS LED LIGHT-CURRENT CHARACTERISTICS The ”internal” optical power is proportional to the number of injected charge Pint = ηint (hν ν) (I/q) The optical power Popt = ηext Pint = ηext ηint (hν ν) (I/q) The(idealized) light flux-current characteristics of LED is linear. In practical units Popt [mW] = ηextηint (1,24/λ λ[µ µm]) I [mA] LED QUANTUM EFFICIENCY An other characteristic parameter of a LED is the total quantum efficiency ηtot, which equals the ratio of the emitted light power to the input electrical power. Pel = IUd, where Ud is the voltage drop on the diode, Ud ≈ hν/q = hc/λq. ηtot = ηextηint (hν ν/qUd) ≈ ηextηint E.g. for a 1.3 µm LED, ηextηint ∼5 %, Popt/I = ∼0,05 mW/mA SURFACE EMITTING LED EDGE EMITTING LED STRUCURE OF BLUE GaN LED Aktív réteg Kontaktus réteg P kontaktu s GaN:Mg ~4 µm GaN:Si GaN: Si Leválasztó réteg Zafír alap ( ~ 100 µm) n kontaktus NEW APPLICATION OF PHOTOLUMINESCENCE: ”WHITE LED” Producing white light: Blue LED + phosphor Yellow or green + red phosphor UV-LED + phosphor Stokes shift (energy shift between the exciting and emitted photon) THREE METHODS OF GENERATING WHITE LIGHT BLUE LED + YELLOW PHOSPHOR: WHTE Cross section of a blue LED with the light emitting phosphor powder in the package (Y3Al5O12:Ce3+) pcLED 60 White light is obtained in a such way that a fraction of the LED’s light excites the phosphor’s yellow band radiance, a.u. 50 40 30 20 10 0 400 500 600 700 nm 800 TWO-PHOSPHOR LED: WHITE LIGHT Blue: InGaN/GaN LED (460 nm) 460nm+TG:Eu+SrS:Eu 50 Green: SrGa2S4 = TG:Eu2+ 7560 K 2930K: Ra=93 45 3060K: Ra=93 40 Red: SrS:Eu2+ 2930 K 3350K: Ra=93 35 3970K: Ra=93 radiance 30 5620K: Ra=93 25 7560K: Ra=90 20 15 10 5 0 400 500 600 700 nm 800 ORGANIC LIGHT EMITTING DIODE: OLED OLED ENERGY LEVEL STRUCTURE singlet triplet singlet Level scheme and colour depends on the chemical structure of the polimer. OLED STRUCTURE < 1µm thick < 1 µm thick EXERCISES AlGaAs LED emitter. An AlGaAs LED emitter for use in optical fiber network has the output spectrum shown in the figure. It is designed for a peak emission at 820 nm at 25 oC. a. What is the linewidth ∆λ between half power points at the three indicated temperatures? What is the empirical relationship between ∆λ and T and how does this compare with ∆(hν) ≈ (2.5-3)kBT ? b. Why does the peak emission wavelength increase with temperature? c. What is the bandgap of AlGaAs in this LED ? d. The bandgap EG of the ternary alloy AlxGa1-xAs follows the empirical expression EG(eV) = 1.424 + 1.266x + 0.266x2 What is the composition of AlGaAs in this LED ? EXERCISES Relative spectral output power The output spectrum from an AlGaAs LED. Values normalized to a peak emission at 25 oC. END OF LECTURE
© Copyright 2024 ExpyDoc