Development of Nanosecond Range Light Sources for Calibration of Astroparticle Cherenkov Detectors Omar Veledar PhD 2007 Development of Nanosecond Range Light Sources for Calibration of Astroparticle Cherenkov Detectors Omar Veledar The thesis is submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy University of Northumbria at Newcastle School of Computing, Engineering and Information Sciences May 2007 Abstract In this thesis the development of light emitting diodes (LED) is reviewed. The em- phasis is put on devices emitting at the blue region of the spectrum. The physical characteristics of these devices are considered. The main interest is based around the ability of blue LEDs to generate nanosecond range optical flashes. The fast pulsing electronic circuits capable of driving the devices are also reviewed. These are complemented by the potentially exploitable techniques that could provide further benefits for required fast optical pulse generation. The simple, compact and inexpensive electronic oscillator for producing nanosecond range pulses is developed. The circuitry is adapted for generation of pulses necessary to switch on and assist with the turn off of blue InGaN based LEDs. The resulting nanosecond range blue optical pulses are suitable for, but not limited to, the calibration of scintillation counters. These devices used in neutrino detection experiments could provide a better understanding of cosmology and particle physics. Contents List of Symbols v List of Acronyms viii Acknowledgements ix Author’s Declaration x Publications as a Result of Work on this Thesis xi 1 Introduction 1 1.1 CurrentStateoftheArt .......................... 3 1.2 PresentApplications ............................ 3 1.3 Objectives.................................. 5 1.4 ScopeoftheThesis............................. 6 1.5 Contributions ................................ 6 1.6 ThesisStructure............................... 6 2 The Light Emitting Diode 8 2.1 LEDDevelopment ............................. 8 2.1.1 Presentdevices ........................... 9 2.1.2 FutureDevices ........................... 16 2.2 LEDCharacteristics ............................ 18 2.2.1 ChargeProperties. .. .. 18 2.2.2 TheP-NJunction.......................... 20 2.2.3 ElectricalProperties . 23 2.2.4 OpticalProperties . .. .. 28 2.3 Advanced Structures - High Brightness LEDs . ... 29 2.3.1 SingleHeterojunction. 30 2.3.2 DoubleHeterojunction . 31 2.3.3 Single Quantum Well - InGaN Based LEDs . 31 2.3.4 ManufactureofHighBrightnessLEDs . 33 i 2.4 Applications................................. 35 2.5 Chapter2Summary ............................ 37 3 Pulse Shaping Techniques and LED Pulse Response 38 3.1 TypesofLEDDrivers ........................... 38 3.1.1 Multivibrators............................ 39 3.1.2 EmitterCoupledMonostable . 39 3.1.3 AvalancheTransistors . 40 3.1.4 Complementary Transistor Pair Regenerative Switch ............................... 41 3.1.5 Standard Telecommunication Techniques . .. 42 3.2 Overview of Standard Pulse Shaping Techniques ................................. 42 3.2.1 Differentiation............................ 42 3.2.2 StepRecoveryDiode . .. .. 43 3.2.3 Clipping and High Speed Comparators . 43 3.2.4 Non-SaturatingSwitch . 44 3.2.5 Speed up Commutating Capacitor for Transistor Switching... 45 3.2.6 Shorted Turn - Theory and Application . 46 3.3 LEDSwitchingParameters . 50 3.4 Basic Switching Principles . 52 3.5 Chapter3Summary ............................ 53 4 LED Modelling 54 4.1 OrCADModels ............................... 55 4.1.1 IdealDiodeStaticModel. 55 4.1.2 RealDiodeStaticModel . 56 4.1.3 LargeSignalModel......................... 59 4.2 BehaviouralModels............................. 60 4.3 Chapter4Summary ............................ 61 5 Experimental and Modelled LED Characteristics 62 5.1 MethodsandResults ............................ 62 5.1.1 Capacitance - Voltage Relationship . 63 5.1.2 Current-voltagerelationship . 71 5.1.3 LEDOutputSpectrum . .. .. 73 5.1.4 Current - Output Light Intensity Relationship . .... 74 5.1.5 LEDasaPhotodetector . 75 5.1.6 Correlation ............................. 77 5.2 ModellingResults ............................. 81 ii 5.2.1 OrCADModelEditor ....................... 81 5.2.2 OrCADBehaviouralModel . 82 5.2.3 MATLAB Behavioural Model . 84 5.2.4 ModelComparison ......................... 84 5.3 Chapter5Summary ............................ 86 6 Optical Pulse Generation 88 6.1 Investigation of the Current Arrangement . .... 88 6.2 Totem-PoleDrivers ............................ 90 6.3 SingleOutputConfigurations . 92 6.3.1 SwitchingConfiguration . 92 6.3.2 PulseShaping............................ 93 6.3.3 Measurement Technique . 95 6.3.4 ResultsandDiscussion . 96 6.3.5 ErrorTreatment .......................... 99 6.4 MultipleOutputConfiguration . 100 6.4.1 CircuitArrangement . 101 6.5 Chapter6Summary ............................ 104 7 Conclusions and Recommendations for Further Work 105 7.1 Conclusions ................................. 105 7.2 FurtherWork ................................ 106 A Practical Diode Equation Analysis 108 A.1 Analysisbyinspection . .. .. 108 A.2 MathematicalAnalysis . 109 A.2.1 ManualSolution . .. .. 110 B SPICE Diode Model Parameters 113 C LED Capacitance Analysis 115 C.1 Inverse Capacitance Squared Versus Voltage Plots . ....... 115 C.2 DepletionCapacitanceFit . 115 C.3 DiffusionCapacitanceFit . 116 C.4 HumpCapacitanceFit . .. .. 117 C.5 FittingErrors ................................ 118 D LED Current - Voltage Analysis 122 E LED Output Spectrum Analysis 125 F LED Intensity Analysis 126 iii G Using LED as a Photodetector 127 H Blue LED Model Netlist - Using OrCAD Model Editor 130 iv List of Symbols The following is a list of symbols used in the thesis: Symbol Unit Description A m2 cross sectional area 2 Ac m core area B T Magnetic Flux Density BV V breakdown voltage CD F diode capacitance Cd F diffusion or injection capacitance Cj F junction or depletion capacitance 2 −1 Dn/p m s diffusion coefficients Ec1,2,3... eV energy of conduction band level Eg(well) eV well material energy band gap EgQW eV Quantum well energy band gap Ev1,2,3... eV energy of valance band level f Hz frequency FWHM(observed) s measured FWHM FWHM(optical) s optical signal FWHM FWHM(system) s measuring system FWHM h Js Planck’s constant (6.6260755 × 10−34Js) H Am−1 Magnetic Field Strength I or i A current I(r,g) A generation-recombination current ID(ideal) A ideal diode current In A electron current Is A saturation current −2 JCOND Am sum of all drift and diffusion current densities −2 Jdiff Am diffusion current density −2 Jdrift Am drift current density −2 Jn Am electron current density −2 Jn(diff) Am electron diffusion current density −2 Jn(drift) Am electron drift current density −2 Jp Am hole current density −2 Jp(diff) Am hole diffusion current density −2 Jp(drift) Am hole drift current density −2 Js Am saturation current density k NmK−1 Boltzmann’s constant (1.3806568 × 10−23JK−1) l m carrier mean free path v Symbol Unit Description lc m core mean length lQW m quantum well length L H inductance L0 H inductance in a coil with an air core Ln m electron minority carrier diffusion length Lp m hole minority carrier diffusion length ∗ me kg electron effective mass ∗ mh kg hole effective mass ∗ mr kg reduced effective mass n m−3 electron concentration −3 ni m intrinsic carrier concentration −3 nn0 m majority electron carrier equilibrium concentration −3 np m minority electron carrier concentration −3 np0 m minority electron carrier equilibrium concentration N number of coil turns −3 NA m acceptor impurity concentration −3 NB m impurity concentration of the lightly doped side −3 ND m donor impurity concentration p m−3 hole concentration −3 pn m minority hole carrier concentration −3 pn0 m minority hole carrier equilibrium concentration −3 pp0 m majority hole carrier equilibrium concentration q C Electron charge (1.60217733 × 10−19C) Q C charge Qd C diffusion charge - due to minority carrier injection QD C charge stored by a diode Qj C depletion charge - due to doping atoms concentration Qn C stored (electron) charge per unit area Qp C stored (hole) charge per unit area R rate of direct recombination (radiation efficiency) R(f) LED frequency response Rp Ω diode parallel resistance Rs Ω diode series resistance t s time tfall s fall time trise s rise time T K Temperature vbi V built in voltage −1 vi ms electron individual drift velocity −1 vn ms average electron drift velocity −1 vp ms average hole drift velocity −1 vth ms carrier thermal velocity V V Voltage VBE V transistor base-emitter voltage VEB V transistor emitter-base voltage Vt V diode thermal voltage W m depletion layer width vi Symbol Unit Description x m distance from the junction xn m distance from the junction into n side xp m distance from the junction into p side β m3s−1 radiation constant of proportionality ε NC−1 electric field −1 ǫS F m semiconductor dielectric permittivity η ideality factor or emission coefficient −1 µ0 Hm permeability in free space 2 −1 µn m (Vs) electron mobility 2 −1 µp m (Vs) hole mobility −1 µr Hm relative permeability ν Hz frequency of light ξ V emf τc s mean free time (minority carrier lifetime) τf(P MT ) s PMT fall time τf(scope) s oscilloscope fall time τf(system) s measuring system fall time τn s excess minority electron carriers’ lifetime τp s excess minority hole carriers’ lifetime τr(P MT ) s PMT rise time τr(scope) s oscilloscope rise time τr(system) s measuring system rise time φ W b magnetic flux The