OPTOELECTRONIC and PHOTOVOLTAIC DEVICES Outline 1. Introduction to the (semiconductor) physics: energy bands, charge carriers, semiconductors, p-n junction, materials, etc. 2. Light emitting diodes Light emitting devices 3. Semiconductor lasers (displays, illumination) 4. Photodetectors Light absorbing devices 5. Photovoltaic solar cells (Photodiodes) Principles of operation (physical background) Design and structure Fabrication http://www.lht.ovgu.de/studium/lehrveranstaltungen.html 1
OPTOELECTRONIC and PHOTOVOLTAIC DEVICES Literature recommendation [1] B. G. STREETMANN, S. K. BANERJEE: Solid state electronic devices 6th ed. [2] S. M. SZE: Physics of semiconductor devices [3] Ch. KITTEL: Introduction to solid state physics Technology [4] J. D. PLUMMER, M. D. DEAL, P. B. GRIFFIN: Silicon VLSI technology: fundamentals, practice and modeling [5] S. K. GHANDHI: CLSI fabrication principles: silicon and gallium arsenide [6] S. A. CAMPBELL: The science and engineering of microelectronic fabrication [7] P.MOTTIER: LEDs for lighting applications http://www.lht.ovgu.de/studium/lehrveranstaltungen.html 2
4.1. Basics Semiconductor devices that can detect optical signals through electronic processes. Operation: basically three processes: 1. carrier generation by incident light 2. carrier transport and/or multiplication by current gain mechanism (if present) 3. extraction of carriers as terminal current to provide the output signal For optical-fiber communication: demodulation of optical signals REQUIREMENTS: 1.) high sensitivity at operating wavelength 2.) high response speed 3.) minimum noise 4.) compact, low biasing voltage/current, reliable 3
4.1. Basics TWO CLASSES: 1) thermal detectors : detection of temperature rise after light energy absorption; far IR 2) photon detectors: based on quantum photoelectric effect (excitation of carriers) photoelectric effect based on photon energy hν (E = h ν; ν = c λ ) wavelength is related to energy transition (ΔE): λ = h c ΔΔ = 1240 ΔΔ ee (nm), Usually hν > ΔE wavelength limit for detection depending on type transition energy: 1.) energy gap 2.) barrier height 3.) transition between impurity level and band edge 4
4.1. Basics absorption indicated by absorption coefficient determines: whether light can be absorbed where light can be absorbed high value near surface! determines quantum efficiency [2] 5
4.1. Basics Speed of photodetector important for fiber optic communication response fast enough for digital transmission data rate on/off > 40Gb/s shorter carrier lifetime faster response BUT also higher dark current depletion width should be minimized short transit time BUT depletion width small high capacitance trade off needed 6
4.1. Basics Sensitivity signal of photocurrent should be maximized high sensitivity Quantum efficiency (carriers per photon) η = I ph = I pp ( h ν ) q Φ q P ooo I ph photocurrent, Φ photon flux (number of photons per time unit) P opt /hν, P opt optical power [W] Losses by a) recombination b) incomplete absorption c) reflection Responsivity R (similar metric): R = I pp P ooo = η q h ν [A/W] 7
4.1. Basics Gain improvement of signal by internal gain possible BUT high gain high noise Charge-couple device (CCD) [2] 8
4.1. Basics Noise: Low noise determination of minimum detectable signal strength signal to noise ratio Factors contributing to noise: dark current: leakage current when photodetector under bias but not exposed to light source one limitation: temperature thermal energy should be smaller than photon energy (kt< hν) (RT: kt = 0.026eV λ = 47.7µm) background radiation: black body radiation from detector housing (+ straylight) 9
4.1. Basics Noise: internal device noise: 1) thermal noise (Johnson noise) random thermal agitation of carriers in any resistive device 2) shot noise due to discrete single events of photoelectric effect with statistical fluctuations important at low light intensity 3) flicker noise (1/ f noise) due to random effects associated with surface fraps more pronounced at low frequencies 4) generation recombination noise from fluctuations of these events (generation noise from optical and thermal processes) all noises independent addition to total noise FIGURE of merit: noise equivalent power (NEP) 10
4.1. Basics Noise: noise equivalent power (NEP) corresponds to rms optical power required to produce a signal to noise ratio of 1 in 1 Hz bandwidth minimum detectable light power Detectivity (D * ) D* = A B NNN (cm Hz1/2 /W) A area, B - bandwidth = signal to noise ratio when 1W of light power on 1cm² detector area and noise measured over 1Hz bandwidth, (detector sensitivity, spectral response, noise) function of wavelength, modulation frequency, bandwidth 11
4.2. Photoconductive devices devices that change their resistance when exposed to light applications: automatic night light (on/off) at home measurement of illumination levels (exposure meters) detectors in photoelectric barriers choice of materials: with respect to optical sensitivity time response sensitive wavelength most sensitive to photons with energies equal to bandgap or slightly more hν < E g no absorption, hν E g absorption very close to surface little contribution to bulk conductivity 12
4.2. Photoconductive devices Some photoconductors respond to excitation of carriers from impurity levels within bandgap sensitive to photons of less than band gap E extrinsic photoconductor [2] 13
4.2. Photoconductive devices CdS (E g = 2.42eV) visible range Ge (0.67eV) InSb (0.18eV) infrared (cooled: 77K, 4.2K) λ 5µm HgCdTe 10µm [2] 14
4.2. Photoconductive devices Device: slab of semiconductor (bulk or thin film) with ohmic contacts intrinsic : σ = q (µ n n + µ p p), σ conductivity, µ = v D mobility E Vs n and p change by light photon flux on surface with area A = W L number of photons : P opt /hν thickness D > light penetration depth 1/α full absorption Steady state: carrier generation rate = recombination rate ccc g e = δn τ = η P ooo/(h ν) W L D [2] τ mean time of each excess carrier in exited state before recombination, δn excess carrier density 15
4.2. Photoconductive devices Concentration much larger than background doping: δn = g e τ Light is taken off: (carrier lifetime important!) δn t = δn 0 e t/τ Photocurrent Intrinsic (δn = δp) I p = σ E W D = (μ p + μ n )δδ q E W D I p = q(η P ooo hν ) (μ p+μ n )E τ L I ph I ph = q η P ooo hν photocurrent gain G a : primary photocurrent G a = I p I ph = µ p + µ n 16
4.2. Photoconductive devices Gain: depends on ratios of carriers lifetimes (τ) to transit time ( L µ n E ) high gain: - long lifetime - short electrode spacing - high mobilities 1000 10 6 possible BUT response time determined by lifetime! Trade off between gain and speed Limit: maximum field at breakdown 17
4.2. Photoconductive devices Sweep-out effect: minority carrier sweep-out moderate field: majority carriers (e - ) have higher mobility: t rn < τ minority carrier (holes) slower: t rp > τ e - swept out quickly, but holes demand charge neutrality more electrons supplied by other electrode gain (loop) high field: also t rp < τ G a 1 photoconductors: attractive for simple structure, low cost, rugged features high dynamic range, good performance for high level detection (IR photodetectors) BUT low level, microwave frequencies diodes 18
4.3. Photodiodes Important attributes for detectors: Sensitivity, speed, noise, size, reliability, T sensitivity, ease of use, cost junction device: improved speed of response and sensitivity optical or high-energy radiation 19
4.4. Photodiodes: Current and Voltage in an illuminated junction p-n junction : diffusion of carriers due to concentration gradients, drift of carriers due to electric field transition region, width W D drift of minority carriers across junction generation current 1) carriers generated within W D separated by junction field e - collected in n region, h + collected in p region + 2) carriers generated within diffusion length of each side of transition region diffuse to depletion region swept by E [2] 20
4.4. Photodiodes: Current and Voltage Junction uniformly illuminated by photons (hν > E g ) added generation rate g opt [ EEEE ] participates in current ccc s A L p g ooo : number of holes created per s within diffusion length of transition region on n side A L n g ooo A W g ooo :on p side :within W D n τ n [1] 21
4.4. Photodiodes: Current and Voltage A L p g ooo A L n g ooo A W g ooo : number of holes created on n side :on p side :within W I ooo = q A g ooo L p + L n + W current due to collection of optically generated carriers [1] 22
4.4. Photodiodes: Current and Voltage Diode equation: I = I tt qq kk e 1 I ooo total reverse current with illumination I tt = q A p n concentration of holes in n region (minority) L p τ p p n L n τ n n p thermally generated current I V curve lowered by amount proportional to g opt [1] 23
4.4. Photodiodes: Current and Voltage device short circuited (V = 0): diode terms cancel : short circuit current from n to p equal I opt I V curve cross I- axis at negative values proportional to g opt open circuit (I = 0) V = V oc V oo = kt q ll I ooo I tt + 1 = kk q ll L p + L n + W L p /τ p p n + L n /τ n n p g ooo + 1 24
4.4. Photodiodes: Current and Voltage in an illuminated junction open circuit (I = 0) in the case of symmetrical junction, p n = n p and τ p = τ n thermal generation rate: g tt = p n (equilibrium thermal generation recombination rate) τ n neglecting generation within W V oo kt q ll g ooo g tt for g opt >>g th minority carrier concentration increased by optical generation of EHP lifetime τ n becomes shorter p n τ n becomes larger (p n is fixed for given N d and T) V oc cannot increase indefinitely 25
4.4. Photodiodes: Current and Voltage open circuit (I = 0) V oc cannot increase indefinitely with increased generation rate! limit on V oc is contact potential V o expected, since Vo is maximum forward bias that can appear across a junction forward voltage across illuminated junction: photovoltaic effect! solar cells [1] 26
4.4. Photodiodes: Current and Voltage Depending on application, photodiode can be operated in either third or fourth quarter of I V characteristic power to the device: current and junction voltage both positive or negative (1st and 3rd quadrants) power from device: junction voltage positive current negative (4 th quadrant) - as in battery: current flows from negative side of V to positive side photodetector: reverse bias (3rd quadrant) solar cells [1] 27
4.4. Photodiode devices photodetector: reverse bias (3rd quadrant) current essentially independent of voltage but proportional to optical generation rate For many applications: detector s speed of response (or bandwidth) critical! Example: series of light pulses 1ns apart generated minority carriers must diffuse to junction and swept across in less than 1ns! diffusion time consuming! to avoid W large enough most photons absorbed there! (not in neutral n and p regions) drift in E field: FAST! [1] 28
4.4. Photodiode devices p i n photodetector controlling of width by thickness of i region (lightly doped) high resistivity applied voltage almost entirely across i region carrier lifetime long compared to drift time most carriers collected no current gain η max = 1 for low level signals: gain (required) desirable [1] 29
4.4. Photodiode devices avalanche photodiode (APD) operation of diode in avalanche region of its characteristic (high reverse voltage) avalanche multiplication gain: external quantum efficiency > 100% intrinsic detectors: sensitive to photons with energies near band gap energy: hν < E g no absorption hν E g absorption very near surface recombination rate high! choice of diode material with band gap corresponding to spectrum required extrinsic detectors: sensitive to longer wavelength by excitation of e - out of or into impurity levels BUT: sensitivity much less than intrinsic detectors 30
4.4. Photodiode devices Tailoring of band gaps by using lattice matched multilayers of compound semiconductors wide band gap material windows for light transmission to absorbing region In x Ga 1-x As with In of 53% (x=0.53):(bandgap 0.75eV 1.65µm close to 1.55µm for fiber optics!) absorption can be grown epitaxially on InP with excellent lattice match In x Al 1-x As x = 0,52: wider gap + lattice match to InP windows avalanche photodiodes [1] 31
4.4. Photodiode devices lattice matched multilayers: narrow band gap material for absorption wider band gap material for avalanche region (higher fields possible!) separation of absorption and multiplication (SAM) avoids excessive leakage currents of reversed - biased junctions in narrows - band - gap materials + addition of doped InAlAs charge layer to optimize (decrease) E field between M an A layer SACM APD some APSs: graded alloy composition between lower gap absorption region and high band gap multiplication region to avoid any band edge discontinuities (carrier trapping!) [1] 32
4.4. Photodiode devices lattice matched multilayers Photocurrent AND dark current increase with bias because of avalanche multiplication maximize difference between photocurrent I p and dark current I d Application: optical communication systems important: sensitivity (depends on gain) response time (bandwidth) [1] 33
4.4. Photodiode devices lattice matched multilayers important: sensitivity (depends on gain) response time (bandwidth) gain bandwidth characteristics limited by transit time of carriers through structure gain increase bandwidth decreased gain bandwidth product as figure of merit [1] 34
4.4. Photodiode devices p i n diode: no gain mechanism ( 1) gain bandwidth product determined by bandwidth (frequency response) dependent on width of depletion region noise is considerably lower than in photoconductor higher dark current, lower dark resistance high thermal noise! and APDs noise multiplication APD: noise by random fluctuations in avalanche process reduced if impact ionization is due to only one type of carrier Si: ability of e - to create EHPs much higher than of holes BUT Si not to be used for optical fibers In 0.55 Ga 0.47 As better BUT higher noise (same ionization rates for electrons and holes) better: InGaAs N Multiplication by holes high effective mass of e - strong scattering of e - [1] 35
4.5. Waveguide structure excellent performance in both high sensitivity and bandwidth light strikes perpendicular to the current transport absorption region can be made quite long (along the photon path light sensitivity) + photogenerated carriers traverse short distance in perpendicular direction short transit time high bandwidth [1] 36
4.6. Fiber optic communications transmission enhanced waveguide = light pipe 5 25µm flexible (relative) distances km Fused silica (SiO 2 ) outer layer Ge doped glass (high index of refraction) - core attenuation coefficient: windows 1.55µm and 1.3µm [1] 37
4.6. Fiber optic communications Fused silica (SiO 2 ) Ge doped glass (high index of refraction) attenuation coefficient: windows 1.55 µm and 1.3 µm Rayleigh (small random inhomogeneities) fluctuations of n IR absorption [1] 38