Array detectors. Hawaii array 2000 x 2000; 17µm; SFD CdHgTe for λ p =2.2µm 40mm x 40mm array (4 attached readout circuits

Transcription

1 Array detectors QWIP array 640 x 512; 25µm; DI λ p = 8.75µm Hawaii array 2000 x 2000; 17µm; SFD CdHgTe for λ p =2.2µm 40mm x 40mm array (4 attached readout circuits 12

2 Optical Detectors Photonic detectors Measure the number of photons Photodiodes: p-n, p-i-n, Schottky Photoconductors Quantum wells Avalanche photodiodes Photomultipliers Thermal detectors Measure the energy Bolometers or thermistors (resistance) Thermocouples or thermopiles (voltage) Golay cells (pressure) Pyrodetectors (electrical charge) 13

3 Two families of detectors Thermal Photonic IR Flux Absorbant Thermometer Thermal insulator Readout Modest sensitivity High time of response Room temperature Low cost IR Flux Semiconductor Readout High sensitivity Short time of response Low temperature (80K) High cost 14

4 Quantum well: Principle and fabrication TEM picture MBE growth GaAs substrate GaAs well Ga 1-x Al x As barriers Al Ga As x 1-x Ga As Al Ga As x 1-x Modulated conduction band Thermal stability Uniformity 3, 4, 6 substrates hν hn d +++ Silicium Silicon! DE c Discrete states in the well Silicon doping e - majority carriers 15

5 QW / QWIP: Choice of the spectral gauge 40 % Al Ga As x 1-x GaAs E 2 E - E (mev) % d E 2 E LIE p µ ( m) E ETENDU 2 10 % X = 5 % d ( Å ) Example for a detector ay 9 µm: d = 5.4 nm ; x = 25 % 16

6 QWIP: Electro-optical principle A QWIP detector can be modeled with a photoconductor approach électron photoexcited electron photoexcité in a quasi-bound dans state un état "quasi-lié" émetteur emitter photocourant photocurrent énergie energy distance +++ donneurs donors Si collecteur collector Blocking emitter contact Inhomogeneous field distribution. 17

7 Parameters of a QWIP pixel Etched grating in GaAs: period, form factor, depth Active layer wells doping number of wells Contact: thickness doping AR coating IR flux Residual substrate 18

8 Coupling grating + concentrator y(µm) x(µm) Thomas Antoni s PhD thesis 19

9 Main parameters of IR detectors SPECTRAL RESPONSE QUANTUM EFFICIENCY NOISE NEP (Noise Equivalent Power) DETECTIVITY NETD (Noise Equivalent Temperature Difference) BLIP PERFORMANCE 20

10 NOISE: definitions rms noise = square root of the variance of a random signal s n = # % $ & 2 S"S( = S 2 2 "S >0 ' The noise associated to independent events sum up in power Main noise sources Thermal (Johnson or Nyquist) Quantum (Shot or Shottky) Generation-Recombination (GR) 1/f (technology) 21

11 REPONSE : R = Q ph /P (.../W) Q ph is the output quantity (voltage, current, etc.), corresponding to the detection of a power P. For a given R, the best detector is the one with lowest output noise q n ( ), i.e. with the lowest input noise equivalent power. NOISE-EQUIVALENT POWER : NEP = q n /R (W) DETECTIVITY : D = 1/NEP (W -1 ) Figures of merit for detectors R, NEP, D, D*, NETD NEP = input power that gives (S/N) out =1 D depends on the square root of the surface A and the bandwidth B of the detector. One can define a D* that allows comparing different detectors D * = A " B NEP (cm Hz 1 2 W #1 ou Jones) NETD : Temperature difference between an object and its environment, which results in a signal equal to the rms noise of the system 22

12 Background Limited Infrared Photodetection BLIP is when all the noise stems from the random fluctuation of the number of photons coming from the background and collected per unit time (Poisson s statistical low) Physical limitation The best IR detectors are close to BLIP A detector all the more close to BLIP that its intrinsic electronic noise (dark current, readout circuit) is small with respect to photonic noise For a given spectral band, the higher D* BLIP (cold background, small aperture), the harder that a real detector can reach it. For a room-temperature background and a sufficient angle of view (ρ>15 ), the 3-5 µm or 8-12 µm detectors are close BLIP D * réel > 50%D * BLIP 23

13 Readout circuit and integration time K 1 K 2 Integration: K 1 on / K 2 off Readout: K 1 off / K 2 on 50 I tot C int NETD (mk) ! pic = 9!m ; "! = 2!m pixel = 25!m ; # = 2 D * = Jones ; T B = 300 K V pol Integration time is limited by: Video frequency Integration capacitance t int (s) 24

14 Quantum detectors: main materials Temperature Spectral band 1 to 3 µm 3 to 5 µm 8 to 12 µm 295 K 1.2 µm : Si 1.8 µm : Ge, InGaAs PbS, HgCdTe PbSe Too noisy 200 to 250 K PbS (up to 3.5 µm) PbS PbSe HgCdTe 77 K No interest HgCdTe InSb QWIP PtSi QWIP HgCdTe PbSnTe < 35 K GeHg CuHg Si : As 25

15 Formation of indium micro-spheres (1) After Indium deposition (2) After reflow process (3) Bump diameter =15 µm ; pitch 25µm; good uniformity on 3 inch wafer 26

16 Hybridation focal plane array on readout integrated circuit QWIP FPA In Bump Si ROIC Before hybridation (Detail of QWIP array) After hybridation (section view) 27

17 Example of focal plane Images Sofradir 28

18 Cryogenics, or the price to pay for performance How to reduce the thermal agitation of the detector and the collected environmental radiation? Cool the detector Limit the field of view by an enclosure with a cooled diaphragm Stirling Cryogenerator T limit > 60 K Joules-Thomson Cryogenerator T limit > 90 K Peltier cooling: TE cooler T limit > 200 K 29

19 QWIP Performances f/1.6 and T CN =300K Spectral response (A/W) D*(100 K) = cm.hz 1/2 /W D*(77 K) = cm.hz 1/2 /W D*(77 K) = cm.hz 1/2 /W D*(50 K) = cm.hz 1/2 /W Wavelength (!m) High performances on the whole IR spectrum 30

20 Thales Optronics Thermal Imagers in 4 Segments 31

21 SOPHIE: Night vision googles Detector: 288 x 4TDI CMT module Δλ = µm F/3, Θ = 4 et 8 288x430 IFOV (elementary fields) NETD = 120mK (noise equivalent temperature difference) Performances Iintegration time: 20µs Frame Frequency: 25Hz NETp = 70mK (temperature difference that can be resolved) 32

22 Semiconductor Lasers Milestones 1962 First GaAs laser diode (pulsed operation, cryogenic temperature) (General Electric Research Labs) 1970 AlGaAs / GaAs DH laser diode (CW, 300K) (Ioffe Institute, Bell Labs) 1974 AlGaAs / GaAs DFB laser diode Diode laser 1976 GaInAsP / InP DH laser diode at 1.2µm (CW, 300K) (Lincoln Labs) 1977 InGaAsP / InP QW laser (Urbana University) n+ GaAs p+ GaAs 1978 AlGaAs / GaAs QW laser (Urbana University) GaAs homojunction laser diode (1962) 1979 InGaAsP / InP VCSEL (pulsed operation, 77K) (Tokyo Institute of Technology) 1984 InGaAs / AlGaAs strained QW laser 1988 AlGaAs / GaAs VCSEL (CW, 300K) (Tokyo Institute of Technology) 33

23 Semiconductor Lasers Milestones Quantum cascade laser 1994 InGaAs / AlInAs / InP Quantum Cascade Laser (pulsed operation, cryogenic temperatures) (Bell Labs) 1995 InGaN/AlGaN/GaN blue laser diode (pulsed operation, cryogenic temperatures) (Nichia Chemicals) INJECTOR 3 2 ELECTRON STOPBAND 1 MINIBAND ACTIVE REGION 1996 InGaN/AlGaN/GaN blue laser diode (CW, 300K) (Nichia Chemicals) 1998 AlGaAs / GaAs Quantum Cascade Laser (pulsed operation, cryogenic temperatures) (Thomson-CSF) 2002 InGaAs / AlInAs / InP Quantum Cascade Laser (CW, 300K) (University of Neuchâtel) Unipolar GaAs/AlGaAs Quantum Cascade Laser (1998) 2002 GaAs / AlAs THz Quantum Cascade Laser (CW, 30K, crossover lasers transistor ) (Scuola Normale Superiore, Pisa) 2007 InAs/AlSb quantum cascade lasers at 2.8µm (low temperature, crossover diode and QC lasers) (University of Montpellier) 34

24 The emission spectrum of semiconductor laser? II-VI compounds? Diode lasers GaN/AlInGaN GaAs/GaInAs GaAs/AlGaInP InP/GaInAsP Wavelength ( µ m) PbTe/PbSe IV-VI GaSb/ AlGaInSbAs QC lasers GaAs/AlGaAs GaInAs/AlInAs/InP THz and mm wave Wavelength ( µ m) 35

25 Basic differences between LD et QCL Population inversion n 2 n2 n 1 n1 Question: how to obtain population inversion? Inversion is not natural, it has to be designed 36

26 Light generation in semiconductors Bipolar recombination (intraband) Photons are the result of the recombination across the gap Electron flow Hole flow Unipolar recombination (Intersubband) Photons are the result of the recombination between quantised states in the conduction band 3 Electron flow 2 1 Electron flow Semiconductor quantum well 37

27 QCL principles of operation 0-order approximation 1 st -order approximation No electron-hole recombination!! 38

28 QCLs are in mid- to far-ir GaInAs/AlInAs Δ Ec =520meV GaInAs 820meV Lateral quantum confinement increase the effective gap and reduces the available energy in the quantum well 39

29 Why the QC laser is in the mid-ir? It is not a choice, but a constrain imposed by the material! è The conduction band discontinuity (ΔE c ) has a very limited value in the technological proven III-V semiconductor GaAs and InP (~ 0.5 mev). è The exploitable energy to produce stimulated photon emission at high temperature is only between 1/2 to 1/3 of the ΔE c 3 E b -E 3 necessary to prevent electron ionisation into the continuum 2 1 ~ 500 mev 0-point energy D 21 necessary for fast evacuation of electron from the n=2 state 40

30 GaP AlAs Energy Gap (ev) GaAs InP GaInAs AlInAs AlSb GaSb InAs 0.0 InSb Lattice Constant (A) 41

31 Quantum wells in III-V semiconductor compounds AlSb/InAs//InAs or GaSb GaAs/AlGaAs//GaAs AlInAs/GaInAs//InP ~ 2000 mev AlSb ~ InAs ~ 390 mev GaAs AlGaAs x=45% 520 mev GaInAs AlInAs m * e = m 0 GaAs/AlAs ~ 1000meV Indirect discontinuity Γ - X only 200meV!! m * e = m 0 AlInAs/GaInAs/InP strain compensated ~ 600/700 mev Even in strain compensated materials no evidence of lateral valleys m * e = m 0 Exact position of the lateral minima is unknown 42

32 Active region: electrons interacting Minigap Active region Injector (doped) Solve first this part Minigap AlInAs barriers InGaAs wells ΔE c =0,52 ev 43

33 Envelope function approximation: the wavefunction is the product of an envelope function and the Bloch periodic part? > = f env >? Bloch> " = f env u Bloch Matrix elements involve mostly the envelope part Intersubband transition in QW s Textbook 1D potential - Electron wavelength (~10nm) "averages" the interface (0.3nm) - Charges are free in the plane -> no Coulomb charging effects - Dominant non-radiative scattering are one particule effect Optical transitions in QW : Kane k p approach at k // =0 Effective mass 44