Photodetectors and their Spectral Ranges

Transcription

1 Photodetectors and their Spectral Ranges SINGLE ELEMENT IMAGE - photoemission devices vacuum photodiode pickup tubes (or external gas photodiode image intensifiers photoelectric devices) photomultiplier and converters - internal photoelectric semiconductor photodiode CCDs devices avalanche photodiode phototransistor (BJT, FET) photoresistance vidicon - thermal detectors thermocouple (or photopile) thermistor (or bolometer) uncooled IR FPA pyroelectric IR vidicon - weak interaction photon drag, Golay cell detectors photoelectromagnetic point contact diode 0.1µm 1µm µm 0µm (λ) photoemission internal photoelectric effect thermal

2 Detectors based on Photoelectric effect P V bb DET C R I V Power collected P = hν F is a flux F of photons of energy hν Output current I = e F is a flux F of electrons of charge e Then, current is proportional to power, I/P= σ = e F / hν F = η (e /hν) where η = F /F is quantum efficiency (electrons-to-photons) and σ = I/P= η (λe /hc) = η (λ /1.) [A/W] is spectral sensitivity (current out -to-power in)

3 Detectors based on Photoelectric effect To trade photons for electrons we need a material requiring an energy not larger than the photon energy, so hν E cc, where energy E cc for the charge carrier generation is E W (work function) in external and E G (bandgap) in internal photoemission. This is the threshold condition: hc/λ E cc or λ λ t = hc/ee cc = 1. / E cc (ev) In alkaline antimonides, E W ev, and λ t 1-0. µm (blue to NIR) ternaries (InGaAs) E G 0.7 ev, λ t 1. µm InSb E G 0. ev, λ t µm (MIR) HgCdTe E G 0.0 ev, λ t 1 µm (FIR)

4 Detectors based on Photoelectric effect σ = I/P (A/W) threshold η=1 1.0 η=0. real response 1. λ t λ general response curve of a quantum detector: at P=cons, current increases linearly with λ, then sharply decreases to 0 at the photoelectric threshold a real detector has a curve rather than a triangle

5 Detectors based on Photoelectric effect Once produced, we shall remove charge carriers fast, so we need very thin layers to cross or a favorable electric field helping collection photocathodes pn junction in a diode base-collector junct.of BJT gate-drain junct in a FET depleted layer in a MOS rd junct in a SCR applied f ield in a resistance

6 TYPES OF PHOTOCATHODES REFLECTION PHOTOCATHODE (-) electrode photocathode anode input window photocathode TRANSMISSION PHOTOCATHODE transparent electrode (-) anode photocathode z d win dow

7 PHOTOEMISSION PROCESS i) photon absorption and generation of an electron-hole pair ii) diffusion of the electron to the surface iii) emission of the electron in the vacuum pair production level vacuum level E p conduction band E A hν E g +E A E W hν E g λ t = hc/(e g +E A ) λ t [µm] = 1./E [ev ] Fermi level valence band E p >> E A P = P(0) exp -αz 7

8 ABSORPTION and DIFFUSION hν (ev) CsNaKSb Cs Te GaInAs 0.1 α (cm-1) 1 L att (µm) Cs Sb WAVELENGTH λ (µm) (a) (b) (c) (d) absorption diff usion to the surface

9 Photocathode responses SPEC TRAL SENSITIVITY σ (ma/w) 0 T RANSMISSION (mm window) MgF Cs I REFLECT TRANSM fused silica UV glass CsTe η=0% Ni, W borosilicate glass CsSb (S11) AgOCs (S1) WAVELENGTH NaKSb (S) GaAs:Cs InGaAs :Cs CsNaKSb (S0 ERMA) λ (nm)

10 EFFICIENCY CALCULATION p(l) = (1/Λ) exp -L/Λ ; p(θ)= 1/π p(z) = gauss (z,λ) = [1/ (π)λ] exp -z /Λ l = l f ( E/ e f ) Π(z) = escape probability η e = 0- Π(z) α exp -αz dz, p 1 (E,z) = p(e- e) gauss(z,λ) p (E,z) = p(e-k e) gauss(z, Λ)... p k (E,z) = p(e-k e) gauss(z, kλ) Π(z) = EA- de z- [Σ k=0- p k (E,z')] dz' Π(d-z) = EA- de (d-z)- [Σ k=0- p k (E,z')] dz' (reflection photocathode) (transmission photocathode)

11 BAND BENDING AT THE SURFACE E A E p E A E p (a) (b) p : n n : n n : p p : p (c) (d) (e) 11

12 NEGATIVE AFFINITY TUNNEL TUNNEL VACUUM VACUUM E A E g E A E g Cs (<1nm) p - GaAs Cs O ( 1nm) p - GaAs 1

13 PHOTOCATHODE PARAMETERS material E g E A E E p λ s α η max J dark SD (ev) (ev) (ev) (ev) (µm) (µm -1 ) (%) (A/cm ) Na Sb <..7 0 K Sb < Rb Sb Cs Sb S f Na K Sb S <0.1f [Cs]Na K Sb S f Ag-O-Cs S p Cs Te GaAs [Cs O] 1. < f Ga x In 1-x As [Cs] 1.1 < f other semiconductors: Si Ge Notes: SD = standard international (EIA) designation of spectral response and window type; η max = quantum peak efficiency (at λ=λ max ) for reflection photocathodes; α = optical absorp-tion coefficient at λ=λ max ; J = dark current density, in pico- or femto-ampere per cm of photocathode surface (at 00 K). 1

14 TRANSMISSION PHOTOCATHODES SPECTRAL SENSITIVITY σ (ma/w) 0 1 η=0% CsTe Cs I (glass) CsSb (S11) WAVELENGTH λ (nm) CsNaKSb (S0) AgOCs (S1) CsNaKSb (S0 ERMA) NaKSb (S)

15 REFLECTION PHOTOCATHODES: UV-cutoff of a -mm thick window σ (ma/w) 0 (fused silica) η=0% NaKSb CsSb (S1) CsNaKSb CsNaKSb (S0 ERMA) TRANSMISSION (mm) window 0.1 MgF fused silica UV glass borosilicate glass AgOCs WAVELENGTH λ (nm) 1

16 REFLECTION PHOTOCATHODES SPECTRAL SENSITIVITY σ (ma/w) 0 1 CsTe Cs I η=0% CsNaKSb (S0 ERMA) WAVELENGTH CsSb (S19) λ (nm) GaAs:Cs CsNaKSb (S-0) InGaAs :Cs AgOCs

17 TEMPERATURE COEFFICIENT OF SPECTRAL SENSITIVITY α σ = (1/σ) dσ/dt α temperature coefficient (%/ C) σ CsTe NaKSb (S-) CsNaKSb (S-0) WAVELENGTH λ (nm) 17

18 DARK CURRENT vs WORKFUNCTION λ (nm) t DARK CURRENT DENSITY (A/cm ) 1p 1f 1a S E +E g InGaAs A (ev) S-0 S dark photoelectron rate (s cm ) 1

19 DARK CURRENT TEMPERATURE COEFFICIENT DARK CURRENT DENSITY (A/cm ) 1p 1f 1a InGaAs S-0 S- 1 dark photoelectron rate -1 - (s cm ) TEMPERATURE ( C) α J = (1/J d ) dj d /dt = (+ E W /kt)/t 0. (+ E W /kt) [%/ C] 0 19

20 PHOTOCATHODE FABRICATION Common features: a high-vacuum process ( - torr) surface contaminants control very critical medium-temperature thin film deposition Bi- and tri-alkaline fabrication: Sb evaporated first, ( nm in transm. photocath), K in the stoichiometric ratio (K SB) ** then Na adding K and Sb in turn to have Na KSb ** last Cs or Cs-O ** ** = maximizing photoresponse 0

21 PHOTOCATHODE FABRICATION Typical apparatus for photocatode fabrication Picture to be added 1

22 PHOTOTUBES (or vacuum photodiodes) PT with hemicylindrical reflection photocathode (left) and with transmission photocathode on a plane input window

23 PHOTOTUBES space charge regime J a = (ε 0 /9d ) (e/m) 1/ V ak / saturation regime P = 0. lm i V/I characteristics Anode current I ( µa) 1 R=0 M Ω bias and eqv circuit Anode voltage V (V) ak + V bb R A PH A σp C R PH 0

24 PHOTOTUBES: speed of response Transit time: τ d = d (m/ev ak ) 1/ =.7 ns d [cm] (V ak ) -1/ Dispersion: τ is a fraction of τ d Frequency cutoff: f =0./ τ (intrinsic cutoff), or f =1/πRC a (extrinsic cutoff) 00 TIME (ps) τ d τ ANODE VOLTAGE (V)

25 TYPICAL FAST PHOTOTUBE A fast phototube (rise time 0 ps or bandwidth GHz) with transmission photocathode (S-1, S-11 or S-0) on a glass or quartz window and 0-Ohm output electrode. Top: device structure; bottom: bias circuit. With the field grid, speed of response is limited by the dispersion t rather than by the transit time τ d

26 GAS PHOTOTUBE Ionization in a low-pressure gas filling the tube is a mechanism to increase photoelectron number. Internal gain is typically G=-0 P = 0.0 mw 0.0 µa ) Anode current I ( M Ω M Ω Anode voltage V (V) ak 10 Gas phototubes are used in industrial f lame control