Photodetectors and their Spectral Ranges · from:”Photodetectors”, by S.Donati, Prentice Hall...

from:”Photodetectors”, by S.Donati, Prentice Hall 2000

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 10µm 100µm (λ) ___|_____________|_____________|_____________|_____________|_ __photoemission ____

____internal photoelectri c effect _____

_____________________thermal________________________________

Photodetectors and their Spectral Ranges

from:”Photodetectors”, by S.Donati, Prentice Hall 2000 2

Detectors based on Photoelectric effect

DET

I

C Rbb

P

VV

Power collected P = hν F

is a f lux F of photons of energy hν

Output current I = e F’

is a f lux 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 eff iciency (electrons-to-photons)

and σ = I/P= η (λe /hc) = η (λ /1.24) [A/W] is spectral sensitivity (current out -to-power in)


Detectors based on Photoelectric effect 2

To trade photons for electrons we need a materialrequiring an energy not larger than the photonenergy, so hν≥Ecc , where energy Ecc for the chargecarrier generation is EW (work function) in externaland EG(bandgap) in internal photoemission.This is the threshold condition: hc/λ≥Ecc or λ ≤ λt = hc/eEcc = 1.24 / Ecc (eV)

In alkaline antimonides, EW ≈1.2-3.0 eV, and λt ≈1-0.4 µm (blue to NIR)ternaries (InGaAs) EG ≈0.75 eV, λt ≈1.8 µm InSb EG ≈0.25 eV, λt ≈5 µm (MIR) HgCdTe EG ≈0.08 eV, λt ≈16 µm (FIR)



λλ t

σ = I/P(A/W)

1.24

1.0

η=1

η=0.5

real response

threshold

general response curve of a quantum detector:at P=cons, current increases linearly with λ, thensharply decreases to 0 at the photoelectric thresholda real detector has a curve rather than a triangle



Once produced, we shall remove charge carriersfast, so we need very thin layers to cross or afavorable electric fieldhelping collection photocathodes

pn junction in a diode

base-collector junct.of BJT

gate-drain junct in a FET

depleted layer in a MOS

3rd junct in a SCR

applied field in a resistance


input windowelectrode

d

z

TRANSMISSIONPHOTOCATHODE

transparentelectrode

(-)

(-)

window

REFLECTIONPHOTOCATHODE

photocathode

anode

anode

photocathode

photocathode

TYPES OF PHOTOCATHODES


PHOTOEMISSION PROCESS

vacuum level

conduction band

valence band

Fermi level

Eg

E

EA

p

WE

P = P(0) exp -αz

hν

pair production level

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

hν≥Eg+EA

λt = hc/(Eg+EA)λt [µm]= 1.24/E[eV]

Ep >> EA


ABSORPTION and DIFFUSION

0.2 0.3 0.4 0.5 0.7 1.0 1.5 2.0

1.5 1.023456

10

10

10

10

10

10

10

100

1000

0.1

1

0.01

1

2

3

4

6

5

hν (eV)

λ (µm)

Cs Sb3

Cs Te

CsNaKSb

GaInAs

0.7

(cm )α

-1

Latt

(µm)

WAVELENGTH

(a) (b) (c) (d)

diffusion to the surfaceabsorption


Photocathode responses

WAVELENGTH λ (nm)

100

10

1

0.1

2

5

3

4

6

8

2

5

3

4

6

8

2

5

3

4

6

8

100 150 200 300 500 1000 1200

1

10

0.025

25

5

2.5

0.5

0.25

600 800

0.1

0.05

Cs I

CsTe

NaKSb(S24)

CsSb(S11)

CsNaKSb(S20 ERMA)

AgOCs(S1)

η=50%

SP

EC

TR

AL

SE

NS

ITIV

ITY

σ

(mA

/W)

InGaAs :Cs

GaAs:Cs

1500 1700

100 150 200 300

1.0

0.1

TR

AN

SM

ISS

ION

UV glass

borosilicate glass

(3m

m w

ind

ow

)

MgF fused silica

2

5

3

4

6

8

400

REFLECTTRANSM

2

Ni, W


EFFICIENCY CALCULATI ON

p(L) = (1/Λ) exp -L/Λ ; p(θ)= 1/2π p(z) = gauss (z,Λ) = [1/√(2π)Λ] exp -z2/2Λ2

⟨l⟩ = lf √(∆E/∆ef)

ηe = ∫0-∞ Π(z) α exp -αz dz,

p1(E,z) = p(E-∆e) gauss(z,Λ)

p2(E,z) = p(E-k∆e) gauss(z,√2Λ)

………...

pk(E,z) = p(E-k∆e) gauss(z,√kΛ)

Π(z) =∫EA-∞ dE ∫z-∞ [Σk=0-∞ pk(E,z')] dz'

(transmission photocathode)

(reflection photocathode)

Π(d-z) =∫EA-∞ dE ∫(d-z)-∞ [Σk=0-∞ pk(E,z')] dz'

Π(z) = escape probability


BAND BENDING AT THESURFACE

(a) (b)

(e)(d)(c)

Ep AE

EA

p : n n : pn : np : p

Ep


NEGATIVE AFFINITY

Cs O (≈1nm)

2 Cs(<1nm)

E A

TUNNEL

VACUUM

p - GaAs

EgE

A

TUNNEL

p - GaAs

VACUUM

Eg


PHOTOCATHODE PARAMETERSPHOTOCATHODE PARAMETERS_________________________________________________________________________________________________

material Eg E

A E

E

p λs α ηmax Jdark

SD (eV) (eV) (eV) (eV) (µm) (µm-1) (%) (A/cm2)_________________________________________________________________________________________________

Na3 Sb 1.1 2.2 3.3 <4.3 .37 60 2K3 Sb 1.1 1.5 2.6 <3.7 .48 30 7Rb3 Sb 1.0 1.2 2.2 3.0 .57 30 10Cs3 Sb S-11 1.6 0.45 2.05 2.0 .60 50 25 1 fNa2 K Sb S-24 1.0 1.0 2.0 3.0 .62 100 30 <0.1f[Cs]Na2K Sb S-20 1.0 0.55 1.55 3.0 .80 100 35 1 fAg-O-Cs S-1 ≈ 1 1.2 1 1 pCs2Te 3.7 5.0 .31 30GaAs [Cs2O] 1.42 <0 1.4 .87 25 0.3 fGaxIn1-xAs [Cs] 1.1 <0 1.1 1.1 10 5 fother semiconductors:Si 1.1 4.1 5.2 1.8 0.04Ge 0.7 4.5 5.2 1.5-2 0.08_________________________________________________________________________________________________

Notes: SD = standard international (EIA) designation of spectral response and window type; ηmax = quantum peak efficiency (at λ=λmax) for reflection photocathodes; α = optical absorp-tion coeffic ient at λ=λmax; J = dark current densi ty, in pico- or femto-ampere per cm2 of photocathode surface (at 300 K).


TRANSMISSION PHOTOCATHODES

100

10

1

0.1

2

5

34

68

2

5

34

68

2

5

34

68

100 150 200 300 400 500 1000 1200

1

10

0.025

25

5

2.5

0.5

0.25

600 800

0.1

0.05

Cs I

CsTe

NaKSb(S24)

CsSb(S11)

CsNaKSb(S20)

CsNaKSb

AgOCs(S1)

(gla

ss)

SP

EC

TR

AL

SE

NS

ITIV

ITY

σ

(m

A/W

)

WAVELENGTH λ (nm)

η=50%

(S20 ERMA)


REFLECTION PHOTOCATHODES: UV-cutoff of a 3-mm thick window

CsSb(S13)

CsNaKSb

CsNaKSb(S20 ERMA)

AgOCs

NaKSb100

10

2

5

34

68

2

5

34

68

300 400 500 1000 1200

1

10

0.025

25

5

2.5

0.5

0.25

600 800

0.1

0.05

2

5

34

68

1.0

0.1

TR

AN

SM

ISS

ION

(fu

se

d s

ilica

)

UV

gla

ss

bo

rosi

lica

te g

lass

100 150 200

WAVELENGTH λ (nm)

(3

mm

) w

ind

ow

MgF

2

η=50%

σ

(m

A/W

)

fuse

d s

ilica


REFLECTION PHOTOCATHODES

SP

EC

TR

AL

SE

NS

ITIV

ITY

σ

(m

A/W

)

WAVELENGTH λ (nm)

InGaAs

GaAs:Cs100

10

1

0.1

2

5

34

68

100 150 200 300 400 500 1000 1200

1

10

0.025

25

5

2.5

0.5

0.25

600 800

0.1

0.05

2

5

34

68

2

5

34

68

η=50%

Cs I

CsTeCsSb(S19)

CsNaKSb(S-20)

AgOCs

CsNaKSb(S20 ERMA)

:Cs


TEMPERATURE COEFFICIENT OFSPECTRAL SENSITI VI TY

0.5

100 150 200 300 400 500 1000600 800

WAVELENGTH λ (nm)

1200

1.0

-0.5

0

α

te

mp

era

ture

co

eff

icie

nt

(

%/°

C)

σ

CsTeNaKSb(S-24)

CsNaKSb(S-20)

ασ = (1/σ) dσ/dT


DARK CURRENT vs WORKFUNCTION

DA

RK

CU

RR

EN

T D

EN

SIT

Y (

A/c

m )2

1p

1f

1a

da

rk p

ho

toe

lect

ron

ra

te (

s

cm

)-1

-2

10

103

6

1.21.0 1.4 1.81.6

E +E (eV)Ag

1100 1000 900 800 700

λ (nm)t

InGaAs

S-24

S-20

S-1

1


DARK CURRENT TEMPERATURE COEFFICIENT

DA

RK

CU

RR

EN

T D

EN

SIT

Y

(

A/c

m )

2 1p

1f

1a

da

rk p

ho

toe

lect

ron

ra

te

(

s

cm

)

-1 -2

10

10

1

3

6

TEMPERATURE (°C)

-50 -30 -10 3010

InGaAs

S-24

S-20

50

αJ = (1/Jd) dJd/dT = (2+ EW/kT)/T ≈ 0.34 (2+ EW/kT) [%/°C]


PHOTOCATHODE FABRICATI ON

Common f eatures: a high-vacuum process (10-6 torr) surface contaminants control very critical medium-temperature thin f ilm deposition

Bi- and tri-alkaline fabrication:

Sb evaporated f irst, (6 nm in transm. photocath), K in the stoichiometric ratio (K3SB) ** then Na adding K and Sb in turn to have Na2KSb ** last Cs or Cs-O ** ** = maximizing photoresponse


PHOTOCATHODE FABRICATI ON

Typical apparatus for photocatode f abrication

Picture

to be

added


PHOTOTUBES (or PHOTOTUBES (or vacuum photodiodesvacuum photodiodes))

PT with hemicylindricalreflection photocathode

(left) and wi thtransmission

photocathode on a planeinput window


PHOTOTUBESPHOTOTUBES

V /I characteristics

space charge regime

Ja = (4ε0/9d2) (2e/m)1/2 V ak3/2

C RσP

PH

A

PH

A

Vbb

R

+

bias and eqv circuit

saturation regime

iP = 0.6 lm

0.4

0.2

0

0 40 80 120

akAnode voltage V (V)

Ano

de c

urre

nt I

(µ A

)

3

2

1

0

R=40 M Ω


PHOTOTUBES: PHOTOTUBES: speed speed of of responseresponse

Transit time:

τd = d (2m/eVak)1/2 = 33.7 ns d[cm] (Vak)-1/2

Dispersion: ∆τ is a fraction of τd

Frequency cutoff: f2=0.44/∆τ (intrinsic cutoff), or f2=1/2πRCa (extrinsic cutoff)

40

60

80

100 200 500 1000 2000

100

200

400

ANODE VOLTAGE (V)

TIM

E

(ps)

20

∆τ

τ d


TY PICAL FAST PHOTOTUBETY PICAL FAST PHOTOTUBE

A fast phototube (rise time 100 ps or bandwidth 3 GHz) wi th transmission photo-cathode (S-1, S-11 or S-20) on a glass or quartz window and 50-Ohm output

electrode. Top: device structure; bottom: bias circuit. With the field grid, speed ofresponse is l imited by the dispersion ∆t rather than by the transit time τd


GAS PHOTOTUBEGAS PHOTOTUBE

Ionization in a low-pressure gas f illi ng the tube is a mechanism to increasephotoelectron number. Internal gain is typically G=5-20

Gas phototubes are used in industrial flame control

0.04

0.02

0

0 40 80 120

akAnode voltage V (V)

Ano

de c

urre

nt I

(

µA

)

6

4

2

0

5 MΩ

160

P = 0.06 mW

10 MΩ

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Photodetectors and their Spectral Ranges · from:”Photodetectors”, by S.Donati, Prentice Hall...

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