factors affecting qe and dark current in alkali cathodes...good qe (4% 12% @ 532 nm, >30% @ 355nm)...
TRANSCRIPT
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Factors Affecting QE and Dark Current in Alkali Cathodes
John SmedleyBrookhaven National Laboratory
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Outline• Desirable Photocathode Properties
– Low light detection– Accelerator cathodes
• Factors Affecting Performance• Practical Experience with K2CsSb
– Monte Carlo modeling– Cathode studies
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Photoinjector
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What makes a good photocathode?Photoinjector
• High QE at a convenient λ• Low dark current
– Dominated by field emission• Spatially Uniform• Long lifetime in challenging
vacuum environment– Chemical poisoning– Ion bombardment
• Low intrinsic energy spread (thermal emittance)
• Typical pulse length of 1050 ps
• Peak current density can be >10kA/cm2
Photodetector• High QE in range of interest• Low dark current
– Dominated by thermal emission
• Spatially Uniform• Large area• Low response to “stray”
light• Reproducible• Long lifetime in sealed
system• Cheap, easily manufactured
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Energy
Medium Vacuum
Φ
Three Step Model Semiconductors
Filled StatesEm
pty States
hν
1) Excitation of eReflection, Transmission,
InterferenceEnergy distribution of excited e
2) Transit to the Surfaceelattice scattering
mfp ~100 angstromsmany events possible
ee scattering (if hν>2Eg)Spicer’s Magic Window
Random WalkMonte CarloResponse Time (subps)
3) Escape surface Overcome Electron Affinity
Laser
No States
Eg
Ea
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Factors Affecting QE
Reflection Choice of polarization and angle of incidenceLight traps (microstructures)
Nonproductive absorption Semiconductor cathodes (especially NEA materials)Narrow valence bandWork function reduction (Schottky effect, dipole layers)
Electron scattering(electron mfp)
Stay within the “magic window”, φ
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Factors Affecting Dark Current
Field emission Electric field at cathodeSurface morphology (field enhancement)Work function
Thermal emission TemperatureWork FunctionI = A T2 exp[eφ/(kT)]
Ion bombardment VacuumWork function
Low work function reduces the threshold photon energy and improvesQE, especially near threshold
But, it increases dark current=> Optimal work function depends on application
Hamamatsu Tech Note
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K2CsSb (Alkali Antimonides)
D. H. Dowell et al., Appl. Phys. Lett., 63, 2035 (1993)C. Ghosh and B.P. Varma, “J. Appl. Phys., 49, 4549 (1978)A.R.H.F. Ettema and R.A. de Groot, Phys. Rev. B 66, 115102 (2002)
Work function 1.92.1eV, Eg= 1.11.2 eVGood QE (4% 12% @ 532 nm, >30% @ 355nm)
Deposited in K>Cs)Cs deposition used to optimize QEOxidation to create CsO dipoleCodeposition increases performancein tubes
Cathode stable in deposition system (after initial cooldown)
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Laser Propagation and Interference
2 10 7 4 10 7 6 10 7 8 10 7 1 10 6
0.2
0.4
0.6
0.8
Vacuum K2CsSb200nm
Copper
543 nm
Laser energy in media
Not exponential decay
Calculate the amplitude of the Poynting vector in each media
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Thickness dependence @ 543 nm
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 50 100 150 200 250
Thickness (nm)
Tran
smis
sion
/Ref
lect
ion
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.1
QE
ReftransTotal QEQE w/o R&T
Monte Carlo Modeling
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Monte Carlo Modeling
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Deposition System
Sequential deposition with retractable sources (prevents crosscontamination)Cathode mounted on rotatable linearmotion armTypical vacuum 0.02 nTorr (0.1 nTorr during Sb deposition)
Sb K Cs
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Substrate & Recipe
Copper Substrate
Polished Solid Copper
~30 nm Copper Sputtered on Glass
RecipeFollowing D. Dowell (NIM A356 167)
Cool to room temperature as quickly as possible (~15 min)
Stainless Steel Shield
Stainless Section
Substrate Temperature
100 Å Sb 150 C200 Å K 140 C
Cs to optimize QE
135 C
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100
105
110
115
120
125
130
135
140
0
0.5
1
1.5
2
2.5
3
3.5
4
0 5 10 15 20 25 30
Subs
trate
Tem
pera
ture
(C)
Phot
ocur
rent
(μA)
Time (min)
Current Cs Deposition Temp
10 min to cool to 100CLose 15% of QE
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Spectral Response
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Temperature Dependence
0.0001
0.001
0.01
0.1
1.8 2.3 2.8 3.3 3.8 4.3 4.8 5.3 5.8 6.3
QE
hv (eV)
On Shield (8 days after HC test)
On Shield (8 days after HC test)
On Shield (8 days after HC test T= 77C)
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0
1
2
3
4
5
6
7
8
0
5
10
15
20
25
30
35
40
460 470 480 490 500 510
Phot
ocur
rent
Tr
ansm
issio
n (
μA)Phot
ocur
rent
(μA)
Position (mm)
Initial Scan24 hrs20 daysAfter OxygenationTransmission ModeSS Cath
Window Cu Cath
SS Shield
Cu transmission ~20%
Position Scan (532 nm)
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0.0001
0.001
0.01
0.1
190 290 390 490 590 690
QE
Wavelength (nm)
On Fork (24 hrs)
On Cathode (24 hrs)
On Shield (48 hrs)
On Shield (7 days)
High current
47.7 mW @ 532 nm0.526 mA
Copper vs Stainless
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Summary• Alkali Antimonide cathodes have good QE in the visible and near
UV– Narrow valance band from Sb 5p level– Band gap depends on which alkali metals used– Work function depends on surface termination (and metals
used)– May be room for improvement by growing better crystals
• Optimal work function depends on wavelength range of interest• For thin cathodes, it may be possible to enhance the QE by tailoring
the thickness to improve absorption near emission surface • Practical aspects, such a choice of substrate material, surface finish
of substrate, and cooling rate after deposition can have a dramatic effect on the QEThanks for your attention!
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Additional Slides
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Photoinjector BasicsWhy use a Photoinjector?Electron beam properties
determined by laser– Timing and repetition
rate– Spatial Profile– Bunch length and
temporal profile (Subps bunches are possible)
High peak current density 105 A/cm2
Low emittance/temperature
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Time
Elec
tric
Fiel
d
Time
Elec
tric
Fiel
d
PhotoinjectorSW, TM010f = 1298.07726 MHzQ0 = 7.07x109Pd = 5.1W
Bmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulationSW, TM010f = 1298.07726 MHzQ0 = 7.07x109Pd = 5.1W
Bmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulation
SW, TM010f = 1298.07726 MHzQ0 = 7.07x109Pd = 5.1W
Bmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulationSW, TM010f = 1298.07726 MHzQ0 = 7.07x109Pd = 5.1W
Bmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulation
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Thin Cathode
QE in reflection mode
00.20.40.60.8
11.21.4
465 470 475 480 485 490 495Position in mm
QE %
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QE Decay, Small Spot
0.01
0.012
0.014
0.016
0.018
0.02
0.022
0.024
0.026
0 4 9 14 19 24 28 33 38 43
QE
Hours
1kV bias
2kV bias
3kV bias
1.3 mA/mm2 average current density (ERL goal)
80 µm FWHM spot on cathode (532 nm)
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Linearity and Space Charge
80 µm FWHM spot on cathode