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Factors Affecting QE and Dark Current in Alkali Cathodes
John SmedleyBrookhaven National Laboratory
Outline
• Desirable Photocathode Properties– Low light detection– Accelerator cathodes
• Factors Affecting Performance• Practical Experience with K2CsSb
– Monte Carlo modeling– Cathode studies
Photoinjector
What makes a good photocathode?
Photoinjector• High QE at a convenient • 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 10-50 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
Energy
Medium Vacuum
Φ
Three Step Model - Semiconductors
Filled S
tatesE
mpty S
tates
h
1) Excitation of e-
Reflection, Transmission, Interference
Energy distribution of excited e-
2) Transit to the Surfacee--lattice scattering
mfp ~100 angstromsmany events possible
e--e- scattering (if hν>2Eg)Spicer’s Magic Window
Random WalkMonte CarloResponse Time (sub-ps)
3) Escape surface Overcome Electron Affinity
Laser
No S
tates
Eg
Ea
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”, <E<2Egap
Minimize photon absorption length (surface plasmons)Good crystals – minimize defect and impurity scattering
Deposition parameters Substrate material, cathode thickness, sequential vs co-deposition, substrate temperature, cooling time, oxide layer formation
Vacuum environment Ion back-bombardment, electron stimulated desorption, chemical poisoning
Operating environment Thermal stability, space charge
Factors Affecting Dark Current
Field emission Electric field at cathodeSurface morphology (field enhancement)Work function
Thermal emission TemperatureWork FunctionI = A T2 exp[-e/(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
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 1.9-2.1eV, Eg= 1.1-1.2 eV
Good QE (4% -12% @ 532 nm, >30% @ 355nm)
Deposited in <10-10 Torr vacuumTypically sequential (Sb->K->Cs)Cs deposition used to optimize QEOxidation to create Cs-O dipoleCo-deposition increases performancein tubes
Cathode stable in deposition system (after initial cooldown)
Laser Propagation and Interference
210-7 410-7 610-7 810-7 110-6
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
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)
Tra
nsm
issi
on
/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
Ref
trans
Total QE
QE w/o R&T
Monte Carlo Modeling
Monte Carlo Modeling
Deposition System
Sequential deposition with retractable sources (prevents cross-contamination)Cathode mounted on rotatable linear-motion armTypical vacuum 0.02 nTorr (0.1 nTorr during Sb deposition)
Sb K Cs
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 C
200 Å K 140 C
Cs to optimize QE 135 C
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
trat
e Te
mpe
ratu
re (C
)
Phot
ocur
rent
(μA
)
Time (min)
Current Cs Deposition Temp
10 min to cool to 100CLose 15% of QE
Spectral Response
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)
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
issi
on (
μA)
Phot
ocur
rent
(μA
)
Position (mm)
Initial Scan
24 hrs
20 days
After Oxygenation
Transmission ModeSS Cath
Window Cu Cath
SS Shield
Cu transmission ~20%
Position Scan (532 nm)
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
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 QE
Thanks for your attention!
Additional Slides
Photoinjector BasicsWhy use a Photoinjector?
Electron beam properties determined by laser– Timing and repetition rate– Spatial Profile– Bunch length and temporal
profile (Sub-ps bunches are possible)
High peak current density 105 A/cm2
Low emittance/temperature<0.2 µm-rad
G. Suberlucq, EPAC04, 64JACoW.org
Cathode/Injector PropertiesQuantum Efficiency (QE)
Lifetime: time (or charge) required for QE to drop to 1/e of initial
Response Time: time required for excited electrons to escape
Peak Current:
P
Ih
eQE
incident
emitted
#
#
bunch
bunchp
QI
Time
Ele
ctr
ic F
ield
Time
Ele
ctr
ic F
ield
PhotoinjectorSW, TM010f = 1298.07726 MHzQ0 = 7.07x109
Pd = 5.1WBmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulationSW, TM010f = 1298.07726 MHzQ0 = 7.07x109
Pd = 5.1WBmax/Emax = 2.2 mT/(MV/m)Emax/Ecathode = 1.048
SUPERFISH simulation
Thin Cathode
QE in reflection mode
0
0.2
0.4
0.6
0.8
1
1.2
1.4
465 470 475 480 485 490 495
Position in mm
QE
%
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)
Linearity and Space Charge
80 µm FWHM spot on cathode