Transcript Factors Affecting QE and Dark Current in Alkali Cathodes

Factors Affecting QE and Dark
Current in Alkali Cathodes
John Smedley
Brookhaven 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
Photodetector
• High QE in range of interest
• 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
– Dominated by thermal emission
•
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•
•
•
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Spatially Uniform
Large area
Low response to “stray” light
Reproducible
Long lifetime in sealed system
Cheap, easily manufactured
Three Step Model - Semiconductors
1) Excitation of eEmpty States
Reflection, Transmission,
Interference
Energy distribution of excited e-
h
2) Transit to the Surface
Eg
Filled States
Energy
No States
Φ
Ea
e--lattice scattering
mfp ~100 angstroms
many events possible
e--e- scattering (if hν>2Eg)
Spicer’s Magic Window
Random Walk
Monte Carlo
Response Time (sub-ps)
3) Escape surface
Overcome Electron Affinity
Medium
Vacuum
Factors Affecting QE
Reflection
Choice of polarization and angle of incidence
Light traps (microstructures)
Nonproductive absorption
Semiconductor cathodes (especially NEA materials)
Narrow valence band
Work 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 codeposition, 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 cathode
Surface morphology (field enhancement)
Work function
Thermal emission
Temperature
Work Function
I = A T2 exp[-e/(kT)]
Ion bombardment
Vacuum
Work function
Low work function reduces the
threshold photon energy and improves
QE, especially near threshold
But, it increases dark current
=> Optimal work function depends on application
Hamamatsu Tech Note
K2CsSb (Alkali Antimonides)
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 vacuum
Typically sequential (Sb->K->Cs)
Cs deposition used to optimize QE
Oxidation to create Cs-O dipole
Co-deposition increases performance
in tubes
Cathode stable in deposition system (after initial cooldown)
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)
Laser Propagation and Interference
Laser energy in media
0.8
Calculate the amplitude of the
Poynting vector in each media
0.6
Not exponential decay
0.4
543 nm
0.2
2 10
Vacuum
K2CsSb
200nm
-7
4 10
Copper
-7
6 10
-7
8 10
-7
1 10
-6
Monte Carlo Modeling
Thickness dependence @ 543 nm
0.1
0.6
Ref
0.09
trans
0.08
Total QE
0.5
QE w/o R&T
0.4
0.07
0.06
0.05
0.3
0.04
0.03
0.2
0.02
0.1
0.01
0
0
50
100
150
Thickness (nm)
200
0
250
QE
Transmission/Reflection
0.7
Monte Carlo Modeling
Deposition System
Sb K Cs
Sequential deposition with retractable sources (prevents cross-contamination)
Cathode mounted on rotatable linear-motion arm
Typical vacuum 0.02 nTorr (0.1 nTorr during Sb deposition)
Substrate & Recipe
Copper Substrate
Recipe
Following D. Dowell (NIM A356 167)
Substrate
Temperature
Polished Solid
Copper
Stainless
Section
~30 nm Copper
Stainless Steel Sputtered on Glass
Shield
100 Å Sb
150 C
200 Å K
140 C
Cs to optimize QE
135 C
Cool to room temperature as quickly as
possible (~15 min)
4
140
Current
Cs Deposition
Temp
3.5
135
10 min to cool to 100C
Lose 15% of QE
130
2.5
125
2
120
1.5
115
1
110
0.5
105
0
100
0
5
10
15
Time (min)
20
25
30
Substrate Temperature (C)
Photocurrent (μA)
3
Spectral Response
Temperature Dependence
0.1
QE
0.01
On Shield (8 days - after HC test)
On Shield (8 days - after HC test)
0.001
On Shield (8 days - after HC test - T= -77C)
0.0001
1.8
2.3
2.8
3.3
3.8
4.3
hv (eV)
4.8
5.3
5.8
6.3
Position Scan (532 nm)
40
8
24 hrs
30
Photocurrent (μA)
7
Initial Scan
6
20 days
SS Cath
25
After Oxygenation
SS Shield
5
Transmission Mode
20
4
Window
Cu Cath
15
3
10
2
5
1
0
0
460
470
480
Cu transmission ~20%
490
Position (mm)
500
510
Photocurrent - Transmission (μA)
35
Copper vs Stainless
0.1
47.7 mW @ 532 nm
0.526 mA
QE
0.01
On Fork (24 hrs)
On Cathode (24 hrs)
0.001
On Shield (48 hrs)
On Shield (7 days)
High current
0.0001
190
290
390
490
Wavelength (nm)
590
690
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 Basics
Why 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
Cathode/Injector Properties
Quantum Efficiency (QE)

# eemitted
I
QE 
 h
#  incident
P
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: I p 
Qbunch
 bunch
G. Suberlucq, EPAC04, 64
JACoW.org
SUPERFISH simulation
SW, TM010
f = 1298.07726 MHz
Q0 = 7.07x109
Pd = 5.1W
Bmax/Emax = 2.2 mT/(MV/m)
Emax/Ecathode = 1.048
Electric Field
Photoinjector
Emax/Ecathode = 1.048
Bmax/Emax = 2.2 mT/(MV/m)
Pd = 5.1W
Q0 = 7.07x109
f = 1298.07726 MHz
SW, TM010
SUPERFISH simulation
Time
Thin Cathode
QE in reflection mode
QE %
1.4
1.2
1
0.8
0.6
0.4
0.2
0
465
470
475
480
Position in mm
485
490
495
QE Decay, Small Spot
0.026
80 µm FWHM spot on cathode (532 nm)
0.024
1kV bias
0.022
2kV bias
3kV bias
QE
0.02
0.018
0.016
0.014
1.3 mA/mm2 average current density (ERL goal)
0.012
0.01
0
4
9
14
19
24
Hours
28
33
38
43
Linearity and Space Charge
80 µm FWHM spot on cathode