K 2 CsSb Cathode Development
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K 2 CsSb Cathode Development John Smedley Triveni Rao Andrew Burrill BNL ERL needs – 50 mA, 7 mm dia., 1.3 mA/mm 2
description
K 2 CsSb Cathode Development. John Smedley Triveni Rao Andrew Burrill BNL ERL needs – 50 mA, 7 mm dia., 1.3 mA/mm 2. K 2 CsSb (Alkali Antimonides). Work function 1.9eV, E g = 1.2 eV Good QE (4% -12% @ 532 nm, >30% @ 355nm) Deposited in 10 -11 Torr vacuum - PowerPoint PPT Presentation
Transcript of K 2 CsSb Cathode Development
K2CsSb Cathode Development
John SmedleyTriveni Rao
Andrew Burrill
BNL ERL needs – 50 mA, 7 mm dia., 1.3 mA/mm2
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 1.9eV, Eg= 1.2 eVGood QE (4% -12% @ 532 nm,
>30% @ 355nm)Deposited in 10-11 Torr vacuum
Typically sequential (Sb->K->Cs); Cs deposition used to optimize QEOxidation to create Cs-O dipoleCo-deposition increases performance in tubes
Cathode stable in deposition system (after initial cooldown)
0.01
0.1
1
2.2 2.4 2.6 2.8 3 3.2
QE
photon energy [eV]
QE vs Cathode ThicknessMonte Carlo Model
50 nm 200 nm
20 nm 20 nm
10 nm
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
Deposition System
Laser access from bottom during depositionfrom front and back for measurementAnode for HV bias (2.5 cm gap, up to 5 kV)
Substrate
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
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
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 (μ
A)
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
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 12 24 36 48 60 720.019
0.02
0.021
0.022
0.023
0.024
Hours
QE
(532
nm
)QE Decay
500 V bias3 mm diameter spotCurrent ~ 60µA0.07 nTorr
Linearity and Space Charge
80 µm FWHM spot on cathode
QE Decay, Small Spot
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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 current density1.5x105 C/mm2
80 µm FWHM spot on cathode
Conclusions
• Cathodes have better QE on Stainless than on Cu• QE decreases 15% during cathode cool-off• QE at 355 nm is 19%, better than the 10% required• ERL current density has been met (1.3 mA/mm2)• QE does not change in storage (even illuminated)
– Ion bombardment seems to limit lifetime under bias• QE at -77C same as at 20C• Charge density of 1.5x105 C/mm2 extracted from a
small (80 µm fwhm) spot; 15 C extracted from a 3 mm fwhm spot; both with small but measurable QE loss
Thanks!
Deposition Notes• Initial vacuum <0.1 nTorr
– Primary contaminants H2 and Ar• Source to substrate distance of ~45 mm• Antimony and Potassium deposition rates set by
crystal monitor– Crystal monitor to source spacing ~70mm
• QE measured (@ 532 nm) during Cs deposition• Substrate takes 15 min to reach room temperature• QE drops rapidly during this period
– Initial QE decay occurs prior to QE map, so we may miss peak QE
57:07 00:00 02:52 05:45 08:38 11:31 14:24 17:16 20:09
3.33E-14
3.33E-13
3.33E-12
3.33E-11
Time (min)
O2
Parti
al P
ress
ure
(Tor
r)
Alkali Sources
• Initial attempts with Saes Getters Chromate sources– Could not obtain evaporation below 10A (should be <6.5A)
• Used Alvasources– Pure Alkali sources, with In seal– Two stage heating cycle– Two geometries – line and V– Ar fill gas produces gas load on initial heating
0.00001
0.0001
0.001
0.01
460 470 480 490 500 510 520
QE
pos (mm)
Vert position 7.3
Vert position 8.3
Vert position 9.3
Vert position 10.3
Reflection QE (543 nm) vs Position
SS Guard
TransparentSubstrate
Cu Cathode
468 470 472 474 476 478 480 4820.0001
0.001
Vert pos 3.97Vert pos 4.97Vert pos 5.97Vert pos 6.97
pos (mm)
QE
afte
r los
s in
Cu
Transmission QE (543 nm) vs Position
470 475 480 485 490 495 500 505 510 515 5200
200
400
600
800
1000
1200
1400
1600
240 nm
Position (mm)
Phot
ocur
rent
(pA
)
Energy
Medium Vacuum
Φ
Three Step Model - Semiconductors
Filled StatesEm
pty States
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 States
Eg
Ea
A.R.H.F. Ettema and R.A. de Groot, Phys. Rev. B 66, 115102 (2002)
Thickness dependence @ 543 nm
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0.1
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0.3
0.4
0.5
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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
Ref
trans
Total QE
QE w/o R&T
QE vs Field
