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