GaAs Photocathode Activities at

Daresbury Laboratory

Narong Chanlek

Ph.D Student,University of Manchester

Accelerator Science and Technology Centre

Science & Technology Facility Council, UK

Ultra Bright Electron Sources Workshop, 29th June 2011

GaAs photocathodes

High QE over the entire spectrum of visible light once it

is activated to the negative electron affinity (NEA) state.

Some issues

Lifetime

Response time

2

p-type GaAs

EVAC

Vacuum

ECBM

EgEVB

Vbb

Vbb

Vdipole

eff

EF

Wbb

Cs,Op-type GaAs

EVAC

ECBM

EVBEF

Eg

Vacuum

NEAPEA

29th June 2011

ALICE Gun

3

Anode plate

Ceramic Cathode Ball

GaAs cathode

Current ALICE cathode:

VGF Bulk GaAs cathode,

Zn doped, 31 mm diameter

and 625 m thick

Consistently achieving 3%

QE @ 532 nm

No cathode change

possible.

29th June 2011

Photocathode preparation facility (PPF)

4

Provide a rapid and reliable

way to exchange the

photocathode.

Prepare the photocathods

in a controllable and

reproducible way.

Remove the activation

process from the gun.

Gun

Photocathode Preparation Facility

500 kV power supply

29th June 2011

Photocathode preparation facility (PPF)

5

Loading chamber

Atomic hydrogen chamber

Preparation chamber

Dummy gun

29th June 2011

629th June 2011

RM GaAs photocathode

New epitaxial grown GaAs cathodes – 10 mm diameter:

Reflection mode (RM) bound to molybdenum substrate

Transmission mode (TM) bound to glass or sapphire

substrate

Different thickness’ of active layer to enable a fast

measurement of time response.

Photocathodes

729th June 2011

Cathode preparation

Chemically cleaned with HCl/IPA solution in

N2 purged glove box.

Transported to the loading chamber without

exposure to the air.

Heat cleaned and activated in the

preparation chamber.

Degraded photocathodes are heat cleaned

and re-activated.

When it is not possible to fully recover the

QE, it is cleaned by atomic hydrogen

cleaning.

8

Achieved QE and lifetime

29th June 2011

Consistently achieving 14 - 19% QE.

Lifetime in the order of 1000 hours.

0 10 20 30 40 50 60 70 80

0.0

1.0x10-10

2.0x10-10

3.0x10-10

Pre

ss

ure

(m

ba

r)

Elapsed time (minutes)

0

4

8

12

16

Photocurrent

Cs current

QE

(%

)

0

2

4

6

8

10

C

s c

urr

en

t (A

)

A typical activation curve with O2

0 100 200 300 400 5000.8

0.9

1.0

1.1

QE = e-t/688.81

R2 = 0.95

QE

(a.u

.)

Elapse time (hours)

Dark lifetime in the preparation

chamber (9 10-12 mbar)

1/e lifetime = 1/0.001 hrs= 1000 hrs

929th June 2011

Cathode preparation

QE vs the number of activations with O2 and NF3

1 2 3 4 5 6 7 80

4

8

12

16

QE

(%

) @

63

5 n

m

Number of heat clean and activation cycles

1 2 3 4 5 60

4

8

12

16

QE

(%

) @

63

5 n

m

Number of heat clean and activation cycles

Repeated heat cycles and

activations with O2 lead to a

degradation in achieved QE.

Cathode needs to be removed

for chemical cleaning in order to

achieve high QE again.

NF3 provides a more stable QE

after repeated activations.

1029th June 2011

Surface science analysis

Surface analysis

chamberPreparation

chamber

Loading chamberAtomic hydrogen

cleaning chamber

1200 1000 800 600 400 200

Initial surface

Cleaned surface

Cs 3d5/2

Cs 3d3/2

Binding energy (eV)

Activated surface

Inte

nsit

y (

C/s

)

As L3M

45M

45

As L2M

45M

45

C 1sGa L3M

45M

45

O 1sO KL3/2

L3/2

Cs M5N

45N

45

O KL1L

3/2

Ga 2p3/2

Ga 2p1/2

1129th June 2011

Gas exposures after activation with O2 and NF3

0.00 0.05 0.10 0.15

0.0

0.2

0.4

0.6

0.8

1.0

QE

(a

.u.)

Exposure (L)

H2, CH

4 and N

2

CO

CO2

O2

0.00 0.05 0.10 0.15

0.0

0.2

0.4

0.6

0.8

1.0

QE

(a

.u.)

Exposure (L)

NF3 activation exposing to CO

O2 activation exposing to CO

NF3 activation exposing to CO

2

O2 activation exposing to CO

2

NF3 activation exposing to O

2

O2 activation exposing to O

2

O2 activation Compared with NF3 activation

12

Future work - Cold cathode energy

distribution measurements

29th June 2011

Future experiments are to use existing

systems to measure the 3D energy

distribution of a low energy (10-20V)

electron beam. The GaAs cathode will

be at liquid nitrogen temperatures.

Use existing PPF for cathode

preparation.

13

Future work - Cold cathode energy

distribution measurements

29th June 2011

Energy distribution measured as a function of QE decay.

Cathode port

Laser port

MCP drive

MCP

Cathode at LN2

temperature

14

Future work - 160 KV photoinjector for time response

and emittance measurements (cold cathode)

29th June 2011

Experiment will make use of GaAs cathodes with different thicknesses

of active layer for time response measurements.

Diagnostic line will be used for

emittance measurements.

The aim is to measure both

parameters as a function of QE

decay at LN2 temperatures.

15

Acknowledgements

29th June 2011

I would like to thank the following people for their

contributions to this work:

Daresbury Laboratory ISP Novosibirsk

Lee Jones Alex Terekhov

Ian Burrows Sergey Kosolobov

Barry Fell Heinrich Schreibler

Ryan Cash

Julian McKenzie

Adrian Hannah

Mark Pendleton

Keith Middleman

Boris Militsyn

Thank You!