secondary-electron emission from hydrogen-terminated diamond · lifetime of diamond amplifier 12.5...

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Secondary-electron emission from hydrogen-terminated diamond Erdong Wang Brookhaven National Laboratory 30-May-12 1

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Page 1: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Secondary-electron emission from hydrogen-terminated diamond

Erdong WangBrookhaven National Laboratory

30-May-12 1

Page 2: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Outline

• Motivation-High average current electron source requirement-Diamond amplifier concept

• Experiments-Hydrogenation optimization-High gain emission test-Lifetime and uniformity

• Emission models development-Effective NEA surface-Energy Spread-Emission gain depend on pulse width

• Summary

30-May-12 2

Page 3: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Motivation

•High average current, high-brightness, low emittance injector requirement:

-ERL light source-E-cooling

•Desired cathode:-High peak current-High average current-Low transverse emittance-Fast temporal response-Small energy spread-Simple installation expose to air, no load-lock mechanism requirement-Long life time-Should not contaminate gun

30-May-12 3

Page 4: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Diamond amplifier concept

Laser

Photocathode

Metal coating

RF cavity

Hydrogenated surface

Primary beam

-10kV

Diamond

Secondary beam

Gap

Focusing

Metal photocathode: QE 0.1%,long life time (months)Semiconductor photocathode: QE 1%~10%, short life time (weeks)Diamond amplifier, high current, long lifetime

Diagram courtesy of D.A.Dimitrov (Tech-X Crop.)30-May-12 4

Page 5: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Why we choose diamond?

• Wide band gap– Diamond has a band gap of 5.47 eV;

indirect band gap– Material with larger band gap will have the

ability of tolerant stronger electric fields.Prevent field emission and dark current.

– Could have negative electron affinity(NEA) with hydrogen termination on thesurface, due to the large band gap

• Best rigidity– Diamond window will protect the gun from

cathode material.• High thermal conductivity

-- Easily handle heat power from primarycurrent deposited

• Very high Mobility and Saturation Velocity– Saturation velocity is 2.7×105 m/s. It have

high frequency application

30-May-12 5

Page 6: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Diamond amplifier fabrication

•Single crystal CVD diamond (Element 6); X ray topography.

•Surface cleanThe diamond sample was cleaned ultrasonically in acetone and then in alcohol for 15 minutes.

•Metal Pt SputteringSputter 30nm Pt layer

•HydrogenationGenerate NEA surface,determine the high gain of diamond

(110) X ray topography

primary metal

e h

E EG

E −

−=

30-May-12 6

Page 7: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Hydrogenation

1E-06

1E-05

0.0001

0.001

0.01

0.1190 200 210 220 230 240 250 260 270 280

QE

Wavelength(nm)

1E-06

1E-05

0.0001

0.001

0.01

0.1

190 200 210 220 230 240 250 260 270 280

QE

wavelength(nm)

1. Heat to 800 ⁰C for 30 minute.2. Cool down and measure background

emission spectrum, then heat to 800 ⁰C.3. Expose to hydrogen atoms with10-6 torr.4. Monitor the photocurrent increase once

turn off the heater.5. After the photocurrent reached its peak,

Turned off the hydrogen atoms6. Measured the emission spectrum after

sample cool down.

After hydrogenation

Before hydrogenation

Good sample

Contaminated sample

30-May-12 7

Page 8: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Hydrogenation optimization

High temperature hydrogenation decay

RM temperature hydrogenation decay

•Slow decay after high temperature Hydrogenation (4.8 hrs)•Twin decay after RM temperature Hydrogenation (0.25hr,4.8hrs)•Slow decay be recovered by baking ,but not in quick decay

RM temperature hydrogenation

0

5

10

15

20

25

0 10 20 30 40 50

Phot

ocur

rent

(nA)

Minutes

High temperature hydrogenationHydrogenation stop

0

5

10

15

20

200 300 400 500 600 700 800

Phot

ocur

rent

(nA)

Temperature(⁰C)

Optimized temperature

~200

4030-May-12 8

Page 9: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Lifetime of diamond amplifier

12.5

13

13.5

14

14.5

15

15.5

16

0 2 4 6 8 10 12 14

Phot

ocur

rent

(nA)

Hours

Decay current

Lamp off/HV on

Lamp on/HV off

Off axis lamp on/HV on

Lamp off/HV off

Total 18%

Residual gas 1.9%

UV light 9.8%

Bias 0.7%

Ion back bombardment(emission)

4.2%

Ion back bombardment(UV)

1.4%•Hydrogenated diamond exposed to atmosphere for 30 minute, then gain drop 5%•New diamond amplifier gain is 135•One month later, gain still above 90•Exposed to atmosphere four months, gain 130 after re-hydrogenation

Hydrogen-termination is robust!

30-May-12 9

Page 10: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Emission gain test

Hydrogenated surface Diamond

0- to 10-keV Electron beam

A

CCD camera

Phosphor ScreenFocusing Channel Pt metal coating

Anode with holes

H.V. pulse generator

Primary current

HV pulse

Secondary current

30-May-12 10

Page 11: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Secondary electron beam

Frequency:1000Hz Pulse voltage:3000VPulse width:10usCurrent:169nA

After focusing <500um

The highest bunch charge: 500 pC @1 mm2

Current density:70 μA/mm2

30-May-12 11

Page 12: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Gain measurement

0

20

40

60

80

100

120

140

160

0 0.5 1 1.5 2 2.5 3

Gain

Gradient(MV/m)

0.2us

0.6us

1us

2us

4us

6us

10us

20us

50us

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

Gain

Pulse width(us)

500V

1000V

1500V

0.0 0.5 1.0 1.5 2.0 2.5MV�m50

100

150

200

250

300Gain

•Diamond biased to negative voltage, thermionic gun is -10 kV. Primary electrons energy drop when the voltage of diamond increase, so gain drop•The gain drop due to trapping electron accumulate on the surface when the pulse width increase•Without the surface trapping,ideally gain is 280

30-May-12 12

Page 13: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Secondary emission uniformity

•Emission diameter 2.8 mm•Maximum gain178•Center gain143•Edge gain increase due to thin metal layer

A

A’

B

B’

A-A’

B-B’

30-May-12 13

Page 14: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Reproducible

30-May-12 14

Page 15: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Emission probability

Diamond Vacuum

Metal coating Hydrogenated surface Anode

Ei

Ld

Ee

Lg

Ip Ii(t) Ie(t)Is it

p

IGI

=

Unmeasureable:P=Ie/Ii Measureable when pulse width tend to infinitely small:P=Ia/(Ii ∙w∙ f ) =Ge/Gt

0( )

1/

w

ea

I t dtI

f= Ge =

Ia

I p

1f ⋅ w

Defined a parameter independent on primary beam, metal coating:Emission probability

Average emission gain:

Transmission gain:

30-May-12 15

Page 16: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Effective negative electron affinity

No barrier on the surface, totally emission

Barrier on the surface, some electrons be trapped

The hydrogenated diamond (100) surface is effective NEA

Effective NEA True NEA

30-May-12 16

Page 17: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Schottky effect on diamond surface

Vc

Vv

φ1φmφs

Diamond Vacuum

_ 00.0318 ( / )[ ]sch dia E MV m eVφ =

2

2( )

2

2( )

2

m

ef x

ϕσ

πσ

−−

=

11 ( )2 2

m sP Erfc ϕ ϕσ

+= −

Schottky effect:

Distribution:

Emission probability:

Energy spread

Gauss peak to vacuum level

•Prepared four nearly identical samples and measured the emission probability• Obtain σ and φm by data fitting•Energy spread estimated 0.12±0.01eV

30-May-12 17

Page 18: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Gain depend on the pulse width

0

1( ) (1 ) ( , ( ) )te

i p t p ir

EE t P I G E t d tSε ε

= − − × ×× E

3000V

5000V

3000V

5000V

30-May-12 18

Page 19: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Summary & prospective

1. Hydrogen terminated diamond was demonstrated toamplify the electron beam from traditional cathode.

2. The standard procedure was developed after systemically hydrogenation study. The high quality diamond amplifier is reproducible now.

3. The highest measured gain was 200.4. Developed an emission model on the effective NEA

surface and the energy spread was estimated.5. The emission gain decrease when the HV pulse

width increase. This phenomena is explained well by the surface trapping model.

6. The 112MHz SRF gun for diamond amplifier test is under development.

7. Thermal emittance measurement is under going.

30-May-12 19

Page 20: Secondary-electron emission from hydrogen-terminated diamond · Lifetime of diamond amplifier 12.5 13 13.5 14 14.5 15 15.5 16 0 2 4 6 8 101214 Photocurrent(nA) Hours Decay current

Acknowledgements

Special thanks to the diamond amplifier group:

Ilan Ben-zviTriveni Rao

Sergey BelomestnykhXiangyun Chang

Dimitre .A. DimitrovQiong Wu

John smedleyErik MullerTianmu Xin

Mengjia Gaowei

30-May-12 20

Thanks for your attention!