01.10.2008v. tioukine, inst. of nuclear physics, mainz, germany atomic hydrogen cleaning of super...

01.10.2008 V. Tioukine, Inst. of Nuclear Physics, Mainz, Germany

Atomic Hydrogen Cleaningof Super Lattice photo cathodes


• Introduction

• Long term storage problem of photo cathodes

• Hydrogen Cleaning

• QE/Polarization investigations

• Conclusion


MAMI overview

beam parameters• 855MeV• max. 100A cw current• h=8 nm rad• ca. 6000h – 7000h operation / year

MAMI B

beam parameters• 1508.4MeV• max. 100A• h=10 nm rad

MAMI C


Our ‘standard’ way from cathode package to work

position in source:

• 1. sample from wafer is unpacked under nitrogen atmosphere, inserted in cathode holder and placed in transport vessel.

• 2. Transport vessel is connected to load-lock chamber and pumped to below 10-7 torr.

• 3. Cathode holder is transferred through valve from load-lock chamber to preparation chamber.

• 4. Preparation chamber at ~a few 10-11 torr. Cathode is heat cleaned and NEA-activated.

• 5. Activated photo cathode be placed into source (at probably even lower pressure).


MAMI standard photocathodes

• For test used: bulk GaAs (Wafer Tech. LTD., England, U.K.)

• For the beam production strained layer (up to 2004)

• Since then: Super lattice cathodes (Sankt Petersburg State Technical University, Russia)

• Example: ‘S-45’ piece of wafer SL5-998

As cup

GaAs

Highly doped with Be6 nm

30 alternating layer of

In0.16Al0.2Ga0.64As

Al0.28Ga0.72As

Form super lattice structure

112.5 nm

Buffer – Layer

Al0.4Ga0.6As1250 nm

GaAs(100) Substrate 0.5 mm


Preparation.Q.E. – Trend: History of the super

lattice cathode S-45

B C D E F G H I J K L M N0

5

10

15

20

25

30

35

40

The story one of superlatice cathode S-45maximum Q.E.from Juni 2006 till now

Q.E

. [ m

kA /

mW

]

Preparation Number

nm

final state worse than it looks….


New DBR and non-DBR photocathodes

As coating

Cap GaAs doped with Be 6 nm

2x12+1=25 alternating layer of

In0.2Al0.19Ga0.61As // Al0.4Ga0.6As


92 nm

Buffer Al0.35Ga0.65As 580 nm

Buffer GaAs 12 nm

Buffer GaAs ( to DBR ) 20 nm

2x22=44 alternating layer of

Al0.19Ga0.81As // AlAs

Form DBR structure

2830 nm

GaAs(100) Substrate0.5 mm

As coating

Cap GaAs doped with Be 6 nm

2x12+1=25 alternating layer of

In0.2Al0.19Ga0.61As // Al0.4Ga0.6As


92 nm

Buffer Al0.35Ga0.65As 580 nm

Buffer GaAs 12 nm

GaAs(100) Substrate0.5 mm

DBR type 7-396 Non-DBR-type 7-395


‘non-DBR’-wafer SL 7-395

550 600 650 700 750 800 850 900

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

QE

, %

, nm

QE-1, SL 7-395 T=300K Tht=400C 07,05,2007 QE-2, SL 7-395 T=300K Tht=450C 08,05,2007 QE-3, SL 7-395 T=300K Tht=500C 11,05,2007

P-1, SL 7-395 T=300K Tht=400C 07,05,2007 P-2, SL 7-395 T=300K Tht=450C 08,05,2007 P-3, SL 7-395 T=300K Tht=500C 11,05,2007

Pol

ariz

atio

n,

%

Data measured directly after wafer production at SPSTU


Disappointment: QE is much too small, unpleasant nonlinearity.

First results of new SL’s at our installation


Oxide problem?

• Probably due to insufficient As-passivation of surface • Deterioration seems to appear after typical storage

times of months• Possible Reason: oxide transfer AsGa. Maybe not

avoidable even if sample is stored under vacuum. • Investigated ,e.g., by D. A. Allwood et al. for GaAs ‘epi-

ready’ surfaces. (Thin solid films, 412 (2002) 76-83)• Allwood suggests slowing down oxide transfer by cooling

to -20C: too late for our stock. • Oxides not removed by conventional heating.• Attempted solution: Atomic Hydrogen cleaning


Atomic Hydrogen Sources

• 1. Radio frequency source. Atomic hydrogen cleaning of polarized GaAs photocathodes was successfully applied to strained GaAs cathodes used for producing highly polarized electrons. (see for example T. Maruyama et al. APL, 82,23 (2003) 4184)

• 2. Thermal cracking atomic beam sources are used successfully to remove native oxidation from GaAs and provide extremly good surface quality. See for example V. Andreev et al: Proc. Spin 2000, p.901.

• Open question: Polarisation after super lattice treatment? Note: 6 nm thin functional structure in SL top layer


Hydrogen Cleaning: HABS

• H2 dissociation typically 80-98% depending on operational conditions • Atomic hydrogen flux density up to 1*1016/(cm2 s) • No high-energy neutrals or ions • Low power consumption (P < 200 W) • Integrated water cooling, low thermal load on other experimental equipment

Hydrogen Atomic

Beam Source

(commercial system by

Dr. Eberl

MBE-components GmbH)


Atomic Hydrogen Cleaning Installation

Preparation Chamber

UHV transport vessel

AtomicHydrogen

Source


Preparation with / without AHC

0

2

4

6

8

10

12

1 6 11 16 21 26 31

Preparation

Qu

antu

m E

ffic

ien

cy, [

%] AHC ON

SL7-395 (PKA1)

SL7-396 DBR

SL5-998 ( used S-45)

SL7-395

SL7-395

GaAs

GaAs

=680nm


Cathode transport from HABS to test source PKAT

Preparation Chamber

UHV transport vessel

Photo:

E. Riehn


Results SL 7-395

700 725 750 775 800 825 8501E-3

0,01

0,1

1

10

Fresh Long term storage Hydrogen cleanedQ

ua

ntu

m E

ffic

ien

cy, [

%]

Wavelength, [nm] Improves *5 at low intensities+absence of saturation! (*50 improvement for high intensities at MAMI)


700 725 750 775 800 825 850 875 900 9250

2

4

6

8

10

12

14

16

18

20

700 725 750 775 800 825 850 875 900 9250

2

4

6

8

10

12

14

16

18

20

SL 7-395H

Asy

mm

etr

y, [%

]

Wavelength, [nm]

SL 7-395

Structure SL 7-395

395H: second activation 17.7% at 802nm difference insignificant. (other 395 H sample achieves 85±3 % of Polarisation at MAMI)


550 600 650 700 750 800 850 900 950

10-3

10-2

10-1

100

101

0

20

40

60

80

Fresh Long Term Hydrogen Cleaned

QE

, [%

]

Wavelength, [nm]

P

Pol

ariz

atio

n,

%

DBR Structure SL 7-396H

Extended P, QE datasets measured directly after production by Y. Yashin, SPSTU

Highest Q.E values ever measured at high polarization in our lab (1.2%)Stands 5 times more incident power than conventional GaAs cathode (preliminary!)


Conclusion

• GaAlAs / InAlGaAs Super lattice photo cathodes are the standard type at our facility.

• ‘Storage’ problem present in some (not all) wafers• Atomic hydrogen cleaning by thermal cracker results in

dramatically improved surface condition • no significant polarization loss.• Typical quantum efficiency 3-6 μA/mW at working point

of high polarization (P=85%), Operation at accelerator started, now observing long term behaviour

• Promising first results from hydrogen cleaned DBR super lattice (7-12 μA/mW at max P).


The End


Appendix


Cathode lifetime under different operating conditions.

Mode of operation and current Life time, [hours]

No operation, Stand by

(valves closed)1200

A2 Collaboration Operation, I=0.05 µA 850



High Current Test, I=200 µA 160


Muster Title

• Based on GaAs strained layer – muster text

• Quantum Efficiency

• Wave length

• And go on.


Story of MAMI• 1972 - 1975 Project studies about Microtrons • 1975 Proposal of a Race Track Microtron • 1979 First stage 14 MeV beam of MAinz MIcrotron MAMI• 1983 Second stage at 183 MeV energy, maximal beam current 30µA • 1990 Third stage 855 MeV beam of MAMI B• 1991 Beam from distant upstairs polarized electron source • 1992 First acceleration of polarised electrons to full energy• 1999 Approval of the 1.5GeV Harmonic Double Sided Microtron

(HDSM) as a fourth stage of MAMI• Dec. 19, 2006, Beam through HDSM ! 1508MeV reached ! • Feb. 23, 2007 until Mar. 05., 2007 Start the first production beam time

with 10µA polarized beam polarization 84% at 1.508GeV • Feb.27, 2007 performed a high current test and with reasonable

radiation level in the HDSM halls 50µA beam current (75.4kW beam power)

• Oct. 5, 2007 Inauguration ceremony of MAMI C.


EKAN

PKAT

PKA2

PKA1


Vacuum chamber handling

• All vacuum components from 304 stainless steel, • Vacuum and beam line downstream to differential stage

bake able to 250 °C • Using continuously bake-out procedure.• Heating elements – taps and special ordered jackets• Heating 200 °C during one week.• One of test source (PKA2) is coved now by NEG, under

investigations.• For example of chamber handling in CEBAF: Stutzmann

et al. NIM A 574 (2007) 213-220.


Preparation.Example Cathode S-45

0 10 20 30 40 50 600

5

10

15

20

25

30

35

40

O2 let in

Cs switch on

The story one of superlatice cathode S-45from Juni 2006 till now

B C D E F G H I J K L M N

Q.E

., [ m

kA /

mW

]

Preparation Time, [ min ]

2. Cooling 45 min

3. Switch on Cs

1. Thermal heating 30 min, P~100 W, T~550-600°C

4. Waiting ~10 min., before photocurrent

5. Let in O2 pressure ~2x10-9 torr

6. Control maximum rise velocity of current

7. Stop after ~ 45 min


PKA1 Main Source of MAMI

Chamber High voltage insulator

NEG Pump

Load Lock Chamber

Preparation Chamber

Manipulator

Alpha Magnet

Differential Stage

Cathode Position

Ion Pump

Ion Pump

Spin Rotator


2005: 6140h operation, 68% with polarised beam2006: 5950h operation, 65% with polarised beam2007: 7100h operation, 50% with polarised beam2008: yet more then 50 % with polarised beam

MAMI overview.Polarised electron source

Polarised beam produced by photo cathodes based on A3B5 semiconductors by illuminating by circular polarised laser light. Activation by Cs:O Layers.

Polarised beam means:

1. High quantum efficiency.

2. High degree of the polarisation

01.10.2008v. tioukine, inst. of nuclear physics, mainz, germany atomic hydrogen cleaning of super...

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