XIIth International Workshop on Polarized Sources, Targets & Polarimetry

Highly Effective Polarized Electron Sources Based on Strained Semiconductor

Superlattice with Distributed Bragg Reflector

Leonid G. GerchikovLaboratory of Spin-Polarized Electron Spectroscopy

Department of Experimental PhysicsState Polytechnic University

St.-Petersburg, Russia

Highly Effective Polarized Electron Sources Based on Strained Semiconductor

Superlattice with Distributed Bragg Reflector DBR

CollaboratorsCollaboratorsDepartment of Experimental Physics, St.Petersburg State Polytechnic University, Russia, Yurii A. Mamaev, Yurii P.Yashin, Vitaly V. Kuz’michev, Dmitry A. Vasiliev, Leonid G. Gerchikov

Stanford Linear Accelerator Center, Stanford, CA, USA, James E. Clendenin , Takashi Maruyama

A.F. Ioffe Physicotechnical Institute RAS, Russia, Viktor M. Ustinov, Aleksey E. Zhukov, Vladimir S. Mikhrin, Alexey P. Vasiliev

Department of Electronic and Electrical Engineering, University of Sheffield, UK, John S. Roberts

Institute of Nuclear Physics, Mainz University, Mainz, Germany, Kurt Aulenbacher

• Introduction– High-Energy spin physics requirements– Photocathodes based on strained

semiconductor superlattices – Optical resonator with DBR

• Design of photocathode

• Strain-compensated superlattice photocathode with DBR

• Superlattice with strained QW and DBR

• Summary & Outlook

OUTLINEOUTLINE

High-Energy physics requirementsHigh-Energy physics requirements

• High electron polarization, P > 80%

Accelerator P, % Beam

MAMI 85% QE > 1%

eRHIC at BNL 70% 50-250 mA, QE > 0.5%

ILC 80% QE > 0.5%

90% is better

• High QE for large beam currents

•Large electronic current requirement•Light energy limitations:•Surface charge saturation•Heating

High QE

High polarization of electron emission from High polarization of electron emission from strained semiconductor SL at the expense of QEstrained semiconductor SL at the expense of QE

0.00 0.04-0.04

-0.02

1.4

1.5

1.6

lh1

hh1

e1

k001

, A -1

Ene

rgy,

eV

k100

, A -1

valence band

bandbendingregion

electronemission

electrongenerationheavy hole

miniband

light holeminiband

InG aAs AlGaAs

E c

E v

550 600 650 700 750 800 850 900

10-5

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

100

QE

QE, %

, nm

Polarization

Pol

ariz

atio

n, %

SL Al0.2 In0.155 Ga0.65As(5.1nm)/Al0.36Ga0.64As(2.3nm)

Spectra of electron emission: Polarization P and Quantum Efficiency QE

•Polarization is maximal at photoabsorption threshold where QE is small.•Strain relaxation does not allow to produce thick photocathode with high QE.•Rise of the vacuum level increases P and decreases QE

Best photocathodesBest photocathodes

Sample Composition Pmax QE(max) Team

SLSP16 GaAs(3.2nm)/ GaAs0.68P0.34 (3.2nm)

92% 0.5% Nagoya University,

2005SL5-777 GaAs(1.5nm)/

In0.2Al0.23Ga0.57As(3.6nm) 91% 0.14% SPbSPU, 2005

SL7-307 Al0.4Ga0.6As(2.1nm)/In0.19Al0.2Ga0.57As(5.4nm)

92% 0.85% SPbSPU, 2007

h

GaAsSubstrate Buffer BBR SL

e

RGaAs

= 0.3h

GaAsSubstrate DBR Buffer BBR SL

e

RDBR

= 1

Goal: considerable increase of QE at the main polarization maximum.Method: Resonance enhancement of photoabsorption in SL integrated into Fabry-Perot optical cavity. Photoabsorption in the working layer:L 1, - photoabsorbtion coefficient,L - thickness of SL

Resonant enhancement by factor 2/(1-(RDBRRGaAs) 1/2)2

Resonant enhancement of polarized electron Resonant enhancement of polarized electron emission from strained semiconductor layeremission from strained semiconductor layer

T. Saka, T.Kato, T.Nakanishi, M.Tsubata, K.Kishino, H.Horinaka, Y.Kamiya, S.Okumi, C.Takahashi, Y.Tanimoto, M.Tawada, K.Togawa, H.Aoyagi, S.Nakamura, Jpn. J. Appl. Phys. 32, L1837 (1993).

Resonant enhancement of polarized electron Resonant enhancement of polarized electron emission from strained semiconductor layeremission from strained semiconductor layer

J. C. Groebli, D. Oberli, F. Meier, A. Dommann, Yu. Mamaev, A. Subashiev and Yu. Yashin, Phys. Rev. Lett. 74, 2106 (1995).

Optimization of Photocathode structureOptimization of Photocathode structure

Buffer

GaAsSubstrate

DBR

SL

BBR

760 780 800 820 840 860 880 9000.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength, nm

• SL structure: layers composition and thickness are chosen to assure Eg= for P()=Pmax Ehh-lh > 60meV for high polarization Ee1 > 40meV for effective electron transport

• DBR structure: 20x(AlAs(/4)/ (AlxGa1-xAs(/4)) Layer thickness l = /4n() for Bragg reflection x 0.8 for large reflection band width = 2n/n

• Fabry-Perot resonance cavity: BBR + SL + buffer layer Effective thickness = k /2 for QE() = QEmax Effective thickness of BBR+SL /4

Simulation of resonant photoabsorptionSimulation of resonant photoabsorption

• SL’s energy band structure, photoabsorption coefficient, polarization of photoelectrons.

Method: kp – method within 8-band Kane model. A.V. Subashiev, L.G. Gerchikov, and A.I. Ipatov. J. Appl. Phys., 96, 1511

(2004).

• Distribution of electromagnetic field in resonance cavity, reflectivity, QE.

Method: transfer matrixes. M.Born and E.Wolf. Princeples of Optics, Pergamon Press, New York,

1991

• Even small in-plane anisotropy leads to resonant polarization losses. High quality structure of Fabry-Perot cavity is required.

• The optical thickness of Fabry-Perot cavity can not be adjusted after fabrication.

Problems of fabricationsProblems of fabrications

Ehh-lh

Eg

Evl1

Evh1

Evl2

Evh2

Ec2

Ec1

e1

lh1hh1

AlInGaAsGaAsP

8

6

Strain-compensated SLStrain-compensated SL

Features:• No strain relaxation

• Thick working layer without structural defects

• Large deformation splitting in each SL layer

Tensiled barrierab < a0

GaAs Substrate

Buffer Layera0 - latt. const

GaAs BBR

Stressed QWaw > a0

Stressed QWaw > a0

Tensiled barrierab < a0

SL

DBR

Composition Thickness DopingAs cover

GaAs QW 60 A 11019 cm-3 ZnGaAs0.83P0.17

SL60 A

41017 cm-3 Zn(In0.16Al0.84 )0.82Ga0.68As 40 A

Al0.35Ga0.65As Buffer 0.5 m 61018 cm-3 Zn

GaAsSL

710 A41017 cm-3 Zn

Al0.19Ga0.81As 580 A

p-GaAs substrate, Zn doped

MOVPE grown AlInMOVPE grown AlInGaAs-GaAsP strained-compensated SLsGaAs-GaAsP strained-compensated SLs

“Resonance Enhancement of Spin-Polarized Electron Emission from Strain Compensated AlInGaAs GaAsP Superlattices” J.S. Roberts, Yu.P. Yashin, Yu. A. Mamaev, L.G.Gerchikov,T. Maruyama, D.-A. Luh, J.E. Clendenin, Proceedings of the 14th international conference “Nanostructures: Physics and Technology”, St.Petersburg, 26-30 June 2006.

ReflectivityReflectivity

Experiment, QT 1890 DBR Theory , QT 1890 DBR

780 800 820 840 860 880 9000.0

0.2

0.4

0.6

0.8

1.0

Ref

lect

ivity

Wavelength, nm

550 600 650 700 750 800 850 900 950

10-5

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

QE

, %

W avelength, nm

QE-4, SL QT 1890 non DBR QE-2, SL QT 1890 DBR

P-4, SL QT 1890 non DBR P-2, SL QT 1890 DBR

Pol

ariz

atio

n, %SPTU data

Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())

550 600 650 700 750 800 850 900 950

10-5

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

QE

, %

W avelength, nm

QE, Experiment SL QT 1890 DBR QE, Theory SL QT 1890 DBR

P, Experiment SL QT 1890 DBR P, Theory SL QT 1890 DBR

Pol

ariz

atio

n, %SPTU data

Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())

Resonant enhancement of QE Resonant enhancement of QE

550 600 650 700 750 800 850 900 9500

2

4

6

8

10

0

20

40

60

80

QE

Enc

hanc

emen

t

W avelength, nm

QE enchancement

SPTU data

P-4, SL QT 1890 non DBR P-2, SL QT 1890 DBR

Pol

ariz

atio

n, %

550 600 650 700 750 800 850 900 9500

2

4

6

8

10

0

20

40

60

80

QE

Enc

hanc

emen

t

W avelength, nm

QE enchancement, Experiment QE enchancement, Theory

SPTU data

P, Experiment SL QT 1890 DBR

Pol

ariz

atio

n, %

Resonant enhancement of QE Resonant enhancement of QE

Unstrained barrierab = a0

Strained-well SLStrained-well SL

Feature:• Large valence band splitting due to combination of deformation and quantum confinement effects in QW

Unstrained barrierab = a0

GaAs Substrate

Buffer Layera0 - latt. const

GaAs BBR

Strained QWaw > a0

Strained QWaw > a0

SL

DBR

Composition Thickness Doping

As cap

GaAs QW 60 A 71018 cm-3 Be

Al0.4Ga0.6As SL

21 A31017 cm-3 Be

In 0.2Al 0.19Ga 0.61As 54 A

Al0.35Ga0.65As Buffer 0.58 m 61018 cm-3 Be

AlAs DBR

682 A31017 cm-3 Be

Al0.19Ga0.81As 604 A

p-GaAs substrate

MBE grown AlInGaAs/AlGaAs strained-well superlatticeMBE grown AlInGaAs/AlGaAs strained-well superlattice

SPTU & FTI, St.Petersburg, 2006

ReflectivityReflectivity

750 800 850 900 9500.0

0.2

0.4

0.6

0.8

1.0

Experiment, SL 7-396 DBR Theory, SL 7-396 DBRR

efle

ctiv

ity

W avelength, nm

Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())

550 600 650 700 750 800 850 900

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

W avelength, nm

QE

, %

QE, SL 7-396 DBR QE, SL 7-395 no DBR

P, SL 7-396 DBR P, SL 7-395 no DBR

Pol

ariz

atio

n, %

550 600 650 700 750 800 850 900

10-4

10-3

10-2

10-1

100

101

0

20

40

60

80

W avelength, nm

QE

, %

QE, Experiment SL 7-396 DBR QE, Theory SL 7-396 DBR

P, Experiment SL 7-396 DBR P, Theory SL 7-396 DBR

Pol

ariz

atio

n, %

Spectra of electron emission, P(Spectra of electron emission, P(), QE(), QE())

Resonant enhancement of QE Resonant enhancement of QE

750 800 850 9000

10

20

30

W avelength, nm

QE

Enc

hanc

emen

t

Experiment SL 7-396 DBR Theory SL 7-396 DBR

Summary & OutlookSummary & Outlook

• We have developed a novel type photocathode based on strain compensated superlattices integrated into a Fabry-Perot optical cavity of high structural quality.

• We demonstrate a tenfold enhancement of quantum efficiency without polarization losses due to the multiple resonance reflection from DBR layer.

• The obtained results demonstrate the advantages of the developed photocathode as a perspective candidate for spin polarized electron sources.

AcknowledgmentsAcknowledgments

This work was supported by • Russian Ministry of Education and

Science under grant N.P. 2.1.1.2215 in the frames of a program “Development of the High School scientific potential”

• Swiss National Science Foundation under grant SNSF IB7420-111116

SLAC data