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