Transcript High performance polarized electron photocathodes based on InAlGaAs/AlGaAs superlattices Yu.A.Mamaev, A.V.Subashiev, L.G.Gerchikov, Yu.P.Yashin, St.

High performance polarized electron
photocathodes based on
InAlGaAs/AlGaAs superlattices
Yu.A.Mamaev, A.V.Subashiev, L.G.Gerchikov, Yu.P.Yashin,
St. Petersburg State Polytechnic University, Russia
T. Maruyama, D.-A. Luh, J.E. Clendenin
Stanford Linear Accelerator Center, USA
OUTLINE
1. Introduction
2. Highly strained InAlGaAs/AlGaAs SL
structures
3. InAlGaAs/GaAs SL structures with
minimized conduction band offset
4. Photocathode lifetime improvement.
5. Summary & Outlook
Since the figure of merit for asymmetry measurements
is
P2I,
where
I
is the beam intensity, even a small
increase in P will result in a significant improvement in
the efficiency of electron accelerators. In addition the
physics reach of a collider is significantly enhanced by
any improvement in the beam polarization.
Polarized Photocathode R&D at St. Petersburg Polytechnic
University
Experimental setup
Experimental Facilities
Gun Test Lab
Cathode Test Lab
• QE and Polarization at 100 V
• QE and Polarization at 120 kV
under accelerator condition
Polarized photoemission
Optical spin orientation in A3B5 semiconductor layers:
1 - Band spectrum near -point of the Brillouin zone in unstressed (a)
and in stressed (b) crystal. 2 - The arrows indicate interband optical
transitions under illumination by the circularly polarized + or - light
Reducing the cubic GaAs lattice symmetry to tetragonal symmetry by
applying a biaxial stress in the (100) plane due to lattice mismatch
between the buffer and overlayer
Problem of strained layer
photocathodes:
Strain relaxation
Thick stressed layer with large
deformation is hardly possible
Possible solution:
Periodical heterostructure
- Superlattice
Goals of Starined Superlattice:
•Thick high quality working layer
(>0.1 μm )
•Large valence band splitting
(>50meV)
•High polarization of photoemission
(>90%)
Superlattice based photocathode
with negative electron affinity
InGaAs AlGaAs
electron
emission
Ec
heavy hole
miniband
electron
generation
Ev
light hole
miniband
valence band
band
bending
region
Spin-Orientation of
Photoelectrons in SL
Al0.4Ga0.6As
GaAs
Ec
e2
c=0.28eV
e1
6
Eg=1.42eV
8
Unstrained GaAs/AlxGa1-xAs SL
v=0.23eV
Ev
9nm
9nm
9nm
Band energy spectrum
GaAs(9nm)/Ga0.6Al0.4As(9nm)
1,7
No hh-lh mixture at k||=0
e2
1,5
e1
hh1
Energy, eV
Main Transitions:
hh1 – e1 lh1 - e1
hh2 - e2 lh2 - e2
1,6
lh1
hh2
-0,05
hh3
-0,10
lh2
hh4
-0,15
Kz , A
-1
0,00
0,05
K||, A
-1
0,10
Polarization Losses
Working
layer
1. Photoabsorbtion stage:
• Indirect optical transitions
•Smearing of valence band edge
•Photoabsorption in BBR
2. Transport stage
3. Emission stage
4 - 6%
5 - 10%
Surface Charge Limit
•
•
•
•
•
Photon absorption excites
electrons to conduction band
Electrons can be trapped near the
surface; electron escape prob.
 20%
Electrostatic potential from
trapped electrons raises affinity
Affinity recovers after electron
recombination
Increasing photon flux
counterproductive at extremes
Cesium plus Oxygen (or Fluorine) to achieve
NEA surface state
QE, Lifetime
Low doping to suppress depolarization
in the course of transport
Very high doping for
better QE and to overcome
surface charge limit
Arsenic cap to protect the surface
from the air contamination
Coherent strain to keep high splitting
Electrons are mostly
trapped at BBR prior
emission
High valence band splitting
(60 – 100 meV) for high Initial
polarization
Activation
coverage
Top
Structure (bulk)
Strained-superlattice
T. Maruyama
1000 A
25mm
25mm
Active Region
GaAs0.64P0.36
Buffer
GaAs(1-x)Px Graded
Layer
GaAsP
30 A
Strained GaAs
40 A
GaAsP
Strained GaAs
GaAsP
Strained GaAs
GaAs Substrate
Polarization and QE
T. Maruyama
1
w = 3 nm, b = 3 nm
w = 4 nm, b = 4 nm
w = 4 nm, b = 3 nm
w = 5 nm, b = 3 nm
60
QE (%)
Polarization (%)
80
X = 0.35
0.1
40
0.01
20
660
680
700
720
740
760
Wavelength (nm)
780
800
820
• Peak polarization 85%
• QE ~ 0.8 – 1%
• Wavelength dependence
is consistent with the
simulation.
Highly strained
AlxInyGa1-x-yAs/AlzGa1-zAs
SL structures
New InAlGaAs/AlGaAs SL structures with thin (close to 2 nm) quantum
well layers and considerably high (up to 35 %) concentration of In
within the quantum wells were developed and tested. The studies of
polarization spectra obtained at lowered activation temperatures
revealed a rather wide plateau in the vicinity of the maximum
polarization (about 85%) and sharp edge of the quantum yield
spectrum, which indicates the good structural qualities of these
samples. Record high values of strain splitting are reproducibly
obtained.
MBE grown AlInGaAs/AlGaAs strain superlattice samples
Composition
Thickness
Doping
As cap
GaAs QW
AlxGa 1-xAs
50 A
SL
In yAl zGa 1-y-zAs
Al0.4Ga0.6As
11019 cm-3 Be
60 A
41017 cm-3 Be
40 A
Buffer
1.25 mm
51018 cm-3 Be
p-GaAs substrate, Zn doped
5-337 SL thickness:
15 periods
Band edges: In 0.16Al0.2Ga0.64As (a) ; Al 0.28GaAs 0.72 (b=1000)
a
eh1
eh2
evl1
evl2
ec1
ec2
Without strain
40
0.000 -0.102 -0.064
-0.093
1.513 1.687
Eg1
Eg2
1.378
1.731
1.513
1.780
In 0.16Al0.2Ga0.64As/Al 0.28GaAs 0.72 SL, Room temperature
90
1
10
QE, %
70
0
10
60
50
-1
10
40
30
-2
10
20
650
700
750
Wavelength, nm
800
Polarization, %
80
Photocurrent,
mA
8
6
4
2
0
0
10
20
Light power
30
40
50
mJ/pulse
In 0.16Al0.2Ga0.64As/Al 0.28GaAs 0.72 SL, Room temperature.
Red – just after cesiation. Blue – a week after cesiation.
5-830 miniband spectrum (q
;k
)
Al0.28In0.32Ga0.4As(2nm)/Ga0.77Al0.23As(4nm)
Energy, eV
1,9
1,8
1,7
1,6
e1
hh1
-0,05
lh1
-0,10
hh2
-0,15
-0,4
0.4
0,0
-1
q, nm
0,4
-1
k, nm
0,8
5-830; 20 periods of SL: In0,32Al0,27Ga0,4As(2nm thick) / Al0,23Ga0,77As (4nm thick)
P @ Room
P @ T=130 K
QE @ Room,
QE @ T=130 K
90
1
10
80
70
0
10
60
-1
10
-2
40
10
30
-3
10
20
-4
10
10
0
-5
10
550
600
650
700
750
Wavelength, nm
800
850
900
P, %
QE, %
50
Sample
Superlattice
GaAs QW
x
y
Th
(nm)
z
Th
(nm)
Doping
Th
(nm)
Doping
5-830
20 pairs
0.27
0.32
2
0.23
4
5*1017 cm-3
6
6*1018 cm-
5-832
20 pairs
0.27
6-041
20 pair
0.25
6-042
22 pairs
0.25
3
0.32
2
0.28
4
5*1017 cm-3
6
6*1018 cm3
0.32
2
0.28
4
3*1017 cm-3
6
3*1018 cm3
0.32
2
0.30
3
3*1017cm-3
6
3*1018 cm3
QE, SL 6-042, Tht=500C, T=300K, 02.11.2004
QE, SL 6-041, Tht=500C, T=300K, 26.10.2004
QE, SL5-832, T=300C, Tht=500C, 12.03.2004
QE, SL 5-830, T=300C, Tht=400C, 03.03.2004
P, SL 6-042, Tht=500C, T=300K, 02.11.2004
P, SL 6-041, Tht=500C, T=300K, 26.10.2004
P, SL 5-832, T=300C, Tht=500C, 12.03.2004
P, SL 5-830, T=300C, Tht=400C, 17.03.2004
1
10
80
0
60
-1
QE, %
10
-2
10
40
-3
10
20
-4
10
0
-5
10
550
600
650
700
750
800
Wavelength, nm
850
900
950
Polarization, %
10
AlxInyGa1-x-yAs/GaAs SL
structures with minimized
conduction band offset
A new set of the InAlGaAs/GaAs SL structures
with minimized conduction band offset was
designed and tested. The Al content determines
the formation of a barrier in the conduction band,
while adding In leads to conduction band lowering,
so the conduction band offset can be completely
compensated, while barriers for the holes remain
uncompensated.
Unstrained wells and strained barriers
GaAs/AlxInyGa1-x-yAs SL
GaAs
6
Ec2
e1
Ec1
Al0.2In0.18Ga0.62As
8
Ev1
hh1
Evh2
Evl2
lh1
SL: Al0.21In0.2Ga0.59As (40Å)/Ga As(dQW )
Minibands edges (eV)
dQW(Å)
10
15
20
25
40
E hh2
E lh1
E hh1
E e1
-0.2054E+00
-0.1752E+00
-0.1542E+00
-0.1389E+00
-0.9484E-01
-0.1160E+00
-0.1041E+00
-0.9352E-01
-0.8409E-01
-0.6199E-01
-0.5120E-01
-0.4413E-01
-0.3794E-01
-0.3267E-01
-0.2145E-01
1.429
1.429
1.428
1.428
1.427
Miniband Edges, eV
EV10.00
70
60
-0.05
EVh2
50
hh1
lh1
Ehh1-Elh1 40
-0.10
30
EVl2
-0.15
20
2
4
QW (GaAs) width, nm
6
Valence Band Splitting, meV
GaAs/Al0.2In0.21Ga0.59As (4nm) SL
P-1, SL 5-777, Tht=400C, run 1, T=300K, 24.08.2004
P-1, SL 5-777, Tht=400C, run 2, T=300K, 25.08.2004
P-1, SL 5-777, Tht=540C, run 1, T=300K, 06.09.2004
QE-1, SL 5-777, run 1, Tht=400C, T=300K, 24.08.2004
QE-1, SL 5-777, run 2, Tht=400C, T=300K, 25.08.2004
QE-1, SL 5-777, run 1, Tht=540C, T=300K, 06.09.2004
100
1
10
0
-1
QE, %
10
60
-2
10
40
-3
10
20
-4
10
0
-5
10
550
600
650
700
750
Wavelength, nm
800
850
900
Polarization, %
80
10
18.5 periods of SL: Al0.21In0.2Ga0.59As (4nm)/Ga As(1.5nm); Room temperature
1
10
80
0
60
-1
QE, %
10
-2
10
40
-3
10
20
-4
10
0
-5
10
550
600
650
700
750
800
Wavelength, nm
850
900
Polarization, %
10
(5-777) 18.5 periods of SL: Al0.21In0.2Ga0.59As (4nm)/Ga As(1.5nm)
Room temperature
Ehh1-E lh1 = 60 meV, E barrier = 78meV
100
Experiment
Theory
Polarization, %
80
60
40
20
1,4
1,5
1,6
1,7
1,8
Photon energy, eV
1,9
1.25 μm Al0.35Ga0.65As Buffer
18.5 periods of SL: Al0.21In0.2Ga0.59As (4nm)/Ga As(1.5nm)
GaAs QW (6 nm)
As cap
Sample #
GaAs (100)
substrate
SL doping
GaAs QW doping
5 - 777
p-doped
4*10 17cm -3
homogeneous doping
7*1018 cm-3
6 - 035
semi-insulating
6 - 037
semi-insulating
4*10 17cm -3
the only barriers are doped
2*1019 cm-3
2*10 17cm -3
the only barriers are doped
5*1019 cm-3
100
10
Ioffe 6-035
80
60
QE(%)
Polarization (%)
1
40
0.1
Heat Cleaning Temperature
500°C (SLAC)
550°C (SLAC)
586°C (SLAC)
605°C (SLAC)
540°C
20
0
0.01
650
700
750
800
Wavelength (nm)
850
900
Photocathode lifetime
improvement.
The possibility to protect the activation
layer by an additional co deposition of Sb
on the Cs-O activated surface was
investigated. Several types of coadsorption processes with antimony
addition to oxygen during Yo-Yo- cycles as
well as Sb co-deposition at the final stage
of the activation cycles were tested. The
quantum efficiency attenuation and the
lifetime of the cathodes was measured for
the varying activation conditions.
QE without SB SL 5-830 T=300K Tht = 500C 24,03,2004
QE with SB SL 5-830 T=300K Tht = 500C 26,03,2004
P without SB SL 5-830 T=300K Tht = 500C 24,03,2004
P with SB SL 5-830 T=300K Tht = 500C 26,03,2004
1
10
80
0
60
-1
QE, %
10
-2
10
40
-3
10
20
-4
10
0
-5
10
550
600
650
700
750
800
Wavelength, nm
850
900
Polarization, %
10
Normal activation (without Sb)
Activation with Sb (at the end)
2,0
Phorocurrent, m
1,5
1,0
0,5
0,0
0
10
20
30
40
Time, min
50
60
70
80
Normal activation (without Sb)
Phorocurrent, m
2.0
O2 off
1.5
Cs
1.0
Cs
O2 on
0.5
0.0
56.0
56.5
57.0
57.5
58.0
Time, min
58.5
59.0
59.5
60.0
Activation with Sb (at the end)
1,6
Phorocurrent, a.u.
SL 5-830
Sb on
1,4
Sb off
O2 off
Cs off
1,2
1,0
73
74
75
Time, min
76
e
0
Iph, a.u.
Lifetime without Sb
Lifetime with Sb
e
-1
SL 5-830
e
-2
0
50
100
Time, a.u.
150
200
250
Conclusions
• New structures based upon highly strained InAlGaAs/AlGaAs SL
and InAlGaAs/GaAs SL with minimized conduction band offset have
been developed and tested. InAlGaAs/AlGaAs SL structures are
favorable candidates for photocathodes since they can be grown by
a standard MBE technology and the structures are well controlled
and reproduced during the growth.
• A new technology of surface protection in MBE growth leads to
considerable reduction of the heat-cleaning temperature (not more
than 4500C). At these lowered cleaning temperatures, the thermal
degradation of the working structure parameters is avoided. As a
result a polarization P of up to 91% at corresponding quantum
efficiency (QE) of 0.3% was achieved at room temperature.
• For the studied activation process with antimony deposition at the
final activation stage it was found that highly strained
InGaAs/AlGaAs SL photocathodes have about 2 times smaller
quantum efficiency, but result in a 50% increase in the photocathode
lifetime.
Outlook
Optimized photocathode structure
- Optimal layer composition and thickness providing highest
possible valence band splitting
- High quality SL providing minimal band edge smearing
- Special doping profile with minimal doping level in QW
layers and maximal in BBR providing low hh-lh mixture and
thin BBR
- Maximal photoabsorption in working layer with respect to
photoabsorption in BBR layer.
Pmax of up to 95%
Acknowledgments
This work was supported by
- Russian Fond for Basic Research under grant
04-02-16038
- Russian Ministry of industry, science and technology
under contracts # 40.012.1.1.1152 and #
40.072.1.1.1175
- CRDF under grant RP1-2345-ST-02
- NATO under grant PST.CLG 979966
- the US Department of Energy under grant
DE-AC02-76SF00515