Transcript Polarized Photocathode R&D in Saint-Petersburg Alexander V. Rochansky Laboratory of Spin-Polarized Electron Spectroscopy Saint-Petersburg State Technical University.

Polarized Photocathode R&D
in Saint-Petersburg
Alexander V. Rochansky
Laboratory of Spin-Polarized Electron Spectroscopy
Saint-Petersburg State Technical University
Polarized Photocathode R&D at St. Petersburg Technical University
OUTLINE
1. Introduction
2. Experimental setup
3. Tuning of strained GaAsP photocathodes
for matching the wavelength of the
excitation light
4. New strain-balanced SL photocathodes
5. Optical Orientation in strained-barrier
GaAs-AlInGaAs SLs with controlled parity
of atomic layers
Polarized Photocathode R&D at St. Petersburg Technical University
SPESLAB history
1975 – Spin Polarized Electron Spectroscopy Laboratory
(SPESLab) was established in Leningrad Politechnical
Institute by dr. Yury Mamaev
1982 – First run of experimental setup “Matilda”, the first
experimental setup for polarized electron photoemission
studying in exUSSR
Since 1992 – Collaboration with scientists teams from SLAC
(USA), Laboratorium fur Festkorperphysik, ETH,
Honggerberg, Zurich, (Switzerland), Institute fur Physik,
Universitat Mainz, Mainz, (Germany) and Ecole
Polytechnique, Palaiseau, (France)
1998 – First run of new experimental setup with load-lock
system and full computer control
Polarized Photocathode R&D at St. Petersburg Technical University
Spin Polarized Electron Spectroscopy Laboratory
www.speslab.spbstu.ru
Laboratory head, professor Yury
Mamaev
Chief theorist professor Arsen
Subashiev
Chief experimenter dr. Yury Yashin
Polarized Photocathode R&D at St. Petersburg Technical University
Experimental setup
Polarized Photocathode R&D at St. Petersburg Technical University
Experimental setup
Polarized Photocathode R&D at St. Petersburg Technical University
Light source
5
1
7
2
3
4
10
8
6
9
11
12
1 – Xe lamp, 2 – Condenser, 3 – Filters, 4 – Monochromator,
5 – Glan-Tompson prism, 6 – Photoelastic modulator,
7 – Dumper, 8 – Lens, 9 – Chamber, 12 – Sample.
Electron optic elements: 10 - 90 - deflector, 11 –electron
optic lenses
Polarized Photocathode R&D at St. Petersburg Technical University
Light source
Polarized Photocathode R&D at St. Petersburg Technical University
7
5
P
2
3
4
6
8
7
P
1
Electron optic system
1 - Sample, 2 – Entrance lenses, 3 – 900 - Deflector,
4 – electrodes, 5 – Light, 6 – Gold foil, 7 – Channeltrons.
Polarized Photocathode R&D at St. Petersburg Technical University
Tuning of strained GaAsP photocathodes for matching the
wavelength of the excitation light
Composition
Thickness
Doping
As cover
GaAs1-xPx
Strained
overlayer
GaAs1-yPy
Buffer
1.0 mm
GaAs0.55P0.45
SL
10 pairs
7 nm
GaAs0.85P0.15
Gradient doping of up
0.12 mm
to 1*1019 cm-3 Mg
7 nm
GaAs0.68P0.32
0.5 mm
GaAs0.8P0.2
0.5 mm
GaAs0.9P0.1
0.5 mm
5*1017 cm-3 Mg
uniform doping
p-GaAs (100) substrate
Structure of strained GaAs1-YPY/GaAs1-XPX sample
Polarized Photocathode R&D at St. Petersburg Technical University
80
Quantum Efficiency, %
1
70
10
10
-1
60
50
-2
40
10
10
10
-3
30
-4
20
10
-5
600
650
700
750
800
Electron beam Polarization, %
90
10
850
Light wavelength, nm
Electron spin polarization (solid symbols) and quantum yield (open
symbols) as a function of excitation energy for the GaAsPx/GaAsPy
strained samples with various phosphorous fractions “x” and “y”.
Room temperature.
Sample 1 (x=0.06, y=0.38) - circles, sample 2 (x=0.12, y=0.36) - squares,
sample 3 (x=0.18, y=0.37) – up triangles.
Polarized Photocathode R&D at St. Petersburg Technical University
Composition
GaAs0.91P0.09
GaAs0.68P0.32
GaAs0.55P0.45
Thickness
Doping
As cover
Strained
Gradient doping of
0.12 mm
overlayer
up to 1*1019 cm-3 Mg
Buffer
SL
10 pairs
1.0 mm
7 nm
GaAs0.85P0.15
7 nm
5*1017 cm-3 Mg
uniform doping
GaAs0.68P0.32
0.5 mm
GaAs0.8P0.2
0.5 mm
GaAs0.9P0.1
0.5 mm
p-GaAs (100) substrate
MOCVD grown GaAsP photocathode without GaAs quantum well
Polarized Photocathode R&D at St. Petersburg Technical University
Composition
Thickness
Doping
As cover
GaAs
QW
20 nm
Gradient doping of up
to 5*1019 cm-3 Mg
GaAs0.91P0.09
Strained
overlayer
0.14 mm
Uniform doping
5*1018 cm-3 Mg
GaAs0.68P0.32
Buffer
1.0 mm
GaAs0.55P0.45
SL
10 pairs
7 nm
GaAs0.85P0.15
7 nm
GaAs0.68P0.32
0.5 mm
GaAs0.8P0.2
0.5 mm
GaAs0.9P0.1
0.5 mm
5*1017 cm-3 Mg
uniform doping
p-GaAs (100) substrate
MOCVD grown GaAsP photocathode with GaAs quantum well
Polarized Photocathode R&D at St. Petersburg Technical University
90
10
80
70
-1
60
10
50
-2
10
40
-3
30
10
20
-4
10
Polarization, %
Quantum Yield, %
1
10
-5
10
0
550
600
650
700
750
800
850
900
Excitation light wavelength, nm
Electron spin polarization (solid symbols) and quantum yield
(open symbols) as a function of excitation energy for the GaAsP
strained sample with (green) and without (red) thin GaAs
quantum well at room temperature.
Polarized Photocathode R&D at St. Petersburg Technical University
Polarized Photocathode R&D at St. Petersburg Technical University
Electron beam Polarization, %
86
84
82
 = 827 nm
Room temperature
80
78
10
-1
10
-2
Quantum Yield, %
10
-3
Polarization evolution upon the degradation of the
GaAs/GaAs0.91P0.09/GaAs0.68P0.32 sample at room
temperature. Excitation light wavelength 827 nm.
Polarized Photocathode R&D at St. Petersburg Technical University
Tuning of strained GaAsP photocathodes for matching
the wavelength of the excitation light
Conclusions
1. By varying of the phosphorous contents
"x“ and "y" at the GaAs1-xPx/GaAs1-yPy
cathodes they can be tuned to the
wavelength, corresponding to maximum light
power of the certain accelerator laser system
Polarized Photocathode R&D at St. Petersburg Technical University
Tuning of strained GaAsP photocathodes for matching
the wavelength of the excitation light
Conclusions
2. The value of quantum efficiency of strained
GaAsP sample has been improved by
incorporating of a thin heavily doped GaAs layer.
At the polarization maximum the efficiency
enhancement of up to ten times has been
achieved.
Polarized Photocathode R&D at St. Petersburg Technical University
Polarized Photocathode R&D at St. Petersburg Technical University
Design of stain-balanced SLs
First set
Composition
GaAs QW
GaAs0.65P0.35
In0.12Al0.12Ga0.76As
Thickness
As cover
80 A
40 A
SL
40 A
Dopping
11019 cm-3 Be
41017 cm-3 Be
Al0.3Ga0.7As
Buffer
0.5 mm
61018 cm-3 Be
p-GaAs substrate, Zn doped
SL thickness: 17 periods
Polarized Photocathode R&D at St. Petersburg Technical University
P T=300K
P T=130K
60
10
50
1
10
10
40
-1
30
-2
Polarization, %
Quantum efficiency, %
QE T=300K
QE T=130K
20
10
-3
10
10
-4
550
600
650
700
750
800
850
900
0
Wavelength, nm
Polarization and quantum yield spectra of the emitted
photoelectrons at T=130 K (open dots) and at T=300 K
(closed dots) for InAlGaAs-GaAsP strain-balanced
superlattice (first set).
Polarized Photocathode R&D at St. Petersburg Technical University
Design of stain-balanced SLs (second set)
Composition
Thickness
Dopping
As cover
GaAs QW
GaAs0.75P0.25
40 A
SL
In0.16Al0.14Ga0.7As
Al0.3Ga0.7As
11019 cm-3 Be
60 A
41017 cm-3 Be
40 A
Buffer
0.5 mm
61018 cm-3 Be
p-GaAs substrate, Zn doped
SL thickness:
SL 1  8 periods
SL - 2  12 periods
Band edges: In0.16Al0.14Ga0.7As (a); GaAs0.75P0.25 (b = 40)
a
eh1
eh2
evl1
evl2
ec1
ec2
Without strain
40
0.000 -0.131 -0.070
-0.069
1.435 1.539
Eg1
Eg2
1.378
1.731
1.435
1.608
Polarized Photocathode R&D at St. Petersburg Technical University
QE QT1669A (8 periods) T=300K
QE QT1669B (12 periods) T=300K
P QT1669A (8 periods) T=300K
P QT1669B (12 periods) T=300K
80
70
1
10
10
10
10
10
60
-1
50
40
-2
30
-3
20
-4
Polarization, %
Quantum efficiency, %
10
10
-5
0
550
600
650
700
750
800
850
900
950
Wavelength, nm
Electron spin polarization and quantum yield as a
function of excitation wavelength for the 8 periods
(blue) and 12 periods (red) SLs at room temperature.
Polarized Photocathode R&D at St. Petersburg Technical University
P T=300K
P T=130K
QE T=300K
QE T=130K
80
10
70
10
10
60
-1
50
40
-2
30
10
10
10
P, %
QE, %
1
-3
20
-4
10
-5
0
550
600
650
700
750
800
850
900
950
Wavelength , nm
Electron spin polarization and quantum yield as a function of
excitation energy at room temperature (red) and 130 K (blue).
Polarized Photocathode R&D at St. Petersburg Technical University
Polarization and quantum yield spectra of the emitted
photoelectrons for In16Al14Ga0.7As/GaAs0.75P0.25 superlattice are
compared to calculated electron polarization at the excitation (lines)
and the absorption coefficient energy dependence (scaled).
Polarized Photocathode R&D at St. Petersburg Technical University
Polarized electron emission from strain-compensated
superlattices
Conclusions
1. New structures based on InAlGaAs-GaAsP straincompensated superlattices with high structural
quality has been grown and studied as candidates
for highly polarized electron emission.
Polarized Photocathode R&D at St. Petersburg Technical University
Polarized electron emission from strain-compensated
superlattices
Conclusions
2. Allowing the electron spin relaxation and
nonlinear dependence of the hydrostatic
pressure coefficients, calculated polarization
spectra are in a good agreement with the
observed excitation spectra of polarized
electron photoemission.
Polarized Photocathode R&D at St. Petersburg Technical University
Polarized electron emission from strain-compensated
superlattices
Conclusions
3. Since there is still discrepancy between the
experiment and the theory in the positions of
the polarization maxima we believe that the
optimization of the new strain balanced
structures, is still possible.
Polarized Photocathode R&D at St. Petersburg Technical University
Optical orientation in strained-barrier GaAs/AlInGaAs
SLs with controlled parity of atomic layers
The nearest neighbors of an As interface atom. The point
symmetry C2v of a single heterojunction contains the
twofold rotation axis C2 parallel to the growth direction
and two mirror planes (110) and (110).
Polarized Photocathode R&D at St. Petersburg Technical University
Optical orientation in strained-barrier GaAs/AlInGaAs SLs
with controlled parity of atomic layers
GaAs
G6
e1
Conduction band offset
appears to be minimized for
x=1.1*y
hh1
x, y = 0.18-0.20 are found to
be close to an optimum
Ec
Ec1
2
AlxInyGa1-x-yAs
G8
Ev1
Evh2
Evl2
lh1
x=0.2, y=0.18
Band edge position in AlxInyGa1-x-yAs/GaAs SL. The electrons
and holes minibands e1 and hh1, lh1 are shown by thin lines.
Polarized Photocathode R&D at St. Petersburg Technical University
As cover
GaAs
In0.18Al0.2Ga0.62As
GaAs
Al0.4Ga0.6As
11019 cm-3 Be
60 A
SL
N Mono layers
M Mono layers
1.25 mm
Buffer
41017 cm-3 Be
61018 cm-3 Be
p-GaAs substrate
Structure of sample In0.18Al0.2Ga0.62As/GaAs SL cathode
Sample
1 (5-165)
2 (5-166)
3 (5-167)
4 (5-168)
n
14
13
15
16
m
14
15
17
16
Number of periods
12.5
12.5
11.5
11.5
d, nm
104
104
109
109
SLs design
Polarized Photocathode R&D at St. Petersburg Technical University
QE 5-165
QE 5-167
QE 5-166
QE 5-168
P 5-165
P 5-167
P 5-166
P 5-168
90
10
80
70
10
10
10
60
-1
50
40
-2
P, %
QE, %
1
30
20
-3
10
10
-4
0
550
600
650
700
750
800
850
900
Wavelength, nm
Polarization and quantum yield spectra for samples with controlled
parity of atomic layers number
Sample
1 (5-165)
2 (5-166)
3 (5-167)
4 (5-168)
Maximum polarization, %
77.6
80.1
76.2
81.5
QE (Maximum polarization), %
0.5
0.22
0.15
0.6
Polarized Photocathode R&D at St. Petersburg Technical University
P 14x14 experiment
P 15x17 experiment
P 14x14 calculation
P 15x17 calculation
QE 14x14
QE 15x17
10
1
70
-1
60
10
-2
50
10
40
-3
10
Polarization, %
Quantum efficiency, %
80
30
-4
10
1,30
1,35
1,40
1,45
1,50
1,55
1,60
1,65
1,70
1,75
1,80
1,85
hv, eV
Polarization and quantum efficiency spectra for samples
with controlled parity of atomic layers. Dotted and dashed
lines depict calculation results.
Polarized Photocathode R&D at St. Petersburg Technical University
Vl,h = Ul,h  {Jx Jy } (z – zl )
zl - the heteroboundary coordinate Jx , Jy - the momentum operators
 takes two values (+1 and -1) varying from one heterointerface to
another
The additional term in the SL Hamiltonian describing the
mixing of the light and heavy hole states at the SL interfaces
VГ,Х = TГ,Х ( zl ) (z – zl )
TГ,Х - a constant of the mixing strength
( zl ) - phase factor which changes its sign from one monolayer to
another
The interaction Hamiltonian for this mixing for the Г – Х
mixing in the conduction subbands at the well boundaries
Polarized Photocathode R&D at St. Petersburg Technical University
14x14 Calculation
14x14 Experiment
15x17 Calculation
15x17 Experiment
60
Polarization, %
50
40
30
20
1,45
1,50
1,55
1,60
1,65
1,70
1,75
1,80
hv, eV
Polarization and quantum efficiency spectra for samples
with controlled parity of atomic layers near second
polarization maximum
Polarized Photocathode R&D at St. Petersburg Technical University
Optical orientation in strained-barrier GaAs/AlInGaAs SLs
with controlled parity of atomic layers
Conclusion
1. Set of structures based on InAlGaAs-GaAs shortperiod strain-barrier superlattices with controlled
number of atomic layers has been grown and
studied by spin-polarized photoemission technique
Polarized Photocathode R&D at St. Petersburg Technical University
Optical orientation in strained-barrier GaAs/AlInGaAs SLs
with controlled parity of atomic layers
Conclusion
2. Theoretical investigation of these structures
with allowance of hole mixing at the interface
and electron tunneling through X- valleys is
fulfilled.
Polarized Photocathode R&D at St. Petersburg Technical University
Optical orientation in strained-barrier GaAs/AlInGaAs SLs
with controlled parity of atomic layers
Conclusion
3. The mixing effects can give substantial contribution
in the polarization losses in the excitation moment
and must be taken into account in the photocathode
characterization and design.