1. high performance photocathodes for microscopy and accelerators
8th Oct. 2012
15:15~15:40
Photocathode Physics for Photoinjectors (P3) 2012　　
Development of
high performance photocathodes
for microscopy and accelerators
○ X.G. Jin1, F. Ichihashi1, A. Mano1, N. Yamamoto1,
Y. Takeda1, M. Yamamoto2, T. Miyajima2, Y. Honda2,
T. Uchiyama2, S. Matsuba3
1Nagoya University
2High Energy Accelerator Research Organization (KEK)
3Japan Atomic Energy Agency

2. Outline Transmission-type spin-polarized photocahtode for microscopy
GaInP photocathode for ERL

3. Outline Transmission-type spin-polarized photocahtode for microscopy
GaInP photocathode for ERL

4. Introduction
Spin-polarized electron beams are drawing much attention in several types of electron microscopy for magnetic image.
Spin-polarized LEEM observation
Electron beam for microscopy requires
two properties:
High spin-polarization,
(for high image contrast)
High brightness
(for short exposure time)
A : Magnetic contrast M P   A  P
: Spin-polarization of electrons
SPLEEM Image 
M : Magnetization of surface P  M
W(110)
Co : 4 ML
Polarized
electrons
2 m
Conventional electron beam for SPLEEM:
Spin-polarization=20~30%
Brightness=1103 Acm-2sr-1
Exposure time=1-10 s
Real-time imaging with a high contrast:
Spin-polarization >80%
Brightness >3105 Acm-2sr-1
Exposure time< 0.03 s

5. Semiconductor photocathode
Vacuum
Electron
Conduction band
Light
Valence band
Negative electron
affinity surface

6. Semiconductor photocathode for spin-polarization
GaAs-type semiconductor
Strained
superlattice
Bulk
Conduction band
Circularly-polarized
light ｘ
Band splitting
Heavy hole band
Γ-point
Γ-point
Light hole band
Up-spin electron Down-spin electron
In GaAs type semiconductor, the heavy hole band and the light hole band degenerate
at -point and both up- and down-spin electrons are excited simultaneously by
circularly polarized light.
If the valence bands are split, one type of electrons can be excited selectively.
The band splitting allows excitation of one type of spin-polarized electrons.

7. GaAs/GaAsP strained superlattice
Spin-polarization result
Spin-polarized
electrons
Circularly
polarized light
92% spin-polarization
GaAs/GaAsP
superlattice 12 pair
GaAsP buffer layer
GaAs substrate
GaAs/GaAsP strained
superlattice structure
T. Nakanishi, Proceedings of LINAC (2002) 813.
Using GaAs/GaAsP strained superlattice grown on GaAs substrate, we have achieved the highest spin-polarization of 92%.

8. by highly focused laser light irradiation
High brightness
by highly focused laser light irradiation I
I: Electron beam current
S: Electron generation area
: Electron beam solid angle
Brightness =
S
High brightness is obtained by reducing electron generation area.
Conventional reflection-type
New transmission-type
Diameter 100 m
Diameter 1~2 m
The pump laser light is difficult to be focused, and the electron generation
area is large.
The pump laser light can be highly focused, and the electron generation
area is very small.

9. GaP substrate Conventional reflection-type New transmission-type
GaAs substrate
Band gap energy: 1.42 eV
GaP substrate
Band gap energy: 2.26 eV
Pump laser light energy: 1.4~1.8 eV
GaAs band gap energy is smaller than the pump laser light energy, and irradiation through the substrate is impossible.
On the other hand, GaP is transparent to pump laser light, and transmission-type photocathode is possible.

10. Transmission-type photocathode
photocathode structure
Growth conditions
Growth method: MOVPE,
Growth temperature： 660C,
Reactor pressure: 76 Torr,
V/III: 15,
Source materials： TEG, TBP, TBA.
Flow rates:
For GaAs,
TEG: 9.5 mol/min,
TBA: 143 mol/min.
GaAs/GaAsP
superlattice
GaAsP buffer layer
GaAs or AlGaAs
inter-layer
For GaAsP,
TEG: 9.5 mol/min,
TBA: 28 mol/min,
TBP: 114 mol/min.
GaP substrate
X.G. Jin et al. J. Appl. Phys. 108 (2010) #094509

11. Transmission-type electron gun
20 kV Electron Gun
Laser light
Photocathode
Brightness measurement
chamber
Electron beam
Electron beam
Mott Analyzer：
Spin-polarization measurement
Transmission-type electron gun was designed and
fabricated in Nagoya University

12. Effect of inter-layer ＴＥＭ image ＴＥＭ image Without inter-layer
50 nm
GaAs
GaAsP
g [004]
GaAs/GaAsP
superlattice
GaAsP buffer
Thickness modulation
GaP substrate
100 nm
With inter-layer
ＴＥＭ image
GaAs/GaAsP
superlattice
g [004]
GaAsP buffer
GaAs inter-layer
GaP substrate
100 nm

13. Spin-polarization measurement
With inter-layer
90%
Spin-polarization (%)
Without inter-layer 64％
Wavelength (nm)
The maximum spin-polarization of 90% was achieved in transmission-type photocathode.

14. Brightness measurement=
r2 L2
(R-r)2
I: electron beam current
r: electron beam source radius on photocathode
R: electron beam radius
L: length between photocathode and knife
Laser spot image on
photocathode back side
Laser light
Laser spot profile
Intensity (a.u)
FWHM
1.3 m
L=531 mm
1.9 mm
10 m
Position (m)
Laser spot diameter: 1.3 m, Electron beam current: 3.2 A,
Electron beam diameter: 1.9 mm.
The brightness is 1.3107 Acm-2sr-1 kV).　It is by times higher than that of the conventional reflection-type photocathode.

15. Progression of Photocathode
2008
2010
2011
Spin-polarized
electrons
Spin-polarized
electrons
Spin-polarized
electrons
GaAs/
GaAsP
Strain
compen
sated SL
GaAs/
GaAsP SL
GaAs/
GaAsP SL
GaAs layer
AlGaAs layer
GaP Sub.
GaP Sub.
GaP Sub.
A.R.
coating
Laser light
Laser light
Laser light
Spin-polarization: 90%;
Brightness: ~107 Acm-2sr-1
Quantum efficiency: 0.1%
Spin-polarization: 90%;
Brightness: ~107 Acm-2sr-1
Quantum efficiency: 0.4%
Spin-polarization: 90%;
Brightness: ~107 Acm-2sr-1
Quantum efficiency: 0.5%
(without A.R. coating)
X.G. Jin et al. APEX 1 (2008) #045002
X.G. Jin et al. JJAP 51 (2012) #108004
X.G. Jin et al. submitted for publication

16. New SPLEEM Transmission-type electron gun system LEEM Electron gun
Screen
Spin rotator
Sample

17. SPLEEM images
Specimen : Co 10 ML on W(110) substrate
View area: 6 m
Exposure time :
0.02 sec sec sec
1 m
M. Suzuki et al. Appl. Phys. Express 3 (2010) #026601
Clear magnetic image observed with exposure time 0.02sec.
Comparison: 1~10 s in conventional spin-polarized LEEM.

18. Spin-polarized TEM image
Transmission-type Gun
M. Kuwahara, S. Kusunoki, X.G. Jin et al.
Appl. Phys. Lett. 101 (2012) #
TEM with transmission-type spin-polarized photocathode is being developed at Nagoya University.

19. Conclusion
Transmission-type GaAs/GaAsP superlattice photocathodes were designed and fabricated.
A super-high brightness (1.3107 Acm-2sr-1 ) and a high spin-polarization (90%) of electron beam was achieved.
Using SPLEEM with transmission-type photocathode,
clear magnetic image was observed with exposure
time 0.02 s.
TEM with transmission-type spin-polarized photocathode is being developed at Nagoya University.

20. Outline Transmission-type spin-polarized photocahtode for microscopy
GaInP photocathode for ERL

21. ERL image
500 kV Electron Gun
cERL facility has been developed in KEK.

22. Targets for electron beam @Gun
High average current: 10~100 mA
Low emittance: 0.1~1 mm.mrad
Fast response: t90<10 ps
Drive laser:
532 nm 16 ps(FWHM) pulse Laser

23. Semiconductor photocathode
Vacuum e-
Energy
relaxation
Conduction band e- e-
Photons: h
Band
gap
(Eg)
Valence band
Energy
relaxation
Negative electron
affinity surface
(h - Eg) contributes to heat semiconductor photocathode.

24. III-V semiconductors Energy gap (eV) Lattice constant (Å)
532 nm (2.33 eV)
Ga0.52In0.48P
Eg: 1.9 eV
Energy gap (eV)
Eg: 1.42 eV
Lattice constant (Å)
Lattice constant (Å)

25. GaInP photocathodes 5-nm GaAs 5-nm GaAs 5-nm GaInP GaAs Activity layer
SI GaAs (001)
Substrate
SI GaAs (001)
Substrate
SI GaAs (001)
Substrate
Zn dopant concentration of GaAs, GaInP cap layer: 6.01019 cm-3.
Zn dopant concentration of GaAs, GaInP activity layer: 1.51018 cm-3.

26. Measurement system Emittance measuremet: Gun voltage: 100 kV
DC gun
Emittance measuremet:
Gun voltage: 100 kV
Measurement method: Waist scan
Current of electron beam: 2~3 nA
Response measurement:
Gun voltage: 100 kV
Pulse duration of the laser: 2 ps
Current of electron beam: 2~3 nA

27. GaInP activity layer photocathodes showed higher values QE.
Quantum efficiency
QE measurement:
523 nm
Pump laser
100 V
Electron beam
Photo-
cathode
GaAs activity
with GaAs cap
GaInP activity
with GaAs cap
GaInP activity
with GaInP cap QE
@523 nm
9% % %
GaInP activity layer photocathodes showed higher values QE.

28. Mean Transverse Energy
MTE （meV)
GaAs with
GaAs cap
GaInP with
GaAs cap
GaInP with
GaInP cap

29. Electron scattering Photocathode Vacuum CaP Activity layer layer
Conduction band
Photons: h
Valence band
CaP
layer
Activity layer
Activity and cap layers have different effects on excited electrons.

30. Excited electrons were distributed on constant energy surface.
In activity layer KY
Constant energy
surface
E = (h - Eg) KZ  KX
Excited electrons were distributed on constant energy surface.

31. Electrons lose energy and approach to  point.
In activity layer KY KZ  KX
Electrons lose energy and approach to  point.
Absorption length and thickness of activity layer affect MTE.

32. In cap layer NEA value [meV]
D.A. Orlov et al., APL 78 (2001) 2721
In band bending region, electrons were scattered elastically,
and emitted with larger transverse energy.

33. Band effect on MTE GaAs activity layer with GaAs cap layer
Absorption constant for 544 nm
GaAs: 8104 /cm, GaInP: 2.5  104 /cm
Conduction
band
0.12 eV
Band
bending
Band
bending
Vacuum
level
Eg:1.42 eV
Eg:1.90 eV
Vacuum
level
Valence
band
0.36 eV
GaAs
GaAs
cap
GaInP
GaInP
cap
GaAs activity layer with
GaAs cap layer
GaInP activity layer with
GaInP cap layer

34. Surface layer effect on MTE
Conduction
band
Band
bending
Band
bending
Eg:1.90 eV
Eg:1.90 eV
Vacuum
level
Valence
band
GaInP
GaAs
cap
GaInP
GaInP
cap
GaInP activity layer with
GaAs cap layer
GaInP activity layer with
GaInP cap layer

35. Response time GaInP activity layer with GaAs cap layer
Wavelength:
532 nm
Fitting functions: Af
Tauf (ps) As
Taus (ps)
Tzero (ps)
SigmaT (ps)
t90 (ps)
6.5165

36. Conclusion GaInP based photocathodes were developed.
GaInP activity layer with GaAs cap layer photocathode showed a higher QE.
The GaInP based photocathode showed a small value
of mean transverse energy that is almost the same with
That of GaAs photocathode.
A short pico-second electron pulse was achieved from GaInP based photocathode.

38. Comparison among electron sources Reflection-type GaAs bulk
Transmission-type
GaAs/GaAsP
107
LaB6 emitter
(applied to LEEM)
Brightness (Acm-2sr-1)
105
Reflection-type GaAs bulk
(applied to Spin-Polarized LEEM)
Reflection-type
GaAs/GaAsP
103 20 40 60 80
100
Spin-polarization (%)

40. Response time 600 nm GaInP with GaAs cap layer Fitting functions: Af
Tauf (ps) As
Taus (ps)
Tzero (ps)
SigmaT (ps)
t90 (ps)
6.5165