Doctoral Dissertation

Photo-cathode studies for High
Performance Linear Accelerator

Lei Guo
Graduate School for Advanced Sciences ofMatters, Hiroshima University, Higashihiroshima,
Hiroshima 739-8530, Japan

Supervisor: Professor Masao Kuriki

07 2017

Contents
List of tables

3

List of figures

7

Abstract

8

Acknowledgments

91 Introduction

10

2 Photo cathode in modern accelerators
2.1 ERL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 FEL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Linear Colliders . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16
16
19
21

3 Photocathode3.1 Metal cathode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 CsK2 Sb photocathode . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 NEA-GaAs photocathode . . . . . . . . . . . . . . . . . . . . . . .

24
24
25
27

4 Substrate dependence of CsK2 Sb cathode4.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2 Substrate surface treatment . . . . . . . . . . . . . . . . . . . . . .
4.3 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

31
31
36
38
44

5 Eﬀect of Gas Adsorption on NEA GaAs
5.1 Experimental Setup . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

47
47
48
51

6 Summary57
1Appendix

65

A NEA GaAs as a polarized electron source

66

B Atomic arrangement model of unit cell

68

2

List of Tables
2.1
2.2
2.3
2.4
2.5

Electron beam parameter of ERL project [21][22][37] . . . . . . . .
Parameters of electron gun for ERL[21][22] . . . . . . . . . . . . . .
Parameters of the electron beam for LCLS[43] . . . . . . . . . . . .
Parameters of the electron beam for LCLS II.[44] . . . . . . . . . .
The electron beam parameters at the gun exit of ILC.[50] . . . . .

18
19
20
21
23

4.1

The maximum QE of the CsK2 Sb photo-cathode on Mo(100), Mo(amorphous),Si(100), Si (111), and GaAs (100) substrates at 532 nm are summarized. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.1

Mass to charge ratio and ion current of the major peaks of the mass
spectrum by QMS with CO2 introduction. The rightest row shows
the candidate molecule. The inside of the parenthesis is the mothermolecule. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

B.1 The maximum QE of the CsK2 Sb photo-cathode with ζ of CsK2 Sb(100)and CsK2 Sb(111) surface at 532 nm are summarized. . . . . . . . . 70

3

List of Figures
1.1

Livingston plot, in which the horizontal axis represents year andthe vertical axis represents acceleration energy, indicating that theenergy increased rapidly in time.[1] . . . . . . . . . . . . . . . . . . 10

2.1 An overview graphic of the ERL.[35]. . . . . . . . . . . . . . . . . . 17
2.2 The conceptual diagram of FEL.[42] . . . . . . . . . . . . . . . . . . 20
2.3 A schematic overview of ILC layout based on the accelerator design
described in Technical Design Report.[47] . . . . . . . . . . . . . . . 22
3.1 Schematic drawing of the photo-electron emission from the NEA
GaAs cathode. Electron excited by laser to the conduction band isthermalized. Because the state of the conduction bottom has higher
energy than that of the vacuum, electron can be emitted throughthe surface thin barrier. [20] . . . . . . . . . . . . . . . . . . . . . . 28
3.2 A schematic diagram of the electric dipole made by Cs vapor-depositedon the GaAs surface. Electric dipoles are formed on the GaAs surface, and the surface potential is lowered. . . . . . . . . . . . . . . 29
4.1 The picture of the evaporation chamber.This evaporation chamber
is made from SUS 304 chemically polished on the inner surface.
The vacuum was maintained with an ion pump (ULVAC PST-100
100 l/s pumping speed)[102] and NEG (Non Evaporative Getter, SAES Capacitorr-D400 480 l/s )[103] and was kept at UHV (atypical pressure was 1.0e-8Pa). . . . . . . . . . . . . . . . . . . . . . 32
4.2 3D drawing of the CsK2 Sb evaporation chamber. Up to 3 cathodepucks can be stored in the chamber. The cathode can be transfered through the gate valve to another vacuum chamber (vacuum
suitcase) for cathode transportation. . . . . . . . . . . . . . . . . . 33
4.3 The drawing of the cathode puck. For each cathode puck, a 15 mm
× 15 mm substrate is placed on a molybdenum base plate. . . . . . 33

4

4.4

The picture of the cathode holder. Up to 3 cathode pucks can be
set. The holder can be moved vertically to adjust the position forevaporation, photo-electron emission, and transfer. . . . . . . . . . .
4.5 Si(100) and GaAs(100) substrates. Si(100) and Si(111) wafers witha of ≤ 0.002 Ωcm−1 and GaAs(100) p-type wafer are employed as
the substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6 The substrates mounted on the puck. The substrate is soldered on
the the base with indium and fixed by a tantalum cup. The puckis mounted on the cathode holder during evaporation and photoelectron emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7 The picture of the evaporation sources. The evaporation sources
are mounted on a linear stage in the chamber. K and Cs can be
set each of three in the evaporation sources. The Sb pellets wereresistively heated in a tungsten evaporation basket. . . . . . . . . .
4.8 A vertical view of the arrangement around the cathode puck. The
thickness monitor and the cathode puck were placed in symmetrical
positions to the evaporation source. The cathode puck temperature
is controlled by a heater from the backside. . . . . . . . . . . . . . .
4.9 Etching procedure of Si substrate. The Si(100) and Si(111) substrates were processed with a 5% HF solution for about 5 min.[106]4.10 Etching procedure of GaAs substrate. The GaAs surface was processed with a H2 SO4 :H2 O2 :H2 O (4:1:1) solution for about 1 min[90]and to make As rich surface. . . . . . . . . . . . . . . . . . . . . .
4.11 Heat cleaning. The substrate is heated up to 600◦ C in vacuum.
Temperature is shown red line and vacuum pressure is shown blueline during the heat cleaning. . . . . . . . . . . . . . . . . . . . . .
4.12 QE evolution and progress of thickness of Sb, K, and Cs. QE evolution is shown red circle and progress of thicknesses of Sb, K, andCs are shown blue triangle. QE appears in K evaporation and increases rapidly in Cs evaporation. The thickness are 10nm, 70nm
and 1400nm for Sb, K and Cs, respectively. . . . . . . . . . . . . .
4.13 The maximum QE is shown as a function of temeperature during
the evaporation with blue squares and red circles for the Si(100) andSi(111) substrates, respectively. The error on the horizontal axis isfluctuation during evaporation and the error on the vertical axis isstatistical error by 3 times measurements. . . . . . . . . . . . . . .

5

34

34

35

35

36
37

37

38

39

40

4.14 The maximum QE is shown as a fucntion of Sb thickness withblue squares and red circles for the Si(100) and Si(111) substrates,
respectively. The error on the horizontal axis and error on thevertical axis are only statistic. . . . . . . . . . . . . . . . . . . . .
4.15 The comparison of QE evolution during the cathode evaporation
for the cleaned substrates are shown with red circles, blue triangles,
and green squares for the cleaned Si(100), Si(111), and GaAs(100)substrates, respectively. QE appears in K evaporation and increases
rapidly in Cs evaporation. K evaporation started on 48, 45 and 45
min for the cleaned Si(100), Si(111), and GaAs(100) substrates,
respectively. Cs evaporation started on 183, 175 and 135 min forthe cleaned Si(100), Si(111), and GaAs(100) substrates, respectively.
4.16 The comparison of QE evolution during the cathode evaporationis shown with red circles, blue circles, and green circles for the asreceived Si(100), Si(111), and GaAs(100) substrates, respectively.
In the blue, green, and yellow shaded regions, Sb, K, and Cs were
evaporated, respectively. The QE appears in the K evaporation andincreases rapidly in the Cs evaporation. . . . . . . . . . . . . . . . .
4.17 Atomic arrangement of CsK2 Sb(100) on the Mo(100), GaAs(100)and Si(100) surfaces. The lattice constants of Mo, Si, and GaAs are
3.15Å, 5.43Å, and 5.65Å, respectively [111]. . . . . . . . . . . . . .
4.18 Atomic arrangement of CsK2 Sb(111) on the Si(111) surface. . . . .
A schematically drawing of the sample gas introduction system.
The system and the main chamber are connected by a leak valve tocontrol the gas introduction rate. The gas species can be changed
by toggling valves to each gas bombs. The system is evacuated bya rough pump for every gas switching to avoid gas mixture. . . . .
5.2 QE evolution during the activation process by Cs and O2 . The firstpeak is made by Cs only. In other peaks, QE is increased by O2
and decreased by Cs. . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3 QE evolution of the activation process with Cs and CO2 . The activation procedure was same as that with O2 except the introduced
gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4 QE evolution during the activation process with Cs and CO. QE did
not rise with CO. After t=0.9 h, the NEA activation was performedby Cs and O2 . The solid line is that with CO gas introduction andthe dashed line is that by O2 . . . . . . . . . . . . . . . . . . . . . .

41

42

43

45
46

5.1

6

48

49

50

51

5.5

QE evolution of the activation process with Cs and N2 . After t=1.05
h, the NEA activation was performed with Cs and O2 . The solidline is that with N2 gas introduction and the dashed line is thatwith O2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
5.6 The height of each peak in Fig.5.2is shown as a function of O2 gas
exposure. The red solid line is a fitting curve by assuming Eq. (5.4). 53
5.7 The height of each peak in Fig.5.3is shown as a function of CO2 gas
exposure. The red solid line is a fitting curve by assuming Eq. (5.3). 55
A.1 The band structure of GaAs and the state of excitation by photons
to the conduction band. (a) shows bulk GaAs crystal, (b) is a diagram of distorted GaAs crystal. Degeneracy of angular momentum
3/2 state is solved by distortion.[20] . . . . . . . . . . . . . . . . . . 67
B.1 Atomic arrangement of CsK2 Sb(100) and CsK2 Sb(111) on the Mo(100),
GaAs(100), Si(100), and Si(111) surfaces. The lattice constants of
Mo, Si, and GaAs are 3.15Å, 5.43Å, and 5.65Å, respectively [111]. . 68
B.2 ζ is obtained with the atomic positions of the substrate and CsK2 Sb
by Eq. B.1. By rotating angle θ, we take the minimum value as arepresentative matching index. . . . . . . . . . . . . . . . . . . . . . 69
B.3 ζ(100) is calculated as a function of the rotation angle θ for CSK2 Sb(100)and the various substrates. . . . . . . . . . . . . . . . . . . . . . . . 71
B.4 ζ(110) is calculated as a function of the rotation angle θ for CSK2 Sb(110)
and the various substrates. . . . . . . . . . . . . . . . . . . . . . . . 72
B.5 ζ(111) is calculated as a function of the rotation angle θ for CSK2 Sb(111)
and the various substrates. . . . . . . . . . . . . . . . . . . . . . . . 73

7

Abstract
The accelerator has contributed greatly to develop many scientific displine, e.g.
elementary particle/nuclear physics, synchrotron radiation sciences including material physics, bio-science, industrial applications, medical applications, etc. Although the electron storage ring has been widely used as the advanced accelerator
for the electron and positron collider and the third generation light source, thebeam performance of these ring accelerators is strongly limited by two aspects by
the synchrotron radiation. One is the large energy loss limiting the beam energy
practically up to 120 GeV. Another is the equilibrium state limiting the beam emittance, especially in horizontal direction. To obtain the better beam performance
breaking the limits by the ring, Linac (Linear Accelerator) is revived. The electron
source is one of the most important device in Linac, because the performance of theaccelerated beam strongly depends on the initial beam quality. A photo-cathodeis able to generate a high-performance electron beam with a large operability. The
thesis addresses a couple of studies for the advanced photo-cathode. The first is a
study of CsK2 Sb cathode. This cathode is driven by a green light laser (532 nm)with high quantum eﬃciency(QE) 10%. It is fabricated as a thin film by vacuum
evaporation on a substrate. We found that the performance strongly depends not
only on the substrate material and its chemical state (oxidized, etc.), but also on
the surface direction of crystal substrate. This is the first experimental evidence
in the world. The second is the NEA (Negative Electron Aﬃnity) activation of
GaAs photo-cathode with N2 , CO, and CO2 gases. NEA-GaAs cathode is able togenerate highly spin polarized electron beam more than 90% and the NEA activation is performed usually with Cs and O2 or NF3 . We found that CO2 activatedthe cathode, but N2 and CO did not. By analyzing CO2 activation, CO degrades
the NEA activated surface, and the NEA activation ability of atomic O is almosta half of that of O2 molecule. The NEA activation with CO2 and measuring theactivation ability of CO2 , O2 , and atomic O are the first experimental eﬀorts.

8

Acknowledgments
First, I would like to thank my supervisor, Dr. Masao Kuriki, for his continuous
support.
I would also like to thank Dr. Hiromi Okamoto, Dr. Hiroyuki Higaki and Dr.
Kiyokazu Ito for his technical guidance on experiments throughout my research.
I want to thank the other members of the Beam Physics Laboratory at Hiroshima University.
Finally, I thank my wife for supporting me.
And this work is partly supported by Photon and Quantum Basic Research
Coordinated Development Program from the the Ministry of Education, Culture,
Sports, Science, and Technology, Japan and Cooperative supporting Program for
Research and education in University by KEK(High Energy Accelerator Research
Organization), Japan.

9Chapter 1
Introduction

Figure 1.1: Livingston plot, in which the horizontal axis represents year and thevertical axis represents acceleration energy, indicating that the energy increased
rapidly in time.[1]Accelerator is a device that boosts the particle energy as its name suggests.
It is no exaggeration to say that the first half of the particle accelerator history10

is the history of elementary particle and nuclear physics. Figure 1.1 is Livingston
plot[1], in which the horizontal axis represents year and the vertical axis represents
acceleration energy, indicating that the energy increased rapidly in time. Particle
physics has been developed as the accelerator energy has been increased. Electron
and positron colliders with storage rings have been a powerful tool to explore theparticle physics since 1960s. The examples are VEPP[2], SPEAR[3], DORIS[4],TRISTAN[5], LEP[6], CESR[7], BEPC[8], PEP[9], KEKB[10], etc. In the 1960s, many ”elementary particles” were discovered by accelerators. Later, they were
understood as composite particles of quarks and gluons. The deep inelastic scattering of high energy electrons with nucleon revealed the structure of the nucleus
and that was an experimental evidence of the quark model. In the 1970s, the discovery of J/Ψ particles, which is the binding state of c quarks, and in the 1980s,
W Boson and Z Boson which are gauge particles that mediate weak interaction
were discovered. In 1990s, LEP (Large Electron Positron collider)[6] which is ane+ e- ring collider, and SLC (SLAC Large Collider)[11] which was the first e+ elinear collider, studied the detail of Z Boson property. In 2012, the Higgs Boson
which was the last piece of the standard model was discovered by LHC (Large
Hadron Collider) at CERN. Since electron and positron are elementary particles
having no structure and are anti-particle to each other, it is possible to study any
energy state reachable by CME (Center of Mass Energy) with the well-defined initial state. The e+ e- colliders greatly contribute to establish the standard modelof particle physics.
After top quark was discovered by TEVATRON at Fermi national laboratory,
USA, Higgs particle was “the last pierce” of the standard model. It is theoretically
predicted, but it was not discovered yet. Although LEP had tried to find Higgs
particle by scanning its energy up to 209 GeV CME, it could not. Finally, Higgs
particle was discovered by LHC which is a proton-proton collider with 8 TeV CME.
The proton collider is reachable such high energy region, but the initial state is
not fixed because the proton is a composite particle. To study the detail property
of not only Higgs, but also top quark and any new particles, we need e+ ande- collider in several 100 GeV CME or higher. CME energy of Higgs production
through ZH process is peaked at 250 GeV. Top pair production is on 350 GeV
CME. On the other hand, it is not realistic to increase the energy of the e+ ande-ring collider further to this energy region, because the synchrotron energy loss
is proportional to the fourth order of the beam energy. The energy loss of LEP
at 209 GeV CME was 2 GeV per turn. If we consider 500 GeV CME, the energy
loss becomes 66 GeV per turn. Obviously, it is not manageable by considering thehuge space for accelerator in the ring or the huge electricity.
11Instead of the ring e+ e- collider, e+ e- linear collider has been considered since
1980s as the next generation e+ e- collider. In linear collider, the beam is accelerated by linear accelerator. The beam orbit is in line and the synchrotron radiationis negligible. High energy physics society and ICFA (International Committee
for Future Accelerator) anounced to promotes International Linear Collider (ILC)
project[12] as the next high energy physics project in 2004. ILC is an electronpositron collider with superconducting linear accelerators. In linear collider, determination of the initial state as precise as possible is essential to maximize thephysics ability. That leads that the spin state of electron and positron shouldbe aligned to one direction, polarized. A highly spin polarized beam is strongly
demanded by linear colliders.
Another big usage of accelerator than the particle physics is the synchrotron
radiation science. synchrotron radiation was theoretically predicted in 1946. SR
is emitted when a charged particle receive any force. It has been experimentallyconfirmed in an electron synchrotron in the following 1947. The synchrotron radiation power is proportional to the fourth power of the beam energy and rapidly
increases as energy increases. The synchrotron radiation is merely an energy loss
from the viewpoint of increasing the energy of particles as we mentioned, but it
is applicable as a light source, because it has high brightness, high directive, polarization, pulse characteristics, etc. The use of synchrotron radiation began as aparasite. It was called as the first generation light source. The second generation
light source is a dedicated accelerator (electron storage ring) as the synchrotron
radiation source. The examples are NSLS[13], BESSY[14], Photon Factory[15].
The third generation light source is a storage ring incorporating a magnet (wiggler or undulator) dedicated and optimized to generate the synchrotron radiationand the storage ring was optimized for low emittance. The third generation lightsource is a completed scheme of the Synchrotron Radiation source based on anelectron storage ring. The examples are ESRF[16] and NSLS II[17], Spring-8[18],and MAX IV[19].
Although demands for the synchrotron radiation light is improved in time, suchas short pulse, high coherence, becomes higher and higher, the storage ring based
light source has limitations on the performance in principle. The pulse lengthand emittance of the beam are determined as an equilibrium state of quantum
excitation and radiation damping in a storage ring. The pulse length(σ) of thebeam stored in a storage ring is determined as[20],√√2πcηc E0 σE(1.1)
σ=ωrev heV cos ϕs E
where ωs is the revolution frequency, σE is energy spread, ηc is momentum com12

paction factor which is the ratio of the variation of the revolution period to themomentum, V is the RF voltage, E is the beam energy. To obtain a short pulse,
the RF voltage for re-acceleration should be large, but it has technical limitation.
The practical limit on the bunch length is in order of 10s ps. The ultra-short pulse
such as ps or sub-ps cannot be generated with the third generation light source.
Coherence is a typical property of waves. High coherence light is useful invarious contexts. For example, by using phase information of light, structure
analysis for a huge molecule such as protein is possible without crystallization.
To make high coherent light as the synchrotron radiation, the electron bunch size
should be “point like” comparing to the wavelength of the light. That means shortpulse and low emittance. We have already mentioned that the storage ring has
limitation on the bunch length. The emittance in the storage ring is expressed
as[20]Cq γ 2 1 ∫Hds
ϵx =Jh 2πρ2(1.2)

where Cq is a constant, Jh is damping partition number, γ is Lorentz factor, ρ is the∫radius of curvature of the bending electromagnet, Hds is a function of momentum
dispersion. To minimize the emittance, the momentum dispersion should be small,but there is a limitation in principle. There is a practical limitation on the orbit
radius, too. The practical limit on the emittance of the storage ring can be inorder of 0.1 nm·rad. In short, it is diﬃcult to obtain the coherent hard X-ray withthe third generation light source.
Linac based synchrotron radiation facility can realize such short pulse andhigh coherent light. The linear accelerator is that accelerates the charged particle
with periodic RF structures aligned in line. The beam geometry in the linear
accelerator is independently determined from the lattice structure. Then, if one
can generate the low emittance beam and can accelerate the beam without asignificant emittance growth, an extremely low emittance beam can be obtained.
That is also true for a short pulse beam. In Linac, higher electricity consumption
per beam current is an issue, because the beam is not re-used (circulated). Energy
Recovery Linac (ERL)[21][22] is a solution to cure the problem. In ERL, the beamenergy is transferred from the used beam to the new beam. The beam is always
fresh, but most of the beam energy is re-used. ERL generates a high intensity
beam with a small emittance and short pulse length in CW. ERL can be an ideal
facility for users which requests an extremely small emittance and short pulse such
as Laser-Compton X-ray source.
A highly coherent synchrotron light can be obtained with FEL (Free Electron
Laser)[23] based on Linac. FEL consists from a Linac and undulator which is a13

device making the transverse magnetic field changing the sign periodicaly. If a low
emittance beam with a large peak current passes through the undulator, a dense
synchrotron radiation light will be emitted. If the light has a small coherence, theelectro-magnetic field by the light modulates the electron bunch intensity which
increases the coherence. The improved coherence on the beam enhances the coherence and power of the synchrotron light. This positive feedback process between
the bunch intensity modulation and coherent synchrotron radiation results a highintensity coherent synchrotron radiation. This is called as FEL.
As we explained, Linac is expected to be a break-through in the particle physics
and synchrotron radiation science. In Linac, the beam emittance is determined by
the initial beam emittance from the electron source and emittance growth eﬀect
such as space charge eﬀect, coherent synchrotron radiation in bending magnet,
etc. To minimize the beam emittance of a linear accelerator, the initial emittanceshould be small enough with good emittance growth compensation. The beamsource is one of the most important subsystem in linear accelerator.
There are four physical process to generate electron beam; thermionic emission, photo-electro emission, field emission, and secondary emission. Among them,
thermal emission and photo-electro emission are popular as the electron source foraccelerator. By employing a short pulse laser, photo-electro emission can generatea short pulse electron beam directly with a large operability. The electron sourceby the photo-electro emission with the short pulse laser is considered to be the advanced technology for the electron beam generation[24]. The material to generate
the photo-electron is called as photo-cathode.
Among various photo-cathode materials, CsK2 Sb and NEA GaAs cathodes are
the most advanced one.
CsK2 Sb is a semi-conductor fabricated as a thin film by vacuum evaporation
technique. The material can be driven by visible green light (532 nm) for thephoto-emission with more than 10% QE[25][26]. The material is robust against toa large emission current[27]. Due to these characters, CsK2 Sb is one of the best
candidate for high brightness electron source (a large current density). This is thebaseline design of LCLS II project[28], which is the first CW X-ray FEL projectin US. A large current up to 0.3 mA is required with 0.4 mm·mrad emittance.
It is well known that the cathode performance depends on the substrate material and surface condition[25] [29] [30] [31]. In this thesis, we studied the cathode
performance dependence systematically on the substrate material, surface state,and surface direction to improve the performance and its reproducibility.
NEA GaAs photo cathode is a unique device which is able to generate highly
spin polarized electron beam up to 90% and an extremely small emittance beam.
14

The photo-electron emission is possible with IR (Infrared Red) and visible lightand the QE can be more than 20%. This is the only solution of highly spinpolarized electron source for accelerator[32][33]. The spin polarized electron beam
has a crucial role in ILC project[12].
The NEA surface is a surface state in which the energy level of the vacuum islower than the lowest level of the conduction band. The surface is artificially madeby an activation process with Cs and Oxygen orNF3 evaporation on a cleaned
P-type GaAs. The NEA surface is fragile and easily damaged by residual gas
adsorption or beam extraction. At present, there are some hypotheses, but thestructure of the NEA surface is not understood well. To improve our understanding
for the NEA surface, we performed the NEA activation process with other gases
than O2 .
In Chapter 2, we explain the advanced accelerator projects based on Linac
from the technical point of view of electron beam generation.
In Chapter 3, the general aspects of photo cathode are explained.
In Chapter 4, the experiment and analysis for the substrate dependence ofCsK2 Sb are explained.
In Chapter 5, the experiment and analysis for the NEA GaAs activation withother gas than O2 are explained.
In Chapter 6, we summarize our studies and discuss possible impacts on theaccelerator science.

15

Chapter 2
Photo cathode in modern
accelerators
As we explained in the previous chapter, there are a bunch of the next generation
accelerator projects based on Linac. Because Linac has an open topology andthis is a simple response system, a good beam as the input is required in order toobtain a good beam as the output. Because the beam performance of the input
is determined by the cathode and the gun configuration, the gun is much more
important in Linac than that in the ring accelerator. In this Chapter, the cathodein the advanced accelerators is reviewed.

2.1

ERL

ERL is a new accelerator concept initially proposed by M. Tigner for linear collider[34].
The advantage of Linac is the beam quality, but the dis-advantage is large electricity consumption per beam current. The required wall plug power by Linac PLinac
is
IE
PLinac =,(2.1)
η
where I is the average beam current, E is the beam energy in eV, and η is thepower eﬃciency converting from the wall plug power to the beam power. The
eﬃciency depends on the technology and the maturity, but it is in order of 10%.
The wall plug power is several times of the beam power. On the other hand, thewall plug power in a ring accelerator, Pring isPring =

IE
,
ητ

(2.2)

where τ is the lifetime of the stored beam. The stored beam in a ring is lost by
several eﬀects such as beam scattering with residual gas and the beam current is
16

exponentially decayed. For the ring, the electricity is determined by not the beam
current (stored current), but the lost current, leading the much better eﬃciency ofthe electricity per the beam current. Please note that the synchrotron radiation
loss is ignored here. If the beam energy is large, so that the synchrotron radiationenergy loss becomes significant, the advantage of the ring is lost.

Figure 2.1: An overview graphic of the ERL.[35].

To compensate the electricity of Linac, ERL concept is considered. The electron
beam is accelerated and the accelerated beam is injected to the accelerator again
through a return path, but it is on the decelerated phase. The beam is decelerated
along the accelerator and the energy is transferred from the beam to RF powerin the cavity. In ERL, the beam is continuous, i.e. the accelerated beam and thedecelerated beam exist in the accelerator simultaneously. The power extracted
from the decelerated beam is moved to the accelerated beam. The electricity ismuch suppressed as
IχE
PERL =,(2.3)
η
where χ is the fraction of the injection energy to the full beam energy, because
the injection energy is not recovered. The topology of ERL is same as Linac if we
follow the electron beam trajectory, but the topology is torus (ring) if we follow thepower flow. Here we ignore the power dissipated in RF accelerator. To maximize
the merit of ERL, a super-conducting RF accelerator is the choice.
Because the beam is always fresh, ERL can generates a short pulse and coherent light. ERL performance surpasses that of third generation light source with an
17

Table 2.1: Electron beam parameter of ERL project [21][22][37]3GeVERL radiation source
cERL
Orbiting energy
3 GeV
60-85 MeV
Average current
10-100 mA
10-100 mA
Normalized emittance
0.06-1 πmm·mrad
0.1-1 πmm·mrad
Electronic bunch length
0.1-3 ps
0.1-3 psElectron bunch charge amount
7.7-77 pC
7.7-77 pC
Repetition frequency
1.3 GHz
1.3 GHzequivalent power consumption. For example, an X-ray light source using ERL cangenerate X-rays having spatial coherence and ultra-short pulse X-rays in femtosecond which opens up new X-ray sciences. Gamma ray source by the laser-Compton
scattering basen on ERL realizes a large flux that exceeds the existing gamma ray
source by 6 to 8 orders of magnitude, and has a great impact on the research suchas photo-nuclear science and industrial application such as non-destructive isotope
analysis[21][22][36].
Figure 2.1 shows an example of ERL designed for synchrotron radiation facility
which is capable to generate X-ray in the diﬀraction limit. It consists from a 5-15
MeV injector, a 400 m long super-conducting linac, two 800 m return arcs, and abeam dump line. The beam is dumped after it is decelerated down to the injectionenergy. The injection beam energy is equal to that in beam dump in principle,
which limits the power eﬃciency.
The electricity is much compensated by employing ERL, but the injector has toprovide a large current which is equivalent to the beam current. Because providing
a high performance beam which is superior to that of a storage ring is the aim of
ERL, the electron source of ERL has to provide a large current, short pulse, andsmall emittance beam.
Currently, the development of the next-generation synchrotron source by ERL
is under way in Japan and US[21][37]. In Japan, KEK, QST(National Institute forQuantum and Radiological Science and Technology), University of Tokyo, Nagoya
University, and Hiroshima University are jointly developing a test machine[37].
Tables 2.1 and 2.2 show parameters of an example of ERL facility and theelectron gun, respectively. The electron bunch length is 0.1-3 ps which is 10-100
times shorter than that of Spring-8[38]. Emittance is several or several ten times
lower than that of Srping-8 whose horizontal normalize emittance is 48 πmm·mrad,with an equivalent beam current.
18

Table 2.2: Parameters of electron gun for ERL[21][22]Electron gun for ERL
Energy500 kV
Average current
10-100 mA
Normalized emittance
0.1-1 πmm·mrad
Electronic bunch length
0.1-3 psElectron bunch charge amount
7.7-77 pC
Repetition frequency
1.3 GHzThe ERL gun has to provide 100 mA beam current with a high quality continuously. This is 104 times more than that for a conventional electron source foraccelerator. That leads several issues on the electron source which will be discussed
in detail later.

2.2

FEL

As briefly explained in 1, FEL is a kind of laser whose lasing medium is electrons
moving freely through undulator field[39][40]. FEL can achieve very high peak
power, tunability (covering from microwave to X-ray), coherence which are desirable in many disciplines of synchrotron sciences. The conceptual diagram of FEL
is shown in Figure 2.2.
In FEL, a small intensity modulation along the electron bunch in a wavelengthis amplified through FEL process. The initial modulation is noise in Self-Amplified
Spontaneous Emission (SASE) mode, but it is artificially generated in pre-bunched
FEL mode. The intensity modulation leads a high power radiation with a coherence and the high power radiation enhances the intensity modulation. This results
exponential growth of the power of the coherent radiation. High power radiation
is obtained finally[41].
Table 2.3 shows the parameters of the Linac Coherent Light Source (LCLS)
which is an X-ray FEL in operation at SLAC. Comparing to the 3rd generation
light source, the average current of LCLS is quite low, but the peak current is highwhich is essential for FEL. LCLS employs a metal photo cathode, Cu. It requires
a high power UV light for electron generation, but an ordinal laser technology isable to deliver such laser pulse, because the reputation is low.
LCLS, Euro XFEL(DESY), and SACLA(Spring-8) are pulse XFEL, i.e. Xray laser is provided in pulse. To improve the X-ray laser power, CW operationis the next step. LCLS II in SLAC[28] which is a CW X-ray FEL, is under19

Figure 2.2: The conceptual diagram of FEL.[42]Table 2.3: Parameters of the electron beam for LCLS[43]LCLSElectron energy
4-14 GeV
Bunch charge
0.1-1.0 nC
Bunch repetition rate
120 HzPeak current
3.4 kA
Average current< 1 µA
Normalized emittance 1.2 πµm·rad
Bunch length
230 fsconstruction. LCLS-II is constructed from the success of LCLS. It delivers photons
between 200 eV and 5 keV at repetition rates as high as 1 MHz (929 kHz) using
a superconducting RF (SCRF) Linac.[28].
LCLS II employ 186 MHz (the 7th sub-harmonics of 1.3GHz) RF Gun which
is originally developed by LBNL (Laurence Berkeley National Laboratory), USA.
The gun has to provide high current and low emittance beam continuously. Table
2.4 shows the parameters of the LCLS-II. The metal cathode is robust, but it is
not feasible for the high current generation, because an extremely high power UV
laser is required to generate the beam, since the QE is in order of 1e-4. Cs2 Te
semi-conductor cathode can be a candidate, because 1% QE can be maintained
for a long period, but it is not ideal, because it requires still UV light for photoemission. CsK2 Sb is the best candidate for the cathode because it is driven by agreen laser. The green light can be generated as SH(Second Harmonic) of a solidlaser a much better eﬃciency than that of UV laser. The 10% high QE is also a
big advantage. CsK2 Sb is feasible for the continuous high current operation since
it is robust enough to generate in order of 10mA beam current. In Chapter 3, we
discuss the status and issues of the cathode.

20

Table 2.4: Parameters of the electron beam for LCLS II.[44]LCLS IIElectron energy at gun exit
750 keVElectron energy
4 GeV
Bunch charge
100 pC
Bunch repetition rate
1 MHz
Peak current
10 A
Average current
0.062 mA
Normalized emittance
0.4 πµm·rad
Average CW RF gradient
16 MV/m

2.3

Linear Colliders

To study the detail property of not only Higgs, but also top quarks and any newparticles, we need e+ and e- collider of 250 GeV or higher CME. It is not realistic toincrease the energy of the e+ and e-ring collider further, because the synchrotron
energy loss is proportional to the fourth order of the beam energy. Instead, e+
e- linear collider has been considered since 1980’s as the next generation e+ ecollider. In linear collider, determination of the initial state as precise as possible
is essential to maximize the physics ability. That leads that the spin state ofelectron and positron should be aligned to one direction, polarized. A highly spinpolarized beam is strongly demanded by linear colliders.
ILC is a 250-500 GeV CME (extendable to 1TeV) electron-positron collider,based on 1.3 GHz superconducting radio-frequency (SCRF) linear accelerator as
shown in Figure 2.3. Kitakami mountain area, Iwate prefecture of northern Japan,
is the candidate site of ILC[45]. ILC is 30 km long for 500 GeV CME and 50 km
for 1 TeV CME [46]. ILC can be characterized as• A polarised electron source based on a photocathode DC gun.
• A polarised positron source obtained from pair creation by circularly polarized gamma ray generated by helical undulator.
• 3.2 km 5 GeV electron and positron damping rings (DR) for the extremely
low emittance beam in vertical direction.
• Two 11 km main linacs with1.3 GHz SCRF accelerator operating in 31.5
MV/m.

21

• Final focus system which focuses the beam size down to 7 nm in verticaldirection for the high luminosity.

Figure 2.3: A schematic overview of ILC layout based on the accelerator design
described in Technical Design Report.[47]Compared to LHC[48] which is a high enegy proton-proton collider, the CME(Center of Mass Energy) of LHC and ILC are up to 14 TeV and 1 TeV, respectively.
Because LHC is a proton-proton collider, only a part of the CME contributes to thereaction and there are many background events. CME of each event is statistically
distributed for proton collider and the initial state is basically not decidable. On
the other hand, ILC CME is lower, but CME is fully used for reaction and there
are few background events. The initial state of the interaction is well defined.
Therefore, ILC can search any phenomena in a clean experimental environment
with a high accuracy, even if the final state is invisible.
The electron beam parameters at the gun exit of ILC are shown in Table 2.5.
Since the extremely low emittance beam is formed by DR (Damping Ring), emittance of the electron beam from the electron gun doesn’t have to be small. The
most important parameter is the bunch charge (3.2 nC) and polarization (morethan 80% ). This large bunch charge is essential to obtain a large luminosity
together with the extremely small emittance at the interaction point. The polarization is essential to define the initial state and decrease the background event
such as W-W scattering to perform the accurate measurement[49]. The spin polarization is defined asP =N+ − N−
N+ + N−(2.4)

NEA-GaAs cathode is the only option to meet the ILC requirements, i.e. morethan 80% spin polarization. In ILC[12], the GaAs/GaAsP strained super-lattice22

Table 2.5: The electron beam parameters at the gun exit of ILC.[50]Electron gun for ILC
Polarization degree>80%
Bunch charge
3.2 nC
Normalized emittance
10 µm·rad
Electronic bunch length
1 ns
Bunch interval
308 nsMacro pulse length
0.9 msAverage current
5.2 mA (in pulse)

crystal is employed. The NEA surface is essential to generate the highly spinpolarized electron. Because the NEA surface is sensitive to residual gas pollution
and ion-back bombardment, this cathode is not compatible to RF gun and isused only in a DC electron gun. The structure of NEA surface is not understoodwell. Understanding the NEA surface and improving the cathode robustness areimportant not only from the fundamental physics point view, but also the practicalview point. NEA GaAs cathode is also useful as a high brightness (large current
and small emittance) electron beam, due to the small beam emittance[51]. In
Chapter 3, we explain the NEA-GaAs cathode in detail.

23

Chapter 3
PhotocathodeAs explained in the previous chapter, several advanced accelerators based on Linac
has been constructed and proposed to break the limitations of the ring accelerator. The photo-cathode has a crucial role in these accelerators. The initial beam
property is more important in Linac than that in ring, because the accelerator
performance depends on the initial beam. CsK2 Sb and NEA-GaAs cathode are
the key components of CW FEL/ERL and ILC, respectively. In this chapter, we
discuss general aspects of photo cathode materials and property of CsK2 Sb and
NEA GaAs regarding to the beam performance in detail.

3.1

Metal cathodeThe most popular photo-cathode material is a metal such as Cu[52], Mg[53], orAg[54]. Metal photocathode is used in a high gradient RF guns due to the highrobustness, such as S-band BNL gun IV[55]. LCLS employs the S-band RF Gun
with Cu photo-cathode to provide a high bunch charge and low-emittance beamfor X-ray FEL. The LCLS gun is operated for over a year without exchange thecathode[56]. The metal cathode is robust, so that it can be transferred in air
without any significant performance degradation. Due to the large work functionof the metal which is typically more than 3 eV, ultra-violet (UV) light is requiredfor photo-electron emission. The QE of photo-electron emission is typically inorder of 0.01% or less. Therefore, it requires a high power UV laser to generatea high current beam. It is diﬃcult to generate UV light directly and harmonic
generation technique is employed[57]. The UV light is obtained as the third or
fourth harmonics of the fundamental laser light such as Ti:Saphire or Nd:YAG
lasers. The harmonic generation is performed with a birefringence crystal. The
conversion eﬃciency is usually much less than unity, and the laser system becomes24complex. The harmonic generation is a non-linear phenomena enhancing the powerfluctuation in principle, and it degrades stability.
Metal has a limited penetration depth of photon to inside the material. Thatis a reason of the low QE, but it leads the extremely fast response to the photoemission. It is as fast as cannot be measured accurately.
The beam emittance from a metal cathode is [20],
R
ε=
2

√

h̄ω − ϕ
3mc2

(3.1)

where R is the beam size, h̄ is Planck constant, ω is angular frequency of laser,
ϕ0 is work function of the material, mc2 is electronic static mass. The emittancecan be minimized by adjusting the laser frequency to the work function. Please
note that the thermal contribution to the emittance is ignored in Eq. 3.1. If thelaser contribution is negligible, the thermal contribution becomes significant, but
the contribution is usually small.
QE as a function of the laser frequency is expressed asη(h̄ω) ∼ η0 (h̄ω − ϕ0 )2 ,(3.2)

which means the QE is approaching to zero by the adjustment. The high QE andthe low emittance are not compatible in metal cathode.
According to the property, metal cathode is feasible to generate a short pulse
electron beam, but low repetition rate. A good emittance beam can be generated,
but it requires a more laser power. Metal cathode is not feasible as cathode for
ERL or CW FEL due to the low QE. LCLS uses Cu cathode, but LCLS II which
is CW FEL, will employ CsK2 Sb.

3.2

CsK2Sb photocathodeCsK2 Sb is called as multi-alkali cathode because it composed from more than
one alkali metal. CsK2 Sb is a direct transition type semi-conductor with 1.2 eVband-gap energy [58]. It is formed as thin film layer with vacuum evaporation
technique[59]. It has been used as photo-cathodes in photo-multiplier tubes[60]and in a few electron guns[61][62].
CsK2 Sb forms a DO3 cubic structure[63]. The unit cell contains four formula√units and is represented by four face-centered sub-lattices shifted by a 3/4 (ais a lattice constant of the primitive translation vector) along the body diagonal[63][64]. The lattice constant a is 8.61 Å[63][65][66].

25CsK2 Sb cathode is fabricated as a thin film on a substrate by evaporation in
an ultra-high vacuum environment. Typically the CsK2 Sb cathode was formed bysequential evaporation of Sb, K, and Cs on a substrate[67].
The CsK2 Sb cathode showed relatively high QE as 7-10% with 532nm laser
light[25][26][68]. The cathode is robust enough for a high current beam generation
as demonstrated by Cornell University (60 mA about 30 hours)[69]. The CsK2 Sb
photocathode can reliably exhibit high QE like NEA-GaAs, and has a slightly
larger thermal emittance as compared to the NEA-GaAs[70], which would aﬀect
the maximum brightness of its electron beam, but it is considered to be a prompt
emitter because of its PEA(Positive Electron Aﬃnity) nature, producing shorter
electron bunches than GaAs. CsK2 Sb cathode is the most promising candidate forhigh brightness electron source due to high QE, green excitation and long life.
Since CsK2 Sb cathode has PEA, it is diﬃcult to extract the polarized electrons
as same as metal cathode.
Because CsK2 Sb cathode is generated by vacuum evaporation and is oxidized
immediately in air due to the high chemical activity of alkali metal, the material
property is not well understood. For example, many materials have been used as
the substrate of the evaporation, glass[71], Cu(amorphous)[72][29], SUS[29][30][31],Mo(amorphous)[29][31], Mo(100)[25], Si(100)[26], GaAs(100)[68], where the numbers in parenthesis are the surface direction of the crystalline substrate.
Cs3 Sb cathode has proven to depend strongly on the chemistry of substratesurface[59]. XPS studies for the CsK2 Sb cathode[31][73] suggest that the cathode
performance strongly depends on the substrate surface state (oxidation, etc.).
Cathodes fabricated on the amorphous substrates (glass, Cu, SUS, and Mo)
showed relatively low QE as 1-5% with 532nm laser light[71][72][29][30][31]. In contrast, cathodes fabricated on crystalline substrates (Si(100), Mo(100) and GaAs(100))
showed relatively high QE as 7-10% with 532nm laser light[25][26][68]. These results suggest that the substrate crystallinity has an impact on the cathode performance.
CsK2 Sb crystal direction was studied with XRD[74][75]. In the first study[74],the crystal was grown in (200) direction on Si(100) at room temperature(RT) andgrown in (220) direction at 100 ◦ C. On the other hand, in the second study[75],the crystal was grown in (200) direction on Si(100) at RT and 100 ◦ C. The QE ofthe cathode in these studies was relatively low (3 %) and the cathode performance
dependence on the surface direction of the substrate or CsK2 Sb crystal was not
significantly observed. One possibility is that the cathode evaporation condition
was not fully optimized in these studies.
Although the cathode performance depends on the substrate material as de26

scribed, any universal reason is not established yet. To establish the universal
understanding for the dependence is one of the purposes of this thesis.
In Chapter 4, we performed cathode evaporation on Si(100), Si(111), andGaAs(100) substrates to evaluate the performance dependence on the substratematerial and surface state systematically.

3.3

NEA-GaAs photocathode

GaAs is a III-V semiconductor having a direct band gap of 1.4 eV at room temperature. Fermi level is in the middle of the forbidden band, and the work function is
4.3 eV. The first formation of NEA on the GaAs surface was reported adsorbing
Cs on p type GaAs(110) by Scheer and Laar in 1965[76]. Since then, researches
on GaAs cathode have actively been made.
The NEA surface is a surface state in which the energy level of the vacuum islower than the lowest level of the conduction band. The NEA surface on GaAs isartificially generated by a so-called ”Yo-Yo method” in which Cs and Oxygen/NF3
are applied on a clean p-type GaAs semiconductor surface. At present, there aresome hypotheses, but the structure of the NEA surface is not determined. One
example is the electric dipole hypothesis; Cs and oxygen form an electric dipole
layer on the surface [77].
Figure 3.1 shows the electronic level of the NEA GaAs cathode and the photoelectron emission. Electrons excited to the conduction band by the laser photon.
The electron diﬀuses thermally through the conduction band while losing energy.
Since the energy level of the conduction band bottom is higher than that of vacuum,
these electrons are also released into the vacuum and a relatively high QE isrealized.
GaAs was originally developed for spin polarized electron source at SLAC
(Stanford Linear Accelerator Center)[78] in late 1970’s. It has been used as polarized electron source for Stanford 2-mile linear accelerator[79] and provided opportunity to study nuclear spin physics[80]. The spin crisis that the sum of the spin
content of partons (quark and gluon) are much less than that of nucleon spin, is
still one of the hottest issue in the nuclear physics. The polarized electron beam
is powerful tool for the particle physics, since each spin state is belong to diﬀerent category in SU(2) x U(1) gauge interaction[81]. The right handed electron issinglet, but the left handed electron is belong to doublet together with neutrino.
If the right handed electron is used for a particle physics experiment, the weakinteraction is prohibited. By observing asymmetry of interaction by switching
the polarization, we can study the detail of the structure of particle interaction.
27

SLC[82] which was a first linear collider in the world, was operated on Z0 pole
(90 GeV) at 1990’ at SLAC. SLC has greatly contributed to establish the standard model of particle physics, together with LEP at CERN[6]. There is no doubtthat the spin polarized electron beam has a crucial role in the next high energy
program, ILC, too.

Figure 3.1: Schematic drawing of the photo-electron emission from the NEA GaAs
cathode. Electron excited by laser to the conduction band is thermalized. Because
the state of the conduction bottom has higher energy than that of the vacuum,
electron can be emitted through the surface thin barrier. [20]The spin polarized electron beam is also useful to study magnetization of material. Spin Polarized Low Energy Electron Microscopy[83] is developed to observe
the magnetization.
Spin polarized electron is generated by photo-emission from NEA GaAs withcircularly polarized laser corresponding to a band gap energy. Since the excitedelectron does have no kinetic energy in the conduction band, NEA surface is necessary for photo-electro emission to vacuum. The spin polarized electron emission
from NEA-GaAs is explained in the Appendix A for detail.
The polarization is principally limited up to 50% with a bulk-GaAs crystal
since it is determined by matrix element of the transition as expressed with ClebshGordon coeﬃcient. By breaking the degeneration of the spin 3/2 valence states,28

a specific transition is enhanced giving 90% polarization[84]. There are several
techniques to break the degeneration and the strained super-lattice crystal is thestate of the art[85]. In ILC[12], the GaAs/GaAsP strained super-lattice crystal isemployed as the photo-cathode.
Because the conduction band at the NEA surface is higher than the vacuum,
electrons with tiny momentum is extracted to vacuum resulting an extremely small
emittance. It is basically possible to obtain such small emittance beam with PEA
cathode by adjusting the laser wave length, we have to be patient for small QE.
In NEA case, QE is kept high, because electrons with a small momentum are also
extracted. NEA GaAs cathode is useful for high brightness (large current andsmall emittance) electron source[51][86]. On the other hand, because thermalized
electrons in the conduction band is extracted to vacuum, the time response of theemission is quite slow, in order of several 10 ps. NEA GaAs is not feasible as theshort pulse electron source.

Figure 3.2: A schematic diagram of the electric dipole made by Cs vapor-depositedon the GaAs surface. Electric dipoles are formed on the GaAs surface, and thesurface potential is lowered.

In contrast to the large improvement on the spin polarization and the good
performance of the beam from the NEA cathode, the structure of the NEA surface
is not understood well. The NEA surface is artificially formed by Cs and O2 [87][88]or NF3 [89] evaporation on a cleaned GaAs surface[90] as explained. Figure 3.2
shows a schematic diagram of the electric dipole made by Cs vapor-deposited on
the GaAs surface according to the hypothesis. Electric dipoles are formed on the
GaAs surface, and the surface potential is lowered. According to an investigationof the NEA surface by STM, the work function of p-GaAs(110) was lowered by Cs
adsorption on its surface[91], but the role of Oxygen or NF3 in the NEA activation
has not been understood well. By analyzing spectra of QE of NEA GaAs, it is29

suggested that the thickness of the surface potential is decreased by progress of
Yo-Yo process[92]. According to an investigation of the NEA surface by X-ray
absorption fine structure(XAFS), the structure of the NEA surface may be O-CsAs(Ga)[93] or Cs-O-As(Ga)[94] with Cs and O2 activation.
The NEA surface can be easily damaged by residual gas adsorption [95] [96][97]and ion back-bombardment[98][99]. For example, the 1/e lifetime of NEA surface
with the average current of 50 µA is only 100 hours with 200 kV electron gun(NPES - 3)[100]. The lifetime is further shortened with a large current extraction.
10% initial QE is degraded to down to less than 1% by 20mA 1.5 h operation[101].
Understanding the NEA surface and improving the cathode robustness are important issues not only from the fundamental physics point view, but also thepractical view point.
In Chapter 5, we studied the activation process of NEA surface with severalgas(CO2 , CO, and N2 ) species to improve our understanding for the NEA surface.

30

Chapter 4
Substrate dependence of CsK2Sb
cathodeIn this chapter, we describe the experiment for the substrate dependence of CsK2 Sb
photo-cathode and the analysis. CsK2 Sb cathode was fabricated on Si(100),
Si(111), and GaAs(100) and the performance was compared. To observe the dependence significantly, the cathode evaporation condition was optimized at first.
For each substrate, the as-received and cleaned surfaces were also compared.

4.1

Experimental Setup

CsK2 Sb cathode is formed on a substrate with evaporation in a vacuum chamber.
This evaporation chamber is made from SUS 304 chemically polished on the innersurface. The picture and 3D drawing are shown in Figure 4.1 and Figure 4.2,
respectively. The vacuum was maintained with an ion pump (ULVAC PST-100 100
l/s pumping speed)[102] and NEG (Non Evaporative Getter , SAES CapacitorrD400 480 l/s )[103] and was kept at UHV (a typical pressure was 1.0e-8Pa). The
cathode (substrate) is fixed on a puck made from molybdenum and SUS (cathodepuck) and the chamber accommodates up to three pucks. Figure 4.3 shows thedrawing of the cathode puck.
The puck is mounted on the cathode holder during evaporation and photoelectron emission. Three cathode pucks are placed vertically in line on the holder.
The Figure 4.4 shows the picture of the cathode holder. The holder can be moved
vertically to adjust the position for evaporation, photo-electron emission, andtransfer. For each cathode puck, a 15 mm × 15 mm substrate is placed on amolybdenum base plate. The substrate is soldered on the base with indium andfixed by a tantalum cup. Figure 4.5 shows Si and GaAs substrates and Figure 4.6

31

shows the substrates mounted on the puck. The substrate is soldered on the thebase with indium and fixed by a tantalum cup.

Figure 4.1: The picture of the evaporation chamber.This evaporation chamber ismade from SUS 304 chemically polished on the inner surface. The vacuum wasmaintained with an ion pump (ULVAC PST-100 100 l/s pumping speed)[102] andNEG (Non Evaporative Getter , SAES Capacitorr-D400 480 l/s )[103] and was
kept at UHV (a typical pressure was 1.0e-8Pa).

The evaporation sources are mounted on a linear stage in the chamber. Figure
4.7 shows the picture of the evaporation sources. The high-purity (99.9999%) Sb
beads were resistively heated in a tungsten evaporation basket. The K and Cs
sources are dispensers provided by SAES Getters Co., Ltd.[104] . The amount ofmaterial on the substrate was monitored with a quartz thickness monitor (INFICON Q-pod Quartz Crystal Monitor). The cross-sectional view of the arrangement
around the cathode puck is shown in Figure 4.8. The thickness monitor is placedin a symmetrical position to the cathode substrate in order to absorb an equivalent amount of vapor. The source to substrate distance was maintained at about
12 mm during the evaporation. To control the cathode temperature, a tungsten
heater is used. The heater is mounted on the head of the linear mover, which
can be inserted from back side of the cathode puck, as shown in Figure 4.8. The
32

Figure 4.2: 3D drawing of the CsK2 Sb evaporation chamber. Up to 3 cathodepucks can be stored in the chamber. The cathode can be transfered through thegate valve to another vacuum chamber (vacuum suitcase) for cathode transportation.

Figure 4.3: The drawing of the cathode puck. For each cathode puck, a 15 mm ×
15 mm substrate is placed on a molybdenum base plate.

33

Figure 4.4: The picture of the cathode holder. Up to 3 cathode pucks can be
set. The holder can be moved vertically to adjust the position for evaporation,
photo-electron emission, and transfer.

Figure 4.5: Si(100) and GaAs(100) substrates. Si(100) and Si(111) wafers with aof ≤ 0.002 Ωcm−1 and GaAs(100) p-type wafer are employed as the substrates.

34

Figure 4.6: The substrates mounted on the puck. The substrate is soldered on thethe base with indium and fixed by a tantalum cup. The puck is mounted on thecathode holder during evaporation and photo-electron emission.

Figure 4.7: The picture of the evaporation sources. The evaporation sources aremounted on a linear stage in the chamber. K and Cs can be set each of three
in the evaporation sources. The Sb pellets were resistively heated in a tungsten
evaporation basket.

35cathode puck temperature is measured with a thermocouple.

Figure 4.8: A vertical view of the arrangement around the cathode puck. The
thickness monitor and the cathode puck were placed in symmetrical positions to
the evaporation source. The cathode puck temperature is controlled by a heaterfrom the backside.
The QE was measured with a 532 nm laser. The 1.0 mW light of 532 nm
emitted from laser transmits through the ND - filter for laser power adjustment.
In fact, it is possible to install multiple ND filters, and the power is adjusted to
20 to 500 µW. A manual shutter controls the irradiation to the cathode. A simple
2D scan can be taken with a manual stage. The cathode was biased at -100 V,
and the photo-current was measured as the current of the bias supplier.

4.2

Substrate surface treatment

According to previous studies[25][72][105], the QE depends strongly on the substrate surface state, i.e., oxidation, roughness, etc. In this study, Si(100) andSi(111) p-type wafers with a resistivity of ≤ 0.002 Ωcm−1 and GaAs(100) p-type
wafers are employed as the substrates. Si wafer was etched, single side polished andfinally washed with RCA method by NILACO Co. Ltd. GaAs wafer was etched,double-side polished, and finally washed with RCA method, as well. These substrate surfaces were oxidized again because it was treated in air. To control thesurface state of the substrate, we perform the etching of the substrate sample prior
to the experiment as follows.

36Figure 4.9: Etching procedure of Si substrate. The Si(100) and Si(111) substrates
were processed with a 5% HF solution for about 5 min.[106]The Si(100) and Si(111) substrates were processed with a 5% HF solution
for about 5 min to remove the surface oxidized layer[106] as the process shownin Figure 4.9. The GaAs surface was processed with a H2 SO4 :H2 O2 :H2 O (4:1:1)
solution for about 1 min to remove the surface oxidized layer[90] and to make As
rich surface. The As layer is removed by heating in the vacuum chamber to obtainthe clean GaAs surface. The process is called as heat cleaning. The process isshown in Figure 4.10.

Figure 4.10: Etching procedure of GaAs substrate. The GaAs surface was processed with a H2 SO4 :H2 O2 :H2 O (4:1:1) solution for about 1 min[90] and to makeAs rich surface.

According Ref.[106] , the surfaces of Si(100) and Si(111) are equivalent except
37

the surface direction after the HF treatment. This cleaned surface is kept for 3
and 50 hours in air for Si(100) and Si(111), respectively [107] . In our experiment,
after the cleaning, the Si sample was kept in a desiccator (typical pressure was3Pa) and the air exposure duration during the sample transfer was totally less than
30 minutes and the re-oxidation on the substrates are negligible. Therefore, theSi(100) and Si(111) substrates in out experiment are kept clean and are equivalent
except the surface atomic arrangement.
To examine the eﬀect of the surface oxidation, we perform the cathode evaporation also on the as-received substrates[108][109] . The atomic arrangement of the
as-received surface is disturbed by the oxidation layer[110] , and it is considered
to be amorphous[109][110]. That could be also true for GaAs.

4.3

Experimental Results

Figure 4.11: Heat cleaning. The substrate is heated up to 600◦ C in vacuum.
Temperature is shown red line and vacuum pressure is shown blue line during theheat cleaning.
Before the evaporation, in order to remove impurities on the substrate surface,
we perform the heat cleaning. The substrate is heated up to 450–600◦ C in vacuum.
This process is performed not only for a virgin substrate which was newly installed38

to the experimental chamber, but also for an used substrate. In every cathodeevaporation, we perform the process. Figure 4.11 shows temperature (red line)
and vacuum pressure (blue line) evolutions during the heat cleaning.

Figure 4.12: QE evolution and progress of thickness of Sb, K, and Cs. QE evolution
is shown red circle and progress of thicknesses of Sb, K, and Cs are shown blue
triangle. QE appears in K evaporation and increases rapidly in Cs evaporation.
The thickness are 10nm, 70nm and 1400nm for Sb, K and Cs, respectively.
CsK2 Sb cathode was formed by sequential evaporation of Sb, K, and Cs on asubstrate. The evaporation procedure is summarized as follows:
1. Heat cleaning.
2. The temperature is lowered down to 100◦ C. This temperature was maintained during the evaporation.
3. A 10-nm-thick layer of Sb is evaporated on the substrate surface.
4. K is evaporated. Simultaneously, QE is observed by illuminating the cathodewith 532-nm laser light. The evaporation is stopped when QE is saturated,which usually occurs between 0.3–0.8%. The typical thickness of K was 60
nm.

39

5. The Cs layer is evaporated. The Cs evaporation is controlled with the samemanner as that for the K evaporation. The QE was typically between 1–10%
and the thickness was 140 nm.
6. The substrate is cooled down to room temperature.
Figure 4.12 shows an example of QE evolution with red circle and progressof thicknesses of Sb, K, and Cs with blue triangle. The laser power was
adjusted to 0.8mW during the K evaporation and 0.36mW during the Cs
evaporation to avoid the space charge limitation. The laser spot size was
0.5 mm2 , and the typical photo-current was 1 − 3µA for K evaporation and3 − 9µA for Cs evaporation.

Figure 4.13: The maximum QE is shown as a function of temeperature during theevaporation with blue squares and red circles for the Si(100) and Si(111) substrates,
respectively. The error on the horizontal axis is fluctuation during evaporation andthe error on the vertical axis is statistical error by 3 times measurements.
QE of CsK2 Sb depend on the evaporation condition even the substrate is same[74]. In order to eliminate such eﬀect, we fixed the evaporation condition (Sbthickness and substrate temperature during the evaporation) giving a maximum
QE and less QE fluctuation regarding to condition variation. Amount of K and Cs
were automatically determined giving the maximum QE after each evaporations.
The maximum QE of CsK2 Sb as a function of substrate temperature is shown40

in Figure 4.13 for Si(100) and Si(111) substrates, respectively. We repeated theevaporation several times for each substrates. The error on the horizontal axis istemperature fluctuation during Cs evaporation and error on the vertical axis isstatistical error. We found that QE is maximized around 100◦ C on Si(100) andSi(111) with a good reproducibility. Figure 4.14 shows that the QE as a functionof Sb thickness showing a weak dependence.

Figure 4.14: The maximum QE is shown as a fucntion of Sb thickness with blue
squares and red circles for the Si(100) and Si(111) substrates, respectively. The
error on the horizontal axis and error on the vertical axis are only statistic.
According to these results, the evaporation was performed at 100◦ C with 10 nm
Sb thickness. Amount of K and Cs are automatically determined to maximize QE
after each evaporation, i.e. we stoped the evaporation whenever QE is saturated.
We examined Si(100), Si(111), and GaAs(100) substrates. The comparisonof QE evolution during the cathode evaporation is shown in Figure 4.15 for thecleaned substrates and Figure 4.16 for the as-received substrates. Figure 4.15 shows
red circles, blue triangles, and green squares for the cleaned Si(100), Si(111), andGaAs(100) substrates, respectively. QE appears in K evaporation and increases
rapidly in Cs evaporation. K evaporation started on 48, 45 and 45 min for thecleaned Si(100), Si(111), and GaAs(100) substrates, respectively. Cs evaporation
started on 183, 175 and 135 min for the cleaned Si(100), Si(111), and GaAs(100)substrates, respectively. Figure 4.16 shows that the comparison of QE evolution
41

during the cathode evaporation is shown with red circles, blue circles, and green
circles for the as-received Si(100), Si(111), and GaAs(100) substrates, respectively.
In the blue, green, and yellow shaded regions, Sb, K, and Cs were evaporated,
respectively. The QE appears in the K evaporation and increases rapidly in theCs evaporation.

Figure 4.15: The comparison of QE evolution during the cathode evaporation forthe cleaned substrates are shown with red circles, blue triangles, and green squares
for the cleaned Si(100), Si(111), and GaAs(100) substrates, respectively. QE appears in K evaporation and increases rapidly in Cs evaporation. K evaporation
started on 48, 45 and 45 min for the cleaned Si(100), Si(111), and GaAs(100)substrates, respectively. Cs evaporation started on 183, 175 and 135 min for thecleaned Si(100), Si(111), and GaAs(100) substrates, respectively.
The results (maximum QE) are summarized in Table 5.1 including the those of
preceding studies. We performed the same experiment five times for each substrate.
After every experiment, the substrate was heat-cleaned to initialize the surface.
The error is obtained as the standard deviation of the five measurements, and it isstatistical only. The QE of the as-received substrate was around 2.5–5.5% at 532
nm. The cleaned Si (100) and GaAs(100) substrates showed a good QE as highas 10%. These results are similar to those of the cleaned Mo(100)[25] and Si(100)substrates obtained by Cornell University[26] . In contrast, the QE of the cleaned
Si(111) was higher than that of the as-received Si(111), but it is much lower than42

that of the cleaned Si(100) and GaAs(100) substrates.

Figure 4.16: The comparison of QE evolution during the cathode evaporation isshown with red circles, blue circles, and green circles for the as-received Si(100),
Si(111), and GaAs(100) substrates, respectively. In the blue, green, and yellow
shaded regions, Sb, K, and Cs were evaporated, respectively. The QE appears in
the K evaporation and increases rapidly in the Cs evaporation.

According results shown in Table 5.1, the cathode performance developed on
the cleaned substrate was significantly higher than that on the as-received substrate for all cases. The cathode formed on the Si(111) substrate showed the least
performance in the sample substrates in both the cleaned and as-received cases.
The diﬀerence in the cathode performance on the Si(100) and Si(111) substrates
provide direct evidence that the cathode performance depends strongly on the surface direction, because the material properties of Si(100) and Si(111) are exactly
same otherwise. A similar conclusion was also obtained by comparing QE of thecrystalline Mo(100)[25] and amorphous Mo[29][31] , because the physical propertyof the crystalline and amorphous Mo other than the surface atomic arrangements,
are almost same. Mo(100) and Mo(amorphous) substrates in Reference [25][29][31]
are considered to be cleaned because it was polished and sputtered.
The results can be summarized as• By comparing the as-received and cleaned substrates, the cathode on the43

Table 4.1: The maximum QE of the CsK2 Sb photo-cathode on Mo(100),Mo(amorphous), Si(100), Si (111), and GaAs (100) substrates at 532 nm are summarized.
Substrate
Surface treatment QE[%]@532nm
Mo(100)Polished+sputter
10.0[25]
Mo(amorphous) Polished+sputter
2-5[29][31]Si(100)as-received
4.8 ± 0.6
Si(100)5%HF
9.4 ± 0.7
Si(100)
5%HF
7 − 10[26]Si(111)
as-received
1.6 ± 0.1
Si(111)5%HF
2.3 ± 0.3
GaAs(100)
as-received
5.5 ± 0.2
GaAs(100)
H2 SO4 :H2 O2 :H2 O
10.0 ± 0.2

cleaned substrates shows better performance.
• By comparing amorphous and crystalline substrates, the cathode on the
crystalline substrates shows better performance.
• By comparing Si(100) and Si(111), the cathode on Si(100) shows better
performance.
By considering these facts, the cathode performance depends not only on cleanness and crystallinity, but also on surface direction of the crystalline substrate.
This is the first experimental evidence that CsK2 Sb cathode performance depends
on the crystal surface direction of the substrate. In the following section, we discussthese results.

4.4

Discussion

CsK2 Sb forms a DO3 cubic structure [63][65] as explained already. Figure 4.17
and Figure 4.18 shows that the surface atomic arrangements of CsK2 Sb(100),
CsK2 Sb(111), Mo(100), GaAs(100), Si(100), and Si(111) surfaces. By considering the matching between the atomic arrangements among CsK2 Sb (100), (111),
Si(100), Mo(100), and GaAs(100) surfaces, CsK2 Sb are grown in (100) direction
on Si(100), Mo(100), and GaAs(100) surfaces, and in (111) direction on Si(111)
direction. Atomic arrangement model of unit cell is explained in the Appendix B
for detail.
44

Figure 4.17: Atomic arrangement of CsK2 Sb(100) on the Mo(100), GaAs(100) andSi(100) surfaces. The lattice constants of Mo, Si, and GaAs are 3.15Å, 5.43Å, and
5.65Å, respectively [111].

The reason for the reduced performance of the as-received substrates comparing
to the cleaned substrates could be the less quality of CsK2 Sb crystal. Oxidation
distorts the atomic arrangement of the substrate surface resulting a poor qualityof CsK2 Sb crystals leading the reduced performance.
The cathode on the cleaned Si (100) and GaAs(100) substrates showed a good
QE as high as 10%. These results are similar to those on the cleaned Mo(100)and Si(100) substrates obtained by the Cornell University[25]. On the other hand,the cathode performance on Si (111) is less than the others. These results canbe explained with the band structure of CsK2 Sb. In References [63][65], the bulk
band dispersion of CsK2 Sb was calculated. The point on the boundary surface of
the Brillouin region is called the K, L, and X points in the (110), (111), and (100)directions, respectively. According to the bulk band dispersion, CsK2 Sb is a directtransition type at the Γ point with 1.1 eV bandgap. The bandgap is about 2.1, 3.1,
and 1.4eV at K, L, and X points, respectively. If we consider the photo-electron
emission in (100) direction, not only electrons at Γ point, but also electrons at X
point contributes to the emission, because the bandgap at X point is similar tothat at Γ. On the other hand, for the photo-electron emission in (111) direction,
there is no contribution at L point, because the bandgap energy (3.1 eV) is larger

45

than the laser photon energy (2.3 eV at 532 nm). The photo-electron emission of
CsK2 Sb in (111) direction is possible only at Γ point and this is the reason why
the QE of (111) surface is less than that of (100) surface.

Figure 4.18: Atomic arrangement of CsK2 Sb(111) on the Si(111) surface.

By a similar consideration, the QE of (110) surface should be less than thatof (100) surface. In the preceding studies[74][75], CsK2 Sb was grown in (200) or(220) directions on Si (100) substrate, depending on the case and the cathode
performance of (200) and (220) were similar[74]. We consider that the cathodeevaporation conditions in these experiments were not fully optimized, because
the QE is only 3% for 532 nm light in both cases. That is why CsK2 Sb crystal
direction depends on the case and there was no significant diﬀerence on the cathode
performance grown in (100) and (110) directions in these studies.
In our case, the cathode evaporation condition was carefully optimized and thecathode performance reproducibility was quite good. The cathode performance
dependence on the substrate crystallinity and the surface direction was confirmed
based on the reliable experiments.

46

Chapter 5Eﬀect of Gas Adsorption on NEA
GaAs
To improve our understanding for the NEA surface, we studied the activation
process of NEA surface. In this study, we activated the NEA surface of GaAs
with diﬀerent gas species and compared the cathode performance. We examined
CO2 , CO, and N2 as test gases and compared them with O2 . In this chapter, we
describe the experiment followed by the results and discussion.

5.1

Experimental Setup

NEA activation experiments were carried out with a test chamber described inRef. [112]. The vacuum was maintained with a 160 l/s ion pump and 310 l/s
non-evaporative getter (NEG) pump and was kept at ultra-high vacuum (UHV).
The typical pressure was 6.0×10−9 Pa. The chamber is made from SUS and theinner surface was finished by chemical polishing.
1.0-inch GaAs wafer is cut to 4 pieces. One piece is placed on a molybdenum
base plate. It is soldered on the base with indium and fixed by a tantalum cup. In
this study, the p-type, Zn-doped bulk GaAs(100) was used as a cathode sample.
The doping density is 4.0 × 1019 cm−3 . This high doping causes the band bendingat the surface, which lowers the eﬀective work function. The GaAs surface wasprocessed with a H2 SO4 :H2 O2 :H2 O (4:1:1) solution for about 1 min to remove thesurface oxidized layer[90] as same as substrate process in Chap. 4. The temperature of the cathode mount can be controlled with a tungsten heater embeddedin the mount. The GaAs cathode temperature is measured with a thermocouple
attached to the cathode mount.
The sample gas introduction system is shown in Figure 5.1. To provide several

47

sample gas species, three gas cylinders are connected to a leak valve attached tothe main chamber. The gas species can be switched by toggling several valves.
To avoid mixture of undesired gas species, the sample gas introduction system is
evacuated by a rough pump (molecular pump) while every switching.
A quadrupole mass spectrometer, QMS (M101QA-TDM-W, ANELVA) is used
for the gas analysis. The sample gas contains not only the specific molecule, butalso fragment components and contaminations. The gas content is obtained byanalyzing the mass-to-charge ratio (m/z) spectrum up to 100 m/z measured by
QMS.

Figure 5.1: A schematically drawing of the sample gas introduction system. The
system and the main chamber are connected by a leak valve to control the gas
introduction rate. The gas species can be changed by toggling valves to each gas
bombs. The system is evacuated by a rough pump for every gas switching to avoid
gas mixture.

5.2

Experimental Results

The NEA GaAs cathode was formed by alternate evaporations of Cs and gas (O2as reference) on a cleaned GaAs substrate (yo-yo method)[113]. The procedure isgiven as follows:
1. The GaAs substrate is heated to 600◦ C and held at that temperature for one
hour to clean the surface (heat cleaning process).
48

Figure 5.2: QE evolution during the activation process by Cs and O2 . The firstpeak is made by Cs only. In other peaks, QE is increased by O2 and decreased byCs.

2. The GaAs wafer is cooled down to the room temperature.
3. Cs is evaporated on the GaAs. Usually, a small enhancement of QE isobserved, but it is shortly disappeared (Cs peak).
4. Gas (O2 or NF3 ) introduction and Cs evaporation are alternately performedon the GaAs. In a usual activation process, QE is increased when the gas is
applied, but it is saturated at some moment. We switched to Cs evaporation
and QE is rapidly decreased down to zero. This process is repeated whenever
the height of the peak is increased.
QE is simultaneously observed by illuminating the cathode with 633nm He-Ne
laser. We examined CO2 , CO and N2 as the test gas. Addition to these gas species,
O2 was used as the reference. The QE evolutions during the NEA activation with
O2 , CO2 , CO and N2 are shown in Figures 5.2-5.5, respectively.
Figure 5.2 shows the QE evolution of the NEA activation with O2 gas. The
first peak is made by Cs evaporation. In other peaks, QE is increased by O2 anddecreased by Cs. If the height of the peak is almost same as that of the previous
peak, as the sixth peak in Figure 5.2, we terminate the NEA activation process.
Figure 5.3 shows the QE evolution of an activation with CO2 . The first peak49

Figure 5.3: QE evolution of the activation process with Cs and CO2 . The activation procedure was same as that with O2 except the introduced gas.

is made by Cs evaporation. In other peaks, QE is increased by CO2 and decreased
by Cs. The evolution is similar to that with O2 , but the maximum QE value isabout 2.3% which is significantly lower than that with O2 .
Figure 5.4 shows the QE evolution of an activation with CO. After the firstpeak was made by Cs, CO gas was introduced up to t=0.8 h, but nothing happened,
i.e. QE was not increased at all. CO did not activate GaAs cathode. To confirm
the health of the GaAs sample and eﬀect of CO on GaAs, O2 gas was introduced
after t=0.9 h, i.e. the usual NEA activation was performed. In Figure 5.4, QE
evolutions of the NEA activation with CO and O2 are drawn with a solid line anda dashed line, respectively. By introducing O2 gas, QE was increased as same as
the usual NEA activation with O2 and the final maximum QE was more than 9%which is almost same as that in Figure 5.2. From this result, we can conclude that
the GaAs sample was healthy (there was no trouble on the preparation) and CO
does not activate GaAs cathode. CO gas introduction prior to the process does
not disturb NEA activation with O2 , because the NEA surface was activated with
O2 even after CO gas was applied.
Figure 5.5 shows the QE evolution of an activation with N2 . After the first peakby Cs, N2 gas was introduced up to t=0.95 h, but N2 gas did not activate GaAs
cathode at all, as same as that with CO gas. A nominal NEA activation with O2
50

Figure 5.4: QE evolution during the activation process with Cs and CO. QE did
not rise with CO. After t=0.9 h, the NEA activation was performed by Cs and O2 .
The solid line is that with CO gas introduction and the dashed line is that by O2 .

gas was performed after t=1.05 h and the cathode was activated as same as thenominal NEA activation with O2 . In Figure 5.5, QE evolutions of the activations
with N2 and O2 are drawn with a solid line and a dashed line, respectively. Fromthese experiments with CO and N2 , we suggest that CO and N2 are not adsorbedon the surface of Cs-GaAs. Even CO or N2 are introduced in advance, NEA
activation with Cs and O2 is not disturbed, as long as the partial pressure of CO
and N2 is even low during the activation.

5.3

Discussion

Here we consider the diﬀerence of the NEA activation with O2 and CO2 . It is
known that when oxygen atoms or molecules are adsorbed on the Cs-GaAs surface,
they accept electrons from Cs atoms and are chemisorbed inside the activation
layer forming the NEA surface[114].
Figure 5.6 shows the height of each peak in the Figure 5.2 (NEA activation
with O2 ) as a function of exposure of O2 gas in Langmuir (1L = 1.33 × 10−4 Pa · s).
According to the result, the height is linearly increased as the O2 gas exposure.
Figure 5.7 shows the height of each peak in the Figure 5.3 (NEA activation with51

Figure 5.5: QE evolution of the activation process with Cs and N2 . After t=1.05
h, the NEA activation was performed with Cs and O2 . The solid line is that with
N2 gas introduction and the dashed line is that with O2 .

52

CO2 ) as a function of exposure of CO2 gas. It is quite diﬀerent from Figure 5.6.
The QE height in Figure 5.6 is proportional to the exposure of O2 , but the relationof that with CO2 in Figure 5.7 is not linear.

Figure 5.6: The height of each peak in Fig.5.2 is shown as a function of O2 gas
exposure. The red solid line is a fitting curve by assuming Eq. (5.4).

According to our experiment of NEA activation with CO, CO molecule does
not contribute to the NEA activation and does not disturb the NEA activation process. On the other hand, CO degrades the NEA GaAs activated withO2 (nominal
NEA activation process), according to Ref. [115]. By considering this degradation eﬀect of CO molecule, the saturation observed in Figure 5.7, i.e. less QE of
the NEA GaAs activated withCO2 can be explained. Because a fraction of CO2
molecule fragments to CO and O (atomic oxygen), we assume that O (atomic
oxygen) activates the NEA surface and CO degrades the activated NEA surface,
simultaneously. The QE(η) evolution can be expressed asdη= α1 f1 − βf3 η
dt

(5.1)

where f1 is the partial pressure of atomic oxygen , f3 is the partial pressure of CO,
α1 is the activation ability and β is the degradation ability. The first term of RHS
shows activation by atomic oxygen, and the second term shows degradation by
CO. We assume the ratio of f1 and f3 is constant during experiment as µf3 = f1 .
53

Table 5.1: Mass to charge ratio and ion current of the major peaks of the mass
spectrum by QMS with CO2 introduction. The rightest row shows the candidate
molecule. The inside of the parenthesis is the mother molecule.
m/z Ion current[A] Molecule daughter (mother)
12
3.32e-11
C(CO2 ), C(CO)14
3.98e-11
N(N2 )
16
9.00e-11
O, O(CO2 ), O(CO)28
7.27e-10CO(CO2 ), N2 , CO44
4.81e-10
CO2

It leadsdη= α1 µf3 − βf3 η
dt

(5.2)

We got a solution with η=0 at t=0 as,η=α1 µ[1 − exp(−βx3 )],
β

(5.3)

∫

where x3 = 0t f3 dt, i.e. exposure of CO. This model can explain not only the lower
QE of the activation with CO2 than that with O2 , but also the non-linearity of
the curve in Figure 5.7 as we will see soon.
The content of CO2 , CO, and atomic oxygen in the experimental chamber were
measured with QMS. The measured mass spectrum (m/z) is a convolution of thegas content, sensitivity coeﬃcient and fragmentation pattern coeﬃcient of eachcomponent[116][117]. The ion current of the major peaks measured by QMS with
CO2 gas introduction is shown in Table 5.1. The rightest row in Table 5.1 is thecandidate molecule. The inside of the parenthesis is the mother molecule. We
assume O, C, CO, N, N2 and CO2 as the content. By analyzing the data withthe pattern coeﬃcient of QMS[116], the partial pressure of these molecules during
the experiment can be extracted. For example, the peak at 44 is composed by
CO2 only. Because a part of CO2 fragments into CO(m/z=28), O(m/z=16), andC(m/z=12) , these components has to be accounted. The ion current by CO2 isthe total of these components as
ICO2 = I44 + 0.0885I44 + 0.0805I44 + 0.058I44 = 5.90 × 10−100.0885 is the pattern coeﬃcient of CO from CO2 by QMS, and so on.
Because the peak at 28 contains not only CO, but also CO as the fragmentation
component of CO2 and N2 , the ion current of CO can be estimated by subtracting
54these components from I28 as
ICO = 1.04(I28 − I14 /(0.0790) − 0.0885I44 ) = 1.87 × 10−10
N2 contribution is extracted from the height of the peak at 14 which contains only
N contribution. The factor 1.04 is the correction, because a part of CO fragments
into C and atomic oxygen.

Figure 5.7: The height of each peak in Fig.5.3 is shown as a function of CO2 gas
exposure. The red solid line is a fitting curve by assuming Eq. (5.3).

The peak at 16 is composed not only by atomic oxygen, but also fragmentationsas O(CO2 ) and O (CO). Although the fragmentation components of CO2 and CO
can be estimated from the heights of these peaks at 44 and 28, respectively, but
the peak at 28 contains contributions from CO(CO2 ) and N2 as we discussed. We
have to subtract these components from I28 . The ion current of atomic oxygen isestimated as
IO = I16 − 0.0805I44 − 0.0102(I28 − I14 /(0.0790) − 0.0885I44 ) = 0.49 × 10−10
From these ion currents, the partial pressure p for each molecule is obtained
with
p=

I
βS
55

where I is the ion current for each molecule, β is the sensitivity coeﬃcient[117],
and S is relative sensitivity to nitrogen. The fraction of CO2 , CO, and O during
the experiment was estimated to be 0.75:0.18:0.07. µ is 0.39.
The ratio of CO2 , CO, and O is diﬀerent from the natural ratio, because oursystem has an ionization vacuum gauge which stimulates CO2 molecule. QMS
was in operation only when QMS spectrum was measured and the period was
short comparing to the experimental period. Then, the gas contains the fragmentcomponents by the ionization gauge, but the fragmentation by QMS occurred only
in QMS spectrum measurement. The partial pressure ratio during the experiment(ionization gauge on, QMS oﬀ) was obtained by analyzing the QMS spectrum withthe pattern coeﬃcient of QMS ion source.
As same as that for CO2 case, the content was estimated with the same manner,when O2 gas was introduced. O2 molecule composes peaks at 32 (O2 ) and 16 (O)as the fragment product, but atomic oxygen composes only a peak at 16. The
fraction of O2 molecule and atomic oxygen during the experiment was estimatedto be 0.75:0.25.
To determine both activation ability of the O2 molecule and atomic oxygen
independently, the NEA activations with CO2 and O2 gas were analyzed together.
The expected evolution of the QE η for the activation with O2 isη = α1 x1 + α2 x2 ,(5.4)

where α2 is the activation ability of O2 molecule and x2 is the exposure of O2
molecule.
Three parameters, α1 , α2 , and β can be determined with the data of activations
with O2 and CO2 . The results are shown in Figure 5.6 and 5.7 as the fitting curves.
α1 and α2 (activation abilities of atomic oxygen and O2 molecule) were estimatedto be 7.7 ± 0.1 L−1 and 15.4 ± 0.1 L−1 , respectively. The error of α1 and α2 areestimated by three measurements with CO2 . The center value is the average andthe error was half of the peak-to-peak value. We found that O2 molecule hasan activation ability that is approximately twice of that of atomic oxygen. This
fact can be explained as that the activation ability is simply proportional to thenumber of atomic oxygen provided to GaAs-Cs surface. The degradation ability
β of CO was estimated to be 0.91 ± 0.09 L−1 which can be compared to 0.84 L−1
in Ref. [115]. These numbers are consistent to each other within the error. The
error of β is estimated with the same manner for α.

56

Chapter 6
Summary
The accelerator has been a powerful tool to “accelerate” the scientific studies andapplications. Especially, the ring accelerator (e+ e- ring collider and the third
generation synchrotron light source) is playing an important role in the last 30
years in the high energy physics and synchrotron science discipline. On the other
hand, the ring accelerator is reaching the performance limits in two contexts. One
is the beam energy which is limited by huge energy loss by synchrotron radiation.
Another is the beam emittance which is limited by equilibrium state of the stored
beam. Many new accelerator projects based on Linac are proposed and constructedto break the limitations. In Linac, the particle source is taking a crucial role todetermine the accelerator performance and the photo-cathode is the state of theart.
In this thesis, we studied a couple of subjects for the advanced photo-cathode.
One is substrate dependence of CsK2 Sb cathode which is the best cathode for highbrightness electron beam at this moment. Another is NEA GaAs activation withother gas species than O2 . NEA GaAs is the only solution for highly spin polarized
electron source.
CsK2 Sb photo-cathode was fabricated on several substrates as the test sample.
GaAs(100), Si(100), and Si(111) were examined. We found that the cleaned substrates resulted in higher performance than the as-received substrates for all materials. By comparing cathodes on GaAs(100), Si(100), Si(111), and Mo [25][29][31],
we found that the cathode on GaAs(100), Si(100), and Mo(100) had significantly
better performance than that on Si(111) and Mo(amorphous). It showed thatthe cathode performance depends strongly not only on the substrate material andsurface state, but also on the crystallinity and the surface direction. This is thefirst experimental evidence for substrate surface direction dependence of CsK2 Sb
photo-cathode performance. The diﬀerence can be explained that the band structure of CsK2 Sb. The result is significant showing importance of substrate choice57

on material, crystallinity and surface direction. This fact has an impact on theexperimental physics with accelerators, because there is some potential to improvea thin-film cathode performance by revisiting the substrate crystallinity and surface direction of the substrate. The high QE has a large merit to generate a highbrightness electron beam by relaxing requirements for the drive laser. The fewer
requirements for the laser make the system reliable, and stable. The availabilityof the system is improved and the eﬀective cost of the project (cost per operation time) becomes less. Our result has a potential of the large impact on theaccelerator science from this point of view.
We examined the NEA activation of GaAs cathode with O2 , CO2 , CO and N2
to understand property of the NEA GaAs cathode. In a nominal activation with O2gas, we found that the height of each yo-yo peak are proportional to O2 exposure.
CO and N2 did not activate the NEA GaAs cathode at all. CO2 activated the
NEA GaAs cathode, but the QE was 2.3% which was much lower than that with
O2 . We found that the activation ability of O2 molecule and atomic oxygen were15.4±0.1 L−1 and 7.7±0.1 L−1 , respectively. The low QE of the cathode activatedwith CO2 can be explained with the degradation by CO to the NEA surface. The
degradation ability of CO was estimated to be 0.91 ± 0.09 L−1 , which is consistent
to 0.84 by a preceding study[115]. This eﬀort is the first experiment to performthe NEA activation with CO2 and the first measurement for the activation abilityof O2 and O, separately.
Although these experimental facts provide useful information for better understanding of the NEA surface, it is still not suﬃcient. Establishing a robust NEA
GaAs cathode is a future issue.

58

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65

Appendix A
NEA GaAs as a polarized
electron source
Polarized electrons are obtained by irradiating laser light circularly polarized(Ti:Sa wavelength 700 to 800 nm) into a semiconductor GaAs crystal and extracting
electrons generated by the photoelectric eﬀect with a DC electric field. Figure A.1(a) shows the band structure of bulk GaAs crystal. The GaAs crystal has a charged
zone with an angular momentum of 3/2 in the outermost shell. Valence electrons
are excited by photons corresponding to the energy diﬀerence between this bandand the conduction band, and taken out. Free electrons in the conduction band
can only take state of helicity ±1, so if the photon is circularly polarized, thetransition becomes selective from the law of conservation of angular momentum.
Let’s assume excitation by right-handed photons. The transition indicated by thesolid line in Figure A.1 (a) is due to the right-handed photon. Since the transition
from the level of J = 1/2 is strongly suppressed when the energy of the photon
is set to 1.43 eV or more and less than 1.77 eV, only transition from m = -3/2,-1/2 state of J = 3/2 is possible. The state density of J = 3/2 level is 3: 1 for m= -3/2 and m = -1/2 by ClebschGordan coeﬃcient. Therefore, 75% of electronsobtained in the conduction band are m = -1/2, and 25% are m = + 1/2 state.
The polarization degree of the beam isP =N+ − N−
N+ + N−(A.1)

Where N± is the density of the electron’s helicity state. Therefore, the degree ofpolarization of electrons is 50% by irradiation of polarized photon. The polarization degree of the electron beam is determined by the density of the J = 3/2
state which is the composite state of the spin-orbital angular momentum. Thisis impossible to change, because it is decided by quantum mechanics. Therefore,
6650% polarization is the theoretical limit value and it is impossible to exceed thisas long as bulk GaAs is used.
A polarization degree exceeding 50% was obtained by breaking the symmetryof the GaAs crystal and solving the degeneracy of the J = 3/2 state. Two methods
as a strained thin film and a superlattice thin film by SLAC and Nagoya University
were developed as a method to solve the degeneracy. Figure A.1 (b) shows thatthe band structure of the GaAs crystal in a state where the degeneracy is resolved.
An energy gap is generated between m = ±3/2 and m = ±1/2 at J = 3/2 dueto the degeneracy being resolved. The transition from the m = ±1/2 state canbe suppressed by adjusting the energy of the photon. In principle, a degree ofpolarization of 100% is possible, and polarizations of around 90% are also obtainedin actual beams[85].

Figure A.1: The band structure of GaAs and the state of excitation by photons
to the conduction band. (a) shows bulk GaAs crystal, (b) is a diagram of distorted GaAs crystal. Degeneracy of angular momentum 3/2 state is solved by
distortion.[20]67Appendix B
Atomic arrangement model ofunit cell
In chapter 4, we explained that the growth direction is diﬀerent depending by theatomic arrangement between the CsK2 Sb and the substrate. We explain in detailhere with atomic arrangement model of unit cell.

Figure B.1: Atomic arrangement of CsK2 Sb(100) and CsK2 Sb(111) on theMo(100), GaAs(100), Si(100), and Si(111) surfaces. The lattice constants of Mo,
Si, and GaAs are 3.15Å, 5.43Å, and 5.65Å, respectively [111].

68

Figure B.2: ζ is obtained with the atomic positions of the substrate and CsK2 Sb
by Eq. B.1. By rotating angle θ, we take the minimum value as a representative
matching index.

Figure B.1 shows that the surface atomic arrangements of CsK2 Sb(100), CsK2 Sb(111), Mo(100), GaAs(100), Si(100), and Si(111) surfaces. To examine the atomic
arrangement matching, we introduce the matching index ζ asζ(hkl) =n
1 ∑(xi − yi )2
n − 1 i=1
x2i

(B.1)

where hkl are the Miller indices defining the plane of the CsK2 Sb crystal, n is
the number of atoms used to evaluate the matching, and ⃗xi and ⃗yi are the atomic
position vectors for CsK2 Sb and the substrate, respectively. Figure B.2 shows
the schematic calculation of the matching index. One atom of the substrate andCsK2 Sb overlap at the origin, which is why the index is divided by n−1. Moreover,ζ is proportional to the square of the displacement between each atom of CsK2 Sb
and the nearest atom in the substrate, normalized by the distance of the atomfrom the origin. The red and blue solid circles represent the atoms of substrate
and CsK2 Sb, respectively. The rotation angle θ is a degree of freedom on ζ, asshown in FigureB.2. We take the minimum value as a representative ζ by rotating
the angle.
Therefore, we calculated ζ for several surface directions of the CsK2 Sb crystal.
Figure B.3, Figure B.4 and Figure B.5 show ζ(100), ζ(110) and ζ(111) for Mo(100),
GaAs(100), Si(100), and Si(111). Each ζ value is plotted as a function of the69

Table B.1: The maximum QE of the CsK2 Sb photo-cathode with ζ of CsK2 Sb(100)and CsK2 Sb(111) surface at 532 nm are summarized.
Substrateζ (100) ζ (110) ζ (111) QE[%]@532nm
cleaned Mo(100)1.3e-3
1.5e-21.0e-210.0[25]
cleaned GaAs(100)5.0e-32.4e-24.4e-210.0 ± 0.2
as-received GaAs(100)5.5 ± 0.2
cleaned Si(100)1.1e-22.0e-23.5e-29.4 ± 0.7
as-received Si(100)4.8 ± 0.6
cleaned Si(111)3.5e-27.2e-36.0e-32.3 ± 0.3
as-recieved Si(111)1.6 ± 0.1

rotation angle θ. For ζ(100), the rotation angle of π/4 gives the representativevalue for Mo(100), GaAs(100), Si(100), and Si(111). For ζ(110), the representativevalues for Mo(100), GaAs(100), Si(100) and Si(111) were given at the angles of 0,
π/4, π/4, and π/6, respectively. For ζ(111), the representative values for Mo(100),
GaAs(100), Si(100) and Si(111) were given at the angles of π/6, π/12, π/12, andπ/6, respectively. The matching between CsK2 Sb(100) and Si(100), Mo(100) or
GaAs(100) is best. The matching between CsK2 Sb(111) and Si(111) is best.
The results are summarized in Table B.1. Two facts are obvious from theseresults.
• The QE of the cleaned substrate is almost twice that of the as-received
substrate for all materials.
• The QE of Mo(100), GaAs(100), and Si(100) showed a similar value of QE ∼10% for the cleaned substrate. On the other hand, Si(111) showed a lower
QE value than the other substrates.

70Figure B.3: ζ(100) is calculated as a function of the rotation angle θ for
CSK2 Sb(100) and the various substrates.

71

Figure B.4: ζ(110) is calculated as a function of the rotation angle θ for
CSK2 Sb(110) and the various substrates.

72

Figure B.5: ζ(111) is calculated as a function of the rotation angle θ for
CSK2 Sb(111) and the various substrates.

73