Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FROH04

NEGATIVE ELECTRON AFFINITY GAAS CATHODE ACTIVATIONWITH CSKTE THIN FILM
M. Kuriki∗ , K. Masaki, Hiroshima University, Higashihiroshima, Japan
Abstract
Negative Electron Affinity (NEA) GaAs cathode is anunique device which can generate a highly polarized electron beam with circularly polarized light. The NEA surface
which is necessary for spin polarization, is conventionallymade by Cs and O/NF3 adsorption on the cleaned p-doped
GaAs crystal, but the robustness of the cathode is very limited, so that the electron emission is easily lost by residual
gas adsorption, ion back-bombardment, etc. To improve thecathode robustness, NEA activation with a stable thin-film
on GaAs surface according to Hetero junction hypothesis
has been proposed by the author. An experiment of the NEA
activation with CsKTe thin film was carried out at Hiroshima
University and a significant electron emission with 1.43 eV
photon was observed which strongly suggested NEA activation. The cathode showed 16 to 20 times improvement oflifetime comparing to GaAs activated with Cs and O.

INTRODUCTION
Spin is one of the fundamental quantum number of electron and sensitive to magnetic structure of material. Spinpolarization is then important to define the state in the nuclear and particle physics and observe the magnetic structure
in material sciences. Spin polarized electron beam has important roles in ILC (International Linear Collider) which isthe next project of high energy physics; By suppressing thebackground events and improving the effective luminosity,
it increases the sensitivity to new physics. NEA (NegativeElectron Affinity) GaAs photo-cathode has been developed
as a highly spin polarized electron source since 1970’s [1].
The spin polarization is defined as the ratio of the differenceof number of right handed and left handed electrons to the total number of electrons. Although it is limited fundamentallyup to 50% with a bulk GaAs crystal, the strain super-lattice
crystal of GaAs [2] realize 92% polarization by breaking thedegeneration of the valence band. The strain compensatedsuper-lattice GaAs [3] improves the crystal quality leading
the quantum efficiency 1.6% with 90 % polarization.
In contrast to the many improvements of polarization andquantum efficiency, the robustness of the cathode has beenvery limited. The polarized electron emission requires NEA
surface where the conduction band minimum state has higher
energy than the vacuum state, because the electron excited tothe conduction band have almost no momentum. The NEA
surface is made by adsorption of Cs-O/NF3 on a cleaned
GaAs surface and the procedure making NEA surface iscalled as activation. In NEA surface, Cs and oxygen orNF3 are weakly bound on GaAs surface and the electron∗ mkuriki@hiroshima-u.ac.jp

emission is easily lost by residual gas pollution [4], thermal
desorption [5], Ion back bombardment [6].
An exact mechanism of NEA activation with Cs-O/NF3
is not understood, but an hypothesis is electric dipole ofthe surface layer decreasing the energy state of vacuum.
A possible NEA activation with CsTe was suggested bySugiyama [7]. The author proposed NEA activation ofGaAs with a thin layer of semi-conductor. It is called asNEA activation with hetero-junction of a thin semiconductor. The first NEA activation based on the hetero-junction
hypothesis was confirmed with CsTe thin layer reported inRef. [8, 9] and the lifetime improvement has been demonstrated in Ref. [10]. Figure 1 explains the NEA activation
with the hetero-junction hypothesis. The horizontal axis
shows normal direction of the cathode (𝑧) and the verticalaxis is the energy of states. From the left, the regions areGaAs, surface thin layer, and vacuum. The horizontal dashedline shows Fermi level. GaAs has a band gap with 𝐸𝐵𝐺 andFermi level is at the bottom of the band gap in p-doped case.
If the work function of the semiconductor is less than 𝐸𝐵𝐺 ,the vacuum level is less energy than the conduction band
minimum of GaAs, i.e. the surface state becomes NEA. The
size of NEA is 𝜒 in the figure.
EGaAs

Conduction
BandSurface
Layer

VacuumConduction
Bandχ

EBG
EF
ValenceBand
ValenceBandzFigure 1: NEA surface state by the hetero-junction hypothesis. The horizontal and vertical axes show the normal direction of the cathode surface and energy state, respectively.
From the left, the regions are GaAs, surface thin layer, andvacuum. The horizontal dashed line shows Fermi level. Ifthe work function of the surface thin layer is less than 𝐸𝐵𝐺 ,the surface state becomes NEA.
In the following sections, we present the experimental
study of NEA activation of GaAs with CsKTe thin layer.
CsKTe [11] is known as a thin film photo-cathode made byevaporation as same as Cs2 Te. The study is important toconfirm the hypothesis of NEA activation with the heterojunction.

- 183 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 FROH04

The experiment was carried out at Hiroshima University.
Two inches bulk Zn-doped (5 × 1019 cm−3 ) p-type GaAs
(100) wafer was used. A quarter of the wafer was placedon a cathode mount in a vacuum chamber. Prior to theinstallation, the wafer was treated with a solution of H2 SO4 ,H2 O2 , and H2 O, and another solution of HCl and C3 H8 OH(iso-propanol)to remove the oxidized surface layer and makean As rich surface [12]. These processes are done in air andthe treated sample was stored in a desiccator at 0.1 Pa.
After the sample is installed in the vacuum chamber, thechamber is baked to established an ultra-high vacuum condition. A typical base pressure was 3.6 × 10−8 Pa. The vacuum
chamber was pumped by ion pump and NEG pump. Before
the experiment, the sample was heated up to 450 ∘ C to obtainthe cleaned surface.
Te beads is mounted on tungsten basket. By applying thecurrent to the tungsten basket, Te vapor is generated. K andCs vapor were generated with dispensers by SAES Co. Thesevapor sources were mounted on a movable head and theposition can be adjusted to the sample position. The vapor
source was designed to generate vapor symmetrically to theGaAs sample and a quartz thickness monitor to measure theamount of the evaporated metal. The amount of the metalon GaAs is controlled with the thickness monitor.
QE (quantum efficiency) is defined as the ratio of numberof photo-electrons to number of photons. To measure QEof the cathode, monochromatic light from Xe lump with agrating monochromator is introduced to the chamber througha silica viewing port. The cathode mount is biased with -100
to measure the photo-current. The space charge effect isnegligible with this condition.
After the sample was cooled down to RT (room temperature), the NEA activation was done with the following
process.
1. Evaporate Te with a determined thickness.
2. Observe QE in the range from 250 nm to 1000 nm with
10 or 50 nm step.
3. Evaporate K with some thickness.
4. Evaporate Cs with the same thickness.
5. Repeat QE measurement and K and Cs evaporation.
6. If QE with 250 nm photon is saturated, we stop theprocess.
The condition established with these processes is called asthe optimum condition. In this condition, QE with 250 nm
photon is maximized. CsKTe cathode should have a highquantum efficiency in the UV region [11] and we expectthat a good CsKTe thin layer is formed with this condition.
Figure 2 shows the alkali metal thickness (sum of Cs andK thicknesses) of the optimum condition as a function ofTe thickness. The line is a linear fit curve to the data points
except one of 0 Te thickness. The thickness of the alkali

metal is proportional to Te thickness, i.e. the ratio of thealkali-metal and Te is constant for different Te thickness.
This is an evidence of that a compound of Cs, K, and Te isformed on GaAs sample. Please note that the point of 0 Tethickness is off the line, because this is a compound of Csand K (no Te) and should be a different compound.

Cs + Te (A)NEA ACTIVATION

600

500

400300200

1000010203040506070
Te (A)

Figure 2: Total thickness of alkali metal (Cs + K) at theoptimum condition as a function of Te thickness is plotted.
Except zero thickness of Te, Total thickness of alkali metal
is proportional to Te thickness.
Figure 3 shows the evolution of QE spectrum during theactivation experiment. As mentioned already, Cs and K
were evaporated repeatedly after Te was evaporated on GaAs
surface with same amount. The number in legend is the sumof Cs and K thickness. In this case, Te thickness was 14 Å.
10 Å Cs and K were evaporated every time. 0 thickness Csand K data (closed circle) shows zero quantum efficiency
for less than 2.5 eV. By increasing the Cs and K thickness,
QE in 1.4 - 2.5 eV region is increased and saturated for 80
Å or more Cs and K thickness. The spectra with Cs and Kthickness more than 30 Å have a significant value at 1.43
eV which corresponds to GaAs band gap energy. QE at 1.43
eV with 100Å Cs and K was 1.5 × 10−5 . Background levelis estimated as less than 1.0 × 10−6 in QE.
Electron emission with 1.43 eV photon is not from CsKTestates [11]. Surface intermediate states is a possibility toexplain this emission, but the steep drop of QE at less than
1.43 eV is not consistent. The emission with 1.43 eV photon is consistent to the hetero-junction structure of GaAsand CsKTe thin film. 1.43 eV photon can penetrate theCsKTe thin film because the energy is less than the band
gap of CsKTe. The photon excites the valence band electronof GaAs to the conduction band. Because the electron in
the conduction band has almost no momentum, it moves
to CsKTe by diffusion. Electrons in CsKTe obtains somemomentum due to the energy difference between GaAs andCsKTe. Electrons in CsKTe are thermalized and some electrons whose momentum is less than the vacuum potential,
are captured in CsKTe conduction band. A fraction of electrons can escapes to vacuum. This description is consistent- 184 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

1

10
20

10−1

10−1

4060
80
100

10−2

10−2120

Quantum efficiency

Quantum efficiency

PASJ2019 FROH04

14010−310−310−44.96 eV

1010−510−510−610−61.522.533.5

10−744.55Photon energy (eV)Figure 3: QE spectrum of GaAs activated with CsKTe (14Å Te). Number of legend is the sum of Cs and K thickness.
to the steep drop of QE at less than 1.43 eV and the small
QE at 1.43 eV. Considering these phenomena, the emission
with 1.43 eV photon from the CsKTe-GaAs cathode strongly
suggests the NEA activation of GaAs with CsKTe thin film.
Figure 4 shows QE as a function of Te thickness. For eachTe thickness condition, alkali metal (Cs and K) are appliedto maximize QE with 4.96 eV photon. The red filled circle
shows QE with 1.43 eV photon, and the green filled squareshows QE with 4.96 eV. According to the discussion in theprevious paragraph, the emission with 1.4 eV photon is fromGaAs and it with 4.96 eV photon is from CsKTe. 4.96 eVdata shows QE increasing monotonically with thicker CsKTe
layer and a saturation for more than 40 ÅTe thickness. This
property is consistent to that the emission with 4.96 eV photon is from CsKTe layer. QE with 1.43 eV photon ie peakedaround 10-20 Åand decreased for larger Te thickness. It isconsistent that the emission with 1.43 eV is from GaAs. The
decay of QE with thicker Te is light or electron absorption
by the CsKTe layer.

Lifetime Measurement
Lifetime of the CsKTe-GaAs cathode was measured. After the activation, QE evolution in time was observed. Thistime, we measured the lifetime without beam emission, i.e.
the beam current is negligibly small. In this case, the cathodeQE is decreased by residual gas poisoning. The effect shoulddepend on the vacuum pressure and gas species. By assuming the gas species does not change during the measurement,
QE evolution 𝜂(𝑡) can be expressed as∫ 𝑃𝑑𝑡

𝜂(𝑡) = 𝜂0 𝑒− 𝜏 ,(1)010203040Te thickness (A)

5060

70

Figure 4: QE with 4.96 eV photon (green filled square) and1.43 eV (red filled circle) are shown as a function of Tethickness. For each Te thickness, alkali metal (Cs and K)
are applied to maximize QE with 4.96 eV photon.

Quantum efficiency

10−7

1.43 eV−410−310−410−5012345678910Exposure [1e-3 Pa.sec]

Figure 5: QE with 2.06 eV photon as a function of exposure(10−3 Pa.sec) is plotted. By fitting with the data > 2.0 ×
10−3 Pa.sec, 1/e lifetime in exposure is obtained as 6.49 ±0.01 × 10−3 Pa.sec.
axis show the exposure in 10−3 Pa.sec. The line is a fitting
curve according to Eq. (1). 𝑡 = 0 is the end of evaporation.
The data points < 2.0 × 10−3 𝑃𝑎.𝑠𝑒𝑐 are not included toevaluate the fitting curve. 1/e lifetime is evaluated as 6.49 ±0.01 × 10−3 Pa.sec. This number should be compared to thepreceding studies [13, 14] which are summarized in Table 1.
The lifetime of CsKTe-GaAs cathode is 16 - 22 times longer

where 𝜂0 is a constant, 𝑃 is vacuum pressure, and 𝜏 is thelifetime. The dimension of 𝜏 is Pa.sec.
Figure 5 shows QE evolution for 2.06 eV photon. Te
thickness was 37Å for this data set. Because the preceding
researches evaluated the lifetime with this photon energy, weused the same photon energy for comparison. The horizontal- 185 -Table 1: Lifetime of NEA GaAs Cathodes
CathodeCsKTe-GaAs
CsO-GaAs
CsO-GaAs

Lifetime[10−3 Pa.sec]
6.49 ± 0.01
0.29 ± 0.03 [13]0.40 ± 0.02 [14]

Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FROH04

than that of CsO-GaAs cathode.
The lifetime with 1.43 eV photon of CsKTe-GaAs cathode was 2.75 ± 0.05 × 10−3 Pa.sec. 1.43 eV photon is usedfor spin-polarized electron generation. According to thisnumber, operational lifetime under various condition can be
estimated. If we assume pressure in an RF gun and DC gun
as 1.0 × 10−7 Pa and 5.0 × 10−10 Pa, the expected operational
lifetimes are 7.8 hours and 1540 hours for RF and DC guns,
respectively. The results are summarized in Table 2.
Table 2: Expected Operational Lifetime of CsKTe-GaAs
CathodesGun

Pressure[Pa] Operational lifetime [h]RF gunDC gun1.0 × 10−7
5.0 × 10−10

hetero-junction hypothesis and improving the robustness ofthe NEA GaAs cathode. It suggests a possibility of polarized
RF electron gun with this cathode.

ACKNOWLEDGEMENTS
This work is partly supported by Japan-US Cooperative
grant for scientific studies, Grant aid for scientific study by
MEXT Japan (KAKENHI) Kiban B 17H02898.

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[5] M. Kuriki et al., Nucl. Inst. and Meth. A, 637(2011)S87-S90.
[6] J. Grames et al., Proc. of PAC 2005(2005)WPAP045.

CONCLUSION

We perform the experimental study of NEA activation
with CsKTe thin film. Electron emission with 1.43 eV photon
was observed and the result strongly suggested the NEA
activation with CsKTe. The lifetime was measured with thiscathode and the lifetime was 6.49 ± 0.01 × 10−3 Pa.sec with
2.06 eV photon which was 16 - 22 times longer than thatwith CsO-GaAs cathode. The expected operational lifetimeof 1.43 eV photon with RF gun is 7.7 hours. The resultdemonstrated the effectiveness of NEA activation with the[7] H. Sugiyama et al.,J. Phys. Conf. Ser. 298(2011)012014.
[8] M. Kuriki et al., Proc. of IPAC 2015(2015) TUPWA062.
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