Cs-K-Te ヘテロ接合によるGaAsカソードの高耐久化
栗木雅夫A, 正木一成A, 郭磊BA)広島大学先端物質科学研究科
B)名大シンクロ

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Contents
 Introduction,
 NEA Activation with Cs-K-Te
 Summary.

Introduction

Cs-O NEA-GaAs Photo-cathodep-type GaAs
PEA
VacuumConduction
BandFermi level

Valence
BandCs-O• Electron excited by circularly
polarized laser is spinpolarized.
• The polarized electron can beextracted to vacuum by NEAsurface.

NEAVacuumDegradation of Cs-O NEA surface
Thermal desorption Gas adsorption
Cs

heatCsNEA
GaAsCsCs

Ion Feedback

H2 ONEAGaAsO2NEA
GaAs++CO2

1/e lifetime
3.0 – 4.0 x 10-4 Pa.sec

C. Shonaka, Master thesis, Hiroshima U. (2009)

Robust NEA GaAs cathode Cs-O NEA GaAs has a limited robustness.

 Requires UHV (<1.0e-9 Pa).
 Only compatible with DC biased electron gun,~5 MV/m.
 Limited bunch intensity, long bunch, largeemittance.

 If a robust NEA surface on GaAs isdeveloped, Less requirement on vacuum.
 Compatible with RF electron gun, ~ 100MV/m.
 High bunch intensity, short bunch, smallemittance.

NEA Activation withCs-K-Te

NEA surface with Hetero Junction
p-type GaAsSurface
filmvacuum

Conduction
BandEGaAs

Valence
BandEfilmeΦ
Fermilevel 𝐸𝐺𝑎𝐴𝑠 > 𝜑 for NEA state.
 𝐸𝑓𝑖𝑙𝑚 > 𝐸𝐺𝑎𝐴𝑠 for transparency.

Apparatus
TeK

Chamber

Cs2

Cs1

Evaporation head

 Chemically polished SUS chamber with NEA and Ion
pumps. Typical vacuum pressure is1.5e-8 Pa.
 Cs, K, and Te evaporation.
 Heater for cleaning.
 Quartz thickness monitor.
 Quantum efficiency measurement with Xe lump.

Calibration of Te thickness

Several samples of Te evaporated on GaAs substrate.
Calibrated with Ellipsometry measurement.

Use the thickness with the ellipsometry for further
analysis.

Experiment Evaporate Te until the decided thickness.
 Evaporate K and Cs repeatedly.
 Measure the QE spectrum after each K and Cs
evaporation.
 The optimum thickness of Cs and K is defined as thatgiving the maximum QE at 4.9 eV.

1.43 eVThickness VS. QE
Te 13.6Å

 QE at 4.96 eV is saturat
ed with more Cs-K and Te.
 QE at 1.43 eV is peaked
at some Te and Cs-K thickness.
 These results are consis
tent that electron emissions with 4.96 and 1.43
eV photon are from CsKTe and GaAs,
respectively.

Lifetime of CsKTe NEA GaAs
𝜂 𝑃 ∙ 𝑡 = 𝜂0 exp −‫𝑡𝑑𝑃 ׬‬𝜏𝑎𝜏𝑎 = (6.50±0.01)× 10−3 Pa ∙ sec

CathodesLifetime
𝜏𝑎 [10−3 Pa ∙ sec]

CsKTe/GaAs

6.50±0.01

Cs-O/GaAs

0.29±0.03 [1]Cs-O/GaAs

0.40±0.02 [2][1]K. Miyoshi, M. Thesis, Hiroshima U. (2013)[2]G. Lei, M. Thesis, Hiroshima U. (2014)

E-Gun

Pressure[Pa]Life[h]DC-gun

5.0×10-101535.6

RF-gun1.0×10-7

7.7

Summary We found that GaAs is activated with CsKTe.
 QE was 2.0~3.0e-4 with 15-20 Å Te thickness.
 Thicker Te gave less QE.
 We found that the CsKTe GaAs has a longer lifetime(darklife) than that of Cs-O GaAs cathode.
 It is operable in RF gun, even a frequent cathodeactivation is needed.

NEA Activation with CsKTe• Thin film photo-cathode byevaporation.
• High quantum efficiency
at 4.75 ～ 3.0 eV.
• Work function and bandgap?

Fig : Quantum effiency of Cs-K-Te, Cs2Te, K-Te : Appl. Phys. Lett. 70, 1491 (1997)
D. Bisero and B. M. van Oerle et al., “High efficiency photoemission from Cs–K–Te“)Ellipsometry
𝜃𝑖Reflectivity and phasedifference for S and P
polarization is measured.

𝜃𝑗𝜌 = tan(ψ)𝑒 𝑖Δ
Injection

Reflection
𝜃𝑟𝑝𝜓=𝑟𝑠𝑟𝑝01 + 𝑟𝑝12 𝑒 −𝑖2𝛽
1 + 𝑟𝑝01 𝑟𝑝12 𝑒 −𝑖2𝛽𝜌=𝑟𝑠01 + 𝑟𝑠12 𝑒 −𝑖2𝛽
1 + 𝑟𝑠01 𝑟𝑠12 𝑒 −𝑖2𝛽𝑟01𝑛0
film 𝑛1
substrate 𝑛2

𝑟12

Δ = 𝜑𝑝 − 𝜑𝑠dr : reflectivityd : thickness𝑑𝛽 = 2𝜋 𝑛1 𝑐𝑜𝑠𝜃1𝜆