1. Scattering controlled photoelectron escape from NEA – photocathodes
V.V. Bakin, D.V. Gorshkov, S.A. Rozkov, S.N. Kosolobov, H.E. Scheibler, A.S. Terekhov
Rzhanov Institute of Semiconductor Physics Siberian Branch of Russian Academy of Sciences, Novosibirsk, Russia
1. Introduction.
2. Atomic structure of p-GaAs(100)/(Cs,O) – interface.
3. Energy diagram of p-GaAs(100)/(Cs,O) – interface.
4. Photoelectron elastic and inelastic scattering at
NEA-photocathode – vacuum interface.
5. Summary.
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2. • Photocathodes manufacturing and study • Tubes manufacturing
Experimental
• Photocathodes manufacturing and study
• Tubes manufacturing
Knowledge
Diodes
Spectrometers
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3. Atomic structure of NEA-surface
• Оxygen sticking probability (Pox) at p-GaAs(Cs,O) surface
[V.V. Bakin et al. JETP Letters, v. 101, 380 (2015)]
Non local model
Non local model : [B. Hellsing, Phys. Rev. B 40, 3855 (1989)]
Pox ~ exp(-Ф/Ф*)
Ф* = 0.32 eV
Local model : [J.E. Ortega et al., Phys. Rev. B 36, 6213 (1987)]
Local model
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4. Atomic structure of NEA -surface
• Prolonged activation
[V.V. Bakin et al. Appl. Phys. Lett., 106, (2015)]
Bulk density
Work function
Cs/O ratio
Material type Cs
1.9 g/cm3
1.8 eV
1:0
metal
Cs7O
2.2 g/cm3
1:0.14
Cs4O
2.6 g/cm3
1:0.25
Cs11O3
1:0.27
Cs3O
2.7 g/cm3
1:0.33
Cs2O
4.7 g/cm3
11.17 eV
1:0.5
semi-conductor
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5. Are we the first? No, Michael G. Clark, UK is!
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6. Energy diagram of NEA-surface
2D – quantum bands e c*
~0.2 0.3 эВ
BBR eC
eVAC eF
image potential eV
Cs,O - layer
~0.5 nm z
~ 10 nm
~ 10 nm
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7. Does band bending in p-GaAs(Cs,O) depend
on (Cs,O) – composition?
• Yes!
V.L. Alperovich, A.G. Paulish, A.S.Terekhov, Phys. Rev. B 50, 5480 (1994)
V.L. Alperovich, A.G. Paulish, H.E. Scheibler, A.S. Terekhov, Appl. Phys. Lett. 66, (1995)
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8. Ballistic and diffusive photoelectron escape from p-GaAs(Cs,O)- photocathode.
● Ne(e,) -spectrometer
● Theory
photocathode
collector
V.E. Andreev et.al., Journal of Inversed and Ill-Posed problems, v.7, No.5, p.427 (1999)
● Experiment
V.V. Bakin et. al., JETP Letters v.77, p.167 (2003)
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9. Photoemission from p-GaN(Cs,O)
• Photoelectron elastic scattering
Pox, arb. units
• Photoelectron inelastic scattering [JETP Letters, in press]
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10. Summary
p - GaAs(Cs,O) – photocathodes with maximal QEs have the “dipole – type” activation layer with the metal character, which consists of Cs enriched (CsxO) – clusters with x ≈ 3. During activation, both composition and the lateral density (CsxO) – clusters are optimized, while the thickness of (CsxO) – layer does not exceed 1 ML.
p - GaAs(Cs,O) – photocathodes with maximal │ χ* │ have the “heterojunction – type” activation layers of metal (or semiconductor?) character. Thickness of (Cs,O) - layers for these photocathodes exceeds ~ 2 ML and x ≤ 2.
Energy diagrams of both p - GaAs(Cs,O) – and p - GaN(Cs,O) – photocathodes with maximal QEs correspond to the energy diagram of semiconductor - metal structure ( Schottky barrier).
4. The size quantization of electron spectra within potential wells with ~ 10 nm widths at both sides of NEA - surface is the inevitable consequence of quantum mechanics.
5. Photoelectrons during their escape from NEA photocathode are elastically scattered by the potential step at the semiconductor surface, covered with the partially disordered (Cs,O) – layer.
6. Photoelectrons during their transport across both BBR and the image potential well at the “vacuum side” of photocathode lose their energy due to emission of both bulk and (or) interface optical phonons. If hole concentration in the photocathode bulk exceeds ~ 1018 cm-3 , photoelectrons lose their energy because of emission of interface plasmons also.
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