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. EWPPAA - 2016

• Photocathodes manufacturing and study • Tubes manufacturing Experimental • Photocathodes manufacturing and study • Tubes manufacturing Knowledge Diodes Spectrometers EWPPAA - 2016

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 EWPPAA - 2016

Atomic structure of NEA -surface • Prolonged activation [V.V. Bakin et al. Appl. Phys. Lett., 106, 183501 (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 EWPPAA - 2016

Are we the first? No, Michael G. Clark, UK is! EWPPAA - 2016

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 EWPPAA - 2016

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, 2122 (1995) EWPPAA - 2016

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) EWPPAA - 2016

Photoemission from p-GaN(Cs,O) • Photoelectron elastic scattering Pox, arb. units • Photoelectron inelastic scattering [JETP Letters, in press] EWPPAA - 2016

Summary 1. 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. EWPPAA - 2016