Materials Considerations in Photoemission Detectors

1. Materials Considerations in Photoemission Detectors S W McKnight C A DiMarzio

2. Energy Bands in Solids Energy Forbidden electron energies (Energy Gap) Eg2 Allowed electron energies (Energy Band) Eg1

3. Energy Bands and Gaps • Metals, insulators, and semiconductors all have energy bands and gaps • Difference is due to electron filling of bands • Metals: highest band with electrons in it is part-filled. • Insulators: highest band with electron in it is completely filled. (Filled band carries no net current.)

4. Electron Fermi Energy • Pauli Exclusion Principle (“fermions”): each electron state can be occupied by no more than one electron per spin state • Fermi Energy (Ef) separates occupied states from unoccupied states at T=0K • Ef is halfway between highest filled state and lowest empty state

5. Metal/Insulator Band Structure Energy Ef Ef Metal Insulator

6. Semiconductor Band Structure Ef Ef Ef Eg electrons “holes” Extrinsic Semiconductor (n-type) Extrinsic Semiconductor (p-type) Intrinsic Semiconductor (Eg≤ ~100 kT)

7. Surface Energies Vacuum Level (Evac) Ea Ec Vacuum Level (Evac) Фo Ef Ea = electron affinity = Evac - Ec Фo= work function = Evac - Ef Metal Insulator

8. Work Function of Elements

9. Photomultiplier Tubes • Vacuum photoemissive device • Window • End-on, side-looking • Photocathode • Insulator/semiconductor materials (better η than metals) • Spectral response from UV to Near IR • Moderate quantum efficiency (< 0.3) • Dynode chain • Gain ~106 through secondary electron emission

10. PMT Concept

12. Window Materials

13. Photocathode • Quantum efficiency (ηq) • ηq= (# emitted photoelectrons/# of incident photons) • Photon absorbed • Photoelectron created • Photoelectron escapes surface • Wavelength limits • hν > Eg + Ea • UV tubes: CsI, CsTe “solar blind” (<300-200 nm) • IR tubes: multi-alkali materials (Sb-Na-K-Cs)

14. Photocathode Band Models

15. Photocathode Quantum Efficiency η = PA Pν Pt Ps PA = Probability that photon will be absorbed by material = (1-R) Pν = Probability that light absorption will excite electron above vacuum level Pt = Probability that electron will reach surface PS = Probability that electron reaching surface will be released into vacuum

16. dx

17. Probability of absorption between x and x+dx =

18. Probability of Electron Reaching Surface

19. Probability of absorption between x and x+dx and electron escaping to surface = P(x) = k e-kx dx e-x/L P(x) = k e –(kx + x/L) dx Total probability of absorption and electron escaping to surface = P(x1) + P(x2) + P(x3) + …

20. Photocathode Quantum Efficiency Pν = Probability that light absorption will excite electron above vacuum level PS = Probability that electron reaching surface will be released into vacuum R= Surface reflectivity k= photon absorption coefficient L= mean escape length of electrons

21. Photocathode Materials • Cs-Te: UV “solar blind” • Sb-Cs: UV-Vis • Bialkali (Sb-Rb-Cs, Sb-K-Cs): UV-Vis • Multialkali (Sb-Na-K-Cs): UV-IR • Ag-O-Cs: Vis-IR • GaAs(Cs), InGaAs(Cs): UV-IR

22. Bialkali Cs-Te Sb-Cs

23. Dynode Chain • Amplification of photoelectrons by secondary electron emission • δ = (# of secondary electrons) / (# of primary electrons) • Gain: G~(δ)n (for n-stage dynode chain)

24. Secondary Electron Emission E Primary Electron Collision Process x Secondary Electrons Ea Vacuum Level Ec Eg Electron-Hole Pairs Valence Band Surface Insulator/Semiconductor

25. Secondary Electron Emission • Primary electron loses energy to electrons in solid • Metals: electron-electron interactions • Insulators: electron-hole creation • Penetration depth proportional to primary electron energy • Secondary electrons travel to surface • Electron-electron or electron-phonon collisions reduce energy and facilitate recombination • Greater chance of collision if created deeper • More electron-electron collisions in metals than insulators • Secondary electrons emitted into vacuum • Requires kinetic energy > electron affinity (Ea) • Secondary emission coefficient (σ) = (# of secondaries)/ (number of primaries)

26. Electron-Electron Scattering Vacuum Level (Evac) Vacuum Level (Evac) Ea Ec Фo Ef Electrons Ea = electron affinity = Evac - Ec Фo= work function = Evac - Ef Holes Metal Insulator Many final states available Few final states available

27. Secondary Electron Emission Coefficient

28. Secondary Emission Coefficients From Handbook of Physics and Chemistry

29. Secondary Emission Ratios

30. Types of Electron Multipliers

31. Characteristics of Dynode Types

32. PMT Timing Measurements

33. Timing Data for PMT Dynode Types

34. Microchannel-Plate PMT

35. MCP-PMT Construction

36. MCP-PMT • High gain/compact size • 2D detection with high spatial resolution • Fast time response • Stable in high magnetic fields • Low power consumption and light weight

37. MCP-PMT Gain

38. Photomultiplier Limitations • Dark current • Drift • Response time • Saturation: space charge limit • Tube damage at high illumination (anode current limit)

39. Dark Current vs. Temperature

42. Anode/Cathode Sensitivity • Radiant Sensitivity: photocurrent per incident radiant flux at given wavelength (A/W) • Luminous Sensitivity: photocurrent per incident luminous flux from tungsten lamp at 2856K (A/lm)

43. Luminous Sensitivity