1. Second Workshop on Photocathodes: 300nm-500nm
June 29-30, 2012
University of Chicago
Enrico Fermi Institute, The University of Chicago High Energy Physics Division at ANL

2. Goals of the Workshop The proposed goals of the workshop are to:
Understand deeply the chemistry and physics of alkali photocathodes.
Discuss/propose experiments, measurements and theory that would answer remaining questions;
Foster collaborative efforts to bring a broad and powerful variety of photodetectors with high QE into the application areas.
Or Google: University of Chicago Photocathode
11/8/2018
2nd Photocathode Workshop

3. Neutrino Physics
Need: lower the cost and extend the reach of large neutrino detectors
H. Nicholson
Large area Cheap High QE over broad spectrum Compatible with MCP assembly process

4. Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes
Quantum Efficiency
Quantum efficiency (QE) of a sensor is defined as the ratio
QE = N(ph.e.) : N(photons)
Conversion of a photon into ph.e. is a purely binomial process (and not poisson !)
Assume N photons are impinging onto a photocathode and every photon has the same probability P to kick out a ph.e..
Then the mean number of ph.e.s is N x P and the Variance is equal to N x P x (1 – P)
29th of June 2012, Univ. Chicago
Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

5. QE: Short Historical Excursion
Sommers found the „multialkali“ effect: combination of Cs-K-Na-Sb has high QE in the visible spectrum.
Also were discovered
Cs3Sb on MnO (S11, lpeak 400nm, QE ~ 20%)
(Cs)Na2KSb (S20, lpeak 400nm, QE ~ 30%)
K2CsSb (lpeak 400nm, QE ~ 30%)
K2CsSb(O) (lpeak 400nm, QE ~ 35%)
29th of June 2012, Univ. Chicago
Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

6. How it shall be possible to boost the QE and who is interested in it ?
Use of highly purified materials for photo cathode (provides lower scattering for e- (low recombination probability)
→ e- kicked out from deeper (top) layers can reach photo cathode-vacuum junction ¬_ and „jump“ into it (→ thicker cathode is possible).
Optimal tuning of the photo cathode thickness
……of the structure and material composition
……of the anti-reflective layer
……?
29th of June 2012, Univ. Chicago
Razmik Mirzoyan, Max-Planck-Munich: Highest QE PhotoCathodes

7. Parameters, and how to affect them
Reflectivity depends on angle of incidence and cathode thickness. Though already small, structuring of the photocathode can further reduce loss due to reflection. Increasing the electron MFP will improve the QE. Phonon scattering cannot be removed, but a more perfect crystal can reduce defect and impurity scattering: A question to consider: Why can CsI (another ionic crystal, PEA cathode) achieve QE>80%? Large band gap and small electron affinity play a role, but, so does crystal quality.
R. Downey, P.D. Townsend, and L. Valberg, phys. stat. sol. (c) 2, 645 (2005)
T.H. Di Stefano and W.E. Spicer, Phys. Rev. B 7, 1554 (1973)
J. Smedley

8. Inès Montaño
Sandia National Laboratories

12. ~1-2 nm of K & Sb per layer – subcrystaline

14. Metallic Cesium Source Construction
Charlie Sinclair

15. In Situ Measurement Techniques
3 Talks (Matt Highland and Jeffrey Elam of ANL, and Miguel Ruiz Osés of Stony Brook)
XRD (and CTR), XRR, XPS, XRF, XAS, AFM, Elipsometry
Want to measure thickness, roughness, stoichometry, crystalline phase, grain size and orientation, chemical purity

16. Probing the Growth Environment
Synchrotron x-rays are capable to penetrating the MOCVD environment and yield structural and elemental details in real time
In-situ MOCVD reactor at sector 12ID-D of the Advanced Photon Source
Diffraction from GaN surfaces and InN crystals
X-ray Fluorescence from deposited Indium
Measurements reveal a very complex growth behavior
Fluorescence
Detector
Movie
camera
Visible illumination
Synchrotron x-rays
Scattering
Detector 16
Matt Highland 16

17. In-InN Phase Boundaries
By monitoring InN and In liquid formation we can map out an indium condensation phase diagram
Bare GaN
surface
In liquid
droplets
pNH3= 27 Torr
InN crystals
Upon increasing TMI flow
At higher temp, elemental In liquid condenses
At lower T, relaxed InN solid particles grow
F. Jiang, et al. PRL 101, (2008)

18. Oscillatory Growth and Decomposition
Near phase boundaries system can spontaneously oscillate
Inter-conversion between InN and liquid In
AFM of quenched samples shows microstructure of distinct surface species
Epitaxial InN islands
Elemental In droplets
F. Jiang, et al. PRL 101, (2008)

19. Oscillatory Growth Mechanism
NH3 cracks on the GaN of InN surface and forms the intermediate species that allow InN to grow
Critical amount of liquid In metal condenses which accelerates conversion of NH3to N2 and InN starts to decompose
Liquid In metal evaporates to expose GaN surface and InN growth starts again

20. The Fundamental Problem
At desirable growth temperatures required nitrogen activity is equivalent to kilobars (~104 psi) of N2
During MOCVD growth nitrogen activity provided by cracking ammonia
Reaction we want to avoid:
Ambacher et al., JVST B 14, 3532 (1996) 20

21. Charlie Sinclair Cornell University (ret.)
Alkali Metal Sources for Photocathodes and their Quantification – Getter Sources vs. Metallic Evaporation Sources
Charlie Sinclair
Cornell University (ret.)
11/8/2018
UChicago Photocathode Workshop

22. Operation of Metallic Cs Source
Maintain delivery tube and nozzle at ~ 250 C, valve at about 230 C, and Cs tube to give desired vapor pressure.
Temperature maintained continuously, resulting in fairly quick response (seconds) to opening the valve
Delivered ~ 1 monolayer (for GaAs cathode) in ~ 1 minute
Used to make uniform QE on 35 mm GaAs by bouncing Cs off heated plate
11/8/2018
UChicago Photocathode Workshop

23. Quartz Crystal Microbalance
Resonant frequency shifts downward as mass is added. For our 6 MHz crystal, 10 Hz is ~ 100 ngm ~ 1 monolayer of Cs.
Uncertainties are the number of atoms per monolayer, and the sticking coefficient (~1 for alkalis)
Measured at ~ 290 K and 77K, getting same result, implying a sticking coefficient ~ 1
11/8/2018
UChicago Photocathode Workshop