1. In-situ Growth and X-ray Characterization of Alkali Antimonide Photocathodes
Presenter: Zihao Ding
Department of Materials Science & Engineering,
Stony Brook University

2. Introduction
Alkali Antimonide (K2CsSb, Cs3Sb, …) – excellent photocathode candidate for future light source
High quantum efficiency
Low emittance
Good lifetime
Fast response …
D.H. Dowell et al. / Nuclear Instruments and Methods in Physics Research A 622 (2010) 685–697

3. Introduction – Roughness From Traditional Recipe
QE~1%
QE(%) t K Cs
QE during growth (532 nm laser)
RMS roughness of 25nm over 100 nm spatial period on a K2CsSb photocathode grown by sequential evaporation.
S. G. Schubert, et al, APL Mater. 1, (2013)

4. Field dependent emittance growth: 20 nm amplitude, 80 nm period
Introduction – Effect of Roughness on Thermal Emittance
D. Xiang et al. Proceedings of PAC07, Albuquerque, New Mexico, USA
To quantify
Room temperature thermal
Iimit is microns /mm rms
- requires roughness < 1 nm
Field dependent emittance growth: 20 nm amplitude, 80 nm period
Emittance growth at 20 MV/m, period 4 x amplitude.

5. Introduction – In-operando Growth System
Turret evaporator
Incident X-ray
Cam1:
XRD, XRR, GiSAXS
XRF
Cam2:
WAXD
Thickness monitor
XRF
Triple evaporator
Sample
Chamber at Beamline G3, CHESS

6. Basics of X-ray Reflectivity
Surface roughness
Film thickness
Film density
Interface roughness
Film
Substrate
X-ray reflectivity: a fast and non-destructive technique to characterize thin film properties

7. Basics of X-ray Reflectivity
Parratt recursion, Parratt, 1954.
Parratt, L. G. (1954). Phys. Rev. 95, 359.
X-ray and Neutron Reflectivity, J. Daillant, A. Gibaud

8. Previous Study Critical Sb-crystallization thickness: 4nm
Crystal structure evolution of K2CsSb photocathode grown by traditional recipe
Critical Sb-crystallization thickness: 4nm
Surface roughening occurs due to transition from crystalline Sb film (Sub nm)to crystalline K3Sb film (a few nm) according to XRR analysis.
Ruiz-Osés et al. APL Mater. 2, (2014)

9. Previous Study K Crystal structure evolution in sequential deposition
Ts = room temperature : K step: mostly hexagonal. Cs step: never converts to CsK2Sb
Ts = room temperature : K step: mostly cubic K3Sb phase, hexagonal appears if over-potassiated. Cs step: cubic phase converts to CsK2Sb quickly.
Room temperature
100 C Cs
S. Schubert et al., J. Appl. Phys. 120, (2016)
Room temperature
100 C

10. Previous Study 100 C substrate temperature 15 nm Sb + 70 nm K
Mixed phase of KxSb, lower QE
90 nm Cs sufficient to fully convert to K2CsSb
QE = 6.7% at 532 nm.
S. Schubert et al., J. Appl. Phys. 120, (2016)

11. Experimental Details – Roughness Improvement
Idea: to prevent Sb from crystallizing
Yo-yo sequential deposition
Substrate: Si (100)
125 ̊C
+3nm Sb Sb
+K until photocurrent peaks
KxSb
+Cs until photocurrent peaks
CsxKySb
2nd layer: +5nm Sb
XRR experimental data and fitted data

12. Results and Analysis Yo-yo alkali co-deposition Sub Sb 1st 2nd 3rd 4th
Substrate: MgO
~120 ̊C
+3 nm Sb Sb
+K, Cs
Deposition rate ratio 2:1
CsxKySb
Repeat growth (6 layers)
XRR experimental data and fitted data
XRF: final stoichiometry: K0.35Cs0.89Sb

13. Comparison on roughness vs. thickness
Results and Analysis
Comparison on roughness vs. thickness K Sb Cs
K+Cs
Layer:
Sb+(K,Cs)
4th
5th
6th
4.9%
0.88%
0.16%
3.1%
1.2%
1.7%
4.2%
3.7%
4.5%
Yo-yo sequential deposition
Yo-yo alkali co-deposition

14. Effusion cell configuration in chamber (right side)
Recent Activities
RF sputtering system: in collaboration with Radiation Monitoring Devices.
Effusion cells:
Inherited from Jlab
Provides “unlimited” amount of K & Cs.
Effusion cell configuration in chamber (right side)

15. Results L019 XRD results K-Cs codep 95nm Sb K-Cs codep
Photocurrent vs time
Sb (202)
(422)
(420)
Sb (110)
(400)
(222)
Stoichiometry vs time
MgO substrate, with Cs cleaning. Substrate heater set to 6%, around 150 C. Sb deposition: 95nm. K-Cs co-effusion: rate 0.15~0.45 Å/s. Starting to show texture at thickness = 86nm. 
Final QE (532 nm) ~ 4%

16. Thank you for your attention!
Summary
Traditional recipe ends up in rough photocathodes which are unfavorable for the application in electron gun.
In-situ growth system with X-ray characterization capabilities help to better understand the growth mechanism of alkali antimonides, including crystal structure, QE and roughness evolution, etc…
Much smoother photocathode surface can be achieved using modified recipe, such as yo-yo sequential growth, yo-yo co-deposition, as well as sputtering & effusion (Next talk by Mengjia!)
Acknowledgements
Thank Mengjia Gaowei, John Sinsheimer, John Walsh, Erik Muller and my adviosr John Smedley from BNL, Siddharth Karkare, J. Xie, ANL, Jun Feng and Howard Padmore from LBL. Susanne Schubert. Martin Schmeisser, Julius Kuehn From HZB.
Thank all staff at CHESS, Cornell University.
Thank you for your attention!