Study of bialkali antimonide photocathode on NB substrate
Study of bialkali antimonide photocathode on NB substrate at Jlab Abdullah Mamun P 3 Workshop : Photocathode Physics for Photoinjectors, October 17 -19, 2016 Thomas Jefferson National Accelerator Facility, Newport News, Virginia, USA M. A. Mamun, P 3 Workshop, October 17 -19, 2016
Estimation of Spectral Response at 2 K for Bialkali Antimonide on Nb Substrate M. A. Mamun, P 3 Workshop, October 17 -19, 2016 2
Motivation • The future Jefferson Lab Electron Ion Collider would require to implement electron cooling of the proton beam. • A superconducting radio frequency (SRF) photogun is an ideal candidate electron source for this application, provided that the photocathode can give good yield or quantum efficiency (QE) at cryogenic temperature. • The focus of this project is to make bialkali antimonide photocathode on a niobium substrate that can be used in an SRF photogun. • The goal of this work is to estimate the spectral response at liquid helium temperature (2 K) by investigating the photocathode QE at different temperatures: Room temperature (295 K) Water ice(273 K) Dry ice (195 K) Liquid nitrogen (77 K) temperatures. M. A. Mamun, P 3 Workshop, October 17 -19, 2016 3
Deposition chamber a) Photograph of the bialkaliantimonide photocathode deposition chamber with effusiontype alkali dispenser (shown in the inset), b) Schematic of the substrate holder assembly with substrate heater that acted as a cryostat, c) Photograph of the QE scanner system with the mirrors attached to the stepper-motor-controlled translation stages, d) Photograph of the Ga. As substrate secured to the sample holder using an annular Ta cup, and e) Schematic of the effusion-type alkali dispenser used for coevaporation of K and Cs species (with permission from Lawrence S. Cardman). M. A. Mamun, C. Hernandez-Garcia, M. Poelker, and A. A. Elmustafa, “Effect of Sb thickness on the performance of bialkali-antimonide photocathodes, ” Journal of Vacuum Science & Technology A 34 , 021509 (2016). M. A. Mamun, P 3 Workshop, October 17 -19, 2016 4
Bialkali Antimonide Photocathode by Codeposition • Sources: • • • Sb pellets (99. 9999% purity, from Alfa Aesar) K ampoules (99. 95% purity, 1 gram in argon) from Espi-metals Cs ampoules (99. 9þ% purity, 1 gram in argon) from Strem Chemicals For the baked system, the evacuated chamber was baked at 180 C for 60 h Post bake vacuum reached ~1. 4 10 -9 Pa Sb evaporation via resistive heating by 32. 7 A supply to the Tungsten heater Sb was deposited for ~30 min on Nb substrate at 200 C Alkali coevaporation was controlled by adjusting heater power and gas flow through the effusion source. , in the following ranges: • Inlet tube: • Dispensing tube: • Reservoir tube: 381 -462 C 232 -294 C 153 -281 C Used a residual gas analyzer (RGA) to monitor the chemical’s vapor pressure during deposition. Representative partial pressures observed and maintained as: • Sb PP: 7. 3(± 0. 6) 10 -9 Pa; Cs PP: 1. 8(± 0. 1) 10 -8 Pa K PP: 4. 2(± 0. 5) 10 -9 Pa Alkali was deposited at substrate temperatures falling from 120 C to 80 C Alkali deposition continued as long as the photocurrent showed increasing trend QE was monitored using a 532 nm green laser with cathode biased at Vb= - 284 V M. A. Mamun, C. Hernandez-Garcia, M. Poelker, and A. A. Elmustafa, “Effect of Sb thickness on the performance of bialkali-antimonide photocathodes, ” Journal of Vacuum Science & Technology A 34 , 021509 (2016). M. A. Mamun, P 3 Workshop, October 17 -19, 2016 5
Photocathode QE at Cryogenic Condition • • • Photocathode QE is expected to decrease at lower temperature due to the change in the emission threshold (i. e. , Eg+EA) of the semiconductor. The QE is also expected to decrease if chemicals adsorb onto the cold photocathode surface. These chemicals serve as contaminants that increase the surface work function. We investigated QE change as a function of temperature for two vacuum conditions of the deposition chamber (pre-bake and post-bake) and observed that: − QE reduction due to contamination of the photocathode surface via adsorption can be minimized by reducing the amount of water vapor and oxygen within the gun chamber. M. A. Mamun, P 3 Workshop, October 17 -19, 2016 6
Vacuum Conditions We investigated QE change as a function of temperature for two vacuum conditions of the deposition chamber (pre-bake and post -bake) 1 E-09 Partial Pressure, Torr 6/10 (Pre-bake) 7/21 (Post-bake) H 2 O 1 E-10 1 E-11 1 E-12 1 3 5 7 9 11 13 15 17 19 21 23 25 27 Atomic Mass Unit 29 31 M. A. Mamun, P 3 Workshop, October 17 -19, 2016 33 35 37 39 41 43 45 7
Distinguishing QE Drop due to Contamination QE (non-baked chamber) 6 Pre bake vacuum ~1. 8 10 -8 Pa Brief heating 170 - 200 °C, H 2 O 34% QE, % 5 RT 4 Brief heating 170 - 200 °C, H 2 O 26% QE Recovery Brief heating 170 - 200 °C, H 2 O 37% QE Recovery LN 2 RT (Post Heating) 3 2 5, 33 5, 12 4, 63 4, 61 2, 25 1 1, 55 1, 29 0, 55 0 RT 1 LN 1 RT 2 -a post LN 1 LN 2 1, 52 0, 56 RT 3 post RT 4 post LN 2 heating LN 3 RT 5 post RT 6 post LN 3 heating 1, 17 LN 4 1, 60 RT 7 post RT 8 post LN 4 heating • Systematic decrease in QE at RT in repeated cooling events • Considerable increase in water partial pressure during brief heating was observed • Rejuvenation of QE by brief heating at 170 -200 C was consistently observed in the non-baked system which can be considered as a signature of water adsorption M. A. Mamun, P 3 Workshop, October 17 -19, 2016 8
Distinguishing QE Drop due to Contamination QE (baked chamber) 12 Post bake vacuum ~1. 4 10 -9 Pa Heating 170 - 200 °C, H 2 O 7. 1% 10 RT QE, % 8 LN 2 RT (Post Heating) 6 10, 31 4 10, 22 NO QE Recovery 6, 69 2 4, 04 2, 84 1, 34 0 RT 1 LN 1 RT 2 LN 2 RT 3 RT 5 post heating • NO Systematic decrease in QE at RT in repeated cooling events • NO QE Rejuvenation from brief heating at 170 -200 C in the baked system • minimal water adsorption • QE degradation due to photocathode lifetime in chamber M. A. Mamun, P 3 Workshop, October 17 -19, 2016 9
Reproducible QE in Cryocooled Photocathode QE (baked chamber) 14 RT 12 LN 2 RT (Post Heating) QE, % 10 8 12, 89 6 11, 27 9, 83 4 6, 78 6, 20 RT (1) 77 K (1) 11, 05 7, 45 6, 50 7, 29 2 0 • • RT (2) post 77 K (1) 77 K (2) RT (3) post 77 K (2) 77 K (3) RT (5) post 77 K (3) 77 K (4) RT (7) The QE at RT did not show any decrease between repeated cooling events QE at Liquid N 2 temperature (77 K)showed similar QE every time it was cooled Initial QE rise due to stoichiometric optimization We did our spectral response and temperature analysis from this phocathode. M. A. Mamun, P 3 Workshop, October 17 -19, 2016 10
A shift in spectral response when cooled 35 RT Avg 30 A shift equivalent to 35. 6 nm in wavelength LN avg (original) 25 QE, % LN Avg (shifted) 20 15 10 5 0 400 450 500 550 600 650 700 750 800 Wavelength, nm A change in emission threshold equivalent to a horizontal wavelength shift in spectral response when the photocathode is cooled to liquid N 2 temperature (77 K) M. A. Mamun, P 3 Workshop, October 17 -19, 2016 11
Analysis Approach • The experimental spectral response data were fitted according to R. H. Fowler, Phys. Rev. 38, 45 (1931): • Correlate Vo and B with temperature and obtain temperature dependent functions Vo(T) and B(T) • Evaluate Vo(2 K) and B(2 K), and use the values to simulate spectral response for photocathode at 2 K M. A. Mamun, P 3 Workshop, October 17 -19, 2016 12
-8 Dry Ice -11 -13 -12 -15 -14 Vo 1. 66 B -19. 69 -17 -19 -21 -23 -25 50 100 hν/κBT Vo 1. 70 B -19. 80 -17 -19 -21 150 -25 -16 Vo 1. 76 B -20. 09 -18 -20 -22 -23 276. 95 K Fowler Fit Liq N 2 -10 ln(QE/T 2) Applying Fowler’s Model to the Experimental Data Allows to Estimate QE at 2 K 194. 65 K Fowler Fit 50 150 hν/κBT 77 K Fowler Fit -24 -26 250 200 400 600 hν/κBT -1 Temp, K 194. 65 K (Fit) 77 K (Fit) -11 Estimation at 2 K ln(QE/T 2) Vo = Eg + EA, e. V -6 276. 95 K (Fit) 2 K (Sim) -16 -21 -26 4 E+01 4 E+02 4 E+03 hν/κBT M. A. Mamun, P 3 Workshop, October 17 -19, 2016 13
Estimated QE Spectral Response at 2 K 10 QE, % 1 0, 01 276. 95 K (Sim) 2 K (Sim) 194. 65 K (Sim) 77 K (Sim) 0, 001 1, 82 1, 92 2, 02 2, 12 2, 22 2, 32 2, 42 2, 52 2, 62 2, 72 2, 82 2, 92 hν, e. V 10 QE, % 1 λ Hν QE (%) nm e. V 440 2. 82 23. 38 20. 75 15. 16 4. 10 530 2. 34 10. 78 8. 88 4. 86 1. 14 600 2. 07 2. 76 1. 92 1. 15 0. 27 650 1. 91 1. 07 0. 68 0. 33 0. 04 670 1. 85 0. 71 0. 44 0. 10 0. 01 276. 95 K 194. 65 K 77 K 2 K (sim) 0, 1 0, 001 276. 95 K (Sim) 2 K (Sim) 194. 65 K (Sim) 77 K (Sim) 420 440 460 480 500 520 540 560 580 600 620 640 660 680 Wavelength, nm M. A. Mamun, P 3 Workshop, October 17 -19, 2016 14
Conclusions • A bialkali-antimonide photocathode (Cs and K)was grown on a niobium substrate. • QE at LN 2 temperature decreased as expected due to the change in the bandgap energy and electron affinity of the semiconductor. • We used Fowler’s model to fit the experimental data and determined the temperature dependent Vo (T) and B (T) • Fowler’s model and our experimental results extrapolate to an estimated QE for this cathode of ~1% at 2 K and 530 nm, compatible with an SRF gun versus 10. 8% QE at 277 K. M. A. Mamun, P 3 Workshop, October 17 -19, 2016 15
Thank you Q&A M. A. Mamun, P 3 Workshop, October 17 -19, 2016 16
Backup Slides M. A. Mamun, P 3 Workshop, October 17 -19, 2016 17
Pre-bake M. A. Mamun, P 3 Workshop, October 17 -19, 2016 18
Post-bake M. A. Mamun, P 3 Workshop, October 17 -19, 2016 19
Spectral Response 35 30 25 RT 3 post LN 2 QE, % 20 LN 3 post RT 3 RT 4 post LN 3 15 LN 4 post RT 4 10 RT 5 post LN 4 5 0 400 450 500 550 600 650 700 750 800 Wavelength, nm M. A. Mamun, P 3 Workshop, October 17 -19, 2016 20
After ~70 days of activation of K 2 Cs. Sb photocathode - QE did not recover with temperature reversal or rejuvenation effort by brief heating. M. A. Mamun, P 3 Workshop, October 17 -19, 2016 20
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