Introduction

1. Preparation/Characterization of Atomically Flat and Clean Mo (100) Surfaces and Thermal Emittance/Response Time Measurements of Cs3Sb PhotocathodesMatt NicholsAdvisor: Luca Cultrera

2. Introduction Energy Recovery LINAC: DC gun operates on laser based photoemission of electrons Performance of ERL related to brightness of electron beam Beam brightness is limited by intrinsic properties of the photocathode in the DC gun Want 100 mA average current and 2 ps bunch length To achieve this, we need cathodes with high QE (in visible), small response time, low thermal emittance, and long lifetime

3. Introduction Cont… (a) Quantum Efficiency vs. Wavelength for the CsK2Sb photocathode • CsK2Sb cathodes grown on Si (100) substrates here at Cornell have shown QEs that are a factor of ~2 lower than expected Need QE> 5% (in visible), response time < 1 ps, and MTE of ~120 meV GaAs photocathodes work well, but have a short lifetime (hours) and need pressure of 10-11 Torr CsK2Sb photocathodes have recently shown similar success but with added effect of longer lifetime

4. Substrate-Cathode Interface • Mo is a body centered cubic crystal with a lattice parameter of 3.14 Å • The lattice parameter of CsK2Sb is 8.61 Å The substrate may actually play a more important role in photocathode properties than originally thought Mismatch of the Si and CsK2Sb crystal structures may lead to various crystal defects: line defects, plane defects, etc… These defects could lower the mean free path of excited electrons by acting as recombination centers Lower mean free path means lower QE It has been proposed that using Mo (100) (Molybdenum) substrates could improve the crystalline lattice mismatch to as low as 3%

5. Substrate-Cathode Interface Cont… 45 deg rotation of film with respect to substrate Film-substrate boundary • Matching the lattice parameters of the substrate and film could reduce the density of defects • A smaller number of defects should theoretically increase the mean free path of electrons • A larger MFP means more electrons can escape which leads to an improved QE • This project aims to test this hypothesis by preparing clean and atomically flat Mo (100) surfaces for deposition of CsK2Sb thin films

6. Project Summary The vacuum system before the rebuilding began • Assembled UHV system for surface diagnostics and Mo sample prep • Wrote software to operate LEED/AES system devices and save data • Did preliminary system tests • Prepared Mo substrates • Analyzed effect of preparation process on substrate and effect of substrate on photocathode

7. Experimental Setup/Preparation • System alreadycontained LEED/AES optics, 300 L/S ion pump, Ar leakvalve, and RGA • Thingsaddedto system: • -Heater/bellows and powersupply • -TSP and TSP controller • -Load lock system • -Coldcathode gauge on load lock • -Gatevalve • -New viewports • -Hangerassembly (thermocouple, cradle, etc…) • -Camera and mount • -Ar ion gun • -Measurementdevices (pico-ammeter, thermocouple box, ADC/DAC) • Performed 48 hourbakeout at 150 C

8. LEED/AES System Software

9. LEED/AES Software • Software usedto monitor variousparameters: hangertemperature; chamberpressure; currentthrough puck; heatercurrent, power, and voltage; partialpressures and residual gas massspectra • Thisinvolvesmeasuringanalog output signals of thermocouple box, coldcathode gauge, and pico-ammeterwithan ADC • Alsousedtooperateheaterpowersupply, RGA, and camera • RGA software editedtoallowfor total pressuremeasurements and abiltiytoturnon/off CEM at will

10. Preliminary Tests

11. Thermocouple Calibration • Weneedtomeasure the temperature of the puck/substrateduringannealing • Mustcalibrate the hangerthermocouple in ordertofind the temp of the puck • Attached a thermocoupleto a Mo test puck • Increasedheatercurrent in smallsteps and measuredtemperature of boththermocouplesafterequilibration at eachstep • Foundanaveragecalibration factor of about 2.7 • Calibration factor maybe off dueto puck notfitting in hanger/thermocouplebreaking

12. LEED Unit Cell LEED will be used as a binary test to determine whether or not the Mo (100) surface is clean and flat • LEED: Low-Energy Electron Diffraction • Useful for determining 2-D surface structure since electrons do not penetrate deeply • Diffraction condition depends on reciprocal crystal lattice and is very sensitive to condition of first ~4 monolayers of crystal

13. Si Crystal Cleaving and LEED Test • Tested LEED using a Si crystaltipsurroundedby a BeOscreen • BeOscreenusedoriginallyto line up beam, butBeObuilt up chargewhichcauseddiffractionpatternstovarywith time • Removed BeOscreen and cleaved Si crystalonmagneticarm • Tested LEED oncleavedcrystal and obtainedtie-dyepatterns • Thisindicatedthattheremaybesome residual magnetic/electricfieldproblems • This test warrantedtheneedto test LEED on a differenttype of crystal, GaAs, whichhadpreviouslyproducedsuccesful LEED patterns

14. Experimental Issues • Tested LEED on clean, well-defined crystals: freshly cleaved GaAs and Si (with and without BeO screen) • Expected typical LEED pattern (several bright intense spots) • Obtained tie-dye like pattern which changed with both time and incident electron energy • Tie-dye phenomenon may be due to presence of residual magnetic fields (YAG screen test, and hall probe test of LEED optics and puck hanger) • May also have charge building up on LEED screen insulators or view ports (a) GaAs LEED pattern (b) GaAs LEED pattern (c) Si LEED pattern (d) Si LEED pattern

15. GaAs Cleaving and LEED Test • BeforeperformingGaAs test, the LEED optics and thehangerweredegaussed, thoughstrongfields (5-15 gauss) werestillpresent, evenafterdegaussing • CleavedGaAscrystal in samemanner as for Si toprovide a clean, well-definedcrystalsurface • Thismethodhadproduced a good LEED patternonanothersystem • Obtainedtie-dyepattern once again

16. Ar Ion Sputtering • Neededto test theabilities of the ion gun • UsedanoxidizedGaAswaferwhichallowedustoclearlyseetheeffects of thesputtering • Sputtergunrequiressystembebackfilledwith Ar to 5e-05 Torr (can be time consuming) • Sputteringoccured at near normal incidence (allignedbyeye) forapproximately 20 minutes in one spot (also time consuming) • Sputteringparameterswere 20 mAemissioncurrent and 2 kVfilamentvoltage

17. Mo Sample Preparation • Prepared 2 Mo substratesfrom 99.9% pure Mo foil • Cut Mo foil (~2 mm thick) to disk shape • Handpolished disk using 30 μm diamond suspension followed by 15 μm sand paper • 4 or 5 deep scratches remained which couldn’t be removed • Moved to 400 grade silicon carbide sand paper (with water) followed by 600 grade, then 9 µm, 6 µm, 3 µm, 1 µm, 0.25 µm, and 0.1 µm diamond suspension pastes (in that order) • Between each polishing, the surface was cleaned using coconut-oil soap and isopronal • Following polishing, the two substrates were taken to AFM to analyze RMS roughness

18. Mo Surface Roughness (Pre-Annealing) (a) Surface of hand polished Mo substrate (Luca). 1 micron scale, RMS Roughness: 2.27 nm (b) Surface of hand polished Mo substrate (Luca). 1 micron scale, RMS Roughness: 2.64 nm (c) Surface of hand polished Mo substrate (Luca). 10 micron scale, RMS Roughness: 5.32 nm (d) Surface of hand polished Mo substrate (Matt). 10 micron scale, RMS Roughness: 18.95 nm (e) Surface of hand polished Mo substrate (Matt). 1 micron scale, RMS Roughness: 3.52 nm

19. Thermal Annealing and Ion Sputtering • After AFM imaging, samplesunderwent a sputtering/annealingprocess • Theflattersubstrateunderwent a 3-stage process and theotherunderwent a 2-stage process • 1 stage of theprocessconsisted of at least 20 minutes of ion sputtering in thesamelocationfollowedbyannealingfor at least 30 minutes at temperatures >870 C • Reached a peaktemperature of 950-1000 C ontheflatter puck • Foundthatcurrentsthroughtheheater of >12 A cause enoughheatto cause accumulation of meltedindiumsolderwhichholds puck in place

20. Results

21. SEM/EDX Measurements

22. SEM/EDX Measurements Cont…

23. More SEM • The EDX spectra indicate that carbon is in fact present on the surface: diamond from the suspensions may have lodged itself in the Mo surface • There are other elements which may have come from dust or polishing (soap, suspensions, etc…) • There are also clearly still large pits, holes, and scratches in the surface, even after annealing

24. AFM Measurements After Annealing (a) Surface of hand polished Mo substrate (Luca). 10 micron scale, RMS Roughness: 7.43 nm (b) Surface of hand polished Mo substrate (Luca). 10 micron scale, RMS Roughness: 7.46 nm (c) Surface of hand polished Mo substrate (Luca). 50 micron scale, RMS Roughness: 4.05 um

25. Spectral Response of CsK2Sb Photocathode with Mo (100) Substrate

26. Thermal Emittance and Response Time Measurements • Emittance measured using beam profile after solenoid and solenoid transfer matrix • Used 0.5mm, 1.0mm, and 1.5mm laser aperature sizes and 473 nm, 532 nm, and 405 nm laser wavelengths • Response time measuredusing deflector cavity in ERL injector • Measured<1ps for the Cs3Sb response time (limitedbyresolution of equipment) These measurements resemble closely those of CsK2Sb (to within error bars)

27. Future Work The improved QE obtained using the Mo substrate warrants a more detailed investigation into the preparation of the Mo substrates for the multi-alkali photocathodes and their overall effect on the cathode’s performance A bi-alkali photocathode on a Mo substrate still needs to be tested in the DC gun (need to fix the melted indium solder problem) A more detailed investigation must be undergone to fix the problems associated with LEED

28. Acknowledgements Thanks to Luca Cultrera, Ivan Bazarov, Tobey Moore, Joseph Conway, Richard Merluzzi, John Dobbins, Karl Smolenski, SiddharthKarkare, Adam Bartnik, and the whole ERL injector team Thanks also to the NSF and DOE http://www.wellesley.edu/Chemistry/Chris/LEED.html http://www.lns.cornell.edu/~ib38/research.html