1. Preparation of Clean III-V Semiconductor Surfaces for NEA Photocathodes Yun Sun 1, 2, Zhi Liu 3, Francisco Machuca 3, Piero Pianetta 1 and William E. Spicer 1 1 Stanford Synchrotron Radiation Lab, Stanford, CA 2 Department of Chemistry, Stanford University, CA 3 Center for Integrated Systems, Stanford University, CA Work is funded by Intevac, and is carried out at Stanford Synchrotron Radiation Lab (Department of Energy, Office of Basic Energy Sciences)

2. Importance of InP Cleaning InP(100) based Negative Electron Affinity (NEA) photocathode –Surface cleanness critical for the performance –Chemical cleaning of InP(100) not well understood Other applications –Critical for MBE and CVD growth, etc.

3. Analysis Technique: Photoelectron Spectroscopy using Synchrotron Radiation A systematic study by SR-PES –hv: 60 - 600eV, P2p, In4d, F1s, VB –High resolution --> resolve chemical shift –Short escape depth(~5Å) --> high surface sensitivity A controlled etching environment –Ar purged glove bag around load lock

4. 1.2. 3. 4. 5. 6. 7. 1.Argon/Nitrogen purged glove box 2.Connected Loadlock 3.XYZ-theta Stage with integrated heater in UHV chamber 4.Beam line (Radiation) 5.Hemispherical Energy Analyzer, Energy Resolution, 0.2eV. 6.Leak Valve for Gases 7.Alkali Metal doser 4.

5. Typical One Step Cleaning Methods Hydrogen Peroxide based solutions*: –H 2 SO 4 : H 2 O 2 : H 2 O(4:1:100) –H 2 SO 4 : H 2 O 2 : H 2 O(4:1:1) –NH 3 : H 2 O 2 : H 2 O(10:2:100) –etc. Other: Br-CH 3 OH, HF, etc. *H 2 SO 4 : 98%, H 2 O 2 30%, NH 3 30%

6. One Step Chemical Cleaning CHEMICAL ETCHING VACUUM ANNEAL INERT(Ar) ATMOSPHEREUHV Not effective for InP(100) ! H 2 SO 4 : H 2 O 2 : H 2 O (4:1:100)

7. One Step Chemical Cleaning H 2 SO 4 : H 2 O 2 : H 2 O(4:1:100) K. E. / eV Intensity Oxide ~0.5ML K. E. / eV Oxide Intensity Etched Etch + 360 o C Etched Etch + 360 o C P2p hv = 165eV In4d hv = 70eV

8. One Step Chemical Cleaning In oxide Kinetic Energy / eV In4d for bulk InP Intensity In4d hv = 70eV In in InP

9. One Step Chemical Cleaning Not Enough Surface is left with oxide, which can not be removed completely by vacuum heating Add another step to remove the oxide –36% HCl : H 2 O (1:3) –HF (1%)

10. Two Step Chemical Cleaning CHEMICAL ETCHING VACUUM ANNEAL OXIDE REMOVAL INERT(Ar) ATMOSPHERE UHV H 2 SO 4 :H 2 O 2 :H 2 O 4:1:100 HCl(36%): H 2 O 1:3 HF 1%

11. HCl Solution as Second Step Surface is Hydrophobic

12. HCl Clean oxide gone r.t. 360 o C P in InP ~0.42ML

13. HCl Clean r.t. 360 o C

14. One Step Two Step +330 o C K.E. / eV One Step Two Step +330 o C < 0.1ML C1s O1s C1s And O1s Spectra at different stages Intensity hv = 340eV hv = 600eV

15. HF Solution as Second Step Surface is Hydrophilic

16. HF Clean

17. In4d Fit after HF Clean r.t. 120 o C. 180 o C.230 o C. In in InP

18. HF Clean

19. In-F and F Coverage

20. HCl Clean v.s. HF Clean

21. GaAs(100) Cleaning recipe H 2 SO 4 :H 2 O 2 :H 2 O * 4:1:100 + Vacuum annealing at 500C *H 2 SO 4 98%; H 2 O 2 30%; NH 3 30%

22. After chemical clean Peak found on sample after chemical clean

23. After thermal annealing Peak found on sample after annealing

24. C 1s and O 1s Intensity [Arbitrary Units] 7570656055 Kinetic Energy O 1s Sample as received After chemical Clean After annealing Kinetic Energy Intensity [Arbitrary Units] 6058565452504846 C 1s After annealing After chemical clean Sample as received

25. Result of chemical cleaning

26.  Chemical Cleaning (Ex situ) Optimize the sulfuric acid cleaning used for other III-V’s for GaN --> 4:1 H 2 SO 4 :H 2 O 2 (~90 o C)  Annealing Ambient (In situ) Testing the use of ammonia back pressure in comparison to vacuum annealing --> Anneal in vacuum better  Annealing Temperature (In situ) Find an effective temperature --> 700 o C

27. 9.

28. 10.

29. H 2 SO 4 /H 2 O 2 H C H C H H H C GaN O C O C O O O C Anneal GaN Carbons 1s spectroscopy shows a conversion of hydrocarbons to more volatile oxides of carbon. Carbon contamination reduced to 1% and Oxygen 8% of a monolayer. 12.

30. I.A sulfuric acid/hydrogen peroxide treatment followed by a vacuum anneal at 700C reduces carbon and oxygen concentrations to a few percent of a monolayer. II.An ammonia annealing ambient is worse than a vacuum ambient for the thermal desorption of carbon and oxygen at temperatures at or below 740C. III.The chemical state is predominantly a volatile oxide of carbon. IV. We have a reproducible and clean GaN surface for our photocathode research achieved by a non-destructive sulfuric acid cleaning technique. 13.

31. More on GaN Quantum yield 20-30 % by Cs 40-50 % by Cs+O Works in Transmission Mode with Sapphire/AlN/GaN Looking for collaboration: fmachuca@stanford.edu Electron Gun Test Column Current density and brightness measurement Energy spread measurement

32. VB after HCl Cleanof InP(100)

33. Valence Band after Two Step cleaning and Annealing for InP(100)

34. Surface Etching(2) H2O2H2O2 InPO O OH O 3/2 InP H+H+ In 3+ PO OH SO 4 2- H+H+ H+H+ H+H+ H2O2H2O2 Grow oxide: InP + H 2 O 2 + H +  In(HPO 4 ) 3/2  Etch oxide: In(HPO 4 ) 3/2 + H +  In 3+ + H 3 PO 4 InPO O OH O 3/2

35. Oxide Removal InPO O OH O 3/2 InP H+H+ In 3+ PO OH SO 4 2- H+H+ Oxide Removal: In(HPO 4 ) 3/2 + H +  In 3+ + H 3 PO 4 H+H+ H+H+ InP P Elemental P Generation: H 3 PO 4 + InP + H +  In 3+ + P  In 3+

36. Hydrophilic Surface after HF Clean In In In In In In In F F F F F F F H H O O O O H H Hydrogen Bonds InP

37. GaAs(100) Valence Band