Physics and Technology of NEA Photocathodes

1. Physics and Technology of NEA Photocathodes Charles Sinclair Cornell University (retired) BNL PPP Workshop 10/10

2. A Little History • Until recently, NEA cathodes for accelerators were developed only for polarized electrons • The emphasis was on high polarization, high QE, and cathode operational life, not e and t • Poor cathode operational lifetime (for many years!) led to the development of load-lock systems for faster cathode replacement • Lifetime considerations also resulted in many ultra and extreme high vacuum improvements to photoemission electron guns BNL PPP Workshop 10/10

3. The basic model of an NEA cathode • Direct bandgap semiconductor • maximize optical absorption • High crystalline quality • maximize electron transport • High p-type doping • reduce thickness of depletion region • Atomically clean surface • optimize surface dipole • Surface dipole (alkali plus oxidation) • lower the vacuum level BNL PPP Workshop 10/10

4. Common Practical Realization • Substrate quality GaAs wafer, ~ 20 off 100 orientation, p-doped to ~ 1019 cm2 • Surface cleaning by chemistry, in-vacuum heating and/or atomic hydrogen exposure • Surface dipole formed by the “yo-yo” process using cesium and an oxidant, typically O2 or NF3 BNL PPP Workshop 10/10

5. Typical NF3 Activation BNL PPP Workshop 10/10

6. Typical (Good) Result • High quantum efficiency - ~10% in the near IR to ~20% at 530 nm (green) • QE dark lifetime in an actively pumped electron gun is very long (> 22,000 hours) • QE lifetime with beam is limited only by ion back bombardment to ~1 x 106 C/cm2 • en, thermal and t are strongly wavelength dependent – long l gives small en, thermal and large t, and vice-versa at shorter l BNL PPP Workshop 10/10

7. The Mysterious Surface Dipole • Originally thought of in terms of stoichiometric Cs compounds – Cs2O, CsF • Pianetta group found significant bound nitrogen in NF3 activated cathodes (Liu et al., APL 92, 241107 (2008)) • Cornell REU student demonstrated XeF2 activation, and consistently observed significant NH3 desorption from either NF3 or XeF2 activations (B. Henderson REU report, Cornell University, unpublished) BNL PPP Workshop 10/10

8. Cornell then activated with Nitrogen!! BNL PPP Workshop 10/10

9. Surface Dipole, cont’d. • Mulhollan and Bierman report improved chemical stability on GaAs cathodes activated with Cs, Li, and NF3 (JVST A26, 1195 (2008)) • Pianetta group deduced a complex structure for Cs-Li-NF3 cathodes (Sun et al., APL 95, 174109 (2009)) • We clearly do NOT fully understand the surface dipole, or how to best form it BNL PPP Workshop 10/10

10. GaAs activated with Cs, Li, and NF3 from Sun et al. paper BNL PPP Workshop 10/10

11. GaAs Cathode QE Lifetime • First, get to the point where the dark lifetime is very long (> 22,000 hours at Jlab) – measured over four months). This is basically a vacuum quality issue – i.e. no chemically active gases. • Verify that with full voltage on the gun, but no illumination, the lifetime is still very long • Verify that with no voltage but full laser power, the lifetime is still very long • Eliminate large radius electrons • At this point, the life is limited only by ion back bombardment BNL PPP Workshop 10/10

12. Evidence of Ion Back Bombardment The figures show the QE degradation resulting from illumination of three separate spots over a period of many weeks. The damage appears only along a line joining the illuminated spot and the electrostatic center of the gun. The damage is characterized (imperfectly) in terms of the Coulombs/cm2 delivered from the illuminated spot. BNL PPP Workshop 10/10

13. Jlab results up to 10 mA in 100 kV Gun Must eliminate large radius electrons to achieve these results Clearly factors other than the charge delivered per unit area are involved. Whether these are fundamental or not isn’t clear. From Grames et al., in Proceedings of the 17th International Spin Symposium, Kyoto, Japan, 2006 BNL PPP Workshop 10/10

14. Aspects of Ion Bombardment Damage • The damage may arise from implantation, crystal damage, or sputtering of the activation layer • Ions can originate from downstream of the anode aperture. Predicted by Pozdeyev, the effect has been clearly shown in measurements at JLab. • The damage is not permanent – heat cleaning and re-activation completely restores the QE • Sputtering the surface dipole comes close to explaining damage, even though the ions are hydrogen BNL PPP Workshop 10/10

15. XRF to examine Cs loss at illuminated spot • Scanned a severely ion damaged cathode with XRF – hoping to observe cesium depletion at the illuminated spot. • Found – COPPER!! • Ultimately understood the source of this • XRF is a powerful tool to examine the spatial distribution of very small quantities of elements • Light element observation requires vacuum BNL PPP Workshop 10/10

16. Constant Current QE Lifetime Where Qois the charge density lifetime, A is the illuminated area, l is the wavelength in m, and P is the laser power in W. For a Qoof 106 C/cm2 you can deliver 100 mA for 100 hours with 10 W of laser power at 527 nm on a 10% initial QE cathode, from a 1.8 mm diameter spot. BNL PPP Workshop 10/10

17. Complete Energy Distribution at 90K From Orlov et al. APL 78, 2721 (2001) BNL PPP Workshop 10/10

18. Angular Distribution Measurement From Liu et al., JVST B 23, 2758 (2005) BNL PPP Workshop 10/10

19. Distributions aren’t Emittance • Unfortunately, the measured distributions don’t translate directly into emittance • Contradictions amongst measurements: • Orlov et al. “It was shown that elastic and inelastic scattering are of crucial importance in the electrons transfer through the GaAs(Cs, O)-vacuum interface.” • Liu et al. “This result further confirms our earlier statement that the scattering from the activation layer is not significant.” BNL PPP Workshop 10/10

20. en Measured at Cornell by Three Techniques - with Excellent Agreement BNL PPP Workshop 10/10

21. Measured en from GaAs From Bazarov et al., JAP 103, 154901 (2008) Agrees well with earlier measurements by Dunham et al., PAC 1995, p. 1030 BNL PPP Workshop 10/10

22. GaAsEthermal calculated from en EX EL Eg BNL PPP Workshop 10/10

23. Temporal Response Measurements • High quality measurements require an RF deflection cavity • Three measurements with cavities: - Aleksandrov et al., Phys. Rev. E 51, 1449 (1995) Had insufficient temporal resolution • Hartmann et al., JAP 86, 2245 (1999) Studied only near the bandgap • Bazarov et al., JAP 103, 054901 (2008), and PRST-AB 11, 040702 (2008) Used two techniques, and measured versus l BNL PPP Workshop 10/10

24. Temporal Response • A solution to the diffusion equation for a delta-function optical pulse gives a fast initial component and a long tail. 57% of the pulse is contained within t/t = 1, with t = 1/a2D; a is the absorption coefficient, and D is the diffusion constant. BNL PPP Workshop 10/10

25. GaAs is prompt at 520 nm Cross correlation measurement of 520 nm laser beam RF deflection cavity measurement of electron beam BNL PPP Workshop 10/10

26. Face Dependence of GaAs QE The width of the depletion region depends on the square root of the band bending potential divided by the dopant density Both the face dependence and the optimal dopant density need to be systematically explored BNL PPP Workshop 10/10

27. Graded Bandgap Cathode to Improve t Jones et al. (Spin 2009, p. 1057) reported on a graded bandgap transmission mode cathode (on sapphire) to improve the temporal response with good QE and e. The graded region is Ga(1-x)AlxAs with 0≤x≤0.1. This creates an electric field of ~ 4 x 106 V/m, accelerating electrons into the GaAs emitting layer. They estimate a < 10 ps response time with QE ~ 20%. BNL PPP Workshop 10/10

28. Few Materials Explored to Date • Almost all NEA cathodes used on accelerator guns are GaAs. Some work has been reported on GaxAl(1-x)As and GaAsxP(1-x). • Selecting the optimal bandgap, or grading the bandgap, using alloys, may offer an avenue for optimizing the tradeoff between e and t. • InP looks interesting – its L and X minima are much higher in the conduction band w.r.t. Gthan in the case of GaAs, and it has 50 to 60% higher absorption coefficient. BNL PPP Workshop 10/10

29. Additional Issues • At high average current, the optical energy deposited in the cathode becomes important. Green illumination of GaAs deposits ~ 28% of the laser energy in the crystal as heat, which must be removed. • Heat cleaning at high temperature is known to cause 110 faceting on 100 and 111 faces of GaAs – which will impact the emittance. • Thermionic dark current at 25 C is tiny and is dramatically reduced on modest cooling BNL PPP Workshop 10/10

30. Summary • Detailed understanding of the surface dipole and its formation is lacking, but the Cs-NF3 yo-yo process gives consistent good results • The QE can be well parameterized within the three step model (a, L, P) • Thermal emittance from 100 GaAs is well characterized, and at room temperature near the bandgap is close to kTat room temperature (~ 25 meV) BNL PPP Workshop 10/10

31. Summary, cont’d • Emittanceshould be measured at a significantly lower temperature – e.g. 77 K • The effect of the surface dipole on the emittance is unclear (but likely not large?) • Faceting is known to occur under intense heat cleaning, and may impact emittance • The temporal response appears to be well understood in terms of a and D BNL PPP Workshop 10/10

32. Summary, cont’d. • Exploring different alloys, or possibly InP, to give an optimal tradeoff between e and t is likely worthwhile • The optimal dopant density and emission face should be explored • In the distant future – don’t forget that emission from GaAs has been done from a pn junction, and by making the GaAs on either a LED or a laser. BNL PPP Workshop 10/10