1. MBE Growth of Graded Structures for Polarized Electron Emitters
Aaron Moy
SVT Associates, Eden Prairie, Minnesota
in collaboration with
SLAC Polarized Photocathode Research Collaboration (PPRC):
T. Maruyama, F. Zhou and A. Brachmann
Acknowledgements:
US Dept. of Energy SBIR
contract #DE-FG02-07ER86329 (Phase I)
contract #DE-FG02-07ER86330 (Phase I and II)

2. Outline Introduction to Molecular Beam Epitaxy GaAsP Photocathode
AlGaAsSb Photocathode
AlGaAs/GaAs Internal Gradient Photocathode
Conclusion

3. Epitaxy Growth of thin film crystalline material where crystallinity
is preserved, “single crystal”
Atomic Flux
Bare (100) III-V surface,
such as GaAs
Deposition of crystal source
material (e.g. Ga, As atoms)

4. Result: Newly grown thin film, lattice structure maintained
Starting surface

5. Molecular Beam Epitaxy (MBE)
Growth in high vacuum chamber
Ultimate vacuum < torr
Pressure during growth < 10-6 torr
Elemental source material
High purity Ga, In, Al, As, P, Sb ( %)
Sources individually evaporated in high temperature cells
In situ monitoring, calibration
Probing of surface structure during growth
Real time feedback of growth rate

6. Molecular Beam Epitaxy
Growth Apparatus:

7. MBE- In Situ Surface Analysis
Reflection High Energy Electron Diffraction (RHEED)
High energy (5-10 keV) electron beam
Shallow angle of incidence
Beam reconstruction on phosphor screen
RHEED image of GaAs (100) surface

8. H-Plasma Assisted Oxide Removal
External view of
ignited H-Plasma
RHEED image of oxide
removal from GaAs Substrate
Regular oxide removal with GaAs occurs at ~ 580 °C
With H-plasma, clean surface observed at only 460 °C

9. MBE System Photo

10. MBE- Summary Ultra high vacuum, high purity layers
No chemical byproducts created at growth surface
High lateral uniformity (< 1% deviation)
Growth rates micron/hr
High control of composition and thickness
Lower growth temperatures than MOCVD
In situ monitoring and feedback
Mature production technology

11. MBE Grown GaN Photocathodes
Unpolarized emission
Very efficient, robust
Can be grown on SiC

12. MBE Grown GaAsP SL greater than 1% QE achieved 86% polarization
material specific spin depolarization mechanism
US Dept. of Energy SBIR Phase I and II
contract #DE-FG02-01ER83332

13. Antimony-based SLs for Polarized Electron Emitters
Develop structure based on AlGaAsSb/GaAs material
Sb has 3 orders lower diffusivity than Ga
Sb has higher spin orbit coupling than As

14. Antimony-based SLs for Polarized Electron Emitters
X-ray
Low QE measured for test samples (< 0.2%)
Confinement energy too high --> electrons trapped in quantum wells
Band Alignment

15. Internal Gradient SLs for Polarized Electron Emitters
Photocathode active layers with internal accelerating field
Internal field enhances electron emission for higher QE
Less transport time also reduces depolarization mechanisms
Gradient created by varied alloy composition or dopant profile

16. Internal Gradient SLs for Polarized Electron Emitters
With accelerating field
No accelerating field
Order of magnitude decrease in transport time
Increased current density
Projected increase of 5-10% in polarization

17. Internal Gradient GaAs/AlGaAs SLs for Polarized Electron Emitters
35% to 15% Aluminum grade
Non-graded control

18. Internal Gradient GaAs/AlGaAs SLs for Polarized Electron Emitters
X-ray Characterization
Simulation
Measured Data

19. Internal Gradient GaAs/AlGaAs SLs
Polarization decreased as aluminum gradient increased
Due to less low LH-HH splitting at low aluminum %
QE increased 25% due to internal gradient field
Peak polarization of 70 % at 740 nm, shorter than nm of GaAs

20. SBIR Phase II Internal Gradient SLs
Next Steps:
Further graded AlGaAs/GaAs photocathodes
Linear grading versus step grading
Doping gradient
Vary the doping level throughout the active region to generate the accelerating field
Doping gradient applied to GaAsP SL structure

21. Conclusion
Applying capabilities of MBE to polarized photocathode emitters
AlGaAsSb photocathodes
SBIR Phase II for internal gradient photocathodes
Increase current extraction
Increase polarization