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)
• Introduction to Molecular Beam Epitaxy
• GaAsP Photocathode
• AlGaAsSb Photocathode
• AlGaAs/GaAs Internal Gradient Photocathode
• Conclusion
Outline
Epitaxy
Bare (100) III-V surface,such as GaAs
Deposition of crystal sourcematerial (e.g. Ga, As atoms)
Growth of thin film crystalline material where crystallinityis preserved, “single crystal”
Atomic Flux
Result: Newly grown thin film, lattice structure maintained
Starting surface
• Growth in high vacuum chamber• Ultimate vacuum < 10-10 torr• Pressure during growth < 10-6 torr
• Elemental source material• High purity Ga, In, Al, As, P, Sb (99.9999%)• Sources individually evaporated in high temperature cells
• In situ monitoring, calibration• Probing of surface structure during growth • Real time feedback of growth rate
Molecular Beam Epitaxy (MBE)
Molecular Beam Epitaxy
Growth Apparatus:
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
H-Plasma Assisted Oxide Removal
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
External view of ignited H-Plasma
MBE System Photo
MBE- Summary
• Ultra high vacuum, high purity layers• No chemical byproducts created at growth surface• High lateral uniformity (< 1% deviation)• Growth rates 0.1-10 micron/hr • High control of composition and thickness• Lower growth temperatures than MOCVD• In situ monitoring and feedback• Mature production technology
MBE Grown GaN Photocathodes
• Unpolarized emission• Very efficient, robust• Can be grown on SiC
US Dept. of Energy SBIR Phase I and IIcontract #DE-FG02-01ER83332
MBE Grown GaAsP SL
• greater than 1% QE • achieved 86% polarization
• material specific spin depolarization mechanism
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
Antimony-based SLs for Polarized Electron Emitters
Band Alignment
X-ray
• Low QE measured for test samples (< 0.2%)
• Confinement energy too high --> electrons trapped in quantum wells
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
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
Internal Gradient GaAs/AlGaAs SLsfor Polarized Electron Emitters
Non-graded control
35% to 15% Aluminum grade
Internal Gradient GaAs/AlGaAs SLsfor Polarized Electron Emitters
Simulation Measured Data
X-ray Characterization
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 875 nm of GaAs
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
Conclusion
• Applying capabilities of MBE to polarized photocathode emitters
• AlGaAsSb photocathodes
• SBIR Phase II for internal gradient photocathodes• Increase current extraction• Increase polarization