MBE Growth of Graded Structures for Polarized Electron Emitters
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Transcript of MBE Growth of Graded Structures for Polarized Electron Emitters
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