MBE Growth of Graded Structures for Polarized Electron Emitters

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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 Acknowledgeme
nts US Dept. of Energy SBIR contract
DE-FG02-07ER86329 (Phase I) contract
DE-FG02-07ER86330 (Phase I and II)
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Outline

• Introduction to Molecular Beam Epitaxy
• GaAsP Photocathode
• AlGaAsSb Photocathode
• AlGaAs/GaAs Internal Gradient Photocathode
• Conclusion

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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)
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Result Newly grown thin film, lattice structure
maintained
Starting surface
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Molecular Beam Epitaxy (MBE)

• Growth in high vacuum chamber
• Ultimate vacuum lt 10-10 torr
• Pressure during growth lt 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

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Molecular Beam Epitaxy
Growth Apparatus
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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
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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

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MBE System Photo
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MBE- Summary

• Ultra high vacuum, high purity layers
• No chemical byproducts created at growth surface
• High lateral uniformity (lt 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

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MBE Grown GaN Photocathodes

• Unpolarized emission
• Very efficient, robust
• Can be grown on SiC

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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
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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

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Antimony-based SLs for Polarized Electron
Emitters
X-ray

• Low QE measured for test samples (lt 0.2)
• Confinement energy too high --gt electrons
  trapped in quantum wells

Band Alignment
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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

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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

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Internal Gradient GaAs/AlGaAs SLs for Polarized
Electron Emitters
35 to 15 Aluminum grade
Non-graded control
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Internal Gradient GaAs/AlGaAs SLs for Polarized
Electron Emitters
X-ray Characterization
Simulation
Measured Data
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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

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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

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Conclusion

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