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