Ultra-Thin PhotocathodesCollaboration Meeting 12/9/11

Overview• Introduction: GaAs Review• Ultra-Thin Photocathode

– Momentum and Mass– Wavelength Tuning– Field Guiding– Electron Discrimination

• In Literature• Conclusion

Why GaAs?

873 nm

413.2 nm

248nm

Reflection vs Transmission : Thick vs Thin

[3],Graph Courtesy of Zeke Insepov(ANL)

Timeline• Explore physics and design for III-V

photocathode• Evaluate GaAs material quality• Create fabrication process for device• Evaluate bonding material quality• Confirm Ultra-Thin PC Properties

Experimentally• Test photocathode in reflection mode• Test photocathode in transmission mode

Ultra-Thin Photocathode

• Semiconductor ultra-thin transmission photocathodes are much improved over thick photocathodes– Broad Spectral Range– High Quantum Efficiency– Improved Short-Wavelength

Response– Application-Specific Tunability

• Thickness of photocathode is relative to application intended

DIffusion Guided

Field Guided

Near NEA Surface

Reflection vs Transmission : Thick vs Thin

GaAs

ElectricField

+-

Vacuum

}

Momentum and Mass

• Γ-valley photoelectrons for direct materials have vanishing momentum and low effective mass (low curvature)

• High energy transitions (X or L – valley) have built in momentum and high effective mass that can be potentially used for photoelectron guiding

• Change in mass from crystal to vacuum can effect emission angle (momentum conservation

• Surface orientation can be chosen for high or low emittance, depending on available directions residing on surface

GaAs Band Structure Zincblende Brillouin Zone

Wavelength Tuning

• Compound (lesser extent elemental) semiconductor bandstructures scale (non-linearly) with varying alloy composition

• Band structure can be tailored to target wavelength utilizing application specific binary, ternary and quaternary alloys

• “Indirect” materials can find use for high energy applications

Field GuidingElectric Field

• In addition to, or with low momentum photoelectrons, built-in fields can be create to guide electrons to the surface

• Graded doping or alloy profiles can be used for guided fields, as well as backside field for “wrong-way” electrons

• Enough granularity in doping spread to create a built in field and induce band bending at the surface for emission

Electron Discrimination• NEA surfaces allow electrons

(dark and light) to escape easily, lowering SNR

• For thick transmission mode cathodes, electrons usually thermalize (depending on scattering) before reaching the surface

• High energy electrons, from non-Γ valleys, can be extracted before they thermalize from thin cathodes

• Less band bending (thus higher surface potential) is needed and thermalized (dark) electrons can be filtered out

Band Bending vs Cs Coverage

Hot vs Thermalized Electrons

Ultra-Thin in Literature: PETE

• Photon-Enhanced Thermionic Emission (PETE) concept utilizes an ultrathin photocathode for solar energy generation

• Combines thermal and photo electrons for energy harvesting• Theoretically higher efficiency than single junction SC

J. W. Schwede, I. Bargatin, D. C. Riley, B. E. Hardin, S. J. Rosenthal, Y. Sun, F. Schmitt, P. Pianetta, R. T. Howe, Z.-X. Shen, and N. A. Melosh, "Photon-enhanced thermionic emission for solar concentrator systems," Nat Mater, 2010.

• In normal SC, waste heat and losses during carrier transport degrade efficiency

• Improved efficiency resultant from high quantum efficiency during transport and use of thermal energy to overcome potential at surface

Conclusion• Ultra-Thin PC have increased advantages

over thick PC for transmission mode operation

• Ultra-Thin PC allows for electron control through momentum/mass and/or field guiding

• Application tolerance for either high QE or low emittance for high SNR

Questions?

Thank you for your time!