Ultra-Thin Photocathodes Collaboration 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 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 Electric Field + - 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 StructureZincblende 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 Guiding Electric 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

Potential Fabrication 1.Grow thin layer of sacrificial AlGaAs 2.Growth of photocathode, with layers inverted 3.Deposition of intermediate bonding layers (i.e. SiO 2, S ix N x ) 4.Wafer bonded to a glass substrate predeposited bonding layers 5.Bulk of substrate is etched/CMP away 6.Sacrificial layer removed 7.Photocathode ready for activation Glass substrate

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!