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Advanced Photocathode Development

Advanced Photocathode Development. Klaus Attenkofer & cathode development group ANL. Overview. The Basic Principles of Photocathodes The Three Steps of Absorption: Requirements on the Material What does “Novel” mean in Photocathode- Development

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Advanced Photocathode Development

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  1. Advanced Photocathode Development Klaus Attenkofer & cathode development group ANL

  2. Overview • The Basic Principles of Photocathodes • The Three Steps of Absorption: Requirements on the Material • What does “Novel” mean in Photocathode-Development • The Two Level of Rational Design: Basic Concept and Materials Optimization • NEA Versus Field-Enhancement • Optimizing Materials for Photon-Absorption-Bandpass • How does Nanosciences Play a Role • Details of Materials • GaAs • GaN • Multi-Alkali • What to Do Next • Materials – cathode property catalog (especially surface) • The Setup • How to Get Materials Large Area Detector Project: Tuesday Meeting

  3. The Basic Principles of PhotocathodesReconstruction of Surface Surface results: • Reconstruction of surface • Dipole-layer Large Area Detector Project: Tuesday Meeting

  4. The Basic Principles of PhotocathodesSurface States and Work Function Structure of dipole layer / fermi-level in SC determines work function Small changes on surface -> large influence on work function Large Area Detector Project: Tuesday Meeting

  5. The Basic Principles of PhotocathodesInfluence of External Field Increase of bias: Increasing of “depletion layer” Problem: Emission of carrier? Large Area Detector Project: Tuesday Meeting

  6. The Three Steps of Absorption:Requirements on the Material Emission Layer • Three Step Model: • Absorption layer • Electron/hole separation and transport layer • Electron emission layer • Possible, if • Scattering cross section is small • Recombination probability small (low carrier concentration) • Electron-capturing by defects small (exciton …) Transport Layer Absorption Layer Electrode Materials quality determines design concept Large Area Detector Project: Tuesday Meeting

  7. What does “Novel” mean in Photocathode-DevelopmentThe Two Level of Rational Design:Basic Concept and Materials Optimization Concept (for example electric field enhancement) Iteration Process: Macroscopic modeling of doping concentrations & carrier behavior Microscopic Theory Growth of film system Macroscopic and microscopic Proof of Concept (positive or negative and reason why) Large Area Detector Project: Tuesday Meeting

  8. What does “Novel” mean in Photocathode-DevelopmentNEA Versus Field-Enhancement • Will require intrinsic materials • Was demonstrated with intrinsic diamond • It will be essential to control surface states(crystal cut, surface reconstruction,....) • Effect will dramatically depend on transparentelectrode (n+ doping) • Effect can be enhanced by geometry Dark current Large Area Detector Project: Tuesday Meeting

  9. What does “Novel” mean in Photocathode-DevelopmentOptimizing Materials for Photon-Absorption-Bandpass Absorption bandpass adjusted by ML-structure and bias field Large Area Detector Project: Tuesday Meeting

  10. What does “Novel” mean in Photocathode-DevelopmentHow does Nanosciences Play a Role • Novel materials combinations • Reduction of strain and therefore defects • Manipulation of crystal structure • Surface morphology J Johansson et al., Crystal Growth & Des. 9 (2009) 766 Large Area Detector Project: Tuesday Meeting

  11. Details of MaterialsGaAs-Family The Challenge • Largest family • Growth on GaAs substrate • GaAs too much red! • GaAsP large strain (Similar to GaInN) • Alternative: AlGaAs/GaAs multilayer • No NEA system known for AlGaAs • Finding best bonding or transfer printing technique • Optimizing AlGaAs/GaAs film structure and doping profile • Surface doping & NEA layer • Delta-doping? Xiuling Li and colleagues (UIUC) The Research Program Large Area Detector Project: Tuesday Meeting

  12. Details of MaterialsGaN-Family Jim Buckley & Daniel Leopold (Wash University) The Challenge • Largest variation in band-gap • Growth on a-Al2O3 (sapphire) • GaN NEA-layer exist • GaN is UV active • Perfect combination would be GaxIn(x-1)N, but:large strain -> high defect density -> large losses • Direct growth on ALD coated a-Al2O3 (sapphire) glass • InN/GaN multilayer system to adjust band-gap and minimize strain • Cascade structures? • Optimizing surface reconstruction (growth direction, temperature, coating) The Research Program Large Area Detector Project: Tuesday Meeting

  13. Details of MaterialsMulti-Alkali-Family • Understanding of defect structure and growth conditions • Influence of surface morphology • Band-bending optimization • “growth under stoichometric conditions” • Transparent electrode – cathode optimization • Optimization of surface states Large Area Detector Project: Tuesday Meeting

  14. What to Do Next?Materials – cathode property catalog (especially surface) • Macroscopic measurements (easy to determine indicators for production process) • In-plane resistivity (surface states) • Perpendicular resistivity (bulk-defects) • Temperature dependent resistivity and field emission (dopant characterization) • Optical absorption measurement • QE-measurements • Microscopic measurements • Surface symmetry • Surface morphology (islands, size, strain, reconstruction….) • Surface adsorbants & chemisorbants • Kind • Amound • symmetry • Electronic level and density system of surface states Large Area Detector Project: Tuesday Meeting

  15. What to Do Next?The setup • On Air/inert-atmosphere • Wet cleaning system • Plasma cleaning • “dust-free-cleaning” • Vacuum cleaning • Heating (up to 800C) • Ion etching? • Chemical etching (HCl) • Characterization • Optical characterization • Resistivity • LEED/Auger • UPS/XPS? (may be able to do extern) • In-situ activation • Cs- source • O-source Large Area Detector Project: Tuesday Meeting

  16. What to Do Next?How to Get Materials • GaN-Family: • GaAs-Family: • Nano-Structures: Jonas Johansson (University of Lund) • Characterization: Ernesto Indacochea (UIC) Jim Buckley & Daniel Leopold (Wash University) Xiuling Li and colleagues (UIUC) (student support) Large Area Detector Project: Tuesday Meeting

  17. Conclusion: • Novel design of cathode will require iterative: • Concept • Modeling • Growth & activation • Characterization • Design concepts are based on: • Field enhancement • Absorption optimization • Creation of internal electric fields • Proposal will require: • Growth facilities (external resources) • Simulation & theory contributions • Internal activation & characterization facility • Delivery: • Proof of principle • Fundamental understanding of obstacles and optimization options Large Area Detector Project: Tuesday Meeting

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