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Advanced Photocathode Development

This overview discusses the fundamentals of photocathode technology, detailing the crucial steps of absorption and the importance of material properties in optimizing performance. It explores novel advances in photocathode development, including the role of nanosciences, material combination strategies, and approaches to surface state control. Key materials like GaAs, GaN, and multi-alkali systems are thoroughly reviewed, alongside their challenges and growth techniques. The content highlights rational design concepts and the significance of maximizing photon absorption efficiency in large-area detector applications.

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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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