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 –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 2 11/3/09

The Basic Principles of Photocathodes Reconstruction of Surface 11/3/09 Large Area Detector Project: Tuesday Meeting 3 Surface results: Reconstruction of surface Dipole-layer

The Basic Principles of Photocathodes Surface States and Work Function 11/3/09 Large Area Detector Project: Tuesday Meeting 4 Structure of dipole layer / fermi-level in SC determines work function Small changes on surface -> large influence on work function

The Basic Principles of Photocathodes Influence of External Field 11/3/09 Large Area Detector Project: Tuesday Meeting 5 Increase of bias: Increasing of “depletion layer” Problem: Emission of carrier?

The Three Steps of Absorption: Requirements on the Material  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 …) 11/3/09 Large Area Detector Project: Tuesday Meeting 6 Electrode Absorption Layer Transport Layer Emission Layer Materials quality determines design concept

Iteration Process: What does “Novel” mean in Photocathode-Development The Two Level of Rational Design: Basic Concept and Materials Optimization 11/3/09 Large Area Detector Project: Tuesday Meeting 7 Concept (for example electric field enhancement) Macroscopic modeling of doping concentrations & carrier behavior Growth of film system Macroscopic and microscopic Microscopic Theory Proof of Concept (positive or negative and reason why)

What does “Novel” mean in Photocathode-Development NEA 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 transparent electrode (n+ doping)  Effect can be enhanced by geometry 11/3/09 Large Area Detector Project: Tuesday Meeting 8 Dark current

What does “Novel” mean in Photocathode-Development Optimizing Materials for Photon-Absorption-Bandpass 11/3/09 Large Area Detector Project: Tuesday Meeting 9 Absorption bandpass adjusted by ML-structure and bias field

What does “Novel” mean in Photocathode-Development How does Nanosciences Play a Role  Novel materials combinations  Reduction of strain and therefore defects  Manipulation of crystal structure  Surface morphology 11/3/09 Large Area Detector Project: Tuesday Meeting 10 J Johansson et al., Crystal Growth & Des. 9 (2009) 766

Details of Materials GaAs-Family 11/3/09 Large Area Detector Project: Tuesday Meeting 11  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? The Challenge The Research Program Xiuling Li and colleagues (UIUC)

Details of Materials GaN-Family 11/3/09 Large Area Detector Project: Tuesday Meeting 12  Largest variation in band-gap  Growth on  -Al 2 O 3 (sapphire)  GaN NEA-layer exist  GaN is UV active  Perfect combination would be Ga x In (x-1) N, but: large strain -> high defect density -> large losses  Direct growth on ALD coated  -Al 2 O 3 (sapphire) glass  InN/GaN multilayer system to adjust band-gap and minimize strain  Cascade structures?  Optimizing surface reconstruction (growth direction, temperature, coating) The Challenge The Research Program Jim Buckley & Daniel Leopold (Wash University)

Details of Materials Multi-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 11/3/09 Large Area Detector Project: Tuesday Meeting 13

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 11/3/09 Large Area Detector Project: Tuesday Meeting 14

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) 11/3/09 Large Area Detector Project: Tuesday Meeting 15 In-situ activation Cs- source O-source

What to Do Next? How to Get Materials  GaN-Family:  GaAs-Family:  Nano-Structures: Jonas Johansson (University of Lund)  Characterization: Ernesto Indacochea (UIC) 11/3/09 Large Area Detector Project: Tuesday Meeting 16 Xiuling Li and colleagues (UIUC) (student support) Jim Buckley & Daniel Leopold (Wash University)

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 11/3/09 Large Area Detector Project: Tuesday Meeting 17