Advanced Photocathode Development Klaus A(enkofer & cathode development group ANL
Overview The Basic Principles of Photocathodes The Three Steps of Absorp@on: Requirements on the Material What does Novel mean in Photocathode- Development The Two Level of Ra@onal Design: Basic Concept and Materials Op@miza@on NEA Versus Field- Enhancement Op@mizing Materials for Photon- Absorp@on- Bandpass How does Nanosciences Play a Role Details of Materials GaAs GaN Mul@- Alkali What to Do Next Materials cathode property catalog (especially surface) The Setup How to Get Materials 2
The Basic Principles of Photocathodes Reconstruction of Surface Surface results: Reconstruc@on of surface Dipole- layer 3
The Basic Principles of Photocathodes Surface States and Work Function Structure of dipole layer / fermi- level in SC determines work func@on Small changes on surface - > large influence on work func@on Large Area Detector Project: Tuesday Mee@ng 4
The Basic Principles of Photocathodes Influence of External Field Increase of bias: Increasing of deple@on layer Problem: Emission of carrier? Large Area Detector Project: Tuesday Mee@ng 5
The Three Steps of Absorption: Requirements on the Material Emission Layer Transport Layer Three Step Model: Absorp@on layer Electron/hole separa@on and transport layer Electron emission layer Possible, if Sca(ering cross sec@on is small Recombina@on probability small (low carrier concentra@on) Electron- capturing by defects small (exciton ) Absorp@on Layer Electrode Materials quality determines design concept 6
What does Novel mean in Photocathode-Development The Two Level of Rational Design: Basic Concept and Materials Optimization Concept (for example electric field enhancement) Itera@on Process: Microscopic Theory Macroscopic modeling of doping concentra@ons & carrier behavior Growth of film system Macroscopic and microscopic Proof of Concept (posi@ve or nega@ve and reason why) 7
What does Novel mean in Photocathode-Development NEA Versus Field-Enhancement Will require intrinsic materials Was demonstrated with intrinsic diamond It will be essen@al to control surface states (crystal cut, surface reconstruc@on,...) Effect will drama@cally depend on transparent electrode (n+ doping) Effect can be enhanced by geometry Dark current 8
What does Novel mean in Photocathode-Development Optimizing Materials for Photon-Absorption-Bandpass Absorp@on bandpass adjusted by ML- structure and bias field 9
What does Novel mean in Photocathode-Development How does Nanosciences Play a Role Novel materials combina@ons Reduc@on of strain and therefore defects Manipula@on of crystal structure Surface morphology J Johansson et al., Crystal Growth & Des. 9 (2009) 766 Large Area Detector Project: Tuesday Mee@ng 10
Details of Materials GaAs-Family Xiuling Li and colleagues (UIUC) The Challenge Largest family Growth on GaAs substrate GaAs too much red! GaAsP large strain (Similar to GaInN) Alterna@ve: AlGaAs/GaAs mul@layer No NEA system known for AlGaAs The Research Program Finding best bonding or transfer prin@ng technique Op@mizing AlGaAs/GaAs film structure and doping profile Surface doping & NEA layer Delta- doping? 11
Details of Materials GaN-Family Jim Buckley & Daniel Leopold (Wash University) The Challenge Largest varia@on in band- gap Growth on α- Al 2 O 3 (sapphire) GaN NEA- layer exist GaN is UV ac@ve Perfect combina@on would be Ga x In (x- 1) N, but: large strain - > high defect density - > large losses The Research Program Direct growth on ALD coated α- Al 2 O 3 (sapphire) glass InN/GaN mul@layer system to adjust band- gap and minimize strain Cascade structures? Op@mizing surface reconstruc@on (growth direc@on, temperature, coa@ng) 12
Details of Materials Multi-Alkali-Family Understanding of defect structure and growth condi@ons Influence of surface morphology Band- bending op@miza@on growth under stoichometric condi@ons Transparent electrode cathode op@miza@on Op@miza@on of surface states Large Area Detector Project: Tuesday Mee@ng 13
What to Do Next? Materials cathode property catalog (especially surface) Macroscopic measurements (easy to determine indicators for produc@on process) In- plane resis@vity (surface states) Perpendicular resis@vity (bulk- defects) Temperature dependent resis@vity and field emission (dopant characteriza@on) Op@cal absorp@on measurement QE- measurements Microscopic measurements Surface symmetry Surface morphology (islands, size, strain, reconstruc@on.) Surface adsorbants & chemisorbants Kind Amound symmetry Electronic level and density system of surface states 14
What to Do Next? The setup On Air/inert- atmosphere Wet cleaning system Plasma cleaning dust- free- cleaning Vacuum cleaning Hea@ng (up to 800C) Ion etching? Chemical etching (HCl) Characteriza@on Op@cal characteriza@on Resis@vity LEED/Auger UPS/XPS? (may be able to do extern) In- situ ac@va@on Cs- source O- source 15
What to Do Next? How to Get Materials GaN- Family: Jim Buckley & Daniel Leopold (Wash University) GaAs- Family: Xiuling Li and colleagues (UIUC) (student support) Nano- Structures: Jonas Johansson (University of Lund) Characteriza@on: Ernesto Indacochea (UIC) 16
Conclusion: Novel design of cathode will require itera@ve: Concept Modeling Growth & ac@va@on Characteriza@on Design concepts are based on: Field enhancement Absorp@on op@miza@on Crea@on of internal electric fields Proposal will require: Delivery: Growth facili@es (external resources) Simula@on & theory contribu@ons Internal ac@va@on & characteriza@on facility Proof of principle Fundamental understanding of obstacles and op@miza@on op@ons 17