Advanced Photocathode Development. Klaus A(enkofer & cathode development group ANL

Transcription

1 Advanced Photocathode Development Klaus A(enkofer & cathode development group ANL

2 Overview The Basic Principles of Photocathodes The Three Steps of Requirements on the Material What does Novel mean in Photocathode- Development The Two Level of Design: Basic Concept and Materials NEA Versus Field- Enhancement Materials for Photon- 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

3 The Basic Principles of Photocathodes Reconstruction of Surface Surface results: of surface Dipole- layer 3

4 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

5 The Basic Principles of Photocathodes Influence of External Field Increase of bias: Increasing of layer Problem: Emission of carrier? Large Area Detector Project: Tuesday 5

6 The Three Steps of Absorption: Requirements on the Material Emission Layer Transport Layer Three Step Model: layer Electron/hole and transport layer Electron emission layer Possible, if Sca(ering cross is small probability small (low carrier Electron- capturing by defects small (exciton ) Absorp@on Layer Electrode Materials quality determines design concept 6

7 What does Novel mean in Photocathode-Development The Two Level of Rational Design: Basic Concept and Materials Optimization Concept (for example electric field enhancement) Process: Microscopic Theory Macroscopic modeling of doping & carrier behavior Growth of film system Macroscopic and microscopic Proof of Concept or and reason why) 7

8 What does Novel mean in Photocathode-Development NEA Versus Field-Enhancement Will require intrinsic materials Was demonstrated with intrinsic diamond It will be to control surface states (crystal cut, surface Effect will depend on transparent electrode (n+ doping) Effect can be enhanced by geometry Dark current 8

9 What does Novel mean in Photocathode-Development Optimizing Materials for Photon-Absorption-Bandpass bandpass adjusted by ML- structure and bias field 9

10 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

11 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) AlGaAs/GaAs No NEA system known for AlGaAs The Research Program Finding best bonding or transfer technique AlGaAs/GaAs film structure and doping profile Surface doping & NEA layer Delta- doping? 11

12 Details of Materials GaN-Family Jim Buckley & Daniel Leopold (Wash University) The Challenge Largest 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

13 Details of Materials Multi-Alkali-Family Understanding of defect structure and growth Influence of surface morphology Band- bending growth under stoichometric Transparent electrode cathode of surface states Large Area Detector Project: Tuesday 13

14 What to Do Next? Materials cathode property catalog (especially surface) Macroscopic measurements (easy to determine indicators for process) In- plane (surface states) Perpendicular (bulk- defects) Temperature dependent and field emission (dopant measurement QE- measurements Microscopic measurements Surface symmetry Surface morphology (islands, size, strain, Surface adsorbants & chemisorbants Kind Amound symmetry Electronic level and density system of surface states 14

15 What to Do Next? The setup On Air/inert- atmosphere Wet cleaning system Plasma cleaning dust- free- cleaning Vacuum cleaning (up to 800C) Ion etching? Chemical etching (HCl) LEED/Auger UPS/XPS? (may be able to do extern) In- situ Cs- source O- source 15

16 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) Ernesto Indacochea (UIC) 16

17 Conclusion: Novel design of cathode will require Concept Modeling Growth & Design concepts are based on: Field enhancement of internal electric fields Proposal will require: Delivery: Growth (external resources) & theory Internal & facility Proof of principle Fundamental understanding of obstacles and 17