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Advanced Photocathode Development
Klaus Attenkofer & cathode development group ANL
Large Area Detector Project: Tuesday Meeting
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
11/3/09
Large Area Detector Project: Tuesday Meeting
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The Basic Principles of PhotocathodesReconstruction of Surface
11/3/09
Surface results:
•Reconstruction of surface•Dipole-layer
Large Area Detector Project: Tuesday Meeting
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The Basic Principles of PhotocathodesSurface States and Work Function
11/3/09
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
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The Basic Principles of PhotocathodesInfluence of External Field
11/3/09
Increase of bias:Increasing of “depletion layer”
Problem:Emission of carrier?
Large Area Detector Project: Tuesday Meeting
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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
Electrode
Absorption Layer
Transport LayerEmission Layer
Materials quality determines design concept
Large Area Detector Project: Tuesday Meeting
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Iteration Process:
What does “Novel” mean in Photocathode-DevelopmentThe Two Level of Rational Design: Basic Concept and Materials Optimization
11/3/09
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)
Large Area Detector Project: Tuesday Meeting
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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 transparent
electrode (n+ doping) Effect can be enhanced by geometry
11/3/09
Dark current
Large Area Detector Project: Tuesday Meeting
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What does “Novel” mean in Photocathode-DevelopmentOptimizing Materials for Photon-Absorption-Bandpass
11/3/09
Absorption bandpass adjusted by ML-structure and bias field
Large Area Detector Project: Tuesday Meeting
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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
11/3/09
J Johansson et al., Crystal Growth & Des. 9 (2009) 766
Large Area Detector Project: Tuesday Meeting
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Details of MaterialsGaAs-Family
11/3/09
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)
Large Area Detector Project: Tuesday Meeting
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Details of MaterialsGaN-Family
11/3/09
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 Challenge
The Research Program
Jim Buckley & Daniel Leopold (Wash University)
Large Area Detector Project: Tuesday Meeting
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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
11/3/09
Large Area Detector Project: Tuesday Meeting
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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
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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
In-situ activationCs- sourceO-source
Large Area Detector Project: Tuesday Meeting
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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
Xiuling Li and colleagues (UIUC) (student support)
Jim Buckley & Daniel Leopold (Wash University)
Large Area Detector Project: Tuesday Meeting
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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