Advanced Photocathode Development Klaus Attenkofer & cathode development group ANL

Advanced Photocathode Development

Klaus Attenkofer & cathode development group ANL

Large Area Detector Project: Tuesday Meeting

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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

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The Basic Principles of PhotocathodesReconstruction of Surface

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Surface results:

•Reconstruction of surface•Dipole-layer


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The Basic Principles of PhotocathodesSurface States and Work Function

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Structure of dipole layer / fermi-level in SC determines work function

Small changes on surface -> large influence on work function


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The Basic Principles of PhotocathodesInfluence of External Field

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Increase of bias:Increasing of “depletion layer”

Problem:Emission of carrier?


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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 …)

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Electrode

Absorption Layer

Transport LayerEmission Layer

Materials quality determines design concept


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Iteration Process:

What does “Novel” mean in Photocathode-DevelopmentThe Two Level of Rational Design: Basic Concept and Materials Optimization

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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)


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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

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Dark current


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What does “Novel” mean in Photocathode-DevelopmentOptimizing Materials for Photon-Absorption-Bandpass

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Absorption bandpass adjusted by ML-structure and bias field


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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

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J Johansson et al., Crystal Growth & Des. 9 (2009) 766


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Details of MaterialsGaAs-Family

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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)


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Details of MaterialsGaN-Family

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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)


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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

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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

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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)

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In-situ activationCs- sourceO-source


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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)

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Xiuling Li and colleagues (UIUC) (student support)

Jim Buckley & Daniel Leopold (Wash University)


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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

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Documents

Advanced Photocathode Development Klaus Attenkofer & cathode development group ANL