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  

Large  Area  Detector  Project:  Tuesday  Mee@ng  

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The Basic Principles of Photocathodes Reconstruction of Surface

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Surface  results:  

• Reconstruc@on  of  surface  • Dipole-­‐layer  

The Basic Principles of Photocathodes Surface States and Work Function

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Structure  of  dipole  layer  /  fermi-­‐level  in  SC  determines  work  func@on  

Small  changes  on  surface  -­‐>  large  influence  on  work  func@on  

The Basic Principles of Photocathodes Influence of External Field

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Increase  of  bias:  Increasing  of  “deple@on  layer”  

Problem:  Emission  of  carrier?  

The Three Steps of Absorption: Requirements on the Material

  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  …)    

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Electrode  

Absorp@on  Layer  

Transport  Layer  Emission  Layer  

Materials  quality  determines  design  concept  

Itera@on  Process:  

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

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Concept    (for  example  electric  field  enhancement)  

Macroscopic  modeling  of  doping  concentra@ons  &  carrier  behavior  

Growth  of  film  system  

Macroscopic  and  microscopic    

Microscopic  Theory  

Proof  of  Concept    (posi@ve  or  nega@ve  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  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  

11/3/09  

Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Dark  current  

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

11/3/09  

Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Absorp@on  bandpass  adjusted    by  ML-­‐structure  and  bias  field  

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  

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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

Details of Materials GaAs-Family

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  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  

  Finding  best  bonding  or  transfer  prin@ng  technique  

  Op@mizing  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

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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  Largest  varia@on  in  band-­‐gap  

  Growth  on  α-­‐Al2O3  (sapphire)  

  GaN  NEA-­‐layer  exist      GaN    is  UV  ac@ve  

  Perfect  combina@on  would  be  GaxIn(x-­‐1)N,  but:  large  strain  -­‐>  high  defect  density  -­‐>  large  losses  

  Direct  growth  on  ALD  coated  α-­‐Al2O3  (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)  

The  Challenge  

The  Research  Program  

Jim  Buckley  &  Daniel  Leopold  (Wash  University)    

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  

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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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  

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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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)  

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Large  Area  Detector  Project:  Tuesday  Mee@ng  

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In-­‐situ  ac@va@on  Cs-­‐  source  O-­‐source  

What to Do Next? How to Get Materials

  GaN-­‐Family:  

  GaAs-­‐Family:  

  Nano-­‐Structures:        Jonas  Johansson    (University  of  Lund)  

  Characteriza@on:          Ernesto  Indacochea  (UIC)  

11/3/09  

Large  Area  Detector  Project:  Tuesday  Mee@ng  

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Xiuling  Li    and  colleagues  (UIUC)  (student  support)  

Jim  Buckley  &  Daniel  Leopold  (Wash  University)    

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:  •  Growth  facili@es  (external  resources)  •  Simula@on  &  theory  contribu@ons  •  Internal  ac@va@on  &  characteriza@on  facility  

  Delivery:  •  Proof  of  principle  •  Fundamental  understanding  of  obstacles  and  op@miza@on  op@ons    

11/3/09  

Large  Area  Detector  Project:  Tuesday  Mee@ng  

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