NEEP 541 Displacements in Silicon

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NEEP 541Displacements in Silicon

• Fall 2002
• Jake Blanchard

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Outline

• Displacements in Silicon
• Carrier lifetime
• Carrier concentration and mobility
• Annealing

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Displacement Effect

• In addition to ionization, displacements can
  affect the performance of semiconductor materials
• Displacements tend to affect the carrier
  lifetime, the carrier concentration, and the
  carrier mobility

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Carrier Lifetime

• Defined as the average time a carrier survives
  before recombination
• Determined by carrier concentrations, defect
  concentrations, and carrier mobility
• Irradiation reduces lifetime because it
  introduces defects which act as recombination
  sites
• Clusters do this more effectively (on a
  per-defect basis) than point defects

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Carrier Concentration and Mobility

• Equilibrium conductivity depends on carrier
  concentration and mobility
• Displacements decrease equilibrium concentration
  by enhancing recombination at defects
• Defects also decrease the carrier mobility,
  largely by the production of ionized impurities
  leading to enhanced scattering
• Again, clustering enhances these effects

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Annealing

• Defects can be removed by heating the
  semiconductor
• Defects recombine and crystal moves towards its
  unirradiated state
• Time scales are microseconds to hours, depending
  on temperature
• We must reach about 650 K to get significant
  defect removal

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Notes

• Threshold energy for electron irradiation is
  about 150 keV
• Defect production rates increase rapidly above
  about 300 keV
• Displacement energy for Si is about 12.9 eV
• Properties are affected by interstitial O in a Si
  vacancy
• There is no known effect from Si interstitials
• Photons (Elt5 MeV) create defects via Compton
  electrons