NEEP 541 Displacements in Silicon Fall 2002 Jake Blanchard

Outline Displacements in Silicon Carrier lifetime Carrier concentration and mobility Annealing

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

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

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

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

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 (E<5 MeV) create defects via Compton electrons