NEEP 541Displacements in Silicon

Fall 2002Jake 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