NEEP 541 – Defects

Fall 2003Jake Blanchard

Outline Irradiation Induced Defects

Definitions Particles Cascades Depleted zones Thermal Spikes

Definitions PKA=atom knocked from its lattice

site by bombarding particle Formation of PKA produces 1

vacancy and one interstitial (Frenkel pair)

Minimal energy to create PKA is displacement energy - Ed

Displacement energy is 20-50 eV (depends on crystal direction)

Bombarding particles To create PKA, neutrons need > 1

keV, electrons > 1 MeV, ions > 100 eV Energy transfer leads to local

temperature rise Some of energy lost to inelastic

collisions and to electrons Look at separately at processes

associated with each of these particles

Cascades Collection of Frenkel pairs from

single PKA with E>10s of eV is called a collision cascade

Time frame is <0.1 ps When PKA energy is sufficiently

low, the distance between collisions drops and becomes comparable to interatomic spacing

Depleted Zones When PKA energy is low, the

deposited energy is concentrated over small volume (10,000 atoms or so)

Irregular vacancy clusters are formed, surrounded by interstitials

This is a “depleted zone” Two models exist: displacement

spike and thermal spike

Cascade

Depleted Zone

Displacement Spike

Displacement Spike During the cascade dynamic,

collective atom motion occurs Relocation is random Focusing and channeling are key

elements (transfer over large distances)

Atoms transferred ~ 20 nm, so recombination is unlikely

Thermal Spike Energy transfer below displacement

energy heats lattice Heat pulse is “thermal spike” Local temperature can exceed

Tmelt Heat is gradually dissipated through

electron collisions as atoms vibrate

Thermal Spike Evolution A spike follows a cascade It is a local region where atoms are in

“violent motion” Intensity of motion decreases as spike

cools (10 ps or so) Spike contains central region where

cascade effects are converted to heat Interstitials are injected into

surrounding lattice by focusing, channeling, etc.

Thermal Spike Evolution (cont.) In core of spike, “liquid drop

forms” then solidifies Process is rapid solidification of

undercooled liquid core Void is created in center Hence, we get a depleted zone

Cascade Development Ballistic effects occur over ~10 ps

(primary damage state) Other processes are diffusional and

thus slower (~10 ns) Latter changes microstructure and

leads to property changes