Lattice Defect
Fundamental damage events in crystalline materials begin with the displacement of an atom from its lattice site. A frenkel pair is a point defect consisting of a self-interstitial atom and its corresponding vacant lattice site. This defect represents the basic unit of radiation-induced structural degradation in semiconductors.
Creation Energy
Displacement of a lattice atom occurs when a collision imparts energy exceeding the threshold displacement energy of the material. In silicon, this threshold is approximately twenty to thirty electronvolts depending on the crystallographic direction. The newly formed frenkel pair exists initially in a close configuration where the interstitial and vacancy are separated by a few lattice constants.
This initial separation is a critical factor in determining whether the defect will persist or spontaneously recombine.
Recombination Rate
Thermal energy drives the recovery process by allowing mobile vacancies and interstitials to annihilate one another. The majority of newly created frenkel pairs recombine within microseconds of the initial collision. Defects that escape annihilation migrate through the crystal lattice to form stable complexes with dopants and impurities.
Device Consequence
Accumulation of stable defect complexes leads to a marked change in semiconductor electrical parameters. Charge carriers are trapped by these defects, causing a reduction in carrier lifetime and a change in majority carrier concentration. Devices such as imaging sensors and diodes show higher dark current and lower responsivity as a result of the accumulated defects.
