Defect Exchange
Radiation-induced atomic displacements in doped semiconductors can initiate highly mobile point defects that interact with stationary impurities. The watkins replacement mechanism describes the process where a mobile silicon interstitial atom replaces a substitutional impurity atom, pushing the impurity into an interstitial position. This interaction is particularly efficient for group three acceptor impurities such as boron or aluminum.
Energy Barrier
Displacement of the dopant atom occurs rapidly even at cryogenic temperatures. This indicates that the watkins replacement mechanism possesses an extremely low activation energy for migration and reaction. The silicon interstitial is highly mobile, and when it encounters a substitutional boron atom, the exchange of positions is energetically favorable.
This rapid exchange effectively converts a relatively stable substitutional dopant into a highly mobile interstitial dopant. Such newly created interstitial atoms can migrate further to form more complex defect centers even at low temperatures.
Metrological Observation
Direct verification of this atomic exchange was achieved using electron paramagnetic resonance. This spectroscopic technique allows researchers to identify the specific spin states and local environments of the newly created interstitial impurities. Measurements are performed at temperatures below four Kelvin to freeze the mobile interstitial boron atoms and prevent further reaction.
Electrical Impact
Conversion of dopants from substitutional to interstitial positions reduces the net active carrier concentration of the semiconductor. The newly created interstitial boron atoms act as donor-like defects rather than acceptors, leading to carrier compensation. This transformation is a major factor in the rapid degradation of silicon-based detectors under high radiation fluxes.
