Ionization-Enhanced Migration of Defects Produced in Silicon at Cryogenic Temperatures.

This dissertation describes the study of ionization -enhanced migration of lattice defects in silicon. The enhanced migration process has been studied for two simple lattice defects in silicon, viz., the isolated lattice vacancy and the aluminum interstitial. The defects were produced in silicon by in situ electron irradiation (1.5 -2.5 MeV) at cryogenic temperatures (4.2 K or 20.4 K). The experimental technique used in these studies was Electron Paramagnetic Resonance (EPR). Ionization-enhanced migration of the isolated lattice vacancy in silicon was observed during photoexcitation with near band-gap light (h(nu) = 1.165 eV) and also during in situ electron irradiation, at temperatures ranging between 4.2 K and 20.4 K. The enhanced vacancy migration process is essentially athermal with the vacancy making one diffusional jump for every (TURN)40 photons incident on the sample during 1.165 eV photoexcitation. Direct excitation of the vacancy is the dominant process during optical illumination. It is about twenty times more effective in causing migration enhancement than the recombination of electron-hole pairs generated by in situ electron irradiation. The athermal vacancy migration indicates that the electronic excitation supplies enough energy to overcome the migrational barriers of the vacancy (ranging between 0.18 eV and 0.45 eV for different charge states). The EPR studies of ionization-enhanced migration of the aluminum interstitial in silicon were used to complement the more detailed Deep Level Transient capacitance Spectroscopy (DLTS) studies performed by J. R. Troxell. The EPR observations unambiguously demonstrate enhanced migration of the aluminum interstitial during photoexcitation with 1.165 eV light at 320 K. The aluminum interstitial is normally stable up to (TURN)200(DEGREES)C during thermal anneal. The disappearance of the E(,v) + 0.17 eV electrical level under minority carrier injection at (TURN)300 K is correlated to the enhanced migration of interstitial aluminum by comparing the EPR and DLTS observations. The enhanced migration process is quite efficient with the aluminum interstitial making approximately one diffusional attempt for every electron -hole pair recombination. Finally, the EPR studies also indicate that the isolated lattice vacancy and the aluminum interstitial are both positively charged (V('+) or V('++) for the vacancy and Al(,i)('+) for the interstitial) during enhanced migration under photoexcitation.