Diffusivity of the double negatively charged mono-vacancy in silicon

Abstract

Lightly-doped silicon (Si) samples of n-type conductivity have been irradiated with 2.0 MeV ions at a temperature of 30 K and characterized in situ by deep level transient spectroscopy (DLTS) measurements using an on-line setup. Migration of the Si mono-vacancy in its double negative charge state (V2−) starts to occur at temperatures above ∼70 K and is monitored via trapping of V2− by interstitial oxygen impurity atoms (), leading to the growth of the prominent vacancy-oxygen () center. The center gives rise to an acceptor level located at ∼0.17 eV below the conduction band edge (Ec) and is readily detected by DLTS measurements. Post-irradiation isothermal anneals at temperatures in the range of 70 to 90 K reveal first-order kinetics for the reaction in both Czochralski-grown and Float-zone samples subjected to low fluences of ions, i.e. the irradiation-induced V concentration is dilute (1013 cm−3). On the basis of these kinetics data and the content of , the diffusivity of V2− can be determined quantitatively and is found to exhibit an activation energy for migration of ∼0.18 eV with a pre-exponential factor of ∼ cm2 s−1. The latter value evidences a simple jump process without any entropy effects for the motion of V2−. No deep level in the bandgap to be associated with V2− is observed but the results suggest that the level is situated deeper than ∼0.19 eV below Ec, corroborating results reported previously in the literature.