A Comparative Electron-Spin-Resonance Study of the Ground State and a Photoconverted Metastable State of theMg+Donor in Silicon

Magnesium diffused into silicon forms a deep-double-donor state. Depending on the compensation, the distinct valence states ${\mathrm{Mg}}^0$, ${\mathrm{Mg}}^{+}$, or ${\mathrm{Mg}}^{++}$ are possible. EPR measurements have been performed at 55 GHz on the paramagnetic valence state ${\mathrm{Mg}}^{+}$ at liquid-helium temperatures. In thermal equilibrium, with the sample in the dark, the EPR absorption of ${\mathrm{Mg}}^{+}$ is consistent with the ${\mathrm{Mg}}^{+}$ ion occupying the tetrahedral interstitial position, with $1s\left(A_1\right)$ as the electronic ground state. This confirms previous optical-absorption work. With near-infrared light incident on the sample ($1\lesssim \lambda \lesssim 2\mu$), this paramagnetic center is converted to a metastable state ${\left({\mathrm{Mg}}^{+}\right)}^{*}$, where the ${\mathrm{Mg}}^{+}$ ion is displaced from $T_d$ to an interstitial position along the $〈111〉$ axis of the Si unit cell. Although only a small $g$ shift of ${\mathrm{Mg}}^{+}$ results from such a conversion, large changes are observed in both the contact interaction with the magnesium nucleus and the spin-lattice relaxation time. The optical conversion is total below $\sim 14$ °K, where the characteristic lifetime is of the order of minutes. In the dark, ${\left({\mathrm{Mg}}^{+}\right)}^{*}$ decays back to (${\mathrm{Mg}}^{+}:T_d$) with second-order kinetics. This optical conversion can be explained either by an ionic-motion model or by an electron transfer process involving ${\mathrm{Mg}}^{++}$ ions associated with a substitutional lattice defect.