Excitation Spectra and Piezospectroscopic Effects of Magnesium Donors in Silicon

Excitation spectra of magnesium impurities diffused into undoped silicon as well as into silicon doped with group-III acceptors have been measured. In the former, magnesium is a heliumlike neutral donor (${\mathrm{Mg}}^0$) with excited states similar to those of group-V donors and close to the effective-mass positions; its ionization energy at liquid-helium temperature is 107.50±0.04 meV. In specimens containing group-III impurities, with the magnesium partially compensated, excitation spectra are observed similar to those of group-V donors and that of ${\mathrm{Mg}}^0$ except that the spacings between corresponding lines are approximately four times larger and the $1s\left(A_1\right)\to 2p_{\pm }$ transition is a closely spaced doublet, 0.2 meV apart. These features are consistent with a singly ionized heliumlike magnesium donor (${\mathrm{Mg}}^{+}$) and a small chemical splitting of the $2p_{\pm }$ state; the ionization energy is 256.47±0.07 meV at liquid-helium temperature. The excitation spectrum of ${\mathrm{Mg}}^{+}$ was also observed in specimens containing ${\mathrm{Mg}}^0$ subjected to high-energy electron irradiation. Study of the piezospectroscopic effects shows that both ${\mathrm{Mg}}^0$ and ${\mathrm{Mg}}^{+}$ occupy a $T_d$-symmetry site with $1s\left(A_1\right)$ as the ground state. A value of 8.7±0.2 eV has been deduced for the shear-deformation-potential constant ${\Xi }_u$ of the $〈100〉$ conduction-band minima of silicon.