Infrared absorption spectrum of magnesium double donors in silicon

It is well known that magnesium behaves likes an interstitial double donor in silicon and the ionization energies at liquid helium temperature are 107.50 and 256.47 meV for neutral and singly ionized magnesium donors, respectively. Recently, we have found several spectral lines unobserved before from the high-resolution infrared absorption spectra of magnesium-doped silicon measured at liquid helium temperature demonstrating the existence of other neutral and singly ionized magnesium donors in silicon. Compared with the well known donors mentioned previously, they are shallower donors with smaller ionization energies. Our study clearly indicates that there are multiple neutral and singly ionized magnesium donors in silicon. We have also observed some unusual spectra surprisingly inconsistent with typical donor spectra in silicon.