Electron Paramagnetic Resonance of the Aluminum Interstitial in Silicon

Electron-paramagnetic-resonance spectra of the ${\mathrm{Al}}^{++}$ interstitial (Si-$G18\mathrm{}$) produced in aluminum-doped ($p$-type) silicon by room temperature or 4°K electron irradiations are presented and show that the ${\mathrm{Al}}^{++}$ is located in the tetrahedral interstitial site. The hyperfine interactions with the ${\mathrm{Al}}^{27}$ and surrounding ${\mathrm{Si}}^{29}$ atoms indicate that the wave function for the paramagnetic electron is localized mostly within ∼5 Å of the ${\mathrm{Al}}^{++}$ and that within this region the surrounding lattice has $T_d$ symmetry. From a qualitative description of this wave function in terms of a molecular orbital, an attempt has been made to assign the ${\mathrm{Si}}^{29}$ superhyperfine lines observed in the Si-$G18\mathrm{}$ spectrum to particular lattice sites. The ground-state energy level for this paramagnetic electron is on or below the substitutional aluminum acceptor level. From energy arguments, it appears that this electron is in a localized resonant state within the valence band or in a bound state in the band gap below the bottom of the valence band.