Impurity doping in SiO2: Formation energies and defect levels from first-principles calculations

Substitutional doping properties of silicon dioxide are investigated systematically with ab initio calculations. The dopants include a range of elements from group-III, group-V, and group-VII. We find that SiO2 has a relatively symmetric doping profile in terms of its ionization energies, i.e., relatively shallow acceptor levels and donor levels are both predicted, despite its wide experimental band gap of 9.65 eV. The best candidates for p-type and n-type doping are Al-Si and P-Si with calculated ionization energy of 0.86 eV and 0.74 eV, respectively, both being less than 10% of the total band gap. Larger doping asymmetry exists in terms of the impurity formation energy: under optimum (O-rich) growth conditions, the shallowest acceptor, AlSi, and donor, PSi, have formation energies of 1.75 eV and 3.05 eV, respectively. These results provide theoretical insights on how to make this previously considered absolute insulator work as a wide-gap semiconductor at elevated temperatures.