Ab initioelectronic structure calculations for metallic intermediate band formation in photovoltaic materials

A metallic isolated band in the middle of the band gap of several III-V semiconductors has been predicted as photovoltaic materials with the possibility of providing substantially enhanced efficiencies. We have investigated the electronic band structures and lattice constants of ${\mathrm{Ga}}_n{\mathrm{As}}_m\mathrm{M}$ and ${\mathrm{Ga}}_n{\mathrm{P}}_m\mathrm{M}$ with $\mathrm{M}=\mathrm{Sc},$ Ti, V, and Cr, to identify whether this isolated band is likely to exist by means of accurate calculations. For this task, we use the SIESTA program, an ab initio periodic density-functional method, fully self consistent in the local-density approximation. Norm-conserving, nonlocal pseudopotentials and confined linear combination of atomic orbitals have been used. We have carried out a case study of ${\mathrm{Ga}}_n{\mathrm{As}}_m\mathrm{Ti}$ and ${\mathrm{Ga}}_n{\mathrm{P}}_m\mathrm{Ti}$ energy-band structure including analyses of the effect of the basis set, fine k-point mesh to ensure numerical convergence, structural parameters, and generalized gradient approximation for exchange and correlation corrections. We find the isolated intermediate band when one Ti atom replaces the position of one As (or P) atom in the crystal structure. For this kind of compound we show that the intermediate band relative position inside the band gap and width are sensitive to the dynamic relaxation of the crystal and the size of the basis set.