Structure and electronic properties of FeSi2

The nature of the band gap in the semiconducting material beta-FeSi2 is still under some dispute. Although most experimental results indicate the band gap to be direct, nb initio work generally reports the material to be an indirect semiconductor with the direct transition a few tens of millielectron volts higher than the indirect gap. However, beta-FeSi2 is commonly grown epitaxially on a diamond-structure Si substrate, and as a consequence, the beta-FeSi2 unit cell is strained. Here we report the results of ab initio density-functional calculations, which we have performed on beta-FeSi2 where its lattice parameters are constrained according to the heteroepitaxial system beta-FeSi2(100)/Si(001). This forms two types of lattice matching: (A) beta-FeSi2[010] parallel to Si (110)and (B) beta-FeSi2[010] parallel to Si(001). We find that the beta-FeSi2 band gap is highly sensitive to its lattice parameters and therefore to the orientation at which the material is grown on silicon. We find that type A favors a more direct band gap, while type B has an indirect gap. [S0163-1829(98)05039-5].