Interface phase transition as observed in ultra thin FeSi2 epilayers

Because its optical gap is close to 0.85 eV, the semiconducting orthorhombic disilicide β-FeSi2 has been extensively studied since it might find applications in integrated optoelectronic devices. During the course of these investigations several unexpected pseudomorphic phases of ultra thin FeSi2 layers have been observed on top of the Si substrates. The growth of these phases and their physicochemical properties have been investigated for a large variety of preparation methods and surface techniques. In this paper, particular attention will be paid to the mechanisms of stabilization of such metastable phases due to their epitaxy on Si substrates, and to their transition, both electronic and structural, towards the β-FeSi2 phase. Recent findings concerning the growth at low temperature of the metallic α-FeSi2 phase will also be discussed since they might shed light on the hierarchy of these phase transitions and offer a useful way to achieve, by a subsequent post-annealing, high quality semiconducting β-FeSi2 grains.