Abstract
The structural and optical properties of Ion Beam Synthesised (IBS) semiconducting FeSi2 (beta-FeSi2) have been studied by transmission electron microscopy and optical transmission measurements. A number of different orientation relationships between the beta grains and the silicon substrate have been observed, which can be attributed to the existence of a variety of parallel plane pairs with the available small lattice mismatches. It is suggested that the IBS beta-FeSi2 is incoherent with the silicon matrix. The internal streaking contrast of the beta grains results from the interfaces between coexistent beta order domains which are 90° related to one another around [200]beta. The interface of the adjacent order domains is (200)beta. The mechanism for the formation of these order domain boundaries has been investigated. Face-centred cubic gamma-FeSi2 was observed in the as-implanted sample and the samples annealed at temperatures up to 600°C. It is suggested that the formation of the equilibrium beta-phase is preceded via the transition gamma-phase due to the better lattice match with the silicon substrate. Photon energy dependence of the absorption coefficient has indicated the existence of a minimum direct band gap in the IBS beta-FeSi2 with an energy of 0.847eV at room temperature. The absorption below the fundamental edge can be mainly attributed to the presence of an exponential Urbach tail with the width of ~47meV at room temperature. Measurement temperature dependence of both the direct band gap energy and the Urbach tail width has been analyzed using the Einstein Model, demonstrating the effect of thermal disorder on the band edge parameters. The variation of the band edge parameters with annealing temperature reflects the influence of structural disorder including bond distortion at the grain boundaries in the IBS beta-FeSi2. Fe+ dose dependence of the band edge parameters has revealed the existence of the metallic FeSi phase in the samples with doses higher than 4x1017 Fe+/cm 2, leading to the slightly small direct band gap energy and relatively large Urbach tail width.