Buried (Fe_{1 −x} Co _x )Si₂ layers with variable band gap formed by ion beam synthesis

Abstract

Buried layers of (Fe_{1 −x} Co _x )Si₂ were prepared by sequent implantation of iron and cobalt into (100) silicon. The depth distributions of iron and cobalt and the atomic concentration ratio silicon/metal were determined by Auger Electron Spectroscopy (AES) and Rutherford Backscattering Spectrometry (RBS). The phase composition and the microstructure of the silicide layer were studied by X-ray diffraction and electron microscopy. The band gap energy was evaluated from Infrared (IR) reflection and transmission experiments. The semiconductingβ-FeSi₂ structure remains stable up to a cobalt fraction ofx = 0.2 if the iron silicide is stabilized by an intermediate annealing between iron and cobalt implantation. With increasing cobalt content, the electrical resistivity as well as the energy of the direct band gap of the (Fe_{1 −x} Co _x )Si₂ layer decrease. In this way, a band gap tuning between 0.84 and 0.70 eV is possible. The gap energy is found to vary quadratically with the Co content in the composition range 0 <x < 0.15.