Characterisation by photoemission of the surface structure of epitaxial CoSi 2 grown on Si(111) by various preparation techniques

Abstract

The nature of the surface phases formed on epitaxial CoSi 2 layers grown on Si(111) is investigated by angle-resolved X-ray (ARXPS) and ultra-violet (ARUPS) photoemission, low energy electron diffraction (LEED) and work function measurements. Various preparation techniques, such as solid phase epitaxy (SPE) and high temperature annealing (≥ 500°C), Co and Si codeposition in the CoSi 2 ratio and simultaneous or subsequent annealing at 360°C or above, and sequential deposition of equal amounts (≤10 ML) of Co and Si and subsequent heating to 400°C, all invariably lead to the Si rich high temperature phase labeled CoSi 2(111)-Si. Co-rich surface phases with peculiar structures generally intermediate between two limiting geometries labeled CoSi 2(111)-Co and CoSi 2(111) can be obtained either by a ∼ 2 ML Co deposit onto CoSi 2 films prepared with the techniques mentioned above and annealing at low temperature (≤ 400°C) or directly on films grown by reactive molecular beam epitaxy (reactive MBE). In the latter method a template layer is first prepared by evaporating 3 ML of Co onto the Si substrate held at room temperature and subsequent annealing at 360°C for 5 min. Co is then deposited at a low rate (∼ 0.5 ML/min) onto this template layer, maintained at 360°C, up to the desired film thickness. The Co richest pure CoSi 2(111)-Co geometry can be formed by a 2 ML Co deposit on films prepared by reactive MBE and subsequent short anneal (≤ 3 min) at 360°C. This structure is basically metastable and converts progressively into the low temperature stable CoSi 2(111) form upon annealing at 400°C.