First stage of the formation of refractory metal thin films on Si(111)

Abstract

XPS, work-function and LEED have been used to study the interface formation of the heavy refractory metals (Nb, Mo, Ta and W) with Si substrates. Metal evaporation has been carried out in uhv, on clean Si(111) (7 × 7) surfaces maintained at room temperature. Deposit thickness θ has ranged from a fraction of a monolayer (ML) to ≈ 100 ML. Intensity variations of the substrate and deposit XPS signals, work-function changes and core level binding energy shifts as a function of θ agree so as to indicate the different natures of the interfaces, and different growth modes of the thin films. Mo and W form abrupt interfaces with Si(111) and grow layer-by-layer. On the contrary, Nb and Ta give very reactive interfaces; interdiffusion takes place, after the completion of one 2D disordered chemisorbed metal layer, leading to mixed (1 metal: 2 Si) phases until ≈ 6 ML of Nb or ≈ 4 ML of Ta are deposited. Then, pure Nb condenses on top of the mixed layer, while the Ta overlayer presents a gradually decreasing concentration of Si. These different interfacial reactions can be explained by a qualitative model which takes into account both the Hiraki electrostatic screening mechanism and the Brillson metal-semiconductor bond strength concept.