Phase stabilities and surface morphologies of (Ti_1-xZr _x)Si₂ thin films on Si(100)

The effects of zirconium content on the phase stabilities and the surface morphologies of Ti-Zr alloy disilicide thin films were examined. The (Ti _1-xZr_x)Si₂ thin films were grown by codeposition of Ti and Zr alloys (200 Å) onto Si(100) surfaces in ultrahigh vacuum and followed by in situ thermal annealing at temperatures between 650 and 1000°C. The structural properties of films were studied by in situ Raman scattering experiments and ex situ x-ray diffraction experiments. The (Ti_1-xZr_x)Si₂ films with low Zr contents (x<0.1) initially form a base-centered orthorhombic structure (C49 phase) and transform to a face-centered orthorhombic structure (C54 phase) at higher temperatures. The C49-to-C54 phase transition temperature increases with increasing Zr content. When the Zr content, x, is higher than 0.1, the C49 phase is stable in the temperature range of 700-1000°C, and the C49-to-C54 phase transition is not observed. We propose that increasing the Zr content in the alloy silicide films results in a decreased enthalpy difference between these two crystalline structures. An analysis based on the classical theory of nucleation indicates that the nucleation energy barrier for the formation of a C54 cluster is increased. The surface morphologies of the alloy disilicide films were studied using scanning electron microscopy, and the surface roughnesses were quantified using atomic force microscopy. Substrate coverages of the C49 (Ti_1-xZr_x)Si₂ thin films on Si are larger compared to the C54 TiSi₂ thin films with the same thicknesses. The agglomeration of the C49 alloy silicide thin films is less severe than for the C54 TiSi₂ thin films, because the C49 (Ti_1-xZr _x)Si₂ silicide thin films have lower surface and interface free energies than C54 TiSi₂.