M. Iannuzzi

Electronic origin of the stability trend in TiSi2 phases with Al or Mo layers

Through a tight-binding rigid-band approach we show that changes in the relative stability of the C54, C49, and C40 phases of TiSi2, with electrons per atom ratio, are produced by the corresponding differences in the electronic density of states at the Fermi level. In particular, by increasing this ratio the stable phase evolves from C49 to C54, and then to C40. Our microscopic model provides a straightforward interpretation of very recent experimental findings concerning the sizeable variations in the transition temperature between C49 and C54 TiSi2 in the presence of Al or Mo layers.

Supersoft elastic parameters and low melting temperature of the C49 phase in TiSi2 by Brillouin scattering and molecular dynamics

In this letter, we show that the polymorphic C49 form of TiSi2 has much smaller elastic constants than those of the C54 bulk-stable structure and that its melting temperature is about 300 °C lower. These issues supply intriguing hints in explaining the kinetic advantage of the C49 over the C54 phase and in understanding the role of the elastic energy in the phase transformation from C49 to C54.

Role of the substrate in the C49–C54 transformation of TiSi2

In this work we report the results of the crystallographic and morphological characterization of TiSi2 films grown on a patterned and on a blanket Si substrate. We show that the C49 films grown on crystalline silicon exhibit a rougher surface with respect to that grown on polycrystalline substrate. The different surface morphology is maintained after transformation to the C54 phase. This latter is always (040) textured in the case of patterned films, whereas in blanket films texturing occurs only on a polysilicon substrate. The C49 phase displays (200) texturing when the film is grown on single crystalline Si(100), while random orientation of the grains is detected in the other cases. The experimental indications are associated with semiempirical total energy estimations of the surface energies for the two competing phases. From this comparison it turns out that, in the case of texturing, the preferential surfaces facing the substrate are the most energetic.

Point defects and stacking faults in TiSi2 phases by tight binding molecular dynamics

Tight binding molecular dynamics is used to predict the structure and the total energy of the most relevant intrinsic point defects in C54 and C49 TiSi2. The comparison between the relative formation energies of point defects of the two phases in contact with a Si substrate suggests that the metastable C49 form has a higher concentration of point defects. In particular, we point out that Si vacancies and (010) stacking faults should be quite common in the C49 structure. This issue could be important in explaining the kinetic advantage of the latter phase in film growth by solid state reaction.

Silicon diffusion in competitive TiSi 2 phases by molecular dynamics simulations

Abstract After a brief review of the basic properties of TiSi 2 phases recently obtained by molecular dynamics simulations, we show the self-diffusing species at high temperatures in both the C54 and C49 structures to be silicon, by hopping through a first neighbours network. The kinetic parameters and the diffusion paths for Si diffusion by vacancies and interstitials are reported, pointing out that diffusion in the C49 structure by SiI vacancies is strongly favoured, due to the very low formation energy they display.

Texturing, surface energetics and morphology in the C49-C54 transformation of TiSi2

In this paper we analyse the extent of grain orientation before and after the C49–C54 transformation in TiSi2, depending on the substrate microstructure and the lateral dimensions of the film. In the former case, for blanket configuration, we make a comparison to the corresponding evolution in surface roughness, both by AFM and by light scattering measurements. In the second case, an interpretation of the strong texturing occurring in narrow lines, independently of substrate microstructure, is given on the basis of surface energy calculations.

Self-diffusion of silicon in TiSi2 competing phases by tight-binding molecular dynamics

Abstract Tight-binding (TB) potentials allow for a straightforward separation of the binding and repulsive contributions in the total energy. This issue can be used in molecular dynamics runs to correlate the local atomic structure to the diffusion paths and the activation energies of silicon mobility in TiSi 2 , which is the most common material for metallisations and local interconnects in microelectronic devices. In this work we focus on the self-diffusion process at high temperature and a relation between the crystal structures and the diffusion paths for the two competing phases is drawn.

Molecular dynamics characterization of crystalline and amorphous TiSi 2 phases

Molecular dynamics simulations by a tight binding potential provide new interesting information on the ground state properties of the TiSi2 phases. In particular, we have compared some structural, elastic, thermodynamic and electronic properties of the C49, the C54 and the amorphous phases. It turns out that the C49 structure is much softer than the C54, also displaying a melting temperature some 300–400 K below the one for the C54, in agreement to very recent experimental results. The amorphous phase is energetically and structurally more akin to the C49 than to the C54 phase. On the basis of these results we suggest the higher formation kinetics of the former to be related to an intrinsic advantage in the growth stage.

Estimation of the critical radius for the nucleation of the C54 phase in C49 TiSi2: role of the difference in density

We discuss the rather scattered measurements of the lattice parameters for C49 TiSi 2 , which are reported in literature, along with new and accurate X-ray diffraction measurements and ab-initio calculations. Both agree in indicating that the density of the metastable C49 structure cannot be much smaller than the one for the polymorphic C54 phase, as it is commonly reported. We conclude by demonstrating that only in the case of such a smaller difference in density between the two phases, the elastic strain contribution to the nucleation energy of the C54 structure in the C49 matrix can be neglected. The estimation of the critical radius strongly depends on this issue.

Brillouin scattering of TiSi 2 : elastic constants and related thermodynamic parameters

Abstract Elastic constants of the C49 and C54 phases of TiSi 2 thin films have been measured by surface Brillouin spectroscopy; the results were exploited to estimate other thermodynamic parameters such as the melting and Debye temperatures and surface free energies of both phases. The C49 form displays much softer elastic parameters than the C54 bulk-stable structure. Furthermore, the estimated thermodynamic parameters of C49 appear to be lower than those of C54.