Abstract
We have carried out first principles calculations of the vibrational and thermodynamic behavior in NiSi and isostructural compound NiGe. Phonon density of states has also been measured in NiSi using inelastic neutron scattering techniques. We find that the vibrational spectra of the two compounds are very different, due to the difference in the size and mass of Si and Ge. Interesting anomalous thermal behavior of NiSi due to anharmonic phonons is brought out well in our calculations, particularly the negative thermal expansion (NTE) along the b-axis of the orthorhombic unit cell. Large difference in thermal expansion behavior of NiSi and NiGe is very well reproduced by the calculations. Additionally, calculations enable to identify the phonon modes which lend major contribution to the negative thermal expansion behavior in NiSi, and reasons for negligible NTE in NiGe. Such typical representative modes at the zone-boundary along b-axis involve transverse vibrations of Si/Ge along c-axis.PACS numbers: 78.70.Nx, 63.20.-e, 65.40.–b.
Highlights
Transition metal silicides are considered important in microelectronics industry
The lattice parameter as well as volume of NiGe is larger in comparison to that of NiSi. This is reflected in the phonon density of states in the two compounds
We find that the range of phonon energies contributing to NTE along b- axis is significantly different for NiSi and NiGe; and this leads to significant difference in the magnitude of negative thermal expansion along b- axis in these compounds
Summary
Transition metal silicides are considered important in microelectronics industry. These materials are used to reduce contact resistance at source, drain and gate terminals in standard complementary metal oxide semiconductor (CMOS) devices and very-large-scale integration (VLSI) technology. The low energy peak in the partial density of states of Ge in NiGe is at 8 meV in comparison to that at 16 meV in Si spectrum of NiSi. The shift in the peak position may be mainly due to the mass difference of both the atoms in respective compounds.
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