Thermal decomposition mechanisms of hafnium and zirconium silicates at the atomic scale

The hafnium and zirconium silicates, $(MO2)x(SiO2)1−x$⁠, with $M=Hf∕Zr$⁠, are being considered as high-$k$ gate dielectrics for field-effect transistors as a compromise between high permittivity and thermal stability during processing. Using atomic-scale models of silicates derived from hafnon/zircon, stability before and after simulated thermal annealing is calculated within a density-functional approach. These silicates are found to be thermodynamically unstable with respect to decomposition into $SiO2$ and $MO2$ $(M=Hf∕Zr)$⁠. Segregation mechanisms on the atomic scale are studied leading to an insight as to an why $SiO2$-rich mixtures undergo spinodal decomposition and why, by contrast, $MO2$-rich phases are metastable, decomposing below typical process temperatures.