The hafnium and zirconium silicates, (MO2)x(SiO2)1x, with M=HfZr, 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=HfZr). 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.

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