Short-range thermal and structural properties of Ge nanocrystals

Ge nanocrystals were produced in SiO₂ by ion implantation and thermal annealing. Size and depth distributions as well as short-range structural and thermal properties of the nanocrystals were analysed by RBS, TEM, SAXS and EXAFS. From temperature-dependent EXAFS measurements analysed using a correlated anharmonic Einstein model and thermodynamic perturbation theory it was verified that the thermal properties of Ge nanocrystals differ significantly from both bulk crystalline and amorphous Ge. For the first shell of nearest neighbours, the increase in interatomic distance with temperature for the nanocrystals was observed to be smaller than for bulk crystalline Ge. It was also observed that the first shell Ge-Ge bonds were stiffer in the nanocrystals than in both the amorphous and crystalline bulk. Such differences are ascribed to the increased surface to volume ratio of the nanocrystalline phase and the presence of the surrounding SiO₂ matrix.