Formation mechanism of Ge nanocrystals embedded in SiO₂ studied by fluorescence x-ray absorption fine structure

This paper reports that the Ge nanocrystals embedded in SiO_2 matrix are grown on Si(100) and quartz--glass substrates, and the formation mechanism is systematically studied by using fluorescence x-ray absorption fine structure (XAFS). It is found that the formation of Ge nanocrystals strongly depends on the properties of substrate materials. In the as-prepared samples with Ge molar content of 60%, Ge atoms exist in amorphous Ge (about 36%) and GeO_2 (about 24%) phases. At the annealing temperature of 1073 K, on the quartz--glass substrate Ge nanocrystals are generated from crystallization of amorphous Ge, rather than from the direct decomposition of GeO_2 in the as-deposited sample. However, on the Si(100) substrate, the Ge nanocrystals are generated partly from crystallization of amorphous Ge, and partly from GeO_2 phases through the permutation reaction with Si substrate. Quantitative analysis reveals that about 10% of GeO_2 in the as-prepared sample are permuted with Si wafer to form Ge nanocrystals.