PROPERTIES OF Ge NANOCRYSTALS DETERMINED BY EXAFS AND SAXS

We report on the atomic level structural properties of Ge nanocrystals (Ge NCs) grown by ion implantation and thermal annealing. The synchrotron-based techniques of extended X-ray absorption fine structure (EXAFS) spectroscopy and small angle X-ray scattering (SAXS) were applied to characterize the Ge NCs. Four different size distributions of Ge NCs were produced in amorphous SiO2 matrices, with mean diameters ranging from 4.0 to 9.0 nm as verified by SAXS measurements. The decrease in size, or increase in surface-to-volume ratio, for the NCs leads to a decrease in the crystalline-like features in the X-ray absorption near-edge structure (XANES) and EXAFS regions of the X-ray absorption spectra. It also yields a decrease in coordination number and an increase in mean interatomic distance, structural disorder, and asymmetry of the interatomic distance distribution. By comparison with bulk standards, the NCs are described as a combination of crystalline Ge core and surrounding amorphous Ge-like surface layer.