GeSi intermixing in Ge nanostructures on Si(111): An XAFS versus STM study

We report a detailed investigation of interdiffusion processes that occur during the growth of germanium nanostructures on the (111)-oriented surface of silicon. In particular, X-ray Absorption Fine Structure (XAFS) measurements performed ex situ show that a Ge(1-x)Si(x) alloy forms during deposition, with average composition x varying between 0.25 and 0.50, depending on substrate temperature and total coverage. By fitting the Si nearest-neighbor numbers around Ge as a function of the deposited thickness with a simple model, the effective vertical composition profile in the growth direction has been estimated. The latter has been described with a static effective diffusion length of (10.0 +/- 1.5) nm at 530 degrees C and (5 +/- 1) nm at 450 degrees C, which is interpreted as the dominance of surface transport processes in the intermixing dynamics. The analysis of the data on Ge-Ge bond length indicates a decrease of the Ge-Ge atomic distances with increasing Ge fraction, confirming previous theoretical predictions for strained epilayers. The XAFS results are compared to morphological information obtained by scanning tunneling microscopy investigations carried out in situ, yielding a satisfactory description for the epitaxy of this system.