Fe diffusion in amorphous Si studied using x-ray standing wave technique

Diffusion of Fe impurity in amorphous Si at the intermediate concentration range of 25 at.% Fe has been studied. A combination of x-ray standing wave technique and secondary ion mass spectrometry provides unambiguous determination of the concentration profiles of the constituent species with sub-nanometer depth resolution. X-ray standing waves are generated using total external reflection from an underlying W layer. It is found that up to 573 K, Fe diffusivity is less than 10−23m2/s. This is in stark contrast to isolated Fe impurity diffusion in Si or to the interdiffusion at Fe/Si interface, which are orders of magnitude higher. An interesting phenomenon is observed, when a Pt buffer layer is used instead of W for generating standing waves: With thermal annealing, as the Pt atoms move into Si layer and cross the marker layer containing Fe atoms, Fe atoms also move along. This results in an upwards shift of the concentration profile of Fe.