Fluctuation Electron Microscopy Analysis of Medium Range Order in Amorphous Thin Films

Lakshminarayana Nittala, Yeonwoong Jung, Sanjay Khare, Ray Twesten, John Abelson

Materials Research Laboratory, U. Illinois at Urbana-Champaign

Fluctuation Electron Microscopy (FEM) is sensitive to 3- and 4-body atomic correlation functions, and therefore to the presence of medium-range structural ordering in amorphous materials. By contrast, conventional diffraction experiments are sensitive primarily to the 2-body function. Our FEM studies have shown the presence of considerable MRO in vapor deposited amorphous silicon (a-Si:H) thin films, whereas the predictions of the continous covalent random network (CRN) model entirely fail to match the data. The increased order is attributed to the presence of strained nanocrystallites of 1-2 nm diameter, called “paracrystallites” in the amorphous film.

Here we report the influence of kinetic processes on the degree of MRO in a-Si:H films. The experimental variables include substrate temperature and fast particle bombardment of the film growth surface, and post-growth thermal annealing or light irradiation. The data indicate that MRO can either coarsen or diminish (meaning the material can relax towards a CRN structure) depending on the processing conditions. We suggest that the paracrystallites can be modeled as nuclei which, as in capillarity theory, may be either supercritical or subcritical in size. We also present molecular dynamics simulations of the paracrystalline structure that have been used as the input to simulations of FEM data. Quantitative agreement has been reached, which allows us to estimate the volume fraction and characteristic size of the ordered domains.