Size analysis of nanoscale order in amorphous materials by variable-resolution fluctuation electron microscopy

Variable resolution fluctuation electron microscopy can reveal the characteristic decay length over which nanoscale order persists in amorphous materials. In the fluctuation method, four-atom positional correlations within the resolution (coherent diameter) of the electron beam dominate the experimental data. Such correlations occur when the sample contains topologically ordered regions within a random network matrix. By changing the resolution, a decay length is obtained under the simplifying assumption that the correlations decay in an exponential manner. We have developed a simple lens configuration on a STEM that affords high quality variable resolution data and does not compromise the stability of the instrument. To test this method, we analyze the structural order in amorphous silicon thin films grown under vastly different deposition conditions and determine decay lengths ranging between 0.4 and 0.6 nm. We then compare the data with model simulations to obtain an estimate of the diameter of the ordered regions.