Measuring the Characteristic Length Scale of Medium Range Order in Amorphous Silicon Using Variable Resolution Fluctuation Electron Microscopy

Abstract

Recent studies using fluctuation electron microscopy [1] have revealed the presence of medium range structural order (MRO) in amorphous silicon (a-Si) thin films. Such order is not predicted by the continuous random network model [2], and may have implications for the nature of electronic transport in hydrogenated a-Si, which is used as the semiconductor in large-area electronics [3]. The paracrystalline model ascribes this MRO to the presence of nanoscale regions that are topologically crystalline but may be strained [4]. Here we analyze the length scale of these ordered regions in a-Si using the variable resolution mode of fluctuation electron microscopy implemented on a JEOL 2010F TEM [5]. To accomplish this, we developed a set of operating conditions that can be used on a routine basis.Fluctuation microscopy is a probe-based scattering technique that quantifies structural order on the 1-3 nm length scale in disordered materials [6]. The statistical variance V of the scattered intensities from many nanoscale volumes is calculated as a function of the scattering vector k. Previous fluctuation microscopy experiments in a TEM used hollow cone dark field imaging at a fixed real space resolution to measure V(k). However, determining the paracrystallite size in a-Si requires V(k,Q) data, where Q is the objective (or probe-forming) aperture radius in reciprocal space, and the real space resolution R = 0.61/Q. Measuring V(Q) at fixed k is called variable resolution fluctuation electron microscopy (VR-FEM) [5]. Qualitatively, if the probe size is exactly matched to the size of the paracrystallites, we expect a maximum in V(Q). Real materials are likely to contain a distribution of sizes, but systematic measurements as a function of probe size will still provide a characteristic scale of the size distribution.Following Voyles and Muller [5], we have implemented VR-FEM on a Schottky emission JEOL 2010F. The JEOL 2010 has a condenser minilens capable of forming electron probes with varying convergence angles. Highly coherent diffraction-limited electron probes with a full width at half maximum in the range 1-4 nm were obtained using a very high demagnification of the source and a range of settings for the condenser minilens. Two different condenser apertures, 4µm and 10µm, were used to define the probe sizes. The objective lens setting was fixed at the value optimized for high resolution imaging. The probes were focused on the sample using Ronchigrams and the condenser aperture was centered on the coma free axis. Nanodiffraction patterns were recorded using a Gatan CCD camera. The acquisition of nanodiffraction patterns was automated to systematically vary the condenser deflectors and scan the sample area in steps of 5 nm in a square grid of 10x10 points. The exposure time was set to ~ 6 seconds and a total of ~ 500 patterns were collected in five sets of 10x10 patterns from different areas of the sample to get a statistically significant representation and a good signal to noise ratio. The nanodiffraction patterns were ...