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The phase change material Ge2Sb2Te5 is widely investigated for use in nonvolatile memories. It has been reported that the crystallization speed depends on the thermal history, indicating that structural differences exist between amorphous states. The authors apply fluctuation electron microscopy to quantify differences in the nanometer-scale structural order between several amorphous states of Ge2Sb2Te5. All as-deposited films are found to contain ordered regions. Thermal annealing below the crystallization threshold increases the nanoscale order, and such samples crystallize slightly more rapidly. The authors hypothesize that the nanoscale ordered regions act as the nuclei for crystallization, with the largest regions being the most significant.

The nanometer-scale (medium range) structural order in hydrogenated polymorphous silicon films is analyzed using fluctuation electron microscopy. The polymorphous growth regime occurs under relatively high gas pressure during plasma-enhanced chemical vapor deposition, such that small aggregates and nanocrystals form in the gas phase and impinge on the film surface. All polymorphous samples appear completely amorphous in diffraction or Raman scattering analyses. In fluctuation microscopy, carried out in the transmission electron microscope, the statistical variance V in the dark field image intensity is acquired as a function of the scattering vector k at a chosen resolution Q. Theory shows that V is quantitatively related to the three- and four-body atomic correlation functions, and thus to the nanometer scale order, in the material. Unlike typical hydrogenated amorphous silicon, the variance V is a strong function of growth conditions and displays a maximum at a silane pressure of 1.4–1.8Torr. The images also reveal the presence of a small number of unusually bright spots, roughly 5nm in diameter, only in samples grown at 0.8 and 1.4Torr; we interpret that these correspond to nanocrystallites. The observation of enhanced structural order as revealed by the variance V is consistent with previous, but less conclusive, analyses of hydrogenated polymorphous silicon.

We use fluctuation electron microscopy to determine changes in the medium-range structural order of un-hydrogenated amorphous silicon thin films after they are exposed to atomic hydrogen at a substrate temperature of 230 °C. The films are deposited by magnetron sputtering at either 230 or 350 °C substrate temperature to obtain starting states with small or large initial medium-range order, respectively. The in-diffusion of atomic hydrogen causes the medium-range order to decrease for the small initial order but to increase for the large initial order. We suggest that this behavior can be understood in terms of classical nucleation theory: The ordered regions of small diameter are energetically unstable and can lower their energy by evolving towards a continuous random network, whereas the ordered regions of large diameter are energetically stable and can lower their energy by coarsening towards the nanocrystalline state.

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 ...