Electron Irradiation on Amorphous Silicon
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Electron Irradiation on Amorphous Silicon Ju-Yin Cheng1 and J. Murray Gibson2 1 Department of Material Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, U.S.A. 2 Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, U.S.A. ABSTRACT For the first time we use fluctuation electron microscopy to study the effect of electron irradiation on amorphous silicon. Before showing the results, we compare two variable coherence methods, hollow cone and dark field. These techniques have been successfully implemented in the Philips CM12, JEOL 4000 and Hitachi 9000 microscopes. The image fluctuations for ion-implanted and vacuum-deposited amorphous silicon can be reliably measured under the conditions used in these microscopes. INTRODUCTION Fluctuation electron microscopy is a practical and powerful technique for the study of amorphous states. In principle, it measures dark-field intensity from both coherently and noncoherently scattering regions of 1–2 nm. By observing fluctuations in the intensity, we can distinguish the coherently scattering regions from the other, in other words, revealing mediumrange ordering in a random phase. There are two ways to do the microscopy: variable coherence and variable resolution. Variable coherence probes the lattice spacing of the medium-range order structure, and variable resolution measures the correlation length of such a structure. The theory of fluctuation microscopy can be found elsewhere [1,2]. Here we will focus on variable coherence with dark field and hollow cone illuminations. Dark field in transmission electron microscopy is produced by tilting the incident beam and having the objective aperture centered around the optic axis. On the other hand, hollow cone is generated electronically by two existing deflectors (x and y tilt coils), whose voltages are modulated with a wave generator [3]. The difference between dark field and hollow cone is the coherence of the source. Dark-field is an individual coherent mode (more precisely, it is partially coherent), while hollow-cone is made by many spatial modes that produce images independently. Such a difference shows the advantage of hollow cone: the averaging that occurs in the image can reduce the random effect (i.e. noise arising from uncorrelated atoms can be averaged out). Nonetheless, in dark field a crystal can simply scatter 10 times more strongly than a random structure [4], so it is not necessary to use hollow cone illumination for variable coherence microscopy. Theoretically, the incoherent illumination introduces a damping oscillation to the outgoing waves at the exit plane (the specimen plane). This reduces the overall scattering intensity, and therefore the image variance. So the variance obtained from dark field is presumably higher than that from hollow cone. Indeed, this prediction is confirmed by our experiments, which were done separately in the JEOL 4000 and Philips CM12 microscopes (we ever attempted to use one microscope to do both hollow cone and dark field,
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