Fluctuation Microscopy Studies of Medium-range Order Structures in Amorphous Tetrahedral Semiconductors
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Fluctuation Microscopy Studies of Medium-range Order Structures in Amorphous Tetrahedral Semiconductors Xidong Chen * , J. Murray Gibson * , John Sullivan ** , Tom Friedmann** and Paul Voyles*** *
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 Nanostructure and Semiconductor Physics Department, Sandia National Laboratories, Albuquerque, NM 87185-1421 *** Department of Physics, University of Illinois, 1110 West Green St., Urbana, IL 61801 and NEC Research Institute. **
Abstract We applied fluctuation microscopy technique to study medium-range order in tetrahedral semiconductor materials, such as amorphous silicon, amorphous diamond-like carbon films. It is shown that this technique is very sensitive to local structure changes in the medium range order and promises solutions to open questions that cannot be answered by current techniques. For asdeposited amorphous germanium and silicon, we previously identified a fine-grain para-crystallite structure [1, 2], which will be relaxed into a lower-energy continuous random network structure after thermal annealing. With the same fluctuation microscopy technique, we however found that thermal annealing introduces medium-range order in amorphous diamond-like carbon films. Future studies will be focused on modeling and systematic exploration of annealing effects.
Introduction Medium-range order is any structure order that falls into the range from 10 Å to 300 Å. There has been abundant evidence of the existence of such an order but there have not been any experimental technique that can directly characterize such structures in a quantitative way. However, medium-range structures do affect properties as we will show later this paper. In this paper, we apply fluctuation microscopy technique to directly measure medium range order structures in amorphous tetrahedral semiconductor materials, particularly amorphous diamond like carbon films. We will compare our results on amorphous carbon with our previous results on amorphous germanium and amorphous silicon and explain why we see seemingly different thermal annealing behaviors. Hydrogen-free amorphous diamond-like carbon films have stimulated great interest because of their useful properties, such as high hardness, chemical inertness, thermal stability, wide optical gap, and negative electron affinity [3]. Consequently, they may have various potential applications in mechanical and optical coatings, MEMS systems, chemical sensors and electronic devices. Amorphous diamond-like carbon films often contains significant amounts of four-fold or sp3 bonded carbon, in contrast to amorphous carbon films prepared by evaporation or sputtering which consist mostly of three-fold or sp2 bonded carbon. The ratio and the structure configurations of these three-fold and four-fold carbon atoms certainly decide the properties of these amorphous diamond-carbon films. Although the ratio of three-fold and four-fold carbon has been studied with Raman spectroscopy and electron-loss-energy spectroscopy, very little has been u
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