Stress relaxation and medium-range order in diamond-like amorphous carbon films
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Stress relaxation and medium-range order in diamond-like amorphous carbon films Xidong Chen *, John Sullivan **, Tom Friedmann**, Dean Miller*** *
Cedarville University, Cedarville, OH 45314 & Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 ** Nanostructure and Semiconductor Physics Department, Sandia National Laboratories, Albuquerque, NM 87185-1421 *** Materials Science Division, Argonne National Laboratory, Argonne, IL 60439
Abstract In this paper, we have studied the relationship between medium-range order structures in and stress relaxation in amorphous diamond-like carbon films with fluctuation microscopy. Our preliminary results show strong correlation between stress relaxation and medium-range order. Our previous results showed that annealing films that had been through stress relaxation procedures caused great increase of medium-ranger order. In this paper, we have found that the increase of medium-range order in films that have been annealed before going through stress relaxation through removal of substrates is less pronounced than that of films that have been annealed after removal of substrates. We will discuss interpretations and implications of these results.
Introduction Hydrogen-free amorphous diamond-like carbon (DLC) 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 [1]. Consequently, they may have various potential applications in mechanical and optical coatings, MEMS systems, chemical sensors and electronic devices. An important application of hydrogen-free amorphous diamond-like carbon (DLC) films is found in MEMS systems. Understanding mechanical behaviors is critical to successes in developing reliable MEMS systems with DLC films. Structures of DLC films certainly play a central role in affecting their mechanical behaviors. To get a comprehensive picture of structures in those amorphous films however demands information on medium-range order (MRO), a kind of structure order that involves more than two-body correlation. Short-range order involving only two-body correlation has been shown inadequate to address the richness of properties in DLC films [2].
Theory Through the history of studying amorphous materials, the radial distribution function (RDF) has played an important role. While it is equivalent to an atomic pair correlation function, its importance relies on the fact that this function can be directly measured from diffraction experiments. The RDF gives good quantitative descriptions of short-range order (2 – 5 Å). However, it is almost impossible to interpret the third and the fourth peaks in the RDF. This is because the number of atoms filling a shell of r increases significantly as r increases. At a very large r, the RDF is approaching to the average number density of atoms. Therefore, the RDF is an ensemble average of the short-range order structures (2 – 5 Å) of the whole sample. It can not provide information
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