Microstructure dependence of low-temperature elastic properties in amorphous diamond-like carbon films

摘要

We have studied the internal friction and the relative change in the speed ofsound of amorphous diamond-like carbon films prepared by pulsed-laserdeposition from 0.3 K to room temperature. Like the most of amorphous solids,the internal friction below 10 K exhibits a temperature independent plateau.The values of the internal friction plateau, however, are slightly below theuniversal ``glassy range'' where the internal frictions of almost all amorphoussolids lie. Similar observations have been made in our earlier studies in thethin films of amorphous silicon and amorphous germanium, and the behavior couldbe well accounted for by the existence of the low-energy atomic tunnelingstates. In this work, we have varied the concentration of sp^3 versus sp^2carbon atoms by increasing laser fluence from 1.5 to 30 J/cm^2. Our resultsshow that both the internal friction and the speed of sound have a nonmonotonicdependence on sp^3/sp^2 ratio with the values of the internal friction plateauvarying between 6x10^-5 and 1.1x10^-4. We explain our findings as a result of apossible competition between the increase of atomic bonding and the increase ofinternal strain in the films, both of which are important in determining thetunneling states in amorphous solids. In contrast, no significant dependence oflaser fluence is found in shear moduli of the films, which vary between 220 and250 GPa. The temperature dependence of the relative change in speed of sound,although it shows a similar nonmonotonic dependence on laser fluence as theinternal friction, differs from those found in thin films of amorphous siliconand amorphous germanium, which we explain as having the same origin as theanomalous behavior recently observed in the speed of sound of thinnanocrystalline diamond films.