In situ RBS and Raman spectroscopy study of nickel-catalyzed amorphous carbon graphitization

Session: Thin films type of contribution: Poster
Metal-induced crystallization with and without layer exchange (MIC w/o LE) is a method to decrease the crystallization temperature of amorphous group 14 elements (G14E) by up to several hundred degrees. In situ experiments are expected to provide new insights into thin film evolution and elementary process steps of MIC w/o LE and to improve existing models of this type of phase transformation. While MIC w/o LE has been widely studied for Si and Ge in contact with catalytic metals, there exist only a few studies for the crystallization of amorphous carbon. Therefore, in this contribution in situ Rutherford backscattering spectrometry (RBS), Raman spectroscopy and spectroscopic ellipsometry studies were performed during annealing of amorphous carbon/nickel (a-C/Ni) layer stacks at temperatures up to 750°C.
Due to its small lattice mismatch with the basal plane of graphite and high diffusivity of C atoms, Ni is a suitable catalyst for the growth of graphene and crystalline graphitic nanostructures. During the annealing of an a-C/Ni layer stack covalent bonds between the carbon atoms at the catalyst interface are weakened. Liberated carbon atoms can move along the interface and diffuse along the grain boundaries into the Ni layer towards the catalyst surface, where nucleation and grain growth of graphitic crystallites occur. Our in situ studies showed a change in the stacking sequence between C and Ni layers under defined experimental conditions. According to in situ Raman measurements, this mechanism occurs independent of the stacking sequence, while the velocity of the LE differs significantly. As observed in time and temperature resolved Raman spectra, the position of the G peak and the I(D)/I(G) ratio changed according to the Three-Stage-Model by Ferrari and Robertson, confirming the transformation of amorphous carbon to nc-graphite. With the in situ RBS measurements more insight into LE was given. Here peak positions of C and Ni were shifted, indicating a change of the energy of the scattered ions for both layers respectively and proving the combination of the observed graphitization process with LE during annealing. The thickness of the synthesized crystalline graphitic layer is controlled by the finite carbon source – the deposited a-C film, which is a decisive advantage of this process compared to CVD. It is demonstrated that the structure and the crystallite size of the metallic catalyst layer has a strong influence on the crystallite size and the quality of the graphitic film.
LE is potentially interesting for industrial applications, as it allows the formation of polycrystalline thin films of G14E at much lower temperatures - than during thermal annealing without the metallic catalyst. Depending on the initial stacking sequence, the crystalline graphitic film can be deposited on a suitable device-ready substrate or transferred to another substrate after the dissolution of the transition metal catalyst.