Evaluating the Ge: C ratio on the bonding structure, hardness, and residual stress of Ge1-x-Cx coatings fabricated by the PE-CVD method

Abstract

In the present research, the effect of the Ge: C ratio was evaluated and explained on the bonding structure, the hardness, and residual stress of germanium-carbon (Ge1-x-Cx) coatings fabricated by plasma-enhanced chemical vapor deposition (PECVD) method. For characterizing the coatings, a field emission scanning electron microscope (FE-SEM) equipped with an energy dispersive spectrometer (EDS), transmission electron microscope (TEM), Raman spectrometer, Rutherford backscattering spectrometer (RBS), elastic recoil detection (ERD) analysis, X-ray photoelectron spectrometer (XPS), nanoindentation test, and cohesion test were used. by increasing the flow rate ratio of CH4:GeH4 and as a result of decreasing the ratio of Ge: C, the fraction of sp2 C-C and sp3 Ge-C bonds were increased and the fraction of Ge-Ge bonds was decreased. The hardness of the Ge1-x-Cx coatings was heavily reliant on changes in the Ge: C ratio and, consequently, the fraction of the bonds incorporated within the coating structure. To achieve a hard coating, the selection of a moderate carbon content is very important such that it would lead to the formation of maximum sp3 Ge-C bonds alongside slight amounts of sp2 C-C and hydrogen bonds. For very low carbon concentrations, the existence of dominant Ge-Ge bonds would create the coating structure as germanium-like; the excessive rise of the carbon concentration promotes the undesirable graphite-like structures. In the optimum conditions, the hardness of the coating was obtained at 10.1 GPa. The coatings presented a very low residual stress (∼98-240 MPa) despite having suitable hardness.