Influence of topological constraints on ion damage resistance of amorphous hydrogenated silicon carbide

Radiation damage in materials is an important reliability issue in applications ranging from microelectronic devices to nuclear reactors. However, the influence of atomic structure and specifically topological constraints on the ion damage resistance of amorphous dielectrics has until recently been largely neglected. We have investigated the 120 keV He+ ion damage resistance for a series of amorphous hydrogenated silicon carbide (a-SiC:H) thin films. Changes in elemental composition and atomic structure induced by He+ ion irradiation were monitored using nuclear reaction analysis, Rutherford backscattering spectroscopy, transmission Fourier-transform infrared spectroscopy, and transmission electron microscopy while changes in mechanical properties were investigated using nanoindentation measurements. We show that for 120 keV He+ ion doses producing up to one displacement per atom, significant hydrogen loss, bond rearrangement, film shrinkage, and mechanical stiffening were induced for films with mean atomic coordination (〈r〉) ≤ 2.7, while comparatively minor changes were observed for films with 〈r〉 > 2.7. The observed radiation hardness threshold at 〈r〉rad > 2.7 is above the theoretically predicted rigidity percolation threshold of 〈r〉c = 2.4. Based on the observed elimination of terminal CH bonds and SiCH2Si linkages, the higher radiation hardness threshold is interpreted as evidence that these bonds are too weak to function as constraints in high-energy ion collisions. Eliminating these constraints increased 〈r〉c to 2.7, in agreement with the observed 〈r〉rad = 2.7. These results demonstrate the key role of topological constraints in ion damage resistance and provide additional criteria for the design of ion-damage-resistant amorphous materials.