Narrowing of the Boolchand intermediate phase window for amorphous hydrogenated silicon carbide

Intermediate phases represent the so called “sweet spot” in amorphous material systems where the bond stretching and bond bending constraint forces are equally balanced by the total degrees of freedom in the system. They are sometimes also referred to as “Boolchand” phases in recognition of the seminal contributions of Dr. Punit Boolchand to the study of these materials. In a prior publication (King et al., J. Non-Cryst. Solids 379 (2013) 67), we presented possible evidence for the existance of an intermediate (i.e. “Boolchand”) phase in amorphous hydrogenated silicon carbide (a-SiC:H) with a wide range in mean atomic coordination (〈r〉 = 2.4 − 2.7). Support for such a wide phase window was based primarily on a correlation between the post plasma deposition bi-axial film stress and 〈r〉. However, we demonstrate in the present article that the apparent width of the Boolchand intermediate phase window in the prior study was inflated due to two competing film stress contributions, and that the true range in 〈r〉 for the phase window is substantially narrower. Specifically, opposing tensile and compressive film stress components were identified to arise during the plasma enhanced chemical vapor deposition (PECVD) of a-SiC:H. The tensile film stress component was attributed to film/substrate thermal contraction mismatch on cooling from the deposition temperature, while the compressive stress component was attributed to ion bombardment of the a-SiC:H growth surface during PECVD. In the prior study, the energy of the ions during PECVD was primarily modulated by the addition of a low frequency bias to the plasma and was intentionally utilized to produce films with varying compressive stress and 〈r〉. In the present study, the low frequency bias was removed from the PECVD of a-SiC:H and instead the deposition pressure was varied to produce films with varying 〈r〉 and exclusively tensile stress. Through detailed analysis of the tensile bi-axial film stress and Young's modulus dependence on 〈r〉, we found conclusive evidence that the film-substrate thermal contraction mismatch was the dominant film stress component in this case and that the magnitidue of the tensile stress was purely due to rigidity percolation in the a-SiC:H film. We therefore conclude that in our previous study the balance of the tensile and compressive stress components resulted in the deceptive appearance of a potentially wide Boolchand intermediate phase window for a-SiC:H. Based on the a-SiC:H films generated in the present study where only the tensile stress component was significant, we have found the window for a possible intermediate phase to be greatly narrowed at Δ〈r〉 ≤ 0.05.