Hypersonic shock waves and hybridization of a-C:H thin films

Abstract

A distributed electron cyclotron resonance plasma reactor powered by a microwave generator operating at 2.45 GHz was used to deposit a-C:H films at room temperature on rf biased 〈100〉 Si substrates. Modifying substrate bias, substrate current density and composition of the precursor gas enabled the average deposited energy density to be varied. The physical properties of a-C:H were investigated using atomic force microscopy (AFM), x-ray photoelectron spectroscopy, and electron energy loss spectroscopy (EELS). The experimental results were correlated with the predictions of the binary collision theory. The influence of the deposited energy density on the nucleation and growth processes was investigated using both pure C2H2 and C2H2 mixed with Ar. The sp3 nucleation process is shown to be stimulated by high energy density cascades generated by (C2H2)+ and Ar+ ions. For the pure acetylene plasma, the AFM topography displays a random network of circular, crater-like objects close to 1 μm in diameter. These objects are associated with plastic flow of a-C:H and are attributed to the transversal hypersonic shock waves generated by overlapping binary collision cascades. EELS analysis shows that an increasing ion current density applied under constant substrate bias leads to an increased sp3 hybridized carbon fraction. The effect is attributed to interference between the shock waves triggered by individual ions and the corresponding high pressure transients. The probability Pi of a dynamic overlap of order i between shock waves is estimated under the assumption that in order to modify the quantum state and bonding type, the overlap must occur during the wave propagation time τ. The observed evolution of the sp3 hybridized fraction is consistent with theoretical predictions for i=2 and a propagation time τ≈1 ps, indicating that shock waves are generated during the cascade’s lifetime. Analysis of the AFM images shows that once the shock wave comes to rest, the subsequent nucleation of the sp3 hybridized component is controlled by the tensile stress-mediated nucleation process.