Average energy deposited per atom: A universal parameter for describing ion-assisted film growth?

The average energy deposited per atom, 〈Ed〉=Ei(Ji/JMe), where Ei is the ion energy and Ji/JMe is the ratio of the accelerated-ion to deposited-thermal-particle fluxes incident at the growing film, has been shown to be one of a set of parameters useful for describing the effects of low-energy ion irradiation on film microstructure during ion-assisted deposition. Recently, however, 〈Ed〉 has often been treated as if it were a fundamental, or universal, parameter. We have carried out experiments in which Ei (20–100 eV) and Ji/JMe (1–10) were varied independently during the deposition, at constant temperature, of polycrystalline Ti0.5Al0.5N films onto amorphous SiO2 substrates by ultrahigh vacuum reactive magnetron sputtering in pure nitrogen. Ion-irradiation-induced changes in film microstructure, texture, phase composition, and nitrogen-to-metal ratio were found to follow distinctly different mechanistic pathways depending upon whether Ei or Ji/JMe was varied, resulting in quite different properties for the same value of 〈Ed〉. Thus, 〈Ed〉 is clearly not a universal parameter.