On the role of ions in the formation of cubic boron nitride films by ion-assisted deposition

We have investigated how ion irradiation can selectively promote the formation of dense sp³-bonded cubic boron nitride (cBN) over the graphite-like sp²-bonded phases. We have conducted a series of experiments using ion-assisted pulsed laser deposition in which either the ion mass (m_ion) or ion energy (E) was varied in conjunction with the ratio of ion flux to depositing atom flux (J/a). For a fixed ion energy and mass, there is a critical J/a above which cBN formation is initiated, a window of J/a values in which large cBN percentages are obtained, and a point at which J/a is so large that the resputter and deposition rates balance and there is no net film deposition, in agreement with Kester and Messier. As do Kester and Messier, we find that cBN formation is controlled by a combination of experimental parameters that scale with the momentum of the ions. However, unlike Kester and Messier, we do not find that cBN formation scales with the maximum momentum that can be transferred in a single binary collision, as either incorrectly formulated by Targove and Macleod and used by Kester and Messier, or as correctly formulated. Instead we observe that cBN formation best scales with the total momentum of the incident ions, (m_ionE)_1/2. We also consider the mechanistic origins of this (m_ionE)_1/2 dependence. Computer simulations of the interaction of ions with BN show that cBN formation cannot be simply scaled to parameters such as the number of atomic displacements or the number of vacancies produced by the ion irradiation. A critical examination of the literature shows that none of the proposed models satisfactorily accounts for the observed (m_ionE)_1/2 dependence. We present a quantitative model that describes the generation of stress during ion-assisted film growth. The model invokes a kinetic approach to defect production and loss. We apply a simplified version of the model to cBN synthesis, and find that it predicts an approximate (m_ionE)_1/2 dependence for cBN formation.