CYLINDRICAL SPIKE MODEL FOR THE FORMATION OF DIAMOND-LIKE THIN-FILMS BY ION DEPOSITION

We present a new model for the formation of diamondlike films by ion d eposition. In particular we model the observed ion energy dependence f or the formation of tetrahedral amorphous carbon (ta-C). Ion depositio n is treated as a cylindrical thermal spike, with energy loss along th e ion track, collision cascade effects, and conversion of energy into phonons and electronic excitations taken into account. Spike-induced a tomic rearrangements appear to be crucial for the evolution of a diamo ndlike phase, but do not lead to density relaxation. For the measured deposition conditions best suited to grow ta-C our model reveals compl ete rearrangement of the spike volume, resembling a liquidlike phase w hich is rapidly quenched. We introduce the ratio n(T)/n(S) of n(T) rea rrangements and ns atoms in the spike volume as the crucial parameter characterizing the ability of a given ion-target combination to achiev e complete rearrangement of the spike volume. n-(T)/n(S) > 1 is the op timum condition for diamond-like film growth. For a-C films the ion en ergy dependence of n(T)/n(S) agrees well with the measured sp(3) bond fraction. For Ar+-ion-assisted deposition of a-C we find n(T)/n(S) > 1 above 50 eV with no pronounced ion energy dependence. Furthermore, ou r model predicts optimum conditions for the formation of cubic boron n itride between 50 eV and 3 keV.