MOLECULAR-DYNAMICS STUDY OF DIAMOND SILICON(001) INTERFACES WITH AND WITHOUT GRAPHITIC INTERFACE LAYERS/

A theoretical study of diamond/silicon (001) interface structures with and without graphitic interlayers using a density-functional based ti ght-binding molecular-dynamics method is presented. The study is motiv ated by recent progress towards diamond heteroepitaxy on Si (001) usin g the bias technique. The proposal by Robertson that an initial graphi tic deposit with the graphite planes normal to the Si surface (resulti ng from carbon subplantation) provides a well matched compliant buffer layer and hence may lower the interface energy is examined. It is fou nd here that the models with ''graphitic'' or sp(2) bonded interlayers have, in fact, higher energy than those without and that the energy i ncreases with the number of graphitic layers. Residual strain in the g raphite planes is found to be one of the reasons for this energetic di sadvantage. Si surface dimerization was assumed in the initial structu res. The diamond surface layer is found to dimerize spontaneously in b oth cases (with and without graphite interlayers). In models with an o dd number of sp(2) layers, the last sp(2) bonded atoms next to diamond become the diamond dimerized termination layer. Because of the dimeri zation, compatibility is required between the 3-on-2 epitaxy and the ( 2 x 1) dimerizations of both the Si and C surfaces joined together at the interface. The various ways in which the diamond and Si dimer arra ys can be juxtaposed are analyzed and structural relaxation calculatio ns are carried out for each to determine the optimum energy interface structure. The relative energies of the various structures are tentati vely explained in terms of favorable and unfavorable local structural units. The optimum structure found has a stepped diamond surface with the lowermost carbon layer dimers orthogonal to the Si dimers.