Carbon dimers on the diamond (100) surface: Growth and nucleation

We use a density-functional based tight-binding method to study diamond growth by C-2 on a nonhydrogenated diamond (100)-(2x1) surface. The study is motivated by advances in the growth of ultrananocrystalline diamond (UNCD) films under hydrogen-poor conditions. We identify and classify stable adsorbate configurations formed above dimer rows and troughs on the reconstructed surface. We also investigate adsorption and migration barriers using the nudged elastic band method. We find viable adsorption pathways leading to chain growth and step advancement. Initial depositions proceed without barriers into topologically imperfect configurations. The most stable configuration is a growth position that bridges two adjacent surface dimers along a dimer row. It is reached over a barrier of 1.2 eV and has an adsorption energy of -6.9 eV. Many other configurations exist that have adsorption energies differing by up to 2.7 eV. By comparison, analogous structures for silicon are fewer in number and closer in energy because Si lacks pi bonding, which is important for C-2 on diamond. Migration barriers for ad-dimers are in the range of 2-3 eV due to relatively large differences in the energies of intermediate local minima. Comparing our results with previous studies on the (110) surface, we note that barriers leading to growth are higher and pathways are more complex on the (100) surface. The barriers suggest that reactions leading to both growth and re-nucleation are possible, which helps to understand the small observed grain sizes in UNCD.