Mechanisms of monovacancy annihilation and type-A defect creation on Si(0 0 1)-2 × 1

Based on first principles quantum mechanics (DFT/GGA with pseudopotentials) calculations, we propose a new mechanism for monovacancy annihilation and single missing dimer creation. Our study shows that an isolated monovacancy can exist fairly stable, rather than liberating the remaining atom of the 'defect' dimer readily. The liberation barrier is calculated to be 1.3 eV. However, the monovacancy can diffuse rapidly along a dimer row by overcoming a barrier of 0.3 eV, leading to vacancy-vacancy pairing at elevated temperatures. These results suggest that the vacancy-vacancy pairing may play a major role in creating a single missing-dimer vacancy (which is ≈1.8 eV more stable than two isolated monovacancies). We also present the pathways and barriers of (i) the remaining atom hopping between the buckled-up and down site of the 'defect' dimer and (ii) the vacancy diffusion into the subsurface layer.