Molecular dynamics study of atomic displacements and subsequent lattice relaxation in 3C- and 4H-SiC

Ion implantation is considered to be a very suitable means for selective electrical doping of SiC. However, ion irradiation produces defects which can prevent the electrical activation of the implanted dopants. The understanding of ion-beam-induced defect generation and evolution in SiC is therefore extremely important. It consists of three stages: (i) atomic displacements during ballistic processes, (ii) formation of metastable defects after fast relaxation of the crystalline lattice, and (iii) long-term thermally induced defect rearrangement, migration, recombination and reduction. The present work deals with elementary processes occurring in the first two stages. Classical molecular dynamics (MD) simulations using a modified Tersoff potential are performed to investigate the conditions for defect formation by a single Si or C primary knockon atom (PKA) at T = 300 K. The threshold PKA energy for defect formation as well as the resulting defect configuration and its formation energy are determined. This study is limited to cases where the PKA starts parallel or antiparallel to the [0001] direction, which is identical to [111] in the cubic 3C-SiC. However, certain general results obtained are also valid for other examples. In contrast to previous publications, the more detailed investigations performed in this work reveal, that for a well-defined start direction neither for a Si nor for a C PKA a fixed threshold PKA energy for defect formation exists. Instead, a transition region having a width of 2.5 – 20 eV is found in which the probability for defect formation increases from 0 to 1. Moreover, for the same PKA energy and start direction, different defect configurations may be obtained. Both results are due to the fact that lattice vibrations influence the dynamics of atomic displacement and relaxation processes, in particular its initial conditions. Most of the defects found are different configurations of single pairs of vacancies (V) and self-interstitials (I). Since the lattice structure of 4H-SiC is more complicated than that of 3C-SiC, a greater variety of different defect types is found in this polytype. At 300 K the probability for overcoming the energetic barriers for V-I recombination or between different metastable defect states is very small. Therefore, within 20 – 50 ps after the PKA is started, present MD simulations do not yield a further transformation of the defect configurations formed by fast relaxation processes.