Reactive Force Field Molecular Dynamics Study of the Effects of Gaseous Species on the Composition and Crystallinity of Silicon–Germanium Thin Films

We simulated the growth of a silicon–germanium (SiGe) film using reactive force field molecular dynamics (ReaxFF MD) in combinations of SiH₃, SiH₂, GeH₃, and GeH₂ radicals to evaluate the effects of gaseous species on thin-film composition and crystallinity and to understand the growth mechanisms. The film compositions could be estimated in these combinations because of the linear increase in the Ge content of the films. The average crystallinity grown by SiH₃ was higher than that by SiH₂ radicals. The crystallinity of the film grown by SiH₃ radicals tends to be drastically decreased by GeH₂ radicals. The growth mechanisms for XH₃ and XH₂ (X = Si or Ge) radicals were compared. XH₃ radicals abstracted surface H atoms, and then more XH₃ radicals chemisorbed onto the formed dangling bonds, resulting in film growth through a two-step reaction known as the Eley–Rideal-type (ER-type) mechanism. The ER-type mechanism grows the film with a low hydrogen content and high crystallinity. In contrast, XH₂ radicals displayed not only the ER-type mechanism but also a one-step reaction, the H-capturing mechanism, which incorporates surface H atoms into the gaseous species. The H-capturing mechanism results in film growth with high hydrogen content and low crystallinity. The growth mechanisms are influenced by high/low H-coverage. The surface H atoms thermally move around the bonded atoms and give their kinetic energy to the diffusing gaseous species. Excess surface H atoms promote desorption. Our results from the ReaxFF MD suggested experimental settings and conditions that would enable the growth of high-quality films. Our results also suggested that SiH₃ and GeH₃ radicals should be mainly generated in the gas phase for high-quality SiGe film growth.