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

Reactive Force Field Molecular Dynamics Studies of the Initial Growth of Boron Nitride Using BCl₃ and NH₃ by Atomic Layer Deposition

A new ReaxFF reactive force field for the atomic layer deposition (ALD) of boron nitride (BN) thin film growth using BCl3 and NH3 has been developed, and the initial stage of the BN growth is numerically demonstrated by ReaxFF reactive force field-based molecular dynamics (ReaxFF MD). Based on density functional theory, the ReaxFF parameters were carefully trained to describe BCl3 geometries and simulate surface reactions with BCl3 and NH3, forming BN films and HCl. The ALD process was simulated by repeating four steps: (1) BCl3 pulse, (2) first purge, (3) NH3 pulse, and (4) second purge. The film growth simulation indicates that BN thin films are grown through five steps: (i) BCl3/NH3 surface diffusion, (ii) BN cluster formation/growth, (iii) HCl formation, (iv) HCl surface diffusion, and (v) HCl desorption. Through the 5 cycles of ALD simulation, we found a mixed growth mechanism of three-dimensional growth in the form of clusters and two-dimensional growth in the form of thin films. The substrate temperature strongly affects the initial growth behavior and the resulting thickness of the BN thin film. A moderate temperature favors the formation and growth of BN clusters, while too high temperature hinders the growth of thin films because of the desorption of gas molecules and BN clusters on the surface. Through our simulation, we show that the ReaxFF MD is capable of approaching nanoscale surface reactions and clarifying the mechanisms of ALD with an atomic scale, which should be a powerful method to realize a wafer-scale ALD simulation by combining with macroscale methods