Molecular dynamics studies of indentation and sliding processes on 6H-SiC(0001) surface

Abstract

The frictional force is the sum of the forces required to break the real contact points produced by the adhesion of the asperities of two facing objects. It is known that the real contact area increases as the load increases. [1] However, the motion of atoms during a growth process of the real contact areas has not been fully discussed yet. Therefore, in this study, we discuss a structural change during indentation process and stick-slip motion on the 6H-SiC (0001) substrate surface, which has recently attracted much attentions as a power-semiconducting material that can significantly reduce heat loss and electrical energy.

The simulation model comprised of 6H-SiC(0001) substrate and a virtual rigid tip with a radius of curvature of 1.5 nm composed of helium atoms was adopted. The periodic boundary condition was applied to the unit cell within a lateral plane. As model potentials, Lennard-Jones and Tersoff potentials were used. The temperature, the time step, and the tip sliding velocity were set as T=100 K, t=0.1 fs, and v=10 m/s, respectively. The velocity-verlet method was adopted. First, the static optimized structure was prepared using the conjugate gradient method. Next, the thermal equilibrium state of the substrate surface was achieved using molecular dynamics simulation without tip for 10 ns. Then simulations of vertical and horizontal tip scans were performed. For the vertical scan, the tip was brought close to the substrate surface at a speed of 10 m/s to simulate the relationship between the loading force and the indentation depth of the tip apex atom. For the horizontal scan, the initial surface structure was prepared simulating the thermal equilibrium state a under a constant loading condition of 100 nN for 10 ns before the lateral scan of the tip was performed. Then, the lateral scan of the tip for the scan length of 3 nm was performed.

The simulated results are described below. First, under the initial thermal equilibrium state without a tip, the mean thermal oscillation amplitude of the surface atoms was reduced to 1 pm under T=100 K. Figure 1 shows the relationship between the loading force and the z-height of the tip atom during the vertical indentation process. An observed trend is that the loading force increases with decreasing tip height. However, once the loading force reached a peak value at the tip height of 0.173 nm, it rapidly decreases with a decrease of the tip height. The reason for this can be explained as follows: As the surface-atom height directly below the tip atom becomes lower, the number of substrate surface atoms except for those directly below the tip atom in contact with the tip increases, and whole the number of atoms supporting the tip increases. During the horizontal scan process, a friction force curve with a periodicity of approximately 0.3 nm was obtained, as shown in Figure 2. It is clarified that this period of stick-slip motion corresponds to the lattice constant of 6H-SiC(0001) of 0.308 nm.

[1] J. H. Dieterich and B. D. Kilgore, Pure and Appl. Geophys. 143, 283 (1994).