Subsurface channeling of keV ions between graphene layers: Molecular dynamics simulation

Using molecular dynamics simulation, we study the impact of 3 keV Xe ions at glancing incidence on a $\beta$-SiC (111) surface covered by graphene. On top of a full graphene layer covering the substrate, we add a graphene half-layer; the step forming where the half-layer terminates allows the entrance of glancing-incidence ions into a subsurface channel between graphene layers. We find a high channeling probability which leads to only little sputtering and damage formation. Typically, vacancy defects are formed at periodic intervals when the ion hits the uppermost graphene layer from below. Extended damage occurs when the ion hits the step edge itself. There we find several kinds of defects varying from adatoms over the formation of $s{p}^1$-bonded chains to hillocks.

Figure 1

Sketch of the target surface, showing (a) top view and (b) side view. Brown circles show the uppermost Si atoms of the Si-terminated SiC (111) substrate. Full black circles denote C atoms of the fist graphene layer; open black circles denote C atoms of the second half-layer. The C atoms of the SiC substrate are not shown. The impact area is outlined with blue borders. $\Delta h=3.354\text{Å}$ is the graphene interlayer distance.

Figure 2

Sputter yield of a 3 keV ${\mathrm{Xe}}^{+}$ ion impinging at ${83}^{\circ }$ on a graphene-covered SiC crystal as a function of the ion impact point $\xi$ in front of the step. Only C atoms originating from the graphene cover are sputtered.

Figure 3

Channeling probability of 3 keV ${\mathrm{Xe}}^{+}$ ions impinging at ${83}^{\circ }$ on a graphene-covered SiC crystal as a function of the ion impact point $\xi$ in front of the step.

Figure 4

Top and side view of a direct-hit event. Colors indicate height above the substrate. The projectile trajectory is indicated in silver.

Figure 5

Top and side view of an indirect-hit event. Colors indicate height above the substrate. The projectile trajectory is indicated in silver.

Figure 6

Top and side view of a channeling event. Colors indicate height above the substrate. The projectile trajectory is indicated in silver.

Figure 7

Top and side view of a hyperchanneling event. Colors indicate height above the substrate. The projectile trajectory is indicated in silver. The creation of two 5–7 defects has been marked.

Figure 8

Evolution of the projectile kinetic energy as function of the path length traveled. $r=0$ indicates the ion position immediately above the step edge. (a) Direct-hit and indirect-hit events shown in Figs. 4 and 5, respectively. (b) Channeling and hyperchanneled events shown in Figs. 6 and 7, respectively. The dashed line is a fit assuming a constant energy loss to $dE/dx=5.73$ eV/Å.

Figure 9

Average number of over- and undercoordinated defect atoms created by ion impact at a distance $\xi$ from the step.

Figure 10

Perspective view of the surface showing defect formation close to the step edge. Colors denote coordination numbers and differentiate $s{p}^1$-bonded (yellow) and $s{p}^3$-bonded (red) defect atoms from ordinary $s{p}^2$-bonded atoms (gray). Defect atoms are emphasized by showing them by larger balls.

Figure 11

Perspective and side view showing buckling of the upper graphene layer after impact. Colors denote coordination numbers as in Fig. 10.

Figure 12

Top view snapshots showing the emergence of a surface wave in the graphene top layer, taken at 0.15, 0.23, 0.37, 0.67, and 1.20 ps after ion impact. Colors denote height above the substrate. The white point indicates the projectile.