Multiple Time Scale Simulations of Metal Crystal Growth Reveal the Importance of Multiatom Surface Processes

A method for extending atomistic computer simulations of solids beyond the nanosecond time scale was used to simulate metal crystal growth on the time scale of laboratory experiments. Transitions involving concerted motion of multiple atoms on the crystal surface are found to lead to remarkably smooth growth of pure Al(100). Cu(100) is found to grow with a rougher surface, consistent with experiments. Not only is the activation energy of the multiatom Al processes surprisingly low, but vibrational entropy also favors processes where many atoms are displaced.

Figure 1

Multiatom processes seen in aluminum crystal growth simulations at 80 K include the four-atom concerted displacement process (a). The most commonly observed adatom descent process (b) also involves concerted displacement of several atoms. More than 20% of the activated processes involved three or more atoms such as the six atom concerted descent process shown in (c).

Figure 2

The upper panel shows the number of atoms in each deposited layer; the lower panel gives roughness [26].

Figure 3

Temperature dependence of the roughness of aluminum and copper surfaces after five layers have been deposited at ten layers per second. At low temperature the two metals have similar roughness, but as the surface temperature is increased and long time scale processes become active, copper becomes rougher while aluminum grows smoother. At the slower deposition rate of six layers per minute used in the experiments of Egelhoff and Jacob [23] the copper surface (★) grows smoother with more prevalent activated events.

Figure 4

Most surface transitions observed in simulations of aluminum growth at 80 K involve two or more atoms, while single atom processes are most common in Cu. The transition rate has a larger prefactor, i.e., more favorable entropy contribution, when more atoms are involved in the process (see filled circles and the vertical axis to the right).