Breaking News: Surface Crowdions

Wei Xiao, P. Alex Greaney, and D. C. Chrzan

Supported by the National Science Foundation under grant EEC-0085569

Many proposed processing routes for the self-assembled growth of nanostructured materials employ thin film growth techniques and periodically strained substrates. In order to develop reliable models of these growth processes, one must understand how strain influences adatom diffusion processes. To investigate this aspect of the process, we have studied the diffusion of Cu on Cu(001). Specifically, we have used the embedded atom method nudged elastic band method to identify saddle points and compute energy barriers for the two competing diffusion mechanisms: simple adatom hopping, and a mechanism

Surface of energy barriers to adatom hopping and kick-out and crowdion formation plotted as a function of strain.

Images show the adadtom, initially on the substrate surface, then in the metastable crowdion configuration, and finally the crowdion having re-emited an atom to the surface after moving 4 atomic spaces.

first identified by G. Kellogg and P. Feibelman at Sandia National Laboratories, referred to here as “kick-out.” In the kick-out diffusion process, the adatom “burrows” into the surface, and the displaced surface atom becomes the new adatom. In the course of our studies, we have identified a new, metastable adatom configuration: a surface crowdion. This crowdion (show on the left), is extremely mobile in the direction in which it is extended, and can lead to long ranged motion of the adatom. The crowdion behaves like a particle whose effective mass can be estimated from the atom displacements assosiated with the defect. Assuming the crowdion is born with room temperature kinetic energy, and estimating the effective mass to be 0.07 times the mass of a Cu atom, the crowdion would propagate at speeds of ~30 meters per second. This means the crowdion would move on the order of 100 inter-atomic spacings in a nanosecond. The motion of the crowdion can be be seen in the animations (bellow), where the crowdion is formed, moves 4 atomic spacings and then emits an atom to the surface. MPEG animation (3.5 MB): crowdion.mpg Quicktime animation (0.5 MB): crowdion.mov The figure above displays the energy barrier for simple adatom hopping and for the kick-out and crowdion adatom diffusion mechanisms. (The x and y directions are taken to be the [110] and [-110] directions of the crystal.) Tensile biaxial strains favor kickout, compressive biaxial strains favor simple hopping, and principle strains of opposite sign favor the formation of the surface crowdions. The factors driving the above trends have been identified, and we are in the process of exploring which other materials might also display this newly identified diffusion mechanism. Further, crowdion formation is certain to impact nucleation and growth processes mediated by surface diffusion. We are presently constructing Monte Carlo simulations to investigate these processes.