Introduction
The diffusion of atoms on a solid surface is a fundamental problem in surface science that has attracted attention over many years. Also from a technological point of view surface diffusion is a very important topic because it plays a key role in processes such as crystal growth and etching. The evolution of surface morphology depends on the detailed interplay of a variety of atomic-scale kinetic processes. Among them surface diffusion without any doubt plays a key role. A good understanding of surface diffusion will lead to better control over the growth conditions and tailor these to obtain atomically sharp interfaces, for instance. Imaging techniques that can achieve a resolution at the atomic scale, such as Field Ion Microscopy (FIM) and Scanning Tunneling Microscopy (STM), have provided us a remarkable look at the variety of ways in which atoms, clusters and molecules diffuse on surfaces. We show two recent results from our group.
Influence of dimer buckling on dimer diffusion1
Ge has a diamond crystal lattice. The Ge(001) surface minimizes the free energy by the formation of dimer bonds. Figure 1 shows a dimer resolved STM image of a clean and defect free Ge(001) surface. The inset of figure 1 shows a atomistic model of the Ge(001) surface dimers.
Figure 1.
Actually, dimers are observed in two apparently different forms, called symmetric and asymmetric dimers. An asymmetric dimer (e.g. in for B) has one atom that buckles out of the surface plane and one atom that buckles inwards. The dimers which appear symmetric in the STM image (row A) are in fact rapidly switching between two equivalent asymmetric geometries, in which the role of the out- and inwards buckled atoms is reversed.
We studied the influence of the dimer buckling of the substrate dimer rows on the diffusion of an ad-dimer over the substrate rows. In order to study the diffusion behaviour of an ad-dimer we collected images of the same area of the surface for more than 12 hours, while keeping the temperature stable at 298 K. The successive images are shown as a video in figure 2.
Figure 2.
As can be seen with the naked ey, the diffusion of an ad-dimer along an asymmetric (buckled) dimer row turns out to be significantly slower than along an seemingly symmetric substrate dimer row.
Dynamics and Energetics of Ge(001) dimers2
The dynamics of surface dimers can be studied by positioning the STM tip above the dimer and measure the tunneling current over time. Sato, Iwatsuki and Tochihara were the first who used this method to measure the flip-flop motion of dimers3. We used this method to study the dynamics and energetics Ge(001) dimers in symmetric and asymmetric dimers rows, respectively. Figure 3 shows an STM image of the studied surface.
Figure 3.
Two defects in a asymmetric and symmetric row respectively are highlighted dotted white circles. The dimer rows that contain the missing dimer defects have a noise appearance, indicative of the rapid flip-flopping motion of the dimers. Interestingly, the flickering is not only observed in the dimer row that appears symmetric, but also in the one that appears asymmetric. The inset shows a current versus time plot of the tunneling current at the indicated positions. The two level pattern shows clearly the flip-flop motion.
The flip-flop motion can be explained by the presence of phase defects in the antiferromagnetic dimer alignment (phasons). The dimer under the tip is flipped each time a phason makes an in-plane traversal of the tip surface junction. Figure 4 shows an animation of such a traveling phason in a dimer row.
Figure 4.
Moreover, the frequency of the flip-flop motion is used to probe the local strain fields near surface defects.
References
[1] J. Huijben, A. van Houselt, H.J.W. Zandvliet, B. Poelsema, Phys. Rev. B. 73 (2006), 073311
[2] A. van Houselt, R. van Gastel, B. Poelsema, H.J.W. Zandvliet, Phys. Rev. Lett. 97 (2006), 266104
[3] T. Sato, M. Iwatsuki, H. Tochihara, J. Electron. Microsc. 48 (1999), 1
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