Mountain-Climbing Atoms

Atoms that are deposited on crystal surfaces, through a method known as molecular beam epitaxy, often form surfaces covered with numerous small mounds rather than smooth layers, if the substrate temperature is sufficiently low. For higher temperatures, an atom near the top of a mound can often move about and diffuse down toward the crystal surface. Conventional wisdom holds that upward diffusion, on the other hand, is essentially negligible. Recently, however, a collaboration of researchers at the INFM-Università di Genova in Italy, the Chinese Academy of Sciences, and Oak Ridge National Laboratory has found that deposited atoms may sometimes diffuse upward spontaneously, forming faceted mountains that tower over the surrounding crystal plane. Although the formation of faceted nanocrystals has been observed before, these were generally thought to be due to a mismatch between a crystal substrate and the crystal structure of the deposited atoms (for example, when germanium atoms are deposited on silicon, differences in the spacing of the two types of crystals lead to a strain that encourages the growth of large, hut-shaped crystals). The new research, by contrast, reveals for the first time that the crystalline mountains (see image) can form even when the deposited atoms and the substrate crystal consist of the same element, and no strain energy is involved. Specifically, aluminum atoms deposited on an aluminum crystal substrate may diffuse upward into crystal structures that rise upward as much as ten times higher than the thickness of the surrounding planes.

Computer simulations seem to indicate that the growth may be caused by processes thought to be insignificant in previous deposition studies. In particular, an atom sitting at the inner corner near the base of a crystal protrusion may jump out of place and onto the crystal facet, or a pair of atoms can conspire to exchange positions as they leapfrog up a crystal slope. The counterintuitive formation of tall nanocrystals via upward diffusion of aluminum atoms only occurs within a temperature window of about 330 K to 500 K, when the total crystal surface coverage exceeds critical values of about 10 or more deposited layers, depending on the specific temperature. The researchers (Francesco Buatier de Mongeot, +39-10-3536324, and Zhenyu Zhang) predict that the often neglected processes leading to upward atom diffusion are likely to be important for other crystals grown via molecular beam epitaxy, leading to much richer dynamics in the growth of thin films than previously suspected. (F. Buatier de Mongeot et al., Physical Review Letters, upcoming article, probably 4 July)