Atomic Slide Puzzles

Vacancies in crystal surfaces are holes where atoms are missing from otherwise regular and uniform crystal lattices. Scientists have suspected for some time that vacancies are responsible for motion in crystals as the holes trade places with atoms, leading to atom-sized bubbles that percolate across crystal facets.

Now, researchers in the Netherlands (R. van Gastel, Universiteit Leiden, 011-3171-527-5700, gastel@phys.leidenuniv.nl) have managed to measure vacancy motion in a copper crystal, and they found that the holes are surprisingly mobile. The discovery has important implications for the semiconductor industry and technologies that rely on tiny surface structures that may be gradually destroyed through vacancy mediated motion.

The researchers used a scanning tunneling microscope (STM) to study vacancy motion by monitoring the positions of Indium atoms embedded in a copper lattice. Because vacancies move rapidly, changing places with atoms roughly a hundred million times each second at room temperature, comparatively slow STMs cannot image vacancies directly. Instead, the researchers calculated vacancy motion by tracking the positions of the indium atoms. From one image to another, indium atoms exhibited long jumps which result from multiple vacancy interactions. Essentially, the indium atoms move across the copper crystal in much the same way that individual pieces may be maneuvered from one place to another in toys known as slide puzzles (see image at Physics News Graphics).

Although high vacancy mobility may be bad news for manufacturers of microstructures, the new insights will potentially help to optimize crystal growing procedures vital to the semiconductor industry. In future work, the researchers plan to create vacancies artificially by selectively removing atoms from a chilled crystal surface. Provided that crystal is sufficiently cooled, the vacancies should move slowly enough to show up in STM images. (R. van Gastel et al., Physical Review Letters, 19 February 2001; text at Physics News Select).