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
in the Netherlands (R. van Gastel, Universiteit Leiden, 011-3171-527-5700,
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
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
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).