Measuring the Smallest Flaws

Semiconductors can be foiled by the tiniest flaws in the crystals from which they are made--but thanks to a new technique, researchers hope to ferret out so-called stacking faults with an accuracy down to a trillionth of a meter. Existing methods, which rely on conventional electron microscopes, are some ten times less precise. Yimei Zhu and his colleagues at Brookhaven National Laboratory pioneered the measuring method, which they describe in this weeks issue of Physical Review Letters.

Their secret weapon lies in using a class of electron microscope that has only been commercially available for some five years. Unlike the filaments in traditional electron microscopes, a sharp tungsten tip in the newer equipment generates a coherent beam of electrons. The researchers aimed this synchronized beam just in front of a known fault in a thin crystal of a high temperature superconductor, Bi₂Sr₂CaCu₂O₈. The beam cast a shadow image behind the crystal, including a large number of diffraction spots that looked like striped disks: each contained oscillating wave interference patterns, created as the electrons passed through the offset crystal from either side (see image).

They resolved the interference patterns out to the 31st disk from the center, and compared the data to computer simulations. In the end, they concluded that the space between crystal planes in one fault was 0.319 nanometers shorter than the normal spacing. To compare the technique to traditional electron microscopy, Zhu tested the same samples in the same machines. Coherent electron microscopes are still very expensive, but the scientists hope the method will help them better understand crystal interfaces and how they affect a material's properties.