The nanoscale
structural and compositional features of molecular beam epitaxy
(MBE)-grown
GaAs1−yBiy
films have been successfully characterized with unprecedented precision by
researchers of the Wisconsin MRSEC
using high-resolution x-ray diffractometry
and high-resolution high-angle annular dark field (HAADF
or “Z-contrast”) imaging in a scanning transmission electron microscope. The
spots on the left side of the paired bright spots in the image below are
identified as the Ga
columns and those on the right side the As columns in the
projection. The subtle yet quantifiable difference in column brightness that is
detectable by viewing the image is confirmed in intensity line scans obtained
along lines like those superimposed on the image. Contrary to expectations
based on the behavior of other alloy systems, the image suggests that at this
concentration the Bi solute is rather evenly distributed over the As sites,
rather than being strongly clustered over a few nearest neighbors. An
intriguing observation is the bright spot on the lower line scan where the
intensity of the Ga
column exceeds that of its neighbor As column. This reversal of intensity
across the pair and three additional pairs, revealed in quantitative analysis
of the measured intensities across the image, suggests that Bi can substitute
on Ga
sites as well as on As sites. This application of HAADF
to a highly immiscible alloy system yields unique information on the
atomic-scale structure of the material that is important to understanding its
optical and electronic properties and is unattainable by other means.
