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 <110> 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.
