Atomic-resolution structure

Of thin films and interfaces has been directly determined using a new x-ray diffraction technique. Coherent Bragg rod analysis (COBRA) was developed by a multi-institutional team and has now been applied to a gadolinium oxide (Gd₂O₃) film grown on a gallium arsenide (GaAs) substrate. The diffraction patterns consist of coherent contributions from the substrate and the film. COBRA provides both the amplitude and the phase of the complex scattering factors, which in turn yield a three-dimensional image of the film and interface structure at subangstrom resolution. Shown here are (left) the eighth GaAs monolayer beneath the interface and (right) the ninth Gd₂O₃ monolayer above the interface. The physicists discovered that the first layers of Gd atoms are displaced in order to lock in to the positions of the GaAs. Moreover, the Gd₂O₃ layers do not stack up as in bulk Gd₂O₃, but rather continue to conform to the substrate. Yizhak Yacoby says that the COBRA technique is very general and can even be used on films made with large organic molecules. (Y. Yacoby et al., Nat. Mater. 1 , 99, 2002 http://dx.doi.org/10.1038/nmat735 .)