X-Ray scattering at the periphery : forbidden bragg reflections and silicon nanowire arrays

In this work, we developed two separate unconventional X-ray scattering techniques that enable us to extract novel information from distinct systems at disparate length scales. We examine the formation of Si nanowire (SiNW) arrays during Ag metal-assisted chemical etching (MACE). The exceptionally rough surface of these nanowire arrays result in very low optical reflectivity, which also makes it extremely challenging to measure the X-ray specular reflection can be measured and utilized to obtain unique structural information about the composition profile of both Ag and Si during the formation of the SiNWs. Secondly, we have discovered a novel interference effect between the crystal surface and the bulk forbidden Bragg reflection that allows - for the first time - both the amplitude and phase of the forbidden reflection to be resolved. The newly attained amplitude and phase information permits one to extract the structure of the non-spherical electron density distribution from diamond crystal structures. We employed X-ray specular reflectivity, a technique conventionally used to study interfaces and surfaces, to examine the non-spherical electron density distribution of bulk Si. We have discovered and demonstrated that the structure of the covalent bond can be ascertained from the weak scattering between the Bragg reflections along the crystal truncation rod. This discovery significantly expands the experimental capability of examining the structure of electron densities, and especially the valence electrons in crystals. Using these novel and unconventional X-ray scattering techniques, we were able to extract valuable information about each system from exceptionally faint signals.