Fig. 1. Picture of RHEED-TRAXS Chamber in Interface Engineering Lab.
Thin film heterostructures of functional oxides (e.g. magnetostrictive, ferroelectric) integrated on semiconductor platforms can enable tuned coupling between the oxide layers for the development of next-generation multifunctional electronic devices. Multi-element oxides exhibit measurably different functional properties with structure changes or stoichiometry changes of less than one percent. The goal of fabricating multifunctional heterostructures by Molecular Beam Epitaxy (MBE) is to take advantage of this sensitivity to tune properties by controlling the stoichiometry and structure of each layer and the interfaces.
While the use of Reflection High Energy Electron Diffraction (RHEED) is well established for real-time structure monitoring of thin films in MBE, there is a need for real time stoichiometry monitoring and control when growing these mulit-element oxides where the same structure could still mean a different stoichiometry. RHEED - TRAXS can potentially provide real-time monitoring of elemental composition and thus assist the study of growth mechanisms and achieve better control of the MBE process.
Incident RHEED electrons in the range of 12-20 keV interact with the sample surface to emitt x-ray photons characteristic of the surface atoms. The x-ray intensity is proportional to the composition and thickness of the film, and exhibits a strong angular dependence at near critical-angle conditions. By using a geometry that measures these x-rays at the total reflection angle, RHEED-TRAXS was reported to probe the top 2 to 3 nm of material, thus probing the surface chemistry of growing layers.