Interfacial and microstructural evolution of Ge/C multilayers with different period thicknesses

Liquid gallium (Ga) X-ray source holds the potential to replace traditional copper (Cu) source, owing to their high brightness and long lifespan. To effectively reflect Ga's Kα line, the use of multilayers is essential. Around the Ga-Kα line, germanium/carbon (Ge/C) multilayer has the ideal theoretical reflectivity. In this study, a systematic investigation is carried out on the interfacial and microstructural evolution of Ge/C multilayers. A series of multilayers with period thicknesses ranging from 2.31 nm to 10.39 nm are fabricated via magnetron sputtering. Combining the grazing incidence X-ray reflectivity (GIXR), X-ray scattering (XRS), X-ray diffraction (XRD), and transmission electron microscopy (TEM) analyses, it is observed that as the period thickness increases, the Ge layer transits from an amorphous state to a quasi-crystalline state. Concomitantly, interfacial diffusion decreases while interface roughness increases. Atomic force microscopy (AFM) revealed that the surface roughness of these multilayers fluctuates slightly from 0.10 nm to 0.13 nm. This result indicates that the microstructural changes at the interface have a minimal impact on the surface roughness of the Ge/C combination. Such small surface roughness is beneficial for the efficient reflection of liquid Ga X-ray source. • Ge/C multilayers have the excellent theoretical optical performance at Ga-Kα line. • The interfacial microstructure of Ge/C multilayers changed due to the crystalline state of Ge changed. • The microstructure change has little impact on surface roughness in Ge/C multilayer systems.