Microstructural Transformation of Nanoscale Be Layers in the Mo/Be and Be/Mo Periodic Multilayer Mirrors Investigated by Raman Spectroscopy

Raman scattering studies were carried out for the investigation of bonding and microstructural properties of dimensionally confined nanoscale Be layers in the periodic Mo/Be and Be/Mo multilayer structures. The amorphous phase of alternate periodic Be nanolayers was partially transformed into the crystalline phase at the critical thickness of 2.44 nm. Crystallinity was further improved with the increase in the thickness of the periodic Be layers, which was associated with the decline of the surface-to-volume ratio of Be-Be bonding. However, upon thermal annealing, the bond-order defects in the crystalline Be phase gradually increased due to thermal stretching of Be-Be bonding. Beyond the critical annealing temperature of 573 K, a complete transformation of crystalline to ultrashort-ranged amorphous Be phase was observed in the multilayer structures. At critical thicknesses, the transformation of amorphous Mo and Be nanolayers into the polycrystalline structure was confirmed by the combination of X-ray diffraction and Raman scattering studies, respectively.