Role of ultra thin SiO_x layer for epitaxial growth of YSZ/SiO_x/(001) Si thin films

The role of an ultra thin SiO_x layer for the epitaxial growth of the YSZ (Y stabilized ZrO₂) thin film on (001) Si substrate through the SiO_x layer was investigated by high resolution transmission electron microscope (HRTEM) method, in-situ heating transmission electron microscope method and nano-beam diffraction methods. An ultra thin SiO_x layer 1-2 nm in thickness has the effects of inducing crystallization strain relaxation at the YSZ/Si interface; therefore an ultra thin YSZ layer in the very initial stage of the thin film growth could be epitaxially crystallized. The nano-beam (0.4 nm) diffraction analysis, focused on the SiO_x layer of the (001) epitaxial YSZ/SiO_x/Si thin film, showed a no complete halo pattern, but the diffraction spots convoluted into a halo pattern. The diffraction pattern was not so sharp than YSZ and Si, however there existed an epitaxial relationship the YSZ, SiO_x and Si layers. On the other hand, an amorphous-like phase contrast was observed by high coherent HRTEM imaging. This indicates that the ultra thin SiO_x layer formed just above the Si surface has a nano-crystalline structure below 1 nm order. Our results indicate that the SiO_x layer plays two important roles: (a) it enables strain relaxation in the crystallization process of YSZ layer with increasing the crystallinity of the YSZ layer; (b) it enables the transmission of an epitaxial relationship from the Si (semiconductor) toward the YSZ (oxide) layer.