Introduction Oxidation of SiGe alloy has been a target of research for both fundamental and technological reasons, as in the case of Si oxidation. It has been expected as the fabrication process for the gate oxide of SiGe channel metal oxide semiconductor field effect transistors (MOSFETs), which allows higher carrier mobility than that of Si channel MOSFETs [1]. In recent years, it has also been employed in the fabrication of SiGe-oninsulator and Ge-on-insulator substrates [2]. These substrates are employed to realize high-speed MOSFETs with channel materials of strained Si, SiGe, and Ge. However, the oxidation mechanism of SiGe alloy is still open to question, due to the complexity caused by lattice strain and diffusion of Ge atoms. The most characteristic phenomenon of SiGe oxidation is that Ge atoms are ejected from the interface between the surface oxide and the SiGe layer without the incorporation of Ge atoms into the oxide layer. Ge atoms accumulate at the SiO2/SiGe interface and diffuse into the SiGe layer [1,2]. In the case of a strained SiGe layer epitaxially grown on a Si substrate, the effects of strain relaxation and defect generation during the oxidation process must be taken into account. It is important to investigate the atomic structure of the thermally oxidized layer, because it allows us to understand what really happens at the oxidized interface. In the case of the thermally oxidized layer of Si, the Si atoms in the oxide layer still maintain order, which originates from the diamond structure of the parent Si crystal, although the structure appears to be amorphous at a glance. This provides an actual picture of the oxidation process and explains the almost perfect properties obtained by electric measurements of the SiO2/Si interface. In this study, the residual order in the thermal oxide layer on a fully strained SiGe alloy was investigated.