Semiconductor device and method of manufacturing the same

[0073](First Embodiment)

[0074]FIGS. 3A-3C are cross-sectional views of a semiconductor substrate for explaining a method of manufacturing a semiconductor substrate according to a first embodiment of the present invention.

[0075]In the first place, as shown in FIG. 3A, an Si oxide film 2 is formed on an Si substrate (base substrate) 1. The Si oxide film 2 is formed by any method widely employed, e.g., a thermal oxide film such as a dry oxide film or a wet oxide film, a CVD (Chemical Vapor Deposition) oxide film, or a wet oxide film formed by treating with a solution.

[0076]Subsequently, as shown in FIG. 3B, an SiGe thin film 4 is formed on another Si substrate 21. To improve the performance, the Ge content of the SiGe thin film 4 near the Si substrate 21 is set at a value larger than 30 atomic % and the SiGe thin film 4 is basically undoped with impurities. If the Ge content is larger than 30 atomic %, the mobility of electrons in the strained Si layer can be increased to the maximum. The

[0097](Second Embodiment)

[0098]FIGS. 4A-4C are cross-sectional views showing a method of manufacturing a semiconductor substrate according to a second embodiment of the present invention. In this embodiment, after an epitaxial Si layer 6 is formed on the Si substrate 21, an SiGe layer 4 is stacked, and further an Si oxide film 9 is formed on the SiGe layer 4. This is used as one of the two substrates to be bonded.

[0099]In the first place, as shown in FIG. 4A, an Si oxide film 2 is formed on an Si substrate 1 in the same manner as in the first embodiment. Subsequently, as shown in FIG. 4B, an epitaxial Si layer 6 serving as an element formation layer (device layer) is formed on a CZ Si substrate 21. Then, an SiGe thin film 4 is formed on the epitaxial Si layer 6 in the same manner as in the first embodiment. The Ge content of the SiGe layer 4 near the epitaxial Si layer 6 is larger than 30 atomic %. Thereafter, an Si oxide film 9 is formed on the SiGe thin film 4.

[0100]Subsequently, two

[0107](Third Embodiment)

[0108]FIGS. 5A-5C are cross-sectional views of a semiconductor substrate showing a method of manufacturing a semiconductor substrate according to a third embodiment of the present invention.

[0109]The third embodiment is characterized in that the content of a SiGe layer 7 varies in the thickness direction. More specifically, as shown in FIG. 6, a crystal is grown in such a manner that the Ge concentration of the SiGe thin film 7 is gradually increased from a portion near the Si substrate 1 to a portion near the strained Si layer 8. More specifically, the Ge content of the SiGe layer 7 is controlled such that a Ge concentration near the Si substrate 1 falls within the range of 0 to 30 atomic % and the Ge concentration near the strained Si layer 8 is larger than 30 atomic %.

[0110]By controlling the content of SiGe layer 7, the dislocation produced from the interface between the Si oxide film 2 and the SiGe layer 7 grows so as to form a loop in the SiGe layer 7. The