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Strained silicon is exactly what the name implies: silicon that is elastically deformed, as a rubber band is elastically deformed when it is extended. Strained silicon can be strained compressively (i.e., the atoms are being squeezed together) or tensilely (i.e., the atoms are being stretched apart). The most established and proven method of straining silicon involves the deposition of a silicon-germanium (SiGe) thin film on top of a traditional silicon wafer, which then acts as an atomic template for deposition of a subsequent thin film of silicon. The thin film of silicon conforms to the atomic spacing of the underlying SiGe layer, which has a larger atomic spacing, and assumes a state of biaxial tension (i.e., it is being pulled in stretched apart in two orthogonal directions). A schematic of how the crystalline structure of the strained silicon thin film is distorted is shown in Figure 1.
Although it is not readily apparent that the mechanical deformation of silicon would have any positive impact on the electrical properties of silicon, there are indeed benefits to straining silicon. Specifically, the distortion of the crystalline structure changes the properties of the charge carriers in silicon, allowing the carriers to move more quickly in response to an applied voltage. These charge carriers, electrons (negative charge carriers) and holes (positive charge carriers), are said to have a higher mobility when silicon is strained. The higher the Ge content in the SiGe alloy, the greater the strain and the corresponding increase in the mobility of electrons and holes. AmberWave's strained silicon solution targets a Ge content between 15-20%, where the exact target specification is determined based on customer requirements.