Tensile strained silicon (Si) enhances electron mobility by lifting the conduction band degeneracies, reducing carrier scattering and increasing the population of carriers in sub-bands with lower transverse effective mass. A strain relaxed buffer (SRB) is an important element when fabricating strained channel CMOS transistors. As an example, a SiGe SRB can be used when growing on a Si substrate a tensile strained Si channel for nFET devices and compressively strained Ge or high Ge percentage SiGe for pFET devices. Typically a thick (e.g., about one micron) SRB layer is needed to ensure a low defect density in a channel region of the nFET and pFET devices.
However, SiGe has a lower thermal conductivity than Si. For example, the thermal conductivity of Si1-xGex (x=0.50) is only about 10% of the thermal conductivity of pure Si. The presence of the relatively thick SiGe SRB, in conjunction with the low thermal conductivity exhibited by SiGe, results in a poor heat dissipation capability of the SRB layer that in turn can negatively impact the on-chip power budget due to difficulties in power dissipation.
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