Giant piezoresistance in silicon-germanium alloys

We use first-principles electronic structure methods to show that the piezoresistive strain gauge factor of single-crystalline bulk $n$-type silicon-germanium alloys at carefully controlled composition can reach values of $G=500$, three times larger than that of silicon, the most sensitive such material used in industry today. At cryogenic temperatures of 4 K we find gauge factors of $G=135000$, 13 times larger than that observed in Si whiskers. The improved piezoresistance is achieved by tuning the scattering of carriers between different ($\Delta$ and $L$) conduction band valleys by controlling the alloy composition and strain configuration.

Figure 1

Diamond lattice structure shown along the $\left[111\right]$, $\left[1\overline{1}0\right]$, and $\left[11\overline{2}\right]$ directions.

Figure 2

Room temperature piezoresistive strain gauge factor $G$ vs Ge composition $x$ and $\left[111\right]$ strain. (a) Strain applied in the $\left[111\right]$ direction, with current in the $\left[111\right]$ direction. (b) Cantilever-type stress, with strain applied in the $\left[111\right]$ direction (clamped in the $\left[1\overline{1}0\right]$ direction and relaxed in the $\left[11\overline{2}\right]$ direction), with current in the $\left[1\overline{1}0\right]$ direction.