Piezoresistance of silicon and strained Si0.9Ge0.1

We present experimentally obtained results of the piezoresistive effect in p-type silicon and strained Si0.9Ge0.1. Today, strained Si1−xGex is used for high speed electronic devices. This paper investigates if this area of use can be expanded to also cover piezoresistive micro electro mechanical systems (MEMS) devices. The measurements are performed on microfabricated test chips where resistors are defined in layers grown by molecular beam epitaxy on (0 0 1) silicon substrates. A uniaxial stress along the [1 1 0] direction is applied to the chip, with the use of a four point bending fixture. The investigation covers materials with doping levels of NA = 1018 cm−3 and NA = 1019 cm−3, respectively. The results show that the π66 piezoresistive coefficient in strained Si0.9Ge0.1 is approximately 30% larger than the comparable π44 piezoresistive coefficient in silicon at a doping level of NA = 1018 cm−3. Thus, strained Si0.9Ge0.1 holds promise for use in high sensitivity MEMS devices.