Abnormal hole mobility of biaxial strained Si

The strain effect on the hole mobility is investigated by bulk Si field-effect transistor, substrate-strained Si devices, and these devices under biaxial tensile mechanical strain. The hole mobility along 〈110〉 direction on (001) Si substrate degrades at small biaxial tensile strain (<∼0.3%) but enhances at the biaxial tensile strain larger than ∼0.3%. This abnormal behavior can be understood in terms of the effective hole conductive mass which is the population average of heavy-hole and light-hole masses. The effective mass is more heavy-hole-like at small strain, since the heavy-hole band has a larger density of state than light-hole band. As the biaxial tensile strain increases, the hole population in the light-hole band increases due to the upshift and crossover of the light-hole band above the heavy-hole band. Therefore, the effective mass with larger biaxial tensile strain decreases significantly due to the small mass of light hole. The effective hole mass, which increases at small strain, then decreases at large strain, is responsible to the abnormal hole mobility behavior.