The embodiments described above provide mechanisms of forming contact structures with low resistance. A strained material stack with multiple sub-layers is used to lower the Schottky barrier height (SBH) of the conductive layers underneath the contact structures. The strained material stack includes a SiGe main layer, a graded SiG layer, a GeB layer, a Ge layer, and a SiGe top layer. The GeB layer moves the Schottky barrier to an interface between GeB and a metal germanide, which greatly reduces the Schottky barrier height (SBH). The lower SBH, the Ge in the SiGe top layer forms metal germanide and high B concentration in the GeB layer help to reduce the resistance of the conductive layers underneath the contact structures.