Modelling of hole mobilities in heavily doped strained SiGe

Abstract

Low-field hole mobilities have been calculated for heavily relaxed and strained doped SiGe alloy layers with Ge contents varying from 0 to 50%, using a novel semi-analytical bandstructure model which incorporates the effects of strain on the valence band of the alloy. We obtain poor results compared with experiment for mobilities in heavily doped Si, and attribute this to (i) a failure of the Born approximation at low carrier energies and (ii) the omission of additional effects associated with heavy doping and high carrier concentrations. For the strained doped and intrinsic alloy we observe that both the in-plane and out-of-plane hole drift mobilities increase with increasing Ge content relative to those for Si. These enhancements are due mainly to the effects of strain, and to a lesser extent due to alloying with Ge, but are offset by the presence of alloy scattering. Our results are sensitive to the details of the models used for scattering by ionized impurities; however, the large uncertainties and scatter of the experimental data preclude accurate estimates of the alloy potential. We find that an alloy potential of 2.0 eV gives an in-plane mobility consistent with experimental data for intrinsic material. Our calculations for heavily doped layers are affected by uncertainties in the value of the alloy potential and highlight a need for a better quantitative understanding of the scattering processes which are important in heavily doped alloy layers.