Effects of increased compressive strain on hole effective mass and scattering mechanisms in strained Ge channels

Effects of increased compressive strain on the hole effective mass in the strained Ge channel structures was systematically studied. The effective mass was found to increase very sensitively with the hole density, and hence accurate strain dependence was evaluated by means of extrapolation of experimentally obtained mass values to a certain hole density (2.0 × 1012 cm-2). As a result, it was demonstrated that the effective mass monotonically decreased from 0.195 down to 0.150 m0 with the increase in the compressive strain from 0.8% up to 2.8%, indicating that introduction of the larger strain into the Ge channel can enhance the hole mobility further. We also studied effects of the increased strain on dominant scattering mechanisms through evaluation of Dingle ratios. It was found that with increasing strain in the Ge channel, the interface roughness scattering became more significant in addition to the impurity scattering. We revealed that the roughness at the top Ge/SiGe heterointerface brought by the increased strain is responsible for the scattering and that this roughness scattering can be effectively suppressed by inserting a modulation-doping layer underneath the channel layer.