Channel Width Dependence of Mobility in Ge Channel Modulation-Doped Structures

We systematically studied channel width dependence of mobility in Ge channel modulation- doped structures fabricated by solid-source molecular beam epitaxy using the low-temperature buffer technique. This technique made it possible to obtain high-quality strain-relaxed Si 1- x Ge x buffer layers having a very smooth surface (∼5 nm). It was found that the mobility had a maximum around the channel width ( W ch ) of 7.5 nm and that it reached 13000 cm 2 /Vs at 20 K and 1175 cm 2 /Vs at room temperature (RT). The decrease in mobility with decreasing channel width was attributed to interface roughness scattering, since its influence increased as W ch decreased. On the other hand, the decrease in mobility for wider channels was considered to come from strain relaxation of Ge channel layers. In fact, high-resolution X-ray diffraction measurements revealed that strain relaxation of Ge channel layers occurred in the sample with W ch =20 nm. By lowering the growth temperature of Ge channel layers to suppress the strain relaxation, the mobility of 1320 cm 2 /Vs at RT was achieved.