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

Abstract

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 Si1-xGex buffer layers having a very smooth surface (∼5 nm). It was found that the mobility had a maximum around the channel width (Wch) of 7.5 nm and that it reached 13000 cm2/Vs at 20 K and 1175 cm2/Vs at room temperature (RT). The decrease in mobility with decreasing channel width was attributed to interface roughness scattering, since its influence increased as Wch 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 Wch=20 nm. By lowering the growth temperature of Ge channel layers to suppress the strain relaxation, the mobility of 1320 cm2/Vs at RT was achieved.