Role of Si1−xGex buffer layer on mobility enhancement in a strained-Si n-channel metal–oxide–semiconductor field-effect transistor

Strained-Si n-channel metal–oxide–semiconductor field-effect transistors (MOSFETs) were fabricated on molecular-beam epitaxially grown strained Si with various ${\mathrm{Si}}_{\text{1−x}}{\mathrm{Ge}}_x$ buffer layers. Effective electron mobility in n-MOSFETs with a ${\mathrm{Si}}_{\text{1−x}}{\mathrm{Ge}}_x$ $\text{(x=0.2,}$ 0.3) graded buffer layer was 60% higher than that in an unstrained-Si n-MOSFET. However, mobility of samples with a ${\mathrm{Si}}_{\text{1−x}}{\mathrm{Ge}}_x$ buffer layer on a low-temperature grown Si buffer layer was not increased as much as that of samples on a graded buffer layer. Atomic-force microscopic observation suggests that the power spectrum of surface roughness of the strained-Si layer varies according to the buffer layer, and this variation may affect the enhancement of mobility.