How Can Si/Ge Core/Shell Nanowires Outperform Their Pure Material Counterparts?

Si/Ge core/shell nanowires have been demonstrated to be a promising candidate for the next-generation transistor channel. The tunable electron and hole transport paths improve their mobilities significantly. Here, we comparatively investigate the electronic properties of the Si-core-Ge-shell and Ge-core-Si-shell nanowires and their pure material counterparts using the first-principles method. The effects of core/shell ratio, atom number, and cross-sectional shape are studied. The Si-core-Ge-shell nanowires show excellent transport path separation and small carrier effective mass properties when the core/shell ratios exceed 0.5. For smaller ratios, the energies of the off-Γ valleys in the Ge shells are lowered and become the conduction band minimum (CBM) because of the intrinsic strains along the radial direction, enlarging the electron effective masses significantly. To restrain this effect, the uniaxial strain engineering is adopted and the best performance is exhibited when 2%-3% tensile strains are applied. In the Ge-core-Si-shell nanowires, both electrons and holes are easily confined to the core region. Compared with the Si-core-Ge-shell nanowires, their energy gaps and electron effective masses are smaller and larger, indicating poor transportability. The above discoveries are further validated by changing the cross-sectional shapes of the nanowires and device performance simulations, confirming that the Si-core-Ge-shell nanowires can outperform their pure material counterparts in high-speed transistor designs.