Experimental Study on Surface-Orientation/Strain Dependence of Phonon Confinement Effects and Band Structure Modulation in Two-Dimensional Si Layers

We have experimentally studied the surface orientation/strain effects on quantum mechanical confinement (QMC) in two-dimensional (2D) Si layers with thicknesses less than the Si lattice constant for future metal--oxide--semiconductor (MOS) devices. By UV--Raman spectroscopy, we have demonstrated that the quantum phonon confinement effects (PCEs) rapidly increase with decreasing 2D Si thickness $T_{\text{S}}$, but is almost independent of surface orientation and strain. Thus, electron saturation velocity of the 2D Si is degraded by the reduced phonon energy owing to the PCEs. On the other hand, photoluminescence (PL) emitted from the only (100)-surface 2D Si layers, depends on the excitation photon energy $h\nu$ ($2.33\leq h\nu\leq 3.81$ eV), and PL intensity increases with decreasing $T_{\text{S}}$. The PL data can be explained by simple PL models considering the electron/hole pair recombination mechanism. Consequently, it is necessary to reconstruct the device design for future Si devices, considering the $T_{\text{S}}$ dependence of the 2D Si properties.