First-Principles Mobility Calculations and Atomic-Scale Interface Roughness in Nanoscale Structures

Calculations of mobilities have so far been carried out using approximate methods that suppress atomic-scale detail. Such approaches break down in nanoscale structures. Here we report the development of a method to calculate mobilities using atomic-scale models of the structures and density functional theory at various levels of sophistication and accuracy. The method is used to calculate the effect of atomic-scale roughness on electron mobilities in ultrathin double-gate silicon-on-insulator structures. The results elucidate the origin of the significant reduction in mobility observed in ultrathin structures at low electron densities.

Figure 1

Supercell containing a 5 Å-thick UTSOI silicon channel with ideal, abrupt $\mathrm{Si}\mathrm{\text{−}}{\mathrm{SiO}}_2$ interfaces.

Figure 2

Schematics of an $\mathrm{Si}\mathrm{\text{−}}{\mathrm{SiO}}_2$ interface showing elementary interface roughness defects: suboxide bond (left) and oxygen protrusion (right).

Figure 3

Plot showing plane-averaged scattering potentials for oxygen protrusions (top) and suboxide bonds (middle), along with carrier density (bottom). The $z$ axis is perpendicular to the $\mathrm{Si}\mathrm{\text{−}}{\mathrm{SiO}}_2$ interface. Black circles mark the defect centers.

Figure 4

Calculated electron mobilities due to scattering from suboxide bonds and oxygen protrusions for channel thicknesses of 10, 15, and 20 Å at 300 K.

Figure 5

Plane-averaged wave function at the bottom of the conduction band in a 10 Å-thick UTSOI channel. The conduction electron density in the channel is $5.6\times {10}^{11}{\mathrm{e}}^{-}/{\mathrm{cm}}^2$. The $z$ axis is perpendicular to the $\mathrm{Si}\mathrm{\text{−}}{\mathrm{SiO}}_2$ interface. A black circle marks the center of the oxygen protrusion.