Quantum Size Effects on Dielectric Constants and Optical Absorption of Ultrathin Silicon Films

The size dependence of the dielectric constants and optical absorption for silicon nanostructured films are investigated using density-functional theory. A critical thickness of 4.3 nm is observed for Si (100)-oriented thin films. Within this critical thickness, the dielectric function and optical absorption show remarkable size dependence, and a large reduction of static dielectric constant (from 10.8 to 4.4) is observed. This is in contrast to the weak dependence of dielectric constant on film thickness in silicon dioxide thin films. The pronounced dependence and large critical thickness demonstrate a quantum-confinement effect on optical properties, which is of great importance to nanophotonics.