Spectroscopic ellipsometry characterization of ultrathin silicon-on-insulator films

Spectroscopic ellipsometry is used to characterize the optical response of a series of ultrathin silicon-on-insulator (SOI) films with thickness ranging from 50to2nm. Due to the potential barrier formed by the buried oxide layer and the interface with air, the electrons in the top silicon layer of the thinner samples experience dimensional confinement effects. Transition energies were extracted for the E1 and E2 critical points using reciprocal space analysis. This method offers improved accuracy over conventional regression-based techniques, such as line shape fitting, by eliminating error due to correlation among multiple fit parameters. It is shown that dimensional confinement increases the E1 transition energy of ∼3.375eV for bulk silicon to 3.38, 3.393, 3.42, and 3.45eV for the ∼10, ∼7, ∼4, and ∼2nm SOI films, respectively. This trend resembles the 1∕L2 law predicted by quantum mechanical considerations for a confined particle. The dependence of these critical point transitions is investigated as a function of film thickness L. Additionally, the E2 critical point transition energy is found to be much less affected by quantum confinement, contrary with theoretical predictions.