Quantum size effects on the optical properties of nc-Si QDs embedded in an a-SiOx matrix synthesized by spontaneous plasma processing

Abstract

Quantum confinement effects on optical transitions in ensembles of nc-Si QDs in an a-SiO_x matrix has become evident by simultaneously considering the dielectric function dispersions obtained by optical modeling with spectroscopic ellipsometry, the absorption edge, and the photoluminescence peak. Diminution of the peak amplitude in the ε₂-spectra for reducing the diameter of nc-Si QDs could arise due to the disappearance of excitonic effects in the E₁ transition, while the peak broadening indicates an amplification of disorder in Si QDs. An energy blue shift happens to take place in an analogous fashion for all the characteristic parameters, upon decreasing the size of the nc-Si QDs for diameters in the range 6.5 < d < 2.0 nm. The band gap widening with the reduction of QD size is well supported by the first-principles calculations based on quantum confinement, while studies on the Stokes shift in the optical gap from the PL data could provide an understanding of the imperfect passivation of the surface defects on tiny nc-Si QDs. Low dimensional nc-Si QDs (∼2 nm in diameter) assembled in a large density (∼2.3 × 10¹² cm⁻²) embedded in an a-SiO_x matrix synthesized by spontaneous and low-temperature (300 °C) RF plasma processing, compatible to CMOS technology, are highly conducive for device applications. Systematic changes in composition and characteristics, including the thickness, of the individual sub-layers of the nc-Si QD thin films can be comprehensively pursued through a nondestructive process by ellipsometric simulation which could, thereby, enormously contribute to the precise optimization of the deposition parameters suitable for specific device fabrication e.g., all-silicon tandem solar cells and light emitting diodes, using silicon nanotechnology.