Abstract
Optical and structural properties of Si nanocrystals (Si-nc) in silica films are described. For the SiOx (x < 2) films annealed above 1000 °C, the Raman signal of Si-nc and the absorption coefficient are proportional to the amount of elemental Si detected by X-ray photoelectron spectroscopy. A good agreement is found between the measured refractive index and the value estimated by using the effective-medium approximation. The extinction coefficient of elemental Si is found to be between the values of crystalline and amorphous Si. Thermal annealing increases the degree of Si crystallization; however, the crystallization and the Si–SiO2 phase separation are not complete after annealing at 1200 °C. The 1.5-eV PL quantum yield increases as the amount of elemental Si decreases; thus, this PL is probably not directly from Si-nc responsible for absorption and detected by Raman spectroscopy. Continuous-wave laser light can produce very high temperatures in the free-standing films, which changes their structural and optical properties. For relatively large laser spots, the center of the laser-annealed area is very transparent and consists of amorphous SiO2. Large Si-nc (up to ~300 nm in diameter) are observed in the ring around the central region. These Si-nc lead to high absorption and they are typically under compressive stress, which is connected with their formation from the liquid phase. By using strongly focused laser beams, the structural changes in the free-standing films can be made in submicron areas.
Highlights
Many limitations of modern electronic devices may be overcome by implementing photonics into electronics [1,2,3,4]
For 0.5-μm-thick Si/SiO2 SL (2-nm-thick Si and SiO2 layers), no Si clusters are detected in bright-field scanning transmission electron microscopy (TEM) (BF-STEM) images, and energy-dispersive X-ray spectroscopy (EDS) indicates an increase of x in the central region compared to the pristine film [141,142]
Si nanocrystals (Si-nc), SiOx (x < 2) films are annealed in a furnace at temperatures up to 1200 °C
Summary
Many limitations of modern electronic devices may be overcome by implementing photonics into electronics [1,2,3,4]. For a fixed Si content, the absorption coefficient of films prepared by magnetron sputtering and PECVD decreases with the increasing annealing temperature, which is due the amorphous-to-crystalline transition of Si in the samples [33]. Another method of measuring the optical properties of SiOx films is based on the PL-filtering effect. In agreement with the experimental observations, the theoretical simulations of isolated hydrogen-passivated Si nanostructures show that the band gap and the PL peak position change according to QC [13,19] The behavior of these parameters is different for oxidized Si-nc [99].
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