Sol-gel silica coatings for enhanced silicon emission: microstructural origins and optical implications

Silicon is an attractive platform for optoelectronic integration, but its indirect bandgap makes it a weak light emitter. Here, we demonstrate that sol-gel-derived silica (SiO₂) coatings, when thermally annealed, can significantly boost silicon bandgap photoluminescence (PL). Samples annealed at 900 °C exhibit a more than fourfold increase in emission intensity near 1160 nm compared to as-deposited films. High-resolution Transmission Electron Microscopy (TEM), combined with geometric phase analysis (GPA), revealed that as-deposited samples exhibit relatively uniform interfacial strain of less than ±0.1%, whereas annealing at 900 °C introduces local strain fluctuations of up to ±2.0%. These nanoscale strain variations correlate directly with the observed PL enhancement. Detailed analysis using high-resolution TEM (HRTEM) and scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM-EELS) reveals the reduction of annealing-induced defects in Si and the densification of the silica layer. These modifications alter the electronic states at the interface, as reflected in changes in the joint density of states, thereby enabling more efficient radiative recombination. Finite difference time domain (FDTD) simulations suggest that the enhanced PL signal near 1160 nm is partly attributable to annealing-induced morphological changes in silica. Together, these findings demonstrate that sol-gel-derived SiO₂/Si stacks provide a simple and low-cost route to enhance silicon emission through strain engineering, offering strong potential for integration into CMOS-compatible photonic systems and the development of future on-chip light sources and optical interconnects.