Thermal annealing of Si/SiO2 materials: Modification of structural and photoluminescence emission properties

We comparatively study two conventional types of ${\mathrm{Si/SiO}}_2$ materials, ${\mathrm{Si/SiO}}_2$ superlattices (SLs) and Si-rich silica $({\mathrm{SiO}}_x)$ films, prepared with a molecular beam deposition method. Raman scattering, photoluminescence (PL), ultraviolet-visible-infrared absorption, and x-ray photoelectron spectroscopies are employed to characterize the samples. The results show clear parallelism in microstructure and emitting properties of ${\mathrm{Si/SiO}}_2$ SLs and ${\mathrm{SiO}}_x$ films. The as-grown material is amorphous, and disordered Si areas are seen in Raman spectra for samples with higher Si contents. Annealing at 1150 °C in nitrogen atmosphere leads to ordering of the Si grains and the typical crystalline size is estimated to be 3–4 nm. For all samples, an annealing-induced increase of PL at ∼1.6 eV is observed, and its resulting position is quite independent of the initial sample architecture. Furthermore, this PL is practically identical for continuous wave and pulsed excitation at 488 nm as well as for pulsed excitation at various wavelengths (266–488 nm), and the order of PL lifetimes is 1–10 μs. No correlation between the crystallite concentration and the PL intensity for the annealed samples is found, and the strongest PL was obtained for two samples with less defined crystallization. The origin of the annealing-induced 1.6 eV PL band is discussed.