The correlation between the structural (average size and density) and optoelectronic properties [band gap and photoluminescence (PL)] of Si nanocrystals embedded in ${\mathrm{SiO}}_2$ is among the essential factors in understanding their emission mechanism. This correlation has been difficult to establish in the past due to the lack of reliable methods for measuring the size distribution of nanocrystals from electron microscopy, mainly because of the insufficient contrast between Si and ${\mathrm{SiO}}_2.$ With this aim, we have recently developed a successful method for imaging Si nanocrystals in ${\mathrm{SiO}}_2$ matrices. This is done by using high-resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Then, by varying the time of annealing in a large time scale we have been able to track the nucleation, pure growth, and ripening stages of the nanocrystal population. The nucleation and pure growth stages are almost completed after a few minutes of annealing time at 1100 °C in ${\mathrm{N}}_2$ and afterward the ensemble undergoes an asymptotic ripening process. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing at 1100 °C. Forming gas postannealing considerably enhances the PL intensity but only for samples annealed previously in less time than that needed for PL saturation. The effects of forming gas are reversible and do not modify the spectral shape of the PL emission. The PL intensity shows at all times an inverse correlation with the amount of $P_b$ paramagnetic centers at the ${\mathrm{Si–SiO}}_2$ nanocrystal–matrix interfaces, which have been measured by electron spin resonance. Consequently, the $P_b$ centers or other centers associated with them are interfacial nonradiative channels for recombination and the emission yield largely depends on the interface passivation. We have correlated as well the average size of the nanocrystals with their optical band gap and PL emission energy. The band gap and emission energy shift to the blue as the nanocrystal size shrinks, in agreement with models based on quantum confinement. As a main result, we have found that the Stokes shift is independent of the average size of nanocrystals and has a constant value of 0.26±0.03 eV, which is almost twice the energy of the Si–O vibration. This finding suggests that among the possible channels for radiative recombination, the dominant one for Si nanocrystals embedded in ${\mathrm{SiO}}_2$ is a fundamental transition spatially located at the ${\mathrm{Si–SiO}}_2$ interface with the assistance of a local Si–O vibration.

The ability of surface passivation to enhance the photoluminescence (PL) emission of Si nanocrystals in ${\mathrm{SiO}}_2$ has been investigated. No significant increase of the average nanocrystal size has been detected for annealings at $\text{1100\hspace{0.05em}°}\mathrm{C}$ between 1 min and 16 h. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing time. Such behavior shows an inverse correlation with the amount of Si dangling bonds ${\mathrm{(P}}_b$ centers) at the interface between Si nanocrystals and the ${\mathrm{SiO}}_2$ matrix. A postannealing at $\text{450\hspace{0.05em}°}\mathrm{C}$ in forming gas enhances the PL intensity and lifetime, due to a reduction in $P_b$ concentration, without modifying the spectral shape of the PL emission.

The size distribution, band gap energy, and photoluminescence of silicon nanocrystals embedded in ${\mathrm{SiO}}_{\mathrm{2}}$ have been measured by direct and independent methods. The size distribution is measured by coupling high-resolution and conventional electron microscopy in special imaging conditions. The band gap is calculated from photoluminescence excitation measurements and agrees with theoretical predictions. Their correlation allows us to report the experimental Stokes shift between absorption and emission, which is $\text{0.26±0.03\hspace{0.05em}}\mathrm{eV},$ independent of average size. This is almost exactly twice the energy of the Si–O vibration (0.134 eV). These results suggest that the dominant emission is a fundamental transition spatially located at the ${\mathrm{Si–SiO}}_{\mathrm{2}}$ interface with the assistance of a local Si–O vibration.

Field effect induced luminescence has been achieved by alternate tunnel injection of electrons and holes into Si nanocrystals. The emitting device is a metal-oxide-semiconductor structure with a semitransparent polycrystalline Si contact $\sim 250\mathrm{nm}$ thick and a silicon-rich silicon oxide layer of about $40\mathrm{nm}$ deposited on a $p$ -type Si substrate by plasma-enhanced chemical vapor deposition. The electroluminescence is optimized for a Si excess of 17% and annealing at $1250°\mathrm{C}$ for $1\mathrm{h}$ in nitrogen-rich atmosphere. The pulsed emission presents typical decay times of $\sim 5\mu \mathrm{s}$ and external quantum efficiencies of $\sim 0.03\%$ .

We review in this paper our recent results on the correlation between the structural and the optoelectronic properties of Si nanocrystals (Si-nc) embedded in SiO₂. We describe as well the development of both materials and technology approaches that have allowed us to successfully produce efficient and reliable LEDs by using only CMOS processes. Si-nc were synthesised in SiO₂ by ion implantation plus annealing and display average diameters from 2.5 to 6 nm, as measured by electron microscopy. By varying the annealing time in a large scale we have been able to track the nucleation, pure growth and Ostwald ripening stages of the nanocrystal population. The most efficient structures have Si-ncs with average size of 3 nm and densities of about 10¹⁹ cm⁻³. We have estimated band-gap energies, lifetimes (20–200 μs) and absorption cross-sections (10⁻¹⁵–10⁻¹⁶ cm²) as a function of size and surface passivation. Based on these results, we propose a mechanism for exciton recombination based on the strong coupling of excitons with the heterointerfaces. From highly luminescent Si-nc, LEDs consisting of MOS capacitors were fabricated. Stable red electroluminescence has been obtained at room temperature and the I–V characteristics prove that the current is related to a pure tunnelling process. Fowler–Nordheim injection is not observed during light emission for electric fields below 5 MV/cm. Thus, hot carrier injection is avoided and efficient and reliable devices are obtained.