Influence of average size and interface passivation on the spectral emission of Si nanocrystals embedded in SiO2

Abstract

The correlation between the structural ͑average size and density͒ and optoelectronic properties ͓band gap and photoluminescence ͑PL͔͒ of Si nanocrystals embedded in 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 SiO 2 . With this aim, we have recently developed a successful method for imaging Si nanocrystals in 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 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 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 0.26Ϯ0.03 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 SiO 2 is a fundamental transition spatially located at the Si-SiO 2 interface with the assistance of a local Si-O vibration.

Stability of Ag nanocrystals synthesized by ultra-low energy ion implantation in SiO2 matrices

“…11,12 Annealing of the layers at low temperature (400 C) under N 2 allows a partial recovering of the implantation damage even if the total recovering of a stoichiometric SiO 2 needs higher temperature. 34 This annealing step is sufficient to avoid, or at the minimum, to retard the penetration of Orich complexes and therefore the oxidation process, without modifying the particle population. In addition, Ag 2 O is not stable and decomposes into Ag and oxygen at temperature T > 194 C. For this reason, there is no particle oxidation during this annealing and only matrix healing.…”

Role of vacancy-type defects in the formation of silicon nanocrystals

“…3͑b͒ reveal that the peak wavelength, an indication of the Si-nc size, does not change significantly beyond 2 min of annealing. Garrido Fernandez et al 20 . The apparent blueshift in the peak PL wavelength from 10 s to 2 min for the 20% excess silicon is due to the progressive increase in detector noise above 800 nm, which skews the peak PL wavelength to a higher value for very low intensity samples.…”

Optical and structural properties of silicon‐rich silicon oxide films: Comparison of ion implantation and molecular beam deposition methods

“…The light‐emitting centers have been suggested to contain non‐bridging oxygen (SiO) 12. Si‐nc in silica can be produced by annealing of silicon‐rich silicon oxide SiO x ( x < 2) prepared by plasma enhanced chemical vapor deposition (PECVD) 9, 13–15, magnetron sputtering 13, 14, molecular beam deposition (MBD) 10, 11, 16, 17, and Si‐ion implantation 4, 18. The optical and structural properties of SiO x films prepared by PECVD and magnetron sputtering have recently been compared 13, 14.…”