Khomenkova et al. Nanoscale Research Letters 2011, 6:172 http://www.nanoscalereslett.com/content/6/1/172 content or pure ones can be used for low-temperature deposition approach. Considering the above results, different types of the structures, such as HfO2/SRSO/HfO2(SiO2) and HfSiO/ SRSO/HfSiO, were fabricated and their electrical prop- erties were studied versus annealing treatment. Since SRSO single layers with embedded Si-ncs are considered as charge storage layers, they will be analyzed prior to describe the properties of the trilayer structures. SRSO single layers The most common method to form Si-ncs entails the deposition of a thick SRSO monolayer, in which the for- mation of Si-ncs occurs due to phase separation on Si and SiO2 stimulated by high-temperature annealing. The observation of the bright photoluminescence (PL) emis- sion in the visible spectral range is the evidence of the presence of Si-ncs. Unfortunately, the size distribution of Si-ncs in composite SRSO layers is usually broad. Thus, for the fine control of Si-ncs size the multilayer (ML) approach, where SRSO layers are alternated by SiO2 ones, can be applied. In this case, the control of Si-ncs occurs by means of precise thickness for SRSO layer [11,15]. In the present study, the [SRSO/SiO2] MLs were grown with the aim of obtaining optimal con- ditions for Si-ncs formation, which can be applied in future memory structures. Each ML contained 20 [SRSO/SiO2] periods. For all the stacks, the thickness of SiO2 layer was 3 nm, whereas the thickness of SRSO layer varied from 2 to 6 nm for different MLs. It is known that the high-temperature annealing at about 1,100°C is used to form Si-ncs required for optoe- lectronic application [9,11,23]. Grown MLs were Page 4 of 8 annealed at 1,100°C for 60 min in nitrogen flow and were analyzed by means of XRD and PL methods to determine the formation and evolution of Si-ncs. XRD patterns taken in grazing geometry revealed the appear- ance of the Si-related (111) XRD peak at about 28° to 29° that confirmed the formation of Si-ncs inside the layers (Figure 2). As evident from the inset of Figure 2, the samples exhibit strong PL emission, which further confirms the formation of Si-ncs. The brightest emission was observed for the MLs with the 2-nm thickness of SRSO layer. The increase of the thickness of SRSO layer, leading to the increase of Si-ncs average size, results in the shift of PL peak position to the higher wavelength side (inset of Figure 2). It is worth to note that pure HfO2 material does not conserve amorphous structure upon an annealing at high temperatures (900°C to 1,100°C). Such treatment results in the formation of monoclinic HfO2 phase in the single layers [21]. The appearance of grain bound- aries can significantly degrade electrical properties of the structures. Thus, this dictates the elaboration of the lower thermal budget conditions for the formation of Si-ncs. In this regard, grown SRSO/SiO2 MLs were also annealed at relatively lower temperatures (800°C to 950° C) for 10 to 15 min. For all the cases, PL emission was obtained. However, the brightest light emission was found for [2-nm-SRSO/SiO2]20 ML structure (Figure 2b). Its PL spectrum is narrower than that observed for [6-nm-SRSO/SiO2]20 ML. It is obvious that the phase separation can occur easily for thinner SRSO layers due to smaller Si diffusion path, and this can explain the narrower PL band for [2-nm-SRSO/SiO2]20 ML, confirming the narrower size distribution of Si-ncs. Figure 2 XRD patterns and PL spectra of SRSO/SiO2 multilayers. (a) GI-XRD patterns measured for [2-nm-SRSO/SiO2]20 and [3-nm-SRSO/ SiO2]20 multistacks annealed at 1,100°C for 60 min. Inset, PL spectra of the same MLs. The thickness of SRSO layer for each ML is mentioned in the figure. (b) PL properties of the [2-nm-SRSO/SiO2]20 ML versus annealing temperature; annealing time is 15 min.