Structural and luminescence properties of vapour‐etched porous silicon and related compounds

Abstract

Porous silicon (PS) is usually prepared by the electrochemical anodization or the stain etching techniques. Recently, a new method consisting of exposing silicon substrates to acid vapours issued from a mixture of HNO 3 /HF was employed to generate highly luminescent PS layers. The so-called HNO 3 /HF vapour etching (VE) technique can be easily applied in various large area of silicon-based devices. Depending on the HNO 3 /HF volume ratio, we found that VE silicon results in the formation of PS and/or a luminescent (NH 4 ) 2 SiF 6 powder-like phase. FTIR spectra of VE-based PS layers formed at HNO 3 /HF volume ratio ranging between 1/100-1/3 contain N-H and Si-F bonds related to NH 4 + and SiF 6 2-ions in addition to the conventional SiH x species. These nitride and fluoride groups were identified to be associated to the (NH 4 ) 2 SiF 6 powder-like phase which in turn contains small Si nanoparticles embedded in a SiO x matrix. The presence of such structures was explained as being the product of the VE technique itself. For vapours rich in HNO 3 (HNO 3 /HF volume ratio > 1/4), the VE method can produce almost only the luminescent (NH 4 ) 2 SiF 6 compound. The VE-based PS is essentially composed of dot-like Si particles with sizes not exceeding 5 nm and emitting a photoluminescence (PL) band around 1.93 eV. The PL band of the VE-based PS presents a shoulder at 2.09 eV, which becomes more significant after oxidation in air. This shoulder at 2.09 eV was attributed to an excitonic emission from the energy levels of the SiO x surrounding the smallest Si nanocrystallites. The PL band emission of the (NH 4 ) 2 SiF 6 powder presents two peaks. The first one was attributed to Si nanocrystallites emitting at 1.98 eV. The second peak could be associated to the smallest nanocrystallites (≤ 1.5 nm). For these crystallites, excitons are trapped on the SiO x energy levels, leading to a maximum PL band emission around 2.1 eV. This PL band seems to have the same origin than the small shoulder observed in the PL emission of PS.

Silicon Porosification: State of the Art

“…According to Saadoun et al 231 PSi was found to be a major phase above a HNO 3 volume ratio of 1/9, while the WP phase becomes major at HNO 3 /HF volume ratio ranging between 1/6 and 1/2. Similar results were obtained by Aouida et al 239 Besides, Saadoun et al 231 have found that the increase of etching time increases the contents of WP. At an early stage of formation, this powder is formed by very small particles, visible only by SEM.…”

Self-organization of ammonium silicon hexafluoride complex low-dimensional structures on Silicon

“…The intermediate reaction products are not measured but are based on assumptions and chemical balance equations [10,12]. 2) suggests that an interfacial oxide layer is produced in addition to the ASF layer and this is what was observed at the interface as evidenced from the FTIR through the presence of Si-O stretching band at 1100 cm −1 (Fig.…”

“…Recent advances for the production of Si based nanostructures from electroless electrochemical etching in general can be found in a review paper [9]. For the formation mechanism of the ASF, the SiF 4 and H 2 SiF 6 intermediate reaction products have been proposed [10]. Moreover, the possible release of oxygen has been investigated by Winkler technique and no sufficient amount of oxygen was found in the process of the ASF formation [11].…”

New type of dual macro and nano fractal structure of reaction induced vapor phase stain etched porous silicon

“…The NO 2 vapor, serving as a carrier gas for other solution components, emanates from the solution and provides etching of the silicon sample placed above the beaker with the reaction mixture for the formation of porous layer. The full chemical reactions are described elsewhere [10,12]. The exposure time varied from 20 sec to 3 min.…”