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 HNO3/HF was employed to generate highly luminescent PS layers. The so-called HNO3/HF vapour etching (VE) technique can be easily applied in various large area of silicon-based devices. Depending on the HNO3/HF volume ratio, we found that VE silicon results in the formation of PS and/or a luminescent (NH4)2SiF6 powder-like phase. FTIR spectra of VE-based PS layers formed at HNO3/HF volume ratio ranging between 1/100-1/3 contain N-H and Si-F bonds related to NH4+ and SiF62– ions in addition to the conventional SiHx species. These nitride and fluoride groups were identified to be associated to the (NH4)2SiF6 powder-like phase which in turn contains small Si nanoparticles embedded in a SiOx matrix. The presence of such structures was explained as being the product of the VE technique itself. For vapours rich in HNO3 (HNO3/HF volume ratio >1/4), the VE method can produce almost only the luminescent (NH4)2SiF6 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 SiOx surrounding the smallest Si nanocrystallites. The PL band emission of the (NH4)2SiF6 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 SiOx 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. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)