Role of defects in Si/SiO 2 quantum wells Elena Degoli * , Stefano Ossicini Istituto Nazionale per la Fisica della Materia INFM), Dipartimento di Fisica, Universita` di Modena e Reggio Emilia, Via Campi 213/A, Modena 41100, Italy Abstract The optical properties of Si=SiO 2 superlattices SLs) as a function of the Si layer thickness have been, for the ®rst time, theoretically investigated. Through ab initio calculations we consider fully passivated structures, the presence of O vacancy at the Si=SiO 2 interface or in the SiO 2 matrix. We ®nd that quantum con®ned states and O-related defect states play a key role in the experimentally observed visible luminescence in Si=SiO 2 con®ned systems. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Density functional calculations; Low dimensional systems; Oxygen vacancies; Electronic and optical properties; Quantum con®nement 1. Introduction Bulk crystalline silicon c-Si), by far the most important semiconductor material, does not ex- hibit good optoelectronic properties because of the indirect gap in the infrared region, and of the low probability of radiative electron±hole recombina- tion. Nevertheless the surprising discovery by Canham [1] of bright, visible light emission from porous silicon PS), and its attribution to quantum con®nement QC) ®rst, and the prediction [2] and observation [3] of ecient photoluminescence PL) in Si±CaF 2 quantum wells QW) and Si±SiO 2 superlattices SLs) [4] then has enticed many re- searchers to reactivate studies of the optical properties of silicon-based nanostructures. Here, we are interested, in particular, in the electronic and optical properties of Si/SiO 2 QW. It is an in- teresting fact that PS samples aged in air, in which Si/SiO 2 interfaces are formed, increase their PL properties. In the last years, a lot of experimental works have been done on the optical properties of Si=SiO 2 QW and SLs. Nevertheless the situation is still not clear. The PL spectra are today quite discussed. There are, essentially, three peaks of interest: one positioned at 1.65 eV, another one that moves from 1.4 to 1.9 eV changing the thickness of the Si layer and the last one posi- tioned at around 2.2 eV. Mulloni et al. [5], for example, have observed in more than 3 nm thick Si layers embedded in SiO 2 a weak PL band cen- tered at 1.9 eV with a shoulder at 2.3 eV: the ®rst band has been attributed to Si nanocrystals in the poly-Si ®lms and the second to O-related de- fects within the SiO 2 matrix. Moreover, in samples with Si thickness 6 2.5 nm they have found a new broad PL band at around 1.65 eV, that has been attributed to QC eects within the Si layer. Kanemitsu and Okamoto [6], indeed, have studied the PL properties of c-Si=SiO 2 single Q ®nding an asymmetric PL spectra that can be ®tted by two Optical Materials 17 2001) 95±98 www.elsevier.nl/locate/optmat * Corresponding author. Tel.: +39-59-2055297; fax: +39-59- 367488. E-mail address: degoli@unimo.it E. Degoli). 0925-3467/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0925-346701)00027-1