Oxygen role on the optoelectronic properties of silicon nanodots Marcello Luppi a, *, Stefano Ossicini b a INFM-S 3 , Dipartimento di Fisica, Universita ` di Modena e Reggio Emilia, Via Campi 213/A, I-42100 Modena, Italy b INFM-S 3 , DISMI, Universita ` di Modena e Reggio Emilia, Via Allegri 13, I-42100 Reggio Emilia, Italy Abstract The optoelectronic properties of Si nanodots have been investigated using ab initio total energy calculations within the density functional theory. Structural relaxations have been considered. We have studied two types of nanodots: isolated clusters covered by H, studying the substitution of Si à /H bonds with different Si à /O bonds, and nanocrystals embedded in SiO 2 matrix. In the first case, we find that the optoelectronic properties strongly depend on the type and the number of Si à /O bonds, especially for the gap value and the arrangement of the energy levels. In the second case, the close interplay between chemical and structural effects is pointed out. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Silicon nanostructures; Density functional theory; Electronic and optical properties 1. Introduction Semiconductor nanodots (ND) show quite different properties from those of their bulk counterparts. For example, when Si is reduced to nanometer dimensions, as for porous Si (PSi), it exhibits visible photolumines- cence (PL) [1,2]. Moreover, for Si nanocrystals (NC) embedded in SiO 2 optical gain has been demonstrated [3], thus opening a way for the use of Si for photonics that would enable the full integration of optical and electronic technologies. From a theoretical point of view, physical properties calculations for ND are very helpful in understanding size-related effects. Most of the works are devoted to H-covered Si ND and to the oxidation effects to understand the differences in PL between freshly etched and aged PSi [2]. Different schemes, within density functional theory (DFT), have been recently adopted for the discussion of the excited states [4 /6]. Here, we present ab initio total energy (GO) calculations for Si ND; we discuss not only the effects on the optoelectronic properties of the substitution of Si à /H bonds with different types and numbers of Si Ã/O bonds, but also the structural and optoelectronic properties of Si NC in SiO 2 . 2. Methods GO ab initio calculations on isolated Si Ã/H Ã/O ND (Section 3) are based on DFT in the local-density approximation (LDA) using the FHI98md code [7]. The electron  /ion interaction is described via norm- conserving pseudopotentials in the Kleinman  /Bylander (KB) form [8] with Martin  /Trouillier rules [9]. The cut- off energy is set at 680 eV. Because 3D periodic boundary conditions are employed to ensure a good isolation, a large cubic supercell with a side length of 1.614 nm is built up. GO calculations on Si NC in SiO 2 matrix (Section 4) are based on the CASTEP code [10]. Our choice for treating exchange and correlation is the GGA-PBE approach [11]. Ultrasoft pseudopotential, [12] in a separable KB form is used. The energy cut-off is set at 265 eV. The optical properties are obtained within LDA. Only direct transitions have been considered. In both cases, all the atoms are allowed to relax until the residual forces are less than 0.05 eV A ˚ 1 . 3. Si à /H à /O ND: structural and optoelectronic results The starting structure, to study different types of Si Ã/ H à /O ND, is a small cluster (Si 14 H 20 ) of 14 Si atoms (dimensions: 0.37 /0.57 /0.67 nm 3 ) with a crystalline T d interstitial (T d -i) structure and covered by 20 H * Corresponding author. Tel.:  /39-059-2055323; fax:  /39-059- 2055235. E-mail address: luppim@unimo.it (M. Luppi). Materials Science and Engineering B101 (2003) 34  /38 www.elsevier.com/locate/mseb 0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00700-6