Photoluminescence features on the Raman spectra of quasistoichiometric SiC nanoparticles: Experimental and numerical simulations A. Kassiba, 1, * M. Makowska-Janusik, 1,2 and J. Boucle ´ 1,3 1 Laboratoire de Physique de l’Etat Condense ´ – UMR-CNRS N°6087, Faculte ´ des Sciences, Universite ´ du Maine, Avenue O.Messiaen, 72085 Le Mans, Cedex 09, France 2 Institute of Physics, Pedagogical University of Czestochowa Al.Armii Krajowej, PL42-201 Czestochowa-Poland J. F. Bardeau and A. Bulou Laboratoire de Physique de l’Etat Condense ´ – UMR-CNRS N°6087, Faculte ´ des Sciences, Universite ´ du Maine, Avenue O.Messiaen, 72085 Le Mans, Cedex 09, France N. Herlin-Boime 3 Laboratoire Francis Perrin-SPAM-DRECAM-CEA-Saclay, 91191 Gif/Yvette-France Received 24 April 2002; published 14 October 2002 Visible photoluminescence PL broad bands are observed in the Raman spectra of SiC nanoparticles np- SiC with diameters ranging from 10 to 25 nm. The phenomenon is studied versus the particle size, chemical composition, annealing, and oxidation treatments. In the case of quasistoichiometric np-SiC, excitation by 514-nm radiation gives rise to broad red PL emissions mainly enhanced by the amorphous fraction of the particles and by the surface chemical disorder induced by oxidation. The PL spectra are quantitatively analyzed using numerical methods based on cluster approaches. PL bands are calculated as a function of the cluster geometry and defects carbon and silicon vacancies, as well as the oxygen location within np-SiC sites. The relevance of this numerical analysis is discussed to account for the main features of the PL broad structure. The PL signature in SiC nanopowders can be used to monitor the physical organization of the np-SiC and to point out their amorphous structure fraction, surface states, and the defect contents. DOI: 10.1103/PhysRevB.66.155317 PACS numbers: 78.67.Bf, 78.55.-m, 71.35.Cc I. INTRODUCTION SiC thin films are currently used in a wide range of ap- plications such as high-power electronics and photovoltaic and optoelectronic devices. 1,2 Particularly, the search for a blue emitting device from bulk SiC was initiated two de- cades ago 3 and an improvement of the process was under- taken by the use of porous SiC. 4–7 In these nanoscopic or mesoscopic systems, beyond a higher photoluminescence PL efficiency than that in the case of bulk SiC, the PL spectra undergo large redshifts far from energies involved in interband transitions in crystalline SiC. These emissions are attributed to mechanisms mainly governed by defect states, as also shown in polycrystalline SiC or in amorphous a -SiC:H thin films giving rise to red-green luminescence at room temperature. 8 Recent works mainly focused on nano- sized systems such as individual nanoobjects 9 or those cre- ated in porous matrices. 6–10 At this nanoscale, quantum con- finement effects modify the electronic band structures, the vibronic states and the optical emission with respect to the bulk material. 11–13 The present work concerns the analysis of PL broad bands superimposed to the Raman signals from SiC nanoparticles np-SiC. The involved nanocrystallites nc-SiC exhibit a versatile character through the wide range of the electronic gap, modulated by crystalline polytypes, i.e., E g 2.2 eV for 3 C -SiC and 3.33 eV for 2 H -SiC), and by the nc-SiC nanos- cale size. 14 Moreover, a relaxation of chemical bonds, and the existence of defects required for the thermodynamic sta- bility of the particle, 15,16 are expected to generate PL signals. However, instead of well-defined PL bands, only broad fea- tures are observed in the SiC nanopowders. It is therefore necessary to clarify the exact origin of the PL in np-SiC with regard to the amorphous fraction and the defects within the nanoparticles. The second interest in these nanopowders lies in the fonc- tionalization of SiC-based nanocomposites such as polymer-nc-SiC. 17 Indeed, the nanocrystalline size modu- lates the band energy structures and the interface states. So, when associated with suitable matrices, promising potentiali- ties appear in nonlinear optic 17 as well as electro-optical properties which are under examination. 18 In these SiC-based composites, the interfaces play a key role in the main physi- cal responses and the challenge lies in the control of the particle surface states. So PL studies are able to monitor the physical organization of the nanoparticles including struc- tural features, surface states, and the involved defects. In this context, nc-SiC, with sizes ranging from 10 to 25 nm are investigated. The synthesis method is based on the CO 2 laser pyrolysis of gaseous silane and acetylene reactants. 19 The crystalline structure, composition, and np-SiC sizes can be modulated in a large extent by using appropriate reactant fluxes and laser power as well as post thermoannealing under argon or oxygen. The spectral studies are performed with confocal Raman spectrometer coupled with an argon-krypton laser for excitation. The 514-nm laser excitation is well adapted to probe the defect levels within the gap and also the emission from cubic SiC structure ( E g =2.2 eV). To account for the PL phenomena quantitatively, numerical methods based on cluster approaches are developed. They consist in PHYSICAL REVIEW B 66, 155317 2002 0163-1829/2002/6615/1553177/$20.00 ©2002 The American Physical Society 66 155317-1