Photoluminescence of Er-doped silicon nanoparticles from sputtered SiO x thin films Daniel Biggemann, Danilo Mustafa, Leandro R. Tessler * Instituto de Fı ´sica ‘‘Gleb Wataghin’’, UNICAMP, C.P. 6165, 13083-970 Campinas SP, Brazil Available online 9 November 2005 Abstract We present a study of the Er 3+ photoluminescence from Er-doped thin SiO x films prepared by reactive RF sputtering from a silicon target partially covered by metallic erbium platelets in an Ar + O 2 atmosphere. Annealing at 1250 °C induces the formation of silicon nanocrystals and modifies the Er 3+ luminescence spectrum due to changes in the Er 3+ environment. The photoluminescence efficiency decreases by two orders of magnitude with nanoparticle formation. This decrease may be due to less efficient energy transfer processes from the nanocrystals than from the amorphous matrix, to the formation of more centro-symmetric Er 3+ sites at the nanocrystal surfaces or to very different optimal erbium concentrations between amorphous and crystallized samples. Ó 2005 Elsevier B.V. All rights reserved. 1. Introduction Erbium-doped silicon nanocrystals have been inten- sively studied in the last few years. Characteristic intra-4f level Er 3+ luminescence due to the 4 I 13/2 ! 4 I 15/2 transition can be easily achieved in this class of materials. This tran- sition corresponds to an emission at 1.54 lm, the most widely used wavelength for photonic communications because it corresponds to the minimum loss in commercial optical fibers. Erbium excitation is thought to occur through coupling of an exciton in the silicon nanocrystals [1–4] with an excited state within the Er 3+ ion. Although this model has been widely accepted, recent experimental results have shown that different excitation processes that do not require nanocrystals may actually be more efficient [5,6]. In most of the published studies the silicon nanocrys- tals were obtained from the annealing of a silicon rich sub- oxide SiO x with x close to 1. Due to the complexity associ- ated with using an erbium precursor during the SiO x deposition by PECVD, erbium was normally introduced by ion implantation onto the already formed SiO x films. In these cases, thermal treatment is always required to recover the implantation damage even if the Si nanocrys- tals were already formed in the films. In the very few stud- ies where erbium was co-deposited by PECVD the samples were annealed first at low temperatures to effuse hydrogen (always present from the PECVD precursor gases) and then annealed at high temperatures to form the nanoparti- cles. Two papers reported that in fact the Er 3+ excitation efficiency was higher when silicon was in the form of amor- phous clusters rather than in nanocrystals [5,6]. In this paper we present results obtained from co-sputtered erbium-doped SiO x films with 0.8 < x < 1.2. We compared the Er 3+ photoluminescence efficiency in the as-deposited amorphous samples with the same samples annealed at 1250 °C, when silicon nanocrystals are formed. The results indicate that although there is efficient 1.54 lm lumines- cence in the annealed samples, its efficiency in the as-depos- ited samples is two orders of magnitude higher. This indicates that either the energy transfer process from the amorphous sub-oxide to the Er 3+ ions is more efficient than the nanocrystals exciton–Er 3+ coupling or the local envi- ronment of Er 3+ at the nanocrystals is more symmetric than in the amorphous film making the 4 I 13/2 ! 4 I 15/2 tran- sition less permitted. 0925-3467/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.09.036 * Corresponding author. Tel.: +55 19 37885380; fax: +55 19 37885376. E-mail address: tessler@ifi.unicamp.br (L.R. Tessler). www.elsevier.com/locate/optmat Optical Materials 28 (2006) 842–845