Physics Letters A 372 (2008) 1508–1511 www.elsevier.com/locate/pla
Structural and luminescent properties of silicon nanoparticles incorporated into zirconia matrix J. Klangsin a , O. Marty a,∗ , J. Munguía a , V. Lysenko a , A. Vorobey a , M. Pitaval a , A. Céreyon b , A. Pillonnet b , B. Champagnon b a Institut des Nanotechnologies de Lyon (INL), UMR-5270 CNRS-UCBL-INSA-ECL, Bâtiment Léon Brillouin, Université Claude Bernard-Lyon 1,
43 Bd du 11 Novembre 1918, 69622 Villeurbanne cedex, France b Laboratoire de Physico-Chimie des Matériaux Luminescents, CNRS-UMR 5620, Bâtiment Kastler, Université Claude Bernard-Lyon 1,
43 Bd du 11 Novembre 1918, 69622 Villeurbanne cedex, France Received 15 September 2007; received in revised form 2 October 2007; accepted 3 October 2007 Available online 9 October 2007 Communicated by V.M. Agranovich
Abstract An impact of mechanical stresses on structural and photoluminescent properties of Si nanoparticles (NPs) incorporated into the zirconia thin films is reported. The stresses are found to be responsible for important structural modifications of the NPs. The zirconia matrix doped with the NPs exhibits bright red photoluminescence (PL) at room temperature due to efficient quantum confinement of the photogenerated carriers in the nanoscale Si particles. Spectral position of the PL picks depends on: mean dimension of the NPs, their concentration, stress induced deformation and order degree of the near-surface region. In general, zirconia matrix appears as a robust and reliable host media for Si NPs. © 2007 Elsevier B.V. All rights reserved. Keywords: Silicon nanoparticles; Zirconia; Transmission electron microscopy; Raman spectroscopy; Photoluminescence
1. Introduction Fabrication of the efficient silicon-based luminescent materials is an important challenge because it will allow combination of silicon integrated circuits technologies with optoelectronic applications. However, due to its indirect energy band gap, bulk silicon exhibits a very poor radiative efficiency and produces light outside the visible range. Different approaches have been investigated in order to form low dimensional silicon nanostructures that exhibit new physical properties, and particularly, an enhanced rate of electron–hole radiative recombination leading to visible light emission [1]. Various silicon nanostructures, such as: porous silicon [2] and nanoparticles [3] have been reported to display strong room-temperature luminescence in the visible spectral region. For different electronic applications [4] as well as to limit aging effects [5], Si nanoparticles (NPs) are * Corresponding author.
E-mail address:
[email protected] (O. Marty). 0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.10.008
often incorporated inside a dielectric thin layer matrix. However, an influence of the host matrix on the structural and photoluminescent properties of the Si NPs incorporated inside the matrix is not yet studied in details. For example, a recent paper of Santana et al. [6] reported about the influence of the host matrix containing oxygen atoms on PL spectra of the incorporated Si NPs. In particular, according to the authors, the main mechanisms responsible for blue-shift and intensity increase of the PL spectra are: (i) quantum confinement of photogenerated charge carriers and (ii) chemical passivation of the NP surface states. In this Letter we describe an important impact of mechanical stresses ensured by a host matrix on structural and PL properties of the incorporated Si NPs. In particular, stress induced structural modifications of the Si NPs incorporated into zirconia (ZrO2 ) matrix at different concentrations are reported. A direct correlation between structural state of the nanoparticles inside the zirconia matrix and their PL properties at room temperature was established. Zirconia films prepared by sol–gel technique were chosen in our work as a new host matrix for the Si NPs (in
J. Klangsin et al. / Physics Letters A 372 (2008) 1508–1511
1509
contrast to already used and relatively known SiOX [7], SiX NY [6] and polymer matrixes [8]) because it is an interesting candidate for replacing SiO2 in nanoscale electronic devices [9]. Indeed, zirconia has a high dielectric constant and its band gap is still large in comparison to the band gap of silicon. 2. Experiment First of all, to fabricate Si NPs, highly porous Si was formed by electrochemical etching of (100)-oriented moderately doped by boron (3–10 cm) silicon wafer in a mixture of hydrofluoric acid and ethanol in a 1 : 1 volume ratio with a current density of 54 mA/cm2 . Once PS was formed, it was dried in air and then removed from the etched Si wafer. After that, mechanical grinding of the PS nanostructures was performed in a planetary ball milling machine RETSCH-PM100 at rotation velocity of 300 rpm during 3 minutes to decompose the porous nanostructures down to homogeneous nanopowder constituted by the separated elementary NPs. The obtained PS powder was then dispersed in ethanol with different concentrations (3, 9 and 15 g/l) and the formed solutions were treated in ultrasonic bath for 5 minutes to ensure the best homogeneous dispersion of the NPs in the ethanol solutions. The sol of zirconia was prepared by mixing of zirconium n-propoxide [Zr(OC3 H7 )4 ] with isopropanol [10]. Acetic acid was added to prevent moisture induced precipitation of ZrO2 and to maintain a stable sol. A resulting yellow transparent sol was mixed with silicon nanopowder at following concentration values: 1, 3 and 5 g/l. Then, these mixtures were dried in a recipient at 90 ◦ C during 3 days. After the drying, the solvents were eliminated and the solid ZrO2 matrix with the Si NPs trapped inside was formed. At the end, the solid matrix containing the NPs was grinded in the planetary ball milling machine at rotation velocity of 300 rpm during 10 minutes to obtain a homogeneous powder. The resulting fine powder of ZrO2 doped with Si NPs was again annealed at 200 ◦ C for 1 hour to ensure complete evaporation of all solvents. 3. Experimental results and discussion A Transmission Electron Microscopy (TEM) picture given in Fig. 1(a) shows Si NPs obtained after mechanical grinding of the PS nanostructures before their incorporation into the zirconia matrix. Numerous separated quasi-spherical NPs with dimensions
