Ion Beam Induced Conversion of Si-Based Polymers and Gels Layers into Ceramics Coatings

Transcription

1 Journal of Sol-Gel Science and Technology 26, , 2003 c 2003 Kluwer Academic Publishers. Manufactured in The Netherlands. Ion Beam Induced Conversion of Si-Based Polymers and Gels Layers into Ceramics Coatings J.C. PIVIN Centre de Spectrométrie Nucléaire et de Spectrométrie de Masse, Bâtiment 108, Orsay Campus, France P. COLOMBO Università di Bologna, Dipartimento di Chimica Applicata e Scienza dei Materiali, viale Risorgimento 2, Bologna, Italy A. MARTUCCI Università di Padova, Departimento di Ingegneria Meccanica, Settore Materiali, via Marzolo 9, Padova, Italy G.D. SORARÙ Università di Trento, Dipartimento di Ingegneria dei Materiali, via Mesiano 77, Trento, Italy E. PIPPEL Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D Halle, Germany M. SENDOVA-VASSILEVA Central Laboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria Abstract. The transformations induced by ion irradiation in films of polycarbosilane and gels derived from tetraethoxysilane, methyltriethoxysilane and triethoxysilane have been studied. Hydrogen is released selectively under the effect of electronic excitations at a rate depending on the probability of H radicals to combine for forming molecules inside the film. The radiolysis efficiency of C H bonds is the lowest and that of Si H bonds the highest. Beside the conversion of the polymeric structure into an amorphous ceramic, ion irradiation induces the precipitation of C or Si clusters, according to the precursor, leading to a noticeable luminescence in the visible. This precipitation has been put into evidence for the first time by energy filtered electron microscopy. Keywords: oxy-carbides, ion irradiation luminescence 1. Introduction Ion irradiation effects in organic polymers are well documented and interesting for the treatment of films. In comparison to a thermal processing or to electron and UV irradiation ion-induced ionizations provide a higher efficiency in breaking and cross-linking of bonds. Ion-irradiation of Si-based polymers and alkoxide-derived siloxanes has been undertaken more recently with the purpose of demonstrating that this processing of thin films offers several advantages with respect to the thermal route [1, 2]. The most

2 252 Pivin et al. characteristic features of the conversion process into ceramics by ion irradiation are the selective release of H atoms, and the fact that films undergo more compaction and hardening than when heat treated and that they remain adherent to their substrate and show no cracking. The segregation of part of the C atoms into diamond-like clusters during this conversion is responsible for the hardening. These clusters exhibit also an intense luminescence when their size and volume density is limited, so that electrons are confined in quantum dots and capable of exhibiting radiative transitions. The purpose of the present paper is to study in more detail the process of H release and precipitation of C or other particles in some precursors selected for their different types of H bonding configurations. The studied materials are three hydroxide gels derived from (i) tetraethoxysilane (TEOS) constituted only of Si O chains and OH side groups, (ii) methyltriethoxysilane (MTES) with organic CH 3 side groups, (iii) triethoxysilane (TH) with Si H bonds and liable to give Si clusters. A polycarbosilane (PCS), containing the same methyl side groups as MTES and some Si H bonds (when there is no branching of chains), is also studied. The interest of getting more insight in the physical mechanism of H evolution is that this mechanism is still debated in other fully organic polymers [3 6]. The yield of H release affects all other properties, especially the size and density of C, Si clusters (or M clusters in sol-gel precursors mixed in solution with metallic salts) and their electronic properties. 2. Experimental Procedure Gel films derived from TEOS, MTES and TH were prepared from ethanolic solutions of the precursors with a dilution of 1 / 2, hydrolyzed by addition of 0.01 mole HCl and 1.5 mole H 2 O per mole of alkoxide, while polycarbosilane (PCS, Dow Corning X9-6348) was dissolved in HPLC hexane. The products of the alkoxides hydrolysis (herafter referred to as polysiloxanes, with same abbreviations as their precursors, for simplifying terms) and PCS were spun at 1500 to 3000 rpm on silicon susbtrates for obtaining films with a thickness of 400 to 800 nm. The study of pristine films by Rutherford Backscattering Spectrometry (RBS), Nuclear Reactions Analysis (NRA), Elastic Recoil Detection Analysis (ERDA) and FTIR indicates that their network is noticeably condensed: SiC 1.95 O 0.05 H 4.5 (PCS), SiO 2 C 0.1 H 1.5 (TEOS), SiC 1.0 O 1.9 H 3.9 (MTES), SiO 1.40 C 0.05 H 1.0 (TH). Figure 1. (Top) kinetics of H release in the various polymers under He irradiation and fit of the initial loss with a simple exponential law ρ = ρ 0 exp( σ S e ); (bottom) Comparison of the kinetics of release in TH under irradiation in the most different conditions with the law proposed by Adel et al. [6]. The films were irradiated with ions of increasing masses (He, C, Si, Cu, Au) at the same energy of 3 MeV, with a current density limited to 0.1 µa/cm 2 and a sample holder cooled with water in order to prevent target heating. In the case of the 3 MeV He irradiation the beam used for ERDA analyses provided for H depletion also. The H concentration, ρ 0, in pristine films was measured accurately by adding spectra

3 Ion Beam Induced Conversion of Si-Based Polymers 253 recorded from about 20 different areas with very low He doses (0.05 µc/30 mm 2 ), because the initial decrease of the concentration, ρ, is very steep. Changes in the bonding configuration of C and Si atoms were assessed by Raman spectroscopy, using a DILOR X-Y microspectrometer and with scanning of the laser source of excitation (514.5 nm Ar line) over 75 µm, in order to avoid heating of the films. For the photoluminescence (PL) experiments, a double SPEX 1403 monochromator and an EMI 9863B photomultiplier were used. The PL was excited wih the 488 nm line of an Ar + ion laser with power density below 1 W/cm 2 on the sample. Microstructure and nanochemical investigations were carried out using a high resolution Philips CM 20 FEG field emission electron microscope, run at 200 kv and equipped with a Gatan Imaging Filter (GIF 200). Besides electron energy loss spectroscopy at an energy resolution of 0.8 to 1 ev, this filter enables the imaging of a specific element with a spatial resolution better than 1nm (energy filtered electron microscopy, EFTEM). 3. Results and Discussion 3.1. H Evolution It has first to be noted that NRA and RBS analyses show that the concentration of Si, O and C atoms changes less than 5% after irradiation with large ion fluences. The kinetics of H evolution from the polymers irradiated in situ by He ions in alternation with ERDA analyses is displayed in Fig. 1. The H concentration is plotted versus the electronic stopping power, S e, multiplied by the ion fluence, (number of ions/cm 2 ), because the release of H in polymers under high energy ion irradi- ation is driven mainly by the ionizations. S e increases from 30 to 300 ev/10 15 atoms/ion/cm 2 with the mass of ions from 4 (He) to 197 (Au) in the studied materials and the nuclear stopping power S n (energy transferred in atomic collisions) varies from 0.1 to 300 ev/10 15 atoms/ion/cm 2. In spite of numerous studies over the past 30 years of the release of hydrogen in organic polymers under ion irradiation, there is still no consensus on the precise mechanism. The most simple model is based on the hypothesis that the rate of H release is determined by the probability of breaking bonds between H and other atoms X and that this rate depends only on the concentration, ρ i, and strength of each type of X H bonds in the material. As many exponential functions of the ion fluence,, as the number of H sites can be found successively in the kinetics of H evolution, ρ( ) [3]: ρ = ρ i exp( σ i S e ), in which σ i is the radiolytic (breaking) efficiency for a type of bonds i (in ev 1 ) and σ i S e the cross section (cm 2 ). In a second type of model, ions are supposed to produce a complete release of H within the track core and the law would thus be a single exponential function, exp( σ S e ), if the size of this core was well defined [4]. In fact the cross section σ S e changes when tracks overlap [5]. The last type of model is a two step process in which two H radicals are produced in a track and must combine for diffusing as molecular H 2 towards the surface; in this case the cross section σ r S e is the product of the probability to form two radicals by the probability for these radicals to be within a given recombination distance [6]. The concentration ρ varies as: 1 ρ = 1 ( ) exp( σ r S e ) ρ f ρ 0 ρ f Figure 2. EFTEM images of a MTES film irradiated with ions/cm 2. (See text for details).

4 254 Pivin et al. wherer ρ f is the minimum concentration reached at saturation of the irradiation effect (when all H radicals are apart by more than the threshold distance) and ρ 0 the concentration in pristine films. As shown by Fig. 1 the decrease of ρ is not a simple exponential function of S e for TEOS containing a single type of H site nor a sum of exponentials for the other polymers. The decay of ρ can be fitted by an exponential only in a limited range of fluences. A better agreement is found with the model involving the recombination of H radicals (bottom of Fig. 1). The initial slope σ as well as σ r values found for irradiations with the various ion species indicate that Si H and O H bonds are much more easily broken than C H ones. The radiolytic efficiencies measured for MTES are exactly the average of those found for TEOS and PCS. The difference between TEOS, MTES and PCS diminishes when they are irradiated with heavy ions) because of an increase of the size of tracks and recombination volumes Precipitation of C and Si Clusters Two-dimensional distributions of C (using the C-K edge at 284 ev) and Si atoms (using the Si-L 23 edge at 99 ev) are shown on the EFTEM images of Fig. 2 for MTES irradiated with 3 MeV Au ions at a fluence of ions/cm 2. The images clearly evidence that part of the C atoms segregates into clusters. The observed degree of segregation is already noticeable for a fluence of Au (S e = 15 ev/atom) and increases with the fluence. A similar segregation has been observed in films irradiated by ions producing only ionizations [7]. Raman spectra of the same MTES specimens are displayed in Fig. 3(a). All the Stokes Raman peaks of C in these films are centred at cm 1 like those of amorphous C coatings with a noticeable degree of sp 3 hybridization. Whatever the ion species used for the irradiation and the fluence, they are never split into two components at 1350 and 1580 cm 1 like in the spectra of evaporated C, pyrolyzed polymers and other types of turbostratic graphite. The Raman spectrum of MTES irradiated with Au ions/cm 2 is a weak bump on a continuous background of luminescence, which has been subtracted in the figure. At this fluence the intensity of the photoluminescence peak, centred at 2.1 ev, is close to its maximum value, reached for a fluence of ions/cm 2 (Fig. 3(b)). The same trend is observed for all polysiloxanes and polycarbosilanes: the PL goes through a maximum for a deposited energy Figure 3. (a) Raman spectra of MTES films irradiated with 3 MeV Au ions at the indicated fluences compared to those of defected diamond and graphite, (b) PL spectra of the same films. of the order of ev/atom then vanishes when the concentration of free C exceeds a threshold (of 15% free C per SiO 2 molecule according to measurements on mixtures of TEOS with MTES and various silicone resins [8]). In the case of TH films, irradiation induces a precipitation of Si clusters according to Raman spectrometry (Fig. 4(a)) and PL measurements (Fig. 4(b)). Their Raman peak is shifted by 40 cm 1 with respect to that of well crystallized clusters of same size in understoichiometric silicon oxide films annealed at 1000 C,

5 Ion Beam Induced Conversion of Si-Based Polymers 255 clusters with different sizes [9] the size of the clusters is estimated to be close to 3 nm. 4. Conclusion The evolution of hydrogen from Si-based polymers or gels depends on the nature of bonds exchanged by H atoms and also on the probability for H radicals to combine into molecules before leaving the surface. The radiation resistance of C H bonds is much stronger than that of hydroxyl groups. Energy filtered electron microscopy permitted to obtain a direct evidence of the formation of C clusters in this class of polymers, confirming our interpretation of the high hardness of the obtained ceramics (twice that of silica and comparable to that of amorphous SiC [1, 2]) and of their yellow green luminescence. Ceramic films derived from TH could however be more interesting for applications in opto-electronics because materials free of C are preferred by electronic engineers and Si clusters are studied much more. Acknowledgments Thanks are due to G. Sagon of LADIR-CNRS who recorded the Raman spectra and to D. Nesheva from ISSP, Bulgarian Academy of Sciences, who deposited the Si:O understoichiometric film. References Figure 4. (a) Raman spectrum of a TH film irradiated with 3 MeV Au ions at the indicated fluences and an evaporated film of understoichiometric silicon oxide annealed at 1000 C (containing well crystallized Si clusters with a diameter of 3 4 nm according to HRTEM images, (b) PL spectra of TH films irradiated with 3 MeV Au ions or annealed 1 h at 1000 C (intensity divided by 10 for the annealed film). because they are amorphous. The intensity of the luminescence does not decrease at high fluence under the effect of the growth of Si clusters and is even higher in films converted by thermal treatment, contrary to the case of C clusters. According to calculations of the efficiency of radiative transition and energy of excitons in 1. J.C. Pivin and P. Colombo, J. Mat. Sci. 32, 6163 and 6175 (1997). 2. J.C. Pivin, P. Colombo, and G.D. Soraru, J. Am. Ceram. Soc. 83, 713 (2000). 3. D Boutard, B.M.U. Scherzer, and W. Möller, J. Appl. Phys. 65, 3833 (1989). 4. A. Davenas, X.L. Xu, C. Khoidi, M. Treilleux, and G. Steffan, Nucl. Instr. and Meth. B 7/8, 513 (1985). 5. V.K. Mittal, S. Lotha, and D.K Avasthi, Rad. Eff. Def. Solids 147, 199 (1996). 6. M.E. Adel, O. Amir, R. Kalish, and L.C. Feldman, J. Appl. Phys. 66, 3248 (1989). 7. J.C. Pivin, E. Pippel, J. Woltersdorf, D.K. Avasthi, and Sanjev Kumar, Zeit. Metallkunde 92/7, 712 (2001). 8. J.C. Pivin, M. Sendova-Vassileva, P. Colombo, and A. Martucci, Mat. Sci. and Engineer. B 69, 574 (2000). 9. C. Delerue, M. Lannoo, and G. Allan, Phys. Rev. Lett. 76, 3039 (1996).