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Effect of carbon implantation on visible luminescence and composition of Si-implanted SiO2 layers
Surface & Coatings Technology 203 (2009) 2658–2663
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effect of carbon implantation on visible luminescence and composition of Si-implanted SiO2 layers D.I. Tetelbaum a,b,⁎, A.N. Mikhaylov a,b, V.K. Vasiliev a,b, A.I. Belov b, A.I. Kovalev a, D.L. Wainstein a, Yu.A. Mendeleva b, T.G. Finstad c, S. Foss c, Y. Golan d, A. Osherov d a
Surface Phenomena Research Group, 9/23 2nd Baumanskaya street, CNIICHERMET, 105005, Moscow, Russia Physico-Technical Research Institute, University of Nizhny Novgorod, Gagarin prospect 23/3, 603950, Nizhny Novgorod, Russia Department of Physics, University of Oslo, N-0316, Blindern, Norway d Department of Materials Engineering and the Ilse Katz Institute for Nanoscience and Nanotechnology, Ben-Gurion University of the Negev, 84105, Beer-Sheva, Israel b c
a r t i c l e
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Available online 4 March 2009 PACS: 61.46.Bc 61.46.Hk 85.40.Ry 78.55.-m 79.20.Uv 82.80.Pv 87.64.Lg 87.64.Hd Keywords: Silicon nanocrystals Carbon-rich clusters Silicon dioxide Ion implantation Photoluminescence Composition
a b s t r a c t Ion implantation is a convenient tool for synthesis of silicon nanocrystals in SiO2 matrix which exhibit strong red/near-IR luminescence at room temperature. Ion beams can be successfully used also for controllable modification of the nanostructures by introducing defects or changing their phase composition. In the present work, both possibilities are tested by implantation of carbon into thermal SiO2 films either containing pre-fabricated Si nanocrystals or as-implanted with Si. In the carbon-free SiO2 films, Si nanocrystals were synthesized at typical Si excess of about 10 at.% and annealing temperatures of 1000 and 1100 °C. It is shown that carbon ion irradiation of the Si-implanted and then annealed SiO2 films completely quenches a luminescence band at 650–850 nm related to Si nanocrystals and gives rise to emission in the range of 350– 650 nm that originated from oxygen-deficient defects. The subsequent final thermal treatment reduces concentration of the matrix defects, but only partially recovers photoluminescence from Si nanocrystals depending on the C dose. The latter is explained by the hindering effect of implanted carbon on growth and crystallization of Si phase inclusions. Strong “white” emission in the spectral range of 350–800 nm is observed only for the equal Si and C concentrations, irrespective of the sequence of carbon introduction and Si nanocrystal formation. This broad spectral band consists of the three sub-bands at around 400, 500 and 620 nm attributed to inclusions of SiC, C and Si phases, respectively. Electron spectroscopy analysis of the annealed layers confirms the presence of silicon carbide phase and carbon precipitates showing the diamond-like sp3, not the graphitic sp2 hybridization. The Si- and C-rich nanoclusters with a size of 3–5 nm were identified in the co-implanted films by electron microscopy as amorphous nanoparticles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction One of the new applications of ion implantation is the ion-beam synthesis of light-emitting nanocrystals (quantum dots) in wide-band (dielectric) matrices, transparent in visible and near-infrared spectral ranges. This approach presents considerable actuality for optoelectronics owing to prospective of such systems in fabrication of lightemitting diodes, lasers, planar optical amplifiers and waveguides, and also for nanoelectronics — in fabrication of nonvolatile memory devices with distributed charge storage in ultrathin gate dielectric of MOS-transistor. In particular, this would permit to include silicon, as indirect-bandgap semiconductor, into a family of light-emitting materials. Great efforts have been already devoted to ion-beam formation and investigation of Si nanocrystals in SiO2 matrix, which possess luminescence in red and near-IR spectral regions due to a ⁎ Corresponding author. 23/3 Gagarin avenue, 603950 Nizhny Novgorod, Russia. Tel.: +7 831 4656914; fax: +7 831 4659366. E-mail address: [email protected] (D.I. Tetelbaum). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.087
quantum-size effect [1,2]. At the same time, it would be of practical importance to develop the Si-based nanostructured systems with the luminescence covering the whole visible range of spectrum. One of the principal approaches could be the synthesis of nanoinclusions of silicon carbide [3], which has such advantages as wide bandgap (2.3–3.3 eV depending on polytype [4]), high melting point, high thermal conductivity and chemical stability. These distinctive properties of SiC make it a technologically important material for electronic devices especially operating at high values of temperature, power, and frequency [5]. The method of co-implantation of Si+ and С+ in SiO2 successfully employed in several works [6–10] provides fabrication of layers with intense photo- and electroluminescence almost in the whole visible domain, tentatively ascribed to nanoclusters of Si, SiC, C or to SixCyOz complexes. The blue-green luminescence is characterized by small lifetimes, which are valuable for telecommunication applications where fast switching is required. However, the relation between the observed luminescent properties of the implanted SiO2:Si:C nanostructures and conditions of their fabrication remains weakly studied.
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The present work is aimed at the investigation of impact of such factors as the ratio of concentrations of silicon and carbon coimplanted into SiO2 films, the sequence of implantation and hightemperature annealing (intermediate and final), as well as the annealing temperature, on the properties related to defects and nanoclusters synthesized in oxide matrix. The knowledge of these factors will further allow employing more comprehensively the advantages of ion implantation, e.g. the capability of additional impurity doping (that has been shown to improve the luminescent properties of Si nanocrystals in SiO2 [11]). The methods of room-temperature photoluminescence (PL) in the range of 350–900 nm, electron spin resonance spectroscopy (ESR), X-ray photoelectron spectroscopy (XPS), spectroscopy of extended energy loss fine structure (EELFS), and cross-sectional transmission electron microscopy (XTEM) were used for characterization of the layers.
2. Experimental Amorphous SiO2 films with a thickness of 800 nm thermally grown at 1100 °C in wet oxygen on p-type Si substrates were used as an original material. Si+ ions were implanted with an energy of 100 keV and a dose of 7·1016 cm− 2 to reach Si supersaturation at the projected ion range of about 10 at.%. The C+ ions were implanted with an energy
Fig. 2. XTEM image of SiO2 layer implanted with Si+ (7 U1016 cm− 2) and postannealed at 1100 °C coincided with the TRIM-calculated Si ion profile. The insets 1 and 2 show the high-resolution TEM image and the SAD picture of the layer, respectively.
of 50 keV in a wide range of doses (7·1013–7·1016 cm− 2) into either SiO2 films as-implanted with Si+ or the same films embedded with Si nanocrystals pre-synthesized by furnace annealing at 1100 °C for 2 h in dry N2. The energies of Si+ and C+ ions were selected to provide best coincidence of their depth profiles calculated by the TRIM code [12]. The C+ implantation was followed by the final two-hour annealing at 1000 °C or 1100 °C. Thus, the three series of samples were prepared: Series 1: SiO2 → Si+ → C+ (various doses) → 1000 °C; Series 2: SiO2 → Si+ → C+ (various doses) → 1100 °C; Series 3: SiO2 → Si+ → 1100 °C → C+ (various doses) → 1100 °C. PL was excited by a pulse N2-laser (337 nm, 1 mW, 10 ns, 25 Hz), detected by an Acton SP-150 grating monochromator and a Hamamatsu R-928 photomultiplier tube at room temperature in the spectral range of 350–900 nm. ESR spectra were measured at 77 K on a modified EPA-2M spectrometer (9 GHz). The g-values were determined relative to a MgO:Mn2+ standard. XPS was realized on an ESCALAB MK2 system (VG) with monochromatized Al Kα X-ray source (1486.6 eV). The samples in the initial state and after Ar+ ion etching were investigated. The spectra were calibrated using Ag reference. The charge effect was suppressed by charge neutralizer — a flood gun EMU-50. The conditions for electronic spectra acquisition were selected in the best way to provide energy resolution of the spectrometer better than 0.6 eV. The treatment and deconvolution of photoelectron spectra were carried out by using the “UNIFIT 2007” software. EELFS spectra were registered in the same spectrometer using the electron gun LEG200, accelerating voltage 1.5 kV in CAE mode (pass energy of 2 eV, C3 slit of 1.5 mm). The spectra recording parameters were the following: range 250 eV, 1200 channels, 10 min/ scan, 10 scans. XTEM micrographs and selected area diffraction (SAD) patterns were obtained by using JEOL 2010F (at University of Oslo) and JEOL FasTem-2010 (at Ben-Gurion University) transmission electron microscopes operating at 200 kV. 3. Results and discussion Fig. 1. Influence of C+ irradiation with various doses on PL spectra of SiO2 films as-implanted with Si+ ions to a dose of 7·1016 cm− 2 (a) and postannealed at 1100 °C (b). Artificial shifts in base line were introduced to improve readability of the spectra.
PL measurements at room temperature in the range of 350– 900 nm were performed at each stage of implantation and annealing.
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The PL spectra of the Si+-implanted and postannealed SiO2 films after irradiation of various doses of C+ are presented in Fig. 1. The thermal SiO2 film as-implanted with Si+ (Fig. 1a, upper curve), is characterized by the three PL bands in the range of 350–650 nm. According to literature data, this PL originates from defects arising at radiation damage of the oxide and accumulation of Si excess atoms. Namely, the emission in the range of 400–500 nm is attributed to neutral oxygen mono- and divacancies [13–17], and the emission at around 620 nm — to paramagnetic non-bridging oxygen hole center (NBOHC) [18,19]. High-temperature annealing after Si+ implantation leads to synthesis of Si nanocrystals, evidently responsible for the intense PL observed at 650–850 nm (upper curve in Fig. 1b) [1,20]. XTEM picture (Fig. 2) of the carbon-free Si+-implanted SiO2 film after annealing at 1100 °C reveals an ensemble of Si nanocrystals at the depth region expected from the calculated Si ion profile. The mean size of nanocrystals is estimated as 3.3 ± 0.8 nm. Si lattice planes are clearly visible in the high-resolution image (see inset 1). The random orientation of Si nanocrystals in amorphous matrix is confirmed by the SAD picture shown in inset 2. Irradiation with C+ (without postannealing) quenches completely the PL emission at 650–850 nm (Fig. 1b) of the pre-synthesized Si
Fig. 4. PL spectra of SiO2 layers implanted with Si+ (7·1016 cm− 2) and С+ (various doses) after the final annealing (a — series 1, b — series 2, c — series 3). Artificial shifts in base line were introduced to improve readability of the spectra.
Fig. 3. Influence of C+ irradiation with various doses on ESR spectra of SiO2 films asimplanted with Si+ ions to a dose of 7·1016 cm− 2 (a) and postannealed at 1100 °C (b). Artificial shifts in base line were introduced to improve readability of the spectra.
nanocrystals even at the lowest dose. This is in agreement with numerous reports on radiation hardness of Si nanocrystals probed through their PL properties. Indeed, it was shown [21,22] that even a
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single displacement in the Si nanocrystal should completely suppress radiative recombination of excitons. The intensity of PL in the range of 350–650 nm (Fig. 1a, b) depends non-monotonously on the dose of C+ ions. Some increase in intensity is observed at lower doses followed by the decrease at the dose of 7·1015 cm− 2, and a certain enhancement of PL bands at around 500 and 620 nm is seen for the maximum carbon dose. Such behavior may be caused by an interplay between two processes. The first one is the accumulation of the mentioned above light-emitting centers. The carbon atoms introduced with high concentration into the SiO2 network can substitute Si atoms (contribute to Si excess) and, as a consequence, to the concentration of oxygen-deficient defects responsible for the short-wave PL. The second process is the PL quenching due to generation of radiation defects (like E'-centers [23]) acting as the centers of non-radiative recombination in an oxide layer. ESR data shown in Fig. 3 along with the literature values of g-factors [23] (indicated by the dashed lines) demonstrate increase in concentration of E' γ (g = 1.9990–2.0018), E' δ (g = 2.0019), EP2 (g = 2.0045) centers, associated with well known variants of the non-radiative E' center, and EX (g = 2.00246) center upon C+ irradiation of the as-implanted with Si+ (Fig. 3a) and postannealed (Fig. 3b) SiO2 films. The NBOHC (g = 2,0090) responsible for the PL at 600–650 nm [18] is also identified, but sensitivity is not enough to monitor its evolution with C+ dose. The final annealing at 1000 or 1100 °C makes ESR signals lower than noise for all defects either in Si+implanted or Si+ and С+ co-implanted layers for all sequences of implantation and annealing procedures (not shown). This indicates thermal healing of the defects and recovery of stoichiometry of the SiO2 matrix. Effect of the final annealing at 1000 and 1100 °C on PL spectra of SiO2 films implanted with Si+ and С+ is illustrated in Fig. 4 compared to the carbon-free films. There is a sharp difference of PL spectra for the SiO2:Si: C films with carbon concentration lower than Si excess (C+ doses of 7·1013–7·1015 cm− 2) and the same films with approximately equal Si excess and C concentrations (C+ dose of 7·1016 cm− 2). In the first case, PL spectrum almost reproduces the spectrum of the carbon-free film (except for some reduction of PL at 650–850 nm), and there is no indication of PL which might be attributed to the C-rich phases or the Ccontaining complexes [8,9]. In the second case, the PL band at around 700 nm is absent at all, but the strong “white” emission appears composed of the three well resolved PL bands at around 400, 500 and 625 nm. We can rule out light-emitting defects in the oxide matrix as an origin of this emission as the defects should be annealed out at the given temperatures according to the ESR data. Following the authors of [9], the first two PL bands can be related to C-rich precipitates with a composition close to stoichiometric SiC and to elemental carbon, respectively. Association of the green PL (at 500 nm) with carbon precipitates is supported by the fact that the corresponding transition energy of 2.5 eV is a typical optical gap of the diamond-like (sp3-rich) amorphous carbon films [24]. At the same time, the bandgap of silicon carbide depends strongly on its polytype and also matches a wide range (2.3–3.3 eV) [4], so the exact nature of intense blue-green luminescence at around 500 nm of the Si+, C+ co-implanted and annealed SiO2 films is a subject of debate. The band at around 625 nm may originate from radiative recombination in small Si nanocrystals or non-crystalline Si clusters [25–28] and is also slightly noticeable for the carbon-free SiO2:Si films annealed at 1000 °C (Fig. 4a, upper curve). Thus, one can assume that the C-rich phases do not form at lower carbon content, whereas the Si inclusions do not grow up to the typical size or do not crystallize at the presence of high carbon concentration. The absence of carbon-rich precipitates at low C+ dose can be explained as follows. The rate of a new phase nucleation is proportional to probability of meeting the corresponding atoms, which, in turn, is determined (at the equal diffusivities) as a square of atomic concentration for nuclei composed of one-type atoms and a
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product of concentrations for a nucleation process from equal amounts of different-type atoms. This implies that, for other conditions being equal, the probability of meeting C and excess Si atoms is an order of magnitude lower than the probability of meeting two Si atoms, when the carbon concentration is one order of magnitude lower. Respectively, the probability of meeting two C atoms is two orders of magnitude lower. Moreover, usually no phase separation is expected for excess concentrations lower than 1 at.% [29], that is a case for carbon dose of 7·1015 cm− 2. Therefore, it is not surprising, that, in the C+ dose range of 7·1013–7·1015 cm− 2, the whole Si excess is consumed by the formation of Si nanocrystals (or Si nanoclusters). The carbon atoms implanted in this dose range are dissolved in matrix and (or) Si nanocrystals. Incorporation of carbon atoms into nanocrystals should produce high level of mechanical stress due to a considerable size difference of Si and C atoms (and, thus, breaking the bonds in the core and/or at the interfaces of nanocrystals [9], i.e. formation of non-radiative centers). This may be a reason for the observed tendency of degradation of the PL intensity at 650–850 nm with a rising carbon dose (Fig. 4). In the case of equal concentrations of excess Si and C atoms, the initial probabilities of meeting two Si excess atoms, Si excess and C atoms, and two C atoms would be equal, if both components had equal diffusion coefficients. In this case, the dominant formation of the C-
Fig. 5. The C1s photoelectron spectra of SiO2 layers co-implanted with equal doses of Si+ and С+ (7·1016 cm− 2) after the final annealing (a — series 1, b — series 2, c — series 3).
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rich phase is an indication of either higher diffusivity of C atoms or higher rate of the Si + C and C + C reactions as compared to the Si + Si reaction (at meeting the corresponding atoms). The data on SiO2 films containing pre-synthesized Si nanocrystals (series 3), for which the PL at 650–850 nm completely disappears at the maximum C concentration, speak in favor of the higher rate of the Si + C reaction. The presence, at the highest C concentration, of small Si nanocrystals or non-crystalline Si clusters (if we assume them as an origin of PL at around 625 nm) can be explained by the C-induced slowing down the growth kinetics of Si inclusions [9]. Another explanation is that carbon alloying hinders the crystallization process of the amorphized Si inclusions (as known for amorphized bulk silicon layers [30]). The above given interpretation is partially confirmed by the XPS data received at the depth of maximum ion concentrations for the samples of each series implanted with approximately equal Si+ and C+ doses. The C1s spectra shown in Fig. 5 are consistently decomposed into three Gaussian components: one is at 283.3–283.7 eV and corresponds to C-Si bonds, the other at 285.0 eV corresponds to C–C sp3-type bonds [31], and the third at 286.5–286.7 eV probably relates to C–O interfacial states. These results demonstrate simultaneous formation of SiC and elemental carbon phases. It is also seen that the relative intensity of the C–C line (proportional to amount of elemental carbon) in the case of providing an intermediate annealing between Si+ and C+ implantations (Fig. 5c) is higher than that without the intermediate annealing (Fig. 5b). The possible reason is the more complicated growth of SiC nanoinclusions in the first case, because this process requires the reaction of implanted carbon with silicon atoms constituent the presynthesized nanocrystals. It is interesting that C–C bonds reveal the sp3 character of hybridization typical of a diamond (in contrast to [9]). The interatomic lengths in the SiO2 film implanted successively with equal doses of C+ and Si+ after annealing at 1100 °C (series 2) were investigated by using EELFS spectroscopy. The EELFS Fourier transform (an analog of Radial Distribution Function) is shown in Fig. 6. Two phases are found to co-exist in SiO2 matrix: SiC nanoclusters and diamond-like carbon clusters: the nearest interatomic bond lengths of Si–C and C–C (0.189 nm and 0.155 nm) are equal to the reference atomic bond lengths in these phases [32]. The C–C bonds with a length of 0.142 nm typical of sp2 atomic configuration (graphite) were not found. This observation suggests that the diamond-like phase is thermodynamically favorable in low-dimensional carbon particles. The XTEM analysis results (Fig. 7) show that the SiO2 layer coimplanted with equal doses of Si+ and C+, after annealing at 1000 °C, contains particles with a size of 3–5 nm distributed in the depth region corresponding to the calculated ion distributions. According to the high-resolution image (inset 1) and the SAD picture (inset 2), these particles, assumed to be Si- and C-rich precipitates in the oxide
Fig. 6. Fourier transform of EELF spectrum for SiO2 film successively implanted with equal doses of Si+ and С+ (7·1016 cm− 2) after annealing at 1100 °C (series 2).
Fig. 7. XTEM image of SiO2 layer co-implanted with equal doses of Si+ and С+ (7·1016 cm− 2) and postannealed at 1000 °C coincided with the TRIM-calculated ion profiles. The insets 1 and 2 show the high-resolution the TEM image and the SAD picture of the layer, respectively.
matrix, have amorphous structure, what is in agreement with the reported data [9] and the present argumentation. 4. Conclusions The room-temperature PL in the range of 350–900 nm of thermally grown SiO2 films implanted with Si+ and С+ ions to synthesize lightemitting layers with silicon carbide nanoinclusions was investigated. Silicon was implanted to a fixed dose of 7·1016 cm− 2 at 100 keV (corresponding to Si excess of about 10 at.%), and a dose of 50-keV carbon ions was varied from 7·1013 to 7·1016 cm− 2. In one scheme, the annealing at 1000 or 1100 °С was performed after successive (Si+ + C+) implantation and, in another scheme, Si+ and C+ implantations were mediated by the annealing at 1100 °С. To interpret the PL data, the methods of ESR, XPS, EELFS, and XTEM were applied. Irradiation with C+ ions leads to some enhancement of the PL in the visible range related to oxygen-deficient centers in Si+-implanted SiO2 and completely quenches the emission at 650–850 nm originated from Si nanocrystals pre-synthesized at the intermediate annealing. As a result of final annealing of the SiO2 films implanted with equal Si excess and C concentrations, strong “white” PL was observed with sub-bands at around 625, 500 and 400 nm attributed to Si nanoclusters, elemental (C-rich) and alloy (SiC) phase inclusions, respectively. The presence of Si- and C-rich amorphous nanoclusters was confirmed in these films by the methods of electron spectroscopy and electron microscopy. It was also found that carbon precipitates revealed sp3 (diamond-like) hybridization, and the amount of elemental carbon was higher in the case of carbon introduction into the SiO2 layers with the pre-synthesized Si nanocrystals. For the C+ doses ranging from 7·1013 to 7·1015 cm− 2, no features related to the C-rich phases were found in PL spectra, that was explained by the low nucleation rate of the C-containing phases compared to the Si phase at such low carbon content. Meanwhile, the
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carbon atoms weaken the intensity of PL at 650–850 nm due to reduction of structural perfection of the pre-synthesized Si nanocrystals and hindering the growth and (or) crystallization of amorphous Si nanoclusters. Acknowledgements The work was partially supported by the European Commission through project SEMINANO (contract NMP4-CT-2004-505285), Russian Ministry for Education and Science (RNP Programme), RFBR (0803-00105), CRDF (BRHE REC-001, Y4-Р-01-05), and the grant of the President of Russian Federation (MK-3877.2007.2). References [1] B. Garrido, M. Lopez, A. Perez-Rodrıguez, C. Garcıa, P. Pellegrino, R. Ferre, J.A. Moreno, J.R. Morante, C. Bonafos, M. Carrada, A. Claverie, J. de la Torre, A. Souifi, Nucl. Instr. Meth. Phys. Res. B 216 (2004) 213. [2] A.N. Mikhaylov, D.I. Tetelbaum, O.N. Gorshkov, A.P. Kasatkin, A.I. Belov, S.V. Morozov, Vacuum 78 (2005) 519. [3] J.Y. Fan, X.L. Wu, P.K. Chu, Prog. Mater. Sci. 51 (2006) 983. [4] V.V. Afanas'ev, M. Bassler, G. Pensl, M.J. Schulz, E. Stein von Kamienski, J. Appl. Phys. 79 (1996) 3108. [5] J.S. Shor, L. Bemis, A.D. Kuttz, I. Grimberg, B.Z. Weiss, M.F. MacMillian, W.J. Choyke, J. Appl. Phys. 76 (1994) 4045. [6] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, H.Z. Wang, G.Q. Yang, Appl. Phys. Lett. 73 (1998) 1838. [7] O. Gonzalez-Varona, A. Perez-Rodriguez, B. Garrido, C. Bonafos, M. Lopez, J.R. Morante, J. Montserrat, R. Rodriguez, Nucl. Instr. Meth. Phys. Res. B 161–163 (2000) 904. [8] L. Rebohle, T. Gebel, H. Frob, H. Reuther, W. Skorupa, Appl. Surf. Sci. 184 (2001) 156. [9] A. Perez-Rodriguez, O. Gonzalez-Varona, B. Garrido, P. Pellegrino, J.R. Morante, C. Bonafos, M. Carrada, A. Claverie, J. Appl. Phys. 94 (2003) 254.
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / s u r f c o a t
Effect of carbon implantation on visible luminescence and composition of Si-implanted SiO2 layers D.I. Tetelbaum a,b,⁎, A.N. Mikhaylov a,b, V.K. Vasiliev a,b, A.I. Belov b, A.I. Kovalev a, D.L. Wainstein a, Yu.A. Mendeleva b, T.G. Finstad c, S. Foss c, Y. Golan d, A. Osherov d a
Surface Phenomena Research Group, 9/23 2nd Baumanskaya street, CNIICHERMET, 105005, Moscow, Russia Physico-Technical Research Institute, University of Nizhny Novgorod, Gagarin prospect 23/3, 603950, Nizhny Novgorod, Russia Department of Physics, University of Oslo, N-0316, Blindern, Norway d Department of Materials Engineering and the Ilse Katz Institute for Nanoscience and Nanotechnology, Ben-Gurion University of the Negev, 84105, Beer-Sheva, Israel b c
a r t i c l e
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Available online 4 March 2009 PACS: 61.46.Bc 61.46.Hk 85.40.Ry 78.55.-m 79.20.Uv 82.80.Pv 87.64.Lg 87.64.Hd Keywords: Silicon nanocrystals Carbon-rich clusters Silicon dioxide Ion implantation Photoluminescence Composition
a b s t r a c t Ion implantation is a convenient tool for synthesis of silicon nanocrystals in SiO2 matrix which exhibit strong red/near-IR luminescence at room temperature. Ion beams can be successfully used also for controllable modification of the nanostructures by introducing defects or changing their phase composition. In the present work, both possibilities are tested by implantation of carbon into thermal SiO2 films either containing pre-fabricated Si nanocrystals or as-implanted with Si. In the carbon-free SiO2 films, Si nanocrystals were synthesized at typical Si excess of about 10 at.% and annealing temperatures of 1000 and 1100 °C. It is shown that carbon ion irradiation of the Si-implanted and then annealed SiO2 films completely quenches a luminescence band at 650–850 nm related to Si nanocrystals and gives rise to emission in the range of 350– 650 nm that originated from oxygen-deficient defects. The subsequent final thermal treatment reduces concentration of the matrix defects, but only partially recovers photoluminescence from Si nanocrystals depending on the C dose. The latter is explained by the hindering effect of implanted carbon on growth and crystallization of Si phase inclusions. Strong “white” emission in the spectral range of 350–800 nm is observed only for the equal Si and C concentrations, irrespective of the sequence of carbon introduction and Si nanocrystal formation. This broad spectral band consists of the three sub-bands at around 400, 500 and 620 nm attributed to inclusions of SiC, C and Si phases, respectively. Electron spectroscopy analysis of the annealed layers confirms the presence of silicon carbide phase and carbon precipitates showing the diamond-like sp3, not the graphitic sp2 hybridization. The Si- and C-rich nanoclusters with a size of 3–5 nm were identified in the co-implanted films by electron microscopy as amorphous nanoparticles. © 2009 Elsevier B.V. All rights reserved.
1. Introduction One of the new applications of ion implantation is the ion-beam synthesis of light-emitting nanocrystals (quantum dots) in wide-band (dielectric) matrices, transparent in visible and near-infrared spectral ranges. This approach presents considerable actuality for optoelectronics owing to prospective of such systems in fabrication of lightemitting diodes, lasers, planar optical amplifiers and waveguides, and also for nanoelectronics — in fabrication of nonvolatile memory devices with distributed charge storage in ultrathin gate dielectric of MOS-transistor. In particular, this would permit to include silicon, as indirect-bandgap semiconductor, into a family of light-emitting materials. Great efforts have been already devoted to ion-beam formation and investigation of Si nanocrystals in SiO2 matrix, which possess luminescence in red and near-IR spectral regions due to a ⁎ Corresponding author. 23/3 Gagarin avenue, 603950 Nizhny Novgorod, Russia. Tel.: +7 831 4656914; fax: +7 831 4659366. E-mail address: [email protected] (D.I. Tetelbaum). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.02.087
quantum-size effect [1,2]. At the same time, it would be of practical importance to develop the Si-based nanostructured systems with the luminescence covering the whole visible range of spectrum. One of the principal approaches could be the synthesis of nanoinclusions of silicon carbide [3], which has such advantages as wide bandgap (2.3–3.3 eV depending on polytype [4]), high melting point, high thermal conductivity and chemical stability. These distinctive properties of SiC make it a technologically important material for electronic devices especially operating at high values of temperature, power, and frequency [5]. The method of co-implantation of Si+ and С+ in SiO2 successfully employed in several works [6–10] provides fabrication of layers with intense photo- and electroluminescence almost in the whole visible domain, tentatively ascribed to nanoclusters of Si, SiC, C or to SixCyOz complexes. The blue-green luminescence is characterized by small lifetimes, which are valuable for telecommunication applications where fast switching is required. However, the relation between the observed luminescent properties of the implanted SiO2:Si:C nanostructures and conditions of their fabrication remains weakly studied.
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The present work is aimed at the investigation of impact of such factors as the ratio of concentrations of silicon and carbon coimplanted into SiO2 films, the sequence of implantation and hightemperature annealing (intermediate and final), as well as the annealing temperature, on the properties related to defects and nanoclusters synthesized in oxide matrix. The knowledge of these factors will further allow employing more comprehensively the advantages of ion implantation, e.g. the capability of additional impurity doping (that has been shown to improve the luminescent properties of Si nanocrystals in SiO2 [11]). The methods of room-temperature photoluminescence (PL) in the range of 350–900 nm, electron spin resonance spectroscopy (ESR), X-ray photoelectron spectroscopy (XPS), spectroscopy of extended energy loss fine structure (EELFS), and cross-sectional transmission electron microscopy (XTEM) were used for characterization of the layers.
2. Experimental Amorphous SiO2 films with a thickness of 800 nm thermally grown at 1100 °C in wet oxygen on p-type Si substrates were used as an original material. Si+ ions were implanted with an energy of 100 keV and a dose of 7·1016 cm− 2 to reach Si supersaturation at the projected ion range of about 10 at.%. The C+ ions were implanted with an energy
Fig. 2. XTEM image of SiO2 layer implanted with Si+ (7 U1016 cm− 2) and postannealed at 1100 °C coincided with the TRIM-calculated Si ion profile. The insets 1 and 2 show the high-resolution TEM image and the SAD picture of the layer, respectively.
of 50 keV in a wide range of doses (7·1013–7·1016 cm− 2) into either SiO2 films as-implanted with Si+ or the same films embedded with Si nanocrystals pre-synthesized by furnace annealing at 1100 °C for 2 h in dry N2. The energies of Si+ and C+ ions were selected to provide best coincidence of their depth profiles calculated by the TRIM code [12]. The C+ implantation was followed by the final two-hour annealing at 1000 °C or 1100 °C. Thus, the three series of samples were prepared: Series 1: SiO2 → Si+ → C+ (various doses) → 1000 °C; Series 2: SiO2 → Si+ → C+ (various doses) → 1100 °C; Series 3: SiO2 → Si+ → 1100 °C → C+ (various doses) → 1100 °C. PL was excited by a pulse N2-laser (337 nm, 1 mW, 10 ns, 25 Hz), detected by an Acton SP-150 grating monochromator and a Hamamatsu R-928 photomultiplier tube at room temperature in the spectral range of 350–900 nm. ESR spectra were measured at 77 K on a modified EPA-2M spectrometer (9 GHz). The g-values were determined relative to a MgO:Mn2+ standard. XPS was realized on an ESCALAB MK2 system (VG) with monochromatized Al Kα X-ray source (1486.6 eV). The samples in the initial state and after Ar+ ion etching were investigated. The spectra were calibrated using Ag reference. The charge effect was suppressed by charge neutralizer — a flood gun EMU-50. The conditions for electronic spectra acquisition were selected in the best way to provide energy resolution of the spectrometer better than 0.6 eV. The treatment and deconvolution of photoelectron spectra were carried out by using the “UNIFIT 2007” software. EELFS spectra were registered in the same spectrometer using the electron gun LEG200, accelerating voltage 1.5 kV in CAE mode (pass energy of 2 eV, C3 slit of 1.5 mm). The spectra recording parameters were the following: range 250 eV, 1200 channels, 10 min/ scan, 10 scans. XTEM micrographs and selected area diffraction (SAD) patterns were obtained by using JEOL 2010F (at University of Oslo) and JEOL FasTem-2010 (at Ben-Gurion University) transmission electron microscopes operating at 200 kV. 3. Results and discussion Fig. 1. Influence of C+ irradiation with various doses on PL spectra of SiO2 films as-implanted with Si+ ions to a dose of 7·1016 cm− 2 (a) and postannealed at 1100 °C (b). Artificial shifts in base line were introduced to improve readability of the spectra.
PL measurements at room temperature in the range of 350– 900 nm were performed at each stage of implantation and annealing.
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The PL spectra of the Si+-implanted and postannealed SiO2 films after irradiation of various doses of C+ are presented in Fig. 1. The thermal SiO2 film as-implanted with Si+ (Fig. 1a, upper curve), is characterized by the three PL bands in the range of 350–650 nm. According to literature data, this PL originates from defects arising at radiation damage of the oxide and accumulation of Si excess atoms. Namely, the emission in the range of 400–500 nm is attributed to neutral oxygen mono- and divacancies [13–17], and the emission at around 620 nm — to paramagnetic non-bridging oxygen hole center (NBOHC) [18,19]. High-temperature annealing after Si+ implantation leads to synthesis of Si nanocrystals, evidently responsible for the intense PL observed at 650–850 nm (upper curve in Fig. 1b) [1,20]. XTEM picture (Fig. 2) of the carbon-free Si+-implanted SiO2 film after annealing at 1100 °C reveals an ensemble of Si nanocrystals at the depth region expected from the calculated Si ion profile. The mean size of nanocrystals is estimated as 3.3 ± 0.8 nm. Si lattice planes are clearly visible in the high-resolution image (see inset 1). The random orientation of Si nanocrystals in amorphous matrix is confirmed by the SAD picture shown in inset 2. Irradiation with C+ (without postannealing) quenches completely the PL emission at 650–850 nm (Fig. 1b) of the pre-synthesized Si
Fig. 4. PL spectra of SiO2 layers implanted with Si+ (7·1016 cm− 2) and С+ (various doses) after the final annealing (a — series 1, b — series 2, c — series 3). Artificial shifts in base line were introduced to improve readability of the spectra.
Fig. 3. Influence of C+ irradiation with various doses on ESR spectra of SiO2 films asimplanted with Si+ ions to a dose of 7·1016 cm− 2 (a) and postannealed at 1100 °C (b). Artificial shifts in base line were introduced to improve readability of the spectra.
nanocrystals even at the lowest dose. This is in agreement with numerous reports on radiation hardness of Si nanocrystals probed through their PL properties. Indeed, it was shown [21,22] that even a
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single displacement in the Si nanocrystal should completely suppress radiative recombination of excitons. The intensity of PL in the range of 350–650 nm (Fig. 1a, b) depends non-monotonously on the dose of C+ ions. Some increase in intensity is observed at lower doses followed by the decrease at the dose of 7·1015 cm− 2, and a certain enhancement of PL bands at around 500 and 620 nm is seen for the maximum carbon dose. Such behavior may be caused by an interplay between two processes. The first one is the accumulation of the mentioned above light-emitting centers. The carbon atoms introduced with high concentration into the SiO2 network can substitute Si atoms (contribute to Si excess) and, as a consequence, to the concentration of oxygen-deficient defects responsible for the short-wave PL. The second process is the PL quenching due to generation of radiation defects (like E'-centers [23]) acting as the centers of non-radiative recombination in an oxide layer. ESR data shown in Fig. 3 along with the literature values of g-factors [23] (indicated by the dashed lines) demonstrate increase in concentration of E' γ (g = 1.9990–2.0018), E' δ (g = 2.0019), EP2 (g = 2.0045) centers, associated with well known variants of the non-radiative E' center, and EX (g = 2.00246) center upon C+ irradiation of the as-implanted with Si+ (Fig. 3a) and postannealed (Fig. 3b) SiO2 films. The NBOHC (g = 2,0090) responsible for the PL at 600–650 nm [18] is also identified, but sensitivity is not enough to monitor its evolution with C+ dose. The final annealing at 1000 or 1100 °C makes ESR signals lower than noise for all defects either in Si+implanted or Si+ and С+ co-implanted layers for all sequences of implantation and annealing procedures (not shown). This indicates thermal healing of the defects and recovery of stoichiometry of the SiO2 matrix. Effect of the final annealing at 1000 and 1100 °C on PL spectra of SiO2 films implanted with Si+ and С+ is illustrated in Fig. 4 compared to the carbon-free films. There is a sharp difference of PL spectra for the SiO2:Si: C films with carbon concentration lower than Si excess (C+ doses of 7·1013–7·1015 cm− 2) and the same films with approximately equal Si excess and C concentrations (C+ dose of 7·1016 cm− 2). In the first case, PL spectrum almost reproduces the spectrum of the carbon-free film (except for some reduction of PL at 650–850 nm), and there is no indication of PL which might be attributed to the C-rich phases or the Ccontaining complexes [8,9]. In the second case, the PL band at around 700 nm is absent at all, but the strong “white” emission appears composed of the three well resolved PL bands at around 400, 500 and 625 nm. We can rule out light-emitting defects in the oxide matrix as an origin of this emission as the defects should be annealed out at the given temperatures according to the ESR data. Following the authors of [9], the first two PL bands can be related to C-rich precipitates with a composition close to stoichiometric SiC and to elemental carbon, respectively. Association of the green PL (at 500 nm) with carbon precipitates is supported by the fact that the corresponding transition energy of 2.5 eV is a typical optical gap of the diamond-like (sp3-rich) amorphous carbon films [24]. At the same time, the bandgap of silicon carbide depends strongly on its polytype and also matches a wide range (2.3–3.3 eV) [4], so the exact nature of intense blue-green luminescence at around 500 nm of the Si+, C+ co-implanted and annealed SiO2 films is a subject of debate. The band at around 625 nm may originate from radiative recombination in small Si nanocrystals or non-crystalline Si clusters [25–28] and is also slightly noticeable for the carbon-free SiO2:Si films annealed at 1000 °C (Fig. 4a, upper curve). Thus, one can assume that the C-rich phases do not form at lower carbon content, whereas the Si inclusions do not grow up to the typical size or do not crystallize at the presence of high carbon concentration. The absence of carbon-rich precipitates at low C+ dose can be explained as follows. The rate of a new phase nucleation is proportional to probability of meeting the corresponding atoms, which, in turn, is determined (at the equal diffusivities) as a square of atomic concentration for nuclei composed of one-type atoms and a
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product of concentrations for a nucleation process from equal amounts of different-type atoms. This implies that, for other conditions being equal, the probability of meeting C and excess Si atoms is an order of magnitude lower than the probability of meeting two Si atoms, when the carbon concentration is one order of magnitude lower. Respectively, the probability of meeting two C atoms is two orders of magnitude lower. Moreover, usually no phase separation is expected for excess concentrations lower than 1 at.% [29], that is a case for carbon dose of 7·1015 cm− 2. Therefore, it is not surprising, that, in the C+ dose range of 7·1013–7·1015 cm− 2, the whole Si excess is consumed by the formation of Si nanocrystals (or Si nanoclusters). The carbon atoms implanted in this dose range are dissolved in matrix and (or) Si nanocrystals. Incorporation of carbon atoms into nanocrystals should produce high level of mechanical stress due to a considerable size difference of Si and C atoms (and, thus, breaking the bonds in the core and/or at the interfaces of nanocrystals [9], i.e. formation of non-radiative centers). This may be a reason for the observed tendency of degradation of the PL intensity at 650–850 nm with a rising carbon dose (Fig. 4). In the case of equal concentrations of excess Si and C atoms, the initial probabilities of meeting two Si excess atoms, Si excess and C atoms, and two C atoms would be equal, if both components had equal diffusion coefficients. In this case, the dominant formation of the C-
Fig. 5. The C1s photoelectron spectra of SiO2 layers co-implanted with equal doses of Si+ and С+ (7·1016 cm− 2) after the final annealing (a — series 1, b — series 2, c — series 3).
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rich phase is an indication of either higher diffusivity of C atoms or higher rate of the Si + C and C + C reactions as compared to the Si + Si reaction (at meeting the corresponding atoms). The data on SiO2 films containing pre-synthesized Si nanocrystals (series 3), for which the PL at 650–850 nm completely disappears at the maximum C concentration, speak in favor of the higher rate of the Si + C reaction. The presence, at the highest C concentration, of small Si nanocrystals or non-crystalline Si clusters (if we assume them as an origin of PL at around 625 nm) can be explained by the C-induced slowing down the growth kinetics of Si inclusions [9]. Another explanation is that carbon alloying hinders the crystallization process of the amorphized Si inclusions (as known for amorphized bulk silicon layers [30]). The above given interpretation is partially confirmed by the XPS data received at the depth of maximum ion concentrations for the samples of each series implanted with approximately equal Si+ and C+ doses. The C1s spectra shown in Fig. 5 are consistently decomposed into three Gaussian components: one is at 283.3–283.7 eV and corresponds to C-Si bonds, the other at 285.0 eV corresponds to C–C sp3-type bonds [31], and the third at 286.5–286.7 eV probably relates to C–O interfacial states. These results demonstrate simultaneous formation of SiC and elemental carbon phases. It is also seen that the relative intensity of the C–C line (proportional to amount of elemental carbon) in the case of providing an intermediate annealing between Si+ and C+ implantations (Fig. 5c) is higher than that without the intermediate annealing (Fig. 5b). The possible reason is the more complicated growth of SiC nanoinclusions in the first case, because this process requires the reaction of implanted carbon with silicon atoms constituent the presynthesized nanocrystals. It is interesting that C–C bonds reveal the sp3 character of hybridization typical of a diamond (in contrast to [9]). The interatomic lengths in the SiO2 film implanted successively with equal doses of C+ and Si+ after annealing at 1100 °C (series 2) were investigated by using EELFS spectroscopy. The EELFS Fourier transform (an analog of Radial Distribution Function) is shown in Fig. 6. Two phases are found to co-exist in SiO2 matrix: SiC nanoclusters and diamond-like carbon clusters: the nearest interatomic bond lengths of Si–C and C–C (0.189 nm and 0.155 nm) are equal to the reference atomic bond lengths in these phases [32]. The C–C bonds with a length of 0.142 nm typical of sp2 atomic configuration (graphite) were not found. This observation suggests that the diamond-like phase is thermodynamically favorable in low-dimensional carbon particles. The XTEM analysis results (Fig. 7) show that the SiO2 layer coimplanted with equal doses of Si+ and C+, after annealing at 1000 °C, contains particles with a size of 3–5 nm distributed in the depth region corresponding to the calculated ion distributions. According to the high-resolution image (inset 1) and the SAD picture (inset 2), these particles, assumed to be Si- and C-rich precipitates in the oxide
Fig. 6. Fourier transform of EELF spectrum for SiO2 film successively implanted with equal doses of Si+ and С+ (7·1016 cm− 2) after annealing at 1100 °C (series 2).
Fig. 7. XTEM image of SiO2 layer co-implanted with equal doses of Si+ and С+ (7·1016 cm− 2) and postannealed at 1000 °C coincided with the TRIM-calculated ion profiles. The insets 1 and 2 show the high-resolution the TEM image and the SAD picture of the layer, respectively.
matrix, have amorphous structure, what is in agreement with the reported data [9] and the present argumentation. 4. Conclusions The room-temperature PL in the range of 350–900 nm of thermally grown SiO2 films implanted with Si+ and С+ ions to synthesize lightemitting layers with silicon carbide nanoinclusions was investigated. Silicon was implanted to a fixed dose of 7·1016 cm− 2 at 100 keV (corresponding to Si excess of about 10 at.%), and a dose of 50-keV carbon ions was varied from 7·1013 to 7·1016 cm− 2. In one scheme, the annealing at 1000 or 1100 °С was performed after successive (Si+ + C+) implantation and, in another scheme, Si+ and C+ implantations were mediated by the annealing at 1100 °С. To interpret the PL data, the methods of ESR, XPS, EELFS, and XTEM were applied. Irradiation with C+ ions leads to some enhancement of the PL in the visible range related to oxygen-deficient centers in Si+-implanted SiO2 and completely quenches the emission at 650–850 nm originated from Si nanocrystals pre-synthesized at the intermediate annealing. As a result of final annealing of the SiO2 films implanted with equal Si excess and C concentrations, strong “white” PL was observed with sub-bands at around 625, 500 and 400 nm attributed to Si nanoclusters, elemental (C-rich) and alloy (SiC) phase inclusions, respectively. The presence of Si- and C-rich amorphous nanoclusters was confirmed in these films by the methods of electron spectroscopy and electron microscopy. It was also found that carbon precipitates revealed sp3 (diamond-like) hybridization, and the amount of elemental carbon was higher in the case of carbon introduction into the SiO2 layers with the pre-synthesized Si nanocrystals. For the C+ doses ranging from 7·1013 to 7·1015 cm− 2, no features related to the C-rich phases were found in PL spectra, that was explained by the low nucleation rate of the C-containing phases compared to the Si phase at such low carbon content. Meanwhile, the
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carbon atoms weaken the intensity of PL at 650–850 nm due to reduction of structural perfection of the pre-synthesized Si nanocrystals and hindering the growth and (or) crystallization of amorphous Si nanoclusters. Acknowledgements The work was partially supported by the European Commission through project SEMINANO (contract NMP4-CT-2004-505285), Russian Ministry for Education and Science (RNP Programme), RFBR (0803-00105), CRDF (BRHE REC-001, Y4-Р-01-05), and the grant of the President of Russian Federation (MK-3877.2007.2). References [1] B. Garrido, M. Lopez, A. Perez-Rodrıguez, C. Garcıa, P. Pellegrino, R. Ferre, J.A. Moreno, J.R. Morante, C. Bonafos, M. Carrada, A. Claverie, J. de la Torre, A. Souifi, Nucl. Instr. Meth. Phys. Res. B 216 (2004) 213. [2] A.N. Mikhaylov, D.I. Tetelbaum, O.N. Gorshkov, A.P. Kasatkin, A.I. Belov, S.V. Morozov, Vacuum 78 (2005) 519. [3] J.Y. Fan, X.L. Wu, P.K. Chu, Prog. Mater. Sci. 51 (2006) 983. [4] V.V. Afanas'ev, M. Bassler, G. Pensl, M.J. Schulz, E. Stein von Kamienski, J. Appl. Phys. 79 (1996) 3108. [5] J.S. Shor, L. Bemis, A.D. Kuttz, I. Grimberg, B.Z. Weiss, M.F. MacMillian, W.J. Choyke, J. Appl. Phys. 76 (1994) 4045. [6] J. Zhao, D.S. Mao, Z.X. Lin, B.Y. Jiang, Y.H. Yu, X.H. Liu, H.Z. Wang, G.Q. Yang, Appl. Phys. Lett. 73 (1998) 1838. [7] O. Gonzalez-Varona, A. Perez-Rodriguez, B. Garrido, C. Bonafos, M. Lopez, J.R. Morante, J. Montserrat, R. Rodriguez, Nucl. Instr. Meth. Phys. Res. B 161–163 (2000) 904. [8] L. Rebohle, T. Gebel, H. Frob, H. Reuther, W. Skorupa, Appl. Surf. Sci. 184 (2001) 156. [9] A. Perez-Rodriguez, O. Gonzalez-Varona, B. Garrido, P. Pellegrino, J.R. Morante, C. Bonafos, M. Carrada, A. Claverie, J. Appl. Phys. 94 (2003) 254.
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