Availableonlineatwww.sciencedirect.com SCIENCE DIRECT pp surface science ELSEVIER pplied Surface Science 252(2006)3342-3351 elsevier. com/locate/apsusc Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies G Brauer,", W. Anwand, F. Eichhorn, W Skorupa", C Hofer C. Teichert,J. Kuriplach, J Cizek, I. Prochazka P.G. Coleman, T Nozawa, A. Kohyama Institut fiir lonenstrahlplrysik und Materialforschung, Forschungszentrum Rossendorf e V PF 510119,. D-01314 Dresden, Germany e b Institut fiir Physik, Montanuniversitat Leoben, Franz Josef Str: 18, A-8700 Leoben, austria epartment of Low Ter ure Physics, Faculry of Mathematics and Physics, Charles University, V Holesovickach 2, CZ-180 00 Prague, Czech Republic d Department of Physics, University of Bath, Bath BA27AY,UK Metals and Ceramics Division, Oak Ridge National Laboratory, PO. Box 2008, MS6151 Oak Ridge, TN 37831-615/, USA Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Available online 21 October 2005 Abstract A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C-SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented. C 2005 Elsevier B V. All rights reserved. Keywords: SiC/SiC composite; Graphite; X-ray diffraction; Atomic force microscopy; Slow positron spectroscopy; Positron lifetime; Positron affinity; Positron re-emission 1. Introduction Silicon carbide(SiC) fibre-reinforced SiC matrix Corresponding author. Tel. +49 351 2602117: composite materials(SiC/SiC)are considered to be fax:+493512603285. the attractive candidates as materials for advanced E-mail address: g brauer@fz-rossendorf de(G. Brauer) energy systems, such as high performance combustion 0169-4332/S- see front matter c 2005 Elsevier B V. All rights reserved. doi:10.1016/ apsac.200508.096
Characterization of a SiC/SiC composite by X-ray diffraction, atomic force microscopy and positron spectroscopies G. Brauer a, *, W. Anwand a , F. Eichhorn a , W. Skorupa a , C. Hofer b , C. Teichert b , J. Kuriplach c , J. Cizek c , I. Prochazka c , P.G. Coleman d , T. Nozawa e , A. Kohyama f a Institut fu¨r Ionenstrahlphysik und Materialforschung, Forschungszentrum Rossendorf e.V., PF 510119, D-01314 Dresden, Germany b Institut fu¨r Physik, Montanuniversita¨t Leoben, Franz Josef Str. 18, A-8700 Leoben, Austria c Department of Low Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, CZ-180 00 Prague, Czech Republic d Department of Physics, University of Bath, Bath BA2 7 AY, UK e Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, MS6151, Oak Ridge, TN 37831-6151, USA f Institute of Advanced Energy, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan Available online 21 October 2005 Abstract A SiC/SiC composite is characterized by X-ray diffraction, atomic force microscopy and various positron spectroscopies (slow positron implantation, positron lifetime and re-emission). It is found that besides its main constituent 3C–SiC the composite still must contain some graphite. In order to better interpret the experimental findings of the composite, a pyrolytic graphite sample was also investigated by slow positron implantation and positron lifetime spectroscopies. In addition, theoretical calculations of positron properties of graphite are presented. # 2005 Elsevier B.V. All rights reserved. Keywords: SiC/SiC composite; Graphite; X-ray diffraction; Atomic force microscopy; Slow positron spectroscopy; Positron lifetime; Positron affinity; Positron re-emission 1. Introduction Silicon carbide (SiC) fibre-reinforced SiC matrix composite materials (SiC/SiC) are considered to be the attractive candidates as materials for advanced energy systems, such as high performance combustion www.elsevier.com/locate/apsusc Applied Surface Science 252 (2006) 3342–3351 * Corresponding author. Tel.: +49 351 2602117; fax: +49 351 2603285. E-mail address: g.brauer@fz-rossendorf.de (G. Brauer). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.08.096
G. Brauer et al. / Applied Surface Science 252(2006)3342-335 3343 systems, fuel-flexible gasification systems, fuel Positron annihilation spectroscopy(PAS)is gen cell/turbine hybrid systems, nuclear fusion reactors erally suited to detect, distinguish, and eventually nd high temperature gas-cooled fission reactors [1]. identify open volume defects in solids, including Recently, a review of state-of-the-art achievements in semiconductors [8]. Slow positron implantation production and application of SiC/SiC composites wa spectroscopy (SPIS), based on the generation, published [2] plantation and subsequent annihilation of mono It has long been known that Sic is a polytypic energetic positrons in a sample, is well suited to study substance. But the formation of a phase diagram is depth dependent vacancy-type damage in silicon very difficult, for annealing is slow; different forms carbide [9]. In addition, atomic force microscopy may grow under almost identical conditions, and even (AFM)[10, 11] is a suitable method to investigate the small quantities of impurities may have significant surface morphology of a sample effects. From previous studies, it was found that Recently, systematic SPIS and AFM studies of especially the cubic form grows under conditions various 6H-SiC samples, differing in their conductiv- where one of the hexagonal polytypes is more stable. ity type, crystal quality, ion implantation conditions First explanations of this fact were given in terms of a and annealing, were conducted in order to see if and stacking reversal at a surface and bulk polytype how these parameters may influence the formation of energies [3]. A later extension of this idea was based continuous long furrows(undulations)running in one on a distinction between the two different(0001) direction across the wafer surface [12]. It was found surfaces and application of bulk-derived parameters at that the observed changes in surface morphology are a surface [4 primarily the result of thermal activation during The positron affinity is a fundamental bulk quantity annealing and thus occur independent of conductivity of a solid, which does not depend on the surface type, crystal quality and ion implantation. Moreover, it orientation of a crystalline sample, and it has already was observed that the changes in surface morphology been calculated for 3C-Sic and 6H-Sic polytypes have no influence on the defect depth profiling by 5]. At the same time, an experimental estimation of SPIs the electron work function of 6H-SiC. combined with Based on the experience in studying basic proper independent positron work-function measurements on ties and near surface defects in single-crystalline 6H- the same specimen, allowed the evaluation of the Sic [5,6, 9, 12], it is challenging to investigate a SiC/ positron affinity and its comparison with the SiC composite made from nano-crystalline 3C-SiC. theoretical value. This comparison has prompted Following the specification of the preparation condi- suggestions for improvements in the theoretical tions of such a composite, results of various calculations to be confirmed by future work experimental investigations, namely X-ray diffraction The observation of copious positron re-emission (XRD), AFM, SPIS and the re-emission of positrons, from crystalline 6H-SiC, due to a negative positron will be presented and discussed. In addition, some ork function and with no pre-treatment and without experimental and theoretical results for graphite are the need for ultra-high vacuum conditions, suggests presented to complement these discussions. Concl this material may form the basis of an important new sions are drawn at the end of the paper moderator for the production of monoenergetic positron beams [6] Furthermore, SiC in monocrystalline, hexagonal 2. Preparation of a SiC/SiC composite polytype form is a very interesting material for a wide class of novel applications in electronics [7].An he preparation of a sample having the dimensions essential step in most of the state-of-the-art technol- 10 mm x 10 mm x l mm was performed by the ogies is ion implantation, which is used to confine the nano-infiltration transient eutectic phase sintering on a substrate, to be modified. Therefore, the detection selection of a 3C-SiC nano-powder (30 nm d: lateral dimensions of an area of a crystal wafer, or film (NITE) process [13] in four steps as follows: nd characterization of lattice defects is an essential meter: Marketech International Inc. Port Townsend/ need and challenge for materials science. WA, USA, as determined by XRD and transmission
systems, fuel-flexible gasification systems, fuel cell/turbine hybrid systems, nuclear fusion reactors and high temperature gas-cooled fission reactors [1]. Recently, a review of state-of-the-art achievements in production and application of SiC/SiC composites was published [2]. It has long been known that SiC is a polytypic substance. But the formation of a phase diagram is very difficult, for annealing is slow; different forms may grow under almost identical conditions, and even small quantities of impurities may have significant effects. From previous studies, it was found that especially the cubic form grows under conditions where one of the hexagonal polytypes is more stable. First explanations of this fact were given in terms of a stacking reversal at a surface and bulk polytype energies [3]. A later extension of this idea was based on a distinction between the two different (0 0 0 1) surfaces and application of bulk-derived parameters at a surface [4]. The positron affinity is a fundamental bulk quantity of a solid, which does not depend on the surface orientation of a crystalline sample, and it has already been calculated for 3C–SiC and 6H–SiC polytypes [5]. At the same time, an experimental estimation of the electron work function of 6H–SiC, combined with independent positron work-function measurements on the same specimen, allowed the evaluation of the positron affinity and its comparison with the theoretical value. This comparison has prompted suggestions for improvements in the theoretical calculations to be confirmed by future work. The observation of copious positron re-emission from crystalline 6H–SiC, due to a negative positron work function and with no pre-treatment and without the need for ultra-high vacuum conditions, suggests this material may form the basis of an important new moderator for the production of monoenergetic positron beams [6]. Furthermore, SiC in monocrystalline, hexagonal polytype form is a very interesting material for a wide class of novel applications in electronics [7]. An essential step in most of the state-of-the-art technologies is ion implantation, which is used to confine the lateral dimensions of an area of a crystal wafer, or film on a substrate, to be modified. Therefore, the detection and characterization of lattice defects is an essential need and challenge for materials science. Positron annihilation spectroscopy (PAS) is generally suited to detect, distinguish, and eventually identify open volume defects in solids, including semiconductors [8]. Slow positron implantation spectroscopy (SPIS), based on the generation, implantation and subsequent annihilation of monoenergetic positrons in a sample, is well suited to study depth dependent vacancy-type damage in silicon carbide [9]. In addition, atomic force microscopy (AFM) [10,11] is a suitable method to investigate the surface morphology of a sample. Recently, systematic SPIS and AFM studies of various 6H–SiC samples, differing in their conductivity type, crystal quality, ion implantation conditions and annealing, were conducted in order to see if and how these parameters may influence the formation of continuous long furrows (undulations) running in one direction across the wafer surface [12]. It was found that the observed changes in surface morphology are primarily the result of thermal activation during annealing and thus occur independent of conductivity type, crystal quality and ion implantation. Moreover, it was observed that the changes in surface morphology have no influence on the defect depth profiling by SPIS. Based on the experience in studying basic properties and near surface defects in single-crystalline 6H– SiC [5,6,9,12], it is challenging to investigate a SiC/ SiC composite made from nano-crystalline 3C–SiC. Following the specification of the preparation conditions of such a composite, results of various experimental investigations, namely X-ray diffraction (XRD), AFM, SPIS and the re-emission of positrons, will be presented and discussed. In addition, some experimental and theoretical results for graphite are presented to complement these discussions. Conclusions are drawn at the end of the paper. 2. Preparation of a SiC/SiC composite The preparation of a sample having the dimensions 10 mm 10 mm 1 mm was performed by the nano-infiltration transient eutectic phase sintering (NITE) process [13] in four steps as follows: (1) selection of a 3C–SiC nano-powder (30 nm diameter; Marketech International Inc., Port Townsend/ WA, USA, as determined by XRD and transmission G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 3343
G. Brauer et al. /Applied Surface Science 252(2006)3342-3351 electron microscopy (TEM)) and sintering additives XRD clearly shows the existence of the 3C-SiC (Al2O3+Y2O3=12 wt %(Al2O3: Y2O3=60: 40)and polytype, and no indication of another Sic polytype is SiO2=3 wt %);(2) preparation of a matrix slurry observed. The cubic polytype has the highest 3C-SiC nano-powder and sintering additives in symmetry and, therefore, shows the lowest number ethylene solution);(3)infiltration of matrix slurry of diffraction lines. All lower symmetric SiC into the fabrics(fibre: 500 nm near-stoichiometric polytypes give diffraction lines near to and between SiC continuous Tyranno-SA Grade-3, Ube Indus- the positions of the 3C-SiC diffraction lines. However, tries Ltd, Ube, Japan, covered with pyrolytic carbon; no intensity is found at the""between position",e.g.at architecture: unidirectional cross-plies; fibre volume =342°,38.2°,658°and73.6°for6H-SiC.The fraction: 40%):(4) sintering(1.780C, Ar atmo- diameter of the Sic crystallites is M60 nm,as sphere, 20 MPA pressure, I h) calculated from the line width after deconvolution of the measured data with the instrumental resolution This indicates the size of regions showing coherent 3. Results and discussion scattering. As this value is about twice the diameter of the nano-particles availed for sintering, this is another 3.1. X-ray diffraction indication of the perfectness of the sintered body. In particular, no real grain boundaries are indicated but A standard phase analysis was performed by XRD just a disturbance of translational symmetry in one in Bragg-Brentano geometry using a D8-Advance crystal direction(here, the surface normal of the instrument(Bruker AXS). Fig. I shows the diffraction 10 mm x 10 mm sample face), whereas the other pattern measured with Cu Ko radiation crystal directions may keep translational symmetry (=0.154 nm). The positions of the diffraction lines Furthermore, only the most intense graphite line according to the powder diffraction database(PDF (002)was measured because other lines have a more with (hk)=(111),(200),(220),(31 1)and than one order less intensity in randomly oriented (222)are indicated for 3C-SiC(PDF 29-1129)and graphite: the intensity of the next intense( 0 1) line is graphite with (hk D=(002)(PDF 41-1487). Non- only 6% of the intensity of the(00 2) line indicated lines are formed by the sintering additive YAlO3(PDF 38-0222) 3. 2. Atomic force microscopy surface morphology of the SiC/SiC sample was nvestigated by AFM using a closed-loop scanner, -.-SiC sintered which allows high precision measurements on the nanometer scale. All measurements were recorded under ambient conditions in Tapping Mode[14, 15 品 typical tip radius smaller than 10 nm and an opening angle of less than 20 were applied. The scanner's large measurement range of 15 um in the vertical direction was in particular beneficial for this investigation since the sample exhibits-due to the manufacturing process-a gnificant root mean square(RMs) roughness of RMS N 300 nm and maximum height differences of Scattering angle2e(°) about 1.5 um on a 10 um x 10 um image The AFM results are summarized in Fig. 2 showing Fig. 1. Normalized scattering intensity from XRD of the SiC/Sic composite sample as a function of scattering angle 20 measured with representative images ranging from 10 um x 10 um Cu Ka radiation (=0.154 nm). The (1 11).(200),(220).(310 to I um x I um scan size. The large area scan and (222) peaks of 3C-SiC as well as the(002) peak of graphite (Fig. 2a) leaves the overall impression that the surface haracterized by two main morphological features
electron microscopy (TEM)) and sintering additives (Al2O3 + Y2O3 = 12 wt.% (Al2O3:Y2O3 = 60:40) and SiO2 = 3 wt.%); (2) preparation of a matrix slurry (3C–SiC nano-powder and sintering additives in ethylene solution); (3) infiltration of matrix slurry into the fabrics (fibre: 500 nm near-stoichiometric SiC continuous TyrannoTM-SA Grade-3, Ube Industries Ltd., Ube, Japan, covered with pyrolytic carbon; architecture: unidirectional cross-plies; fibre volume fraction: 40%); (4) sintering (1.780 8C, Ar atmosphere, 20 MPA pressure, 1 h). 3. Results and discussion 3.1. X-ray diffraction A standard phase analysis was performed by XRD in Bragg-Brentano geometry using a D8-Advance instrument (Bruker AXS). Fig. 1 shows the diffraction pattern measured with Cu Ka radiation (l = 0.154 nm). The positions of the diffraction lines according to the powder diffraction database (PDF) with (hkl) = (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) are indicated for 3C–SiC (PDF 29-1129) and graphite with (hkl) = (0 0 2) (PDF 41-1487). Nonindicated lines are formed by the sintering additive YAlO3 (PDF 38-0222). XRD clearly shows the existence of the 3C–SiC polytype, and no indication of another SiC polytype is observed. The cubic polytype has the highest symmetry and, therefore, shows the lowest number of diffraction lines. All lower symmetric SiC polytypes give diffraction lines near to and between the positions of the 3C–SiC diffraction lines. However, no intensity is found at the ‘‘between position’’, e.g. at 2u = 34.28, 38.28, 65.88 and 73.68 for 6H–SiC. The diameter of the SiC crystallites is 60 nm, as calculated from the line width after deconvolution of the measured data with the instrumental resolution. This indicates the size of regions showing coherent scattering. As this value is about twice the diameter of the nano-particles availed for sintering, this is another indication of the perfectness of the sintered body. In particular, no real grain boundaries are indicated but just a disturbance of translational symmetry in one crystal direction (here, the surface normal of the 10 mm 10 mm sample face), whereas the other crystal directions may keep translational symmetry. Furthermore, only the most intense graphite line (0 0 2) was measured because other lines have a more than one order less intensity in randomly oriented graphite: the intensity of the next intense (1 0 1) line is only 6% of the intensity of the (0 0 2) line. 3.2. Atomic force microscopy The surface morphology of the SiC/SiC sample was investigated by AFM using a closed-loop scanner, which allows high precision measurements on the nanometer scale. All measurements were recorded under ambient conditions in ‘Tapping Mode’ [14,15]. Silicon-tips with a typical tip radius smaller than 10 nm and an opening angle of less than 208 were applied. The scanner’s large measurement range of 15 mm in the vertical direction was in particular beneficial for this investigation since the sample exhibits – due to the manufacturing process – a significant root mean square (RMS) roughness of RMS 300 nm and maximum height differences of about 1.5 mm on a 10 mm 10 mm image. The AFM results are summarized in Fig. 2 showing representative images ranging from 10 mm 10 mm to 1 mm 1 mm scan size. The large area scan (Fig. 2a) leaves the overall impression that the surface is characterized by two main morphological features: 3344 G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 Fig. 1. Normalized scattering intensity from XRD of the SiC/SiC composite sample as a function of scattering angle 2u measured with Cu Ka radiation (l = 0.154 nm). The (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) peaks of 3C–SiC as well as the (0 0 2) peak of graphite are indicated.
G. Brauer et al. / Applied Surface Science 252(2006)3342-335 3345 (d) 0 1.0 Fig. 2. AFM images of the SiC/SiC composite a)10 um x 10 um image representing the overall sample morphology. The grey scale range is 500 nm. (b)2.5 um x 2.5 um AFM ima ifferent sample position. The grey scale range is 200 nm. The 3D crystallites are eithe arranged along the furrows on a fibre ply, or are distributed on the surface. (c)I um x I um AFM image showing a small fraction of a fibre ply in the left part of the image. The arrow a larger ridge with a step. The grey scale range is 50 nm. ( d)Section analysis along the line indicated in(c) first, small three-dimensional (3D) crystallites and crystallites are rather uniform. second, unidirectional oriented elongated structures sized 30+ 5 nm in diameter can I nd in irregular The latter can clearly be addressed to the Sic fibres of arrays over a few percent of arger the composite sample. Whereas individual isolated crystallites with 75+ 15 nm diameter, occupyin fibres are rarely observed, a large fraction of them about 15%o of the area of the surface, are frequently occurs in densely packed two-dimensional(2D) observed along the furrows between the fibres arrays. It seems that such 2D-ply fibres are embedded (Fig. 2b). Due to the overall roughness of the sample in the matrix of 3D crystallites. One ply usually it was impossible to reveal the 3D shape(facets)of the extends over an area of about 5 um x 5 um(Fig 2a) crystallites with frayed edges. From the line section presented In addition to the crystallites and fibre plies Fig 2d, we can assume that the fibres have a circular larger ridges are occasionally observed-indicated cross-section. Their diameters range between 40 and by arrows in Fig. 2c and d. These ridges have a 55 nn ateral size of about 250 nm and are on average The 3D crystallites have an average diameter of 50 nm high. The line section in Fig. 2d reveals a about 50 nm ranging from 25 to almost 100 nm. This second step on top of the larger ridge, which is about finding is in excellent agreement with the results from 12 nm high. The nature of these features is not yet Xrd discussed above. Within certain areas the clear
first, small three-dimensional (3D) crystallites and second, unidirectional oriented elongated structures. The latter can clearly be addressed to the SiC fibres of the composite sample. Whereas individual isolated fibres are rarely observed, a large fraction of them occurs in densely packed two-dimensional (2D) arrays. It seems that such 2D-ply fibres are embedded in the matrix of 3D crystallites. One ply usually extends over an area of about 5 mm 5 mm (Fig. 2a) with frayed edges. From the line section presented in Fig. 2d, we can assume that the fibres have a circular cross-section. Their diameters range between 40 and 55 nm. The 3D crystallites have an average diameter of about 50 nm ranging from 25 to almost 100 nm. This finding is in excellent agreement with the results from XRD discussed above. Within certain areas the crystallites are rather uniform. Small crystallites sized 30 5 nm in diameter can be found in irregular arrays over a few percent of the surface. Larger crystallites with 75 15 nm diameter, occupying about 15% of the area of the surface, are frequently observed along the furrows between the fibres (Fig. 2b). Due to the overall roughness of the sample it was impossible to reveal the 3D shape (facets) of the crystallites. In addition to the crystallites and fibre plies, larger ridges are occasionally observed—indicated by arrows in Fig. 2c and d. These ridges have a lateral size of about 250 nm and are on average 50 nm high. The line section in Fig. 2d reveals a second step on top of the larger ridge, which is about 12 nm high. The nature of these features is not yet clear. G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 3345 Fig. 2. AFM images of the SiC/SiC composite sample. (a) 10 mm 10 mm image representing the overall sample morphology. The grey scale range is 500 nm. (b) 2.5 mm 2.5 mm AFM image of a different sample position. The grey scale range is 200 nm. The 3D crystallites are either arranged along the furrows on a fibre ply, or are irregularly distributed on the surface. (c) 1 mm 1 mm AFM image showing a small fraction of a fibre ply in the left part of the image. The arrow indicates a larger ridge with a step. The grey scale range is 50 nm. (d) Section analysis along the line indicated in (c).
3346 G. Brauer et al. /Applied Surface Science 252(2006)3342-3351 preferential positron annihilation in graphite com pared to 3C-SiC, a linear dependency may be dislocation loop Composite =(1-m)S3C-Sic msgraphite (1) From the comparison of bulk S values shown in Fig. 2a it is found that m=016+0.04. Then. a similar o。o assumption can be made regarding the measured positron diffusion length L+ of the composite provided the two phases form parallel channels to the surface E(kev) Composite=(1-m)L3c-Sic mLgraphi Fig 3. Lineshape parameter S for different silicon carbide samples Inserting all numbers, one obtains atrue' value of and pyrolytic g function of incident positron energy For comparison, alues of dislocation loops and Si +C L+ N 213 nm for 3C-SiC. This remarkably large num- ber indicates that defect-free grains should have a diameter of at least twice this value. i.e. a426 nn but most probably are formed much larger in size by 3.3. Slow positron implantation spectroscopy the sintering process. Indeed, this may be the case as the fibres and some particles have a size similar to the SPIS results of the SiC/SiC sample are presented in estimated effective diffusion length. Thus, grain Fig. 3. For comparison, SPIS data from a very clean boundaries, which might perhaps act as trapping sites pyrolytic graphite sample (of unknown origin) and for positrons too, are evenly distributed through the crystalline 6H-SiC (provided by CREE Research Inc, macroscopic sample made up of defect-free grains and Durham, NC, USA;(000 1)-oriented (3.5 off), Si- do not play any significant role faced, n-type) are given Although dislocation loops and Si+C divacancies A positron diffusion length of L+=186+ 6 nm is (V2) have S values above the bulk value of 6H-SiC and calculated for the composite, which needs to be the composite(see Fig 3), they are unlikely to exist in compared to the values of epi-6H-SiC (L+=157 the composite due to its preparation from 3C-Sic t 36 nm) and crystalline 6H-SiC (L+=54+ 3 nm), nano-crystalline material at high temperature condi- respectively [12]. This comparison already suggests tions To judge whether they exist or not, one option is that the composite has the lowest defect concentration. an improved approach for the analysis of SPis However, it seems to be a contradiction that at higher Doppler broadening data introduced by using a positron implantation energies the bulk value of the combination of Doppler broadening lineshape para composite is found to be slightly above the bulk value meters S and W[17, 18]. These results are shown in of the 6H-SiC sample. On the other hand, from Fig. 4 previous positron lifetime calculations [16] it became Any material state, like the bulk or a certain defect, clear that differences in the structure of the Sic is characterized by a given set of lineshape parameters, polytypes 3C, 4H and 6H are below the detection limit i.e.(S, W) values, which are required to be deduced in of ordinary PAS, and thus the same bulk value for 3c the same way from experimental data. In case of and 6H has to be expected. Therefore, a most natural silicon carbide, from previous work(see ref. [9] and explanation of the results from Fig. 3 is that positron references therein) it became possible to include in annihilation is occurring partly in graphite, which is Fig. 4 the(S, W)values for dislocation loops and the definitely still contained in the composite sample V2 defect, both being of 'open volume type'although according to XRd results to different extents. When only two distinct annihila From the results shown in Fig. 3, a positron tion characteristics, described by (SI, W1)and diffusion length of L+=42+9 nm is calculated for (S2, W2), contribute to a set of experimental data
3.3. Slow positron implantation spectroscopy SPIS results of the SiC/SiC sample are presented in Fig. 3. For comparison, SPIS data from a very clean pyrolytic graphite sample (of unknown origin) and crystalline 6H–SiC (provided by CREE Research Inc., Durham, NC, USA; (0 0 0 1)-oriented (3.58 off), Sifaced, n-type) are given. A positron diffusion length of L+ = 186 6 nm is calculated for the composite, which needs to be compared to the values of epi-6H–SiC (L+ = 157 36 nm) and crystalline 6H–SiC (L+ = 54 3 nm), respectively [12]. This comparison already suggests that the composite has the lowest defect concentration. However, it seems to be a contradiction that at higher positron implantation energies the bulk value of the composite is found to be slightly above the bulk value of the 6H–SiC sample. On the other hand, from previous positron lifetime calculations [16] it became clear that differences in the structure of the SiC polytypes 3C, 4H and 6H are below the detection limit of ordinary PAS, and thus the same bulk value for 3C and 6H has to be expected. Therefore, a most natural explanation of the results from Fig. 3 is that positron annihilation is occurring partly in graphite, which is definitely still contained in the composite sample according to XRD results. From the results shown in Fig. 3, a positron diffusion length of L+ = 42 9 nm is calculated for the pyrolytic graphite. Supposing that there is no preferential positron annihilation in graphite compared to 3C–SiC, a linear dependency may be assumed: Scomposite ¼ ð1 mÞS3CSiC þ mSgraphite (1) From the comparison of bulk S values shown in Fig. 2a it is found that m = 0.16 0.04. Then, a similar assumption can be made regarding the measured positron diffusion length L+ of the composite provided the two phases form parallel channels to the surface: Lcomposite ¼ ð1 mÞL3CSiC þ mLgraphite (2) Inserting all numbers, one obtains a ‘true’ value of L+ 213 nm for 3C–SiC. This remarkably large number indicates that defect-free grains should have a diameter of at least twice this value, i.e. 426 nm, but most probably are formed much larger in size by the sintering process. Indeed, this may be the case as the fibres and some particles have a size similar to the estimated effective diffusion length. Thus, grain boundaries, which might perhaps act as trapping sites for positrons too, are evenly distributed through the macroscopic sample made up of defect-free grains and do not play any significant role. Although dislocation loops and Si + C divacancies (V2) have S values above the bulk value of 6H–SiC and the composite (see Fig. 3), they are unlikely to exist in the composite due to its preparation from 3C–SiC nano-crystalline material at high temperature conditions. To judge whether they exist or not, one option is an improved approach for the analysis of SPIS Doppler broadening data introduced by using a combination of Doppler broadening lineshape parameters S and W [17,18]. These results are shown in Fig. 4. Any material state, like the bulk or a certain defect, is characterized by a given set of lineshape parameters, i.e. (S, W) values, which are required to be deduced in the same way from experimental data. In case of silicon carbide, from previous work (see ref. [9] and references therein) it became possible to include in Fig. 4 the (S, W) values for dislocation loops and the V2 defect, both being of ‘open volume type’ although to different extents. When only two distinct annihilation characteristics, described by (S1, W1) and (S2, W2), contribute to a set of experimental data, a 3346 G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 Fig. 3. Lineshape parameter S for different silicon carbide samples and pyrolytic graphite as a function of incident positron energy E. For comparison, the S values of dislocation loops and Si + C divacancies (V2) in 6H–SiC are given.
G. Brauer et al. / Applied Surface Science 252(2006)3342-335 3347 confirmed at atomic level by Pas because any possible than in the crystalline 6H-SiC Samp be at a lower remaining open volume fraction should 094 3.4. Positron affinity The positron affinity A+ as a bulk quantity is e sic y sIC defined by [20] A+=-++=-(中-+中+) A dislocation loop Here, p+ and - are the positron and electron work functions, and u- and A+ are the electron and positron chemical potentials, respectively. First-principles Fig. 4. Normalized lineshape parameters w/wb vs. S/S, plot for electronic structure and positron-state calculations different silicon carbide samples, defect states in 6H-SiC and for perfect and defected 3C-SiC and 6H-SiC were pyrolytic graphite. An untreated Si(1 00) sample served as already calculated by assuming that u_corresponds to reference(Sb. Wb)for normalization. the top of the valence band [5]. On the other hand when comparing positron affinities of two materials in contact(SiC and ere), one should be aware straight line is obtained in the Sw representation where the endpoints represent the two st of actual position of electron chemical potentials 17, 18]. If(SI, Wi) is taken to represent the 6H-SiC ( Fermi levels)of both materials (cf [21]). Neverthe less, we adopt here the same approach as in bulk, and(S2, W2) to represent the V2 defect, then a because the actual position of the Fermi level in the traight line connecting both states should contain the Sic composite is unknown. However, this simplifica (S, W) values of all defects having an open volume less tion does not influence conclusions given below than the v2 defect. Indeed, the (s, w value representing dislocation loops is found to be locate The positron affinity is a very useful materials property to judge whether positrons become trapped correctly regarding its wvalue but shifted slightly ins by precipitates. This concept implies an even towards the value of bulk pyrolytic graphite(see distribution of precipitates in a host matrix. Then, a Fig. 3). This could be an indication that in the positron will be trapped by a spherical precipitate if dilatation part of the dislocation loops observed in AI* the difference AA, between the positron affinity of the implanted 6H-Sic(see ref. [9] and references therein) host and the precipitate is positive and the radius of the the positron annihilates preferentially at carbon atoms From Al implantation into 4H-SiC [19], it wa precipitate exceeds a critical radius re given by [20] concluded that excess Si interstitials, being generated in a substitutional process upon annealing, form the (4) (△A4) dislocation loops For the composite, the bulk w value is negligibly different from the corresponding value The proportionality constant has the dimension for 6H-SiC, which is an indication that trapping at nm(ev), AA+ is given in eV and re is given in grain boundaries can be neglected. However, the nm. This 'positron affinity concept'was successfully noticeable shift in S towards the graphite value is a applied to consider, e. g. irradiation-induced precipi- direct confirmation of positron annihilation in graphite tates in reactor pressure vessel steels [22], and cluster- still contained in the composite ing of Ge [16 and B [23 in 6H-Sic due to ion The SPis results are another indication of the plantation and annealing perfectness of the composite in agreement with the Here, the positron affinity of graphite is of interest. findings from XRD described above, and furthermore The application of different calculational methods has a very nice confirmation of a perfectly sintered body been described already in detail elsewhere [5]. Here, we already found by TEM [13]. Now the TEM results are mention that we employ the linear-muffin-tin-orbital
straight line is obtained in the S–W representation where the endpoints represent the two states itself [17,18]. If (S1, W1) is taken to represent the 6H–SiC bulk, and (S2, W2) to represent the V2 defect, then a straight line connecting both states should contain the (S, W) values of all defects having an open volume less than the V2 defect. Indeed, the (S, W) value representing dislocation loops is found to be located correctly regarding its W value but shifted slightly in S towards the value of bulk pyrolytic graphite (see Fig. 3). This could be an indication that in the dilatation part of the dislocation loops observed in Al+ implanted 6H–SiC (see ref. [9] and references therein) the positron annihilates preferentially at carbon atoms. From Al+ implantation into 4H–SiC [19], it was concluded that excess Si interstitials, being generated in a substitutional process upon annealing, form the dislocation loops. For the composite, the bulk W value is negligibly different from the corresponding value for 6H–SiC, which is an indication that trapping at grain boundaries can be neglected. However, the noticeable shift in S towards the graphite value is a direct confirmation of positron annihilation in graphite still contained in the composite. The SPIS results are another indication of the perfectness of the composite in agreement with the findings from XRD described above, and furthermore a very nice confirmation of a perfectly sintered body already found by TEM [13]. Now the TEM results are confirmed at atomic level by PAS because any possible remaining open volume fraction should be at a lower than in the crystalline 6H–SiC sample. 3.4. Positron affinity The positron affinity A+ as a bulk quantity is defined by [20]: Aþ ¼ m þ mþ ¼ ðF þ FþÞ: (3) Here, F+ and F are the positron and electron work functions, and m and m+ are the electron and positron chemical potentials, respectively. First-principles electronic structure and positron-state calculations for perfect and defected 3C–SiC and 6H–SiC were already calculated by assuming that m corresponds to the top of the valence band [5]. On the other hand, when comparing positron affinities of two materials in contact (SiC and graphite here), one should be aware of actual position of electron chemical potentials (Fermi levels) of both materials (cf. [21]). Nevertheless, we adopt here the same approach as in [5] because the actual position of the Fermi level in the SiC composite is unknown. However, this simplification does not influence conclusions given below. The positron affinity is a very useful materials property to judge whether positrons become trapped by precipitates. This concept implies an even distribution of precipitates in a host matrix. Then, a positron will be trapped by a spherical precipitate if the difference DA+ between the positron affinity of the host and the precipitate is positive and the radius of the precipitate exceeds a critical radius rc given by [20]: rc ¼ 0:31 ðDAþÞ 1=2 (4) The proportionality constant has the dimension nm(eV)1/2, DA+ is given in eV and rc is given in nm. This ‘positron affinity concept’ was successfully applied to consider, e.g. irradiation-induced precipitates in reactor pressure vessel steels [22], and clustering of Ge [16] and B [23] in 6H–SiC due to ion implantation and annealing. Here, the positron affinity of graphite is of interest. The application of different calculational methods has been described already in detail elsewhere [5]. Here, we mention that we employ the linear-muffin-tin-orbital G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 3347 Fig. 4. Normalized lineshape parameters W/Wb vs. S/Sb plot for different silicon carbide samples, defect states in 6H–SiC and pyrolytic graphite. An untreated Si(1 0 0) sample served as a reference (Sb, Wb) for normalization
3348 G. Brauer et al. /Applied Surface Science 252(2006)3342-3351 Table I estimated order of the positron diffusion length in Positron lifetimes(r)and affinities(A+)for graphite calculated using 3C-SiC should remain correct different computational methods and approaches(see text) to elec- tron-positron correlations LMTO 3.5. Positron lifetime ATSUR t(ps) In addition to the positron affinity, the positron lifetime is also calculated, and both the lmto and atomic superposition(ATSUP)[27, 28] methods are used for this purpose(see Table 1). The lifetime results obtained using these two methods differ non- (LMTO)method [24]. In the framework of this method, negligibly. This is probably due to LMTO limitations ne often needs to incorporate empty spheres(ES)into to describe properly the interstitial space. The ATSUP- the studied structure in order to describe properly the Bn results compare well with other calculated (electron and positron) charge distribution in the lifetimes presented in the literature [29,30), on the interstitial space [24. We tried several choices of the other hand, our ATSUP-GC value agrees well with ES sizes and positions and the results presented in 209 ps given [31] and obtained using a different Table I correspond to the most realistic case. Boronski- approach. Nieminen(BN) scheme [25] and gradient correction Positron lifetime measurements on the pyrolytic (GC)approach of Barbiellini et al. [26] were employed graphite and SiC/SiC composite samples were per- to treat electron-positron correlation effects. formed using a spectrometer of 160 ps time resolution The positron affinity of graphite is found to be (FWHM at" -Na window settings )which is described in positioned well below the one of 3C-Sic detail elsewhere [32]. The measured positron lifetime (A+=-557ev 5] independent of the various spectrum of graphite was decomposed into two potentials chosen in the particular calculations and components: [1=93+ 4 ps and t2= 242 t 4 ps with Iso independent of the position of the 3C-SiC Fermi corresponding intensities 11=23+ 1% and 12=77 level. This indicates that graphite precipitates (or 19. The components ti and t2 are attributed to regions)imbedded in a 3C-SiC host matrix would be delocalized and trapped positrons, respectively. When attractive to positrons. A formal application of Eq (4) the two state trapping model is considered, the and inserting numbers from Table I would give a corresponding bulk positron lifetime amounts to critical radius of the order re N0.1674-0 1876 nm. tb=177+ 1 ps, which corresponds reasonably well The lattice parameters of graphite can be found from to the calculated lifetimes given in Table 1, the LMTO- XRD(PDF 41-1487)thus giving the lattice constants N number being the closest one Measured positron 0. 24704 nm and c=0.67244 nm. Because carbon lifetimes presented in literature(see [29-31] and atoms may touch each other at the utmost within the references therein)range from 195 to 215 ps but were (0001) face, the atomic radiusof a carbon atom measured with time resolutions being worse than the should be less equal a/2, i.e. 0. 1235 nm. Taking this presently used. From the above two components, we number, another formal calculation gives the result also calculated the mean positron lifetime Tav= 208 hat a ' graphite precipitate'able to trap a positron t3 ps that falls into this range. This indicates that a inside a 3C-SiC matrix should contain at least three to lifetime of about 200 ps usually measured in graphite four carbon atoms. Certainly, such a consideration is corresponds to a mixture of delocalized and localized not applicable to the given composite sample because positrons. However, the nature of positron trapping sites the carbon is not evenly distributed inside the 3C-Sic is not fully certain, these could be monovacancies and/ matrix. From the production process described above, or small vacancy clusters on the basis of calculation it is more probable to have maybe continuous carbon iven in 311 threads after the sintering process. Anyway, the As for the SiC/SiC composite sample, we decided attractiveness of graphite overestimates the carbon to fix one of lifetime components to tav for graphite ratio m in Eqs.(1) and (2), i.e. the assumed linear as given above- because our sample contains graphite dependency does not really exist. Nevertheless, the regions'(as indicated by SPIs measurements) which
(LMTO) method [24]. In the framework of this method, one often needs to incorporate empty spheres (ES) into the studied structure in order to describe properly the (electron and positron) charge distribution in the interstitial space [24]. We tried several choices of the ES sizes and positions and the results presented in Table 1 correspond to the most realistic case. Boronski– Nieminen (BN) scheme [25] and gradient correction (GC) approach of Barbiellini et al. [26] were employed to treat electron–positron correlation effects. The positron affinity of graphite is found to be positioned well below the one of 3C–SiC (A+ = 5.57 eV [5]) independent of the various potentials chosen in the particular calculations and also independent of the position of the 3C–SiC Fermi level. This indicates that graphite precipitates (or ‘regions’) imbedded in a 3C–SiC host matrix would be attractive to positrons. A formal application of Eq. (4) and inserting numbers from Table 1 would give a critical radius of the order rc 0.1674–0.1876 nm. The lattice parameters of graphite can be found from XRD (PDF 41-1487) thus giving the lattice constants a = 0.24704 nm and c = 0.67244 nm. Because carbon atoms may touch each other at the utmost within the (0 0 0 1) face, the ‘atomic radius’ of a carbon atom should be less equal a/2, i.e. 0.1235 nm. Taking this number, another formal calculation gives the result that a ‘graphite precipitate’ able to trap a positron inside a 3C–SiC matrix should contain at least three to four carbon atoms. Certainly, such a consideration is not applicable to the given composite sample because the carbon is not evenly distributed inside the 3C–SiC matrix. From the production process described above, it is more probable to have maybe continuous carbon threads after the sintering process. Anyway, the attractiveness of graphite overestimates the carbon ratio m in Eqs. (1) and (2), i.e. the assumed linear dependency does not really exist. Nevertheless, the estimated order of the positron diffusion length in 3C–SiC should remain correct. 3.5. Positron lifetime In addition to the positron affinity, the positron lifetime is also calculated, and both the LMTO and atomic superposition (ATSUP) [27,28] methods are used for this purpose (see Table 1). The lifetime results obtained using these two methods differ nonnegligibly. This is probably due to LMTO limitations to describe properly the interstitial space. The ATSUPBN results compare well with other calculated lifetimes presented in the literature [29,30], on the other hand, our ATSUP-GC value agrees well with 209 ps given [31] and obtained using a different approach. Positron lifetime measurements on the pyrolytic graphite and SiC/SiC composite samples were performed using a spectrometer of 160 ps time resolution (FWHM at 22Na window settings) which is described in detail elsewhere [32]. The measured positron lifetime spectrum of graphite was decomposed into two components: t1 = 93 4 ps and t2 = 242 4 ps with corresponding intensities I1 = 23 1% and I2 = 77 1%. The components t1 and t2 are attributed to delocalized and trapped positrons, respectively. When the two state trapping model is considered, the corresponding bulk positron lifetime amounts to tb = 177 1 ps, which corresponds reasonably well to the calculated lifetimes given in Table 1, the LMTOBN number being the closest one. Measured positron lifetimes presented in literature (see [29–31] and references therein) range from 195 to 215 ps but were measured with time resolutions being worse than the presently used. From the above two components, we also calculated the mean positron lifetime tav = 208 3 ps that falls into this range. This indicates that a lifetime of about 200 ps usually measured in graphite corresponds to a mixture of delocalized and localized positrons. However, the nature of positron trapping sites is not fully certain, these could be monovacancies and/ or small vacancy clusters on the basis of calculations given in [31]. As for the SiC/SiC composite sample, we decided to fix one of lifetime components to tav for graphite – as given above – because our sample contains graphite ‘regions’ (as indicated by SPIS measurements) which 3348 G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 Table 1 Positron lifetimes (t) and affinities (A+) for graphite calculated using different computational methods and approaches (see text) to electron–positron correlations Theory LMTO ATSUP t (ps) A+ (eV) t (ps) BN 174 9.0 185 GC 186 8.3 206
G. Brauer et al. / Applied Surface Science 252(2006)3342-335 3349 re attractive to positrons-as follows from positron ffinity results. Then, the fitting procedure results in three components T1=141+ 6 ps, T2=208 T3=320+ 6 ps with the following intensities 1=65±9%,12=15±7%andl3=20±7%.This further confirms the presence of graphite in the Sic 0.15 Sic composite sample though the corresponding annihilation fraction calculated from lifetime results (considering the three state trapping model) amounts to 5%o only, which is somewhat less than 16% obtained from sPIs data (based on an assumed linear dependency of change in S values). Considering further the trapping model, the bulk lifetime E( keV calculated is 168 ps, which is somewhat too high Fig. 5. Re-emitted positron yields from the SiC/SiC composite and compared to the experimental bulk lifetime of 3C a crystalline 6H-SiC sample. Solid lines are fits using diffusion Sic (138 ps [16]). These findings indicate that our lengths of 45 nm(6H-SiC) and I nm(epithermal) and 200 nI interpretation of the lifetime measurement is not (thermal positrons)(composite) perfect, but at this moment we do not have enough nowledge about the studied sample to suggest a better one corresponds to a positron diffusion length of L+=200 nm (as suggested by the sparameter 3.6. Re-emission of positrons measurements of Fig. 3)and a zero-energy yield of 0.0l-about 30 times smaller than the same yield for The persistence of positron re-emission from a 6H-SiC. In comparison, a piece of 6H-SiC shows ample up to positron implantation energies of several typical work-function re-emission, albeit in this case kiloelectron volts is characteristic of work-function with a rather small diffusion length of L+ N 45 nm. emission [34]. Thus, an estimation of from re- The data are thus consistent with the picture of a very measurements relies on the fact of being low branching ratio for work-function re-emission a negative quantity and has been successfully applied from the composite. It could be that any work-function to determine for 6H-Sic [6]. However, in case of re-emission that does occur is from the small (30 nm p+ being a positive quantity another experimental diameter) crystallites seen by AFM, as described in method employing positrons was already published Section 3. 2, and that the surface fibres- perhaps 33 coated with graphite- and the larger crystallites Results of positron re-emission measurements at (which are buried in surface furrows) do not re-emit the Sic/Sic sample in comparison with a crystalline thermalized positrons efficiently 6H-SiC sample are presented in Fig. 5. For the Re-emitted positron spectra of the SiC/SiC sample omposite, the dependence of re-emitted positron in comparison with a crystalline 6H-SiC sample are yield( the fraction of incident positrons re-emitted at presented in Fig. 6. These data were taken for both low energies) on incident positron energy E is samples by measuring annihilation gamma count rates characteristic of epithermal positron emission, becom- from the samples as a stopping potential was ramped ing significant only at incident energies below I keV from 2 to-5 V. As the potential becomes increasingly with a fitted effective positron diffusion length of negative, more re-emitted positrons are returned to the <I nm. Because the re-emitted positron fraction is sample and are annihilated there. Only annihilation measurably non-zero above 1 ke V, in conflict with the events in the sample are observed; a thick lead slit is epithermal emission model used to fit the data, it is placed between the sample and detector. To obtain possible that for the composite there is a very small, acceptable counting statistics, the sintered Sic data longtail of work-function re-emission extending to were taken for an incident positron energy of 0.5 ke V several kiloelectron volts; the line on the graph The shapes of the two spectra are essentially the same
re attractive to positrons—as follows from positron affinity results. Then, the fitting procedure results in three components t1 = 141 6 ps, t2 = 208 ps and t3 = 320 6 ps with the following intensities: I1 = 65 9%, I2 = 15 7% and I3 = 20 7%. This further confirms the presence of graphite in the SiC/ SiC composite sample though the corresponding annihilation fraction calculated from lifetime results (considering the three state trapping model) amounts to 5% only, which is somewhat less than 16% obtained from SPIS data (based on an assumed linear dependency of change in S values). Considering further the trapping model, the bulk lifetime calculated is 168 ps, which is somewhat too high compared to the experimental bulk lifetime of 3C– SiC (138 ps [16]). These findings indicate that our interpretation of the lifetime measurement is not perfect, but at this moment we do not have enough knowledge about the studied sample to suggest a better one. 3.6. Re-emission of positrons The persistence of positron re-emission from a sample up to positron implantation energies of several kiloelectron volts is characteristic of work-function emission [34]. Thus, an estimation of F+ from reemission measurements relies on the fact of F+ being a negative quantity and has been successfully applied to determine F+ for 6H–SiC [6]. However, in case of F+ being a positive quantity another experimental method employing positrons was already published [33]. Results of positron re-emission measurements at the SiC/SiC sample in comparison with a crystalline 6H–SiC sample are presented in Fig. 5. For the composite, the dependence of re-emitted positron yield (the fraction of incident positrons re-emitted at low energies) on incident positron energy E is characteristic of epithermal positron emission, becoming significant only at incident energies below 1 keV with a fitted effective positron diffusion length of 1 nm. Because the re-emitted positron fraction is measurably non-zero above 1 keV, in conflict with the epithermal emission model used to fit the data, it is possible that for the composite there is a very small, long ‘tail’ of work-function re-emission extending to several kiloelectron volts; the line on the graph corresponds to a positron diffusion length of L+ = 200 nm (as suggested by the S parameter measurements of Fig. 3) and a zero-energy yield of 0.01—about 30 times smaller than the same yield for 6H–SiC. In comparison, a piece of 6H–SiC shows typical work-function re-emission, albeit in this case with a rather small diffusion length of L+ 45 nm. The data are thus consistent with the picture of a very low branching ratio for work-function re-emission from the composite. It could be that any work-function re-emission that does occur is from the small (30 nm diameter) crystallites seen by AFM, as described in Section 3.2, and that the surface fibres – perhaps coated with graphite – and the larger crystallites (which are buried in surface furrows) do not re-emit thermalized positrons efficiently. Re-emitted positron spectra of the SiC/SiC sample in comparison with a crystalline 6H–SiC sample are presented in Fig. 6. These data were taken for both samples by measuring annihilation gamma count rates from the samples as a stopping potential was ramped from 2 to 5 V. As the potential becomes increasingly negative, more re-emitted positrons are returned to the sample and are annihilated there. Only annihilation events in the sample are observed; a thick lead slit is placed between the sample and detector. To obtain acceptable counting statistics, the sintered SiC data were taken for an incident positron energy of 0.5 keV. The shapes of the two spectra are essentially the same, G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 3349 Fig. 5. Re-emitted positron yields from the SiC/SiC composite and a crystalline 6H–SiC sample. Solid lines are fits using diffusion lengths of 45 nm (6H–SiC) and 1 nm (epithermal) and 200 nm (thermal positrons) (composite).
3350 G. Brauer et al. /Applied Surface Science 252(2006)3342-3351 4. Conclusions o SiC/SiC, 500 eV It has been experimentally demonstrated by XRD that a macroscopic SiC/SiC composite sintered from nano-crystalline 3C-Sic consists exclusively 3C-SiC containing still some graphite inclusions AFM measurements on different areas of the composite sample reveal that micrometer sized 2D plies of Sic fibres are embedded in a matrix of crystallites with diameters in the range between 30 and SPIS investigations underline the perfectness of the Stopping paene,a 4 -5 composite at an atomic size level due to the sintering process used, and demonstrate that depth profiling of Fig. 6. Re-emitted positron spectra from the Sic/Sic composite and defects is not hindered by a large surface roughness a crystalline 6H-SiC sample as a function of stopping potential. From positron affinity calculations it becomes clear Solid line fit to the data. the two arrows indicate zero and that graphite embedded in 3C-Sic is attractive to maximum positron energies, yielding a value for the positron work function=3.0±0.3 Positron lifetime measurements indicate also the to within statistical uncertainty. This supports the studied view that there may be low-level work-function re- Appreciable re-emission from the Sic/SiC sample emission from the sintered sample at 0.5 keV. is only observed for incident positron energies below However, the unavoidable experimental scatter I kev, characteristic of epithermal positron emission he data may obscure a small epithermal tail to higher energies(on the right of the plot). The work function suggested by these measurements is +=3.0 Acknowledgement +0.3 ev, close to the previously measured values The authors express their gratitude to Dr V. Heera SPis data of the Sic/Sic sample and a 6H-SiC(FZ Rossendorf) for valuable discussions of various sample, taken just for comparison at the Bath, UK aspects of this work positron beamline, give diffusion lengths of 186 and 45 nm for the composite and crystalline samples respectively, when analysed by VEPFIT.These diffusion lengths are consistent with the curves drawn References on Fig. 5 for work-function re-emission, although for [1] L.L. Snead, R.H. Jones, P Fenici, A Kohyama. J Nucl. Mater. the composite sample epithermal re-emission dom- 233-237(1996)26. inates at low energies. The bulk S value is found to be [2] Advanced SiC/SiC ceramic composites: developments and higher for the SiC/SiC sample, in agreement with plications in energy systems, in: A. Kohyama, M. Singh, results presented in Fig. 3 -T. Lin, Y. Katoh(Eds ), Ceramic Transactions, vol. 144. In summary, the positron re-emission measure- merican Ceramic Society, Westerville, OH, 2002. ments(Figs. 5 and 6)-in combination with co- [3] V. Heine, C. Cheng, R. Needs, J. Am. Ceram Soc. 74(1991) 2630 mparative SPIS studies- suggest that, although the [4] MJ. Rutter, V. Heine, J Phys. Condens. Matter 9(1997) diffusion length for thermalized positrons in 82 composite is rather long, the probability for positroN [5 G. Brauer, w. Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. re-emission by the 3 ev work function Wagner, P.G. Coleman, M.J. Puska, T Korhonen, Phys. Rev. B 54(1996)2512 small--at most 3% of that for a single-crystal 6H- [6]J Stormer, A. Goodyear, w. Anwand, G. Brauer, P G. Cole- man,w. Triftshauser, J Phys. Condens Matter 8(1996)L89
to within statistical uncertainty. This supports the view that there may be low-level work-function reemission from the sintered sample at 0.5 keV. However, the unavoidable experimental scatter in the data may obscure a small epithermal tail to higher energies (on the right of the plot). The work function suggested by these measurements is F+ = 3.0 0.3 eV, close to the previously measured values [6]. SPIS data of the SiC/SiC sample and a 6H–SiC sample, taken just for comparison at the Bath, UK positron beamline, give diffusion lengths of 186 and 45 nm for the composite and crystalline samples, respectively, when analysed by VEPFIT. These diffusion lengths are consistent with the curves drawn on Fig. 5 for work-function re-emission, although for the composite sample epithermal re-emission dominates at low energies. The bulk S value is found to be higher for the SiC/SiC sample, in agreement with results presented in Fig. 3. In summary, the positron re-emission measurements (Figs. 5 and 6) – in combination with comparative SPIS studies – suggest that, although the diffusion length for thermalized positrons in the composite is rather long, the probability for positron re-emission by the 3 eV work function is very small—at most 3% of that for a single-crystal 6H– SiC sample. 4. Conclusions It has been experimentally demonstrated by XRD that a macroscopic SiC/SiC composite sintered from nano-crystalline 3C–SiC consists exclusively of 3C–SiC containing still some graphite inclusions. AFM measurements on different areas of the composite sample reveal that micrometer sized 2D plies of SiC fibres are embedded in a matrix of 3D crystallites with diameters in the range between 30 and 90 nm. SPIS investigations underline the perfectness of the composite at an atomic size level due to the sintering process used, and demonstrate that depth profiling of defects is not hindered by a large surface roughness. From positron affinity calculations it becomes clear that graphite embedded in 3C–SiC is attractive to positrons. Positron lifetime measurements indicate also the presence of graphite in the SiC/SiC composite studied. Appreciable re-emission from the SiC/SiC sample is only observed for incident positron energies below 1 keV, characteristic of epithermal positron emission. Acknowledgement The authors express their gratitude to Dr. V. Heera (FZ Rossendorf) for valuable discussions of various aspects of this work. References [1] L.L. Snead, R.H. Jones, P. Fenici, A. Kohyama, J. Nucl. Mater. 233–237 (1996) 26. [2] Advanced SiC/SiC ceramic composites: developments and applications in energy systems, in: A. Kohyama, M. Singh, H.-T. Lin, Y. Katoh (Eds.), Ceramic Transactions, vol. 144, American Ceramic Society, Westerville, OH, 2002. [3] V. Heine, C. Cheng, R. Needs, J. Am. Ceram. Soc. 74 (1991) 2630. [4] M.J. Rutter, V. Heine, J. Phys.: Condens. Matter 9 (1997) 8213. [5] G. Brauer, W. Anwand, E.-M. Nicht, J. Kuriplach, M. Sob, N. Wagner, P.G. Coleman, M.J. Puska, T. Korhonen, Phys. Rev. B 54 (1996) 2512. [6] J. Sto¨rmer, A. Goodyear, W. Anwand, G. Brauer, P.G. Coleman, W. Triftsha¨user, J. Phys.: Condens. Matter 8 (1996) L89. 3350 G. Brauer et al. / Applied Surface Science 252 (2006) 3342–3351 Fig. 6. Re-emitted positron spectra from the SiC/SiC composite and a crystalline 6H–SiC sample as a function of stopping potential. Solid line: fit to the data. The two arrows indicate zero and maximum positron energies, yielding a value for the positron work function = 3.0 0.3 eV.
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