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 transmissionsystems, 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 technol￾ogies 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 gen￾erally 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 mono￾energetic 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 conductiv￾ity 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 proper￾ties 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 condi￾tions 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. Conclu￾sions 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 dia￾meter; 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