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 re￾emission 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, becom￾ing 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).