Experimental investigation of slow-positron emission from 4H-SiC and 6H-SiC surfaces

Abstract

Slow-positron emission from the surfaces of as-grown n-type 4H-SiC and 6H-SiC (silicon carbide) with a conversion efficiency of ∼10 −4 has been observed. After 30 min of 1000 • C annealing in forming gas, the conversion efficiency of the n-type 6H-SiC sample was observed to be enhanced by 75% to 1.9 × 10 −4 , but it then dropped to ∼10 −5 upon a further 30 min annealing at 1400 • C. The positron work function of the n-type 6H-SiC was found to increase by 29% upon 1000 • C annealing. For both p-type 4H-SiC and p-type 6H-SiC materials, the conversion efficiency was of the order of ∼10 −5 , some ten times lower than that for the n-type materials. This was attributed to the band bending at the p-type material surface which caused positrons to drift away from the positron emitting surface.

Positronium formation at 4H SiC(0001) surfaces

“…The positron affinity is in good agreement with the previous report [31]. The positron work function also agrees with the experimental values determined so far [32][33][34][35].…”

“…Previous positron re-emission experiments on SiC(0001) surfaces were performed mostly using n-type SiC with no surface treatments [32][33][34][35][36][37]. Thus, the surfaces were probably covered with natural oxide layers, although sufficient positron reemission was observed.…”

Efficient positron trapping and extraction with a center-hole SiC remoderator

“…The distribution parameters were derived to be µ =2.1 ± 0.2 eV and σ =1.5 ± 0.3 eV, respectively. The value of µ is comparable to the absolute value of the positron work function for SiC [22][23][24][25][26]. The energy spread (σ) is slightly larger than the reported value (∼1 eV) for an incident energy of 1 keV [26].…”

“…Note that the incoming positron beam may have been affected by collisions between it and the CF 4 gas; however, the effect was considered negligible in our gas-pressure range [29]. A fraction of the positrons impinging on the SiC was remoderated and then re-emitted from the surface with an energy of approximately 2 eV (due to the positron work function) [24][25][26]. The re-emitted positrons were trapped in the potential barrier formed in front of the SiC with the CF 4 cooling gas.…”

“…A different approach to accumulate the LINAC-based beam is now under development by the GBAR Collaboration using a silicon carbide (SiC) remoderator instead of a nitrogen buffer gas [21]. Since n-type SiC crystals have negative positron work functions of approximately −2 eV, and the positron diffusion lengths in these materials are remarkably long, positrons impinging upon SiC are moderated and then efficiently re-emitted from the surface with a low energy of approximately 2 eV [22][23][24][25][26]. The efficiency of this re-emission process for n-type SiC can reach 60% for positrons of less than 1 keV incident energy [23,26], although the epithermal positron component may be included [25].…”