ANNIHILATION OF POSITRONS TRAPPED AT THE (100) AND (111) SURFACES OF SI

We present results of theoretical studies of positron surface states a nd positron annihilation characteristics at the clean non reconstructe d(100) and (111) surfaces of Si performed within the modified atomisti c, superposition method. It is found that in the case of non reconstru cted semiconductor surfaces, the positron surface state is localized m ainly on the vacuum side of the topmost layer. The computed positron s urface state energies E-b at the (100) and (111) surfaces of Si are - 2.81 and - 2.69 eV. In addition, calculations of the positron work fun ctions with respect to the vacuum for bulk Si(100) and Si(lll) yielded 2.34 and 2.23 eV, respectively demonstrating the stability of positro n surface state on these surfaces. The positron surface state lifetime as well as probabilities for a positron trapped in a surface state to annihilate with relevant core-level electrons are computed for both s urfaces, and compared with available experimental data.