Annihilation of positrons trapped at the (100) and (111) surfaces of Si

We present results of theoretical studies of positron surface states and positron annihilation characteristics at the clean non reconstructed(100) and (111) surfaces of Si performed within the modified atomistic, superposition method. It is found that in the case of non reconstructed semiconductor surfaces, the positron surface state is localized mainly on the vacuum side of the topmost layer. The computed positron surface 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 functions with respect to the vacuum for bulk Si(100) and Si(lll) yielded 2.34 and 2.23 eV, respectively demonstrating the stability of positron 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 surfaces, and compared with available experimental data.