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.