X-ray photoemission from small mercury clusters on II-VI semiconductor surfaces

The presence of small Hg clusters (R=5–20 Å) on ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te samples grown by molecular-beam epitaxy has been deduced from a careful analysis of the x-ray-induced photoemission spectra. The positive binding-energy shift measured for these clusters is explained by the appearance of a positive charge on the clusters during the photoemission process. (The experimental results are compared with the calculated $e^2$/2R behavior.) The apparent spin-orbit splitting for the Hg 5d levels is reduced, compared to bulk Hg and to isolated Hg atoms. This is attributed to the repulsion between the Cd 4d and Hg 5d orbitals. It is shown that Hg out-diffusion is the main reason for the formation of these clusters.