It is shown that growth of homoepitaxial CdTe films on CdTe (110) substrates by the atomic layer epitaxy method can lead to high structural perfection. The electronic structure of these films with the thickness of approximately 100 Å was studied at room temperature by means of angle-resolved photoemission. In addition to several bulklike features observed in the spectra an intrinsic surface state is believed to be found at the energy of 0.33 eV below the valence-band maximum for the first time in epitaxially grown II-VI compound semiconductor films. The work function of the films was found to be 5.75 eV±0.05 eV independent of whether the growth was terminated by a Cd pulse or Te2 pulse.
The positron and positronium emission yields are measured as a function of temperature at Cu(111) and Al(110) surfaces having a negative positron work function, and at the positive-workfunction surfaces Ag(100) and Ag(111). At the Cu(111) positron (e+) emission yield is reduced at low temperatures, vanishing as T~O K. The positronium (Ps) emission exhibits a similar temperature dependence at Cu(111) and Al(110). The dominating temperature-dependent feature at these negative-e+-work-function surfaces is the reflection of the e+ wave from the surface potential. Using a simple one-dimensional model we obtain estimates for the transition rates for surface trapping, e+, and Ps emission to be of the order of 10' m/s. Ps-emission probabilities for Ag(100) andAg(111) surfaces show drastically different temperature behavior. In order to explain these results we propose a new surface-trapping mechanism, the acoustic-phonon-mediated trapping. This would imply a weak temperature dependence of Ps emission at Ag surfaces.
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