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Photoluminescence ͑PL͒ and electroluminescence ͑EL͒ of blue and green SrS:Cu thin films prepared by atomic layer epitaxy and molecular beam epitaxy, and blue SrS:Ag,Cu,Ga thin films prepared by sputtering were studied in a temperature range of 80-320 K. Two bands are present both in the PL and EL spectra of SrS:Cu films. The low energy band ͑L band͒ at about 520 nm can be observed throughout the temperature range studied. The high energy band ͑H band͒ at about 460 nm is seen only at higher temperatures and it disappears at 80 K. Decay studies at 80 K reveal at least two types of centers having lifetimes of 100 and 17 s with a 10% variation. Several luminescence processes are proposed for further discussions. The L band may be attributed to the emission from an isolated Cu ϩ ion replacing the host Sr 2ϩ ion but at an off-center site, and it undergoes a redshift caused by aggregated Cu centers. The H band is tentatively described as due to the Cu ϩ ion at different site symmetry. Blue luminescence of SrS:Ag,Cu,Ga most probably originates from Ag ϩ pairs. There is no direct evidence of energy transfer from Cu ϩ to Ag ϩ but Cu does contribute to the EL.

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We show that thermal-positron reemission and positronium formation at clean Cu(lll) and Al(llO) surfaces are strongly reduced at low temperatures. This is explained by quantum-mechanical reflection of the positron wave function from the surface potential. Analysis of the data yields first estimates of the different transition rates for positrons at the surface. Information on the nature of the positron-surface interaction can be obtained. We discuss the implications of our findings to other surface emission and sticking processes.