Photo- and electroluminescence of SrS:Cu and SrS:Ag,Cu,Ga thin films

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Abstract
Photoluminescence (PL) and electroluminescence (EL) 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 band) at about 520 nm can be observed throughout the temperature range studied. The high energy band (H band) 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 μs 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+ ion replacing the host Sr2+ 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+ ion at different site symmetry. Blue luminescence of SrS:Ag,Cu,Ga most probably originates from Ag+ pairs. There is no direct evidence of energy transfer from Cu+ to Ag+ but Cu does contribute to the EL.

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