Luminescence due to dimer type self-trapped excitons in lead halides

Abstract

Recombination luminescence in lead halides (PbCl 2 and PbBr 2) has been investigated by the VUV light excitation above the band gap energy. An intense blue-green (BG) emission band is observed at 2.62 eV both in PbCl 2 and in PbBr 2. The same BG emission is also observed under illumination by the red light from a HeNe laser on the crystals pre-irradiated with X-ray. The temperature dependence of the BG emission intensity shows rather complicated behavior. As raising temperature, a prominent hump appears around 85 K, which is attributed to thermo-luminescence resulting from recombination of thermally activated holes with STEL centers. The intensity of the BG emission decreases at temperatures above 140 K, corresponding to thermal instability of self-trapped electron (STEL) centers. Time dependence of the luminescence after pulsed light excitation comprises two stages, namely, rather quick decay in the μs region and long afterglow up to several tens ms. Two channels are proposed for the BG emission; one is the direct recombination process between the STEL and the hole center, the other radiative annihilation of the STE of the [(Pb 2) 3+-STEL+hole] type.