Optical properties of amorphous As2S3 doped with lanthanum oxide and erbium oxide

The photoluminescence (PL) spectra of amorphous As2S3 samples are investigated in a wide temperature interval by using the optical and ellipsometric methods. PL spectra under radiation with excitation energy smaller than Tauc's edge energy allows us to determine all defect states in the forbidden band of amorphous As2S3. The ellipsometric investigation of a - As2S3 on reflection indicates precise anomaly of real and imaginary parts of the dielectric function around the transition edge corresponding to the inter-band transition, determined from photoluminescence measurements. PL investigation of La2O3, Er2O3 and As2S3: (La2O3)(Er2O3) compounds from nearest ultra-violate up to infra-red (IR) energy interval shows a coincidence of many absorption lines of La2O3 and Er2O3, furthermore surviving of these lines in As2S3 : (La2O3)(Er2O3). These facts allow us to conclude that S atoms in As2S3 : (La2O3)(Er2O3) replace O atoms, and that the discrete PL lines observed as in La2O3, Er2O3 as well as in As2S3 : (La2O3)(Er2O3)belong to La and Er atoms. Comparison of PL spectra lines of La2Os and As2S3 : (La2O3) 5% with those of Er2O3 and As2S3 : (Er2O3) 5% shows that the intensity of the former lines is 2-3 order stronger than those of the latter samples' lines. Nevertheless this picture is reversed for amorphous As2S3 samples simultaneously doped with La2O3 and Er2O3, which confirms that a cross relaxation from excited states of La to Er takes place. It is worthy to note that the PL spectra in IR-region were firstly measured in this work in order to get a complete information on the localized states density in the forbidden band of a - As2S3.