Effect of visible light on the structural and optical properties of (Ge2Sb2Te5)100−x Ag x (x = 0, 1 and 3) thin films

Abstract

In this work, the effect of visible light on the structural, morphological and optical properties of $$(\hbox {Ge}_2\hbox {Sb}_2\hbox {Te}_5)_{100-x}\hbox {Ag}_x$$ ( $$\hbox {x} = 0, 1$$ and 3) thin films prepared by thermal evaporation was studied. Deposited thin films were exposed to visible light of intensity $$10^{4}$$ and $$10^{5}$$ Lux for the duration of 2, 4, 8 and 20 h. Prepared samples were characterized with X-ray diffraction (XRD), Scanning electron microscope (SEM) and UV–Vis–NIR spectrophotometer. The amorphous nature of as-deposited and visible light exposed thin films was confirmed by XRD. The SEM images did not show significant change in the morphology of thin films after visible light exposure. The optical transmission of as-deposited and exposed thin films was measured by UV–Vis–NIR spectrophotometer in the wavelength range of 500–3300 nm. Optical transmission was used to calculate the absorption coefficient and optical band gap. Photo-darkening (decrease in optical band gap) with light exposure was observed in $$(\hbox {Ge}_2\hbox {Sb}_2\hbox {Te}_5)_{100-x}\hbox {Ag}_x$$ ( $$\hbox {x} = 0, 1$$ and 3) thin films. Photo-darkening may be attributed to the enhanced lone pair-lone pair interactions which broaden the valence band and hence decrease the optical band gap. Thus, change in optical parameters is because of change in local structure of deposited thin films.