Photoinduced effect in Ga-Ge-S based thin films

Glassy films of Ga10Ge25S65 with 4 μm thickness were deposited on quartz substrates by electron beam evaporation. Photoexpansion (PE) (photoinduced increase in volume) and photobleaching (PB) (blue shift of the bandgap) effects have been examined. The exposed areas have been analyzed using perfilometer and an expansion of 1.7 μm (ΔV/V ≈ 30%) is observed for composition Ga10Ge25S65 exposed during 180 min and 3 mW/cm2 power density. The optical absorption edge measured for the film Ge25Ga10S65 above and below the bandgap show that the blue shift of the gap by below bandgap photon illumination is considerable higher (ΔEg = 440 meV) than ΔEg induced by above bandgap illumination (ΔEg = 190 meV). The distribution of the refraction index profile showed a negative change of the refraction index in the irradiated samples (Δn = −0.6). The morphology was examined using a scanning electron microscopy (SEM). The chemical compositions measured using an energy dispersive analyzer (EDX) indicate an increase of the oxygen atoms into the irradiated area. Using a Lloyd's mirror setup for continuous wave holography it was possible to record holographic gratings using the photoinduced effects that occur in them. Diffraction efficiency up to 25% was achieved for the recorded gratings and atomic force microscopy images are presented.