Dielectric spectroscopic investigation of reversible photo-induced changes in amorphous Ge2Sb2Se5 thin films

Abstract

Broadband dielectric spectroscopy (BDS) was used to study thin films of Ge2Sb2Se5 exposed to low fluence UV-irradiation that resulted in thermally reversible photo-induced changes (PICs) on the film surfaces. Changes were characterized by reversible changes in film morphology and changes in the microwave scattering signature prior to and following irradiation. The PIC resulting from irradiation formed a low modulus and electrically resistive layer at the film surface. Modest heating of the photo-exposed material to about 60 °C anneals out the photo-formed layer to leave behind a very thin layer of smooth, thermally stable material at the film surface. Re-exposure of the material to UV-light recreated a foamy layer, which anneals away with modest heating over several cycles. The broadband microwave insertion loss (S21) increased with the film morphology changes after UV light exposure and decreased with thermal anneal over several cycles. These changes are correlated with transformations in the film's surface morphology and possibly structural modifications in the amorphous film. Thus, the BDS analysis provided interesting new insights into the nature of photo-induced processes in chalcogenide films, such as the electrical resistance consequences of morphological/structural changes.