Quantitative characterization of photodoping phenomena in amorphous chalcogenide GeS_2 film

摘要

When metal layers like Ag or Cu deposited on the amorphous chalcogenide films such as As_2S_3, As_2Se_3, GeS_2, and GeSe_2, are illuminated by light, the metal elements diffuse abnormally into the amorphous chalcogenide layer. This abnormal diffusion of metal was found by Kostyshin in Russia in 1965 and called photodoping. The large modulation of refractive index is realized by the doping of large amount of Ag atoms into the amorphous network of chalcogenide film which gives the possibility for various photonic device applications. In this study, the quantitative characterization of photodoping phenomena was carried out for amorphous GeS_2 films using Ag as a doping metal element to obtain the basic information to the photonic device fabrication. Quantum efficiency to reach the saturation of the doping was derived using the laser diodes with different wavelengths which cause a photodoping phenomenon. As for the wavelength dependence of the photodoping, it was suggested that the quantum efficiency was enhanced for the irradiated photon energy near the optical gap energy (ca. 3.3eV) of a-GeS_2 and showed the tendency similar to the absorption spectrum of the amorphous GeS_2 film. Photodoping rate was studied for various irradiated power densities of a He-Cd laser (441.5 nm) and the photodoping rate was proportional to the number of incident photons at low intensity. For the intensity over 10mW/cm~2, the enhancement of photodoping rate was observed due to some extra effects like a thermal effect.