Study of isotopic silver dissolution in vitreous GeSex thin films from secondary ion mass spectrometry measurements

Abstract

Silver dissolution in vitreous GeSex thin films was investigated by secondary ion mass spectrometry (SIMS) measurements with 109Ag and 107Ag isotopes. Three compositions were used: x = 3, 4.5 and 5.5. After dissolution of the first 107Ag layer evaporated on the GeSex film, an 109Ag layer was deposited and dissolved. The expected profile was either a first 109Ag doped region near the surface followed by a deeper region doped by 107Ag or the opposite. Surprisingly, 107Ag and 109Ag have the same depth profiles and their concentrations are proportional to the initial silver thickness, as long as the saturation threshold is not reached. These data are qualitatively explained by a two-step model: silver ions begin to move along paths originating from the metal-chalcogenide interface defects (dendritic phase). Simultaneously, lateral diffusion of silver from these dendrites occurs with a low rate, which gives a step-like concentration profile Cp0. The first phase does not provide a SIMS signal because the silver concentration is too low to decrease the resistivity of the doped region (charge phenomenon). When dendrites reach the GeSexSi interface, the silver concentration at this interface increases up to Cp0 because of the numerous dangling bonds in this region. So an intermediary region poor in silver is observed between this region and the first region. Then, if the silver metal layer is not yet exhausted, the silver concentration of the whole chalcogenide film increases up to the saturation threshold Cp0 and the silver-poor intermediary region disappears. Furthermore, SIMS data obtained from negative secondary ions revealed the existence of an oxygen-rich region at the GeSexSi substrate interface.