Impact of Sn on the Ti/Ge solid-state reaction: Phase formation sequence, morphological and electrical properties

Abstract

The impact of Sn on the solid-state reaction between Ti thin films and Ge(Sn) layers was studied. Thanks to in-situ X-ray diffraction (XRD) analyses, the phase formation sequence was first studied. Ti5Ge3 (resp. Ti5(GeSn)3) phase was formed first. This phase was consumed to the benefit of Ti6Ge5 (resp. Ti6(GeSn)5). At higher temperatures, TiGe2 (resp. Ti(GeSn)2) was the last phase to appear. The phase formation sequence appeared to be strongly impacted by the presence of Sn into the system. With Sn addition, the formation of the Ti6Ge5 phase was delayed while the Ti5Ge3 and TiGe2 phases formation was promoted at lower temperatures. These phenomena were linked to the fact that the Ti6Ge5 (resp. Ti6(GeSn)5) formation was limited by diffusion-controlled reaction while the TiGe2 formation was controlled by nucleation.The morphological evolution of Ti/Ge and Ti/GeSn systems was comparable. For a given temperature, GeSn-based samples were however slightly rougher than Ge-based samples. It was linked to Sn segregation (i.e. long-scale diffusion of Sn). On the other hand, the electrical properties evolution differed from one system to the other. For the Ti/Ge system, Rsh values remained relatively constant over the temperature range studied. Meanwhile, for the Ti/GeSn system and beyond 450 °C, a significant decrease of the resistivity was observed. It was linked to the Sn segregation and the subsequent formation of metallic Sn.