Local structure of the crystalline and amorphous states ofGa2Te3phase-change alloy without resonant bonding: A combined x-ray absorption andab initiostudy

Abstract

Phase-change memories are usually associated with GeTe-${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ quasibinary alloys, where the large optical contrast between the crystalline and amorphous phases is attributed to the formation of resonant bonds in the crystalline phase, which has a rocksalt-like structure. The recent findings that tetrahedrally bonded ${\mathrm{Ga}}_{2}{\mathrm{Te}}_{3}$ possesses a similarly large property contrast and very low thermal conductivity in the crystalline phase and undergoes low-energy switching [H. Zhu et al., Appl. Phys. Lett. 97, 083504 (2010); K. Kurosaki et al., Appl. Phys. Lett. 93, 012101 (2008)] challenge the existing paradigm. In this work we report on the local structure of the crystalline and amorphous phases of ${\mathrm{Ga}}_{2}{\mathrm{Te}}_{3}$ obtained from x-ray absorption measurements and ab initio simulations. Based on the obtained results, a model of phase change in ${\mathrm{Ga}}_{2}{\mathrm{Te}}_{3}$ is proposed. We argue that efficient switching in ${\mathrm{Ga}}_{2}{\mathrm{Te}}_{3}$ is due to the presence of primary and secondary bonding in the crystalline phase originating from the high concentration of Ga vacancies, whereas the structural stability of both phases is ensured by polyvalency of Te atoms due to the presence of lone-pair electrons and the formation of like-atom bonds in the amorphous phase.