Phase transformation through metastable structures in atomically controlled Se/Sb Multilayers

Multilayer films composed of individual layers of [Sb(8.84 Å)/Se(12.6 Å)](Sb4Se6), [Sb(8.84 Å)/Se(7.2 Å)]- (Sb4Se4), and [Sb(15.4 Å)/Se(7.2 Å)](Sb6Se4) were synthesized using effusion cells controlled at the subatomic scale. After an annealing process, the Sb4Se6 multilayered film with an Sb₂Se₃ orthorhombic structure had a high resistance and a clean valence band edge similar to that for a band shape of a semiconductor, whereas the Sb6Se4 film with an Sb rhombohedral structure and an Sb₂Se₃ orthorhombic structure had a low resistance and a band tail that originated from their metallic characteristics in the near Fermi level. In the case of Sb4Se4, a metastable Sb ₄Se₄ monoclinic structure was induced at an annealing temperature of 200 °C because of the unstable, local, and anisotropic distribution of each element in the vertical direction of multilayer films with a specific stoichiometry. Moreover, the nonbonding states originating from a band-gap state were generated in the film with a metastable structure. When the annealing process was conducted at 256 °C, the linear diffusion of elements in the film induced the most stable crystal structure with a stable stoichiometry. That is, the multilayered Sb4Se4 film underwent a steplike resistance change through a two-level phase change process. The findings indicate that a multilayered system with an atomically controlled thickness can be utilized to control the electrical resistance, metastable phase formation, and the valence band structure in an Sb-Se alloy system.