Relation between density and optical contrasts upon crystallization in Cr₂Ge₂Te₆ phase-change material: Coexistence of a positive optical contrast and a negative density contrast

In this study, the relation between density contrast and optical contrast for Cr₂Ge₂Te₆ (CrGT) was studied. The CrGT showed negative density contrast between its as-deposited amorphous and crystalline phases. We confirmed that the CrGT satisfies the linear relation between the density contrast and optical contrast at wavelength of 630 nm, consistent with the behaviors of reported phase-change materials (PCMs). In CrGT films annealed at temperatures below 350 °C, the films exhibited a negative density contrast and negative optical contrast at 630 nm, consistent with the Clausius-Mossotti relationship. However, the 380 °C-annealed film showed a positive optical contrast at 630 nm due to a drastic increase in reflectance even with the negative density contrast. The absorption coefficient measurements as a function of wavelength indicated that there is no electron delocalization in the crystalline CrGT phase. It was found that the crystalline CrGT exhibited a drastic increase in the refractive index upon annealing at 380 °C at a wavelength range of visible light, causing an increase in the reflectance. Similar behavior was also observed in ultraviolet (330 nm) and infrared light (1000 nm) ranges. Such a large difference in refractive index in a wide wavelength range covering from ultraviolet to infrared light ranges introduces the possibility to realize the CrGT-based metasurface.