Investigating the Influence of Resonant Bonding on the Optical Properties of Phase Change Materials (GeTe)xSnSb2Se4

Thin film samples of Ge 9 SnSb 2 Te 9 Se 4, Ge 4.5 SnSb 2 Te 4.5 Se 4, Ge 2.5 SnSb 2 Te 2.5 Se 4, and GeSnSb 2 TeSe 4 were prepared via co-sputtering of GeTe and SnSb 2 Se 4 and compared to the well-investigated phase change material GeTe. All samples were obtained in an amorphous state. Temperature-dependent in situ X-ray diffraction experiments reveal a crystallization temperature that increases with an increasing SnSb 2 Se 4 content, leading to a higher stability of the amorphous phase. The electrical contrast between the amorphous and metastable crystalline state investigated via the van der Pauw method is as large as 4 orders of magnitude up to a 4.5:1 GeTe:SnSb 2 Se 4 ratio. Increasing the SnSb 2 Se 4 content leads to a decrease in the electrical contrast. Investigations of the samples by applying Fourier transform infrared spectroscopy and variable incident angle spectroscopic ellipsometry show that the optical properties of the amorphous phase are not affected by changes in stoichiometry. In striking contrast, the impact of SnSb 2 Se 4 on the optical properties of the crystalline phases is significant: all optical constants decrease because of the reduction in the level of resonant bonding, which leads to a reduced absolute reflectivity of the crystalline phase resulting in a decrease in the optical contrast. These results support the assumption that resonant bonding is crucial for successful optical phase change memory materials.