We study the diffusion of metal atoms into phase change chalcogenides. Recently, phase change materials have been investigated for active photonics applications. They have been applied in tuneable polarisation-independent perfect absorbers, optical imaging devices, nano-displays, active nano-photonics, and reconfigurable optical circuits. However, many recent publications do not consider reactions and diffusion at the interface between the metal and chalcogenide layers, which may occur. The diffusion influences the properties of the phase change layer, such as the crystallisation kinetics and optical constants[1-2]. Here, we study the interface between the phase change material, Ge2Sb2Te5, and different metal layers using X-ray reflectivity (XRR) and reflectometry of metal/phase change chalcogenide stacks. We find that a diffusion barrier layer, such as Si3N4, can help to prevent the interfacial diffusion of the metal and Ge2Sb2Te5 layers.
We are particularly interested in phase change material tuned plasmonic structures, where metals such as Au, Ag, and Al are commonly interfaced with Ge2Sb2Te5. Our XRR analysis showed that heating Si/Au/Ge2Sb2Te5 samples results in severe interfacial roughening. However, inter-diffusion is readily prevented by a 10 nm thick Si3N4 layer deposited between the Au and Ge2Sb2Te5 layers. We find that in this structure the XRR fringes are present even after annealing at a temperature of 573 K. This indicates that Si3N4 prevents the interfacial damage between metal layers and the Ge2Sb2Te5. Since metallic diffusion into Ge2Sb2Te5 may affect the crystallisation temperature and optical properties, we expect this structure to exhibit stable and cycleable optical switching.
We conclude that Al, Au, and Ag readilly diffuse into Ge2Sb2Te5 when the structure is heated to 573 K. However, the diffusion can be prevented by adding a Si3N4 diffusion barrier layer, which indicates that the diffusion barriers must be included in phase change material tuned plasmonic devices.
This research work was supported by the Singapore Ministry of Education project T1MOE1703 “Advanced Intelligent Materials (AIM)” and the A-star Singapore-China joint research program (grant: 1520203155).
 Pandian R, Kooi B, De Hosson J, Pauza A. Influence of capping layers on the crystallization of doped SbxTe fast-growth phase-change films. Journal of Applied Physics. 2006;100(12).
 Kozyukhin S, Kudoyarova V, Nguyen H, Smirnov A, Lebedev V. Influence of doping on the structure and optical characteristics of Ge2Sb2Te5amorphous films. physica status solidi (c). 2011;8(9):2688-2691.