Measurement of anisotropy in the thermal conductivity of Ge2Sb2Te5 films

This work measures both the out-of-plane and the in-plane thermal conductivities of Ge₂Sb₂Te₅ (GST) films in the amorphous (a), face-centered-cubic (c), and hexagonal close packed (h) phases using the 3¿ method. The out-of-plane thermal conductivity, k_z, is first measured from a wide heater. The data include 0.17 W/mK for a-GST, 0.55 W/mK and 1.01 W/mK for c-GST of annealing temperature 150°C and 200°C respectively, and 2.85 W/mK for h-GST. The GST film is rf-sputtered with film thickness nearly 600 nm to capture significant lateral spreading from a narrow heater. The measured out-of-plane thermal conductivity is used in the multi-layer heat diffusion equation to deduce the in-plane thermal conductivity, k_r. The measurements from the varying heater width reveal that the anisotropy ratio (¿ = k_r/k_z) to be 1 for amorphous GST and ~0.6 for different forms of crystalline GST. Electrothermal simulation demonstrates that the thermal conductivity anisotropy possesses potential impact on the designs of novel phase change memory that are based on lateral structure.