Thermal conductivity of chalcogenide glass phases governed by disparity in phonon group velocity

Germanium telluride (GeTe) can change its electrical resistance by up to eight orders of magnitude from just a small temperature change, making it a promising choice for memory storage. However, little is known about germanium telluride’s thermal properties at the nanoscale. A new article reports findings from a systematic study on the thermal conductivity of GeTe thin-films with thickness ranging from 30 to 1200 nm.

Warzoha et al. found that in germanium telluride, the heat energy carriers experience similar scattering mechanisms in both the crystalline and amorphous phases, despite significant disparity in their atomic structures.

They measured the thin-film samples using frequency-domain thermoreflectance. The samples were first coated with an 80 nm thick gold layer that acted as a transducer, before being irradiated with a 405 nm laser modulated at between 4 kHz and 20 MHz. A 532 nm laser was used to probe the reflectance of the gold surface to obtain the temperature of the system.

The authors were surprised to find that, unlike the electrical properties, the thermal properties of the thin films did not vary as much between phases. This suggests that the scattering mechanisms governing heat energy carriers are similar in both phases. Although one might expect the long range order of the crystalline phase would result in a higher thermal conductivity, phonon scattering mitigates against that. Instead, the authors identified the phonon group velocity as the likely culprit for the difference in thermal conductivity across the phase transition.

Their calculations of the phonon mean-free path agree with their experimental results, giving values of 63 nm for crystalline and 45 nm for the amorphous phase. Given the similar mean free paths, differences in thermal conductivity can be attributed to disparities in phonon group velocity.

Source: “Nanoscale thermal transport in amorphous and crystalline GeTe thin-films,” by Ronald J. Warzoha, Brian F. Donovan, Nicholas T. Vu, James G. Champlain, Shawn Mack, and Laura B. Ruppalt, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5098334 .