Thermal transport of amorphous phase change memory materials using population-coherence theory: a first-principles study

Abstract

Thermal conduction plays a vital role in applications of phase change memory (PCM) materials. Phonon-based theory and the Wiedemann–Franz–Lorenz rule have been widely utilized to describe the thermal transport in crystalline PCM materials, while the understanding of heat conduction in the amorphous phase remains insufficient. Here, we quantify the contributions of the coherences (coupling of vibrational modes) and populations (phonon-like) to the thermal conduction of amorphous Ge2Sb2Te5 (GST) and GeTe4, two kinds of typical PCM materials. The contributions of the coherences and populations are calculated using the theory proposed by Allen and Feldman (AF theory) and the single-mode relaxation time approximation of the Boltzmann transport equation based on first-principles calculations. Our results demonstrate that coherences contribute more than 97% of the total thermal conductivities for both amorphous GST and GeTe4 above Debye temperature, while the populations’ contribution is negligible. Besides, the temperature dependence of the thermal conductivities is predicted and analyzed, allowed by the AF theory with the mode linewidths-dependent broadening method introduced in this paper. The predicted positive temperature dependence of amorphous GeTe4 above Debye temperature, in good agreement with the experimental results, is due to the unique nature of coherences, i.e. larger contribution to heat conduction from stronger couplings between different normal modes at a higher temperature. Our calculation provides new insight into thermal transport in amorphous PCM materials and reveals the physical mechanisms of temperature-dependent thermal conductivities above Debye temperature, and the calculation framework can also be extended to other disordered systems.