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 Ge 2 Sb 2 Te 5 (GST) and GeTe 4 , 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 GeTe 4 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 GeTe 4 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.

Assessing the thermal conductivity of amorphous SiN by approach-to-equilibrium molecular dynamics

“…This undermines any assumption on a phonon-like mode of propagation. In the amorphous Ge-based GeTe [58], GeTe 4 and Ge 2 Sb 2 Te 5 [59] on the other hand, the above models concur with a negligible contribution of the heat propagation, in favor of a diffuson-based process. This last result is in contrast with the length dependence of the thermal conductivity observed by AEMD for amorphous GeTe 4 , Ge 2 Sb 2 Te 5 and SiN, which extends to hundreds of nanometers and is more in favor of propagation, since it closely follows This is the author's peer reviewed, accepted manuscript.…”

Bonding‐Enhanced Interfacial Thermal Transport: Mechanisms, Materials, and Applications

“…To be exact, although phonon successfully describes energy transfer in crystals, its validity cannot be extended in water and many amorphous solids, like polymers, gels, a-Si, and glass, due to lack of periodic long-range order. Wave-like tunneling conduction mechanism, named diffusons [148,149] or coherences, [150] has been proved to be dominant in amorphous solids and non-negligible in low-thermal conductivity complex crystals. The tunneling conduction is from coherences between pairs of vibrational modes, assuming great importance when the mode linewidths are comparable to or larger than the interbranch spacings, which is beyond the well-established phonon Boltzmann transport equation.…”

Thermal smart materials and their applications in space thermal control system

“…(GST)系合金, 可以在室温临界点下在不同相态间 切换 [22,23] . 低温下, GST为无定型相, 声子振动模 态间的耦合为热传导做出了绝大部分贡献 [24] , 热 导率低; 随温度升高, GST发生相转变, 变为六方 相, 电子对导热的贡献增加, 热导率也得到提高, 由此可以实现r = 2的调控幅度.…”