Understanding Thermal Transport in Amorphous Si Using Wigner Transport Equation

Amorphous materials are indispensable in various technological domains, powering nanoscale electronics [1] and facilitating affordable solar energy solutions [2]. Their thermal transport properties significantly influence the efficiency and durability of associated devices. However, understanding heat carriers in these materials often relies on the decades-old yet incomplete Allen-Feldman (AF) theory [3] and the conventional classification of three thermal carriers, leaving considerable room for improvement.

Despite notable progress in recent decades, ambiguities in classifying these heat carriers persist, complicating the assessment of their relative contributions to thermal conductivity. A commonly used approach involves identifying a frequency crossover (w_c) to differentiate propagons and diffusons. However, the lack of a precise definition for w_c often leads to the adoption of arbitrary criteria, resulting in inconsistencies across studies. For instance, varying w_c definitions have caused substantial discrepancies in estimating the contribution of propagons to the total thermal conductivity of amorphous silicon (a-Si), with reported values ranging from 25% to 90% at room temperature.

To address these challenges, we conducted numerical simulations on a-Si using the Stillinger-Weber (SW) potential, a framework validated in prior studies. We employed the Wigner transport formalism [4], which explicitly incorporates anharmonic effects. Our findings reveal that amorphous Si are not computationally equivalent to crystals with large, disordered primitive cells as suggested by an earlier study on amorphous SiO₂ [5]. The two-mode terms in the Wigner transport formalism can effectively characterize the thermal transport properties of all vibrational modes, suggesting an alternative to the traditional mode-classification schemes required by the AF theory.