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From atom to system: first-principles thermal modeling with ALMA-BTE
Keywords: thermal modeling, multiscale, first-principles
Heat in contemporary electronic devices is often generated by submicron active regions and/or must find its way towards ambient through intricate multi-layered substrates. The thermal performance of such structures can substantially deviate from conventional predictions because the fundamental carriers of heat energy (phonons) no longer obey the basic assumptions that underpin classic Fourier diffusion. One-micron thick silicon films, for example, are about 40% less conductive than bulk substrates due to the increased phonon scattering by the film boundaries. Advanced theories such as the Boltzmann transport equation (BTE) therefore become necessary to obtain realistic estimates of thermal device performance. Our presentation will give an overview of the latest developments in this field through findings obtained with our 'ALMA-BTE' solver [1]. This free and open-source software, currently in public beta with full release set for December, already outperforms its well established ‘ShengBTE’ predecessor in terms of both computational efficiency and scope. At its heart, the program employs first-principles (parameter-free) phonon dispersions and scattering rates to enable multiscale thermal simulations for a wide array of semiconductor materials. I will illustrate technical capabilities, as well as how some of the generated insights can be directly incorporated into conventional modeling approaches, through several industry-inspired case studies including (1) experimentally validated thermal conductivity calculations of InGaAs superlattices used in quantum dot lasers and solar cells; (2) extraction of cross-plane and in-plane thermal conductivities for a variety of compounds as a function of film thickness in compact parametric form; and (3) Monte Carlo simulations of multilayer substrates for GaN-based high electron mobility transistors (HEMTs) and light emitting diodes (LEDs). The GaN study in particular highlights the nondiffusive nature of thermal transport in microscale devices and the dominant role played by material interfaces therein. [1]
Bjorn Vermeersch, Research Engineer
Grenoble, Rhone-Alpes

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