Abstract
Ultraviolet Nanosecond Laser Annealing (LA) is a powerful tool for both fundamental investigations of ultrafast, nonequilibrium phase-change phenomena and technological applications (e.g., the processing of 3D sequentially integrated nano-electronic devices) where strongly confined heating and melting is desirable. Optimizing the LA process along with the experimental design is challenging, especially when involving complex 3D-nanostructured systems with various shapes and phases. To this purpose, it is essential to model critical nanoscale physical LA-induced phenomena, such as shape changes or formation and evolution of point and extended defects. To date, LA simulators are based on continuum models, which cannot fully capture the microscopic kinetics of a solid–liquid interface. In this work a fully atomistic LA simulation methodology is presented, based on the parallel coupling of a continuum, finite elements, μm-scale electromagnetic-thermal solver with a super-lattice Kinetic Monte Carlo atomistic model for melting. Benchmarks against phase-field models and experimental data validate the approach. LA of a Si(001) surface is studied varying laser fluence and pulse shape, assuming both homogeneous and inhomogeneous nucleation, revealing how liquid Si nuclei generate, deform and coalesce during irradiation. The proposed methodology is applicable to any system where the atom kinetics is determined by a strongly space- and time-dependent field, such as temperature or strain.
Highlights
The ability of heating and melting solid materials over small space- and time scales allows for accessing the early stages of the melting phenomenon, characterized by the nucleation of the molten phase and the ultra-rapid liquid nuclei kinetics[1,2]
Self-consistent, fully open-source simulation tool which enables a seamless coupling of a continuum mesoscale finite element method (FEM) electromagnetic-thermal problem, solved using the FENICS computing platform[29], with the kinetic Monte Carlo (KMC) scheme implemented in the MulSKIPS code[30–32]
The procedure is based on a self-consistent coupling between the continuum and atomistic models, contrary to sequential coupling approaches[13], where the thermal problem is first solved over the whole pulse duration via, e.g., a phase-field formalism, and coupling with KMC occurs only afterwards, through mapping of the space- and time-dependent temperature into the KMC
Summary
The ability of heating and melting solid materials over small space- and time scales allows for accessing the early stages of the melting phenomenon, characterized by the nucleation of the molten phase and the ultra-rapid liquid nuclei kinetics[1,2]. The main technological challenge is to design the nanosecond laser annealing (LA) process together with the device design, in order to optimize the topography and the materials’ choice, in 3D sequentially integrated architectures, and in other complex patterned structures, featuring low-dimensional, nm-sized elements with various shapes and phases[12–14] Tackling this challenge requires predictive simulation tools, with reliable calibrations for the materials of interest, able to model the relevant physical phenomena at various length and time scales while keeping the computational cost affordable. In this work we present a tool for fully atomistic simulations of phase transitions occurring during an LA process of group IV elemental or compound semiconductors, such as Si, Ge, SiGe, and more It is based on a multiscale algorithm that seamlessly couples a continuum model, based on the finite element method (FEM), for self-consistently solving the electromagnetic (Maxwell) and heat diffusion (Fourier) problem of an irradiated 3Dnanostructured system with an atomistic super-lattice kinetic Monte Carlo (KMC) model.
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