Towards an integrated modeling of the plasma-solid interface

Abstract

Solids facing a plasma are a common situation in many astrophysical systems and laboratory setups. Moreover, many plasma technology applications rely on the control of the plasma-surface interaction, i.e. of the particle, momentum and energy fluxes across the plasma-solid interface. However, presently often a fundamental understanding of them is missing, so most technological applications are being developed via trial and error. The reason is that the physical processes at the interface of a low-temperature plasma and a solid are extremely complex, involving a large number of elementary processes in the plasma, in the solid as well as fluxes across the interface. An accurate theoretical treatment of these processes is very difficult due to the vastly different system properties on both sides of the interface: quantum versus classical behavior of electrons in the solid and plasma, respectively; as well as the dramatically differing electron densities, length and time scales. Moreover, often the system is far from equilibrium. In the majority of plasma simulations surface processes are either neglected or treated via phenomenological parameters such as sticking coefficients, sputter rates or secondary electron emission coefficients. However, those parameters are known only in some cases and with very limited accuracy. Similarly, while surface physics simulations have often studied the impact of single ions or neutrals, so far, the influence of a plasma medium and correlations between successive impacts have not been taken into account. Such an approach, necessarily neglects the mutual influences between plasma and solid surface and cannot have predictive power.In this paper we discuss in some detail the physical processes a the plasma-solid interface which brings us to the necessity of coupled plasma-solid simulations. We briefly summarize relevant theoretical methods from solid state and surface physics that are suitable to contribute to such an approach and identify four methods. The first are mesoscopic simulations such as kinetic Monte Carlo (KMC) and molecular dynamics (MD) that are able to treat complex processes on large scales but neglect electronic effects. The second are quantum kinetic methods based on the quantum Boltzmann equation that give access to a more accurate treatment of surface processes using simplifying models for the solid. The third approach are ab initio simulations of surface process that are based on density functional theory (DFT) and time-dependent DFT. The fourths are nonequilibrium Green functions that able to treat correlation effects in the material and at the interface. The price for the increased quality is a dramatic increase of computational effort and a restriction to short time and length scales. We conclude that, presently, none of the four methods is capable of providing a complete picture of the processes at the interface. Instead, each of them provides complementary information, and we discuss possible combinations. PACS numbers:

Ion Impact Induced Ultrafast Electron Dynamics in Finite Graphene‐Type Hubbard Clusters

“…), where quantum transitions inside the projectile are taken into account. However, the uncertainties in the quality of the adiabatic LDA and the model parameters in the Newns‐Anderson model, respectively, as well as the neglect of correlation effects in the material make it desirable to develop an independent many‐body approach to this problem.…”

“…In low‐temperature plasma physics, this process is routinely used to clean surfaces from adsorbates or modify them via sputtering for example ref. or as a source of secondary electrons . On the other hand, ions impacting a solid can be used as a diagnostic tool of the electronic structure of the material by measuring the energy loss (or stopping power or stopping range) as a function of impact energy …”

Microplasmas for Advanced Materials and Devices

“…With the advent of radically new concepts and approaches to describing and quantifying elementary processes on plasma‐solid surfaces, at atomic‐to‐microscales such as the “plasma interface,” there is a clear need to apply these and other concepts and methods to better understand and control microplasma–NP interactions, ideally from the moment of initial nuclei formation. The ability to describe specific phase formation and extend this description to multi‐material, multi‐phase systems, represents a significant challenge.…”

Time‐dependent simulation of ion stopping: Charge transfer and electronic excitations

“…On the other hand, NEGF simulations of charge transfer have recently been attempted for simple model systems (1D Hubbard chains) in Bonitz et al [12,75] . Even though charge transfer is most likely not relevant for the kinetic energy loss of the projectile, it may well affect the electronic processes in the target, including doublon excitations.…”

“…Even though charge transfer is most likely not relevant for the kinetic energy loss of the projectile, it may well affect the electronic processes in the target, including doublon excitations. This will be particularly important for the treatment of Auger-type processes in the interaction of an ion or atom with the surface which are straightforwardly described within NEGF already with second-order Born selfenergies, for example, Bonitz et al [12] and Covito et al [82] These combinations of TDDFT and NEGF are expected to be of high importance for the development of accurate simulations of low-temperature plasma-surface interaction. However, presently none of the two methods is capable of incorporating the missing effect.…”