The influence of palladium impurities on vacancy diffusion in cubic silicon carbide
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The influence of palladium impurities on vacancy diffusion in cubic silicon carbide Guido Roma1 1 CEA, DEN, Service de Recherches de Métallurgie Physique, F-91191 Gif sur Yvette, France.
ABSTRACT The basic properties of palladium impurities in silicon carbide, such as solubility or diffusion mechanisms, are far from being well understood. In a recent paper I presented a systematic study of stability and kinetic properties of Pd in cubic silicon carbide using first principles calculations. In this paper I focus on the effect of the presence of palladium in silicon carbide, even in very low concentrations, on the kinetic properties of carbon vacancies. I apply a model of Pd diffusion through a vacancy mechanism on the carbon sublattice and extract the correlation factors leading to an enhancement of vacancy migration, due to the coupling of diffusion fluxes between vacancies and palladium impurities. INTRODUCTION The role of palladium impurities in silicon carbide is relevant for several applications, encompassing contacts for high power/high frequency semiconductor devices, catalysis, hydrogen sensing, and nuclear fuel cladding for helium cooled fission reactors. However, the stability and kinetics of palladium impurities in silicon carbide are still largely unknown. In order to shed light on the role of palladium, in particular concerning the formation kinetics of palladium silicides, I recently performed a first principles study of the stability of Pd in cubic SiC and of its kinetic behaviour [1]. The main results were that Pd in SiC preferentially occupies substitutional positions, either a carbon (PdC) or a silicon site (PdSi), depending on silicon-rich or carbon-rich conditions. In spite of the difficulties of standard density functional theory approaches for the description of charged defects, we can safely assume that Pd is mostly in a doubly positive charge state when occurring as PdC, and neutral when PdSi. It is expected to be preferentially neutral when sitting in an interstitial position surrounded by carbons. Concerning kinetics, the migration energy of an interstitial palladium is much lower than that of a substitutional one, if one looks at direct exchange mechanisms. However, some mechanisms for the migration of substitutional Pd assisted by intrinsic defects, intertitials and vacancies, can lead to effective migration energies that are comparable to those of interstitial Pd impurities (between 1 and 2 eV, according to the charge state). In particular, a vacancy assisted mechanism on the carbon sublattice, as described by the classical 5 frequency model, leads to effective migration energies of 1.2 and 1.7 eV for neutral and +2 charge states respectively. This is particularly relevant under irradiation, where the concentration of vacancies and interstitials can be much higher than the equilibrium ones. In this paper I will focus on the consequences of the vacancy assisted mechanism on the migration of the vacancy itself. I will show how even quite low concentrations of palladium impurit
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