Abstract
Transition metals (TMs) implanted in oxides with rock-salt crystal structures (for example MgO and BaO) are assumed to substitute cations (Mg in case of MgO) from the lattice sites. We show that not all implanted TMs substitute cations but can be stable in interstitial sites as well. Stability of TM (Sc–Zn) dopants in various charge states in MgO and BaO has been investigated in the framework of density functional theory. We propose an effective way to calculate stability of implanted metals that let us predict site preference (interstitial or substitution) of the dopant in the host. We find that two factors govern the preference for an interstitial site: (i) relative ionic radius and (ii) relative oxygen affinity of cation and the TM dopants. If the radius of the cation is much larger than TM dopant, as in BaO, TM atoms always sit at interstitial sites. On the other hand, if the radius of the cation is comparable to that of the dopant TM, as in case of MgO, the transition of the preferred defect site, from substituting lattice Mg atom (Sc to Mn) to occupying interstitial site (Fe to Zn) is observed. This transition can be attributed to the change in the oxygen affinity of the TM atoms from Sc to Zn. Our results also explain experiments on Ni and Fe atoms implanted in MgO. TM dopants at interstitial sites could show substantially different and new properties from substitutionally doped stable compounds.
Highlights
Dopants in semiconductors cause significant changes in their electronic and optical properties, as required for their industrial applications
We show that transition metals (TMs) dopants can occupy interstitial site, in contrary to the common belief that TM dopants will invariably substitute cations in the oxide
Our results suggest that the site preference of TM atoms in rock-salt oxides depends on relative ionic radii and oxygen affinity of the host cation and the dopant
Summary
Dopants in semiconductors cause significant changes in their electronic and optical properties, as required for their industrial applications. Apart from various chemical routes available for creating defects in MgO38–41, there have been instances where MgO is implanted with different ions These include, Au for modifying refractive index of MgO and creating quantum antidots[42,43,44,45], Cu and Ni for modifying optical properties[45,46], He, Ar, Fe, Cr for photoluminiscence[47,48], Ne, Ar, Zr, Ru, Si, Cr and Fe for enhancing secondary electron emission yield[25,49,50]. Studies on ion-implanted rock-salt oxides assume that the dopants occupy only the substitutional sites replacing host lattice cations without any exploration of the other possible sites including interstitials.
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