Resolving mass spectral overlaps in atom probe tomography by isotopic substitutions - case of TiSi¹⁵N

Mass spectral overlaps in atom probe tomography (APT) analyses of complex compounds typically limit the identification of elements and microstructural analysis of a material. This study concerns the TiSiN system, chosen because of severe mass-to-charge-state ratio overlaps of the ¹⁴N⁺ and ²⁸Si²⁺ peaks as well as the ¹⁴N₂⁺ and ²⁸Si⁺ peaks. By substituting ¹⁴N with ¹⁵N, mass spectrum peaks generated by ions composed of one or more N atoms will be shifted toward higher mass-to-charge-state ratios, thereby enabling the separation of N from the predominant Si isotope. We thus resolve thermodynamically driven Si segregation on the nanometer scale in cubic phase Ti_1-xSi_x¹⁵N thin films for Si contents 0.08 ≤ x ≤ 0.19 by APT, as corroborated by transmission electron microscopy. The APT analysis yields a composition determination that is in good agreement with energy dispersive X-ray spectroscopy and elastic recoil detection analyses. Additionally, a method for determining good voxel sizes for visualizing small-scale fluctuations is presented and demonstrated for the TiSiN system.