Substrate-dependent thermally driven morphological evolution of Pt${}_{0.9}$Ni${}_{0.1}$ thin films on sapphire and langasite

Abstract

We examine the thermal evolution of 100-nm-thick Pt${}_{0.9}$Ni${}_{0.1}$ alloy films on sapphire and langasite substrates with a 10-nm Zr adhesion layer. Sequential vacuum annealing from room temperature to 800 ${}^{\circ }$C produces a substrate-dependent morphological pathway that is quantified by atomic force microscopy, image segmentation, height-distribution analysis, and effective coarsening metrics. Both types of films remain densely granular up to 400 ${}^{\circ }$C, undergo a sharp coarsening transition between 400 and 600 ${}^{\circ }$C, and evolve into coalesced faceted crystallites by 800 ${}^{\circ }$C. The mean projected feature area increases from $7.84\times 10^2$ to $1.09\times 10^5$ nm${}^2$ on langasite and from $3.32\times 10^2$ to $1.67\times 10^5$ nm${}^2$ on sapphire. Sapphire therefore develops the larger faceted crystallites, while langasite exhibits the larger roughness maximum at 600 ${}^{\circ }$C. Area distributions reveal that the 400-600 ${}^{\circ }$C transition does not uniformly shift the initial granular population but instead replaces it with a distinct large-feature ensemble. Height and roughness measurements show a pronounced maximum at 600 ${}^{\circ }$C, followed by partial smoothing at 800 ${}^{\circ }$C, consistent with rapid coalescence followed by faceting-limited growth. An effective Arrhenius analysis based on $\Delta ⟨r^2⟩$, with $⟨r^2⟩=⟨A⟩/\pi$, identifies the 400-600 ${}^{\circ }$C interval as the dominant coarsening window. These results show that the thermal stability of PtNi/Zr films on sapphire and langasite is governed by the coupled influence of alloy mobility, interfacial energetics, and substrate-dependent morphological selection.

8 pages, 5 figures in the main manuscript; 4 pages, 5 figures in the Supplemental Material