Strain-driven amorphization in Al1−xYxN thin films and lattice-engineering stabilization illustrated by In1−xYxN

Aluminum yttrium nitride (Al1−xYxN) is known to grow amorphous at relatively low Y concentrations (x ∼0.3), but the exact structural origin of this transition has not been examined at the nanoscale. In this work, we deposited a series of Al1−xYxN thin films (x ∼0.08, 0.21, and 0.5) on an AlN buffer layer (∼200 nm) grown on silicon (111) substrates in an ultra-high-vacuum sputtering system and performed a detailed transmission electron microscopy (TEM) investigation complemented by x-ray diffraction and depth-resolved time-of-flight secondary ion mass spectrometry (ToF-SIMS). ToF-SIMS confirmed a uniform distribution of Al and Y throughout each layer, eliminating compositional segregation as a possible cause of crystalline quality degradation. TEM revealed the onset of partial amorphization at Y concentrations as low as x = 0.21, and a fully amorphous film at x = 0.5. By contrast, an analogous series of In1−xYxN films, deposited by magnetron sputtering, maintains its wurtzite-type structure up to x = 0.5, in agreement with density functional theory predictions and confirming that a larger, softer host lattice suppresses the strain-driven amorphization observed in Al1−xYxN. These results demonstrate that internal strain arising from the Al–Y atomic-radius mismatch is likely the primary factor destabilizing the wurtzite lattice of Al1−xYxN and that strain and host-lattice engineering offers a viable route to stabilize highly mismatched nitride alloys.