Near‐Perfect Absorption Based on Atomically Crystalline–Amorphous Hybrid AlN Films

Strong light absorption in a thin semiconductor film plays a key role in a variety of optoelectronic applications. Most ultrathin absorbers use plasmonic nanocavities or complex nanostructures to maximize absorption, which limits the practical large‐scale and the weight efficiency of the device. Here, we show the theoretical and experimental realization of a planar aluminum nitride (AlN) ultrathin film absorber based on a crystallinity‐controlled growth strategy. High‐quality AlN films were epitaxially grown on single‐crystalline aluminum substrates via plasma‐enhanced atomic layer deposition (PEALD), with gradual tuning of optical dispersion through engineered crystallinity. Our analysis reveals a systematic evolution in the dispersion relationship as the film thickness increases from 5.5 to 20.5 nm, accompanied by a rise in the refractive index and a clear amorphous‐to‐crystalline phase transition. By optimizing this transition, a peak absorption efficiency of 98.6% at 157 nm wavelength was experimentally achieved using a 5.5 nm‐thick AlN film with a hybrid amorphous‐crystalline structure. Concurrently, theoretical numerical simulations project that a near‐perfect absorption of 99.9% can be expected at 153 nm with a hypothetical 1 nm single‐crystalline AlN heterostructure. This study establishes a scalable, growth‐directed strategy for ultra‐thin photonic devices, offering a practical route toward high‐performance UV optoelectronics without relying on complex meta‐structures.