Nanoscale-Thick Thin Films of High-Density HfO₂ for Bulk-like Optical Responses

The high refractive index, broadband transparency, and low spectral absorption of HfO2 (hafnia) make it a suitable choice for thermally stable nanoscale optical filters, UV mirrors, and antireflection coatings. However, achieving dense thin films of HfO2 with bulk-like optical properties has been a challenge due to differences in film stoichiometric and spatial uniformity at nanoscale thicknesses. Here, we assess HfO2 thin films (i.e., <200 nm thicknesses) prepared using pulsed laser deposition (PLD) at different substrate temperatures (20–675 °C) and the associated chemical, structural, and optical properties are reported. X-ray diffraction analysis reveals that nanoscale-thick thin films of HfO2 deposited at an ambient substrate temperature (20 °C) using PLD are amorphous with embedded nanocrystallites, whereas films deposited on heated substrates are polycrystalline monoclinic HfO2. For the films deposited on heated substrates, further analysis shows that the nanocrystalline phase does not change with increasing substrate temperatures; however, the texture of the crystalline orientation changes to favor (111) at 300–675 °C from the initial (002) orientation at ambient substrate temperature. Such differences in nanoscale-thick thin-film HfO2 PLD process-dependent amorphousness, crystallinity, and surface textures discussed here exhibit minimal influence on the resulting broadband optical properties (250 nm–30 μm). Furthermore, we show that the complex refractive index of high-density nanoscale-thick thin-film HfO2 prepared at an ambient substrate temperature using PLD resembles that of bulk HfO2 responses. The achievement of fully stoichiometric, high-density, thin-film HfO2 from PLD is expected to further enable thermally robust nanoscale photonic device integration involving low-dimensional high-performance optical and optoelectronic applications.