Optical films are crucial in integrated optics, optical instruments, and laser systems; they offer functions such as high reflection, anti-reflection, and polarization control. To realize these functional optical films, materials with varying refractive indices must be used; however, the availability of natural materials with a specific refractive index is limited. A promising approach to modulate the refractive index is by using material mixtures. Silicon dioxide (SiO₂) and hafnium dioxide (HfO₂) are widely recognized as low and high refractive index materials, respectively. In the domain of laser thin films, the laser damage threshold can be significantly enhanced by optimizing the mixing ratio of HfO₂ and SiO₂ in HfO₂-SiO₂ hybrid films. This study aims to establish a comprehensive correlation among the microscopic properties—such as chemical composition, binding energy, and porosity—and the macroscopic properties, including density, hardness, crystallization temperature, and mixing ratio. By analyzing these relationships, we seek to elucidate the effect of material composition on the performance characteristics of HfO₂-SiO₂ hybrid thin films.

Hafnia-silica (HfO₂-SiO₂) mixed coatings with various ratios are fabricated on fused silica substrates via electron beam co-evaporation. The density, hardness, chemical composition, and crystalline state of the HfO₂-SiO₂ mixed coatings are analyzed via X-ray diffraction (XRD), nanoindentation, X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS).

The results show that a SiO₂ atomic fraction of 13% in the HfO₂-SiO₂ mixed coatings yields the highest hardness and density among the seven samples measured (Table 2, Fig. 2, and Fig. 3). At this atomic fraction, only physical mixing is observed in the mixed coatings. However, when the SiO₂ atomic fraction exceeds 13%, the hardness and density decrease with increasing SiO₂ atomic fraction. Analysis reveals the presence of physical mixing and the formation of the HfSiO₄ compound in the mixed coating (Figs. 3, 6, and 7). Furthermore, minimal silicon doping does not significantly elevate the crystallization temperature of the samples, although the crystallization temperature increases gradually with the SiO₂ atomic fraction (Fig. 5).

The hardness and density of HfO₂-SiO₂ hybrid films are intricately linked to the microscopic columnar structure of HfO₂. When the atomic fraction of SiO₂ is 13%, smaller SiO₂ particles infiltrate the pores of HfO₂, thus increasing the film density and achieving the maximum hardness and density for the hybrid films. However, beyond this atomic fraction threshold of 13% , the hardness decreases as the SiO₂ content continues to increase.

Furthermore, the crystallization temperature of pure SiO₂ exceeds that of pure HfO₂, thereby indicating that incorporating SiO₂ can elevate the crystallization temperature of the hybrid films. Notably, a small proportion of SiO₂ does not significantly affect the crystallization temperature. At 13% (SiO₂ atomic fraction), the mixed film exhibits a physical amalgamation of HfO₂ and SiO₂. However, when the SiO₂ content surpasses a certain percentage, the mixed film transitions from being merely a physical mixture to one that incorporates the silicate compound HfSiO₄.

The binding energy peaks for hafnium (Hf), silicon (Si), and oxygen (O) in the HfO₂-SiO₂ hybrid films vary depending on the proportions of the constituent materials. This phenomenon is due to changes in the electron cloud density surrounding each element, which is reflected in the shifts of the binding-energy peak positions. As the Si content decreases and the Hf content increases, the binding energies of Hf, Si, and O diminish, which suggests an increase in the electron cloud density around these elements, thereby indicating a greater number of electrons.