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

Abstract

The high refractive index, broadband transparency, and low spectral absorption of HfO 2 (hafnia) make it a suitable choice for thermally stable nanoscale optical filters, UV mirrors, and antireflection coatings. However, achieving dense thin films of HfO 2 with bulk-like optical properties has been a challenge due to differences in film stoichiometric and spatial uniformity at nanoscale thicknesses. Here, we assess HfO 2 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 HfO 2 deposited at an ambient substrate temperature (20 °C) using PLD are amorphous with embedded nanocrystallites, whereas films deposited on heated substrates are polycrystalline monoclinic HfO 2 . 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 HfO 2 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 HfO 2 prepared at an ambient substrate temperature using PLD resembles that of bulk HfO 2 responses. The achievement of fully stoichiometric, high-density, thin-film HfO 2 from PLD is expected to further enable thermally robust nanoscale photonic device integration involving low-dimensional high-performance optical and optoelectronic applications.

Optical dielectric properties of HfO2-based films

“…Considering the significance of reporting dielectric properties of HfO2-based materials at optical frequencies, future directions should be centered around films grown on more prevalent substrates in industry such as silicon, 33 which will provide crucial information for studies on the HfO2-based devices for optoelectronic applications.…”

Effective Optical Properties of Laterally Coalescing Monolayer MoS2

“…34,35 eliminate uncertainty toward eventual integration of necessary in situ ultrathin film metrology, which is routinely utilized during the physical vapor deposition of comparative ultrathin films. 36 The utility of our multisample EMA optical analysis is further demonstrated in Figure 3a comparing image estimated and EMA derived MOCVD MoS 2 surface coverage from AFM and ellipsometry, respectively, to the normalized effective ε 2 at 1.24 eV (∼1000 nm). Our optical dispersion data analysis assumed constant monolayer thickness in the model (i.e., ∼0.63 nm for 2D MoS 2 ), which is consistent with the selected growth conditions.…”

“…Ex situ ellipsometry was intentionally selected for this study to permit variable angle optical characterization of representative stepwise growth conditions. The ability to characterize such films under multiple angles improves the robustness of the fit parametrization, which is not possible with in situ ellipsometry reactor configurations fixed at one angle. , This study aids to eliminate uncertainty toward eventual integration of necessary in situ ultrathin film metrology, which is routinely utilized during the physical vapor deposition of comparative ultrathin films …”

Controlling the optical properties of hafnium dioxide thin films deposited with electron cyclotron resonance ion beam deposition

“…Hafnium oxide, also known as Hafnia (HfO 2 ), is one of the most attractive high refractive index materials, high-k dielectric, with excellent thermal and chemical stability [1] that are widely used in optical coating applications. They are also commonly used in multilayer optical coatings as the high index material, alongside low index material like silica, where it can be utilized in interference filters, anti-reflective coatings, metal-oxide-semiconductor transistors, and cameras that can be utilized for space applications [2][3][4].…”

“…They are also commonly used in multilayer optical coatings as the high index material, alongside low index material like silica, where it can be utilized in interference filters, anti-reflective coatings, metal-oxide-semiconductor transistors, and cameras that can be utilized for space applications [2][3][4]. Other applications that HfO 2 films has been utilized aside for optical coatings are memory applications [1,5,6], ferroelectrics transistors which can be used for in-memory computing devices, as well as neuromorphic devices [7][8][9] and as HfO 2 based nanoagent in clinical trials for radiosensitized tumor therapy [10]. HfO 2 has optical transparency over a wide spectral range, from ultraviolet (UV) to mid-infrared (mid-IR) region, due to its wide bandgap of 5.3 -5.7 eV [11,12], alongside high laser induced damage threshold (LIDT), allowing it to often be utilized as the coating for optics in high power laser systems.…”