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All issues Volume 22 / No 1 (2026) J. Eur. Opt. Society-Rapid Publ., 22 1 (2026) 33 Full HTML
EOSAM 2025
Open Access
Issue
J. Eur. Opt. Society-Rapid Publ.
Volume 22, Number 1, 2026
EOSAM 2025
Article Number 33
Number of page(s) 5
DOI https://doi.org/10.1051/jeos/2026036
Published online 07 May 2026

© The Author(s), published by EDP Sciences, 2026

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1 Introduction

Hafnium oxide (HfO2) has attracted significant attention over the past two decades as a functional material in both microelectronics and photonics. In microelectronic devices, HfO2 is well established as a high-permittivity (high-k) dielectric replacing SiO2 in advanced metal-oxide-semiconductor field-effect transistors [1–3]. Beyond its electronic functionality, the optical properties of HfO2, namely its high refractive index, wide optical bandgap, and high transparency from the ultraviolet to the infrared, render it attractive for optical coatings, interference filters, and dielectric metasurfaces [4–7]. In particular, the demand for optical components such as wire grid polarizer [8, 9], meta lenses [10] or diffractive optical elements [11] or grating couplers in photonic integrated circuits [12] operating in the ultraviolet and vacuum ultraviolet spectral ranges has increased substantially due to applications in spectroscopy – especially in quantum technologies [13–15] – space optics, and high-resolution microscopy. For these applications, materials must combine low optical absorption with high refractive index contrast and excellent environmental stability. HfO2 fulfills these requirements and offers superior laser-induced damage threshold (LIDT) [16, 17] as well as resistance to radiation-induced damage [18, 19] and chemical degradation and corrosion compared to many alternative high-index oxides. A large variety of deposition techniques have been employed for HfO2 thin films, such as electron beam evaporation [20, 21], magnetron sputtering [22], ion beam sputtering [23, 24], sol-gel [25–27] and atomic layer deposition (ALD) [28–33]. Among these, ALD is particularly suitable for the fabrication of HfO2 thin films with precisely controlled thickness and excellent structural conformality. The self-limiting surface reactions inherent to ALD enables uniform coatings on both planar as well as three-dimensional nanostructured substrates, which is essential for emerging meta-optical concepts relying on subwavelength structuring.

Numerous studies have been reported on the optical and structural properties of ALD-grown HfO2, demonstrating its applicability in optical thin films across a wide spectra range, for instance as anti-reflection coatings at 266 nm, 355 nm, 532 nm and/or 1064 nm [31, 33], or as high-reflective mirrors at 355 nm and 532 nm [32]. Nevertheless, comprehensive experimental data on its optical constants over an extended spectral range, particularly toward the vacuum ultraviolet (VUV) remain comparatively scarce.

The present work aims to expand the accessible optical database by providing detailed optical properties of PEALD HfO2 thin films down to a wavelength of 140 nm. By combining multiple complementary measurement techniques [34, 35], we extract reliable optical constants from the visible to the VUV range. The resulting dataset provides a solid foundation for the optical design and simulation of HfO2-based coatings and meta-optical devices.

2 Thin film deposition

Hafnium oxide thin films are commonly deposited by atomic layer deposition using a variety of hafnium precursor chemistries in combination with either thermal or plasma-assisted oxidation steps. In thermal ALD, both inorganic precursors such as hafnium tetrachloride (HfCl4) [36] and metal-organic precursors tetrakis(dimethylamido)hafnium (TDMAH), tetrakis(ethylmethylamido)hafnium (TEMAH) [37], or β-diketonate-based [38, 39] compounds have been widely employed together with water as the oxidant. While HfCl4-based processes typically require higher deposition temperatures and may lead to halogen-related impurities, metal-organic precursors enable lower-temperature growth and improved process flexibility. In plasma-enhanced ALD (PEALD), the molecular oxidant is replaced by an oxygen plasma, which enhances ligand removal and film densification [40]. PEALD processes using TDMAH or TEMAH have been shown to produce dense, stoichiometric HfO2 films with reduced impurity content at comparatively low substrate temperatures [30–32], making them especially attractive for optical coatings and nanophotonic applications.

Depending on the chosen precursor chemistry and oxidation scheme, the resulting films may exhibit significant differences in growth per cycle, density, impurity concentration, and ultimately optical material properties. The HfO2 thin films investigated in this study were deposited at 100 °C using plasma-enhanced atomic layer deposition in an OpAL reactor (Oxford Instruments Plasma Technology). Tetrakis(dimethylamido)hafnium (TDMAH, Hf[N(CH3)2]4) served as the metal precursor, while an oxygen plasma was employed as the oxidizing reactant. Each ALD cycle consisted of a TDMAH precursor pulse (0.4 s), followed by a purge (8 s) and pump-down (2 s) sequence to remove excess precursor and by-products, a gas stabilization period (3 s) followed by an oxygen plasma exposure (10 s) and a final purge (10 s). The details are summarized in Table 1.

Table 1

ALD parameter for Hafnia deposition using TDMAH and oxygen plasma in an OPAL-tool.

The deposition was performed on fused silica substrates in order to enable transmission measurements over a wide spectral range. To ensure sufficient optical sensitivity in ellipsometry measurements while maintaining negligible interference from the substrate backside, a total film thickness of approximately 200 nm was targeted. The deposition process yielded smooth, homogeneous films with excellent macroscopic uniformity.

3 Optical characterization methods

The optical properties of the HfO2 films were characterized using a combination of spectroscopic ellipsometry and spectrophotometry to cover a broad spectral range. Vacuum ultraviolet and ultraviolet ellipsometric measurements were performed using a Jobin Yvon UVISEL2 ellipsometer at photon energies ranging from 1.5 to 8.7 eV at a fixed angle of incidence of 70°. Additional ellipsometric data in the range from 0.6 to 6.5 eV were acquired with a Jobin Yvon UVISEL ellipsometer using multiple angles of incidence between 55° and 75°. Infrared ellipsometry measurements covering the range from 300 to 6500 cm-1 were conducted using a Woollam IR-VASE ellipsometer, again employing variable angles of incidence. Complementary transmission and reflection spectra were recorded using a Perkin Elmer Lambda 1050 spectrophotometer and a Bruker Vertex 80v Fourier-transform infrared spectrometer. For reflectance measurements, illumination from both the film and substrate side was used to enhance sensitivity to interface effects. The experimental data were analyzed using the universal dispersion model implemented in the newAD2 software package [34]. All relevant parameters, including film thickness, dispersion coefficients, and interface roughness, were treated as free fitting parameters. This approach ensured a consistent description of the optical response across all spectral regions and measurement techniques.

4 Results and discussion

The combined optical analysis yielded film thickness values between 217 and 221 nm, depending on the specific measurement configuration and spot size. This small variation indicates a high degree of thickness uniformity across the sample. The surface roughness of the top interface was determined to be approximately 1.51 nm (root mean square), with an autocorrelation length of about 5.3 nm. These values correspond to dense, smooth oxide layers produced with PEALD and are comparable to HfO2 thin films on silicon reference samples deposited under similar conditions with 0.8 nm rms (XRR) [29]. The low surface roughness is critical for optical applications, as elevated topography would introduce scatter losses and degrade coating performance in the ultraviolet spectral range.

To evaluate film quality and optical functionality, the extracted refractive index was compared with a previous PEALD-HfO2 study. The thin film examined in this work is designated Type A, while the comparison sample is designated Type B (compare Fig. 1 and Table 2). To minimize the influence of impurity contributions on the optical properties, we compare only processes performed with the same precursor (TDMAH) same oxidizing agent (O2-plasma) and at the same deposition temperature (100 °C), since these are the main parameters affecting carbon and nitrogen levels [30]. The low deposition temperature was chosen to reduce crystallization in the thin films, even though lower temperatures often lead to increased carbon contamination because of incomplete precursor oxidation.

Thumbnail: Fig. 1 Refer to the following caption and surrounding text. Fig. 1

Wavelength dependence of the refractive index of the fabricated hafnia thin films.
Table 2

Compared HfO2 thins films prepared by PEALD with different ALD tools.

The refractive index of both HfO2 thin films exhibits a pronounced wavelength dependence, with a normal dispersion i.e. high values in the visible and near-ultraviolet spectral range and a gradual decrease toward longer wavelengths. In the ultraviolet region, the refractive index increases significantly as the photon energy approaches the optical band edge, while absorption remains negligible over a wide spectral range down to 230 nm (compare band edge of 5.6 eV [33]). This behaviour is highly advantageous for optical coatings and metasurfaces requiring strong phase modulation with minimal optical losses.

While both HfO2 thin films demonstrate excellent optical properties and comparable transparent regions, Type A and Type B show slight deviations in refractive index. These differences can be attributed to two distinct sources: differences in metrology and deposition conditions.

For Type A thin films, the universal dispersion model (UDM) was employed to extract optical constants [34]. In contrast, optical constants of Type B thin films were extracted from VUV transmittance and reflectance spectra using the Lorentz Calculator (LCalc) [41]. These different analytical methods can introduce systematic deviation, particularly in the VUV region where extinction is dominated by complex inter-band transitions. Nevertheless, both methods are sufficiently mature to provide reliable data and inherently capture the material response, including any influence of i.e. residual impurities.

Typically, different deposition parameters are the main source of such differences. The dominant factor separating the two film types are the different PEALD reactor configuration and associated plasma parameters, which are known to strongly influence HfO2 film density and refractive index [31]. Type A films were deposited using an OpAL PEALD reactor with a remote Inductively Coupled Plasma (ICP) source, while Type B samples were produced on a SILAYO-ICP330 system (Sentech Instruments GmbH), equipped with a direct ICP configuration featuring a Planar Triple Spiral Antenna (PTSA) source. These distinct plasma architectures lead to markedly different energy input and ion bombardment characteristics. The SILAYO PTSA-ICP source delivers higher plasma power density and enhanced ion flux, enabling more effective energetic assistance during film growth. This results in denser films: Type B achieves 8.25 g/cm3 compared to 7.90 g/cm3 for Type A (Table 2). Film density is directly reflected in the refractive index; the denser Type B films exhibit higher n-values across the transparent spectral range. This density-dependent optical property improvement is a well-established phenomenon in oxide thin films and confirms that plasma parameter optimization is a critical control lever for tailoring optical performance in PEALD-HfO2 systems.

X-ray diffraction measurements conducted at the deposition temperature (100 °C) indicate that both Type A and Type B films are X-ray amorphous (not shown here), consistent with low-temperature PEALD processes. However, X-ray amorphous coatings can contain nanocrystalline domains, short-range crystalline order, or point defects that influence optical properties. This may also explain the moderate deviations, especially observed below the absorption edge between the two film sets. The higher density of type B can encourage slight structural rearrangement, such as localized crystallization or structural densification, and affect the absorption profile below the band gap. Despite these structural subtleties, both films maintain low absorption in the transparent region and preserve well-defined optical bandgaps, indicating that any nanocrystalline or defect-state contributions remain minimal and do not compromise optical performance.

The results of both HfO2 thin films conclusively confirm that PEALD enables the fabrication of optically dense HfO2 layers with properties suitable for advanced optical designs, including applications in ultraviolet meta-optics.

5 Conclusion

In summary, we have presented a comprehensive optical characterization of hafnium oxide thin films deposited by plasma-enhanced atomic layer deposition. By combining spectroscopic ellipsometry and spectrophotometry across a wide spectral range, reliable optical constants from the infrared to the vacuum ultraviolet were obtained. The films exhibit high refractive indices, low surface roughness, and excellent optical homogeneity. These findings underscore the suitability of ALD-grown HfO2 for demanding optical applications, particularly in the ultraviolet spectral range and in nanostructured meta-optical systems. The presented dataset provides a valuable reference for optical modeling and design and supports the further development of HfO2-based optical components.

Funding

The research has been supported by the Deutsche Forschungsgemeinschaft (DFG) (Emmy-Noether-Project SZ253/1-1, 287542364 and Collaborative Research Center (CRC/SFB) 1375), the European Space Agency (ESA) (Contract No. 4000109161/13/NL/RA), under Grant No. 13 N16897 GRADIENT, and BMFTR, Förderprogramm Fusion2040 – Forschung auf dem Weg zum Fusionskraftwerk, FKZ: 13F1009I, SHARP and ATIQ (Fkz: 13N16116), Cluster of excellence QuantumFrontiers (ExC-2123-390837967) as well as from the EMPIR programme 20IND04 ATMOC.

Conflicts of interest

The authors declare that they have no competing interests to report.

Data availability statement

The data that support the findings of this study will be made publicly available in the refractiveindex.info database (https://refractiveindex.info/) upon publication.

Author contribution statement

Thomas Siefke contributed to writing (original draft), formal analysis, funding acquisition, and resources. Kristin Gerold contributed to writing (original draft), visualization, formal analysis, and investigation. Svetlana Shestaeva contributed to writing (review and editing), formal analysis, and investigation. Pallabi Paul contributed to formal analysis, and investigation. Shawon Alam contributed to formal analysis, and investigation. Daniel Franta contributed to formal analysis and investigation. Adriana Szeghalmi contributed to funding acquisition and writing (review and editing). Sven Schröder contributed to writing (review and editing). Stefanie Kroker contributed to funding acquisition and writing (review and editing).

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All Tables

Table 1

ALD parameter for Hafnia deposition using TDMAH and oxygen plasma in an OPAL-tool.

In the text
Table 2

Compared HfO2 thins films prepared by PEALD with different ALD tools.

In the text

All Figures

Thumbnail: Fig. 1 Refer to the following caption and surrounding text. Fig. 1

Wavelength dependence of the refractive index of the fabricated hafnia thin films.
In the text

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