© The Author(s), published by EDP Sciences, 2023

1 Introduction

The reversible semiconductor to metal transition (SMT) of vanadium dioxide (VO₂) at a critical temperature of T _crit = 68 °C was first found in 1959 [1]. Since then, many potential applications were described, including, but not limited to, smart windows [2], adaptive optics [3], and optical switches [4, 5]. VO₂ thin films can be fabricated on different conductive and non-conductive materials [6]. The SMT is accompanied by a structural phase transition from the monoclinic low-temperature VO₂(M) to the rutile high-temperature VO₂(R) crystal structure [7], and several changes in physical properties, like the change in electrical resistivity, refractive index, and extinction coefficient, among others. However, the focus of most work is mainly on applications using non-patterned VO₂ or considering THz-radiation [8].

For this article, VO₂ thin films were deposited on silicon and fused silica substrates using reactive ion beam deposition (RIBD). On both types of samples, the VO₂ thin film was then lithographically patterned by reactive ion etching. The structural and optical properties of the films and the resulting gratings were studied using variable angle spectroscopic ellipsometry (VASE), Raman spectroscopy, transmission and reflection measurements.

We hereby propose the design for an actively switching reflector for the near infrared wavelength range with high switching ratio, that is based on guided mode resonances (GMR) in a lithographically patterned VO₂ grating structure. We use a one-dimensional grating with a period of 1 μm on a stack of conductive and nonconductive films (Fig. 1). The underlying layers serve as a resistive heating element to achieve the phase transition of vanadium dioxide. The heating element consists of a thin film of 20 nm ITO covered with 100 nm chromium contacts for the electrical connection to a regulated power supply. On top of the heating element is an electrically non-conductive film of 100 nm silicon dioxide (SiO₂). It insulates the heating element from the electrically conductive vanadium dioxide (VO₂) grating.

Fig. 1 (a) Design of the electrically switchable GMR-filter element. The ITO layer serves as resistive heating element underneath the VO2 grating, which itself forms the waveguide structure. (b) Cross-sectional profile of GMR-filter element with dimensions.

The aim of the optical design of the grating is to achieve a high switching ratio for reflected TE polarized light with a wavelength of λ = 1550 nm. At room temperature (below T _crit = 68 °C), incoming light is coupled into the waveguide structure formed by the grating itself and is absorbed.

2 Manufacturing

We developed a reactive ion beam deposition (RIBD) process for reproducibly manufacturing VO₂(M) layers of specified thicknesses ranging from less than 100 nm to over 400 nm. Using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), we were able to determine the stoichiometric composition and ratio of vanadium to oxygen of films depending on the oxygen partial pressure during the deposition. The partial pressure of oxygen was controlled by variation of the oxygen flow rate. After deposition, an annealing of the layer stack for 30 min at 520 °C in an atmosphere composed of nitrogen and oxygen in a 100:1 ratio and a total pressure of 10 mbar was performed. The formation of the phase VO₂(M) was validated by Raman spectroscopy. For all VO₂ thin films produced for this study we used this RIBD process to produce a consistent thickness after annealing of 300 nm for the resonant reflection grating and 265 nm for all other samples.

We fabricated four types of samples for our study. Two of these were non-patterned VO₂ thin films, one on a single side polished silicon wafer with <100> orientation and the other on a fused silica wafer coated with 20 nm ITO using a sputtering process. Additionally, we made two lithographically patterned VO₂ grating structures. One on a silicon substrate with no other thin films applied, and the other on fused silica substrate which had the proposed layer stack of Figure 1 applied. The chromium and silicon oxide films were deposited by ion beam deposition (IBD) and reactive ion beam deposition (RIBD), respectively. Both layers (Cr and SiO₂) were patterned individually by a lift-off process utilizing the negative tone resist AZ®nLOF 2070 (MicroChemicals). Afterwards, the VO₂ was deposited. To ensure, that the VO₂ is only present on the insulating SiO₂ areas we used another lift-off process prior to the annealing.

The VO₂ grating structures on both samples were created by first applying and patterning a chromium mask using e-beam lithography with positive tone resist FEP-171 (Fujifilm) and chlorine-based reactive ion etching (RIE). And afterwards by inductively coupled plasma (ICP) etching of the VO₂ layer against a chromium hard mask using carbon tetrafluoride. The chromium was later removed from the top of the ridges with RIE.

Each of the four samples were analysed by different means: ellipsometry, Raman and SEM analysis for the non-patterned thin film on silicon, temperature-dependent transmission measurements for the non-patterned thin film on fused silica, Raman analysis for the VO₂ grating on silicon, and reflection analysis for the VO₂ grating on fused silica.

3 Results

3.1 Complex refractive index of VO₂

The complex refractive index of the realized VO₂ thin films was measured using variable angle spectroscopic ellipsometry on samples deposited using RIBD on a bare silicon substrate. The refractive index (n) and extinction coefficient (k) were recorded at 25 °C and 90 °C, with temperature controlled by a regulated hot plate. The results of the measurements are shown in Figures 2a and 2b, respectively. It was observed that both, the refractive index and especially the extinction coefficient show a significant change for near infrared wavelengths.

Fig. 2 (a) Refractive index n and (b) extinction coefficient k of prepared VO2 film below and above the SMT.

3.2 Morphological changes

Scanning electron microscopy (SEM) images were analysed to detect structural and morphological changes of the deposited VO₂ thin films after the annealing and etching processes. The image in Figure 3a shows an amorphous layer directly after deposition. However, after the annealing process (see Fig. 3b), there is a formation of crystal grains with an average diameter of 56 nm with a σ = 1.6 nm, indicating a reorganization of the crystal structure. The annealing also led to a change in the thickness of the thin films from 226 nm before annealing to 265 nm after. This calculates to a film thickness increase of approximately 17%. While the first two images show samples with VO₂ on silicon substrate, the latter image shows the sample used for the reflection measurement that is described below. The etching depth into the thin film (see FIB-cut in Fig. 3c) was found to be not fully complete, with the grating not reaching its intended depth. Visible below the VO₂, there are 100 nm SiO₂ and 20 nm ITO. Please note, that for the preparation of the FIB-cut images there is chromium (mask material) and an additional layer of platinum on top of the grating. This was removed prior to other optical measurements.

Fig. 3 SEM images of a cross-section of a VO2 thin film on silicon substrate, (a) before annealing and (b) after annealing, indicating the growth of thickness. Image (c) shows an etched VO2 grating with additional layers above and below. ITO is the thin bright layer above the substrate.

3.3 Crystalline phase

Raman spectroscopy was performed to identify the crystalline phase of VO₂ thin films on two different silicon samples. One after deposition as well as after annealing, and the other sample after etching the grating structure. The spectra (see Fig. 4) measured before and after the annealing process show the appearance of many characteristic peaks in accordance with other publications [9], implying the reconfiguration of the thin film to form the crystalline phase of VO₂(M).

Fig. 4 Raman spectra of VO2 films on Silicon: after deposition, after annealing and after etching (lines from bottom to top).

Most prominent are two sharp peaks of VO₂(M) at 194 cm⁻¹ and 224 cm⁻¹ and the broader peak at 612 cm⁻¹. The peak at 520 cm⁻¹ is due to the Si substrate material. This peak was particularly pronounced after etching, as the grating structure was opened to the underlying silicon. Other smaller peaks that are characteristic for VO₂(M) are marked with diamond (◆) shapes. The Raman spectroscopy measurements confirmed the formation of the monoclinic phase VO₂(M) after annealing and indicate that this phase was not changed by the etching process.

3.4 Transmittance

The transmission was measured at λ = 1550 nm on a non-patterned VO₂ sample with thickness of 265 nm (Fig. 5) on a fused silica substrate coated with an ITO heating layer. The sample was heated by applying a current to the resistive heating element directly in contact to the VO₂ thin film. The temperature T was calculated from the coupled electrical power assuming an ohmic resistance, a thermal system in equilibrium and two known points of reference. For no heating power, the temperature was 25 °C and the temperature of the SMT was found from previous experiments on a regulated hotplate and in accordance with literature [1] at 68 °C. At thermal equilibrium, the coupled power P _heat and the thermally radiated power P _radiate are equal, whereby the proportionality P ~ T ⁴ applies.

Fig. 5 Transmitted amplitude of the optical element in relation to the calculated temperature from heating power inserted into the resistive heating film. Each measurement was taken after thermal equilibrium was reached.

This way, a hysteresis with a width of ΔT ≈ 4 K could be observed. Depending on the applied heating current and resulting temperature, the transmittance could be actively switched between t = 43.6% and t = 0.17% multiple times, resulting in a transmission ratio of c _t  > 250, which is in good agreement with the expected switching ratio of c _t  = 375 according to RCWA.

3.5 Reflectance

The reflection properties of a VO₂ grating structure on fused silica substrate with additional layers as shown in Figure 1 were measured between 1460 nm and 1600 nm (see Fig. 6). This measurement was conducted below and above the SMT. By heating the sample to T ≈ 90 °C, the reflectance can be actively controlled to a highly reflective state with r = 51.7% at a wavelength of λ = 1580 nm. At a temperature below the SMT (T ≈ 30 °C), the reflectance significantly drops to r = 0.77%. This results in a switching ratio of c _r  = 67. The theoretical reflection calculated from the model in Figure 1 using RCWA is shown as dashed lines in Figure 6. Overall, the trend is followed closely, with deviations resulting mostly from the VO₂ not being fully opened, thereby degrading its performance. Despite the imperfect etching process, it could be demonstrated that VO₂ still exhibits thermochromic phase change after ICP-etching and can be utilized to actively switch micro-optical devices.

Fig. 6 Reflection of the guided mode resonance grating at 30 °C and 90 °C, respectively. The dashed lines show the results of RCWA simulation. The calculated reflection switching ratio c r is shown in black.

4 Conclusion

In summary, we successfully fabricated and patterned VO₂(M) thin films using reactive ion beam deposition, e-beam lithography, and inductively coupled plasma etching, demonstrating the potential of VO₂ in micro-structuring, nano-optics, and guided mode resonant gratings. This article presented the design and fabrication of an actively switchable reflector based on guided mode resonances. The thermochromic properties of the material were verified through ellipsometry, transmission and reflection measurements and Raman spectroscopy measurements, which confirmed the presence of the monoclinic phase of VO₂(M) after annealing and showed its stability after ICP-etching.

However, deviations between the measured and theoretical reflection was observed, indicating that the VO₂ was not fully opened during etching. This shows the need for an optimized fabrication process to achieve the full potential of VO₂ in optical applications. One possible solution is to replace the SiO₂ layer with a more etch-resistant Al₂O₃ layer, which can serve as an effective etch-stop.

Despite the observed deviations, the thermochromic phase change and the ability to actively switch micro-optical devices after ICP-etching demonstrates the significant and versatile role of VO₂ in optical technologies. This creates new opportunities for the use of VO₂ in various micro-optical devices and systems, such as optical switches, filters, modulators, and more, making it a promising material for future advancements in the field of active optics and it paves the way for future studies in this field.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgments

The authors acknowledge the financial support by the Federal Ministry of Education and Research (BMBF) of Germany and VDI (funding no. 13N15175). This work is funded through the project 20IND04 ATMOC within the Programme EMPIR. The EMPIR initiative is co-founded by the European Union’s Horizon 2020 research and innovation program and the EMPIR Participating Countries.

References

All Figures

	Fig. 1 (a) Design of the electrically switchable GMR-filter element. The ITO layer serves as resistive heating element underneath the VO2 grating, which itself forms the waveguide structure. (b) Cross-sectional profile of GMR-filter element with dimensions.
In the text

	Fig. 2 (a) Refractive index n and (b) extinction coefficient k of prepared VO2 film below and above the SMT.
In the text

	Fig. 3 SEM images of a cross-section of a VO2 thin film on silicon substrate, (a) before annealing and (b) after annealing, indicating the growth of thickness. Image (c) shows an etched VO2 grating with additional layers above and below. ITO is the thin bright layer above the substrate.
In the text

	Fig. 4 Raman spectra of VO2 films on Silicon: after deposition, after annealing and after etching (lines from bottom to top).
In the text

	Fig. 5 Transmitted amplitude of the optical element in relation to the calculated temperature from heating power inserted into the resistive heating film. Each measurement was taken after thermal equilibrium was reached.
In the text

	Fig. 6 Reflection of the guided mode resonance grating at 30 °C and 90 °C, respectively. The dashed lines show the results of RCWA simulation. The calculated reflection switching ratio c r is shown in black.
In the text