Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

. Vanadium dioxide (VO 2 ) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO 2 thin ﬁ lms with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO 2 thin ﬁ lms fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and re ﬂ ection measurements. The results show that the refractive index and extinction coef ﬁ cient exhibit signi ﬁ cant changes for near-infrared wave-lengths when heated above 68 (cid:1) C, con ﬁ rming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO 2 (M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO 2 -layers were successfully realized demonstrating the potential of VO 2 as a material for active micro-optical devices, such as guided mode resonance ﬁ lters with switchable re ﬂ ec-tance. The results suggest that VO 2 has great potential as a promising material for actively switched optical elements and micro-optical devices.


Introduction
The reversible semiconductor to metal transition (SMT) of vanadium dioxide (VO 2 ) 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]. VO 2 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 VO 2 (M) to the rutile hightemperature VO 2 (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 VO 2 or considering THz-radiation [8].
For this article, VO 2 thin films were deposited on silicon and fused silica substrates using reactive ion beam deposition (RIBD). On both types of samples, the VO 2 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 VO 2 grating structure. We use a one-dimensional grating with a period of 1 lm 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 (SiO 2 ). It insulates the heating element from the electrically conductive vanadium dioxide (VO 2 ) grating.
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 k = 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.

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We developed a reactive ion beam deposition (RIBD) process for reproducibly manufacturing VO 2 (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 VO 2 (M) was validated by Raman spectroscopy. For all VO 2 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 VO 2 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 VO 2 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 SiO 2 ) were patterned individually by a lift-off process utilizing the negative tone resist AZÒnLOF 2070 (MicroChemicals). Afterwards, the VO 2 was deposited. To ensure, that the VO 2 is only present on the insulating SiO 2 areas we used another lift-off process prior to the annealing.
The VO 2 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 VO 2 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 VO 2 grating on silicon, and reflection analysis for the VO 2 grating on fused silica.

Complex refractive index of VO 2
The complex refractive index of the realized VO 2 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.

Morphological changes
Scanning electron microscopy (SEM) images were analysed to detect structural and morphological changes of the deposited VO 2 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 r = 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 VO 2 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 VO 2 , there are 100 nm SiO 2 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.

Crystalline phase
Raman spectroscopy was performed to identify the crystalline phase of VO 2 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 VO 2 (M).
Most prominent are two sharp peaks of VO 2 (M) at 194 cm À1 and 224 cm À1 and the broader peak at 612 cm À1 . The peak at 520 cm À1 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 VO 2 (M) are marked with diamond (r) shapes. The Raman spectroscopy measurements confirmed the formation of the monoclinic phase VO 2 (M) after annealing and indicate that this phase was not changed by the etching process.

Transmittance
The transmission was measured at k = 1550 nm on a nonpatterned VO 2 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 VO 2 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.
This way, a hysteresis with a width of DT % 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.

Reflectance
The reflection properties of a VO 2 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 k = 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 VO 2 not being fully opened, thereby degrading its performance. Despite the imperfect etching process, it could be demonstrated that VO 2 still exhibits thermochromic phase change after   ICP-etching and can be utilized to actively switch microoptical devices.

Conclusion
In summary, we successfully fabricated and patterned VO 2 (M) thin films using reactive ion beam deposition, e-beam lithography, and inductively coupled plasma etching, demonstrating the potential of VO 2 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 VO 2 (M) after annealing and showed its stability after ICP-etching.
However, deviations between the measured and theoretical reflection was observed, indicating that the VO 2 was not fully opened during etching. This shows the need for an optimized fabrication process to achieve the full potential of VO 2 in optical applications. One possible solution is to replace the SiO 2 layer with a more etch-resistant Al 2 O 3 layer, which can serve as an effective etch-stop.
Despite the observed deviations, the thermochromic phase change and the ability to actively switch microoptical devices after ICP-etching demonstrates the significant and versatile role of VO 2 in optical technologies. This creates new opportunities for the use of VO 2 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.