Fabrication and Properties of Epitaxial VO₂ Thin Film on m-Al₂O₃ Substrate

Abstract

A thin film of thermochromic VO₂ was prepared on m-Al₂O₃ substrate using a radio frequency (RF) magnetron sputtering technique. The epitaxial growth of the monoclinic M₁ phase of VO₂ on the m-Al₂O₃ substrate was confirmed through synchrotron X-ray diffraction (XRD) measurements. The transformation of this monoclinic M1 phase into a rutile phase at ~68 °C was reflected in the temperature-dependent XRD measurements of the VO₂ thin film. The temperature-dependent electrical resistance measurements of this sample also revealed an abrupt metal-to-insulator transition at ~68 °C, which is reversible in nature. Temperature-dependent X-ray absorption (XAS) measurements at V L-edge and O K-edge were performed to study the electronic structure of the epitaxial VO₂/m-Al₂O₃ thin film during the metal-to-insulator (MIT) transition.

1. Introduction

In recent years, vanadium oxide compounds have been the focus of attention because of their rich fundamental physics and the potential for device applications [1,2,3,4]. Among these, the monoclinic M₁ phase of vanadium dioxide (VO₂) is particularly attractive, owing to its dramatic change in physical properties exhibited during phase transition [5,6]. It undergoes metal-to-insulator transition (MIT), along with structural phase transition at a critical temperature (T_C) of ~68 °C [1,7]. VO₂ transforms from a low temperature monoclinic insulating phase into a metallic rutile phase at high temperatures through a first-order phase transition [8]. Below T_C, the insulator phase is transparent to near infra-red (NIR) rays, while above the T_C, it is opaque to NIR rays [6,8]. In addition to thermal activation, the phase transition in VO₂ can be triggered by electrical, optical, and mechanical excitations. Because of these unique features, VO₂ has spurred diverse application in many areas including optical switches, smart windows, Mott memories, THz devices, or field effect transistors [1,9,10,11]. From the device perspective, it is imperative to grow high-quality thin film VO₂. This is the reason why the researchers have been continuously striving to determine the most viable and optimized method for synthesizing high-quality thin films of VO₂ since the phase transition was first observed in this material. The synthesis of phase-controlled stochiometric VO₂ thin films is slightly tricky as a small variation in the oxygen content can lead to several other compounds of vanadium oxide [12]. Moreover, VO₂ itself can exist in several polymorphs [12]. So, the phase-specific growth of VO₂ thin film with the desired orientation is challenging. The epitaxial growth of VO₂ thin films is desired to realize the single crystal-like properties for device applications. Until now, researchers have widely studied the epitaxial growth of VO₂ thin films mainly on substrates such as c-Al₂O₃ and TiO₂, but studies focused on the epitaxial growth of VO₂ films on m-Al₂O₃ substrates are limited [12,13,14,15]. This warrants the study of the fabrication and comprehensive characterization of epitaxial VO₂ thin film prepared on other substrates to achieve the desired orientation and/or properties. Recently, the MIT performance was studied in epitaxial VO₂ thin film prepared on an m-Al₂O₃ substrate using the moisture assisted chemical solution approach [16]. Zhao et al. reported mixed-orientation growth of VO₂ film on an m-Al₂O₃ substrate by using a sputtering technique [14]. Recently Choi et al. successfully grew epitaxial VO₂ film on an m-Al₂O₃ substrate by using RF sputtering, but the roughness (16.5 nm) of their sample is on the slightly higher side [15].

In the present work, we have stabilized epitaxial VO₂ thin film on m-Al₂O₃ using the RF magnetron sputtering technique and investigated its temperature-dependent structural, electrical, and electronic properties. The detailed study of the structural, electrical, and electronic properties of epitaxial VO₂ thin films is significant in order to achieve device realization and the control of the functionalities of VO₂ thin films.

2. Materials and Methods

Thin films of VO₂ were prepared by means of an RF magnetron sputtering technique using a commercially purchased VO₂ target (Crystal Bank at Pusan National University, Busan, South Korea) with a diameter of 76.2 mm and a thickness of 3 mm. A chemically cleaned m-Al₂O₃ (10$\overline{1}$0) substrate was mounted in the deposition chamber. After achieving the base pressure of 1 × 10⁻⁶ Torr in the deposition chamber, the VO₂ target was pre-sputtered for 5 min to clean its surface before the actual deposition. During the deposition, the chamber pressure and substrate temperature were kept at ~0.5 mTorr and ~50 °C, respectively. Ar gas was used for plasma, and the sputtering power was maintained at 200 W. This was followed by the external annealing of a grown sample in a rapid thermal annealing (RTA) furnace for 30 min in vacuum at a temperature of 530 °C [8,17]. The thickness of the grown samples was estimated to be ~112 nm by X-ray reflectivity measurements. The structural characterization of VO₂ thin film prepared on a m-Al₂O₃ substrate was studied using the BL5A beamline of Pohang Light source-II, Pohang, Republic of Korea. X-rays of 11.57 keV were used to obtain synchrotron X-ray diffraction (XRD) measurements, and the obtained data were converted to Cu K_α energy to be shown in the present manuscript. The surface morphology of the prepared sample was studied by atomic force microscopy (AFM) using the Veeco Dimension 3100 SPM instrument (Veeco Instruments Inc. 112 Robin Hill Road Santa Barbara CA 93117, Goleta, CA, USA) at room temperature. The Raman spectrum was obtained using a micro-Raman spectrometer (Witec Alpha 300 RA, Ulm, Germany) with a 532 nm wavelength and a 1 μm spot size for the laser excitation. The temperature-dependent resistance of VO₂ thin film grown on the m-Al₂O₃ substrate was measured using the Keithley 2612B (Keithley, Cleveland, OH, USA) source-meter in terms of four-probe geometry. X-ray absorption (XAS) measurements at V K-edge were performed in the fluorescence mode measured using the 8C beamline of the Pohang Light Source II (PLS II). XAS measurements at V L-edge and O K-edge in the total electron yield (TEY) mode were taken using the 2A MS undulator beamline of Pohang Light source-II, Pohang, Republic of Korea.

3. Results

The surface topography and surface roughness of the VO₂/m-Al₂O₃ film analyzed by using the AFM image are shown in Figure 1a,b, respectively. The surface of the grown film is quite homogenous and continuous. In the AFM image, the nano-scale granular surface is evident, and the root mean square (RMS) roughness is estimated to be ~1.2 nm. The RMS roughness value in the present sample is significantly lower than that of the recently reported epitaxial VO₂ film on an m-Al₂O₃ substrate [15]. Room temperature Raman spectra of the VO₂/m-Al₂O₃ thin film sample and bare m-Al₂O₃ substrate are shown in Figure 1c. The observed Raman modes at 141 cm⁻¹, 194 cm⁻¹, 224 cm⁻¹, 309 cm⁻¹, and 611 cm⁻¹ are in good agreement with the earlier reports on VO₂, and this advocates the monoclinic M1 phase of VO₂ in the current sample [14,15,16,18,19,20]. Figure 1d shows the room temperature synchrotron XRD data (measured in θ–2θ geometry) of the VO₂ film deposited on the m-Al₂O₃ substrate. Apart from the substrate peak in the XRD data, a single diffraction peak appearing at ~64.87° in the film corresponding to the ($\overline{4}$02) plane of the monoclinic M1 phase of VO₂ is observed. Further, no other phase peak was detected in the XRD data of the grown sample, indicating the epitaxial growth of the monoclinic M₁ phase of VO₂ on the m-Al₂O₃ substrate. The epitaxial relationship between the VO₂ film and the m-Al₂O₃ substrate was further examined with Φ-scans (Figure 1e) of the off-axis planes of VO₂ and the substrate. The off-axis (11$\overline{2}$0) plane peak of the m-Al₂O₃ substrate exhibits a two rotational symmetry, with 180° in Φ-scan. The off-axis ($\overline{2}$11) plane peak of VO₂ exhibits a four-fold rotational symmetry with 90°, confirming the epitaxial relationship between the VO₂ thin film and m-Al₂O₃ substrate.

Temperature-dependent XRD measurements in the θ–2θ geometry were further carried out at several temperatures at a range of 31 °C–90 °C in order to check the monoclinic M₁ → rutile structural phase transition in the VO₂/m-Al₂O₃ thin film sample. The measured temperature-dependent XRD data are shown in Figure 2a,b for the thermal cycles of heating and cooling, respectively. During heating, the ($\overline{4}$02) plane peak of the monoclinic M₁ phase of VO₂ remains stable below 53 °C. During further heating, an additional peak corresponding to the (002) plane peak of the rutile VO₂ phase emerges at a higher 2θ value. At a temperature of 68 °C and above, the ($\overline{4}$02) plane peak of the monoclinic M₁ phase disappears. The observed behavior of the ($\overline{4}$02) plane and (002) plane peaks in the temperature-dependent XRD data signifies the monoclinic M₁ → rutile structural phase transition in the VO₂ thin film sample. The reversibility of this structural phase transition is checked by taking temperature-dependent XRD measurements during the cooling cycle, as shown in Figure 2b. Upon cooling, the ($\overline{4}$02) plane peak of the monoclinic M₁ phase starts to reappear at ~60 °C, and a rutile → monoclinic M₁ structural phase transition was witnessed.

The contour plots of the temperature-dependent XRD data during the heating and cooling cycles are together plotted in Figure 3 to display the reversible nature of the monoclinic M₁ ←→ rutile structural phase transition in the VO₂ thin film sample. The transformation of the ($\overline{4}$02) M₁ diffraction peak to (002) R during heating, and vice versa, during cooling is clearly visible. Moreover, the structural phase transition temperature is lower in the cooling cycle as compared to that during the heating cycle, representing a hysteresis associated with first-order phase transition.

Figure 4a shows the normalized resistance (R (T)/R (T = 27 °C)) of the VO₂ film sample as a function of temperature during the heating and cooling cycles, while Figure 4b depicts the temperature derivative of the resistance. In the studied temperature range, a four-order change in resistance is seen for the VO₂ thin film prepared on the m-Al₂O₃ substrate, and the abrupt changes in resistance are associated with the MIT transition. The VO₂ thin film behaves as an insulator at room temperature and becomes metallic above the electrical transition temperature. Hysteresis among the resistance curves during heating and cooling was also noticed. This thermal hysteresis is associated with the latent heat of the first-order phase transition, which is driven by the microstructure of the thin film [21]. The electrical transition temperatures observed during the heating and cooling cycles are 68 °C and 59.5 °C, respectively. The MIT temperature (during the heating cycle) for the current epitaxial VO₂ thin film sample is close to that of single crystal VO₂ [22]. Earlier, Zhao et al. reported an MIT temperature of 64.5 °C with a thermal hysteresis width of 6.1 °C for the VO₂ thin film grown on the m-Al₂O₃ substrate with a mixed orientation [14].

In order to obtain further insights on the crystal structure and the electronic structure of the VO₂/m-Al₂O₃ thin film sample, XAS measurements were carried out. Figure 5a shows the V K-edge XANES (X-ray absorption near edge spectroscopy) spectrum of the VO₂ thin film sample measured at room temperature. The spectral shape of the studied sample is similar to that of the XANES spectra of the VO₂ samples reported previously [8,17,23].

As per earlier reports, six distinct features, namely B₁, B₁*, B₂, B₃, B₄, and B₅, can be assigned to the XANES spectrum of the VO₂ thin film sample. It is well known that the pre-edge peak gives information about the geometry of the absorber atom [24,25,26]. The inset of Figure 5a depicts the enlarged view of the pre-edge region. In monoclinic M₁ phase of VO₂, pre-edge spectra are characterized by two peaks, B₁ and B₁^*. Features B₁ and B₁^* arise due to 1s → 3d (V)/2p (O) transitions, which are mainly t_2g (B₁) and e_g (B₁*) in character [26]. Feature B₁^* appears in the insulating monoclinic M₁ phase of VO₂ and is suppressed in the metallic rutile phase of VO₂. Therefore, the spectral features in the room-temperature XANES spectrum point to the stoichiometry of the VO₂ thin film sample with the +4 oxidation state of V and confirm the monoclinic M1 phase at room temperature. V L-edge and O K-edge XAS spectra were further recorded at 27 °C and 90 °C (Figure 5b) in order to study the electronic structure changes in the insulating monoclinic M₁ phase and metallic rutile phase of the VO₂ thin film. In the room-temperature XAS spectrum, the spectral features at ~518 eV and ~524 eV belongs to the L₃ and L₂ edges of V, respectively. V 2p core-level (spin-orbit split 2p_3/2 and 2p_1/2) transition into the unoccupied V 3d states are responsible for these spectral features [8,27]. The additional features in the V L-edge at ~515.7 eV and ~522.3 eV originate due to the transitions to the crystal-field-split t_2g part of the V 3d states [8,17]. Based on the existing literature, the spectral features in the O K-edge XAS spectrum appearing around ~529.6eV, 530.4 eV, and 532.2 eV can be linked to transitions into the π*, d_||*, and σ* hybridization band, respectively [8,17,27,28]. The shape and energy positions of the spectral features observed in V L-edge and O K-edge XAS spectrum (at 27 °C), as well as the spin orbit splitting values of the L₃ and L₂ edges of V, are consistent with the earlier report of VO₂ single crystal and thin films [8,28,29,30].

The lowest conduction band (π*) in the O K-edge XAS spectrum accounts for the conductivity of the material, while the d_||* band stipulates the structural distortion and is related with the V-V dimerization along the rutile c axis [31,32]. The d_||* states are associated with V-V dimerization in the low-temperature-insulating phase, and therefore, these are considered to be a fingerprint of the monoclinic M₁ structure. These d_||* states are likely to become suppressed in the rutile metallic phase of VO₂ due to the absence of V-V dimers. As expected, the d_||* states disappear in the XAS spectrum recorded at 90 °C. Additionally, a shift of π* states towards a lower photon energy value is seen at 90 °C, indicating the metallic nature of VO₂ thin film at this temperature. So, the spectral feature changes in the O K-edge XAS spectrum measured below and above the MIT temperature are very well correlated to the temperature-dependent structural and electrical changes in the VO₂ thin film prepared on the m-Al₂O₃ substrate.

4. Conclusions

In summary, VO₂ thin film was prepared on an m-Al₂O₃ substrate using a RF magnetron sputtering technique, and its uniform coverage on the substrate was confirmed by AFM. The room-temperature monoclinic M₁ phase of the grown VO₂ film was established through Raman and synchrotron XRD measurements. Phi-scan measurements confirmed the epitaxial growth of VO₂ on the m-Al₂O₃ substrate. The VO₂ thin film displays a monoclinic M₁ → rutile structural phase transition at ~68 °C. The MIT transition in VO₂ thin film was also noticed at ~68 °C. The reversible nature of the structural phase transition and electrical transition was seen in the temperature-dependent XRD and electrical resistance measurements, respectively, during the heating and cooling cycles. XAS spectra at the V K-edge, V L-edge, and O K-edge further confirm the monoclinic M₁ phase and stoichiometry of the VO₂ thin film sample at room temperature. Moreover, the O K-edge XAS spectrum depicts a shifting of the π* states towards lower photon energy values, and the suppression of d_||* states at 90 °C signifies the existence of a metallic rutile phase in the VO₂ thin film at this temperature.