Study of the phase evolution, metal-insulator transition, and optical properties of vanadium oxide thin films

Taixing Huang; Lin Yang; Jun Qin; Fei Huang; Xupeng Zhu; Peiheng Zhou; Bo Peng; Huigao Duan; Longjiang Deng; Lei Bi

doi:10.1364/OME.6.003609

1. Introduction

VO₂ thin films showing a first order metal-insulator phase transition (MIT) at 68 °C have attracted great research interest recently [1–3]. The phase of VO₂ can be reversibly changed between a high-temperature tetragonal rutile phase and a low-temperature monoclinic phase by MIT [4–7]. During this process, the material shows 3 to 5 orders of resistivity change. The optical constants in a wide wavelength range from visible to THz also varies significantly, making this material highly attractive for photonic applications, such as uncooled microbolometers [8], optical switch [9, 10], smart windows [11, 12], infrared camouflage [13] and optical-phased array applications [14,15]. A variety of fabrication methods including pulsed laser deposition [16], sputtering [17], molecular beam epitaxy [18] and atomic layer deposition [19, 20] have been reported for VO₂ thin film preparation. Due to the multi-valence state characteristics of the vanadium ions, oxygen stoichiometry has been also observed to significantly influence phase formation and the properties of vanadium oxides [21–27]. On one hand, at different fabrication oxygen partial pressures, different phases of vanadium oxides including V₂O₃, V_nO_2n-1 (Magneli phase), VO₂, V_nO_{2n + 1} (Wadsley phase) and V₂O₅ phases may co-exist in the thin film [28,29]; on the other hand, different valence states of the vanadium ions can even be present in the same phase such as VO₂, which cause significant variation of the electrical and optical properties of the material [30–32]. These complications make the study of structure-property relationship very challenging for this system. Material characterization methods such as X-ray diffraction, Transmission Electron Microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) methods have been widely applied for vanadium oxide thin film characterization. Yet they show limited capability to access the phase and valence information in thin films both locally and on a large scale. Confocal Raman microscopy on the other hand, show excellent spatial resolution down to several hundred nanometers defined by the laser wavelength and Rayleigh diffraction limit [33, 34]. It also shows the capability to resolve the characteristic bonds from different bonding states, therefore providing a useful way to understand the structure-property relationship in vanadium oxides. In previous studies, this method was mostly used for analyzing single point material properties of vanadium oxide thin films. Combining with the spatial mapping capability and joining with optical spectroscopic analysis, this method can provide more information such as phase/valence state distribution, MIT process, structure-optical property relationship etc. in a large sample area [35].

In this work, we report the study of phase evolution, MIT and optical properties of vanadium oxide thin films prepared by controlled oxidation of pulsed laser deposited VO_x thin films. In particular, we investigate the phase evolution of vanadium oxide thin films fabricated under different oxygen stoichiometry by confocal Raman microscopy mapping. By investigating the intensity distribution of the characteristic Raman peaks, area distribution of pure VO₂, V₂O₅ and V⁴⁺/V⁵⁺ mixed valence state regions is observed for films with different oxygen stoichimotries. The structural influence on the optical properties and MIT process is studied by fitting the optical constants of the vanadium oxide thin films using a Drude-Lorentz model with the effective medium theory for both high temperature and low temperature phases. It is found that the thin films with composite phases show different resonator oscillator energies compared to pure VO₂ and V₂O₅ phases, which is attributed to the different optical properties of the mixed valence state regions.

2. Experimental details

VO₂ films were deposited on a SiO₂ (2 μm)/Si substrate by pulsed laser deposition using a Twente Solid State Technology (TSST) manufactured laser MBE system equipped with a Compex Pro 205 KrF pulsed laser operating at 248 nm wavelength. The substrates were cleaned by acetone, ethanol and de-ionized water using ultrasonic cleaning before deposition. A metallic vanadium target (Alfa Aesa, 99.99%) was used for laser ablation. Before deposition, the chamber was pumped down to a base pressure of 5 × 10⁻⁶ Pa. The oxygen partial pressure and substrate temperature was kept at 0.67 Pa and room temperature respectively during deposition. The laser fluence was maintained at around 1 J/cm². A target to substrate distance of 5.5 cm and laser repetition rate of 10 Hz lead to deposition rate of 4.2 nm/min of the VO_x thin films. After deposition, the films were transferred to a radiative heating chamber and pumped to 2 × 10⁻³ Pa. The films were then annealed at 500 °C for 1 hour at different oxygen partial pressues of 60 Pa, 90 Pa, 150 Pa, 250 Pa, 350 Pa and 500 Pa respectively to form vanadium oxide thin films with different oxidation states.

Phase identification of the VO₂ thin films was carried out by X-ray diffraction (XRD) on a Shimazu XRD-7000 X-ray diffractometer with Cu Kα radiation (λ = 0.1542 nm). All film thickness is determined to be 50 nm on a JEOL7600F field emission scanning electron microscope (SEM). The morphology and surface roughness is characterized by atomic force microscopy (AFM). The surface resistance is characterized using a four point probe equipped with a temperature stage. Visible to near infrared optical constants below and above the MIT temperature of the vanadium oxide thin films are measured using spectroscopic ellipsometry (SE). A Drude-Lorentz model was used to model the optical constants of the vanadium oxide thin films. Confocal Raman microscopy study of the phase transition process was carried out on a Witec Alpha300R Raman microscopy equipped with a nitrogen protected temperature stage during a phase transition cycle from room temperature 25 °C to 80 °C. A continuous wave laser operating at 532 nm wavelength is used for Raman spectrum study. Raman mapping of the phase evolution as a function of annealing oxygen partial pressure was carried out. The mapping resolution is 500 nm within a 10 μm by 10 μm mapping area. To prevent laser induced MIT in VO₂ thin films, the power of the 532 nm laser was kept below 2 mW.

3. Results and discussion

3.1 Structure and MIT behavior of the vanadium oxides annealed at different PO₂

Figure 1(a) shows the XRD spectra of vanadium oxide thin films deposited by PLD and annealed at different oxygen partial pressures (PO₂). A clear phase evolution from V₂O₃ to VO₂ to V₂O₅ is observed with increasing the PO₂. For films annealed at 60 Pa, dominant V₂O₃ phase is observed which shows strong (211) diffraction peaks at 2θ = 24.31°. With increasing PO₂ to 90 Pa, (011) diffraction of monoclinic VO₂(M) phase appears, and the film becomes a composite of V₂O₃ and VO₂(M). Further increase the pressure to 150 Pa, the film becomes almost single phase of VO₂(M) showing textured (011) diffraction peak at 2θ = 27.85°, which agrees well with previous reports [7,36]. Continuing increasing the annealing PO₂, the film changes to a composite of VO₂(M) and V₂O₅ phases, and finally at 500 Pa, the film is fully oxidized to V₂O₅. The room temperature (300 K) Raman spectra of films annealed at 150 Pa, 250 Pa and 350 Pa are shown in Fig. 1(b). For the 150 Pa annealed sample with the pure VO₂(M) phase, characteristic B_g mode at 144 cm⁻¹ and A_g modes at 193 cm⁻¹, 223 cm⁻¹, 311 cm⁻¹, 389 cm⁻¹ and 613 cm⁻¹ are observed [37,38]. For films annealed at 250 Pa and 350 Pa, strong Raman scattering peaks of the V₂O₅ phase are observed at 143 cm⁻¹, 283 cm⁻¹ and 703 cm⁻¹ respectively [39]. The peak intensity increases with increasing the annealing PO₂, suggesting a higher phase volume concentration of V₂O₅. Interestingly, a Raman peak at 166 cm⁻¹ is observed in the 250 Pa annealed sample. This peak not belonging to any lattice vibration modes in pure VO₂(M) and V₂O₅ phases, was actually observed in vanadate nanotubes (VONTS) with similar Raman shift wave number of 162 cm⁻¹ [40–42]. According to the previous reports [40–43], this mode is strongly associated to the mixed valence state of V⁴⁺ and V⁵⁺, which disappeared with oxidizing the sample to pure V₂O₅. Therefore, the appearance of this peak is an indication of V⁴⁺ and V⁵⁺ mixed valence states in the samples. Detailed study of the distribution mapping of Raman peaks with PO₂ will be discussed later in this paper. The surface morphology of the 150 Pa annealed sample show homogenous granular structure with a surface roughness of around 5 nm, as shown in the AFM image in Fig. 1(c).

Fig. 1 (a) XRD spectra of vanadium oxide thin films annealed at different oxygen partial pressures. (b) Raman spectra of the vanadium oxide thin films oxidized at 150 Pa, 250 Pa and 350 Pa. (c) Surface morphology of a VO2 thin film (150 Pa anneal) studied by AFM (d) The temperature-dependence of the electrical resistivity of vanadium oxide thin films oxidized at various oxygen partial pressures.

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Electrical-resistivity versus temperature curve for the 150 Pa, 250 Pa and 350 Pa annealed samples are shown in Fig. 1(d). About 3 orders of resistivity change is observed in the 150 Pa annealed sample, suggesting the VO₂(M) phase purity of this film. The MIT temperature (T_MIT) for the 150 Pa annealed film is around 58 °C, which is lower than the bulk VO₂ T_MIT of 68 °C, suggesting this material is slightly oxygen deficient [44,45]. The oxygen vacancies create donor levels between the V 3d_|| bands and the O 2p π* bands. Thermally excited free electrons from the donors occupy the π* bands, therefore lowering the MIT temperature [44]. With increasing the oxygen partial pressure, the T_MIT approaches the bulk value. The hysteresis becomes noticeably wider, the resistance change amplitude due to MIT becomes clearly smaller, and the film resistance increases both for the high temperature and low temperature phases. Such behavior has been attributed to the change of major point defects from donors to acceptors with increasing PO₂, which eliminates the oxygen vacancy donors and the excess electrons in the π* bands [46,47]. The higher resistivity, and smaller resistivity change amplitude has been attributed to the semiconducting V₂O₅ incorporation with no MIT effect and higher resistivity [48]. The origin of wider hysteresis has been attributed to multiple origins, including the formation of V₂O₅ phases impeding the MIT process [27,48], the distribution of the VO₂ grain sizes and a consequentially size controlled nucleation and growth process during MIT [49]. However, the microsctructural origin of the above mentioned hysteresis variations as a function of PO₂ has not been reported. As will be shown in the following section, we use confocal Raman microscopy mapping to provide insights to the structural details of the phase evolution and MIT process of the vanadium oxide thin films as a function of PO₂.

3.2 Confocal Raman microscopy mapping study of the phase evolution, metal insulator transition in vanadium oxide thin films

Figure 2 shows the confocal Raman microscopy mapping of three characteristic Raman peak intensities, i.e. 193 cm⁻¹ for VO₂(M), 143 cm⁻¹ for V₂O₅ and 166 cm⁻¹ for the mixed valence regions, for 3 samples annealed at 150 Pa, 250 Pa and 350 Pa respectively. In all figures we arranged the color coding, so that Raman scattering at 193 cm⁻¹ from the pure VO₂(M) phase is indicated by bright, flat regions, whereas non-VO₂ phases or mixed valence regions are indicated as dark, high peaks. That is, for the 193 cm⁻¹ peak from VO₂(M), we plot the low intensity regions as dark and high peaks; whereas for the 143 cm⁻¹ and 166 cm⁻¹ peaks from V₂O₅ and V⁴⁺/V⁵⁺, we plot the high intensity regions as dark and high peaks. Characterizations of the 3 peaks are taken at the same 10 μm × 10 μm regions for the same sample, with a spatial resolution of 500 nm and integration time of 2 seconds for each point. For the 150 Pa annealed sample, the 193 cm⁻¹ peak intensity shows homogeneous distribution ranging from 900 to 1400 CCD counts across the mapping area as shown in Fig. 2(a) The intensity variation may come from the polycrystalline nature of the film, as well as the slight laser focal point and sample temperature variations during the test. No strong 143 cm⁻¹ or 166 cm⁻¹ peak is observed across the scanning area(below 240 and 80 CCD counts respectively), indicating few V₂O₅ phases or mixed valence state regions are present in this sample.

Fig. 2 Confocal Raman microscopy mapping of the intensities of characteristic Raman scattering peaks at 193 cm⁻¹, 143 cm⁻¹ and166 cm⁻¹ for vanadium oxide thin films oxidized at various oxygen partial pressures.

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For the 250 Pa annealed sample, clear non-VO₂ region is observed in the 193 cm⁻¹ peak intensity mapping in Fig. 2(b), accompanied by clear 143 cm⁻¹ and 166 cm⁻¹ peaks shown in Figs. 2(d) and 2(f). Interestingly, the peak distribution does not match with the 143 cm⁻¹ peak distribution from V₂O₅ as shown in Fig. 2(d), indicating that a simple VO₂-V₂O₅ composite scenario is not enough to describe the microsctructure of this thin film. For the 143 cm⁻¹ peak mapping in Fig. 2(d), the peak intensity is clearly enhanced across the scanning area, suggesting V₂O₅ is homogeneously distributed across the sample. On the other hand, the low intensity 193 cm⁻¹ peak positions match well with the high intensities of the 166 cm⁻¹ peaks, as shown in Fig. 2(f). In fact, the high intensity regions of the 166 cm⁻¹ peak shown in Fig. 2(f) also match with the low intensity regions of the 143 cm⁻¹ peak in Fig. 2(d). Interestingly, these mixed valence state regions form micron scale island structures in the film. The intensity mapping of the 143 cm⁻¹, 166 cm⁻¹ and 193 cm⁻¹ peaks in Fig. 2(b), 2(d) and 2(f) indicate a composite material scenario of the 250 Pa annealed sample with pure VO₂(M), V₂O₅ and mixed V⁴⁺/V⁵⁺ valence state regions. Since XRD in Fig. 1(a) shows only VO₂(M) or V₂O₅ phases in this sample, the V⁴⁺/V⁵⁺ valence state regions may be present either in VO₂ or V₂O₅ phases by adopting the following defect chemistry reactions:

\begin{array}{l} 4 V_{V}^{\times} + O_{2} \overset{V O_{2}}{\to} 2 O_{o}^{\times} + V_{V}^{4'} + 4 V_{V}^{•} (i n V O_{2}) \\ O_{O} \overset{V_{2} O_{5}}{\to} \frac{1}{2} O_{2} + V_{O}^{• •} + V_{V}^{'} (i n V_{2} O_{5}) \end{array}

where the roman type “V” in the above formulas stands for vacancies, and the italic type “V” stands for vanadium. The different equilibrium oxygen partial pressure of VO₂ and V₂O₅ phases make above reaction possible at intermediate annealing PO₂ [50]. The vanadium vacancies in VO₂ and oxygen vacancies in V₂O₅ creates acceptor and donor levels in the host material lattice, whereas the holes and electrons are trapped by the V⁴⁺ or V⁵⁺ ions to form V⁵⁺ and V⁴⁺ ions respectively. Further discussion on the influence of this material microstructure on the optical properties will be presented later in this paper.

With further increasing the annealing PO₂ to 350 Pa, non-VO₂ phase region with low 193 cm⁻¹ peak intensity continues increasing, as shown in Fig. 2(c). Meanwhile, the intensity of the 143 cm⁻¹ Raman peak from the V₂O₅ phase significantly increases as shown in Fig. 2(e). However, some mixed valence regions with strong 166 cm⁻¹ peak intensity can still be observed as shown in Fig. 2(g). Therefore the film is also a VO₂/V₂O₅/mixed valence state composite with higher volume concentration of the V₂O₅ phase. These Raman mapping results suggest the following phase evolution process in vanadium oxide thin films when increasing PO₂: With excess oxygen incorporation in the lattice, the V⁵⁺ valence ions are firstly formed in the VO₂ lattice. As further increasing PO₂, V₂O₅ phases nucleate and grow from V⁵⁺ rich regions together with part of vanadium ions in the V⁴⁺ valence state due to a relatively low PO₂ for V₂O₅ growth. In a wide PO₂ range, VO₂(M) and V₂O₅ phases coexist, whereas V⁴⁺/V⁵⁺ mixed valence state regions are also present in these phases. When PO₂ is high enough, the material transforms to a pure V₂O₅ phase.

To elucidate the influence of different phases and valence states on the MIT process of VO₂, we characterized the Raman spectrum during MIT of the 250 Pa annealed sample in a heating process from 28 °C to 80 °C, as shown in Fig. 3. Due to temperature variation induced focal point variation, we could not obtain accurate intensity mapping results during the temperature ramping process with a high magnification objective lens (NA = 0.9, 100 × , Zeiss). Instead, we collected the Raman spectrum from a larger sample area with a lower magnification lens (NA = 0.75, 50 × , Zeiss) to cover all the different phases and valence state regions. As shown in Fig. 3, with increasing temperature across the T_MIT, the 193 cm⁻¹ and 223 cm⁻¹ peaks from the VO₂ (M) phase diminish due to the monoclinic to tetragonal phase transition in this material, consistent with other reports [51,52]. The 143 cm⁻¹ peak mostly from V₂O₅ show little intensity variation as no MIT is taking place in this phase. The 166 cm⁻¹ peak partly decreases its intensity with increasing the temperature above T_MIT. However it is not consistent with the 193 cm⁻¹ peak intensity variation. For example from 45 °C to 64 °C the intensity of the 193 cm⁻¹ peak significantly decreases, whereas the intensity for the 166 cm⁻¹ peak remains comparable. This peak is also not diminished at 85 °C like other peaks from the VO₂ phase. Measuring the Raman spectrum of the sample after cooling down to room temperature show much lower intensity of the peak at 166 cm⁻¹ (data not shown), indicating a decomposition process of the V⁴⁺/V⁵⁺ structure instead of phase transition caused intensity variation. This phenomenon has actually been reported before in vanadium nanotubes, where oxidation of the V⁴⁺ ions to V⁵⁺ at high temperatures was observed [53]. Therefore, from temperature dependent Raman spectrum study, both the V₂O₅ phase and the V⁴⁺/V⁵⁺ mixed valence state regions do not show MIT, which impedes the nucleation and growth of the metallic rutile VO₂(T) phases. The inset of Fig. 3 shows the 193 cm⁻¹ peak intensity variation as a function of sample temperature during heating. A wide phase transition temperature range is observed, which matches with the sheet resistance versus temperature curve of this sample in Fig. 1(d).

Fig. 3 Temperature dependent Raman spectrum for vanadium oxide thin film oxidized at 250 Pa at the range of 120 cm⁻¹ to 280 cm⁻¹ during heating the sample from 45 °C to 85 °C. The inset shows the peak intensity versus characterization temperature for the 193 cm⁻¹ peak.

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3.3 Optical properties of vanadium oxide thin films annealed under different PO₂

Figure 4(a) and 4(b) shows the index of refraction and extinction coefficient of vanadium oxide thin films oxidized under different PO₂ measured by spectrometric ellipsometry. Optical constants at both 25 °C (insulator phase) and 85 °C (metal phase) are characterized. Both the refractive index and the extinction coefficient show first increase, then decrease behavior with increasing the annealing PO₂.The refractive index decreases for the metallic state, whereas the extinction coefficient spectrum shift to longer wavelengths in the metallic state. Higher extinction coefficient is observed near the infrared wavelengths for the metallic state [54]. The amplitude of phase transition induced refractive index and extinction coefficient change decreases with increasing the annealing PO₂, due to the presence of non-phase changing V₂O₅ phases and mixed valence state regions for films prepared at high PO₂.

Fig. 4 (a) Index of refraction and (b) extinction ratio of vanadium oxide thin films fabricated under different oxygen partial pressures, for both the insulator (25 °C) and the metal (85 °C) states.

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To further understand the optical properties of these films, we used Drude-Lorentz model and effective medium theory to fit the optical constants of the films prepared under different PO₂. In particular, we fit the optical constants of the end members: the V₂O₅ thin film prepared at PO₂ = 500 Pa and the VO₂ thin film prepared at PO₂ = 150 Pa, as well as the film with mixed phases prepared at PO₂ = 250 Pa. For the pure VO₂ phase, we use the following classical Drude-Lorentz model:

ε (ω) = ε_{1} - i ε_{2} = ε_{\infty} - \frac{ω_{p}^{2}}{ω^{2} - i ω γ_{p}} + \sum_{k} \frac{S_{k} ω_{k}^{2}}{ω_{k}^{2} - ω^{2} + i ω Γ_{k} ω_{k}}

where

ε_{\infty}

is the dielectric constant under high-frequencies, ω_p is the plasma frequency, γ_p is the collision frequency of free carriers, ω_k is the k^th oscillator frequency, S_k is the resonant amplitude factor intensity of the k^th oscillator, Γ_k is the damping factor of the k^th oscillator. For pure V₂O₅, we use the Lorentz model as follows:

ε (ω) = ε_{1} - i ε_{2} = ε_{\infty} + \sum_{j} \frac{S_{j} ω_{j}^{2}}{ω_{j}^{2} - ω^{2} + i ω Γ_{j} ω_{j}}

where, ω_j, Γ_j and S_j correspond to the j^th oscillator frequency, damping factor and resonance amplitude factor for the V₂O₅ phase respectively. For the mixed phase thin film prepared at PO₂ = 250 Pa, we use the effective medium theory [55] with the following expression for its dielectric constants:

ε (ω) = ε_{\infty} + f [\sum_{k} \frac{S_{k} ω_{k}^{2}}{ω_{k}^{2} - ω^{2} + i ω Γ_{k} ω_{k}} - \frac{ω_{p}^{2}}{ω^{2} - i ω γ_{p}}] + (1 - f) \sum_{j} \frac{S_{j} ω_{j}^{2}}{ω_{j}^{2} - ω^{2} + i ω Γ_{j} ω_{j}}

where f is a fitting parameter which is related to the volume fraction of VO₂ in the mixed phase, whereas (1-f) is related to the V₂O₅ phase volume fraction. The experimental dielectric constants for these three samples are obtained by using the following equation:

\begin{array}{l} ε_{1} = n^{2} - k^{2} \\ ε_{2} = 2 n k \end{array}

The fitting results are shown in Fig. 5 above. As shown in all figures, the experimental results and the fitted dielectric constants match well with each other both for the high temperature metallic state and the low temperature insulator state. The optical constants of pure VO₂ and V₂O₅ phases also match with previous reports [56, 57]. The fitting parameters are summarized in Table 1. Compared to previous reports of VO₂ [58] and V₂O₅ thin films [59], the oscillator frequencies and damping factors match very well for films with pure VO₂ phases (150 Pa) and V₂O₅ phases (500 Pa). For example, the high frequency dielectric constant is 4.128 and 3.957 for VO₂ at the metal and insulator state respectively, matching with previous reported values of 4.26 and 3.95 for a 100 nmVO₂ thin film on a sapphire substrate. Detailed comparison can be made between Table 1 and Fig. 5, Fig. 6 for VO₂ in ref. 58 and Fig. 3 for V₂O₅ in ref.59.

Fig. 5 Experimental and Drude-Lorentz model fitted spectrum of real and imaginary parts of the dielectric constant for (a) V₂O₅ (b),(c) VO₂ fabricated at PO₂ = 150 Pa and (d),(e) vanadium oxide with VO₂ and V₂O₅ composite phases fabricated at PO₂ = 250 Pa, for both the metal and insulator states.

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Table 1. Parameters of Drude-Lorentz Model of VO2 and V2O5

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Fig. 6 Comparison of the resonance oscillator energies used for fitting the pure VO₂ (150 Pa) and V₂O₅ (500 Pa) phases (black dots) and for the vanadium oxide with VO₂ and V₂O₅ composite structure (250 Pa) (red circles) for both the (a) insulator and (b) metal states.

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Next, we focus on the optical properties of the film fabricated at PO₂ = 250 Pa with both VO₂ and V₂O₅ phases. The fitting parameters are summarized in Table 2, and the comparison between the resonator oscillator frequencies are shown in Fig. 6. The f parameter related to the volume concentration of the VO₂ phase is fitted to be 0.8065, indicating most of the film is still the VO₂ phase, consistent with Raman intensity mapping results shown in Fig. 2. It is clear that the optical property of the film with both VO₂ and V₂O₅ phases can be well fitted using the effective medium theory described by Eq. (4) for both T<T_MIT and T>T_MIT cases. However the resonance frequency show differences compared to pure VO₂ and V₂O₅ phase. At T<T_MIT, the oscillator frequencies at 5.72 eV for the VO₂ phase shift to lower energies of 4.87 eV, and the oscillator frequencies at 2.98 eV and 4.32 eV for the V₂O₅ phase shift to 2.57 eV and 3.62 eV respectively. There are also slight frequency changes for other resonance oscillators of the VO₂ phase. These differences may indicate the influence of the mixed valence state regions as measured in Fig. 2. At T>T_MIT, the oscillator frequencies almost match with pure metallic VO₂ phase, whereas the oscillator frequency at 5.33 eV for the V₂O₅ phase shifts to 7.21 eV. The less variation of the optical property of the metallic VO₂ phase further suggests that the mixed valence state regions do not participate in the MIT process, which show less influence to the optical properties of the metallic VO₂ but stronger influence to the optical property of the V₂O₅ phase at higher temperatures.

Table 2. Parameters of Drude-Lorentz Model of vanadium oxide composite

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4. Conclusion

In summary, vanadium oxide thin films with different oxygen stoichiometries are fabricated by pulsed laser deposition and oxidation. A clear phase transition from V₂O₃ to VO₂(M) to V₂O₅ phases is observed with increasing the annealing oxygen partial pressure. Confocal Raman microscopy mapping is demonstrated to be a useful tool for analyzing the phase distribution, and metal insulator phase transition process of the vanadium oxide thin films. A new Raman peak at 166 cm⁻¹ is observed, which is attributed to the presence of V⁴⁺/V⁵⁺ mixed valence state regions in VO₂ and V₂O₅ phases. These regions do not participate in the metal insulator phase transition process as indicated by its persistent peak intensity up to high temperatures. The optical property of the vanadium oxide thin films fitted by Drude-Lorentz model and effective medium theory confirms the influence of V₂O₅, VO₂ and mixed valence state regions to the material optical properties. These findings provide insights for understanding the phase evolution, MIT process and optical properties of the vanadium oxide thin film systems with different oxygen stoichiometries.

Funding

National Natural Science Foundation of China (61475031, 51302027, 51522204); the Fundamental Research Funds for the Central Universities (ZYGX2013J028, ZYGX2014Z001); the Science Foundation for Youths of Sichuan Province (2015JQO014); and Open Foundation of Key Laboratory of Multi-spectral Absorbing Materials and Structures, Ministry of Education (ZYGX2014K009-3).

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Resonance oscillator		1	2	3	4	5	6	7
	Sk	0.545	0.427	0.683	0.628	1.543	1.340	1.030
Sample 1 VO2(T<Tc)	ωk[eV]	1.250	1.436	1.683	3.036	3.444	4.040	5.729

	Γk[eV]	0.536	0.483	2.683	0.386	0.353	0.461	0.533
		$ε_{\infty} = 4.128$
	Sk	3.683	1.121	1.192	1.394
Sample 1 VO2(T<Tc)	ωk[eV]	0.888	2.939	3.420	4.454
Sample 1 VO2(T<Tc)
	Γk[eV]	1.069	0.318	0.334	0.247
	$ε_{\infty} = 3.957$		$ω_{p} = 3.479 eV$		$γ_{p} = 0.296 eV$
	Sk	1.228	0.795	0.086	0.176
Sample 2
V2O5	ωk[eV]	2.988	3.368	4.329	5.332
	ωk[eV]	2.988	3.368	4.329	5.332

Resonance oscillator		1	2	3	4	5	6	7	8	9	10	11
	Sk	1.642	0.673	0.699	0.302	1.058	1.372	0.843	0.970	0.759	0.700	0.681
	ωk[eV]	0.962	0.970	1.196	2.878	3.205	4.371	4.865	2.566	3.545	5.604	3.625
Composite
	Γk[eV]	0.686	0.502	0.651	0.520	0.218	0.377	0.034	0.949	0100	0.521	1.058
(T<Tc)
	$ε_{\infty} = 3.944$		$f = 0.8065$
	Sk	1.641	0.599	1.436	0.994	1.298	0126	0.732	0.567
	ωk[eV]	0.818	3.082	3.379	4.213	3.317	3.477	4.579	7.219
Composite
	Γk[eV]	0.767	0.624	0.388	0.562	0.277	0.529	0.064	0.502
(T>Tc)
	$ε_{\infty} = 3.883$		$ω_{p} = 3.620 eV$		$f = 0.8065 eV$

Resonance oscillator		1	2	3	4	5	6	7
	Sk	0.545	0.427	0.683	0.628	1.543	1.340	1.030
Sample 1 VO2(T<Tc)	ωk[eV]	1.250	1.436	1.683	3.036	3.444	4.040	5.729

	Γk[eV]	0.536	0.483	2.683	0.386	0.353	0.461	0.533
		$ε_{\infty} = 4.128$
	Sk	3.683	1.121	1.192	1.394
Sample 1 VO2(T<Tc)	ωk[eV]	0.888	2.939	3.420	4.454
Sample 1 VO2(T<Tc)
	Γk[eV]	1.069	0.318	0.334	0.247
	$ε_{\infty} = 3.957$		$ω_{p} = 3.479 eV$		$γ_{p} = 0.296 eV$
	Sk	1.228	0.795	0.086	0.176
Sample 2
V2O5	ωk[eV]	2.988	3.368	4.329	5.332
	ωk[eV]	2.988	3.368	4.329	5.332

Resonance oscillator		1	2	3	4	5	6	7	8	9	10	11
	Sk	1.642	0.673	0.699	0.302	1.058	1.372	0.843	0.970	0.759	0.700	0.681
	ωk[eV]	0.962	0.970	1.196	2.878	3.205	4.371	4.865	2.566	3.545	5.604	3.625
Composite
	Γk[eV]	0.686	0.502	0.651	0.520	0.218	0.377	0.034	0.949	0100	0.521	1.058
(T<Tc)
	$ε_{\infty} = 3.944$		$f = 0.8065$
	Sk	1.641	0.599	1.436	0.994	1.298	0126	0.732	0.567
	ωk[eV]	0.818	3.082	3.379	4.213	3.317	3.477	4.579	7.219
Composite
	Γk[eV]	0.767	0.624	0.388	0.562	0.277	0.529	0.064	0.502
(T>Tc)
	$ε_{\infty} = 3.883$		$ω_{p} = 3.620 eV$		$f = 0.8065 eV$

Study of the phase evolution, metal-insulator transition, and optical properties of vanadium oxide thin films

Abstract

1. Introduction

2. Experimental details

3. Results and discussion

3.1 Structure and MIT behavior of the vanadium oxides annealed at different PO₂

3.2 Confocal Raman microscopy mapping study of the phase evolution, metal insulator transition in vanadium oxide thin films

3.3 Optical properties of vanadium oxide thin films annealed under different PO₂

4. Conclusion

Funding

References and links

Cited By

Figures (6)

Tables (2)

Equations (5)

Optical Materials Express

Abstract

1. Introduction

2. Experimental details

3. Results and discussion

3.1 Structure and MIT behavior of the vanadium oxides annealed at different PO2

3.2 Confocal Raman microscopy mapping study of the phase evolution, metal insulator transition in vanadium oxide thin films

3.3 Optical properties of vanadium oxide thin films annealed under different PO2

4. Conclusion

Funding

References and links

Cited By

Figures (6)

Tables (2)

Equations (5)

Optical Materials Express

3.1 Structure and MIT behavior of the vanadium oxides annealed at different PO₂

3.3 Optical properties of vanadium oxide thin films annealed under different PO₂