1. Introduction

Vanadium dioxide (VO₂) films have been extensively investigated due to the significant insulator-metal transition (IMT) around 68 °C [1], which is fully reversible between a monoclinic transparent phase at room temperature and a rutile opaque phase at high temperature in IR and terahertz (THz) range. The transition can be excited by thermal, electrical field, and optical excitation [2, 3]. These particular properties enabled VO₂ to be applied on tunable metamaterial devices [4], switching and modulating applications [5].

Nowadays, great attentions about VO₂ films have been paid on the application of IR and far-IR/THz region. For example, N. Davila et al. developed variable optical attenuators through the self-sensing transition properties in VO₂ films. Moreover, combining with artificial metamaterials, metamaterial-VO₂ films can be acted as tunable functional devices. Dicken et al. deposited specific patterns of split ring resonators onto identically patterned VO₂ films, and achieved the range of 110 nm by tuning resonant frequency in near IR frequency [6]. Rensberg et al., using ion irradiation to defect-engineer the IMT of VO₂ as tunable metamaterial absorbers and tunable polarizers, demonstrated widely tunable optical materials in mid-IR region [7]. Also, VO₂ films alleviated many bandwidth-related restriction of metamaterial application with dynamic tuning in far-infrared resonance frequency [8].

However, some infrared-active phonons as absorption peaks restricted VO₂ modulation amplitude, which sporadically locates in mid-IR and far-IR/THz band [9, 10]. Transforming the crystalline structure could be a feasible solution to fix it. Enlighted by a previous report revealing four types of crystal structure of V_0.985Al_0.015O₂ at different temperature [11], including monoclinic M1 and M2, triclinic, and rutile structures, Al-doped VO₂ films exhibit great potential to improve optical THz properties in our previous research [12]. Although aluminum doping altered the crystal structure, the impaction degree of VO₂ crystal needs to estimate. Therefore, we performed several crystalline vibration spectra of Al-doped VO₂ films to analyze its lattice variations in mid-IR and far-IR/THz regions by dopants introducing.

2. Experimental

VO₂ films were deposited on high-purity single-crystal silicon substrates by DC reactive magnetron sputtering. The experimental details were described in our report elsewhere [12]. The crystal information, surface morphology, composition, and Al contents ratio were determined using X-Ray diffractometer (XRD, DX 2700 Dandong), field-emission scanning electron microscopy (FE-SEM, FEI Inspect F), X-ray photoelectron spectroscopy (XPS, XSAM 800), and energy dispersive spectrometer (EDS, OXFORD X-Max). A Raman microscope spectrometer (Raman, Renishaw Invia Reflex) was employed to obtain phonon vibration by the backscattering configuration using a 514.5 nm laser excitation with a power of 2 mW focusing on the samples. Infrared transmittances were measured by Fourier transform infrared spectroscopy (FT-IR/FT-FIR, PerkinElmer Spectrum 400). A standard four point probe method attaching a heating unit were employed to investigate the temperature dependent sheet resistance of the films between 25 °C and 90 °C.

3. Result and discussion

3.1 Composition and morphology

The XPS spectrum of high resolution scans of V2p were performed in Fig. 1. Fitted by Shirley function with software XPS peak 4.1, two valence states of vanadium were detected. The relatively high concentration of V⁵⁺ in all samples was related to the surface oxidation of VO₂ films [13]. When adding Al element into VO₂ films, the relatively low state of Al³⁺ captured less electrons comparing to element vanadium. Thus, this change caused the oxygen escaping form VO₂ crystallite, reduced the oxidation state of the film, and generated the fraction of + 4 valence vanadium rising as shown in Fig. 1(c). Simultaneously, the peak positions of + 4 and + 5 valences state of V2p_3/2 core levels slightly changed, such as V⁴⁺ rising from 515.90 to 515.97 eV, indicating an interaction enhancement between elements of vanadium and oxygen.

 

Fig. 1 XPS spectrum (left) and surface morphology (right) of VO₂ films.

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Figure 1 also showed the surface morphology of VO₂ films, which exhibited the quasi-spherical grains aggregating into striped bars with few pores. Distinct boundaries were observed and grains merged into an overlapped film in undoped sample. Doped with aluminum, grain size was clearly reduced, which provided a smoother and denser surface. The minor grains often accompanied with smaller crystallite, which would enlarge the composition of boundaries and degenerate the transition performance of VO₂ films.

3.2 Electrical and optical IMT properties of VO₂ films

Recorded by four-point probe method, the electrical phase transition properties of undoped and Al-doped VO₂ films were presented in Fig. 2(a). The resistivity of undoped sample abruptly dropped by almost 3 orders of magnitude from 12.30 Ω·cm at room temperature to 0.02 Ω·cm at 90 °C, indicating the competitive quality of film after the sputtering deposition. Due to the clear grain boundary and the porosity [14, 15], the hysteresis width of undoped VO₂ film has reached 18.2 °C, suggesting the polycrystalline structure of the film. After the introduction of Al dopants, the hysteresis width narrowed down, which was caused by the cooling branch significantly shifting to higher temperature. Added with the slightly high-temperature shift in heating braches, the phase transition temperature of Al-doped VO₂ films increased. The elevating transition temperature could be attributed to the combined effect of tight V-O binding energy and great boundary barrier. They have been revealed in the slightly increase of peak positions of + 4 and + 5 valences of V2p_3/2 core levels and the fraction expansion of grain boundaries by grains shrinking. Also, the transition magnitudes of VO₂ films were slightly degenerated by aluminum introduction.

 

Fig. 2 The electrical IMT properties (a) and far-IR/THz transmittance (b) of VO₂ films.

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Figure 2(b) presented the far-IR/THz transmittance of VO₂ films across the abruptly phase transition. The peak at 172 cm⁻¹ resulted from the correction of silicon background information. In the range of 133-233 cm⁻¹ (4-7 THz), VO₂ films presented sharp changes in THz transmittance across IMT. The average values of THz transmittance in this band at room temperature were over 93%, and dramatically decreased after heating to 90 °C. The average amplitude modulations for sample undoped, 1.7%, and 2.7% Al-doped were about 64.1%, 58.3%, and 47.3% with similar thickness about 210 nm measured by cross section SEM image (as shown in Fig. 1 inset). The increasing fraction of low-valence-state vanadium in XPS spectra suggested a transformation of VO₂ crystal and its stoichemistry deviation thus the modulation amplitude of VO₂ films reduced. Otherwise, as the undoped sample shown, two remarkable peaks at 316 cm⁻¹ (or 9.48THz) and 275 cm⁻¹ (or 8.25THz) were attributed to infrared-active phonon modes [10]. When doped with aluminum, these absorptions disrupted and the vibrations impaired. The disruption of phonon peaks by Al doping resulted in the enhancement of modulating performance in resonant bands, i.e. the increases of 6.9% at 9.48 THz and 5.8% at 8.25 THz. This rapid suppression of phonon vibration in far/THz region indicates a great potential to modify VO₂ properties to acquire practical application.

3.3 Information of phonon vibrations

However, it should be involved whether Al doping influences all the phonon vibration modes in VO₂ films and what is the mechanism of suppressing far-IR/THz resonant peaks, thus Raman shift and FTIR spectrum were employed to clarify the variation of phonon vibrations. Raman spectra of VO₂ films were presented in Fig. 3. The Raman spectrum of undoped sample indicated a predominantly monoclinic VO₂ structure and matched previous reports [16]. Introducing Al into VO₂ films, the motivating bands of Al doped films deviated monoclinic structure. Especially the peaks at 192 and 612 cm⁻¹ shifted to high frequencies of 202 and 641 cm⁻¹, respectively. The two insets in Fig. 4 presented the gradually high-frequency shift process. Noted that Parker et al. reported similar shifts by oxidizing VO₂ single crystal [17], thus Al doping exhibited an analogously oxidizing effect on VO₂ crystal. In detail, the bands of 192 and 223 cm⁻¹ were assigned to pairing and tilting of Vanadium cations [18], and peak at 612 cm⁻¹ was due to V-O stretching mode [16]. Since resonant modes for short and strong bonds were motivated in higher frequency by Raman laser, the stretching mode high-frequency shifted to 643 cm⁻¹ indicating that the length of V-O bonds were shortened with Al introducing. After this transformation, VO₂ crystal with Raman peaks at 202 and 641 cm⁻¹ has turned into another monoclinic structure—M2 phase, which could be verified by the identical M2 monoclinic spectra of previous reports [19, 20]. Moreover, the variation in Raman shifts was consistent with the peak position increment of V2p core level in XPS spectrum indicating the interactions between vanadium and oxygen intensified.

 

Fig. 3 Raman spectra of undoped and Al-doped VO₂ films.

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Fig. 4 The Mid-IR spectra of VO₂ films at room temperature.

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Figure 4 exhibited the Mid-IR transmittance spectra of VO₂ films. The undoped VO₂ films exhibited distinct phonon vibration peaks at 610 and 512 cm⁻¹, attributing to the V-O-V octahedral bending modes. After Al doping, although the magnitude were slightly impaired, the vibration peaks remained remarkable. Besides, 610 cm⁻¹ shifted to higher wavenumbers of 643 cm⁻¹, which is consistence with Raman spectra. Also, the broadening of 512 cm⁻¹ resulted from the increase of defection and distortion caused by the dopants ion in the lattice [21]. It is remindful that the broadening mainly extended to low-frequency orientation, which is agreed with the whole variation of Raman peaks in the range of 300-500 cm⁻¹, suggesting a side effect of reduction when oxidizing the V-O bond in O-V-O chains.

Both Raman shift and mid-IR spectra of Al-doped VO₂ films presented similar high-frequency shift. As for the undoped film, the lattice of VO₂ crystal endured a uniform ambient initially. Considering the difference valence of doping atom from the original vanadium, it can be inferred that when Al replaced native V position, the original electrostatic field disturbed by Al only providing three outermost electrons. Due to the surrounding O atoms not attracting enough electrons form aluminum, the interaction of V-O bonds became stronger and further shortened the bond length of adjacent V-V dimers. Consequently, the stronger V-O bonds and shortened V-V dimers result to the high-frequency shifts of phonon vibrations.

There is a pronounced difference that Al-doping suppressed far-IR/THz phonons but mid-IR phonons remained strong, although few changes were observed in mid-IR region. Notice that far-IR peaks were associated with the rotational-vibrational structures or some groups’ vibrations in crystal, but mid-IR peaks were originating from the fundamental vibrations. In Al-doped VO₂ films, the dopants transformed the crystalline structure and changed some bonds, especially altering the V-V dimers, which were identified by the peak variation of Raman shift. This alteration represented that the group structures of VO₂ crystal have been modified, and thus changed the vibration of group structures, which exhibited as the suppression in far-IR/THz band. As for the mid-IR phonon vibration, although it impaired by Al introduction, the majority of V-O bonds, which attributed to the polarizability of high-frequency transverse optical phonons associating with vibrations of the oxygen cages surrounding the V atoms, still remained [22]. Consequently, the intensity of mid-IR phonons remained strong.

Therefore, Al doping not only distorted the crystal vibration mode and transformed the monoclinic VO₂ into M2 phase, which could also be found in the reports by Strelcov et al. [23], but also suppressed the phonon vibration modes in far-IR/THz region. Mid-IR and Raman spectra showed that some modes of phonon vibrations endured high-frequency shifts attributing to oxidizing effect by Al doping while few negligible low-frequency shifts appeared. The valence difference of doping atoms adjusted the originally periodical structure of VO₂ crystal, and diminished V-V dimers so that the transition magnitude of VO₂ films were reduced [22]. Consequently, the distortion suppressed THz resonance and improved the modulation performance of THz resonant frequency that consisted huge proportions of high THz region. However, the modulation performance in un-resonant band was decreasing after Al doping. According to Strelcov report that Al doping has facilitated the transformation at the ratio of 0.25% (the minimum in this report is Al/V = 1.7%) [23], suitable Al concentration would efficiently utilize the advantages of well modulation performance and the reduction of phonon vibration to apply in THz regime.

4. Conclusion

Al dopants introducing into VO₂ films transformed the crystalline structure into M2 phase, exhibiting the reduction of the oxidation states of vanadium, the shrinkage of grain sizes, and the significant enhancement of amplitude modulation at THz phonon resonant band. High-frequency shifts in Raman spectra by Al doping presented the evidence of VO₂ crystalline modification. As for the other phonon characterization of mid-infrared spectra, although high-frequency shifting and peak broadening were observed in V-O-V bending modes, its resonances remained strong. We attribute the difference to the distinct origins of phonon vibrations that group-rotational peaks rapidly erased with crystalline deformation by Al doping whereas the bending modes of VO₂ crystal kept sturdy.