1. Introduction

Monoclinic vanadium dioxide (VO₂(M)) thin films have received considerable research interest, due to its metal-insulator phase transition (MIT) properties at 68 °C [1–4]. Significant variation of optical properties of VO₂ takes place in a wide wavelength range covering from visible to THz frequencies, bringing wide applications such as smart windows [5,6], uncooled micro-bolometers [7], infrared camouflage and thermal radiation control applications [8]. However, the first-order phase transition process of VO₂ is accompanied with large temperature hysteresis. In polycrystalline VO₂ materials, this hysteresis can be as large as 10 °C or above [9]. The large thermal hysteresis width observed in the phase evolution of VO₂(M) causes different electrical and optical constants of VO₂(M) at the same temperature during increasing or decreasing temperature, which prevents device applications at the intermediate phase states of VO₂(M). For example, for infrared camouflage applications, a narrow phase transition hysteresis is desired because it allows one to one correlation between the radiation temperature and the actual temperature [10]. Therefore, the hysteresis width of VO₂(M) phase transition has to be reduced for such applications.

Some previous reports show that transition metal ion doping in vanadium oxides can decrease the thermal hysteresis width of VO₂ [11,12] during MIT. For instance, Shi Chen et al. doped Ti into VO₂(M) nanoparticles and obtained a thermal hysteresis width of 6.6 °C, which is much lower than that of 24.9 °C for undoped VO₂(M) [13]; Zhang Jiao et al. also obtained a narrow hysteresis width of 5 °C by doping W into pure VO₂(M) [14]. However, the hysteresis width reduction is still not enough. Meanwhile, previous reports did not perform a systematic investigation on the structure of doped VO₂(M) during MIT. Several mechanisms for reducing the hysteresis width in Ti doped VO₂ materials are proposed, such as increased nucleation sites or reduced energy barrier difference between phase transitions. Therefore, it is important to explore more material systems and carry out detailed structural characterizations to provide more insights to this phenomenon. In this work, we report hafnium doped (Hf-doped) VO₂(M) polycrystalline thin films with narrow thermal hysteresis width (only 1.9-2.7 °C). The sheet resistance, transmittance and Raman spectra as a function of temperature were investigated to systemically during the MIT process. Finally, thermal infrared radiation images of Hf-doped VO₂(M) samples were characterized, showing negligible hysteretic behavior of the radiation temperature during the MIT process, indicating promising potential for the application of Hf-doped VO₂ in infrared camouflage and thermal radiation control devices.

2. Experimental details

Pulsed laser deposition (PLD) with a Compex Pro 205 KrF excimer laser operating at 248 nm wavelength was used for VO₂ thin films deposition on SiO₂ substrates. The base pressure of the PLD chamber was pumped down to 5 × 10⁻⁶ Pa before deposition. Combinatorial deposition of Hf-doped VO₂ thin films was carried out by ablating alternatively a V(Alfa Aesa, 99.99%) and a HfO₂ (Alfa Aesa, 99.99%) target with different number of laser pulses. The resulting Hf concentrations were 0 at.%, 1 at.%, 3 at.%, 5 at.%, and 8 at.% namely VO₂, HVO1, HVO3, HVO5 and HVO8 respectively, as measured by energy dispersive spectroscopy (EDS). The thin film deposition rate was 4.2 nm/min. The laser fluence was 1 J/cm². The laser repetition rate was 10 Hz. And the target to substrate distance was 5.5 cm. During deposition, the oxygen partial pressure (PO₂) was kept at 0.67 Pa, and the substrate temperature was at room temperature. After deposition, the thin films were annealed at 500 °C, PO₂ = 150 Pa for 1 hour in a chamber equipped with a radiative heater.

X-ray diffraction (XRD, Shimazu XRD-7000) with Cu Kα radiation (λ = 0.1542 nm) was carried out to identify the phase of the samples. Atomic force microscope (AFM, Benyuan BY-3000 SPM) images were measured to analyze the surface morphology. The thickness of all samples was around 50 nm, as measured by a field emission scanning electron microscope (FESEM, JEOL7600F). Four-point probe measurement system equipped with a heater stage was used to characterize the sheet resistance of the samples during MIT. Optical properties involving transmittance of all samples were characterized by ultraviolet-visible spectroscopy (UV-VIS, LAMBDA 750UV/VIS/NIR). Raman spectra were collected using a Reinishaw Invia Reflex Raman microscope operating at 514 nm wavelength. Thermal radiation images were obtained using a thermal camera (FLIR Systems, FLIR T10xx) to characterize the infrared radiation property of samples during phase transition.

3. Results and discussion

3.1 Structural properties

X-ray diffraction (XRD) patterns were firstly employed to characterize the phase of samples as shown in Fig. 1(a). For Hf doping concentrations of 1-3 at.%, all three samples show a diffraction peaks at 2θ~28°, which corresponds to the (011) plane of the monoclinic VO₂(M) phase. However, samples show a phase change to monoclinic VO₂(B) structure with Hf doping concentration up to 5-8 at.% (see supplementary file), suggesting a solubility limit of Hf doping concentration of around 3 at.%. The (011) peak position for 0 at.% is located at 2θ = 27.86°. With Hf doping, the (011) peak positions shifted to 2θ = 27.97° and 2θ = 28.02° for 1 at.% and 3 at.% dopant concentrations respectively, indicating a reduced (011) lattice spacing (d ₍₀₁₁₎) upon Hf doping. The d ₍₀₁₁₎ and grain size (D) of samples are calculated respectively using the Bragg law 2d × sinθ = nλ, where n = 1 and λ = 0.154 nm corresponding to the Cu Kα radiation wavelength, and Debye-Scherrer formula D = K λ/Bcosθ where K = 0.9 and B meaning the full width at half maximum (FWHM) of (011) diffraction peaks. As shown in Fig. 1(b), the grain size of the Hf-doped VO₂(M) thin films (18.20 nm for 1 at.% and 16.68 nm for 3 at.%) is obviously smaller than 20.04 nm of pure VO₂(M), suggesting Hf doping leads to smaller grains of the sample.

 

Fig. 1 (a) XRD patterns of Hf:VO₂ samples at different Hf doping concentrations; (b) the grain size and lattice spacing d ₍₀₁₁₎ values of samples at different Hf doping concentrations.

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Figure 2 shows the Atomic Force Microscope (AFM) images of Hf-doped VO₂ samples at different doping concentrations. AFM images of samples without Hf doping show clear morphology with uniform size of grains, homogeneous grain distribution and obvious grain boundaries (Fig. 2(a)), revealing good crystallinity. However, in Hf-doped VO₂ samples, the smaller grain size causes the decrease of surface roughness from 60 nm to 30 nm, even worse, the crystal grains for HVO1 and HVO3 samples cannot be resolved clearly (Fig. 2(b-c)), indicating deteriorated crystallinity in these two samples. The AFM images results are in good agreement with XRD diffraction patterns.

 

Fig. 2 AFM 2D and 3D images (insets) of Hf-doped VO2 samples with different Hf doping concentrations (a) 0 at.%, (b) 1 at.% and (c) 3 at.%.

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3.2 Sheet resistance during MIT

To evaluate the phase transition process, temperature dependent sheet resistance curves were measured for Hf-doped VO₂(M) thin films, as shown in Fig. 3a. All samples show clear resistance change due to MIT (samples with Hf doping concentrations of 5-8 at.% with VO₂(B) phase show no MIT property, see supplementary materials). For VO₂, 3 orders of magnitude resistance change is observed during MIT, whereas about 2 orders of resistance change was observed in HVO1 and HVO3 samples. A drastic difference is observed in the hysteresis width. The hysteresis width of HVO1 and HVO3 is much narrower than pure VO₂, despite of a similar phase transition temperature range for all samples. Further, it can be seen that the sheet resistance of samples with Hf doping is lower by ~5 times at 30 °C compared to that of pure VO₂ thin films.

 

Fig. 3 (a) Sheet resistance curves of HVO samples at different doping concentrations; Also shown are their Gauss fit curves using d(lg(R))/d(T) for (b) VO₂, (c) HVO1 and (d) HVO3 samples.

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To quantitatively evaluate the phase transition temperatures, we applied Gaussian curve fitting to the first order derivatives of d(lg(R))/d(T), where R is sheet resistance and T is temperature, as shown in Fig. 3(b-d) [15]. Notice that the MIT temperature of the pure VO₂ sample during the heating process is 67.9 °C, which agrees well with the reported 68 °C of bulk VO₂(M). It also shows a large hysteresis width of 8.3 °C (Fig. 3(b)). The MIT temperatures during the heating process for HVO1 and HVO3 samples are 62.2 °C and 62.9 °C respectively, as shown in Fig. 3(c) and 3(d). The comparable MIT temperature to pure VO₂ is due to the equal valence state of Hf⁴⁺ ions compared to V⁴⁺ ions. Meanwhile, the hysteresis width drastically decreased from 8.3 °C to 1.9 °C and 2.7 °C for HVO1 and HVO3 respectively. It should be noted that the d(lg(R))/d(T) curves are noisier than the R-T curves due to the error created by our temperature control stage ($\pm$0.5 °C). The drastic decrease of hysteresis width is clearly observed in both Hf-doped VO₂ samples. However it is hard to compare the hysteresis width between HVO1 and HVO3 because they are comparable within the error range of our test set-up. It should be noted that grain size has been reported to strongly influence the thermal hysteresis in sputter deposited VO₂ thin film with flake-like grains [2]. However the mechanism may not apply for this work as the grain size variation is very small between our samples. In previously reported Ti doped VO₂ thin films, the mechanism of reduced hysteresis has been attributed to a change of phase transition activation energies between different phases of VO₂ [13]. Future study is needed to reveal the underlying mechanism of reduced hysteresis of phase transition in Hf doped VO₂ thin films.

3.3 Optical transmittance

To study the optical properties of Hf doped VO₂ during the phase transition process, the optical transmittance curves in the ultraviolet to near infrared spectrum range of Hf-doped VO2 samples were characterized at different temperatures during the heating and cooling process, as shown in Fig. 4. The transmittance values of all samples decrease and increase respectively with increasing or decreasing temperatures, indicating an MIT process in all samples. For VO₂ (Fig. 4(a)), the transmittance curves in the near infrared show a fast decrease during heating from 65 °C to 80 °C, but a slow recovery during cooling from 80 °C to 65 °C, due to the hysteretic behavior of MIT process. Nevertheless, the transmittance curves for samples with Hf doping in the heating process (Fig. 4(b) and 4(c)) show similar transmittance at the same temperature compared to that during the cooling process, showing no obvious hysteretic behavior. To show this more clearly, optical transmittance versus temperature curves at the wavelength of 1500 nm are plotted in Fig. 4(d). It can be seen from Fig. 4(d) that the widest hysteresis width is observed for the VO₂ sample with a temperature span of ~8 °C, whereas the hysteresis width decreases dramatically after doping Hf, consistent with the resistivity characterizations. It should be noted from Fig. 4(d) that the hysteresis widths decrease from ~9 °C to ~2.5 °C and ~2 °C for Hf doping concentrations from 0 at.% to 3 at.%, comparable to resistivity measurements in Fig. 3. Notice the difference between HVO1 and HVO3 is small and comparable to experimental errors of the temperature control stage, indicating both films show very narrow hysteresis compared to undoped VO₂ thin films. These results indicate that the metal-insulator transition process dominate the optical properties of Hf doped VO₂ thin films.

 

Fig. 4 Optical transmittance curves measured at different temperatures during the heating and cooling process of Hf-doped VO2 samples for (a) VO₂ (b) HVO1 and (c) HVO3 respectively. (d) Optical transmittance at 1500 nm versus temperature for all 3 samples. Notice the feature labeled by “*” is due to changing lamps in the UV-Vis characterizations.

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3.4 Temperature dependent Raman spectroscopy

To study the structural evolution during the MIT process of Hf-doped VO₂ samples, temperature dependent Raman spectra characterizations were carried out. Raman spectra of samples with doping concentrations of 0-3 at.% were obtained by setting various temperature points between 30 °C and 80 °C during the heating and cooling processes, as shown in Fig. 5. Three obvious peaks at 193 cm⁻¹, 223 cm⁻¹ and ~616 cm⁻¹ are observed in all samples at low temperatures, and two weak peaks at ~306 cm⁻¹ and ~386 cm⁻¹ only appear in the spectra of the VO₂ sample (Fig. 5(a) and 5(b)). Previous reports have assigned the low frequency phonons at 193 cm⁻¹ and 223 cm⁻¹ to lattice phonon Ag modes of V-V bonds in VO₂(M) phase [16–19], whereas the peaks at ~306 cm⁻¹, ~386 cm⁻¹ and ~616 cm⁻¹ are related to Ag vibrational modes of V-O bonds in the VO₂(M) phase [19,20]. The intensity of Raman peaks for all samples decreases and increases during the heating and cooling process respectively, which rely closely on the MIT process of the VO₂(M) phase [16,20]. With increasing Hf⁴⁺ concentration, formation of Hf-O bonds reduces the number of V-O bonds, hence the weak phonon modes at ~306 cm⁻¹ and ~386 cm⁻¹ vanish for HVO1 and HVO3 thin films. In addition, the strong peak at ~616 cm⁻¹ shifts gradually to higher frequencies with increasing Hf concentrations. The peak shifts from 616 cm⁻¹ for VO₂ to 626 cm⁻¹ for HVO1, and to 630 cm⁻¹ for HVO3 samples. The higher phonon energy of V-O vibration modes indicate clear distortion of Hf doping to the VO₂ lattice, which is also different from Ti doped VO₂ in previous reports [21]. The structural transition during the MIT process can be observed by plotting the intensity of Raman peaks at 193 cm⁻¹ and ~616 cm⁻¹ as a function of temperature, as shown Fig. 5(g) and Fig. 5(h).Wide and narrow hysteresis is observed for VO₂ and Hf-doped VO2 samples, in agreement with that of resistance and transmittance spectra shown in Fig. 3 and Fig. 4. This observation demonstrate that the narrow hysteresis in resistivity and optical transmittance is mainly due to the phase transition process difference of VO₂ and Hf doped VO₂ unit cell, rather than due to the mesoscopic crystalline and morphology difference as observed in the XRD and AFM measurements.

 

Fig. 5 The temperature dependent Raman spectra during the heating and cooling stages of samples at different doping concentrations, (a) and (b) 0 at.%, (c) and (d) 1 at.%,(e) and (f) 3 at.%; (g) peak intensity versus temperature curves of the Raman peak at 193 cm⁻¹; (h) peak intensity versus temperature curves of the Raman peak at ~616 cm⁻¹.

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3.5 Thermal radiation images

The unique narrow hysteresis in Hf doped VO₂ thin films enable them as good candidates for thermal radiation control applications. We used thermal imaging camera to measure the infrared radiation property of Hf-doped VO2 samples during the heating and cooling process across the MIT temperatures. The thermal infrared radiation images of the VO₂, HVO1 and HVO3 samples were recorded, with an interval of 5 °C both for the ranges of 30 °C −55 °C and 70 °C −80 °C, and an interval of 1 °C for the range of 55 °C −70 °C . The thermal radiation temperature (IRT, assuming a constant emissivity as pure VO₂(M) at 30 °C) versus the actual background temperature(BGT) read on the heater stage is plotted, as shown in Fig. 6 [10].Thanks to the MIT process of VO₂(M) thin films, the emissivity of VO₂ decreases with increasing BGT as indicated by IRT of the samples rapidly deviating away from BGT after 68 °C, 62 °C and 63 °C for VO₂, HVO1 and HVO3 samples respectively. The MIT temperatures (68 °C, 62 °C and 63 °C) agree well with the sheet resistance characterization results shown in Fig. 3. The IRT curves of the VO₂ sample show obvious thermal hysteresis property of the infrared radiation (Fig. 6(a) and (b)). Meanwhile, benefitted from the very weak phase transition hysteresis, both HVO1 and HVO3 samples show almost no hysteretic behavior of infrared radiation during heating and cooling (Fig. 6(c), (d), (e) and (f)), therefore the sample temperature and the radiation temperature is one to one corresponded. These results demonstrate that Hf doped VO₂ thin films show a promising potential for infrared camouflage applications [22].

 

Fig. 6 The thermal infrared radiation temperature(IRT) versus the background temperature (BGT) during the heating and cooling process for samples with different Hf doping concentration, (a) VO₂, (c) HVO1 and (e) HVO3, respectively. Several thermal radiation images and the IRT reading at different BGT were also presented in (b), (d) and (f).

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4. Conclusions

In summary, Hf-doped VO₂(M) thin films are deposited by pulsed laser deposition with a solid solution solubility up to 3 at.%. VO₂(M) doped with Hf (1-3 at.%) is found to significantly reduce the metal-insulator transition (MIT) hysteresis width compared to that of pure VO₂(M) thin films. The sheet resistance of 0-3 at.% Hf-doped VO₂(M) thin films show clear MIT property with 2-3 orders of resistance change. A significant reduction of the MIT hysteresis width down to 1.9 °C and 2.7 °C is observed in 1 at.% and 3 at.% Hf doped VO₂(M) thin films respectively. The temperature dependent optical transmittance curves also agree well with the resistance hysteresis. Raman spectroscopy shows Hf doping induced clear blue shift of the V-O vibration modes, whereas little change was observed in V-V vibration phonon modes. Thermal radiation images of 1 at.% and 3 at.% Hf-doped VO2 samples show almost no hysteretic behavior during the heating and cooling process, indicating a promising potential for such materials in infrared camouflage and thermal radiation control applications.

Appendix Supplementary materials

Characterizations of 5 at.% and 8 at.% Hf doped VO₂ thin films

Figure 7 shows the XRD patterns of 5 at.% and 8 at.% Hf doped VO₂ thin films. As the doping concentration increases from 1 at.% to 3 at.%, the increase in the impurity defect concentrations deteriorates the crystallization quality of samples, resulting in the decreased intensity of diffraction peak at (011) plane of VO₂(M) thin films. As shown in Fig. 7, the intensity of (011) peak of sample at doping concentration 5 at.% shows a further decline, and the peak at 2θ=26.9° corresponding to (110) plane of VO₂(M) also appears, meanwhile, the XRD pattern also indicates the emergence of the metastable monoclinic phase VO₂(B) with a series of weak peaks at 2θ=28.86°, 44.08° and 45.54° respectively, matching well with those planes at (002), (003) and ($\overline{5}$11) presented in PDF^#31-1438 [23]. With the Hf doping concentration up to 8 at.%, the VO₂(M) phase of the sample vanishes completely, instead diffraction intensity from the VO₂(B) phase increases. Its two peak positions match well with the results reported by B. Guo et al., and corresponds to (002) and (003) planes of VO₂(B) in PDF^#81-2392 [24,25].

 

Fig. 7 XRD patterns of samples with Hf doping concentrations of 5 at.% and 8 at.%.

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The change in phase might be due to following: with Hf doping concentration of 5 at.% and 8 at.%, a large amount of Hf atoms are embed into the substitutional lattice sites of VO₂(M) lattice to distort the crystal structure, causing remarkable increase of structural defect concentrations and total system energy [26], so that VO₂(M) phase eventually becomes unstable. And the samples changes from VO₂(M) phase to the monoclinic VO₂(B) phase.

Figure 8 shows the sheet resistance of 5 at.% and 8 at.% Hf doped VO₂ thin films. Samples with doping concentrations of 0-3 at.% show 2-3 orders of resistance change. But it can be seen from Fig. 8 that resistance of samples with doping concentrations of 5 at.% and 8 at.% changed only by 2-4 times with temperature. Previous studies have revealed that metal-insulator phase transition (MIT) exists in VO₂(M) phase but not in VO₂(B) phase [27]. Therefore we focused our discussion on 0 at.% to 3 at.% Hf doped VO₂ thin films in the manuscript.

 

Fig. 8 The sheet resistance curves of samples at doping concentrations of 5 at.% and 8 at.%.

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Funding

National Natural Science Foundation of China (61475031, 51522204); 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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