Room Temperature Optical Constants and Band Gap Evolution of Phase Pure M 1-VO 2 Thin Films Deposited at Different Oxygen Partial Pressures by Reactive Magnetron Sputtering

Spectroscopic ellipsometry study was employed for phase pure VO 2 (M 1 ) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO 2 (M 1 ) thin films have been determined in a photon energy range between 0.73 and 5.05 eV. The near-infrared extinction coefficient and optical conductivity of VO 2 (M 1 ) thin films rapidly increase with decreasing O 2 -Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge (E 1 ) at variedO 2 -Ar ratios are almost the same (∼2.0 eV); consequently, the absorption edge is not significantly changed. However, the optical band gap corresponding to semiconductor-to-metal phase transition (E 2 ) decreases from 0.53 to 0.18 eV with decreasing O 2 -Ar ratios.


Introduction
Vanadium dioxide (VO 2 ), one of the most interesting transition metal oxides, exhibits a reversible first-order semiconductor-to-metal phase transition (SMT) at a critical temperature   = 68 ∘ C (for bulk single crystal VO 2 ) [1].VO 2 has a tetragonal rutile structure with the P4 2 /mnm space group (R phase) above the phase transition temperature, where the partially filled d // band localized at the Fermi level and the rutile phase is metallic [2].Below the phase transition temperature, it transforms to a monoclinic structure with the P2 1 /c space group (M 1 phase), in which the partially filled d // band splits into an unoccupied part being pushed past the  * band and a filled part with the d // band dropping below the Fermi level, thus opening up a bandgap of ∼0.6 eV between  * and the filled part of d // band [2].Dramatic changes in the electrical and optical properties across the SMT make VO 2 thin films suitable for many applications, such as switching devices, sensors, and smart windows [3][4][5][6].
It has been noted that oxygen partial pressure has effects on the structural and resistivity transition behaviors of VO 2 [7].Although the optimized oxygen partial pressure to fabricate VO 2 films on glass and the optical properties of those samples were investigated [8], the optical constants, especially extinction coefficient , which is crucial in understanding band structures, are not involved.Moreover, two energy gaps  1 and  2 are not distinguished as well.
Low visible transparency and unfavorable yellowish colour, which are correlated with absorption edges, limit the application of VO 2 smart windows.For most practical applications the phase transition temperature   needs to be in the vicinity of room temperature (∼25 ∘ C) and the   may be assumed to be correlated with the optical band gap  2 .Consequently, the distinguishment of  1 and  2 plays an important role in improving the performance of VO 2 .
In this research, we thoroughly investigated the effects of oxygen partial pressures on the optical constants and  the electronic transition behaviors of phase pure VO 2 (M 1 ) thin films deposited on quartz glass by reactive magnetron sputtering.Moreover, two electronic transitions related to absorption edge ( 1 ) and SMT ( 2 ) were distinguished.

Experimental Section
VO 2 thin films with a thickness of ∼70 nm were deposited using a reactive rf magnetron sputtering system with a watercooled vanadium metal target (50 mm in diameter, 99.9% purity).Quartz glasses (20 × 20 × 1 mm) were used as substrates and they were ultrasonically cleaned in acetone and subsequently in ethanol for 15 min, respectively, and then dried with pure nitrogen flow.After being pumped down to a base pressure of 5 × 10 −4 Pa, the deposition chamber was filled with high purity (99.999%)Ar and O 2 mixture gas.The O 2 -Ar ratio was fixed as 1.0 : 49.0, 1.5 : 48.5, and 2.0 : 48.0, respectively (the unit is sccm).The total gas pressure was maintained at ∼1.0 Pa.An rf power of 200 W was applied to the V target.During deposition, the substrate temperature was kept at 450 ∘ C for the better crystallinity of VO 2 thin films.To improve the film homogeneity, the substrates were rotated along the vertical axis at a speed of 10 rpm.
The structure of the films was characterized by Raman spectrometer (Renishaw inVia Raman microscope) using a 514.5 nm laser.The optical transmittance was measured at a photon energy range of 0.73-5.05eV at 26 ∘ C and 95 ∘ C by a spectrophotometer (Hitachi Corp., Model UV-4100).Temperature was measured using a PT100 temperature sensor in contact with the films and was controlled via a temperature controlling unit.Heating was controlled through a temperature-controlling unit.Hysteresis loops were measured by collecting the transmittance of films at a fixed photon energy (0.83 eV) at a temperature interval of ∼2.0 ∘ C. Spectroscopic ellipsometry (SE J. A. Woollam M-2000) measurements were carried out between photon energies of 0.73 and 5.05 eV at 75 ∘ angle of incidence and the results were modeled using a commercial software.

Results and Discussion
3.1.Structural Characterization.Raman measurements were conducted to examine the effect of O 2 -Ar ratio on the microstructure of VO 2 (Figure 1(a)).The Raman spectrum at room temperature shows characteristic vibration modes for the M 1 semiconducting phase of VO 2 .Comparing to previous works, Raman peaks are identified as 194(Ag), 223(Ag), 262(Bg), 307(Bg), 391(Ag), 492(Ag), and 618(Ag) cm −1 [9][10][11].No Raman shifts for other kinds of vanadium oxides and any types of other polymorphs of VO 2 (M 2 /T) [12,13] were identified within the measurement accuracy (±0.2 cm −1 ).The XRD spectra are shown in Figure 1(b).All peaks can be indexed to VO 2 (M) and (011) was the prominent plane for VO 2 thin film.No reflections due to other VO  phases such as V 4 O 7 , V 6 O 13 and V 3 O 7 were observed [14,15].Thermooptical hysteresis curves were deduced by measuring the transmittance at 0.83 eV with varying temperatures, which are shown in Figure 2(c).For comparison, the vertical axis of this figure has been normalized as .

Optical Properties of the VO 2
From the -temperature () data, a plot of / was obtained, yielding one peak with a well-defined maximum.Each of the / curves has been analyzed with a Gaussian function using the single peak fitting module of Originpro 8.0 software.The temperature corresponding to the maximum of / was defined as the phase transition temperature during a heating/cooling cycle;  1 and  2 represent the SMT temperature of heating and cooling branches, respectively.The SMT temperature was defined as   = (  temperature was increased to 67 ∘ C, contributions from the Drude response become more prominent.The extinction coefficient  and the optical conductivity   values at the NIR region rapidly increase with increasing temperature.In this research, electron concentration increases with decreasing O 2 -Ar ratios and the Drude response is also responsible for the increased  and   value at the NIR region (Figure 3).However, refractive index  was not significantly changed.

Band Gap.
The indirect OBG can be estimated using the power law: where  = 4/ is the absorption coefficient and   is the OBG energy.The   value is extrapolated by the linear portion of the plot to () 1/2 = 0.
Two electronic transitions can be uniquely assigned, as shown in Figures 4(a) and 4(b).The gaps correlated with absorption edge ( 1 ) at varied O 2 -Ar ratios are almost the same (∼2.0eV); consequently, the absorption edge was not significantly changed, as shown in Figure 2(a).However, the optical band gap corresponding to SMT ( 2 ) decreases from 0.53 to 0.18 eV with decreasing O 2 -Ar ratios, as shown in Figures 4(b) and 4(d).The OBG  2 of semiconducting VO 2 can be assigned to the indirect transition from the top of filled d // bands to the bottom of empty  * band, as shown using a red arrow in Figure 4(c).Note that  2 at an O 2 -Ar ratio of 2.0 : 48.0 is similar to that from a previous report by theoretical calculation (0.6 eV) [18] and experimental results by photoemission spectroscopy (0.6 eV) [2].Besides, the 0.41 eV band gap of the VO 2 film prepared at an O 2 -Ar ratio of 1.5 : 48.5 agrees to a calculated intermediate structure at 339.8 K (0.36 eV) [18]. 2 at 1.0 : 49.0 O 2 -Ar ratio further decreased to 0.18 eV.When decreasing O 2 -Ar ratios, the filled d // bands and the empty  * band are shifted to the higher and lower energy, respectively.Both the d // and  * bands gradually moved closely, resulting in a redshift of  2 .The decreasing  2 results in a decrement in the SMT energies; therefore the SMT temperature decreased with decreasing O 2 -Ar ratios.Moreover, with decreasing the O 2 -Ar ratio, the NIR transmittance of the film evidently decreases.This behavior is because the bandgap  2 of the film, which is narrowing at low O 2 -Ar ratios, is different at the distinct O 2 -Ar ratio regions.It can enhance the electronic transitions and results in more interband absorptions at lower O 2 -Ar ratios, as shown in Figure 2

Conclusions
To summarize, diverse phase transformation properties are reported for phase pure VO 2 (M 1 ) thin films grown at different oxygen partial pressures by reactive magnetron sputtering.The transmittance and absorptivity spectra below phase transition temperatures are closely related to O 2 -Ar ratios.The phase transition temperature   decreased from 66.4 ∘ C to 46.2 ∘ C as the O 2 -Ar ratio decreased from 2.0 : 48.0 to 1.0 : 49.0.The optical constants of the VO 2 (M 1 ) thin films have been determined between 0.73 and 5.05 eV.The near-infrared extinction coefficient  and optical conductivity   increase with decreasing O 2 -Ar ratios.Moreover, two electronic transitions were identified.The energy gaps correlated with absorption edge at varied O 2 -Ar ratios are almost the same, while the optical band gap corresponding to semiconductor-to-metal phase transition decreases from 0.53 to 0.18 eV with decreasing O 2 -Ar ratios.
Films.Figures 2(a) and 2(b) show the transmittance and absorptivity spectra of the prepared VO 2 thin films at different O 2 -Ar ratios.The SMT transition is clearly observed with a dramatic change in the infrared transmittance with varied temperature ranges.The absorption edge, luminous (lum) transmittance ( lum , 1.64-3.27eV), and near-infrared (NIR) transmittance (0.73-1.64 eV) at the high temperature of 95 ∘ C were almost the same for all of the samples studied here.However, low temperature (below 26 ∘ C) phase VO 2 (M 1 ) showed a gradually decreased NIR transmittance but increased absorptivity with decreasing O 2 -Ar ratios.