# Room Temperature Optical Constants and Band Gap Evolution of Phase Pure M1-VO2 Thin Films Deposited at Different Oxygen Partial Pressures by Reactive Magnetron Sputtering

Authors: Meng Jiang; Yamei Li; Shaotang Li; Huaijuan Zhou; Xun Cao; Shanhu Bao; Yanfeng Gao; Hongjie Luo; Ping Jin
Journal: Journal of Nanomaterials
 (2014)

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## Abstract

Spectroscopic ellipsometry study was employed for phase pure VO2(M1) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO2(M1) 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 VO2(M1) thin films rapidly increase with decreasing O2-Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge (
E
1
) at varied O2-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 O2-Ar ratios.


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## Body

## 1. Introduction

Vanadium dioxide (VO2), one of the most interesting transition metal oxides, exhibits a reversible first-order semiconductor-to-metal phase transition (SMT) at a critical temperature T
c
=
68°C (for bulk single crystal VO2) [1]. VO2 has a tetragonal rutile structure with the P42/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 P21/c space group (M1 phase), in which the partially filled d// band splits into an unoccupied part being pushed past the π
* band and a filled part with the d// band dropping below the Fermi level, thus opening up a bandgap of ~0.6 eV between π
* and the filled part of d// band [2]. Dramatic changes in the electrical and optical properties across the SMT make VO2 thin films suitable for many applications, such as switching devices, sensors, and smart windows [3–6].It has been noted that oxygen partial pressure has effects on the structural and resistivity transition behaviors of VO2 [7]. Although the optimized oxygen partial pressure to fabricate VO2 films on glass and the optical properties of those samples were investigated [8], the optical constants, especially extinction coefficient k, which is crucial in understanding band structures, are not involved. Moreover, two energy gaps E
1 and E
2 are not distinguished as well.Low visible transparency and unfavorable yellowish colour, which are correlated with absorption edges, limit the application of VO2 smart windows. For most practical applications the phase transition temperature T
c needs to be in the vicinity of room temperature (~25°C) and the T
c may be assumed to be correlated with the optical band gap E
2. Consequently, the distinguishment of E
1 and E
2 plays an important role in improving the performance of VO2.In this research, we thoroughly investigated the effects of oxygen partial pressures on the optical constants and the electronic transition behaviors of phase pure VO2(M1) thin films deposited on quartz glass by reactive magnetron sputtering. Moreover, two electronic transitions related to absorption edge (E
1) and SMT (E
2) were distinguished.


## 2. Experimental Section

VO2 thin films with a thickness of ~70 nm were deposited using a reactive rf magnetron sputtering system with a water-cooled vanadium metal target (50 mm in diameter, 99.9% purity). Quartz glasses (2
0
×
2
0
×
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 of5
×
10
-
4 Pa, the deposition chamber was filled with high purity (99.999%) Ar and O2 mixture gas. The O2-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 VO2 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 0.73–5.05 eV 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.


## 3. Results and Discussion

### 3.1. Structural Characterization

Raman measurements were conducted to examine the effect of O2-Ar ratio on the microstructure of VO2 (Figure 1(a)). The Raman spectrum at room temperature shows characteristic vibration modes for the M1 semiconducting phase of VO2. 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 [9–11]. No Raman shifts for other kinds of vanadium oxides and any types of other polymorphs of VO2 (M2/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 VO2(M) and (011) was the prominent plane for VO2 thin film. No reflections due to other V
O
x phases such as V4O7, V6O13 and V3O7 were observed [14, 15].Raman (a) and XRD (b) spectra for VO2 films deposited at different O2-Ar ratios.
(a)
(b)


### 3.2. Optical Properties of the VO2 Films

Figures2(a) and 2(b) show the transmittance and absorptivity spectra of the prepared VO2 thin films at different O2-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 (T
lum, 1.64–3.27 eV), 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 VO2(M1) showed a gradually decreased NIR transmittance but increased absorptivity with decreasing O2-Ar ratios.(a) Transmittance spectra of VO2 films deposited at different O2-Ar ratios. Solid and dashed lines refer to low temperature (26°C) and high temperature (95°C), respectively. (b) Low temperature (M1 phase) absorptivity spectra. Thermal hysteresis loops of optical transmittance at photon energy of 0.83 eV and phase transition temperature T
c are shown in (c) and (d).
(a)
(b)
(c)
(d)Thermooptical hysteresis curves were deduced by measuring the transmittance at 0.83 eV with varying temperatures, which are shown in Figure2(c). For comparison, the vertical axis of this figure has been normalized as X. From the X-temperature (T) data, a plot of d
X
/
d
T was obtained, yielding one peak with a well-defined maximum. Each of the d
X
/
d
T 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 d
X
/
d
T was defined as the phase transition temperature during a heating/cooling cycle; T
1 and T
2 represent the SMT temperature of heating and cooling branches, respectively. The SMT temperature was defined as T
c
=
(
T
1
+
T
2
)
/
2. As shown in Figure 2(d), the SMT temperatures were 46.2°C, 59.8°C, and 66.4°C for VO2 films deposited at the O2-Ar ratio of 1.0 : 49.0, 1.5 : 48.5, and 2.0 : 48.0, respectively. The phase transition temperature T
c consistently decreased as the O2-Ar ratio decreased. It was pointed out that vanadium interstitials and/or oxygen vacancies could reduce the SMT temperature of VO2 [16], which is also responsible for the decreased SMT temperatures at low O2-Ar ratios of this work. Low O2-Ar ratios can introduce both extra electrons and internal strains in nonstoichiometric VO2 (related data was revealed in a paper submitted to Thin Solid Films, Manuscript Number: TSF-D-13-00641).


### 3.3. Optical Constant

The electronic transitions, optical constants, and optical band gap (OBG) in the vicinity of the phase transition temperature have been investigated by Li et al. [17]. For a standard bulk VO2 sample, when temperature was increased to 67°C, contributions from the Drude response become more prominent. The extinction coefficient k and the optical conductivity σ
r values at the NIR region rapidly increase with increasing temperature. In this research, electron concentration increases with decreasing O2-Ar ratios and the Drude response is also responsible for the increased k and σ
r value at the NIR region (Figure 3). However, refractive index n was not significantly changed.Refractive indexn, extinction coefficient k, and optical conductivity σ
r of low temperature phase VO2 deposited at different O2-Ar ratios.
(a)
(b)
(c)


### 3.4. Band Gap

The indirect OBG can be estimated using the power law:(1)
(
α
E
)
1
/
2
∝
(
E
-
E
g
)
,
where α
=
4
π
k
/
λ is the absorption coefficient and E
g is the OBG energy. The E
g value is extrapolated by the linear portion of the plot to (
α
E
)
1
/
2
=
0.Two electronic transitions can be uniquely assigned, as shown in Figures4(a) and 4(b). The gaps correlated with absorption edge (E
1) at varied O2-Ar ratios are almost the same (~2.0 eV); consequently, the absorption edge was not significantly changed, as shown in Figure 2(a). However, the optical band gap corresponding to SMT (E
2) decreases from 0.53 to 0.18 eV with decreasing O2-Ar ratios, as shown in Figures 4(b) and 4(d). The OBG E
2 of semiconducting VO2 can be assigned to the indirect transition from the top of filled d// bands to the bottom of empty π
* band, as shown using a red arrow in Figure 4(c). Note that E
2 at an O2-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 VO2 film prepared at an O2-Ar ratio of 1.5 : 48.5 agrees to a calculated intermediate structure at 339.8 K (0.36 eV) [18]. E
2 at 1.0 : 49.0 O2-Ar ratio further decreased to 0.18 eV.(a) and (b)(
α
E
)
1
/
2 versus E which is determined from extinction coefficient k and the extrapolating dashed lines are drawn to obtain the value of the band gap E
1 and E
2, respectively. (b) is the enlarged low photon energy portion of (
α
E
)
1
/
2 versus E. (c) Schematic diagram of electronic band structures in the VO2 film. (d) Variation of optical band gap E
2 of VO2 thin films deposited at different O2-Ar ratios.
(a)
(b)
(c)
(d)When decreasing O2-Ar ratios, the filled d// bands and the empty π
* band are shifted to the higher and lower energy, respectively. Both the d// and π
* bands gradually moved closely, resulting in a redshift of E
2. The decreasing E
2 results in a decrement in the SMT energies; therefore the SMT temperature decreased with decreasing O2-Ar ratios. Moreover, with decreasing the O2-Ar ratio, the NIR transmittance of the film evidently decreases. This behavior is because the bandgap E
2 of the film, which is narrowing at low O2-Ar ratios, is different at the distinct O2-Ar ratio regions. It can enhance the electronic transitions and results in more interband absorptions at lower O2-Ar ratios, as shown in Figure 2(b).


## 3.1. Structural Characterization

Raman measurements were conducted to examine the effect of O2-Ar ratio on the microstructure of VO2 (Figure 1(a)). The Raman spectrum at room temperature shows characteristic vibration modes for the M1 semiconducting phase of VO2. 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 [9–11]. No Raman shifts for other kinds of vanadium oxides and any types of other polymorphs of VO2 (M2/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 VO2(M) and (011) was the prominent plane for VO2 thin film. No reflections due to other V
O
x phases such as V4O7, V6O13 and V3O7 were observed [14, 15].Raman (a) and XRD (b) spectra for VO2 films deposited at different O2-Ar ratios.
(a)
(b)


## 3.2. Optical Properties of the VO2 Films

Figures2(a) and 2(b) show the transmittance and absorptivity spectra of the prepared VO2 thin films at different O2-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 (T
lum, 1.64–3.27 eV), 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 VO2(M1) showed a gradually decreased NIR transmittance but increased absorptivity with decreasing O2-Ar ratios.(a) Transmittance spectra of VO2 films deposited at different O2-Ar ratios. Solid and dashed lines refer to low temperature (26°C) and high temperature (95°C), respectively. (b) Low temperature (M1 phase) absorptivity spectra. Thermal hysteresis loops of optical transmittance at photon energy of 0.83 eV and phase transition temperature T
c are shown in (c) and (d).
(a)
(b)
(c)
(d)Thermooptical hysteresis curves were deduced by measuring the transmittance at 0.83 eV with varying temperatures, which are shown in Figure2(c). For comparison, the vertical axis of this figure has been normalized as X. From the X-temperature (T) data, a plot of d
X
/
d
T was obtained, yielding one peak with a well-defined maximum. Each of the d
X
/
d
T 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 d
X
/
d
T was defined as the phase transition temperature during a heating/cooling cycle; T
1 and T
2 represent the SMT temperature of heating and cooling branches, respectively. The SMT temperature was defined as T
c
=
(
T
1
+
T
2
)
/
2. As shown in Figure 2(d), the SMT temperatures were 46.2°C, 59.8°C, and 66.4°C for VO2 films deposited at the O2-Ar ratio of 1.0 : 49.0, 1.5 : 48.5, and 2.0 : 48.0, respectively. The phase transition temperature T
c consistently decreased as the O2-Ar ratio decreased. It was pointed out that vanadium interstitials and/or oxygen vacancies could reduce the SMT temperature of VO2 [16], which is also responsible for the decreased SMT temperatures at low O2-Ar ratios of this work. Low O2-Ar ratios can introduce both extra electrons and internal strains in nonstoichiometric VO2 (related data was revealed in a paper submitted to Thin Solid Films, Manuscript Number: TSF-D-13-00641).


## 3.3. Optical Constant

The electronic transitions, optical constants, and optical band gap (OBG) in the vicinity of the phase transition temperature have been investigated by Li et al. [17]. For a standard bulk VO2 sample, when temperature was increased to 67°C, contributions from the Drude response become more prominent. The extinction coefficient k and the optical conductivity σ
r values at the NIR region rapidly increase with increasing temperature. In this research, electron concentration increases with decreasing O2-Ar ratios and the Drude response is also responsible for the increased k and σ
r value at the NIR region (Figure 3). However, refractive index n was not significantly changed.Refractive indexn, extinction coefficient k, and optical conductivity σ
r of low temperature phase VO2 deposited at different O2-Ar ratios.
(a)
(b)
(c)


## 3.4. Band Gap

The indirect OBG can be estimated using the power law:(1)
(
α
E
)
1
/
2
∝
(
E
-
E
g
)
,
where α
=
4
π
k
/
λ is the absorption coefficient and E
g is the OBG energy. The E
g value is extrapolated by the linear portion of the plot to (
α
E
)
1
/
2
=
0.Two electronic transitions can be uniquely assigned, as shown in Figures4(a) and 4(b). The gaps correlated with absorption edge (E
1) at varied O2-Ar ratios are almost the same (~2.0 eV); consequently, the absorption edge was not significantly changed, as shown in Figure 2(a). However, the optical band gap corresponding to SMT (E
2) decreases from 0.53 to 0.18 eV with decreasing O2-Ar ratios, as shown in Figures 4(b) and 4(d). The OBG E
2 of semiconducting VO2 can be assigned to the indirect transition from the top of filled d// bands to the bottom of empty π
* band, as shown using a red arrow in Figure 4(c). Note that E
2 at an O2-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 VO2 film prepared at an O2-Ar ratio of 1.5 : 48.5 agrees to a calculated intermediate structure at 339.8 K (0.36 eV) [18]. E
2 at 1.0 : 49.0 O2-Ar ratio further decreased to 0.18 eV.(a) and (b)(
α
E
)
1
/
2 versus E which is determined from extinction coefficient k and the extrapolating dashed lines are drawn to obtain the value of the band gap E
1 and E
2, respectively. (b) is the enlarged low photon energy portion of (
α
E
)
1
/
2 versus E. (c) Schematic diagram of electronic band structures in the VO2 film. (d) Variation of optical band gap E
2 of VO2 thin films deposited at different O2-Ar ratios.
(a)
(b)
(c)
(d)When decreasing O2-Ar ratios, the filled d// bands and the empty π
* band are shifted to the higher and lower energy, respectively. Both the d// and π
* bands gradually moved closely, resulting in a redshift of E
2. The decreasing E
2 results in a decrement in the SMT energies; therefore the SMT temperature decreased with decreasing O2-Ar ratios. Moreover, with decreasing the O2-Ar ratio, the NIR transmittance of the film evidently decreases. This behavior is because the bandgap E
2 of the film, which is narrowing at low O2-Ar ratios, is different at the distinct O2-Ar ratio regions. It can enhance the electronic transitions and results in more interband absorptions at lower O2-Ar ratios, as shown in Figure 2(b).


## 4. Conclusions

To summarize, diverse phase transformation properties are reported for phase pure VO2(M1) 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 O2-Ar ratios. The phase transition temperature T
c decreased from 66.4°C to 46.2°C as the O2-Ar ratio decreased from 2.0 : 48.0 to 1.0 : 49.0. The optical constants of the VO2(M1) thin films have been determined between 0.73 and 5.05 eV. The near-infrared extinction coefficient k and optical conductivity σ
r increase with decreasing O2-Ar ratios. Moreover, two electronic transitions were identified. The energy gaps correlated with absorption edge at varied O2-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 O2-Ar ratios.



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*Source: 183954-2014-01-02.xml*