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Annealing effect on phase transition and thermochromic properties of VO2 thin films
Superlattices and Microstructures 137 (2020) 106335
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Annealing effect on phase transition and thermochromic properties of VO2 thin films Manish Kumar a, *, Jitendra Pal Singh a, b, Keun Hwa Chae b, Jaehun Park a, Hyun Hwi Lee a, ** a b
Pohang Accelerator Laboratory, POSTECH, Pohang, 37673, South Korea Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul, 02792, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Vanadium dioxide Thin film Phase transition
Vanadium dioxide (VO2) is one kind of desired thermochromic material for many smart devices because of its notable temperature-responsive infrared modulation and metal insulator transi tion with underlying structural phase transition (SPT). Understanding on the tuning of these properties in an anticipated manner is essential to accomplish device realization. Here, we report the annealing time induced modifications in SPT and the thermochromic properties of VO2 thin films. Using RF sputtering deposition, VO2 thin films were grown at room temperature and their ex-situ annealing was carried out at 600 � C for different time from 2 min to 60 min. Structural, electronic and thermochromic properties of these films were investigated. VO2 thin films samples annealed for longer time exhibit larger crystallite size, higher surface roughness and SPT tem perature, and the reduced hysteresis width of SPT during heating and cooling cycle. X-ray ab sorption spectroscopy results indicate that the variation in the annealing time do not alter the electronic structure significantly. Nevertheless, VO2 thin film samples with prolonged annealing display higher IR transmittance modulation across the SPT.
1. Introduction Since the Morin’s pivotal work on the phase transition of vanadium dioxide (VO2), this new frontier has attracted much attention from the scientific community across the globe for several decades [1]. Thermochromic VO2 undergoes a structural phase transition (SPT) at a critical temperature (Tc) of ~68 � C, which is accompanied by a metal-to-insulator transition (MIT) [2,3]. VO2 possess the monoclinic structure at low temperatures, which is insulator and has high infrared transmittance. At higher temperature (higher than Tc), it transforms to a rutile structure, which is metallic and almost opaque to infrared rays. The closeness of Tc to room temperature, ultrafast thermochromic switching and the optical contrast between the two states of VO2, make it a promising candidate for appli cations such as energy efficient smart windows, memory devices, optical sensors, electronic switch devices etc [2–6]. For a series of applications, it is important to tailor Tc, resistivity and the optical transmittance. A wide variety of optical and electrical switching in VO2 can be induced by stabilizing it in thin film and nanostructure forms along with doping by right dopant [2, 7]. However, the commercial utilization of VO2 is limited by factors such as large-scale synthesis at low cost, specific phase control, and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Kumar), [email protected] (H.H. Lee). https://doi.org/10.1016/j.spmi.2019.106335 Received 4 September 2019; Received in revised form 16 October 2019; Accepted 5 November 2019 Available online 6 November 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
S3
Intesnity (Arb. Units)
30 °C 100 °C
23
15
20
2 (degree)
JCPDS No. 065-2358 071-0565 25
30
(c)
52 50 48 46 44
(220) (022)
(220) / (211)
24
2 (degree)
S1 25 R(110)
D(nm)
S1
S2 (300) / (112)
(-311) / (-122) (102) / (121)
S2
R(110) M(011)
Intesnity (Arb. Units)
S3
10
(b)
30 °C 100 °C
(211)
(a)
M(011) 1
10
Annealing Time (Min.)
Fig. 1. (a) Grazing incidence X-ray diffraction patterns at 30 � C and 100 � C temperature for VO2 films annealed for different time. (b) Enlarged view of the diffraction data for VO2 samples shown in figure (a). (c) Crystallite size calculated for the high and low temperature phase of VO2 thin film samples.
homogeneous doping of suitable ions in favor of Tc control. In case of doping, dopant induced unintentional modifications in the fundamental properties of VO2 cannot be ruled out. In this scenario, it is imperative to tailor the properties of VO2 just by controlling the synthesizing parameters. Until now, various methods have been employed for preparing VO2 thin films, which include pulsed laser deposition, sol-gel spin coating, chemical deposition etc [7–14]. Among these methods, sputtering has an edge as it offers to grow large area films with uniformity. Sputtering at room temperature with ex-situ annealing is a plus for large-scale production. In past, attempts were made to stabilize the VO2 films by tuning annealing parameters [15,16]. However, the systematic studies on the structural transition of samples annealed for different time at fixed annealing temperature are missing in literature. Understanding of any annealing time induced modulations will be crucial for the device applications. In this work, VO2 thin films were stabilized at room temperature and post deposition annealing treatment was given for different time duration. The structural, electronic and infrared transmittance properties of these thin film samples were explored. It is realized that variation in annealing time allow us to modulate the structural phase transition temperature as well as the optical transmittance. The understanding of the annealing time dependent modulation in properties of VO2 thin films can help to stabilize VO2 with desired functional properties. 2. Experimental VO2 thin film samples were fabricated by radio frequency (RF) magnetron sputtering from a commercial VO2 target. Before deposition, base pressure of 1 � 10 6 Torr was achieved and then the target was pre-sputtered for 3 min for its surface cleanliness. This was followed by the room temperature deposition of VO2 films on c-cut Al2O3 substrate with 200 W deposition power. During sput tering, the pressure of sputtering Ar gas (5 mTorr) and the target to substrate distance (7 cm) were kept fixed for all the samples. The thickness of the samples was ~70 nm. After the deposition, the thin film samples were annealed in a rapid thermal annealing (RTA) furnace for different time at temperature of 600 � C in vacuum (~2mTorr) environment. The films were rapidly heated to 600 � C (within a minute) and were kept at that temperature for different time duration followed by natural cooling in vacuum. Three samples namely S1, S2 and S3 were annealed for 2 min, 10 min and 60 min, respectively. The structural and electronic characterization of the grown samples were carried out at three different beamlines of Pohang Light Source-II in Korea [17]. Three beamline namely BL5A beamline for X-ray diffraction (XRD) measurements, BL10D beamline for X-ray absorption spectroscopy (XAS) measurements in total electron yield (TEY) mode and BL1D beamline for X-ray absorption near-edge structure (XANES) measurements in fluorescence mode were used. Using the synchrotron light source (λ ¼ 0.69265 Å), XRD measurements at room temperature and higher temperature were performed in ambience and in grazing incidence mode (incidence angle ¼ 1� ). Atomic force microscopy (AFM) was employed to study the surface morphology of the samples. The infrared transmittance was measured with Varian 7000e Fourier-transform infrared spectroscopy (FTIR) spectrometer.
2
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
Intensity (Arb. Units)
(b)
S1
M(011) R(110)
(d)
S2
(f)
S3
30
40
50
60
70
80
Temperature ( C)
90
100
Fig. 2. (a),(c) and (e) Contour maps of the temperature dependent XRD data for VO2 samples S1, S2 and S3. (b),(d) and (f) XRD intensity variation at M(011) and R(110) VO2 peak position as a function of temperature for studied samples. Solid and dotted lines/dots represent the heating and cooling cycle, respectively.
74
H Tc
Temperature (
)
72 70 68 66 4 2 0
S2
S1
S3
Fig. 3. Structural phase transition temperature (Tc) and hysteresis width (ΔH) of the first order structural transition estimated from the diffraction intensity of M(011) peak for VO2 samples annealed for different time.
3. Results and discussion To understand the crystal structure of the grown VO2 thin samples, XRD measurements were performed at room temperature and 100 � C (above the VO2 transition temperature). Fig. 1(a) depicts the obtained XRD results. The intensities from the standard mono clinic VO2 (JCPDS card No. 065–2358) and tetragonal VO2 (JCPDS card No. 071–0565) phases are also shown in this figure. The obtained XRD patterns for samples S1, S2 and S3 at room temperature could be readily indexed into monoclinic VO2, while that for 100 � C assigned into the tetragonal rutile VO2. Room temperature XRD measurements signifies the single-phase growth of all samples. At higher temperature (100 � C), the shifting of certain peaks such as M(011) [(011) plane of monoclinic VO2] to R(110) [(110) plane of rutile VO2] peak in 12� � 2θ � 13� is observed which is a diagnostic feature for the structural transition from monoclinic to tetragonal VO2 phase. Fig. 1(b) displays the enlarged view of the XRD data shown in Fig. 1(a). The shifting of XRD peaks at higher temperature is clearly visible. On can also notice the vanishing of monoclinic VO2 ( 311/-122) and (102/121) plane peaks at higher temperature indicating the occurrence of the structural phase transition. The crystallite size (D) was estimated from the measured XRD data of VO2 thin films, using the Debye Scherrer’s formula [18,19]. Fig. 1(c) displays the variation of crystallite size in VO2 films with annealing temperature estimated from the M(011) and R(110) peaks. It is noted that crystallite size increases with the annealing time. This trend is similar for the lower as well as high temperature phase of VO2. For understanding the structural transition and the transformation of VO2(M) (011) to VO2(R) (110), temperature dependent XRD measurements for all the samples were carried out at several temperatures from 30 � C to 100 � C during heating and cooling run. Fig. 2 3
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
Table 1 Parameters obtained from temperature dependent XRD data of the VO2 thin films annealed for different time. Sample
Tc(011)m (� C)
Tc(110)R (� C)
ΔH (� C)
S1 S2 S3
67.7 71.5 72.3
64.2 68.3 70.0
3.6 3.2 2.3
Fig. 4. Room temperature AFM images of VO2 thin film samples.
(a) and (c) and 2(e) depict the contour plots of XRD intensity measured at different temperatures for all samples. In these figures, normalized XRD intensities were plotted as functions of temperature (horizontal axes) and diffraction angle 2θ (vertical axes). M(011) peak of VO2 shift to lower angles upon heating, indicating the transformation to R(110). After the subsequent cooling, R(110) peaks shift to higher angles and M(011) peaks recover to their original high angles. This kind of trend is shown by all the studied samples. The reversibility of monoclinic to rutile phase change confirms the thermochromic nature of prepared VO2 thin film samples. Fig. 2(b), (d) and 2(f) show the normalized XRD intensity variation at M(011) and R(110) peak position as a function of temperature for the studied VO2 thin film samples. For sample S1, intensity of M(011) starts to decrease after 65 � C and turns constant after 75 � C. Almost opposite trend is shown by intensity of R(110). Moreover, XRD intensity of respective planes do not overlap in heating and cooling cycle and depicts a hysteresis. Such hysteresis in cooling and heating curve is a characteristic feature of first order phase transition. Tc and the hysteresis width ΔH (temperature difference in Tc during heating and cooling) were estimated from the measured XRD data and the same are plotted in Fig. 3. The parameters obtained from temperature dependent XRD data of the VO2 thin films annealed for different time are tabulated in Table 1. A decrease in ΔH values is observed with increase in the annealing time suggesting the annealing has sharpened the first order phase transition. Moreover, modulation in the transition temperature for samples annealed with different 4
Superlattices and Microstructures 137 (2020) 106335
Normalised Absorption (Arb. Units)
M. Kumar et al.
(a)
d
e
S3 S2 S1
b a
5460 Normalised Intensity (Arb. Units)
c
5475
(b)
5490
5505
5520
Photon Energy (eV)
V
5535
V L-edge and O K-edge
S3 S2
L2
L3
S1 510
520
530
540
Photon Energy (eV)
550
Fig. 5. (a) Normalized XANES spectra measured in fluorescence mode at V K-edge for VO2 films. (b) X-ray absorption spectra of these films recorded in TEY mode at V L-edge and O K-edge. For clarity, the X-ray absorption spectra for different samples are vertically shifted.
time was noticed. The transition temperature of thin film annealed for 2 min has shown the lowest transition temperature while the film annealed for 60 min displayed highest transition temperature. Possibly; this is driven by the crystallinity and the microstructure of the thin film samples. AFM pictures of the VO2 thin film samples are shown in Fig. 4. Root mean square (RMS) roughness observed in sample S1 is 4 nm. However, enhanced RMS roughness is observed in samples S2 (6.8 nm) and S3 (9.3 nm) as compared to sample S1. VO2 sample annealed for 2 min (sample S1) exhibits a uniform granular structure. Higher grain size is noticed in samples S2 and S3, which are annealed for longer time. These samples display varying grain size compared to sample S1. It appears that nanograins evolve pro gressively into larger grains upon prolonged annealing. The increase in crystallite size with annealing time was also indicated in the XRD results. Therefore, the AFM results are in agreement with the XRD results. Previous works argued that crystallinity, grain size and grain boundaries play a vital role in MIT of VO2 [12,20–22]. In case of smaller grains, the large density of grain boundaries lead to a less intense and less abrupt transition. In contrast, the larger grains with lower density of grain boundaries lead to a stronger and sharper MIT. Considering these facts, the observed variation of Tc and ΔH in the studied VO2 thin film samples can be ascribed to variation in grain size and grain boundaries. The different microstructure of the samples may change their functional properties. To get further insight on the crystal structure and the electronic arrangement in the grown films of VO2, XAS measurements were carried out. Fig. 5(a) displays the V K-edge XANES data of the studied VO2 thin films measured at room temperature. Based on the literature, we can assign the distinctive features in the obtained XANES data [11,23]. The pre-edge absorption feature (peak a) is derived by the dipole-forbidden 1s→3d transition. Overlapping of the V 3d orbitals with the V 4p and O 2p orbitals is responsible for the occurrence of pre-edge absorption peak in VO2. The intensity and position of pre-edge peak strongly depend on the local coor dination environment and oxidation state, respectively. The main absorption edge (peak b) corresponds to the dipole allowed V 1s to 4p transition. The near edge features (feature c, d and e) above the absorption edge can be attributed to the multiple scattering contributions and the dipole-allowed excitation of a V core 1s electron to a localized 4p state. The energy positions of the pre-edge and main absorption peaks are used to recognize the oxidation state of the vanadium. For our thin film samples, the XANES results infer þ4 oxidation state of vanadium confirming the VO2 stoichiometry, and a distorted VO6 octahedral geometry around the V sites. 5
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
100
30 C 80
Transmission (%
(
S3 S2
60
S1
40
100 C
20 0
2000
2500
3000
3500
Wavelength (nm)
4000
Fig. 6. IR transmittance spectra of VO2 thin film samples measured at 30 � C (solid lines) and 100 � C (dotted lines).
Fig. 5(b) depicts V L-edge and O K-edge XAS spectra of the VO2 thin films measured in TEY mode at room temperature. The strict dipole selection rules and energy constraints of XAS make certain that one can get elemental and orbital selectivity. V L-edge (2p→3d) exhibit 2 prominent maxima near 518 and 524 eV, which are mainly linked to the electron excitations from spin-orbit split-levels 2p3/2 and 2p1/2, respectively. The spin-orbit splitting value of 6.5 eV displayed by 2p levels remains intact in the studied VO2 thin film samples. The observed value of spin-orbit splitting is identical to the value derived from XAS studies of VO2 single crystals [24]. The XAS spectrum at O K-edge probes unoccupied density of states in conduction band through the transitions from O 1s → 2p states, which are hybridized with adjacent metals [25–29]. Therefore, selectivity of XAS led to V L-edge transition primarily into dk orbitals, while O K-edge comprises signatures of all three bands near fermi level π*, σ* and dk. Based on the literature, we attribute the spectral features in the O K-edge ron (~ 525–545 eV) as associated with the π* (~529 eV) and σ* (~531 eV) states. It is noticed that the shape of the V L-edge and O K-edge XAS spectra and the energy position of spectral features are almost identical in our VO2 thin film samples. Moreover, the observed XAS data matches well to previous work on monoclinic VO2 thin film [27,28]. As no substantial changes in the electronic structure of studied samples are observed, we can assume that prolonged annealing has not introduced significant oxygen vacancies in studied samples. Moreover, oxygen vacancies in VO2 thin films may reduce Tc due to extra free electrons [30]. Never theless, here we noticed enhancement in Tc with increase in annealing time. Therefore, it is likely that oxygen content in studied samples is not much influenced by prolonged annealing. In order to check the annealing time dependence on the thermochromic property, we measured the IR transmittance spectra of VO2 thin film samples at 30 � C and 100 � C and the obtained results are shown in Fig. 6. At room temperature, VO2 films are in monoclinic insulating phase and display good transparency in the IR range. Interestingly, the IR transmittance at room temperature was found to increase with annealing time. Among the studied samples, the sample annealed for 2 min showed least IR transmittance at room temperature that further drops to ~10% upon heating to higher temperature (100 � C). At higher temperature, the IR transmittance for all the VO2 samples drops significantly. When we compare the modulation in transmittance at lower and higher temperature (at 2500 nm), it is found that modulation in sample S3 is highest while in S1 is lowest. On the other hand, IR transmittance in sample S1 at 100 � C is lowest among the studied samples. We can recall here that prolonged annealing of VO2 thin films led to better crystallinity and higher roughness but the electronic structure is not much influenced. Hence, the variation in the thermochromic performance of VO2 thin films annealed for different time can be correlated to their microstructures. 4. Conclusions In summary, we have prepared thermochromic VO2 films by employing RF sputtering at room temperature and giving them post deposition annealing treatment for different time duration ranging from 2 min to 60 min. Their structural, morphological, electronic and thermochromic properties were explored. Room temperature XRD results confirms all the VO2 thin film samples stabilized in monoclinic phase. The transformation from monoclinic phase to rutile phase in these samples is realized through temperature dependent XRD measurements. An increase in crystallite size, surface roughness and Tc is observed with increase in the annealing time. Longer annealing also led to smaller ΔH. Annealing for different time has also brought a modulation in the thermochromic perfor mance of VO2 thin films. The present work conclude that the functional properties of VO2 thin film can be tailored by controlling the post depositing annealing. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (Grant No. NRF-2019K1A3A7A09033398 and NRF-2015R1A5A1009962). SR-XRD measurements at PLS-II (Pohang Light Source II, POSTECH) were supported in part by MSIT, Korea. 6
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
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Stefanovich, Electrical switching and Mott transition in VO2, J. Phys. Condens. Matter 12 (2000) 8837–8845. [7] J. Nag, R.F. Haglund, Synthesis of vanadium dioxide thin films and nanoparticles, J. Phys. Condens. Matter 20 (2008) 264016. [8] D. Vernardou, P. Paterakis, H. Drosos, E. Spanakis, I.M. Poveyim, M.E. Pemble, E. Koudoumas, N. Katsarakis, A study of the electrochemical performance of vanadium oxide thin films grown by atmospheric pressure chemical vapour deposition, Sol. Energy Mater. Sol. Cells 95 (2011) 2842–2847. [9] J.C. Orlianges, J. Leroy, A. Crunteanu, R. Mayet, Electrical and optical properties of vanadium dioxide containing gold nanoparticles deposited by pulsed laser deposition, Appl. Phys. Lett. 101 (2012) 133102. [10] J. Yoon, C. Park, S. Park, B.S. Mun, H. Ju, Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method, Appl. Surf. Sci. 353 (2015) 1082–1086. [11] S.S. Majid, D.K. Shukla, F. Rahman, S. Khan, K. Gautam, A. Ahad, S. Francoual, R.J. Choudhary, V.G. Sathe, J. Strempfer, Insulator-metal transitions in the T phase Cr doped and M1 phase undoped VO2 thin films, Phys. Rev. B 98 (2018), 075152. [12] C. Ba, S.T. Bah, M. D’Auteuil, P.V. Ashrit, R. Vall�ee, Fabrication of high-quality VO2 thin films by ion-assisted dual ac magnetron sputtering, ACS Appl. Mater. Interfaces 5 (2013) 12520–12525. [13] Manish Kumar, Sunita Rani, Hyun Hwi Lee, Correlation between surface morphology and electrical properties of VO2 films grown by direct thermal oxidation method, J. Korean Phys. Soc. 75 (2019) 519–522. [14] Manish Kumar, Sunita Rani, Hyun Hwi Lee, Thermochromic VO2 thin films: growth and characterization, AIP Conf. Proc. 2142 (2019), 080007. [15] L. Kang, Y. Gao, Z. Zhang, J. Du, C. Cao, Z. Chen, H. Luo, Effects of annealing parameters on optical properties of thermochromic VO2 films prepared in aqueous, J. Phys. Chem. C 114 (2010) 1901–1911. [16] G.H. Fu, A. Polity, N. Volbers, B.K. Meyer, Annealing effects on VO2 thin films deposited by reactive sputtering, Thin Solid Films 515 (2006) 2519–2522. [17] Hyun Hwi Lee, Manish Kumar, Hyun-Joon Shin, Current status of the 3rd generation Korean synchrotron light source, PLS-II, Phys. Adv. Technol. 26 (6) (2017) 7, https://doi.org/10.3938/PhiT.26.023. [18] A.L. Patterson, The Scherrer formula for X-Ray particle size determination, Phys. Rev. 56 (1939) 978. [19] V. Devi, M. Kumar, R. Kumar, B.C. Joshi, Effect of substrate temperature and oxygen partial pressure on structural and optical properties of Mg doped ZnO thin films, Ceram. Int. 41 (2015) 6269–6273. [20] R. Lopez, T.E. Haynes, L.A. Boatner, L.C. Feldman, R.F. Haglund Jr., Size effects in the structural phase transition of VO2 nanoparticles, Phys. Rev. B 65 (2002) 224113. [21] Jie Jian, Wenrui Zhang, Clement Jacob, Aiping Chen, Han Wang, Jijie Huang, Haiyan Wang, Roles of grain boundaries on the semiconductor to metal phase transition of VO2 thin films, Appl. Phys. Lett. 107 (2015) 102105. [22] D. Brassard, S. Fourmaux, M. Jean-Jacques, J.C. Kieffer, M.A. El Khakani, Grain size effect on the semiconductor-metal phase transition characteristics of magnetron-sputtered VO2 thin films, Appl. Phys. Lett. 87 (2005), 051910. [23] Y.F. Wu, L.L. Fan, S.M. Chen, S. Chen, C.W. Zou, Z.Y. Wu, Spectroscopic analysis of phase constitution of high quality VO2 thin film prepared by facile sol-gel method, AIP Adv. 3 (2013), 042132. [24] M. Abbate, F.M.F. deGroot, J.C. Fuggle, Y.J. Ma, C.T. Chen, F. Sette, A. Fujimori, Y. Ueda, K. Kosuge, Soft-x-ray-absorption studies of the electronic-structure changes through the VO2 phase transition, Phys. Rev. B 43 (1991) 7263. [25] Manish Kumar, Jitendra Pal Singh, Keun Hwa Chae, Jong Hyun Kim, Hyun Hwi Lee, Structure, optical and electronic structure studies of Ti:ZnO thin films, J. Alloy. Comp. 759 (2018) 8–13. [26] Manish Kumar, R.J. Choudhary, D.K. Shukla, D.M. 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7
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Annealing effect on phase transition and thermochromic properties of VO2 thin films Manish Kumar a, *, Jitendra Pal Singh a, b, Keun Hwa Chae b, Jaehun Park a, Hyun Hwi Lee a, ** a b
Pohang Accelerator Laboratory, POSTECH, Pohang, 37673, South Korea Advanced Analysis Center, Korea Institute of Science and Technology (KIST), Seoul, 02792, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Vanadium dioxide Thin film Phase transition
Vanadium dioxide (VO2) is one kind of desired thermochromic material for many smart devices because of its notable temperature-responsive infrared modulation and metal insulator transi tion with underlying structural phase transition (SPT). Understanding on the tuning of these properties in an anticipated manner is essential to accomplish device realization. Here, we report the annealing time induced modifications in SPT and the thermochromic properties of VO2 thin films. Using RF sputtering deposition, VO2 thin films were grown at room temperature and their ex-situ annealing was carried out at 600 � C for different time from 2 min to 60 min. Structural, electronic and thermochromic properties of these films were investigated. VO2 thin films samples annealed for longer time exhibit larger crystallite size, higher surface roughness and SPT tem perature, and the reduced hysteresis width of SPT during heating and cooling cycle. X-ray ab sorption spectroscopy results indicate that the variation in the annealing time do not alter the electronic structure significantly. Nevertheless, VO2 thin film samples with prolonged annealing display higher IR transmittance modulation across the SPT.
1. Introduction Since the Morin’s pivotal work on the phase transition of vanadium dioxide (VO2), this new frontier has attracted much attention from the scientific community across the globe for several decades [1]. Thermochromic VO2 undergoes a structural phase transition (SPT) at a critical temperature (Tc) of ~68 � C, which is accompanied by a metal-to-insulator transition (MIT) [2,3]. VO2 possess the monoclinic structure at low temperatures, which is insulator and has high infrared transmittance. At higher temperature (higher than Tc), it transforms to a rutile structure, which is metallic and almost opaque to infrared rays. The closeness of Tc to room temperature, ultrafast thermochromic switching and the optical contrast between the two states of VO2, make it a promising candidate for appli cations such as energy efficient smart windows, memory devices, optical sensors, electronic switch devices etc [2–6]. For a series of applications, it is important to tailor Tc, resistivity and the optical transmittance. A wide variety of optical and electrical switching in VO2 can be induced by stabilizing it in thin film and nanostructure forms along with doping by right dopant [2, 7]. However, the commercial utilization of VO2 is limited by factors such as large-scale synthesis at low cost, specific phase control, and
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Kumar), [email protected] (H.H. Lee). https://doi.org/10.1016/j.spmi.2019.106335 Received 4 September 2019; Received in revised form 16 October 2019; Accepted 5 November 2019 Available online 6 November 2019 0749-6036/© 2019 Elsevier Ltd. All rights reserved.
Superlattices and Microstructures 137 (2020) 106335
M. Kumar et al.
S3
Intesnity (Arb. Units)
30 °C 100 °C
23
15
20
2 (degree)
JCPDS No. 065-2358 071-0565 25
30
(c)
52 50 48 46 44
(220) (022)
(220) / (211)
24
2 (degree)
S1 25 R(110)
D(nm)
S1
S2 (300) / (112)
(-311) / (-122) (102) / (121)
S2
R(110) M(011)
Intesnity (Arb. Units)
S3
10
(b)
30 °C 100 °C
(211)
(a)
M(011) 1
10
Annealing Time (Min.)
Fig. 1. (a) Grazing incidence X-ray diffraction patterns at 30 � C and 100 � C temperature for VO2 films annealed for different time. (b) Enlarged view of the diffraction data for VO2 samples shown in figure (a). (c) Crystallite size calculated for the high and low temperature phase of VO2 thin film samples.
homogeneous doping of suitable ions in favor of Tc control. In case of doping, dopant induced unintentional modifications in the fundamental properties of VO2 cannot be ruled out. In this scenario, it is imperative to tailor the properties of VO2 just by controlling the synthesizing parameters. Until now, various methods have been employed for preparing VO2 thin films, which include pulsed laser deposition, sol-gel spin coating, chemical deposition etc [7–14]. Among these methods, sputtering has an edge as it offers to grow large area films with uniformity. Sputtering at room temperature with ex-situ annealing is a plus for large-scale production. In past, attempts were made to stabilize the VO2 films by tuning annealing parameters [15,16]. However, the systematic studies on the structural transition of samples annealed for different time at fixed annealing temperature are missing in literature. Understanding of any annealing time induced modulations will be crucial for the device applications. In this work, VO2 thin films were stabilized at room temperature and post deposition annealing treatment was given for different time duration. The structural, electronic and infrared transmittance properties of these thin film samples were explored. It is realized that variation in annealing time allow us to modulate the structural phase transition temperature as well as the optical transmittance. The understanding of the annealing time dependent modulation in properties of VO2 thin films can help to stabilize VO2 with desired functional properties. 2. Experimental VO2 thin film samples were fabricated by radio frequency (RF) magnetron sputtering from a commercial VO2 target. Before deposition, base pressure of 1 � 10 6 Torr was achieved and then the target was pre-sputtered for 3 min for its surface cleanliness. This was followed by the room temperature deposition of VO2 films on c-cut Al2O3 substrate with 200 W deposition power. During sput tering, the pressure of sputtering Ar gas (5 mTorr) and the target to substrate distance (7 cm) were kept fixed for all the samples. The thickness of the samples was ~70 nm. After the deposition, the thin film samples were annealed in a rapid thermal annealing (RTA) furnace for different time at temperature of 600 � C in vacuum (~2mTorr) environment. The films were rapidly heated to 600 � C (within a minute) and were kept at that temperature for different time duration followed by natural cooling in vacuum. Three samples namely S1, S2 and S3 were annealed for 2 min, 10 min and 60 min, respectively. The structural and electronic characterization of the grown samples were carried out at three different beamlines of Pohang Light Source-II in Korea [17]. Three beamline namely BL5A beamline for X-ray diffraction (XRD) measurements, BL10D beamline for X-ray absorption spectroscopy (XAS) measurements in total electron yield (TEY) mode and BL1D beamline for X-ray absorption near-edge structure (XANES) measurements in fluorescence mode were used. Using the synchrotron light source (λ ¼ 0.69265 Å), XRD measurements at room temperature and higher temperature were performed in ambience and in grazing incidence mode (incidence angle ¼ 1� ). Atomic force microscopy (AFM) was employed to study the surface morphology of the samples. The infrared transmittance was measured with Varian 7000e Fourier-transform infrared spectroscopy (FTIR) spectrometer.
2
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Intensity (Arb. Units)
(b)
S1
M(011) R(110)
(d)
S2
(f)
S3
30
40
50
60
70
80
Temperature ( C)
90
100
Fig. 2. (a),(c) and (e) Contour maps of the temperature dependent XRD data for VO2 samples S1, S2 and S3. (b),(d) and (f) XRD intensity variation at M(011) and R(110) VO2 peak position as a function of temperature for studied samples. Solid and dotted lines/dots represent the heating and cooling cycle, respectively.
74
H Tc
Temperature (
)
72 70 68 66 4 2 0
S2
S1
S3
Fig. 3. Structural phase transition temperature (Tc) and hysteresis width (ΔH) of the first order structural transition estimated from the diffraction intensity of M(011) peak for VO2 samples annealed for different time.
3. Results and discussion To understand the crystal structure of the grown VO2 thin samples, XRD measurements were performed at room temperature and 100 � C (above the VO2 transition temperature). Fig. 1(a) depicts the obtained XRD results. The intensities from the standard mono clinic VO2 (JCPDS card No. 065–2358) and tetragonal VO2 (JCPDS card No. 071–0565) phases are also shown in this figure. The obtained XRD patterns for samples S1, S2 and S3 at room temperature could be readily indexed into monoclinic VO2, while that for 100 � C assigned into the tetragonal rutile VO2. Room temperature XRD measurements signifies the single-phase growth of all samples. At higher temperature (100 � C), the shifting of certain peaks such as M(011) [(011) plane of monoclinic VO2] to R(110) [(110) plane of rutile VO2] peak in 12� � 2θ � 13� is observed which is a diagnostic feature for the structural transition from monoclinic to tetragonal VO2 phase. Fig. 1(b) displays the enlarged view of the XRD data shown in Fig. 1(a). The shifting of XRD peaks at higher temperature is clearly visible. On can also notice the vanishing of monoclinic VO2 ( 311/-122) and (102/121) plane peaks at higher temperature indicating the occurrence of the structural phase transition. The crystallite size (D) was estimated from the measured XRD data of VO2 thin films, using the Debye Scherrer’s formula [18,19]. Fig. 1(c) displays the variation of crystallite size in VO2 films with annealing temperature estimated from the M(011) and R(110) peaks. It is noted that crystallite size increases with the annealing time. This trend is similar for the lower as well as high temperature phase of VO2. For understanding the structural transition and the transformation of VO2(M) (011) to VO2(R) (110), temperature dependent XRD measurements for all the samples were carried out at several temperatures from 30 � C to 100 � C during heating and cooling run. Fig. 2 3
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Table 1 Parameters obtained from temperature dependent XRD data of the VO2 thin films annealed for different time. Sample
Tc(011)m (� C)
Tc(110)R (� C)
ΔH (� C)
S1 S2 S3
67.7 71.5 72.3
64.2 68.3 70.0
3.6 3.2 2.3
Fig. 4. Room temperature AFM images of VO2 thin film samples.
(a) and (c) and 2(e) depict the contour plots of XRD intensity measured at different temperatures for all samples. In these figures, normalized XRD intensities were plotted as functions of temperature (horizontal axes) and diffraction angle 2θ (vertical axes). M(011) peak of VO2 shift to lower angles upon heating, indicating the transformation to R(110). After the subsequent cooling, R(110) peaks shift to higher angles and M(011) peaks recover to their original high angles. This kind of trend is shown by all the studied samples. The reversibility of monoclinic to rutile phase change confirms the thermochromic nature of prepared VO2 thin film samples. Fig. 2(b), (d) and 2(f) show the normalized XRD intensity variation at M(011) and R(110) peak position as a function of temperature for the studied VO2 thin film samples. For sample S1, intensity of M(011) starts to decrease after 65 � C and turns constant after 75 � C. Almost opposite trend is shown by intensity of R(110). Moreover, XRD intensity of respective planes do not overlap in heating and cooling cycle and depicts a hysteresis. Such hysteresis in cooling and heating curve is a characteristic feature of first order phase transition. Tc and the hysteresis width ΔH (temperature difference in Tc during heating and cooling) were estimated from the measured XRD data and the same are plotted in Fig. 3. The parameters obtained from temperature dependent XRD data of the VO2 thin films annealed for different time are tabulated in Table 1. A decrease in ΔH values is observed with increase in the annealing time suggesting the annealing has sharpened the first order phase transition. Moreover, modulation in the transition temperature for samples annealed with different 4
Superlattices and Microstructures 137 (2020) 106335
Normalised Absorption (Arb. Units)
M. Kumar et al.
(a)
d
e
S3 S2 S1
b a
5460 Normalised Intensity (Arb. Units)
c
5475
(b)
5490
5505
5520
Photon Energy (eV)
V
5535
V L-edge and O K-edge
S3 S2
L2
L3
S1 510
520
530
540
Photon Energy (eV)
550
Fig. 5. (a) Normalized XANES spectra measured in fluorescence mode at V K-edge for VO2 films. (b) X-ray absorption spectra of these films recorded in TEY mode at V L-edge and O K-edge. For clarity, the X-ray absorption spectra for different samples are vertically shifted.
time was noticed. The transition temperature of thin film annealed for 2 min has shown the lowest transition temperature while the film annealed for 60 min displayed highest transition temperature. Possibly; this is driven by the crystallinity and the microstructure of the thin film samples. AFM pictures of the VO2 thin film samples are shown in Fig. 4. Root mean square (RMS) roughness observed in sample S1 is 4 nm. However, enhanced RMS roughness is observed in samples S2 (6.8 nm) and S3 (9.3 nm) as compared to sample S1. VO2 sample annealed for 2 min (sample S1) exhibits a uniform granular structure. Higher grain size is noticed in samples S2 and S3, which are annealed for longer time. These samples display varying grain size compared to sample S1. It appears that nanograins evolve pro gressively into larger grains upon prolonged annealing. The increase in crystallite size with annealing time was also indicated in the XRD results. Therefore, the AFM results are in agreement with the XRD results. Previous works argued that crystallinity, grain size and grain boundaries play a vital role in MIT of VO2 [12,20–22]. In case of smaller grains, the large density of grain boundaries lead to a less intense and less abrupt transition. In contrast, the larger grains with lower density of grain boundaries lead to a stronger and sharper MIT. Considering these facts, the observed variation of Tc and ΔH in the studied VO2 thin film samples can be ascribed to variation in grain size and grain boundaries. The different microstructure of the samples may change their functional properties. To get further insight on the crystal structure and the electronic arrangement in the grown films of VO2, XAS measurements were carried out. Fig. 5(a) displays the V K-edge XANES data of the studied VO2 thin films measured at room temperature. Based on the literature, we can assign the distinctive features in the obtained XANES data [11,23]. The pre-edge absorption feature (peak a) is derived by the dipole-forbidden 1s→3d transition. Overlapping of the V 3d orbitals with the V 4p and O 2p orbitals is responsible for the occurrence of pre-edge absorption peak in VO2. The intensity and position of pre-edge peak strongly depend on the local coor dination environment and oxidation state, respectively. The main absorption edge (peak b) corresponds to the dipole allowed V 1s to 4p transition. The near edge features (feature c, d and e) above the absorption edge can be attributed to the multiple scattering contributions and the dipole-allowed excitation of a V core 1s electron to a localized 4p state. The energy positions of the pre-edge and main absorption peaks are used to recognize the oxidation state of the vanadium. For our thin film samples, the XANES results infer þ4 oxidation state of vanadium confirming the VO2 stoichiometry, and a distorted VO6 octahedral geometry around the V sites. 5
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100
30 C 80
Transmission (%
(
S3 S2
60
S1
40
100 C
20 0
2000
2500
3000
3500
Wavelength (nm)
4000
Fig. 6. IR transmittance spectra of VO2 thin film samples measured at 30 � C (solid lines) and 100 � C (dotted lines).
Fig. 5(b) depicts V L-edge and O K-edge XAS spectra of the VO2 thin films measured in TEY mode at room temperature. The strict dipole selection rules and energy constraints of XAS make certain that one can get elemental and orbital selectivity. V L-edge (2p→3d) exhibit 2 prominent maxima near 518 and 524 eV, which are mainly linked to the electron excitations from spin-orbit split-levels 2p3/2 and 2p1/2, respectively. The spin-orbit splitting value of 6.5 eV displayed by 2p levels remains intact in the studied VO2 thin film samples. The observed value of spin-orbit splitting is identical to the value derived from XAS studies of VO2 single crystals [24]. The XAS spectrum at O K-edge probes unoccupied density of states in conduction band through the transitions from O 1s → 2p states, which are hybridized with adjacent metals [25–29]. Therefore, selectivity of XAS led to V L-edge transition primarily into dk orbitals, while O K-edge comprises signatures of all three bands near fermi level π*, σ* and dk. Based on the literature, we attribute the spectral features in the O K-edge ron (~ 525–545 eV) as associated with the π* (~529 eV) and σ* (~531 eV) states. It is noticed that the shape of the V L-edge and O K-edge XAS spectra and the energy position of spectral features are almost identical in our VO2 thin film samples. Moreover, the observed XAS data matches well to previous work on monoclinic VO2 thin film [27,28]. As no substantial changes in the electronic structure of studied samples are observed, we can assume that prolonged annealing has not introduced significant oxygen vacancies in studied samples. Moreover, oxygen vacancies in VO2 thin films may reduce Tc due to extra free electrons [30]. Never theless, here we noticed enhancement in Tc with increase in annealing time. Therefore, it is likely that oxygen content in studied samples is not much influenced by prolonged annealing. In order to check the annealing time dependence on the thermochromic property, we measured the IR transmittance spectra of VO2 thin film samples at 30 � C and 100 � C and the obtained results are shown in Fig. 6. At room temperature, VO2 films are in monoclinic insulating phase and display good transparency in the IR range. Interestingly, the IR transmittance at room temperature was found to increase with annealing time. Among the studied samples, the sample annealed for 2 min showed least IR transmittance at room temperature that further drops to ~10% upon heating to higher temperature (100 � C). At higher temperature, the IR transmittance for all the VO2 samples drops significantly. When we compare the modulation in transmittance at lower and higher temperature (at 2500 nm), it is found that modulation in sample S3 is highest while in S1 is lowest. On the other hand, IR transmittance in sample S1 at 100 � C is lowest among the studied samples. We can recall here that prolonged annealing of VO2 thin films led to better crystallinity and higher roughness but the electronic structure is not much influenced. Hence, the variation in the thermochromic performance of VO2 thin films annealed for different time can be correlated to their microstructures. 4. Conclusions In summary, we have prepared thermochromic VO2 films by employing RF sputtering at room temperature and giving them post deposition annealing treatment for different time duration ranging from 2 min to 60 min. Their structural, morphological, electronic and thermochromic properties were explored. Room temperature XRD results confirms all the VO2 thin film samples stabilized in monoclinic phase. The transformation from monoclinic phase to rutile phase in these samples is realized through temperature dependent XRD measurements. An increase in crystallite size, surface roughness and Tc is observed with increase in the annealing time. Longer annealing also led to smaller ΔH. Annealing for different time has also brought a modulation in the thermochromic perfor mance of VO2 thin films. The present work conclude that the functional properties of VO2 thin film can be tailored by controlling the post depositing annealing. Acknowledgments This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (Grant No. NRF-2019K1A3A7A09033398 and NRF-2015R1A5A1009962). SR-XRD measurements at PLS-II (Pohang Light Source II, POSTECH) were supported in part by MSIT, Korea. 6
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