- Email: [email protected]
The effects of vacuum annealing on the structure of VO2 thin films
Surface & Coatings Technology 201 (2007) 6772 – 6776 www.elsevier.com/locate/surfcoat
The effects of vacuum annealing on the structure of VO2 thin films Y.L. Wang ⁎, M.C. Li ⁎, L.C. Zhao Key Laboratory of Precision Hot-Forming for National Defense Science and Technology, School of Material Science and Engineering, Harbin Institute of Technology, P.O. Box 428, Harbin 150001, China Available online 16 November 2006
Abstract Noncrystalline VOx thin films were deposited onto p-doped Si (100) substrates at 400 °C using magnetron sputtering. By vacuum annealing, we obtained polycrystalline VO2 thin films with two different structures under a variety of annealing conditions. With the annealing temperature increasing and the annealing time developing, structures of the films underwent the following transformation: amorphous structure→metastable VO2 (B)→VO2 (B) + VO2 (M). Vacuum annealing is useful of acquiring VO2 thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful, which make the surface degenerate. © 2006 Elsevier B.V. All rights reserved. PACS: 81.1577.Cd; 36.20.Ng; 81.40.Ef Keywords: Vanadium dioxide; Vacuum annealing; Infrared absorption; Structure
1. Introduction In 1959, Morin discovered a semiconductor-metal transition in bulk vanadium dioxide (VO2) [1]. This structure transition near ambient temperature (≈ 68 °C) is accompanied by drastic changes in optical and electrical properties, which offers a potential for applications. The change of volume accompanied by structure transition is a disaster to bulk vanadium dioxide, but to film which is not a puzzle; therefore, much effort has been devoted to preparation and characterization of corresponding high quality thin films. The vanadium dioxides are unique as they exhibit different polymorphic structures. Till now, four polymorphs of VO2 [2] are known. Various techniques have been employed for depositing VO2 thin films such as sputtering [3,4], evaporation [5], sol–gel [6,7] and pulsed laser deposition (PLD) [8,9], etc. Post annealing treatment [10] is often used as an intermediate reduction step to acquire VO2 thin films. Despite all that, it is still difficult to produce a VO2 thin film for the complexity of vanadium oxides. In this paper, VO2 thin films with two different polymorphic structures have been successfully achieved according to ⁎ Corresponding authors. Tel.: +86 451 86412505; fax: +86 451 86415083. E-mail addresses: [email protected] (Y.L. Wang), [email protected] (M.C. Li). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.097
effectively combining magnetism sputtering with post vacuum annealing. 2. Experimental details VOx thin films were fabricated on p-Si (100) substrates at 400 °C by reactive magnetron sputtering in an argon–oxygen atmosphere. The target material is a bulk vanadium disk (100 mm in diameter) with a high purity of 99.99 wt.%. To achieve VO2 thin films, vacuum annealing of as-deposited VOx thin films was carried out in vacuum tubes with pressure lower than 10− 2 Pa. The vacuum annealing temperature range is between 420 °C and 500 °C, and the vacuum annealing time is varied from 5 h to 10 h. The structure of the samples was measured by XRD using a Rigaku-D/max-rB diffractometer and CuKa radiation. To acquire the information in regard to the composition and crystal state of the films, vibrational modes of the films were analyzed in vacuum with a pressure of 300 Pa by a Fourier transform infrared spectrometer (IFS 66V/S). To study the surface morphology of prepared thin films, AFM, Dimension 3100 SPM, was conducted. The mean particle size and the size distribution were evaluated from the AFM images. Roughness (Rms) of the thin film was analyzed using the software accompanied with AFM.
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
6773
3. Results and discussion 3.1. XRD analysis Annealing effects on the noncrystalline VOx thin films were analyzed by XRD, FT-IR and AFM. Fig. 1 shows the XRD pattern of VOx thin film as-deposited at 400 °C by reactive magnetron sputtering. From this figure, it is noted that the film deposited at 400 °C is amorphous. Besides the peaks of impurity, one V2O5 peak exists in this film. The peaks unidentified in the XRD spectrum represent some unknown phases in the amorphous VOx thin film, and for which no matching data can be found. They could be caused by some nonstoichiometric vanadium oxides which are easy to grow in the process of oxygen evolution. To acquire VO2 thin films, noncrystalline VOx thin films were annealed in vacuum with different parameters. Fig. 2 displays the XRD results of VOx thin films annealed for 8 h at various temperatures. After vacuum annealing treatments with noncrystalline VOx thin films at various temperatures, polycrystalline films of vanadium dioxide with two different structures are obtained. In Figs. 2 and 3, subscript B and M indicate VO2 (B) and VO2 (M), respectively. VO2 (B) and VO2 (M) (the polymorphs of VO2) appear simultaneously in the thin films annealed (Fig. 2). Among the four kinds of VO2 polymorphs [2] (Table 1), VO2 (M) is a slightly distorted rutile structure (monoclinic structure) exists under 68 °C; above 68 °C, it is a normal rutile structure – VO2 (R), which is considered the most stable. VO2 (B) is a monoclinic structure. VO2 (A) has a tetragonal structure. VO2 (B) and VO2 (A) are all metastable structure and cannot be found in phase diagram. The transitions of VO2 (B) and VO2 (A) to VO2 (R) are irreversible; while, VO2 (M) transiting to VO2 (R) is reversible. In fact, the semiconductor–metal transition Morin discovered is just the reversible transition VO2 (M)↔VO2 (R). In this work, polycrystalline VO2 thin films with VO2 (B) + VO2 (M) mixed structures are obtained by vacuum annealing on amorphous VOx thin films. In order to detect the annealing
Fig. 1. XRD pattern of VOx thin film as-deposited at 400 °C.
Fig. 2. XRD patterns of VO2 thin films annealed for 8 h at 420 °C(a), 450 °C(b) and 500 °C(c).
effects on the thin films, the diffraction profiles of VO2 (M) have been analyzed and compared with those of VO2 (B). From Fig. 2, we can see that VO2 (B) crystallizes preferentially into (001) plane while VO2 (M) does not. With annealing temperature increasing, Bragg peak intensity of VO2 (M) is increasing and the full width at half maximum (FWHM) of Bragg peaks of VO2 (M) is narrowing, while the trend of VO2 (B) peaks is just the reverse. According to comparing two couples of peaks—(011)M/(002)B and (210)M/(003)B, it can be concluded that metastable VO2 (B) is decreasing while VO2 (M) is just reverse with the annealing temperature ascending. This conclusion can also be made by observing the trend of other Bragg peaks such as (001)B and (220)M. Fig. 3 shows XRD patterns of VOx thin films annealed at 450 °C for different hours. The film annealed at 450 °C for 5 h is single VO2 (B) structure (Fig. 3a), which is different from the films annealed with other parameters (Figs. 2a–c and 3b–c). It
Fig. 3. XRD patterns of VO2 thin films annealed at 450 °C for 5 h (a), 10 h (b) and 15 h (c).
6774
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
is reported that VO2 (B) can be acquired by annealing the V2O5 thin film (prepared by vacuum evaporative plating) in vacuum at 350 °C for 7.5 h [11]. While in this work, single VO2 (B) has been achieved by amorphous VOx thin film annealed at 450 °C for 5 h. The Bragg peaks trend of VO2 thin films annealed at 450 °C for different times is similar to that of VO2 thin films annealed for 8 h at different temperatures. Judging from the XRD patterns shown in Figs. 2 and 3, it can be seen that vacuum annealing appears to be very efficient in crystallizing VO2 thin films. With the increasing of annealing temperature and annealing time, metastable VO2 (B) is gradually transformed into VO2 (M). It is reported that single VO2 (M) can be obtained according to heating the VO2 (B) powder in a quartz tube under Ar atmosphere above 600 °C [12]. We can deduce a conclusion from above results that the thin film with single VO2 (M) structure will be achieved as long as optimum annealing temperature and annealing time are selected. 3.2. FT-IR analysis An IFS 66V/S spectrometer was used to analyze vibrational modes of the films, for the sake of acquiring the information about the composition and crystal state of the thin films. The change of structure can be estimated according to the modification of absorption bands. Fig. 4 shows the transmission FT-IR spectrums of VO2 thin films. The substrate p-Si (100) has good absorption characteristic at the wavelength coverage of mid and far infrared. In Fig. 4, the bands at 610 cm− 1 and 739 cm− 1 are intrinsic absorption bands of silicon crystal, their vibrational modes are optical and acoustical vibration. Besides the intrinsic absorption bands of silicon crystal itself, a series of extrinsic absorption bands (the extrinsic vibrational modes of impurity) appear in the transmission FT-IR spectrums of silicon for being of impurity-carbon (intermiscible type impurity in silicon) and oxygen (replaceable type impurity in silicon). Among these extrinsic absorption bands, the 1106 cm− 1 band with a half width of ∼ 32 cm− 1 representing ν3 (inversion symmetry stretching vibration of Si–O–Si quasimolecule with an angle ∼ 160°) is the strongest absorption band of oxygen impurity in silicon; the 604.9 cm− 1 band with a half width of ∼ 8 cm− 1 is the strongest absorption band of carbon impurity in silicon. In the range of measurement, the extrinsic absorption bands of vanadium oxides were weak compared to the strong absorption bands of silicon. A weak absorption band near 400 cm− 1 that appears in all the spectrums corresponds to shear distortion mode of V–O bond. During vacuum annealing, the
Table 1 Polymorphic structures of VO2
670 cm− 1 vibrational band (existed at low annealing temperature (420 °C, Fig. 4b) and for short time (5 h, Fig. 4e)) becomes thinner gradually (Fig. 4c and f) and disappears at last (Fig. 4d and g). In fact, it appears at 630 cm− 1 to reach the value of 690 cm− 1 at the end of transition VO2(B)→VO2(M). But in our spectrums, the 690 cm− 1 band was too weak to be seen. Meanwhile, a new band at 717 cm− 1 appears (Fig. 4g), indeed, which is the characteristic of first “rutile packing” of octahedral and is the band of VO2 (M). In addition, it is noticed that two bands near 490 cm− 1 and at 840 cm− 1 exist in the spectrum of the thin films before annealed (Fig. 4a). The two bands correspond to the vibrational modes of vanadium oxides with high valence (V5+). After annealing, the V5+ ions are reduced to V4+ ions, so the 490 cm− 1 and 840 cm− 1 band disappeared (Fig. 4b–g). In the spectrum of amorphous VOx thin film, the band at the position of 553 cm− 1 occur (Fig. 4a), which might be caused by the nonstoichiometric VOx phase. It disappears after annealing (Fig. 4b–g). A new band appears at 440 cm− 1 when the annealing time is 15 h, and the 880 cm− 1 band disappeared at the same time (Fig. 4g). This phenomenon is consistent with the result of Valmalette [12]. It is explained that these two features are attributed to some disordering in the initial VO2 (B) octahedral arrangement: the stretching modes associated with the corner and edges sharing of octahedral are broadened, giving rise to intermediate vibrational states which slowly disappeared up to the final VO2 (M) spectrum. The VO2 thin films have active absorption characteristic of infrared according to the FT-IR spectrums. The results of FT-IR are consistent with those of XRD. 3.3. AFM analysis
VO2 polymorphs
a (nm)
b (nm)
c (nm)
β (°)
Space group
VO2 VO2 VO2 VO2
0.575 1.203 0.455 0.844
0.542 0.369 0.455 0.844
0.538 0.642 0.288 0.768
122.6 106.6 / /
P21/c C2/m P42/mmm P42/nmc
(M) (B) (R) (A)
Fig. 4. Transmittance FT-IR spectrums of VO2 thin films, as-deposited at 400 °C (a), annealed at 420 °C for 8 h (b), at 450 °C for 8 h (c), at 500 °C for 8 h (d), at 450 °C for 5 h (e), at 450 °C for 10 h (f) and at 450 °C for 15 h (g).
AFM was employed to study the surface morphology of both thin films, as-deposited and annealed (Fig. 5). The values of Rms are shown in Table 2. The surface of VO2 thin films prepared by magnetron and vacuum annealing is in high quality. Mean
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
6775
Fig. 5. AFM images of VO2 thin films, as-deposited at 400 °C (a), annealed at 420 °C for 8 h (b), at 450 °C for 8 h (c), at 500 °C for 8 h (d), at 450 °C for 5 h (e), at 450 °C for 10 h (f) and at 450 °C for 15 h (g).
6776
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
Table 2 Rms values of VOx thin films prepared in different conditions (nm) As-deposited Annealed 400 °C
420 °C, 8h
450 °C, 8h
500 °C, 8h
450 °C, 5h
450 °C, 10 h
450 °C, 15 h
10.447
7.459
6.970
14.925
6.375
6.222
9.341
crystallite size densely range from 70 to 390 nm. Observing Fig. 5 and Table 2, we can conclude that vacuum annealing is good method to acquire VO2 thin films with high surface quality (Fig. 5b, c, e and f), but too high annealing temperature and too long time are harmful and can make the surface degenerate (Fig. 5d and g). The trends of surface roughness changed by condition are consistent with the films annealed in vacuum under Ar atmosphere [11]. The reasons of this phenomena above are analyzed as follows: on the one hand, the diffusion of atoms is encouraged and the atoms of vanadium and oxygen react sufficiently at high annealing temperature (450 °C) and for long annealing time (10 h), which make the sizes of particles smaller and the surface smoother; on the other hand, the recombination of VO2 molecules and the particle growth make the composition in partial area relatively uniform during thermal diffusion, and the increasing thermal stress (among substrate and film) resulted in the larger particle sizes and the looser structure of thin films as the films were annealed at too high temperature (500 °C) and for too long time (15 h). 4. Summary VO2 thin films with two different polymorphic structures have been successfully achieved by effectively combining magnetism sputtering with post vacuum annealing. With the
increasing annealing temperature and the developing annealing time, structures of the films underwent the following transformation: amorphous structure→metastable VO2 (B)→VO2 (M) + VO2 (B). The FT-IR results show that the VO2 thin films have active absorption of infrared and accord with those of XRD. Vacuum annealing is a good method to acquire VO2 thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful and make the surface degenerate. The VO2 thin films prepared have smooth surface with high qualities and Rms of ∼ 10 nm. Acknowledgements This study is supported in part by both project 50272019 supported by the National Natural Science Foundation of China and project HIT.2001.15 supported by the Scientific Research Foundation of Harbin Institute of Technology. References [1] F.J. Morin, Phys. Rev. Lett. 3 (1959) 34. [2] G. Nihoul, Ch. Leroux, V. Madigou, J. Durak, Solid State Ionics 117 (1999) 105. [3] Y.L. Wang, M.C. Li, L.C. Zhao, Rare Met. Mater. Eng. 7 (2005) 1077. [4] A. Atrei, T. Cecconi, B. Cortigiani, et al., Surf. Sci. 513 (2002) 149. [5] M.D. Negra, M. Sambi, G. Granozzi, Surf. Sci. 494 (2001) 213. [6] Z.O. Crnjak, I. Muševic, Nanostruct. Mater. (1999) 399. [7] J. Livage, F. Beteille, C. Roux, et al., Acta Mater. (1998) 743. [8] M. Nagashima, H. Wada, Thin Solid Films 312 (1998) 61. [9] C.V. Ramana, R.J. Smith, O.M. Hussain, C.M. Julien, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 111 (2004) 218. [10] J.C. Valmalette, J.R. Gavarri, P. Satre, Eur. J. Solid State Inorg. Chem. 32 (1995) 71. [11] W. Zhou, L.X. Zhu, J. Chongqing Univer. Posts Tel. 12 (2000) 24. [12] J.C. Valmalttte, J.R. Gavarri, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 54 (1998) 168.
The effects of vacuum annealing on the structure of VO2 thin films Y.L. Wang ⁎, M.C. Li ⁎, L.C. Zhao Key Laboratory of Precision Hot-Forming for National Defense Science and Technology, School of Material Science and Engineering, Harbin Institute of Technology, P.O. Box 428, Harbin 150001, China Available online 16 November 2006
Abstract Noncrystalline VOx thin films were deposited onto p-doped Si (100) substrates at 400 °C using magnetron sputtering. By vacuum annealing, we obtained polycrystalline VO2 thin films with two different structures under a variety of annealing conditions. With the annealing temperature increasing and the annealing time developing, structures of the films underwent the following transformation: amorphous structure→metastable VO2 (B)→VO2 (B) + VO2 (M). Vacuum annealing is useful of acquiring VO2 thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful, which make the surface degenerate. © 2006 Elsevier B.V. All rights reserved. PACS: 81.1577.Cd; 36.20.Ng; 81.40.Ef Keywords: Vanadium dioxide; Vacuum annealing; Infrared absorption; Structure
1. Introduction In 1959, Morin discovered a semiconductor-metal transition in bulk vanadium dioxide (VO2) [1]. This structure transition near ambient temperature (≈ 68 °C) is accompanied by drastic changes in optical and electrical properties, which offers a potential for applications. The change of volume accompanied by structure transition is a disaster to bulk vanadium dioxide, but to film which is not a puzzle; therefore, much effort has been devoted to preparation and characterization of corresponding high quality thin films. The vanadium dioxides are unique as they exhibit different polymorphic structures. Till now, four polymorphs of VO2 [2] are known. Various techniques have been employed for depositing VO2 thin films such as sputtering [3,4], evaporation [5], sol–gel [6,7] and pulsed laser deposition (PLD) [8,9], etc. Post annealing treatment [10] is often used as an intermediate reduction step to acquire VO2 thin films. Despite all that, it is still difficult to produce a VO2 thin film for the complexity of vanadium oxides. In this paper, VO2 thin films with two different polymorphic structures have been successfully achieved according to ⁎ Corresponding authors. Tel.: +86 451 86412505; fax: +86 451 86415083. E-mail addresses: [email protected] (Y.L. Wang), [email protected] (M.C. Li). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.09.097
effectively combining magnetism sputtering with post vacuum annealing. 2. Experimental details VOx thin films were fabricated on p-Si (100) substrates at 400 °C by reactive magnetron sputtering in an argon–oxygen atmosphere. The target material is a bulk vanadium disk (100 mm in diameter) with a high purity of 99.99 wt.%. To achieve VO2 thin films, vacuum annealing of as-deposited VOx thin films was carried out in vacuum tubes with pressure lower than 10− 2 Pa. The vacuum annealing temperature range is between 420 °C and 500 °C, and the vacuum annealing time is varied from 5 h to 10 h. The structure of the samples was measured by XRD using a Rigaku-D/max-rB diffractometer and CuKa radiation. To acquire the information in regard to the composition and crystal state of the films, vibrational modes of the films were analyzed in vacuum with a pressure of 300 Pa by a Fourier transform infrared spectrometer (IFS 66V/S). To study the surface morphology of prepared thin films, AFM, Dimension 3100 SPM, was conducted. The mean particle size and the size distribution were evaluated from the AFM images. Roughness (Rms) of the thin film was analyzed using the software accompanied with AFM.
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
6773
3. Results and discussion 3.1. XRD analysis Annealing effects on the noncrystalline VOx thin films were analyzed by XRD, FT-IR and AFM. Fig. 1 shows the XRD pattern of VOx thin film as-deposited at 400 °C by reactive magnetron sputtering. From this figure, it is noted that the film deposited at 400 °C is amorphous. Besides the peaks of impurity, one V2O5 peak exists in this film. The peaks unidentified in the XRD spectrum represent some unknown phases in the amorphous VOx thin film, and for which no matching data can be found. They could be caused by some nonstoichiometric vanadium oxides which are easy to grow in the process of oxygen evolution. To acquire VO2 thin films, noncrystalline VOx thin films were annealed in vacuum with different parameters. Fig. 2 displays the XRD results of VOx thin films annealed for 8 h at various temperatures. After vacuum annealing treatments with noncrystalline VOx thin films at various temperatures, polycrystalline films of vanadium dioxide with two different structures are obtained. In Figs. 2 and 3, subscript B and M indicate VO2 (B) and VO2 (M), respectively. VO2 (B) and VO2 (M) (the polymorphs of VO2) appear simultaneously in the thin films annealed (Fig. 2). Among the four kinds of VO2 polymorphs [2] (Table 1), VO2 (M) is a slightly distorted rutile structure (monoclinic structure) exists under 68 °C; above 68 °C, it is a normal rutile structure – VO2 (R), which is considered the most stable. VO2 (B) is a monoclinic structure. VO2 (A) has a tetragonal structure. VO2 (B) and VO2 (A) are all metastable structure and cannot be found in phase diagram. The transitions of VO2 (B) and VO2 (A) to VO2 (R) are irreversible; while, VO2 (M) transiting to VO2 (R) is reversible. In fact, the semiconductor–metal transition Morin discovered is just the reversible transition VO2 (M)↔VO2 (R). In this work, polycrystalline VO2 thin films with VO2 (B) + VO2 (M) mixed structures are obtained by vacuum annealing on amorphous VOx thin films. In order to detect the annealing
Fig. 1. XRD pattern of VOx thin film as-deposited at 400 °C.
Fig. 2. XRD patterns of VO2 thin films annealed for 8 h at 420 °C(a), 450 °C(b) and 500 °C(c).
effects on the thin films, the diffraction profiles of VO2 (M) have been analyzed and compared with those of VO2 (B). From Fig. 2, we can see that VO2 (B) crystallizes preferentially into (001) plane while VO2 (M) does not. With annealing temperature increasing, Bragg peak intensity of VO2 (M) is increasing and the full width at half maximum (FWHM) of Bragg peaks of VO2 (M) is narrowing, while the trend of VO2 (B) peaks is just the reverse. According to comparing two couples of peaks—(011)M/(002)B and (210)M/(003)B, it can be concluded that metastable VO2 (B) is decreasing while VO2 (M) is just reverse with the annealing temperature ascending. This conclusion can also be made by observing the trend of other Bragg peaks such as (001)B and (220)M. Fig. 3 shows XRD patterns of VOx thin films annealed at 450 °C for different hours. The film annealed at 450 °C for 5 h is single VO2 (B) structure (Fig. 3a), which is different from the films annealed with other parameters (Figs. 2a–c and 3b–c). It
Fig. 3. XRD patterns of VO2 thin films annealed at 450 °C for 5 h (a), 10 h (b) and 15 h (c).
6774
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
is reported that VO2 (B) can be acquired by annealing the V2O5 thin film (prepared by vacuum evaporative plating) in vacuum at 350 °C for 7.5 h [11]. While in this work, single VO2 (B) has been achieved by amorphous VOx thin film annealed at 450 °C for 5 h. The Bragg peaks trend of VO2 thin films annealed at 450 °C for different times is similar to that of VO2 thin films annealed for 8 h at different temperatures. Judging from the XRD patterns shown in Figs. 2 and 3, it can be seen that vacuum annealing appears to be very efficient in crystallizing VO2 thin films. With the increasing of annealing temperature and annealing time, metastable VO2 (B) is gradually transformed into VO2 (M). It is reported that single VO2 (M) can be obtained according to heating the VO2 (B) powder in a quartz tube under Ar atmosphere above 600 °C [12]. We can deduce a conclusion from above results that the thin film with single VO2 (M) structure will be achieved as long as optimum annealing temperature and annealing time are selected. 3.2. FT-IR analysis An IFS 66V/S spectrometer was used to analyze vibrational modes of the films, for the sake of acquiring the information about the composition and crystal state of the thin films. The change of structure can be estimated according to the modification of absorption bands. Fig. 4 shows the transmission FT-IR spectrums of VO2 thin films. The substrate p-Si (100) has good absorption characteristic at the wavelength coverage of mid and far infrared. In Fig. 4, the bands at 610 cm− 1 and 739 cm− 1 are intrinsic absorption bands of silicon crystal, their vibrational modes are optical and acoustical vibration. Besides the intrinsic absorption bands of silicon crystal itself, a series of extrinsic absorption bands (the extrinsic vibrational modes of impurity) appear in the transmission FT-IR spectrums of silicon for being of impurity-carbon (intermiscible type impurity in silicon) and oxygen (replaceable type impurity in silicon). Among these extrinsic absorption bands, the 1106 cm− 1 band with a half width of ∼ 32 cm− 1 representing ν3 (inversion symmetry stretching vibration of Si–O–Si quasimolecule with an angle ∼ 160°) is the strongest absorption band of oxygen impurity in silicon; the 604.9 cm− 1 band with a half width of ∼ 8 cm− 1 is the strongest absorption band of carbon impurity in silicon. In the range of measurement, the extrinsic absorption bands of vanadium oxides were weak compared to the strong absorption bands of silicon. A weak absorption band near 400 cm− 1 that appears in all the spectrums corresponds to shear distortion mode of V–O bond. During vacuum annealing, the
Table 1 Polymorphic structures of VO2
670 cm− 1 vibrational band (existed at low annealing temperature (420 °C, Fig. 4b) and for short time (5 h, Fig. 4e)) becomes thinner gradually (Fig. 4c and f) and disappears at last (Fig. 4d and g). In fact, it appears at 630 cm− 1 to reach the value of 690 cm− 1 at the end of transition VO2(B)→VO2(M). But in our spectrums, the 690 cm− 1 band was too weak to be seen. Meanwhile, a new band at 717 cm− 1 appears (Fig. 4g), indeed, which is the characteristic of first “rutile packing” of octahedral and is the band of VO2 (M). In addition, it is noticed that two bands near 490 cm− 1 and at 840 cm− 1 exist in the spectrum of the thin films before annealed (Fig. 4a). The two bands correspond to the vibrational modes of vanadium oxides with high valence (V5+). After annealing, the V5+ ions are reduced to V4+ ions, so the 490 cm− 1 and 840 cm− 1 band disappeared (Fig. 4b–g). In the spectrum of amorphous VOx thin film, the band at the position of 553 cm− 1 occur (Fig. 4a), which might be caused by the nonstoichiometric VOx phase. It disappears after annealing (Fig. 4b–g). A new band appears at 440 cm− 1 when the annealing time is 15 h, and the 880 cm− 1 band disappeared at the same time (Fig. 4g). This phenomenon is consistent with the result of Valmalette [12]. It is explained that these two features are attributed to some disordering in the initial VO2 (B) octahedral arrangement: the stretching modes associated with the corner and edges sharing of octahedral are broadened, giving rise to intermediate vibrational states which slowly disappeared up to the final VO2 (M) spectrum. The VO2 thin films have active absorption characteristic of infrared according to the FT-IR spectrums. The results of FT-IR are consistent with those of XRD. 3.3. AFM analysis
VO2 polymorphs
a (nm)
b (nm)
c (nm)
β (°)
Space group
VO2 VO2 VO2 VO2
0.575 1.203 0.455 0.844
0.542 0.369 0.455 0.844
0.538 0.642 0.288 0.768
122.6 106.6 / /
P21/c C2/m P42/mmm P42/nmc
(M) (B) (R) (A)
Fig. 4. Transmittance FT-IR spectrums of VO2 thin films, as-deposited at 400 °C (a), annealed at 420 °C for 8 h (b), at 450 °C for 8 h (c), at 500 °C for 8 h (d), at 450 °C for 5 h (e), at 450 °C for 10 h (f) and at 450 °C for 15 h (g).
AFM was employed to study the surface morphology of both thin films, as-deposited and annealed (Fig. 5). The values of Rms are shown in Table 2. The surface of VO2 thin films prepared by magnetron and vacuum annealing is in high quality. Mean
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
6775
Fig. 5. AFM images of VO2 thin films, as-deposited at 400 °C (a), annealed at 420 °C for 8 h (b), at 450 °C for 8 h (c), at 500 °C for 8 h (d), at 450 °C for 5 h (e), at 450 °C for 10 h (f) and at 450 °C for 15 h (g).
6776
Y.L. Wang et al. / Surface & Coatings Technology 201 (2007) 6772–6776
Table 2 Rms values of VOx thin films prepared in different conditions (nm) As-deposited Annealed 400 °C
420 °C, 8h
450 °C, 8h
500 °C, 8h
450 °C, 5h
450 °C, 10 h
450 °C, 15 h
10.447
7.459
6.970
14.925
6.375
6.222
9.341
crystallite size densely range from 70 to 390 nm. Observing Fig. 5 and Table 2, we can conclude that vacuum annealing is good method to acquire VO2 thin films with high surface quality (Fig. 5b, c, e and f), but too high annealing temperature and too long time are harmful and can make the surface degenerate (Fig. 5d and g). The trends of surface roughness changed by condition are consistent with the films annealed in vacuum under Ar atmosphere [11]. The reasons of this phenomena above are analyzed as follows: on the one hand, the diffusion of atoms is encouraged and the atoms of vanadium and oxygen react sufficiently at high annealing temperature (450 °C) and for long annealing time (10 h), which make the sizes of particles smaller and the surface smoother; on the other hand, the recombination of VO2 molecules and the particle growth make the composition in partial area relatively uniform during thermal diffusion, and the increasing thermal stress (among substrate and film) resulted in the larger particle sizes and the looser structure of thin films as the films were annealed at too high temperature (500 °C) and for too long time (15 h). 4. Summary VO2 thin films with two different polymorphic structures have been successfully achieved by effectively combining magnetism sputtering with post vacuum annealing. With the
increasing annealing temperature and the developing annealing time, structures of the films underwent the following transformation: amorphous structure→metastable VO2 (B)→VO2 (M) + VO2 (B). The FT-IR results show that the VO2 thin films have active absorption of infrared and accord with those of XRD. Vacuum annealing is a good method to acquire VO2 thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful and make the surface degenerate. The VO2 thin films prepared have smooth surface with high qualities and Rms of ∼ 10 nm. Acknowledgements This study is supported in part by both project 50272019 supported by the National Natural Science Foundation of China and project HIT.2001.15 supported by the Scientific Research Foundation of Harbin Institute of Technology. References [1] F.J. Morin, Phys. Rev. Lett. 3 (1959) 34. [2] G. Nihoul, Ch. Leroux, V. Madigou, J. Durak, Solid State Ionics 117 (1999) 105. [3] Y.L. Wang, M.C. Li, L.C. Zhao, Rare Met. Mater. Eng. 7 (2005) 1077. [4] A. Atrei, T. Cecconi, B. Cortigiani, et al., Surf. Sci. 513 (2002) 149. [5] M.D. Negra, M. Sambi, G. Granozzi, Surf. Sci. 494 (2001) 213. [6] Z.O. Crnjak, I. Muševic, Nanostruct. Mater. (1999) 399. [7] J. Livage, F. Beteille, C. Roux, et al., Acta Mater. (1998) 743. [8] M. Nagashima, H. Wada, Thin Solid Films 312 (1998) 61. [9] C.V. Ramana, R.J. Smith, O.M. Hussain, C.M. Julien, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 111 (2004) 218. [10] J.C. Valmalette, J.R. Gavarri, P. Satre, Eur. J. Solid State Inorg. Chem. 32 (1995) 71. [11] W. Zhou, L.X. Zhu, J. Chongqing Univer. Posts Tel. 12 (2000) 24. [12] J.C. Valmalttte, J.R. Gavarri, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 54 (1998) 168.