Thermochromic VO2 thin films studied by photoelectron spectroscopy

ELSEVIER

Thin Solid Films 287 (1996) 134-138

Thermochromic VO2 thin films studied by photoelectron spectroscopy T. Christmann, B. Felde, W. Niessner, D. Schalch, A. Scharmann L Physikalisches institut, Justus-Liebig-Universitiit Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany Received4 October 1995; accepted 24 January 1996

Abstract Thermochromic vanadium dioxide films which are discussed as "intelligent" window coatings were deposited by reactive rf-sputtering. The films are grown on quartz polycrystalline and unidirectional. Photoelectron spectroscopy (XPS, UPS) was applied to study the density of states distribution in the valence band regime below and above the phase transition temperature. It is shown that the band structure of the polycrystalline films bears resemblance to single-crystallinevanadium dioxide to a large extent, e.g. bandgap, valence band energy position, and valence band structure and width in the semiconducting phase. In contrast to single crystals, e.g. the metal-semiconductor transition in the films occurs in a broad temperature range of about 20 to 30 K, dependent mainly on deposition temperature. Correlations between optical, electronical, and structural properties of the film are evident. Keywords: Opticalcoatings;Phasetransitions;Photoelectronspectroscopy;Vanadiumoxide

1, Introduction Over the last decade vanadium dioxide has been discussed as a thermochromic window coating, by the aid of which the solar energy transmission can be controlled passively [ !-3 ]. Due to a reversible metal-semiconductor phase transition at a temperature Tt-341 K, vanadium dioxide exhibits significant switching characteristics particularly in the infrared. Below Tt vanadium dioxide is a shallow gap (0.7 eV) semiconductor with a monoclinic structure, above Tt it has a tetragonal rutile (TiOa) structure and has metallic properties. In 1959 the vanadium dioxide phase transition was observed by Morin [4] for the first time. In the following years its properties in the single-crystalline state have been studied intensively. Goodenough [5] gave the first theoretical description of the phenomenon in terms of the so-called "metal-insulator transition" (MIT), applying crystal field and molecular orbital (Me) theory. In the tetragonal high-temperature phase the V ++ ion with one 3d electron per atom is in the center of an O octahedron (Fig. I (a)). Due to this symmetry the degenerated five 3d t atomic orbitals of V are split into doubly degenerated e8 levels and triply degenerated t28 levels. The % orbitals are directed towards the O iigands and are strongly hybridized with the O 2p orbitals. They form the or and or* bands together with the O 2p orbitals. The t2g orbitals which point in between the ligands, form the 'tr and ~'* bands and also the d z band which results from the V 3d~ orbitals 0(M0-60901961515.00© 1996ElsevierScienceS.A. All rightsreserved PilS0040.6090 (96) 08770-6

along the c axis. Due to the overlap of the dlt and the ~* bands at the Fermi level (Fig. l(b)), the high-temperature phase of VO2 exhibits metallic properties. Below the transition temperature V-V pairing occurs along the c axis together with small displacements of V atoms perpendicular to the c axis, resulting in a monoclinic crystal structure. The V--O hybridization is changed, the V-V bonding is stronger and thus the ~r* band rises above the Fermi level. The d, band is split into an empty and a filled one (Fig. l(b)). Goodenough's description of the MIT in VO2 agrees qualitatively well with experiments. But the experimentally proven large d, band splitting of about 2.5 eV [ 6,7] cannot be explained. This effect has been the subject of further theoretical efforts (see, for example, Refs. [ 8-10] ), considering electron-electron correlation and electron-phonon interaction or a combination of both effects. Up to now, the MIT is not understood in detail. So far, all experimental investigations aimed at the understanding of the band structure of VO2 below and above Tt and the MIT itself have been done on single-crystalline material. Since Granqvist [2] proposed thermochromic VO2 films as "intelligent" window coatings, many efforts have been undertaken to deposit suitable films, e.g. by reactive r.f., d.c., and magnetron sputtering, reactive thermal and electron beam evaporation, CVD techniques, oxidation of deposited metal films, and sol-gel techniques. Up to now, all these investigations did not result in coatings which sufficiently accomplish the demands for economical application, e.g. high

T. Christmann et al. / Thin Solid Films 287 (1996) 134-138

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determined by X-ray diffraction (XRD). Photoelectron spectroscopy (XPS, UPS) was carried out using a VG Instruments equipment. XPS was performed with the Mg Ka line at 1253.6 eV with a line width of 0.7 eV, for UPS measurements we used the He I line at 21.21 eV with a line width of 0.05 eV. The electron energy analyzer was a CLAM-type hemispherical one. The mu metal analysis chamber was equipped with an ion pump and a liquid nitrogen-cooled titanium sublimation pump, thus enabling a residual gas pressure of less than 10 -s Pa. The samples can be conditioned by heat treatment up to about 1000 K. From our first measurements we have learned that sputter-cleaning of samples deteriorates the MIT. Optical transmittance and reflectance measurements were performed using a Varian 2300 spectrophotometer.

3. Results and discussion

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Fig, 1, (a) VO2 untt cell of the futile structure above the transition temperature Tt; @, vanadium; O, oxygen. (b) Schematic sketch of the band structure of single,crystalline VO2 in the metallic and the semiconducting phase according to Goodenough [ 5 ].

transmission in the visible spectral range below and above the MIT, good switching characteristics in the near infrared, and a Tt near room temperature. The subject of this work is to apply further analytical tools on VOz films in addition to optical and electrical methods as reported in the literature until now. It is expected that a better understanding of the band structure of the films, and its dependence on the deposition parameters will enable the deposition of films of higher quality. First results are presented in the following.

2. Experimental The films were deposited by reactive r.f. sputtering from a vanadium target in an argon/oxygen atmosphere. The gases had typical purities of 99.999%, the vanadium target had a purity of 99.8%. Previous to film deposition the sputter chamber was evacuated to about 10-5 Pa using a turbomolecular pump together with a liquid nitrogen trap. (110) Si discs and quartz glasses were used as substrates, the latter ones mainly for optical measurement. The temperature of substrates was varied between about 500 and 800 K. Typical deposition parameters were as follows: total sputter gas pressure, 10- i Pa; oxygen to argon ratio, 0.16; r.f. power, 200-400 W; deposition rate, 2-6 nm min- ~. Film thicknesses were typically about 100 nm. The polycrystalline structure of films, which strongly depends on the substrate temperature during sputtering, was

Depending on the sputter parameters, particularly on substrate temperature and sputter rate, the VOz films exhibit a more or less polycrystalline structure. In Fig. 2 an XRD record from a film deposited at about 800 K on a quartz plate is presented. Besides the broad halo due to the SiO2 substrate, only one sharp reflex near 2 0 = 28 ° with a half width of 0.2 ° (FWHM) is observed. It can be attributed to the (002) orientation of microerystals. Since no other reflexes appear in the full range of reflection angles (not shown here), it is evident that the film has a unidirectional polycrystalline structure. At lower substrate temperatures the (002) reflexes become appreciably broader and their intensities decrease, but no additional reflexes appear. This temperature-dependent film structure can be correlated with the optical switching characteristics of the films and also with the number of occupied states at the Fermi level in the metallic phase above the transition temperature Tt (see discussion below).

3.2. Photoelectron spectroscopy XPS spectra were recorded for determining the stoichiometry of the vanadium oxide films by analyzing the core

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20 25 30 20 / degrees Fig. 2. XRD record at room temperature of a well-switching VO2 film on a quartz substrate (section only) deposited at a substrate temperature of 800 K.

136

T. Christmann et al. / Thin Solid Fi!.... 287 (1996) 134-138

Table 1 Binding energies and half widths of V02 core levels Peak

Temp.

O Is V 2ptl2 V 2P3/2

Ref. [I0] "

This work

(K) 393 298 393 298 393 298

Ref. [11]

Pos. (eV) b

Width (eV) c

Pos. (eV) b

Width (eV) c

Pos. (eV) b

Width (eV) c

529.9 530.1 523.8 523.8 516.4 516.3

1.9 1.6 3.2 2.7 3.4 2.5

529.7 529.9

2.0 1.8

529.4 529.6

3.0 2.4

523.3 523.5 515.9 516.2

5.0 4.5 4.1 3.2

515.7 515.7

4.2 3.1

• Measuredat T= 273 K and T= 373 K, resp. t, Bindingenergywithrespectto the Fermilevel. c I~VHM.

level emission intensities, and they also give information on the change of the distribution of states below the valence band edge at temperatures below and above the phase transition. UPS analysis proved valuable to measure changes in the distribution of states in the vicinity of the bandgap, though the low-energy XPS data give qualitative information in this energy range, too, by taking a larger number of spectral scans. Fig. 3 shows two XPS spectra of deep core levels below and above Tt. The peak positions are in good agreement with results from single-crystalline VO2 (Table 1). The numbers given in Table 1 are averages over measurements on different well-switching films deposited at about 800 K. The mean deviation of measured values from the average is 0.05 eV for the peak positions and 0.1 eV for the half widths. The broadening of the peaks above 7", is 0.3 eV (O Is) and 0.5.-0.9 eV (V 2p), and is therefore not attributed to phonon broadening but to the instantaneous interaction of a hole in the core and the V 3d band in the final state [ 10-12]. From the intensities (peak areas) of the O Is and the V 2p3/2 peaks the stoichiometry of deposited films was determined by applying element-specific sensitivity factors [ 13] and the relation no:nv-lo/$o:lv/$v, no:nv is the atomic ratio of O and V in the film, lo and Iv are the intensities of emitted photoelectrons (peak areas), So and Sv are the sensitivity factors. The mean O:V ratio of well-switching films was determined to 2.08 + 0.02. From the small mean devia-

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52S 520 515 510 binding enet3y / eV Fig. 3. XPS spectra (section only) from a VO2 film in the corn level regime below ( ) and above ( . . . ) the transition temperature Tt, deposition temperature of the film 800 K. S35

S30

tion of a single measured value it can be concluded that films can be deposited with fairly good reproducibility if the sputter parameters are suitably chosen. The deviation from the real O/V ratio of 2 is due to the well-known fact that sensitivity factors depend to a certain extent on the experimental equipment used. So we cannot expect to find the exact atomic ratio of 2 when applying the sensitivity factors of Wagner et al. [ 13 ] who used Perkin-Elmer instrumentation. XPS spectra from the valence band regime of a VO2 film, where more significant changes at the MIT are expected, are presented in Fig. 4. The structure of the O 2p peak and the small differences of intensities below and above Tt, and also its half width of about 5 eV are similar to those reported from single-crystalline material [ 10,11 ]. A strong shift of the V 3d band at 1.1 eV of about 0.6 eV towards the Fermi level is observed in the metallic phase above Tt, in accordance with XPS literature data from single-crystalline VO2 [14]. The inset shows two UPS spectra in the V 3d range, recorded from the same film, which demonstrate more clearly the metallic character of the film above Tt by showing a steep Fermi edge. This is due to the smaller line width of He I UV line of 0.05 eV, in comparison to the Mg Ka source line width of 0.7 eV. The V 3d peak position is now 0.95 eV in the semiconducting phase, and the upper edge of the 3d band is 0.15 eV below the Fermi level as determined from the low energy flank. Because of the better resolution of UPS within this energy range we consider these values as more realistic than those

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binding energy / eV Fig. 4. XPS and UPS (inset) spectraof a well-switcMngfilmin the valence band regimebelow (. • . ) and above ( ) the transitiontemperature T,. depositiontemperatureof the film800 K.

!37

T. Christmann et al. I Thin Solid Films 287 (1996) 134-138

ones determined by XPS. Shin et al. [6] have reported 0.9 eV and 0.2 eV, respectively, from UPS measurements on single-crystalline material. Two interesting features of the UPS spectra (Fig. 4, inset) are worth mentioning: the UPS intensity (which resembles approximately the density of occupied initial states [ 15] ) does not disappear in the vicinity of the Fermi level in the semiconducting phase below Tt. This may be attributed to a strong V 3d band tailing into the bandgap, caused by defects in the polycrystalline film. Secondly, in the metallic phase above T~the intensity in the range of the V 3d band does not disappear and remains higher than the intensity at the Fermi level, where a maximum of the intensity should be expected in the metallic phase. This is in contrast to results from singlecrystalline materials [ 6], and leads to the assumption that a more or less strong dll band splitting exists even in the metallic phase, and only a smaller part of the available electrons becomes delocalized in our polycrystalline films than in VOw, single crystals. Fig. 5 shows a comparison of our XPS and UPS results with theoretical calculations of the density of states by applying the APW-LCAO [ 16], and the FPLAPW method [ 17] for the metallic phase of single-crystalline VOz. The obvious different shapes of the XPS and UPS spectra are due to the dependence of photoionization cross-sections on, f~r example, the energy of the ionizing radiation and the kind of initial electronic states. XPS is more sensitive for d states whereas UPS is more sensitive for p states. From theoretical considerations it was deduced (see, for example, Ref. [ 16] ), th~,t the character of the O 2p band is indeed dominated by the O 2p orbitals, but on account of the covalent V--O binding there occurs a mixing of p and d states. It is assumed that binding pdo" orbitals are responsible for the lower part of the O 2p band, and binding pdp orbitals lbr the upper region. Furthermore an overlap of both kind of orbitals occurs. The upper edge of the O 2p band is believed to be formed by non-binding

O 2plr orbitais. Following these assumptions and combining the XPS and UPS intensities (Fig. 5(b) ) additively there is a fairly good qualitative accordance with the calculated density of states (Fig. 5 (a)). The only significant discrepancies are, as mentioned above, the non-zero intensities between the low-energy edge of the V 3d band and the Fermi level and the comparative low intensities at the Fermi level which are found experimentally. The upper edge of the O 2p band is about 2.5 eV below the Fermi level as determined by extrapolation of the low-energy O 2p flank of the UPS curve (Fig. 5 (b)). This agrees fairly well with experimental results of Shin et al. [6] on VO2 single crystals, and also with the calculation of Caruthers at al. [ 16] (Fig. 5(a) ). Fig. 6 shows UPS spectra of two VO2 films which were deposited at different substrate temperatures of about 800 K (Fig. 6(a)) and 500 K (Fig. 6(b)). The spectra from each film were recorded successively at different fixed tempera.tures. The striking feature of both sets of spectra is the more or less small decrease of intensity in the V 3d band range and the simultaneously increasing intensity at the Fermi level over a more or less broad temperature interval, i.e. the phase transition does not occur suddenly at a certain temperature. From the comparison of the two sets of spectra it is also evident that the phase transition in the film which was sputtered at about 500 K (Fig. 6(b)) is smeared out over a larger temperature interval. This correlates with measurements on optical switching (see below), and is attributed to the poorer polycrystalline order in films which were deposited at lower temperatures. Furthermore, the spectra from the film sputtered at about 500 K show smaller intensities at the Fermi

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binding energy/ eV Fig. 5. Calculateddensitiesof states: (a) accordingto Carutherset al. [ 15], solid curve, and Nikolaevet al. [ 16], shadedarea,comparedwith measured photoelectron spectra; (b) above the transitiontemperatureTt; solid curve. UPS; dottedcurve, XPS;depositiontemperatureof the film 800 K.

20

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binding energy / eV Fig. 6. Sets of UPS spectra of two switching films recorded at different fixed temperatures, the curves are normalized with respect to background counl rates, deposition temperature of the films: (a) 800 K, (b) 500 K.

7'. Christmann et aL /Thin Solid Films 287 (1996) 134-138

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Fig. 7(b) shows the optical switching hysteresis of the two films recorded at a wavelength of 1500 rim. The temperature was varied with a rate of 5 deg min- ~ between 303 and 353 K. Both curves demonstrate that the phase transition in the films does not occur at a fixed temperature, particularly not at 341 K where the transition in VO2 single crystals is observed. The temperature interval within which the transition takes place corresponds well with the temperature dependence of UPS spectra in Fig. 6. Since the MIT in VO2 films proceeds over a more or less broad temperature range, its characterization by a single transition temperature Tt as it is customary in single-crystal investigations, is meaningless. We believe it would be better to use the center of the hysteresis together with the width of the transition interval, e.g. T+AT, when describing film properties. At least it should be mentioned that a comparison of results from optical measurements with those fi'om photoelectron ~pectroscopy is somewhat crucial. With optical methods the volume of a film is studied, with XPS and UPS only a thin surface layer. Therefore we have controlled the stoichiometric homogeneity of our films by SIMS depth profiling [20].

341 K 1

l

3 0

3 0

3 0

34~

Temperature / K Fig. 7. (a) Optical transmittance below and above the transition temperature Tt of two different films, deposition temperatures 800 K ( ), 500 K (- - -). (b) Correspondent switching hysteresis of two films at a wavelength of 1500 nm.

level, i.e. less electrons are delocalized at elevated temperatures.

3.3. Optical measurements The results of optical measurements on two films which were deposited at different temperatures are presented exemplarily in Fig. 7. The film sputtered at about 800 K (Fig. 7(a), pair of solid curves) exhibits the typical switching characteristics which are well-known from the literature (see, for example, Refs. [ 18,19] ). The lower transmission curve was recorded at a constant temperature of 303 K, the upper one at a constant temperature of 353 K. The film deposited at about 500 K (pair of dashed curves) has nearly the same transmittance as the 800 K film at room temperature, but above Tt its transmittance remains higher. Considering the UPS results (Fig. 6) we attribute this higher transmittance above Tt to the smaller number of delocalized electrons in the metallic phase, which are responsible for the metallic reflection in the infrared.

References C.M. Lampert, Prec. SPIE, 234 (1982) I. C.G. Granqvist, Phys. Scr., 32 (1985) 401. G.V. Jorgensen and J.C. Lee, Sol. Energy Mater., 14 (1986) 205. FJ. Morin, Phys. Rev. Lett., $ (1959) 34. J.B. Goodenough, J. Solid State Chem., 3 (1971) 490. S. Shin, S. Sugn, M. Taniguchi, M. Fujisawa, H. Kanzaki, A. Fujimori, H. Damon, Y. Ueda, K. Kosuge and S. Kachi, Phys. Rev. B, 41 (1990) 4993. [7] M. Abbate, F.M.F. de Greet, J.C, Fuggle, Y.C. Ma, C.T. Chen, F. Sette, A. Fujlmori, Y. Ueda and K. Kosuge, Phys. Rev. B, 43 ( 1991 ) 7263. [8] A. Zylberstejn and N.F. Mort, Phys. Rev. B, !1 (1975) 4383. [9] D. Paquet and P. Lemux-Hugon, Phys. Rev. B, 22 (1980) 5284. [ 10] G.A. Savatzky and D. Post, Phys. Rev. B, 20 (1979) 1546. [ 1! ] C. Blaauw, F. Leenhouts, F. van der Woude and G.A. Savatzky, J. Phys. C: Solid State Phys., 8 (1975) 459. [ 12] A. Bianconi, Phys. Rev. B, 26 (1982) 2741. [ 13] C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.H. Raymond and L.H. Gale, Su~ inte~ Anal., 3 ( 1981 ) 211. [ 14l G.K. Wertheim, in D. Briggs and M.P. Sheah (eds.), PracticaISurface Analysis by Auger and X.ray Photoelectron Spectroscopy, Wiley, New York, 1983, p. 123. [ 15] H. LUth, Surfaces and Interfaces of Solids, Springer, New York, 1993. [ 16] E. Caruthers, L. Kleinman and H.I. Zhang, Phys. Rev. B, 7 (1973) 3753. [ 17] A.V. Nikolaev, Y.N. Kostrubov and B.V. Andreev, Soy. Phys. Solid State, 34 (1992) 1614. [ 18] S.M. Babulanam, T.S. Eriksson, G.A. Nicklasson and C.G. Granqvist, Sol. Energy Mater., 16 (1987) 347. [ 19] S. Lu, L. Hoe and F. Gan, J. Mater. Sci., 28 (1993) 2169. [20] T. Christmann, W. Kriegseis, A. Niessner, D. Schalch, A. Scharmann and M. Werling, Vak. Forsch. Praxis, 7 (1995) 257. [ 1] [2] [3] [4] [5] [6]