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In-situ Fourier transform infrared spectroscopy gas phase studies of vanadium (IV) oxide coating by atmospheric pressure chemical vapour deposition using vanadyl (IV) acetylacetonate
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Thin Solid Films 516 (2008) 4502 – 4507 www.elsevier.com/locate/tsf
In-situ Fourier transform infrared spectroscopy gas phase studies of vanadium (IV) oxide coating by atmospheric pressure chemical vapour deposition using vanadyl (IV) acetylacetonate D. Vernardou a , M.E. Pemble b,⁎, D.W. Sheel a a
Institute for Materials Research, University of Salford, Cockroft Building, Salford, Manchester, M5 4WT, UK b Tyndall National institute, Lee Maltings, Prospect Row, Cork, Ireland Available online 14 June 2007
Abstract This paper describes the use of in-situ Fourier transform infrared spectroscopy to monitor the gas phase reactions of the formation of VO2 thin films from VO(acac)2 under atmospheric pressure chemical vapour deposition conditions. In the absence of O2, it is found that anhydride species may form, while there is also some evidence of ester species. In the presence of O2, the spectra obtained are almost identical to those in the absence of O2. However in this case, there is also some indication for the enhanced production of CO and the suppression of the formation of C–H species. A possible mechanism for the formation of VO2 is proposed, which involves the release of two C3H4 molecules and the decomposition of vanadyl (IV) acetylacetonate into VO(CH3COO)2, which then further decomposes to yield (CH3CO)2O and VO2. However, while spectroscopic evidence for the formation of these species is presented, the mechanism proposed cannot be confirmed on the basis of these data alone. © 2007 Elsevier B.V. All rights reserved. Keywords: In-situ monitoring; Fourier transform infrared spectroscopy; Atmospheric pressure chemical vapour deposition; Vanadyl (IV) acetylacetonate; VO2
1. Introduction In spite of the many successes of chemical vapour deposition (CVD) in terms of its use for the growth of a wide range of technologically important coatings, the fundamental chemical mechanisms underlying CVD processes are generally only poorly understood. Since atmospheric pressure CVD (APCVD) growth is usually mass transport controlled, gas phase reactions are likely to play an important role in influencing film growth rates and other film characteristics. For this reason, a fundamental understanding of the mechanisms of gas phase precursor reactions and reactive intermediate production in CVD is ideally required in order to gain some insight into the mechanisms of film growth by APCVD. The characterization techniques used should ideally have a high time resolution and a high spatial resolution [1]. Some real-time studies of surface processes involved in various CVD reactions have been undertaken, although these have usually required the use of single crystal semiconducting substrates (e.g. reflectance ⁎ Corresponding author. E-mail address: [email protected] (M.E. Pemble). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.026
anisotropy spectroscopy [2–4], surface photo absorption [5,6], interferometry [7–9]). The need for rapid measurements is less critical when the CVD process in question is operated under pseudo-steady state conditions such as are normally employed, where once the growth reaction chemistry has been initiated, rapid variations in reactant and product concentrations would not be expected. Accurate gas phase measurements during the CVD process require non-invasive, in-situ monitoring techniques. Invasive, ex-situ techniques such as mass spectrometry [10] or gas chromatography [11] have been applied, and although extremely sensitive, cannot be relied upon to give an accurate picture of events occurring in-situ in the CVD reactor when gas samples must be extracted for the measurement to take place. Optical monitoring techniques are perhaps most suitable for this challenge and several methods have already been applied to the gas phase. Other vibrational spectroscopic techniques, such as Raman [12] are often too insensitive to detect gas phase species, especially on short time-scale (of the order of one second). However, non-linear Raman techniques such as coherent anti-stokes Raman spectroscopy [13] are showing considerable promise as point measurements can be made remotely. Furthermore, cavity ring-down spectroscopy
D. Vernardou et al. / Thin Solid Films 516 (2008) 4502–4507
[14] has been developed as a tool that is capable of measuring gas phase kinetics on the same time-scale as the loss from the cavity. Finally, ultraviolet and visible absorption spectroscopy, although widely used in an ex-situ manner either up-stream or down-stream of the reactor, suffer from broad, overlapping absorption peaks and are also limited by the fact that not all molecular species absorb in this spectral region [15]. Fourier transform infrared spectroscopy (FTIR) is probably the most versatile of all the available spectroscopic methods as the majority of the species of interest have IR active vibrations, and several species can be detected simultaneously [16–22]. Some previous examples of our work on the use of FTIR in the study of CVD and metalorganic CVD (MOCVD) processes include the decomposition of tertiary butylphospine (TBP), trimethyl indium (TMIn) and mixtures of TBP and TMIn [23]. A further example concerns the study of the decomposition of arsine at atmospheric pressure, which has been examined for gallium arsenide (GaAs) in a MOCVD reactor [24]. More recent work has focused on the use of FTIR methods to determine specific wavelengths of interest in the study of particular species involved in a CVD process, and then to monitor only at these wavelengths using diode laser spectroscopy in the near IR regime [25–27]. These methods possess the potential for rapid, real-time measurements of fast CVD processes. One of the APCVD processes that we are currently studying with FTIR methods is that of the formation of thermochromic vanadium (IV) oxide (VO2) thin films from vanadyl (IV) acetylacetonate (VO(acac)2). Thermochromic VO2 thin films have been proposed as possible low cost routes to active energy efficient glazing since they undergo a temperature dependent phase transition, which results in the higher temperature phase increasing its coefficient of reflection towards solar radiation in the IR region, without undergoing any major changes in terms of its visible optical properties. CVD routes to the production of such films may be particularly appropriate from an industrial perspective. With this in mind, we have been active in exploring such routes, since it is apparent that a good understanding of the fundamental chemistry occurring in the CVD processes is important to optimize film performance. In this paper we describe the use of in-situ FTIR gas phase studies to monitor the formation thermochromic monoclinic VO2 from VO(acac)2 under APCVD conditions, both in the absence and in the presence of an O2 flow. The role of O2 is specifically examined since in a recent study we have shown that for the APCVD growth of VO2 from VO(acac)2, it is necessary to introduce O2 to the reactive gas mixture in order to obtain single phase monoclinic VO2 [28]. In terms of previous studies of metal acetylacetonate complexes using IR methods, the IR spectra of metal (II) acetylacetonate and metal (III) acetylacetonate type complexes have been studied extensively [29–34]. Theoretical band assignments were first made by Nakamoto and Martell [29], who carried out normal coordinate analysis on the 1:1 model of copper (II) acetylacetonate. Mikami et al. [30] performed normal coordinate analysis on the 1:2 (square-planar) and 1:3 (octahedral) models of various acetylacetonate complexes. Other researchers have investigated the thermal decomposition
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of metal acetylacetonate compounds, including lanthanum (III) acetylacetonate [31], samarium (III) acetylacetonate [32], zinc (II) acetylacetonate [33], iron (III) acetylacetonate [33], zirconium (IV) acetylacetonate [34] and have found that the acetylacetonate ligand initially decomposes to give a metal ethanoate and propyne (C3H4) and then the ethanoate breaks down to yield metal oxycarbonate and acetone (C3H6O). In contrast, the decomposition of the VO(acac)2 complex has not been examined before. We present data which, although are far from conclusive, suggest that while a metal ethanoate may form in the decomposition of VO(acac)2 as seen for other metal acetylacetonate complexes, there is also evidence for the formation of ethanoic anhydride ((CH3CO)2O) during the decomposition process. 2. Experimental The APCVD reactor employed for the growth and in-situ monitoring of the films was of in-house design and construction, which is described in detail elsewhere [28]. Briefly, a flow of N2 gas was used as the process gas in all reactions, while high purity O2 gas was used as the oxidation source during some of the reactions. The precursor bubbler was heated at 200 °C, while the gas lines were heated at 220 °C to enhance transport of precursor and to prevent condensation in the lines. The deposition time was 10 min and the N2 flow rate through the bubbler was 1.4 L min− 1 for a total gas-flow rate of 12 L min− 1. The substrates used were commercial SiO2-precoated glass (Pilkington, UK), of dimensions 220 mm × 85 mm × 3 mm. Immediately before coating, all substrates were cleaned with H2O and detergent, rinsed thoroughly with H2O and deionised H2O, then allowed to dry. A set of in-situ monitoring experiments was performed by varying the substrate temperature from 200 °C to 600 °C using both 0 L min− 1 and 0.2 L min− 1 O2 flow rates. Fig. 1 shows a schematic presentation of the experiment where the IR beam falls on mirror 1, which is on the level of the machine. The beam is then directed upwards to mirror 2, which is on the level of the reactor. This directs the beam through windows 1 and 2 of the reactor tube, and then onto mirror 3, which focuses the beam onto the detector. In-situ monitoring measurements were performed using an Equinox 55 spectrometer under specific operating conditions: resolution 1 cm− 1, with 800 scans both for the acquisition of the sample and the background spectra from 500–4000 cm− 1 as
Fig. 1. Schematic representation of the set up employed for the FTIR monitoring experiments. The arrows indicate the direction of the IR beam.
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D. Vernardou et al. / Thin Solid Films 516 (2008) 4502–4507
3. Results and discussion
Fig. 2. FTIR spectrum of solid VO(acac)2 recorded at room temperature with the sample in the form of a pellet.
shown in Fig. 1. The beamsplitter was KBr. It was also necessary to use KBr windows, as the silica walls of the reactor are partially opaque to the wavelengths in question. We found that it was necessary to use heating tapes around the windows, maintained at a temperature of 200 °C in order to avoid condensation of the precursor on the KBr windows. An IR spectrum of the solid form of VO(acac)2 as pellet was also obtained using a Vector 22 FTIR with a resolution of 4 cm− 1, 30 scans both for the acquisition of the sample and the background spectra from 500 to 4000 cm− 1. All experiments were performed at least three times for consistency and reproducibility.
Fig. 2 depicts the spectrum of the VO(acac)2 precursor recorded from the solid precursor at room temperature with likely peak assignments. These agree with previous studies of compounds of the type metal (x) acetylacetonate, where x = 2 or 3 [29–34]. This spectrum was used in the analysis of the temperature-dependent spectra presented later. IR spectra of the gas phase present in the reactor obtained at different substrate temperatures, 250 °C, 400 °C and 600 °C, without the addition of O2 are shown in Fig. 3. It is apparent that even at the lowest temperature employed (250 °C, similar with the one at 200 °C), the spectrum obtained cannot be attributed to unreacted precursor alone. Table 1 lists the bands observed with some possible assignments, based on the spectrum shown in Fig. 2, together with the use of conventional IR group frequencies. The spectra depicted in Fig. 3 are quite clearly complex, and thus a method for the analysis of the spectra was devised aimed at analysing the spectra in a systematic manner. This method involved the labelling of each peak, the determination of peak area as a function of temperature, and then the correlation of the trend observed with those trends obtained for other peaks in the spectra (Fig. 4(a)–(d)). From Fig. 3, and a comparison with Figs. 2 and 4, it may be seen that the bands labelled A, B, C, E, F and H, all show very little variation in intensity with increasing temperature, up to ca. 500 °C–600 °C. These bands may all be attributed to either unreacted VO(acac)2 precursor or to species very similar in structure to the acetylacetonate ligand that contains the softened C–O bonds associated with this type of ligand operating in a bidentate fashion. In this respect we cannot determine whether or not the V atom is still present because by comparison with the spectrum of the solid precursor shown in Fig. 2, it is apparent that the V_O band seen in Fig. 2 would be too weak to be observed in Fig. 3, even if the precursor was intact. These bands are similar to those observed by other
Table 1 IR peak positions, relative intensities and possible assignments, for the decomposition of VO(acac)2 as a function of temperature, extracted from the spectra shown in Fig. 3 Band position/ cm− 1
Fig. 3. IR spectra at 250 °C, 400 °C and 600 °C with 0 L min− 1 O2 flow rate and a total gas-flow rate of 12 L min− 1 for the mid-IR region.
Label Intensity Possible assignment
1490–1550 A
s
1550–1610 B
s
1330–1450 C 3000–3400 D
m vw
1250–1300 E 1130–1230 F
w m
1730–1850 G
m
1000–1050 H
m
C–O stretching, bidentate carboxylic acid grouping, where the carbonyl stretch is softened C–O stretching, bidentate carboxylic acid grouping, where the carbonyl stretch is softened CH2 or CH3 deformation C–H stretching vibrations, broad features associated with alkynes or carboxylic acid derivatives C–C stretch or C–H deformation C–O and/or C–C asymmetric stretch, carbonyl species Carbonyl vibrations, characteristic of an anhydride species C–O stretching vibrations
s = strong, m = medium, w = weak, vw = very weak.
D. Vernardou et al. / Thin Solid Films 516 (2008) 4502–4507
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Fig. 4. Area under the peaks as a function of temperature (200–600 °C) for bands A and B (a), C (b), E and H (c), F (d) with 0 L min− 1 O2 flow rate for a total gas-flow rate of 12 L min− 1.
researchers, who have assigned these features to ethanoate species [31–34]. The only new bands detected (apart from those attributed to the miscancellation of bands due to atmospheric gases) are bands G and D. Band G lies in the spectral region associated with a carbonyl species, usually one found in acid anhydride species. Although difficult to be certain, it appears that there may be two bands present in this region. This would be entirely consistent with the formation of an anhydride species. Band D would also be consistent with this assignment since it would be assigned to the characteristic broad C–H stretching modes associated with species such as (CH3CO)2O, although the behaviour of this band with increasing temperature does not appear to be exactly the same as that observed for band G. In this respect it is noteworthy that band D appears most prominently in the spectrum recorded at 400 °C. This spectrum also shows the appearance of a vibration–rotation spectrum for a small, linear species, as evidenced by the weak P and R branches centred near 2145 cm− 1. This band is assigned to the formation of carbon monoxide (CO) [35] resulting from the decomposition of either the VO(acac)2 itself or the decomposition of possible products arising from the original precursor such as the ethanoate species or the anhydride species.
Strong OH stretching bands due to H2O [35] as well as carbon dioxide (CO2) [35] stretching bands were seen at all substrate temperatures as shown in Fig. 3. Regarding CO2 it is observed that some of the peaks have different direction than the rest on the spectra at 200 °C, 250 °C and 400 °C at 690 cm− 1 and 2375 cm− 1. These are caused by small fluctuations in the external gas environment, which the IR beam passes through, leading to miscancellations between background and sample spectra. As a result of these interferences it was not possible to ascertain whether H2O and CO2 are detected as decomposition products. Fig. 5 depicts the IR spectra of the gas phase recorded from the CVD reactor in the presence of VO(acac)2 using an O2 flow of 0.2 L min− 1 as a function of temperature, 250 °C, 400 °C and 600 °C. The spectra obtained are clearly very similar to those obtained in the absence of O2, shown in Fig. 3. Bands A, B, C, E, F and H have the same trend, in that the area under the peaks stays essentially constant with increasing temperature until ca. 500 °C, after which a decrease in intensity is observed. This behaviour correlates well with the behaviour of the similarly labelled bands shown in Fig. 3 and as a result, these peaks are again assigned to either unreacted VO(acac)2 precursor or to species similar in structure to the acetylacetonate complex, such as vanadyl ethanoate (VO(CH3COO)2).
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has also been observed for the decomposition of other acetylacetonate complexes [31–34]. This is then followed by the decomposition of VO(CH3COO)2 and further reaction of this species to yield (CH3CO)2O and VO2: VOðC5 H7 O2 Þ2 →VOðCH3 COOÞðC5 H7 O2 Þ þ C3 H4
ðiÞ
VOðCH3 COOÞðC5 H7 O2 Þ→VOðCH3 COOÞ2 þ C3 H4
ðiiÞ
VOðCH3 COOÞ2 →ðCH3 COÞ2 O þ VO2
ðiiiÞ
Although apparently simple, it is not clear at all whether such a mechanism actually occurs and much more work is necessary in order to unambiguously identify the true mechanism that operates for this particular CVD process. 4. Conclusions
Fig. 5. IR spectra at 250 °C, 400 °C and 600 °C using 0.2 L min− 1 O2 flow rate and 12 L min− 1 total gas-flow rate for the mid-IR region.
As compared to the spectrum of the unreacted precursor (Fig. 2) the only new features are again that of band G, previously assigned to the split carbonyl bands associated with an anhydride species, possibly (CH3CO)2O and band D, previously assigned to the C–H stretching vibrations of either the ethanoate or the anhydride species, which is significantly suppressed. This is in contrast to the spectra recorded in the absence of O2. Also evident from a comparison of Figs. 3 and 5 is that in the presence of O2, the production of CO appears to be enhanced, with the small vibration–rotation band near 2145 cm− 1 being significantly more prominent in Fig. 5 as compared to Fig. 3. Overall by comparison with Fig. 3, the data in Fig. 5 suggest that addition of O2 does not result in any dramatic change in the observed chemistry, other than a suppression of species containing C–H bonds and a possible enhancement in the production of CO. The rest of the spectrum is largely unchanged. It is therefore possible that, in the presence of O2, some oxidation of reaction products occurs possibly catalysed by the VO2 deposits themselves [36]. However, the differences in the spectra are small and given the nature of the run-to-run variations that are always observed in this type of experiment, they cannot be regarded as reliable without further corroboration. Given the uncertainty in the band assignments it is not possible to define a mechanism for the decomposition of VO (acac)2 to VO2 based solely on the spectra obtained. However, given that the only bands observed in the spectra that cannot be explained in terms of the vibrations of the original precursor would be consistent with the formation of an acid anhydride, it is possible to speculate as to possible decomposition routes for the VO(acac)2 species: Eqs. (i)–(iii) reveal how VO(CH3COO)2 may first form via a simple intramolecular rearrangement of the VO(acac)2 precursor resulting in the release of two C3H4 molecules. In this respect it is noteworthy that C3H4 formation
The gas phase chemistry of the VO2 formation from VO (acac)2 with and without the presence of O2 has been studied using in-situ FTIR in an APCVD reactor. For the case where O2 is not present in the reactor, apart from bands attributable to the original precursor itself or very similar species, bands associated with the formation of anhydride species have been detected. In the presence of O2 the spectra are essentially unchanged apart from some evidence for enhanced CO production and the possible suppression of C–H containing species. A possible mechanism for the formation of VO2, has been proposed. This involves as a first step the elimination of C3H4, a step that has been observed for acetylacetonate complexes previously [31–34] to yield an ester species, possibly VO (CH3COO)2. Secondly this species subsequently decomposes to yield an acid anhydride, most likely (CH3CO)2O and VO2. Acknowledgements The authors would like to thank the EPSRC for financial support, Pilkington Glass, UK for supply of the glass substrates, Dr Martin Davies and Dr Tom Paterson for the set up of the system as well as for the useful discussions. References [1] R.J. Holdsworth, P.A. Martin, D. Raisbeck, J. Rivero, H.E. Sanders, D. Sheel, M.E. Pemble, Chem. Vap. Depos. 7 (2001) 39. [2] M.N. Simcock, L. He, M.E. Pemble, J. Phys., IV 12 (2002) Pr4-25. [3] H. Juergensen, Mater. Sci. Semicond. Process. 4 (6) (2001) 467. [4] G. Apostolopoulos, J. Herfort, W. Ulrici, L. Daweritz, Inst. Phys. Conf. Ser. 166 (2000) 31. [5] Y. Kobayashi, N. Kobayashi, Jpn. J. Appl. Phys., Part 1 31 (1992) 3988. [6] N. Kobayashi, Y. Kobayashi, K. Uwai, J. Cryst. Growth 170 (1997) 225. [7] K.P. Killeen, W.G. Breiland, J. Electron. Mater. 23 (1994) 179. [8] S.J.C. Irvine, A. Hartley, A. Stafford, J. Cryst. Growth 221 (2000) 117. [9] A. Stafford, S.J.C. Irvine, K.L. Hess, J. Bajaj, J. Electron. Mater. 28 (1999) 712. [10] V.S. Ban, J. Cryst. Growth 17 (1972) 19. [11] J. Heinrich, S. Hemeltjen, G. Marx, Mikrochim. Acta 133 (2000) 209. [12] J. Smith Jr, E. Sedwick, Thin Solid Films 40 (1977) 1.
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E. Demirba, R. Devonshire, Vib. Spectrosc. 37 (2005) 141. A.J. Alexander, Chem. Phys. Lett. 393 (2004) 138. W.G. Tong, R.W. Shaw, Appl. Spectrosc. 40 (1986) 494. C. Giunta, D. Strickler, R. Gordon, J. Phys. Chem. 97 (1993) 2275. J. Slifirski, G. Huchet, A. Reynes, A. Marty, F. Teyssandier, Chem. Mater. 7 (1995) 622. D.R.T. Zahn, Phys. Status Solidi 152 (1995) 179. Y. Gao, Thin Solid Films 346 (1999) 73. V. Hopfe, W. Grahlert, O. Throl, J. Phys., IV 9 (1999) 995. V. Hopfe, D.W. Sheel, C.I.M.A. Spee, R. Tell, P. Martin, A. Beil, M.E. Pemble, R. Weiss, U. Vogt, W. Graehlert, Thin Solid Films 442 (2003) 60. V. Hopfe, D.W. Sheel, W. Graehlert, O. Throl, Surf. Coat. Technol. 142 (2001) 328. G.H. Fan, R.D. Hoare, M.E. Pemble, I.M. Povey, A.G. Taylor, J.O. Williams, J. Cryst. Growth 124 (1992) 49. S.R. Armstrong, R.D. Hoare, M.E. Pemble, I.M. Povey, A. Stafford, A.G. Taylor, J.O. Williams, J. Cryst. Growth 124 (1992) 10. J.E. Butler, N. Bottka, R.S. Sillmon, D.K. Gaskill, J. Cryst. Growth 77 (1986) 163.
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[26] R.J. Holdsworth, P.A. Martin, D. Raisbeck, M.E. Pemble, J. Phys., IV 9 (1999) Pr8-109. [27] V. Hopfe, D.W. Sheel, D. Raisbeck, J.M. Rivero, W. Graehlert, O. Throl, A.M.B. van Mol, C.I.M.A. Spee, J. Phys., IV 11 (2001) Pr3-1153. [28] D. Vernardou, M.E. Pemble, D.W. Sheel, Chem. Vap. Depos. 12 (2006) 263. [29] K. Nakamoto, A.E. Martell, J. Chem. Phys. 32 (1960) 588. [30] M. Mikami, I. Nakagawa, T. Shimanouchi, Spectrochim. Acta 23 (A) (1967) 1037. [31] G.A.M. Hussein, H.M. Ismail, Powder Technol. 84 (1995) 185. [32] H.M. Ismail, Colloids Surf., A 97 (1995) 247. [33] H.M. Ismail, J. Anal. Appl. Pyrolysis 21 (1991) 315. [34] H.M. Ismail, Powder Technol. 85 (1995) 253. [35] C.N. Banwell, Fundamentals of Molecular Spectroscopy, 2nd Edn. McGraw-Hill Book Company Ltd, London, 1972. [36] E.V. Kondratenko, O. Ovsitser, J. Radnik, M. Schneider, R. Kraehnert, U. Dingerdissen, Appl. Catal., A Gen. 319 (2007) 98.
Thin Solid Films 516 (2008) 4502 – 4507 www.elsevier.com/locate/tsf
In-situ Fourier transform infrared spectroscopy gas phase studies of vanadium (IV) oxide coating by atmospheric pressure chemical vapour deposition using vanadyl (IV) acetylacetonate D. Vernardou a , M.E. Pemble b,⁎, D.W. Sheel a a
Institute for Materials Research, University of Salford, Cockroft Building, Salford, Manchester, M5 4WT, UK b Tyndall National institute, Lee Maltings, Prospect Row, Cork, Ireland Available online 14 June 2007
Abstract This paper describes the use of in-situ Fourier transform infrared spectroscopy to monitor the gas phase reactions of the formation of VO2 thin films from VO(acac)2 under atmospheric pressure chemical vapour deposition conditions. In the absence of O2, it is found that anhydride species may form, while there is also some evidence of ester species. In the presence of O2, the spectra obtained are almost identical to those in the absence of O2. However in this case, there is also some indication for the enhanced production of CO and the suppression of the formation of C–H species. A possible mechanism for the formation of VO2 is proposed, which involves the release of two C3H4 molecules and the decomposition of vanadyl (IV) acetylacetonate into VO(CH3COO)2, which then further decomposes to yield (CH3CO)2O and VO2. However, while spectroscopic evidence for the formation of these species is presented, the mechanism proposed cannot be confirmed on the basis of these data alone. © 2007 Elsevier B.V. All rights reserved. Keywords: In-situ monitoring; Fourier transform infrared spectroscopy; Atmospheric pressure chemical vapour deposition; Vanadyl (IV) acetylacetonate; VO2
1. Introduction In spite of the many successes of chemical vapour deposition (CVD) in terms of its use for the growth of a wide range of technologically important coatings, the fundamental chemical mechanisms underlying CVD processes are generally only poorly understood. Since atmospheric pressure CVD (APCVD) growth is usually mass transport controlled, gas phase reactions are likely to play an important role in influencing film growth rates and other film characteristics. For this reason, a fundamental understanding of the mechanisms of gas phase precursor reactions and reactive intermediate production in CVD is ideally required in order to gain some insight into the mechanisms of film growth by APCVD. The characterization techniques used should ideally have a high time resolution and a high spatial resolution [1]. Some real-time studies of surface processes involved in various CVD reactions have been undertaken, although these have usually required the use of single crystal semiconducting substrates (e.g. reflectance ⁎ Corresponding author. E-mail address: [email protected] (M.E. Pemble). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.06.026
anisotropy spectroscopy [2–4], surface photo absorption [5,6], interferometry [7–9]). The need for rapid measurements is less critical when the CVD process in question is operated under pseudo-steady state conditions such as are normally employed, where once the growth reaction chemistry has been initiated, rapid variations in reactant and product concentrations would not be expected. Accurate gas phase measurements during the CVD process require non-invasive, in-situ monitoring techniques. Invasive, ex-situ techniques such as mass spectrometry [10] or gas chromatography [11] have been applied, and although extremely sensitive, cannot be relied upon to give an accurate picture of events occurring in-situ in the CVD reactor when gas samples must be extracted for the measurement to take place. Optical monitoring techniques are perhaps most suitable for this challenge and several methods have already been applied to the gas phase. Other vibrational spectroscopic techniques, such as Raman [12] are often too insensitive to detect gas phase species, especially on short time-scale (of the order of one second). However, non-linear Raman techniques such as coherent anti-stokes Raman spectroscopy [13] are showing considerable promise as point measurements can be made remotely. Furthermore, cavity ring-down spectroscopy
D. Vernardou et al. / Thin Solid Films 516 (2008) 4502–4507
[14] has been developed as a tool that is capable of measuring gas phase kinetics on the same time-scale as the loss from the cavity. Finally, ultraviolet and visible absorption spectroscopy, although widely used in an ex-situ manner either up-stream or down-stream of the reactor, suffer from broad, overlapping absorption peaks and are also limited by the fact that not all molecular species absorb in this spectral region [15]. Fourier transform infrared spectroscopy (FTIR) is probably the most versatile of all the available spectroscopic methods as the majority of the species of interest have IR active vibrations, and several species can be detected simultaneously [16–22]. Some previous examples of our work on the use of FTIR in the study of CVD and metalorganic CVD (MOCVD) processes include the decomposition of tertiary butylphospine (TBP), trimethyl indium (TMIn) and mixtures of TBP and TMIn [23]. A further example concerns the study of the decomposition of arsine at atmospheric pressure, which has been examined for gallium arsenide (GaAs) in a MOCVD reactor [24]. More recent work has focused on the use of FTIR methods to determine specific wavelengths of interest in the study of particular species involved in a CVD process, and then to monitor only at these wavelengths using diode laser spectroscopy in the near IR regime [25–27]. These methods possess the potential for rapid, real-time measurements of fast CVD processes. One of the APCVD processes that we are currently studying with FTIR methods is that of the formation of thermochromic vanadium (IV) oxide (VO2) thin films from vanadyl (IV) acetylacetonate (VO(acac)2). Thermochromic VO2 thin films have been proposed as possible low cost routes to active energy efficient glazing since they undergo a temperature dependent phase transition, which results in the higher temperature phase increasing its coefficient of reflection towards solar radiation in the IR region, without undergoing any major changes in terms of its visible optical properties. CVD routes to the production of such films may be particularly appropriate from an industrial perspective. With this in mind, we have been active in exploring such routes, since it is apparent that a good understanding of the fundamental chemistry occurring in the CVD processes is important to optimize film performance. In this paper we describe the use of in-situ FTIR gas phase studies to monitor the formation thermochromic monoclinic VO2 from VO(acac)2 under APCVD conditions, both in the absence and in the presence of an O2 flow. The role of O2 is specifically examined since in a recent study we have shown that for the APCVD growth of VO2 from VO(acac)2, it is necessary to introduce O2 to the reactive gas mixture in order to obtain single phase monoclinic VO2 [28]. In terms of previous studies of metal acetylacetonate complexes using IR methods, the IR spectra of metal (II) acetylacetonate and metal (III) acetylacetonate type complexes have been studied extensively [29–34]. Theoretical band assignments were first made by Nakamoto and Martell [29], who carried out normal coordinate analysis on the 1:1 model of copper (II) acetylacetonate. Mikami et al. [30] performed normal coordinate analysis on the 1:2 (square-planar) and 1:3 (octahedral) models of various acetylacetonate complexes. Other researchers have investigated the thermal decomposition
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of metal acetylacetonate compounds, including lanthanum (III) acetylacetonate [31], samarium (III) acetylacetonate [32], zinc (II) acetylacetonate [33], iron (III) acetylacetonate [33], zirconium (IV) acetylacetonate [34] and have found that the acetylacetonate ligand initially decomposes to give a metal ethanoate and propyne (C3H4) and then the ethanoate breaks down to yield metal oxycarbonate and acetone (C3H6O). In contrast, the decomposition of the VO(acac)2 complex has not been examined before. We present data which, although are far from conclusive, suggest that while a metal ethanoate may form in the decomposition of VO(acac)2 as seen for other metal acetylacetonate complexes, there is also evidence for the formation of ethanoic anhydride ((CH3CO)2O) during the decomposition process. 2. Experimental The APCVD reactor employed for the growth and in-situ monitoring of the films was of in-house design and construction, which is described in detail elsewhere [28]. Briefly, a flow of N2 gas was used as the process gas in all reactions, while high purity O2 gas was used as the oxidation source during some of the reactions. The precursor bubbler was heated at 200 °C, while the gas lines were heated at 220 °C to enhance transport of precursor and to prevent condensation in the lines. The deposition time was 10 min and the N2 flow rate through the bubbler was 1.4 L min− 1 for a total gas-flow rate of 12 L min− 1. The substrates used were commercial SiO2-precoated glass (Pilkington, UK), of dimensions 220 mm × 85 mm × 3 mm. Immediately before coating, all substrates were cleaned with H2O and detergent, rinsed thoroughly with H2O and deionised H2O, then allowed to dry. A set of in-situ monitoring experiments was performed by varying the substrate temperature from 200 °C to 600 °C using both 0 L min− 1 and 0.2 L min− 1 O2 flow rates. Fig. 1 shows a schematic presentation of the experiment where the IR beam falls on mirror 1, which is on the level of the machine. The beam is then directed upwards to mirror 2, which is on the level of the reactor. This directs the beam through windows 1 and 2 of the reactor tube, and then onto mirror 3, which focuses the beam onto the detector. In-situ monitoring measurements were performed using an Equinox 55 spectrometer under specific operating conditions: resolution 1 cm− 1, with 800 scans both for the acquisition of the sample and the background spectra from 500–4000 cm− 1 as
Fig. 1. Schematic representation of the set up employed for the FTIR monitoring experiments. The arrows indicate the direction of the IR beam.
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3. Results and discussion
Fig. 2. FTIR spectrum of solid VO(acac)2 recorded at room temperature with the sample in the form of a pellet.
shown in Fig. 1. The beamsplitter was KBr. It was also necessary to use KBr windows, as the silica walls of the reactor are partially opaque to the wavelengths in question. We found that it was necessary to use heating tapes around the windows, maintained at a temperature of 200 °C in order to avoid condensation of the precursor on the KBr windows. An IR spectrum of the solid form of VO(acac)2 as pellet was also obtained using a Vector 22 FTIR with a resolution of 4 cm− 1, 30 scans both for the acquisition of the sample and the background spectra from 500 to 4000 cm− 1. All experiments were performed at least three times for consistency and reproducibility.
Fig. 2 depicts the spectrum of the VO(acac)2 precursor recorded from the solid precursor at room temperature with likely peak assignments. These agree with previous studies of compounds of the type metal (x) acetylacetonate, where x = 2 or 3 [29–34]. This spectrum was used in the analysis of the temperature-dependent spectra presented later. IR spectra of the gas phase present in the reactor obtained at different substrate temperatures, 250 °C, 400 °C and 600 °C, without the addition of O2 are shown in Fig. 3. It is apparent that even at the lowest temperature employed (250 °C, similar with the one at 200 °C), the spectrum obtained cannot be attributed to unreacted precursor alone. Table 1 lists the bands observed with some possible assignments, based on the spectrum shown in Fig. 2, together with the use of conventional IR group frequencies. The spectra depicted in Fig. 3 are quite clearly complex, and thus a method for the analysis of the spectra was devised aimed at analysing the spectra in a systematic manner. This method involved the labelling of each peak, the determination of peak area as a function of temperature, and then the correlation of the trend observed with those trends obtained for other peaks in the spectra (Fig. 4(a)–(d)). From Fig. 3, and a comparison with Figs. 2 and 4, it may be seen that the bands labelled A, B, C, E, F and H, all show very little variation in intensity with increasing temperature, up to ca. 500 °C–600 °C. These bands may all be attributed to either unreacted VO(acac)2 precursor or to species very similar in structure to the acetylacetonate ligand that contains the softened C–O bonds associated with this type of ligand operating in a bidentate fashion. In this respect we cannot determine whether or not the V atom is still present because by comparison with the spectrum of the solid precursor shown in Fig. 2, it is apparent that the V_O band seen in Fig. 2 would be too weak to be observed in Fig. 3, even if the precursor was intact. These bands are similar to those observed by other
Table 1 IR peak positions, relative intensities and possible assignments, for the decomposition of VO(acac)2 as a function of temperature, extracted from the spectra shown in Fig. 3 Band position/ cm− 1
Fig. 3. IR spectra at 250 °C, 400 °C and 600 °C with 0 L min− 1 O2 flow rate and a total gas-flow rate of 12 L min− 1 for the mid-IR region.
Label Intensity Possible assignment
1490–1550 A
s
1550–1610 B
s
1330–1450 C 3000–3400 D
m vw
1250–1300 E 1130–1230 F
w m
1730–1850 G
m
1000–1050 H
m
C–O stretching, bidentate carboxylic acid grouping, where the carbonyl stretch is softened C–O stretching, bidentate carboxylic acid grouping, where the carbonyl stretch is softened CH2 or CH3 deformation C–H stretching vibrations, broad features associated with alkynes or carboxylic acid derivatives C–C stretch or C–H deformation C–O and/or C–C asymmetric stretch, carbonyl species Carbonyl vibrations, characteristic of an anhydride species C–O stretching vibrations
s = strong, m = medium, w = weak, vw = very weak.
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Fig. 4. Area under the peaks as a function of temperature (200–600 °C) for bands A and B (a), C (b), E and H (c), F (d) with 0 L min− 1 O2 flow rate for a total gas-flow rate of 12 L min− 1.
researchers, who have assigned these features to ethanoate species [31–34]. The only new bands detected (apart from those attributed to the miscancellation of bands due to atmospheric gases) are bands G and D. Band G lies in the spectral region associated with a carbonyl species, usually one found in acid anhydride species. Although difficult to be certain, it appears that there may be two bands present in this region. This would be entirely consistent with the formation of an anhydride species. Band D would also be consistent with this assignment since it would be assigned to the characteristic broad C–H stretching modes associated with species such as (CH3CO)2O, although the behaviour of this band with increasing temperature does not appear to be exactly the same as that observed for band G. In this respect it is noteworthy that band D appears most prominently in the spectrum recorded at 400 °C. This spectrum also shows the appearance of a vibration–rotation spectrum for a small, linear species, as evidenced by the weak P and R branches centred near 2145 cm− 1. This band is assigned to the formation of carbon monoxide (CO) [35] resulting from the decomposition of either the VO(acac)2 itself or the decomposition of possible products arising from the original precursor such as the ethanoate species or the anhydride species.
Strong OH stretching bands due to H2O [35] as well as carbon dioxide (CO2) [35] stretching bands were seen at all substrate temperatures as shown in Fig. 3. Regarding CO2 it is observed that some of the peaks have different direction than the rest on the spectra at 200 °C, 250 °C and 400 °C at 690 cm− 1 and 2375 cm− 1. These are caused by small fluctuations in the external gas environment, which the IR beam passes through, leading to miscancellations between background and sample spectra. As a result of these interferences it was not possible to ascertain whether H2O and CO2 are detected as decomposition products. Fig. 5 depicts the IR spectra of the gas phase recorded from the CVD reactor in the presence of VO(acac)2 using an O2 flow of 0.2 L min− 1 as a function of temperature, 250 °C, 400 °C and 600 °C. The spectra obtained are clearly very similar to those obtained in the absence of O2, shown in Fig. 3. Bands A, B, C, E, F and H have the same trend, in that the area under the peaks stays essentially constant with increasing temperature until ca. 500 °C, after which a decrease in intensity is observed. This behaviour correlates well with the behaviour of the similarly labelled bands shown in Fig. 3 and as a result, these peaks are again assigned to either unreacted VO(acac)2 precursor or to species similar in structure to the acetylacetonate complex, such as vanadyl ethanoate (VO(CH3COO)2).
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has also been observed for the decomposition of other acetylacetonate complexes [31–34]. This is then followed by the decomposition of VO(CH3COO)2 and further reaction of this species to yield (CH3CO)2O and VO2: VOðC5 H7 O2 Þ2 →VOðCH3 COOÞðC5 H7 O2 Þ þ C3 H4
ðiÞ
VOðCH3 COOÞðC5 H7 O2 Þ→VOðCH3 COOÞ2 þ C3 H4
ðiiÞ
VOðCH3 COOÞ2 →ðCH3 COÞ2 O þ VO2
ðiiiÞ
Although apparently simple, it is not clear at all whether such a mechanism actually occurs and much more work is necessary in order to unambiguously identify the true mechanism that operates for this particular CVD process. 4. Conclusions
Fig. 5. IR spectra at 250 °C, 400 °C and 600 °C using 0.2 L min− 1 O2 flow rate and 12 L min− 1 total gas-flow rate for the mid-IR region.
As compared to the spectrum of the unreacted precursor (Fig. 2) the only new features are again that of band G, previously assigned to the split carbonyl bands associated with an anhydride species, possibly (CH3CO)2O and band D, previously assigned to the C–H stretching vibrations of either the ethanoate or the anhydride species, which is significantly suppressed. This is in contrast to the spectra recorded in the absence of O2. Also evident from a comparison of Figs. 3 and 5 is that in the presence of O2, the production of CO appears to be enhanced, with the small vibration–rotation band near 2145 cm− 1 being significantly more prominent in Fig. 5 as compared to Fig. 3. Overall by comparison with Fig. 3, the data in Fig. 5 suggest that addition of O2 does not result in any dramatic change in the observed chemistry, other than a suppression of species containing C–H bonds and a possible enhancement in the production of CO. The rest of the spectrum is largely unchanged. It is therefore possible that, in the presence of O2, some oxidation of reaction products occurs possibly catalysed by the VO2 deposits themselves [36]. However, the differences in the spectra are small and given the nature of the run-to-run variations that are always observed in this type of experiment, they cannot be regarded as reliable without further corroboration. Given the uncertainty in the band assignments it is not possible to define a mechanism for the decomposition of VO (acac)2 to VO2 based solely on the spectra obtained. However, given that the only bands observed in the spectra that cannot be explained in terms of the vibrations of the original precursor would be consistent with the formation of an acid anhydride, it is possible to speculate as to possible decomposition routes for the VO(acac)2 species: Eqs. (i)–(iii) reveal how VO(CH3COO)2 may first form via a simple intramolecular rearrangement of the VO(acac)2 precursor resulting in the release of two C3H4 molecules. In this respect it is noteworthy that C3H4 formation
The gas phase chemistry of the VO2 formation from VO (acac)2 with and without the presence of O2 has been studied using in-situ FTIR in an APCVD reactor. For the case where O2 is not present in the reactor, apart from bands attributable to the original precursor itself or very similar species, bands associated with the formation of anhydride species have been detected. In the presence of O2 the spectra are essentially unchanged apart from some evidence for enhanced CO production and the possible suppression of C–H containing species. A possible mechanism for the formation of VO2, has been proposed. This involves as a first step the elimination of C3H4, a step that has been observed for acetylacetonate complexes previously [31–34] to yield an ester species, possibly VO (CH3COO)2. Secondly this species subsequently decomposes to yield an acid anhydride, most likely (CH3CO)2O and VO2. Acknowledgements The authors would like to thank the EPSRC for financial support, Pilkington Glass, UK for supply of the glass substrates, Dr Martin Davies and Dr Tom Paterson for the set up of the system as well as for the useful discussions. References [1] R.J. Holdsworth, P.A. Martin, D. Raisbeck, J. Rivero, H.E. Sanders, D. Sheel, M.E. Pemble, Chem. Vap. Depos. 7 (2001) 39. [2] M.N. Simcock, L. He, M.E. Pemble, J. Phys., IV 12 (2002) Pr4-25. [3] H. Juergensen, Mater. Sci. Semicond. Process. 4 (6) (2001) 467. [4] G. Apostolopoulos, J. Herfort, W. Ulrici, L. Daweritz, Inst. Phys. Conf. Ser. 166 (2000) 31. [5] Y. Kobayashi, N. Kobayashi, Jpn. J. Appl. Phys., Part 1 31 (1992) 3988. [6] N. Kobayashi, Y. Kobayashi, K. Uwai, J. Cryst. Growth 170 (1997) 225. [7] K.P. Killeen, W.G. Breiland, J. Electron. Mater. 23 (1994) 179. [8] S.J.C. Irvine, A. Hartley, A. Stafford, J. Cryst. Growth 221 (2000) 117. [9] A. Stafford, S.J.C. Irvine, K.L. Hess, J. Bajaj, J. Electron. Mater. 28 (1999) 712. [10] V.S. Ban, J. Cryst. Growth 17 (1972) 19. [11] J. Heinrich, S. Hemeltjen, G. Marx, Mikrochim. Acta 133 (2000) 209. [12] J. Smith Jr, E. Sedwick, Thin Solid Films 40 (1977) 1.
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