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Thin films of FeVO4 obtained by annealing under room atmosphere of Fe and V layers sequentially deposited
Materials Chemistry and Physics 63 (2000) 208±212
Thin ®lms of FeVO4 obtained by annealing under room atmosphere of Fe and V layers sequentially deposited a
E. Baba Alia, J.C. BerneÁdea,*, A. Barreaub
Equipe couches minces et mateÂriaux nouveaux, E.P.S.E.-F.S.T.N. 2, rue de la HoussinieÁre, BP 92208, 44322 Nantes, Cedex 3, France Centre Commun de Microscopie Electronique aÁ Balayage F.S.T.N. 2, rue de la HoussinieÁre, BP 92208, 44322 Nantes, Cedex 3, France
b
Received 11 May 1999; received in revised form 21 July 1999; accepted 28 September 1999
Abstract Metallic thin ®lms Fe/V/Fe/V... /V sequentially deposited under vacuum have been annealed half an hour in room atmosphere. The samples have been characterised by X-rays diffraction (XRD), scanning electron microscopy (SEM), microprobe analysis (EDX), X-ray photoelectron spectroscopy (XPS) and optical absorption. The annealing temperature and the under layer material have been taken as parameters. The results have shown that triclinic phase of FeVO4, homogeneous thin ®lms can be synthesised by this technique. The best results have been obtained when the glass substrate used was coated with SnO2 thin ®lm. The results were improved when, before air-annealing, a ®rst annealing under vacuum was proceeded in order to homogenise the metal distribution in the sample. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Metallic thin ®lms; Room atmosphere; X-rays diffraction; FeVO4; Annealing treatment
1. Introduction
2. Experimental techniques
One of the prospective applications for solid-state lithium batteries would be as on-chip power backup, where the battery is an integral part of an integrated circuit device. Such batteries should be made with conventional thin ®lm techniques such as vapour deposition or sputtering in order to meet the production requirements of microelectronics industry. In general, the polycrystalline ®lms show electrochemical performance exceeding that of bulk materials [1,2]. Authors have recently opened new avenues of research towards this goal by showing that several Li-vanadate-based electrodes, when discharged to voltages lower than 0.2 V, can reversibly intercalate up to 7 Li leading to 800± 900 Ah kgÿ1 capacities, about two and a half times greater than present graphite electrodes [3]. With its open framework structure, bulk FeVO4 [4±8] for instance, was initially studied and seen as potential candidate for cathode materials [9]. In the present report, we shows that a very simple physical vapour deposition technique followed by an annealing in room air can be used to grow such oxide in thin ®lm form.
The substrates used were bare glass and SnO2 coated glass. Metallic thin ®lms Fe/V/Fe/V.../V/Fe , sequentially deposited under vacuum by electron beam evaporation (Edwards), have been annealed at ®rst in secondary vacuum at 850 K and then in room atmosphere at 863 K for a quarter of an hour and half an hour, respectively. Structural investigations were carried out with an X-ray diffractometer (DIFFRACT AT V3.1 SIEMENS), using monochromatic CuKa radiation.1 Detailed morphological analysis of the ®lms was carried out by scanning electron microscopy (SEM) using a JEOL 6400F ®eld-emission scanning electron microscope. Electron microscope, the JEOL 5800LV, equipped with a PGT X-rays microanalysis system, in which X-rays were detected by a germanium crystal has been used for quantitative microprobe analysis. The XPS measurements were performed with a magnesium X-ray source (1253.6 eV) operating at 10 kV and 10 mA. Data acquisition and treatment were realised through a computer and a standard program. The quantitative studies were based on the determination of the O1s, V2p3/2, Fe2p3/2 peak areas. The homogeneity in depth of the
* Corresponding author. Tel.: 33-0251-1255-30; fax: 33-0251-135528. E-mail address: [email protected] (J.C. BerneÁde).
1 XRD measurments have been done at the Institute of Materials of Nantes.
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 2 0 - 5
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
209
Fig. 1. XRD diagram of FeVO4 film first annealing under vacuum and second annealing in room atmosphere.
®lms was checked by recording successive XPS spectra obtained after argon ion etching for short periods. Sputtering was accomplished at pressures of less than 5 10ÿ4 Pa a 10 mA emission current and a 3 kV beam energy using an ion gun. With these experimental conditions all the surface of the samples was sputtered.2 3. Experimental results After annealing the ®lms, which had metallic appearance Ð highly re¯exive and grey coloured Ð after deposition, were brown red coloured with a granular morphology visible with naked eyes. It can also be seen that the ®lms obtained on bare glass substrates are highly inhomogeneous with a lot of pinholes, cracks and voids visible, while ®lms grown on SnO2 coated substrates were homogeneous. Therefore, only the latter ®lms have been carefully studied. Moreover, the peaks present in the X-ray diagrams (XRD) are more intensive when, before air-annealing, the samples were heated half an hour under vacuum. This annealing allows to homogenise the metallic ®lms, by interdiffusion. A typical XRD diagram obtained from such annealed sample is presented in Fig. 1. For comparison the diagram of a reference powder is presented in Fig. 2. Moreover, in Fig. 1 is reported the SnO2 typical diagram issued form JCPDS data (5-0467) in order to discriminate between the SnO2 peaks and those of FeVO4. When the diffractograms of Fig. 1 are compared to reference data (JCPDS no 25-0418 and reference powder Fig. 2) it can be seen that all the more
2 XPS measurments have been done with a Leybold spectrometer (University of Nantes-CNRS).
intensive peaks are present in the diagram with the expected relative intensity, which means that there is no preferential orientation of the crystallites in the layers, since these results are systematically obtained. The surface visualisation of the sample by scanning electron microscopy allows to see that the crystallites are clearly visible (Fig. 3). They are well faceted with parallelepipedic shape as well as the grains of the FeVO4 powder (Fig. 4), It should be noted that the averaged grain size of the ®lms (0.3 mm) is far smaller than that of the powder. Moreover, the crystallite size distribution in the ®lms is narrower than that of the powder. It can be seen that the grain dimensions vary strongly from one grain to another one in the case of the powder, while the ®lms are more homogeneous. The cross section of sample FeVO4/SnO2/glass is shown in Fig. 5. It can be seen that the ®lms are adherent to the SnO2 coated substrate. Moreover, it appears that, even after annealing, the ®lm has kept in memory, at least partly, the deposition sequence. This is probably related to the fact that usually the initialisation of the crystallisation takes place at the interfaces. Probably, after deposition, some interfaces between the metallic layers are suf®ciently inhomogeneous to initiate crystallisation. The same behaviour is expected when a substrate coated with a SnO2 ®lm is used. It can be seen Fig. 5 that this ®lm is polycrystalline, which induces some surface roughness. These relief, distributed all-over the surface of the ®lm, will acts, not only as crystallisation sites, but also as anchorage point for the upper FeVO4 ®lm, which explains that the ®lms grown on such rough SnO2 substrate are more homogeneous than those grown on bare and smooth amorphous glass substrates. All the results described above have been obtained at an annealing temperature of 863 K for half an hour.
210
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
Fig. 2. XRD diagram of FeVO4 film powder.
A systematically study of the optimum annealing conditions has shown that, when the annealing temperature is kept constant at T 863 K, the intensity of the
XRD peaks increases during the ®rst 30 min and then stabilises. Therefore, annealing of half an hour have been used. Also there is no difference when the gaze used is pure oxygen or room air atmosphere. The annealing temperature is limited by the substrate. The soda-lime glass substrates which is stable up to 873 K, therefore, the maximum temperature used was 863 K. It has been shown that a minimum temperature of 833 K is necessary
Fig. 3. Visualisation of the surface of a FeVO4 film by SEM.
Fig. 4. Visualisation of the FeVO4 powder by SEM.
Fig. 5. Cross section of a FeVO4 film by SEM.
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
211
Fig. 6. XPS spectra of FeVO4 A: C1s; B: O1s and V2p; C: Fe2p. (a) Before etching; (b) After 1 min etching.
to initiate the crystallisation of the ®lms. However, the crystallisation is improved when the annealing temperature is increased up to 863 K.
The energy of the V Ka ray of the vanadium is 0.510 keV, that of the O Ka ray of the oxygen is 0.523 keV. Therefore, quantitative micoprobe analysis has been done on the
212
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
Table 1 XPS quantitative analysis Sample FeVO4
theory no etching 1 min etching 6 min etching
Fe
V
O
16.66 10 15 20
16.66 14 18 19
66.66 76 67 61
samples mainly before crystallisation, because the superposition of the VLa peak with the OKa peak induces a very broad uncertainty on qualitative measurement after oxide formation. However, the composition of the sample estimated, during deposition, with the help of the vibrating quartz method has been checked after deposition by microprobe analysis. It has been shown that, in the uncertainty range of the apparatus (1%), the ®lms are homogeneous from one point to another one. The average measured values are Fe 47 1 and V 53 1. It can be concluded that the atomic ratio M/V 1 expected is nearly obtained. XPS analysis are reported in Fig. 6, Table 1 (quantitative analysis) and Table 2 (qualitative analysis). It can be seen that there is some surface contamination by oxygen and carbon. After etching the contamination has nearly disappeared, it is probably issued from room air environment. The etching rate of oxygen being higher than that of the metals, its relative atomic concentration decreases when the etching time increases. However, the atomic ratio F/V stays nearly equal to one, which shows that after annealing the metallic ®lms have interdiffused. The binding energies of Fe2p3/2, V2p3/2 and O1s peaks are reported in Table 2. Also reference values for iron and vanadium are presented, it can be seen in Table 2 that there is a good agreement between Table 2 XPS qualitative analysis Materials
Binding energy (eV) Fe2p3/2
Fe FeO Fe2O3 V VO2 V2 O 5 FeVO4 Thin film No etching 1 min etching
706.75 710.30 710.70
711.30 710.70
V2p3/2
O1s
512.00 516.00 [25] 517.00 [25]
529.80 [25]
516.80 516.70
529.80 530.10
the measured values and that of the element in the same oxidation state. It can be seen Fig. 6B that, after etching, the width of the XPS peaks increase and that there is a shift of the binding energy of V2p3/2 towards smaller values. During the etching V±O bonds are destroyed because the sputtering yield of O is far higher than that of V. Therefore, the binding energy of V decreases while the disorder, induced by the sputtering, increases the width of the V peaks. The same discussion can explains the modi®cations of the Fe doublet after etching (Fig. 6C). 4. Conclusion In the present work, FeVO4 has been elaborated in polycrystalline thin ®lm form and studied. It has been shown that a very simple physical vapour deposition technique followed by twice annealings at temperature of 863 K for half an hour each, ®rst under vacuum and second in room atmosphere can be used to growth such oxides. These oxides ®lms have been grown on transparent conductive oxide, which is very promising for the future application as negative electrode in thin ®lms batteries. Acknowledgements The authors wish to thank Mr. Guyomard for helpful discussions. References [1] F.K. Shokoohi, J.M. Tarascon, B.J. Wilkens, Appl. Phys. Lett. 59(10) (1991) 2. [2] G.R. Gruzalski, J.B. Bates, Sold. State. Division, Oak Ridge National Laboratory, Oak Ridge, Tennesee, 37837±6030. [3] S. Denis, E. Baudrin, M. Touboul, J.M. Tarascon, J. Electrochem. Soc. 144(12) (1997) 12. [4] Oka Yoshio, Yao Takeshi, Yamamoto Naoichi, Ueda Yutaka, Kawasaki Shuji, Azuma Masaki, Takano Mikio, J. Solid State Chem. 123 (1996) 54±59. [5] J. Muller, J.C. Joubert, J. Solid State Chem. 123 (1996) 54±59. [6] A.P. Young, C.M. Schwartz, Acta Crystallogr. 15 (1962) 1305. [7] B. Robertson, E. Kostiner, J. Solid State Chem. 4 (1972) 29. [8] F. Laves, Acta Crystallogr. 17 (1964) 1476. [9] M. Sugawara, M. Fugiwara, K. Matsuki, Denki Kagaku 61 (1993) 224.
Thin ®lms of FeVO4 obtained by annealing under room atmosphere of Fe and V layers sequentially deposited a
E. Baba Alia, J.C. BerneÁdea,*, A. Barreaub
Equipe couches minces et mateÂriaux nouveaux, E.P.S.E.-F.S.T.N. 2, rue de la HoussinieÁre, BP 92208, 44322 Nantes, Cedex 3, France Centre Commun de Microscopie Electronique aÁ Balayage F.S.T.N. 2, rue de la HoussinieÁre, BP 92208, 44322 Nantes, Cedex 3, France
b
Received 11 May 1999; received in revised form 21 July 1999; accepted 28 September 1999
Abstract Metallic thin ®lms Fe/V/Fe/V... /V sequentially deposited under vacuum have been annealed half an hour in room atmosphere. The samples have been characterised by X-rays diffraction (XRD), scanning electron microscopy (SEM), microprobe analysis (EDX), X-ray photoelectron spectroscopy (XPS) and optical absorption. The annealing temperature and the under layer material have been taken as parameters. The results have shown that triclinic phase of FeVO4, homogeneous thin ®lms can be synthesised by this technique. The best results have been obtained when the glass substrate used was coated with SnO2 thin ®lm. The results were improved when, before air-annealing, a ®rst annealing under vacuum was proceeded in order to homogenise the metal distribution in the sample. # 2000 Elsevier Science S.A. All rights reserved. Keywords: Metallic thin ®lms; Room atmosphere; X-rays diffraction; FeVO4; Annealing treatment
1. Introduction
2. Experimental techniques
One of the prospective applications for solid-state lithium batteries would be as on-chip power backup, where the battery is an integral part of an integrated circuit device. Such batteries should be made with conventional thin ®lm techniques such as vapour deposition or sputtering in order to meet the production requirements of microelectronics industry. In general, the polycrystalline ®lms show electrochemical performance exceeding that of bulk materials [1,2]. Authors have recently opened new avenues of research towards this goal by showing that several Li-vanadate-based electrodes, when discharged to voltages lower than 0.2 V, can reversibly intercalate up to 7 Li leading to 800± 900 Ah kgÿ1 capacities, about two and a half times greater than present graphite electrodes [3]. With its open framework structure, bulk FeVO4 [4±8] for instance, was initially studied and seen as potential candidate for cathode materials [9]. In the present report, we shows that a very simple physical vapour deposition technique followed by an annealing in room air can be used to grow such oxide in thin ®lm form.
The substrates used were bare glass and SnO2 coated glass. Metallic thin ®lms Fe/V/Fe/V.../V/Fe , sequentially deposited under vacuum by electron beam evaporation (Edwards), have been annealed at ®rst in secondary vacuum at 850 K and then in room atmosphere at 863 K for a quarter of an hour and half an hour, respectively. Structural investigations were carried out with an X-ray diffractometer (DIFFRACT AT V3.1 SIEMENS), using monochromatic CuKa radiation.1 Detailed morphological analysis of the ®lms was carried out by scanning electron microscopy (SEM) using a JEOL 6400F ®eld-emission scanning electron microscope. Electron microscope, the JEOL 5800LV, equipped with a PGT X-rays microanalysis system, in which X-rays were detected by a germanium crystal has been used for quantitative microprobe analysis. The XPS measurements were performed with a magnesium X-ray source (1253.6 eV) operating at 10 kV and 10 mA. Data acquisition and treatment were realised through a computer and a standard program. The quantitative studies were based on the determination of the O1s, V2p3/2, Fe2p3/2 peak areas. The homogeneity in depth of the
* Corresponding author. Tel.: 33-0251-1255-30; fax: 33-0251-135528. E-mail address: [email protected] (J.C. BerneÁde).
1 XRD measurments have been done at the Institute of Materials of Nantes.
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 2 0 - 5
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
209
Fig. 1. XRD diagram of FeVO4 film first annealing under vacuum and second annealing in room atmosphere.
®lms was checked by recording successive XPS spectra obtained after argon ion etching for short periods. Sputtering was accomplished at pressures of less than 5 10ÿ4 Pa a 10 mA emission current and a 3 kV beam energy using an ion gun. With these experimental conditions all the surface of the samples was sputtered.2 3. Experimental results After annealing the ®lms, which had metallic appearance Ð highly re¯exive and grey coloured Ð after deposition, were brown red coloured with a granular morphology visible with naked eyes. It can also be seen that the ®lms obtained on bare glass substrates are highly inhomogeneous with a lot of pinholes, cracks and voids visible, while ®lms grown on SnO2 coated substrates were homogeneous. Therefore, only the latter ®lms have been carefully studied. Moreover, the peaks present in the X-ray diagrams (XRD) are more intensive when, before air-annealing, the samples were heated half an hour under vacuum. This annealing allows to homogenise the metallic ®lms, by interdiffusion. A typical XRD diagram obtained from such annealed sample is presented in Fig. 1. For comparison the diagram of a reference powder is presented in Fig. 2. Moreover, in Fig. 1 is reported the SnO2 typical diagram issued form JCPDS data (5-0467) in order to discriminate between the SnO2 peaks and those of FeVO4. When the diffractograms of Fig. 1 are compared to reference data (JCPDS no 25-0418 and reference powder Fig. 2) it can be seen that all the more
2 XPS measurments have been done with a Leybold spectrometer (University of Nantes-CNRS).
intensive peaks are present in the diagram with the expected relative intensity, which means that there is no preferential orientation of the crystallites in the layers, since these results are systematically obtained. The surface visualisation of the sample by scanning electron microscopy allows to see that the crystallites are clearly visible (Fig. 3). They are well faceted with parallelepipedic shape as well as the grains of the FeVO4 powder (Fig. 4), It should be noted that the averaged grain size of the ®lms (0.3 mm) is far smaller than that of the powder. Moreover, the crystallite size distribution in the ®lms is narrower than that of the powder. It can be seen that the grain dimensions vary strongly from one grain to another one in the case of the powder, while the ®lms are more homogeneous. The cross section of sample FeVO4/SnO2/glass is shown in Fig. 5. It can be seen that the ®lms are adherent to the SnO2 coated substrate. Moreover, it appears that, even after annealing, the ®lm has kept in memory, at least partly, the deposition sequence. This is probably related to the fact that usually the initialisation of the crystallisation takes place at the interfaces. Probably, after deposition, some interfaces between the metallic layers are suf®ciently inhomogeneous to initiate crystallisation. The same behaviour is expected when a substrate coated with a SnO2 ®lm is used. It can be seen Fig. 5 that this ®lm is polycrystalline, which induces some surface roughness. These relief, distributed all-over the surface of the ®lm, will acts, not only as crystallisation sites, but also as anchorage point for the upper FeVO4 ®lm, which explains that the ®lms grown on such rough SnO2 substrate are more homogeneous than those grown on bare and smooth amorphous glass substrates. All the results described above have been obtained at an annealing temperature of 863 K for half an hour.
210
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
Fig. 2. XRD diagram of FeVO4 film powder.
A systematically study of the optimum annealing conditions has shown that, when the annealing temperature is kept constant at T 863 K, the intensity of the
XRD peaks increases during the ®rst 30 min and then stabilises. Therefore, annealing of half an hour have been used. Also there is no difference when the gaze used is pure oxygen or room air atmosphere. The annealing temperature is limited by the substrate. The soda-lime glass substrates which is stable up to 873 K, therefore, the maximum temperature used was 863 K. It has been shown that a minimum temperature of 833 K is necessary
Fig. 3. Visualisation of the surface of a FeVO4 film by SEM.
Fig. 4. Visualisation of the FeVO4 powder by SEM.
Fig. 5. Cross section of a FeVO4 film by SEM.
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
211
Fig. 6. XPS spectra of FeVO4 A: C1s; B: O1s and V2p; C: Fe2p. (a) Before etching; (b) After 1 min etching.
to initiate the crystallisation of the ®lms. However, the crystallisation is improved when the annealing temperature is increased up to 863 K.
The energy of the V Ka ray of the vanadium is 0.510 keV, that of the O Ka ray of the oxygen is 0.523 keV. Therefore, quantitative micoprobe analysis has been done on the
212
E.B. Ali et al. / Materials Chemistry and Physics 63 (2000) 208±212
Table 1 XPS quantitative analysis Sample FeVO4
theory no etching 1 min etching 6 min etching
Fe
V
O
16.66 10 15 20
16.66 14 18 19
66.66 76 67 61
samples mainly before crystallisation, because the superposition of the VLa peak with the OKa peak induces a very broad uncertainty on qualitative measurement after oxide formation. However, the composition of the sample estimated, during deposition, with the help of the vibrating quartz method has been checked after deposition by microprobe analysis. It has been shown that, in the uncertainty range of the apparatus (1%), the ®lms are homogeneous from one point to another one. The average measured values are Fe 47 1 and V 53 1. It can be concluded that the atomic ratio M/V 1 expected is nearly obtained. XPS analysis are reported in Fig. 6, Table 1 (quantitative analysis) and Table 2 (qualitative analysis). It can be seen that there is some surface contamination by oxygen and carbon. After etching the contamination has nearly disappeared, it is probably issued from room air environment. The etching rate of oxygen being higher than that of the metals, its relative atomic concentration decreases when the etching time increases. However, the atomic ratio F/V stays nearly equal to one, which shows that after annealing the metallic ®lms have interdiffused. The binding energies of Fe2p3/2, V2p3/2 and O1s peaks are reported in Table 2. Also reference values for iron and vanadium are presented, it can be seen in Table 2 that there is a good agreement between Table 2 XPS qualitative analysis Materials
Binding energy (eV) Fe2p3/2
Fe FeO Fe2O3 V VO2 V2 O 5 FeVO4 Thin film No etching 1 min etching
706.75 710.30 710.70
711.30 710.70
V2p3/2
O1s
512.00 516.00 [25] 517.00 [25]
529.80 [25]
516.80 516.70
529.80 530.10
the measured values and that of the element in the same oxidation state. It can be seen Fig. 6B that, after etching, the width of the XPS peaks increase and that there is a shift of the binding energy of V2p3/2 towards smaller values. During the etching V±O bonds are destroyed because the sputtering yield of O is far higher than that of V. Therefore, the binding energy of V decreases while the disorder, induced by the sputtering, increases the width of the V peaks. The same discussion can explains the modi®cations of the Fe doublet after etching (Fig. 6C). 4. Conclusion In the present work, FeVO4 has been elaborated in polycrystalline thin ®lm form and studied. It has been shown that a very simple physical vapour deposition technique followed by twice annealings at temperature of 863 K for half an hour each, ®rst under vacuum and second in room atmosphere can be used to growth such oxides. These oxides ®lms have been grown on transparent conductive oxide, which is very promising for the future application as negative electrode in thin ®lms batteries. Acknowledgements The authors wish to thank Mr. Guyomard for helpful discussions. References [1] F.K. Shokoohi, J.M. Tarascon, B.J. Wilkens, Appl. Phys. Lett. 59(10) (1991) 2. [2] G.R. Gruzalski, J.B. Bates, Sold. State. Division, Oak Ridge National Laboratory, Oak Ridge, Tennesee, 37837±6030. [3] S. Denis, E. Baudrin, M. Touboul, J.M. Tarascon, J. Electrochem. Soc. 144(12) (1997) 12. [4] Oka Yoshio, Yao Takeshi, Yamamoto Naoichi, Ueda Yutaka, Kawasaki Shuji, Azuma Masaki, Takano Mikio, J. Solid State Chem. 123 (1996) 54±59. [5] J. Muller, J.C. Joubert, J. Solid State Chem. 123 (1996) 54±59. [6] A.P. Young, C.M. Schwartz, Acta Crystallogr. 15 (1962) 1305. [7] B. Robertson, E. Kostiner, J. Solid State Chem. 4 (1972) 29. [8] F. Laves, Acta Crystallogr. 17 (1964) 1476. [9] M. Sugawara, M. Fugiwara, K. Matsuki, Denki Kagaku 61 (1993) 224.