- Email: [email protected]
Initial stages of TiO2 thin films MOCVD growth studied by in situ surface analyses
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e1263–e1268 www.elsevier.com/locate/jcrysgro
Initial stages of TiO2 thin films MOCVD growth studied by in situ surface analyses A. Brevet, P.M. Peterle´, L. Imhoff, M.C. Marco de Lucas, S. Bourgeois LRRS, UMR 5613 CNRS-Universite´ de Bourgogne, BP 47870, 21078 Dijon Cedex, France Available online 9 December 2004
Abstract In situ chemical surface analyses using X-ray photoelectron spectroscopy (XPS) were performed to understand the initial stages of TiO2 thin-film MOCVD growth. Deposits on Si (1 0 0), a few nanometres thick, were obtained at a fixed temperature of 650 1C and for two different pressures, 2.9 and 0.05 mbar, using titanium tetraisopropoxide (TTIP) as precursor. Pressure lowering led to a higher deposit growth rate. Reduction of titanium with respect to stoichiometric titanium dioxide and oxidation of the wet-cleaned silicon substrate are observed from decomposition of the Ti 2p and Si 2p peaks. The formation of a TiSixOy mixed oxide is also pointed out and confirmed by the presence of a characteristic component in the O 1 s peak. r 2004 Elsevier B.V. All rights reserved. PACS: 82.33.Ya; 82.80.Pv Keywords: A1. Electron spectroscopy for chemical analysis; A3. Metalorganic chemical vapor deposition; B1. Titanium dioxide
1. Introduction Many physico-chemical properties of thin films depend strongly on the quality of the interfacial region, which in turn is mainly determined by the initial stages of the growth. It is thus of great importance to perform in situ characterisations to clarify the growth mechanism and control properties of thin films. Chemical surface analysis using electron spectroscopies is thus helpful to achieve Corresponding author. Fax: +33 3 80 39 38 19.
E-mail address: [email protected] (A. Brevet).
this. Even though MOCVD could be performed from atmospheric pressure (APCVD) [1] to UHV pressure [2,3], to the best of our knowledge no studies were carried out coupling a MOCVD deposition reactor to surface spectroscopic analysis UHV chambers. As TiO2/Si, TiO2/SiO2 or TiO2/SiO2/Si systems are widely used and still investigated in many high technological fields due to their outstanding properties [1,4–6], in the present work we will describe preliminary results on the first stages of TiO2 thin-film MOCVD growth on Si (1 0 0) performed in an original experimental set-up.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.081
ARTICLE IN PRESS e1264
A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
2. Experimental Titanium dioxide thin films were deposited by MOCVD with titanium tetraisopropoxide (TTIP), Ti(OCH(CH3)2)4, used as both titanium and oxygen precursor. The deposition chamber is a home-made one and is coupled to UHV chambers devoted to surface analysis techniques (AES, XPS and LEED). Among them only X-ray photoelectron spectroscopy (XPS) experiments will be presented here. This original system will be described in a forthcoming paper. 2.1. Growth process The growths were performed on (1 0 0) oriented n-type silicon substrates, polished on one side. To remove native oxide, wafers were ex situ cleaned as follows. Each wafer was dipped for 5 min in H2SO4:H2O2:H2O (6:1:1), rinsed with deionised water (DI) and then dipped for 10 s in HF:H2O, 10% diluted. After having been rinsed in DI water, each wafer was dried under N2 blow and immediately introduced into the UHV system. The liquid precursor TTIP was maintained in a thermal bath at 40 1C which gave a vapour pressure of about 0.50 mbar. Since TTIP is very volatile at low temperature, it was not necessary to use any carrier gas to increase mass transportation. All growth parameters were kept identical for the growths presented here, the substrate temperature was fixed at 650 1C and deposition time at 15 min, except for the working pressure which was fixed at two values, 2.9 and 0.05 mbar, respectively, for growths A and B.
emerging photoelectrons analyses were performed with angles of detection varying from 15 to 901 with respect to the surface. Deposit thicknesses were thus estimated using the model proposed by Fadley [7]. Ex situ characterisations were carried out by scanning electron microscopy (SEM) using a JEOL JSM 6400F at 23 keV to obtain morphological information. 2.2.2. XPS spectra analysis procedure All performed XPS experiments were analysed using the following procedure. All spectra were referenced to the Si0 2p3/2 peak positioned at 99.3 eV according to the literature [8]. Quantifications were performed by correcting peak areas with sensitivity factors obtained using Lindau cross sections [9]. The Si 2p spectra with several oxidation states of silicon were decomposed with the following procedure: correct fits of the Si0 2p line were obtained taking into account a splitting of 0.6 eV between 1/2 and 3/2 components of the Si 2p line, measured with a monochromatic Al Ka radiation (resolution of 0.2 eV) in good agreement with previous work [10]. The Si 2p spectra were decomposed with silicon suboxides Si4+, Si3+, Si2+ shifted to higher binding energies by 3.6, 2.7 and 1.8 eV from Si0, respectively [10]. Peak FWHM were constrained to fixed values, higher for Six+ than Si0 and higher for Si 2p1/2 than Si 2p3/2. The Ti 2p3/2 spectra were decomposed into three components corresponding to Ti4+, Ti3+, Ti2+ oxidation states equally spaced by 1.7 eV [11].
2.2. Deposits characterisations
3. Results and discussion
2.2.1. Experimental techniques Information on film composition and on the oxidation states of the different elements involved in the process were determined by in situ XPS using a VG Microtech CLAM 4 MCD analyser system. Experiments presented here were carried out with non-monochromatised Al Ka radiation and with electron detection normal to the surface. Furthermore, when it was interesting to get some insight on in-depth composition of the deposits,
Fig. 1 presents a typical XPS Si 2p spectrum obtained just before growth for ex situ cleaned silicon substrates. It should be noticed that only one peak is present at a binding energy of 99.3 eV for the bulk Si0 2p3/2 component [8]. In particular, no peaks at higher binding energies which should be attributed to silicon oxides are present, in good agreement with the literature concerning clean silicon [12]. According to this, the presence of oxygen and carbon (Table 1) can be mainly
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
attributed to the contamination by carbonaceous species during sample introduction in the UHV system. Even so, this demonstrates the good Si°
Intensity (arb. units)
Substrate
Si2+ Si3+
(a)
Si4+
(b) 105
104
103
102 101 100 99 Binding Energy (eV)
98
97
Fig. 1. Si 2p spectra with normalised intensity obtained for the clean substrate before deposition, for deposit A grown at 650 1C, 2.9 mbar (a) and for deposit B grown at 650 1C, 0.05 mbar (b). The different components of the line obtained taking into account the spin-orbit splitting and using the decomposition procedure explained in the text are also shown.
e1265
efficiency of our wet cleaning procedure on native silicon oxide removal. The presence of titanium oxide on the silicon substrate after the deposition process is at first evidenced by Auger electron spectroscopy. In the same way, a decrease of Si 2p intensity and an increase of Ti 2p intensity are observed simultaneously in XPS as shown from the global quantification presented in Table 1. Moreover, when the working pressure decreases, the substrate signal is more attenuated: the relative contribution of silicon is much higher for deposit A processed at 2.9 mbar than for B processed at 0.05 mbar. Deposit thicknesses were estimated from angledependent XPS analyses to 1 nm for deposit A and 6 nm for deposit B. This gives deposition rates of about 0.7 and 4 A˚ min1, respectively. Then the lower the pressure is, the higher the deposition rate. This result is in agreement with the basic kinetic theories of chemical vapor deposition [13]. Besides, after MOCVD titanium dioxide deposition, silicon oxides are clearly observed in both XPS spectra deposits (Fig. 1). The presence of an interfacial silicon oxide for TiO2 or other metal oxide thin-film growth on bare silicon substrate by CVD techniques was still reported by other authors [3,14]. The presence of the oxidation states Si2+, Si3+ in addition to Si4+ reveals nonstoichiometric silicon oxide. From the relative intensities ratio of these oxidized species, the
Table 1 XPS quantification for Si 2p, O 1 s, Ti 2p and C 1 s lines, in agreement with Figs. 1–4, on a cleaned substrate and deposits A and B grown at 650 1C, 2.9 mbar and 650 1C, 0.05 mbar, respectively Sample
Si 2p (at%) 4+
Substrate A B
O 1s (at%)
Si
Si
10 17
2 8
3+
2+
0
Si
Si
Global
Si–O–Ti
Ti–O–Ti
Global
4 1
100 84 74
71 48 19
81 42
19 58
8 31 34
Ti 2p (at%) Ti Substrate A B
4+
77 90
C 1s (at%) 3+
2+
Ti
Ti
Global
15 8
8 2
2 8
C1
88
C2
Global
12
21 19 39
The percentage of the different components in each peak is indicated as well as the global percentage of each element.
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
e1266
stoichiometry was calculated to be SiO1.7 in deposit A and SiO1.8 in deposit B. Moreover, the relative intensities ratio of silicon suboxides to bulk silicon ððSSixþ Þ=Si0 Þ reveals a greater silicon oxidation for the thickest deposit (B) as it is evidenced in Table 1 and Fig. 1. Fig. 2 presents the Ti 2p3/2 peaks which are found at 458.7 eV for both deposits in agreement with the Ti4+ binding energy in TiO2 given in the literature [8,15]. Peaks show a small broadening at low binding energies due to the presence of reduced species Ti3+ and Ti2+. The stoichiometry was estimated to TiO1.85 and TiO1.95 for deposits A and B, respectively. As titanium is at oxidation state +IV in the titanium tetraisopropoxide precursor molecule, reduction of titanium occurs with silicon oxidation at the first stages of titanium dioxide growth. It should be noticed that the deposit global oxidation state increases with deposit thickness. Consequently, the formation of a
Ti4+
growing and more and more oxidized interfacial TiSixOy layer before stoichiometric TiO2 growth may be inferred and could be compared to the literature results [3]. The presence of two components in the O 1 s line is evidenced in Fig. 3. The first one at the lowest binding energy increases with deposit thickness at the expense of the second one positioned at a higher binding energy. According to Ti 2p and Si 2p intensities evolution, the component at 530.0 eV for both deposits is attributed to oxygen atoms bound to titanium [15] and the component around 531.6 eV to oxygen bound to silicon. As this value is much lower than oxygen binding energy in stoichiometric SiO2, it was attributed to Si–O–Ti bonds in a mixed TiSixOy oxide [16]. Furthermore, a slight shift of 0.2 eV of this component to lower binding energies is noticed when the amount of titanium increases as it was observed by Stakheev et al. [15]. These
(a) Ti3+ Ti2+ Intensity (arb. units)
Intensity (arb. units)
Si-O-Ti (a)
Ti-O-Ti
Ti-O-Ti Si-O-Ti
(b)
461
460
459 458 457 456 Binding Energy (eV)
455
454
Fig. 2. Ti 2p3/2 spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). The different components of the line obtained using the decomposition procedure explained in the text are also shown.
534
533
532 531 530 Binding Energy (eV)
(b)
529
528
Fig. 3. O 1 s spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). The two components of the line corresponding, respectively, to oxygen bound to silicon or to titanium as explained in the text are also shown.
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
Intensity (arb. units)
results are in good agreement with the previous model proposed for the growth. Residual contamination of the deposits is revealed by the presence of carbon species in the XPS spectra for the two deposits (Table 1, Fig. 4). Based upon results obtained on a few hundred nanometres thick TiO2 layers grown in another home-made MOCVD set-up in a previous work of Babelon et al. [17], carbon amounts given in Table 1 are relatively higher than those expected. This fact can perhaps be explained by the initial carbon contamination of the cleaned substrates. It is worth noting that the carbon amount increases with the deposit thickness, so it can be inferred that it is incorporated also during the growth. This could be due to a lower desorption rate of TTIP residual fragments compared to the growth rate. The C 1 s peak of the thickest deposit B is
(a)
e1267
decomposed into two components. The main contribution at 284.0 eV is attributed to C–C or C–H bonds and the component spaced of about 1.5 eV on the higher binding energies side is characteristic of C–O bonds [18]. The broad C 1 s line in the thinnest deposit A (FWHM of 2.2 eV versus 1.6 eV for deposit B) has to be decomposed into more than two components. A further study of the C 1 s peak will be performed on the forthcoming deposits using the monochromatic Al Ka X-ray source in order to be able to perform decompositions in a correct way, which was impossible using the non-monochromatised Xray source. The morphology and an overview of the surface coverage were carried out ex situ using SEM. A homogeneous deposit composed of about 20 nm large grains is observed on deposit B (Fig. 5). This is in the same order of size as the one obtained in the previous work of Babelon et al. for the highest flow rate of N2 carrier gas [17]. The thickness and morphology contrasts of the thinnest deposit (A) were so low that the SEM resolution limit was reached and prevented surface imaging. It should be mentioned that grains are not spherical as they seem to be in the SEM image since the deposit thickness was estimated to 6 nm. We may infer that these grains are the starting point of the growing columnar structure reported by Babelon et al. [17].
C1
(b)
C2
288
287
286 285 284 283 Binding Energy (eV)
282
281
Fig. 4. C 1 s spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). Components C1 and C2 in (b) are attributed, respectively, to C–C or C–H and C–O bonds in residual metalorganic precursor fragments as discussed in the text.
Fig. 5. SEM picture of deposit B grown at 650 1C, 0.05 mbar.
ARTICLE IN PRESS e1268
A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
4. Conclusion Few nanometres thick TiO2 deposits were carried out by MOCVD on wet-cleaned Si (1 0 0) substrates in an experimental set-up allowing in situ characterisations of the early stages of the growth. Growth rate is increased when pressure is lowered, according to CVD kinetic laws. The substrate surface was covered homogeneously with grains of about 20 nm large for the thickest deposit. The presence of reduced species of titanium with respect to stoichiometric TiO2 and silicon at several oxidation states were revealed using XPS analyses. The O 1 s peak was decomposed into two components attributed to Ti–O–Ti and Si–O–Ti bonds from binding energy values. The last one is shifted by 0.2 eV towards the first one as the Ti/Si0 ratio increases. All these results can be accounted for by the growth of a TiSixOy mixed oxide at first stages. This interlayer becomes more and more oxidized as its thickness increases before stoichiometric TiO2 growth. References [1] B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, C.B. Honsberg, Solar Energy 76 (2004) 269. [2] J.-P. Lu, R. Raj, J. Mater. Res. 6 (1991) 1913.
[3] A. Sandell, M.P. Anderson, Y. Alfredsson, M.K.-J. Johansson, J. Schnadt, H. Rensmo, H. Siegbahn, P. Uvdal, J. Appl. Phys. 92 (2002) 3381. [4] F. Fabreguette, L. Imhoff, O. Heintz, M. Maglione, B. Domenichini, M.C. Marco de Lucas, P. Sibillot, S. Bourgeois, M. Sacilotti, Appl. Surf. Sci. 175–176 (2001) 685. [5] A. Rothschild, F. Edelman, Y. Komem, F. Cosandey, Sens. Actuators B 67 (2000) 282. [6] E. Puzenat, P. Pichat, J. Photochem. Photobiol. A 160 (2003) 127. [7] C.S. Fadley, Prog. Surf. Sci. 16 (1984) 275. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Physical Electronics Division, 1979. [9] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) pp.1–155. [10] F. Rochet, Ch. Poncey, G. Dufour, H. Roulet, C. Guillot, F. Sirotti, J. Non-Cryst. Solids 216 (1997) 148. [11] S. Pe´tigny, H. Moste´fa-Sba, B. Domenichini, E. Lesniewska, A. Steinbrunn, S. Bourgeois, Surf. Sci. 410 (1998) 250. [12] R. Larciprete, E. Borsella, J. Electron. Spectrosc. Relat. Phenom. 76 (1995) 607. [13] D.M. Dobkin, M.K. Zuraw, Principles of Chemical Vapor Deposition, Kluwer Academic Publishers, 2003. [14] W. Tsai, et al., Microelectron. Eng. 65 (2003) 259. [15] A.Y. Stakheev, E.S. Shpiro, J. Apijok, J. Phys. Chem. 97 (1993) 5668. [16] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233. [17] P. Babelon, A.S. Dequiedt, H. Moste´fa-Sba, S. Bourgeois, P. Sibillot, M. Sacilotti, Thin Solid Films 322 (1998) 63. [18] D. Briggs, J.T. Grant, Surface Analysis by Auger and Xray Photoelectron spectroscopy, Surface Spectra, IMP Publications, 2003.
Journal of Crystal Growth 275 (2005) e1263–e1268 www.elsevier.com/locate/jcrysgro
Initial stages of TiO2 thin films MOCVD growth studied by in situ surface analyses A. Brevet, P.M. Peterle´, L. Imhoff, M.C. Marco de Lucas, S. Bourgeois LRRS, UMR 5613 CNRS-Universite´ de Bourgogne, BP 47870, 21078 Dijon Cedex, France Available online 9 December 2004
Abstract In situ chemical surface analyses using X-ray photoelectron spectroscopy (XPS) were performed to understand the initial stages of TiO2 thin-film MOCVD growth. Deposits on Si (1 0 0), a few nanometres thick, were obtained at a fixed temperature of 650 1C and for two different pressures, 2.9 and 0.05 mbar, using titanium tetraisopropoxide (TTIP) as precursor. Pressure lowering led to a higher deposit growth rate. Reduction of titanium with respect to stoichiometric titanium dioxide and oxidation of the wet-cleaned silicon substrate are observed from decomposition of the Ti 2p and Si 2p peaks. The formation of a TiSixOy mixed oxide is also pointed out and confirmed by the presence of a characteristic component in the O 1 s peak. r 2004 Elsevier B.V. All rights reserved. PACS: 82.33.Ya; 82.80.Pv Keywords: A1. Electron spectroscopy for chemical analysis; A3. Metalorganic chemical vapor deposition; B1. Titanium dioxide
1. Introduction Many physico-chemical properties of thin films depend strongly on the quality of the interfacial region, which in turn is mainly determined by the initial stages of the growth. It is thus of great importance to perform in situ characterisations to clarify the growth mechanism and control properties of thin films. Chemical surface analysis using electron spectroscopies is thus helpful to achieve Corresponding author. Fax: +33 3 80 39 38 19.
E-mail address: [email protected] (A. Brevet).
this. Even though MOCVD could be performed from atmospheric pressure (APCVD) [1] to UHV pressure [2,3], to the best of our knowledge no studies were carried out coupling a MOCVD deposition reactor to surface spectroscopic analysis UHV chambers. As TiO2/Si, TiO2/SiO2 or TiO2/SiO2/Si systems are widely used and still investigated in many high technological fields due to their outstanding properties [1,4–6], in the present work we will describe preliminary results on the first stages of TiO2 thin-film MOCVD growth on Si (1 0 0) performed in an original experimental set-up.
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.081
ARTICLE IN PRESS e1264
A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
2. Experimental Titanium dioxide thin films were deposited by MOCVD with titanium tetraisopropoxide (TTIP), Ti(OCH(CH3)2)4, used as both titanium and oxygen precursor. The deposition chamber is a home-made one and is coupled to UHV chambers devoted to surface analysis techniques (AES, XPS and LEED). Among them only X-ray photoelectron spectroscopy (XPS) experiments will be presented here. This original system will be described in a forthcoming paper. 2.1. Growth process The growths were performed on (1 0 0) oriented n-type silicon substrates, polished on one side. To remove native oxide, wafers were ex situ cleaned as follows. Each wafer was dipped for 5 min in H2SO4:H2O2:H2O (6:1:1), rinsed with deionised water (DI) and then dipped for 10 s in HF:H2O, 10% diluted. After having been rinsed in DI water, each wafer was dried under N2 blow and immediately introduced into the UHV system. The liquid precursor TTIP was maintained in a thermal bath at 40 1C which gave a vapour pressure of about 0.50 mbar. Since TTIP is very volatile at low temperature, it was not necessary to use any carrier gas to increase mass transportation. All growth parameters were kept identical for the growths presented here, the substrate temperature was fixed at 650 1C and deposition time at 15 min, except for the working pressure which was fixed at two values, 2.9 and 0.05 mbar, respectively, for growths A and B.
emerging photoelectrons analyses were performed with angles of detection varying from 15 to 901 with respect to the surface. Deposit thicknesses were thus estimated using the model proposed by Fadley [7]. Ex situ characterisations were carried out by scanning electron microscopy (SEM) using a JEOL JSM 6400F at 23 keV to obtain morphological information. 2.2.2. XPS spectra analysis procedure All performed XPS experiments were analysed using the following procedure. All spectra were referenced to the Si0 2p3/2 peak positioned at 99.3 eV according to the literature [8]. Quantifications were performed by correcting peak areas with sensitivity factors obtained using Lindau cross sections [9]. The Si 2p spectra with several oxidation states of silicon were decomposed with the following procedure: correct fits of the Si0 2p line were obtained taking into account a splitting of 0.6 eV between 1/2 and 3/2 components of the Si 2p line, measured with a monochromatic Al Ka radiation (resolution of 0.2 eV) in good agreement with previous work [10]. The Si 2p spectra were decomposed with silicon suboxides Si4+, Si3+, Si2+ shifted to higher binding energies by 3.6, 2.7 and 1.8 eV from Si0, respectively [10]. Peak FWHM were constrained to fixed values, higher for Six+ than Si0 and higher for Si 2p1/2 than Si 2p3/2. The Ti 2p3/2 spectra were decomposed into three components corresponding to Ti4+, Ti3+, Ti2+ oxidation states equally spaced by 1.7 eV [11].
2.2. Deposits characterisations
3. Results and discussion
2.2.1. Experimental techniques Information on film composition and on the oxidation states of the different elements involved in the process were determined by in situ XPS using a VG Microtech CLAM 4 MCD analyser system. Experiments presented here were carried out with non-monochromatised Al Ka radiation and with electron detection normal to the surface. Furthermore, when it was interesting to get some insight on in-depth composition of the deposits,
Fig. 1 presents a typical XPS Si 2p spectrum obtained just before growth for ex situ cleaned silicon substrates. It should be noticed that only one peak is present at a binding energy of 99.3 eV for the bulk Si0 2p3/2 component [8]. In particular, no peaks at higher binding energies which should be attributed to silicon oxides are present, in good agreement with the literature concerning clean silicon [12]. According to this, the presence of oxygen and carbon (Table 1) can be mainly
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
attributed to the contamination by carbonaceous species during sample introduction in the UHV system. Even so, this demonstrates the good Si°
Intensity (arb. units)
Substrate
Si2+ Si3+
(a)
Si4+
(b) 105
104
103
102 101 100 99 Binding Energy (eV)
98
97
Fig. 1. Si 2p spectra with normalised intensity obtained for the clean substrate before deposition, for deposit A grown at 650 1C, 2.9 mbar (a) and for deposit B grown at 650 1C, 0.05 mbar (b). The different components of the line obtained taking into account the spin-orbit splitting and using the decomposition procedure explained in the text are also shown.
e1265
efficiency of our wet cleaning procedure on native silicon oxide removal. The presence of titanium oxide on the silicon substrate after the deposition process is at first evidenced by Auger electron spectroscopy. In the same way, a decrease of Si 2p intensity and an increase of Ti 2p intensity are observed simultaneously in XPS as shown from the global quantification presented in Table 1. Moreover, when the working pressure decreases, the substrate signal is more attenuated: the relative contribution of silicon is much higher for deposit A processed at 2.9 mbar than for B processed at 0.05 mbar. Deposit thicknesses were estimated from angledependent XPS analyses to 1 nm for deposit A and 6 nm for deposit B. This gives deposition rates of about 0.7 and 4 A˚ min1, respectively. Then the lower the pressure is, the higher the deposition rate. This result is in agreement with the basic kinetic theories of chemical vapor deposition [13]. Besides, after MOCVD titanium dioxide deposition, silicon oxides are clearly observed in both XPS spectra deposits (Fig. 1). The presence of an interfacial silicon oxide for TiO2 or other metal oxide thin-film growth on bare silicon substrate by CVD techniques was still reported by other authors [3,14]. The presence of the oxidation states Si2+, Si3+ in addition to Si4+ reveals nonstoichiometric silicon oxide. From the relative intensities ratio of these oxidized species, the
Table 1 XPS quantification for Si 2p, O 1 s, Ti 2p and C 1 s lines, in agreement with Figs. 1–4, on a cleaned substrate and deposits A and B grown at 650 1C, 2.9 mbar and 650 1C, 0.05 mbar, respectively Sample
Si 2p (at%) 4+
Substrate A B
O 1s (at%)
Si
Si
10 17
2 8
3+
2+
0
Si
Si
Global
Si–O–Ti
Ti–O–Ti
Global
4 1
100 84 74
71 48 19
81 42
19 58
8 31 34
Ti 2p (at%) Ti Substrate A B
4+
77 90
C 1s (at%) 3+
2+
Ti
Ti
Global
15 8
8 2
2 8
C1
88
C2
Global
12
21 19 39
The percentage of the different components in each peak is indicated as well as the global percentage of each element.
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
e1266
stoichiometry was calculated to be SiO1.7 in deposit A and SiO1.8 in deposit B. Moreover, the relative intensities ratio of silicon suboxides to bulk silicon ððSSixþ Þ=Si0 Þ reveals a greater silicon oxidation for the thickest deposit (B) as it is evidenced in Table 1 and Fig. 1. Fig. 2 presents the Ti 2p3/2 peaks which are found at 458.7 eV for both deposits in agreement with the Ti4+ binding energy in TiO2 given in the literature [8,15]. Peaks show a small broadening at low binding energies due to the presence of reduced species Ti3+ and Ti2+. The stoichiometry was estimated to TiO1.85 and TiO1.95 for deposits A and B, respectively. As titanium is at oxidation state +IV in the titanium tetraisopropoxide precursor molecule, reduction of titanium occurs with silicon oxidation at the first stages of titanium dioxide growth. It should be noticed that the deposit global oxidation state increases with deposit thickness. Consequently, the formation of a
Ti4+
growing and more and more oxidized interfacial TiSixOy layer before stoichiometric TiO2 growth may be inferred and could be compared to the literature results [3]. The presence of two components in the O 1 s line is evidenced in Fig. 3. The first one at the lowest binding energy increases with deposit thickness at the expense of the second one positioned at a higher binding energy. According to Ti 2p and Si 2p intensities evolution, the component at 530.0 eV for both deposits is attributed to oxygen atoms bound to titanium [15] and the component around 531.6 eV to oxygen bound to silicon. As this value is much lower than oxygen binding energy in stoichiometric SiO2, it was attributed to Si–O–Ti bonds in a mixed TiSixOy oxide [16]. Furthermore, a slight shift of 0.2 eV of this component to lower binding energies is noticed when the amount of titanium increases as it was observed by Stakheev et al. [15]. These
(a) Ti3+ Ti2+ Intensity (arb. units)
Intensity (arb. units)
Si-O-Ti (a)
Ti-O-Ti
Ti-O-Ti Si-O-Ti
(b)
461
460
459 458 457 456 Binding Energy (eV)
455
454
Fig. 2. Ti 2p3/2 spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). The different components of the line obtained using the decomposition procedure explained in the text are also shown.
534
533
532 531 530 Binding Energy (eV)
(b)
529
528
Fig. 3. O 1 s spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). The two components of the line corresponding, respectively, to oxygen bound to silicon or to titanium as explained in the text are also shown.
ARTICLE IN PRESS A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
Intensity (arb. units)
results are in good agreement with the previous model proposed for the growth. Residual contamination of the deposits is revealed by the presence of carbon species in the XPS spectra for the two deposits (Table 1, Fig. 4). Based upon results obtained on a few hundred nanometres thick TiO2 layers grown in another home-made MOCVD set-up in a previous work of Babelon et al. [17], carbon amounts given in Table 1 are relatively higher than those expected. This fact can perhaps be explained by the initial carbon contamination of the cleaned substrates. It is worth noting that the carbon amount increases with the deposit thickness, so it can be inferred that it is incorporated also during the growth. This could be due to a lower desorption rate of TTIP residual fragments compared to the growth rate. The C 1 s peak of the thickest deposit B is
(a)
e1267
decomposed into two components. The main contribution at 284.0 eV is attributed to C–C or C–H bonds and the component spaced of about 1.5 eV on the higher binding energies side is characteristic of C–O bonds [18]. The broad C 1 s line in the thinnest deposit A (FWHM of 2.2 eV versus 1.6 eV for deposit B) has to be decomposed into more than two components. A further study of the C 1 s peak will be performed on the forthcoming deposits using the monochromatic Al Ka X-ray source in order to be able to perform decompositions in a correct way, which was impossible using the non-monochromatised Xray source. The morphology and an overview of the surface coverage were carried out ex situ using SEM. A homogeneous deposit composed of about 20 nm large grains is observed on deposit B (Fig. 5). This is in the same order of size as the one obtained in the previous work of Babelon et al. for the highest flow rate of N2 carrier gas [17]. The thickness and morphology contrasts of the thinnest deposit (A) were so low that the SEM resolution limit was reached and prevented surface imaging. It should be mentioned that grains are not spherical as they seem to be in the SEM image since the deposit thickness was estimated to 6 nm. We may infer that these grains are the starting point of the growing columnar structure reported by Babelon et al. [17].
C1
(b)
C2
288
287
286 285 284 283 Binding Energy (eV)
282
281
Fig. 4. C 1 s spectra with normalised intensity obtained for deposit A grown at 650 1C, 2.9 mbar (a) and deposit B grown at 650 1C, 0.05 mbar (b). Components C1 and C2 in (b) are attributed, respectively, to C–C or C–H and C–O bonds in residual metalorganic precursor fragments as discussed in the text.
Fig. 5. SEM picture of deposit B grown at 650 1C, 0.05 mbar.
ARTICLE IN PRESS e1268
A. Brevet et al. / Journal of Crystal Growth 275 (2005) e1263–e1268
4. Conclusion Few nanometres thick TiO2 deposits were carried out by MOCVD on wet-cleaned Si (1 0 0) substrates in an experimental set-up allowing in situ characterisations of the early stages of the growth. Growth rate is increased when pressure is lowered, according to CVD kinetic laws. The substrate surface was covered homogeneously with grains of about 20 nm large for the thickest deposit. The presence of reduced species of titanium with respect to stoichiometric TiO2 and silicon at several oxidation states were revealed using XPS analyses. The O 1 s peak was decomposed into two components attributed to Ti–O–Ti and Si–O–Ti bonds from binding energy values. The last one is shifted by 0.2 eV towards the first one as the Ti/Si0 ratio increases. All these results can be accounted for by the growth of a TiSixOy mixed oxide at first stages. This interlayer becomes more and more oxidized as its thickness increases before stoichiometric TiO2 growth. References [1] B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, C.B. Honsberg, Solar Energy 76 (2004) 269. [2] J.-P. Lu, R. Raj, J. Mater. Res. 6 (1991) 1913.
[3] A. Sandell, M.P. Anderson, Y. Alfredsson, M.K.-J. Johansson, J. Schnadt, H. Rensmo, H. Siegbahn, P. Uvdal, J. Appl. Phys. 92 (2002) 3381. [4] F. Fabreguette, L. Imhoff, O. Heintz, M. Maglione, B. Domenichini, M.C. Marco de Lucas, P. Sibillot, S. Bourgeois, M. Sacilotti, Appl. Surf. Sci. 175–176 (2001) 685. [5] A. Rothschild, F. Edelman, Y. Komem, F. Cosandey, Sens. Actuators B 67 (2000) 282. [6] E. Puzenat, P. Pichat, J. Photochem. Photobiol. A 160 (2003) 127. [7] C.S. Fadley, Prog. Surf. Sci. 16 (1984) 275. [8] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Physical Electronics Division, 1979. [9] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) pp.1–155. [10] F. Rochet, Ch. Poncey, G. Dufour, H. Roulet, C. Guillot, F. Sirotti, J. Non-Cryst. Solids 216 (1997) 148. [11] S. Pe´tigny, H. Moste´fa-Sba, B. Domenichini, E. Lesniewska, A. Steinbrunn, S. Bourgeois, Surf. Sci. 410 (1998) 250. [12] R. Larciprete, E. Borsella, J. Electron. Spectrosc. Relat. Phenom. 76 (1995) 607. [13] D.M. Dobkin, M.K. Zuraw, Principles of Chemical Vapor Deposition, Kluwer Academic Publishers, 2003. [14] W. Tsai, et al., Microelectron. Eng. 65 (2003) 259. [15] A.Y. Stakheev, E.S. Shpiro, J. Apijok, J. Phys. Chem. 97 (1993) 5668. [16] X. Gao, I.E. Wachs, Catal. Today 51 (1999) 233. [17] P. Babelon, A.S. Dequiedt, H. Moste´fa-Sba, S. Bourgeois, P. Sibillot, M. Sacilotti, Thin Solid Films 322 (1998) 63. [18] D. Briggs, J.T. Grant, Surface Analysis by Auger and Xray Photoelectron spectroscopy, Surface Spectra, IMP Publications, 2003.