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Surface preparation influence on the initial stages of MOCVD growth of TiO2 thin films
Thin Solid Films 515 (2006) 687 – 690 www.elsevier.com/locate/tsf
Surface preparation influence on the initial stages of MOCVD growth of TiO2 thin films A. Monoy a, A. Brevet a, L. Imhoff a,*, B. Domenichini a, E. Lesniewska b, P.M. Peterle´ a, M.C. Marco de Lucas a, S. Bourgeois a b
a Laboratoire de Recherches sur la Re´activite´ des Solides (LRRS)—UMR 5613, France Laboratoire de Physique de l’Universite´ de Bourgogne (LPUB)—UMR 5027, CNRS—Universite´ de Bourgogne, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France
Available online 7 February 2006
Abstract In situ chemical surface analyses using X-ray photoelectron spectroscopy (XPS), completed by ex situ atomic force microscopy (AFM) analyses, were performed in order to compare the initial stages of MOCVD growth of TiO2 thin films on two different surface types. The first type was a silicon native oxide free hydrogen terminated surface and the second one was a silicon dioxide surface corresponding to a thin layer of 3.5 nm thick in situ thermally grown on silicon substrate. Si(100) was used as substrate, and the growths of TiO2 thin films were achieved with titanium tetraisopropoxide (TTIP) as precursor under a temperature of 675 -C, a pressure of 0.3 Pa and a deposition time of 1 h. Whatever the surface is, the deposited titanium amount was globally the same in the two cases. On the contrary, the deposit morphology was different: a covering layer composed of a SiO2 and TiO2 phases mixture on the hydrogen terminated surface, and small TiO2 clusters homogeneously spread on the SiO2 surface. D 2005 Elsevier B.V. All rights reserved. PACS: 82.33.Ya; 82.80.Pv Keywords: MOCVD; XPS; In situ analyses; Titanium dioxide
1. Introduction Many physical and/or chemical properties of thin films strongly depend on the quality of the interfacial region, which in turn is mainly determined by the initial stages of the growth. It is thus of high importance, in order to study these stages, to perform in situ characterizations. Chemical surface analysis using electron spectroscopies is thus helpful to achieve this, these techniques making possible also some morphologic characterization such as film morphology determination. Although such analyses are carried out in numerous works in the cases of physical vapour deposition growths [1 –3], few works have been carried out for chemical vapour deposition growths.
* Corresponding author. Tel.: +33 3 80 39 61 61; fax: +33 3 80 39 38 19. E-mail address: [email protected] (L. Imhoff). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.237
In this way, an original experimental set-up composed of a MOCVD reactor coupled to a UHV chamber devoted to chemical surface analyses (XPS, AES, LEED) has been developed. First, we have focused our works on TiO2/Si, TiO2/SiO2 and TiO2/SiO2/Si systems, because they are widely used and still investigated in many high technological fields due to their outstanding properties [4– 7]. In a previous study [8], using XPS in situ analyses, we succeeded in observing the initial stages of TiO2 thin films growth on silicon substrate, with titanium tetraisopropoxide as precursor. Especially, the formation of TiSix Oy mixed oxide at the interface between silicon and few nanometers thick stoichiometric TiO2 films was pointed out. In this work, we will compare the initial growth stages for two types of surface: on the one hand a silicon native oxide free hydrogen terminated surface, and on the other hand a silicon dioxide surface corresponding to a thin layer of 3.5 nm thick in situ thermally grown on silicon substrate. Moreover, the films are also ex situ characterized by atomic force microscopy in order to
688
A. Monoy et al. / Thin Solid Films 515 (2006) 687 – 690
complete the results obtained by in situ spectroscopic surface analyses, especially the substrate coverage. 2. Experimental 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 Xray photoelectron spectroscopy (XPS) experiments will be presented here. 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 liquid precursor TTIP was maintained in a thermal bath at 40 -C which gave a vapour pressure of about 50 Pa. The growths were performed on (100) oriented n-type silicon substrates, polished on one side. To remove native oxide for hydrogen terminated surface samples, 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 surface oxidized silicon samples were obtained with the following way: they were first cleaned with the same ex situ procedure and then oxidized in situ with a temperature of 710 -C and an oxygen pressure of 100 Pa during 1 h. The TiO2 growths were carried out with temperature fixed at 675 -C, TTIP pressure at 0.3 Pa and deposition time at 1 h. 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 monochromatised Al Ka radiation with electron detection normal to the surface, or with nonmonochromatised Al Ka radiation with angles of detection varying from 15- to 90- with respect to the surface (ARXPS: angular resolved XPS), when it was interesting to get some insight on in-depth composition of the deposits. Deposit thicknesses and average coverages were thus estimated using the model proposed by Fadley [9]. Ex situ characterisations were carried out by atomic force microscopy (AFM) to investigate the morphology of TiO2 deposited onto SiO2 surface. A commercial AFM (Nanoscope IIIa quadrexed, Veeco Inst.) was used in oscillating contact mode (Tappingi mode) investigation. 3. Deposition on oxidized silicon surface First, the substrate surface was studied before deposition by XPS through the Si2p and O1s lines, no C1s line being observed on survey spectrum. Si2p lines are composed of two peaks at 99.3 and 103.4 eV. Such binding energies are characteristic of Si0 and SiO2, respectively [10]. Moreover, the O1s line is composed of a component only at 532.6 eV which can be attributed to thick SiO2 layer [11]. Besides, the analysis of the ratio of the Si0 and SiO2 components as function
Fig. 1. AFM image of an oxidized silicon surface after MOCVD deposition.
of the angle of electron detection according to the Fadley model [9] showed that the silicon oxide wets totally the surface, is homogeneous and presents a thickness of 3.5 nm. After TiO2 deposition, no real change occurs in the Si2p lines. However, a new component occurs in the O1s line at 530.0 eV. This one can be attributed to titanium oxide formation because in the same time, titanium can be detected, especially from the Ti2p lines. Those are characterized by Ti4+ components only at 458.8 eV (Ti2p3/2) and 464.4 eV (Ti2p1/2). Although it is impossible to carry out a quantification from O1s new component and Ti2p too small line intensities, the binding energies [12] and the shapes of Ti2p lines are characteristic of TiO2. Moreover, these too small intensities make impossible the determination of the surface fractional coverage and the thickness of the deposit. Because the film seemed composed of just TiO2 and SiO2 which are phases stable under air, the morphological characterization was carried out by ex situ AFM. By comparison with AFM images before and after deposition (Fig. 1), it is clear that the TiO2 deposit is composed of small clusters homogeneously spread on the SiO2 surface. The average height of these clusters is about 10 nm while their average width is about 20 nm. The fractional coverage is about 5%. 4. Deposition on hydrogen terminated silicon surface Before deposition, the clean surface was investigated by XPS only. Indeed, this surface is not stable under air and cannot thus be characterized by ex situ AFM. On the XPS data, only a very low amount of carbon (through C1s line) and oxygen (thought O1s line) can be detected. Besides, Si2p spectrum obtained just before deposition is composed of one peak at 99.3 eV, characteristic of Si0 component [13]. Because no clear peaks at higher binding energies which could be attributed to silicon oxides are present, the presence of tiny amounts of oxygen and carbon can be mainly attributed to the contamination by carbonaceous species during sample introduction in the UHV vessel, demonstrating the good efficiency on the one hand of the used cleaning recipe on native silicon oxide removal, and on the other hand of the surface stabilisation by hydrogen during air exposure.
A. Monoy et al. / Thin Solid Films 515 (2006) 687 – 690
689
Fig. 2. After MOCVD deposition carried out on hydrogen terminated silicon surface; (a): Si2p photoemission spectra as function of the take-off angle; O1s (b) and Ti2p (c) spectra recorded at normal detection angle. These spectra were normalised in order to show the same intensity of the bulk component.
After deposition, new components appear on the Si2p spectrum at higher binding energy revealing silicon oxidation (Fig. 2(a) 10-). Si2p spectrum was then decomposed taking into account the two Si02p components as well as sub-oxides Si4+, Si3+, Si2+ components [14]. The presence of two components in the O1s line was evidenced (Fig. 2(b)). The first component at 530 eV can be attributed to oxygen atoms bound to titanium in titanium dioxide [15] whereas the component around 532.1 eV is due to oxygen atoms bound to silicon. This value is lower than oxygen binding energy in stoichiometric thick layer SiO2 (532.6 eV) and higher than those find for mixed Tix Siy Oz oxide (531.6 eV) [16,8]. It should thus be attributed to SiOx suboxide [17].
Fig. 3. Evolutions as function of the detection angle with respect to the surface of the intensity ratios between: SiOx component of the Si2p lines and Ti2p line (r); Si – O and Ti – O components of the O1s line (q); SiOx component of the Si2p lines and Si – O component of the O1s line (g); Ti2p line and Ti – O component of the O1s line (?).
The stoichiometry of this oxide seems constant through the thickness of the film. Indeed, the shape and the position of the peak containing all the Six+ component do not change as the detection take off angle changes, i.e. as the scanned depth changes (Fig. 2(a)). Moreover, from Si2p line decomposition and the relative intensities ratio of each oxidized species, an average stoichiometry can be determined for this silicon suboxide: SiO1.95. After deposition, titanium was well detected at the surface sample, especially through Ti2p line (Fig. 2(c)). As in the previous case, two peaks at 458.8 eV and 464.4 eV characterize these lines without any broadening at low binding energies. Besides, peak FWHM (1.8 eV) are also in agreement with the presence, in each line, of Ti4+ component only [18]. The morphology of the film was studied by an ARXPS experiment involving the peak intensity evolution for Ti2p, Si0 and Six+ components of Si2p line as well as TiO2 and SiOx components of O1s line. Evolutions of several ratios were plotted (Fig. 3) as the angle of electron detection with respect to the surface decreases, i.e. when the analysis is more and more sensitive to the surface. Especially, the ratio (closed triangle) between the SiOx component of the Si2p lines and the Ti2p intensity were plotted as well as the ratio (open triangle) between the SiOx and TiO2 component of the O1s line (Fig. 3(a)). These two ratios increase as the detection angle with respect to the surface increases, showing that there is more titanium dioxide at the surface than at the interface. However, the detection of titanium dioxide phase from intensities of either Ti2p lines or TiO2 component of the O1s line give different results: the ratio (closed circle) between these two intensities is not independent of the electron detection angle (Fig. 3(b)). Equivalent remark can be expressed from the comparison (open square) between the intensity of SiOx component of the Si2p lines and those of SiOx component of the O1s line (Fig. 3(b)). Actually, these observations reveal that there is less and less titanium detected from Ti2p line than from TiO2 component of the O1s line when the analysis concerns
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A. Monoy et al. / Thin Solid Films 515 (2006) 687 – 690
more and more the surface, i.e. when the analysis concerns more and more the part of the material where there is mainly TiO2. Besides, there is more oxidized silicon detected from Si2p line than from SiOx component of the O1s line when the analysis concerns less the interfacial part of the material, i.e. the part of the material where there is mainly SiOx . A simple interpretation of these points is non-negligible presence of both titanium in SiOx phase and silicon in TiO2 phase. Besides, fitting attempts of different intensities ratios were carried out, especially those related to the simple film models such as TiO2 film grown on a SiOx interfacial layer or TiO2 clusters grown on a layer composed by a homogeneous distribution of TiO2 and SiOx phases. However, no try gave coherent results and a gradient of TiO2 and SiOx phases distribution should be considered. Besides, on the basis of this hypothesis and using average mean free paths of electrons in the TiO2 / SiOx mixture, the total thickness of the mixed phase oxide film could then be estimated from the evolution of the ratio of intensities of O1s components by Si0 component. This total thickness was found equal to 3.4 nm.
Such reaction leads to TTIP breaks, inducing TiO2 formation as well as silicon oxidation. Besides, as the oxidizing power induced by TTIP presence is rather low, the silicon oxidation cannot lead to SiO2 but to a silicon sub-oxide. This process occurs at a rather high temperature which can lead to some interdiffusion between the two phases, inducing a low content of titanium in SiOx phase as well as a low content of silicon in TiO2 phase.
5. Discussion
References
The growth mode revealed on the two studied substrates are quite different: growth of TiO2 clusters on SiO2 surface and appearance of an interphase region which can be described as (TiO2)y (SiOx )z where y and z continuously change from the interface to surface, on hydrogen terminated silicon surface. In other words, the structure of the interface region is strongly influenced by the initial state of the substrate, especially its chemical composition. However, according to the results of the characterisations (size and surface density of the TiO2 islands for the former case and oxide layer thickness as well as average titanium content in the film for the latter case), the amounts of titanium carried out inside the two deposits are globally the same. This point seems to indicate a flux-limited growth of titanium dioxide (characterized by low growth rate) rather than a reaction-limited growth, as previously demonstrated in similar conditions on Si(111) substrates [19]. Indeed, in case of the latter growth, the amount of titanium deposited could be far different. The difference in growth mode as function of the substrate surface should come from the difference in reactivity of precursor molecule with surface. Indeed, it is possible to reckon that TTIP molecules do not well react with inert SiO2 surface. In such case, these molecules can easily diffuse on the surface at 675 -C in order to reach nucleation points, this mechanism involving small TiO2 clusters formation. In case of hydrogen terminated silicon surface, the reaction between surface and substrate is not negligible, especially due to the oxygen affinity of silicon and the oxidizing power of TTIP.
6. Conclusions The substrate surface state (especially its chemical composition) plays a major role in the first state of a TiO2 MOCVD growth carried out from TTIP at very low pressure (fluxlimited condition). Growth carried out on SiO2 terminated surface leads to TiO2 clusters whereas growth on hydrogen terminated silicon surface leads to the formation of an interphase region containing TiO2 and SiOx phases. These differences can be understood as differences in the interfacial reactions between precursor and substrate surface.
[1] Q.-H. Wu, A. Thissen, W. Jaegermann, M. Liu, Appl. Surf. Sci. 236 (2004) 473. [2] G. Soto, Appl. Surf. Sci. 230 (2004) 254. [3] S. Virtanen, P. Schmuki, H.S. Isaacs, Electrochim. Acta 47 (2002) 3117. [4] B.S. Richards, S.F. Rowlands, A. Ueranatasun, J.E. Cotter, C.B. Honsberg, Sol. Energy 76 (2004) 269. [5] 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. [6] A. Rothschild, F. Edelman, Y. Komem, F. Cosandey, Sens. Actuators, B, Chem. 67 (2000) 282. [7] E. Puzenat, P. Pichat, J. Photochem. Photobiol., A Chem. 160 (2003) 127. [8] A. Brevet, P.M. Peterle´, L. Imhoff, M.C. Marco de Lucas, S. Bourgeois, J. Cryst. Growth 275 (2005) e1263. [9] C.S. Fadley, Prog. Surf. Sci. 16 (1984) 275. [10] B. Xie, G. Montano-Miranda, C.C. Finstad, A.J. Muscat, Mater. Sci. Semicond. Process. 8 (2005) 231. [11] R.P. Netterfield, P.J. Martin, C.G. Pacey, W.G. Sainty, D.R. McKenzie, G. Auchterlonie, J. Appl. Phys. 66 (1989) 1805. [12] Y. Gao, S. Thevuthasan, D.E. McCready, M. Engelhard, J. Cryst. Growth 212 (2000) 178. [13] 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, Eden Prairie, Minnesota, 1979. [14] F. Rochet, Ch. Poncey, G. Dufour, H. Roulet, C. Guillot, F. Sirotti, J. Non-Cryst. Solids 216 (1997) 148. [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] W.J. Yang, T. Sekino, K.B. Shim, K. Niihara, K.H. Auh, J. Non-Cryst. Solids 216 (1997) 148. [18] P.R. McCurdy, L.J. Sturgess, S. Kohli, E.R. Fisher, Appl. Surf. Sci. 233 (2004) 69. [19] P.G. Karlsson, J.H. Richter, M.P. Andersson, J. Blomquist, H. Siegbahn, P. Uvdal, A. Sandell, Surf. Sci. 580 (2005) 207.
Surface preparation influence on the initial stages of MOCVD growth of TiO2 thin films A. Monoy a, A. Brevet a, L. Imhoff a,*, B. Domenichini a, E. Lesniewska b, P.M. Peterle´ a, M.C. Marco de Lucas a, S. Bourgeois a b
a Laboratoire de Recherches sur la Re´activite´ des Solides (LRRS)—UMR 5613, France Laboratoire de Physique de l’Universite´ de Bourgogne (LPUB)—UMR 5027, CNRS—Universite´ de Bourgogne, 9 avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France
Available online 7 February 2006
Abstract In situ chemical surface analyses using X-ray photoelectron spectroscopy (XPS), completed by ex situ atomic force microscopy (AFM) analyses, were performed in order to compare the initial stages of MOCVD growth of TiO2 thin films on two different surface types. The first type was a silicon native oxide free hydrogen terminated surface and the second one was a silicon dioxide surface corresponding to a thin layer of 3.5 nm thick in situ thermally grown on silicon substrate. Si(100) was used as substrate, and the growths of TiO2 thin films were achieved with titanium tetraisopropoxide (TTIP) as precursor under a temperature of 675 -C, a pressure of 0.3 Pa and a deposition time of 1 h. Whatever the surface is, the deposited titanium amount was globally the same in the two cases. On the contrary, the deposit morphology was different: a covering layer composed of a SiO2 and TiO2 phases mixture on the hydrogen terminated surface, and small TiO2 clusters homogeneously spread on the SiO2 surface. D 2005 Elsevier B.V. All rights reserved. PACS: 82.33.Ya; 82.80.Pv Keywords: MOCVD; XPS; In situ analyses; Titanium dioxide
1. Introduction Many physical and/or chemical properties of thin films strongly depend on the quality of the interfacial region, which in turn is mainly determined by the initial stages of the growth. It is thus of high importance, in order to study these stages, to perform in situ characterizations. Chemical surface analysis using electron spectroscopies is thus helpful to achieve this, these techniques making possible also some morphologic characterization such as film morphology determination. Although such analyses are carried out in numerous works in the cases of physical vapour deposition growths [1 –3], few works have been carried out for chemical vapour deposition growths.
* Corresponding author. Tel.: +33 3 80 39 61 61; fax: +33 3 80 39 38 19. E-mail address: [email protected] (L. Imhoff). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.12.237
In this way, an original experimental set-up composed of a MOCVD reactor coupled to a UHV chamber devoted to chemical surface analyses (XPS, AES, LEED) has been developed. First, we have focused our works on TiO2/Si, TiO2/SiO2 and TiO2/SiO2/Si systems, because they are widely used and still investigated in many high technological fields due to their outstanding properties [4– 7]. In a previous study [8], using XPS in situ analyses, we succeeded in observing the initial stages of TiO2 thin films growth on silicon substrate, with titanium tetraisopropoxide as precursor. Especially, the formation of TiSix Oy mixed oxide at the interface between silicon and few nanometers thick stoichiometric TiO2 films was pointed out. In this work, we will compare the initial growth stages for two types of surface: on the one hand a silicon native oxide free hydrogen terminated surface, and on the other hand a silicon dioxide surface corresponding to a thin layer of 3.5 nm thick in situ thermally grown on silicon substrate. Moreover, the films are also ex situ characterized by atomic force microscopy in order to
688
A. Monoy et al. / Thin Solid Films 515 (2006) 687 – 690
complete the results obtained by in situ spectroscopic surface analyses, especially the substrate coverage. 2. Experimental 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 Xray photoelectron spectroscopy (XPS) experiments will be presented here. 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 liquid precursor TTIP was maintained in a thermal bath at 40 -C which gave a vapour pressure of about 50 Pa. The growths were performed on (100) oriented n-type silicon substrates, polished on one side. To remove native oxide for hydrogen terminated surface samples, 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 surface oxidized silicon samples were obtained with the following way: they were first cleaned with the same ex situ procedure and then oxidized in situ with a temperature of 710 -C and an oxygen pressure of 100 Pa during 1 h. The TiO2 growths were carried out with temperature fixed at 675 -C, TTIP pressure at 0.3 Pa and deposition time at 1 h. 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 monochromatised Al Ka radiation with electron detection normal to the surface, or with nonmonochromatised Al Ka radiation with angles of detection varying from 15- to 90- with respect to the surface (ARXPS: angular resolved XPS), when it was interesting to get some insight on in-depth composition of the deposits. Deposit thicknesses and average coverages were thus estimated using the model proposed by Fadley [9]. Ex situ characterisations were carried out by atomic force microscopy (AFM) to investigate the morphology of TiO2 deposited onto SiO2 surface. A commercial AFM (Nanoscope IIIa quadrexed, Veeco Inst.) was used in oscillating contact mode (Tappingi mode) investigation. 3. Deposition on oxidized silicon surface First, the substrate surface was studied before deposition by XPS through the Si2p and O1s lines, no C1s line being observed on survey spectrum. Si2p lines are composed of two peaks at 99.3 and 103.4 eV. Such binding energies are characteristic of Si0 and SiO2, respectively [10]. Moreover, the O1s line is composed of a component only at 532.6 eV which can be attributed to thick SiO2 layer [11]. Besides, the analysis of the ratio of the Si0 and SiO2 components as function
Fig. 1. AFM image of an oxidized silicon surface after MOCVD deposition.
of the angle of electron detection according to the Fadley model [9] showed that the silicon oxide wets totally the surface, is homogeneous and presents a thickness of 3.5 nm. After TiO2 deposition, no real change occurs in the Si2p lines. However, a new component occurs in the O1s line at 530.0 eV. This one can be attributed to titanium oxide formation because in the same time, titanium can be detected, especially from the Ti2p lines. Those are characterized by Ti4+ components only at 458.8 eV (Ti2p3/2) and 464.4 eV (Ti2p1/2). Although it is impossible to carry out a quantification from O1s new component and Ti2p too small line intensities, the binding energies [12] and the shapes of Ti2p lines are characteristic of TiO2. Moreover, these too small intensities make impossible the determination of the surface fractional coverage and the thickness of the deposit. Because the film seemed composed of just TiO2 and SiO2 which are phases stable under air, the morphological characterization was carried out by ex situ AFM. By comparison with AFM images before and after deposition (Fig. 1), it is clear that the TiO2 deposit is composed of small clusters homogeneously spread on the SiO2 surface. The average height of these clusters is about 10 nm while their average width is about 20 nm. The fractional coverage is about 5%. 4. Deposition on hydrogen terminated silicon surface Before deposition, the clean surface was investigated by XPS only. Indeed, this surface is not stable under air and cannot thus be characterized by ex situ AFM. On the XPS data, only a very low amount of carbon (through C1s line) and oxygen (thought O1s line) can be detected. Besides, Si2p spectrum obtained just before deposition is composed of one peak at 99.3 eV, characteristic of Si0 component [13]. Because no clear peaks at higher binding energies which could be attributed to silicon oxides are present, the presence of tiny amounts of oxygen and carbon can be mainly attributed to the contamination by carbonaceous species during sample introduction in the UHV vessel, demonstrating the good efficiency on the one hand of the used cleaning recipe on native silicon oxide removal, and on the other hand of the surface stabilisation by hydrogen during air exposure.
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689
Fig. 2. After MOCVD deposition carried out on hydrogen terminated silicon surface; (a): Si2p photoemission spectra as function of the take-off angle; O1s (b) and Ti2p (c) spectra recorded at normal detection angle. These spectra were normalised in order to show the same intensity of the bulk component.
After deposition, new components appear on the Si2p spectrum at higher binding energy revealing silicon oxidation (Fig. 2(a) 10-). Si2p spectrum was then decomposed taking into account the two Si02p components as well as sub-oxides Si4+, Si3+, Si2+ components [14]. The presence of two components in the O1s line was evidenced (Fig. 2(b)). The first component at 530 eV can be attributed to oxygen atoms bound to titanium in titanium dioxide [15] whereas the component around 532.1 eV is due to oxygen atoms bound to silicon. This value is lower than oxygen binding energy in stoichiometric thick layer SiO2 (532.6 eV) and higher than those find for mixed Tix Siy Oz oxide (531.6 eV) [16,8]. It should thus be attributed to SiOx suboxide [17].
Fig. 3. Evolutions as function of the detection angle with respect to the surface of the intensity ratios between: SiOx component of the Si2p lines and Ti2p line (r); Si – O and Ti – O components of the O1s line (q); SiOx component of the Si2p lines and Si – O component of the O1s line (g); Ti2p line and Ti – O component of the O1s line (?).
The stoichiometry of this oxide seems constant through the thickness of the film. Indeed, the shape and the position of the peak containing all the Six+ component do not change as the detection take off angle changes, i.e. as the scanned depth changes (Fig. 2(a)). Moreover, from Si2p line decomposition and the relative intensities ratio of each oxidized species, an average stoichiometry can be determined for this silicon suboxide: SiO1.95. After deposition, titanium was well detected at the surface sample, especially through Ti2p line (Fig. 2(c)). As in the previous case, two peaks at 458.8 eV and 464.4 eV characterize these lines without any broadening at low binding energies. Besides, peak FWHM (1.8 eV) are also in agreement with the presence, in each line, of Ti4+ component only [18]. The morphology of the film was studied by an ARXPS experiment involving the peak intensity evolution for Ti2p, Si0 and Six+ components of Si2p line as well as TiO2 and SiOx components of O1s line. Evolutions of several ratios were plotted (Fig. 3) as the angle of electron detection with respect to the surface decreases, i.e. when the analysis is more and more sensitive to the surface. Especially, the ratio (closed triangle) between the SiOx component of the Si2p lines and the Ti2p intensity were plotted as well as the ratio (open triangle) between the SiOx and TiO2 component of the O1s line (Fig. 3(a)). These two ratios increase as the detection angle with respect to the surface increases, showing that there is more titanium dioxide at the surface than at the interface. However, the detection of titanium dioxide phase from intensities of either Ti2p lines or TiO2 component of the O1s line give different results: the ratio (closed circle) between these two intensities is not independent of the electron detection angle (Fig. 3(b)). Equivalent remark can be expressed from the comparison (open square) between the intensity of SiOx component of the Si2p lines and those of SiOx component of the O1s line (Fig. 3(b)). Actually, these observations reveal that there is less and less titanium detected from Ti2p line than from TiO2 component of the O1s line when the analysis concerns
690
A. Monoy et al. / Thin Solid Films 515 (2006) 687 – 690
more and more the surface, i.e. when the analysis concerns more and more the part of the material where there is mainly TiO2. Besides, there is more oxidized silicon detected from Si2p line than from SiOx component of the O1s line when the analysis concerns less the interfacial part of the material, i.e. the part of the material where there is mainly SiOx . A simple interpretation of these points is non-negligible presence of both titanium in SiOx phase and silicon in TiO2 phase. Besides, fitting attempts of different intensities ratios were carried out, especially those related to the simple film models such as TiO2 film grown on a SiOx interfacial layer or TiO2 clusters grown on a layer composed by a homogeneous distribution of TiO2 and SiOx phases. However, no try gave coherent results and a gradient of TiO2 and SiOx phases distribution should be considered. Besides, on the basis of this hypothesis and using average mean free paths of electrons in the TiO2 / SiOx mixture, the total thickness of the mixed phase oxide film could then be estimated from the evolution of the ratio of intensities of O1s components by Si0 component. This total thickness was found equal to 3.4 nm.
Such reaction leads to TTIP breaks, inducing TiO2 formation as well as silicon oxidation. Besides, as the oxidizing power induced by TTIP presence is rather low, the silicon oxidation cannot lead to SiO2 but to a silicon sub-oxide. This process occurs at a rather high temperature which can lead to some interdiffusion between the two phases, inducing a low content of titanium in SiOx phase as well as a low content of silicon in TiO2 phase.
5. Discussion
References
The growth mode revealed on the two studied substrates are quite different: growth of TiO2 clusters on SiO2 surface and appearance of an interphase region which can be described as (TiO2)y (SiOx )z where y and z continuously change from the interface to surface, on hydrogen terminated silicon surface. In other words, the structure of the interface region is strongly influenced by the initial state of the substrate, especially its chemical composition. However, according to the results of the characterisations (size and surface density of the TiO2 islands for the former case and oxide layer thickness as well as average titanium content in the film for the latter case), the amounts of titanium carried out inside the two deposits are globally the same. This point seems to indicate a flux-limited growth of titanium dioxide (characterized by low growth rate) rather than a reaction-limited growth, as previously demonstrated in similar conditions on Si(111) substrates [19]. Indeed, in case of the latter growth, the amount of titanium deposited could be far different. The difference in growth mode as function of the substrate surface should come from the difference in reactivity of precursor molecule with surface. Indeed, it is possible to reckon that TTIP molecules do not well react with inert SiO2 surface. In such case, these molecules can easily diffuse on the surface at 675 -C in order to reach nucleation points, this mechanism involving small TiO2 clusters formation. In case of hydrogen terminated silicon surface, the reaction between surface and substrate is not negligible, especially due to the oxygen affinity of silicon and the oxidizing power of TTIP.
6. Conclusions The substrate surface state (especially its chemical composition) plays a major role in the first state of a TiO2 MOCVD growth carried out from TTIP at very low pressure (fluxlimited condition). Growth carried out on SiO2 terminated surface leads to TiO2 clusters whereas growth on hydrogen terminated silicon surface leads to the formation of an interphase region containing TiO2 and SiOx phases. These differences can be understood as differences in the interfacial reactions between precursor and substrate surface.
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