14786
J. Phys. Chem. 1996, 100, 14786-14793
Electron Irradiation Induced Chemical Vapor Deposition of Titanium Chloride on Gold and on Magnesium Chloride Thin Films. Surface Characterization by AES, XPS, and TPD§ E. Magni† and G. A. Somorjai*,†,‡ Department of Chemistry, UniVersity of California, Berkeley, California 94720-1460, and Materials Sciences DiVision, Lawrence Berkeley National Laboratory, UniVersity of California, Berkeley, California 94720 ReceiVed: March 29, 1996; In Final Form: June 18, 1996X
The electron irradiation induced TiCl4 deposition on a gold polycrystalline foil and on a Au (100) single crystal in ultra-high vacuum conditions is reported. Titanium chloride ultrathin films are prepared at 250300 K by irradiation of the substrate with 1 keV electrons in a background pressure of TiCl4 of 1 × 10-8 to 5 × 10-7 Torr. The deposited films are characterized by AES, XPS, and temperature-programmed desorption (TPD). We can grow titanium chloride films of the maximum thickness of about 25 Å under our experimental conditions. The film consists of 3-4 layers of TiCl2 with 1 monolayer of TiCl4 chemisorbed on its surface. A comparison is made with the titanium chloride film grown on MgCl2 ultrathin films by electron irradiation induced TiCl4 deposition as previously reported. This film is very similar to the titanium chloride film deposited on Au and can be described as few layers of TiCl2 deposited on a defective MgCl2-x film. One monolayer of TiCl4 is chemisorbed on the TiCl2 surface.
1. Introduction In the past few years, the interest of a number of surface scientists has been focused on the preparation and study of oxide and halide ultrathin films, grown by gas-phase deposition on different substrates, in controlled conditions. We are interested in the growth and characterization of titanium chloride thin films deposited in ultra-high vacuum (UHV) on different supports. The ultimate object of our project is the preparation and surface characterization of model Ziegler-Natta catalysts, which contain titanium chloride as the main component. The low-temperature deposition of TiCl4 on polycrystalline gold foil and on MgCl2 thin films has been previously reported.1 The TiCl4 molecules stick to the gold surface only at very low temperature. This condensed layer sublimes upon heating at 174 K. The lack of chemical interaction between the TiCl4 molecules and the gold or MgCl2 surfaces prevents the deposition of a stable film at higher temperatures. We have already described the electron irradiation induced reduction of MgCl2 monolayer and multilayer films.2 The irradiation of the MgCl2 films with 1 keV electrons stimulates the preferential desorption of Cl atoms. The halide film is partially reduced and a high concentration of defects is produced. Mg atoms appear at the film surface, as detected by ion scattering spectroscopy (ISS). Upon introduction of TiCl4 in the UHV chamber at a pressure of 1 × 10-8 Torr, a layer of titanium chloride is readily deposited at 330 K. A more effective procedure for the preparation of titanium chloride films is the electron irradiation induced TiCl4 deposition, in which the substrate is bombarded with 1 keV electrons in a background pressure of TiCl4. By this means, we have deposited titanium chloride on MgCl2 monolayer and multilayer films.3 In this paper we describe the electron irradiation induced TiCl4 deposition on gold and on MgCl2 thin films at 250-300 * Corresponding author. FAX: +1 510 643 9668. E-mail: somorjai@ garnet.berkeley.edu. † Department of Chemistry. ‡ Materials Sciences Division, Lawrence Berkeley National Laboratory. § This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. X Abstract published in AdVance ACS Abstracts, August 1, 1996.
S0022-3654(96)00941-0 CCC: $12.00
K. TiCl4 is introduced in the UHV chamber at controlled pressure, while the gold substrate is irradiated with 1 keV electrons. Using this procedure, a titanium chloride film can be deposited on gold in the absence of MgCl2. This film has been characterized by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). We can deposit titanium chloride on the Au substrate up to a maximum uptake, which corresponds to an overlayer film thickness of about 25 Å. Titanium chloride films are also grown on MgCl2 thin films by electron irradiation induced TiCl4 deposition at 300 K. We analyzed the chemical composition depth profile of the titanium chloride films deposited on gold and on magnesium chloride by XPS analysis during cycles of ion sputtering and by angular resolved XPS. These studies show that the titanium chloride films deposited on Au and on MgCl2 are very similar. They both consist of a few layers of TiCl2 deposited on the substrate and 1 monolayer of TiCl4 chemisorbed on the TiCl2 film surface. 2. Experimental Section Apparatus and Procedure. The UHV apparatus used in the present study has been described in detail elsewhere.1-3 The surface of the substrate (gold polycrystalline foil or Au(100) single crystal) is cleaned by Ar-ion bombardment and subsequent heating in UHV to anneal the surface defects. The source of TiCl4 is the vapor in equilibrium with its liquid phase (10 Torr at 293 K). During deposition, TiCl4 is introduced in the UHV chamber at a pressure between 1 × 10-8 and 5 × 10-7 Torr through a leak valve. At the same time, the gold substrate, held at 250-300 K, is irradiated with an electron beam of 1 keV of energy and a current density of 50-100 µA/cm2, which corresponds to a flux of (3-6) × 1014 electrons/(cm2 s). The focused beam is rastered over the entire surface of the sample to obtain uniform exposure. The deposition process can be interrupted by closing the leak of TiCl4 into the UHV chamber and turning off the electron beam. MgCl2 films are prepared by gas-phase deposition on the Au substrate held at 300 K. A Knudsen cell is used for this purpose. The stability of the MgCl2 molecule in the gas-phase guarantees the correct stoichiometry of the magnesium chloride film deposited on the inert substrate. © 1996 American Chemical Society
Electron Irradiation Induced TiCl4 Deposition
J. Phys. Chem., Vol. 100, No. 35, 1996 14787
TABLE 1: Percent of Gaussian Contribution (G), Full Width at Half-Maximum (fwhm), Tail Length (TL), and Tail Scale (TS) for the Generation of Individual XPS Peaks at Different Experimental Conditions TiCl4 p3/2 p1/2
X-ray source/ detector pass energy TL TS
TiCl2 p3/2 p1/2
Ti-Au p3/2 p1/2
14 0.6
18 0.6
13.5 18 0.55 0.5
25 15 0.85 0.7
Mg KR/8.9 eV
G 75 fwhm 1.6
75 1.6
75 1.6
75 1.4
75 2.2
Mg KR/71.5 eV
G 85 85 85 85 fwhm 1.85 1.85 1.85 1.85
Al KR/71.5 eV
G 85 fwhm 2.1
85 1.8
85 2.4
85 2.1
85 2.1
75 1.6
85 2.1
The elemental composition and the oxidation states of the atoms in the deposited films are monitored by AES and XPS. AES spectra are taken by detecting the Auger electrons leaving the sample in the direction of the surface normal. In the XPS analysis, photoelectrons with a take-off angle of 45° are detected, unless otherwise specified. The Mg KR excitation radiation (1253.6 eV) is used for the XPS analysis of the titanium chloride films deposited on gold. The Al KR excitation radiation (1486.6 eV) is used for the XPS analysis of the titanium chloride films grown on MgCl2. The photons of this last radiation have sufficient energy to excite the MgKL23L23 Auger transition. The Au4f7/2 peak at 84.0 eV is taken as a reference for the energy scale. For each of the XPS spectra reported in the following section, an attempt has been made to deconvolute the experimental curve in a series of synthetic peaks that represent the contribution of the photoelectron emission from atoms in different chemical environments. These peaks are described as a mixture of Gaussian and Lorentzian contributions in order to take into consideration the effect of the instrumental error on the peak shape characteristic of the photoemission process. A certain degree of asymmetry is introduced to the peak shape to account for the tail present at the high binding energy side of the peak due to inelastic scattering of the photoelectrons during their transport to the sample surface and due to the possible presence of shakeup features. The mathematical form of the tail is described elsewhere.4 The shape of each synthetic peak is fully identified by the percent of Gaussian contribution (G), the full width at half-maximum (fwhm), the tail length (TL), and the tail scale (TS). The values of the parameters used for the description of the individual peaks are described in Table 1. In all cases, p1/2 peaks present areas exactly one-half of the respective p3/2 peaks. TPD experiments are performed by exposing 0.01 L of n-hexane to the sample held at 110 K. After evacuation of the gas phase, the temperature is ramped at a linear rate of 40 K/s and the desorption of the probe molecule is monitored with a mass spectrometer positioned in front of the sample. The atomic composition depth profile has been studied by XPS analysis between cycles of ion sputtering. A beam of Ar ions of 3 keV of energy and a flux of 1.8 × 1014 ions/(cm2 s) have been used for this purpose. 3. Results 3.1. Electron Irradiation Induced TiCl4 Deposition on Au. 3.1.1. TiCl4 Deposition on Au Monitored by AES. We have already reported that it is not possible to deposit TiCl4 on the surface of a gold polycrystalline substrate at a temperature above 170 K, by exposure of 10 L of TiCl4 to the gold.1 The deposition of titanium chloride on the gold surface is successfully accomplished around room temperature in the simultaneous
Figure 1. Electron irradiation induced TiCl4 deposition on Au polycrystalline foil monitored by AES as a function of the deposition time. The signal intensities have been corrected by the Auger cross section of the different elements.4 Pressure of TiCl4: 5 × 10-8 Torr. Electron flux: 6 × 1014 electrons/(cm2 s). Electron energy: 1 keV. Temperature of substrate: 250 K.
TABLE 2: AES Atomic Composition of the Target Sample during Electron Irradiation Induced TiCl4 Deposition on Gold Foil (TiCl4 Deposition Conditions as in Figure 1) TiCl4 depn time (min)
Au
atomic composition (%) Ti
Cl
0 6 12 17 20 25 40
100 42 16 3 2 2 2
10 15 24 25 24 24
48 69 73 73 74 74
presence of an electron beam irradiating the substrate during deposition. A titanium chloride film can be grown on a gold polycrystalline foil held at 250 K, in a background pressure of 5 × 10-8 Torr of TiCl4, in the presence of a flux of 6 × 1014 electrons/(cm2 s) striking the gold substrate with 1 keV of energy. Figure 1 shows the peak to peak height of the TiLMM (387 eV), ClLMM (181 eV), and AuMNN (239 eV) AES peaks, corrected by the relative cross sections,5 during the electron irradiation induced TiCl4 deposition on gold. The linearity of these uptake curves suggests a constant rate of growth of the titanium chloride film until the maximum thickness of the deposited film is reached. This happens in the first 17 min of deposition in our conditions. For longer deposition time, the Ti, Cl, and Au peak intensities remain constant. The titanium chloride film never grows so thick to prevent the detection of the Au Auger electrons. Table 2 summarizes the AES atomic composition of the sample during the electron irradiation induced TiCl4 deposition calculated from the data of Figure 1. The Cl to Ti atomic ratio is close to 3 in the maximum thickness titanium chloride film. The deposited overlayer is stable under electron beam irradiation, with no evident variation of composition of the titanium chloride film after completion of its growth. We can evaluate the thickness of the deposited overlayer from the attenuation of the intensity of the AuMNN Auger peak, knowing the value of the inelastic mean free path of 239 eV electrons in the titanium chloride film and assuming uniform coverage of the substrate. According to S. Tanuma et al.,6-8 we estimated the mean free path of the AuMNN Auger electrons through the titanium chloride film to be 7.3 Å. With this value we can calculate an average thickness of the maximum coverage overlayer of 25 Å. It is not possible to grow thicker films in our experimental conditions.
14788 J. Phys. Chem., Vol. 100, No. 35, 1996
Magni and Somorjai
Figure 3. Electron irradiation induced TiCl4 deposition on polycrystalline Au. Deconvolution of the Ti2p region of the XPS spectrum in its components. Pressure TiCl4: 5 × 10-7 Torr. Electron flux: 3 × 1014 electrons/(cm2 s). Electron energy: 1 keV. Temperature of substrate: 300 K. Deposition time: 60 min. X-ray source: Mg KR. Detector pass energy: 71.5 eV.
Figure 2. Electron irradiation induced TiCl4 deposition on Au polycrystalline foil monitored by TPD of n-C6H14. The relative coverages are calculated in reference to the AES uptake analysis. TiCl4 deposition conditions as in Figure 1. Adsorption temperature: 110 K. n-C6H14 exposure: 1 × 10-2 L. Temperature ramp: 40 K/s.
3.1.2. TiCl4 Deposition on Au Monitored by n-Hexane TPD. The deposition and growth of the titanium chloride film on the Au foil surface can be monitored by n-hexane TPD. n-Hexane physisorbs molecularly on both gold and titanium chloride at 110 K but shows different peak temperatures upon thermal desorption from the two surfaces. A sub-monolayer of n-hexane adsorbed on the gold polycrystalline substrate upon exposure at 0.01 L desorbs at 243 K. Assuming first-order kinetics, we can determine a heat of adsorption of 13.7 kcal/mol for the submonolayer n-hexane physisorbed on gold. n-Hexane desorbs from the maximum coverage titanium chloride film surface at 190 K, corresponding to a heat of adsorption of 10.6 kcal/mol. The difference in the heat of adsorption of n-hexane on these two surfaces allows us to examine the deposition of titanium chloride on the Au foil by TPD. The thermal desorption spectra obtained at various overlayer coverages are shown in Figure 2. The electron irradiation induced TiCl4 deposition on the gold foil was carried out in the same conditions as described for the experiment displayed in Figure 1. The titanium chloride relative coverage has been calculated in agreement with the results reported in Figure 1, assuming maximum coverage (θmax) after 17 min of deposition. The peak due to n-hexane desorbing from gold shifts to 220 K after about one-third of the surface is covered by titanium chloride. At the same coverage, n-hexane shows a desorption peak from the titanium chloride overlayer at 162 K. For higher coverages, the n-hexane desorption peak from gold gradually vanishes, while the desorption peak from the titanium chloride film shifts toward the final value of 190 K and becomes larger, until it is the only feature present when the overlayer is completed. 3.1.3. Titanium Chloride Film Atomic Composition. Determination of the Ti Oxidation State. The presence of the electron beam is crucial for the deposition of a film of titanium chloride on the gold substrate. It is possible that TiCl4+ molecular ions are formed by electron impact, eventually leading to the dissociation of one or more Ti-Cl bonds; a film of stoichiom-
etry different from TiCl4 can be deposited under this circumstance. We analyzed the chemical composition and the oxidation state of the titanium atoms in the deposited film by XPS. The deposition of a titanium chloride film has been induced by electron irradiation, using a pressure of 5 × 10-7 Torr of TiCl4 in the presence of an electron beam flux of 3 × 1014 electrons/(cm2 s) and 1 keV of energy. After 60 min of deposition on the substrate held at 300 K, the atomic composition of the sample has been analyzed by XPS. From the peak areas corrected by the cross section for the different photoelectron emissions,4 the elemental concentration of the sample has been estimated as follows: Ti ) 25%; Cl ) 73%; Au ) 2%. This is in very good agreement with the result of the AES analysis presented in Table 2. Figure 3 shows the Ti2p region of the XPS spectrum recorded after the deposition of the film in the conditions described above. The same figure shows the deconvolution of the experimental result in four asymmetric Gaussian-Lorentzians; the dotted line represents the sum of the four components, to be compared with the experimental result. Two spin-orbit doublets are present, indicating two different chemical environments for the deposited titanium atoms. The doublet at 458.5 and 464.6 eV is due to the 2p3/2 and 2p1/2 photoelectrons from titanium atoms in TiCl4 in its molecular solid state.9,10 In agreement with our previous study concerning the deposition of titanium chloride on MgCl2 films,3 we identify this state as chemisorbed TiCl4. The doublet at 456.1 and 462.1 eV is due to the 2p3/2 and 2p1/2 photoelectrons from titanium atoms at a lower oxidation state. The assignment of the 456.1 eV band to a specific compound is complicated by the fact that it shows a binding energy of the Ti2p3/2 electrons that is intermediate between the range characteristic of the 2+ and the 3+ oxidation states of Ti in bulk oxides and halides. The Ti2p3/2 photoelectrons excited from TiCl3 have a binding energy between 457.0 and 457.6 eV.9 Bulk Ti2O3 has the Ti2p3/2 peak at 457.5 eV,11 while Ti atoms in the 3+ oxidation state, obtained by weak Ar-ion bombardment of the TiO2 (110) singlecrystal surface, show the Ti2p3/2 peak at 457.6 eV.12 TiO bulk has been reported to give the Ti2p3/2 peak at 455.3 eV.11,13 However, thin titanium oxide films, obtained by oxidation at low oxygen pressure of the Pt3Ti alloy, show the Ti2p3/2 XPS peak in the region between 456.3 and 456.9 eV (ref 14 and references therein). This peak has been assigned to the presence of the TiO phase in these films.14,15 The position of the lower binding energy peak shown in Figure 3 (456.1 eV) falls between
Electron Irradiation Induced TiCl4 Deposition
J. Phys. Chem., Vol. 100, No. 35, 1996 14789
Figure 4. Electron irradiation induced TiCl4 deposition on 10 mL of MgCl2 film monitored by XPS as a function of the deposition time. The peak areas have been corrected by the photoelectron cross section of the different elements.8 Pressure of TiCl4: 1 × 10-8 Torr. Electron flux: 3 × 1014 electrons/(cm2 s). Electron energy: 1 keV. Temperature of substrate: 300 K.
the expected values of the Ti2p3/2 peak in the Ti2+ bulk oxide and in the TiO thin films. We assign this feature to the presence of the TiCl2 phase in the deposited titanium chloride film. 3.2. Electron Irradiation Induced TiCl4 Deposition on 10 Layers of MgCl2 Film. XPS Uptake Curves. The electron irradiation induced TiCl4 deposition on monolayer and multilayer MgCl2 films has been already described,3 along with the characterization of the deposited film by AES, XPS, ISS, and TPD. AES uptake experiments during the deposition of TiCl4 on a 1 monolayer MgCl2 film show a behavior very similar to the uptake of TiCl4 on gold, with a constant rate of deposition of the titanium chloride film up to the maximum coverage reachable. TiCl4 has been deposited on 10 layers of MgCl2 previously grown on the surface of an Au (100) single crystal. The electron irradiation induced TiCl4 deposition has been carried out at a pressure of 1 × 10-8 Torr of TiCl4 in the presence of the usual electron beam, with the substrate held at 300 K. Figure 4 shows the correspondent XPS uptake curves, where the experimental areas of the Ti2p, Cl2p, MgKLL, and Au4f7/2 peaks have been corrected by the value of the respective cross sections.4 It is interesting to note that, while the Au signal stays approximately constant during the deposition, the Mg signal decreases sharply. Also, the Cl signal drops by about 20% during the TiCl4 deposition. The XPS atomic concentration after 60 min of deposition is as follows: Ti ) 27%; Cl ) 69%; Mg ) 3%; Au ) 1%. 3.3. Titanium Chloride Film Deposited on Au. Atomic Composition Depth Profile. To better characterize the nature and composition of the film grown on the Au substrate, the atomic composition depth profile has been studied by XPS analysis between cycles of ion sputtering. The result of this analysis is summarized in Figure 5, which shows the Ti2p region of the XPS spectra recorded at various stages of this experiment. Curve a has been obtained after 60 min of electron irradiation induced TiCl4 deposition on polycrystalline gold held at 300 K, using a pressure of 5 × 10-7 Torr of TiCl4 in the presence of an electron beam of the flux of 3 × 1014 electrons/(cm2 s). The two spin-orbit doublets due to TiCl4 (higher binding energy) and to TiCl2 (lower binding energy) are clearly evident. Curves b-e refer to XPS analysis after different doses of Ar sputtering. Spectrum f has been obtained after deposition of a titanium chloride film by the usual procedure on a gold foil, its temperature treatment at 700 K for 1 min, followed by Ar-ion
Figure 5. Ti2p region of the XPS spectra recorded after deposition of titanium chloride on polycrystalline Au (curve a) and its bombardment with different doses of Ar ions at 3 keV of energy (curves b-e). The deconvolution of the spectra in their components is shown. TiCl4 deposition conditions as in Figure 3. X-ray source: Mg KR. Detector pass energy: 8.9 eV. Curve f: Ti-Au alloy obtained by decomposition of a titanium chloride film deposited on Au, followed by temperature treatment at 700 K for 1 min and Ar-ion bombardment with 1 × 1016 ions/cm2.
bombardment with 1 × 1016 ions/cm2. A Ti-Au alloy was formed in this way, with no Cl left on the sample surface. The 2p3/2 electrons of the Ti atoms of this alloy have a binding energy of 454.5 eV, to be compared with the value of 453.8454.0 eV for the Ti2p3/2 peak of Ti metal.11,15-18 A similar shift toward higher binding energy of the Ti metal peaks has been observed in the Pt3Ti alloy,15,19,20 with the Ti2p3/2 peak at 455.1455.5 eV. The pronounced tail of the Ti2p3/2 XPS peak of the Ti-Au alloy is also in agreement with the shape of the same peak detected in the analysis of the Pt3Ti alloy. From the spectra in Figure 5, we see that the peak due to TiCl4 is the first to disappear during ion sputtering. For larger exposure to the ion beam, Cl atoms (and/or ions) preferentially desorb from the titanium chloride film. The Ti atoms left on the sample surface form an alloy with the Au atoms of the substrate. The Ti and Cl XPS peak areas of the spectra recorded after different exposure to the ion beam are reported in Figure 6, after correction by the relative cross sections.4 The average thickness of the overlayer has been determined from the area of the Au4f7/2 peak, compared to the area of the same peak in the XPS spectrum of the clean Au substrate, prior to the TiCl4 deposition. For this calculation, the electron inelastic mean-free path of the Au4f7/2 photoelectrons with a kinetic energy of 1169.6 eV has been estimated to be 22 Å.6-8 The resulting film thickness is reported in Figure 6 together with the areas of the peaks assigned to TiCl4, TiCl2, and Ti-Au alloy, obtained by deconvolution of the experimental data in asymmetric Gaussian-Lorentzians. The Cl to Ti atomic ratio is approximately equal to 3 for the titanium chloride film as deposited. When most of the TiCl4 has been sputtered away by the ion beam, the Cl/Ti ratio is about 2. For larger exposures of the ion beam, this ratio decreases sharply and most of the Ti in the sample is alloyed with Au. Finally, the titanium chloride film grown on gold has been characterized by angular resolved XPS. We have used an Au (100) single-crystal substrate in this experiment. The flat surface
14790 J. Phys. Chem., Vol. 100, No. 35, 1996
Figure 6. Ar-ion bombardment of the titanium chloride film deposited on polycrystalline Au. The Ti and Cl XPS peak areas are corrected by the photoelectron cross section of the different elements.8 The areas of the TiCl4, TiCl2, and Ti-Au components of the Ti2p peak are indicated. The Cl/Ti atomic ratio is also reported. The overlayer film thickness is calculated from the intensity of the Au4f7/2 peak.
Figure 7. Ti2p region of the angular resolved XPS spectra of a titanium chloride film deposited on Au (100) single crystal. The deconvolution of the spectra in their components is shown. The Cl/Ti atomic ratio as a function of the photoelectrons’ take-off angle is also reported. Pressure of TiCl4: 1 × 10-7 Torr. Electron flux: 3 × 1014 electrons/(cm2 s). Electron energy: 1 keV. Temperature of substrate: 300 K. Deposition time: 60 min. X-ray source: Mg KR. Detector pass energy: 71.5 eV.
of the deposited film allowed us to record XPS spectra that varies significantly at the different photoelectron take-off angles and thus to obtain information about the variation of the titanium oxidation state across the film. The titanium chloride film has been deposited using an electron beam flux of 3 × 1014 electrons/(cm2 s) in a background pressure of 1 × 10-7 Torr of TiCl4, with the substrate held at 300 K. The deposition lasted 60 min. Figure 7 shows the Ti2p region of the XPS spectra recorded at four photoelectron take-off angles. For small angles (θ ) 10° and 15°), where the XPS analysis is most surface sensitive, the Ti2p signal is dominated by the TiCl4 component, while at larger angles (θ ) 45° and 75°), the TiCl2 component is the strongest. In the same figure, the Cl to Ti atomic ratio calculated from the XPS analysis is reported as a function of
Magni and Somorjai
Figure 8. Ti2p region of the XPS spectra recorded after deposition of titanium chloride on 10 mL of MgCl2 film (curve a) and its bombardment with different doses of Ar ions at 3 keV of energy (curves b-c). The deconvolution of the spectra in their components is shown. TiCl4 deposition conditions as in Figure 4. X-ray source: Al KR. Detector pass energy: 71.5 eV.
the take-off angle. At small angles this ratio is close to 4, as in TiCl4, while for angles closer to the surface normal the Cl/Ti ratio is about 3, average between 4 and 2 (as in TiCl2). 3.4. Titanium Chloride Film Deposited on MgCl2. Atomic Composition Depth Profile. For a more complete description of the structure and composition of titanium chloride films deposited on MgCl2, we have further characterized these halide thin films by XPS depth profile, both by ion sputtering and by angular resolved analysis. Curve a of Figure 8 shows the Ti2p region of the XPS spectrum recorded after TiCl4 deposition, as described in section 3.2. This spectrum is quite similar to the one reported in Figure 3, taken after TiCl4 deposition on gold, and it indicates the presence of two spin-orbit Ti2p doublets. The two 2p3/2 peaks have binding energies of 458.5 and 455.8 eV, respectively. The higher energy peak is due to Ti atoms in TiCl4 molecules chemisorbed at the film surface. The other peak corresponds to the lower energy peak of Figure 3. We have already assigned this band to TiCl2. Mixed titanium/magnesium chloride with the titanium in the 2+ oxidation state (TiRMg1-RCl2) might be present in the deposited film, as previously suggested.3 The atomic composition depth profile has been studied by cycles of ion sputtering and XPS analysis. Curves a and b of Figure 8 show the Ti2p region of the XPS spectra recorded after two doses of ion bombardment. The titanium chloride film deposited on 10 layers of MgCl2 behaves similarly to the titanium chloride film deposited on gold upon ion bombardment. The peak due to TiCl4 is the first to disappear during sputtering. Also, the Ti atoms form an alloy with the Au atoms of the substrate for large exposures to the ion beam, as indicated by the appearance of the Ti2p3/2 peak at 454.5 eV. The Ti2p, Cl2p, and MgKLL peak areas, corrected by the relative cross sections,4 are shown in Figure 9 as a function of the exposure to the ion beam. The average thickness of the overlayer has been estimated from the area of the Au4f7/2 peak, considering an electron inelastic mean free path of 25 Å for the Au4f7/2 photoelectrons with a kinetic energy of 1402.6 eV.6-8 The resulting film thickness is indicated in Figure 9. The areas of the peaks assigned to Ti4+, Ti2+, and Ti0 are reported in the same figure. The intensity of
Electron Irradiation Induced TiCl4 Deposition
Figure 9. Ar-ion bombardment of the titanium chloride film deposited on 10 mL of MgCl2 film. The Ti, Mg, and Cl XPS peak areas are corrected by the relative cross sections.8 The areas of the Ti4+, Ti2+, and Ti0 components of the Ti2p peak are indicated. The average number of Cl atoms bound to each Ti atom is calculated considering all the Mg present as MgCl2overlayer film thickness is calculated from the intensity of the Au4f7/2 peak.
the Ti2p signal remains approximately constant during the ion bombardment, while Cl atoms (and/or ions) preferentially desorb from the deposited film. The significant increase of the MgKLL signal indicates that most of the magnesium stays underneath the titanium chloride film upon TiCl4 deposition, in the form of defective MgCl2, as indicated by the MgKLL peak at 1179.6 eV (not reported here). The average number of Cl atoms bound to each Ti atom is also shown in Figure 9. This value is calculated by considering all the detected magnesium present as MgCl2. In average, more than two Cl atoms are bound to each Ti atom in the film as deposited, while only 0.5 Cl atoms are bound to one Ti atom at the end of the sputtering experiment. Angular resolved XPS data put in evidence, once again, the similarity between the titanium chloride films grown on the gold substrate and on the multilayer MgCl2 film. Figure 10 shows the Ti2p region of the XPS spectra recorded at four photoelectron take-off angles, after the TiCl4 deposition as described for the data reported in Figure 4. At the smallest take-off angle (θ ) 5°) the Ti2p signal is dominated by the TiCl4 component. At larger angles (θ ) 15°, 45°, and 75°) the Ti2+ peak is the strongest component of the Ti2p signal. This is in full agreement with what we have seen in the case of TiCl4 deposition on gold. 4. Discussion Electron beam incidence is important for the deposition of titanium chloride films on gold and on MgCl2. AES, XPS, and TPD uptake analysis show that a titanium chloride film can be deposited on the surface of these substrates in the presence of a 1 keV electron beam. However, the rates of growth of the titanium chloride films on the two substrates are different. This implies that the respective mechanisms of TiCl4 deposition in the presence of an electron beam might be different. The electron irradiation induced TiCl4 deposition on MgCl2 films has been described as an interaction of the TiCl4 molecules with Mg atoms exposed at the film surface as a consequence of electron induced Cl desorption from MgCl2.2,3 The chemisorbed TiCl4 is involved in a chemical reaction with the defective magnesium chloride obtained by electron bombardment, with partial reduction of the titanium to the 2+ oxidation state. During the irradiation of a MgCl2 monolayer held at 330 K with 1 keV electrons at a flux of 6 × 1014 electrons/(cm2 s),
J. Phys. Chem., Vol. 100, No. 35, 1996 14791
Figure 10. Ti2p region of the angular resolved XPS spectra of a titanium chloride film deposited on 10 mL of MgCl2 previously deposited on Au (100) single crystal. The deconvolution of the spectra in their components is shown. TiCl4 deposition conditions as in Figure 4. X-ray source: Al KR. Detector pass energy: 71.5 eV.
the titanium chloride film reaches its maximum growth after exposure to 2 L of TiCl4.3 The deposition rate is constant up to the maximum thickness of the titanium chloride film, as indicated by the linear shape of the AES uptake curves. From the attenuation of the intensity of the AuMNN Auger peak, considering the inelastic mean free path of 239 eV electrons to be 7.3 Å, we estimate an average total thickness of about 25 Å for the chlorides overlayer at maximum titanium chloride coverage on the MgCl2 monolayer. The electron irradiation induced TiCl4 deposition on gold is slower than the deposition on a monolayer MgCl2 film. When irradiating a gold foil held at 250 K with 6 × 1014 electrons/ (cm2 s) of 1 keV of energy, the maximum coverage titanium chloride film is reached after the exposure to 50 L of TiCl4. The AES uptake curves of Figure 1 suggest a constant rate of growth of the titanium chloride film up to the maximum coverage. We have already calculated the thickness of this film to be about 25 Å. In our experimental conditions the electron irradiation induced TiCl4 deposition is 25 times faster on a monolayer MgCl2 film held at 330 K than on a gold polycrystalline foil at 250 K. When depositing TiCl4 on Au, it is not possible to invoke any chemical reaction between the halide and the substrate, as we have already shown the lack of interaction with the inert Au.1 It is likely that the TiCl4 molecules are ionized by electron impact; the molecular ions can then generate ionized fragments able to chemically interact with the Au surface. Titanium chloride can be deposited through this mechanism also on the MgCl2 monolayer film, but the deposition by chemical reaction between the TiCl4 molecules and the Mg surface atoms of the electron irradiated, defective MgCl2 film appears to be faster in this case. It is not clear, at present time, why we cannot grow titanium chloride films of thickness greater than the observed 25 Å in our experiments. One possibility is that both the electron induced Cl desorption from MgCl2 films and the electron impact ionization of the TiCl4 molecules are due to low-energy secondary electrons (of the energy of 50 eV or lower), which have a higher cross section for the interaction with matter than the 1 keV primary electrons. The smaller yield of secondary
14792 J. Phys. Chem., Vol. 100, No. 35, 1996 electrons produced by the magnesium and titanium chlorides compared to the gold substrate would induce a slower growth of the titanium chloride film, when the deposited overlayer is thick enough to inelastically scatter most of the substrate secondary electrons. This would explain the maximum overlayer thickness of about 25 Å measured when depositing TiCl4 both on gold and on the monolayer MgCl2 film deposited on gold. The same observation would explain the slower electron irradiation induced TiCl4 deposition on multilayer MgCl2 films as compared to the deposition on a monolayer MgCl2.3 Future studies will aim to clarify the mechanism of the TiCl4 deposition in the presence of electron irradiation on gold and on MgCl2 films. AES and XPS analyses of the maximum coverage titanium chloride film deposited on gold indicate that the Cl to Ti atomic ratio is about 3. The Ti2p region of the XPS spectrum shows the presence of Ti atoms in two different chemical environments: TiCl2 and TiCl4 (Figure 3). The same two oxidation states were detected in titanium chloride films deposited on monolayer and multilayer MgCl2 films.3 The relative amount of Ti2+ and Ti4+ depends on the exact TiCl4 deposition conditions. n-Hexane adsorption and thermal desorption has proven to be useful to monitor the deposition of TiCl4 on Au. It does not chemically react with the gold and halide surfaces, but its heat of adsorption on the Au surface differs from the heat of adsorption on the TiCl4 chemisorbed monolayer. This allows us to determine the relative concentrations of gold and titanium chloride on the exposed surface by TPD. The 243 K TPD peak, characteristic of n-hexane desorbing from the clean Au surface, shifts to 220 K after 35% of the maximum coverage titanium chloride has been deposited (Figure 2). A similar peak shift was noticed when monitoring the deposition of MgCl2 on gold polycrystalline foil by n-hexane TPD.1 When 80% of the maximum coverage titanium chloride overlayer has been deposited, the n-hexane desorption peak from the bare Au surface is still present in the TPD spectrum. No bare Au is left at the sample surface at the maximum titanium chloride coverage. The n-hexane desorption peak from the titanium chloride film gradually shifts from 162 K, after the deposition of one-third of the maximum coverage titanium chloride, to 190 K at maximum overlayer coverage. A similar shift was reported for the temperature of the n-hexane desorption peak from MgCl2 thin films deposited on gold.1 n-Hexane desorbs at 150 K from 1 monolayer of MgCl2 film and at 175 K from 6 layers of MgCl2. In the present case, the shift of the desorption peak from the titanium chloride film begins well before the Au surface is completely covered by the overlayer. As we have already discussed, the AES uptake curves (Figure 1) indicate a constant rate of the titanium chloride film growth until the maximum overlayer thickness is obtained. From this and the result of the TPD study, we suggest that the titanium chloride film grows by the deposition of three-dimensional islands of TiCl2. The rate-determining step for the electron irradiation induced TiCl4 deposition on gold is the deposition of the first molecular layer of TiCl2 on the inert substrate. Once this is formed, the fast deposition of multilayers is possible, up to the maximum TiCl2 film thickness. These three-dimensional islands can then grow laterally until the complete coverage of the Au surface. TiCl4 might be chemisorbed on the surface of the TiCl2 islands at all times during the film growth. Both the deposition of TiCl2 on TiCl2 and the chemisorption of TiCl4 are faster than the deposition of TiCl2 on Au. The electron irradiation induced TiCl4 deposition on MgCl2 multilayers deposited on gold is a process more complex than
Magni and Somorjai the TiCl4 deposition on gold and on 1 monolayer of MgCl2, as indicated by the XPS uptake curves of Figure 4. After the exposure of the 10 layers of MgCl2 film to 36 L of TiCl4 in the presence of an electron beam with the flux of 3 × 1014 electrons/ (cm2 s) and 1 keV of energy, the average total thickness of the titanium chloride/magnesium chloride overlayer is about 76 Å, according to the attenuation of the Au4f7/2 XPS peak. The decrease of the MgKLL signal cannot be accounted for by the simple deposition of a few layers of titanium chloride on the MgCl2 film. From the attenuation of the MgKLL peak intensity, considering the inelastic mean-free path of 22 Å for the 1179.6 eV electrons,6-8 we can estimate an average thickness of the titanium chloride film of 45 Å, in the limit case of no intermixing between the Ti2+ and Mg2+ ions in the two chloride films. If we added this value to the thickness of 10 layers of MgCl2 (5.9 Å per monolayer), we would obtain a total thickness of the titanium chloride/magnesium chloride overlayer of 104 Å. This value largely exceeds the 76 Å measured from the attenuation of the Au substrate signal. From this observation, we conclude that some Mg desorbs from the MgCl2 film during the electron irradiation induced TiCl4 deposition. The desorption of Mg atoms (and/or ions) was not detected during the electron irradiation induced reduction of MgCl2 thin films deposited on gold.2 We also notice that the intensity of the Cl signal decreases during the deposition of TiCl4. This indicates that the extra Cl atoms deposited with the titanium chloride film do not compensate the loss of Cl by electron induced desorption from the 10 layers of MgCl2 film. The atomic concentration depth profile and the distribution of TiCl4 and TiCl2 in the titanium chloride films deposited on gold and on 10 layers of MgCl2 film are best characterized by angular resolved XPS (Figures 7 and 10). In both sets of experiments, the Ti4+ doublet is the dominant feature of the Ti2p XPS signal at low photoelectron take-off angles, where the analysis is most surface sensitive. At large take-off angles, the Ti2+ doublet dominates the Ti2p region of the spectrum. The two samples appear to be very similar in this regard, with TiCl4 being at the outermost monolayer of the film and TiCl2 forming the inner layers. In the case of the titanium chloride film deposited on gold, the variation of the Cl to Ti atomic ratio with the angle of the analyzed photoelectrons confirms the description of the overlayer as a few layers of TiCl2 with TiCl4 chemisorbed at its surface. There cannot be more than 1 monolayer of TiCl4 deposited on the TiCl2 film because of the high vapor pressure of TiCl4 around room temperature. The study of the concentration depth profile by ion sputtering and XPS is difficult because of the preferential desorption of Cl and the Ti-Au alloy formation. These problems appear when studying the titanium chloride films deposited on both gold and MgCl2 multilayers. However, in both cases, the Ti4+ component of the Ti2p XPS spectra is the first feature to disappear during the sputtering experiment, again indicating that the outermost monolayer of the titanium chloride film is composed of TiCl4 (Figures 5 and 8). The atomic concentration depth profile of the titanium chloride film deposited on 10 layers of MgCl2 reported in Figure 9 shows more than a 5-fold increase of the Mg signal after the second dose of bombarding ions. From the position of the MgKLL peak, we can assign this signal to defective MgCl2. It follows that the titanium chloride film is deposited onto the MgCl2 film. From these results it is not possible to speculate about the presence of an interface between the two chloride films composed by a mixed titanium/ magnesium chloride, as previously suggested.3 In the case of the electron irradiation induced TiCl4 deposition on gold, we have estimated the thickness of the maximum
Electron Irradiation Induced TiCl4 Deposition coverage overlayer to be about 25 Å. Bulk TiCl2 crystallizes in the CdI2-type structure, with hexagonal close-packed arrangement of the Cl anions. The c parameter of the unit cell is equal to 5.875 Å.21-22 Incidentally, this crystallographic structure is very similar to the β form of bulk MgCl2, where the c parameter is equal to 5.927 Å.23 The TiCl4 free molecule is tetrahedral, with a Ti-Cl bond distance of 2.170 Å. From these observations we conclude that the titanium chloride film deposited on gold consists of 3-4 layers of TiCl2 with 1 monolayer of TiCl4 chemisorbed on it. 5. Conclusion The electron irradiation induced chemical vapor deposition of TiCl4 can be used for the preparation of titanium chloride ultrathin films. This deposition is successful on substrates as different as gold (polycrystalline foil and Au (100) single crystal) and MgCl2 monolayer and multilayer films. Even though the mechanism of TiCl4 deposition in the presence of an electron beam of 1 keV might be different on the two substrates, the deposited films are very similar. On both substrates, the titanium chloride overlayer consists of a few layers of TiCl2 with 1 monolayer of TiCl4 chemisorbed at the surface. The TiCl4/TiCl2/Au system is easier to characterize than the film deposited on MgCl2. The Ti/Cl atomic ratio from the XPS analysis, its dependence on the take-off angle of the analyzed photoelectrons, and the shape of the Ti2p region of the XPS spectrum allowed us to recognize the presence of TiCl2 in the titanium chloride film deposited on gold. The TiCl2 film can reach a maximum thickness of 3-4 layers on the same substrate. We have already pointed out the similarity between the titanium chloride films deposited on gold and on MgCl2. The TiCl4/TiCl2/MgCl2/Au system has been shown to be an active model catalyst for the Ziegler-Natta polymerization of ethylene and propylene after reaction with an aluminum alkyl.24 We are presently studying the TiCl4/TiCl2/Au system as a possible model catalyst for the same polymerization reactions. Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences,
J. Phys. Chem., Vol. 100, No. 35, 1996 14793 Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. The authors would also like to acknowledge support from MONTELL, Inc. References and Notes (1) Magni, E.; Somorjai, G. A. Appl. Surf. Sci. 1995, 89, 187. (2) Magni, E.; Somorjai, G. A. Surf. Sci. 1995, 341, L1078. (3) Magni, E.; Somorjai, G. A. Surf. Sci. 1996, 345, 1. (4) ESCA Operator’s Reference Manual. ESCA Version 4.0 and MultiTechnique Version 2.0; Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN, 1988. (5) AES Operator’s Reference Manual. Version 2.5/3.0; Perkin-Elmer Corp., Physical Electronics Division: Eden Prairie, MN. (6) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1988, 11, 577. (7) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17, 911. (8) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1991, 17, 927. (9) Mousty-Desbuquoit, C.; Riga, J.; Verbist, J. J. Inorg. Chem. 1987, 26, 1212. (10) Mousty-Desbuquoit, C.; Riga, J.; Verbist, J. J. J. Chem. Phys. 1983, 79, 26. (11) Carley, A. F.; Chalker, P. R.; Riviere, J. C.; Roberts, M. W. J. Chem. Soc., Faraday Trans. 1987, 83, 351. (12) Schierbaum, K. D.; Fischer, S.; Torquemada, M. C.; de Segovia, J. L.; Roma´n, E.; Martı´n-Gago, J. A. Surf. Sci. 1996, 345, 261. (13) Simon, D.; Perrin, C.; Bardolle, J. J. Microsc. Spectrosc. Electron. 1976, 1, 175. (14) Bardi, U. Catal. Lett. 1990, 5, 81. (15) Bardi, U.; Ross, P. N.; Rovida, G. Stud. Surf. Sci. Catal. 1989, 48, 59. (16) Anderson, C. R.; Lee, R. N.; Morar, J. F.; Park, R. L. J. Vac. Sci. Technol. 1982, 20, 617. (17) Lebugle, A.; Axelsson, U.; Nyholm, R.; Ma˚rtensson, N. Phys. Scr. 1981, 23, 825. (18) Nefedov, V. I.; Salyn, Y. V.; Chertkov, A. A.; Padurets, L. N. Zh. Neorg. Khim. 1974, 19, 1443. (19) Paul, J.; Cameron, S. D.; Dwyer, D. J.; Hoffmann, F. M. Surf. Sci. 1986, 177, 121. (20) Beard, B. C.; Ross, P. N. J. Electrochem. Soc. 1986, 133, 1839. (21) Baenziger, N. C.; Rundle, R. E. Acta Crystallogr. 1948, 1, 274. (22) Gal’perin, E. L. Sandler, R. A.; Kristallografia 1962, 7, 217. (23) Bassi, I. W.; Polato, F.; Calcaterra, M.; Bart, J. C. J. Zeitchr. Kristallog. 1982, 159, 297. (24) Magni, E.; Somorjai, G. A. Catal. Lett. 1995, 35, 205.
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