Identification of Defect Sites on SiO2 Thin Films Grown on Mo(112

Dec 12, 2002 - In contrast, UPS did not show any changes within the band gap in the presence of these defects. Extended defect sites on SiO2 are ident...
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Langmuir 2003, 19, 354-357

Identification of Defect Sites on SiO2 Thin Films Grown on Mo(112) Y. D. Kim, T. Wei, and D. W. Goodman* Department of Chemistry, Texas A & M University, College Station, Texas 77842-3012 Received July 15, 2002. In Final Form: October 15, 2002 SiO2 thin films on Mo(112) have been characterized using metastable impact electron and ultraviolet photoelectron spectroscopies (MIES and UPS). The electronic properties of SiO2 thin films with a thickness of 0.7-0.8 nm are identical to those of bulk SiO2. For defective SiO2 surfaces prepared by three different ways (synthesis without an anneal, e-beam bombardment, and Si-deposition), additional features are observed in the band gap using MIES that are consistent with theoretical predictions of additional occupied states in the band gap of SiO2 due to vacancies or excess oxygen. In contrast, UPS did not show any changes within the band gap in the presence of these defects. Extended defect sites on SiO2 are identified using MIES/UPS by a narrowing of the O(2P) features with a reduction in the density of extended defect sites. MIES spectra for adsorbed Xe (MAX) are also used to estimate the density of extended defect sites.

1. Introduction A molecular level understanding of surface phenomena in metal oxides is important to a variety of technologies including electronic devices, heterogeneous catalysis, electrochemistry, and geochemistry. Wide band gap oxides such as MgO, Al2O3, and SiO2 are of particular importance; however, studies of these materials utilizing surfacesensitive charged particle probes are complicated because of charging effects. To circumvent difficulties related to specimen charging, oxide thin films supported on refractory metal surfaces can be employed. By using a suitable substrate, epitaxially grown oxide thin films of MgO, Al2O3, and TiO2 have been synthesized that show essentially the same electronic and chemical properties of the corresponding bulk single crystals.1-4 In recent studies SiO2 single-crystalline thin films have been synthesized on a Mo(112) surface.5,6 The identification and characterization of defect sites on oxide surfaces is important in the manufacture of electronic devices where defects can cause significant degradation.7 Moreover, defects on oxide surfaces play a pivotal role in heterogeneous catalysts as nucleation sites for metal nanoclusters and as sites for reaction. Metastable impact electron spectroscopy (MIES) is an extremely surface-sensitive technique that can be used for the identification of defect sites on oxide surfaces difficult to detect with other surface analytical methods. For example, F/F+-centers and extended defects on MgO(100)/Mo(100) have been detected with MIES.2,3,8,9 It is noteworthy that * Author to whom correspondence should be addressed. Phone: (979) 845-0214. Fax: (979) 845-6822. E-mail: [email protected]. tamu.edu. (1) Goodman, D. W. J. Vac. Sci. Technol. A 1996, 14, 1526-1531. (2) Kim, Y. D.; Stultz, J.; Goodman, D. W. Langmuir 2002, 18, 39994004. (3) Kim, Y. D.; Stultz, J.; Goodman, D. W. Surf. Sci. 2002, 506, 228234. (4) Xu, C.; Goodman, D. W. Chem. Phys. Lett. 1997, 265, 341-346. (5) Schroeder, T.; Hammoudeh, A.; Pykavy, M.; Magg, N.; Adelt, M.; Baumer, M.; Freund, H. J. Solid-State Electron. 2001, 45, 1471-1478. (6) Schroeder, T.; Adelt, M.; Richter, B.; Naschitzki, M.; Baumer, M.; Freund, H. J. Surf. Rev. Lett. 2000, 7, 7-14. (7) Pacchioni, G.; Skuja, L.; Griscom, D. L. Defects in SiO2 and related dielectrics: Science and technology, NATO Science Series 2002. (8) Kim, Y. D.; Stultz, J.; Goodman, D. W. Unpublished results. (9) Kolmakov, A.; Stultz, J.; Goodman, D. W. J. Chem. Phys. 2000, 113, 7564-7570.

ultraviolet photoelectron spectroscopy (UPS) has been shown to be much less sensitive than MIES with respect to F/F+-centers and/or extended defects.8,9 In this work, SiO2 thin films grown on Mo(112) were studied using UPS and MIES. The electronic structures of the SiO2 thin films on Mo(112) with a thickness greater than 0.7-0.8 nm are essentially identical to those of SiO2 bulk crystals, indicating that these films can be used as model catalysts and catalyst supports. Using MIES, additional features for various defect sites created on the SiO2 surfaces are observed in the bad gap. MIES and UPS spectra are used for the identification of extended defects by the narrowing of the O(2P) features with a decrease in the number of extended defect sites. Finally, MIES spectra for adsorbed Xe (MAX) is utilized to estimate the density of extended defects. 2. Experimental Section The experiments were carried out in an ultrahigh vacuum (UHV) system (base pressure 1 × 10-10 Torr) consisting of two interconnected chambers. One chamber is equipped with an iongun for sputtering, low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). The second chamber contains Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and MIES/UPS. MIES/UPS spectra were measured simultaneously using a cold-cathode discharge source10,11 that provides both ultraviolet photon and metastable He 23S (E* ) 19.8 eV) atoms with thermal kinetic energy. Metastable and photon contributions to the signal were separated by a time-of-flight method using a mechanical chopper. MIES and UPS spectra were acquired with the photon/metastable beams incident at 45° with respect to the surface normal and utilizing a double pass cylindrical mirror analyzer (CMA). The resolution of our analyzer, estimated from the width of the Fermi edge, is ca. 0.4 eV. In the following spectra, all binding energies are referenced to the Fermi level of the Mo(112) substrate. The Mo(112) sample, used as a substrate for the SiO2 films, was cleaned by repeated flashing to 2200 K; the sample cleanliness was verified with AES. The clean Mo(112) surface showed a rectangular LEED pattern with low background and high spot intensities. To prepare a low defect SiO2 thin film,5,6 Si was deposited on Mo(112) at room temperature and annealed at 800 K in 1 × 10-7 (10) Mausfriedrichs, W.; Wehrhahn, M.; Dieckhoff, S.; Kempter, V. Surf. Sci. 1990, 237, 257-265. (11) Mausfriedrichs, W.; Dieckhoff, S.; Kempter, V. Surf. Sci. 1991, 249, 149-158.

10.1021/la020634e CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002

Defect Sites on SiO2 Films Grown on Mo(112)

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Figure 2. A magnified view of the MIES spectrum in Figure 1a.

Figure 1. MIES and UPS spectra for a low-defect SiO2 thin film. Torr oxygen. Subsequently, the film was annealed at 1200 K for 5 min in 1 × 10-7 Torr oxygen. LEED for the SiO2 films synthesized as above showed a c(2 × 2) periodicity, indicating the formation of a well-ordered SiO2 network. For these SiO2 thin films, Si4+ features were evident at 76 eV in the AES spectra with no Si0 features at ca. 90 eV, indicating that the Si is completely oxidized.5,6

3. Results and Discussions 3.1. Electronic Properties of Low-Defect SiO2 Thin Films. In Figure 1a, a MIES spectrum for a low-defect SiO2 thin film is shown. Using the attenuation of the Mo peaks in AES, the film thickness is estimated to be 0.7-0.8 nm. The MIES spectra for the SiO2 thin films on Mo(112) show a large feature at 8 eV with a shoulder at 7 eV and a smaller feature at 11 eV. The features at 6-8 eV correspond to the O(2P) nonbonding states, and the 11 eV feature to the Si-O bonding state.12 The UPS spectrum for this SiO2 thin film (Figure 1b) is similar to the MIES spectrum, i.e., the O(2P) and the Si-O features are at identical energy positions. In the UPS data, in contrast to the MIES data, additional features with lower intensities originating from the Mo(112) substrate are apparent between 0 and 4 eV. It should be emphasized that MIES has been shown to be sensitive to the topmost surface layer, whereas UPS integrates over the surface and near-surface region.13 It is also noteworthy that the MIES/UPS spectra for these SiO2 thin films are identical to previously published photoemission spectra for SiO2 bulk crystals,12 indicating that the electronic properties of the SiO2 thin films with a thickness of 0.7-0.8 nm are essentially identical to those of bulk SiO2. Recent scanning tunneling spectroscopy (STS) studies in combination with UPS and electron energy loss spectroscopy (EELS) show that bulklike electronic properties of MgO thin films on Ag(100) develop within the first 2-3 monolayers (ca. 0.5-0.8 nm).14 (12) Dipomponio, A.; Continenza, A.; Lozzi, L.; Passacantando, M.; Santucci, S.; Picozzi, P. Solid State Commun. 1995, 95, 313-317. (13) Harada, Y.; Masuda, S.; Ozaki, H. Chem. Rev. 1997, 97, 18971952.

Figure 3. An enlarged view of a MIES spectrum for an MgO thin film on Mo(100).

3.2. Determination of the Band Gap for SiO2 Thin Films. Figure 2 shows a magnified view for the low-energy portion of the MIES spectrum of Figure 1a, in which the onset of the first filled state of the valence band appears at 4.4 eV. Assuming that the Fermi level of Mo(112) is located at the midpoint between the conduction and valence bands of the SiO2 films,15 the measured band gap of the SiO2 thin films is approximately 8.8 eV, in good agreement with the band gap found for bulk SiO2.16 To check the reliability of the method used for the determination of the band gap of the SiO2 thin films, the band gap of a low-defect MgO thin film on Mo(100) was determined from its MIES spectrum. Previous studies have shown that the electronic and chemical properties of MgO thin films prepared by deposition of Mg in O2 background at 600 K followed by annealing at 1150 K are identical to those of well-ordered MgO(100) single crystals.2,3 The onset of the valence band of the well-ordered MgO thin film is at 2.8 eV (Figure 3). With the same assumptions used for the determination of the SiO2 thin film band gap, the band gap of the MgO thin film is estimated to be 5.6 eV, consistent with the value determined with electron energy loss (EELS) and optical spectroscopies.17 3.3. Identification of Defect Sites using MIES. Previous optical spectroscopy measurements indicate various absorption and luminescence bands for defective SiO2.7,18-24 Theoretical studies, carried out to better (14) Schintke, S.; Messerli, S.; Pivetta, M.; Patthey, F.; Libioulle, L.; Stengel, M.; De Vita, A.; Schneider, W. D. Phys. Rev. Lett. 2001, 8727, art-276801. (15) Ochs, D.; Braun, B.; Maus-Friedrichs, W.; Kempter, V. Surf. Sci. 1998, 417, 406-414. (16) Pacchioni, G.; Ierano, G. Phys. Rev. B 1998, 57, 818-832. (17) Wu, M. C.; Truong, C. M.; Goodman, D. W. Phys. Rev. B 1992, 46, 12688-12694.

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Figure 5. MIES spectra for a SiO2 thin film before and after e-beam bombardments.

Figure 4. MIES and UPS spectra for a defective SiO2 film before and after annealing at 1050 K. The defective film was prepared by deposition of Si onto a well-ordered SiO2 film at room temperature, followed by oxidation at 800 K. Further annealing at 1050 K results in the disappearance of the defect state.

understand the optical absorption and photoluminescence spectra of SiO2,7,16,25-30 have shown that additional occupied and unoccupied states are present in the band gap region16 when various defect sites (oxygen vacancies or excess oxygen) are present, and that these defect sites are responsible for the experimentally observed color centers. Previous MIES studies have shown that small amounts of oxygen vacancies on MgO surfaces can be identified using MIES,9 whereas UPS is not as sensitive as MIES for assessing oxygen vacancies.13 If defect sites on SiO2 surfaces lead to additional occupied states in the band gap, MIES should identify these defect states. An additional occupied state in the band gap, which is not present on low-defect SiO2 surfaces, is observed for a defective SiO2 surface (Figure 4a) prepared by depositing excess Si onto a well-ordered SiO2 surface at room temperature followed by oxidation at 800 K. Following an anneal to 1050 K, the band gap state disappears (Figure 4a), indicating that the anneal reduces the number of defect sites. (18) Trukhin, A. N.; Goldberg, M.; Jansons, J.; Fitting, H. J.; Tale, I. A. J. Non-Cryst. Solids 1998, 223, 114-122. (19) Miller, A. J.; Leisure, R. G.; Mashkov, V. A.; Galeener, F. L. Phys. Rev. B 1996, 53, R8818-R8820. (20) Nishikawa, N.; Miyake, Y.; Watanabe, E.; Ito, D.; Seol, K. S.; Ohki, Y.; Ishii, K.; Sakurai, Y.; Nagasawa, K. J. Non-Cryst. Solids 1997, 222, 221-227. (21) Nishikawa, H.; Watanabe, E.; Ito, D.; Ohki, Y. Phys. Rev. Lett. 1994, 72, 2101-2104. (22) Guzzi, M.; Pio, F.; Spinolo, G.; Vedda, A.; Azzoni, C. B.; Paleari, A. J. Phys.: Condens. Matter 1992, 4, 8635-8648. (23) Boscaino, R.; Cannas, M.; Gelardi, F. M.; Leone, M. J. Phys.: Condens. Matter 1996, 8, L545-L549. (24) Boscaino, R.; Cannas, M.; Gelardi, F. M.; Leone, M. Phys. Rev. B 1996, 54, 6194-6199. (25) Pacchioni, G.; Ierano, G.; Marquez, A. M. Phys. Rev. Lett. 1998, 81, 377-380. (26) Pacchioni, G.; Ierano, G. Phys. Rev. B 1997, 56, 7304-7312. (27) Pacchioni, G.; Ierano, G. J. Non-Cryst. Solids 1997, 216, 1-9. (28) Pacchioni, G.; Ierano, G. Phys. Rev. Lett. 1997, 79, 753-756. (29) Uchino, T.; Takahashi, M.; Yoko, T. Phys. Rev. Lett. 2001, 86, 5522-5525. (30) Uchino, T.; Takahashi, M.; Yoko, T. Phys. Rev. Lett. 2001, 86, 4560-4563.

Figure 6. MIES spectra for a SiO2 thin film as a function of Si deposition at 300 K.

Given that the defective surface in Figure 4 was created by oxygen treatment at 800 K, it is reasonable to assume that the number of oxygen vacancies on this surface is relatively low. Furthermore, no Si0 was detected with AES, consistent with the number of oxygen vacancies being low. Therefore it is likely that defect sites other than oxygen vacancies, such as a peroxyl bridge, a peroxyl radical, or a nonbridging oxygen,16 are responsible for the appearance of the band gap states in Figure 4a. In contrast to MIES, no additional electronic state induced by defect sites can be clearly identified by UPS (Figure 4b). Furthermore, the UPS spectra consist of contributions from the SiO2 film as well as features from the underlying Mo(112) substrate that overlap the band gap states from various defect sites of SiO2. This superposition of the Mo(112) features and the defect states complicate the unambiguous identification of defect states in the band gap using UPS. Additional band gap states were observed on a SiO2 thin film damaged by a 3 kV electron-beam (Figure 5). On MgO surfaces, high-energy electron-beam treatments selectively remove oxygen atoms from the surface, creating F/F+ centers.9 Likewise, electron bombardments induce the formation of oxygen vacancies on SiO2 thin films, which can form additional states in the band gap. Band gap states are also introduced in the SiO2 films by deposition of Si at room temperature. As is apparent in Figure 6, Si deposition onto SiO2 at room-temperature introduces features in the MIES spectrum similar to those found for the defective SiO2 surfaces of Figures 4 and 5. Si deposition onto SiO2 can create an oxygen-depleted SiO2 surface with an electronic structure essentially identical to a surface with oxygen vacancies. Analogously, Mg deposition onto MgO produces additional features in the band gap as measured by EELS, and has been assigned to oxygen vacancies (e.g., neutral F centers).31 3.4 .Extended Defects. Figure 7 shows MIES spectra for two SiO2 thin films prepared by annealing at 1050 and

Defect Sites on SiO2 Films Grown on Mo(112)

Figure 7. MIES for the SiO2 thin films prepared by annealing at 1050 and 1200 K, respectively. An enlarged image of the lower energy region of the above spectra.

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Figure 9. MIES for adsorbed Xe (MAX) for the SiO2 thin films prepared by annealing at 1050 and 1200 K, respectively.

features from the Xe 5P1/2 and 5P3/2 levels are observed. The widths of the Xe 5P peaks reflect the homogeneity of the surface, i.e., the narrow 5P peaks for the 1200 K-annealed SiO2 films indicate that the surface is very uniform. In contrast, the 1050 K-annealed film shows a shoulder at higher binding energies in addition to the doublet features. Most likely this shoulder results from Xe atoms adsorbed at extended defect sites.32,33 From the relative intensity of this shoulder with respect to those of the sharp doublet features, the extended defect density for the 1050 K-annealed SiO2 surface is estimated to be approximately 20-30%. Figure 8. UPS for the SiO2 thin films prepared by annealing at 1050 and 1200 K, respectively.

1200 K, respectively. The 1050 K-annealed film exhibited a c(2 × 2) LEED pattern with weak spots and a high background intensity, suggesting a rough surface with relatively small grains. Annealing at 1200 K leads to an increase in the intensity of the LEED diffraction spots with a concomitant decrease in the background intensity, indicating that a more atomically smooth and ordered surface is formed by the anneal. MIES spectra for the 1200 K-annealed SiO2 surfaces show much narrower O(2P) features in comparison to those from the 1050 K-annealed SiO2 thin films, indicating that the widths of the O(2P) bands are a measure of the extended defect (step, corner, etc.) densities. The energy of the valence band in the MIES spectra for both SiO2 films are identical, showing that the valence band edge is insensitive to surface roughness or grain size. These results are consistent with the band gap states at 3.3 eV corresponding to point defects rather than arising from surface roughness. In Figure 8 , UPS spectra for both SiO2 surfaces are presented. Similar to the MIES results, UPS also shows changes in the O(2P) features with increasing extended defect densities. An additional technique for the identification of extended defect sites is MAX.2 Figure 9 shows MAX spectra from two different SiO2 surfaces collected with a Xe pressure of 5 × 10-5 Torr at a sample temperature of 80 K. Under these conditions, Xe monolayers are formed.2 For the 1200 K-annealed SiO2 surface, sharp doublet (31) Tegenkamp, C.; Pfnur, H.; Ernst, W.; Malaske, U.; Wollschlager, J.; Peterka, D.; Schroder, K. M.; Zielasek, V.; Henzler, M. J. Phys.: Condens. Matter 1999, 11, 9943-9954.

4. Conclusions In this work, UPS and MIES results for various SiO2 thin films grown on Mo(112) are presented. The electronic structure of a 0.7-0.8 nm SiO2 thin film on Mo(112) is essentially identical to that of a SiO2 bulk crystal. The band gap of the SiO2 thin films was determined by MIES to be 8.8 eV, in agreement with the band gap of bulk SiO2. Using MIES, additional features for various defect sites on SiO2 surfaces are observed within the band gap region, whereas UPS shows only minor features that can be related to the presence of defect sites. These results illustrate that relatively low densities of defect sites on oxides surfaces, not detectable using other surface science techniques, can be measured with MIES. Extended defects can be identified by broadening of the O(2P) band in MIES and UPS. In addition, MIES for adsorbed Xe (MAX) is shown to be useful for the identification and quantification of defect sites. In this work, it is demonstrated that a combination of AES, MIES/UPS, LEED, and MAX can provide information regarding various defect sites. Studies on the behavior of metal clusters on various SiO2 thin films are in progress to address the role of defect sites in the modification of electronic and chemical properties of supported metal clusters. Acknowledgment. Funding for this work was provided by the Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, and the Robert A. Welch Foundation. J. Stultz is acknowledged for helpful discussions. LA020634E (32) Kim, Y. D.; Stultz, J.; Wei, T.; Goodman, D. W. J. Phys. Chem. B, submitted. (33) Wandelt. K. Appl. Surf. Sci. 1997, 111, 1.