A series of PM-IRAS spectra collected during step-by- step preparation of SiO 2 films on Mo ͑ 112 ͒ . Films ͑ a ͒ and ͑ b ͒ were 

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... that is not evident in the other UPS spectra. The c ͑ 2 ϫ 2 ͒ LEED pattern of film ͑ c ͒ is very sharp, indicating a high ordered surface. PM-IRAS was also used to investigate the SiO 2 films synthesized with method 2 ͑ Fig. 5 ͒ . Starting with a ϳ 1.3 ML SiO 2 film ͑ a ͒ , silica films ͑ b ͒ – ͑ f ͒ were prepared by successive annealing cycles in the presence of 1 ϫ 10 −7 Torr O 2 at 1140, 1150, and 1200 K for 10 to 25 min. The striking decrease in the intensity of the band at ϳ 1201 cm −1 during annealing is accompanied by the sharpening and growth of the feature at ϳ 1057 cm −1 . Finally, after five successive annealing steps, film ͑ f ͒ with ␪ ϳ 1.0 ML is produced. The film thickness was verified by AES and its order, by a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. For reference, a much thicker SiO 2 film was grown by Si deposition in the presence of O 2 using the procedure of Xu and Goodman. 13 This recipe was used to grow a ϳ 4.8 ML amorphous SiO 2 film on Mo ͑ 112 ͒ ͑ Fig. 6 ͑ a ͒͒ . Next, the coverage of this amorphous film was reduced by annealing at 1250 K. Comparing the spectrum of film ͑ a ͒ with those of films ͑ b ͒ and ͑ c ͒ , it is apparent that the broad asymmetric band at 1165 cm −1 becomes sharper due to annealing at 1250 K. Furthermore, the band shifts to 1234 cm , forming a much more intense feature with a shoulder at 1159 cm −1 . In addition, a small feature at 1048 cm −1 is evident in all of the spectra of Fig. 6, although in film ͑ a ͒ , this feature is convoluted with the broadband at 1159 cm −1 . SiO 2 films on various substrates have been studied exten- sively in the literature using vibrational spectroscopic techniques. 13,15,16,39,40,43–46 Our discussion of SiO 2 films on Mo ͑ 112 ͒ will focus primarily on the phonon structure between 1000 and 1400 cm −1 , i.e., the asymmetric stretching ͑ AS ͒ region, as the vibrational features corresponding to symmetric stretching ͑ϳ 768 cm −1 ͒ and bending ͑ϳ 496 cm −1 ͒ modes of SiO 2 / Mo ͑ 112 ͒ 16 were not accessible due to the cutoff frequencies of the CaF 2 windows used in the experiments. Amorphous SiO 2 films reveal typically two different asymmetric stretching modes: in-phase motion of adjacent O atoms ͑ AS 1 ͒ and out-of-phase motion of adjacent O atoms ͑ AS 2 ͒ . 44 Since the optical phonon band is composed of transverse ͑ TO ͒ and longitudinal ͑ LO ͒ modes, four different AS vibrational features are observed in the IR spectra at ϳ 1076 cm −1 ͑ TO-AS 1 ͒ , ϳ 1160 cm −1 ͑ LO-AS 2 ͒ , ϳ 1200 cm −1 ͑ TO-AS 2 ͒ , and ϳ 1256 cm −1 ͑ LO-AS 1 ͒ . 44 However, the SiO 2 films studied in this work have thicknesses well below the so-called Berreman thickness. 45–47 Therefore TO modes are expected to be fully suppressed whereas LO modes corresponding to the interlinking of the ͓ SiO 4 ͔ tetrahedra and other bands such as the ones corresponding to Si- O - Mo linkages of ͓ SiO 4 ͔ tetrahedra to the metallic Mo ͑ 112 ͒ substrate are anticipated to be observable. 47 Conse- quently, the vibrational features between 1100 and 1250 cm −1 are assigned to LO-AS 1 and LO-AS 2 modes of Si- O - Si linkages whereas vibrational features between 1000 and 1100 cm −1 are attributed to the Si- O - Mo linkages. Note that for SiO 2 films grown on a metal surface, the AS 1 -LO mode was observed at a wide range, 1180– 1250 cm −1 , de- pending on the annealing temperature, 13 1190– 1218 cm −1 as a function of the film thickness ͑ within 3 ML ͒ . 15 During the initial stages of the SiO 2 film growth on Mo ͑ 112 ͒ using method 1, only a single broad vibrational feature at ϳ 1046 cm −1 is apparent ͑ Fig. 3 ͑ a ͒͒ . This feature is associated with the Si- O - Mo linkages of isolated ͓ SiO 4 ͔ tetrahedra at the surface. That there is no evidence of an LO mode at 1100– 1250 cm −1 at this coverage ͑ 0.6 ML ͒ is consistent with there being no Si- O - Si linkages between these isolated ͓ SiO 4 ͔ units. 16 In previous reports, a shoulder at ϳ 980 cm −1 , similar to the one seen in Fig. 3 ͑ a ͒ , was attributed to suboxides in the SiO 2 / Si interface 43 or to the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ . 15 However, after a high-temperature anneal, no OH-related stretching features were apparent. Therefore, the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ can be ruled out, fully consistent with recent work addressing the interaction of water with SiO 2 / Mo ͑ 112 ͒ . 48 Furthermore, it is known from previous AES and XPS data that SiO 2 films grown on Mo ͑ 112 ͒ do not contain suboxides for coverages ␪ Ͻ 1 ML. 14 Thus, the shoulder at ϳ 980 cm −1 is likely due to oxygen atoms bonded to the Mo substrate. According to previous vibrational studies we assign the shoulder at ϳ 980 cm −1 to the stretching mode ͓ ␯ ͑ Mo v O ͔͒ of terminal atop oxygen on the topmost Mo atoms. 49–51 This assignment is consistent with the disappearance of the 980 cm −1 feature and the formation of a sharp and symmetric feature at ϳ 1057 cm −1 upon the formation of a well-ordered SiO 2 film at 1 ML ͑ Fig. 3 ͑ b ͒͒ . After completion of the monolayer film ͑ Fig. 3 ͑ c ͒͒ , the formation of a second and a relatively less ordered SiO 2 layer is observed. This is evident from the appearance of an asymmetric feature at ϳ 1201 cm −1 with a shoulder at ϳ 1170 cm −1 corresponding to LO modes of AS 1 and AS 2 for Si- O - Si linkages, respectively. The appearance of such LO modes after completion of the first layer suggests the formation of Si- O - Si linkages between the ͓ SiO 4 ͔ tetrahedra ini- tially disconnected at lower coverages. 16 Figure 5 illustrates that method 2 also can be used to obtain a highly ordered SiO 2 film by annealing an amorphous Ͼ 1 ML SiO 2 film in oxygen at elevated temperatures. The ordered film with a coverage close to one monolayer obtained at the end of these annealing steps shows only a single sharp vibrational feature at ϳ 1054 cm −1 with a FWHM of ϳ 29 cm −1 . LO modes of bulklike SiO 2 can be addressed by growing relatively thick SiO 2 films where Si- O - Si linkages are known to exist between the ͓ SiO 4 ͔ tetrahedra and shown to exhibit intense LO bands. Figure 6 presents such thick SiO 2 films grown on Mo ͑ 112 ͒ where LO modes are visible at ϳ 1160 cm −1 ͑ LO-AS 2 ͒ and ϳ 1234 cm −1 ͑ LO-AS 1 ͒ . In addition to these bands, the feature at ϳ 1048 cm −1 associated with Si- O - Mo linkages is evident in these spectra. It should be noted that the high-temperature annealing steps lead to ordering of the SiO 2 films and to sharpening and growth of the LO-AS 1 band at ϳ 1234 cm −1 with concurrent attenua- tion of the LO-AS 2 feature at ϳ 1160 cm −1 . Similar intensity changes for LO-AS 1 and LO-AS 2 modes have been reported for thick SiO 2 films on Mo ͑ 110 ͒ 13 and for the transition from amorphous silicon dioxide to ordered ␣ quartz. 44 Likewise the electronic properties of SiO 2 films on various substrates have been addressed previously. 15,17,18,36–42 UPS has been frequently employed to study the valence band of various SiO 2 surfaces while MIES has been used to a lesser extent. For a comparison with previous work using these two spectroscopies, 36,37 we summarize in Table I the positions of the valence bands and the valence band edge. The relative band positions and the widths of the bands of the spectra in Refs. 36 and 37 agree very well. Deviations of the band positions with respect to E F are due to an uncer- tainty in the assignment of the Fermi level in Ref. 36. Note that in the present work the valance band edge of the thick a -SiO 2 films agrees exactly with that reported by Brause et al. It is also noteworthy that all of the spectra are dominated by the O ͑ 2 p ͒ nonbonding band. The Si- O bonding bands are clearly less intense than the O ͑ 2 p ͒ nonbonding bands for both MIES and UPS. Since the thicker SiO 2 films in the present work ͑ Figs. 2 ͑ c ͒ and 4 ͑ a ͒͒ show spectra very similar to those of various SiO 2 surfaces in the literature 36–42 it is appropriate to use these films as a reference where Si- O - Si linkages predomi- nate. Therefore, the broad Si- O bonding feature with a maximum at ϳ 11.5 eV is assigned to a surface consisting of Si- O - Si linkages. In contrast, the spectra observed for films of one monolayer that were prepared by method 1 ͑ Figs. 1 ͑ a ͒ and 2 ͑ a ͒͒ clearly differ from the spectra of the thicker films. This difference arises due to influence of the Mo substrate—a view consistent with the vibrational data ob- tained using PM-IRAS see previously and HREELS. Consequently, we attribute the sharp and intense Si- O bonding band at ϳ 10.5 eV to Si- O - Mo linkages at the surface. This assignment is supported by the AES and work function data in Fig. 1 ͑ b ͒ , where break points indicate the completion of the first ML. It is noteworthy that the exis- tence of Si- O - Mo linkages at the interface is also evident from XPS measurements. 14 ͑ a ͒ ,20 The results shown in Fig. 2 further support the assignment of the features at ϳ 11.5 and ϳ 10.5 eV to Si- O - Si and Si- O - Mo linkages, respectively. These results suggest that within the range of 1 to ϳ 2.5 ML the band in the Si- O bonding region is a convolution of features at ϳ 10.5 and ϳ 11.5 eV. Since the second layer consists of Si- O - Si linkages, the maximum of the Si- O bonding band shifts to higher binding energies as the coverage is increased. At a coverage of ϳ 2.6 ML, the contribution of the feature at ϳ 10.5 eV is no longer obvious with either UPS or MIES. The states close to E F are still detectable with UPS even for a thick SiO 2 film since this technique probes several layers in the near-surface region. According to the results shown in Fig. 2 we conclude that bulklike electronic properties of silica develop within the first 2 ML. These results are consistent with the suggested thickness lower limit of usable SiO 2 gate electric materials observed by Muller et al. 52 Based on EELS in a scanning transmission electron micro- scope these authors found the thinnest, usable SiO ...

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... 2 ϫ 2 ͒ LEED pattern of film ͑ c ͒ is very sharp, indicating a high ordered surface. PM-IRAS was also used to investigate the SiO 2 films synthesized with method 2 ͑ Fig. 5 ͒ . Starting with a ϳ 1.3 ML SiO 2 film ͑ a ͒ , silica films ͑ b ͒ – ͑ f ͒ were prepared by successive annealing cycles in the presence of 1 ϫ 10 −7 Torr O 2 at 1140, 1150, and 1200 K for 10 to 25 min. The striking decrease in the intensity of the band at ϳ 1201 cm −1 during annealing is accompanied by the sharpening and growth of the feature at ϳ 1057 cm −1 . Finally, after five successive annealing steps, film ͑ f ͒ with ␪ ϳ 1.0 ML is produced. The film thickness was verified by AES and its order, by a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. For reference, a much thicker SiO 2 film was grown by Si deposition in the presence of O 2 using the procedure of Xu and Goodman. 13 This recipe was used to grow a ϳ 4.8 ML amorphous SiO 2 film on Mo ͑ 112 ͒ ͑ Fig. 6 ͑ a ͒͒ . Next, the coverage of this amorphous film was reduced by annealing at 1250 K. Comparing the spectrum of film ͑ a ͒ with those of films ͑ b ͒ and ͑ c ͒ , it is apparent that the broad asymmetric band at 1165 cm −1 becomes sharper due to annealing at 1250 K. Furthermore, the band shifts to 1234 cm , forming a much more intense feature with a shoulder at 1159 cm −1 . In addition, a small feature at 1048 cm −1 is evident in all of the spectra of Fig. 6, although in film ͑ a ͒ , this feature is convoluted with the broadband at 1159 cm −1 . SiO 2 films on various substrates have been studied exten- sively in the literature using vibrational spectroscopic techniques. 13,15,16,39,40,43–46 Our discussion of SiO 2 films on Mo ͑ 112 ͒ will focus primarily on the phonon structure between 1000 and 1400 cm −1 , i.e., the asymmetric stretching ͑ AS ͒ region, as the vibrational features corresponding to symmetric stretching ͑ϳ 768 cm −1 ͒ and bending ͑ϳ 496 cm −1 ͒ modes of SiO 2 / Mo ͑ 112 ͒ 16 were not accessible due to the cutoff frequencies of the CaF 2 windows used in the experiments. Amorphous SiO 2 films reveal typically two different asymmetric stretching modes: in-phase motion of adjacent O atoms ͑ AS 1 ͒ and out-of-phase motion of adjacent O atoms ͑ AS 2 ͒ . 44 Since the optical phonon band is composed of transverse ͑ TO ͒ and longitudinal ͑ LO ͒ modes, four different AS vibrational features are observed in the IR spectra at ϳ 1076 cm −1 ͑ TO-AS 1 ͒ , ϳ 1160 cm −1 ͑ LO-AS 2 ͒ , ϳ 1200 cm −1 ͑ TO-AS 2 ͒ , and ϳ 1256 cm −1 ͑ LO-AS 1 ͒ . 44 However, the SiO 2 films studied in this work have thicknesses well below the so-called Berreman thickness. 45–47 Therefore TO modes are expected to be fully suppressed whereas LO modes corresponding to the interlinking of the ͓ SiO 4 ͔ tetrahedra and other bands such as the ones corresponding to Si- O - Mo linkages of ͓ SiO 4 ͔ tetrahedra to the metallic Mo ͑ 112 ͒ substrate are anticipated to be observable. 47 Conse- quently, the vibrational features between 1100 and 1250 cm −1 are assigned to LO-AS 1 and LO-AS 2 modes of Si- O - Si linkages whereas vibrational features between 1000 and 1100 cm −1 are attributed to the Si- O - Mo linkages. Note that for SiO 2 films grown on a metal surface, the AS 1 -LO mode was observed at a wide range, 1180– 1250 cm −1 , de- pending on the annealing temperature, 13 1190– 1218 cm −1 as a function of the film thickness ͑ within 3 ML ͒ . 15 During the initial stages of the SiO 2 film growth on Mo ͑ 112 ͒ using method 1, only a single broad vibrational feature at ϳ 1046 cm −1 is apparent ͑ Fig. 3 ͑ a ͒͒ . This feature is associated with the Si- O - Mo linkages of isolated ͓ SiO 4 ͔ tetrahedra at the surface. That there is no evidence of an LO mode at 1100– 1250 cm −1 at this coverage ͑ 0.6 ML ͒ is consistent with there being no Si- O - Si linkages between these isolated ͓ SiO 4 ͔ units. 16 In previous reports, a shoulder at ϳ 980 cm −1 , similar to the one seen in Fig. 3 ͑ a ͒ , was attributed to suboxides in the SiO 2 / Si interface 43 or to the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ . 15 However, after a high-temperature anneal, no OH-related stretching features were apparent. Therefore, the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ can be ruled out, fully consistent with recent work addressing the interaction of water with SiO 2 / Mo ͑ 112 ͒ . 48 Furthermore, it is known from previous AES and XPS data that SiO 2 films grown on Mo ͑ 112 ͒ do not contain suboxides for coverages ␪ Ͻ 1 ML. 14 Thus, the shoulder at ϳ 980 cm −1 is likely due to oxygen atoms bonded to the Mo substrate. According to previous vibrational studies we assign the shoulder at ϳ 980 cm −1 to the stretching mode ͓ ␯ ͑ Mo v O ͔͒ of terminal atop oxygen on the topmost Mo atoms. 49–51 This assignment is consistent with the disappearance of the 980 cm −1 feature and the formation of a sharp and symmetric feature at ϳ 1057 cm −1 upon the formation of a well-ordered SiO 2 film at 1 ML ͑ Fig. 3 ͑ b ͒͒ . After completion of the monolayer film ͑ Fig. 3 ͑ c ͒͒ , the formation of a second and a relatively less ordered SiO 2 layer is observed. This is evident from the appearance of an asymmetric feature at ϳ 1201 cm −1 with a shoulder at ϳ 1170 cm −1 corresponding to LO modes of AS 1 and AS 2 for Si- O - Si linkages, respectively. The appearance of such LO modes after completion of the first layer suggests the formation of Si- O - Si linkages between the ͓ SiO 4 ͔ tetrahedra ini- tially disconnected at lower coverages. 16 Figure 5 illustrates that method 2 also can be used to obtain a highly ordered SiO 2 film by annealing an amorphous Ͼ 1 ML SiO 2 film in oxygen at elevated temperatures. The ordered film with a coverage close to one monolayer obtained at the end of these annealing steps shows only a single sharp vibrational feature at ϳ 1054 cm −1 with a FWHM of ϳ 29 cm −1 . LO modes of bulklike SiO 2 can be addressed by growing relatively thick SiO 2 films where Si- O - Si linkages are known to exist between the ͓ SiO 4 ͔ tetrahedra and shown to exhibit intense LO bands. Figure 6 presents such thick SiO 2 films grown on Mo ͑ 112 ͒ where LO modes are visible at ϳ 1160 cm −1 ͑ LO-AS 2 ͒ and ϳ 1234 cm −1 ͑ LO-AS 1 ͒ . In addition to these bands, the feature at ϳ 1048 cm −1 associated with Si- O - Mo linkages is evident in these spectra. It should be noted that the high-temperature annealing steps lead to ordering of the SiO 2 films and to sharpening and growth of the LO-AS 1 band at ϳ 1234 cm −1 with concurrent attenua- tion of the LO-AS 2 feature at ϳ 1160 cm −1 . Similar intensity changes for LO-AS 1 and LO-AS 2 modes have been reported for thick SiO 2 films on Mo ͑ 110 ͒ 13 and for the transition from amorphous silicon dioxide to ordered ␣ quartz. 44 Likewise the electronic properties of SiO 2 films on various substrates have been addressed previously. 15,17,18,36–42 UPS has been frequently employed to study the valence band of various SiO 2 surfaces while MIES has been used to a lesser extent. For a comparison with previous work using these two spectroscopies, 36,37 we summarize in Table I the positions of the valence bands and the valence band edge. The relative band positions and the widths of the bands of the spectra in Refs. 36 and 37 agree very well. Deviations of the band positions with respect to E F are due to an uncer- tainty in the assignment of the Fermi level in Ref. 36. Note that in the present work the valance band edge of the thick a -SiO 2 films agrees exactly with that reported by Brause et al. It is also noteworthy that all of the spectra are dominated by the O ͑ 2 p ͒ nonbonding band. The Si- O bonding bands are clearly less intense than the O ͑ 2 p ͒ nonbonding bands for both MIES and UPS. Since the thicker SiO 2 films in the present work ͑ Figs. 2 ͑ c ͒ and 4 ͑ a ͒͒ show spectra very similar to those of various SiO 2 surfaces in the literature 36–42 it is appropriate to use these films as a reference where Si- O - Si linkages predomi- nate. Therefore, the broad Si- O bonding feature with a maximum at ϳ 11.5 eV is assigned to a surface consisting of Si- O - Si linkages. In contrast, the spectra observed for films of one monolayer that were prepared by method 1 ͑ Figs. 1 ͑ a ͒ and 2 ͑ a ͒͒ clearly differ from the spectra of the thicker films. This difference arises due to influence of the Mo substrate—a view consistent with the vibrational data ob- tained using PM-IRAS see previously and HREELS. Consequently, we attribute the sharp and intense Si- O bonding band at ϳ 10.5 eV to Si- O - Mo linkages at the surface. This assignment is supported by the AES and work function data in Fig. 1 ͑ b ͒ , where break points indicate the completion of the first ML. It is noteworthy that the exis- tence of Si- O - Mo linkages at the interface is also evident from XPS measurements. 14 ͑ a ͒ ,20 The results shown in Fig. 2 further support the assignment of the features at ϳ 11.5 and ϳ 10.5 eV to Si- O - Si and Si- O - Mo linkages, respectively. These results suggest that within the range of 1 to ϳ 2.5 ML the band in the Si- O bonding region is a convolution of features at ϳ 10.5 and ϳ 11.5 eV. Since the second layer consists of Si- O - Si linkages, the maximum of the Si- O bonding band shifts to higher binding energies as the coverage is increased. At a coverage of ϳ 2.6 ML, the contribution of the feature at ϳ 10.5 eV is no longer obvious with either UPS or MIES. The states close to E F are still detectable with UPS even for a thick SiO 2 film since this technique probes several layers in the near-surface region. According to the results shown in Fig. 2 we conclude that bulklike electronic properties of silica develop within the first 2 ML. These results are consistent with the suggested thickness lower limit of usable SiO 2 gate electric materials observed by Muller et al. 52 Based on EELS in a scanning transmission electron micro- scope these authors found the thinnest, usable SiO 2 film on a Si substrate to be ϳ 0.7 nm. 52 It is also ...

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... deexcitate at the surface via Auger deexcitation ͑ AD ͒ . In this case, a plot of the intensity of the ejected electrons versus their kinetic energies yields the surface density of states ͑ SDOS ͒ for the topmost layer of the surface. 28,29 In the spectra reported here, the intensities are plotted as a function of binding energy ͑ E B ͒ . The fermi level ͑ E F ͒ of the Mo substrate was used as the reference in all of the UPS and MIES spectra ͑ E B = 0 ͒ . For all SiO 2 thin films considered, AES data were collected concomitant to the UPS/MIES and PM-IRAS measurements. For films of ϳ 1 ML thickness, the prominent features in the AES spectra are the transitions at 187 eV ͑ MNV ͒ and at 223 eV ͑ MVV ͒ of Mo. The feature at 78 eV due to the Si LVV of SiO 230 is less intense than the MNV and MVV features of Mo in both experimental setups, and becomes dominant only for films with ␪ ജ 1.5 ML. The at- tenuation of the Mo MNV ͑ 187 eV ͒ feature 31 was used as a measure of the film thickness “ t .” The formula, t = ln ͑ I 0 / I ͒ ϫ ␭ ϫ cos ͑ ␣ ͒ , was applied, with I 0 and I being the peak intensities of the Mo MNV ͑ 187 eV ͒ feature prior to and after SiO 2 film growth, respectively. Since a CMA was used, the angle ␣ between the sample normal and the detected Auger electrons was ϳ 42°. A mean-free path ͑ ␭ ͒ of 0.95 nm for the 187 eV electrons in SiO 2 was used. 32 The Mo ͑ 112 ͒ crystals were spot welded to U-shaped tan- talum wires for resistive heating. The sample temperature could be varied from 90 K ͑ by cooling with liquid N 2 ͒ to 1300 K ͑ by resistive heating ͒ and to 2200 K ͑ by e-beam heating ͒ ; the temperature was measured by a ͑ W-5%Re/W- 26%Re ͒ thermocouple spot welded to the backside of the sample. The Mo ͑ 112 ͒ crystals were cleaned by multiple flashes to 2100 K and the cleanliness verified with AES. The first method utilized in the current work to synthesize SiO 2 thin films on Mo ͑ 112 ͒ follows the recipe of Chen et al. 16 In this method ͑ method 1 ͒ , the Mo ͑ 112 ͒ substrate was first exposed to oxygen 5 10 Torr at 850 K for ϳ 10 min to produce a p ͑ 2 ϫ 3 ͒ O surface reconstruction. 33 Subsequently, small amounts of Si were deposited, with each deposition step followed by an anneal at 800 K in an oxygen atmosphere for 5 min, then the temperature increased to 1200 K for an additional 5 min. During the annealing pro- cess the oxygen pressure was kept at 1 ϫ 10 −7 Torr. These Si deposition and annealing steps were repeated several times until a coverage of 1 ML was achieved. Method 1 synthesis was followed with UPS ͑ Fig. 1 ͑ a ͒͒ and AES Fig. 1 b measurements. Spectrum 1 in Fig. 1 a is that of the Mo ͑ 112 ͒ - p ͑ 2 ϫ 3 ͒ O surface that exhibit dominant features between 4 and 7 eV and less intense states near E F . 34 The structure between 4 and 7 eV results from the interaction of O ͑ 2 p ͒ electrons with Mo spd hybridized states that are energetically close to the Fermi edge. 35 After depos- iting Si and annealing, spectrum ͑ 2 ͒ show an O ͑ 2 p ͒ band with enhanced intensity and a new band at ϳ 10.5 eV. An increase in the film coverage leads to an increase of both bands. Based on previous reports, the band between 5 and 9 eV corresponds to O ͑ 2 p ͒ nonbonding states of SiO 2 . 36–42 The band at ϳ 10.5 eV is in the region of the Si- O bonding states; 36–42 its position and intensity will be discussed below. Spectrum ͑ 6 ͒ in Figure 1 ͑ a ͒ is attributed to a 1 ML SiO 2 film given the appearance of the break points in the AES ratios and in the work function data ͑ Fig. 1 ͑ b ͒͒ . The corresponding thickness of the film at this point in the synthesis procedure is estimated to be ϳ 0.4 nm, consistent with a coverage of 1 ML. It is noteworthy that this surface exhibits a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. Figure 2 summarizes the UPS and MIES data acquired for SiO 2 films with coverages Ͼ 1 ML prepared by using the 1 ML preparation described above and increasing the coverage using successive Si deposition and oxidation cycles. Only small quantities were deposited and subsequently oxidized at 850 K to prepare stoichiometric SiO 2 films. Therefore, many deposition/oxidation cycles had to be performed to increase the coverage sufficiently. For the 1 ML film, the maximum of the Si- O bonding feature is at ϳ 10.5 eV in UPS and MIES ͑ curves ͑ a ͒͒ . After increasing the coverage to ϳ 1.4 ML ͑ curves ͑ b ͒͒ , the bonding feature is at ϳ 10.8 eV, with a shoulder at ϳ 11.5 eV. At higher coverages, i.e., at ϳ 2.6 ML ͑ curves ͑ c ͒͒ , the bonding band is most intense at ϳ 11.5 eV. Note that this thick silica film is essentially stoichiometric, since almost no states in the bandgap region are detectable, even with MIES. 17,18 In the corresponding AES spectrum, no Si feature at 90 eV was detected, supporting this conclu- sion. A diffuse c ͑ 2 ϫ 2 ͒ LEED pattern with a relatively high background was obtained for this surface. Experiments similar to those summarized in Fig. 2 were carried out using PM-IRAS ͑ Fig. 3 ͒ . Starting with film ͑ a ͒ in the submonolayer range ͑ ␪ ϳ 0.6 ML ͒ , SiO 2 films corresponding to coverages of ϳ 1.0, ϳ 1.1, and ϳ 1.2 ML, were synthesized ͑ curves ͑ b ͒ – ͑ d ͒͒ . Film ͑ a ͒ is characterized by a broadband at ϳ 1046 cm −1 ͑ FWHM= 84 cm −1 ͒ with a shoulder on the low-frequency side at ϳ 980 cm −1 . At about ϳ 1 ML ͑ film ͑ b ͒͒ , a significant sharpening of this single vibrational band is observed ͑ FWHM= 29 cm −1 ͒ in addition to a minor blue shift to ϳ 1057 cm −1 . Other important aspects of the spectrum for film ͑ b ͒ are the symmetry of the feature at ϳ 1057 cm −1 and the disappearance of the shoulder at ϳ 980 cm −1 . For coverages above 1 ML ͑ films ͑ c ͒ and ͑ d ͒͒ , the feature at 1057 cm −1 remains very sharp, although slightly attenuated in intensity. In addition, a new feature at ϳ 1201 cm −1 is apparent in the spectra of the films with ␪ Ͼ 1 ML, which is asymmetric on the low-frequency side with a shoulder at ϳ 1170 cm −1 . With increasing coverage, the intensity of the feature at 1201 cm −1 increases. A second method method 2 for SiO 2 film synthesis also begins with a Mo ͑ 112 ͒ - p ͑ 2 ϫ 3 ͒ O surface. However, in method 2, Si is deposited in a single step without intervening oxidizing cycles. The amount of Si deposited is typically that needed to synthesize 1.5–2 ML of SiO 2 . The sample was oxidized via annealing at 800 K in an oxygen atmosphere ͑ p = 1 ϫ 10 −7 Torr ͒ for a minimum of 10 min. Finally, the sample was further annealed at 1050 K or, in certain cases, to 1150– 1250 K, using the same oxygen pressure. MIES and UPS data obtained for three films synthesized using method 2 are summarized in Fig. 4. Film ͑ a ͒ was prepared utilizing an oxidation step at 800 K for 20 min ͑ ␪ ϳ 1.8 ML ͒ . The MIES spectrum of this film shows a broad O ͑ 2 p ͒ band at ϳ 8 eV and smaller contributions due to Si- O bonding states at ϳ 10.4 and ϳ 11.5 eV. In UPS, the two bands appear at ϳ 7 and ϳ 11.4 eV, respectively, while states originating from the substrate are seen between 0 and 5 eV. With the exception of these Mo states, the UPS and MIES spectra of film a resemble those obtained for a -SiO 2 grown on various Si substrates. 36,37 Films ͑ b ͒ and ͑ c ͒ in Fig. 4 were prepared by anneals at 1100 K ͑ 10 min ͒ and 1150 K ͑ 30 min ͒ , respectively, leading to SiO 2 films with coverages of ϳ 1.7 ML and ϳ 1.2 ML, respectively. The O ͑ 2 p ͒ nonbonding band of film ͑ c ͒ shows a fine structure consisting of two features at ϳ 7.6 and ϳ 6.7 eV, and ϳ 7.6 and ϳ 6.3 eV in MIES and UPS, respectively. Furthermore, the relative contribution of the feature at ϳ 10.5 eV is enhanced in comparison to films ͑ a ͒ and ͑ b ͒ . The latter is more obvious in MIES than in UPS. The appearance of the fine structure for film ͑ c ͒ is accompanied by an increase of the states of the Mo substrate near to E F in UPS. Furthermore, there is a shoulder at ϳ 5.6 eV apparent in the UPS spectrum that is not evident in the other UPS spectra. The c ͑ 2 ϫ 2 ͒ LEED pattern of film ͑ c ͒ is very sharp, indicating a high ordered surface. PM-IRAS was also used to investigate the SiO 2 films synthesized with method 2 ͑ Fig. 5 ͒ . Starting with a ϳ 1.3 ML SiO 2 film ͑ a ͒ , silica films ͑ b ͒ – ͑ f ͒ were prepared by successive annealing cycles in the presence of 1 ϫ 10 −7 Torr O 2 at 1140, 1150, and 1200 K for 10 to 25 min. The striking decrease in the intensity of the band at ϳ 1201 cm −1 during annealing is accompanied by the sharpening and growth of the feature at ϳ 1057 cm −1 . Finally, after five successive annealing steps, film ͑ f ͒ with ␪ ϳ 1.0 ML is produced. The film thickness was verified by AES and its order, by a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. For reference, a much thicker SiO 2 film was grown by Si deposition in the presence of O 2 using the procedure of Xu and Goodman. 13 This recipe was used to grow a ϳ 4.8 ML amorphous SiO 2 film on Mo ͑ 112 ͒ ͑ Fig. 6 ͑ a ͒͒ . Next, the coverage of this amorphous film was reduced by annealing at 1250 K. Comparing the spectrum of film ͑ a ͒ with those of films ͑ b ͒ and ͑ c ͒ , it is apparent that the broad asymmetric band at 1165 cm −1 becomes sharper due to annealing at 1250 K. Furthermore, the band shifts to 1234 cm , forming a much more intense feature with a shoulder at 1159 cm −1 . In addition, a small feature at 1048 cm −1 is evident in all of the spectra of Fig. 6, although in film ͑ a ͒ , this feature is convoluted with the broadband at 1159 cm −1 . SiO 2 films on various substrates have been studied exten- sively in the literature using vibrational spectroscopic techniques. 13,15,16,39,40,43–46 Our discussion of SiO 2 films on Mo ͑ 112 ͒ will focus primarily on the phonon structure between 1000 and 1400 cm −1 , i.e., the asymmetric stretching ͑ AS ͒ region, as the vibrational features corresponding to symmetric stretching ͑ϳ 768 cm −1 ͒ and bending ͑ϳ 496 cm −1 ͒ modes of SiO 2 / Mo ͑ 112 ͒ 16 were not accessible due to the cutoff frequencies of the ...

Context 4

... 1250 K, using the same oxygen pressure. MIES and UPS data obtained for three films synthesized using method 2 are summarized in Fig. 4. Film ͑ a ͒ was prepared utilizing an oxidation step at 800 K for 20 min ͑ ␪ ϳ 1.8 ML ͒ . The MIES spectrum of this film shows a broad O ͑ 2 p ͒ band at ϳ 8 eV and smaller contributions due to Si- O bonding states at ϳ 10.4 and ϳ 11.5 eV. In UPS, the two bands appear at ϳ 7 and ϳ 11.4 eV, respectively, while states originating from the substrate are seen between 0 and 5 eV. With the exception of these Mo states, the UPS and MIES spectra of film a resemble those obtained for a -SiO 2 grown on various Si substrates. 36,37 Films ͑ b ͒ and ͑ c ͒ in Fig. 4 were prepared by anneals at 1100 K ͑ 10 min ͒ and 1150 K ͑ 30 min ͒ , respectively, leading to SiO 2 films with coverages of ϳ 1.7 ML and ϳ 1.2 ML, respectively. The O ͑ 2 p ͒ nonbonding band of film ͑ c ͒ shows a fine structure consisting of two features at ϳ 7.6 and ϳ 6.7 eV, and ϳ 7.6 and ϳ 6.3 eV in MIES and UPS, respectively. Furthermore, the relative contribution of the feature at ϳ 10.5 eV is enhanced in comparison to films ͑ a ͒ and ͑ b ͒ . The latter is more obvious in MIES than in UPS. The appearance of the fine structure for film ͑ c ͒ is accompanied by an increase of the states of the Mo substrate near to E F in UPS. Furthermore, there is a shoulder at ϳ 5.6 eV apparent in the UPS spectrum that is not evident in the other UPS spectra. The c ͑ 2 ϫ 2 ͒ LEED pattern of film ͑ c ͒ is very sharp, indicating a high ordered surface. PM-IRAS was also used to investigate the SiO 2 films synthesized with method 2 ͑ Fig. 5 ͒ . Starting with a ϳ 1.3 ML SiO 2 film ͑ a ͒ , silica films ͑ b ͒ – ͑ f ͒ were prepared by successive annealing cycles in the presence of 1 ϫ 10 −7 Torr O 2 at 1140, 1150, and 1200 K for 10 to 25 min. The striking decrease in the intensity of the band at ϳ 1201 cm −1 during annealing is accompanied by the sharpening and growth of the feature at ϳ 1057 cm −1 . Finally, after five successive annealing steps, film ͑ f ͒ with ␪ ϳ 1.0 ML is produced. The film thickness was verified by AES and its order, by a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. For reference, a much thicker SiO 2 film was grown by Si deposition in the presence of O 2 using the procedure of Xu and Goodman. 13 This recipe was used to grow a ϳ 4.8 ML amorphous SiO 2 film on Mo ͑ 112 ͒ ͑ Fig. 6 ͑ a ͒͒ . Next, the coverage of this amorphous film was reduced by annealing at 1250 K. Comparing the spectrum of film ͑ a ͒ with those of films ͑ b ͒ and ͑ c ͒ , it is apparent that the broad asymmetric band at 1165 cm −1 becomes sharper due to annealing at 1250 K. Furthermore, the band shifts to 1234 cm , forming a much more intense feature with a shoulder at 1159 cm −1 . In addition, a small feature at 1048 cm −1 is evident in all of the spectra of Fig. 6, although in film ͑ a ͒ , this feature is convoluted with the broadband at 1159 cm −1 . SiO 2 films on various substrates have been studied exten- sively in the literature using vibrational spectroscopic techniques. 13,15,16,39,40,43–46 Our discussion of SiO 2 films on Mo ͑ 112 ͒ will focus primarily on the phonon structure between 1000 and 1400 cm −1 , i.e., the asymmetric stretching ͑ AS ͒ region, as the vibrational features corresponding to symmetric stretching ͑ϳ 768 cm −1 ͒ and bending ͑ϳ 496 cm −1 ͒ modes of SiO 2 / Mo ͑ 112 ͒ 16 were not accessible due to the cutoff frequencies of the CaF 2 windows used in the experiments. Amorphous SiO 2 films reveal typically two different asymmetric stretching modes: in-phase motion of adjacent O atoms ͑ AS 1 ͒ and out-of-phase motion of adjacent O atoms ͑ AS 2 ͒ . 44 Since the optical phonon band is composed of transverse ͑ TO ͒ and longitudinal ͑ LO ͒ modes, four different AS vibrational features are observed in the IR spectra at ϳ 1076 cm −1 ͑ TO-AS 1 ͒ , ϳ 1160 cm −1 ͑ LO-AS 2 ͒ , ϳ 1200 cm −1 ͑ TO-AS 2 ͒ , and ϳ 1256 cm −1 ͑ LO-AS 1 ͒ . 44 However, the SiO 2 films studied in this work have thicknesses well below the so-called Berreman thickness. 45–47 Therefore TO modes are expected to be fully suppressed whereas LO modes corresponding to the interlinking of the ͓ SiO 4 ͔ tetrahedra and other bands such as the ones corresponding to Si- O - Mo linkages of ͓ SiO 4 ͔ tetrahedra to the metallic Mo ͑ 112 ͒ substrate are anticipated to be observable. 47 Conse- quently, the vibrational features between 1100 and 1250 cm −1 are assigned to LO-AS 1 and LO-AS 2 modes of Si- O - Si linkages whereas vibrational features between 1000 and 1100 cm −1 are attributed to the Si- O - Mo linkages. Note that for SiO 2 films grown on a metal surface, the AS 1 -LO mode was observed at a wide range, 1180– 1250 cm −1 , de- pending on the annealing temperature, 13 1190– 1218 cm −1 as a function of the film thickness ͑ within 3 ML ͒ . 15 During the initial stages of the SiO 2 film growth on Mo ͑ 112 ͒ using method 1, only a single broad vibrational feature at ϳ 1046 cm −1 is apparent ͑ Fig. 3 ͑ a ͒͒ . This feature is associated with the Si- O - Mo linkages of isolated ͓ SiO 4 ͔ tetrahedra at the surface. That there is no evidence of an LO mode at 1100– 1250 cm −1 at this coverage ͑ 0.6 ML ͒ is consistent with there being no Si- O - Si linkages between these isolated ͓ SiO 4 ͔ units. 16 In previous reports, a shoulder at ϳ 980 cm −1 , similar to the one seen in Fig. 3 ͑ a ͒ , was attributed to suboxides in the SiO 2 / Si interface 43 or to the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ . 15 However, after a high-temperature anneal, no OH-related stretching features were apparent. Therefore, the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ can be ruled out, fully consistent with recent work addressing the interaction of water with SiO 2 / Mo ͑ 112 ͒ . 48 Furthermore, it is known from previous AES and XPS data that SiO 2 films grown on Mo ͑ 112 ͒ do not contain suboxides for coverages ␪ Ͻ 1 ML. 14 Thus, the shoulder at ϳ 980 cm −1 is likely due to oxygen atoms bonded to the Mo substrate. According to previous vibrational studies we assign the shoulder at ϳ 980 cm −1 to the stretching mode ͓ ␯ ͑ Mo v O ͔͒ of terminal atop oxygen on the topmost Mo atoms. 49–51 This assignment is consistent with the disappearance of the 980 cm −1 feature and the formation of a sharp and symmetric feature at ϳ 1057 cm −1 upon the formation of a well-ordered SiO 2 film at 1 ML ͑ Fig. 3 ͑ b ͒͒ . After completion of the monolayer film ͑ Fig. 3 ͑ c ͒͒ , the formation of a second and a relatively less ordered SiO 2 layer is observed. This is evident from the appearance of an asymmetric feature at ϳ 1201 cm −1 with a shoulder at ϳ 1170 cm −1 corresponding to LO modes of AS 1 and AS 2 for Si- O - Si linkages, respectively. The appearance of such LO modes after completion of the first layer suggests the formation of Si- O - Si linkages between the ͓ SiO 4 ͔ tetrahedra ini- tially disconnected at lower coverages. 16 Figure 5 illustrates that method 2 also can be used to obtain a highly ordered SiO 2 film by annealing an amorphous Ͼ 1 ML SiO 2 film in oxygen at elevated temperatures. The ordered film with a coverage close to one monolayer obtained at the end of these annealing steps shows only a single sharp vibrational feature at ϳ 1054 cm −1 with a FWHM of ϳ 29 cm −1 . LO modes of bulklike SiO 2 can be addressed by growing relatively thick SiO 2 films where Si- O - Si linkages are known to exist between the ͓ SiO 4 ͔ tetrahedra and shown to exhibit intense LO bands. Figure 6 presents such thick SiO 2 films grown on Mo ͑ 112 ͒ where LO modes are visible at ϳ 1160 cm −1 ͑ LO-AS 2 ͒ and ϳ 1234 cm −1 ͑ LO-AS 1 ͒ . In addition to these bands, the feature at ϳ 1048 cm −1 associated with Si- O - Mo linkages is evident in these spectra. It should be noted that the high-temperature annealing steps lead to ordering of the SiO 2 films and to sharpening and growth of the LO-AS 1 band at ϳ 1234 cm −1 with concurrent attenua- tion of the LO-AS 2 feature at ϳ 1160 cm −1 . Similar intensity changes for LO-AS 1 and LO-AS 2 modes have been reported for thick SiO 2 films on Mo ͑ 110 ͒ 13 and for the transition from amorphous silicon dioxide to ordered ␣ quartz. 44 Likewise the electronic properties of SiO 2 films on various substrates have been addressed previously. 15,17,18,36–42 UPS has been frequently employed to study the valence band of various SiO 2 surfaces while MIES has been used to a lesser extent. For a comparison with previous work using these two spectroscopies, 36,37 we summarize in Table I the positions of the valence bands and the valence band edge. The relative band positions and the widths of the bands of the spectra in Refs. 36 and 37 agree very well. Deviations of the band positions with respect to E F are due to an uncer- tainty in the assignment of the Fermi level in Ref. 36. Note that in the present work the valance band edge of the thick a -SiO 2 films agrees exactly with that reported by Brause et al. It is also noteworthy that all of the spectra are dominated by the O ͑ 2 p ͒ nonbonding band. The Si- O bonding bands are clearly less intense than the O ͑ 2 p ͒ nonbonding bands for both MIES and UPS. Since the thicker SiO 2 films in the present work ͑ Figs. 2 ͑ c ͒ and 4 ͑ a ͒͒ show spectra very similar to those of various SiO 2 surfaces in the literature 36–42 it is appropriate to use these films as a reference where Si- O - Si linkages predomi- nate. Therefore, the broad Si- O bonding feature with a maximum at ϳ 11.5 eV is assigned to a surface consisting of Si- O - Si linkages. In contrast, the spectra observed for films of one monolayer that were prepared by method 1 ͑ Figs. 1 ͑ a ͒ and 2 ͑ a ͒͒ clearly differ from the spectra of the thicker films. This difference arises due to influence of the Mo substrate—a view consistent with the vibrational data ob- tained using PM-IRAS see previously and HREELS. Consequently, we attribute the sharp and intense Si- O bonding band at ϳ 10.5 eV to Si- O - Mo ...

Context 5

... the two bands appear at ϳ 7 and ϳ 11.4 eV, respectively, while states originating from the substrate are seen between 0 and 5 eV. With the exception of these Mo states, the UPS and MIES spectra of film a resemble those obtained for a -SiO 2 grown on various Si substrates. 36,37 Films ͑ b ͒ and ͑ c ͒ in Fig. 4 were prepared by anneals at 1100 K ͑ 10 min ͒ and 1150 K ͑ 30 min ͒ , respectively, leading to SiO 2 films with coverages of ϳ 1.7 ML and ϳ 1.2 ML, respectively. The O ͑ 2 p ͒ nonbonding band of film ͑ c ͒ shows a fine structure consisting of two features at ϳ 7.6 and ϳ 6.7 eV, and ϳ 7.6 and ϳ 6.3 eV in MIES and UPS, respectively. Furthermore, the relative contribution of the feature at ϳ 10.5 eV is enhanced in comparison to films ͑ a ͒ and ͑ b ͒ . The latter is more obvious in MIES than in UPS. The appearance of the fine structure for film ͑ c ͒ is accompanied by an increase of the states of the Mo substrate near to E F in UPS. Furthermore, there is a shoulder at ϳ 5.6 eV apparent in the UPS spectrum that is not evident in the other UPS spectra. The c ͑ 2 ϫ 2 ͒ LEED pattern of film ͑ c ͒ is very sharp, indicating a high ordered surface. PM-IRAS was also used to investigate the SiO 2 films synthesized with method 2 ͑ Fig. 5 ͒ . Starting with a ϳ 1.3 ML SiO 2 film ͑ a ͒ , silica films ͑ b ͒ – ͑ f ͒ were prepared by successive annealing cycles in the presence of 1 ϫ 10 −7 Torr O 2 at 1140, 1150, and 1200 K for 10 to 25 min. The striking decrease in the intensity of the band at ϳ 1201 cm −1 during annealing is accompanied by the sharpening and growth of the feature at ϳ 1057 cm −1 . Finally, after five successive annealing steps, film ͑ f ͒ with ␪ ϳ 1.0 ML is produced. The film thickness was verified by AES and its order, by a sharp c ͑ 2 ϫ 2 ͒ LEED pattern. For reference, a much thicker SiO 2 film was grown by Si deposition in the presence of O 2 using the procedure of Xu and Goodman. 13 This recipe was used to grow a ϳ 4.8 ML amorphous SiO 2 film on Mo ͑ 112 ͒ ͑ Fig. 6 ͑ a ͒͒ . Next, the coverage of this amorphous film was reduced by annealing at 1250 K. Comparing the spectrum of film ͑ a ͒ with those of films ͑ b ͒ and ͑ c ͒ , it is apparent that the broad asymmetric band at 1165 cm −1 becomes sharper due to annealing at 1250 K. Furthermore, the band shifts to 1234 cm , forming a much more intense feature with a shoulder at 1159 cm −1 . In addition, a small feature at 1048 cm −1 is evident in all of the spectra of Fig. 6, although in film ͑ a ͒ , this feature is convoluted with the broadband at 1159 cm −1 . SiO 2 films on various substrates have been studied exten- sively in the literature using vibrational spectroscopic techniques. 13,15,16,39,40,43–46 Our discussion of SiO 2 films on Mo ͑ 112 ͒ will focus primarily on the phonon structure between 1000 and 1400 cm −1 , i.e., the asymmetric stretching ͑ AS ͒ region, as the vibrational features corresponding to symmetric stretching ͑ϳ 768 cm −1 ͒ and bending ͑ϳ 496 cm −1 ͒ modes of SiO 2 / Mo ͑ 112 ͒ 16 were not accessible due to the cutoff frequencies of the CaF 2 windows used in the experiments. Amorphous SiO 2 films reveal typically two different asymmetric stretching modes: in-phase motion of adjacent O atoms ͑ AS 1 ͒ and out-of-phase motion of adjacent O atoms ͑ AS 2 ͒ . 44 Since the optical phonon band is composed of transverse ͑ TO ͒ and longitudinal ͑ LO ͒ modes, four different AS vibrational features are observed in the IR spectra at ϳ 1076 cm −1 ͑ TO-AS 1 ͒ , ϳ 1160 cm −1 ͑ LO-AS 2 ͒ , ϳ 1200 cm −1 ͑ TO-AS 2 ͒ , and ϳ 1256 cm −1 ͑ LO-AS 1 ͒ . 44 However, the SiO 2 films studied in this work have thicknesses well below the so-called Berreman thickness. 45–47 Therefore TO modes are expected to be fully suppressed whereas LO modes corresponding to the interlinking of the ͓ SiO 4 ͔ tetrahedra and other bands such as the ones corresponding to Si- O - Mo linkages of ͓ SiO 4 ͔ tetrahedra to the metallic Mo ͑ 112 ͒ substrate are anticipated to be observable. 47 Conse- quently, the vibrational features between 1100 and 1250 cm −1 are assigned to LO-AS 1 and LO-AS 2 modes of Si- O - Si linkages whereas vibrational features between 1000 and 1100 cm −1 are attributed to the Si- O - Mo linkages. Note that for SiO 2 films grown on a metal surface, the AS 1 -LO mode was observed at a wide range, 1180– 1250 cm −1 , de- pending on the annealing temperature, 13 1190– 1218 cm −1 as a function of the film thickness ͑ within 3 ML ͒ . 15 During the initial stages of the SiO 2 film growth on Mo ͑ 112 ͒ using method 1, only a single broad vibrational feature at ϳ 1046 cm −1 is apparent ͑ Fig. 3 ͑ a ͒͒ . This feature is associated with the Si- O - Mo linkages of isolated ͓ SiO 4 ͔ tetrahedra at the surface. That there is no evidence of an LO mode at 1100– 1250 cm −1 at this coverage ͑ 0.6 ML ͒ is consistent with there being no Si- O - Si linkages between these isolated ͓ SiO 4 ͔ units. 16 In previous reports, a shoulder at ϳ 980 cm −1 , similar to the one seen in Fig. 3 ͑ a ͒ , was attributed to suboxides in the SiO 2 / Si interface 43 or to the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ . 15 However, after a high-temperature anneal, no OH-related stretching features were apparent. Therefore, the presence of Si- OH groups on SiO 2 / Mo ͑ 112 ͒ can be ruled out, fully consistent with recent work addressing the interaction of water with SiO 2 / Mo ͑ 112 ͒ . 48 Furthermore, it is known from previous AES and XPS data that SiO 2 films grown on Mo ͑ 112 ͒ do not contain suboxides for coverages ␪ Ͻ 1 ML. 14 Thus, the shoulder at ϳ 980 cm −1 is likely due to oxygen atoms bonded to the Mo substrate. According to previous vibrational studies we assign the shoulder at ϳ 980 cm −1 to the stretching mode ͓ ␯ ͑ Mo v O ͔͒ of terminal atop oxygen on the topmost Mo atoms. 49–51 This assignment is consistent with the disappearance of the 980 cm −1 feature and the formation of a sharp and symmetric feature at ϳ 1057 cm −1 upon the formation of a well-ordered SiO 2 film at 1 ML ͑ Fig. 3 ͑ b ͒͒ . After completion of the monolayer film ͑ Fig. 3 ͑ c ͒͒ , the formation of a second and a relatively less ordered SiO 2 layer is observed. This is evident from the appearance of an asymmetric feature at ϳ 1201 cm −1 with a shoulder at ϳ 1170 cm −1 corresponding to LO modes of AS 1 and AS 2 for Si- O - Si linkages, respectively. The appearance of such LO modes after completion of the first layer suggests the formation of Si- O - Si linkages between the ͓ SiO 4 ͔ tetrahedra ini- tially disconnected at lower coverages. 16 Figure 5 illustrates that method 2 also can be used to obtain a highly ordered SiO 2 film by annealing an amorphous Ͼ 1 ML SiO 2 film in oxygen at elevated temperatures. The ordered film with a coverage close to one monolayer obtained at the end of these annealing steps shows only a single sharp vibrational feature at ϳ 1054 cm −1 with a FWHM of ϳ 29 cm −1 . LO modes of bulklike SiO 2 can be addressed by growing relatively thick SiO 2 films where Si- O - Si linkages are known to exist between the ͓ SiO 4 ͔ tetrahedra and shown to exhibit intense LO bands. Figure 6 presents such thick SiO 2 films grown on Mo ͑ 112 ͒ where LO modes are visible at ϳ 1160 cm −1 ͑ LO-AS 2 ͒ and ϳ 1234 cm −1 ͑ LO-AS 1 ͒ . In addition to these bands, the feature at ϳ 1048 cm −1 associated with Si- O - Mo linkages is evident in these spectra. It should be noted that the high-temperature annealing steps lead to ordering of the SiO 2 films and to sharpening and growth of the LO-AS 1 band at ϳ 1234 cm −1 with concurrent attenua- tion of the LO-AS 2 feature at ϳ 1160 cm −1 . Similar intensity changes for LO-AS 1 and LO-AS 2 modes have been reported for thick SiO 2 films on Mo ͑ 110 ͒ 13 and for the transition from amorphous silicon dioxide to ordered ␣ quartz. 44 Likewise the electronic properties of SiO 2 films on various substrates have been addressed previously. 15,17,18,36–42 UPS has been frequently employed to study the valence band of various SiO 2 surfaces while MIES has been used to a lesser extent. For a comparison with previous work using these two spectroscopies, 36,37 we summarize in Table I the positions of the valence bands and the valence band edge. The relative band positions and the widths of the bands of the spectra in Refs. 36 and 37 agree very well. Deviations of the band positions with respect to E F are due to an uncer- tainty in the assignment of the Fermi level in Ref. 36. Note that in the present work the valance band edge of the thick a -SiO 2 films agrees exactly with that reported by Brause et al. It is also noteworthy that all of the spectra are dominated by the O ͑ 2 p ͒ nonbonding band. The Si- O bonding bands are clearly less intense than the O ͑ 2 p ͒ nonbonding bands for both MIES and UPS. Since the thicker SiO 2 films in the present work ͑ Figs. 2 ͑ c ͒ and 4 ͑ a ͒͒ show spectra very similar to those of various SiO 2 surfaces in the literature 36–42 it is appropriate to use these films as a reference where Si- O - Si linkages predomi- nate. Therefore, the broad Si- O bonding feature with a maximum at ϳ 11.5 eV is assigned to a surface consisting of Si- O - Si linkages. In contrast, the spectra observed for films of one monolayer that were prepared by method 1 ͑ Figs. 1 ͑ a ͒ and 2 ͑ a ͒͒ clearly differ from the spectra of the thicker films. This difference arises due to influence of the Mo substrate—a view consistent with the vibrational data ob- tained using PM-IRAS see previously and HREELS. Consequently, we attribute the sharp and intense Si- O bonding band at ϳ 10.5 eV to Si- O - Mo linkages at the surface. This assignment is supported by the AES and work function data in Fig. 1 ͑ b ͒ , where break points indicate the completion of the first ML. It is noteworthy that the exis- tence of Si- O - Mo linkages at the interface is also evident from XPS measurements. 14 ͑ a ͒ ,20 The results shown in Fig. 2 further support the assignment of the features at ϳ 11.5 and ...