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Oxidation of silicon: New electron spectroscopy results
Solid State Communications, Vol. 20, PP. 277—280, 1976.
Pergamon Press.
Printed in Great Britam
OXIDATION OF SILICON: NEW ELECTRON SPECTROSCOPY RESULTS i.E. Rowe and G. Margaritondo* Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and H. Thach and H. Froitzheim Institut für Grenzflachenforschung and Vacuumphysik, Kernforschungslage, Julich, West Germany (Received25 June 1976 by J. Tauc) New X-ray photoemission spectroscopy data and high resolution electron scattering spectroscopy data are presented which indicate that a peroxidelike model of the oxygen chemisorption on silicon surface is correct. These results are discussed in light of a recent double-bonded oxygen atom model due to Ludeke and Koma. ALTHOUGH THE oxidation of Si and Ge have been studied by a number of workers over more than one decade’6 there is stifi some controversy over the initial stage of the oxidation mechanism.46 The most recent work of Ludeke and Koma proposes that the initial oxidation of Si and Ge proceeds via a monoxide doublebond mechanism6 much like a carbon monoxide molecule on the surface of silicon or germanium but with the carbon atom replaced by a host Si or Ge atom. On the other hand the most extensive experimental results obtained previously4’5 suggest a peroxide-like O~ion geometry with the oxygenmolecule not being completely dissociated. The main evidence for this latter model came from surface phonon spectra4 which indicated that there were three stretching vibrational modes and from ultraviolet photoemission spectroscopy5 (UPS) that indicated four O(2p)-like peaks rather than three peaks expected for a single oxygen atom in an asymmetric site or two O(2p) peaks for the double-bond model of Ludeke and Koma based on a CO-like surface molecule configuration. By analogy with gas-phase CO one expects a total of four molecular orbitals (3u, 4o, Sn and lir). However, only two of these, 4a and lir, have an appreciable O(2p) contribution, The arguments presented by Ludeke and Koma6 are based on total energy considerations. However, it is obvious that this type of reasoning is not conclusive for an intermediate metastable phase but is only correct for the most stable phase of any compound. In the present case the most stable phase is Si02 and not chemisorbed oxygen. In this present paper we present additional evidence in favor of the peroxide-like single-bond
*
Fellow of the Italian National Research Council— Gruppo Nazionale di Struttura della Matena.
oxygen chemisorption model, rather than the doublebonded oxygen considered in the recent work of Ludeke and Koma. The two structural models are shown in Fig. 1. The peroxide-like model is shown in Fig. 1(a) with the characteristic feature being that the oxygen—oxygen bond is not completely broken. The main evidence supporting this model comes from the phonon and electronic spectra mentioned above. In contrast the main evidence for the double-bond model of Fig. 1(b) is from total energy arguments and from a comparison of ELS data of chemisorbed oxygen with gas phase SiO and GeO molecules.6 The new experiments reported in this work are X-ray photoemission spectroscopy (XPS) of the O(ls) core level and a new measurement of surface phonon spectra by high resolution electron spectroscopy (HRES) with improved signal-to-background ratio.7 Both of these experiments lend support to the peroxide-like oxidation model in contrast to the double-bond model recently proposed by Ludeke and Koma. The XPS experiments were performed with a PHI-15-250 precision electron energy analyzer of the cylindrical mirror analyzer (CMA) design and PHI X-ray source of unmonochromatized A1K~X-rays. The Si + 02 and SiO 5 2 samples were as described previously and measured in situ byprepared XPS. Typical experimental results are shown in Fig. 2(a) for the bulk SiO 2 phase and in Fig. 2(b) for the surface monoxide phase, Si02. Note that the binding energy of the O(ls) core level is 1.1 eV larger for Si02 than the value for chemisorbed oxygen. This is in strong support of our model of single bonded oxygen for the chemisorbed phase with the Si02 phase being more stable the chemisorbed oxygen. An additional point is the asymmetry of the 0 (is) signal for the monolayer Si + 02 which has been emphasized in
277
278
OXIDATION OF SILICON
Vol. 20, No.3
0 Si
S;02 Fig. 1. Schematic structural models for the peroxide-like bond model (a) and for the double-bond monoxide model (b). Two different oxygen atom potentials are present in (a). While only a single oxygen potential is present in (b). Fig. 2 by performing nonlinear least squares fits (solid lines) to the raw experimental data (shown as points) for each sample. Curve (c) in Fig. 2 shows the first derivatives of curve (a) shown as a dashed line and of curve (b) shown as a solid line shifted in energy by 1.1eV to aligh with curve 2(b). As one can see from Figs. 2(a), (b) and (c) the O(ls) results for the mono-
(3~
—
537I I
I I I 529 I 533 ENERGY (eV) —
—
Fig. 2. X-ray photoemission spectra for the oxygen (is) core level of SiO2 (a) and a monolayer of 02 on Si (111) or (100) (curve b). The experimental results are shown as points along with least-squares fits (solid lines) to a double-component asymmetric Gaussian function. The first derivative of the fitted curves are shown in part (c) for the Si + 02 (solid curve) and for SiO2 (dashed curve shifted to coincide with peak of solid curve).
layer Si + 02 are asymmetnc while the results for SiO2 are nearly symmetric. In the latter case the experimental line width is partly due to instrumental photon and electron resolution components of— 13 eV FWHM Gaussian line shape and partly due to inhomogeneous broadening which has a 0.6 eV splitting, we estimate a splitting of since the SiO2 is amorphous with some (—‘ 10%) variation 0.7—0.8 eV for chemisorbed 02 on Si (ill) surfaces. in bond angles and bond length. The same symmetric This energy is too to be “shake-up” explained asprocess a “shake-up” 8 since the small minimum should response function has been found from least-squares satellite data analysis of Si (2s), Si (2p), Ge (2p) and Ge (3d) have an energy slightly greater than the silicon bandgap XPS signals. For oxygen chemisorbed on metal surfaces of 1.1eV. This asymmetric line shape of the monolayer other workers as well as own unpublished work indicates Si + 02 data is clear evidence against the double-bonded a symmetric line shape corresponding to dissociated, monoxide model of Ludeke and Koma. However, it does atomic oxygen provided that only a single adsorption not imply a symmetric peroxide model as discussed by species is present.8 In contrast, the asymmetric Si + 02 Ibach eta!.4 Thus we propose a new modified peroxide results are sharper on the high energy (right-hand) side model which involves two inequivalent Si—O “single” with partial half-width near that of the instrumental bonds and one 0—0 “single” bond. These bonds on the resolution. The lower energy (left-hand) side of Fig. surface provide a natural explanation of the three sur2(b) is considerably broader than the other side. This face vibrational frequencies as determined by Thach and we take to be an indication of an unresolved doublet due coworkers. to two different O(is) states much like the barely We present in Fig. 3, new experimental data for the resolved doublet of the 02 molecule due to exchange vibrational energies of Si + 02 obtained from HRES. splitting.9 These experiments were performed in a new version of By comparing the observed asymmetry of 0(1 s) the HRES apparatus which was previously used.4’7 This and that of the unresolved spin—orbit doublet Si (2p) new apparatus has the main advantage that the featureless
Vol. 20, No. 3
OXIDATION OF SILICON
x40
____
56
Si 95
-
hE.12.5
0
BACKG~~ UPPER LIMIT
distinction can be made between a symmetric peroxide 4 and the asymmodel proposed for simplicity earlier metric as model suggested by the XPS data. Finally we would like to comment on some of the weaker points of the Ludeke—Koma double-bond model for chemisorption of 02 at Si and Ge surfaces. The total energy is not an important criterion for chemisorption. The energy ofthe Si0 2 phase is obviously
02
130
~J L1,1
+
279
lower than the Si + 02 surface phase so any total energy
180
100 ENERGY LOSS (meVi
200
Fig. 3. Surface phonon spectra obtained by high resolution electron scattering spectroscopy (HRES) showing the Si—Si phonon at 56meV and the Si—O phonons at 95,130 and 180meV. background intensity (due mainly to scattering of the primary elastic electrons within the analyzer) is much reduced over the earlier of HRES experimental Four different phonons vibrational modes equipment. are evident in Fig. 3 which correspond to (1) Si—Si mode at 56meV, (2) Si—O mode at 95meV, (3) Si—O mode at 130meV and (4) 0—0 mode at 180 meV. The 56 meV mode is also present on the clean surface, however, much sharper. The modes at 95 and 130 meV are the strongest and we take this as an indication of the dominant Si—O chemisorption bonds. The broad 180meV mode is observed to be well above the background intensity and is assigned to an 0—0 vibration which is only weakly allowed by electron scattering with electric dipole selection rules because of the surface chemisorption geometry. The 180 neV mode cannot be explained by the model of double-bonded oxygen atoms and its energy is close to that of the free 02 molecule, 196meV. Based on the surface modes no
argument favorable should not predict aminimum double-bonded the Si02 phase chemisorption to be the most monoxide the geometry phase. with The surface activation phase isphase determined energy for by disturbing believe to theand be 0—0 the peroxide-like double bond model ofand gas based 02. both This on we earlier results as well as the XPS HRES results discussed above. On the other hand, the agreement claimed by Ludeke and Koma for gas phase optical absorption and ELS of chemisorbed oxygen may be somewhat misleading. The ELS and optical transitions occur at the same energies only for comparison oftwo gas phase samples. For solids and for solid surfaces the ELS peaks are shifted to higher energies. For example in pure Si the optical E2 peak in absorption occurs at 4.4 eV while in 5 Also ELS thethe same transitions a peak at 4.9 eV. in SmS lowest opticalproduce absorption transition is at 1.1 eV while in ELS it occurs at 2.6 eV.10 Thus the empirical shift is 0.5—1.5 eV and the agreement claimed by Ludeke and Koma could well be fortuitous. It would be useful to compare HRES of electronic transitions for chemisorbed oxygen with the optical data of gas phase SiO and GeO. However, such a detailed comparison is not appropriate in the present comment. In summary, we have presented new XPS and HRES results for the monolayer oxide, Si + 02 which seem to clearly support a peroxide-like model of 02 chemisorption as shown in Fig. 1(a). The double-bond model of Ludeke and Koma [see Fig. 1(b)] does not agree with these new results or with a reinterpretation of their own ELS results. Thus at the present time the peroxide-like model seems to be correct.
REFERENCES 1.
GREENM. &MAXWELLK.H.,J. Phys. Chem. Solids 13, 145 (1960).
2. 3.
GREEN M. & LIBERMAN A., J. Phys. Chem. Solids 23, 1407 (1962). BOOTSMA G.A., Surf Sci. 15, 340 (1969).
4. 5.
IBACH H., HORN K., DORN R. & LOTH H., Surf Sci. 38,433(1973). IBACH H. & ROWE i.E., Phys. Rev. B9, 1951 (1974); ibid. 10, 710 (1974).
6. 7.
LUDEKE R. & KOMA A.,Phys. Rev. Lett. 34, 1170(1975). FROITZHEIM H., IBACH H. & LEHWALD S., Rev. Sci. Instrum. 46, 1325 (1975).
280
OXIDATION OF SILICON
8.
FUGGLE i.C., MADEY T.E., STEINKILBERG M. & MENSEL D., Chem. Phys. Lett. 33,233 (1975) and references cited therein.
9. 10.
Vol. 20, No. 3
SIEGBAHN K. et al., ESCA Applied to Free Molecules. North Holland, Amsterdam (1969). ROWE J.E., CAMPAGNA M., CHRISTMAN S.B. & BUCHER E.,Phys. Rev. Lett. 36, 148 (1976).
Pergamon Press.
Printed in Great Britam
OXIDATION OF SILICON: NEW ELECTRON SPECTROSCOPY RESULTS i.E. Rowe and G. Margaritondo* Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and H. Thach and H. Froitzheim Institut für Grenzflachenforschung and Vacuumphysik, Kernforschungslage, Julich, West Germany (Received25 June 1976 by J. Tauc) New X-ray photoemission spectroscopy data and high resolution electron scattering spectroscopy data are presented which indicate that a peroxidelike model of the oxygen chemisorption on silicon surface is correct. These results are discussed in light of a recent double-bonded oxygen atom model due to Ludeke and Koma. ALTHOUGH THE oxidation of Si and Ge have been studied by a number of workers over more than one decade’6 there is stifi some controversy over the initial stage of the oxidation mechanism.46 The most recent work of Ludeke and Koma proposes that the initial oxidation of Si and Ge proceeds via a monoxide doublebond mechanism6 much like a carbon monoxide molecule on the surface of silicon or germanium but with the carbon atom replaced by a host Si or Ge atom. On the other hand the most extensive experimental results obtained previously4’5 suggest a peroxide-like O~ion geometry with the oxygenmolecule not being completely dissociated. The main evidence for this latter model came from surface phonon spectra4 which indicated that there were three stretching vibrational modes and from ultraviolet photoemission spectroscopy5 (UPS) that indicated four O(2p)-like peaks rather than three peaks expected for a single oxygen atom in an asymmetric site or two O(2p) peaks for the double-bond model of Ludeke and Koma based on a CO-like surface molecule configuration. By analogy with gas-phase CO one expects a total of four molecular orbitals (3u, 4o, Sn and lir). However, only two of these, 4a and lir, have an appreciable O(2p) contribution, The arguments presented by Ludeke and Koma6 are based on total energy considerations. However, it is obvious that this type of reasoning is not conclusive for an intermediate metastable phase but is only correct for the most stable phase of any compound. In the present case the most stable phase is Si02 and not chemisorbed oxygen. In this present paper we present additional evidence in favor of the peroxide-like single-bond
*
Fellow of the Italian National Research Council— Gruppo Nazionale di Struttura della Matena.
oxygen chemisorption model, rather than the doublebonded oxygen considered in the recent work of Ludeke and Koma. The two structural models are shown in Fig. 1. The peroxide-like model is shown in Fig. 1(a) with the characteristic feature being that the oxygen—oxygen bond is not completely broken. The main evidence supporting this model comes from the phonon and electronic spectra mentioned above. In contrast the main evidence for the double-bond model of Fig. 1(b) is from total energy arguments and from a comparison of ELS data of chemisorbed oxygen with gas phase SiO and GeO molecules.6 The new experiments reported in this work are X-ray photoemission spectroscopy (XPS) of the O(ls) core level and a new measurement of surface phonon spectra by high resolution electron spectroscopy (HRES) with improved signal-to-background ratio.7 Both of these experiments lend support to the peroxide-like oxidation model in contrast to the double-bond model recently proposed by Ludeke and Koma. The XPS experiments were performed with a PHI-15-250 precision electron energy analyzer of the cylindrical mirror analyzer (CMA) design and PHI X-ray source of unmonochromatized A1K~X-rays. The Si + 02 and SiO 5 2 samples were as described previously and measured in situ byprepared XPS. Typical experimental results are shown in Fig. 2(a) for the bulk SiO 2 phase and in Fig. 2(b) for the surface monoxide phase, Si02. Note that the binding energy of the O(ls) core level is 1.1 eV larger for Si02 than the value for chemisorbed oxygen. This is in strong support of our model of single bonded oxygen for the chemisorbed phase with the Si02 phase being more stable the chemisorbed oxygen. An additional point is the asymmetry of the 0 (is) signal for the monolayer Si + 02 which has been emphasized in
277
278
OXIDATION OF SILICON
Vol. 20, No.3
0 Si
S;02 Fig. 1. Schematic structural models for the peroxide-like bond model (a) and for the double-bond monoxide model (b). Two different oxygen atom potentials are present in (a). While only a single oxygen potential is present in (b). Fig. 2 by performing nonlinear least squares fits (solid lines) to the raw experimental data (shown as points) for each sample. Curve (c) in Fig. 2 shows the first derivatives of curve (a) shown as a dashed line and of curve (b) shown as a solid line shifted in energy by 1.1eV to aligh with curve 2(b). As one can see from Figs. 2(a), (b) and (c) the O(ls) results for the mono-
(3~
—
537I I
I I I 529 I 533 ENERGY (eV) —
—
Fig. 2. X-ray photoemission spectra for the oxygen (is) core level of SiO2 (a) and a monolayer of 02 on Si (111) or (100) (curve b). The experimental results are shown as points along with least-squares fits (solid lines) to a double-component asymmetric Gaussian function. The first derivative of the fitted curves are shown in part (c) for the Si + 02 (solid curve) and for SiO2 (dashed curve shifted to coincide with peak of solid curve).
layer Si + 02 are asymmetnc while the results for SiO2 are nearly symmetric. In the latter case the experimental line width is partly due to instrumental photon and electron resolution components of— 13 eV FWHM Gaussian line shape and partly due to inhomogeneous broadening which has a 0.6 eV splitting, we estimate a splitting of since the SiO2 is amorphous with some (—‘ 10%) variation 0.7—0.8 eV for chemisorbed 02 on Si (ill) surfaces. in bond angles and bond length. The same symmetric This energy is too to be “shake-up” explained asprocess a “shake-up” 8 since the small minimum should response function has been found from least-squares satellite data analysis of Si (2s), Si (2p), Ge (2p) and Ge (3d) have an energy slightly greater than the silicon bandgap XPS signals. For oxygen chemisorbed on metal surfaces of 1.1eV. This asymmetric line shape of the monolayer other workers as well as own unpublished work indicates Si + 02 data is clear evidence against the double-bonded a symmetric line shape corresponding to dissociated, monoxide model of Ludeke and Koma. However, it does atomic oxygen provided that only a single adsorption not imply a symmetric peroxide model as discussed by species is present.8 In contrast, the asymmetric Si + 02 Ibach eta!.4 Thus we propose a new modified peroxide results are sharper on the high energy (right-hand) side model which involves two inequivalent Si—O “single” with partial half-width near that of the instrumental bonds and one 0—0 “single” bond. These bonds on the resolution. The lower energy (left-hand) side of Fig. surface provide a natural explanation of the three sur2(b) is considerably broader than the other side. This face vibrational frequencies as determined by Thach and we take to be an indication of an unresolved doublet due coworkers. to two different O(is) states much like the barely We present in Fig. 3, new experimental data for the resolved doublet of the 02 molecule due to exchange vibrational energies of Si + 02 obtained from HRES. splitting.9 These experiments were performed in a new version of By comparing the observed asymmetry of 0(1 s) the HRES apparatus which was previously used.4’7 This and that of the unresolved spin—orbit doublet Si (2p) new apparatus has the main advantage that the featureless
Vol. 20, No. 3
OXIDATION OF SILICON
x40
____
56
Si 95
-
hE.12.5
0
BACKG~~ UPPER LIMIT
distinction can be made between a symmetric peroxide 4 and the asymmodel proposed for simplicity earlier metric as model suggested by the XPS data. Finally we would like to comment on some of the weaker points of the Ludeke—Koma double-bond model for chemisorption of 02 at Si and Ge surfaces. The total energy is not an important criterion for chemisorption. The energy ofthe Si0 2 phase is obviously
02
130
~J L1,1
+
279
lower than the Si + 02 surface phase so any total energy
180
100 ENERGY LOSS (meVi
200
Fig. 3. Surface phonon spectra obtained by high resolution electron scattering spectroscopy (HRES) showing the Si—Si phonon at 56meV and the Si—O phonons at 95,130 and 180meV. background intensity (due mainly to scattering of the primary elastic electrons within the analyzer) is much reduced over the earlier of HRES experimental Four different phonons vibrational modes equipment. are evident in Fig. 3 which correspond to (1) Si—Si mode at 56meV, (2) Si—O mode at 95meV, (3) Si—O mode at 130meV and (4) 0—0 mode at 180 meV. The 56 meV mode is also present on the clean surface, however, much sharper. The modes at 95 and 130 meV are the strongest and we take this as an indication of the dominant Si—O chemisorption bonds. The broad 180meV mode is observed to be well above the background intensity and is assigned to an 0—0 vibration which is only weakly allowed by electron scattering with electric dipole selection rules because of the surface chemisorption geometry. The 180 neV mode cannot be explained by the model of double-bonded oxygen atoms and its energy is close to that of the free 02 molecule, 196meV. Based on the surface modes no
argument favorable should not predict aminimum double-bonded the Si02 phase chemisorption to be the most monoxide the geometry phase. with The surface activation phase isphase determined energy for by disturbing believe to theand be 0—0 the peroxide-like double bond model ofand gas based 02. both This on we earlier results as well as the XPS HRES results discussed above. On the other hand, the agreement claimed by Ludeke and Koma for gas phase optical absorption and ELS of chemisorbed oxygen may be somewhat misleading. The ELS and optical transitions occur at the same energies only for comparison oftwo gas phase samples. For solids and for solid surfaces the ELS peaks are shifted to higher energies. For example in pure Si the optical E2 peak in absorption occurs at 4.4 eV while in 5 Also ELS thethe same transitions a peak at 4.9 eV. in SmS lowest opticalproduce absorption transition is at 1.1 eV while in ELS it occurs at 2.6 eV.10 Thus the empirical shift is 0.5—1.5 eV and the agreement claimed by Ludeke and Koma could well be fortuitous. It would be useful to compare HRES of electronic transitions for chemisorbed oxygen with the optical data of gas phase SiO and GeO. However, such a detailed comparison is not appropriate in the present comment. In summary, we have presented new XPS and HRES results for the monolayer oxide, Si + 02 which seem to clearly support a peroxide-like model of 02 chemisorption as shown in Fig. 1(a). The double-bond model of Ludeke and Koma [see Fig. 1(b)] does not agree with these new results or with a reinterpretation of their own ELS results. Thus at the present time the peroxide-like model seems to be correct.
REFERENCES 1.
GREENM. &MAXWELLK.H.,J. Phys. Chem. Solids 13, 145 (1960).
2. 3.
GREEN M. & LIBERMAN A., J. Phys. Chem. Solids 23, 1407 (1962). BOOTSMA G.A., Surf Sci. 15, 340 (1969).
4. 5.
IBACH H., HORN K., DORN R. & LOTH H., Surf Sci. 38,433(1973). IBACH H. & ROWE i.E., Phys. Rev. B9, 1951 (1974); ibid. 10, 710 (1974).
6. 7.
LUDEKE R. & KOMA A.,Phys. Rev. Lett. 34, 1170(1975). FROITZHEIM H., IBACH H. & LEHWALD S., Rev. Sci. Instrum. 46, 1325 (1975).
280
OXIDATION OF SILICON
8.
FUGGLE i.C., MADEY T.E., STEINKILBERG M. & MENSEL D., Chem. Phys. Lett. 33,233 (1975) and references cited therein.
9. 10.
Vol. 20, No. 3
SIEGBAHN K. et al., ESCA Applied to Free Molecules. North Holland, Amsterdam (1969). ROWE J.E., CAMPAGNA M., CHRISTMAN S.B. & BUCHER E.,Phys. Rev. Lett. 36, 148 (1976).