Direct Observation of Valence and Conduction States near the SiO2/Si(100) Interface

Valence and conduction states near the SiO2/Si(100) interface were directly observed using soft x-ray absorption and emission spectroscopy. For the O K-edge absorption spectra, the step-like structures were observed at 531, 533 and 534.5 eV. These step-like structures observed at 531, 533 and 534.5 eV were assigned to an oxygen atom bonding to Si, Si, and Si of the interface, respectively. In the case of O K-edge emission spectra, with decreasing incident photon energy from 535 to 531 eV so as to shift the conduction band minimum of the interface towards Fermi energy, the corresponding valence band maximum was shifted to Fermi energy direction. Thus, the local band gap became narrower with the decrease of the oxidation number of the suboxide species. Further the interface structure was also discussed from the spectrum features. [DOI: 10.1380/ejssnt.2008.209]


I. INTRODUCTION
Metal-oxide-silicon fieled effect transistors (MOSFETs) are the most important fundamental device structures for large scale integration (LSI), i.e., more than 80% of LSI are composed of the MOSFETs.High performance and low cost fabrication of LSI have been achieved through miniaturization, which requires the use of ultrathin gate oxide layers [2,11].In fact, the gate oxide layer has been already thin as ∼ 2 nm [3].For such a thin oxide layer, the interface between silicon oxide and silicon plays a dominant role in determining performance of MOSFETs.Therefore, an atomic-scale understanding of electronic states at the SiO 2 /Si(100) interface has become of paramount importance.
Many studies have been done to probe the electronic states at the interface.Hirose et al. used high-resolution photoelectron spectroscopy and obtained the electronic states of interfacial transition layer of SiO 2 /Si(100) by subtracting a spectrum for 0.61-nm thick SiO 2 from that for 0.83-nm thick SiO 2 [4].Hattori et al. estimated the compositional and electronic transition layers of the SiO 2 /Si(100) interface to be 0.19 nm and more than 0.51 nm, respectively, by using O 1s electron energy loss spectroscopy [5].Muller et al. successfully observed conduction band states at the interface and clarified that electronic transition layer is less than 0.7 nm by means of electron energy loss spectroscopy in transmission electron microscopy (TEM) [6], but they observed the average interfacial electronic structures because the electrons excite several interfacial electronic states through the thick sample for TEM observation.Although extensive studies have been performed on electronic states of the SiO 2 /Si(100) interface, the valence and conduction electronic structures and how to change the band gap near the SiO 2 /Si(100) interface have not been directly observed yet.Thus we require a new method that enables us to probe interfacial electronic states directly and that gives us information on the transition layers and change in band-gap around the interface.
Recently we successfully observed conduction and valence electronic structures at solid-solid interfaces directly using soft x-ray absorption (SXA) and emission (SXE) spectroscopy [7][8][9].This method is based on site-specific photo absorption and emission, which gives us the local valence electronic states at the interface [10,11].In the present study, we investigated the conduction and valence states near the SiO 2 /Si(100) interface using SXA and SXE spectroscopy.Furthermore, we also elucidated the transition layer and changing behavior in the bandgap near the interface.

II. EXPERIMENTAL
SiO 2 /Si(100) samples were prepared from boron-doped p-type Si(100) wafers having a resistivity of 3-5 Ωcm.After standard RCA cleaning, a native oxide layer was etched away by a 1% HF solution.A 1.8 nm oxide layer was prepared in 0.1 MPa of oxygen at 600 K for 5 min, and as a reference 8 nm-thick SiO 2 oxide layer was prepared in 0.1 MPa of oxygen at 1100K for 1 hour.These thicknesses were estimated with Si 2p photoelectron spectroscopy [2].Note that the intensity ratios of Si 1+ : Si 2+ : Si 3+ of the suboxide species in the present sample were 0.264: 0.309: 0.427 respectively, revealed with analysis of Si 2p photoelectron spectra.
The synchrotron radiation experiments were performed using BL-27SU at SPring-8.The oxygen K-edge absorption spectra were measured by detecting O KVV Auger electrons (510 eV) with an instrumental energy resolution of 50 meV.SXE spectroscopy was performed with an 800 meV energy resolution.For SXA and SXE, Fermi energy was determined from the O 1s core-level spectroscopy of SiO 2 /Si(100) structures and the Fermi energy of Au film [8].The design and performance of the SXE spectrometer is described in detail elsewhere [12].

III. RESULTS AND DISCUSSIONS
Figures 1 (a) and (b) show the oxygen K-edge absorption spectra of SiO 2 /Si(100) structures with 1.8 nm and 8 nm thick SiO 2 layers, respectively.Because the mean free path of the Auger electrons is ∼ 1.5 nm at 510 eV [13], the spectrum of 8-nm thick SiO 2 represents the bulk SiO 2 spectrum.The absorption spectrum of the 1.8-nm thick SiO 2 /Si(100) structure is strikingly different from that of the bulk SiO 2 spectrum.The spectrum of the 1.8-nm thick SiO 2 /Si(100) structure has a lower onset with steplike structures at 531, 533 and 534.5 eV.According to Muller et al., the oxygen K-edge was lowered by 3 eV at the interface compared to the case of the bulk SiO 2 [6], indicating that the step-like structures in the spectrum are attributed to unoccupied O 2p states of the interface, and the lowered edge is explained in term of a reduced band gap of the interface.Note that the peak at 537.5 eV is attributed to unoccupied O 2p states that are hybridized with Si 3s and 3p states of bulk SiO 2 [14,15].
As for the interface structure of SiO 2 /Si, it is well known that the interface consists of intermediate oxidation states (namely suboxide), i.e., Si 1+ (SiO 0.5 ), Si 2+ (SiO), and Si 3+ (SiO 1.5 ) [16,17].Wallis  the intermediate states in the amorphous silicon shifts to lower energy as the oxidation number of the adjacent silicon atoms decreases [18].This means that with decreasing the oxidation number of the adjacent silicon atoms, unoccupied states due to oxygen atom shift toward the Fermi energy.Thus, the step-like structures at 531, 533 and 534.5 eV (Fig. 1) were assigned to an O atoms bonding to Si 1+ , Si 2+ , and Si 3+ of the interface, respectively.Hereafter, we denote the O atoms bonding to Si 1+ , Si 2+ , and Si 3+ as P1, P2 and P3, respectively.Accordingly, a site-specific SXE spectrum, that is, a site-specific valence electronic structure of the interface could be obtained using different excitation energy, i.e. 531 eV for P1, 533 eV for P2, and 534.5 eV for P3.It should be noted that when exciting P2 and P3 by using the corresponding edge energies, not only P2 (P3) but also P1 (P2 and P3) are excited.In order to avoid overlapping excitations as low as possible, the excitation energies 533.5 eV for P2 and 535 eV for P3 were used.
Figures 2 (a)-(d) show the O K-edge SXE spectra, that is, the valence states for the 1.8-nm thick SiO 2 /Si(100) structure.For the spectra, electronic states between 6 and 10 eV are attributed to the O 2p non-bonding states while electronic states between 11 and 16 eV are attributable to bonding states between the O 2p and Si sp 3 orbitals [19].As can be seen in Fig. 2, the bonding states hardly change their peak positions while the non-bonding states are different.This indicates that the non-bonding states are http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology Volume 6 (2008) sensitive to the chemical environment.With the incident photon energy of 537.5 eV, the occupied O 2p states of bulk SiO 2 dominantly contribute to the SXE spectrum (Fig. 2(a)).In this spectrum, the peak at 7.1 eV and the shoulder at 8.8 eV are attributed to the non-bonding states, while the peaks at 11.8 and 14.4 eV are due to the bonding states.When the incident photon energy is 535 eV in order to excite P3, the non-bonding states become broader compared to the case of bulk SiO 2 and an edge structure is observed at 5.2 eV (Fig. 2(b)).By exciting P2 (the incident photon energy of 533.5 eV), the nonbonding states also become broader with respect to those of bulk SiO 2 and an edge is detected at 3.2 eV (Fig. 2(c)).When the incident photon energy is 531 eV in order to excite P1, broad and intense states with the lower edge at 2.5 eV are observed (Fig. 2(d)).According to the previous study, these edge structures were attributed to valence band maximums (VBMs) of the suboxide species [7].Thus, the VBMs, conduction band minimums (CBMs), and band-gaps (E g s) of the suboxide species are obtained, which is summarized in Table I.Note that when exciting P2 and P3 in order to obtain the corresponding valence states, multiple excitations should be occurred described above.However the spectrum features observed in the valence states are different.Thus, overlapping excitations must be negligible.From these results, it is safely concluded that with the decrease of the oxidation number of the suboxide species in the interface, the CBM and VBM of the interface are shifted toward the Fermi energy; the local band gap becomes narrower with the decrease of the oxidation number of the suboxide species.
Finally we would like to discuss the structure of the SiO 2 /Si(100) interface.Bell and Ley prepared SiO x (0 < x < 2) alloy films by sputter deposition of Si in Ar and O 2 mixture [20].According to their PES results, when SiO 0.57 (Si 1+ ), SiO(Si 2+ ) and SiO 1.5 (Si 3+ ) alloy films were measured, the VBMs were observed at ∼ 1.9 eV, 2.2 eV and 4.5 eV respectively.By comparing their results with our SXE spectra, the energy positions (the VBMs and the non-bonding states) of the SiO 0.57 (Si 1+ ) and SiO(Si 2+ ) alloy films are similar tendency to those of P1 and P2, respectively (Figs. 2(b) and (c)).In the case of P3, on the other hand, P3 shows similar VBM to bulk SiO 2 in the SXE spectrum, which is not consistent with the result in the SiO x alloy film.Therefore, we consider that Si 3+ atoms are not located at the interface.The location close to the bulk SiO 2 should change the electronic structure compared to the case of the interface.As a result, P3 may show a similar VBM to the bulk SiO 2 .This consideration is supported with the previous core level spectroscopy [21] and the theoretical calculation [22]; the Si 1+ and Si 2+ atoms are considered to be located at the interface while the Si 3+ atoms should not be located at the interface.Thus the SiO 2 /Si(100) interface should not be an entirely abrupt interface and a possible structural model for the SiO 2 /Si(100) interface is shown in Fig. 3.

IV. SUMMARY
Valence and conduction band electronic structures at the SiO 2 /Si(100) interface were investigated using soft x-ray absorption and emission spectroscopy.For the O K-edge absorption spectra, the step-like structures were observed at 531, 533 and 534.5 eV.These step-like structures were assigned to an oxygen atom bonding to Si 1+ , Si 2+ , and Si 3+ of the interface, respectively.In case of O K-edge emission spectra, the valence electronic structure was changed depending on the oxidation number of the suboxide species; VBM was shifted towards the Fermi energy direction with decreasing the oxidation number of the suboxide species.From these results, it was found that with the decrease of the oxidation number of the suboxide species in the interface, the CBM and VBM of the interface were shifted to the Fermi energy; the local band gap becomes narrower with the decrease of the oxidation number of the suboxide species.Furthermore from the observed valence spectra, we concluded that the SiO 2 / Si(100) interface is not an entirely abrupt interface.

FIG. 1 :
Photon Energy (eV) Figures1 (a) and (b) show the oxygen K-edge absorption spectra of SiO 2 /Si(100) structures with 1.8 nm and 8 nm thick SiO 2 layers, respectively.Because the mean free path of the Auger electrons is ∼ 1.5 nm at 510 eV[13], the spectrum of 8-nm thick SiO 2 represents the bulk SiO 2 spectrum.The absorption spectrum of the 1.8-nm thick SiO 2 /Si(100) structure is strikingly different from that of the bulk SiO 2 spectrum.The spectrum of the 1.8-nm thick SiO 2 /Si(100) structure has a lower onset with steplike structures at 531, 533 and 534.5 eV.According to Muller et al., the oxygen K-edge was lowered by 3 eV at the interface compared to the case of the bulk SiO 2[6], indicating that the step-like structures in the spectrum are attributed to unoccupied O 2p states of the interface, and the lowered edge is explained in term of a reduced band gap of the interface.Note that the peak at 537.5 eV is attributed to unoccupied O 2p states that are hybridized with Si 3s and 3p states of bulk SiO 2[14,15].As for the interface structure of SiO 2 /Si, it is well known that the interface consists of intermediate oxidation states (namely suboxide), i.e., Si 1+ (SiO 0.5 ), Si 2+ (SiO), and Si 3+ (SiO 1.5 )[16,17].Wallis et al. revealed using theoretical calculations that the oxygen K-edge of

TABLE I :
Valence band maximums (VBMs), conduction band minimums (CBMs) and band gaps (Egs) of the suboxide species and SiO2.The positions of CBMs are determined from the pre-edge energy positions.The unit is eV.The Fermi energy was set to be the origin of the energy.