Direct observation of the site-specific valence electronic structure at SiO2/Si(111) interface

For today’s silicon based devices, understanding the SiO2/Si(111) interface on atomic level is an important subject for fabricating superior devices. However, despite of many studies on the SiO2/Si(111) interface, the interfacial valence electronic states have been typically evaluated as the average and not as individual states. In the present study, we successfully observed valence electronic states of particular atoms at the SiO2/Si(111) interface for the first time using soft x-ray absorption and emission spectroscopy. In addition, comparing the experimental results to first-principles calculations revealed local interfacial properties. [DOI: 10.1380/ejssnt.2006.280]


I. INTRODUCTION
Over the last 30 years, the active element density in integrated circuits has doubled roughly every 18 months as embodied in Moore's law. The smallest, but crucial, controlling feature of these circuits is the gate oxide (amorphous SiO 2 ) layer, which is the basis for metaloxide-semiconductor field effect transistors (MOSFET's). Currently, industry can produce MOSFET's with a gate shorter than 50 nm and an SiO 2 gate oxide less than 2 nm thick [1]. If the present reductions continue, the thickness will soon be a few atomic layers. This implies that an atomic level understanding of the properties at the SiO 2 /Si interface is important for fabricating devices with an ultra-thin oxide layer. Therefore, we have focused on measuring the occupied electronic states projected at particular atoms of the SiO 2 /Si(111) interface.
Recently, several groups reported that the conduction band edge was lower at the interface compared to bulk SiO 2 [2][3][4]: Muller, et al. reported that the oxygen Kedge was 3 eV lower at the interface using electron energy loss spectroscopy (EELS) in a scanning transmission microscope [2]. Based on this finding, the selective photoabsorption at the interface is realized by tuning the incident photon energy to only allow the electronic excitation from the O 1s level to the lowered conduction band levels. Once the O 1s core level at the interface is excited, the core hole is filled by an electron from an occupied valence level accompanied with soft X-ray emission. Since the O 1s state is localized on a particular oxygen site, the emit-ted photon spectrum allows an atom-specific projection of the valence electronic states. Using this method, we successfully observed site-specific occupied states at the SiO 2 /Si(111) interface for the first time.

II. EXPERIMENTAL
SiO 2 /Si(111) samples were prepared from phosphorusdoped n-type Si(111) wafers that have a resistivity of 0.001 Ωcm. After standard RCA cleaning, a native oxide layer was etched away by a 1% HF solution and then the wafers were immersed in a 40% NH 4 F solution for 15 min to form an atomically smooth silicon surface [Si(111)(1 × 1)-H] [5,6]. A 1.8 nm-oxide-layer was prepared in 0.1 M Pa of oxygen at 600 K for 5 min.
The synchrotron radiation experiments were performed using BL-27SU at SPring-8. The oxygen K-edge absorption spectroscopy was performed by detecting O KVV Auger electrons at 510 eV and an instrumental energy resolution of 50 meV. Soft X-ray emission (SXE) spectra were measured with an 800 meV energy resolution. The design and performance of the SXE spectrometer is described in detail elsewhere [7]. Si 2p core level spectra were measured with incident photon energy of 800 eV and a Gammadata Scienta SES2002.
Atomic force microscopy (AFM) measurements were performed with a JEOL SPM 4200 using contact mode. The H-terminated Si(111) surface and the surface after the etch of the oxide layer of SiO 2 on Si(111) away with a 0.5% HF solution were investigated. The measurements were performed in nitrogen atmosphere.
First-principles calculations based on the density functional theory with a generalized gradient approximated correction were performed to evaluate the local density of states of the valence level and the interface structure e-Journal of Surface Science and Nanotechnology and properties [8]. A six Si-layer thick slab model was used with a periodic boundary condition: the unit cell contains four dangling bonds of a clean Si(111) surface in its initial configuration. The oxidized structures were then obtained by inserting O atoms one by one with structure relaxation. We prepared three models obtained by a different insertion order of oxygen atoms. Figure 1 shows AFM images of the H-terminated Si(111) surface and after the etch of the oxide layer on Si(111) away. After the etch of the oxide layer, steps are clearly observed indicating that the interface is atomically flat after the oxidation. This is consistent with the previous studies [6,9]. The spectrum (c) shows the magnified spectrum of (a). Incident photon angle was 36 • from surface normal and p-polarized photon was used.

III. RESULTS AND DISCUSSION
free path of the Auger electron is ∼1.5 nm at 510 eV [10], which is much shorter than the SiO 2 layer thickness. An absorption spectrum of the 1.8-nm-thick SiO 2 /Si(111) structure is strikingly different from the bulk SiO 2 spectrum. The 1.8 nm spectrum has a lower onset and exhibits edge structures at 530 and 531.5 eV. Muller et al. used EELS and reported that the oxygen K-edge was lowered by 3 eV at the interface compared to the bulk SiO 2 [2]. Thus, the edge structures in the spectrum are explained by unoccupied O 2p states at the interface, and the lowered edge exhibits a reduced band gap at the interface. Note that the peak at 537.5 eV is attributed to unoccupied O 2p states bound with Si 3s and 3p states of bulk SiO 2 [11,12]. Figure 3 shows the Si 2p core level spectrum for the 1.8-nm-thick SiO 2 /Si(111) structure. A peak fitting procedure was referred to the previous studies [5,6,13]. In these figures, Si 0 and Si 4+ denote the Si substrate and SiO 2 respectively, and Si 1+ and Si 3+ denote Si atoms bound to one and three oxygen atoms, respectively. Note that Si 2+ due to an Si atom bound to two oxygen atoms was very weak for the 1.8-nm-thick SiO 2 /Si(111) structure. It is found from emission angle dependence of Si 2p spectra that these suboxide species, i.e. Si 1+ , Si 2+ , and Si 3+ , were dominantly formed at the interface. In addition, analysis of the Si 2p photoelectron spectra for the 1.8-nm-thick SiO 2 /Si(111) structure revealed that the relative intensities of the suboxide species were Si 1+ : Si 2+ : Si 3+ = 0.563 : 0.003 : 0.434. Thus the interface predominantly consists of Si 1+ and Si 3+ and these results are good agreement with the previous results [5].
Wallis, et al. reported using theoretical calculations that the oxygen K-edge of the intermediate states in the amorphous silicon shifted to lower energy as the oxidation number of the adjacent silicon atoms decreased [3]. Thus, the absorption edges at 530 and 531.5 eV are assigned to an O atom bonding to Si 1+ and Si 3+ at the interface, respectively (hereafter, the O atom bonding to Si 1+ and Si 3+ at the interface is denoted to P1 and P3, respectively). Therefore, site-specific SXE spectra of particular oxygen atoms at the interface can be obtained by choosing excitation energies, i.e., 530 eV for P1, 531.5 eV for P3, and 537.5 eV for SiO 2 . Figures 4(a)-(c) show the O K-edge SXE spectra for the 1.8-nm-thick SiO 2 /Si(111) structure. The electronic states at the interface are noticeably different from those of bulk SiO 2 and the interfacial electronic states strongly depend on the intermediate oxidation states at the interface. With a photon energy of 537.5 eV, the occupied O 2p states of bulk SiO 2 predominantly contribute to the SXE spectrum ( Fig. 4(a)). In this spectrum, the peak at 7.25 eV and the shoulder at 8.65 eV are attributed to the O 2p non-bonding states, while the peaks at 11.75 and 14.33 eV are due to the bonding states between O 2p and Si sp 3 orbitals [14]. When the incident photon energy is 533 eV in order to excite P3 (Fig. 4(b)), the peaks due to non-bonding states become broader and shift towards a higher binding energy compared to bulk SiO 2 . In addition, a new, small, broad peak around 4 eV is observed. For P1 (Fig 4. (c)), the peak at 6.50 eV, which is at- tributed to the non-bonding states of O 2p, is very sharp and does not have a shoulder, indicating that an uniform structure is formed at the interface. Note that P1 should be also excited as well as P3 when the incident photon energy is 533 eV because the absorption edge of P3, i.e. 531.5 eV, could overlap with that of P1 (see Fig. 2(c)). However, intensity of P1 decreased as increasing incident photon energy; the relative intensity of P1 was less than 10% judging from the P1 derived peak intensity at 6.50 eV. Thus, the occupied O 2p states of P3 predominantly contribute to the SXE spectrum ( Fig. 4(b)) when the incident photon energy is 533 eV. A straightforward way to quantitatively evaluate the interface states is to combine the experimental results with theoretical calculations, providing the interface properties such as the structure, conducting and dielectric behavior. For the SiO 2 /Si(111) interface, the interface is abrupt, and Si 1+ and Si 3+ species dominate as described above. Therefore, the atomically flat SiO 2 /Si(111) interface only consisting of Si 1+ or Si 3+ is calculated as the interface model. Figures 4(d) tively, calculated with the model structures illustrated in the right. It is clear that the calculated DOS for amorphous SiO 2 produces SXE spectrum of the oxide region well (Figs. 4(a)(d)). The Si-O-Si angle and Si-O bond length in the theoretically optimized structure are predominantly distributed from 140 • to 155 • and 0.155 nm to 0.165 nm, respectively, which are consistent with previous experimental results [15]. As for the P3 interface layer, the Si-O-Si angle is distributed from 131.6 • to 151.6 • and the Si-O bond length from 0.158 nm to 0.176 nm in the theoretically optimized structure. The local variation from the amorphous SiO 2 results in the appearance of a shoul-der at 4 eV and the broadening of the non-bonding state from 7.75 eV to 9.13 eV (Figs. 4(b)(e)). On the other hand, for the P1 interface layer, the Si-O-Si angle and Si-O bond length should be maintained near 180 • and 0.19 nm, respectively, in order to reproduce the non-bonding sharp peak at 6.50 eV (Figs. 4(c)(f)).
Next we investigated how the structure with P1 or P3 interface exhibits the interface properties, which would enable us to understand atomic scale transport properties of carrier. Figure 5 shows the calculated conduction band minimum (CBM) and valence band maximum (VBM) of the structure with P1 or P3 interface in the interfacial region. For the P1 interface, since the Si-O-Si bond angle and the Si-O bond length are near 180 • and 0.19 nm, the orbital of the non-bonding state is along the parallel to the interface. In this case, our calculations clearly show that the non-bonding state of P1 hardly interacts with Si atoms, and the CBM and VBM in the interface region are attributed to penetration from DOS of the silicon substrate. Therefore the P1 interface exhibits a similar band gap to the bulk Si in the interfacial region from 0 nm to 0.2 nm. For the P3 interface, on the other hand, the non-bonding state of the P3 interacts with Si atoms because the Si-O-Si bond angle and the Si-O bond length are distributed as described above. Thus the gap of the P3 interface gradually becomes large in the interfacial region from 0 nm to 0.2 nm. These results indicate that the P1 interface is relatively more conductive. Thus, the interface characteristics strongly depend on the local interface structures, which implies that performance of MOSFET with an extremely small gate width is essentially affected by the suboxide composition at the interface. In future MOSFET, it is determined that controlling the interfacial states on atomic level is crucial for not only SiO 2 less 1nm thick, but high-dielectric constant materials and silicon oxynitride used as alternatives to SiO 2 [16,17]. In this study, we present our approach using SXE, which successfully detects the electronic states of particular atoms at the interface. Therefore, this approach is applicable to interfacial states for various kinds of gate oxides and will be indispensable for evaluating and designing nm-scale superior devices.

IV. CONCLUSIONS
Site-specific valence electronic states at the SiO 2 /Si(111) interface were successfully observed by SXA and SXE spectroscopy. We found that the interfacial valence electronic states were noticeably different from those of bulk SiO 2 and strongly depended on the chemical states of each atom at the interface. In addition, comparison of the experimental results with first-principles calculations quantitatively revealed not only the interface structure but also the interface electronic properties in atomic-scale. by the Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology. The synchrotron radiation experiments were performed at the SPring-8 with the approval of JASRI (Proposal No. 2003B0209-Nsa-np-Na and 2004A0345-Nsa-np-Na).