Si(110) Islands

Surface Science 681 (2019) 9–17

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Local valence electronic states of silicon (sub)oxides on HfO2/Si-(sub)oxide/ Si(110) and HfSi2/Si-(sub)oxide/Si(110) Islands

T

⁎

Takuhiro Kakiuchia, , Kyouhei Ikedaa, Kazuhiko Maseb,c, Shin-ichi Nagaokaa a

Department of Chemistry, Faculty of Science, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan c SOKENDAI (School of High Energy Accelerator Science, The Graduate University for Advanced Studies), 1-1 Oho, Tsukuba 305-0801, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Metal-insulator-semiconductor structure High-dielectric-constant material Local valence electronic states at surface and interface Auger photoelectron coincidence spectroscopy X-ray photoelectron spectroscopy Synchrotron radiation

The effect on the local valence electronic states of Sin+ suboxide components (n = 2, 3, and 4) of hafnium deposited on a low-index Si(110) substrate is investigated by Si-L23VV Auger electron Sin+-2p photoelectron coincidence spectroscopy (Si-L23VV-Sin+-2p APECS), and the chemical states and stabilities are discussed. Hafnium-covered Si(110) is immediately oxidized to HfO2 and SiO2 because hafnium serves as an effective catalyst for Si oxidation. Therefore, a HfO2/Sin+-(sub)oxide/Si(110) [HfO2/Sin+/Si(110)] structure is easily formed (n = 1, 2, 3, and 4). Oxygen diffusion from HfO2 layers toward the Si(110) substrate is promoted by annealing at 923 K. Oxygen atom desorption from the HfO2/Sin+/Si(110) surface occurs after annealing at 1073 K, and HfSi2 islands (i-HfSi2) are formed with a partly exposed Si(110)-16 × 2 double domain (DD) surface. i-HfSi2 shows low reactivity toward O2 molecules, whereas the exposed Si(110)-16 × 2 DD surface is immediately oxidized. Here, a i-HfSi2/Sin+-(sub)oxide/Si(110) (i-HfSi2/Sin+/Si(110)) structure is formed. Furthermore, we measure the Si-L23VV-Sin+-2p APECS spectra of Sin+ in the HfO2/Sin+/Si(110) and the i-HfSi2/ Sin+/Si(110) structures (n = 2, 3, and 4) to evaluate the local valence electronic states of the Sin+ (sub)oxide components. The binding energy at the valence band maximum (BEVBM) of Sin+ in the i-HfSi2/Sin+/Si(110) structure is lower than 1.5 ± 0.7 eV as compared to that in the HfO2/Sin+/Si(110) structure (n = 2, 3, and 4). The local valence electric states of the nearest neighbors and the second neighbors through oxygen of Sin+ are determined to affect those of the Sin+ atom (n = 2, 3, and 4). The Sin+ atoms in the i-HfSi2/Sin+/Si(110) structure can directly bond to hafnium atoms as the nearest neighbors and most commonly have Sim+ atoms in lower ionic valence states as second neighbors (m < 4), whereas the Sin+ atoms in the HfO2/Sin+/Si(110) structure cannot form this bond. In addition, the existence of Hf silicide and Si in lower ionic valence states can reduce the band gap of the HfO2/Si(110) structure.

1. Introduction Over the past couple of decades, high-dielectric-constant (high-k) materials, such as tantalum pentoxide, zirconium dioxide, and hafnium dioxide (HfO2), have attracted significant interest as alternatives to SiO2 gate oxides in the downscaling of metal-oxide-semiconductor field-effect transistors (MOS-FET) [1]. HfO2 is promising and reliable because of several factors such as its high k (∼21 [1,2]), large band gap (∼5.7 eV [3–6]), and good thermal stability on a Si substrate [1,2,7]. The application of ideal HfO2 crystals in MOS-FETs requires increased physical thickness of the gate oxide film in order to reduce the severe current leakage that is generated from the direct tunneling effect. Thus, various Hf deposits on Si(100) and (111) substrates have been studied for their band distributions [3–6], crystal structures, chemical stability ⁎

[7–11], and chemical states at the surface and the interface [12–20]. Previous studies have reported that Hf and Si substrates have various chemical states that are strongly dependent on different factors such as the manufacturing method, vacuum environment, thicknesses of the Hf layer(s), facets of the Si substrate, and atoms and chemicals on the surface [12–20]. In some previous reports on HfO2/Si structures with some SiO2 layers at the interface, the initial Hf–O bonds in the HfO2 layers were converted to Hf–O–Si bonds by annealing at 873–1073 K. Furthermore, the polycrystalline HfOx started to decompose at temperatures over 1027 K [10,11,15,16,19]. On the other hand, in the case of Hf/Si(100) and Hf/Si(111) structures, three-dimensional islands were formed, regardless of the Hf thickness (from monolayer to 50 nm), just after annealing at around 973–1273 K [9,11]. These structures comprised a

Corresponding author. E-mail address: [email protected] (T. Kakiuchi).

https://doi.org/10.1016/j.susc.2018.10.024 Received 27 April 2018; Received in revised form 27 October 2018; Accepted 31 October 2018 Available online 04 November 2018 0039-6028/ © 2018 Elsevier B.V. All rights reserved.

Surface Science 681 (2019) 9–17

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for 10 min. The sample was then quenched slowly to room temperature (RT). The pressure was kept below 1.0 × 10−7 Pa during the cleaning procedure. The temperature of the Si(110) surface in the UHV chamber was monitored with an infrared pyrometer (IR-AHS0, CHINO Co., Ltd.) from outside the chamber. The low-energy electron diffraction (LEED) patterns after cleaning always showed sharp 16 × 2 spots with SD along the [11¯2 ] or [1¯12 ] direction (not shown here) [26]. We used a commercial Hf rod (ϕ : 2 mm, purity of 99.9%, including Zr 3%, Nilaco Co., Ltd.). Hf vapor was generated by means of electron beam bombardment, which is an ideal method because it allows atomic-scale control with minimal impurities. Degassing of the Hf rod was performed by heating to 1300 K for 1 h. Then, Hf vapor was deposited on the clean Si(110)-16 × 2 SD surface in the UHV chamber, which was maintained below 1.2 × 10˗7 Pa. The Hf deposition rate, which was estimated using a commercial quartz crystal unit (Q-pod, INFICON Co., Ltd.) was maintained at (2.5 ± 0.1) × 10˗1 Å/min. As a result, we deposited ultrathin Hf films with a thickness of 9.6 ± 0.5 Å, which corresponds to about four Hf layers because the thickness of a Hf monolayer has been reported to be approximately 2.4 Å [2,10,12]. Although this thickness would not be uniform on the Si(110) surface, we use it as a guide. The 16 × 2 SD LEED pattern of the clean Si(110) surface vanished after the Hf deposition procedure. In addition, no new LEED patterns appeared in our study. In this paper, we denote this sample as the bare Hf/Si(110) surface. The bare Hf/Si(110) surface was exposed to O2 gas (purity of 99.5%) at 200 Langmuir (L, 1 L = 1.33 × 10˗4 Pa s) and annealed at 923 and 1073 K by resistive direct-current heating. The Hf/Si(110) surface after annealing at 1073 K was again exposed to O2 gas at 200 L. The XPS and Si-L23VV-Sin+-2p APECS measurements combined with soft X-ray synchrotron radiation were performed in a UHV chamber at beamline 11D (BL-11D) of the KEK Photon Factory. All samples were irradiated with soft X-rays with p-polarization at an incident angle of 84° from the surface normal. The monochromatic photon energies for Si 2p and Hf 4f (including valence) ionization were fixed at 130 and 70 eV, respectively, with a typical energy resolution (E/ΔE) of ∼2000. We used a coincidence analyzer developed by ourselves to measure all spectra. Our coincidence analyzer consists of several components, including a coaxially symmetric mirror electron energy analyzer (ASMA, electron energy resolution (E/ΔE) of ∼84), and a double-pass cylindrical mirror electron energy analyzer (DP-CMA, E/ΔE of ∼55) [27]. The end of each analyzer was equipped with a unit of microchannel plates (F4655, Hamamatsu Photonics Co., Ltd.) for detection. After subtracting the background using the Tougaard method, all Si 2p1/2, 3/2 photoelectron spectra were fitted by the Voigt functions. The spectra with and without background are shown by red open triangles and open circles, respectively, in all XPS plots. In our peak fitting to the Si 2p1/2, 3/2 peaks, the Lorentzian width (LW), spin–orbit splitting, and the intensity ratio of Si 2p1/2 to Si 2p3/2 were fixed to the typical values of 0.08 eV, 0.6 eV, and 1/2, respectively [24–27]. Although it is well known that the intensity ratio of Si 2p1/2 to 2p3/2 does not take a strict value of 1/2, we cannot determine this value correctly because of the relatively low E/ΔE of our ASMA. The Gaussian widths (GWs) are given in each figure. All chemical shift values were referred to some previous papers. Our references will be given at result parts. All spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/2, 3/2 peak area are summarized in Table 1 in next section. In our paper, all Si 2p1/2, 3/2 photoelectron spectra are drawn on a relative binding energy (ReBE) scale, where the Si 2p3/2 peaks of bulk Si were considered as the zero of energy. Briefly, we describe how to measure the Si-L23VV-Sin+-2p APECS spectrum with our coincidence system [24]. The DP-CMA was tuned to a specific Si 2p photoelectron kinetic energy (PeKE), while the ASMA was swept through the Si L23VV Auger electron kinetic energy (AeKE) range. A multichannel scalar was triggered by the Si 2p photoelectron signals, and Si L23VV Auger electron signals via a 100-ns delay line were recorded as a function of the time-of-flight (TOF) differences between

polycrystalline Hf monosilicide (HfSi) phase after annealing at around 823–1073 K, and phase transition to Hf disilicide (HfSi2) occurred after annealing at around 1023–1073 K [7,9]. Therefore, it is important to reveal the chemical stabilities and the thermodynamics of Hf deposition on basic low-index Si substrates because these factors are key to controlling the dielectric constant. In addition, Si(110) is a significant facet, which is used as a nextgeneration semiconductor substrate in the preparation of multi-MOSFETs because it shows specific superior properties of a long-term 16 × 2 single domain (SD) with an atomic-scale concave–convex surface [21,22] and hole mobility that is nearly two times higher than those of the other facets [23]. Recently, Sakamoto et al. proposed an adatombuckling (AB) model for the clean Si(110)-16 × 2 SD surface structure. In their report, the Si 2p1/2, 3/2 photoelectron spectrum of the clean Si (110)-16 × 2 SD surface showed one bulk component and five surface components, which were assigned to SC1, SC2, SC3, SC4, and SC5 in the increasing order of binding energy. Some band dispersion spectra obtained from high-resolution angle-resolved photoelectron spectroscopy (ARPES) also revealed the presence of four surface states in the bulk band gap, which were assigned to S1, S2, S3, and S4. In addition, scanning tunneling microscope (STM) images and local density of states (LDOS) mapping demonstrated the relationship between SC1–SC5 and S1–S4 [22]. Further details of the assignments of SC1–SC5 and S1–S4 can be found in Ref. [22]. Although both dielectric HfO2 and low-index Si(110) substrates have drawn significant attention as next-generation semiconductor materials, there have been no studies in which they were combined in a unit structure, i.e., HfO2/Si(110) structure. In our study, we deposited an ultrathin Hf film on a clean Si(110)-16 × 2 SD surface. We investigated the Si chemical states at the surface and the interface immediately after exposure to O2 molecules and annealing at low (923 K) and high (1073 K) temperatures using X-ray photoelectron spectroscopy (XPS). Our XPS results indicated that two different samples could be prepared. One was an ultrathin HfO2 film deposited on the Si(110) surface having a Sin+ (sub)oxide layer at the interface and the other contained HfSi2 islands on the Si(110) surface. In the other sample, most of the Si(110) regions among the HfSi2 islands were covered by Sin+ (sub)oxide layers. Here, the oxidation number “n” corresponds to the number of O atoms bonding to a Si atom. In addition, we performed Si-L23VV Auger electron Sin+-2p photoelectron coincidence spectroscopy (Si-L23VV-Sin+-2p APECS) to obtain the Sin+-(sub)oxide-component-selected Si L23VV Auger electron spectra (denoted Si-L23VV -Sin+2p APECS spectra in this paper) [24–26]. The Si-L23VV -Sin+-2p APECS spectra as a function of the two-hole binding energy (BE) in the valence band revealed the angular-integrated local valence electronic states of the Sin+ states of these samples because the Auger electron kinetic A energy (AeKE) is typically given by eKE L23 VV = BESi2p − BEval1 − BEval2 − ϕ − Ueff . Here, BESi 2p, BEval1, and BEval2 are the binding energies (BE) of the Si 2p electrons and valence electron 1 (or 2); ϕ is the work function of the analyzer; and Ueff is the effective hole–hole interaction energy. In addition, we discussed the difference between local valence electronic states in the two prepared samples from the viewpoints of the first and second neighboring atoms of the Sin+ components. 2. Material and methods An n-type Si(110) single crystal wafer with a resistivity of ≤ 0.03 Ω cm was cut along the [11¯2 ] or [1¯12 ] direction, which was used as the long axis. The size was prepared to be about 20 × 3 mm. This piece of Si(110) was attached to our sample manipulator, which heated the Si sample using the direct current method. The sample manipulator was installed in an ultrahigh-vacuum (UHV) chamber with a base pressure of ∼3 × 10−8 Pa. The clean Si(110)-16 × 2 surface was prepared with the following procedure. The Si(110) sample was uniformly flashed at 1523 K and then annealed at 1200 K for 3 s, 930 K for 30 s, and 920 K 10

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Table 1 The spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/2, 3/2 peak area. Hf/Si(110), HfO2/Sin+/Si(110), and HfO2/Sin+/Si(110) annealed at 923 K, and i-HfSi2/Sin+/Si(110) in the sample column indicate an ultrathin deposited Hf film on Si(110), sample after exposure Hf/Si(110) to O2 at 200 L, sample after annealing HfO2/Sin+/Si(110) at 923 K, and sample after annealing HfO2/Sin+/Si(110) at 1073 K. Sample

Hf/Si(110) HfO2/Sin+/Si(110) HfO2/Sin+/Si(110) annealed at 923 K i-HfSi2/Sin+/Si(110)

Each spectral weight to total Si 2p1/2,

3/2

area (%)

Si Bulk

α

β

Si1+

Si2+

Si3+

Si4+

40 33 35 35

17 11 10 11

9 11 12 15

12 14 7 14

9 13 8 11

7 11 18 8

6 7 10 6

the Si 2p photoelectrons and the Si L23VV Auger electrons. When a photoelectron and Auger electron pair emitted by the same event with a photon was detected, a coincidence count was recorded at around 100 ns in the TOF spectrum. The ratio of true coincidence events to accidental ones was 3 to 1. When each integrated coincidence count is plotted as a function of the AeKE, we obtain the Si-L23VV-Si-2p APECS spectrum. Moreover, we describe specific characters of Si-L23VV-Si-2p APECS. First, the escape depth of the APECS [EDAPECS] is given by 1 1 1 = λ cos θ + λ cos θ , where λPe is the inelastic mean free path EDAPECS Pe Pe Ae Ae for the photoelectrons (Pe); λ(Ae) is the inelastic mean free path of the Auger electrons (Ae); and θPe (Ae) is the emission angles for Pe and Ae [26,28]. In the case of the clean Si surface, we have estimated them as follows; λPe ≈ 5.6 Å at PeKE = 26 eV [29], λAe ≈ 5.0 Å at AeKE ≈ 90 eV [30], θPe = 37.5 ± 4°, and θAe = 59.5 ± 11.5° [24]. Therefore, the EDAPECS will be ∼1.3 Å. This value is smaller than the layer spaces of HfO2 (3.64–5.20 Å [2]), the clean Si(110)-16 × 2 surface (1.92 Å [21–23]), and deposited Hf layer (2.4 Å [2,10,12]). Thus, we can obtain very surface-sensitive Si L23VV Auger electron spectra (AES) with SiL23VV-Si-2p APECS. Second, Si-L23VV-Sin+-2p APECS also enables us to obtain information about the local valence electronic states at individual chemical states (such as Sin+, n = 1, 2, 3, and 4) because the AeKE associated with Si L23VV transition can provide information about the twohole BE at the valence band of a specific Sin+ site [28]. 3. Results and discussion Fig. 1(a) and (b) shows the Si 2p1/2, 3/2 and valence photoelectron spectra of a clean Si(110)-16 × 2 SD surface, respectively. The Si 2p1/2, 3/2 peaks in Fig. 1(a) are decomposed into the five surface components (SC1–SC5) in the AB model and a bulk component [22]. The GWs of all components were adjusted to 0.35 eV. The surface core level shifts (SCLSs) of SC1, SC2, SC3, SC4, and SC5 were found to be − 0.8, − 0.29, + 0.25, + 0.47, and + 0.71 eV, respectively, in our fitting based on previous results [22]. Our fitting result can well reproduce the experimental Si 2p1/2, 3/2 photoelectron spectrum. The valence photoelectron spectrum in Fig. 1(b) is drawn against the binding energy, which was obtained from previous results [31]. There are no peaks arising from contamination, such as carbon, nitrogen, oxygen, and hafnium, on the clean Si(110)-16 × 2 SD surface. Fig. 2(a) and 2(b) shows the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra, respectively, including the valence region obtained from the bare Hf/Si(110) surface. The measured Si 2p1/2, 3/2 peaks in Fig. 2(a) can be reproduced by using several Si components. Four Si1+, Si 2+, Si 3+, and Si4+ oxide/ suboxide species at + 0.94 eV (GW: 0.69), + 1.79 eV (0.79), + 2.55 eV (0.85), and + 3.44 eV (0.96) on the ReBE scale are required to reproduce the spectrum on the higher BE side [29]. The spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/2, 3/2 peak area are summarized in Table 1. The existence of unexpected Sin+ oxide species indicates that Hf deposition drastically promotes the reaction of the Si substrate with the residual oxygen species, such as H2O, in UHV

Fig. 1. (a) Si 2p1/2, 3/2 and (b) valence photoelectron spectra obtained from the clean Si(110)-16 × 2 SD surface.

chamber below ∼1.2 × 10˗7 Pa. Previous studies have also reported that Si surfaces covered with metals, such as titanium and copper, show enhanced Si oxidation (to the Si4+ state) at room temperature [32,33]. The deposited Hf film also acts as a catalyst for the oxidation of the Si surface [33]. However, in our fitting, it is difficult to clearly evaluate the appearance of Hf silicate species. Two Si components marked α at − 0.42 (GW: 0.62) and β at + 0.29 eV (0.51) in Fig. 2(a) are needed on both sides of the bulk peak [11,13,14]. The α component has been reported to be mainly HfSi2 [13,14]. However, we consider the possibility that the α component contains a hafnium monosilicide (HfSi) component based on the Pauling electronegativities of Hf (1.3) and Si (1.9). We predict that it is difficult to discriminate the chemical shift difference between HfSi and HfSi2 because the Pauling electronegativity difference between Hf (1.3) and Si (1.9) is smaller than between Si (1.9) and O (3.44). The α component probably contains no HfSi2 because previous studies reported that HfSi2 is formed after annealing at high temperatures (1023–1073 K) [7,9]. The β component was predicted to be either a metallic Hf silicide component [13] or a Hf silicate [14]. 11

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Fig. 2. (a) Si 2p1/2, 3/2 and (b) Hf 4f5/2, 7/2 and valence photoelectron spectra obtained from the sample surface after the exposure of the clean Si(110)-16 × 2 surface to Hf vapor. The thickness of the deposited Hf film was estimated to be 9.6 ± 0.5 Å. (c) Si 2p1/2, 3/2 and (d) Hf 4f5/2, 7/2 and valence photoelectron spectra obtained from a sample surface after exposure of a partially oxidized deposited Hf film on Si(110) to O2 molecules at 200 L [HfO2/Sin+/Si(110) structure]. The BE position of Hf 4f7/2 in metallic Hf is marked by arrows with dashed lines in (b) and (d) as a reference [6,9]. The arrow mark with a solid line in (c) shows the relative Si 2p BE of the trigger in the Si-L23VV-Sin+-2p APECS measurement (Fig. 6(b)).

However, we consider that the β component might be attributed to another Si interface/surface component, such as that observed in SiO2/ Si(001) or (111) structures [34,35], and we discuss this by comparison with the other results obtained from the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra described below. Fig. 2(b) shows the Hf 4f5/2, 7/2 and valence photoelectron spectra. The BE of the most intense Hf 4f7/2 photoelectron peak is 17.3 eV. The Hf 4f7/2 BE in metallic Hf bulk is reported to be 14.2 eV, as marked by an arrow with a dashed line in Fig. 2(b) as a reference [6,9]. The chemical shift of the most intense Hf 4f7/2 peak in comparison with the BE of the metallic Hf bulk is estimated to be 3.1 eV. This chemical shift value is slightly smaller than those (3.2–3.9 eV) obtained from HfO2 bulk on polycrystalline Hf and thick HfO2 films on Si substrates. Therefore, the chemical states of the deposited Hf film are mainly suboxide species of Hfn+ (n = 1, 2, and 3) rather than HfO2 (Hf4+) [6,9,12,15,17–20]. Here, the oxidation number “n” means the number of oxygen atoms bonding to a Hf atom. In the valence region, around 4–10 eV, there is an obvious structure caused by the O 2p level in the HfO2 valence band owing to the charge transfer from Hf 5d to O 2p [3–5]. To promote Sin+ and Hfn+ suboxide states, we exposed the bare Hf/ Si(110) surface to O2 molecules at 200 L. Even after O2 exposure at over 200 L, we hardly observed any changes in the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra. Fig. 2(c) and (d) show the Si 2p1/2, 3/2 and Hf 4 f5/2, 7/2 photoelectron spectra including the valence region of the oxidized Hf/Si(110) surface, respectively. The Si 2p1/2, 3/2 peaks in Fig. 2(c) were also deconvoluted into Sin+ (n = 1, 2, 3, 4), α, β, and bulk components by changing the intensities estimated in Fig. 2(a). The spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/2, 3/2 peak area are summarized in Table 1. The area ratios of the Sin+ components (n = 1, 2, 3, and 4) to the total peak area naturally increased after exposure to O2 at 200 L. In contrast, the ratio of the α component area to the total peak area decreased from 17% to 11%, indicating that the α component, which might be caused by HfSi, had been partially oxidized. The Hf 4f5/2, 7/2 peaks clearly shift to the higher BE side from

17.3 eV in Fig. 2(b) to 17.9 eV in Fig. 2(d). A large chemical shift value of + 3.7 eV from the BE of the metallic Hf state agrees well with those of HfO2 bulk (3.2–3.9 eV), as reported previously [6,9,12,15,17–20]. In addition, the spin–orbit splitting between the Hf 4f5/2 and 4f7/2 peaks in Fig. 2(d) are clearer than that in Fig. 2(b), indicating that most of the Hfn+ suboxide species (n = 1, 2, and 3) have become HfO2. From these results, we determined that the Hf/Si(110) surface promptly reacts with O2 molecules at room temperature and changes from mainly Hfn+ suboxide components (n = 1, 2, and 3) to HfO2 (Hf4+). In addition, the intensity of the valence band arising from the O 2p orbital also becomes stronger than that in Fig. 2(b). Based on the results shown in Fig. 2(c) and (d), we characterize the oxidized Hf/Si(110) surface as a HfO2/Sin+/Si(110) structure (n = 1, 2, 3, and 4). In addition, the Sin+ components (n = 1, 2, 3, and 4) will be dominant between the ultrathin HfO2 film and Si(110) substrate because Si oxidation is promoted by the coexistence of the deposited Hf layer [32]. Next, we will discuss the chemical states after annealing the HfO2/ Sin+/Si(110) surface at lower (923 K) and higher (1073 K) temperatures in the UHV chamber. In previous reports regarding Hf/Si(100) and Hf/Si(111) structures, inhomogeneous HfSi was formed after annealing at 965 K, and only HfSi2 with islands appeared after annealing at 1073 K [7,9,11,14,15]. After annealing our sample at 923 K, we did not observe any LEED spots, which indicates that the surface maintained its amorphous/ polymorphic structure (not shown here) [7]. On the other hand, some changes in chemical state were clearly observed in the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra (Fig. 3(a) and (b)). Fig. 3(a) shows the Si 2p1/2, 3/2 photoelectron spectrum obtained from HfO2/Sin+/Si(110) after annealing at lower 923 K. The fitting curves are also shown. We could reproduce the spectrum by changing the intensities of the Si components estimated in Fig. 2(a) and (c). The spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/ 2, 3/2 peak area are summarized in Table 1. Our fitting results illustrate that the lower ionic valences (Si1+ and Si2+) are partially converted to higher ionic states (Si3+ and Si4+) by annealing at 923 K. Fig. 3(b) 12

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Fig. 3. (a) Si 2p1/2, 3/2 and (b) Hf 4f5/2, 7/2 including valence photoelectron spectra obtained from the sample surface after annealing oxidized deposited Hf film on Si (110) at 923 K. (c) Si 2p1/2, 3/2 and (d) Hf 4f5/2, 7/2 including valence photoelectron spectra obtained from the sample surface after annealing oxidized deposited Hf films on Si(110) at 1073 K. Fitting results of Si 2p1/2, 3/2 photoelectron spectra are also drawn in (a) and (c). The BE position of Hf 4f7/2 in the metallic state is also marked by arrows with dashed lines in (b) and (d) as a reference [6,9].

shows the Hf 4f5/2, 7/2 photoelectron spectrum, showing that the BE position of the most intense Hf 4f peak shifts to the lower BE side from 17.9 to 17.3 eV. Fig. 3(a) and (b) shows that the oxygen atoms in the HfO2 component diffuse into Si(110) substrate without desorption and immediately react with Sin+ in lower oxidation states (n = 0, 1, 2, and 3) [36]. Moreover, it is seen that the spin–orbit splitting between Hf 4f5/2 and 4f7/2 is clearer than that in Fig. 2(b), similarly indicating that the oxygen atoms in higher oxidation state HfO2 easily travel toward the Si(110) substrate at 923 K. Next, we discuss the results obtained after annealing at a higher temperature (1073 K). The LEED pattern after annealing at 1073 K shows the blurred 16 × 2 structure with double domain and streak lines along the [001] direction on a high background (see Fig. 4). This LEED pattern indicates that the Hf atoms are assembled into HfSi2 islands (iHfSi2) on the Si(110) surface [9,10,13]. In the photoelectron spectra [Fig. 3(c) and (d)], we also confirmed the changes in the Si and Hf chemical states. Fig. 3(c) shows the Si 2p1/2, 3/2 photoelectron spectrum obtained from the sample surface after annealing at a high temperature (1073 K). The fitting results are also shown as solid lines. The Si 2p1/2, 3/2 peaks could be well reproduced by adjusting the intensities of the α, β, Si1+, and Si2+ components. There are no contributions from the higher ionic valences of Si3+ and Si4+. The absence of contributions from these states indicates that a large number of oxygen atoms bonded to Sin+ atoms (n = 1, 2, 3, and 4) are desorbed by annealing at 1073 K. In the case of the HfO2/Sin+/Si(110) surface, the desorption of O atoms starts at a lower temperature compared to other Si facets, such as HfOx/Si (100), Hf/SiO2/Si(100), and HfO2/Si(100) structures [10,15,19]. On the other hand, a previous study on SiO2/Si(001) structures reported that the SiO component starts to desorb on annealing at temperatures higher than 873 K [36]. Therefore, it is thought that the O atoms and SiO components desorb from the interface of the HfO2/Sin+/Si(110) structure. In addition, the α component mainly includes the HfSi2 component after annealing at 1073 K because the HfSi component is converted into HfSi2 at temperatures higher than 1023–1073 K. In addition, the intensity of the shoulder structure on the high BE side of the most intense Si 2p1/2, 3/2 peak becomes stronger than those in the other

Fig. 4. LEED pattern obtained from the sample surface after annealing sufficiently oxidized the deposited Hf film on Si(110) at 1073 K. The blurred 16 × 2 structure with double domain and streak lines along with [001] direction is shown on a high background.

spectra, and there were no outstanding changes in the Si 2p linewidth. Therefore, the intensity of the β component remarkably increased in our fitting result. This trend has also been observed in previous results concerning Hf/Si(111) after annealing between 973 and 1233 K [11]. Fig. 3(d) shows Hf 4f5/2, 7/2 and valence photoelectron spectrum. The Hf 4f5/2, 7/2 peaks show a clear spin–orbit doublet, narrow linewidth, and a significant shift to the lower BE side from 17.3 to 14.6 eV. The chemical shift of + 0.4 eV from the BE of metallic Hf bulk (14.2 eV) 13

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3, and 4). Moreover, the intensity of the shoulder structure at the high BE side of the Si 2p1/2, 3/2 peak became weaker than that in Fig. 3(c). As a result, the intensity of the β component between Si1+ and the Si bulk in Fig. 5(a) drastically decreased after exposure to O2 molecules. The spectral weights of each Si 2p1/2, 3/2 component area to the total Si 2p1/ 2, 3/2 peak area are summarized in Table 1. On the other hand, there is no change to the Hf 4f5/2, 7/2 peaks in Fig. 5(b). Therefore, the intensity of the α component, which originates from the HfSi2 component in the Si 2p1/2, 3/2 photoelectron spectra shows no change or shift. The results shown in Fig. 5(a) and (b) indicate that the β component in the Si 2p1/2, 3/2 photoelectron spectra is not related to Hf silicide components (mainly HfSi2). Thus, we consider that the β component is related to the Si interface or surface states among HfSi2 islands. Hereafter, we denoted this sample as i-HfSi2/Sin+/Si(110) (n = 0, 1, 2, 3, and 4). In the valence region over 5.0 eV on the BE scale, the intense oxidation features related to the O 2p level appear again. When comparing the valence structure in Fig. 5(b) with that in Fig. 2(d), two significant differences can be observed. One is that the width of the valence peak, which is centered around 7.1 eV in Fig. 5(b), is narrower than that in Fig. 2(d). This result indicates that valence structures derived from the HfO2 components in Fig. 2(d) are removed by oxygen desorption from the HfO2/Sin+/Si(110) structure (n = 1, 2, 3, and 4). The other is that the BE cutoff in Fig. 5(b) slightly shifts to lower energy in Fig. 2(d), indicating that the valence energy of the HfSi2 component, which appears after annealing at 1073 K, is distributed in the lower BE region compared to that in the HfO2/Sin+/Si(110) structure (n = 1, 2, 3, and 4) [5]. In this study, we prepared HfO2/Sin+/Si(110) and i-HfSi2/Sin+/Si (110) structures (n = 1, 2, 3, and 4). Comparing the two samples, we expected that the local valence electronic structures in the common Sin+ components should be distinct because of the differences between HfO2 and i-HfSi2. Therefore, we examined them by Sin+-site-selected L23VV AES with Si-L23VV-Sin+-2p APECS [24–28]. When we measured the Si-L23VV-Sin+-2p APECS spectra of both samples, we used the same relative BE for the trigger signals. The relative BE was + 3.16 eV, whose positions are marked by arrows with solid lines in Figs. 2(c) and 5(a). The Sin+ spectral weights for the trigger positions of + 3.16 eV are listed in Table 2 (n = 2, 3, and 4) as a reference. The spectral weights of the Si2+, Si3+, and Si4+ components are estimated to be 6%, 56–55%, and 38–39% at + 3.16 eV, respectively. The contributions from the other components, such as the α, β, Si1+, and bulk components, can be ignored because their intensities at + 3.16 eV are negligible in our fitting results. These spectral weights were carefully determined in consideration of the total energy resolution. In the Si 2p1/2, 3/2 photoelectron fitting, the chemical shifts of Si2+, Si3+, and Si4+ were, respectively, estimated to be + 1.79, + 2.55, and 3.44 eV, and the Gaussian widths of Si2+, Si3+, and Si4+ were estimated to be the same 0.81, 0.88, and 0.99 eV, respectively. Besides, these APECS spectra of the HfO2/Sin+/Si(110) and the i-HfSi2/Sin+/Si(110) structures are also shown Sin+ L23VV AES of the Sin+ suboxides which are distributing around surface and interface because of the extremely short EDAPECS. Fig. 6 shows the Si-L23VV-Sin+-2p APECS spectra (filled circles, n = 2, 3, and 4) obtained from the (a) i-HfSi2/Sin+/Si(110) and (b) HfO2/Sin+/Si(110) structures. These Si-L23VV-Sin+-2p APECS spectra were smoothed with the weighted adjacent average method. In our smoothing procedure, the data point number, n, was set to 4. These smoothed data are used as a guide for the scattering APECS data. The singles Si L23VV AES (conventional AES, noncoincidence), which was measured simultaneously with each Si-L23VV-Sin+-2p APECS spectrum (n = 2, 3, and 4), is shown by red solid lines in Fig. 6(a) and (b). All spectra in Fig. 6 are depicted on the relative AeKE scale, where the most intense peak of the singles Si L23VV AES is taken as the origin of the relative AeKE. The accumulation times per datum (1.0 eV) in Fig. 6(a) and (b) were 25 and 20 min, respectively. The most intense peaks of both Si-L23VV-Sin+-2p APECS spectra (n = 2, 3, and 4), which were

agrees well with previous values for the HfSi2 state, which were observed after annealing at 1073 K [7,9,11]. These results indicate that the O atoms bonded to Hf atoms are also desorbed from the HfO2/Sin+/ Si(110) structure, and the Hf in our samples converts to i-HfSi2. The reduction in the valence peak of ∼7.5 eV in BE due to O 2p also supports this interpretation; however, its intensity slightly remains in comparison with Fig. 1(b) because the Sin+ oxidation components could not be removed completely. In addition, the Hf 4f5/2, 7/2 intensities are drastically decreased after annealing at 1073 K. This result indicates that Hf atoms diffuse into the Si(110) substrate. The other possibility, Hf desorption from the surface, can be excluded because the melting point of Hf metal is 2506 K. A similar trend in the reduction in Hf 4f5/2, 7/2 intensities was observed in the case of ultrathin Hf/Si(111) film, which was interpreted as resulting from Hf diffusion into the Si (111) substrate [11]. The behavior observed after annealing the HfO2/Sin+/Si(110) structure at temperatures higher than 1073 K is more similar to that observed in HfO2/Si(111) than HfO2/Si(100) concerning the formation of the HfSi2 component and Hf diffusion into the bulk. Based on Fig. 3(c) and (d), we determined that HfO2/Sin+/Si(110) after annealing at 1073 K has the i-HfSi2/Si(110) structure. We have also estimated the reactivity of the i-HfSi2/Si(110) structure with O2 molecules at room temperature. The blurred 16 × 2 double domain (DD) LEED spots in Fig. 4 completely disappeared after exposure to O2 at 200 L (not shown). The changes in chemical state can only be clearly observed in the Si 2p1/2, 3/2 photoelectron spectrum. Fig. 5(a) and (b), respectively, shows the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra including the valence region obtained after exposing the i-HfSi2/Si(110) surface to O2 molecules at 200 L. As shown, the Sin+ components with higher ionic states appeared again (n = 1, 2,

Fig. 5. (a) Si 2p1/2, 3/2 and (b) Hf 4f5/2, 7/2 and valence photoelectron spectra obtained from the sample surface after the exposure of HfSi2 islands formed on Si(110) to O2 at 200 L [i-HfSi2/Sin+/Si(110) structure]. The arrow with a solid line in (a) shows the relative BE of the trigger signal for Si-L23VV-Sin+-2p APECS measurement [Fig. 6(a) (n = 2, 3, and 4)]. 14

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Table 2 Sin+ spectral weights at a relative BE of + 3.16 eV, which was used for the trigger signals in Si-L23VV-Sin+-2p APECS measurements obtained from the sample surface after exposure of the HfSi2 islands formed on Si(110) to O2 at 200 L [i-HfSi2/Sin+/Si(110) structure] (filled-circles with solid line), and the other sample surface after exposure of a partially oxidized deposited Hf film on Si(110) to O2 molecules at 200 L [HfO2/Sin+/Si(110) structure]. Samples

Spectral weights of Si2+ component (%)

Spectral weights of Si3+ component (%)

Spectral weights of Si4+ component (%)

HfO2/Sin+/Si(110) i-HfSi2/Sin+/Si(110)

6 6

56 55

38 39

L23VV AES calculation are tentative Ueff values [37]. The Ueffs-s, Ueffs-p, and Ueffp-p values have been reported to be 2.3, 2.3, and 0.4 eV, respectively, as fully screened hole–hole Coulomb repulsion [37]. Here, the superscript letters s and p in Ueffs-s, Ueffs-p, and Ueffp-p indicate the Si 3 s and Si 3p orbitals, respectively. Therefore, we would assume that the Ueff values of both samples are approximately 2.0 eV. In our previous SiL23VV-Si-2p APECS study of the clean Si(110)-16 × 2 SD surface, we also estimated the tentative Ueff to be 2.0 eV [26]. Furthermore, we could demonstrate that the specific spectral structures on high KE region which significantly observed in Si-L23VV-Si-2p APECS are originated from the Si L23VV Auger electrons which are emitted by Coulomb repulsions between the electrons at surface states S1-4 localized on specific Si surface components (SC1–SC5) on the clean Si(110)-16 × 2 surface [26]. The Si-L23VV-Sin+-2p APECS spectra clearly show different distributions in the half two-hole BE between i-HfSi2/Sin+/Si(110) and HfO2/Sin+/Si(110), although the Sin+ spectral weights at a relative BE of + 3.16 eV are almost the same (n = 2, 3, and 4, see Table 2). The SiL23VV-Sin+-2p APECS spectrum of the i-HfSi2/Sin+/Si(110) structure is obviously broader than that of the HfO2/Sin+/Si(110) structure (n = 2, 3, and 4). In particular, we paid attention to the cutoff position of the half two-hole BE (two-hole BEcutoff) because it is very important to discuss the band gap of high-k materials. To determine the half twohole BEcutoff in Fig. 7, we fitted the Si-L23VV-Si-2pn+ APECS tails with

estimated from the smoothed spectra, showed a 10-eV shift to the lower AeKE side than those of the singles Si L23VV AES. Each shifted spectrum intrinsically reflects the local valence electronic states of specific Sin+ components on the sample surface. In our previous studies, we also reported similar tendencies in ultrathin SiO2/Si(100) and (111) structures and concluded that the local valence electronic states at Sin+ shift to deeper BE side from Fermi level as the oxidation number, n, increases (n = 0, 1, 2, 3, and 4) [38,39]. Therefore, the results of this study demonstrate that the local valence electronic states at Sin+ in the i-HfSi2/ Sin+/Si(110) and the HfO2/Sin+/Si(110) structures show deeper BEshifts than those of the other Si components, such as Si bulk, HfSi, and HfSi2. As shown in Fig. 7, these Si-L23VV-Sin+-2p APECS spectra (n = 2, 3, and 4) are drawn on a half two-hole BE scale in valence. The half twohole BEs were estimated using Eq. (1):

Half two hole BE

1 × (BEval1 + BEval2) 2 1 = × (BESi2p − AeKE − ϕ − Ueff ) 2 =

(1)

The Si 2p3/2 core energy of bulk Si is 99.2 eV. The work function of our ASMA is regarded as 5.0 eV [26]. On the other hand, to the best of our knowledge the experimental Ueff values of the valence band of both the i-HfSi2/Sin+/Si(110) and the HfO2/Sin+/Si(110) structures have not been reported in the past. Thus, the Ueff values used in the atomic Si

Fig. 6. Si-L23VV-Sin+-2p APECS and singles Si L23VV AES spectra obtained from (a) the sample surface after exposure of the formed HfSi2 islands on Si(110) to O2 at 200 L [i-HfSi2/Sin+/Si(110) structure] and (b) the other sample surface after the exposure of a partially oxidized deposited Hf film on Si(110) to O2 molecules at 200 L [HfO2/Sin+/Si(110) structure]. The AeKE at the intense peak in the singles Si L23VV Auger electron spectrum was 87 eV in both samples.

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Fig. 7. Si-L23VV-Sin+-2p APECS spectra obtained from the sample surface after exposure of formed HfSi2 islands on Si(110) to O2 at 200 L [i-HfSi2/ Sin+/Si(110) structure] (filled-circles with solid line) and the other sample surface after the exposure of the partially oxidized deposited Hf film on Si (110) to O2 molecules at 200 L [HfO2/Sin+/Si(110) structure] (open-circles with solid line), drawn as a function of the two-hole BE scale. The two-hole BEcutoff of i-HfSi2/Sin+/Si(110) and HfO2/Sin+/Si(110) structures are estimated to be 5.1 ± 1.2 and 8.2 ± 0.7 eV, respectively.

Sin+ connected through oxygen. In the i-HfSi2/Sin+/Si(110) structure, only the Sim+–O–Sin+ bonding unit (m = 1, 2, 3, and 4), in which the oxidation number “m” mainly consists of lower ionic valence states from Fig. 5(a), will exist because the Hf 4f5/2, 7/2 peaks in Fig. 5(b) do not show the properties of the HfO2 and suboxides. Therefore, the SiL23VV-Sin+-2p APECS spectrum of the i-HfSi2/Sin+/Si(110) structure reflects the local valence electronic states of Sin+ in the Sim+–O–Sin+ bonding unit (m = 1, 2, 3, and 4; n = 2, 3, and 4). On the other hand, in the HfO2/Sin+/Si(110) structure, the Hf4+–O–Sin+ and the Sim+–O–Sin+ bonding units will co-exist because the Hf 4f5/2, 7/2 peaks in Fig. 2(d) clearly show the existence of the HfO2 component. Thus, the Si-L23VV-Sin+-2p APECS spectrum of the HfO2/Sin+/Si(110) structure will reflect the local valence electronic states of Sin+ in both Hf4+–O–Sin+ and Sim+–O–Sin+ bonding units (m = 1, 2, 3, and 4; n = 2, 3, and 4). Moreover, some previous studies have reported that Si 2p1/2, 3/2 peaks deriving from hafnium silicate are located by BE of 0.7 eV lower than those of the Si4+ 2p1/2, 3/2 peaks [17,20]. Therefore, there is a possibility that the spectrum reflects the local valence electronic states of Sin+ in Hf4+–O–Sin+ bonds in the HfO2/Sin+/Si(110) structure. In our previous APECS studies, we found that the BEVBM values at Si4+ sites decrease by about 1–2 eV as the SiO2 thickness decreases from several SiO2 layers to submonolayer [38,39]. In addition, we concluded that the reduction in BE is attributed to the effect on the local valence electronic states of the second neighbor Sim+ atoms of Sin+ through oxygen atoms (m = 1, 2, and 3). The occupation of Si1+, Si2+, and Si 3+ instead of Si4+ at the second neighbor sites increases as the SiO2 thickness decreases. Other theoretical studies have also reported that the interfacial SiO2 distributed within 5 Å from Si(100) substrate has a narrower band gap than that of the SiO2 bulk, and the Si2+ suboxide is dominant as the second neighbor of Si4+ at the SiO2/Si interface [40]. Thus, the high oxidation state of Hf4+ in the Hf4+–O–Sin+ bonding unit (n = 2, 3, and 4) is favorable, as well as in the case of the Si4+–O–Si4+ bonding unit.

20%–80% intensity in the lower half of the two-hole BE region by the weighted least-squares linear method. In addition, we estimated the half two-hole BEcutoff from the intersection of the x-axis and the fitted linear line (see Fig. 7 and caption). The half two-hole BEcutoff will give us the BE at the valence band maximum (BEVBM). Thus, the BEVBM of the i-HfSi2/Sin+/Si(110) and the HfO2/Sin+/Si(110) structures were estimated to be 2.6 ± 0.6 and 4.1 ± 0.4 eV, respectively. Therefore, the BEVBM of Sin+ in the i-HfSi2/Sin+/Si(110) structure is 1.5 ± 0.7 eV lower than that in the HfO2/Sin+/Si(110) structure (n = 2, 3, and 4). Concerning the BEVBM difference, some extrinsic causes, such as a charge-up, work function, and experimental set-up, can be eliminate because the most intense peaks in the singles Si L23VV AES have the same KE. Therefore, the BEVBM difference could be attributed to the nearest neighbor atoms bonding to Sin+ components and the second neighbor atoms of Sin+ through oxygen atoms (n = 2, 3, and 4). First, we will discuss the influence of the nearest neighbor atoms without oxygen directly bonded to the Sin+ components. In the i-HfSi2/ Sin+/Si(110) structure, there is the possibility that some Hf atoms, in addition to the excess Si atoms, bond to Sin+ as nearest neighbor atoms. Therefore, the Si-L23VV-Sin+-2p APECS spectrum of the i-HfSi2/Sin+/Si (110) structure reflects the local valence electronic states of Sin+ in Hf–Sin+ bonding, as well as that of Si–Sin+ (n = 2, 3, and 4). In contrast, in the HfO2/Sin+/Si(110) structure, Hf atoms do not bond to Sin+ as shown by the fact that the Hf 4f5/2, 7/2 peaks arising from the HfSi2 components are negligible. In addition, most of the Hf atoms are fully oxidized. Therefore, the Si-L23VV-Sin+-2p APECS spectrum of the HfO2/ Sin+/Si(110) structure reflects the local valence electronic states of Sin+ in Si–Sin+ bonds alone. Thus, the broad bands and the lower BEVBM in the spectrum of the i-HfSi2/Sin+/Si(110) structure than those of the HfO2/Sin+/Si(110) structure (n = 2, 3, and 4) could be attributed to the properties of the local valence electronic states of Hf–Sin+bonds. Because of the difference in Pauling electronegativity of Hf (1.3) and Si (1.9), Sin+ in Hf–Sin+ bonds will be more electron-rich than Sin+ in Si–Sin+ bonds. In addition, dangling bonds may be formed around Hf–Sin+ bonds at the interface. Puthenkovilakam and Chang carried out a theoretical study and found that dangling bonds at the Hf/Si interface generate defect states near the Fermi level because of incomplete charge transfer [5]. If there are some Hf dangling bonds around Hf–Sin+ bonds in our iHfSi2/Sin+/Si(110) structure, the distribution of the Si-L23VV-Sin+-2p APECS spectrum on the two-hole BEcutoff will shift to the lower BE side. However, there should not be any dangling bonds in our HfO2/Sin+/Si (110) structure because Hf atoms take the fully oxidized state (Hf4+), as shown in Fig. 2(c). Secondly, we discuss the effects of the second neighbor atoms of

4. Conclusions We prepared ultrathin deposited Hf films on clean Si(110)-16 × 2 SD surfaces by electron bombardment heating. The chemical states and stabilities of the deposited Hf films and Si(110) substrate were investigated using Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 photoelectron spectra. The deposited Hf film was immediately oxidized because of its high reactivity with oxygen species, and the rapid diffusion of oxygen into the bulk also oxidized the Si(110) substrate to the Si4+ state. The Hf deposit served as an effective catalyst for Si oxidation. Therefore, HfO2 on Si (110) substrate was easily formed on exposure to even a small amount 16

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of O2 at room temperature. Upon annealing at 923 K, the oxidation states of the Si(110) substrate progressed from lower oxidation states of Si1+ and Si2+ to higher oxidation states of Si3+ and Si4+ via oxygen diffusion from HfO2 layers. After annealing at 1073 K, on the other hand, the oxygen atoms bonding to both Si and Hf atoms desorbed and HfSi2 islands were formed on the partly exposed Si(110)-16 × 2 DD surface. The HfSi2 islands showed low reactivity with O2, whereas the Si(110)-16 × 2 DD surface immediately reacted with it. Moreover, we investigated the local valence electronic state of Sin+ in the i-HfSi2/Sin+/Si(110) and HfO2/Sin+/Si(110) structures (n = 2, 3, and 4) using Si-L23VV-Sin+-2p APECS. The BEVBM of Sin+ in the i-HfSi2/ Sin+/Si(110) structure (n = 2, 3, and 4) shifted by 1.5 ± 0.7 eV to the lower BE side than that of the HfO2/Sin+/Si(110) structure (n = 2, 3, and 4). To explain this, we examined the differences in the nearest neighbor atoms bonding to Sin+ and the second neighbor atoms of Sin+ through oxygen atoms (n = 2, 3, and 4). Briefly, the excessive presence of Hf–Sin+ bonds and Sim+–O–Sin+ bonding units (m < 4) in the iHfSi2/Sin+/Si(110) structure compared to those in the HfO2/Sin+/Si (110) structure shifted the BE of the local valence electronic states of the Sin+ sites to lower energies. Our results indicated that the existence of Hf silicide components and Sin+ suboxides (n = 1, 2, and 3) at the interface narrows the band gap of the HfO2/Si(110) structure.

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Acknowledgments We express our sincere gratitude to the staff of the Photon Factory for their support. This work was supported by Grants-in-Aid for Scientific Research (Nos. 26870416 and 23760035) from the Ministry of Education, Culture, Sports, Science and Technology–Japan, the Grant Program of the Sumitomo Foundation, and the JGC-S Scholarship Foundation (Saneyoshi Scholarship Foundation). This work was performed with the approval of the Photon Factory Program Advisory Committee (PF-PAC Nos. 2013G019 and 2015G011). References [1] A.I. Kingon, J.-P. Maria, S.K. Streiffer, Alternative dielectrics to silicon dioxide for memory and logic devices, Nature 406 (2000) 1032–1038. [2] X. Zhao, D. Vanderbilt, First-principles study of structural, vibrational, and lattice dielectric properties of hafnium oxide, Phys. Rev. B 65 (2002) 233106-1–233106-4. [3] S. Toyoda, J. Okabayashi, H. Kumigashira, M. Oshima, K. Ono, M. Niwa, K. Usuda, N. Hirashita, Chemistry and band offsets of HfO2 thin films on Si revealed by photoelectron spectroscopy and X-ray absorption spectroscopy, J. Electron Spectrosc. Relat. Phenom 137–140 (2004) 141–144. [4] E. Bersch, S. Rangan, R.A. Bartynski, E. Garfunkel, E. Vescovo, Band offsets of ultrathin high-k oxide films with Si, Phys. Rev. B 78 (2008) 085114-1–085114-10. [5] R. Puthenkovilakan, J.P. Chang, An accurate determination of barrier heights at the HfO2/Si interfaces, J. Appl. Phys. 96 (2004) 2701–2707. [6] S. Suzer, S. Sayan, M.M. Banaszak Holl, E. Garfunkel, Z. Hussain, N.M. Hamdan, Soft X-ray photoemission studies of Hf oxidation, J. Vac. Sci. Technol. A 21 (2003) 106–109. [7] A. de Siervo, C.R. Flüchter, D. Weier, M. Schürmann, S. Dreiner, C. Westphal, M.F. Carazzolle, A. Pancotti, R. Landers, G.G. Kleiman, Hafnium silicide formation on Si(100) upon annealing, Phys. Rev. B 74 (2006) 075319-1–075319-10. [8] Y. Hoshino, Y. Kido, K. Yamamoto, S. Hayashi, M. Niwa, Characterization and control of the HfO2/Si(001) interfaces, Appl. Phys. Lett. 81 (2002) 2650–2652. [9] H.T. Johnson-Steigelman, A.V. Brinck, S.S. Parihar, P.F. Lyman, Hafnium silicide formation on Si(001), Phys. Rev. B 69 (2004) 235322-1–235322-6. [10] J.-H. Lee, Ternary phase analysis of interfacial silicates grown in HfOx/Si and Hf/ SiO2/Si systems, Thin Solid Films 472 (2005) 317–322. [11] M.F. Carazzille, M. Schürmann, C.R. Flüchter, D. Weier, U. Berges, A. de Siervo, R. Landers, G.G. Kleiman, C. Westphal, Structural and electronic analysis of Hf on Si (111) surface studied by XPS, LEED and XPD, J. Electron Spectrosc. Relat. Phenom. 156–158 (2007) 393–397. [12] J.-H. Lee, N. Miyata, M. Kundu, M. Ichikawa, Oxidation of hafnium on Si(001): silicate formation by Si migration, Phys. Rev. B 66 (2002) 233309-1–233309-4. [13] S. Toyoda, J. Okabayashi, H. Kumigashira, M. Oshima, K. Ono, M. Niwa, K. Usuda, G.L. Liu, Chemical analysis of Hf-silicide clusters studied by photoemission spectroscopy, J. Electron Spectrosc. Relat. Phenom 144–147 (2005) 487–490. [14] M.H. Cho, Y.S. Roh, C.N. Whang, K. Jeong, S.W. Nahm, D.-H. Ko, J.H. Lee, N.I. Lee, K. Fujihara, Thermal stability and structural characteristics of HfO2 films on Si(100)

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