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Photoelectron spectroscopy as an in situ contact-less method for studies of MOS properties of ultrathin oxides on Si
Applied Surface Science 353 (2015) 1208–1213
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoelectron spectroscopy as an in situ contact-less method for studies of MOS properties of ultrathin oxides on Si Ana G. Silva a,∗ , Kjeld Pedersen b , Zheshen S. Li c , Per Morgen d a
CeFiTec, Department of Physics, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, DK-9220 Aalborg East, Denmark Institute for Storage Ring Facilities (ISA), Faculty of Science, Aarhus University, Ny Munkegade 120, Building 1520, DK-8000 Aarhus C, Denmark d Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark b c
a r t i c l e
i n f o
Article history: Received 31 May 2015 Received in revised form 29 June 2015 Accepted 4 July 2015 Available online 14 July 2015 Keywords: Electric field Ultrathin oxides Synchrotron Photoelectron spectroscopy Surface nanotechnology
a b s t r a c t The electric field across an ultrathin and uniform Si-oxide layer on a Si (1 1 1) surface and the positions of the valence band edges at the Si-oxide/Si (1 1 1) interface have been probed by high-resolution synchrotron radiation induced photoemission spectroscopy, as an in situ contact-less method. Variation of the “gate bias” is achieved by depositing Sn nanoparticles on the ultrathin oxide surface. These nanoparticles, growing as isolated hemi-spherical islands, attract various quantities of negative charges from the substrate inducing a potential difference between the Sn islands/Si-oxide and Si-oxide/Si (1 1 1) interface. This method allows us to study and extract the locally varying electric field and changes in the positions of the edges of the valence bands by measuring the valence band spectra and the Si 2p and Sn 4d core-levels at different Sn coverages. The ultrathin (0.8 nm thick) Si-oxide layer is grown in a simple and traceable self-limiting thermal process on a clean Si (1 1 1) surface. The oxide grown in this way creates flat bands. The properties of the system of Sn islands grown on this system are also determined. The induced electric field in the oxide varies linearly with the amount of Sn deposited per area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In modern Si-based microelectronic device processing using high-k dielectrics as gate oxides [1], an ultrathin oxide of Si may still often be formed at the interface with the underlying Si, contributing to the total gate voltage drop across the metaloxide-semiconductor (MOS) system. It is therefore important to experimentally establish a technique for evaluating the changes in this part of the total energy band profile around the oxide/Si interface with applied gate voltage. In this work, we achieve this by evaporating Sn onto an ultrathin Si-oxide layer kept at 500 ◦ C, which leads to the growth of variably sized nanoparticles in the form of hemi-spherical islands, not touching each other, and their size depending on the total quantity of Sn arriving at the oxide surface. No sign of a chemical reaction between the Si-oxide layer and the Sn islands was observed. Metallic islands of different sizes attract different amounts of negative charge from the Si surface, tunnelling through the oxide, creating electric field lines in the space between the islands and perpendicular to the Si-oxide
∗ Corresponding author. E-mail address: [email protected] (A.G. Silva). http://dx.doi.org/10.1016/j.apsusc.2015.07.024 0169-4332/© 2015 Elsevier B.V. All rights reserved.
surface. With the use of synchrotron radiation and a sufficiently highly resolving energy spectrometer, the changes in the valence band structure and edges, and intensities and peak positions of core levels, can be measured with the proper resolution and accuracy of intensities for reliable data reduction methods. The core level energies are converted to information about the band bending and other chemical effects while the valence band spectra are examined for structure and positions of the edges. The total variation of the equivalent gate bias, with the present range of Sn coverages below a continuously covering layer, is only active for depletion conditions. The measurements were carried out with highly surface-sensitive experimental conditions, but still allowing us to study in detail the upper surfaces, the distribution of chemically shifted species inside the oxide, and the position of the Si- and Sn atomic level energies in the surface region. Since the inelastic mean free path of low-energy electrons (e.g. 30 eV) in Sn is 0.5 nm even just 2 nm high islands will reduce the signal from the underlying Si to 2%. Photoemission using such low kinetic energies will therefore only record electrons emitted from the top surface of the Sn islands and from the uncovered areas in the space between the islands of the substrate (Si-oxide). In this way, the measurements of the positions of the core lines will refer to the electronic core levels nearest to the surface.
A.G. Silva et al. / Applied Surface Science 353 (2015) 1208–1213
The results obtained for the intensities indicate a systematic variation of the island size with Sn exposure. Regarding the shifts in the peaks positions of the energy levels, we see initially that the growth of the self-limiting oxide on the Si (1 1 1) 7 × 7 surface produces flat bands for this surface, thus unpinning the Fermi level at the surface, giving an interface without charged defects, at the level of accuracy measurable here. Among the thermally processed oxides, although the Si (1 0 0) surface is the most used for industrial purposes, due to its apparently smaller amount of defects at the interface, the Si (1 1 1) 7 × 7 surface is used here, as it offers to the spectroscopist a larger number of parameters to evaluate. In the present work, the magnitude of the normally directed field across the oxide increases linearly with the coverage of Sn up to or above 108 V/m at the highest Sn coverage, corresponding to a maximum value of (0.40 ± 0.01) eV in the band bending of the Si space charge region. This is seen as approaching the value of 0.41 eV of the Si (1 1 1) 7 × 7 surface pinned by the partly occupied surface states before reactions. The variation of the Sn core level positions indicate that the Sn islands after having attained a certain size are in equilibrium with respect to the Fermi level, which means that the affinity of the islands towards extracting electrons from Si becomes constant. 2. Experimental details and results The Si sample, cut from a polished wafer and cleaned with ethanol was inserted in the ultra-high vacuum chamber before pumping down and baking of the system. After the bake-out cycle and reaching a working pressure of around 10−8 Pa, the sample was cleaned by Ohmic heating for several hours at 600 ◦ C and finally shortly to above 1000 ◦ C. Low Energy Electron Diffraction (LEED) and Si 2p photoemission spectra confirmed the perfection of crystalline structure and cleanliness of the resulting Si (1 1 1) 7 × 7 surface structure (Fig. 1(c)). The oxide/Si system was now prepared in situ using a self-limiting procedure to grow a 0.8 nm thick, uniform Si-oxide layer in a direct reaction of a clean, heated Si (1 1 1) surface with 99.998% pure molecular oxygen at 2 × 10−4 Pa for 30 min, at a temperature of 650 ◦ C. The uniformity of the thickness of the Si-oxide layer was confirmed by Si 2p photoemission
Fig. 1. (a) Si 2p spectra from positions across the Sn covered surface recorded with 130 eV photons; (b) corresponding Sn 4d spectra recorded with 63 eV photons. The fitted spectra are shown. The red lines (top for Si 2p and bottom for Sn 4d spectra) are from the low Sn coverage locations, and the blue lines (bottom for Si 2p and top for Sn 4d spectra) are from the high Sn coverage locations. (c) Si 2p spectra from clean (black line) and from oxidized (red line) surface. (d) Examples of experimental and fitted Sn 4d and Si 2p spectra. The Si 2p spectra show the presence of bulk, intermediately and fully oxidized states, while the Sn 4d spectra show no sign of oxidation (no “chemical shifts”). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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measurements across the surface (Fig. 1(c)) and by TEM in separate studies [2]. While keeping the Si surface with oxide, at 500 ◦ C, high-purity (99.99%) Sn was evaporated from an e-beam heated refractory metal crucible onto the surface with the evaporation flux determined by a quartz-crystal microbalance. The sample temperature was measured from outside the vacuum chamber with a spot photometer calibrated for Si and transmission of the window. The deposition of Sn was done in a programmed way by steadily moving the sample into the shadow of a mask while maintaining the evaporated flux constant to produce a systematic and graded variation of the Sn thickness across the surface-oxide layer exposed. The resulting layer of variable and linearly graded amount of Sn deposit is ca. 1.5 cm wide between the “thinnest” and “thickest” edges. The Sn source is equipped with a biased, neutralizing grid to prevent the impact of ionized Sn particles on the sample. The high-resolution surface-sensitive core-level and valenceband photoemission experiments were performed with synchrotron radiation from the ASTRID storage ring at the University of Aarhus (Denmark) on the SGM1 beam line equipped with a spherical grating monochromator and a SCIENTA hemispherical spectrometer with a central electron trajectory radius of 20 cm. The Si 2p and Sn 4d spectra were recorded with photon energies of 130 eV and 63 eV, respectively, at normal emission of the emitted electrons. The synchrotron beam width on the sample is 150 m in the direction of the Sn coverage gradient allowing locally resolved photoemission characterization of the system. In the Si 2p and Sn 4d spectra the energy step in the recorded spectra is set at 20 meV, corresponding to the actual experimental resolution. The valence band spectra were recorded at photon energy of 130 eV, at a lower spectrometer resolution, and with the energy step set at 50 meV, correspondingly. The Si 2p (Fig. 1(a)) and Sn 4d (Fig. 1(b)) spectra of the variably sized Sn islands deposited across the surface, with linearly varying exposure to the Sn source, were recorded in 19 local Si 2p spectra and 28 local Sn 4d spectra with the shortest possible time delays. The current method of preparing the sample with a systematically varied exposure across the surface offers the best method of systematic and rapid spectroscopic studies of surface phenomena under vacuum conditions, as only one preparation cycle is needed. In the spectrum of clean Si (1 1 1) 7 × 7 (Fig. 1(c)) one can distinguish the bulk core level Si0 and two surface core level components for the atoms at the reconstructed surface: S1 due to adatoms and S2 due to rest-atoms [3], shifted in binding energy by +0.27 eV and −0.72 eV respectively relative the bulk Si0 energy. After the Si oxidation (Fig. 1(c)) the intensity of the surface component S1 is reduced significantly and the signal from the rest-atoms (S2 ) has disappeared, while four discrete surface core levels assigned to Si atoms at the surface bonding to one (Si1+ ), two (Si2+ ), three (Si3+ ) and four (Si4+ ) oxygen atoms are now observed [4]. The Si4+ is the bonding configuration of the fully oxidized Si, i.e. SiO2 , while the other three are designated as intermediate oxidation states, originating in the interface [4]. The fully oxidized state Si4+ has its fitted maximum (Si 2p3/2 ) shifted, in binding energy, by +3.65 eV relative to Si 2p3/2 of bulk Si0 . For a more detailed evaluation of the spectra, the original Si 2p and Sn 4d raw spectra are first normalized to the experimental conditions (photon flux, etc.) and afterwards deconvoluted using the program FitXPS (written by D. Adams [5]). Using this program, the Si 2p and Sn 4d spectra are decomposed into peaks of folded Lorentzian/Gaussian line shapes (Fig. 1d). The model used for the Si 2p spectra includes five spin–orbit split doublets; one for pure silicon Si0 , three doublets for the intermediately oxidized states and one for the fully oxidized silicon Si4+ . The value of the spin–orbit splitting is 0.61 eV [6], and the 2p1/2 to 2p3/2 intensity ratio is 1:2. The model used for the Sn 4d spectra includes a single spin–orbit doublet for the bulk Sn 4d of pure Sn0 (splitting: 1.10 eV, Fig. 1(d)).
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Fig. 2. (a) and (b) SEM images of a system of Sn islands on Si (1 1 1) 7 × 7 surface with the 0.8 nm oxide for (a) 5 ML and (b) 10 ML equivalent thickness of Sn. The scale bar is 100 nm. (c) Relative signal intensities of Si 2p and Sn 4d versus Sn relative thickness, normalized according to the SEM image (b). The upper graph (III-stars) is the sum of the two data (I-squares, II-diamonds) fitted with a Hill function (III-continuous curve); the Sn 4d (I) is fitted also with Hill function and Si 2p (II) fitted with the model describing the Si 2p intensity decay. The limiting value of the summed curves is around 1.0. (d) Representation of the electric field strength across the oxide and its behaviour, which is well fitted as a linear function on equivalent ML thickness. (e) And (f) shifts of the peak energies of Si0 , Si4+ , and Sn0 , respectively, across the Sn graded exposure. (g) Scheme of the Sn islands growth as the equivalent thickness increases. (h) Scheme identifying the regions of positive and negative charges as well as the origin of the contributions for Si0 , Si4+ and Sn0 , at a specific coverage.
Unconstrained values of the LFWHM and GFWHM were used in optimizing the fits of the Si 2p and Sn. For the Si 2p peaks the mean value of the Lorentz full widths was 0.22 eV while the Gaussian full width increased systematically from 0.32 eV (Si0 ) to 1.20 eV (Si4+ ). For Sn 4d all spectra were fitted with a LFWHM of 0.22 eV and a GFWHM of 0.30 eV, and a value of 0.099 eV for alpha (the Doniach–Sunjic metallic loss skewness factor [7]), was used in all the fits. The resulting fitted functions are shown (Fig. 1(a) and (b)) for the normalized Si 2p and Sn 4d spectra. To facilitate the interpretation of the dependence of Si 2p and Sn 4d spectra on Sn coverage, SEM images of an identical system were obtained, after Sn exposures of 5 ML (Fig. 2(a)) and 10 ML (Fig. 2(b)). The system was prepared following identical procedures but with handling of the sample in air after deposition for taking SEM images. At exposure of 10 ML an average island size of 50 nm is observed (Fig. 2(b)), with a relatively narrow spread of diameters. Relative intensities of Si 2p and Sn 4d versus relative Sn equivalent thickness, normalized according to the SEM image (Fig. 2(b)), are shown in Fig. 2(c). The maximum intensity for Si 2p corresponds to the Si 2p of the Si clean substrate plus the Si-oxide layer. The Sn 4d normalization is done using the maximum of the Sn 4d intensity at the highest Sn coverage. The results indicate that the Sn 4d intensity is well fitted with a sigmoid-like function, shown as a continuous curve in Fig. 2(c)(I), similar to the functional dependence used earlier to describe the growth of 2D oxide islands on a substrate [8]. This leads us to propose a model for the Sn deposition kinetics and island growth, which could explain the observed trends of the Si 2p and Sn 4d intensity variations. The model is relatively simple and is based on the formation of variable-sized 3D islands of Sn, not touching each other, and with a final fractional (2D) surface coverage, f, of 60%, based on the SEM image (Fig. 2(b)). For 2p photoelectrons of Si0 at 31 eV kinetic energy the inelastic mean free path (IMFP) in Sn is Si = 1.38 ML and for Sn0 at Sn = 1.5 ML (calculated using the code TPP2 M [9]). The model should be able to account for the variation of intensities relative to the maximum intensity for Si 2p and for Sn 4d core levels, as in a 2D growth process, since the emission is normal to the surface. The way these ratios vary is initially dependent on the
size and form of the deposits, and, later, only on the total fractional coverage of Si-oxide by Sn islands larger and thicker than about 3 nm. The Sn signal is, in this latter regime, simply proportional to the area covered, i.e. to f. At thicknesses below 3 nm, this signal should increase approximately linearly with exposure. With these two mechanisms combined, the Sn 4d signal should increase with deposited amount of Sn as a sigmoid function [8]. The Si 2p signal intensity is also determined by two contributions: one contribution due to the electrons that reach the energy analyser without attenuation by passing between the Sn islands and one that would represent electrons being attenuated by passing through the Sn islands. When the island height is larger than 3 nm, all Si 2p photoelectrons are totally absorbed, for normal exit. The first measured coverage of Sn has reduced the Si 2p signal with 17%, corresponding to a fractional random atomic coverage of around 12–14% at this spot. This should give rise to a linear growth of the Sn 4d signal in that part of the process, measured in the same spot. After this, and moving towards higher coverage, the island formation starts. For the later parts of the two graphs (at higher Sn coverage) the two graphs should behave synchronously based on the change of coverage: The Sn 4d signal as proportional to coverage, f, and the Si 2p signal as proportional to 1-f. The Sn 4d signal is used to extract f, which is considered in the calculation of the Si 2p signal as I (Si 2p) = I0 (Si 2p) × (1-f), as shown in Fig. 2(c)(II), after adjustment for relative intensities. The sum graph (Fig. 2(c)(III)) is fitted with a Hill functional behaviour saturating at a value around 1.0. This functional behaviour has also earlier been found to reproduce the present selflimiting growth of oxide [2] via intermediate growth of 2D islands. We interpret the behaviour of the whole process to indicate that the arrival of Sn in the initial and intermediate regimes does not concentrate all the Sn in islands but also as atomic layer deposits, which therefore attenuates the Si 2p signal more than if all Sn atoms were concentrated in islands. At the final coverage obtained here, the SEM images, and the results in the sum graph (Fig. 2(c)(III)), seem to indicate that most of the Sn is present in the islands, and these probably would coalesce to a uniformly covering layer at some higher coverage (as per the Hill function). From the spectral decompositions we determine the Sn 4d and Si 2p peak binding energies for
A.G. Silva et al. / Applied Surface Science 353 (2015) 1208–1213
bulk Si0 , for the fully oxidized Si4+ state (Fig. 2(e)) and for bulk Sn0 (Fig. 2(f)). The Sn0 (j = 5/2) peak shifts uniformly towards lower binding energies with increasing Sn coverage. After the initial steep shift downwards of the energy of this level the position flattens out and becomes constant. This seems a clear indication of the attainment of an equilibrium position of the Sn levels with respect to the Fermi level, and the indication of the attainment of the metallic nature of the larger islands. With a constant position of the Sn energy levels, there is now a constant energy difference between the conduction band in the bulk of Si and the affinity level in Sn, which would be the top of the conduction band in the metallic islands, situated at the Fermi level. The continuing charge transfer to the Sn islands is now happening solely because of the presence of a larger number of Sn atoms in the islands, and the position of the Sn conduction band maximum remains at the Fermi level throughout. We can therefore try to understand the shifts of the Si energy levels based on this observation of the attainment of equilibrium in the system. The fully oxidized Si4+ state peak first shifts rapidly to lower binding energy, like the Si0 peak, for low Sn coverage. This happens while the Sn deposits, or the whole system, have not yet established equilibrium with respect to the Fermi level. When this equilibrium is established, the binding energies of both the Si4+ and Si0 keep varying with Sn deposition, but in opposite directions, in contrast to the case of the Sn 4d levels, which now remain constant. This different tendency of the shifts of Si4+ and Si0 (and Sn0 ) is indicative of the creation of an electric field in the oxide as the metal coverage increases. The shifts of the binding energy of the Si0 2p peak indicate, (1) that the formation of the oxide entails the largest shift of this energy relative to the value on the clean Si (1 1 1) 7 × 7 surface observed in the present investigation, and (2) an increasing band bending takes place with increasing Sn coverage. The variable shifts obtained (not shown here) for the different oxide peaks Sin+ (n = 1, 2, 3) are due to their location in the oxide layer and to the variation of electric field intensity with depth. The highest shifts are obtained for the Si atoms bound to four oxygen atoms (i.e. Si4+ ) since most of these are located nearest to the oxide surface and thus experience the highest change of the potential energy associated with the field (equal to potential energy difference divided by distance from the Si/Si-oxide interface). The values of the strength of the electric field extracted from the values of Si4+ and of Si0 binding energy shifts divided by thickness of oxide are plotted in Fig. 2(d). A valence band spectra of the oxide/Si system recorded with 130 eV photons is shown in Fig. 3. In this spectrum, we find the upper edge of the Si (substrate) valence band at 0.89 eV below the Fermi level, and the upper edge of the oxide valence band at 5.30 eV below the Fermi edge, as indicated in Fig. 3. We have thus obtained a value of 3.47 eV for the CB barrier for the oxide with Si (1 1 1). In a report by Alay and Hirose [10], they also looked experimentally at the valence band alignments using XPS, and deduced a value of 3.42 eV for the CB–CB height studied here. They were looking at thicker oxides and, reduced their data for charging effects, which we do not see here. Thus, the two experiments are probably in relatively good agreement, considering the issues dealt with in Ref. [10], and the nominal value of the oxide band gap used here. The change of Si 2p and Sn 4d peak positions reveal the sign and magnitude of the electric field in the oxide, and in the Si space charge region, due to the varying charges on the metal islands. From our experimental results, we suggest a band diagram (Fig. 4), with proper energy scales of the local energy levels perpendicular to the surface for, (1) the clean surface, (2) after depositing the oxide, and (3) at the maximum Sn coverage. Known parameters for Si at the doping level of Si for the Si/thin Si-oxide interface are
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Fig. 3. This figure shows the valence band spectra of the grown oxide on Si (1 1 1). The positions of the band edges are obtained from the two linear fits to the band edges, where they cross the background baseline. The nominal energy of the photon beam was 130 eV, but an accurate calibration using the second harmonic spectrum gives 131.15 eV. The binding energies in the spectrum are determined from the recorded kinetic energies by subtracting 131.15 eV from these energies.
used, in conjunction with the experimental results obtained here, to construct the diagram [11,12]. For the clean surface (first diagram at the left side in Fig. 4) we have considered for Si a bulk band gap, Eg = 1.12 eV, a dielectric constant εs = 11.9, an energy between the Fermi level EF and the conduction band minimum n = 265 m eV, resulting in an expected valence band maximum at 0.855 eV below Fermi level. From the valence band spectra of the system with flat bands (oxide on top) (Fig. 3) recorded with 130 eV photons, we obtain (0.89 ± 0.05) eV. The initial band bending of the clean Si (1 1 1) 7 × 7 surface (0.41 eV) is estimated from considering a depletion region length ω = 7.3 × 10−7 m and donor concentration, Nd = 1015 cm−3 , for a n-type, 5 cm resistivity silicon wafer. The position of the valence band edge in this case is measured to be (0.40 ± 0.05) eV below the Fermi level. For the Sioxide/Si system we have adopted the value of 9.00 eV for the band gap. The uncertainties in the results for the core levels (i.e. random errors in the experiments) are indicated partially by looking at the core-level-data as plotted in Fig. 2 against a smooth curve. The experimental resolution is around 0.02 eV per measured point, but the use of deconvolution methods with quality indicators of the fits and the systematic variation between the data sets allow us to assume a random error in the results of the order of only 0.01 eV in the values given in Fig. 4, for values of the band bending and the data for the potential differences across the oxide. The values of binding energies are also obtained with nearly the same precision, as the photon energy of the synchrotron beam was measured with two digit resolution by recording the Si 2p spectrum for a clean Si (1 1 1) 7 × 7 surface with the first and second harmonic of the photon beam. An uncertainty in the value of the photon energy might otherwise give rise to a systematic error in the values reported for the absolute core level binding energies, but not in the differences between binding energies. The parameters extracted from the valence band spectra are estimated to within 0.05 eV accuracy. 3. Discussion and conclusions The clean “intrinsic” Si (1 1 1) 7 × 7 surface potential is pinned by the half-filled dangling bonds at adatoms [12] with a band bending of 0.41 eV affecting a space charge region of a depth of 7.3 × 10−7 m.
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Fig. 4. Schematics for the surface electronic properties (MOS-like) of the Sn on Si-oxide/Si (1 1 1) 7 × 7 system, constructed to scale (in energy only). The values deduced from these experiments are represented in blue; the values in black were taken from the literature and in green were estimated. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The binding energy of Si0 (Si 2p3/2 ) measured at the surface is 98.95 eV. After growth of the 0.8 nm thick oxide the band bending is lowered to zero, as a result of removal of the dangling bonds. This is seen from the change in the Si0 binding energy. The field in the oxide is zero in this case (cp. Figs. 2(d) and 4). The change of the value of the E-field due to the maximum Sn island coverage, in relation to the bare oxide, is thus calculated from a 0.10 V potential difference between the Si/oxide interface and the oxide surface, obtained from the relative variations in the distance between the Si0 and Si4+ Si 2p3/2 peaks (see Fig. 2): Emax =
([E(Si4+ ) − E(Si0 )]Sn − [E(Si4+ ) − E(Si0 )]SiOx ) (eV) L
0.10 V = = 1.3 · 108 V/m. 0.8 nm Using L = 0.8 nm as the (nominal) thickness of the oxide the varying values of the field strength in the oxide against Sn coverage are calculated and plotted in Fig. 2 (cp. the dielectric strength of SiO2 , which is ca. 109 V/m [12]). This linear behaviour indicates that the growth of the Sn deposits and especially the formation of the islands result in a linearly increasing metallic surface area (and total surface charge) with Sn coverage, as the affinity for electron transfer to Sn is constant in the latter regime. In summary, this work deals with a very sensitive and accurate in situ method to monitor dynamical changes in a planar metaloxide-semiconductor system under application of a variable bias across the layers, in a contact-less mode. It also helps to characterize the growth of an ultrathin oxide layer in a self-limiting method, which causes the elimination of the original surface states, and no significant amount of new interface defects, creating flat bands at the interface. The variable bias is created by – controlled charging of the surface – depositing nano-sized Sn particles on the oxide surface, which attract different amounts of negative charge through the thin oxide depending on their size. All the experiments are performed under UHV conditions, with methods based on the access to synchrotron radiation and highly resolving spectrometers, and procedures optimized for keeping the surfaces clean during the experiments. The growth of the Sn particles in the form
of hemispherical islands is studied in a parallel experiment with a SEM, and the growth mode is revealed in photoemission as a 2D island growth, seen from above, by fitting a Hill-function model to account well for the Si and Sn signal intensities versus amount of Sn deposited. The field in the oxide created by the charged surface, and the band bending in the Si surface, are monitored with respect to the increasing negative charge accumulated on the Sn islands. The field increases linearly with the amount of Sn deposited in the whole region of coverage obtained in this experiment. The indispensable tools for such studies are high resolution-, and highly surface sensitive photoemission techniques through the choice of photon energies, available with synchrotron radiation, which in this case was provided by the ASTRID facilities at Aarhus University, Denmark. The ultrathin oxide formed in the process described here is of uniform thickness and shows reproducible and stable tunnelling properties. It neutralizes the Fermi level pinning of the clean Si (1 1 1) surface, and it is stable at temperatures up to 800 ◦ C. It may be used as – or studied as a model of – the first interfacial layer for more complex and layered insulator structures in current and future generations of MOSFETs, as a diffusion barrier to atoms, or as a “sacrificial” oxide for a complete conversion into other oxides by reactions with appropriate metals at elevated temperatures, as for example Al [13], creating an ultrathin-Al-oxide/Si interface of very high quality and no intermixing. Acknowledgements Ana Silva received EU (FP7/2007–2013) grants under agreement no. 226716 for this investigation at ISA. The authors are grateful to the staff at ISA, Aarhus, Denmark, for providing support and excellent working conditions. Ana G Silva also acknowledges the Portuguese Foundation for Science and Technology (FCT) the Strategic Program-UI68 2011–2013. Jens Rafaelsen is acknowledged for the SEM images. References [1] M.T. Bohr, R.S. Chau, T. Ghani, K. Mistry, IEEE Spectr. 10 (October) (2007) 29–35. [2] P. Morgen, J. Drews, R. Dhiman, Z.S. Li, Presentation at AVS 59th Symposium Session: EM+SS+AS+NS-ThM3, Tampa, Florida, 2012, Book of Abstracts, p. 199.
A.G. Silva et al. / Applied Surface Science 353 (2015) 1208–1213 [3] K. Takayanagi, Y. Tanishiro, M. Takahashi, S. Takahashi, J. Vac. Sci. Technol. A 3 (1985) 1502–1506. [4] F.J. Himpsel, F.R. McFeely, A. Taleb-Ibrahimi, J.A. Yarmoff, G. Hollinger, Phys. Rev. B 38 (9) (1988) 6084–6096. [5] D. Adams. XPS Fitting Program, University of Aarhus, Denmark, available from Lund University: http://www.sljus.lu.se/download.html [6] G. Le Lay, M. Göthelid, T.M. Grehk, M. Björkquist, U.O. Karlsson, V. Yu Aristov, Phys. Rev. B 50 (19) (1994) 14277–14282. [7] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285–291.
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H. Togashi, Y. Enta, Maki Suemitsu, Appl. Surf. Sci. 252 (2006) 5900–5906. S. Tanuma, C.J. Powell, D.R. Penn, Surf. Interf. Anal. 21 (1994) 165–176. J.L. Alay, M. Hirose, J. Appl. Phys. 81 (3) (1997) 1606–1608. N.V. Richardson, S. Holloway, Electronic structure, in: Handbook of Surface Science, Vol. 2, North-Holland, 2000, pp. 750–758 (K. Horn and M. Scheffler, R. Ludeke, Chapter 11). [12] F.J. Himpsel, G. Hollinger, R.A. Pollak, Phys. Rev. B 28 (12) (1983) 7014–7018. [13] P. Morgen, J.M. Drews, R. Dhiman, P. Nielsen, NATO Sci. Peace Security Series B: Phys. Biophys. 8 (2011) 257–273.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Photoelectron spectroscopy as an in situ contact-less method for studies of MOS properties of ultrathin oxides on Si Ana G. Silva a,∗ , Kjeld Pedersen b , Zheshen S. Li c , Per Morgen d a
CeFiTec, Department of Physics, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, P-2829-516 Caparica, Portugal Department of Physics and Nanotechnology, Aalborg University, Skjernvej 4A, DK-9220 Aalborg East, Denmark Institute for Storage Ring Facilities (ISA), Faculty of Science, Aarhus University, Ny Munkegade 120, Building 1520, DK-8000 Aarhus C, Denmark d Department of Physics, Chemistry, and Pharmacy, University of Southern Denmark, Campusvej 55, DK-5230 Odense M, Denmark b c
a r t i c l e
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Article history: Received 31 May 2015 Received in revised form 29 June 2015 Accepted 4 July 2015 Available online 14 July 2015 Keywords: Electric field Ultrathin oxides Synchrotron Photoelectron spectroscopy Surface nanotechnology
a b s t r a c t The electric field across an ultrathin and uniform Si-oxide layer on a Si (1 1 1) surface and the positions of the valence band edges at the Si-oxide/Si (1 1 1) interface have been probed by high-resolution synchrotron radiation induced photoemission spectroscopy, as an in situ contact-less method. Variation of the “gate bias” is achieved by depositing Sn nanoparticles on the ultrathin oxide surface. These nanoparticles, growing as isolated hemi-spherical islands, attract various quantities of negative charges from the substrate inducing a potential difference between the Sn islands/Si-oxide and Si-oxide/Si (1 1 1) interface. This method allows us to study and extract the locally varying electric field and changes in the positions of the edges of the valence bands by measuring the valence band spectra and the Si 2p and Sn 4d core-levels at different Sn coverages. The ultrathin (0.8 nm thick) Si-oxide layer is grown in a simple and traceable self-limiting thermal process on a clean Si (1 1 1) surface. The oxide grown in this way creates flat bands. The properties of the system of Sn islands grown on this system are also determined. The induced electric field in the oxide varies linearly with the amount of Sn deposited per area. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In modern Si-based microelectronic device processing using high-k dielectrics as gate oxides [1], an ultrathin oxide of Si may still often be formed at the interface with the underlying Si, contributing to the total gate voltage drop across the metaloxide-semiconductor (MOS) system. It is therefore important to experimentally establish a technique for evaluating the changes in this part of the total energy band profile around the oxide/Si interface with applied gate voltage. In this work, we achieve this by evaporating Sn onto an ultrathin Si-oxide layer kept at 500 ◦ C, which leads to the growth of variably sized nanoparticles in the form of hemi-spherical islands, not touching each other, and their size depending on the total quantity of Sn arriving at the oxide surface. No sign of a chemical reaction between the Si-oxide layer and the Sn islands was observed. Metallic islands of different sizes attract different amounts of negative charge from the Si surface, tunnelling through the oxide, creating electric field lines in the space between the islands and perpendicular to the Si-oxide
∗ Corresponding author. E-mail address: [email protected] (A.G. Silva). http://dx.doi.org/10.1016/j.apsusc.2015.07.024 0169-4332/© 2015 Elsevier B.V. All rights reserved.
surface. With the use of synchrotron radiation and a sufficiently highly resolving energy spectrometer, the changes in the valence band structure and edges, and intensities and peak positions of core levels, can be measured with the proper resolution and accuracy of intensities for reliable data reduction methods. The core level energies are converted to information about the band bending and other chemical effects while the valence band spectra are examined for structure and positions of the edges. The total variation of the equivalent gate bias, with the present range of Sn coverages below a continuously covering layer, is only active for depletion conditions. The measurements were carried out with highly surface-sensitive experimental conditions, but still allowing us to study in detail the upper surfaces, the distribution of chemically shifted species inside the oxide, and the position of the Si- and Sn atomic level energies in the surface region. Since the inelastic mean free path of low-energy electrons (e.g. 30 eV) in Sn is 0.5 nm even just 2 nm high islands will reduce the signal from the underlying Si to 2%. Photoemission using such low kinetic energies will therefore only record electrons emitted from the top surface of the Sn islands and from the uncovered areas in the space between the islands of the substrate (Si-oxide). In this way, the measurements of the positions of the core lines will refer to the electronic core levels nearest to the surface.
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The results obtained for the intensities indicate a systematic variation of the island size with Sn exposure. Regarding the shifts in the peaks positions of the energy levels, we see initially that the growth of the self-limiting oxide on the Si (1 1 1) 7 × 7 surface produces flat bands for this surface, thus unpinning the Fermi level at the surface, giving an interface without charged defects, at the level of accuracy measurable here. Among the thermally processed oxides, although the Si (1 0 0) surface is the most used for industrial purposes, due to its apparently smaller amount of defects at the interface, the Si (1 1 1) 7 × 7 surface is used here, as it offers to the spectroscopist a larger number of parameters to evaluate. In the present work, the magnitude of the normally directed field across the oxide increases linearly with the coverage of Sn up to or above 108 V/m at the highest Sn coverage, corresponding to a maximum value of (0.40 ± 0.01) eV in the band bending of the Si space charge region. This is seen as approaching the value of 0.41 eV of the Si (1 1 1) 7 × 7 surface pinned by the partly occupied surface states before reactions. The variation of the Sn core level positions indicate that the Sn islands after having attained a certain size are in equilibrium with respect to the Fermi level, which means that the affinity of the islands towards extracting electrons from Si becomes constant. 2. Experimental details and results The Si sample, cut from a polished wafer and cleaned with ethanol was inserted in the ultra-high vacuum chamber before pumping down and baking of the system. After the bake-out cycle and reaching a working pressure of around 10−8 Pa, the sample was cleaned by Ohmic heating for several hours at 600 ◦ C and finally shortly to above 1000 ◦ C. Low Energy Electron Diffraction (LEED) and Si 2p photoemission spectra confirmed the perfection of crystalline structure and cleanliness of the resulting Si (1 1 1) 7 × 7 surface structure (Fig. 1(c)). The oxide/Si system was now prepared in situ using a self-limiting procedure to grow a 0.8 nm thick, uniform Si-oxide layer in a direct reaction of a clean, heated Si (1 1 1) surface with 99.998% pure molecular oxygen at 2 × 10−4 Pa for 30 min, at a temperature of 650 ◦ C. The uniformity of the thickness of the Si-oxide layer was confirmed by Si 2p photoemission
Fig. 1. (a) Si 2p spectra from positions across the Sn covered surface recorded with 130 eV photons; (b) corresponding Sn 4d spectra recorded with 63 eV photons. The fitted spectra are shown. The red lines (top for Si 2p and bottom for Sn 4d spectra) are from the low Sn coverage locations, and the blue lines (bottom for Si 2p and top for Sn 4d spectra) are from the high Sn coverage locations. (c) Si 2p spectra from clean (black line) and from oxidized (red line) surface. (d) Examples of experimental and fitted Sn 4d and Si 2p spectra. The Si 2p spectra show the presence of bulk, intermediately and fully oxidized states, while the Sn 4d spectra show no sign of oxidation (no “chemical shifts”). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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measurements across the surface (Fig. 1(c)) and by TEM in separate studies [2]. While keeping the Si surface with oxide, at 500 ◦ C, high-purity (99.99%) Sn was evaporated from an e-beam heated refractory metal crucible onto the surface with the evaporation flux determined by a quartz-crystal microbalance. The sample temperature was measured from outside the vacuum chamber with a spot photometer calibrated for Si and transmission of the window. The deposition of Sn was done in a programmed way by steadily moving the sample into the shadow of a mask while maintaining the evaporated flux constant to produce a systematic and graded variation of the Sn thickness across the surface-oxide layer exposed. The resulting layer of variable and linearly graded amount of Sn deposit is ca. 1.5 cm wide between the “thinnest” and “thickest” edges. The Sn source is equipped with a biased, neutralizing grid to prevent the impact of ionized Sn particles on the sample. The high-resolution surface-sensitive core-level and valenceband photoemission experiments were performed with synchrotron radiation from the ASTRID storage ring at the University of Aarhus (Denmark) on the SGM1 beam line equipped with a spherical grating monochromator and a SCIENTA hemispherical spectrometer with a central electron trajectory radius of 20 cm. The Si 2p and Sn 4d spectra were recorded with photon energies of 130 eV and 63 eV, respectively, at normal emission of the emitted electrons. The synchrotron beam width on the sample is 150 m in the direction of the Sn coverage gradient allowing locally resolved photoemission characterization of the system. In the Si 2p and Sn 4d spectra the energy step in the recorded spectra is set at 20 meV, corresponding to the actual experimental resolution. The valence band spectra were recorded at photon energy of 130 eV, at a lower spectrometer resolution, and with the energy step set at 50 meV, correspondingly. The Si 2p (Fig. 1(a)) and Sn 4d (Fig. 1(b)) spectra of the variably sized Sn islands deposited across the surface, with linearly varying exposure to the Sn source, were recorded in 19 local Si 2p spectra and 28 local Sn 4d spectra with the shortest possible time delays. The current method of preparing the sample with a systematically varied exposure across the surface offers the best method of systematic and rapid spectroscopic studies of surface phenomena under vacuum conditions, as only one preparation cycle is needed. In the spectrum of clean Si (1 1 1) 7 × 7 (Fig. 1(c)) one can distinguish the bulk core level Si0 and two surface core level components for the atoms at the reconstructed surface: S1 due to adatoms and S2 due to rest-atoms [3], shifted in binding energy by +0.27 eV and −0.72 eV respectively relative the bulk Si0 energy. After the Si oxidation (Fig. 1(c)) the intensity of the surface component S1 is reduced significantly and the signal from the rest-atoms (S2 ) has disappeared, while four discrete surface core levels assigned to Si atoms at the surface bonding to one (Si1+ ), two (Si2+ ), three (Si3+ ) and four (Si4+ ) oxygen atoms are now observed [4]. The Si4+ is the bonding configuration of the fully oxidized Si, i.e. SiO2 , while the other three are designated as intermediate oxidation states, originating in the interface [4]. The fully oxidized state Si4+ has its fitted maximum (Si 2p3/2 ) shifted, in binding energy, by +3.65 eV relative to Si 2p3/2 of bulk Si0 . For a more detailed evaluation of the spectra, the original Si 2p and Sn 4d raw spectra are first normalized to the experimental conditions (photon flux, etc.) and afterwards deconvoluted using the program FitXPS (written by D. Adams [5]). Using this program, the Si 2p and Sn 4d spectra are decomposed into peaks of folded Lorentzian/Gaussian line shapes (Fig. 1d). The model used for the Si 2p spectra includes five spin–orbit split doublets; one for pure silicon Si0 , three doublets for the intermediately oxidized states and one for the fully oxidized silicon Si4+ . The value of the spin–orbit splitting is 0.61 eV [6], and the 2p1/2 to 2p3/2 intensity ratio is 1:2. The model used for the Sn 4d spectra includes a single spin–orbit doublet for the bulk Sn 4d of pure Sn0 (splitting: 1.10 eV, Fig. 1(d)).
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Fig. 2. (a) and (b) SEM images of a system of Sn islands on Si (1 1 1) 7 × 7 surface with the 0.8 nm oxide for (a) 5 ML and (b) 10 ML equivalent thickness of Sn. The scale bar is 100 nm. (c) Relative signal intensities of Si 2p and Sn 4d versus Sn relative thickness, normalized according to the SEM image (b). The upper graph (III-stars) is the sum of the two data (I-squares, II-diamonds) fitted with a Hill function (III-continuous curve); the Sn 4d (I) is fitted also with Hill function and Si 2p (II) fitted with the model describing the Si 2p intensity decay. The limiting value of the summed curves is around 1.0. (d) Representation of the electric field strength across the oxide and its behaviour, which is well fitted as a linear function on equivalent ML thickness. (e) And (f) shifts of the peak energies of Si0 , Si4+ , and Sn0 , respectively, across the Sn graded exposure. (g) Scheme of the Sn islands growth as the equivalent thickness increases. (h) Scheme identifying the regions of positive and negative charges as well as the origin of the contributions for Si0 , Si4+ and Sn0 , at a specific coverage.
Unconstrained values of the LFWHM and GFWHM were used in optimizing the fits of the Si 2p and Sn. For the Si 2p peaks the mean value of the Lorentz full widths was 0.22 eV while the Gaussian full width increased systematically from 0.32 eV (Si0 ) to 1.20 eV (Si4+ ). For Sn 4d all spectra were fitted with a LFWHM of 0.22 eV and a GFWHM of 0.30 eV, and a value of 0.099 eV for alpha (the Doniach–Sunjic metallic loss skewness factor [7]), was used in all the fits. The resulting fitted functions are shown (Fig. 1(a) and (b)) for the normalized Si 2p and Sn 4d spectra. To facilitate the interpretation of the dependence of Si 2p and Sn 4d spectra on Sn coverage, SEM images of an identical system were obtained, after Sn exposures of 5 ML (Fig. 2(a)) and 10 ML (Fig. 2(b)). The system was prepared following identical procedures but with handling of the sample in air after deposition for taking SEM images. At exposure of 10 ML an average island size of 50 nm is observed (Fig. 2(b)), with a relatively narrow spread of diameters. Relative intensities of Si 2p and Sn 4d versus relative Sn equivalent thickness, normalized according to the SEM image (Fig. 2(b)), are shown in Fig. 2(c). The maximum intensity for Si 2p corresponds to the Si 2p of the Si clean substrate plus the Si-oxide layer. The Sn 4d normalization is done using the maximum of the Sn 4d intensity at the highest Sn coverage. The results indicate that the Sn 4d intensity is well fitted with a sigmoid-like function, shown as a continuous curve in Fig. 2(c)(I), similar to the functional dependence used earlier to describe the growth of 2D oxide islands on a substrate [8]. This leads us to propose a model for the Sn deposition kinetics and island growth, which could explain the observed trends of the Si 2p and Sn 4d intensity variations. The model is relatively simple and is based on the formation of variable-sized 3D islands of Sn, not touching each other, and with a final fractional (2D) surface coverage, f, of 60%, based on the SEM image (Fig. 2(b)). For 2p photoelectrons of Si0 at 31 eV kinetic energy the inelastic mean free path (IMFP) in Sn is Si = 1.38 ML and for Sn0 at Sn = 1.5 ML (calculated using the code TPP2 M [9]). The model should be able to account for the variation of intensities relative to the maximum intensity for Si 2p and for Sn 4d core levels, as in a 2D growth process, since the emission is normal to the surface. The way these ratios vary is initially dependent on the
size and form of the deposits, and, later, only on the total fractional coverage of Si-oxide by Sn islands larger and thicker than about 3 nm. The Sn signal is, in this latter regime, simply proportional to the area covered, i.e. to f. At thicknesses below 3 nm, this signal should increase approximately linearly with exposure. With these two mechanisms combined, the Sn 4d signal should increase with deposited amount of Sn as a sigmoid function [8]. The Si 2p signal intensity is also determined by two contributions: one contribution due to the electrons that reach the energy analyser without attenuation by passing between the Sn islands and one that would represent electrons being attenuated by passing through the Sn islands. When the island height is larger than 3 nm, all Si 2p photoelectrons are totally absorbed, for normal exit. The first measured coverage of Sn has reduced the Si 2p signal with 17%, corresponding to a fractional random atomic coverage of around 12–14% at this spot. This should give rise to a linear growth of the Sn 4d signal in that part of the process, measured in the same spot. After this, and moving towards higher coverage, the island formation starts. For the later parts of the two graphs (at higher Sn coverage) the two graphs should behave synchronously based on the change of coverage: The Sn 4d signal as proportional to coverage, f, and the Si 2p signal as proportional to 1-f. The Sn 4d signal is used to extract f, which is considered in the calculation of the Si 2p signal as I (Si 2p) = I0 (Si 2p) × (1-f), as shown in Fig. 2(c)(II), after adjustment for relative intensities. The sum graph (Fig. 2(c)(III)) is fitted with a Hill functional behaviour saturating at a value around 1.0. This functional behaviour has also earlier been found to reproduce the present selflimiting growth of oxide [2] via intermediate growth of 2D islands. We interpret the behaviour of the whole process to indicate that the arrival of Sn in the initial and intermediate regimes does not concentrate all the Sn in islands but also as atomic layer deposits, which therefore attenuates the Si 2p signal more than if all Sn atoms were concentrated in islands. At the final coverage obtained here, the SEM images, and the results in the sum graph (Fig. 2(c)(III)), seem to indicate that most of the Sn is present in the islands, and these probably would coalesce to a uniformly covering layer at some higher coverage (as per the Hill function). From the spectral decompositions we determine the Sn 4d and Si 2p peak binding energies for
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bulk Si0 , for the fully oxidized Si4+ state (Fig. 2(e)) and for bulk Sn0 (Fig. 2(f)). The Sn0 (j = 5/2) peak shifts uniformly towards lower binding energies with increasing Sn coverage. After the initial steep shift downwards of the energy of this level the position flattens out and becomes constant. This seems a clear indication of the attainment of an equilibrium position of the Sn levels with respect to the Fermi level, and the indication of the attainment of the metallic nature of the larger islands. With a constant position of the Sn energy levels, there is now a constant energy difference between the conduction band in the bulk of Si and the affinity level in Sn, which would be the top of the conduction band in the metallic islands, situated at the Fermi level. The continuing charge transfer to the Sn islands is now happening solely because of the presence of a larger number of Sn atoms in the islands, and the position of the Sn conduction band maximum remains at the Fermi level throughout. We can therefore try to understand the shifts of the Si energy levels based on this observation of the attainment of equilibrium in the system. The fully oxidized Si4+ state peak first shifts rapidly to lower binding energy, like the Si0 peak, for low Sn coverage. This happens while the Sn deposits, or the whole system, have not yet established equilibrium with respect to the Fermi level. When this equilibrium is established, the binding energies of both the Si4+ and Si0 keep varying with Sn deposition, but in opposite directions, in contrast to the case of the Sn 4d levels, which now remain constant. This different tendency of the shifts of Si4+ and Si0 (and Sn0 ) is indicative of the creation of an electric field in the oxide as the metal coverage increases. The shifts of the binding energy of the Si0 2p peak indicate, (1) that the formation of the oxide entails the largest shift of this energy relative to the value on the clean Si (1 1 1) 7 × 7 surface observed in the present investigation, and (2) an increasing band bending takes place with increasing Sn coverage. The variable shifts obtained (not shown here) for the different oxide peaks Sin+ (n = 1, 2, 3) are due to their location in the oxide layer and to the variation of electric field intensity with depth. The highest shifts are obtained for the Si atoms bound to four oxygen atoms (i.e. Si4+ ) since most of these are located nearest to the oxide surface and thus experience the highest change of the potential energy associated with the field (equal to potential energy difference divided by distance from the Si/Si-oxide interface). The values of the strength of the electric field extracted from the values of Si4+ and of Si0 binding energy shifts divided by thickness of oxide are plotted in Fig. 2(d). A valence band spectra of the oxide/Si system recorded with 130 eV photons is shown in Fig. 3. In this spectrum, we find the upper edge of the Si (substrate) valence band at 0.89 eV below the Fermi level, and the upper edge of the oxide valence band at 5.30 eV below the Fermi edge, as indicated in Fig. 3. We have thus obtained a value of 3.47 eV for the CB barrier for the oxide with Si (1 1 1). In a report by Alay and Hirose [10], they also looked experimentally at the valence band alignments using XPS, and deduced a value of 3.42 eV for the CB–CB height studied here. They were looking at thicker oxides and, reduced their data for charging effects, which we do not see here. Thus, the two experiments are probably in relatively good agreement, considering the issues dealt with in Ref. [10], and the nominal value of the oxide band gap used here. The change of Si 2p and Sn 4d peak positions reveal the sign and magnitude of the electric field in the oxide, and in the Si space charge region, due to the varying charges on the metal islands. From our experimental results, we suggest a band diagram (Fig. 4), with proper energy scales of the local energy levels perpendicular to the surface for, (1) the clean surface, (2) after depositing the oxide, and (3) at the maximum Sn coverage. Known parameters for Si at the doping level of Si for the Si/thin Si-oxide interface are
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Fig. 3. This figure shows the valence band spectra of the grown oxide on Si (1 1 1). The positions of the band edges are obtained from the two linear fits to the band edges, where they cross the background baseline. The nominal energy of the photon beam was 130 eV, but an accurate calibration using the second harmonic spectrum gives 131.15 eV. The binding energies in the spectrum are determined from the recorded kinetic energies by subtracting 131.15 eV from these energies.
used, in conjunction with the experimental results obtained here, to construct the diagram [11,12]. For the clean surface (first diagram at the left side in Fig. 4) we have considered for Si a bulk band gap, Eg = 1.12 eV, a dielectric constant εs = 11.9, an energy between the Fermi level EF and the conduction band minimum n = 265 m eV, resulting in an expected valence band maximum at 0.855 eV below Fermi level. From the valence band spectra of the system with flat bands (oxide on top) (Fig. 3) recorded with 130 eV photons, we obtain (0.89 ± 0.05) eV. The initial band bending of the clean Si (1 1 1) 7 × 7 surface (0.41 eV) is estimated from considering a depletion region length ω = 7.3 × 10−7 m and donor concentration, Nd = 1015 cm−3 , for a n-type, 5 cm resistivity silicon wafer. The position of the valence band edge in this case is measured to be (0.40 ± 0.05) eV below the Fermi level. For the Sioxide/Si system we have adopted the value of 9.00 eV for the band gap. The uncertainties in the results for the core levels (i.e. random errors in the experiments) are indicated partially by looking at the core-level-data as plotted in Fig. 2 against a smooth curve. The experimental resolution is around 0.02 eV per measured point, but the use of deconvolution methods with quality indicators of the fits and the systematic variation between the data sets allow us to assume a random error in the results of the order of only 0.01 eV in the values given in Fig. 4, for values of the band bending and the data for the potential differences across the oxide. The values of binding energies are also obtained with nearly the same precision, as the photon energy of the synchrotron beam was measured with two digit resolution by recording the Si 2p spectrum for a clean Si (1 1 1) 7 × 7 surface with the first and second harmonic of the photon beam. An uncertainty in the value of the photon energy might otherwise give rise to a systematic error in the values reported for the absolute core level binding energies, but not in the differences between binding energies. The parameters extracted from the valence band spectra are estimated to within 0.05 eV accuracy. 3. Discussion and conclusions The clean “intrinsic” Si (1 1 1) 7 × 7 surface potential is pinned by the half-filled dangling bonds at adatoms [12] with a band bending of 0.41 eV affecting a space charge region of a depth of 7.3 × 10−7 m.
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Fig. 4. Schematics for the surface electronic properties (MOS-like) of the Sn on Si-oxide/Si (1 1 1) 7 × 7 system, constructed to scale (in energy only). The values deduced from these experiments are represented in blue; the values in black were taken from the literature and in green were estimated. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
The binding energy of Si0 (Si 2p3/2 ) measured at the surface is 98.95 eV. After growth of the 0.8 nm thick oxide the band bending is lowered to zero, as a result of removal of the dangling bonds. This is seen from the change in the Si0 binding energy. The field in the oxide is zero in this case (cp. Figs. 2(d) and 4). The change of the value of the E-field due to the maximum Sn island coverage, in relation to the bare oxide, is thus calculated from a 0.10 V potential difference between the Si/oxide interface and the oxide surface, obtained from the relative variations in the distance between the Si0 and Si4+ Si 2p3/2 peaks (see Fig. 2): Emax =
([E(Si4+ ) − E(Si0 )]Sn − [E(Si4+ ) − E(Si0 )]SiOx ) (eV) L
0.10 V = = 1.3 · 108 V/m. 0.8 nm Using L = 0.8 nm as the (nominal) thickness of the oxide the varying values of the field strength in the oxide against Sn coverage are calculated and plotted in Fig. 2 (cp. the dielectric strength of SiO2 , which is ca. 109 V/m [12]). This linear behaviour indicates that the growth of the Sn deposits and especially the formation of the islands result in a linearly increasing metallic surface area (and total surface charge) with Sn coverage, as the affinity for electron transfer to Sn is constant in the latter regime. In summary, this work deals with a very sensitive and accurate in situ method to monitor dynamical changes in a planar metaloxide-semiconductor system under application of a variable bias across the layers, in a contact-less mode. It also helps to characterize the growth of an ultrathin oxide layer in a self-limiting method, which causes the elimination of the original surface states, and no significant amount of new interface defects, creating flat bands at the interface. The variable bias is created by – controlled charging of the surface – depositing nano-sized Sn particles on the oxide surface, which attract different amounts of negative charge through the thin oxide depending on their size. All the experiments are performed under UHV conditions, with methods based on the access to synchrotron radiation and highly resolving spectrometers, and procedures optimized for keeping the surfaces clean during the experiments. The growth of the Sn particles in the form
of hemispherical islands is studied in a parallel experiment with a SEM, and the growth mode is revealed in photoemission as a 2D island growth, seen from above, by fitting a Hill-function model to account well for the Si and Sn signal intensities versus amount of Sn deposited. The field in the oxide created by the charged surface, and the band bending in the Si surface, are monitored with respect to the increasing negative charge accumulated on the Sn islands. The field increases linearly with the amount of Sn deposited in the whole region of coverage obtained in this experiment. The indispensable tools for such studies are high resolution-, and highly surface sensitive photoemission techniques through the choice of photon energies, available with synchrotron radiation, which in this case was provided by the ASTRID facilities at Aarhus University, Denmark. The ultrathin oxide formed in the process described here is of uniform thickness and shows reproducible and stable tunnelling properties. It neutralizes the Fermi level pinning of the clean Si (1 1 1) surface, and it is stable at temperatures up to 800 ◦ C. It may be used as – or studied as a model of – the first interfacial layer for more complex and layered insulator structures in current and future generations of MOSFETs, as a diffusion barrier to atoms, or as a “sacrificial” oxide for a complete conversion into other oxides by reactions with appropriate metals at elevated temperatures, as for example Al [13], creating an ultrathin-Al-oxide/Si interface of very high quality and no intermixing. Acknowledgements Ana Silva received EU (FP7/2007–2013) grants under agreement no. 226716 for this investigation at ISA. The authors are grateful to the staff at ISA, Aarhus, Denmark, for providing support and excellent working conditions. Ana G Silva also acknowledges the Portuguese Foundation for Science and Technology (FCT) the Strategic Program-UI68 2011–2013. Jens Rafaelsen is acknowledged for the SEM images. References [1] M.T. Bohr, R.S. Chau, T. Ghani, K. Mistry, IEEE Spectr. 10 (October) (2007) 29–35. [2] P. Morgen, J. Drews, R. Dhiman, Z.S. Li, Presentation at AVS 59th Symposium Session: EM+SS+AS+NS-ThM3, Tampa, Florida, 2012, Book of Abstracts, p. 199.
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