* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Electronic Structure and Optical Quality of Nanocrystalline Y2O3
Survey
Document related concepts
Ultraviolet–visible spectroscopy wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Chemical imaging wikipedia , lookup
Chemical thermodynamics wikipedia , lookup
Determination of equilibrium constants wikipedia , lookup
Physical organic chemistry wikipedia , lookup
Reflection high-energy electron diffraction wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Surface properties of transition metal oxides wikipedia , lookup
Transcript
Article pubs.acs.org/JPCC Electronic Structure and Optical Quality of Nanocrystalline Y2O3 Film Surfaces and Interfaces on Silicon E. J. Rubio,† V. V. Atuchin,*,‡,§,∥ V. N. Kruchinin,⊥ L. D. Pokrovsky,‡ I. P. Prosvirin,# and C. V. Ramana† † Department of Mechanical Engineering, University of Texas at El Paso, El Paso, Texas 79968, United States Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia § Functional Electronics Laboratory, Tomsk State University, Tomsk 634050, Russia ∥ Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, Novosibirsk 630090, Russia ⊥ Laboratory for Ellipsometry of Semiconductor Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia # Boreskov Institute of Catalysis, SB RAS, Novosibirsk 630090, Russia ‡ S Supporting Information * ABSTRACT: Nanocrystalline yttrium oxide (Y2O3) thin films were made by sputter deposition onto silicon (100) substrates keeping the deposition temperature fixed at 300 °C. The surface/interface chemistry, Y−O bonding, and optical constants of the Y2O3 film surface and Y2O3−Si interface were evaluated by the combined use of X-ray photoelectron spectroscopy (XPS), depth-profiling, and spectroscopic ellipsometry (SE). XPS analyses indicate the binding energies (BEs) of the Y 3d doublet; i.e., the Y 3p5/2 and Y 3d3/2 peaks are located at 117.0 and 119.1 eV, respectively, characterizing yttrium in its highest chemical oxidation state (Y3+) in the grown films. The optical model is constructed based on the XPS depth profiles, which indicate that the Y2O3//Si heterostructure can be represented with Y2O3 filmYxSiyOz interfacial compoundSi substrate. Such a model accounts for the experimentally determined ellipsometry functions and accurately produces the dispersive index of refraction (n(λ)) of Y2O3 and YxSiyOz. The n(λ) of Y2O3 and YxSiyOz follows Cauchy’s dispersion relation, while the YxSiyOz formation accounts for degradation of optical quality. I. INTRODUCTION Oxide dielectrics have been the subject of numerous investigations for many years due to their possible device integration in a wide range of technologies involving electronics, electro-optics, optoelectronics, and magnetoelectronics.1−5 Yttrium oxide (Y2O3), a stable oxide of yttrium metal, has received significant attention in recent years in view of its possible integration into a wide range of scientific and technological applications.6−12 Y2O3 films exhibit excellent electronic properties such as transparency over a broad spectral range (0.2−8 μm), high dielectric constant (∼14−18), high refractive index (∼2), large band gap (∼5.8 eV), low absorption (from near-UV to IR), and superior electrical breakdown strength (>3 MV/cm).1,8−11,13−15 These properties make Y2O3 films interesting for various electrical and optical devices.16−22 Yttrium oxides were proposed as hosts for rare-earth elements, and efficient thin film phosphors were prepared.23−29 The interface layer formation, however, was detected for several compounds, and structural and chemical parameters of the interface were dependent on the deposition conditions. Therefore, controlled growth and manipulation of micro© 2014 American Chemical Society structure, particularly at the nanoscale dimensions, has important implications for the design and applications of Y2O3 films. Yttrium oxide is a c-type rare-earth oxide.30 The c-type structure of Y2O3 is stable up to 2325 °C in the air. The c-type structure is a modified fluorite-type cubic structure with onefourth of the anion sites vacant and regularly arranged. The structural stability coupled with mechanical properties also make Y2O3 an interesting material for other applications. Specifically, Y2O3 has been in use in the development of functional ceramics for solid oxide fuel cells, nuclear engineering, high-temperature protective coatings, and metal-reinforced composites for high strength structural components.31−35 A wide variety of physical and chemical deposition techniques have been employed to fabricate Y2O3 films. However, in both physical and chemical deposition methods, the ultramicrostructure in terms of the chemical valence state of Received: March 23, 2014 Revised: May 15, 2014 Published: May 30, 2014 13644 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article chamber to ignite the plasma. Once the plasma was ignited the power was increased to 100 W, and oxygen (O2) was released into the chamber for reactive deposition. The flow of the Ar and O2 and their ratio were controlled using MKS mass flow controllers. Before each deposition, the Y-target was presputtered for 10 min using Ar alone with the shutter above the gun closed. The deposition was made keeping the substrate temperature (Ts) at 300 °C, which was found to be optimum to produce nanocrystalline Y2O3 films as reported elsewhere.11,15,20 B. Characterization. X-ray Photoelectron Spectroscopy (XPS). Electronic properties of the Y2O3/Si system were characterized by X-ray photoelectron spectroscopy. Depth profiling of Y2O3 films on Si was performed with the standard procedure, which is a combination of Ar+ ion sputtering/ bombardment followed by XPS data acquisition and analysis.39−41 The XPS valence-band and core-level spectra of Y2O3 were measured using the ultrahigh vacuum (UHV) AnalysisSystem assembled by SPECS (Germany). The system is equipped with a PHOIBOS 150 hemispherical analyzer and Xray monochromator FOCUS-500. The Al Kα irradiation (hν = 1486.74 eV, 200 W) was used as a source of photoelectron excitation. The survey spectrum was recorded at the analyzer pass energy of 50 eV and the narrow spectral regions at 10 eV. The atomic concentration ratios of elements on the sample surface were calculated from the integral photoelectron peak intensities and known atomic sensitivity factors (ASFs).42 The energy scale of the spectrometer was calibrated by setting the measured Au 4f7/2 and Cu 2p3/2 binding energies (BEs) to 84.00 ± 0.05 eV and 932.66 ± 0.05 eV, respectively, in reference to the Fermi level. For the peak fitting procedure a mixture of Lorenzian and Gaussian functions were used together with the Shirley background subtraction method. The film depth profiling was carried out using an argon ion gun (SPECS model IQE 11/35). The energy of Ar+ ions for depth profiling, the current density, and the angle of sputter erosion were 1.05 keV, 4−5 μA/cm2, and 45°, respectively. The depth profiling rate under these conditions was estimated as ∼0.3 nm/min in reference to the Al2O3/Al model film system. Reflection High-Energy Electron Diffraction. The surface crystallography of Y2O3 film was investigated using reflection high-energy electron diffraction (RHEED) measurements. The electron diffraction imaging was performed using a 50 keV electron beam in the EF-Z4-5 (Carl Zeiss, Germany) setup. Spectroscopic Ellipsometry (SE). The dispersive refractive index n(λ) and extinction coefficient k(λ) were determined by means of spectroscopic ellipsometry (SE). Ellipsometric angles Ψ and Δ were measured as a function of λ in the spectral range of λ ∼ 250−1030 nm using an ELLIPS-1771 SA ellipsometer.43 The instrumental spectral resolution was 2 nm, and the recording time of the spectrum did not exceed 20 s. The SE measurements were produced at three angles of incidence of light beam on the sample of 50°, 60°, and 70°. The four-zone measurement method was used with subsequent averaging over all the four zones.44−46 Y and the surface/interface chemistry strongly influence the electronic and optical properties of the grown films and, hence, their device applications. Most importantly, determination of the dispersive optical constants, namely, index of refraction (n) and extinction coefficient (k), is important for Y2O3 films deposited on semiconductor materials when chemical interaction with a substrate surface may influence the interface properties. Furthermore, for thin films, the optical constants are sensitive to the microstructure and are influenced by various factors such as surface/interface structure, crystal quality, packing density, lattice parameters, lattice strain, defect structure, and chemical composition. Additionally, the quality of the evolving film surface and interfacial compounds buried at the film−substrate interface are quite important for oxide dielectrics, such as Y2O3 in this case, for their effective utilization in optical, electronic, and optoelectronic device applications. The chemical reaction between the oxide dielectric and Si substrate during fabrication or the postdeposition annealing process resulting in the formation of interfacial layers (ILs), with silicon oxide (SiO2), or metallic clusters of the respective oxide dielectric, or metal-silicates, or a combination of these has been widely reported.15,20,22,31 Interfacial chemical compounds from the interfacial reaction suppress the effective dielectric constant and optical quality and, hence, device performances.1,8 Therefore, understanding the electronic structure and optical quality of the Y2O3 film surface and interface is quite important. The present work was, therefore, performed on sputter-deposited Y-oxide films made by sputterdeposition onto Si substrates to probe their surface chemistry, Y−O bonding, and optical constants. Combined X-ray photoelectron spectroscopy (XPS) with depth profiling and spectroscopic ellipsometry (SE) measurements, as presented and discussed in this paper, allowed us to determine the surface/interface chemical composition, nature of the interfacial chemical compounds, and optical constants of nanocrystalline Y2O3 films and interfacial compounds at the Y2O3−Si interface. II. EXPERIMENTS A. Fabrication. Yttrium oxide (Y2O3) thin films were deposited onto silicon (Si) (100) wafer substrates by radio frequency (RF) (13.56 MHz) reactive magnetron sputtering. All the substrates were thoroughly cleaned using the RCA (Radio Corporation of America) procedure.36−38 Briefly, the RCA cleaning procedure removes organic, alkali ions, and heavy metal contaminants present on the surface of the substrate. The cleaning procedure has three major steps. (1) Removal of insoluble organic contaminants using 5:1:1 H2O/ H2O2/NH4OH solution. (2) Removal of ionic and heavy metal atomic components using a solution of 6:1:1 H2O/H2O2/HCl solution. (3) Removal of native oxide by buffered oxide etching solution. At the stage, a Kikkuchi pattern was recorded commonly from Si substrates by RHEED measurements. Finally, the substrates were dried with nitrogen before introducing them into the vacuum chamber. After this, the substrates were transferred to a magnetron sputtering chamber through the laboratory air because we do not have an isolated substrate transport system. The vacuum chamber was initially evacuated to a base pressure of ∼10−6 Torr. An yttrium (Y) target (Plasmaterials Inc.) of 2” diameter and 99.95% purity was employed for reactive sputtering. The Y-target was placed on a 2-in. sputter gun, which is placed at the distance of 8 cm from the substrate. A sputtering power of 40 W was initially applied to the target while introducing high purity argon (Ar) into the III. RESULTS AND DISCUSSION The RHEED pattern of Y2O3 films grown at 300 °C is shown in Figure 1. The formation of a well-defined ring structure indicative of Y2O3 film of polycrystalline nature is evident from the RHEED pattern shown in Figure 1. The observed electron diffraction pattern matches the cubic-Y2O3 (JCPDS 43-613).47 The detailed XRD data and RHEED analysis of Y2O3 films as a 13645 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article Figure 1. RHEED pattern recorded from Y2O3/Si film. function of variable Ts were reported elsewhere.20,22 Briefly, the XRD and RHEED analyses indicate that the Y2O3 films grown at 25−100 °C were amorphous (a-Y2O3) in nature. The onset of crystallization with a (222) peak in XRD patterns corresponding to the cubic phase (c-Y2O3) began to appear for the Y2O3 samples grown at Ts = 200 °C. The peak intensity increases with the increasing Ts, indicating an increase in the average grain size with the increasing Ts. The present RHEED data confirm the crystalline nature of the Y2O3 films grown on Si(100) substrates. To obtain the surface and interface chemistry and accurately determine the optical constants, the Y2O3 film was sputtered all along the depth of the film until the signature of the Si substrate, while the surface/interface chemical composition was probed with XPS. The survey photoemission spectrum recorded from the initial surface is shown in Figure S1 (Supporting Information). Besides constituent element core levels and Auger lines related to Y2O3 oxide, the C 1s core level and C KVV Auger line were detected and attributed to adventitious hydrocarbons adsorbed from the air. The detailed photoemission peaks of Y 3d, Si 2p, and O 1s levels are shown in Figures 2−4. All the spectra are shown as a function of sputtering time of the Y2O3//Si heterostructure. Figure 3. Si 2p core-level XPS spectra. Figure 4. O 1s core-level XPS spectra. The integrated sputtering (etching) time is as indicated in the XPS curves in Figures 2−4. The initial time setting (t = 0) corresponds to most of the film top surface (as-grown). When the top surface contaminated layer was sputtered by ion bombardment, in the film bulk, the binding energy (BE) position of the Y 3d doublet (Y 3d5/2 and Y 3d3/2) and O 1s line occurred at 157.0, 159.1, and 529.2 eV, respectively (Figures 2 and 4). The BE values can be compared to those earlier reported for the Y2O3 oxide. However, an observation of available electronic parameters of this functional oxide reveals a drastic scattering of BE values in the literature. The Figure 2. Si 2s−Y 3d core-level XPS spectra. 13646 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article in Figures 2, 3, and 5. The most interesting feature is that, at t > 80 min, the Si 2p (BE ∼ 99.3 eV) and Si 2s (BE ∼ 150.4 eV) lines appear due to the presence of elemental Si0.42,61 Furthermore, the BE value of the Si 2p line remains constant up to the substrate. Simultaneously, at t = 80−90 min, the small intensity signal at 102.8 eV is recorded in the Si 2p core-level spectra shown in Figure 3. This line, in accordance with the energy difference ∼3.5 eV, in reference to the Si 2p (BE ∼ 99.3 eV) line from Si0, should be attributed to Si4+ states.61−63 As for Y 3d5/2 and O 1s levels, at t > 80 min, the lines shift to higher energies. The O 1s band can be subdivided into two components at ∼530.4 and ∼532 eV. The low-energy component indicates the value of ΔBEY = BE (O 1s) − BE (Y 3d5/2) = 372.2 eV, which can be attributed to the Y−O−Si bonding. The higher-energy component in the O 1s band may be related to interfacial carbonate species because the increased C 1s line intensity is found at t = 80−90 min. All film components disappeared at t = 140 min, which relates to a film thickness of ∼42 nm. Y2O3/Si film optical constants were primarily probed by spectroscopic ellipsometry (SE), which measures the relative changes in the amplitude and phase of the linearly polarized monochromatic incident light upon the oblique reflection from the sample surface. Ellipsometric parameters Ψ and Δ are related to the complex Fresnel reflection coefficients by the equation accumulated set of BE values of Y 3d5/2 and O 1s lines in Y2O3, together with BE difference ΔBEY = BE (O 1s) − BE (Y 3d5/2), is shown in Table 1. As found for many chemical classes of Table 1. BE Values of Representative Constituent Element Lines in Y2O3 state BE (Y 3d5/2), eV BE (O 1s), eV ΔBEY, eV ref ceramics film film film film film film film nanocrystals film film 156.7 158.5 155.5 156.5−157.0 156.6 156.0 158.1 158.4 156.7 156.6 156.2 157.0 529.5 530.6 527.7 529.0 529.0 530.2 529.7 529.5 528.4 529.2 372.8 372.1 372.2 372.4 373.0 372.1 371.3 372.8 372.2 372.2 6 6 22 42 48 49 50 51 52 53 54 present study oxides, the BE difference parameters are insensitive to surface charging effects and BE scale calibration methods, and they are particularly suitable for the comparative analysis of XPS results measured by different spectrometers.55−60 In Table 1, the scattering of BE (Y 3d5/2) and BE (O 1s) values is as high as 3.0 and 2.9 eV which is evidently above any physically conditioned range. Comparatively, the scattering range of ΔBEY is as low as 1.7 eV. Moreover, this scattering can be further decreased by simple selection. The highest and lowest ΔBEY values appeared from refs 49 and 51, and this indicates an inexact energy scale calibration. If we exclude the results, the scattering range of ΔBEY becomes as narrow as 0.7 eV. The value obtained in the present study is in good agreement with stoichiometric Y2O3 and characterizes the yttrium ions in their highest oxidation state (Y3+). The calculated atomic concentrations are shown as a function of depth in Figure 5. As is evident from Figures 2−4, over the range of t = 3−60 min, the composition of the film bulk is stable and well related to stoichiometric Y2O3. The change in chemical composition is evident at t > 60 min, where the signals of yttrium and oxygen decrease while the silicon signal increases. The evolution of the Si signal and concentration as a function of time/depth can be clearly seen tg Ψ·eiΔ = Rp Rs (1) where Rp and Rs are the coefficients for p- and s-polarized light waves. To calculate the dependencies of refractive index n(λ) and extinction coefficient k(λ) on optical wavelength λ, the experimental data were processed using the multilayer model. Over the whole spectral range, the spectral dependences of polarization angles were fitted for m points of the spectrum by minimization of the error function m σ2 = 1 · ∑ [(Δexptl − Δcalcd )2 + (Ψexptl − Ψcalcd)2 ] m i=1 (2) Extracting meaningful physical information from ellipsometry requires the construction of an optical model of the sample which generally has a number of distinct layers with individual optical dispersions. Interfaces between these layers are optical boundaries at which light is refracted and reflected according to the Fresnel relations. In the present case, to determine the dispersive optical constants accurately, the optical model of the Y2O3/Si system is constructed based on the XPS depth profiling results.64,65 The model contains the Y2O3 top layer, multicomponent multilayer system, and the Si substrate. The optical model is schematically shown in Figure 6, while the layer composition is summarized in Table 2. The thickness and chemical composition of each layer are basically derived from XPS data with the following key points taken into consideration. As seen from Figure 5, at t < 60 min the film is completely formed by pure Y2O3 oxide with the thickness of ∼18 nm. Between the Y2O3 layer and Si substrate, the bulk of the interface film is modeled as a set of six sublayers with a different composition. The thickness of each sublayer between the Y2O3 film and Si substrate was directly estimated from the XPS sputtering rate. The atomic composition of each layer is taken as the averaged value of boundary XPS measurements. Principally, the whole system is represented as the (Y2O3 Figure 5. Elemental near surface composition as a function of depth. 13647 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article n(λ) = a + b c + 4 2 λ λ ({3}) For the calculations of the multilayer system, the Bruggeman effective medium model was used ⎛ εi − εeff ⎞ ⎟=0 ⎝ εi − 2·εeff ⎠ ∑ ⎜fi i ({4}) where f i and εi are the part and dielectric permittivity of the icomponent and εeff is the effective dielectric permittivity of the medium.68 The calculation results are shown in Figures 7 and 8. Figure 6. Optical model of the reflection system constructed based on the XPS results. Table 2. Thickness and Phase Composition of the Sublayers Considered in Ellipsometric Model layer layer thickness, nm β Si β Y2O3 β YxSiyOz 1 2 3 4 5 6 7 6 3 3 3 3 6 18 0.914 0.862 0.747 0.521 0.290 0.105 0 0.004 0.130 0.218 0.419 0.654 0.870 1 0.082 0.008 0.035 0.06 0.056 0.025 0 film)−(YxSiyOz + Si interfacial composite)−(Si substrate) system. The experimental and calculated Ψ(λ), Δ(λ) curves obtained for the Y2O3/Si multilayered system are shown in Figure 7. Figure 8. Dispersive n(λ) of Y2O3 (1) and YxSiyOz (2). Curve (3) calculated in the homogeneous layer approximation. The best correlation between the experimental and calculated Ψ(λ), Δ(λ) curves (1) in Figure 7 is achieved at the best optical dispersion of YxSiyOz interfacial oxide, as represented by curve (2) in Figure 8. The drastic difference, however, is evident between experimental points and Ψ(λ), Δ(λ) curves (2) in Figure 7 calculated using the model of single homogeneous transparent layer. The refractive index dispersion calculated in the framework of a single homogeneous transparent layer is shown by curve (3) in Figure 8. Thus, the simple reflection model of a single homogeneous layer is principally not applicable for the adequate description of the real Y2O3/Si layered system. The refractive index dispersion in YxSiyOz obtained in the framework of the multilayer reflection model is defined by Cauchy’s polynomial (3) with parameters a = 1.749, b = 6.76 × 103, and c = −8.3 × 107. The refractive index of oxide YxSiyOz is noticeably lower than that of Y2O3 over the spectral range observed. For comparison, the n(λ)curve (3) calculated using the single homogeneous transparent layer model (Figure 8) indicates very low refractive index values, but a little higher than those of pure SiO2. The significance of our present approach and model can be understood as follows. Several optical models were tested for the Y2O3/Si layered system in the literature. The multilayer stack of (surface roughened Y2O3 layer)−(Y2O3 bulk layer)− (SiO2 interface layer)−(Si substrate) was proposed in ref 54. However, a very strange refractive index dispersion has been found for the Y2O3 oxide formed by Y2O3 target sputtering in the (Ar + O2) mixture.54 The dispersion is very strong over the Figure 7. Dispersive ellipsometric parameters Ψ(λ), Δ(λ): (1) recorded for the layer system Y 2O3/Si. Experiment: points. Calculations: curve. (2) Calculated in homogeneous layer approximation. Dispersive optical constants of Si and Y2O3 were taken from refs 66 and 67, respectively. The inverse ellipsometric problem was solved for compound YxSiyOz with a fixed composition to generate the n(λ) function. It was supposed that the n(λ) function of YxSiyOz falls between the n(λ) functions of SiO2 and Y2O3. The n(λ) functions of Y2O3 and YxSiyOz were approximated by Cauchy’s polynomials 13648 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C ■ spectral range of 200−500 nm and is practically absent at longer wavelengths. The results are not at all in correlation with the available dispersion properties of the Y2O3 bulk.67 In addition, the refractive index of Y2O3 films was found to decrease with (Ar + O2) pressure increase and under better oxidative conditions. The SE method was applied for the determination of optical parameters of oxide films prepared by Y2O3 sputtering in ref 69. For the film formed at 700 °C, the refractive index dispersion was found to be close to that of Y2O3 bulk. However, the optical model used for SE calculations was not reported in detail.69 Gibbson et al. have used the multilayer model for the SE determination of optical parameters of thick Y2O3/Si films prepared by molecular beam epitaxy.70 The model was able to account accurately for the thick MBE grown Y2O3 films on Si. Using real-time monitoring, it was found that interface layer accounting is insignificant for a Y2O3 film thickness of ∼133 nm.70 However, as shown in the present study, the interfacial compound formation and interface properties are principal for thin (∼40 nm) oxide films, where the thickness of the Y2O3 bulk of the film is comparable to the interface layer thickness. In the present study, the appearance of silicon in the Si0 state in the interface layer is detected by XPS depth profiling and verified by SE calculations for consistency. It is interesting to consider possible source(s) of silicon incorporated into interfacial oxide YxSiyOz. A SiO2 layer may be formed on the surface of the silicon substrate due to the contact with the air and oxidative atmosphere in a vacuum chamber. Then, at initial stages of Y target sputtering, the silicon oxide layer was partly reduced to the Si0 state by chemical interaction with active yttrium atoms. The silicon atoms agglomerate into nanocrystals leading to the formation of the YxSiyOz + Si interfacial composite. The agglomeration of silicon atoms in nanocrystals in the oxide host medium is a well-known effect in thin-film formation on Si.71−74 As it appears from our data, this reaction dominates at initial film formation stages. In parallel, the top layer of Y2O3 bulk forms by yttrium target reactive sputtering. Further oxidation of the silicon substrate and Si nanocrystals is provided by oxygen diffused through the Y2O3 top layer, and this mechanism offers the YxSiyOz interfacial oxide formation. Article ASSOCIATED CONTENT S Supporting Information * Survey XPS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (V. V. Atuchin). Phone: +7 (383) 3308889. Fax: +7 (383) 3332771. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS Authors at University of Texas at El Paso acknowledge with pleasure the support from NSF; NSF-PREM grant # DMR1205302. This study is partly supported by the Ministry of Education and Science of the Russian Federation. ■ REFERENCES (1) Wilk, G. D.; Wallace, R. M.; Anthony, J. M. High-k Gate Dielectrics: Current Status and Materials Properties Considerations. J. Appl. Phys. 2001, 89, 5243−5275. (2) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (3) Izyumskaya, N.; Alivov, Y.; Morkoc, H. Oxides, Oxides, and More Oxides: High-k Oxides, Ferroelectrics, Ferromagnetics, and Multiferroics. Crit. Rev. Solid State Mater. Sci. 2009, 34, 89−179. (4) Chambers, S. A. Epitaxial Growth and Properties of Doped Transition Metal and Complex Oxide Films. Adv. Mater. 2010, 22, 219−248. (5) Ramana, C. V.; Atuchin, V. V.; Groult, H.; Julien, C. M. Electrochemical Properties of Sputter-Deposited MoO3 Films in Lithium Microbatteries. J. Vac. Sci. Technol. A 2012, 30, 04D105. (6) Durand, C.; Dubourdieu, C.; Vallée, C.; Loup, V.; Bonvalot, M.; Joubert, O.; Roussel, H.; Renault, O. Microstructure and Electrical Characterizations of Yttrium Oxide and Yttrium Silicate Thin Films Deposited by Pulsed Liquid-Injection Plasma-Enhanced MetalOrganic Chemical Vapor Deposition. J. Appl. Phys. 2004, 96, 1719− 1729. (7) Pan, T.-M.; Liao, K.-M. Structural Properties and Sensing Characteristics of Y2O3 Sensing Membrane for pH-ISFET. Sensors Actuators, B 2007, 127, 480−485. (8) Robertson, J. Maximizing Performance for Higher K Gate Dielectrics. J. Appl. Phys. 2008, 104, 124111. (9) Kittl, J. A.; Opsomer, K.; Popovici, M.; Menou, N.; Kaczer, B.; Wang, X. P.; Adelmann, C.; Pawlak, M. A.; Tomida, K.; Rothschild, A.; et al. High-k Dielectrics for Future Generation Memory Devices. Microelectron. Eng. 2009, 86, 1789−1795. (10) Fukabori, A.; Yanagida, T.; Pejchal, J.; Maeo, S.; Yokota, Y.; Yoshikawa, A.; Ikegami, T.; Moretti, F.; Kamada, K. Optical and Scintillation Characteristics of Y2O3 Transparent Ceramic. J. Appl. Phys. 2010, 107, 073501. (11) Mudavakkat, V. H.; Noor-A-Alam, M.; Kamala Bharathi, K.; AlFaify, S.; Dissanayake, A.; Kayani, A.; Ramana, C. V. Structure and AC Conductivity of Nanocrystalline Yttrium Oxide Thin Films. Thin Solid Films 2011, 519, 7947−7950. (12) Back, M.; Boffelli, M.; Massari, A.; Marin, R.; Enrichi, F.; Riello, P. Energy Transfer Between Tb3+ and Eu3+ in Co-Doped Y2O3 Nanocrystals Prepared by Pechini Method. J. Nanopart. Res. 2013, 15, 1753. (13) Nigara, Y. Measurement of the Optical Constants of Yttrium Oxide. Jpn. J. Appl. Phys. 1968, 7, 404−408. (14) Lee, C. K.; Kim, W. S.; Park, H.-H.; Jeon, H.; Pae, Y. H. Thermal-Stress Stability of Yttrium Oxide as a Buffer Layer of MetalFerroelectric-Insulator-Semiconductor Field Effect Transistor. Thin Solid Films 2005, 473, 335−339. IV. CONCLUSIONS The depth profiling of the Y2O3/Si layer system produced by Ar+ ion bombardment and XPS measurements yields detailed information on the chemical composition and thickness needed for the construction of an adequate optical model of the reflection system. This optical model was applied for the calculations of dispersive optical parameters of the interfacial YxSiyOz oxide formed at the substrate temperature of 300 °C. Now, the results can be used as a basis for accurate SE determination of the Y2O3/Si layer thickness and interface control. Furthermore, the formation of a YxSiyOz + Si composite is found by core level photoemission spectroscopy. As it is supposed, the silicon nanocrystals are generated by chemical reaction of yttrium and the SiO2 layer on the substrate surface resulted in silicon reduction up to the Si0 state. This new effect may be of general significance for high-k dielectrics based on Y and rare-earth elements because of similar chemical properties. It is expected that these observations and model made for Y2O3/Si films may be applicable to a large class of similar layered systems, such as rare-earth oxide films grown on Si, SiGe, and Ge substrates. 13649 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article Characterization of Y2O3 ODS-Fe-Cr Model Alloys. J. Nucl. Mater. 2009, 386−388, 449−452. (34) Fard, H. R.; Becker, N.; Hess, A.; Pashayi, K.; Proslier, T.; Pellin, M.; Borca-Tasciuc, T. Thermal Conductivity of Er+3:Y2O3 Films Grown by Atomic Layer Deposition. Appl. Phys. Lett. 2013, 103, 193109. (35) Yao, J. Q.; He, Y. D.; Wang, D.; Peng, H.; Guo, H. B.; Gong, S. K. Thermal Barrier Coating Binded by (Al2O3-Y2O3)/(Y2O3-Stabilized ZrO2) Laminated Composite Coating Prepared by Two-Step Cyclic Spray Pyrolysis. Corros. Sci. 2014, 80, 37−45. (36) Kern, W. The Evolution of Silicon Wafer Cleaning Technology. J. Electrochem. Soc. 1990, 137, 1887−1892. (37) Itano, M.; Kern, F. W.; Miyashita, M.; Ohmi, T. Particle Removal from Silicon Wafer Surface in Wet Cleaning Process. IEEE Trans. Semicond. Manuf. 1993, 6, 258−267. (38) Kern, W. Overview and Evolution of Silicon Wafer Cleaning; Ch. 1, and Gale, G. W.; Small, R. J.; Reinhardt, K. A. Aqueous Cleaning and Surface Conditioning; Ch. 4. In Handbook of Silicon Wafer Cleaning Technology, 2nd ed.; Reinhardt, K. A., Kern, W., Eds.; William Andrew Publishing: Norwich, NY, 2007. (39) Ramana, C. V.; Atuchin, V. V.; Pokrovsky, L. D.; Becker, U.; Julien, C. M. Structure and Chemical Properties of Molybdenum Oxide Thin Films. J. Vac. Sci. Technol. A 2007, 25, 1166−1171. (40) Ramana, C. V.; Vemuri, R. S.; Kaichev, V. V.; Kochubey, V. A.; Saraev, A. A.; Atuchin, V. V. X-ray Photoelectron Spectroscopy Depth Profiling of La2O3/Si Thin Films Deposited by Reactive Magnetron Sputtering. ACS Appl. Mater. Interfaces 2011, 3, 4370−4378. (41) Atuchin, V. V.; Golyashov, V. A.; Kokh, K. A.; Korolkov, I. V.; Kozhukhov, A. S.; Kruchinin, V. N.; Makarenko, S. V.; Pokrovsky, L. D.; Prosvirin, I. P.; Romanyuk, K. N.; et al. E. Formation of Inert Bi2Se3(0001) Cleaved Surface. Cryst. Growth Des. 2011, 11, 5507− 5514. (42) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; PerkinElmer Corporation, Physical Electronics Division, 6509 Flying Cloud Drive: Eden Prairie, Minnesota 55344, USA, 1992. (43) Rykhlitski, S. V.; Spesivtsev, E. V.; Shvets, V. A.; Prokopiev, V. Yu. Spectral Ellipsometric Complex ELLIPS-1771 SA. Instrum. Exp. Tech 2007, 2, 160−161 (in Russian). (44) Ramana, C. V.; Utsunomiya, S.; Ewing, R. C.; Becker, U.; Atuchin, V. V.; Aliev, V. Sh.; Kruchinin, V. N. Spectroscopic Ellipsometry Characterization of the Optical Properties and Thermal Stability of ZrO2 Films Made by Ion-Beam Assisted Deposition. Appl. Phys. Lett. 2008, 92, 011917. (45) Sarkisov, S. Yu.; Atuchin, V. V.; Gavrilova, T. A.; Kruchinin, V. N.; Bereznaya, S. A.; Korotchenko, Z. V.; Tolbanov, O. P.; Chernyshev, A. I. Growth and Optical Parameters of GaSe:Te Crystals. Russ. Phys. J. 2010, 53, 346−352. (46) Atuchin, V. V.; Kruchinin, V. N.; Wong, Y. H.; Cheong, K. Y. Microstructural and Optical Properties of ZrON/Si Thin Films. Mater. Lett. 2013, 105, 72−75. (47) Kevorkov, A. M.; Karyagin, V. F.; Munchaev, A. I.; Uyukin, E. M.; Bolotina, N. B.; Chernaya, T. S.; Bagdasarov, Kh. S.; Simonov, V. I. Y2O3 Single Crystals: Growth, Structure, and Photoinduced Effects. Kristallografiya 1995, 40, 28−32. (48) Choi, S. C.; Cho, M. H.; Whangbo, S. W.; Whang, C. N. Physical Properties of Y2O3 Films Fabricated by the Reactive Ionized Cluster Beam Deposition Technique. J. Korean Phys. Soc. 1997, 31, 144−148. (49) Craciun, V.; Howard, J.; Lambers, E. S.; Singh, R. K.; Craciun, D.; Perriere, J. Low-Temperature Growth of Y2O3 Thin Films by Ultraviolet-Assisted Pulsed Laser Deposition. Appl. Phys. A: Mater. Sci. Process. 1999, 69 (Suppl.), S535−S538. (50) Hayoz, J.; Bovet, M.; Pillo, T.; Schlapbach, I.; Aebi, P. OxygenSegregation-Control Epitaxy of Y2O3 Films on Nb(110). Appl. Phys. A: Mater. Sci. Process. 2000, 71, 615−618. (51) Craciun, V.; Bassim, N.; Howard, J. M.; Singh, R. K. Pulsed Laser Deposition of Y2O3 on Si: Characteristics of the Interfacial Layer. Proc. SPIE 2002, 4762, 93−98. (15) Ramana, C. V.; Mudavakkat, V. H.; Kamala Bharathi, K.; Atuchin, V. V.; Pokrovsky, L. D.; Kruchinin, V. N. Enchanced Optical Constants of Nanocrystalline Yttrium Oxide Thin Films. Appl. Phys. Lett. 2011, 98, 031905. (16) Mahata, C.; Bera, M. K.; Das, T.; Mallik, S.; Hota, M. K.; Majhi, B.; Verma, S.; Bose, P. K.; Maiti, C. K. Charge Trapping and Reliability Characteristics of Sputtered Y2O3 High-k Dielectrics on N- and Spassivated Germanium. Semicond. Sci. Technol. 2009, 24, 085006. (17) Wang, Z. X.; Xu, H. L.; Zhang, Z. Y.; Wang, S.; Ding, L.; Zeng, Q. S.; Yang, L. J.; Pei, T.; Liang, X. L.; Gao, M.; et al. Growth and Performance of Yttrium Oxide as an Ideal High-k Dielectric for Carbon-Based Electronics. Nano Lett. 2010, 10, 2024−2030. (18) Pearce, S. J.; Parker, G. J.; Charlton, M. D. B.; Wilkinson, J. S. Structural and Optical Properties of Yttrium Oxide Thin Films for Planar Waveguiding Applications. J. Vac. Sci. Technol. A 2010, 28, 1388−1392. (19) Dong, N. N.; Yao, Y. C.; Chen, F.; Vazquez de Aldana, J. R. Channel Waveguides Preserving Luminescence Features in Nd3+:Y2O3 Ceramics Produced by Ultrafast Laser Inscription. Phys. Status Solidi RRL 2011, 5, 184−186. (20) Mudavakkat, V. H.; Atuchin, V. V.; Kruchinin, V. N.; Kayani, A.; Ramana, C. V. Structure, Morphology and Optical Properties of Nanocrystalline Yttrium Oxide (Y2O3) Thin Films. Opt. Mater. 2012, 34, 893−900. (21) Rak, Z.; Ewing, R. C.; Becker, U. Electronic Structure and Thermodynamic Stability of Uranium-Doped Yttrium Iron Garnet. J. Phys.: Condens. Matter 2013, 25, 495502. (22) Quah, H. J.; Cheong, K. Y. Effects of Post-Deposition Annealing Ambient on Chemical, Structural, and Electrical Properties of RF Magnetron Sputtered Y2O3 Gate on Gallium Nitride. J. Alloys Compd. 2013, 575, 382−392. (23) Ko, K.-Y.; Lee, Y. K.; Do, Y. R.; Huh, Y.-D. Structural Effect of a Two-dimensional SiO2 Photonic Crystal Layer on Extraction Efficiency in Sputter-deposited Y2O3:Eu3+ Thin-film Phosphors. J. Appl. Phys. 2007, 102, 013509. (24) Coetsee, E.; Terblans, J. J.; Swart, H. C. Characteristic Properties of Y2SiO5:Ce Thin Films Grown with PLD. Phys. B 2009, 404, 4431−4435. (25) Oh, J. R.; Lee, Y. K.; Park, H. K.; Do, Y. R. Effects of Symmetry, Shape, and Structural Parameters of Two-dimensional SiNx Photonic Crystal on the Extracted Light from Y2O3:Eu3+ Film. J. Appl. Phys. 2009, 105, 043103. (26) Deligne, N.; Lamme, J.; Devillers, M. An Easy Route to Pure and Luminescent Eu-doped YVO4 Polycrystalline Films Based on Molecular or Hybrid Precursors. Eur. J. Inorg. Chem. 2011, 23, 3461− 3468. (27) Dlamini, S. T. S.; Swart, H. C.; Terblans, J. J.; Ntwaeaborwa, O. M. The Effect of Different Gas Atmosphere on the Structure, Morphology and Photoluminescence Properties of Pulsed Laser Deposited Y3(Al,Ga)5O12:Ce3+ Nano Thin Films. Solid State Sci. 2013, 23, 65−71. (28) Yousif, A.; Swart, H. C.; Ntwaeaborwa, O. M.; Coetsee, E. Conversion of Y3(Al,Ga)5O12:Tb3+ to Y2Si2O7:Tb3+ Thin Film by Annealing at Higher Temperatures. Appl. Surf. Sci. 2013, 270, 331− 339. (29) Yousif, A.; Swart, H. C.; Ntwaeaborwa, O. M. Effect of Different Annealing Temperatures on the Properties of Y3(Al,Ga)5O12:Tb Thin Films Grown by PLD. Phys. B 2014, 439, 77−83. (30) Ishibashi, H.; Shimomoto, K.; Nakahigashi, K. Electron Density Distribution and Chemical Bonding of Ln2O3 (Ln = Y, Tm, Yb) from Powder X-ray Diffraction by the Maximum-Entropy Method. J. Phys. Chem. Solids 1994, 55, 809−814. (31) Traina, C. A.; Schwartz, J. Surface Modification of Y2O3 Nanoparticles. Langmuir 2007, 23, 9158−9161. (32) Cha, J. W.; Hwang, S. C.; Lee, E. S. Evaluation of Y2O3 Surface Machinability Using Ultra-Precision Lapping Process with IED. J. Mech. Sci. Technol. 2009, 23, 1194−1201. (33) de Castro, V.; Leguey, T.; Munoz, A.; Monge, M. A.; Pareja, R.; Marquis, E. A.; Lozano-Perez, S.; Jenkins, M. L. Microstructural 13650 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651 The Journal of Physical Chemistry C Article (70) Gibbons, B. J.; Hawley, M. E.; Trolier-McKinstry, S.; Schlom, D. G. Real-Time Spectroscopic Ellipsometry as a Characterization Tool for Oxide Molecular Beam Epitaxy. J. Vac. Sci. Technol. A 2001, 19, 584−590. (71) Kachurin, G. A.; Tyschenko, I. E.; Zhuravlev, K. S.; Pazdnikov, N. A.; Volodin, V. A.; Gutakovsky, A. K.; Leier, A. F.; Skorupa, W.; Yankov, R. A. Visible and Near-Infrared Luminescence from Silicon Nanostructures Formed by Ion Implantation and Pulse Annealing. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 122, 571−574. (72) Kachurin, G. A.; Yanovskaya, S. G.; Volodin, V. A.; Kesler, V. G.; Leier, A. F.; Ruault, M. O. Silicon Nanocrystal Formation upon Annealing of SiO2 Layers Implanted with Si Ions. Semiconductors 2002, 36, 647−651. (73) Kesler, V. G.; Yanovskaya, S. G.; Kachurin, G. A.; Leier, A. F.; Logvinsky, L. M. XPS Study of Ion-Assisted Formation of Si Nanostructures in Thin SiO2 Layers. Surf. Interface Anal. 2002, 33, 914−917. (74) Korchagina, T. T.; Gutakovsky, A. K.; Fedina, L. I.; Neklyudova, M. A.; Volodin, V. A. Crystallization of Amorphous Si Nanoclusters in SiOx Films Using Femtosecond Laser Pulse Annealings. J. Nanosci. Nanotechnol. 2012, 12, 8694−8699. (52) Pan, T.-M.; Lee, J.-D. Influence of Oxygen Content on the Physical and Electrical Properties of Thin Yttrium Oxide Dielectrics Deposited by Reactive RF Sputtering on Si Substrates. J. Electron. Mater. 2007, 36, 1395−1403. (53) Han, M.; Li, X.; Li, B. J.; Shi, N.; Chen, K. J.; Zhu, J. M.; Xu, Z. Synthesis, Characterization, and Physicochemical Properties of WellCoupled Y2O3 Nanobelt-Ag Nanocrystals Nanocomposites. J. Phys. Chem. C 2008, 112, 17893−17898. (54) Yan, F.; Liu, Z. T.; Liu, W. T. Structural and Optical Properties of Yttrium Trioxide Thin Films Prepared by RF Magnetron Sputtering. Vacuum 2011, 86, 72−77. (55) Atuchin, V. V.; Kalabin, I. E.; Kesler, V. G.; Pervukhina, N. V. Nb 3d and O 1s Core Levels and Chemical Bonding in Niobates. J. Electron Spectrosc. Relat. Phenom. 2005, 142, 129−134. (56) Atuchin, V. V.; Kesler, V. G.; Pervukhina, N. V.; Zhang, Z. M. Ti 2p and O 1s Core Levels and Chemical Bonding in Titanium-Bearing Oxides. J. Electron Spectrosc. Relat. Phenom. 2006, 152, 18−24. (57) Ramana, C. V.; Atuchin, V. V.; Kesler, V. G.; Kochubey, V. A.; Pokrovsky, L. D.; Shutthanandan, V.; Becker, U.; Ewing, R. C. Growth and Surface Characterization of Sputter-Deposited Molybdenum Oxide Thin Films. Appl. Surf. Sci. 2007, 253, 5368−5374. (58) Atuchin, V. V.; Kesler, V. G.; Pervukhina, N. V. Electronic and Structural Parameters of Phosphorus-Oxygen Bonds in Inorganic Phosphate Crystals. Surf. Rev. Lett. 2008, 15, 391−399. (59) Ramana, C. V.; Atuchin, V. V.; Becker, U.; Ewing, R. C.; Isaenko, L. I.; Khyzhun, O. Yu.; Merkulov, A. A.; Pokrovsky, L. D.; Sinelnichenko, A. K.; Zhurkov, S. A. Low-Energy Ar+ Ion-BeamInduced Amorphization and Chemical Modification of Potassium Titanyl Arsenate (001) Crystal Surfaces. J. Phys. Chem. C 2007, 111, 2702−2708. (60) Atuchin, V. V.; Pokrovsky, L. D.; Khyzhun, O. Yu.; Sinelnichenko, A. K.; Ramana, C. V. Surface Crystallography and Electronic Structure of Potassium Yttrium Tungstate. J. Appl. Phys. 2008, 104, 033518. (61) Himpsel, F. J.; McFeely, F. R.; Taleb-Ibrahimi, A.; Yarmoff, J. A.; Hollinger, G. Microscopic Structure of the SiO2/Si Interface. Phys. Rev. B 1988, 38, 6084−6096. (62) Suwa, T.; Teramoto, A.; Kumagai, Y.; Abe, K.; Li, X.; Nakao, Y.; Yamamoto, M.; Nohira, H.; Muro, T.; Kinoshita, T.; et al. Chemical Structure of Interfacial Transition Layer Formed on Si(100) and Its Dependence on Oxidation Temperature, Annealing in Forming Gas, and Difference in Oxidizing Species. Jpn. J. Appl. Phys. 2013, 52, 031302. (63) Le, T. T. U.; Sasahara, A.; Tomitori, M. Water Wettability of an Ultrathin Layer of Silicon Oxide Epitaxially Grown on a Rutile Titanium Dioxide (110) Surface. J. Phys. Chem. C 2013, 117, 23621− 23625. (64) Ogieglo, W.; Wormeester, H.; Wessling, M.; Benes, N. E. Spectroscopic Ellipsometry Analysis of a Thin Film Composite Membrane Consisting of Polysulfone on a Porous α-Alumina Support. ACS Appl. Mater. Interfaces 2012, 4, 925−943. (65) Ramana, C. V.; Baghmar, G.; Rubio, E. J.; Hernandez, M. J. Optical Constants of Amorphous, Transparent Titanium-Doped Tungsten Oxide Thin Films. ACS Appl. Mater. Interfaces 2013, 5, 4659−4666. (66) Adachi, S. Optical Constants of Crystalline and Amorphous Semiconductors; Kluwer Academic Publishers, Boston/Dordrecht/ London, 1999. (67) Kaminskii, A. A.; Ueda, K.; Konstantinova, A. F.; Yagi, H.; Yanagitani, T.; Butashin, A. V.; Orekhova, V. P.; Lu, J.; Takaichi, K.; Uematsu, T.; et al. Lasing and Refractive Indices of Nanocrystalline Ceramics of Cubic Yttrium Oxide Y2O3 Doped with Nd3+ and Yb3+. Cryst. Rep. 2003, 48, 1041−1043. (68) Handbook of ellipsometry; Tompkins, H. G., Irene, E. A., Eds.; William Andrew Publishing, Springer: New York, 2005. (69) Gaboriaud, R. J.; Pailloux, F.; Guerin, P.; Paumier, F. Yttrium Oxide Thin Films, Y2O3, Grown by Ion Beam Sputtering on Si. J. Phys. D: Appl. Phys. 2000, 33, 2884−2889. 13651 dx.doi.org/10.1021/jp502876r | J. Phys. Chem. C 2014, 118, 13644−13651