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(1-7) Among others, lanthanum oxide, La2O3, is interesting because of its versatility and multifunctionality. Specifically, lanthanum-based compounds ...
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J. Phys. Chem. C 2009, 113, 2911–2918

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Microstructural and Optical Properties Modifications Induced by Plasma and Annealing Treatments of Lanthanum Oxide Sol-Gel Thin Films L. Armelao,*,† M. Pascolini,† G. Bottaro,*,‡ G. Bruno,‡ M. M. Giangregorio,‡ M. Losurdo,‡ G. Malandrino,§ R. Lo Nigro,| M. E. Fragala`,§ and E. Tondello⊥ ISTM-CNR and INSTM, Department of Chemistry, PadoVa UniVersity, Via Marzolo, 1-35131 PadoVa, Italy, IMIP-CNR and INSTM, Department of Chemistry, Bari UniVersity, Via Orabona, 4-70126 Bari, Italy, Department of Chemical Sciences, Catania UniVersity and INSTM, Viale A. Doria, 6-95125 Catania, Italy, IMM-CNR, Stradale Primosole, 50-95121 Catania, Italy, and Department of Chemistry, PadoVa UniVersity and INSTM, Via Marzolo, 1-35131 PadoVa, Italy ReceiVed: NoVember 7, 2008; ReVised Manuscript ReceiVed: December 23, 2008

Lanthanum oxide (La2O3) thin films have been prepared on Si(100) substrates through a sol-gel process from the lanthanum methoxyethoxide (La(OCH2CH2OCH3)3) precursor. To study the effects of different postdeposition treatments on film microstructure and optical properties, the as-deposited layers have been annealed at different temperatures between 200 and 700 °C in air or forming gas (H2 10% in N2) for 1 h. Low-temperature (300 °C) remote O2 plasma processing has also been applied to both as-deposited and previously annealed samples. Films have been fully characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), glancing incidence X-ray diffraction (GIXRD), and atomic force microscopy (AFM). In particular, microstructure and optical properties correlations were accomplished by exploiting spectroscopic ellipsometric investigation. It has been found that thermal annealing at temperatures around 500 °C leads to subcutaneous oxidation of the Si substrate resulting in the formation of a SiO2 layer, and annealing at higher temperature (700 °C) also results in film-substrate intermixing and formation of a lanthanum silicate layer. At variance, these interfacial reactions can be suppressed by low-temperature (300 °C) remote O2 plasma processing of as-deposited films, and optical transparency in the visible range can be strongly improved. 1. Introduction Recently, rare earth (RE) oxides are being intensively investigated due to their excellent chemical, thermal, optical, and electrical properties.1-7 Among others, lanthanum oxide, La2O3, is interesting because of its versatility and multifunctionality. Specifically, lanthanum-based compounds are important in the development of ferroelectric materials. In particular, La-doped Bi4Ti3O12 (BLT) and (PbxZr1-x)TiO3 represent new materials for nonvolatile ferroelectric memories (NVFeRAM).8-13 Moreover, (LaxA1-x)MnO3 (A ) Sr, Ca, Ba,...) systems show colossal magnetoresistance properties (CMR).14-16 La2O3 thin films have also gained attention as a potential high-permittivity dielectric material in complementary metal-oxide-semiconductor (CMOS) devices.17-19 The dielectric permittivity values of La2O3 are in the range of 20-23.20 Lanthanum oxide has been reported to have a large band gap (4.3 eV) and low lattice energy while exhibiting excellent electrical properties.21,22 In addition, La2O3 also has various optical applications such as in IR-transmitting glass ceramics and as an additive to various transparent ceramic laser materials to improve their optical properties.23,24 Furthermore, La2O3-based glasses have been considered as an ideal material for broad-band optical fiber amplifiers.25 Finally, as La2O3 films are photoactive, they have been used as a photo* Corresponding authors. E-mail: [email protected] (L.A.); [email protected] (G.B.). † ISTM-CNR and INSTM, Department of Chemistry, Padova University. ‡ IMIP-CNR and INSTM, Bari University. § Department of Chemical Sciences, Catania University. | IMM-CNR, Catania ⊥ Department of Chemistry, Padova University.

electrode.26 For all those optical and high-κ applications, investigation and optimization of optical properties and their dependence on synthesis and processing conditions represent relevant issues. With respect to this, the complex refractive index, N ) n + ik (where n is the real refractive index and k is the extinction coefficient related to the absorption coefficient, R ) 4πk/λ), is a useful parameter to investigate, since it is linked to other main characteristic of the materials and thin films such as the complex dielectric function, ε(ω) ) ε1(ω) + iε2(ω) ) N2, the dielectric constant, κ, the optical band gap, Eg, and the film density, F. Specifically, it is known that there is a relationship between the refractive index and the optical band gap which has been demonstrated for many materials, and in particular the refractive index decreases with the increase of the band gap according to

n2 - 1 )

4πNe2p2 m/Eg2

where N is the valence band electronic density per unit volume, m* is the effective mass, and h is the Planck’s constant.27 Refractive index (or dielectric constant) and density, being two fundamental materials properties, have been extensively investigated by researchers in diverse disciplines for various purposes. The overall increase of refractive index with increasing density has been well established in literature. Therefore, a way to report and monitor a density variation upon film processing is based on the measurement of the refractive index (n) values as a function of processing conditions and on the application of the Lorentz-Lorenz relation to relate n to density28

10.1021/jp809824e CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

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(n2 - 1) FR ) 3 (n2 + 2) (R being in this expression the polarizability). In this frame, it is important to point out that for various oxide films a higher refractive index and, hence, a higher density (i.e., a more compact structure) are known to be indicative of better barrier properties (e.g., to moisture, etc.). The experimental measurement of the refractive index, and of optical properties in general, is worthy of further comments for La2O3 thin films especially in relation to the deposition methodology. In fact, various deposition techniques, such as chemical vapor deposition, sputtering, thermal oxidation, sol-gel, etc., are being exploited for the synthesis of good quality La2O3 films, which is not a straightforward task.29-32 The main problem is related to the relatively large carbon residue and hydroxyls content in the films, likely due to both a noncomplete precursor decomposition and to the strong tendency of rare earth sesquioxides to adsorb water vapor and carbon dioxide from the atmosphere. In order to obtain better chemical purity and a higher degree of crystallinity, thermal treatments at high temperatures in oxidizing atmospheres (O2 and/or ozone) are commonly employed.30-33 With respect to this, the monitoring of the refractive index variation, possibly in real time during processing, represents an effective way of optimizing processing and annealing treatments of oxide thin films. Furthermore, concerning the growth of metal oxide layers on silicon substrates (e.g., as high-κ films in CMOS), it should also be considered that high-temperature annealing treatments might be responsible for detrimental effects on functional performances because of the formation of interface layers (mostly SiO2) between the high-κ films and the silicon substrate. Thus, to reduce undesired interfacial reactivity, alternative lowtemperature processing methods are needed, aimed at obtaining pure materials with tailored composition and structure and, at the same time, at reducing side effects such as the formation of interlayers due to reactions between films and substrates. In this framework, the present study aims at exploring lowtemperature (300 °C) O2 plasma processing of lanthanum oxide sol-gel thin films. The comparison with conventional thermal annealing processes is carried out to point at the O2 plasma treatment as a valuable low-temperature “chemical annealing” to improve material properties while controlling interfacial reactions. To this aim, the effects on microstructure, composition, and optical properties of the La2O3 thin films induced either by high-temperature annealing performed in air or forming gas or by O2 plasma treatment are analyzed and compared. Lanthanum oxide thin films were prepared by sol-gel dipcoating on (100) silicon substrates starting from alcoholic solutions of lanthanum methoxyethoxide (La(OMT)3, La(OCH2CH2OCH3)3). The films were subsequently heated in air or in forming gas (10% H2 in N2) from 200 up to 700 °C for 1 h. To study the feasibility of a milder alternative annealing processing, both as-deposited and previously annealed films were also subjected to low-temperature (300 °C) O2 remote plasma treatments in order to improve the quality of the films by exploiting the peculiar properties of finite lifetime metastable species and reactive species such as atomic oxygen and singlet ∆ oxygen.34 The chemical composition and microstructural evolution of the samples as a function of the annealing conditions were studied by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and glancing incidence X-ray diffraction (GIXRD), whereas variations in surface morphology were investigated by atomic force microscopy (AFM). Finally, the influence of the annealing conditions

Armelao et al. on the chemical composition and structural properties of the systems has been investigated and correlated to the optical characteristics by in situ ellipsometric studies. It is worth highlighting that remote O2 plasma processing has never been adopted on La-O-based films, and only few reports on its application for oxide-based materials are available in the literature.35-39 To this regard, some patents dealing with the “plasma crystallization process” are reported, but few experimental details are therein described. 2. Experimental Section 2.1. Precursor Solution and Film Deposition. Lanthanum methoxyethoxide was used as the starting compound. The precursor solutions were obtained by diluting La(OMT)3 (ABCR, solution 10-12 wt % in methoxyethanol) with anhydrous ethyl alcohol (C2H5OH, Carlo Erba 99.8%) and adding a controlled amount of deionized water; the final La/ EtOH/H2O molar ratio was 1:2.5:3 (CLa2O3 ) 25 g/L). The precursor solutions were stirred at room temperature for at least 6 h before film deposition. Due to the sensitivity of lanthanum alkoxides toward moisture, controlled amounts of water were added to the starting sols to promote hydrolysis. Both solution preparation and film deposition were carried out under inert atmosphere in a glovebox. Films were prepared by a dip-coating procedure with a controlled withdrawal speed of 25 cm min-1 using n-type (100) silicon wafer (1 × 2 cm2) as a substrate. Before use, silicon slides were cleaned ultrasonically using acetone and isopropyl alcohol. The as-deposited samples that resulted were homogeneous, well-adherent to the substrates, and crack-free. The films were subsequently annealed in air or forming gas (10% H2 in N2) at temperatures between 200 and 700 °C for 1 h. 2.2. Plasma Processing. As-deposited and previously annealed (700 °C in air or forming gas) films were also subjected to a remote O2 plasma processing for 30 min at 300 °C (P ) 0.06 mbar, rf power (13.56 MHz) ) 20 W), in order to study the effects of this peculiar chemical annealing procedure on film composition, microstructure, and optical properties. 2.3. Sample Characterization. The structure and surface morphology of lanthanum oxide thin films were characterized by GIXRD, TEM, and AFM. X-ray diffraction patterns were recorded by means of a Bruker D8 Advance diffractometer equipped with a Go¨bel mirror and a Cu ΚR source (40 kV, 40 mA) in a glancing incidence geometry at a fixed angle of 0.5°. Irrespective of the annealing conditions, the diffraction patterns did not show any sharp peak, suggesting an amorphous or nanocrystalline character of the samples, also with a low degree of structural order because of carbon contamination (see below). TEM characterization was performed on a Hitachi S4500 FE microscope using a 200 keV electron beam. AFM measurements were run in noncontact mode using an AutoProbe CP thermomicroscope; a gold-coated Si cantilever with a resonant frequency of 80 kHz was used. The samples chemical composition was investigated by XPS analysis. Measurements were performed on a Perkin-Elmer Φ 5600ci spectrometer using a monochromatized Al KR radiation (1486.6 eV), at a working pressure lower than 10-9 mbar. The specimens, mounted on steel sample holders, were introduced directly into the XPS analytical chamber by a fast entry lock system. Survey scans were run in the 0-1350 eV range. Detailed spectra were recorded for the following regions: La3d, O1s, C1s, Si2s. The reported binding energies (BEs, standard deviation ) (0.2 eV) were corrected for charging effects assigning to the adventitious C1s line a BE of 284.8 eV.40 The analysis

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involved Shirley-type background subtraction, and whenever necessary, spectral deconvolution which was carried out by nonlinearleast-squarescurvefitting,adoptingaGaussian-Lorentzian sum function. The atomic composition of the samples was calculated by peak integration, using sensitivity factors provided by the spectrometer manufacturer (Φ V5.4A software) and taking into account the geometric configuration of the apparatus. Quantification of Si was performed employing the Si2s peak instead of the most intense Si2p line, because of its severe spectral overlap with the La4d photoelectron peak. Depth profiles were carried out by Ar+ sputtering at 2.5 kV and 0.5 mA cm-2 beam current density, with an argon partial pressure of 5 × 10-8 mbar. Finally, optical characterization was performed by spectroscopic ellipsometry. Measurements were carried out in the range of 0.75-6.5 eV with a phase-modulated spectroscopic ellipsometer (UVISEL-Jobin Yvon) at an angle of incidence of 70° to determine film thickness and optical properties, namely, the refractive index, n, and the absorption coefficient, R, which is related to the extinction coefficient, k, by the relationship R ) 4πk/λ. For each sample various points were tested to verify uniformity. Specifically, ellipsometry measures the ratio (F) of the Fresnel reflection coefficient of the p-polarized (parallel to the plane of incidence of the linearly polarized light beam) and s-polarized (perpendicular to the plane of incidence) light reflected from the surface through the ellipsometric angles Ψ and ∆ defined by the equation

Figure 1. TEM cross-sectional image of the interface structure of lanthanum oxide films on Si(001) substrates annealed at 500 °C for 1 h.

F ) tan Ψ exp(i∆) where tan Ψ ) |Ep|/|Es| and ∆ ) δp - δs represent the amplitude and phase variation of the electric field vector associated with the light electromagnetic wave. F and, hence, Ψ and ∆ are related to the film dielectric function, ε ) ε1 + iε2, and complex refractive index, N ) (n + ik), through the equation

ε ) ε1 + iε2 ) N2 ) (n + ik)2 ) sin2 φ[1 + tan2 φ(1 F)2/(1 + F)2] where φ is the angle of incidence. To derive the spectral dependence of n and R of La2O3 samples from the experimental ellipsometric spectra, a threelayer model was adopted for the films consisting of a double interface layer (as detected by TEM) and of the La2O3 film. Surface roughness was also modeled with a top layer, considered as composed by voids (50%) and the lanthanum oxide film (50%). The energy dispersion of optical properties was parametrized using a single Lorentzian oscillator41 for data analysis

N2 ) (n + ik)2 ) ε ) ε1 + iε2 ) ε∞ +

Ajωj2 ωj2 - ω2 - iγjω

where ε∞ is the high-frequency dielectric constant, ωj, γj, and Aj are the frequency, width, and strength of the j oscillator representing the main optical transition, whose peak energy is outside the experimental range for La2O3. Ellipsometry was also used to acquire spectra as a function of time during various annealing processes and to describe the dynamic of structural-optical modifications. 3. Results and Discussion The structure of the films and the nature of the film-substrate interface were at first characterized by TEM imaging. The crosssectional TEM image of the film annealed in air at 500 °C for

Figure 2. TEM cross-sectional image (a) of the interface structure of lanthanum oxide films on Si(100) substrates annealed at 700 °C for 1 h and its magnification (b).

1 h (see Figure 1) shows an amorphous character. The thickness is uniform, and it has been evaluated to be about 50 nm. Moreover, at the interface with the Si substrate the formation of a thin silicon oxide layer (few nanometers) is clearly observed, whose chemical nature was confirmed to be SiO2 from XPS analysis. At variance, the sample annealed in air at 700 °C for 1 h shows a more complex interface structure (Figure 2a). First, the interfacial SiO2 layer appears thicker (≈10 nm) than in the previous case as a consequence of the higher annealing temperature. Concerning the lanthanum oxide film, ca. 40 nm thick, it displays a small degree of crystallinity and it is likely formed of La2O3-based grains characterized by low dimensionality. Furthermore, as evidenced by the TEM image magnification of the interface region (see Figure 2b), the presence of a further intermediate layer (10 nm) with noncrystalline structure can be clearly seen. It is likely that higher annealing temperatures promote chemical interactions between the film and the silicon substrate, thus leading to the formation of an amorphous lanthanum silicate layer, as also supported by XPS data (see in the below). At this regard, the formation of multiple RE-O/RE-Si-O/SiO2 layer stacks is a well-known phenomenon which has been already observed with other RE elements such as Gd and Pr42-45 as well as in La2O3 thin films

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Figure 3. XPS depth profile for the sample annealed in air at 700 °C for 1 h. In order to better evidence the in-depth distribution of La, O, and Si, quantification was done omitting carbon.

deposited on silicon by both physical vapor deposition (PVD) and chemical vapor deposition (CVD) processes.46 As evidenced by XPS analysis, in addition to oxygen and lanthanum, the annealed samples contain carbon throughout the whole thickness down to the film-substrate interface. Carbon quantification was not easy due to the partial spectral overlap between the C1s line and the high binding energy tail of the La4s photoelectron peak. This effect was further enhanced by the low intensity and the broadness of the two peaks especially in the proximity of the film-substrate interface region. However, at 700 °C carbon was still present in amounts of ca. 10-15 atom %. Figure 3 shows the XPS depth profile for the sample annealed in air at 700 °C for 1 h. Three different regions can be identified: (I) the film surface, (II) the La2O3 layer previously evidenced by TEM analysis, and (III) a third deeper interlayer associated with the film/substrate interface. It is worth noticing as the surface is characterized by an O/La ratio of ca. 3, whereas lanthanum and oxygen in-depth distributions show a parallel profile and the O/La ratio is closer to 1.5. A similar depth profile was also detected for the sample annealed at 700 °C in forming gas which was, however, characterized by a slightly lower oxygen content. In order to optimize any processing methodology for sol-gel deposited thin films, a nondestructive and noninvasive real-time diagnostic would be desirable to monitor in-line an intrinsic film parameter that relates to film microstructure and properties. With respect to this, probing the complex refractive index by spectroscopic ellipsometry is a valuable approach. In order to validate and set the ellipsometric approach and analysis, we have made a comparison of the ellipsometric measurements with TEM and XPS results. Figure 4 shows the experimental and best-fit ellipsometric spectra of the real 〈ε1〉 and imaginary 〈ε2〉 parts of the pseudodielectric function for the samples heated in air at 500 and 700 °C for 1 h (the same samples where TEM analysis was run). Ellipsometric measurements clearly support a multilayer structure for the annealed films. The best-fit models show the layered structure of the films. In particular, a SiO2 interface layer with a thickness of 5 nm is found for the 45 nm La2O3 film upon 500 °C annealing in air. For the 700 °C annealed sample, the best fit has been achieved with two interface layers, i.e., a SiO2 (11 nm) layer followed by a higher refractive index layer (16 nm) attributed to the formation of lanthanum silicate, and a La2O3 film on top 30 nm thick. It is worth noticing the agreement between ellipsometric data and TEM results concerning the thickness of the different layers. Hence, once the ellipsometric analysis approach was validated with TEM analysis, in situ ellipsometry has been used to

Figure 4. Experimental (dots) and best-fit (lines) ellipsometric spectra of the real, 〈ε1〉, and imaginary, 〈ε2〉, parts of the pseudodielectric function of the samples annealed in air for 1 h at (a) 500 and (b) 700 °C. The best-fit models showing the layered structure of films are also reported.

investigate the microstructural changes of the films as a function of the annealing conditions and to correlate them to the optical characteristics. Figure 5 shows the dependence on annealing temperature and atmosphere of the thickness of the interface and La2O3 layers and of the value of the La2O3 refractive index at 2.0 eV. The decrease of La2O3 films thickness and the increase of the refractive index are observed with increasing the treatment temperature. Such variations are associated to the densification and crystallization of the film structure because of the progressive removal of species such as water and solvent molecules as well as of carbon residuals deriving from the lanthanum methoxyethoxide precursor. The thickness decrease is more pronounced when annealing is performed in air than in forming gas, likely due to a more efficient oxidation of carbon residuals and their removal as CO2. Furthermore, a continuous increase of the La2O3 refractive index is found when samples are annealed in forming gas, consistently with data from He et al.47 who reported a variation of n from 1.836 to 1.970 upon annealing of La2O3 films deposited by atomic layer deposition at 400 °C in forming gas. Conversely, when annealing is performed in air a maximum in the refractive index is found at 500 °C (Figure 5), as can also be seen in the energy dispersion spectra shown in Figure 6; a further increase in the temperature leads to a decrease of the refractive index in the presence of oxygen. Various arguments can sustain and explain this trend for annealing in air, e.g., as observed by XPS a further increase of temperature

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Figure 5. Dependence on annealing temperature and atmosphere of the thickness values (a) of the interface layers SiO2 and La-Si-O and (b) of the La2O3 layer thickness and refractive index at 2.0 eV.

Figure 7. Experimental spectra of the imaginary part, 〈ε2〉, of the pseudodielectric function of (a) a sol-gel as-deposited film, (b) after annealing in air at 300 °C, (c) after O2 plasma processing at 300 °C, and (d) after a high-temperature 700 °C air annealing followed by an additional O2 plasma at 300 °C.

Figure 6. Energy dispersion of (a) the refractive index and (b) absorption coefficient of La2O3 films annealed in air and in forming gas at 500 and 700 °C.

in presence of oxygen could yield the formation of lanthanum oxycarbonates competing with CO2 removal48 and a larger film-substrate intermixing not only due to oxygen in-diffusion but also to silicon out-diffusion. Both effects induce a more severe La2O3 contamination which also inhibits structural ordering. Focusing on the formation of the interface layer, annealing in air yields a SiO2 interface thicker than annealing in forming

gas, which does not promote SiO2 formation up to 400 °C. Furthermore, in both cases, a very thin SiO2 layer is the only interface formed up to a temperature of 500 °C. Above 500 °C, the formation of an additional La-Si-O interface layer is observed when annealing is performed in air. Thus, from data in Figures 4-6, it can be inferred that annealing in air at T > 500 °C enhances the interdiffusion of oxygen and the subcutaneous oxidation of the Si substrate, and the out-diffusion of Si thus yields, in addition to interfacial SiO2, the intermixed silicate layer and a more disordered film, in microstructure and composition, with lower refractive index and higher absorption coefficient. To investigate milder alternative

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Figure 8. Energy dispersion of the refractive index, n, and of the absorption coefficient, R, of (a) La2O3 films as-deposited, after a thermal annealing at 300 °C in air for 1 h and after O2 plasma exposure at 300 °C. (b) La2O3 films annealed in air and in forming gas at 500 and 700 °C and exposed for 30 min at 300 °C to a remote O2 plasma. The optical spectra for a film directly exposed to the same O2 plasma without any preliminary thermal annealing are also shown.

film-processing methods, freshly as-deposited films have been directly exposed to a low-temperature (300 °C) remote O2 plasma for 30 min. A remote configuration for the plasma treatment has been adopted in order to avoid any electron bombardment and radiative damage and/or etching of the layers. For comparison, half of the same film was simply annealed in air at 300 °C for 1 h and the same plasma treatment was also adopted on the samples already annealed at 700 °C. Figure 7 compares the experimental ellipsometric spectra of the imaginary part 〈ε2〉 ) 2nk of the pseudodielectric function for (a) an asdeposited sol-gel film (before any treatment), (b) after annealing in air at 300 °C, (c) after O2 plasma processing at 300 °C, and (d) after a high-temperature 700 °C air annealing followed by an additional O2 plasma at 300 °C. The observed changes in the spectra are due to both variations of film thickness and optical response. As for the film thickness, the approximately 110-120 nm thickness of the as-deposited film decreases to 78-80 nm at a temperature of 300 °C (as also shown in Figure 5) for both the air annealed and O2 plasma exposed samples. As for the optical properties, the calculated spectral dependence of the refractive index (n) and of the absorption coefficient (R) are compared in Figure 8. As shown in Figure 8, parts a and b, the O2 plasma processing clearly affects the optical properties of the layers. In particular, an increase of the refractive index, which might be indicative of a more compact and ordered microstructure, is found, presumably steaming from a structural reordering promoted by the further release of the carbon and moisture residual impurities upon reaction with oxygen atoms. This hypothesis and the formation

Figure 9. Reproducibility test. Experimental spectra of the imaginary part, 〈ε2〉, of the pseudodielectric function for (a) three different asdeposited sol-gel films, (b) two different samples after O2 plasma processing at 300 °C, and (c) two different samples after hightemperature air annealing. Small differences are due to slightly different film thickness.

of a better ordered microstructure are also consistent with the observed decrease of the extinction coefficient (k), and of absorption coefficient (R ) 4πk/λ) upon O2 plasma exposure as indicated in Figure 8. Therefore, the O2 plasma also results in the improvement in film transparency in the visible range for all films, but importantly, the highest transparency and refractive index are measured for films directly exposed to the low-temperature O2 plasma without any previous high-temperature thermal annealing. A further important advantage is that no interfacial reactions and subcutaneous oxidation of the silicon substrate are induced at such a low temperature. It is also worthy that all the investigated treatments yield reproducible results when starting from similar samples. Figure 9 shows the 〈ε2〉 experimental spectra obtained for various films either asdeposited or after the different treatments. The observed differences in the spectra measured for three as-deposited samples are mostly due to variations in film thickness. On the other hand, both the O2 plasma processing and high-temperature annealing yield reproducible results for 〈ε2〉 values determined on various sol-gel films. The positive effect of the low-temperature plasma processing in improving the transparency as a consequence of the improved

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J. Phys. Chem. C, Vol. 113, No. 7, 2009 2917 deposited and previously annealed films results in the increase of the refractive index and decrease of the extinction coefficient, probably due to an active oxygen removal of carbon contamination and structural reordering of films. Importantly, the highest transparency and refractive index are measured for as-deposited films directly exposed to plasma. In this case, the employment of low processing temperatures prevents interfacial reactions and the “chemical annealing effect” exerted by the activated oxygen species allows a better structural order, saturation of oxygen vacancies, dangling bonds, and grain boundaries, leading to an actual improvement in film transparency. It is worth highlighting that the proposed synthesis of lanthanum oxide layers, based on the innovative combination of the mild sol-gel route and O2 plasma processing, can be conveniently optimized and extended to various oxide-based films to prepare materials and nanostructures of technological interest with tailored functional properties.

Figure 10. 500 nm × 500 nm AFM topographies of La2O3 films annealed in air and in forming gas at 500 and 700 °C and subsequently exposed for 30 min at 300 °C to a remote O2 plasma and of a film directly exposed to the same O2 plasma without any preliminary thermal annealing. Surface roughness values (rms ) root-mean-square roughness) are also indicated.

Acknowledgment. This work was financially supported by research projects CNR-INSTM PROMO, FIRB-MIUR RBNE033KMA “Molecular Compounds and Hybrid Nanostructured Materials with Resonant and Nonresonant Optical Properties for Photonic Devices”, FISR-MIUR “Inorganic Hybrid Nanosystems for the Development and the Innovation of Fuel Cells”, and CARIPARO 2006 “Multi-Layer Optical Devices Based on Inorganic and Hybrid Materials by Innovative Synthetic Strategies”. The authors also acknowledge the 7th FP European Project NanoCharM (Multifunctional NanoMaterials Characterization Exploiting Ellipsometry and Polarimetry) (NMP3-CA-2007-218570). References and Notes

structural order of films can also be seen in the morphological modification detected by AFM and shown in Figure 10. The film morphology clearly changes from amorphous to nanocrystalline upon plasma processing, the grain size being dependent on overall processing. These observed effects of the lowtemperature O2 plasma treatment can be explained considering that active oxygen species might etch disordered regions resulting in a better structural quality; furthermore, the plasmaactivated oxygen species impinging on the growing surface release a surplus of energy and operate a “chemical annealing effect”, i.e., an increase of mobility of atoms that can rearrange in a better structural order, as already demonstrated for ZnO thin films.49 Additionally, they saturate oxygen vacancies, dangling bonds, and grain boundaries, leading to an actual improvement in film transparency. 4. Conclusions Lanthanum oxide thin films have been prepared through a sol-gel process on Si(100) substrates from ethanolic solutions of the La(OCH2CH2OCH3)3 precursor. As-deposited films have been either conventionally annealed at different temperatures between 200 and 700 °C in air or forming gas (H2 10% in N2) for 1 h or subjected to a low-temperature (300 °C) remote O2 plasma processing. The same O2 plasma processing was also applied to previously annealed layers in order to correlate microstructural and optical properties modifications to postdeposition treatments. TEM and ellipsometry analysis revealed that under 500 °C thermal annealing oxidation of the Si substrate occurs with the growth of a silica layer and heating at higher temperature (700 °C) results in film-substrate reactions and in the formation of a lanthanum silicate layer. At variance, lowtemperature (300 °C) remote O2 plasma processing on as-

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