Subscriber access provided by UNIV OF PADOVA

The Journal of Physical Chemistry C is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036

Article

Microstructural and Optical Properties Modifications Induced by Plasmaand 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

J. Phys. Chem. C, 2009, 113 (7), 2911-2918• Publication Date (Web): 27 January 2009

Downloaded from http://pubs.acs.org on February 13, 2009

More About This Article

Additional resources and features associated with this article are available within the HTML version:

• Supporting Information• Access to high resolution figures

Subscriber access provided by UNIV OF PADOVA

The Journal of Physical Chemistry C is published by the American ChemicalSociety. 1155 Sixteenth Street N.W., Washington, DC 20036

• Links to articles and content related to this article• Copyright permission to reproduce figures and/or text from this article

Microstructural and Optical Properties Modifications Induced by Plasma and AnnealingTreatments 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 andINSTM, 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 processfrom the lanthanum methoxyethoxide (La(OCH2CH2OCH3)3) precursor. To study the effects of differentpostdeposition treatments on film microstructure and optical properties, the as-deposited layers have beenannealed 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 andpreviously 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 correlationswere accomplished by exploiting spectroscopic ellipsometric investigation. It has been found that thermalannealing at temperatures around 500 °C leads to subcutaneous oxidation of the Si substrate resulting in theformation of a SiO2 layer, and annealing at higher temperature (700 °C) also results in film-substrateintermixing and formation of a lanthanum silicate layer. At variance, these interfacial reactions can besuppressed by low-temperature (300 °C) remote O2 plasma processing of as-deposited films, and opticaltransparency in the visible range can be strongly improved.

1. Introduction

Recently, rare earth (RE) oxides are being intensivelyinvestigated due to their excellent chemical, thermal, optical,and electrical properties.1-7 Among others, lanthanum oxide,La2O3, is interesting because of its versatility and multifunc-tionality. Specifically, lanthanum-based compounds are impor-tant in the development of ferroelectric materials. In particular,La-doped Bi4Ti3O12 (BLT) and (PbxZr1-x)TiO3 represent newmaterials for nonvolatile ferroelectric memories (NVFeRAM).8-13

Moreover, (LaxA1-x)MnO3 (A ) Sr, Ca, Ba,...) systems showcolossal magnetoresistance properties (CMR).14-16 La2O3 thinfilms have also gained attention as a potential high-permittivitydielectric 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 reportedto have a large band gap (4.3 eV) and low lattice energy whileexhibiting excellent electrical properties.21,22 In addition, La2O3

also has various optical applications such as in IR-transmittingglass ceramics and as an additive to various transparent ceramiclaser materials to improve their optical properties.23,24 Further-more, La2O3-based glasses have been considered as an idealmaterial for broad-band optical fiber amplifiers.25 Finally, asLa2O3 films are photoactive, they have been used as a photo-

electrode.26 For all those optical and high-κ applications,investigation and optimization of optical properties and theirdependence on synthesis and processing conditions representrelevant issues.

With respect to this, the complex refractive index, N ) n +ik (where n is the real refractive index and k is the extinctioncoefficient related to the absorption coefficient, R ) 4πk/λ), isa useful parameter to investigate, since it is linked to other maincharacteristic of the materials and thin films such as the complexdielectric function, ε(ω) ) ε1(ω) + iε2(ω) ) N2, the dielectricconstant, κ, the optical band gap, Eg, and the film density, F.Specifically, it is known that there is a relationship betweenthe refractive index and the optical band gap which has beendemonstrated for many materials, and in particular the refractiveindex 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, beingtwo fundamental materials properties, have been extensivelyinvestigated by researchers in diverse disciplines for variouspurposes. The overall increase of refractive index with increasingdensity has been well established in literature. Therefore, a wayto report and monitor a density variation upon film processingis based on the measurement of the refractive index (n) valuesas a function of processing conditions and on the applicationof the Lorentz-Lorenz relation to relate n to density28

* Corresponding authors. E-mail: lidia.armelao@unipd.it (L.A.);gregorio.bottaro@cnr.it (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.

J. Phys. Chem. C 2009, 113, 2911–2918 2911

10.1021/jp809824e CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/27/2009

(n2 - 1)

(n2 + 2)) FR

3

(R being in this expression the polarizability). In this frame, itis important to point out that for various oxide films a higherrefractive index and, hence, a higher density (i.e., a morecompact structure) are known to be indicative of better barrierproperties (e.g., to moisture, etc.). The experimental measure-ment of the refractive index, and of optical properties in general,is worthy of further comments for La2O3 thin films especiallyin relation to the deposition methodology. In fact, variousdeposition techniques, such as chemical vapor deposition,sputtering, thermal oxidation, sol-gel, etc., are being exploitedfor the synthesis of good quality La2O3 films, which is not astraightforward task.29-32 The main problem is related to therelatively large carbon residue and hydroxyls content in thefilms, likely due to both a noncomplete precursor decompositionand to the strong tendency of rare earth sesquioxides to adsorbwater vapor and carbon dioxide from the atmosphere. In orderto obtain better chemical purity and a higher degree ofcrystallinity, thermal treatments at high temperatures in oxidizingatmospheres (O2 and/or ozone) are commonly employed.30-33

With respect to this, the monitoring of the refractive indexvariation, possibly in real time during processing, representsan effective way of optimizing processing and annealingtreatments of oxide thin films.

Furthermore, concerning the growth of metal oxide layerson silicon substrates (e.g., as high-κ films in CMOS), it shouldalso be considered that high-temperature annealing treatmentsmight be responsible for detrimental effects on functionalperformances 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 low-temperature processing methods are needed, aimed at obtainingpure materials with tailored composition and structure and, atthe same time, at reducing side effects such as the formation ofinterlayers due to reactions between films and substrates.

In this framework, the present study aims at exploring low-temperature (300 °C) O2 plasma processing of lanthanum oxidesol-gel thin films. The comparison with conventional thermalannealing processes is carried out to point at the O2 plasmatreatment as a valuable low-temperature “chemical annealing”to improve material properties while controlling interfacialreactions. To this aim, the effects on microstructure, composi-tion, and optical properties of the La2O3 thin films induced eitherby high-temperature annealing performed in air or forming gasor by O2 plasma treatment are analyzed and compared.

Lanthanum oxide thin films were prepared by sol-gel dip-coating on (100) silicon substrates starting from alcoholicsolutions of lanthanum methoxyethoxide (La(OMT)3,La(OCH2CH2OCH3)3). The films were subsequently heated inair or in forming gas (10% H2 in N2) from 200 up to 700 °Cfor 1 h. To study the feasibility of a milder alternative annealingprocessing, both as-deposited and previously annealed filmswere also subjected to low-temperature (300 °C) O2 remoteplasma treatments in order to improve the quality of the filmsby exploiting the peculiar properties of finite lifetime metastablespecies and reactive species such as atomic oxygen and singlet∆ oxygen.34 The chemical composition and microstructuralevolution of the samples as a function of the annealingconditions were studied by transmission electron microscopy(TEM), X-ray photoelectron spectroscopy (XPS), and glancingincidence X-ray diffraction (GIXRD), whereas variations insurface morphology were investigated by atomic force micros-copy (AFM). Finally, the influence of the annealing conditions

on the chemical composition and structural properties of thesystems has been investigated and correlated to the opticalcharacteristics by in situ ellipsometric studies. It is worthhighlighting that remote O2 plasma processing has never beenadopted on La-O-based films, and only few reports on itsapplication for oxide-based materials are available in theliterature.35-39 To this regard, some patents dealing with the“plasma crystallization process” are reported, but few experi-mental details are therein described.

2. Experimental Section

2.1. Precursor Solution and Film Deposition. Lanthanummethoxyethoxide was used as the starting compound. Theprecursor solutions were obtained by diluting La(OMT)3

(ABCR, solution 10-12 wt % in methoxyethanol) withanhydrous ethyl alcohol (C2H5OH, Carlo Erba 99.8%) andadding a controlled amount of deionized water; the final La/EtOH/H2O molar ratio was 1:2.5:3 (CLa2O3 ) 25 g/L). Theprecursor solutions were stirred at room temperature for at least6 h before film deposition. Due to the sensitivity of lanthanumalkoxides toward moisture, controlled amounts of water wereadded to the starting sols to promote hydrolysis. Both solutionpreparation and film deposition were carried out under inertatmosphere in a glovebox.

Films were prepared by a dip-coating procedure with acontrolled withdrawal speed of 25 cm min-1 using n-type (100)silicon wafer (1 × 2 cm2) as a substrate. Before use, siliconslides were cleaned ultrasonically using acetone and isopropylalcohol. The as-deposited samples that resulted were homoge-neous, well-adherent to the substrates, and crack-free. The filmswere subsequently annealed in air or forming gas (10% H2 inN2) at temperatures between 200 and 700 °C for 1 h.

2.2. Plasma Processing. As-deposited and previously an-nealed (700 °C in air or forming gas) films were also subjectedto 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 studythe effects of this peculiar chemical annealing procedure onfilm composition, microstructure, and optical properties.

2.3. Sample Characterization. The structure and surfacemorphology of lanthanum oxide thin films were characterizedby GIXRD, TEM, and AFM. X-ray diffraction patterns wererecorded by means of a Bruker D8 Advance diffractometerequipped with a Gobel mirror and a Cu ΚR source (40 kV, 40mA) in a glancing incidence geometry at a fixed angle of 0.5°.Irrespective of the annealing conditions, the diffraction patternsdid not show any sharp peak, suggesting an amorphous ornanocrystalline character of the samples, also with a low degreeof structural order because of carbon contamination (see below).TEM characterization was performed on a Hitachi S4500 FEmicroscope using a 200 keV electron beam. AFM measurementswere run in noncontact mode using an AutoProbe CP thermo-microscope; a gold-coated Si cantilever with a resonant fre-quency of 80 kHz was used.

The samples chemical composition was investigated by XPSanalysis. Measurements were performed on a Perkin-Elmer Φ5600ci spectrometer using a monochromatized Al KR radiation(1486.6 eV), at a working pressure lower than 10-9 mbar. Thespecimens, mounted on steel sample holders, were introduceddirectly into the XPS analytical chamber by a fast entry locksystem. Survey scans were run in the 0-1350 eV range. Detailedspectra 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 tothe adventitious C1s line a BE of 284.8 eV.40 The analysis

2912 J. Phys. Chem. C, Vol. 113, No. 7, 2009 Armelao et al.

involved Shirley-type background subtraction, and whenevernecessary, spectral deconvolution which was carried out bynonlinearleast-squarescurvefitting,adoptingaGaussian-Lorentziansum function. The atomic composition of the samples wascalculated by peak integration, using sensitivity factors providedby the spectrometer manufacturer (Φ V5.4A software) andtaking into account the geometric configuration of the apparatus.Quantification of Si was performed employing the Si2s peakinstead of the most intense Si2p line, because of its severespectral overlap with the La4d photoelectron peak. Depthprofiles were carried out by Ar+ sputtering at 2.5 kV and 0.5mA cm-2 beam current density, with an argon partial pressureof 5 × 10-8 mbar.

Finally, optical characterization was performed by spectro-scopic ellipsometry. Measurements were carried out in the rangeof 0.75-6.5 eV with a phase-modulated spectroscopic ellip-someter (UVISEL-Jobin Yvon) at an angle of incidence of 70°to determine film thickness and optical properties, namely, therefractive index, n, and the absorption coefficient, R, which isrelated to the extinction coefficient, k, by the relationship R )4πk/λ. For each sample various points were tested to verifyuniformity. Specifically, ellipsometry measures the ratio (F) ofthe Fresnel reflection coefficient of the p-polarized (parallel tothe plane of incidence of the linearly polarized light beam) ands-polarized (perpendicular to the plane of incidence) lightreflected from the surface through the ellipsometric angles Ψand ∆ defined by the equation

F) tan Ψ exp(i∆)

where tan Ψ ) |Ep|/|Es| and ∆ ) δp - δs represent theamplitude and phase variation of the electric field vectorassociated 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 theequation

ε) ε1 + iε2 )N2 ) (n+ ik)2 ) sin2φ[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 three-layer model was adopted for the films consisting of a doubleinterface layer (as detected by TEM) and of the La2O3 film.Surface roughness was also modeled with a top layer, consideredas composed by voids (50%) and the lanthanum oxide film(50%). The energy dispersion of optical properties was param-etrized using a single Lorentzian oscillator41 for data analysis

N2 ) (n+ ik)2 ) ε) ε1 + iε2 ) ε∞ +Ajωj

2

ωj2 -ω2 - iγjω

where ε∞ is the high-frequency dielectric constant, ωj, γj, andAj are the frequency, width, and strength of the j oscillatorrepresenting the main optical transition, whose peak energy isoutside the experimental range for La2O3.

Ellipsometry was also used to acquire spectra as a functionof time during various annealing processes and to describe thedynamic of structural-optical modifications.

3. Results and Discussion

The structure of the films and the nature of the film-substrateinterface were at first characterized by TEM imaging. The cross-sectional TEM image of the film annealed in air at 500 °C for

1 h (see Figure 1) shows an amorphous character. The thicknessis uniform, and it has been evaluated to be about 50 nm.Moreover, at the interface with the Si substrate the formationof a thin silicon oxide layer (few nanometers) is clearlyobserved, whose chemical nature was confirmed to be SiO2 fromXPS analysis. At variance, the sample annealed in air at 700°C for 1 h shows a more complex interface structure (Figure2a). First, the interfacial SiO2 layer appears thicker (≈10 nm)than in the previous case as a consequence of the higherannealing temperature. Concerning the lanthanum oxide film,ca. 40 nm thick, it displays a small degree of crystallinity andit is likely formed of La2O3-based grains characterized by lowdimensionality. Furthermore, as evidenced by the TEM imagemagnification of the interface region (see Figure 2b), thepresence of a further intermediate layer (10 nm) with noncrys-talline structure can be clearly seen. It is likely that higherannealing temperatures promote chemical interactions betweenthe film and the silicon substrate, thus leading to the formationof an amorphous lanthanum silicate layer, as also supported byXPS data (see in the below). At this regard, the formation ofmultiple RE-O/RE-Si-O/SiO2 layer stacks is a well-knownphenomenon which has been already observed with other REelements such as Gd and Pr42-45 as well as in La2O3 thin films

Figure 1. TEM cross-sectional image of the interface structure oflanthanum oxide films on Si(001) substrates annealed at 500 °C for1 h.

Figure 2. TEM cross-sectional image (a) of the interface structure oflanthanum oxide films on Si(100) substrates annealed at 700 °C for1 h and its magnification (b).

Processing Treatments of La2O3 Thin Films J. Phys. Chem. C, Vol. 113, No. 7, 2009 2913

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 andlanthanum, the annealed samples contain carbon throughout thewhole thickness down to the film-substrate interface. Carbonquantification was not easy due to the partial spectral overlapbetween the C1s line and the high binding energy tail of theLa4s photoelectron peak. This effect was further enhanced bythe low intensity and the broadness of the two peaks especiallyin the proximity of the film-substrate interface region. However,at 700 °C carbon was still present in amounts of ca. 10-15atom %. Figure 3 shows the XPS depth profile for the sampleannealed in air at 700 °C for 1 h. Three different regions canbe identified: (I) the film surface, (II) the La2O3 layer previouslyevidenced by TEM analysis, and (III) a third deeper interlayerassociated with the film/substrate interface. It is worth noticingas the surface is characterized by an O/La ratio of ca. 3, whereaslanthanum and oxygen in-depth distributions show a parallelprofile and the O/La ratio is closer to 1.5. A similar depth profilewas also detected for the sample annealed at 700 °C in forminggas which was, however, characterized by a slightly loweroxygen content.

In order to optimize any processing methodology for sol-geldeposited thin films, a nondestructive and noninvasive real-timediagnostic would be desirable to monitor in-line an intrinsicfilm parameter that relates to film microstructure and properties.With respect to this, probing the complex refractive index byspectroscopic ellipsometry is a valuable approach. In order tovalidate and set the ellipsometric approach and analysis, we havemade a comparison of the ellipsometric measurements withTEM and XPS results. Figure 4 shows the experimental andbest-fit ellipsometric spectra of the real ⟨ε1⟩ and imaginary ⟨ε2⟩parts of the pseudodielectric function for the samples heated inair at 500 and 700 °C for 1 h (the same samples where TEManalysis was run). Ellipsometric measurements clearly supporta multilayer structure for the annealed films. The best-fit modelsshow the layered structure of the films. In particular, a SiO2

interface layer with a thickness of 5 nm is found for the 45 nmLa2O3 film upon 500 °C annealing in air. For the 700 °Cannealed sample, the best fit has been achieved with twointerface layers, i.e., a SiO2 (11 nm) layer followed by a higherrefractive index layer (16 nm) attributed to the formation oflanthanum silicate, and a La2O3 film on top 30 nm thick. It isworth noticing the agreement between ellipsometric data andTEM results concerning the thickness of the different layers.Hence, once the ellipsometric analysis approach was validatedwith TEM analysis, in situ ellipsometry has been used to

investigate the microstructural changes of the films as a functionof the annealing conditions and to correlate them to the opticalcharacteristics. Figure 5 shows the dependence on annealingtemperature and atmosphere of the thickness of the interfaceand La2O3 layers and of the value of the La2O3 refractive indexat 2.0 eV. The decrease of La2O3 films thickness and the increaseof the refractive index are observed with increasing the treatmenttemperature. Such variations are associated to the densificationand crystallization of the film structure because of the progres-sive removal of species such as water and solvent molecules aswell as of carbon residuals deriving from the lanthanummethoxyethoxide precursor. The thickness decrease is morepronounced when annealing is performed in air than in forminggas, likely due to a more efficient oxidation of carbon residualsand their removal as CO2. Furthermore, a continuous increaseof the La2O3 refractive index is found when samples areannealed in forming gas, consistently with data from He et al.47

who reported a variation of n from 1.836 to 1.970 uponannealing of La2O3 films deposited by atomic layer depositionat 400 °C in forming gas.

Conversely, when annealing is performed in air a maximumin the refractive index is found at 500 °C (Figure 5), as canalso be seen in the energy dispersion spectra shown in Figure6; a further increase in the temperature leads to a decrease ofthe refractive index in the presence of oxygen. Variousarguments can sustain and explain this trend for annealing inair, e.g., as observed by XPS a further increase of temperature

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

Figure 4. Experimental (dots) and best-fit (lines) ellipsometric spectraof the real, ⟨ε1⟩ , and imaginary, ⟨ε2⟩ , parts of the pseudodielectricfunction 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 alsoreported.

2914 J. Phys. Chem. C, Vol. 113, No. 7, 2009 Armelao et al.

in presence of oxygen could yield the formation of lanthanumoxycarbonates competing with CO2 removal48 and a largerfilm-substrate intermixing not only due to oxygen in-diffusionbut also to silicon out-diffusion. Both effects induce a moresevere La2O3 contamination which also inhibits structuralordering.

Focusing on the formation of the interface layer, annealingin 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 onlyinterface formed up to a temperature of 500 °C. Above 500 °C,the formation of an additional La-Si-O interface layer isobserved when annealing is performed in air.

Thus, from data in Figures 4-6, it can be inferred thatannealing in air at T > 500 °C enhances the interdiffusion ofoxygen and the subcutaneous oxidation of the Si substrate, andthe out-diffusion of Si thus yields, in addition to interfacial SiO2,the intermixed silicate layer and a more disordered film, inmicrostructure and composition, with lower refractive index andhigher absorption coefficient. To investigate milder alternative

Figure 5. Dependence on annealing temperature and atmosphere ofthe 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 6. Energy dispersion of (a) the refractive index and (b)absorption coefficient of La2O3 films annealed in air and in forminggas at 500 and 700 °C.

Figure 7. Experimental spectra of the imaginary part, ⟨ε2⟩ , of thepseudodielectric function of (a) a sol-gel as-deposited film, (b) afterannealing 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 anadditional O2 plasma at 300 °C.

Processing Treatments of La2O3 Thin Films J. Phys. Chem. C, Vol. 113, No. 7, 2009 2915

film-processing methods, freshly as-deposited films have beendirectly exposed to a low-temperature (300 °C) remote O2

plasma for 30 min. A remote configuration for the plasmatreatment has been adopted in order to avoid any electronbombardment and radiative damage and/or etching of the layers.For comparison, half of the same film was simply annealed inair at 300 °C for 1 h and the same plasma treatment was alsoadopted on the samples already annealed at 700 °C. Figure 7compares the experimental ellipsometric spectra of the imaginarypart ⟨ε2⟩ ) 2nk of the pseudodielectric function for (a) an as-deposited sol-gel film (before any treatment), (b) after annealingin air at 300 °C, (c) after O2 plasma processing at 300 °C, and(d) after a high-temperature 700 °C air annealing followed byan additional O2 plasma at 300 °C.

The observed changes in the spectra are due to both variationsof film thickness and optical response. As for the film thickness,the approximately 110-120 nm thickness of the as-depositedfilm decreases to 78-80 nm at a temperature of 300 °C (asalso shown in Figure 5) for both the air annealed and O2 plasmaexposed samples. As for the optical properties, the calculatedspectral dependence of the refractive index (n) and of theabsorption coefficient (R) are compared in Figure 8. As shownin Figure 8, parts a and b, the O2 plasma processing clearlyaffects the optical properties of the layers. In particular, anincrease of the refractive index, which might be indicative of amore compact and ordered microstructure, is found, presumablysteaming from a structural reordering promoted by the furtherrelease of the carbon and moisture residual impurities uponreaction with oxygen atoms. This hypothesis and the formation

of a better ordered microstructure are also consistent with theobserved decrease of the extinction coefficient (k), and ofabsorption coefficient (R ) 4πk/λ) upon O2 plasma exposureas indicated in Figure 8. Therefore, the O2 plasma also resultsin the improvement in film transparency in the visible rangefor all films, but importantly, the highest transparency andrefractive index are measured for films directly exposed to thelow-temperature O2 plasma without any previous high-temper-ature thermal annealing. A further important advantage is thatno interfacial reactions and subcutaneous oxidation of the siliconsubstrate are induced at such a low temperature. It is also worthythat all the investigated treatments yield reproducible resultswhen starting from similar samples. Figure 9 shows the ⟨ε2⟩experimental spectra obtained for various films either as-deposited or after the different treatments. The observeddifferences in the spectra measured for three as-depositedsamples are mostly due to variations in film thickness. On theother hand, both the O2 plasma processing and high-temperatureannealing yield reproducible results for ⟨ε2⟩ values determinedon various sol-gel films.

The positive effect of the low-temperature plasma processingin improving the transparency as a consequence of the improved

Figure 8. Energy dispersion of the refractive index, n, and of theabsorption coefficient, R, of (a) La2O3 films as-deposited, after a thermalannealing 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. Theoptical spectra for a film directly exposed to the same O2 plasma withoutany preliminary thermal annealing are also shown.

Figure 9. Reproducibility test. Experimental spectra of the imaginarypart, ⟨ε2⟩ , of the pseudodielectric function for (a) three different as-deposited sol-gel films, (b) two different samples after O2 plasmaprocessing at 300 °C, and (c) two different samples after high-temperature air annealing. Small differences are due to slightly differentfilm thickness.

2916 J. Phys. Chem. C, Vol. 113, No. 7, 2009 Armelao et al.

structural order of films can also be seen in the morphologicalmodification detected by AFM and shown in Figure 10. Thefilm morphology clearly changes from amorphous to nanoc-rystalline upon plasma processing, the grain size being depend-ent on overall processing. These observed effects of the low-temperature O2 plasma treatment can be explained consideringthat active oxygen species might etch disordered regionsresulting in a better structural quality; furthermore, the plasma-activated oxygen species impinging on the growing surfacerelease a surplus of energy and operate a “chemical annealingeffect”, i.e., an increase of mobility of atoms that can rearrangein a better structural order, as already demonstrated for ZnOthin films.49 Additionally, they saturate oxygen vacancies,dangling bonds, and grain boundaries, leading to an actualimprovement in film transparency.

4. Conclusions

Lanthanum oxide thin films have been prepared through asol-gel process on Si(100) substrates from ethanolic solutionsof the La(OCH2CH2OCH3)3 precursor. As-deposited films havebeen either conventionally annealed at different temperaturesbetween 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 alsoapplied to previously annealed layers in order to correlatemicrostructural and optical properties modifications to post-deposition treatments. TEM and ellipsometry analysis revealedthat under 500 °C thermal annealing oxidation of the Si substrateoccurs with the growth of a silica layer and heating at highertemperature (700 °C) results in film-substrate reactions and inthe formation of a lanthanum silicate layer. At variance, low-temperature (300 °C) remote O2 plasma processing on as-

deposited and previously annealed films results in the increaseof the refractive index and decrease of the extinction coefficient,probably due to an active oxygen removal of carbon contamina-tion and structural reordering of films. Importantly, the highesttransparency and refractive index are measured for as-depositedfilms directly exposed to plasma. In this case, the employmentof low processing temperatures prevents interfacial reactionsand the “chemical annealing effect” exerted by the activatedoxygen species allows a better structural order, saturation ofoxygen vacancies, dangling bonds, and grain boundaries, leadingto an actual improvement in film transparency.

It is worth highlighting that the proposed synthesis oflanthanum oxide layers, based on the innovative combinationof the mild sol-gel route and O2 plasma processing, can beconveniently optimized and extended to various oxide-basedfilms to prepare materials and nanostructures of technologicalinterest with tailored functional properties.

Acknowledgment. This work was financially supported byresearch projects CNR-INSTM PROMO, FIRB-MIURRBNE033KMA “Molecular Compounds and Hybrid Nano-structured Materials with Resonant and Nonresonant OpticalProperties for Photonic Devices”, FISR-MIUR “InorganicHybrid Nanosystems for the Development and the Innovationof Fuel Cells”, and CARIPARO 2006 “Multi-Layer OpticalDevices Based on Inorganic and Hybrid Materials byInnovative Synthetic Strategies”. The authors also acknowl-edge the 7th FP European Project NanoCharM (Multifunc-tional NanoMaterials Characterization Exploiting Ellipsom-etry and Polarimetry) (NMP3-CA-2007-218570).

References and Notes

(1) Kakushima, K.; Tsutsui, K.; Ohmi, S.-I.; Ahmet, P.; Rao, V. R.;Iwai, H. Top. Appl. Phys. 2007, 106, 345.

(2) Losurdo, M.; Giangregorio, M. M.; Capezzuto, P.; Bruno, G.; Toro,R. G.; Malandrino, G.; Fragala, I. L.; Armelao, L.; Barreca, D.; Tondello,E.; Suvorova, A. A.; Yang, D.; Irene, E. A. AdV. Funct. Mater. 2007, 17,3607.

(3) Losurdo, M.; Giangregorio, M. M.; Bruno, G.; Yang, D.; Irene,E. A.; Suvorova, A. A.; Saunders, M. Appl. Phys. Lett. 2007, 91, 091914.

(4) Lo Nigro, R.; Raineri, V.; Bongiorno, C.; Toro, R. G.; Malandrino,G.; Fragala, I. L. Appl. Phys. Lett. 2003, 83, 129.

(5) Argall, F.; Johnscher, A. K. Thin Solid Films 1968, 2, 185.(6) Hass, G.; Ramsey, J. B.; Thun, R. J. Opt. Soc. Am. 1959, 49, 116.(7) Bezuidenhout, D. F.; Pretorius, R. Thin Solid Films 1986, 139, 121.(8) Tominaga, K.; Shirayanagi, A.; Taakagi, T.; Okada, M. Jpn. J. Appl.

Phys., Part 1 1993, 32, 4082.(9) Kingon, A. Nature 1999, 401, 658.

(10) Shannigrahi, S. R.; Lee, S.-H.; Jang, H. M. J. Mater. Res. 2002,17, 1884.

(11) Tao, W.; Desu, S. B.; Peng, C. H.; Dickerson, B.; Li, T. K.; Thio,C. L.; Lee, J. J.; Hendricks, W. Mater. Res. Soc. Symp. Proc. 1995, 361,319.

(12) Bu, S. D.; Kang, B. S.; Park, B. H.; Noh, T. W. J. Korean Phys.Soc. 2000, 36, 9.

(13) Kijima, T.; Ushikubo, M.; Matsunaga, H. Jpn. J. Appl. Phys., Part1 1999, 38, 127.

(14) Ahn, G. Y.; Park, S.-I.; Shim, I.-B.; Cho, Y. S.; Kim, C. S. Phys.Status Solidi B 2004, 241, 1561.

(15) Von Helmolt, R.; Wecker, J.; Holzapfel, B.; Schultz, L.; Samwer,K. Phys. ReV. Lett. 1993, 71, 2331.

(16) Nath, T. K.; Rao, R. A.; Lavric, D.; Eom, C. B.; Wu, L.; Tsui, F.Appl. Phys. Lett. 1999, 74, 1615.

(17) Wu, Y. H.; Yang, M. Y.; Chin, A.; Chin, W. J.; Kwei, C. M. IEEEElectron DeVice Lett. 2000, 21, 341.

(18) Guha, S.; Cartier, E.; Gribelyuk, M. A.; Bojarczuk, N. A.; Copel,M. C. Appl. Phys. Lett. 2000, 77, 2710.

(19) Copel, M. C.; Cartier, E.; Ross, F. M. Appl. Phys. Lett. 2001, 78,1607.

(20) Kingon, A. I.; Maria, J. P.; Streiffer, S. K. Nature 2000, 406, 1032.(21) Evarestove, R. A.; Leko, A. V.; Murin, I. V.; Petrov, A. V.;

Veryazov, V. A. Phys. Status Solidi B 1992, 170, 145.(22) Hubbard, K. J.; Schlom, D. G. J. Mater. Res. 1996, 11, 2757.

Figure 10. 500 nm × 500 nm AFM topographies of La2O3 filmsannealed in air and in forming gas at 500 and 700 °C and subsequentlyexposed for 30 min at 300 °C to a remote O2 plasma and of a filmdirectly exposed to the same O2 plasma without any preliminary thermalannealing. Surface roughness values (rms ) root-mean-square rough-ness) are also indicated.

Processing Treatments of La2O3 Thin Films J. Phys. Chem. C, Vol. 113, No. 7, 2009 2917

(23) Ding, J.; Yang, Q.; Tang, Z.; Xu, J.; Su, L. J. Opt. Soc. Am. B2007, 24, 681.

(24) Takebe, H.; Murakami, T.; Kuwabara, M.; Hewak, D. W. J. Non-Cryst. Solids 2006, 352, 2425.

(25) Chen, H.; Liu, Y. H.; Zhou, Y. F.; Zhang, Q. Y.; Jiang, Z. H. J.Non-Cryst. Solids 2005, 351, 3060.

(26) Kale, S. S.; Jadhav, K. R.; Patil, P. S.; Gujar, T. P.; Lokhande,C. D. Mater. Lett. 2005, 59, 3007.

(27) Jellison, G. E.; Burke, H. H. J. Appl. Phys. 1986, 60, 841.(28) Oughstun, K. E.; Cartwright, N. A. Opt. Express 2003, 11, 1541.(29) Nieminen, M.; Putkonen, M.; Niinisto, L. Appl. Surf. Sci. 2001,

174, 155.(30) Shiokawa, Y.; Amano, R.; Nomura, A.; Yagi, M. J. Radioanal.

Nucl. Chem. 1991, 152, 373.(31) Cabanas, M. V.; Ragel, C. V.; Conde, F.; Gonzalez-Calbet, J. M.;

Vallet-Regı, M. Solid State Ionics 1997, 101, 191.(32) Weber, A.; Suhr, H. Mod. Phys. Lett. B 1989, 3, 1001.(33) Gao, Y.-M.; Wu, P.; Dwight, K.; Wold, A. J. Solid State Chem.

1991, 90, 228.(34) Fuller, N. C. M.; Malyshev, M. V.; Donnelly, V. M.; Herman, I. P.

Plasma Sources Sci. Technol. 2000, 9, 116.(35) Ohsaki, H.; Shibayama, Y.; Nakajim, A.; Kinbara, A.; Watanabe,

T. Thin Solid Films 2006, 502, 63.(36) Arimitsu, N.; Nakajima, A.; Kameshima, Y.; Shibayama, Y.;

Ohsaki, H.; Okada, K. Mater. Lett. 2007, 61, 2173.

(37) Jung, C.-K.; Cho, S.-H.; Lee, S.-B.; Kim, T.-K.; Lee, M.-N.; Boo,J.-H. Surf. ReV. Lett. 2003, 10, 635.

(38) Ono, Y.; Ulrich, B. D.; Zhuang, W. W.; Tweet, D. J. U.S. PatentUS20070259127A1, 2007.

(39) Fukuhisa, K.; Nakajima, A.; Shinohara, K.; Watanabe, T.; Ohsaki,H.; Serikawa, T. U.S. Patent US20040241976A1, 2004.

(40) Briggs, D.; Seah, M. P. Practical Surface Analysis; John Wiley:Chichester, U.K., 1990; Vol. 1.

(41) Losurdo, M. Thin Solid Films 2004, 455, 301.(42) Takeuchi, H.; King, T.-J. Appl. Phys. Lett. 2003, 83, 788.(43) Lo Nigro, R.; Toro, R. G.; Malandrino, G.; Condorelli, G. G.;

Raineri, V.; Fragala, I. L. AdV. Funct. Mater. 2005, 15, 838.(44) Fiorenza, P.; Lo Nigro, R.; Raineri, V.; Lombardo, S.; Toro, R. G.;

Malandrino, G.; Fragala, I. L. J. Appl. Phys. 2005, 98, 044312.(45) Lo Nigro, R.; Toro, R. G.; Malandrino, G.; Raineri, V.; Fragala,

I. L. AdV. Mater. 2003, 15, 1071.(46) Niu, D.; Ashcraft, R. W.; Kelly, M. J.; Chambers, J. J.; Klein, T. M.;

Parsons, G. N. J. Appl. Phys. 2002, 91, 6173.(47) He, W.; Schuetz, S.; Solanki, R.; Belot, J.; McAndrew, J.

Electrochem. Solid-State Lett. 2004, 7, G131.(48) Armelao, L.; Barreca, D.; Bottaro, G.; Gasparotto, A.; Maragno,

C.; Tondello, E. Chem. Mater. 2005, 17, 427.(49) Losurdo, M.; Giangregorio, M. M.; Sacchetti, A.; Capezzuto, P.;

Bruno, G.; Malandrino, G.; Fragala, I. L. J. Mater. Res. 2006, 21, 1632.

JP809824E

2918 J. Phys. Chem. C, Vol. 113, No. 7, 2009 Armelao et al.