Surface & Coatings Technology 204 (2010) 2015–2018

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Surface & Coatings Technology

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Structural characterization of TiO2–Cr2O3 nanolaminates grown by atomiclayer deposition

V. Sammelselg a,b,⁎, A. Tarre a, J. Lu c, J. Aarik a, A. Niilisk a, T. Uustare a, I. Netšipailo a, R. Rammula a,R. Pärna a, A. Rosental a

a Institute of Physics, University of Tartu, Riia 142, 51014 Tartu, Estoniab Institute of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estoniac Ångström Microstructure Laboratory, Uppsala University, Box 534, 75121 Uppsala, Sweden

⁎ Corresponding author. Institute of Physics, UniverTartu, Estonia. Tel.: +372 737 4705.

E-mail address: vaino.sammelselg@ut.ee (V. Samme

0257-8972/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2009.11.039

a b s t r a c t

a r t i c l e i n f o

Available online 4 December 2009

Keywords:Protective coatingsTitaniaChromiaNanolaminateAtomic layer deposition (ALD)

TiO2–Cr2O3 nanolaminates were atomic layer deposited on (0 1 2)-oriented sapphire and (1 0 0)-orientedsilicon. The thickness of the alternating layers in the eight-layer laminates grown was close to 10 nm. Thelaminates were characterized by cross-sectional high-resolution transmission electron microscopy, high-resolution scanning electron microscopy, atomic force microscopy, reflection high-energy electrondiffraction, and micro-Raman spectroscopy. A highly oriented growth of the laminate on sapphire and itsgrowth with a very little preferred orientation on silicon were revealed. The laminate grown on sapphirehad, along with better crystallinity, more exactly defined and more planar interphase boundaries. Theamount of indefiniteness of the boundaries increased with the layer distance from the substrate. Thecrystalline phase of titania was rutile in the laminate grown on sapphire and anatase in the laminate grownon silicon, while the crystalline phase of chromia had eskolaite structure. In the laminate grown on sapphire,titania contained numerous twins; compressively strained chromia had in this case more perfect structure.

sity of Tartu, Riia 142, 51014

lselg).

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© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Chromium (III) oxide in the form of α-Cr2O3 (chromia) is amaterial of lively interest, in part due to its applicability in protectivecoatings. The material has high chemical and wear resistance.However, its fundamental weakness lies in the Cr volatility at hightemperatures. In order to reduce the Cr loss, Geng and Zhu haveconsidered placing titania on top of chromia for suppressing thegeneration of volatile Cr oxides in fuel cells [1]. An alternative waycould be the use of multilayer structures (laminates) in which Cr2O3

alternates with TiO2. Some examples of the use of ceramic laminateswith nanometer-range layer thicknesses (nanolaminates) for protec-tive coatings are presented in [2–4]. In case they have to withstandrepeated small mechanical deformations, the elasticity of thematerials is of importance [3]. When designing multilayer coatingsmeant for the high-temperature applications, the difference inthermal expansion of the component materials should be taken intoaccount, since if it is marked, the coatings will crack. The probability ofcracking or fracturing should significantly decrease, however, whenone goes over to nanolaminates. In this work, we investigate thenanolaminates fabricated from TiO2 and Cr2O3.

For growing the nanolaminates we apply atomic layer deposition(ALD) [5]. ALD is amethodwell suited for processing of thin and ultrathinfilms, particularly when a surface having complex shape should beuniformly coated. InALD, the growth results from the saturating reactionscarried out consecutively and separately between the volatilizedprecursors and the surface of the growing film. The method permits aself-controlled submonolayer-by-submonolayer building of solids. Wehave previously designed a route for ALD of TiO2–Cr2O3 laminates fromtwo metal precursors, one for Ti and the other for Cr, and a commonprecursor CH3OH [6]. The route is put to use also in the present work.

The aim of the work is structural characterization of TiO2–Cr2O3

nanolaminates made of 10 nm thick alternating layers by ALD anddemonstration of the substrate effect on the characteristics.

2. Experimental

We grew the nanolaminates at 375 °C on α-Al2O3(0 1 2) (r-cutsapphire) and Si(1 0 0) substrates. Prior to growing, the substrates were,as in [7], subjected to sequential piranha andHF treatments. A laboratoryALD reactor [8]was used. The setup includes an optical reflectance probefor continuous monitoring of the growth [9]. The metal precursors wereTiCl4 (99.9%, Aldrich) and CrO2Cl2 (99.99%, Alfa-Aesar); the common co-precursor was CH3OH (99.99%, Alfa-Aesar). Nitrogen (99.999%, Elme-Messer) performed precursors carrying, supply switching, and purgingfunctions. CrO2Cl2 and CH3OH were evaporated at −20 °C, and TiCl4 at+20 °C. The total gas flow rate was 40 sccm. The pressure in the reactor

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was held at about 10 mbar. To grow TiO2, the TiCl4 pulse, first purge,CH3OHpulse, and second purgewere set at 0.2, 2, 2 and2 s, respectively.In this case, CH3OH was used as a source of oxygen. To grow Cr2O3, thesame pulsing time parameters were used, however, with CrO2Cl2substituted for TiCl4. Now CrO2Cl2 is an oxidising, and CH3OH a reducingagent [10]. It is well to bear in mind that no Cr2O3 growth could beobtained when H2O, an oxygen source, most commonly used in ALDprocessing of oxides, was in combinationwith CrO2Cl2 applied insteadofCH3OH.Thenumber ofALDcycles producinga laminate layer of TiO2was300, while for Cr2O3 this number was 100. The nanolaminates studiedwere grown in the same run and consisted of four TiO2–Cr2O3 doublelayers.

The samples were characterized by cross-sectional high-resolutiontransmission electron microscopy (X-HRTEM), high-resolution scan-ning electron microscopy (HRSEM), atomic force microscopy (AFM),reflection high-energy electron diffraction (RHEED), and micro-Raman

Fig. 1. X-HRTEM images for the nanolaminates consisted of four TiO2–Cr2O3 double layers onAl2the first and last layers in the laminate, respectively) and on Si (b). The arrows in d point to tw

spectroscopy (μRS). Focused ion beam (FIB) milling was used in thepreparation of the samples for X-HRTEM measurements.

X-HRTEManalysiswas performed on a FEI Tecnai F30 STmicroscope(300 keV) in the Ångström Laboratory of Uppsala University. HRSEMmeasurements were done using a FEI Helios NanoLab 600 system. FIBslicing was conducted in the FEI and Carl Zeiss NTS laboratories, andfurther FIB-polished in the Ångström Laboratory using lower densityand low impact angle ion beam. The latter procedure provides thinningthe lamella and removal of the surface layers, which are damaged up tothe amorphism in the FIB slicing process. For the AFM study, a VeecoAutoProbe CPII multimode scanning probe microscope was applied.RHEED patterns were recorded photographically on a SELMI EMR-100electron diffractometer. Additional structural studies were done withμRS using a Renishaw inVia micro-Raman spectrometer. We emphasizehere that, due to sample smallness (2×6 mm), the X-ray diffractionanalysis was inapplicable.

O3 (a, c, d; the latter twopanels depict the images taken after additional FIB-polishing fromins in TiO2, with the fast Fourier transform pattern of the region shown in the insert.

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3. Results and discussion

3.1. Cross-sectional data

Fig. 1 shows cross-sectional TEM images for the nanolaminates. Arather good distinctness of the TiO2 and Cr2O3 phases is seen. Thedistinctness decreased with the increase of the layer distance from thesubstrate. Both phases appear to be polycrystallinewhen the growthwascarried out on Si (Fig. 1(b)) and close to single-crystalline when thegrowth was carried out on Al2O3 (Fig. 1(a,c,d)). Along with a bettercrystallinity theAl2O3-based laminatehadmoreexactlydefinedandmoreplanar interphase boundaries. TiO2 layers in this laminate contained alarge number of twins (Fig. 1(d)). According to Fig. 1(a), the thickness ofthe TiO2 and Cr2O3 layers in the laminate was, respectively, 9 and 8 nm.Between the Si substrate and the laminate an intermediate layer thatmost probably contains SiO2 can be seen (Fig 1(b)). The layer apparentlyresults from the chemical treatment of the substrate [11] and/or from thealien oxide growth on them.

3.2. Surface morphology

Surface morphology of the nanolaminate was determined byHRSEM and AFM. HRSEM measurements revealed that the surface ofthe structure deposited on Al2O3 wasmore finely grained compared to

Fig. 2. HRSEM (a, b) and AFM (c, d) images for the nanolaminates cons

that on Si (Fig. 2(a,b)). AFM data verifies this result (Fig. 2(c,d)) andshows that the RMS roughness (1×1 μm2 area) was 1.7 nm for theformer case and 4.3 nm for the latter. At the same time, the roughnessof a chromia layer on Al2O3 with the thickness comparable to the totalthickness of the nanolaminate was measured to be 2.5 nm, whichproves a favorable influence of lamination on the resulting roughness.

3.3. Structural data relevant to the laminate top

The structure of the subsurface area of the laminates wasascertained by RHEED. Fig. 3(a) shows the diffraction pattern for thelaminate deposited on Al2O3(0 1 2). The reflections labeled with R areassignable to TiO2 rutile and imply that the growth has taken place sothat the (1 0 1) plane of rutile is parallel to the (0 1 2) plane ofalumina. The form of these reflections speaks of highly textured(rather than epitaxial) material. The reflections labeled with E areassignable to (0 0 1)-oriented Cr2O3 eskolaite. The form of the latterindicates that Cr2O3 has grown also highly oriented; however, incomparison to TiO2, it has a less pronounced texture. The intensityanalysis of the reflections suggests that comparable amounts of bothmaterials contributed to diffraction.

Fig. 3(b) shows the diffraction pattern for the laminate depositedon Si substrate. Now the reflections can be assigned to the terminating

isted of four TiO2–Cr2O3 double layers on Al2O3 (a, c) and Si (b, d).

Fig. 3. RHEED patterns for the nanolaminates consisted of four TiO2–Cr2O3 double layerson Al2O3 (a) and Si (b).

Fig. 4. Raman spectra for the nanolaminates consisted of four TiO2–Cr2O3 double layerson Al2O3 (1), Si (2), and SiO2 (3). To measure the spectrum in the second case, thelaminate was made freestanding. E, R, and A denote the bands unmistakably belongingto Cr2O3 eskolaite, TiO2 rutile, and TiO2 anatase, respectively. Asterisks indicate thebands caused by the substrate scattering. Lorentzian curve fitting is shown.

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Cr2O3 eskolaite layer. The pattern indicates that the layer has grownpolycrystalline with very little preferred orientation.

It should be noted that RHEED results for as-grown laminates arein good agreement with the X-HRTEM ones for FIB processedlaminates. Hence it follows that FIB processing insignificantly changedthe inside-slice structure of the laminates.

3.4. Depth-averaged structural data

Depth-averaged crystalline structure of the nanolaminates wasdetermined on the basis of Raman spectra (Fig. 4). In the Ramanexperiments, the informationdepth inprinciple exceeded the thicknessofthe samples. For this reason the spectrumof the laminate grownon the Sisubstrate (Fig. 4, curve 2) was taken from a sample in which the Sisubstrate was chemically removed to get rid of Si-caused strong spectralbackground. ARaman spectrumwas taken also froma laminate grownonamorphous SiO2 (Fig. 4, curve 3). In both cases the bands at 144 cm−1,assignable to the anatase mode of Eg symmetry, and at 550 cm−1,assignable to the eskolaite mode of A1g symmetry, were the strongest.Broadness of the bands suggests the smallness of the crystallites in thelaminate. Twoweakerbands (at 520 cm−1 and638 cm−1)due toanatasewere also revealed. Neither curve 2 nor curve 3 shows the presence ofTiO2 rutile. Thus, the laminates grown on SiO2 and Si containedpolycrystalline fine-grained anatase TiO2 and eskolaite Cr2O3. Smallcrystallite size is themost probable reason for why the RHEED reflectionsfrom TiO2 were not seen on the background of the Cr2O3 reflectionsoriginating from the topmost layer.

Contrary to the above two cases, the spectrum for the laminate onsapphire (Fig. 4, curve 1) did not show any anatase band at 144 cm−1.Instead, the bands at 447 and 606 cm−1 belonging to rutile TiO2 andhaving symmetries Eg and A1g, respectively, appeared. The fact impliesthat TiO2 in the laminate has attained the phase-pure rutile form. TheeskolaiteA1g bandat557 cm−1 is nowblue shiftedby7 cm−1 compared

to the position on curves 2 and 3 and broadened in some degree. Theshift speaks of the in-plane compressive strain in Cr2O3 [12]. The reasonfor the revealed broadening of the Raman band may be the strain thatvaries with the layer distance from the substrate.

4. Conclusions

The data obtained show a highly oriented ALD growth of ananolaminate consisted of four TiO2–Cr2O3 double layers with a singlelayer thickness of ∼10 nmon (0 1 2)-oriented sapphire, and the growthof this nanolaminate with a very little preferred orientation on (0 0 1)-oriented silicon. The laminate deposited on sapphire had, along withbetter crystallinity, more exactly defined and more planar interphaseboundaries. The amount of indefiniteness of the boundaries increasedwith the layer distance from the substrate. The crystalline phase oftitania was rutile in the laminate grown on sapphire and anatase in thelaminate grown on silicon. In the laminate grown on sapphire, titaniacontained numerous twins; compressively strained chromia has in thiscase more perfect eskolaite structure.

Acknowledgments

The participation in the preparation of TEM samples of SteveReyntjens from FEI and Fabian Perez-Willard from Carl Zeiss NTS isgratefully acknowledged. The work belonged to the Estonian govern-ment targeted theme SF0180046s07 andwas partially supported by theEstonian Science Foundation (Grants 6651 and 6999).

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