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Structural characterization of TiO2–Cr2O3 nanolaminates grown by atomic layer

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, Estonia

b Institute of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia

c Ångström Microstructure Laboratory, Uppsala University, Box 534, 75121 Uppsala,

Sweden

Abstract

TiO2–Cr2O3 nanolaminates were atomic-layer deposited on (0 1 2)-oriented sapphire

and (1 0 0)-oriented silicon. The thickness of the alternating layers in the eight-layer

laminates grown was close to 10 nm. The laminates were characterized by cross-

sectional high-resolution transmission electron microscopy, high-resolution scanning

electron microscopy, atomic force microscopy, reflection high-energy electron

diffraction, and micro-Raman spectroscopy. A highly oriented growth of the laminate

on sapphire and its growth with a very little preferred orientation on silicon were

revealed. The laminate grown on sapphire had, along with better crystallinity, more

exactly defined and more planar interphase boundaries. The amount of indefiniteness of

the boundaries increased with the layer distance from the substrate. The crystalline

phase of titania was rutile in the laminate grown on sapphire and anatase in the laminate

grown on 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.

*3. ManuscriptClick here to view linked References

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Keywords:

Protective coatings

Titania

Chromia

Nanolaminate

Atomic layer deposition (ALD)

1. Introduction

Chromium (III) oxide in the form of α-Cr2O3 (chromia) is a material of lively interest, in

part due to its applicability in protective coatings. The material has high chemical and

wear resistance. However, its fundamental weakness lies in the Cr volatility at high

temperatures. In order to reduce the Cr loss, Geng and Zhu have considered placing

titania on top of chromia for suppressing the generation of volatile Cr oxides in fuel

cells [1]. An alternative way could be the use of multilayer structures (laminates) in

which Cr2O3 alternates with TiO2. Some examples of the use of ceramic laminates with

nanometer-range layer thicknesses (nanolaminates) for protective coatings are presented

in [2–4]. In case they have to withstand repeated small mechanical deformations, the

elasticity of the materials is of importance [3]. When designing multilayer coatings

meant for the high-temperature applications, the difference in thermal expansion of the

component materials should be taken into account, since if it is marked, the coatings

will crack. The probability of cracking or fracturing should significantly decrease,

however, when one goes over to nanolaminates. In this work, we investigate the

nanolaminates fabricated from TiO2 and Cr2O3.

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For growing the nanolaminates we apply atomic layer deposition (ALD) [5]. ALD is a

method well suited for processing of thin and ultrathin films, particularly when a

surface having complex shape should be uniformly coated. In ALD, the growth results

from the saturating reactions carried out consecutively and separately between the

volatilized precursors and the surface of the growing film. The method permits a self-

controlled submonolayer-by-submonolayer building of solids. We have previously

designed a route for ALD of TiO2–Cr2O3 laminates from two metal precursors, one for

Ti and the other for Cr, and a common precursor for O [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 and demonstration of the substrate effect on the

characteristics.

2. Experimental

We grew the nanolaminates at 375 °C on -Al2O3(0 1 2) (r-cut sapphire) and Si(1 0 0)

substrates. Prior to growing, the substrates were, as in [7], subjected to sequential

piranha and HF treatments. A laboratory ALD reactor [8] was used. The setup includes

an optical reflectance probe for continuous monitoring of the growth [9]. The metal

precursors were TiCl4 (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 purging functions.

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 was held at about 10 mbar. To grow

TiO2, the TiCl4 pulse, first purge, CH3OH pulse, and second purge were set at 0.2, 2, 2

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and 2 s, respectively. In this case, CH3OH was used as a source of oxygen. To grow

Cr2O3, the same pulsing time parameters were used, however, with CrO2Cl2 substituted

for TiCl4. Now CrO2Cl2 is an oxidising, and CH3OH a reducing agent [10]. It is well to

bear in mind that no Cr2O3 growth could be obtained when H2O, an oxygen source,

most commonly used in ALD processing of oxides, was in combination with CrO2Cl2

applied instead of CH3OH. The number of ALD cycles producing a laminate layer of

TiO2 was 300, while for Cr2O3 this number was 100. The nanolaminates studied were

grown in the same run and consisted of four TiO2–Cr2O3 double layers.

The samples were characterized by cross-sectional high-resolution transmission electron

microscopy (X-HRTEM), high-resolution scanning electron microscopy (HRSEM),

atomic force microscopy (AFM), reflection high-energy electron diffraction (RHEED),

and micro-Raman spectroscopy (μRS). Focused ion beam (FIB) milling was used in the

preparation of the samples for X-HRTEM measurements.

X-HRTEM analysis was performed on a FEI Tecnai F30 ST microscope (300 keV) in

the Ångström Laboratory of Uppsala University. HRSEM measurements were done

using a FEI Helios NanoLab 600 system. FIB slicing was conducted in the FEI and Carl

Zeiss NTS laboratories, and further FIB-polished in the Ångström Laboratory using

lower density and low impact angle ion beam. The latter procedure provides thinning

the lamella and removal of the surface layers, which are damaged up to the amorphism

in the FIB slicing process. For the AFM study, a Veeco AutoProbe CPII multimode

scanning probe microscope was applied. RHEED patterns were recorded

photographically on a SELMI EMR-100 electron diffractometer. Additional structural

studies were done with μRS using a Renishaw inVia micro-Raman spectrometer. We

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emphasize here that, due to sample smallness (2 × 6 mm), the X-ray diffraction analysis

was inapplicable.

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3. Results and discussion

3.1. Cross-sectional data

Fig. 1 shows cross-sectional TEM images for the nanolaminates. A rather good

distinctness of the TiO2 and Cr2O3 phases is seen. The distinctness decreased with the

increase of the layer distance from the substrate. Both phases appear to be

polycrystalline when the growth was carried out on Si (Fig. 1(b)) and close to single-

crystalline when the growth was carried out on Al2O3 (Fig. 1(a,c,d)). Along with a

better crystallinity the Al2O3-based laminate had more exactly defined and more planar

interphase boundaries. TiO2 layers in this laminate contained a large number of twins

(Fig. 1(d)). According to Fig. 1(a), the thickness of the TiO2 and Cr2O3 layers in the

laminate was, respectively, 9 and 8 nm. Between the Si substrate and the laminate an

intermediate layer that most probably contains SiO2 can be seen (Fig 1(b)). The layer

apparently results from the chemical treatment of the substrate [11] and/or from the

alien oxide growth on them.

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3.2. Surface morphology

Surface morphology of the nanolaminate was determined by HRSEM and AFM.

HRSEM measurements revealed that the surface of the structure deposited on Al2O3

was more finely grained compared to that on Si (Fig. 2(a,b)). AFM data verifies this

result (Fig. 2(c,d)) and shows that the RMS roughness (1 × 1 μm2 area) was 1.7 nm for

the former case and 4.3 nm for the latter. At the same time, the roughness of a chromia

layer on Al2O3 with the thickness comparable to the total thickness of the nanolaminate

was measured to be 2.5 nm, which proves a favorable influence of lamination on the

resulting roughness.

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3.3. Structural data relevant to the laminate top

The structure of the subsurface area of the laminates was ascertained by RHEED. Fig.

3(a) shows the diffraction pattern for the laminate deposited on Al2O3(0 1 2). The

reflections labeled with R are assignable to TiO2 rutile and imply that the growth has

taken place so that the (1 0 1) plane of rutile is parallel to the (0 1 2) plane of alumina.

The form of these reflections speaks of highly textured (rather than epitaxial) material.

The reflections labeled with E are assignable to (0 0 1)-oriented Cr2O3 eskolaite. The

form of the latter indicates that Cr2O3 has grown also highly oriented; however, in

comparison to TiO2, it has a less pronounced texture. The intensity analysis of the

reflections suggests that comparable amounts of both materials contributed to

diffraction.

Fig. 3(b) shows the diffraction pattern for the laminate deposited on Si-substrate. Now

the reflections can be assigned to the terminating Cr2O3 eskolaite layer. The pattern

indicates that the layer has grown polycrystalline with very little preferred orientation.

It should be noted that RHEED results for as-grown laminates are in good agreement

with the X-HRTEM ones for FIB processed laminates. Hence it follows that FIB

processing insignificantly changed the inside-slice structure of the laminates.

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3.4. Depth averaged structural data

Fig. 1.

Depth-averaged crystalline structure of the nanolaminates was determined on the basis

of Raman spectra (Fig. 4). In the Raman experiments, the information depth in principle

exceeded the thickness of the samples. For this reason the spectrum of the laminate

grown on the Si substrate (Fig. 4, curve 2) was taken from a sample in which the Si

substrate was chemically removed to get rid of Si-caused strong spectral background. A

Raman spectrum was taken also from a laminate grown on amorphous 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 the

laminate. Two weaker bands (at 520 cm-1

and 638 cm-1

) due to anatase were also

revealed. Neither curve 2 nor curve 3 shows the presence of TiO2 rutile. Thus, the

laminates grown on SiO2 and Si contained polycrystalline fine-grained anatase TiO2 and

eskolaite Cr2O3. Small crystallite size is the most probable reason for why the RHEED

reflections from TiO2 were not seen on the background of the Cr2O3 reflections

originating from the topmost layer.

Contrary to the above two cases, the spectrum for the laminate on sapphire (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 and having symmetries Eg and A1g, respectively, appeared.

The fact implies that TiO2 in the laminate has attained the phase-pure rutile form. The

eskolaite A1g band at 557 cm–1

is now blue shifted by 7 cm-1

compared to the position on

curves 2 and 3 and broadened in some degree. The shift speaks of the in-plane

compressive strain in Cr2O3 [12]. The reason for the revealed broadening of the Raman

band may be the strain that varies with the layer distance from the substrate.

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4. Conclusions

The data obtained show a highly oriented ALD growth of a nanolaminate consisted of

four TiO2–Cr2O3 double layers with a single layer thickness of ~10 nm on (0 1 2)-

oriented sapphire, and the growth of this nanolaminate with a very little preferred

orientation on (0 0 1)-oriented silicon. The laminate deposited on sapphire had, along

with better crystallinity, more exactly defined and more planar interphase boundaries.

The amount of indefiniteness of the boundaries increased with the layer distance from

the substrate. The crystalline phase of titania was rutile in the laminate grown on

sapphire and anatase in the laminate grown on silicon. In the laminate grown on

sapphire, titania contained numerous twins; compressively strained chromia has in this

case more perfect eskolaite structure.

Acknowledgments

The participation in the preparation of TEM samples of Steve Reyntjens from FEI and

Fabian Perez-Willard from Carl Zeiss NTS is gratefully acknowledged. The work

belonged to Estonian government targeted theme SF0180046s07 and was partially

supported by Estonian Science Foundation (Grants 6651 and 6999).

References

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[5] R.L. Puurunen, J. Appl. Phys. 97 (2005) 121301.

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[6] R. Pärna, A. Tarre, A. Gerst, H. Mändar, A. Niilisk, T. Uustare, A. Rosental,

V. Sammelselg, Proc. SPIE 6596 (2007) 659618.

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[10] C. Limberg, Chem Eur. J. 6 (2000) 2083.

[11] V. Sammelselg, J. Karlis, A. Kikas, J. Aarik, H. Mändar, T. Uustare, Atomistic

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Figure captions

Fig. 1. X-HRTEM images for the nanolaminates consisted of four TiO2–Cr2O3 double

layers on Al2O3 (a, c, d; the latter two panels depict the images taken after additional

FIB-polishing from the first and last layers in the laminate, respectively) and on Si (b).

The arrows in d point to twins in TiO2, with the fast Fourier transform pattern of the

region shown in the insert.

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

layers on Al2O3 (a) and Si (b).

Fig. 4. Raman spectra for the nanolaminates consisted of four TiO2–Cr2O3 double layers

on Al2O3 (1), Si (2), and SiO2 (3). To measure the spectrum in the second case, the

laminate was made freestanding. E, R, and A denote the bands unmistakably belonging

to Cr2O3 eskolaite, TiO2 rutile, and TiO2 anatase, respectively. Asterisks indicate the

bands caused by the substrate scattering. Shown is Lorentzian curve fitting.

5. Figure 1aClick here to download high resolution image

5. Figure 1bClick here to download high resolution image

5. Figure 1cClick here to download high resolution image

5. Figure 1dClick here to download high resolution image

5. Figure 2aClick here to download high resolution image

5. Figure 2bClick here to download high resolution image

5. Figure 2cClick here to download high resolution image

5. Figure 2dClick here to download high resolution image

5. Figure 3aClick here to download high resolution image

5. Figure 3bClick here to download high resolution image

5. Figure 4Click here to download high resolution image