Thin Solid Films 545 (2013) 176182

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Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts fAtomic layer deposition of TiO2 from tetrakis(dimethylamino)titanium and H2O

Barbara Abendroth a,, Theresa Moebus a, Solveig Rentrop a, Ralph Strohmeyer a, Mykola Vinnichenko b,1,Tobias Weling c, Hartmut Stcker a, Dirk C. Meyer a

a Institut fr Experimentelle Physik, TU Bergakademie Freiberg, Leipziger Str. 23, 09596 Freiberg, Germanyb Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germanyc Institut fr Physikalische Chemie, TU Bergakademie Freiberg, Leipziger Str. 29, 09596 Freiberg, Germany Corresponding author. Tel.: +49 3731 392773; fax: +E-mail address: barbara.abendroth@physik.tu-freiberg

1 Current affiliation ofMykola Vinnichenko: Fraunhofer Inand Systems IKTS, Winterbergstr. 28, 01277 Dresden, Germ

0040-6090/$ see front matter 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.tsf.2013.07.076a b s t r a c ta r t i c l e i n f oArticle history:Received 4 January 2013Received in revised form 25 July 2013Accepted 26 July 2013Available online 3 August 2013

Keywords:Atomic layer depositionTetrakis(dimethylamino)titaniumTitanium dioxideX-ray diffractionX-ray reflectivityThe atomic layer deposition (ALD) of TiO2 from tetrakis(dimethylamino)titanium (TDMAT) andwater was stud-ied in the substrate temperature (TS) range of 120 C to 330 C.The effect of deposition temperatures on the resulting layer microstructure is investigated. Based on the exper-imental results, possible interaction mechanisms of TDMAT and H2O precursor molecules and the TiO2 surfaceat different temperatures are discussed. The TiO2 layers were characterizedwith respect to microstructure, com-position and optical properties by glancing angle x-ray diffraction and reflectometry, x-ray fluorescence analysis,photoelectron spectroscopy and spectroscopic ellipsometry. A constant layer growth with increasing number ofALD cycles was achieved for all investigated deposition temperatures, if the inert gas purge time after the H2Opulse was increased from 5 s at temperatures below 250 C to 25 s for TS 320 C. In the investigated temper-ature range, the growth per cycle varies between 0.33 and 0.67 /cycle with a minimum at 250 C.The variations of the deposition rate are related to a change from a surface determined decomposition of TDMATto a gas phase decomposition route above 250 C. At the same temperature, themicrostructure of the TiO2 layerschanges from amorphous to predominately crystalline, where both anatase and rutile are present.

2013 Elsevier B.V. All rights reserved.1. Introduction

Titanium dioxide is a wide-gap semiconductor, which is based on itsoptical and electronic properties, widely and versatilely used asgemstone, white pigment and renewable energy material in dye-sensitized solar cells or photocatalytic material for water splitting.Also in the emerging technology of non-volatile resistive switchingmemories TiO2 is used as insulator layer in metal-insulator-metal ca-pacitor structures [1,2]. Many of these applications are based on thinlayers of only a few nanometers on plane substrates or, more likely,on three dimensionally structured substrates. Since atomic layer depo-sition (ALD) facilitates the deposition of thin layers with monolayercontrol of the thickness and allows a homogeneous deposition on 3Dsubstrates, it is a widely used technique for the fabrication of thinoxide layers in microelectronics. It is however, potentially also interest-ing for the deposition of catalytic active oxide materials on porous sub-strates with large specific surfaces.49 3731 394314..de (B. Abendroth).stitute for Ceramic Technologiesany.

ghts reserved.For the atomic layer deposition of TiO2, various metal-organic precur-sors exist, some of the most commonly used are Titanium tetrachloride(TiCl4) (e.g., [36]) and Titanium isopropoxide (Ti[OCH(CH3)2]4) in com-bination with H2O or O3 as oxidant [7,8]. Tetrakis(dimethylamino)titani-um (TDMAT) is a titanium precursor, primarily used for the deposition ofTiN in combination with NH3 as reactant [9]. For the deposition of TiO2,TDMAT has the advantage that precursor and decomposition productsare non-toxic and non-corrosive. Thermal ALD can be realized with H2Oas oxidant [8].

Early reports on the use of TDMAT for ALD of TiO2 have been pub-lished in 2006 [10,11]. Up to date various oxidants have been inves-tigated in combination with TDMAT like ozone [12], O2 or Ar-O2plasma [11,1315] and H2O [8,10,11,13,1620].

A growth regime with linear increase of layer thickness with thenumber of ALD cycles is reported for substrate temperatures as low as50 C [13] ranging to deposition temperatures of up to 300 C [18]. Insome cases, a thermal gas phase decomposition of TDMAT has beenobserved for temperatures above 300 C [18].

In general, the publications cited above, report on the use of theTDMAT ALD process for the synthesis of TiO2 thin films for a specificapplication and report optimization of process parameters with re-spect to a specific property of the TiO2 layer. A systematic study ofthe deposition parameter window is published in reference [13].

http://dx.doi.org/10.1016/j.tsf.2013.07.076mailto:barbara.abendroth@physik.tu-freiberg.dehttp://dx.doi.org/10.1016/j.tsf.2013.07.076http://www.sciencedirect.com/science/journal/00406090http://crossmark.crossref.org/dialog/?doi=10.1016/j.tsf.2013.07.076&domain=pdf

177B. Abendroth et al. / Thin Solid Films 545 (2013) 176182This paper focuses on the comparison of Ti-precursors and oxidants.Regarding the growth of TiO2, here only growth rates are compared.Microstructural investigations are conducted for post depositionannealing and in-situ crystallization XRD studies.

A systematic study of the layermicrostructure of TiO2films as depos-ited from TDMAT and H2O ALD has not been published yet. Hence, thiswork presents the systematic investigation of deposition temperaturesand resulting layer microstructure. Based on the experimental results,possible interaction mechanisms of TDMAT and H2O precursor mole-cules and the TiO2 surface at different temperatures.

Also, the high temperature deposition of TiO2 from TDMAT and H2Ois examined with the aim to use this process for the ALD of ternary tita-nates such as SrTiO3 or BaTiO3 which use a Ba- or Sr- cyclopentadienylprecursor and require deposition temperatures exceeding 250 C[21,22]. Therefore, we focus on the range of deposition temperaturesof 250 C and above.25

30

35

40

Laye

r th

ickn

ess

(nm

)

5 s10 s15 s

TDMAT pulse time: 0.15s

H2O purge time:2. Experimental

TiO2 thin films were deposited in a Savannah S100 ALD tool(Cambridge Nanotech) from TDMAT (obtained from Sigma Aldrich)and H2O precursors. N2 was used as carrier and purge gas at flowrates of 20 sccm resulting in a working pressure of 30 Pa. TheTDMAT precursor was heated in a stainless steel cylinder to 75 C.Monocrystalline (100) oriented silicon wafers with a native SiO2layer of approximately 2 nm were used as substrates. The substratetemperature TS was varied between 120 C and 330 C.

The TDMAT and H2O pulse durations were set to 0.15 s and 0.015 s,respectively, and the purge time after the TDMAT pulse was 8 s. Thesesettingswere kept constant for all deposition temperatures. Tomaintaina linear growth regime with the number of ALD cycles, however, thepurge time after theH2O pulse needed to be increased for increasing de-position temperatures from 5 s below 250 C to 25 s above 300 C. Alldeposition parameters for the tested temperature ranges are summa-rized in Table 1.

Film thickness and optical properties of the layers were determinedby spectroscopic ellipsometry (SE) in the spectral range of 0.85 eV atan angle of incidence of 75 using a Woollam M2000 spectroscopicellipsometer. For ellipsometry data analysis, the Fresnel coefficients ofthe sample are calculated for an optical layer model includingmodel di-electric functions of the substrate and the TiO2 layer. For the TiO2 layer,the model dielectric function parameters and the layer thickness arefitted to reproduce the measured ellipsometric data.

Complementary, x-ray reflectivity (XRR) measurements werecarried out to confirm the SE film thickness and to get additional in-formation on the layer density. XRR analysis was performed with aPhilips X'Pert PW3710 diffractometer using Cu K radiation withparallel beam geometry at an angle of incidence of 04. The analy-sis of the XRR data is carried out analogous to SE data analysis basedon calculation of the Fresnel reflection coefficients from a layermodel. The fit parameters in the model for x-ray reflectivity includeSiO2 layer thickness, TiO2 layer thickness and density as well as sur-face and interface roughness.

Surface roughness and morphology were characterized by atomicforcemicroscopy (AFM) by a VeecoMultimode 5 system. Selected sam-ples were characterized by scanning electron microscopy in secondaryTable 1Optimized process parameters for TiO2 ALD from TDMAT and H2O.

Substratetemperature

TDMAT pulse [s] TDMAT purge [s] H2O pulse [s] H2O purge [s]

120 C250 C 0.15 8 0.015 5250 C300 C 0.15 8 0.015 15300 C330 C 0.15 8 0.015 25electron mode using an FEI Magellan system at an accelerating voltageof 2 keV.

For the identification of crystalline phases in the TiO2 layers, glancingangle X-ray diffraction (GAXRD) under an angle of incidence of 1 wascarried out on the samediffractometer system as used for XRRmeasure-ments. At small angles of incidence the penetration depth of the x-raysis confined to the near surface area; hence, contributions from the sub-strate are mostly suppressed.

The composition of the main constituents Ti and O has beenquantified by wavelength-dispersive x-ray fluorescence spectroscopy(WDXRF) on a Bruker AXS S8 spectrometer. The Bruker AXS softwareML Quant was used to calculate the stoichiometry of the TiO2 layers.Based on a layer model, this software package calculates the re-absorption and secondary fluorescence contributions to the primaryfluorescence yield for a layered and hence vertically inhomogeneoussample.

Residual carbon and nitrogen contents of the films were measuredby x-ray photoelectron spectroscopy (XPS) on a Specs Phoibos 150MCD-9 system with Al K x-ray source.

To distinguish between surface carbon contamination and carbonincorporated in the films during deposition, the surface was cleanedin situ by argon ion sputtering for 30 s at an ion energy of 5 kV. TheXPS peak positions were calibrated using the C-C bond signal witha C 1 s binding energy of 284.5 eV.

3. Results and discussion

3.1. Deposition parameters

ALDwas tested in the temperature range of 120 C up to 330 C. Thepulse duration times of the TDMAT precursor and H2O were set to0.15 s and 0.015 s for all temperatures, which is sufficient to producea homogeneous deposition over the 100-mm diameter of the reactorchamber. It was found that the inert gas purge time following theH2O pulse is a critical parameter to obtain a linear growth regime.Fig. 1 shows the effect of H2O purge time at a TS = 300 C on thelayer thickness as measured by SE. The purge time was increasedfrom 5 s to 15 s. Clearly, insufficient H2O purge times lead to de-creasing deposition rates after approximately 500 ALD cycles or cor-respondingly, for layer thicknesses of more than 25 nm and in a highsurface roughness of the TiO2 layers. Similarly, in reference [10], de-creasing deposition rates were observed for TDMAT/water ALD ofTiO2 for deposition temperatures increasing from 150 C to 210 C.Increasing the H2O purge time to 15 s results in a linear growth.400 500 600 700

20

Number of ALD cycles

TDMAT purge time: 8sH

2O pulse time: 0.015s

Fig. 1. TiO2 layer thickness measured by SE as function of the number of ALD cycles atTS = 300 C. Non-linear growth is obtained for insufficient H2O purge times of 5 s(diamonds) and 10 s (circles). For 15 s H2O purge (triangles), the TiO2 layer thicknessincreases linearly with the number of ALD cycles.

0 100 200 300 400 500 600 700 8000

10

20

30

40

50La

yer

thic

knes

s (n

m)

Number of ALD cycles

Fig. 2. TiO2 layer thicknessmeasured by SE as function of the number of ALD cycles at sub-strate temperatures of 150 C (down triangles), 250 C (diamonds), 300 C (circles) and320 C (up triangles) and the corresponding linear regressions.

3.2

3.4

3.6

3.8

4.0

4.2

4.4

100 150 200 250 300 350

0.3

0.4

0.5

0.6

(a)

(b)

Fig. 3. TiO2 layer density (a) and GPC value (b) for 500 cycles as function of the substratetemperature. Data are obtained fromXRR analysis. The error bars on the values of the layerdensity result from the standard deviation from the least square fitting procedure. Errorbars for the GPC value result from error on the layer thickness data and are less than thesize of the symbols.

100 150 200 250 300 3500.8

1.0

1.2

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2.0

2.2

Cov

erag

e pe

r cy

cle

(101

4 at

oms/

(cm

2 cy

cle)

)

Fig. 4. TiO2 coverage per cycle as function of the deposition temperature. The surface cov-erage is calculated as total deposition of n(Ti + 2 O) atoms per cycle.

178 B. Abendroth et al. / Thin Solid Films 545 (2013) 176182The possible origins of this effect are discussed in Section 4. An effectof the purge time after TDMAT pulses on the deposition rate was notobserved at any temperature.

Hence, the H2O purge times were increased for our processes withincreasing deposition temperatures up to 25 s at 320 C (see Table 1)until a linear growth per cyclewas achieved for each temperature inves-tigated. Fig. 2 shows the linear layer growth as a function of the numberof ALD cycles for deposition temperatures ranging from 150 C up to320 C. Dashed lines are linear regressions to the data, which can be ex-trapolated through the origin for all substrate temperatures. No incuba-tion period delaying the TiO2 growth during the first few ALD cycles isobserved.

Fig. 2 demonstrates that the deposition rate varies with TS. Pre-viously, a decreasing deposition rate for increasing temperatureshas been observed in references [8,13] for constant H2O purge times.Similarly, decreasing growth rates are observed in reference [10]; how-ever, in this work, also a narrow ALD window between 120 C and150 C has been reported for the TDMAT-H2O ALD process.

To further investigate the growth mechanism, the deposition rate iscorrelated with the layer density. Fig. 3 summarizes the results of XRRdata analysis for the TiO2 deposition of 500 cycles at temperaturesranging from 120 C to 330 C. Fig. 3a shows the TiO2 layer densitiesobtained from the angle of total internal reflection together withhorizontal lines indicating the bulk densities of anatase and rutile.Fig. 3b shows the growth per cycle (GPC) values calculated fromthe layer thickness fitted from the XRR oscillations. In general, thelayer thicknesses yielded by SE and XRR data fitting agree within 12 %.

The layer density is low for deposition temperatures below approx-imately 200 C. For temperatures between 200 C and 250 C, the layerdensity equals the density of anatase, which is reported in the range of3.793.98 g/cm3 [23]. For higher temperatures, the density increasesfurther to about 4.1 g/cm3 but does not reach the value of rutile of4.25 g/cm3 [24]. The deposition rate decreases continuously from aGPC value of 0.57 /cycle at 120 C to a minimum of 0.33 /cycle at250 C. At substrate temperatures exceeding 250 C the GPC thenincreases to 0.6 /cycle at 330 C.

If the densities of the layers change with increasing depositiontemperature, a constant deposition rate, which is assumed for anideal ALD window, cannot be expected. To account for an increasinglayer density, an atomic deposition rate or surface coverage of TiO2per cycle is more appropriate. Assuming stoichiometric layers witha molar weight of 79.87 g/mole and neglecting carbon or nitrogencontaminations, an atomic surface coverage per cycle was calculatedfrom the layer density and thickness. The results in Fig. 4 clearlypoint out that the increase in density is not singularly responsiblefor the changes in deposition rate since the rate of surface coveragemoderately decreases with increasing temperatures up to 250 Cand then strongly increases for further increased deposition temper-atures. The minimum of the deposition rate at 250 C indicates thattwo different chemical reaction mechanisms are active for the TiO2growth from TDMAT and H2O, as will be discussed in Section 4.

179B. Abendroth et al. / Thin Solid Films 545 (2013) 1761823.2. Film properties

The TiO2 films have been characterized with respect to composition,microstructure, morphology and optical properties.

3.2.1. CompositionThe composition is quantified regarding the main constituents Ti

and O by WDXRF. The analysis of the Ti K1, Ti K2 and O K1 fluo-rescence line intensities yields for all samples an atomic compositionof Ti: 33.6 0.8 at.% and O: 66.4 0.8 at.%, which represents stoi-chiometric TiO2. Photoelectron spectroscopy was used to detect re-sidual carbon and nitrogen from possibly incomplete precursormolecule dissociation. Since XPS is very sensitive to the surface ofthe specimen, atmospheric contaminations contribute strongly. Toclean the surface from atmospheric carbon-containing species, Arion sputtering was carried out. Due to preferential sputtering, thelayer composition is however altered, and the Ti/O ratio obtainedin this way is not comparable to results yielded by WDXRF. XPS re-sults show that after sputter cleaning carbon is present in all samplesin concentrations of 1 at.% or lower. Nitrogen was detected with in-creasing concentrations for increasing substrate temperatures witha maximum value of about 6 2 at.% at TS = 320 C.

3.2.2. MicrostructureFig. 5 shows the GAXRD data of a 47-nm-thick TiO2 layer deposited

at 320 C. Except the substrate-related reflections, marked by an as-terisk, all reflections can be assigned to anatase [25] or rutile [26]. Bothcrystalline phases can be detected for depositions above 250 C, whichis consistent with the observed increase of the layer density.

As determined by Gaussian profile fitting, the full width at halfmaximum (FWHM) is 0.62 0.06 for the anatase 101 and 1.00 0.17 for the rutile 110 reflection for the data shown in Fig. 5. Withinthe statistics of the measurement, the FWHM of the anatase 101 re-flection does not change with temperature or the number of deposi-tion cycles, whereas the FWHM of the rutile 110 reflection shows nodependence on deposition temperature but reduces from 1.2 to0.75 when the number of ALD cycles is increased from 200 to 700.The Scherrer formula FWHM = ks /L cos(0) is widely used to cal-culate the crystallite size from the broadening of XRD reflections. is the incident wavelength (here 1.542 for Cu K radiation), L thesize of crystallites in one dimension, 0 is the diffraction angle ofthe considered reflection and ks is a correction factor whose valueis near unity [27]. Here, we use ks = 1. Applying the Scherrer30 40 50 60 700

200

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600

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1000

*

112

301

310

002

220

200

211

210

111

200

004

101

110

Inte

nsity

(ar

b. u

nits

)

Diffraction angle 2 (degree)

RutileAnatase

101

*

Fig. 5. Grazing incidence XRD results of TiO2 layers resulting from 700 cycles deposi-tion at 320 C. The reference positions of the anatase [25] (down triangle) and rutile[26] (diamond) reflections are marked as well. Reflections marked by an asterisk inthe region of 3035 and the sharp reflection at 52.5 and the broad reflectionaround 55 originate from the single crystalline silicon substrate.formula to our results, we obtain crystallite sizes of 15 2 nm foranatase. Calculated crystallite sizes for rutile are 10 2 nm for 200and 300 deposition cycles and increasing to 13 2 nm for 700 de-position cycles. Since the crystallite size is however in the order ofthe film thickness, the values are considered as estimates and indic-ative only for a predominantly crystalline growth for substrate tem-peratures above 250 C.

The evolution of microstructure with deposition temperature in-fluences also the surface morphology of the TiO2 films, which can becharacterized by atomic force microscopy. Fig. 6 shows the AFM im-ages of the TiO2 films obtained from 500 ALD cycles at substrate tem-peratures of 250 C, 300 C and 320 C. For depositions below 250 C(not shown here), the layers are smooth and the surface roughness isdefined by the substrate roughness. For deposition at 250 C (Fig. 6a)the AFM image shows still a smooth surface with circular dents ofseveral 100 nm diameter and small clusters with a diameter of 10to 15 nm and an elevation of few nm off the surface. Increasing thedeposition temperature to 300 C and 320 C (Fig. 6b and c), the surfacemorphology shows a fine grained microstructure. At 300 C, the grainsize is uniform and typically 50 nm in diameter and 57 nm in height.At 320 C, the grains have a size distribution,mostly below30 nm in di-ameter and less than 5 nm high, but with few grains of up to 80 nm indiameter and up to 15 nm in height. This grain size cannot be compareddirectly to the crystallite size calculated from broadening of the GAXRDreflections because under an angle of incidence of 1, XRD line broaden-ing is mainly sensitive to the direction perpendicular to the surface,whereas AFM shows the lateral dimensions.

High-resolution scanning electron microscopy images of the layersgrown at 250 C and 300 C, shown in Fig. 7, underline the differencein growth mode, which takes place at 250 C. The SEM secondaryelectron image of the TiO2 layer deposited at 250 C shows, similarto the AFM image, isolated coarse regularly shaped areas and addi-tionally also cracks. The observed cracks run always through themiddle of these areas and meet by an angle of 90, which is typicalfor shrinkage. For the sample deposited at 300 C the SEM imageshows a homogenous fine grained structure which corresponds tothe surface morphology seen in AFM.

From AFM and SEM results, we conclude that at deposition temper-atures near 250 C, the initial growth corresponds to amorphous TiO2.Starting at a layer thickness of about 10 nm, a nucleation of crystallinephases is initiated inside the layer. Due to the higher density of the crys-talline phase compared to the amorphous TiO2, this leads to shrinkageand cracks. Similar observations have been published by Gago et al.[28] for a low temperature (b100 C) physical vapor deposition processwhere, however, particle energies are higher than for ALD. For substratetemperatures above 250 C in ALD, the crystalline phases nucleate di-rectly on the substrate resulting in fully crystalline layers.

3.2.3. Optical propertiesFor ellipsometric data analysis, an optical layer model was used

comprising the Si substrate, native oxide and TiO2 layer. Each com-ponent is described by the corresponding optical constants (refrac-tive index n and extinction coefficient k) and layer thickness. Forthe silicon and silicon oxide, the optical constants were taken fromreference [29]. The TaucLorentz model was used to describe thecomplex refractive indexN = n + i k of the TiO2 layer in the spectralrange of 0.8 to 5 eV, hence including the interband transition [30]. Abroad low-intensity absorption band was detected in the sub-bandgap region, which is included in the dielectric function modelby an additional Lorentz-type oscillator.

The absorption coefficient can be obtained from the model di-electric function of the TiO2 layer by = 4 k / , with being thewavelength of the incident photons in units of cm. Fig. 8 shows as afunction of the incident photon energy for layers obtained from 700ALD cycles for different deposition temperatures. Clearly visible is thebroad absorption band around 1 eV for the layer deposited at 250 C.

Fig. 7. SEM images of the surfacemorphology of TiO2 films deposited at substrate temper-atures of 250 C (a) and 300 C (b). Images were recorded at a magnification of 50000times in secondary electron mode at an accelerating voltage of 2 kV.

Fig. 6. AFM image of the surface morphology of TiO2 films for deposition temperatures of250 C (a), 300 C (b) and 320 C (c).

180 B. Abendroth et al. / Thin Solid Films 545 (2013) 176182Generally, it would be evident to assign this absorption band to C orN defects, originating from the precursor molecule. This absorptionband, however, is detected for all layers, but with a significantlower intensity at all deposition temperatures other than 250 Cand is independent of the measured N concentration in the layers.The strong appearance of the absorption band at this temperature,however, coincides with a change in the TDMAT decompositionmechanism and the beginning of crystallization in the layers.

The position of the optical gap Eg, which corresponds to the onset ofabsorption into extended states in an amorphous semiconductor, can beestimated by plotting

ffiffiffiffiffiffiffiffiffi

Ep

as function of photon energy E. In this so-called Tauc plot, the linear range is extrapolated to zero, and Eg is readfrom the intersection with the abscissa. The Tauc plot is shown inFig. 8b. Independent of the deposition temperature Eg is near 3.4 eV,which is a typical value for nanostructured and amorphous TiO2 (e.g.references [31,32]).

4. Discussion

The experimental results reported here show two general features ofthe TiO2 layer growth fromTDMAT andH2OALD. Thefirst observation isthat the H2O purge times have to be increased for substrate tempera-tures above 250 C. In this temperature range, the thin film surfacechanges from an amorphous TiO2 to anatase and rutile terminationand at the same time the decompositionmechanism of TDMAT changesfrom surface dominated to surface plus gas phase decomposition[33,34]. Both effects may induce changes in the deposition mechanismand in turn of the growth rate, as compared to substrate temperaturesbelow 250 C. This will be discussed in the following.

The adsorption of water on anatase surfaces takes place on twostructurally different sites and desorption from these sites takes place

image of Fig.6

1 2 3 4 50

2x105

4x105

6x105

8x105

1x106

3.0 3.5 4.0 4.5

Abs

orpt

ion

coef

ficie

nt

(cm

-1)

Sqr

t (*

E)

(arb

. uni

ts)

(a)

(b)

Fig. 8. Graph a) shows the absorption coefficient for TiO2 layers deposited at 120 -C(dotted line), 250 C (solid line), 300 C (dashed line) and 320 C (dash-dotted line).Fig. 8b shows the Tauc plot

ffiffiffiffiffiffiffiffiffi

Ep

(eV) and linear regressions to estimate the onset of op-tical band gap.

181B. Abendroth et al. / Thin Solid Films 545 (2013) 176182at about 370 K and 470 K (97 C and 197 C) [35]. On rutile, three ener-getically different adsorption sites for water are reported. Temperatureprogrammeddesorption studies showed thatwater desorption from ru-tile surfaces peaks at 350 K and 500 K (77 C and 227 C). Wateradsorbed to the lower energy site can be dissociated to form isolatedOH groups, which then recombine to H2O and finally desorb into thegas phase at 590 K (317 C) [35].

It is suggested that the combination of a thermally activated andhence delayed desorption of H2O from a rutile surface and a gas phasedecomposition of TDMAT leads to an unwanted chemical reaction inthe gas phase during the TDMAT pulse if the H2O purge time is tooshort. Based on the mechanisms of TDMAT gas phase decompositionsuggested in reference [33], possible gas phase reaction products of aTDMAT/H2O reaction could be either Ti(N(CH3)2)3-OH, which is inertto chemisorption to surface OH groups [8] and hence remains in thegas phase and does not contribute to deposition. In this case, a lowerbut constant deposition rate should be expected. However, as shownin Fig. 1, a non-linear deposition rate is observed with increasing num-ber of ALD cycles.

An alternative explanation could be the gas phase formation a reac-tion product including stable Ti = N bonds, which condenses on theTiO2 surface.

The formation of stable titaniumnitride or oxynitride clusters on thesurface could block the formation of surface OH groups at these sitesand hence inhibit the ongoing TiO2 ALD reaction, which relies on thedocking of TDMAT to a surface OH group. Such amechanism is conformwith the observation of increased N incorporation into the layers and anon-linear deposition rate with increasing number of deposition cyclesas shown in Fig. 1.

The second observation in this paper was that for adjusted purgetimes a linear increase of the layer thickness with the number of ALDcycles can be obtained; however, the growth rates vary with temper-ature. We exclude a strong TDMAT condensation effect at lower sub-strate temperatures since C concentrations are below 1 at% and thelayer densities are close to ideal amorphous TiO2. We assume thateither the adsorption rate of TDMAT to the TiO2 surface or the oxida-tion rate of the chemisorbed TDMAT molecule decreases with in-creasing surface temperatures.

The increase in growth rate for TS above 250 C is attributed to thebeginning of gas phase decomposition of TDMAT and an increasingchemical vapor deposition (CVD) effect.

5. Conclusions

TiO2 thin films have been produced by ALD from TDMAT and H2Oprecursors in the temperature range from 120 C to 330 C. Increas-ing inert gas purge times after the H2O pulse are required to ensure alinear growth regime for temperature above 250 C. This is attribut-ed to changes of the water desorption behavior on amorphous andrutile TiO2 surfaces. For TS 120 C250 C, the TiO2 layer density in-creases with increasing temperature, whereas the atomic depositionper cycle decreases to a minimum at 250 C. For higher TS the depo-sition rate increases due to gas phase decomposition of the TDMATmolecule and an increasing CVD effect. For deposition temperaturesabove 250 C, the layer grow predominantly crystalline, where bothanatase and rutile are detected. Since an atomic layer growth withlinear thickness dependence can be established also for depositiontemperatures above 250 C, the TDMAT-O ALD process is well suit-able for the ALD of ternary oxides like BaTiO3 or SrTiO3, where depo-sition temperatures are generally in the range of 250300.

Acknowledgement

Wewould like to thank Dr. I. Gerstmann of FEI for the scanning elec-tron microscopy images. Part of this work was performed within theCluster of Excellence Functional Structure design of new high perfor-mance materials via atomic design and defect engineering (ADDE)that is financially supported by the European Union Regional Develop-ment Funds and by the Ministry of Science and Art of Saxony(SMWK). Part of this workwas performedwithin the Initiative and Net-working Fund of the German Helmholtz Association, Helmholtz VirtualInstitute VH-VI-442 MEMRIOX.

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Atomic layer deposition of TiO2 from tetrakis(dimethylamino)titanium and H2O1. Introduction2. Experimental3. Results and discussion3.1. Deposition parameters3.2. Film properties3.2.1. Composition3.2.2. Microstructure3.2.3. Optical properties

4. Discussion5. ConclusionsAcknowledgementReferences