Thin Solid Films 519 (2011) 3716–3721

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A theoretical study on initial growth mechanism of ZrO2 film usingcyclopentadienyl-type precursor

Jie Ren a,⁎, Chengxing Cui b, Guangfen Zhou a, Yanchun Liu a, Yongqi Hu a,⁎, Baozhu Wang a

a Colleges of Science and Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, Chinab School of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, Henan 453003, China

⁎ Corresponding authors. Tel./fax: +86 311 8166 853E-mail addresses: renjie@fudan.edu.cn (J. Ren), huyq

0040-6090/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.tsf.2011.01.278

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 February 2010Received in revised form 12 January 2011Accepted 21 January 2011Available online 28 January 2011

Keywords:Density functional theoryDielectricsZirconiaAtomic layer deposition

The initial growth reaction of atomic layer deposition (ALD) of ZrO2 using Cp2Zr(CH3)2 (Cp_C5H5,cyclopentadienyl) as metal precursor on the hydroxylated silicon surface is investigated by using densityfunctional theory (DFT). The ALD cycle is achieved through two types of ligand elimination reactions (i.e., CH4

and CpH elimination reactions). The possible reaction pathways are proposed in order to find the dominantinitial reaction during the Cp2Zr(CH3)2 precursor pulse. DFT calculations show that the CH4 eliminationreaction is energetically more favorable than CpH elimination reaction. As a result, the two CH3 ligands ofCp2Zr(CH3)2 may be dissociated prior to the two Cp rings during the metal precursor pulse. In addition, oneCpH elimination may occurs sequentially following the first CH4 elimination reaction according to activationbarrier analysis during the Cp2Zr(CH3)2 pulse. All the calculated results are in agreement with the ex-perimental findings.

5.@hebust.edu.cn (Y. Hu).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

With the continuous downscaling of metal oxide semiconductor(MOS) devices, silicon dioxide (SiO2) gate dielectric will reach itsphysical limits due to gate leakage and reliability issues [1,2].Currently, high dielectric constant (high-κ) materials have beenintroduced in MOS devices to replace the traditional SiO2 (κ=3.9) asgate dielectric materials, since their higher capacitances with thickerfilms can suppress leakage currents largely [3,4]. Among these high-κcandidates, ZrO2 is a promising material for applications due to itshigh permittivity (κ=17–22), wide band gap, good thermal stabilityand chemical compatibility with silicon [5,6].

By far high-κ oxide films have been extensively fabricated byatomic layer deposition (ALD) technique. Due to the characteristics ofself-limiting surface reactions, ALD can offer a lot of practicaladvantages, such as excellent conformity, accurate thickness controldown to the nanometer level and uniform film properties over a largearea [7,8]. ZrO2 thin films have been grown by ALD most commonlyusing ZrCl4 and H2O as precursor combination [9]. Unfortunately, theZrCl4/H2O process has some distinct drawbacks, such as contamina-tion of chlorine and release of corrosive HCl as the by-product.Therefore alternative precursors are being sought. So far, somechlorine-free precursors have been employed so as to improve thedielectric quality of ALD ZrO2 (Table 1) [9]. Especially, experimentshave recently shown that the organometallic cyclopentadienyl (Cpand C5H5) compounds, e.g. Cp2Zr(CH3)2, together with H2O as an

oxygen source can yield stoichiometric ZrO2 films with extremely lowimpurity contents (below 0.1 atom% for C and H) and promisingelectrical properties [21,22].

As we know that the understanding of reaction mechanism willhelp to optimize the process condition parameters and control theinterface formed between the deposited films and the siliconsubstrate. Usually, the experimental investigation of the ALDmechanism for high-κ oxide can be achieved by capturing theintermediate and final products. At present, density functional theory(DFT) has already been considered as an effective theoretical methodto explore the ALD mechanism. For example, the initial ALD-growthmechanisms of zirconia and hafnia, and the chemical kinetic andthermodynamic data, have always been given by DFT calculations[23–26]. These studies do provide insights into the ALD growthmechanism of high-κ oxides. However, most of these cases focus onthe ALD process of zirconia and hafnia in which metal chloride is usedas metal precursors, and few theoretical investigates have been madeto explore the ALD growth mechanism of these high-κ oxides usingthe cyclopentadienyl-type precursor [27–29].

Experiments [21] show that the ALD ZrO2 using Cp2Zr(CH3)2/H2Ocombination is the self-limiting growth from 210 to 400 °C. Theprecursor pulse duration has no obvious effect on the total amount ofreleased CH4 or CpH. Based on the growthmode of ALD type, we thinkthat the ALD cycle is achieved through two types of ligandelimination reactions of Cp2Zr(CH3)2, i.e. CH4 and CpH eliminationreactions. Therefore, the possible reactions are proposed (Scheme 1)in order to find the dominant initial reaction during the Cp2Zr(CH3)2precursor pulse. Reaction (R1) belongs to CH4 elimination reaction, inwhich one CH3 group of Cp2Zr(CH3)2 firstly reacts with the surface

Table 1Recently published ALD processes for the ZrO2 thin films and the studied depositiontemperature range.

Metal precursor Oxygen source Dep. temp./°C Ref.

Zr(NMeEt)4 N2O or O2 Plasma 240–320 [10]Zr(NMeEt)4 O3 225–275 [11]Zr(NMeEt)4 O2 110–250 [12](RCp)Zr(NMe2)3 (R_H, Me or Et) O3 300 [13]ZrCl2[N(SiMe3)2]2 H2O 150–350 [14](MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) H2O 300–500 [15](MeCp)2ZrMe(OMe) D2O or O3 350 [16](MeCp)2ZrMe(OMe) H2O 350 [17](MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) O3 300 or 350 [18](MeCp)2ZrMe(OBut) or(MeCp)2ZrMe(OMe)

H2O 300–350 [19]

(MeCp)2ZrMe2 or (MeCp)2ZrMe(OMe) O3 250–500 [20]Cp2ZrMe2 D2O 210–440 [21]

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hydroxyl, and R2 belongs to CpH elimination reaction, where one Cpgroup of Cp2Zr(CH3)2 firstly reacts with the surface hydroxyl.Similarly, the following reactions (R3 vs R4and R5 vs R6) are alsodesigned according to CH4 and CpH as the elimination products,respectively.

2. Computational method and models

Elam et al. [17] once used hydrolysis energy calculations to predictthe order of ligand loss of cyclopentadienyl-type precursor in thereactions of ALD-grown ZrO2. Instead, we employed the calculations ofpotential energy surface for each reaction to justify the experimentalfindings in this work. The hybrid DFT approach utilizing Becke's three-parameter exchange functional [30,31] with Lee–Yang–Parr correlationfunctional [32] (B3LYP) was used to optimize the geometries, analyze

Scheme 1. Surface reactions (R1–R6) of Cp2Zr(

the frequencies and obtain the reaction energies of each stationarypoint. We optimized all the structures and the geometrical parametersof reactants, transition states (TSs) and products using the Bernyoptimization algorithm in GAUSSIAN 03 software package [33], usinganalytic first and second derivatives of the energy at every step. Astationary point found by a geometry optimization is a minimum (localor global following its stability) when all the vibrational frequencies arereal. In contrast, it is a TS linking twominimawhen there is an imaginaryfrequency. A mixed basis set scheme was used to save computationalresource. In particular, the LANL2DZ basis set and effective corepotential were employed to describe zirconium atom, 6–31 G(d,p)basis set for chemical active atoms (i.e., the hydroxylated surface siliconatoms, hydroxyls, Cp and CH3 ligands), and 6–31 G for the remnantsilicon and the terminated hydrogen atoms. The B3LYP method andmixed basis set scheme have shown relatively good applications inidentifying the growthmechanisms and energetics for theALDof high-κoxides, such as HfO2 and ZrO2 [25,34–37].

Previously, to explore the surface reaction mechanism on thehydroxylated Si(100)-2×1 surface, Si9H12 one-dimer cluster wasfrequently used to simulate Si(100)-2×1 [23,24,34–36]. In the currentstudy, a larger hydroxylated three-dimer model Si21H24\(OH)2cluster model (Fig. 1a) was employed to diminish the cluster sizeeffect. The cluster approximation method was based on thepredominantly localized bonding of the Si(100)-2×1 surface. TheSi21H24―(OH)2 three-dimer cluster consisted of four layer siliconatoms where the top six silicon atoms composed the surface dimers.The remaining silicon atoms composed three subsurface layers thatwere terminated by hydrogen atoms to prevent unrealistic chargetransfer. The hydroxyls were used to react with the incoming Cp2Zr(CH3)2 precursor as surface sites. The surface hydroxyls used hereindicated that the initial silicon surface had been pretreated by H2Ofirstly. Moreover, only two reactive hydroxyls were kept to avoid theother neighboring hydroxyls intruding on the title surface reactions

CH3)2 on the hydroxylated silicon surface.

Fig. 1. (a) The hydroxylated Si(100)-2×1 cluster modeled by Si21H24―(OH)2 three-dimer cluster. The red, grey and small white balls represent oxygen, silicon andhydrogen atoms, respectively. (b) Cp2Zr(CH3)2 compound.

Fig. 2. Reaction pathway and predicted energetics at 0 K for the adsorption anddecomposition of Cp2Zr(CH3)2 on Si21H24―(OH)2 surface sites. The zero of energy is thesum of total energies of a precursor and a hydroxylated silicon cluster. TS1 and TS2 arethe abbreviation for the first and second transition states, respectively, IP is forintermediate product, PS1 and PS2 are for the first and second physisorbed state,respectively, and FP is for final product.

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which could make them more complicated. The structures of thesecluster models were fully optimized in all calculations. The optimizedstructure of the precursor Cp2Zr(CH3)2 is shown in Fig. 1b. Cp2Zr(CH3)2 contains two η5-Cp and two CH3 ligands. The Zr―CH3 bondlengths are both 2.29 Å and CH3―Zr―CH3 angle is 98.9°. Thedistances from zirconium ion to the centers of Cp rings (Zr―Cp) are2.29 and 2.27 Å, respectively. Individual Zr―C(π) distances to the Cprings are in the range of 2.56 to 2.60 Å. The Cp―Zr―Cp angle is 134.0°.All above calculated values are in close agreement with the crystalstructure data [38].

3. Results and discussion

3.1. Comparison between the reaction pathways of R1 and R2

In the precursor Cp2Zr(CH3)2, both the active groups (i.e., CH3 andCp) can possibly react with the Si―OH* (* denotes the active site)surface sites firstly. Thus there are two possible initial reactionpathways (R1 and R2), which are shown in Fig. 2. In R1, the surfaceSi―OH* group transfers its hydrogen atom to one CH3 group of Cp2Zr(CH3)2, resulting in the elimination of CH4 and forming anintermediate product (IP). The reaction profile at 0 K and thecorresponding geometries along the reaction pathway for R1 areshown in Figs. 2 and 3, respectively. As the starting structure of R1, thereactive CH3 of the precursor is placed above the OH-terminatedsilicon surface (Fig. 3a). After full relaxation, a physisorbed state (PS1in Fig. 2) is obtained with a calculated adsorbed energy of 23.7 kJ/mollower than the separated reactants. Unlike Y(Cp)3 and Mg(Cp)2adsorbed complexes [28,29], the Cp2Zr(CH3)2 adsorbed complex isnot formed through the surface hydroxyl coordinating to the metalcenter. As we can see in Fig. 3a, the zirconium atom cannot approachthe hydroxyl oxygen atom easily due to steric repulsion. As a result,

the adsorbed energy of R1 is lower than those obtained from theadsorption of Y(Cp)3 and Mg(Cp)2 on the Si―OH* surface (36.2 and37.7 kJ/mol, respectively) [28,29]. Moreover, the geometry ofadsorbed Cp2Zr(CH3)2 has not been obviously changed, where theCH3―Zr―CH3 angle is 98.8° and Zr―CH3 bond lengths are 2.30 and2.28 Å, respectively (Table 2). After the adsorption, one CH3 group ofthe adsorbed Cp2Zr(CH3)2 reacts with the H atom of Si―OH* surfaceand forms a four-member-ring TS composed of H―O―Zr―CH3(1)(Fig. 3b), whose energy is 47.3 kJ/mol higher than the PS1 (see Fig. 2).For the TS, Zr―CH3(1) bond length increases to 2.59 Å and CH3―-Zr―CH3 angle increases to 136.0°. At the same time, Zr―O bond isforming with a calculated bond length of 2.42 Å. Over the activationbarrier for hydrogen transfer, the reaction reaches its IP and releasesone by-product CH4 molecule. The overall reaction is exothermic by213.9 kJ/mol, indicating that the CH4 elimination reaction throughhydrogen transfer of Si―OH* to Cp2Zr(CH3)2 is energeticallyfavorable.

As we know that weak adsorption can result in non-ALDdecomposition of the precursor above the surface. Experiment hasshown that non-ALD decomposition occurs at temperature exceeding400 °C[21]. Therefore, the entropy change at 298 K (Δ S298) iscalculated in order to find whether the desorption of the adsorbedCp2Zr(CH3)2 (PS1 in Fig. 2) in R1 is thermodynamically favorable ornot. T ΔS 298 of the desorption products of the surface bound Cp2Zr(CH3)2 is 32.9 kJ/mol, 9.2 kJ/mol larger than the adsorption energy.This indicates that the desorption of the adsorbed Cp2Zr(CH3)2 isfavored in the thermodynamic equilibrium state. However, recenttheoretical studies showed that the activation energy of first CH4 non-ALD decomposition of Cp2Zr(CH3)2 in the gas phase was 147 kJ/mol[39], far higher than the energy (47.3 kJ/mol) obtained from the ALDgrowth (R1) in this study. Therefore, we think that R1 is easier tooccur for the ALD growth before the thermodynamic equilibrium isreached.

In the case of R2, one Cp group of Cp2Zr(CH3)2 abstracts the H atomfrom the surface Si―OH* group, resulting in the elimination of CpHfrom the surface and the forming of the IP state. Similar to R1, thestarting adsorbed structure is modeled by placing the Cp groups ofCp2Zr(CH3)2 above the Si―OH* surface site in R2 (cf. Fig. 4a). After fullgeometry optimization, the energy gain upon adsorption of Cp2Zr(CH3)2 is 18.5 kJ/mol compared to 23.7 kJ/mol for that obtained in R1,

Fig. 3. Geometrical structures along the reaction pathway of R1, R3 and R4: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2 and (e, e′) FP, where (d) and (e) are for R3 and (d′) and (e′) are for R4,respectively. The four-center TS is denoted by blue dashed line.

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which suggests that there exists stronger affinity of CH3 for Si―OH*surface site. Similar to R1, the geometry of the adsorbed Cp2Zr(CH3)2is also hardly changed, in which the Zr―Cp distances are both 2.28 Å

Table 2Representative bond distances (Å) and bond angles (°) in the selected state of reactions (R

State (reactions a) PS(R1) TS(R1) IP(R1) PS(R2) TS(R2) IP(R2) T

Zr―CH3(1) 2.30 2.59 – 2.29 2.28 2.25 –

Zr―CH3(2) 2.28 2.31 2.29 2.29 2.25 2.25Zr―O(1) 4.99 2.42 2.02 4.24 2.26 1.99Zr―O(2) 5.49 4.74 4.85 5.19 4.89 4.18Zr―Cp(1) b 2.29 2.28 2.28 2.28 3.09 –

Zr―Cp(2) b 2.29 2.30 2.28 2.28 2.27 2.28CH3(1)―Zr―CH3(2) 98.8 136.0 – 98.1 83.3 102.8 –

Cp(1)―Zr―Cp(2) b 133.9 129.4 129.9 132.9 115.3 – 1

a The sequence numbers of reactions are defined in Scheme 1.b Strictly speaking, the Cp ring is neither planar nor regular pentagon, so the center of Cp

very approximate value.

and the Cp(1)―Zr―Cp(2) angle is 132.9° (Table 2). After theadsorption, the reaction goes through an activation barrier with anenergy value of 127.1 kJ/mol, which is substantially higher than that

1–R6).

S(R3) FP(R3) TS(R4) FP(R4, R5) TS(R5) TS(R6) PS(R6) FP(R6)

– – – 2.50 2.24 2.25 2.242.64 – 2.25 2.25 2.25 2.24 2.25 2.242.11 2.05 2.04 2.00 2.01 1.99 1.98 1.992.32 2.05 2.17 2.00 2.31 2.18 2.02 1.992.29 2.29 3.12 – – – – –

2.32 2.29 2.29 2.29 2.27 2.97 3.62 –

– – – 82.1 106.6 113.1 109.225.5 123.9 113.6 – – – – –

ring cannot be located accurately. The Zr―Cp and Cp(1)―Zr―Cp(2) is therefore only a

Fig. 4. Geometrical structures along the reaction pathway of R2, R5 and R6: (a) PS1, (b) TS1, (c) IP, (d, d′) TS2, (e′) PS2 and (e, f′) FP,where (d) and (e) are for R5 and (d′), (e′) and (f′) are for R6,respectively. The four-center TS is denoted by blue dashed line.

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of R1. Therefore, the dissociating of the Zr―Cp(1) bond is moredifficult for R2 than that of Zr―CH3(1) for R1 by comparing the energybarriers of R1 and R2. In addition, we can find in Table 2 that theZr―Cp(1) distance is elongated to 3.09 Å and Cp(1)―Zr―Cp(2) angleis decreased to 115.3° in the TS. Finally, R2 finishes along with therelease of one CpH gas molecule. The formation of Zr―O bond is alsocompleted with a calculated bond length of 1.99 Å. The overallreaction is exothermic by 58.3 kJ/mol for R2, far lower than that of R1.In conclusion, the CH4 elimination reaction of Cp2Zr(CH3)2 (R1) isenergetically more favorable than the CpH elimination reaction (R2)on the hydroxylated silicon surface.

3.2. Comparison between the reaction pathways of R3 and R4

After elimination of the first CH4 in R1, the formed Cp2ZrCH3

fragment can react with the neighboring Si―OH* site. We assume thatthe reaction would branch according to either CH3 or Cp as the startingreactive group. One branch is CH4 elimination reaction (i.e., R3 inScheme 1). The other is the CpH elimination reaction (i.e., R4 inScheme 1). The reaction pathways and the corresponding energies atthe stationary points for R3 and R4 are shown in Fig. 2. Thecorrespondinggeometries along thepathways of R3andR4are depicted

in Fig. 3. Starting from the IP state, R3 goes through a four-center TScomposed of H―O―Zr―CH3(2) similar to R1 (Fig. 3d). The activationbarrier (TS2 in Fig. 2) is 99.5 kJ/mol relative to the IP. The Zr―O(1) andZr―O(2) bond distances of the second TS are 2.11 and 2.32 Å,respectively, and the Zr―CH3(2) bond length increases to 2.64 Å.Finally, R3 reaches the final product (FP) with a total exothermicenthalpy (ΔH0) of 318.8 kJ/mol relative to the initial reactants.

As another possible reaction, R4 is also a proton transfer reactionwhere oneCp ringof Cp2ZrCH3 accepts theH atom from theneighboringSi―OH* and releases one CpH molecule. The activation barrier of R4 iscalculated to be 105.0 kJ/mol, which is 5.5 kJ/mol slightly higher thanthat of R3. Also, calculations show that R4 has a decreasingexothermicity with 205.2 kJ/mol as compared to R3. Therefore, we candeduce that it is energetically less favorable for R4 than R3.

3.3. Comparison between the reaction pathways of R5 and R6

After elimination of the first CpH in R2, the adsorbed CpZr(CH3)2fragment reacts with the neighboring hydroxyl on silicon surface,where the reaction network can branch similarly. One alternative isCH4 elimination reaction (i.e., R5 in Scheme 1). The other is the CpHelimination reaction (i.e., R6 in Scheme 1). The reaction pathways and

3721J. Ren et al. / Thin Solid Films 519 (2011) 3716–3721

energetics for R5 and R6 are shown in Fig. 2. The energy of the TSrelative to the IP is calculated to be 39.4 kJ/mol for R5. It could befound that the reaction is exothermic and the enthalpy change is205.2 kJ/mol. In addition, we can see in Fig. 2 that the reactionpathway of R6 is different from R5. The only difference is that oneweak bound by-product CpH is found in R6. Calculation shows that R6must overcome a 3.9 kJ/mol adsorption well (i.e., PS2 in Fig. 2) toreach the FP state. The additional adsorption energy possibly resultsfrom the Zr―C(π) interaction between zirconium ion and Cp(2) ring.The individual Zr―C(π) distances are calculated to be in the range of3.24 to 4.40 Å. As a comparison, the stable physisorption of the by-product CpH on the Si―OH* surface, however, is not found in R2 andR4. Therefore, it can be deduced that the physisorption in R6 is due tothe relief of steric congestion around zirconiummetal center. It shouldbe emphasized that the physisorption is relatively weak. Thus we cansuggest that it is easy to purge the by-product out of the reactionchamber, and it is not necessary to use a long purge time if R6 occursin the actual ALD process.

The activation barrier for the CpH formation in R6 is calculated tobe 88.1 kJ/mol, which is much higher than that of R5. Moreover, R6 isexothermic by 34.3 kJ/mol, which is far lower than that of R5.Therefore, we can conclude that R5 is energetically more favorablethan R6.

3.4. Comparison between the calculations and the experimental findings

As we know that the ALD process of ZrO2 consists of alternatingCp2Zr(CH3)2 precursor and oxygen source pluses. In situ experiments[21] have detected that almost all (90%) of the CH3 ligands werereleased during the Cp2Zr(CH3)2 precursor pulse by using quadrupolemass spectrometer. By comparing the competitive surface reactions(R1–R6), we can argue that the CH4 elimination reaction on theSi―OH* surface site has more favorable thermodynamics than theCpH elimination reaction. In other words, both R1 and R3 are theenergetically more favorable pathways, i.e., the dominant pathways.

Theoretically, the two CH3 ligands of Cp2Zr(CH3)2 can bedissociated prior to the two Cp rings, which is in excellent agreementwith the above-mentioned experimental findings. In addition, theexperiment showed that about 40% of the CpH was released duringthe Cp2Zr(CH3)2 pulse [21], which indicates that there exists a CpHelimination reaction. By comparison of the above-mentioned com-peting reactions, we find that the activation barrier of R2 is 79.8 kJ/mol higher than that of R1, however, in the case of R4, the energybarrier is only 5.5 kJ/mol slightly higher than that of R3 (see Fig. 2).We can therefore conclude that R4 probably occurs sequentiallyfollowing R1 if long pulse duration of Cp2Zr(CH3)2 precursor is appliedin the ALD of ZrO2 films.

4. Conclusion

The initial surface reaction mechanism of ALD-grown ZrO2 on thehydroxylated Si(100)-2×1 surface using Cp2Zr(CH3)2 as metalprecursor has been investigated theoretically. All possible reactionpathways were proposed according to either CH3 or Cp as the startingreactive group when it reacts with the Si―OH* surface site.Calculations show that the CH4 elimination reaction on the Si―OH*surface site has more favorable thermodynamics during the Cp2Zr(CH3)2 precursor pulse. This result explains why almost all (90%) ofthe CH3 ligands were released during the Cp2Zr(CH3)2 precursor pulsein the actual ALD experiments. In addition, the experiment alsoshowed that about 40% of the CpH was released during the metalprecursor pulse. By comparison of the activation barriers of everyreaction, we could argue that the CpH elimination reaction probablyoccurs sequentially following the first CH4 elimination reaction if

long pulse duration of Cp2Zr(CH3)2 precursor is applied in the ALDof ZrO2 films.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (No. 20973052), the Natural Science Foundationsof Hebei Province (No. B2010000845) and the Hebei University ofScience and Technology (No. XL200910).

References

[1] D.A. Muller, T. Sorsch, S. Moccio, F.H. Baumann, K. Evans-Lutterodt, G. Timp,Nature (London) 399 (1999) 758.

[2] P.A. Packan, Science 285 (1999) 2079.[3] G.D. Wilk, R.M. Wallace, J.M. Anthony, J. Appl. Phys. 89 (2001) 5243.[4] E.P. Gusev, C. Cabral, M. Copel, C. D'Emic, M. Gribelyuk, Microelectron. Eng. 69

(2003) 145.[5] R.M. Wallace, G.D. Wilk, Crit. Rev. Solid State Mater. Sci. 28 (2003) 231.[6] J. Robertson, Rep. Prog. Phys. 69 (2006) 327.[7] M. Ritala, M. Leskelä, in: H.S. Nalwa (Ed.), Handbook of Thin Film Materials, Vol. 1,

Academic Press, San Diego, 2002, p. 103.[8] M. Leskelä, M. Ritala, Angew. Chem. Int. Ed. 42 (2003) 5548.[9] L. Niinistö, J. Päiväsaari, J. Niinistö, M. Putkonen, M. Nieminen, Phys. Status Solidi A

201 (2004) 1443.[10] S.-J. Won, J.-Y. Kim, G.-J. Choi, J. Heo, C.S. Hwang, H.J. Kim, Chem. Mater. 21 (2009)

4374.[11] J.-H. Kim, V. Ignatova, P. Kücher, J. Heitmann, L. Oberbeck, U. Schröder, Thin Solid

Films 516 (2008) 8333.[12] S.J. Yun, J.W. Lim, J.-H. Lee, Electrochem. Solid-State Lett. 7 (2004) F81.[13] J. Niinistö, K. Kukli, M. Kariniemi, M. Ritala, M. Leskelä, N. Blasco, A. Pinchart, C.

Lachaud, N. Laaroussi, Z.Y. Wang, C. Dussarrat, J. Mater. Chem. 18 (2008) 5243.[14] W.-H. Nam, S.-W. Rhee, Chem. Vap. Deposition 10 (2004) 201.[15] C.L. Dezelah, J. Niinistö, K. Kukli, F. Munnik, J. Lu, M. Ritala, M. Leskelä, L. Niinistö,

Chem. Vap. Deposition 14 (2008) 358.[16] K. Knapas, M. Ritala, Chem. Mater. 20 (2008) 5698.[17] J.W. Elama, M.J. Pellin, S.D. Elliott, A. Zydor, M.C. Faia, J.T. Hupp, Appl. Phys. Lett. 91

(2007) 253123.[18] K. Kukli, J. Niinistö, A. Tamm, J. Lu, M. Ritala, M. Leskelä, M. Putkonen, L. Niinistö, F.

Song, P. Williams, P.N. Heys, Microelectron. Eng. 84 (2007) 2010.[19] M.G. Jeffrey, A.C. Jones, K. Black, P.R. Chalker, T. Leese, A. Kingsley, R. Odedra, P.N.

Heys, Surf. Coat. Technol. 201 (2007) 9095.[20] J. Niinistö, K. Kukli, A. Tamm, M. Putkonen, C.L. Dezelah, L. Niinistö, J. Lu, F.Q. Song,

P. Williams, P.N. Heys, M. Ritala, M. Leskelä, J. Mater. Chem. 18 (2008) 3385.[21] J. Niinistö, A. Rahtu, M. Putkonen, M. Ritala, M. Leskelä, L. Niinistö, Langmuir 21

(2005) 7321.[22] M. Putkonen, J. Niinistö, K. Kukli, T. Sajavaara, M. Karppinen, H. Yamauchi, L.

Niinistö, Chem. Vap. Deposition 9 (2003) 207.[23] L. Jeloaica, A. Estève, M. Djafari Rouhani, D. Estève, Appl. Phys. Lett. 83 (2003) 542.[24] Y. Widjaja, J.H. Han, C.B. Musgrave, J. Phys. Chem. B 107 (2003) 9319.[25] J.H. Han, G. Gao, Y. Widjaja, E. Garfunkel, C.B. Musgrave, Surf. Sci. 550 (2004) 199.[26] A. Dkhissi, A. Estève, C. Mastail, S. Olivier, G. Mazaleyrat, L. Jeloaica, M. Djafari

Rouhani, J. Chem. Theory Comput. 4 (2008) 1915.[27] M. Nolan, S.D. Elliott, Chem. Mater. 22 (2010) 117.[28] J. Ren, G. Zhou, Y. Hu, D.W. Zhang, Appl. Surf. Sci. 255 (2009) 7136.[29] H.-L. Lu, S.-J. Ding, D.W. Zhang, J. Phys. Chem. A 113 (2009) 8791.[30] A.D. Becke, Phys. Rev. A 38 (1988) 3098.[31] A.D. Becke, J. Chem. Phys. 98 (1993) 5648.[32] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785.[33] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A.

Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J.Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H.Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B.Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R.Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador,J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas,D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G.Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A.Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong, C.Gonzalez, J.A. Pople, Gaussian 03, Gaussian Inc., Pittsburgh PA, 2003.

[34] J. Ren, F.Y. Liu, Y.T. Zhang, D.W. Zhang, Thin Solid Films 515 (2007) 4702.[35] J. Ren, B. Sun, D.W. Zhang, Appl. Surf. Sci. 253 (2007) 9148.[36] J. Ren, H.L. Lu, W. Chen, M. Xu, D.W. Zhang, Appl. Surf. Sci. 252 (2006) 8466.[37] H. Jeon, Y. Won, Appl. Phys. Lett. 93 (2008) 124104.[38] W.E. Hunter, D.C. Hrncir, R.V. Bynum, R.A. Penttila, J.L. Atwood, Organometallics 2

(1983) 750.[39] A. Zydor, S.D. Elliott, J. Phys. Chem. A 114 (2010) 1879.