Computational study on transamination of alkylamides with NH3 ...

ARTICLE IN PRESSY.S. Won et al. / Journal of Crystal Growth 311 (2009) 3587–3591 35892p+Dimethylamine2t42.4 ‡TS42.427.6Intermediate2i28.9 ‡ 56.5TBTDMT0.0Fig. 2. Calculated energetics of the b-hydride elimination of TBTDMT to generate Ta(MeNCH 2 )(NMe 2 )[(N(t-Bu)]. Energies are reported in kcal/mol.3t-a9.43p-aTSTBTDMT+ NH 312.1 ‡+0.0TBTDMT•NH 3 - 2.7 - 3.0Dimethylamine3t-b23t-b1TS231.2TS119.83i-b31.53p-bTBTDMT+ NH 30.022.5TBTDMT•NH 3Intermediate -0.3- 2.72.4+t-butylamineFig. 3. Calculated energetics of the transamination of a dimethylamido ligand (a) and tert-butylamido ligand (b) of TBTDMT with NH 3 . Energies are reported in kcal/mol.Table 1Calculated rate constants for selected individual steps in the decomposition ofTBTDMT at different temperatures.Entry ReactionsaE a ln A a k(s 1 )(kcal/mol)25 1C 3001C 6001C1 TBTDMT-2t 36.7 18.4 2.7 10 19 9.5 10 8 0.42 TBTDMT NH 3 -3t-a 13.2 30.1 2.5 10 3 1.2 10 8 5.6 10 93 TBTDMT NH 3 -3t-b1 23.7 30.2 5.1 10 5 1.2 10 4 1.4 10 74 3i-b-3t-b2 32.2 33.4 7.6 10 10 1.6 10 2 2.8 10 6a E aand A are the activation energy and pre-exponential factor in theArrhenius form, k=A exp( E a /RT), obtained from the plot of the calculated rateconstant vs. temperature.order of magnitude even at an elevated temperature of 600 1C. Incontrast, the dimethylamido ligand exchange was calculated to bekinetically competent for film growth, while tert-butylimidoligand exchange was significantly slower due to its relativelyhigher barrier (Fig. 3b).These calculations are consistent with a switch to a transaminationpathway lowering the growth temperature when TBTDETwas used in the presence of NH 3 (growth at 300–375 1C, comparedto 500–650 1C with TBTDET as a single-source precursor) andexplains the efficient removal of carbon-containing ligands in thepresence of NH 3 compared to the facilitated carbon incorporationin the deposited films via a self-decomposition pathway (Fig. 2)[2–4,24–26]. Similar reactions are likely to be involved in the use

ARTICLE IN PRESS3590Y.S. Won et al. / Journal of Crystal Growth 311 (2009) 3587–3591N2Selected bond lengths and angles4. ConclusionsN1TaTa-N1 (Å) 2.00 (avg.)Ta-N2 ( Å) 1.78N1-Ta-N2 ( ° ) 109.35 (avg.)ΔH ° 298 (kcal/mol)Ta-N1 93.0Ta-N2 138.9Fig. 4. Optimized geometry of Ta(NH 2 ) 3 (NH) following complete transamination.N1 denotes nitrogen in the amido ligand and N2 denotes nitrogen in the imidoligand.of related precursors with NH 3 in TaN ALD executed at even lowertemperature (250 1C) [1,6–8]. However, the relatively higherbarrier for the elimination of tert-butylamine via transaminationis consistent with the observation of residual carbon content(15 at%) in the TaN ALD films using TBTDET and NH 3 at 250 1C[1,8]. The lack of difference in the carbon content (15 at%) in theALD films using the same MO source (TBTDET) with H 2 plasma[1,8] also indirectly supports the slower transamination of tertbutylimidoligand as a cause of residual carbon. ALD experimentsusing PEMAT and NH 3 , in contrast, produced much lower carboncontent (7–8 at%) in the deposited films 1 because the fiveequivalent alkylamido ligands of PEMAT are exchanged easilyvia transamination with NH 3 .To evaluate how Ta–N bonds were affected by transamination,the optimized geometry of the (NH 3 ) 3 Ta(NH), in which all theligands have been exchanged via transamination, and its calculatedbond dissociation enthalpies are shown in Fig. 4. Theincrease of bond dissociation enthalpies by 12–16 kcal/mol upontransamination is consistent with survival of the imido ‘‘Ta–N’’fragment (Ta–N2 in Fig. 4) during deposition but the strongeramido Ta–N bonds (Ta–N1 in Fig. 4) could also have a greatertendency to be carried on the film deposition. A dielectric TaN xfilm (x41, e.g., Ta 3 N 5 ) with very high resistivity would then bemore likely to result if ammonia is employed [2,9,10]. The natureof TaN x has been confirmed to range from highly conductive (Tarich)to insulating (N-rich) according its stoichiometry [9,10,29].As the film grows, NH 2 ligands resulting from transaminationare expected to be easily redistributed over vacant sites on thesurface to provide the ligands for chemisorption of additional Taspecies to the surface. The thermal stability of NH 2 (ad) on the Si(10 0) reconstructured surface was computationally demonstratedby Widjaja and Musgrave [30] using the Si clusterapproximation. The remnant Ta–N bond was then preserved todeposit metallic TaN films. The adatoms, NH 2 (ad)s, however,contribute to the deposition of dielectric N-rich Ta 3 N 5 films [2].The concentration of NH 2 (ad) adatoms on the surface would besufficiently high for them to be incorporated into the depositedfilms in the absence of an additional reducing agent. It is notedthat NH 3 is also a source of NH 2 (ad) adatoms by the dissociationinto NH 2 (ad) and H(ad) [30]. TaN films grown by MOALD as wellas MOCVD generally demonstrated high film resistivity, whichcircumvents their usefulness as diffusion barriers. The ALD of TaNfilms using TBTDET and H 2 plasma (instead of NH 3 ) demonstrateda huge improvement in the film resistivity [1] possibly due to theaddition of active reducing H 2 radicals. The amorphous phaseformation with the use of NH 3 in TaN MOALD [1] is also likelyattributed to the presence of stable NH 2 (ad) adatoms on thesurface especially at the low temperature employed in ALD,occupying active reaction sites and blocking the properly stackedadsorption of Ta MO species.The transamination of alkylamide ligands in MO precursorswith NH 3 for the film growth of transition metal nitrides wascalculated to be facile under MOCVD conditions and would beaccessible at the very low temperature employed in MOALD aswell. Transamination is a likely pathway for removal of carboncontainingligands. 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