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Organometallics 2009, 28, 1838–1844
Synthesis of Zirconium Guanidinate Complexes and the Formation of Zirconium Carbonitride via Low Pressure CVD Stephen E. Potts,† Claire J. Carmalt,*,† Christopher S. Blackman,† Fawzi Abou-Chahine,† David Pugh,† and Hywel O. Davies‡ Materials Chemistry Centre, Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom, and SAFC Hitech Ltd., Power Road, Bromborough, Wirral CH62 3QF, United Kingdom ReceiVed NoVember 1, 2008
Thin films of zirconium carbonitride have been deposited on glass at 600 °C from two novel guanidinate precursors: [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) and [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2) (Cp′ ) monomethylcyclopentadienyl). Both compounds 1 and 2 were structurally characterized by X-ray crystallography. The films grown via low pressure chemical vapor deposition (LPCVD) from compound 1 were mirrorlike with a red-brown sheen whereas those from 2 were gray. Chlorine was present in the films although the levels were no higher than 3 at.%. These two compounds clearly show how the coordination environment around the metal center affects the composition of the film, as films from 1 were nitrogen rich and those from 2 were carbon rich. The films were uniform, adhesive, abrasion resistant, conformal, and hard, being resistant to scratching with steel and brass scalpels. Introduction Electronic devices and gadgets are essential to modern life, but over time, the demand for more functionality and smaller devices is ever increasing. To achieve these requirements, circuitry must be significantly reduced in size. In microcircuitry, a conducting barrier layer is essential between the silicon in components and the metal (usually aluminum or copper) connects, as it prevents the diffusion of the metal into the silicon, which would destroy the functionality of the circuit. As devices such as integrated circuits need to become smaller and more reliable, so must the barrier layer.1-3 The barrier layer can be deposited by physical vapor deposition (PVD), where deposition can be achieved at lower temperatures although the films are poorly adhesive,4 chemical vapor deposition (CVD), which affords higher growth rates, better adhesion and better step coverage, or atomic layer deposition (ALD), which gives greater control over film thickness and growth, as the process is self-limiting.5,6 CVD involves the reaction of precursors either on the substrate or in the gas phase, and the method of precursor introduction can be varied to suit the precursor or application; for example, atmospheric pressure CVD (APCVD) is suited to online processes, aerosol-assisted CVD (AACVD) is best for precursors with low volatility and low pressure CVD (LPCVD)
* To whom correspondence should be addressed. E-mail: c.j.carmalt@ ucl.ac.uk. Tel: +44 (0)207 679 7528. Fax: +44 (0)207 679 7463. † University College London. ‡ SAFC Hitech Ltd. (1) Kim, H. J. Vac. Sci. Technol. B 2003, 21, 2231. (2) McElwee-White, L.; Bochir, O. J.; Johnston, S. W.; Cuadra, A. C.; Anderson, T. J.; Ortiz, C. G.; Brooks, B. C.; Powell, D. H. J. Cryst. Growth 2003, 249, 262. (3) Yong, K.; Lee, B. H. J. Electrochem. Soc. 2004, 151, C594. (4) Hoffman, D. M. Polyhedron 1994, 13, 1169. (5) Gordon, R. G.; Becker, J. S.; Suh, S.; Wang, S. Chem. Mater. 2003, 15, 2969. (6) Gordon, R. G.; Farmer, D. B.; Li, Z.; Lin, Y.; Vlassak, J. Electrochem. Solid-State Lett. 2005, 8, G182.
is applicable to batch production of small area deposition, such as circuitry.7 Zirconium nitride has both conducting (ZrN) and metastable insulating (Zr3N4) forms. Zr3N4 is the result of metal vacancies in the face-centered cubic lattice8,9 and is metastable because it decomposes via the loss of molecular nitrogen at 800 °C to form ZrN.10 However, it is ZrN that is of interest for use in barrier layers as it has a low resistivity of 17-22 µΩ.cm at 20 °C.11 Other applications of ZrN include hardness coatings for machinery12 and decorative alternatives to gold plating.13 Zirconium carbide adopts only the face-centered cubic structure, although the stoichiometries vary widely: ZrCx (x ) 0.55-0.99). It has a similar resistivity to the nitride (35-55 µΩ.cm at 20 °C) but shows less resistance to chemical attack: it reacts readily with halogens, it is dissolved by cold acids, and it will easily form a solid solution with zirconium nitride or zirconium oxide as the lattices are similar.14 Due to their respective resistivities, a solid solution of ZrN and ZrCx would still be suitable as a barrier layer. There are not many examples of precursors for the CVD of zirconium nitride as typically PVD techniques, such as the cathodic arc technique,15 are used for thin film growth. Indeed, (7) Pierson, H. O. Handbook of Chemical Vapour Deposition (CVD), 2nd ed.; William Andrew Publishing: Norwich, NY, 1999. (8) Chhowalla, M.; Emrah Unalan, H. Nat. Mater. 2005, 4, 317. (9) Ivanovskii, A. L.; Medvedeva, N. I.; Okatov, S. V. Inorg. Mater. 2001, 37, 459. (10) Lerch, M.; Fu¨glein, E.; Wrba, J. Z. Anorg. Allg. Chem. 1996, 622, 367. (11) Pierson, H. O. Handbook of Refractory Carbides and Nitrides. 11: Interstitial Nitrides: Properties and General Characteristics; William Andrew Publishing: Norwich, NY, 1996; p 181. (12) Liu, F.; Meng, Y.; Ren, Z.; Shu, X. Plasma Sci. Technol. 2008, 10, 170. (13) Mishra, B.; Grant, W.; Niyomsoan, S.; Olson, D. L. Thin Solid Films 2002, 415, 187. (14) Pierson, H. O. Handbook of Refractory Carbides and Nitrides. 4: Carbides of Group IV; William Andrew Publishing: Norwich, NY, 1996; p 55. (15) Devia, A.; Arango, Y. C.; Arias, D. F. Appl. Surf. Sci. 2006, 253, 1683.
10.1021/om801053y CCC: $40.75 2009 American Chemical Society Publication on Web 02/17/2009
Synthesis of Zirconium Guanidinate Complexes Scheme 1. Canonical Forms of Guanidinate Ligands
precursors which may seem promising for ZrN have been used in the presence of oxygen to form ZrO2 thin films, such as the hydrazido compound [Zr{η2-N(Me)NMe2}4],16 and guanidinate complex [Zr(NEtMe)2{η2-(iPrN)2CNEtMe}2].17 However, there have been some attempts to deposit zirconium nitride, mainly focusing on homoleptic amido compounds such as [Zr(NR2)4] (NR2 ) NMe2, NEtMe, NEt2). Both APCVD18,19 and ALD20 with ammonia have been attempted, yielding Zr3N4, which is not suitable for use as a barrier layer as it is insulating. There are no known examples of zirconium guanidinate complexes that have been used as successful precursors to zirconium carbonitride thin films. Compounds containing guanidinate ligands have received interest as precursors to metal nitride thin films21-26 due to their nitrogen content, potential to increase the volatility of the compound and their ability to stabilize the metal center due to their electronic flexibility.27 The presence of the NR2 moiety in these ligands results in the possibility of a zwitterionic resonance structure (B), as shown in Scheme 1. Donation from the dialkylamido lone pair into the ligand could result in its (16) Hoffman, D. M.; Javed, S.; Lehn, J.-S. M. Chem. Vap. Deposition 2006, 12, 280. (17) Fischer, R.; Devi, A.; Bhakta, R.; Milanov, A.; Hellwig, M.; Barreca, D.; Tondello, E.; Thomas, R.; Ehrhart, P.; Winter, M. Dalton Trans. 2007, 1671. (18) Hoffman, D. M.; Gordon, R. G.; Fix, R. M. J. Am. Chem. Soc. 1990, 112, 7833. (19) Gordon, R. G.; Hoffman, D. M.; Fix, R. Chem. Mater. 1991, 3, 1138. (20) Gordon, R. G.; Becker, J. S.; Kim, E. Chem. Mater. 2004, 16, 3497. (21) Carmalt, C. J.; Newport, A.; O’Neill, S. A.; Parkin, I. P.; White, A. J. P.; Williams, D. J. Inorg. Chem. 2005, 44, 615. (22) McElwee-White, L.; Abboud, K. A.; Reitfort, L. L.; Wilder, C. B. Inorg. Chem. 2006, 45, 263. (23) Fischer, R. A.; Baunemann, A.; Rische, D.; Winter, M. Inorg. Chem. 2006, 45, 269. (24) Fischer, R. A.; Gemel, E.; Parala, H.; Rische, D.; Winter, M. Chem. Mater. 2006, 18, 6075. (25) Fischer, R.; Rische, D.; Parala, H.; Baunemann, A.; Thiede, T. Surf. Coat. Technol. 2007, 201, 9125. (26) Fischer, R. A.; Baunemann, A.; Gemel, C.; Kim, Y.; Milanov, A.; Rische, D.; Winter, M. Dalton Trans. 2005, 3051. (27) Bailey, P. J.; Pace, S. Coord. Chem. ReV. 2001, 214, 91. (28) Newport, A. C.; Bleau, J. E.; Carmalt, C. J.; Parkin, I. P.; O’Neill, S. A. J. Mater. Chem. 2004, 14, 3333. (29) Bleau, J. E.; Carmalt, C. J.; O’Neill, S. A.; Parkin, I. P.; White, A. J. P.; Williams, D. J. Polyhedron 2005, 24, 463. (30) Carmalt, C. J.; Cowley, A. H.; Culp, R. D.; Jones, R. A.; Sun, Y.-M.; Fitts, B.; Whaley, S.; Roesky, H. W. Inorg. Chem. 1997, 36, 3108. (31) Carmalt, C. J.; Cowley, A. H.; Ekerdt, J. G.; Jones, R. A.; Lall, P. S.; McBurnett, B. G.; Whaley, S. R. J. Chem. Soc., Dalton Trans. 1998, 553. (32) Carmalt, C. J.; Newport, A.; Parkin, I. P.; Mountford, P.; Sealey, A. J.; Dubberley, S. R. J. Mater. Chem. 2003, 13, 84. (33) Carmalt, C. J.; Newport, A. C.; Parkin, I. P.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002, 4055. (34) Newport, A.; Carmalt, C. J.; Parkin, I. P.; O’Neill, S. A. J. Mater. Chem. 2002, 12, 1906. (35) Newport, A. C.; Carmalt, C. J.; Parkin, I. P.; O’Neill, S. A. Eur. J. Inorg. Chem. 2004, 4286. (36) Carmalt, C. J.; Mileham, J. D.; White, A. J. P.; Williams, D. J.; Steed, J. W. Inorg. Chem. 2001, 40, 6035. (37) Hasan, P.; Potts, S. E.; Carmalt, C. J.; Palgrave, R. G.; Davies, H. O. Polyhedron 2008, 27, 1041. (38) Potts, S. E.; Carmalt, C. J.; Blackman, C. S.; Leese, T.; Davies, H. O. Dalton Trans. 2008, 5730.
Organometallics, Vol. 28, No. 6, 2009 1839 Scheme 2. Synthetic Route to Compound 1
metal complexes being electron rich when compared to other ligands, such as amidinates. We have been studying a range of nitrogen-containing metal complexes as precursors to metal nitride thin films via CVD, including TaN and NbN,28,29 TiN and TiNxCy,21,30-34 VN,35 GaN,36 ZrN, and HfN37 and WNxCy.38 This study examines and compares two novel zirconium guanidinate compounds: [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) and [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2) (Cp′ ) monomethylcyclopentadienyl) as potential LPCVD precursors to zirconium carbonitride.
Results and Discussion Precursor Synthesis. The synthesis of [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) proceeded via the intermediate compounds: [ZrCl2(NMe2)2(THF)2]39 and [ZrCl2{η2-(iPrN)2CNMe2}2]40 (Scheme 2). Treatment of [ZrCl2{η2-(iPrN)2CNMe2}2] with one equivalent of sodium monomethylcyclopentadienide yielded the monosubstituted product, compound 1, as a glassy orange solid, which formed a yellow powder when it was ground in a pestle and mortar. The formation of compound 1 was confirmed by the elemental analysis and mass spectral data with the latter showing a peak at m/z ) 545, due to the molecular ion. The 1H NMR spectrum of 1 contained five resolved doublets and three overlapping doublets arising from the isopropyl groups. This data is consistent with all four isopropyl moieties being inequivalent. There were also two peaks corresponding to the NMe2 and the methyl substituent on the monomethylcyclopentadienyl ligand. Additionally, for the monomethylcyclopentadienyl ligand, the protons in the 2- and 3-positions were resolved and were assigned to the peaks at 6.39 and 6.22 ppm, respectively, as suggested by Stobart et al. for [MCp′2] (M ) Ge, Sn, Pb).41 The only additional features of the 13C{1H}-NMR spectrum are those of the quaternary carbon on the monomethylcyclopentadienyl group at 115.6 ppm, and the carbon atoms associated with the two CN3 units of the guanidinate ligands at 164.5 and 171.5 ppm. (39) Kempe, R.; Brenner, S.; Arndt, P. Z. Anorg. Allg. Chem. 1995, 621, 2021. (40) Bergman, R. G.; Arnold, J.; Duncan, A. P.; Mullins, S. M. Organometallics 2001, 20, 1808. (41) Stobart, S. R.; Bonny, A.; McMaster, A. D. Inorg. Chem. 1978, 17, 935.
1840 Organometallics, Vol. 28, No. 6, 2009
Potts et al. Scheme 3. Synthetic Route to Compound 2
Figure 1. ORTEP plot of [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) with 50% probability ellipsoids. H atoms omitted for clarity.
Figure 2. ORTEP plot of [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2) with 50% probability ellipsoids. H atoms omitted for clarity. Table 1. Selected Bond Lengths (Å) and Angles (deg) for [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(3) Zr(1)-C(4) Zr(1)-C(5) Zr(1)-Cl(1) Zr(1)-N(1) Zr(1)-N(2) N(1)-Zr(1)-N(2) N(1)-Zr(1)-N(4) N(1)-Zr(1)-N(5) N(2)-Zr(1)-N(4) N(2)-Zr(1)-N(5) N(4)-Zr(1)-N(5) N(1)-Zr(1)-Cl(1) N(2)-Zr(1)-Cl(1) N(4)-Zr(1)-Cl(1) N(5)-Zr(1)-Cl(1) N(1)-C(13)-N(2)
2.583(3) 2.563(4) 2.560(3) 2.586(3) 2.575(3) 2.4961(9) 2.287(2) 2.260(2) 58.38(10) 103.53(10) 81.11(9) 140.07(10) 82.36(9) 58.66(9) 143.52(7) 87.50(7) 94.70(8) 81.93(7) 112.9(3)
Zr(1)-N(4) Zr(1)-N(5) C(13)-N(1) C(13)-N(2) C(13)-N(3) C(22)-N(4) C(22)-N(5) C(22)-N(6) N(1)-C(13)-N(3) N(2)-C(13)-N(3) C(13)-N(3)-C(14) C(13)-N(3)-C(15) C(14)-N(3)-C(15) N(4)-C(22)-N(5) N(4)-C(22)-N(6) N(5)-C(22)-N(6) C(22)-N(6)-C(23) C(22)-N(6)-C(24) C(23)-N(6)-C(24)
2.237(3) 2.273(3) 1.330(4) 1.331(4) 1.408(4) 1.299(4) 1.356(4) 1.421(4) 124.4(3) 122.7(3) 120.0(3) 122.1(3) 116.1(3) 112.6(3) 125.5(3) 121.9(3) 116.8(3) 117.4(3) 115.2(3)
Compound 1 was crystallized from cold (-20 °C) hexane, but the crystals were unstable above 10 °C, when they formed an amorphous glassy solid. However, the crystals were of sufficient quality to enable a low-temperature X-ray crystallographic study to take place (Figure 1). Selected bond lengths and angles are given in Table 1. The structure of compound 1 exhibits a distorted octahedral geometry due to the steric constraints of the guanidinate ligand, assuming the Cp′ moiety occupies one coordination site. This
is shown by the nonlinear angles of trans-ligands subtending the zirconium center, for example, N(2)-Zr(1)-N(4) and N(1)-Zr(1)-Cl(1) are 140.07(10) and 143.52(7)°, respectively. The Zr-C (Cp′) bonds average 2.573(3) Å, which is comparable to other substituted cyclopentadienyl ligands in the literature, although they are at the longer end of the observed range.42-44 As such, the monomethylcyclopentadienyl ligand can be considered to be bound in an η5 mode. The Cp′ ring appears to be somewhat distorted as the C(2) and C(3) carbons are closest to the zirconium center whereas C(1) and C(4) are furthest, with a difference of approximately 0.025(3) Å. C(5) lies at a distance between the two extremes, suggesting that the ring is slightly puckered. However, these observations should be treated with caution when the quality of the crystal is taken into consideration. The Zr-N (bound guanidinate) bond lengths lie in the range 2.23-2.29 Å, which is similar to those in other sterically crowded guanidinate complexes in the literature.25,26,45,46 It is also clear that the two guanidinate ligands are inequivalent, which agrees with the 1H NMR spectrum of 1. The guanidinate nitrogen, N(5), has the longest Zr-N bond length, 2.273(3) Å, due to the trans effect from the Cp′ ring. In contrast, Zr(1)-N(4), on the same ligand, is the shortest Zr-N distance in the molecule at 2.237(3) Å. This leads to the clear difference in NiPr environments in the 1H- and 13C{1H}-NMR spectra. The other guanidinate ligand, N(1)-C(13)-N(2), is more symmetrical, where Zr(1)-N(1) is the longer of the two Zr-N bonds as it lies trans to the chloride ligand, although the difference between the Zr-N distances in this ligand is not as significant as in the other ligand. Precursor [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2) was synthesized via a lithium metathesis route (Scheme 3). [ZrCp′2Cl2] was synthesized from the reaction of 2 equiv of sodium monomethylcyclopentadienide and [ZrCl4(THF)2] and purified by sublimation. The lithium guanidinate salt was prepared in situ using standard procedures by the insertion of lithium dimethylamide into diisopropyl carbodiimide.22 Treatment of [ZrCp′2Cl2] with 1 equiv of the lithium guanidinate salt afforded a beige powder in a good yield. The 1H NMR spectrum showed two different isopropyl environments, which suggests that on the NMR time scale the isopropyl groups are freer to rotate than (42) Petersen, J. L.; Jones, S. B. Inorg. Chem. 1981, 20, 2889. (43) Hey-Hawkins, E.; Lindenburg, F.; Sieler, J. Polyhedron 1996, 15, 1459. (44) Erker, G.; Albrecht, M.; Benn, R.; Dehnicke, S.; Kru¨ger, C.; Mynott, R.; Raabe, E.; Rufin´ska, A.; Sarter, C.; Schlund, R. J. Organomet. Chem. 1990, 382, 89. (45) Richeson, D. S.; Bazinet, P.; Wood, D.; Yap, G. P. A. Inorg. Chem. 2003, 42, 6225. (46) Hey-Hawkins, E.; Lindenburg, F. Z. Naturforsch., B 1993, 48, 951.
Synthesis of Zirconium Guanidinate Complexes
Organometallics, Vol. 28, No. 6, 2009 1841
Table 2. Selected Bond Lengths (Å) and Angles (deg) for [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2) Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(3) Zr(1)-C(4) Zr(1)-C(5) Zr(1)-C(7) Zr(1)-C(8) Zr(1)-C(9) N(1)-Zr(1)-N(2) N(1)-Zr(1)-Cl(1) N(2)-Zr(1)-Cl(1) N(1)-C(19)-N(2) N(1)-C(19)-N(3)
2.598(3) 2.522(3) 2.511(3) 2.523(3) 2.571(3) 2.579(3) 2.550(3) 2.518(3) 57.17(9) 139.05(7) 81.88(7) 111.7(3) 121.3(3)
Zr(1)-C(10) Zr(1)-C(11) Zr(1)-Cl(1) Zr(1)-N(1) Zr(1)-N(2) C(19)-N(1) C(19)-N(2) C(19)-N(3) N(2)-C(19)-N(3) C(19)-N(3)-C(20) C(19)-N(3)-C(21) C(20)-N(3)-C(21)
2.510(3) 2.556(3) 2.5542(3) 2.298(3) 2.274(3) 1.311(4) 1.331(4) 1.422(4) 127.0(3) 116.6(3) 116.6(3) 114.5(3)
Table 3. Crystallographic Data for Compounds 1 and 2 data
1
2
Chemical formula Formula weight T (K) Crystal system Space group a (Å) b (Å) c (Å) γ (deg) V (Å3) Z Fcalc (g cm-3) Refl. collected Unique refl. (Rint) µ (mm-1) R1 [F2 > 2σ] wR2 [all data]
C24H47ClN6Zr 546.35 150(2) Monoclinic P21/n 9.2554(13) 18.498(3) 16.649(2) 90 2823.9(7) 4 1.285 15140 5065 (0.0480) 0.506 0.0470 0.1299
C21H34ClN3Zr 455.18 150(2) Monoclinic P21/n 11.6842(17) 14.005(2) 13.931(2) 90 2199.6(5) 4 1.375 18479 5250 (0.0780) 0.631 0.0478 0.1017
those in compound 1. It is likely this is due to less steric hindrance within the molecule. The methyl groups on the monomethylcyclopentadienyl ligands and on the NMe2 moiety on the guanidinate ligand were assigned to the signals at 2.24 and 2.21 ppm, respectively. It is possible that the peak for their methyl substituents overlap, resulting in one peak. The mass spectrum of compound 2 showed the molecular ion peak at m/z ) 454, although the most intense peak occurred at m/z ) 419, corresponding to the loss of the chloride ligand. The lability of the chloride ligand is promising for future CVD experiments as any chloride contamination in resulting films could potentially be reduced. X-ray quality crystals of compound 2 were grown from cold (-20 °C) hexane. A single crystal X-ray study was carried out (Figure 2). Selected bond lengths and angles are shown in Table 2. On examination of the Zr-C distances of the Cp′ rings, it is apparent that they are both tilted, whereby the carbon bearing the methyl substituent is furthest from the zirconium center. The ring comprising C(7-11) shows the least variation in Zr-C distances, the shortest being 2.510(3) Å and the longest being 2.579(3) Å. This is in the range of M-Cp bond distances in the literature.42-44 In the Cp′ ring comprising C(1-5), the differences are more notable: the longest Zr-C distance is 2.598(3) Å and the shortest is 2.511(3) Å. It was unclear whether the longest Zr-C distance suggested that this Cp′ ligand was bound in an η3 fashion, similar to an allyl group. The distances in compound 2 were compared to those in [W(CO)2(η5-Cp)(η3Cp)], as described by Huttner et al.,48 who reported a W-C (unbound) distance of 2.98(2) Å. This is significantly longer (47) Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, C. J. Chem. Soc., Dalton Trans. 1984, 1349. (48) Huttner, G.; Bejenke, V.; Bell, L. G.; Brintzinger, H. H.; Friedrich, P.; Neugebauer, D. J. Organomet. Chem. 1978, 145, 329.
Figure 3. TGA plots of [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) and [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2).
than Zr(1)-C(1) and therefore it is assumed that both Cp′ rings are bound in an η5 fashion. Again, analysis of the interatomic distances in the guanidinate ligand suggest that there is some contribution to the π-system from the NMe2 moiety. Decomposition Studies. Thermogravimetric analysis (TGA) of compounds 1 and 2 suggest that they begin to decompose at approximately 150-200 °C (Figure 3), although there is a degree of sublimation from each of them starting around 100 °C, which is encouraging for a CVD precursor. They did not decompose significantly further at temperatures above 400 °C. The total mass loss from 1 (74%) was slightly lower than expected for formation of ZrN (calc. 81%) or Zr3N4 (calc. 80%), suggesting incomplete decomposition. When compared to compound 1, the TGA curve for 2 consists of only two steps. The mass loss (85%) is significantly greater than expected for ZrN/ZrC (77%) and even for complete decomposition to zirconium metal (80%). This is due to sublimation of the sample at lower temperatures to an extent that the residual mass is affected, which is visible by the initial shallow curve over the temperature range 80-140 °C. However, this suggests that compound 2 is highly volatile, which is beneficial for CVD but not for accurate assignment of mass losses. Low Pressure Chemical Vapor Deposition. LPCVD of compounds 1 and 2 resulted in the formation of shiny films at 600 °C. Those from 1 were red-brown with a metallic luster, whereas those from 2 were metallic gray. Films from both 1 and 2 were adhesive and could not be removed by Scotch tape. Neither could they be scratched with brass or steel styluses, showing them to be hard. The WDX data for films deposited from 1 and 2 makes clear the case that more nitrogen in the single-source precursor results in more nitrogen in the film. The film formed from compound 1, containing two guanidinate ligands, contained almost three times as much nitrogen as zirconium and almost twice as much carbon compared to those deposited from compound 2. Thus WDX indicated compositions of ZrN2.80C1.82 and ZrN1.31C2.46 from films grown from 1 and 2, respectively. Films from both compounds were conducting, which implies that this was either a result of conducting zirconium carbonitride or graphite. If the former is the case, a significant proportion of the nitrogen and carbon cannot form part of the zirconium carbonitride lattice, as only compounds of the formula ZrNx or ZrCx, where x e 0.5, are electrically conducting. However, it is interesting to note that the Zr:N ratio in the films deposited from 2 is almost equal to that of insulating Zr3N4. The films deposited from these compounds showed chlorine contamination, although this was never higher than 3 at.% for 1 and 1.5 at.% for 2. Oxygen levels in the films from 1 were
1842 Organometallics, Vol. 28, No. 6, 2009
Potts et al.
Figure 4. SEM images of the films formed from (A) [ZrCp′{η2-(iPrN)2CNMe2}2Cl] (1) and (B) [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2). Both images × 75,000, bars ) 100 nm.
remarkably low, not exceeding 2 at.%, which might suggest that this was incorporated after deposition. It was higher in those from 2 (no higher than 12 at.%). However, full quantitative analysis of the oxygen content in the films is difficult because the films were grown on glass. Compound 1 is more sterically crowded so therefore might be less sensitive to oxygen or water than 2, despite efforts to exclude it from the compounds. This might allow compound 1 to be used for larger scale CVD operations. The XRD patterns showed the films to be mainly amorphous, although there were polycrystalline peaks of ZrC present. The zirconium carbide peaks could be indexed to ∼4.73 Å, although this value should be treated with caution due to the broadness of the peaks but it is in agreement with those published previously for ZrCx (x ) 1.1-1.5).49 Peaks corresponding to zirconium nitride were not observed, however, the formation of crystalline ZrN was not expected due to the substrate temperature only being ∼550 °C. The NaCl-type structure adopted by ZrC is the same as that for ZrN, so it is possible that the polycrystalline peaks observed could be attributed to ZrNxCy. However, there is likely some amorphous graphite in the films as well. The reflectance of the films was higher in the infrared for the nitrogen-rich films deposited by compound 1, although for compound 2 the reflectance was more uniform. The films from both compounds were highly transmitting in the infrared, dropping in transparency over the visible and ultraviolet regions of the spectrum. The transmission spectra showed that the films were transmitting across the spectrum, although more so in the infrared region. This is comparable to those reported of titanium nitride.32 SEM images of the films showed small nodules ∼10 nm in diameter (Figure 4). Island-growth was evident and the nodules were approximately 50 nm in diameter. Side-on SEM images showed the films to be 50-200 nm thick.
Conclusions 2
i
[ZrCp′{η -( PrN)2CNMe2}2Cl] (1) was formed from the reaction of [ZrCl2{η2-(iPrN)2CNMe2}2]40 and one equivalent of [NaCp′(THF)], whereas the reaction of [ZrCp′2Cl2] and [Li{(iPrN)2CNMe2}] afforded [ZrCp′2{η2-(iPrN)2CNMe2}Cl] (2). X-ray quality crystals of compounds 1 and 2 were grown from hexane and their crystal structures determined. Both compounds decomposed cleanly, although compound 1 showed a lower (49) Bru¨cker, J.; Ma¨ntyla¨, T. Surf. Coat. Technol. 1993, 59, 166.
mass loss than expected, suggesting that full decomposition had not taken place at 590 °C. The clean decomposition and volatility of these compounds shows promise for their use as CVD precursors. LPCVD of compounds 1 and 2 on glass at 600 °C also gave highly reflective, adhesive, amorphous films of zirconium carbonitride. The XRD patterns for films from both precursors showed some polycrystalline ZrC, although the films are assumed to be largely amorphous. Films deposited by 1 were nitrogen rich and their average composition was ZrN2.80C1.82. Unsurprisingly, films from compound 2 contained more carbon respective to nitrogen, their average composition being ZrN1.31C2.46. Chlorine contamination was present (
