Fabrication of ZrO2 and ZrN Films by Metalorganic Chemical Vapor

Article pubs.acs.org/crystal

Fabrication of ZrO2 and ZrN Films by Metalorganic Chemical Vapor Deposition Employing New Zr Precursors Manish Banerjee,† Nagendra Babu Srinivasan,† Huaizhi Zhu,† Sun Ja Kim,† Ke Xu,† Manuela Winter,† Hans-Werner Becker,‡ Detlef Rogalla,‡ Teresa de los Arcos,§ Daniela Bekermann,# Davide Barreca,∥ Roland A. Fischer,† and Anjana Devi*,† †

Inorganic Materials Chemistry, Faculty of Chemistry and Biochemistry, Ruhr-University Bochum, 44801 Bochum, Germany Dynamitron Tandem Laboratory, Ruhr-University Bochum, 44801 Bochum, Germany § Experimental Physics II, Ruhr-University Bochum, 44801 Bochum, Germany # Department of Chemistry, Padova University and INSTM, 35131 Padova, Italy ∥ CNR-ISTM and INSTM, Department of Chemistry, Padova University, 35131 Padova, Italy ‡

S Supporting Information *

ABSTRACT: The application of new zirconium precursors for the fabrication of ZrO2 and ZrN thin films by metalorganic chemical vapor deposition (MOCVD) is presented. The allnitrogen coordinated Zr precursors exhibit improved thermal properties for vapor phase fabrication of thin films. The growth of ZrO2 thin films was realized by the combination of the Zr complex with oxygen, while the formation of ZrN thin films was achieved for the first time employing a single source precursor (SSP) approach. This was enabled by the presence of nitrogen containing ligands which contributes to the formation of the ZrN phase without the need for any additional nitrogen source in contrast to classical film growth processes for ZrN thin films. In the first step the newly developed precursors were evaluated thoroughly for their use in MOCVD applications, and in the next step they were utilized for the growth of ZrO2 and ZrN thin films on Si(100) substrates. Polycrystalline ZrO2 films that crystallized in the monoclinic phase and the fcc-ZrN films oriented in the (200) direction were obtained, and their structure, morphology, and composition were analyzed by a series of techniques. This work shows the potential of tuning precursors for vapor phase fabrication of Zr containing thin films with a goal of obtaining two different classes of material systems (ZrO2 and ZrN) using one common precursor.

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alkoxides,23 β-diketonates,24−26 fluorinated β-diketonates,27 nitrates,28 cyclopentadienyls,29 and amides,30 there are certain drawbacks associated with these compounds. These are, for example, high deposition temperatures, halide or carbon contamination, low volatility and oligomerization. The Zr amides are another class of precursors mostly suited for atomic layer deposition (ALD) of ZrO2 thin films due to their volatility and reactivity.31,32 Their limited thermal stability and high degree of sensitivity toward air and moisture could be a drawback for CVD, and as a result there are very few reports on the use of the alkyl amides as CVD precursors.30 However, the thermal stability of the parent amides can be improved by introducing the bidentate ligands. Our earlier efforts to stabilize the amide class of Zr precursors employing malonates and guanidinates as chelating ligands led to the formation of mixed amide-malonate or amide-guanidinate complexes, respectively, with promising thermal properties and were used successfully

INTRODUCTION Zirconium-based thin film material systems like zirconium oxide (ZrO2) and zirconium nitride (ZrN) find a range of applications owing to their promising functional properties. While thin films of ZrO2 are used in optical sensors,1 thermal coatings,2 reflectors,3 fuel cells,4 catalysis5 and microelectronics,6,7 ZrN finds applications as hard coatings, diffusion barriers,8,9 gate electrodes,10 and Josephson junctions.11 Thin films of ZrO2 have been primarily deposited by techniques such as sol−gel,12 sputtering,13 pulsed laser deposition (PLD),14 and chemical vapor deposition (CVD).15,16 ZrN thin films have been predominantly deposited by physical vapor deposition (PVD) techniques such as vacuum arc deposition,17 reactive magnetron sputtering,18 ion beam sputtering,19 ion beam assisted deposition,20 and PLD.21 Among the different thin film deposition techniques, CVD offers distinct advantages in terms of large area growth, good composition control, and conformal coverage of substrate with complex topographies. The thin film properties are heavily dependent on the precursors employed. Although several classes of precursors have been employed for the deposition of ZrO2 thin films which include the chlorides,22 © 2012 American Chemical Society

Received: July 19, 2012 Revised: September 4, 2012 Published: September 6, 2012 5079

dx.doi.org/10.1021/cg3010147 | Cryst. Growth Des. 2012, 12, 5079−5089

Crystal Growth & Design

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for MOCVD of ZrO2 thin films.15,33 The mixed amidecyclopentadienyl complexes of zirconium were used for CVD application.34 Compared to ZrO2, there are a limited number of reports on CVD of ZrN. While [Zr(NEt2)4] in combination with ammonia led to the formation of Zr3N4 films by thermal CVD,35 the [Zr(NR2)4] complexes (R = Me, Et) were used with several combinations of carrier gases in plasma-assisted CVD (PACVD) experiments to obtain mainly carbonitride films.36,37 The application of the cyclopentadienyl-guanidinate Zr compounds by a low pressure CVD technique also yielded zirconium carbonitride films.38 The choice of ligands for precursor development depends on the future application of the material. The guanidinate ligand systems can be very efficient in stabilizing coordination complexes, as demonstrated for different metals.15,39−43 Furthermore, guanidinate complexes of various metals proved to be efficient precursors for the deposition of oxide and nitride thin films with desired stoichiometry.15,40,44−46 Owing to the specific decomposition pathway of dimethylamino-guanidinato ligands, metal nitrides thin films such as GdN and DyN were obtained by employing [M{η2-(iPrN)2CNMe2}3] (M = Gd, Dy) as a single source precursor (SSP),45,46 unlike in the case of the closely related amidinates where an additional source of nitrogen, such as ammonia, was required for the formation of the metal nitride. Therefore, it was a logical approach to employ the dimethylamino-guanidinato complex of Zr for the formation of ZrN layers employing the SSP approach. We have reported earlier on the synthesis and characterization of the first mixed amide-guanidinate Zr complex [Zr{η2(iPrN)2CNEtMe}2(NEtMe)2] which was successfully employed for the growth of ZrO2 thin films that were suitable as high-k materials.15 In this work, we have extended our concept of tuning precursor characteristics to the other two parent alkyl amides of Zr, namely, [Zr(NMe2)4] and [Zr(NEt2)4]. The two new heteroleptic Zr amide-guanidinate complexes, [Zr{η 2 (iPrN)2CNMe2}2(NMe2)2] (1) and [Zr{η2(iPrN)2CNEt2}2(NEt2)2] (2), were synthesized and fully characterized to evaluate them as CVD precursors in terms of their thermal and chemical properties. Herein we have successfully utilized the application of the Zr-amide-guanidinate class of precursors for dual application, namely, the formation of ZrO2 in the presence of oxygen and the deposition of ZrN films, which is the first example using the SSP approach.

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MAT spectrometer supplied with an ionizing energy of 70 eV. 1H and C NMR spectra were recorded on the Bruker Advance DRX-250 or DRX-400 spectrometer. 1H NMR decomposition studies were done by dissolving the compounds in toluene-d8 and sealing them in thickwalled NMR tubes which were kept in a preheated oven at temperatures of 100, 120, and 140 °C, for about 7 days, and NMR spectra were measured periodically. For temperature-dependent NMR studies, the compounds were dissolved in toluene-d8 (because of its low freezing point and high boiling point), and NMR measurements were performed by monitoring the temperature within a range of −20 to 80 °C. Thermal analyses were carried out using a Seiko TG/DTA 6300S11 in a N2 atmosphere (flow rate of 300 mL/min, ambient pressure, heating rate 5 °C/min). Isothermal studies were performed using the same instrument under similar conditions by keeping the sample at the required temperature over a span of ∼400 min. Approximately 6 mg of the compounds were taken in aluminum crucibles with a circular opening (diameter 5 mm). Melting points were measured in sealed capillaries under Ar. Crystallographic data were collected on a Xcalibur 2 Oxford using graphite monochromated Mo Kα radiation (k = 0.71073 A°, T = 150 K). The structures were solved using the SHELXL-97 software package and refined by full matrix least-squares methods based on F2 with all observed reflections.48 The CCDC numbers of the compounds 1 and 2 are 880631 and 880632, respectively. Synthesis of [Zr{η2-(iPrN)2CNMe2}2(NMe2)2] (1). Two equivalents of N,N-diisopropylcarbodiimide (1.75 mL, 11.21 mmol) in 20 mL of hexane was added dropwise over a period of 1 h to a solution of [Zr(NMe2)4] (1.50 g, 5.61 mmol) in 20 mL of hexane. During the addition, a slight increase in the temperature was observed. After stirring for 24 h at ambient conditions, the solvent was removed under reduced pressure yielding a white solid. The resulting solid was extracted into hexane, concentrated, and kept at −32 °C for 24 h to afford colorless crystals. Yield 1.39 g (71% based on [Zr(NMe2)4]). Melting point: 118 °C. Elemental analysis: calc. for ZrC22H52N8 (%): C, 50.82; H, 10.08; N, 21.55. Found: C, 50.46; H, 10.49; N, 21.94. 1H NMR (250 MHz, C6D6, 25 °C): δ = 1.26 [d, 24H, (NCH(CH3)2)], 2.53 [s, 12H, CN(CH3)2], 3.31 [s, 12H, Zr(NCH3)2], 3.64 [sept, 4H, NCH(CH3)2] ppm. 13C NMR (250 MHz, C6D6, 25 °C): δ = 25.28 [NC(CH3)2], 40.02 [(CNCH3)], 47.26 [NCH(CH3)2], 172.18 [N3C] ppm. Synthesis of [Zr{η2-(iPrN)2CNEt2}2(NEt2)2] (2). Two equivalents of N,N-diisopropylcarbodiimide (3.63 mL, 23.28 mmol) in 20 mL of hexane was added dropwise over a period of 1 h to a solution of [Zr(NEt2)4] (4.42 g, 11.64 mmol) in 40 mL of hexane. After stirring for 24 h at ambient conditions, the solvent was removed under reduced pressure, yielding a white solid. The resulting solid was extracted into toluene, concentrated, and kept at −32 °C for 24 h to afford colorless crystals. Yield 5.1 g (70% based on [Zr(NEt2)4]). Melting point: 140 °C. Elemental analysis: calc. for ZrC30H68N8 (%): C, 57.00; H, 10.84; N, 17.73. Found: C, 55.32; H, 10.69; N, 18.71. The deviation in carbon value might be due to the incomplete combustion of compound leading to carbide formation. 1H NMR (400 MHz, C6D6, 25 °C): δ = 0.95 [t, 12H, CN(CH2CH3)2], 1.18 [t, 3H, ZrN(CH2CH3)2], 1.24 [d, 6H, NCH(CH3)a], 1.32 [d, 6H, NCH(CH3)b], 1.44 [d, 6H, NCH(CH3)c], 1.48 [d, 6H, NCH(CH3)d], 3.00 [q, 8H, CN(CH2CH3)2], 3.69 [sept, 4H, ZrNCH(CH3)2], 3.83 [q, 8H, ZrN(CH2CH3)2], ppm. 13C NMR (400 MHz, C6D6, 25 °C): δ = 14.13 [CN(CH2CH3)2], 24.82 [ZrN(CH2CH3)2], 26.38 [ZrNCH(CH3)2], 42.86 [NCN(CH2CH3)2], 47.75 [ZrN(CH2CH3)2], 48.18 [ZrNCH(CH3)2], 171.97 [N3C] ppm. Synthesis of [Zr{η2-(iPrN)2CNEtMe}2(NEtMe)2] (3). This compound was synthesized according to the literature procedure,15 and some relevant data are being provided to confirm the formation and reproducibility. Yield 7.4 g (85% based on [Zr(NEtMe)4]). Melting point: 128 °C. Elemental analysis calc. for ZrN8C26H60 (%): C, 54.19; H, 10.50; N, 19.45. Found: C, 53.57; H, 11.21; N, 19.70. 1H-NMR (250 MHz, C6D6, 25 °C): δ = 0.9 (6H, t, CN(CH2CH3), 1.2 (6H, t, ZrN(CH2CH3), 1.3 (24H, d, NCH(CH3)2), 2.5 (6H, s, CNCH3), 2.9 (4H, q, CNCH2CH3), 3.3 (6H, s, ZrNCH3), 3.6 (overlapping, 4H+ 4H, sept + q, NCH(CH3)2 + ZrN(CH2CH3)] ppm. 13C NMR (250 13

EXPERIMENTAL SECTION

General Procedures. All reactions were performed using a conventional vacuum/argon line with standard Schlenk techniques. Samples for all analyses were prepared in argon-filled glove boxes. All solvents were dried and purified by an MBraun solvent purification system. The NMR solvents were degassed and stored over activated molecular sieves. [ZrCl4] (ABCR), nBuLi (Merck), HNEtMe (Fluka), [Zr(NMe2)4] (Aldrich), and N,N-diisopropylcarbodiimide (Aldrich) were used as received. The compounds [Zr(NEt2)4] and [Zr(NEtMe)4] were prepared according to the literature procedure47 with minor variation and further purified by fractional distillation under vacuum. [Zr{η2-(iPrN)2CNEtMe}2(NEtMe)2] (3) was prepared by following the literature procedure15 and analyzed in order to compare it with the analytical data obtained for 1 and 2. Physical Measurements. Elemental analyses were performed by the analytical service center at the Faculty of Chemistry and Biochemistry at Ruhr-University Bochum (CHNSOVario EL 1998). Electronic ionization (EI) mass spectra were recorded using a Varian 5080



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Scheme 1. Synthetic Route for Compounds 1−3

MHz, C6D6, 25 °C): δ = 13.7 (CN(CH2CH3), 14.8 (ZrN(CH2CH3)), 25.5 (NCH(CH3)2), 36.9 (CNCH3), 41.6 (CNCH3), 46.8 (CN(CH2CH3), 47.3 (ZrN(CH2CH3), 50.4 (NCH(CH3)2), 172.3 (N3C) ppm. Thin Film Deposition of ZrO2 and ZrN. A horizontal custombuilt cold-wall MOCVD reactor fabricated for oxide film growth operated under reduced pressure was employed for film deposition with compound 1 as the precursor. Films were grown on 1 × 1.5 cm2 p-type Si(100) substrates (SIMAT) which were ultrasonically cleaned in acetone, isopropanol, and subsequently rinsed with deionized water and dried under an argon gas stream. Nitrogen (flow rate: 50 sccm, purity: 6.0) and oxygen (flow rate: 50 sccm, purity: 4.8) were used as the carrier and reactive gases, respectively. For each deposition, approximately 100 mg of the precursor was filled in a glass bubbler inside a glovebox. Depositions were carried out in the substrate temperature range of 400−700 °C, while the bubbler temperature was maintained at 120 °C. Depositions were carried out for 10 min, and the reactor pressure was maintained at 1 mbar. A similar home-built MOCVD reactor fabricated for nitride film growth operated under reduced pressure was employed for ZrN thin film deposition using the same compound (1) as the precursor. Films were grown on 1.5 × 2 cm2 p-type Si(100) substrates (SIEGERT WAFER). Nitrogen (flow rate: 50 sccm, purity: 6.0) was used as the carrier gas, using the same amount of precursor as above. The substrate temperature was varied in the range 400−800 °C, while the precursor vaporizer was maintained at 130 °C. Depositions were carried out for 20 min at the reactor pressure of 1 mbar. The samples were prone to aerial oxidation and thus they were stored in the glovebox. Film Characterization. The film crystallinity was investigated by X-ray diffraction (XRD) analysis using a Bruker D8 Advance AXS diffractometer [Cu Kα radiation (1.5418 Å)] equipped with a position sensitive detector (PSD). All films were analyzed in the θ-2θ geometry. The surface morphology was analyzed by scanning electron microscopy (SEM) with a LEO Gemini SEM 1530 electron microscope, and the film thickness was determined from cross-section SEM measurements. X-ray photoelectron spectroscopy (XPS) spectra for ZrO2 films were recorded by means of a Perkin−Elmer Φ5600ci spectrometer from Physical Electronics, operating with standard Al Kα (1486.6 eV) radiation. The energy position of each spectrum was calibrated with respect to the Au4f7/2 core level of a clean gold sample at 84.0 eV. The BE shifts were corrected by assigning them to the C1s line of adventitious carbon a value of 284.8 eV. The estimated standard deviation for BEs was ±0.2 eV. Ar+ sputtering was carried out at 3 kV and 0.8 mA × cm−2 beam current density, with an argon partial pressure of 6 × 10−8 mbar. XPS of ZrN thin films was performed in a VersaProbe spectrometer from Physical Electronics, operating with monochromatic Al Kα (1486.6 eV) radiation. The energy position of each spectrum was calibrated with respect to the 4f7/2 core level of a clean gold sample at 83.8 eV. Broad range spectra were recorded with pass energy of 117.4 eV, and the core levels were recorded with a pass energy of 23.5 eV. In situ cleaning of the sample surface and depth profiling of the samples was realized by in situ Ar+ bombardment at 2 kV (ion current density of 0.435 μA × mm−2). Rutherford backscattering spectrometry (RBS) measurements to determine film

composition were carried out with a 2 MeV He beam of the Dynamitron-Tandem accelerator in Bochum with beam intensities of about 10 nA incident to the sample at a tilt angle of 7°. A silicon surface barrier detector with an energy resolution of 15 keV was placed at an angle of 170° with respect to the beam axis. The spectra were analyzed with the RBX program49 by using the stopping powers of the program SRIM. For the nuclear reaction analysis (NRA) measurements, element-specific γ-rays induced by a deuteron beam50 were detected with a high-purity germanium detector operating at 100% relative efficiency. The samples were tilted by 45° toward the detector, which had an angle of 90°, with respect to the beam axis. Typical beam currents on the samples were in the range of 40 nA in an area of ∼1 mm in diameter, while the collected charge for a sample was 25 μC. A polyimide of known chemical composition (Kapton) was used as a standard to obtain the ratios of C, N, and O from the γ-ray yields. Resistivity measurements were carried out using a standard four-point probe instrument (Jandel RM3).

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RESULTS AND DISCUSSION Precursor Synthesis and Characterization. The choice of ligand has a significant influence on the physicochemical properties relevant for a CVD/ALD precursor. In this context, we extended the approach of tuning precursor properties by varying the ligand sphere and in this particular case the influence of the different amide groups on the resulting physicochemical characteristics of the mixed amide-guanidinate class of Zr precursors was investigated. The relevant thermal properties of the new complexes were then compared to the respective parent amides and the results are discussed below. For this purpose, the tetra-coordinated amide complexes of zirconium ([Zr(NR1R2)4]; R1 = R2 = Et and R1 = Et, R2 = Me) were synthesized in large batches (∼15 g) using literature procedures47 and characterized using standard analytical techniques. A facile insertion of the amide groups by the N atoms of the N,N′-diisopropylcarbodiimide ligands led to the formation of the heteroleptic bis(dialkylamido)bis(guanidinato)zirconium complexes [Zr{η2-(iPrN)2CNMe2}2(NMe2)2] (1) and [Zr{η2(iPrN)2CNEt2}2(NEt2)2] (2) (Scheme 1). For comparison, the literature known compound [Zr{η2(iPrN)2CNEtMe}2(NEtMe)2] (3)15 was synthesized and characterized in more detail, especially with respect to the thermal stability of the precursors. Rotational Behavior Studies via NMR Analysis. In order to characterize the compounds 1 and 2 in terms of their spectroscopic purity and solution behavior, room temperature NMR measurements (see Supporting Information, Figure 1) along with variable temperature NMR over a temperature range of −20 to 80 °C were performed. 1 H NMR spectra of 1 and 2, measured at room temperature, reveal that both the compounds are monomeric in solution. 5081



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Variable temperature NMR corroborates the fluxionality of the compounds 1 and 2 in solution. For compound 1, the peak centered at 1.30 ppm is assigned to the isopropyl methyl protons (c) of guanidinate ligands which coalesce into a broad singlet at −20 °C (Figure 1). This temperature is presumed to

The 1H NMR of compound 2 partially resembles compound 1, although four different doublets (cf. one doublet for compound 1) appear for the isopropyl CH3- moieties at 1.24, 1.31, 1.45, and 1.48 ppm (see Supporting Information, Figure 2), attached to the guanidinate ligands indicating that fluxionality of these type of complexes can be arrested even at room temperature by the introduction of sterically demanding groups, which makes the iPr methyl groups nonequivalent on the NMR time scale. For the correct assignment of the peaks, a heteronuclear multiple-quantum coherence (HMQC) spectrum was measured for compound 2 (see Supporting Information, Figure 4). Single Crystal Structural Analysis. The single crystal Xray structures of the compounds 1 and 2 are presented in Figure 2. The crystal data and final agreement factors are listed in Table 1, and the selected bond lengths and angles are presented in Table 2. The compounds 1 and 2 have similar

Figure 1. Temperature-dependent 1 H NMR of [Zr{η 2 (iPrN)2CNMe2}2(NMe2)2] (1) over a temperature range of −20 to 80 °C in toluene-d8.

be near the coalescence temperature of the molecule bearing fluxional groups, where sharp doublets transformed into humps. However at higher temperatures (0−80 °C) such fluxional behavior becomes faster on the NMR time scale which leads to a sharp doublet. It is interesting to note that similar behavior was also observed in an analogous hafnium compound, [Hf{η2(iPrN)2CNEt2}2(NEt2)2],40 which showed the resolution of the peak for the iPr methyl groups on warming the solution from room temperature to 40 °C. The same phenomenon was also observed for compound 315 where the transformation from a broad singlet to a sharp doublet occurred between 30 and 50 °C indicating a higher activation barrier for the rotation of the guanidinate ligands compared to compound 1. The fluxional behavior leads to an identical chemical environment for the iPr methyl protons leading to the appearance of a single doublet, even though the solid state structure exhibits a cis-orientation of the guanidinate ligands. The two singlets, around 2.50 and 3.25 ppm, and the septet around 3.60 ppm represents the methyl groups attached to the amide moiety of the guanidinate ligands (a), amides directly attached to metal (d) and the CH- moiety of the isopropyl groups (b), respectively. Concerning these peaks, the fluxional behavior was not observed implying that the rotation is faster than the NMR time scale as the temperature is as low as −20 °C. No additional peaks appeared when high temperature NMR measurements were performed (maintained at a given temperature for 30 min) which indicates that no carbodiimide deinsertion takes place. This feature is encouraging as the compounds show superior thermal stability at elevated temperatures which can be an advantage for CVD and ALD applications. This observation is in contrast to the carbodiimide deinsertion observed in the case of guanidinate based compounds such as [Al{η2-(iPrN)2CNMe2}3] and [Ga{η2(iPrN)2CNMe2}3].51,52

Figure 2. The molecular structure of [Zr{η2(iPrN)2CNMe2}2(NMe2)2] (1) and [Zr{η2-(iPrN)2CNEt2}2(NEt2)2] (2). All hydrogen atoms have been omitted for clarity. 5082



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which was corrected. The disorder in the molecule 2 (Figure 2) has been removed for clarity. The angle subtended at the metal center by the two amide groups (N31−Zr1−N41) was 90.48°. The bite angles subtended by the two chelating guanidinate ligands at the metal center, N(21)−Zr(1)−N(23) and N(13)−Zr(1)− N(11), were 58.26(8)° and 58.29(8)°, respectively, which were attributed to the angular distortion in the molecule. The presence of bulky isopropyl groups on the guanidinate ligands also contributed to the molecular distortion in order to optimize the repulsive interactions. It was interesting to note that the Zr−N bonds of the guanidinate ligands which were trans to the amide groups were significantly shorter than the adjacent Zr−N bonds of the same guanidinate ligands (Zr1− N41, 2.092(2); Zr1−N23, 2.319(2) Å). This was due to the strong π-donating effect of the amide groups which weakened the opposite bonds. Similar features were observed in the zirconium guanidinate complexes bearing π-donating chloride ligands42 and in the case of compound 3.15 Mass Spectrometric Analysis. Mass spectrometry was employed to gain an insight into the fragmentation of the compounds under electron impact (EI) conditions (see Supporting Information, Figure 5). A very similar fragmentation pattern was observed for compounds 1 and 2, and therefore the fragmentation pattern of compound 1 is discussed in detail while all the observed fragments for compounds 1, 2, and 3 are summarized in Table 3. On the basis of the data obtained, a plausible mechanism is proposed for the fragmentation of the complex under mass spectrometric conditions (Scheme 2). The molecular ion peak for compound 1 was observed at m/z 516.6 with a comparatively low relative intensity value (19%). The isotopic distribution of the metal was observed in the molecular ion peak and the other assigned fragments. The peak at m/z 474.5 can be attributed to the species [M+-NMe2] produced by the dissociation of one NMe2 group, either from the guanidinate ligand or directly from the metal. The predominant peak (relative intensity 100%) is assigned to the species [M+-NMe2CDI] [CDI: N,N′-diisopropylcarbodiimide] at m/z 348.3, produced as a result of the removal of one guanidinate ligand. The peaks of the two species, [M+-NMe2-L] and [M+-L-CDI] [L: {(iPrN)2CNR1R2}; R1= R2 = Me (1), R1 = R2 = Et (2), R1 = Et, R2 = Me (3)] at m/z 303.2 and 218.1 have nearly the same relative intensity. The organic moiety like the guanidinate ligand was not robust enough to withstand the adopted mass

Table 1. Crystallographic Data for Compounds 1 and 2 compound

1

2

empirical formula FW crystal system space group Z a (Å) b (Å) c (Å) V (Å3) dcalc (g/cm3) μ (mm−1) R1 wR2

C22 H52 N8 Zr 519.94 monoclinic C2/c 12 19.688(4) 29.684(3) 17.603(2) 9221(2) 1.248 0.386 0.0397 0.1090

C127 H280 N32 Zr4 2620.71 monoclinic P21/c 2 10.5241(4) 38.9877(14) 18.2981(8) 7476.0(5) 1.164 0.325 0.0511 0.1176

crystallographic features, and therefore, as a representative example, the crystal structure of compound 1 is described in detail. Table 2. Selected Bond Lengths (Å) and Angles (°) for Compounds 1 and 2 compound 1 Zr(1)−N(41) Zr(1)−N(21) Zr(1)−N(23) N(22)−C(22) N(31)−C(32) N(21)−C(22) N(21)−Zr(1)− N(23) N(13)−Zr(1)− N(11) N(31)−Zr(1)− N(11)

compound 2 2.092(2) 2.238(2) 2.319(2) 1.393(3) 1.452(4) 1.343(3) 58.26(8) 58.29(8)

160.85(9)

Zr(1)−N(128) Zr(1)−N(11) Zr(1)−N(111) C(15)−N(16) N(11)−C(15) N(128)−C(131) N(11)−Zr(1)− N(111) N(124)−Zr(1)− N(114) N(133)−Zr(1)− N(114)

2.092(3) 2.244(3) 2.358(3) 1.410(5) 1.352(5) 1.469(5) 58.11(11) 57.93(10) 160.27(11)

Both the compounds were monomeric in the solid state. The zirconium atom occupied the center of the distorted octahedron, with the six corners being occupied by the nitrogen atoms: four atoms from the guanidinate ligands that were arranged in cis-orientation and two from the amide groups that lie adjacent to each other in the coordination sphere of the molecule. Compound 2 possessed a certain degree of disorder

Table 3. The EI-MS Fragmentation Pattern of Compounds 1, 2, and 315a compound 1

compound 2

fragments

mass [m/z]

relative intensity [%]

M+ M+-NMe2 M+-NMe2-CDI M+-NMe2-L M+-L-CDI L+ CDI NMe2/iPr

516.6 474.5 348.3 303.2 218.1 171.0 126.2 43.1

19 28 100 14 15 4 4 39

compound 3

fragments

mass [m/z]


M+ M+-NEt2 M+-2NEt2 M+-L M+-L-NEt2 M+-L-2NEt2 M+-L-2NEt2-iPr M+-L-2NEt2-2iPr iPr

nd 558.5 486.4 432.3 359.2 288.1 245.1 203.0 43.1

nd 36 2 100 6 15 8 4 28

fragments

mass [m/z]


M+ M+-NEtMe M+-L M+-L-NEtMe M+-L-CDI CDI NEtMe iPr

nd 516.5 390.4 331.3 264.2 126.2 58.1 43.1

nd 16 57 4 14 5 13 6

a

M+: molecular ion; L: {(iPrN)2CNR1R2} [R1 = R2 = Me (1), R1 = R2 = Et (2), R1 = Et, R2 = Me (3)]; CDI: N,N′-iisopropylcarbodiimide; nd: not detected. 5083



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Scheme 2. Proposed Fragmentation Pattern of Compound 1 under Mass Spectrometric Conditions (EI-MS, 70 eV)

spectrometry conditions and thus appeared with very low relative intensities compared to the metal-attached-ligand species. Investigation of Thermal Properties. The thermal properties of the compounds were analyzed by thermogravimetric analyses (TGA), isothermal studies, and temperaturedependent NMR decomposition studies in order to evaluate their suitability as precursors. Thermogravimetric Analyses. The TGA curves of compounds 1 and 2 were compared with those of the parent amides to denote the changes in thermal properties (Figure 3A). The temperature onset of volatilization is above 70 °C for the bis-guanidinate compounds (1 and 2) as indicated by the slight mass loss. A gradual mass loss was observed in both the compounds in the temperature range of 200−260 °C. This showed that the temperature window of vaporization of the newly developed compounds (>190 °C) was broadened with respect to the corresponding amides, [Zr(NMe2)4] (∼90 °C) and [Zr(NEt2)4] (∼115 °C). A considerable shift in the temperature, where the one-step volatilization ends, was also observed for 1 and 2 (∼260 °C), compared to the parent amides (150 °C for [Zr(NMe2)4]; 215 °C for [Zr(NEt2)4]). Thus, the tailoring of the amide complexes of Zr to the mixed amide-guanidinates resulted in increased thermal stability and broadened temperature window without any significant compromise with respect to the volatility of the parent amides. To investigate the sublimation behavior of the compounds, isothermal studies were carried out for 1 and 2 (Figure 3B) at different temperatures (under atmospheric pressure and N2 flow). The isothermal TG curves measured at 100 and 120 °C showed a linear weight loss for both the compounds for a long period of time (400 min), which implies that the precursors can be sublimed at a constant evaporation rate at a given temperature. NMR Decomposition Studies. To assess the thermal stability of the precursors 1H NMR decomposition studies were carried out for compounds 1 and 3 at different temperatures. Such studies were difficult for 2 owing to its complex nature of the NMR spectrum. Similar experiments were carried out for

Figure 3. (A) TGA curves for the Zr complexes 1, 2, and their corresponding amides. (B) Isothermal studies for 1 and 2 at different temperatures.

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the corresponding parent amides to compare the stabilities under similar conditions. The compounds were sealed in heavywalled NMR tubes and heated to the required temperatures in the oven, and 1H NMR was recorded periodically for a prolonged period (∼7 days) depending on the rate of decomposition. The integral of the NMR peaks characteristic of the compounds were normalized to the solvent peaks (toluene-d8) and plotted as a function of time (days) to obtain the corresponding half-life of the precursors which are listed in Table 4. Table 4. Half-Life of Compounds 1, 3, and the Corresponding Amides at Different Temperatures half-life (days)

a

compounds

100 °C

120 °C

140 °C

[Zr{η2-(iPrN)2CNMe2}2(NMe2)2] 1 [Zr{η2-(iPrN)2CNEtMe}2(NEtMe)2] 3 [Zr(NMe2)4] [Zr(NEtMe)4]

32 22 3 7

16 19 dec.a dec.

6 5 dec. dec.

Figure 4. Arrhenius plot of ZrO2 films deposited on Si(100) substrates using compound 1 (the lines connecting the individual points are just to guide the eyes).

dec.: decomposed.

At 100 °C, the half-lives of compounds 1 and 3 were a few weeks (32 and 22 days, respectively) as compared to that of their respective parent amides which were less than one week (3 and 7 days, respectively). The half-lives of the parent amides were considerably lower at temperatures as low as 100 °C and at elevated temperatures they decomposed. Upon increasing the temperature beyond 140 °C, which could be a typical temperature for either precursor vaporization or even deposition temperature for an ALD process, the half-life (1 and 3) was about one week. On the basis of the NMR decomposition studies, one can surmise that the guanidinate complexes have a significantly higher thermal stability at a given temperature (Table 4) compared to the parent amides. MOCVD of ZrO2. On the basis of the promising physicochemical properties of the guanidinate complexes, our first attempts were to evaluate their suitability for MOCVD applications and compound 1 was selected as the precursor. Thin films of ZrO2 were grown on Si(100) substrates in the temperature range 400−700 °C using compound 1 in the presence of oxygen in a low pressure horizontal cold-wall reactor. The variation of ZrO2 growth rates with deposition temperature as an Arrhenius plot is shown in Figure 4. The growth rate increased from 2 nm/min to 19 nm/min in the temperature range 400−500 °C, which could be assigned to the kinetically controlled region in an MOCVD process. The maximum growth rate was obtained at 500 °C. In the temperature range from 500 to 650 °C the growth rate was nearly independent of temperature, which could be attributed to the mass transport controlled region. Above 650 °C, the growth rate was slightly reduced and this can probably be explained by the premature decomposition of the precursor or probable gas phase reactions that can occur at elevated temperatures. The crystallinity and morphology of the as-deposited films were evaluated by XRD and SEM, respectively. According to XRD measurements (Figure 5), the films were amorphous in nature at lower temperatures (400 °C). At 500 °C, the formation of polycrystalline ZrO2 in the monoclinic phase was observed as evident from the predominant (200) peak and a

Figure 5. XRD pattern of the ZrO2 films deposited on Si(100) from compound 1 as a function of substrate temperature.

very weak (111) peak (JCPDS card 00-037-1484). As the deposition temperature was increased to 600 °C and above, an additional predominant (020) peak also corresponding to the monoclinic phase appeared. This gives evidence that transition in film orientation for ZrO2 takes place between 500 and 600 °C. The grain size was determined using the Scherrer equation53 and was estimated to be around 20 nm for films grown in the temperature range 500−700 °C. The ZrO2 films were dense and show a closely packed grain structure (Figure 6). The columnar nature of the grains could be seen from the cross section images (inset of Figure 6). The average grain size was to the order of 15−30 nm and these values are consistent with those obtained from XRD analysis discussed earlier. The chemical composition of the ZrO2 films was analyzed using complementary techniques such as RBS, NRA, and XPS. According to RBS analysis illustrated in Figure 7, the Zr and the O signals are clearly evident, whereas other signals such as carbon or nitrogen could not be detected, which were below the RBS detection limit (600 °C) the formation of crystalline fcc-ZrN (JCPDS card 00-031-1493) was clearly evident (Figure 9), and

Figure 11. Wide-scan survey spectra, N1s and Zr3d photoelectron signals for a ZrN sample deposited on Si(100) at 600 °C from 1: (a) surface, (b) after 10 min of Ar+ erosion. Spectra have been vertically shifted for clarity. For the N1s and Zr3d surface peaks, the background and the fit components are plotted together with the raw spectrum.

precursor 1. The presence of carbon at the sample surface (Figure 11, trace (a)) could be mainly traced back to contamination upon air exposure, due to an appreciable intensity reduction of the C1s signal after 10 min Ar+ erosion (trace (b)). Deconvolution of the O1s photopeak (not reported) evidenced the presence of ZrO2 (BE = 529.8 eV) and carbonate/hydroxyl species (BE = 531.5 eV). In agreement with XRD results, the BE positions and shapes of both N1s and Zr3d photoelectron peaks clearly showed the formation of ZrN as the main phase before and after erosion (Figure 11, N1s and Zr3d). The N1s signal was composed by two contributing bands. The main component (I, BE = 397.1 eV, 73.4% of the total nitrogen) was attributed to nitrogen in ZrN,60,61 whereas the one centered at lower BE indicated the formation of Zr oxynitride species (II, BE = 396.3 eV).62 In line with N1s and O1s contributions, the zirconium signal was decomposed by means of three doublets. The main one [I, 50.2% of the overall Zr; BE(Zr3d5/2) = 179.4 eV] confirmed the presence of ZrN, though contributions due to carbide formation could not be ruled out due to the similar BE positions of Zr in ZrN and ZrC.63 The two minor contributions were assigned to zirconium oxynitride [II, BE(Zr3d5/2) = 180.7 eV, 26.4% of

Figure 9. XRD patterns of ZrN thin films grown on Si(100) substrates as a function of deposition temperature.

noticeably, a preferred orientation in the (200) direction was observed. This is in contrast to earlier MOCVD reports on zirconium nitride where only the Zr3N4 was reported.35 The film morphology as revealed by SEM micrographs (Figure 10) shows a densely packed fine grained structure. As in the case of ZrO2, the formation of columnar grains was observed (shown in the inset of Figure 10). ZrN-based surface and in-depth chemical composition was studied by means of XPS measurements. To this regard, Figure 11 shows wide-scan spectra for ZrN grown at 600 °C from

Figure 10. Representative SEM micrographs of ZrN thin films deposited on Si(100) at substrate temperature (a) 600 °C and (b) 800 °C. 5087



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and optical applications, while in the case of ZrN films, the focus will be on growing films free of any oxygen or carbon and then testing them for gate electrodes/barrier coatings and hard coatings. Owing to the reactivity and enhanced thermal stability of these complexes, the ALD of ZrO2 films are currently underway.

the overall Zr content) and ZrO2 [III, BE(Zr3d5/2) = 182.2 eV, 23.4%].61,63 XRD results clearly show the formation of the ZrN, and the XPS analysis revealed the formation of a mixed phase which contains both ZrN and ZrON. It is known from the literature that ZrN based materials are prone to oxidation.62 It should be noted that the films investigated here were exposed to air during post deposition film analysis and this could be one of the factors for the formation of ZrON species. Although the direct comparison of XRD and XPS might be perplexing, the fact that ZrN was formed as the predominant crystalline phase can be concluded combining the results of XRD and XPS (before and after erosion). However, the next step would be to reduce the carbon level and therefore the process needs to be optimized, for example, post deposition annealing or even performing depositions in the presence of ammonia. This will be the focus of our future work. Since ZrN films are appealing as gate electrodes, it was interesting to measure the resistivity of the as-deposited ZrN layers. Four probe measurements were performed on the freshly deposited films and the specific resistivity obtained for the ZrN thin films deposited in the temperature range of 800− 400 °C varied from 40−100 mΩ-cm, which are comparable to the literature reported values.64,65 The resistivity values decreased with an increase in substrate temperature. This could be correlated to the increasing crystallinity of the films as a function of deposition temperature and probably the reduction in carbon content at higher deposition temperatures. However, this needs to be verified by carrying out detailed XPS studies on films grown at different deposition temperatures which are beyond the scope of this work. The formation of ZrN thin films using a single source precursor approach is highly encouraging and the next step is to optimize the CVD process parameters to get a single phase material in terms of chemical composition. Further efforts will be devoted to evaluate the layers as gate electrodes, barrier or hard coatings.

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ASSOCIATED CONTENT

* Supporting Information S

The room temperature 1H NMR of compounds 1 and 2, temperature-dependent 1H NMR and HMQC NMR of compound 2, EI-MS spectrum of compound 1, crystallographic data for compounds 1 and 2, RBS and NRA results of ZrO2 films are available. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under grant agreement number ENHANCE238409.

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REFERENCES

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SUMMARY A dual application in terms of thin film deposition by MOCVD employing a heteroleptic class of Zr precursors was realized. This was achieved by synthesizing an all-nitrogen coordinated Zr-amide-guanidiate class of precursors with the perspective of using them for MOCVD applications. The tuning of the precursor characteristics was performed with a systematic approach. The amide groups were varied and the resulting influence on the physicochemical properties were investigated in detail and compared to their corresponding parent alkyl amides. The new complexes were volatile, less sensitive to air and moisture, and possess enhanced thermal stability when compared to the respective parent alkyl amide rendering them suitable for MOCVD applications. This was demonstrated successfully by the formation of zirconium oxide films on Si(100) substrates in the presence of oxygen. The as-deposited ZrO2 thin films were uniform, dense, and stoichiometric. The same precursor was utilized successfully for the formation of zirconium nitride thin films, employing a SSP approach. This is the first report on the formation of fcc-ZrN films oriented in the (200) direction that were obtained by MOCVD using a single precursor under mild conditions, avoiding any use of additional source of nitrogen for nitridation. On the basis of the promising film characteristics our next approach will be to investigate the functional properties of ZrO2 films for dielectric 5088



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Fabrication of ZrO2 and ZrN Films by Metalorganic Chemical Vapor

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