Article pubs.acs.org/JPCC
Multiphase Structure of Tantalum Oxynitride TaOxNy Thin Films Deposited by Reactive Magnetron Sputtering Christine Taviot-Guého,* Joel̈ Cellier, Angélique Bousquet, and Eric Tomasella Institut de Chimie de Clermont-Ferrand, Université Clermont Auvergne, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6296, ICCF, F-63178 Aubiere, France S Supporting Information *
ABSTRACT: This work deals with the structure and the microstructure of tantalum oxynitride thin films deposited by reactive magnetron sputtering. The local structures of amorphous as-prepared thin films are investigated using the pair distribution function (PDF) technique based on total Xray scattering experiments. The corresponding annealed thin films are analyzed using conventional θ−θ X-ray diffraction technique and full-pattern fitting methods. Rutherford backscattering and X-ray photoelectron spectrometries are used in conjunction with X-ray techniques. As-prepared thin films are nanostructured. The PDF signal is coming from small structural units below 10 Å in diameter, which only maintain nearest-neighbor order and with a composition changing gradually from TaN to δ-TaON and Ta2O5 as the oxygen content in the reactive gas increases. On the other hand, the annealed thin films consist of a mixture of separate crystalline phases with refined cell parameters consistent with the formation of TaN (Fm−3m), β-TaON (P21/c), and Ta2O5 (C2mm) more or less successively as the oxygen content in the reactive gas increases. Information on the size of the coherent domains and the preferential growth orientation are obtained from analysis of anisotropic diffraction line broadening effects in the XRD patterns. The results are in favor of a random bonding model (RBM) in the case of as prepared thin film and random mixture model (RMM) for annealed samples. Methods of preparation of TaOxNy thin films are numerous including electron beam evaporation, sputtering, chemical vapor deposition, sol−gel method, and pulsed laser deposition.18−23The sputtering technique is the most commonly used method for thin films deposition, allowing a monitoring of the percentage of N and O in the sample merely by changing the flow rates of the reactive gases.24−26 Hence, the formation of TaOxNy films with different x, y values have been reported. In comparison with tantalum oxide films, the film properties are strongly affected by the nitrogen content, the optical properties first of all.27−30 This is because the substitution of oxygen by nitrogen atoms increases the refractive index for the metal− nitrogen bonds tend to be less polar than the corresponding metal−oxygen bonds, leading to a higher polarizability for metal nitrides and a decrease of the optical band gap.31 It is otherwise well-known that metallic oxynitrides are denser than their oxide counterparts with less open structures due to the replacement of some oxygen by nitrogen atoms. This higher density is accompanied by a higher refractive index beneficial for optical transparency.24 With a broad excitation band
1. INTRODUCTION Considered as promising multifunctional materials, transitionmetal oxynitrides have received much attention in recent years. Indeed, by adjusting the oxygen-to-nitrogen ratio, it is possible to tune their physical, chemical, and functional properties, making these materials attractive for microelectronic, optical applications, and as biocompatible coatings.1−4 In particular, tantalum oxynitrides TaOxNy are of scientific interest because they benefit from both the properties of tantalum oxides and tantalum nitrides. So, tantalum oxide Ta2O5 has been wellknown for its large band gap (4.5 eV, i.e., ∼280 nm), high refractive index (n ∼ 2.3 at 633 nm), high dielectric constant (εr ∼ 25), and transparency in a wide wavelength range from 300 nm to 2 μm.5,6 As a result, Ta2O5 thin films were found interesting for use as antireflection coatings, optical waveguides, and metal oxide semiconductor devices.7−10 Other applications of Ta2O5 films include corrosion barrier coatings, solid state oxygen sensors, and thin film catalysts.11,12 On the other hand, tantalum nitride is known as a chemically inert, corrosionresistant, and hard ceramic.13−15 Tantalum nitride films are applied for diffusion barriers and resistors in micro- and optoelectronics, heat-resistant layers in mechanical industry, and biocompatible coatings.16,17 © 2015 American Chemical Society
Received: July 30, 2015 Revised: September 23, 2015 Published: September 24, 2015 23559
DOI: 10.1021/acs.jpcc.5b07373 J. Phys. Chem. C 2015, 119, 23559−23571
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The Journal of Physical Chemistry C
precise determination of the unit cell parameters; in some cases, the Rietveld method including a structural model could have been applied. A quantitative crystalline microstructure analysis based on the integral breadth method was also performed. In particular, the average apparent size and shape of the coherent domains were extracted from the use of Thompson, Cox, and Hastings pseudo-Voigt function profile function combined with the anisotropic broadening models as implemented in Fullprof program suite.50 This global modeling approach and its original application to the structural characterization of TaOxNy thin films combined with X-ray photoelectron and Rutherford backscattering spectrometries allowed us to better describe the multiphase structure and microstructure of such films.
extending from the ultraviolet to visible-light range up to ca. 530 nm, a suitable band gap around 2.4 eV, and a good stability in aggressive environments, TaOxNy are good candidates for visible-light responsive photocatalysts, and promising results have been reported in particular as a photoanode for overall water splitting.32 Obviously, the structural changes resulting from compositional changes play a major role in developing the properties. However, the structural characterization of TaOxNy thin films is difficult to address first because as-deposited samples are usually amorphous and second because TaOxNy compounds have widely varying structural properties which depend on the experimental procedures and conditions. Upon heating, crystallization takes place, but phases are often defective structures and deviations from theoretical stoichiometry are quite common. For instance, different polymorphs with the nominal stoichiometry Ta:O:N = 1:1:1 have been reported: the well-characterized monoclinic β-TaON (P21/c) adopting the baddeleyite structure type, 33,34 the hexagonal α-TaON considered as erroneous by Lumey et al.,35 the monoclinic γTaON (C2/m),36 and the tetragonal δ-TaON with the anatase structure.37 The existence of nonstoichiometric phases has also been reported such as Ta1.94O0.15N2.85,38 Ta2ON3,39 and Ta3O6N.40 For a better understanding of their stabilities and electronic structures, TaOxNy have also been the subject of several theoretical investigations, in particular using plane-wave DFT (density functional theory) calculations.41 Because of these complex structural features as well as the occurrence of preferential orientation effects, quite common with thin films, the X-ray structural investigations of TaOxNy thin films may be complicated. Most of the X-ray structural studies reported so far consist mainly in a phase identification, i.e., comparing experiment X-ray diffraction patterns with a database of single-phase reference patterns.21,22,26,42,43 In some of these studies, the unit cell parameters were estimated from the diffraction peak positions; this method is fast, but peak information lacks precision. On the other hand, when a profile fitting method is applied, the lattice parameters and crystallite sizes can be determined more accurately.43 In a recent study, we examined the optical properties of a series of tantalum oxynitride thin films obtained by magnetron sputtering using ellipsometry and UV−vis spectroscopies, and we demonstrated a fine-tuning of both the refractive index and the optical band gap of these films depending on their composition which is otherwise determined by O2/(O2 + N2) flow ratio.44 A preliminary structural study by X-ray diffraction analysis of both as-prepared and annealed (900 °C) thin films was also performed consisting in phase identification by comparing experimental simulated XRD patterns and a microstructural analysis using the Williamson−Hall integral breath method. This latter analysis was only qualitative and revealed anisotropy in the diffraction line broadening for all annealed films with both Lorentz and Gauss contributions. In the present paper, we aimed to go further in the analysis of the X-ray diffraction patterns with a thorough understanding of the crystallographic film growth which is a highly relevant issue for the processing of thin TaOxNy films with optimized optoelectronic properties. In the case of as-deposited thin films, the pair distribution function method was applied giving insight into the nanostructures present. It is worth mentioning that the application of this method to the structural analysis of thin films is quite original.45−49 For annealed films displaying a polycrystalline surface, the Le Bail method was applied for a
2. EXPERIMENTAL METHODS 2.1. Deposition Process. Tantalum oxynitride thin films were deposited by reactive magnetron sputtering at 13.56 MHz frequency using Alcatel SCM 450 sputtering equipment under different Ar−O2−N2 gas atmospheres. The tantalum target (99.999% purity, 100 mm diameter) was used. The target− substrate spacing was fixed at 90 mm. Before deposition, the sputtering chamber was evacuated to 10−5 Pa, and then the target was presputtered in an argon atmosphere at 1 Pa pressure for 30 min to eliminate the surface contamination. For obtaining the gas sputtering mixtures, the argon/oxygen/ nitrogen flow rates (ΦAr, ΦO2, and ΦN2, respectively) were adjusted using mass flow controllers (HFC 302 from Hastings Instruments). The reactive gas composition was characterized by the RF value: RF = ΦO2/(ΦO2 + ΦN2). After some preliminary experiments, the argon and reactive gas concentrations were kept constant: ΦAr = 5 sccm and ΦO2 + ΦN2 = 1.25 sccm. The total pressure (pt) was fixed at 1 Pa. A sputtering power (P) of 250 W and a grounded substrate potential were used. After deposition, rapid thermal annealing (RTA) treatments were made at 900 °C for 10 min under a nitrogen atmosphere. To comply with the different characterization techniques, tantalum oxynitrideTaOxNy thin films were deposited on various substrates: vitreous carbon for Rutherford backscattering spectrometry (RBS), silicon for thickness measurements, X-ray photoelectron spectrometry (XPS), and powder X-ray diffraction (XRD) measurements, and silica for X-ray total scattering and pair distribution function (PDF) analysis. 2.2. Thin Film Characterization. The layer composition was investigated by Rutherford backscattering RBS using 2 MeV alpha particles and 15 nA current intensity; a charge of 10 μC was collected at 165° detection angle. The RBS signal of the graphite substrate appearing at a lower energy than that of the constituents of the film, there is no overlapping. To determine the stoichiometry, experimental spectra were simulated using the SIMNRA program.51 The X-ray photoelectron XPS experiments were carried out in a home-built ultrahigh-vacuum chamber equipped with a hemispherical energy analyzer and using a Mg Kα source (1253.6 eV) operating at 240 W. The base pressure in the chamber was around 10−8 Pa, and measurements were made at room temperature. No cleaning was performed prior to analysis, and all samples were run identically at a takeoff angle of 90° (with respect to the sample surface). Each spectral region was scanned three times to get an acceptable signal-tonoise ratio within a reasonable acquisition time. The binding energies Eb have been corrected for sample charging effect with 23560
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Fullprof program suite.50 The Le Bail method allows obtaining precise unit cell parameters with which one can analyze small unit cell variations.56 The Thompson, Cox, and Hastings (TCH) pseudo-Voigt function was chosen as profile function.57 The background was refined by adjusting the height of preselected points for linear interpolation modeled. The instrumental contribution to peak broadening was estimated using the same TCH profile function by measuring the standard reference sample LaB6 (refined values were U = 7.8 × 10−4, V = 9.8 × 10−3, W = 4.8 × 10−3, X = 3.6 × 10−2, and Y = 2.4 × 10−2). The size of the coherent domains was calculated from the Lorentzian integral breath component using the Scherrer equation.58 For films with a polycrystalline surface, a modeling of the structure by the Rietveld method was performed59 taking as initial structures those reported by Sahnoun et al. for Fm−3m TaN,60 Yashima et al. for P21/c TaON,33,34 and Lehovec et al. for C2mm Ta2O5.61 Only the zero point, the background, the lattice parameters, the profile function parameters, and the preferred orientation were permitted to vary; the positions of the atoms and the atomic displacement parameters were held constant. The contribution of the finite crystallite size to peak broadening was evaluated using a linear combination of spherical harmonics (SPH) for the Lorentzian peak component as well as a mixing Gaussian parameter. The refined spherical harmonic coefficients allow the calculation of the apparent crystallite size in each [hkl] directions. An “average apparent shape” of the crystallites can also be plotted using the GFourier program.62
reference to the C 1s line at 285 eV. A Shirley baseline was chosen for peak fitting. Experimental curves were resolved using a sum of mixed Gauss−Lorentzian line shapes. By iteration, parameters describing the envelope/peak position/ width/Gauss−Lorentzian ratio/intensity were modified within reasonable limits; the fit was refined until the χ2 value converged. Depending on the obtained χ2 value, the curve fitting was stopped, refined, or restarted after addition of a new component. Because the as-deposited thin films are almost amorphous, their structures were investigated by means of the pair distribution functions (PDF) technique. PDF method analyzes the total scattering including Bragg and diffuse scattering and directly provides atom-to-atom distances in the material on length scales from a few angstroms to tens of nanometers depending on the size of the coherent domains.52 What is interesting with PDF is that it enables the characterization of structures without long-range order. The X-ray scattering experiments were performed on a PANalytical X’Pert Pro X-ray powder diffractometer (Bragg−Brentano θ−θ geometry) equipped with an X’Celerator Scientific detector and Ag anticathode (Kα1 = 0.5594 Å, Kα2 = 0.5608 Å). The recording conditions defined hereafter have been validated elsewhere.53 The data were recorded over the range 2°−130°(2θ) using variable divergence slits with a constant irradiated sample length of 10 mm and a step size of 0.01671°. The corresponding accessible maximum scattering vector Q magnitude is 20 Å−1 although 17 Å−1 was used as a cutoff value during the PDF analysis. To ensure good scattering statistics and a high signal-to-noise ratio, several frames were collected resulting in an overall exposure time of 48 h. The summation of the patterns and their conversion to corresponding 0.03° pseudo-fixed-slit data were performed using the PANalytical X’Pert High Score Plus software. The air scattering was taken into account by measuring the empty diffractometer background. The contribution of the amorphous silica substrate was also determined separately. The data were normalized and Fourier transformed to the PDF G(r) using the software PDFgetX3.54 PDF modeling was performed using PDFgui software and the structure of reference materials;55 because the values are not always available, the same isotropic atomic displacement parameters U (Å2) were used in all the simulations, i.e., U(Ta) = U(N) = U(O) = 0.0027(4)Å2 as reported elswhere.33 The structure and the microstructure of annealed thin films were investigated by conventional X-ray diffraction using a PANalyticalX’Pert PRO X-ray diffractometer equipped with a Cu Kα anticathode and X’Celerator linear detector. The investigations were carried out in the Bragg−Brentano θ−θ geometry. The scanning angle of the detector was varied within the range 10°−60° (2θ) with a step size of 0.017° and a counting time of 400 s (continuous mode). In symmetric θ−θ diffraction, the penetration depth is dependent on the beam incidence angle α and the chemical composition of the sample. Using X’Pert High Score program, X-ray penetration depths were calculated, varying from 0.9 μm (α = θ = 5°) to 5.0 μm (α = 30°) for TaN, 1.3 to 7.4 μm for TaON, and 1.8 to 10.2 μm for Ta2O5. These values are larger than the film thicknesses, ranging from 200 to 300 nm, and we therefore expect a scattering contribution from the substrate. Depending on the polycrystalline character of the film, either the Le Bail or the Rietveld methods were applied to analyze the XRD. Both are full-pattern refinement methods and are integrated in the
3. RESULTS 3.1. Composition Analysis by RBS: Influence of the Reactive Gas Ratio RF. As-Deposited Thin Films. Figure 1
Figure 1. Variation of the elemental composition in atomic percentage determined from RBS measurements for as-deposited thin films as a function of gas mixture ratio RF.
shows the evolution of the composition of thin films, determined by RBS, as a function of the reactive flux ratio RF. While Ta concentration is kept constant to approximately 33 at. % over the whole RF range, oxygen and nitrogen contents vary in opposite ways, and two variation zones can be considered. In zone 1 corresponding to 0−0.62 RF range, a slight decrease of nitrogen content from 60 to 42 at. % and an increase of oxygen content from 5 to 28 at. % are observed. In zone 2, i.e. for RF values higher than 0.62, the oxygen content goes up, rapidly reaching a value of 70 at. % at RF = 1; at the same time, the nitrogen content decreases down to 3 at. % at RF = 1. One can note that the same variation zones can be considered when dealing with the deposition rate V d determined by ellipsometry. Indeed, as can be seen in Figure 2, Vd remains relatively constant in zone 1 with a value of 18 nm 23561
DOI: 10.1021/acs.jpcc.5b07373 J. Phys. Chem. C 2015, 119, 23559−23571
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Figure 4. Variation of the O/(O + N) atomic ratio determined by RBS for as-deposited and annealed thin films as a function of gas mixture ratio RF.
Figure 2. Variation of the deposition rate Vd (profilometry, ■) and tantalum atomic flux (RBS measurement ●) as a function of gas mixture ratio RF.
underline that although the trends are similar between asdeposited and annealed films, oxygen content increases in the coating after RTA processing, indicating the occurrence of oxidation despite the use of nitrogen atmosphere. 3.2. Structural Analysis of the Surface by XPS and the Bulk by XRD. The XPS patterns of tantalum oxynitride thin films are presented in Figure 5 for different reactive gas
min−1 and then decreases to 6 nm min−1 in zone 2. The poisoning mechanism, which is known to induce abrupt changes in the discharge properties, might explain this result.63−67 In our conditions, the oxidation of target surface was already reported RF ≥ 0.8.68 A way to understand the poisoning effect consequences is to study the Ta sputtering yield for different target materials, i.e., Ta, TaN, Ta3N5, and Ta2O5. We used SRIM 2010 program to determine the sputtering yield considering argon incident ions with energy in 100−1000 eV range and normal incidence;69 calculations are based on the binary collision approximation for ion beam−solid interaction and the results are presented in Figure 3. We
Figure 3. Sputtering yields of Ta for different target materials Ta, TaN, Ta3N5, and Ta2O5 determined by SRIM.
confirm that in the presence of poisoning phenomenon the sputtering yield of Ta atoms (STa) is less important for tantalum oxide or nitride targets than pure tantalum one, affecting the deposition rate. Hence for RF ≥ 0.8, the target surface oxidation induces a decrease of sputtered tantalum flux. On the other hand, from RBS measurements, the number of Ta atoms composing the total film thickness by area unit can be determined and converted in flux of Ta atoms incorporating the film by area and time units (considering the total deposition time). This Ta atomic flux is plotted in Figure 2. It decreases abruptly in zone 2 due to lower Ta sputtered flux, as predicted with SRIM simulations in the case of oxidized target. Annealed Thin Films. Ta concentration being constant and approximately of 33 at. %, one can plot the variation of the atomic ratio O/(O + N) as a function of the reactive gas composition RF for both as-deposited and annealed films (Figure 4). Trends are similar for both series of samples with a gradual increase of O/(O + N) atomic ratio with RF value up to RF = 0.8 followed then by a plateau. For annealed thin films, the increase is almost proportional indicating a direct relationship between the reactive gas and thin film compositions. We must
Figure 5. XPS spectra of (a) as-deposited (left) and (b) annealed (right) thin films obtained under different gas mixture ratio RF. See Table 1 for peak assignment.
compositions. No etching by argon was realized on the surface before acquisition to avoid modification of the structures. On the basis of published data summarized in Table S1 (Supporting Information), we considered four kinds of Ta 4f doublets (Ta 4f7/2, Ta 4f5/2) for peak separation representing four different bonds between Ta, N, and O: the two peaks are located at 25.8 eV (Ta 4f7/2) and 27.7 eV (Ta 4f5/2) for TaON, at 26.2−26.6 eV and 28.4−28.5 eV in Ta2O5, at 24.8 and 26.7 eV for Ta3N5, and at 23.5 and 24.8 eV for TaN.70−72 As-Deposited Thin Films. At RF = 0 and 0.4, the XPS spectrum was simulated considering four doublets which can be 23562
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diffraction patterns measured using Ag anticathode, for the quartz support and the films obtained at RF = 0, 0.5, and 1.0. Consistent with previously reported results,44 only the film prepared at RF = 0 is partially crystallized displaying broad diffraction peaks attributed to the presence of cubic TaN Fm− 3m. While the value of RF increases, the films become amorphous. For the extraction of PDF data, the normalized structure functions S(Q) were obtained up to 17 Å−1. In Figure 6 are presented the reduced pair distribution functions, i.e.,
attributed to TaN and Ta3N5 as major phases as well as TaON and Ta2O5 as minor phases. The presence of TaON and Ta2O5 results from residual oxygen still present in the chamber atmosphere during the film growth. If one ignores this contamination effect, as-deposited thin film at RF = 0 is mainly composed of TaN and Ta3N5. While the RF value increases, TaN and Ta3N5 contributions decrease and disappear at RF > 0.6 and RF > 0.8, respectively. The TaON contribution reaches a maximum at RF = 0.6. At RF = 1.0, only the contribution of Ta2O5 is observed. Annealed Thin Films. For the corresponding annealed sample at RF = 0, we have to consider an additional Ta 4f doublet contribution attributed to TaNx (x < 1) with those already shown in the case of as deposited thin films (TaN, Ta3N5, TaON, and Ta2O5). TaON and Ta2O5 are also present but in larger amounts than for as-deposited samples. For RF = 0.4, no significant changes are found between as-deposited and annealed samples. While RF increases, TaN and Ta3N 5 disappear to obtain thin film with mainly Ta2O5 bonding at RF = 1. In a previous paper,44 we reported a preliminary structural and microstructural characterization of these thin films which consisted in phase identification by comparing the experimental XRD patterns with the simulated patterns from the JCPDSICDD database and Williamson−Hall analysis to get information on crystallite size and lattice strains. It should be noted that the correspondence between microstrains as determined from XRD line broadening and compressive stress commonly found in films grown by the PVD technique73 has not been demonstrated here. For as-prepared samples, the structural characterization by Xray diffraction was limited because the thin films are mainly amorphous. For RF < 0.7, the growth of a poorly crystalline phase was observed and the XRD pattern recorded at RF = 0 is consistent with the presence of the cubic TaN Fm−3m phase. For annealed samples, by comparison to the database of reference patterns, we showed that the films crystallize in various phases, i.e., cubic TaN Fm−3m (JCPDS 49-1283) at RF = 0, monoclinic P21/c TaON (JCPDS 71-0178) at RF = 0.6, and monoclinic C2mm Ta2O5 (JCPDS 25-922) at RF = 1. In the present study, we aim at a detailed structural analysis of both kinds of films. To explore the structure of as-prepared thin film, we applied the high-energy X-ray scattering and PDF technique. The advantage of the PDF is that it allows studying both the local and the medium-range structure of disordered and crystalline materials.52 Furthermore, it is worth noting that the application of PDF technique to the characterization of thin films is quite original with only a few studies reported so far.45−49 In the case of annealed thin films, a refinement of the full X-ray diffraction patterns was performed that allowed a precise determination of the unit cell parameters, an accurate analysis of crystallite size and preferred orientation effects, and, in some cases, a quantitative phase analysis. For thin films, it must be remembered that the crystallite size is often an anisotropic quantity with a size along the surface normal much larger than the size in the plane of the film. A careful study of the anisotropic character of the microstructure of the films was performed based on the analysis of the anisotropic diffraction line broadening. As-Prepared Thin Films. For the structural characterization of as-prepared samples using the PDF method, the films were deposited on amorphous silica substrates with a thickness of ca. 1 μm. Figure S1 (Supporting Information) shows the X-ray
Figure 6. Experimental PDF of as-prepared thin films obtained at RF = 0, 0.5, 1.0 and for silica substrate alone. To facilitate the comparison, PDF data were normalized by considering the intensity of the first peak at 1.6 Å attributed to the Si−O distance in silica. The inset highlights PDF differences in the 2.5−4.5 Å interatomic distance range.
G(r) obtained by the Fourier transformation of the structure function, and compared to the PDF of the silica substrate alone. As expected, the PDF signal from the films is the sum of the contributions of the film itself and the silica support. Particularly, the first sharp peak at 1.6 Å corresponds to the Si−O distance in silica; other silica-related peaks (Si−Si at ∼3.1 and ∼4.2 Å) are much smaller in magnitude and blend with those of TaOxNy films. To distinguish between the contribution of the deposited thin films and the silica substrate, the scale factors of the PDF were adjusted by considering the intensity of the peak at 1.6 Å. Depending on RF value, the PDF curves are quite different as regards the magnitude of the oscillations. From the extent of these oscillations, it is possible to estimate the grain size. It is noteworthy that because of the limited Q-resolution of our Xray data, we can only determine a lower bound of the grain size. In agreement with the more crystalline character of the films prepared at RF = 0 (presence of Bragg peaks), the oscillations of the PDF curve remain visible up to 40 Å, indicating coherent domains of at least 40 Å in diameter as can be seen in Figure 7. On the contrary, for the films prepared at RF = 0.5 and 1.0, the amplitude of the oscillations on the PDF curves falls off quickly with increasing r, around 10 Å in both cases, and in the same way as for silica support. It can therefore be said that the films prepared at RF = 0.5 and 1.0 are composed of very small nanosized domains 1.81 For instance, the theoretical value associated with the overstoechimetric TaN1.13 is 4.3161 Å. This is also consistent with the film composition TaN1.6 as determined by RBS. From the fwhm (full width at half-maximum) of 111 reflection, corrected from the instrumental broadening measured on LaB6 standard and using Scherrer equation, we were able to quantify the size of TaN crystallites along the [111] growth direction with a value of ∼490 Å. At RF = 0.2, i.e., upon increasing oxygen content in the reactive gas mixture, we observe the growth of TaON with a characteristic peak observed at 29.1° (2θ) assigned to (−111) planes and corresponding to the strongest reflection expected for monoclinic P21/c β-TaON (Figure 9b).33,34 The XRD signal intensity for TaON phase is still too weak to attempt a refinement of the cell parameters, also to determine its proportion with regard to TaN. Contrary to TaN, the growth of TaON crytallites does not seem to be associated with preferential orientation since many hkl reflections are observed. The single peak refinement of the position of 111 TaN reflection leads to a cell parameter a equal to 4.3044(7) Å. The slight difference with the value determined at RF = 0 is not significant if we consider the esd (estimated standard deviation) values given in parentheses and reflecting the precision of the refined unit-cell parameters. Accordingly, one cannot interpret it as resulting from the partial replacement of nitrogen by oxygen in the crystal lattice of TaN as reported elsewhere.21 The average size of TaN crystallites along the [111] growth direction, determined as above from the fwhm and using Scherrer equation, is ∼310 Å. The formation of P21/c β-TaON crystallites is confirmed at RF = 0.4, becoming the major phase (Figure 9c). On the other hand, Fm−3m TaN phase is still present, and the observation of both reflections, i.e. 111 and 200 reflection (not visible at RF = 0 and 0.2), indicates the disappearance of 111 texture for this phase. The net decrease of the intensity of TaN 111 reflection compared to RF = 0.2 is consistent with the disappearance of 111 texture but also may be attributed to a decrease of the overall number of TaN crystallites contributing to the diffraction pattern. On the other hand, the small width of 111 and 200 reflections indicates a large dimension of TaN crystals in both corresponding directions. Owing to the polycrystalline character of both phases, a quantitative phase analysis was attempted considering the structures reported by Sahnoun et al. for TaN60 and by Yashima et al. for TaON.33 The graphical output of the refinement is given in Figure 9c. The refined lattice parameters are a = 4.301(1) Å for TaN and a = 4.976(1) Å, b = 5.035(1) Å, c = 5.188 (1) Å, and β = 99.706(2)° for TaON. Note that the latter values for TaON are close to the unconstrained lattice parameters reported by Weishaupt et al.34 (a = 4.9692 Å, b = 5.0330 Å, c = 5.1821 Å, and β = 99.6820°), thus suggesting the absence of any microstrains. The composition of the crystalline part of the film so determined is 75.0 (1.7) wt % of TaON and 24.9(0.6) wt % of TaN. Using the fwhm and Scherrer equation, the average sizes of TaN crystallites along [111] and [200] directions were determined, 875 and 635 Å, respectively,
Figure 8. Comparison of the experimental PDF for RF = 0 (a), 0.5 (b), and 1.0 (c) with simulated PDF based on the structure of reference materials: Fm−3m TaN,43,75,76 Cmcm Ta3N5,79 C2/m Ta3N5,80 P21/c TaON,33,34 I41/amd TaON,37 I41md TaON,37 C2mm Ta2O5,61 Ibam Ta2O5,77 and Imma Ta2O5.78
to baddeleyite-type β-TaON phase.33,34 The absence of the 002 reflection indicates a preferred orientation of TaN crystallites according to the [111] direction, i.e., with the (111) planes parallel to the surface and therefore a preferential growth orientation along [111] perpendicular to the substrate surface. The unit cell parameter was calculated from the position of this peak by curve fitting with a pseudo-Voigt profile function 23565
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Figure 9. Results of the full pattern fitting of the X-ray diffraction patterns of the annealed thin films using either Le Bail method at (a) RF = 0, (b) RF = 0.2, and (e) RF = 0.8 or the Rietveld method at (c) RF = 0.4, (d) RF = 0.6, and (f) RF = 1.0. The upper continuous red line is the calculated diffraction profile and the lower continuous line the difference between the experimental (black dots) and the calculated profiles. The vertical bars, at the bottom, indicate the Bragg reflection positions expected for Fm−3m TaN,75 P21/c TaON,33,34 and Ta2O5 C2mm.61 Insets give the projections of the average apparent size of crystalline domains of TaON at (c) RF = 0.4 and (d) RF = 0.6 and Ta2O5 at (f) RF = 1.0 on (100), (010), and (001) planes.
deviation obtained by Fullprof represents a measure of the degree of anisotropy, and the quite high value obtained ∼42% (115/270) indicates a pronounced anisotropic grain growth resulting from the preferential growth of TaON crystalline domains in the [001] direction as shown in the inset of Figure 9c. The crystals are thus elongated along the c-axis and of rodlike morphology with an average diameter of ∼185 Å and a length of ∼575 Å. Because we are dealing with thin films, we can also say that a large number of TaON crystallites probably
indicating the formation of larger (but fewer) TaN crystals than at RF = 0.2 as explained above. In the case of TaON component, it should be noted that the application of the anisotropic size-broadening model based on a spherical-harmonics development was essential to get a good fit, consistently with the Williamson−Hall qualitative analysis reported in a previous study.44 The average apparent size of crystalline TaON domains so determined is found to be equal to ∼270 Å with a standard deviation of ∼115 Å. This standard 23566
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The Journal of Physical Chemistry C
Table 1. Results of the Structural and Microstructural Characterization of As-Prepared and Annealed Thin Films Based on X-ray Total Scattering Experiments Using the Pair Distribution Function Technique and X-ray Diffraction Data Using Full-Pattern Fitting Methods As-Prepared Thin Films RF 0 0.5 1.0
TaN Fm−3m Ta3N5 TaON I41amd Ta2O5 Ibam Ta2O5 Imma
3.09 2.86−3.29 3.13 3.49 3.56−3.53 Annealed Thin Films
av size of crystalline domaina (Å) 40
