Hicham Zaid
email: hizaid@ucla.edu

Angel Aleman

Koichi Tanaka

Chao Li

Pascal Berger

Tyson Back

Josh Fankhauser

Mark S Goorsky

Suneel Kodambaka

Influence of ultra-low ethylene partial pressure on microstructural and compositional evolution of sputter-deposited Zr-C thin films

Keywords: transition-metal carbides, reactive magnetron sputtering, Zr-C thin films

Zr-C thin films are grown on single-crystalline MgO(001) substrates via ultra-high vacuum dc magnetron sputtering of Zr target in 10 mTorr Ar-C2H4 gas mixtures with ethylene partial

) between 2 × 10 -7 Torr and 2 × 10 -4 Torr at substrate temperature Ts = 923 K and using p C 2 H 4 = 2 × 10 -6 Torr at 723 K ≤ Ts ≤ 1123 K. The as-deposited layer microstructure and composition are determined using X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. We find that the layers sputter-deposited at Ts = 923 K using the lowest p C 2 H 4 = 2 × 10 -7 Torr are polycrystalline, close-packed hexagonal structured Zr:C solid solutions. At higher p C 2 H 4 = 2 × 10 -6 Torr and 2 × 10 -5 Torr, we obtain films composed of freecarbon (C) and NaCl-structured ZrCx, x  1. The amount of C increases 104% with ten-fold increase in p C 2 H 4 from 2 × 10 -6 Torr to 2 × 10 -5 Torr. At the highest p C 2 H 4 = 2 × 10 -4 Torr, the layers are X-ray amorphous with ~ 49 at.% C. Films grown at 723 K ≤ Ts ≤ 1123 K using constant p C 2 H 4 = 2 × 10 -6 Torr exhibit qualitatively similar microstructures, irrespective of Ts, composed of dense columnar ZrCx grains surrounded by C and corrugated surfaces. Our results

suggest that the compositional and microstructural evolution of Zr-C films during reactive 2 sputter-deposition of Zr is highly sensitive to ethylene partial pressure, with as little as 0.02% of the total pressure sufficient at Ts ≥ 723 K to obtain ZrCx films.

Introduction

Group 4 to 6 transition-metal carbides (TMCs) are highly refractory compounds with excellent thermomechanical and thermochemical properties due to a mixture of ionic, covalent, and metallic bonds between the metal cations and carbon anions. Among the TMCs, ZrC crystallizes in B1 (rock salt) structure (Fm3 ̅ m; space group 225) with a bulk lattice parameter ao = 0.4693 nm at room temperature [START_REF] Morris | Standard x-ray diffraction powder patterns[END_REF]. ZrC is a hard (> 25 GPa), high elastic modulus (> 350 GPa), and high melting point (> 3673 K) solid with good thermal (20.5 W/m.K at 293 K) and electrical conductivities (2326 S/m at 293 K) and excellent resistance to wear, corrosion, and oxidation [START_REF] Pierson | Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing and Applications[END_REF].

ZrC is used as hard protective coatings on cutting tools [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Norgren | Trends in the P/M hard metal industry[END_REF][START_REF] Andersson | Magnetron sputtering of Zr-Si-C thin films[END_REF][START_REF] Toth | Transition metal carbides and nitrides[END_REF][START_REF] Balko | Nanoindentation and tribology of VC, NbC and ZrC refractory carbides[END_REF], as structural components in aerospace vehicles, as coating material for nuclear fuel in nuclear reactors [START_REF] Pierson | Handbook of Refractory Carbides and Nitrides: Properties, Characteristics, Processing and Applications[END_REF][START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Monteverde | Processing and properties of ultra-high temperature ceramics for space applications[END_REF][START_REF] Minato | Fission product release from ZrC-coated fuel particles during postirradiation heating at 1600°C[END_REF][START_REF] Ueta | Fabrication of uniform ZrC coating layer for the coated fuel particle of the very high temperature reactor[END_REF][START_REF] Katoh | Properties of zirconium carbide for nuclear fuel applications[END_REF], and as field-emission cathodes in electronic devices [START_REF] Mackie | Single-crystal zirconium carbide as a high-temperature thermionic cathode material[END_REF].

ZrC thin films have been grown via reactive sputtering of elemental Zr in hydrocarbon/Ar gas mixtures [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF], sputtering of compound ZrC [START_REF] Xu | Photo-thermal conversion and thermal insulation properties of ZrC coated polyester fabric[END_REF][START_REF] Gao | Optical properties and failure analysis of ZrC-ZrOx ceramic based spectrally selective solar absorbers deposited at a high substrate temperature[END_REF] and Zr and C targets [START_REF] Balden | Metal-doped carbon films obtained by magnetron sputtering[END_REF] in pure Ar discharges, sol-gel synthesis [START_REF] Dollé | Synthesis of nanosized zirconium carbide by a sol-gel route[END_REF], chemical vapor deposition [START_REF] Minato | Retention of fission product caesium in ZrC-coated fuel particles for high-temperature gas-cooled reactors[END_REF][START_REF] Reynolds | Chemical vapor deposition of ZrC on pyrocarbon-coated fuel particles[END_REF][START_REF] Won | Equilibrium analysis of zirconium carbide CVD growth[END_REF], and pulsed laser deposition [START_REF] Craciun | Epitaxial ZrC thin films grown by pulsed laser deposition[END_REF][START_REF] D'alessio | Zirconium carbide thin films deposited by pulsed laser ablation[END_REF]. Among all these techniques, reactive sputter-deposition has been the most commonly used method to grow ZrC and other TMC thin films [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF][START_REF] Balden | Metal-doped carbon films obtained by magnetron sputtering[END_REF][START_REF] Portolan | Carbon occupancy of interstitial sites in vanadium carbide films deposited by direct current reactive magnetron sputtering[END_REF][START_REF] Wu | Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Ait Djafer | Deposition and characterization of titanium carbide thin films by magnetron sputtering using Ti and TiC targets[END_REF][START_REF] Li | Microstructure and mechanical properties of hafnium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Zhang | Reactive magnetron sputtering deposition and characterization of niobium carbide films[END_REF][START_REF] Kumar | Investigations on the effect of substrate temperature on the properties of reactively sputtered zirconium carbide thin films[END_REF]. In this approach, one can control both the thin film composition and the growth rate by appropriate choice of the reactive gas and its flux [START_REF] Zoita | Influence of film thickness on the morphological and electrical properties of epitaxial TiC films deposited by reactive magnetron sputtering on MgO substrates[END_REF]. The TMC coatings deposited via this technique typically result in two-phase mixtures, composed of TMC crystallites and free-carbon (C) [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Wu | Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Zhang | Reactive magnetron sputtering deposition and characterization of niobium carbide films[END_REF], with reactive gas pressure-dependent TMC and C phase fractions, crystallinity, and mechanical properties [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF][START_REF] Wu | Microstructure and mechanical properties of vanadium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Li | Microstructure and mechanical properties of hafnium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Zhang | Reactive magnetron sputtering deposition and characterization of niobium carbide films[END_REF][START_REF] Kumar | Reactive magnetron sputtered wear resistant multilayer transition metal carbide coatings: microstructure and tribo-mechanical properties[END_REF]. Recently, Tanaka et al. [START_REF] Tanaka | Effects of ultralow ethylene partial pressure on microstructure and composition of reactively sputterdeposited Ta-C thin films[END_REF] have demonstrated tunability of Ta-C phase composition in thin films sputter-deposited with ultra-low amounts of ethylene partial pressures, p C 2 H 4 (0.0025 ~ 2.5% of total pressure) using a compound TaC target. Here, we focus on the microstructural and compositional evolutions of thin films deposited by sputtering elemental Zr target using similarly low p C 2 H 4 between 0.002 and 2% of the total pressure.

In this paper, we report on the effects of p C with little changes in the C content with increasing Ts. Our experiments reveal that the carbon content in reactively sputter-deposited Zr-C thin films is highly sensitive to the reactive gas pressure.

Experimental details

All the Zr-C films are grown on single-side polished 0.5 × 2 × 10 mm 3 rectangular strips of MgO(001) substrates in a custom-designed ultra-high vacuum (UHV, base pressure < 5.3 × 10 -9 Torr) deposition system following the procedure described in Ref. [START_REF] Fankhauser | Growth and characterization of epitaxial Zr(0001) thin films on Al2O3[END_REF]. Briefly, the MgO(001) substrates are cut from 0.5 × 10 × 10 mm 3 , 99.95% pure crystals (purchased from MTI), cleaned via sonication sequentially in acetone, isopropyl alcohol, and deionized water, and oven-baked in air at 523 K for one hour. The samples are mounted on a pyrolytic boron nitride heating stage [START_REF] Tanaka | Kinetics of Zr-Al intermetallic compound formation during ultra-high vacuum magnetron sputter-deposition of Zr/Al2O3(0001) thin films[END_REF], where they are held by Mo clips, transferred to the main chamber, and degassed by resistively heating to and holding at 1273 K until the chamber pressure is below 6 × 10 -9 Torr.

The sample temperature Ts is then set to the desired value. In our experiments, we estimate that the maximum variation in Ts across the substrate is ± 100 K. The magnetron sputter-deposition is carried out using a 3. Torr). Prior to deposition, the Zr target is sputter-cleaned using the same Ar/C2H4 composition as that used for the film growth for 2 minutes with the sample rotated out-of-sight from the target.

After deposition, the ethylene and Ar gas supplies are shut off and the chamber is evacuated while the sample is passively cooled to room temperature by switching off the current supplied to the substrate heater.

XRD 2θ-ω data are obtained following the procedure described in Ref. [START_REF] Aleman | Ultrahigh vacuum dc magnetron sputter-deposition of epitaxial Pd(111)/Al2O3(0001) thin films[END_REF] using a Jordan

Valley D1 diffractometer with a sealed copper X-ray tube source, a MaxFlux specular mirror to produce a parallel beam, a two bounce channel-cut (2 2 0) Si collimator crystal to select the Cu Kα1 wavelength ( = 0.154056 nm) for the incident beam, and a two bounce channel-cut (2 2 0)

Si analyzer crystal for the third axis optics. The sample and stage are calibrated with respect to ω, ϕ (out-of-plane rotation perpendicular to ω), and χ (in-plane rotation) about the MgO(004) peak, 2θ = 94.0496°. 2θ-ω scans for 2θ values between 20° and 100° are acquired using double-axis diffraction with a step size of 0.02° and a dwell time of 1 s.

Cross-sectional TEM (XTEM) samples are prepared using 30 kV Ga + ions in an FEI Nova 600 NanoLab DualBeam™ scanning electron microscope equipped with a focused ion beam (FIB) source. Prior to milling, the film surface is protected by ~ 150 nm and ~ 1 µm thick Pt layers deposited sequentially, first using electron beams to minimize any damage that may incur during the subsequent ion-beam-assisted deposition of the thicker layers. TEM images of the film and the film-substrate interface are acquired in an FEI Titan 80-300 kV scanning TEM (S/TEM) operated at 300 kV for imaging.

XPS data are obtained using a Kratos Analytical AXIS Ultra DLD at binding energies between 0 and 1200 eV with a pass energy of 160 eV, step size of 1.0 eV, and dwell time of 0.1 s.

High-resolution Zr 3d and C 1s spectra are acquired using a pass energy of 20 eV and a step size of 0.1 eV over the energy ranges 160 -215 eV and 264 -300 eV, respectively, with dwell times of 0.5 s and 2 s. In order to accurately determine the bulk composition and minimize the contribution to the XPS signal from surface contamination, the XPS data are obtained after Ar + ion etching of all but one samples for 1 h and the other sample (grown using p C 2 H 4 = 2 × 10 -7 Torr and Ts = 923 K) for 2.5 h. The relative concentrations of Zr and C in the films are determined from the ratios of the high-resolution Zr 3d and C 1s spectral peak areas, measured using CasaXPS software and defined with a Shirley background type [START_REF] Fairley | Casa XPS User's Manual[END_REF], corrected by their respective relative sensitivity factors, 2.58 and 0.278. In order to assess the accuracy of our quantification procedure, we acquired both XPS and Rutherford backscattering spectrometry (RBS) data (details below) from the same ZrCx film (sputter-deposited on Al2O3(0001) using a ZrC target [START_REF] Fankhauser | Growth and Characterization of Zr and ZrC Thin Films on Al2O3[END_REF]. From the XPS measurements, following the procedure described above, we determined the film composition as C/Zr = 1.44, while the RBS analysis yielded C/Zr = 2.04. The apparent discrepancy in the carbon contents measured using these two techniques could be due to preferential removal of C from the surface during sputter-etching of the samples prior to XPS measurements coupled with the fact that XPS is a surface-sensitive technique while RBS probes the entire cross-section of the film.

RBS-Nuclear Reaction Analysis (RBS-NRA) data, used to determine the film stoichiometry of one sample apart from the set of samples presented in this article, are obtained using 1.2 MeV deuteron beam generated by the nuclear microprobe of CEA Saclay, France. The incident deuteron beam size is 4 × 3 μm 2 . Backscattered particles produced from nuclear reactions are detected with an annular surface barrier particle detector (detection angle 170°). C concentrations are measured from the peak of the nuclear reaction 12 C(d,p0) 13 C and Zr concentration from that of backscattered deuterons. The film composition is deduced by fitting experimental data with simulated spectra using SIMNRA code [START_REF] Mayer | SIMNRA, a simulation program for the analysis of NRA[END_REF], which takes into account experimental beam conditions and related cross-sections of scattering and deuteron induced nuclear reactions.

Results and discussion

Figure 1a shows XRD 2θ-ω scans acquired from Zr-C/MgO( 001) samples (open circles) grown at Ts = 923 K using Ar/C2H4 gas mixtures with p C 2 H 4 between 2 × 10 -7 Torr and 2 × 10 -4

Torr. In Fig. 1a, the solid line is an experimental XRD 2θ-ω scan of a bare MgO(001) substrate, provided as a reference. The higher intensity peaks at 2θ = 42.9° and 41.0° labeled with asterisks (*) correspond to 002 reflections of the MgO single-crystal substrate due to Cu Kα1 ( = 0.154056 nm) and unintentional W Lα (λ = 0.147635 nm) radiations from the X-ray source, respectively.

XRD data obtained from the layers deposited using the lowest p C 2 H 4 (= 2 × 10 -7 Torr) do not show any peaks due to B1-ZrC but reveal reflections at 2θ values corresponding to hcp Zr 0002 and 101 ̅ 1. The lattice constants (aZr, cZr) determined from the XRD data are (0.325 ± 0.003 nm, 0.517 ± 0.003 nm), which are within the measurement uncertainties the same as (0.323 nm, 0.515 nm) of pure Zr [START_REF] Kolesnikov | Neutron scattering studies of ordered gamma -ZrD[END_REF]. In the same XRD pattern, we also note the presence of seemingly high background intensity at 2θ ≈ 35°, highlighted by a dotted circle, between the Zr 0002 and Zr 101 ̅ 1 peaks. We suggest and provide supporting evidence in the following section that this result is likely due to the formation of hexagonal-structured carbides, e.g., Zr3C2 determine a = 0.468 ± 0.004 nm at 2 × 10 -6 Torr and 0.472 ± 0.004 nm at 2 × 10 -5 Torr. The former is, within the measurements uncertainties, the same as ao = 0.4693 nm of the bulk stoichiometric ZrC [START_REF] Morris | Standard x-ray diffraction powder patterns[END_REF], while the latter is ~0.6 % larger than ao, likely due to distortion of the ZrC lattice due to increased incorporation of C in the films [START_REF] Toth | Transition metal carbides and nitrides[END_REF][START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF][START_REF] Li | Microstructure and mechanical properties of hafnium carbide coatings synthesized by reactive magnetron sputtering[END_REF][START_REF] Zhang | Reactive magnetron sputtering deposition and characterization of niobium carbide films[END_REF]. Similar shifts in XRD peak positions have been previously observed with increasing carbon contents in sputter-deposited ZrC [START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF], HfC [START_REF] Li | Microstructure and mechanical properties of hafnium carbide coatings synthesized by reactive magnetron sputtering[END_REF], and NbC [START_REF] Zhang | Reactive magnetron sputtering deposition and characterization of niobium carbide films[END_REF] thin films. The XRD scan from the sample grown with the highest p C 2 H 4 (= 2 × 10 -4 Torr) does not show any peaks, suggesting that the film is X-ray amorphous.

Based on the data in Fig. 1a, we identify p C 2 H 4 = 2 × 10 -6 Torr as the optimal ethylene partial pressure for the growth of B1-ZrC at Ts = 923 K in our deposition system.

We investigated the effect of Ts, another key parameter that can influence the crystallinity of sputter-deposited films [START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF][START_REF] Aguzzoli | Physicochemical and structural characteristics of TiC and VC thin films deposited by DC reactive magnetron sputtering[END_REF]. Fig. 1b shows XRD data obtained from Zr-C layers sputter- -4 Torr. (This is in contrast with the results reported in Ref. [START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF],

deposited
where the deposition rate decreases with increasing reactive gas (methane) partial pressure.) We attribute this result to the increase in the carbon content (supporting evidence provided in the following section) within the films with increasing p C 2 H 4 during sputter-deposition. The TEM images of Zr-C sample deposited using p C 2 H 4 = 2 × 10 -7 Torr reveal that the film is dense with smooth surface and compositionally-abrupt film-substrate interface (see Fig. 2a); these results are in striking contrast with the interfacial reactions observed during sputter-deposition of pure Zr films on Al2O3(0001) [START_REF] Tanaka | Kinetics of Zr-Al intermetallic compound formation during ultra-high vacuum magnetron sputter-deposition of Zr/Al2O3(0001) thin films[END_REF]. Lattice-resolved TEM image (for example, Fig. 2a') and corresponding Fourier transform (FT) (inset in Fig. 2a') show that the Zr-C layers deposited with the lowest p C 2 H 4 = 2 × 10 -7 Torr are crystalline and that the layer is primarily composed of hcpstructured Zr. We have also observed additional reflections that we attribute to trigonal-Zr3C2 0003 ̅ , the presence of which could explain the observed high background XRD intensity at 2θ ≈ 35° in Fig. 1a. This result suggests the coexistence of hcp-Zr and trigonal Zr3C2. Increasing p × 10 -4 Torr used in our experiments, the layers appear to be dense (see Fig. 2d). The XTEM image in Fig. 2d' lacks any diffraction contrast and the corresponding FT shows a broad diffuse ring, commonly associated with amorphous samples. These observations suggest that the Zr-C films deposited using 2 × 10 -4 Torr ethylene are amorphous and are consistent with the XRD data (Fig. 1a). (diamond) hybridized carbon, whose exact contributions to the XPS signal are not resolvable in our measurements [START_REF] Toth | Transition metal carbides and nitrides[END_REF][START_REF] Blume | Characterizing Graphitic Carbon with X-ray Photoelectron Spectroscopy: A Step-by-Step Approach[END_REF]. The peaks observed at binding energies between 178.1 and 180.5 eV correspond to Zr 3d5/2 and Zr 3d3/2, respectively. We find that the Zr peaks are better defined at lower p C 2 H 4 than at higher values, presumably due to the decrease in free-C content. With increasing p C 2 H 4 , we observe a shift (compared to the dashed vertical lines in the plot, which correspond to binding energies in stoichiometric ZrC) in the C-Zr and Zr 3d peak positions to higher binding energies. Based on the increase in C-C, and concomitant decrease in the C-Zr, peak intensities with increasing p C 2 H 4 , we suggest that the observed peak shifts are due to the increase in free-C content in the films.

From the measurements of the Zr 3d, C-C, C-Zr, and C-Zr* peak areas from Fig. 4a, we quantify the concentrations of total carbon, free-C, and bonded-carbon x in ZrCx, plotted as magenta, black, and green curves, respectively, as a function of p C 2 H 4 in Fig. 4b. The label free-C in the plot refers to the elemental carbon that is not bonded to Zr and corresponds to C-C peaks in Fig. 4a. We find that the amount of total C and free-C increase while the fraction of carbon bonded to Zr, i.e. the carbide phase, decreases with increasing p C 2 H 4 during sputter-deposition.

These results are consistent with the observation of increasing fraction of amorphous phase with increasing p C 2 H 4 in the TEM images in Fig. 2 and are commonly observed in sputter-deposited Zr-C films [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF]. Interestingly, the fraction of free-C relative to the amount of carbon in the carbide phase is higher in our films than in the films deposited using three orders of magnitude higher reactive gas (methane) partial pressure [START_REF] Meng | Deposition and characterization of reactive magnetron sputtered zirconium carbide films[END_REF]. Possible reasons for this behavior include: differences in the rates of dissociative chemisorption of carbon due to precursor composition (relatively more stable methane with one C atom/molecule vs. ethylene with two C atoms/molecule), Zr deposition rates, Ts, total gas pressure, plasma characteristics, and residual gas composition and concentration. Among all these samples, based on the XPS data, we note that the films deposited using p C 2 H 4 = 2 × 10 -6 Torr yield the highest fraction of Zr-C phase with the least amount of free-C. In the following section, we present the effect of Ts on the composition of Zr-C films deposited with p C 2 H 4 = 2 × 10 -6 Torr. With increasing Ts, we observe shifts (~ 0.3 eV) in both the Zr 3d and C-Zr peaks to higher binding energies, suggestive of stronger bonding presumably due to decrease in the C-content within the carbide; the magnitude of shift is more pronounced at higher Ts ( 923 K). In all the samples grown at different Ts, C-C peaks are broad, shallow, and changes (if any) in the peak positions are not detectable. The Ts-dependent changes in total C, free-C, and bonded-carbon concentrations are plotted in Fig. 4d. We find that the amounts of total C (magenta curve) and free-C (black curve) increase marginally with increasing Ts, indicating that substrate temperatures over 723 K have little influence on the total C and free-C contents in the Zr-C films sputterdeposited using p C 2 H 4 = 2 × 10 -6 Torr. (These results are opposite to those reported for Zr-C films sputter-deposited using methane gas [START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF], which as mentioned earlier could be not only due to the use of ethylene vs. methane but also system-specific deposition parameters.) Based on these results, we suggest that during reactive sputter-deposition of Zr using low ethylene partial pressures, such as p C 2 H 4 = 2 × 10 -6 Torr, incorporation of C from C2H4 into the growing films occurs readily at all Ts ≥ 723 K.

Conclusions

This

%)

Total C

C 2 H 4 (between 2 × 10 - 7

 42107 Torr and 2 × 10 -4 Torr) and substrate temperature (723 K ≤ Ts ≤ 1123 K) on the composition and crystallinity of Zr-C/MgO(001) thin films sputter-deposited in 10 mTorr Ar-C2H4 gas mixtures. (Here, and in the following sections, we use Zr-C to refer to both Zr:C solid solutions and ZrCx, x  1, compounds.) Using X-ray diffraction (XRD), transmission electron microscopy (TEM), and Xray photoelectron spectroscopy (XPS), we characterized the as-deposited layer microstructure and composition. We find that the films sputter-deposited at Ts = 923 K using p C 2 H 4 = 2 × 10 -7 Torr are polycrystalline with primarily hexagonal close-packed (hcp) Zr:C solid solutions. At p C 2 H 4 = 2 × 10 -6 Torr and 2 × 10 -5 Torr, the films are composed of a two-phase mixture of B1-structured ZrCx and C with an increasing phase fraction of C with increasing p C 2 H 4 . At p C 2 H 4 = 2 × 10 -4 Torr, the film is X-ray amorphous and with a high C content. The Zr-C layers sputter-deposited with p C 2 H 4 = 2 × 10 -6 Torr at Ts between 723 K and 1123 K are all two-phase mixtures of B1-ZrCx and

C 2 H 4 to 2 × 10 - 6

 42106 Torr and 2 × 10 -5 Torr leads to porous films with dendritic columns and facetted surfaces (Figs.2b and c). The corresponding higher resolution XTEM images in Figs.2b' and c' reveal regions with diffraction contrast indicative of crystallinity; surrounding regions without such contrast are likely composed of amorphous C. FTs in Figs. 2b' and c' reveal reflections corresponding to B1-structured 002 and 111 grains in the films, consistent with the XRD results. FTs (not shown here) of the XTEM images acquired from the regions near the film-substrate interfaces in these two samples reveal two sets of identical reflections, indicative of cube-on-cube epitaxial growth of 002-oriented ZrC layers on the MgO(001) substrate. At the highest p C 2 H 4 = 2

Figs 2 ~

 2 Figs. 3a-d and 3a'-d' are series of low-magnification and higher-resolution XTEM images,

Fig. 4c shows

  Fig. 4c shows representative C 1s and Zr 3d spectra obtained from the Zr-C samples grown

Fig. 1 . 2 × 10 - 4

 12104 Fig. 1. 2θ-ω X-ray diffraction (XRD) data obtained from Zr-C/MgO(001) samples sputter-

Fig. 2 .

 2 Fig. 2. Typical bright-field cross-sectional transmission electron microscopy (XTEM) images of

Fig. 3 . 2 × 10 - 6

 32106 Fig. 3. Bright-field XTEM images of Zr-C/MgO(001) samples sputter-deposited using p C 2 H 4 = 2 × 10 -6 Torr at Ts = (a) 723 K, (b) 823 K, (c) 1023 K, and (d) 1123 K. All the samples appear

Fig. 4 .

 4 Fig. 4. (a) X-ray photoelectron spectroscopy (XPS) data acquired around Zr 3d (175 -190 eV)

  Figure 1

  Figure 2

  Figure 3

  2-mm-thick × 50.8 mm diameter Zr target (99.91 wt.% pure with 0.08 wt.%

	Hf from ACI Alloys, Inc.), located at about 30 cm directly above the substrate. The chamber is
	first filled with ethylene (99.999% purity) gas up to the desired pressure p C 2 H 4	between 2 × 10 -7
	Torr and 2 × 10 -4 Torr set using a UHV leak valve. Subsequently, Ar (99.999% purity) gas is let
	in and the total pressure is adjusted to 10 mTorr. Zr-C thin films are deposited using Zr target
	power maintained constant at 50 W for 30 minutes. The target voltage increases with increasing
	p C 2 H 4	from 225 V (2 × 10 -7 Torr) to 227 V (2 × 10 -6 Torr and 2 × 10 -5 Torr), and 234 V (2 × 10 -4

  at 723 K  Ts  1123 K using Ar/C2H4 gas mixtures with p C 2 H 4 = 2 × 10 -6 Torr. At all Ts ≤ 1023 K, we observe two sets of peaks corresponding to B1-structured ZrCx 111 reflection at 2 values between 32.92° and 33.14°, and 002 reflection at around 38.52°. The lattice parameters extracted from the 002 reflection are ~0.5% smaller compared to ao of stoichiometric ZrC[START_REF] Morris | Standard x-ray diffraction powder patterns[END_REF], while those determined from the 111 reflections are ~0.3% larger (for 2 = 32.92°) and ~0.3%

	We now present the microstructures of Zr-C layers sputter-deposited as a function of p C 2 H 4
	and Ts. Figs. 2a-d are typical XTEM images obtained from the Zr-C thin films grown at Ts = 923
	K using p C 2 H 4	= a) 2 × 10 -7 Torr, b) 2 × 10 -6 Torr, c) 2 × 10 -5 Torr, and d) 2 × 10 -4 Torr. We find
	that the film thickness increases with increasing p C 2 H 4	from 87 ± 4 nm at p C 2 H 4	= 2 × 10 -7 Torr to
	245 ± 20 nm at p C 2 H 4	= 2 × 10

smaller (for 2 = 33.14°) than ao. The peaks are however too broad and weak in intensity to accurately determine the temperature-dependent variations, if any, in the carbon content x, grain size, and strain. Interestingly, the XRD data obtained from the ZrCx film grown at the highest Ts = 1123 K showed only one peak corresponding to 002 reflection, indicating that 002 texture is preferred at higher Ts. These results suggest that, in our experiments, 111 texture is promoted in sputter-deposited ZrCx at lower Ts, a likely consequence of kinetic limitations inherent to the deposition process. Similar Ts-dependent 111 texture evolution has been reported in a variety of sputter-deposited TMC thin films

[START_REF] Brückner | Reactive magnetron sputtering of zirconium carbide films using Ar-CH4 gas mixtures[END_REF][START_REF] Kumar | Investigations on the effect of substrate temperature on the properties of reactively sputtered zirconium carbide thin films[END_REF][START_REF] Aguzzoli | Physicochemical and structural characteristics of TiC and VC thin films deposited by DC reactive magnetron sputtering[END_REF][START_REF] Colder | Influence of substrate temperature on growth of nanocrystalline silicon carbide by reactive magnetron sputtering[END_REF][START_REF] Chen | Influence of growth temperature on microstructure and mechanical properties of nanocrystalline zirconium carbide films[END_REF]

.

  report presents the effects of ethylene partial pressure (p C 2 H 4 The amount of free-C incorporated into the films depends sensitively on p C 2 H 4 at Ts = 923 K. Increasing the Ts above 723 K (with p C 2 H 4 = 2 × 10 -6 Torr) has little effect on the carbon incorporation rate. Our results indicate that relatively small amount of ethylene gas

			) and substrate temperature
	(Ts) on the microstructural and compositional evolution of reactively sputter-deposited Zr-C thin
	films on MgO(001) substrates using ultra-high vacuum dc magnetron sputtering of Zr target in
	Ar/C2H4 gas mixtures with fairly low p C 2 H 4	≤ 2% of the total pressure. We find that all the Zr-C
	films, irrespective of the p C 2 H 4	and Ts, are polycrystalline two-phase mixtures composed of
	hexagonal close-packed Zr:C solid solutions (at ultra-low p C 2 H 4	= 0.002%) or B1-ZrCx (at higher
	p C 2 H 4	) and free-C.

(0.02% of the total pressure in our experiments) is sufficient to deposit two-phase mixtures of ZrC and free-carbon. Given that pure, single-phase carbide films are often desirable, we suggest that reactive sputter-deposition of ZrC (and other TMC) films is probably best carried out using ultra-low partial pressures of carbon containing precursors; the exact pressure however depends on the deposition system, substrate temperature, deposition rate, and the precursor.

Acknowledgements

We gratefully acknowledge support from the Air Force Office of Scientific Research (AFOSR, Dr. Ali Sayir) under Grant # FA9550-14-1-0106 and # FA9550-18-1-0050. We thank the Office of Naval Research (Dr. Chagaan Baatar) for the funds received under Grant #N00014-12-1-0518 used to build the UHV deposition system. KT is supported by the Japanese Student Service Organization (L16111111026) and the UCLA department of materials science and engineering for his doctoral study in the United States. AA is supported by the National Science Foundation (NSF CMMI) grant #1563427 (Dr. Kara Peters). We thank Mr. Noah Bodzin and the Nanoelectronics Research Facility in the UCLA Henry Samueli School of Engineering for assistance with focused ion beam milling and acknowledge the use of instruments at the Electron Imaging Center for NanoMachines supported by NIH (1S10RR23057) and the California NanoSystems Institute at UCLA.