Article pubs.acs.org/cm
A Chemical Vapor Deposition Route to Epitaxial Superconducting NbTiN Thin Films Nikolaos Tsavdaris,† Dibyendu Harza,‡ Stéphane Coindeau,† Gilles Renou,† Florence Robaut,† Eirini Sarigiannidou,¤ Manoel Jacquemin,† Roman Reboud,† Max Hofheinz,‡ Elisabeth Blanquet,† and Frederic Mercier*,† †
Université Grenoble Alpes, CNRS, Grenoble INP, SIMaP, F-38000 Grenoble, France INAC/SPSMS, CEA-Grenoble, F-38000 Grenoble, France ¤ Université Grenoble Alpes, CNRS, Grenoble INP, LMGP, F-38000 Grenoble, France ‡
ABSTRACT: The deposition of epitaxial superconducting (Nb,Ti)N thin films is addressed with a new approach, using a chemical vapor deposition technique with in situ production of precursors. Both classic and reactive CVD process are optimized toward (i) the control of crystal structure of the deposited films and (ii) the control of stoichiometry and thus of superconducting properties. Films are chararcterized using thermodynamic, structural, and electrical characterization tools. Precession electron diffraction and electron backscatter diffraction techniques provide the phase and orientation mapping of the films down to the nanometer scale. We demonstrate that the cubic phase (structure with the most prominent superconducting properties) is the thermodynamically stable phase under classic CVD from 800 °C up to 1300 °C. The hexagonal (Nb,Ti)N is formed via a “nitridation like” process under reactive CVD, up to 1200 °C. The composition of the films is controlled by the chemistry of the gas phase, whereas the thickness is regulated down to less than 10 nm due to the low growth rate that can be achieved. This technique allows for the control of the superconducting properties of the films, through the control of Ti composition.
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INTRODUCTION The deposition of Niobium-based films, either as a carbide or a nitride, has an increasing interest in the field of superconducting applications (radio frequency cavities and singlephoton detectors, SSPDs), hard coatings and implants.1−7 In recent years, a ternary compound of (Nb,Ti)N attracts attention both on application and fundamental grounds, mainly due to its interesting superconducting properties. Because of its high critical temperature of superconductivity (Tc) and superconducting energy gap, (Nb,Ti)N can be a potential alternative to the most commonly used material Al, in circuit quantum optics experiments: its high Tc allows one to perform device testing at liquid helium temperature, instead of a more expensive dilution fridge; whereas, its high energy gap gives access to THz frequency rangean unexplored area in circuit QED.8 The high kinetic inductance of (Nb,Ti)N can be explored to design high characteristic impedance resonators9 a necessity to achieve microwave single photon source and photomultiplier, from inelastic Cooper pair tunnelling.10 Out of all the possible crystalline structures that can be stabilized for (Nb,Ti)N, only the cubic rock salt structure undergoes a superconducting transition with high Tc.11−14 Additionally, (Nb,Ti)N thin films have served as model systems for studying the effects of disorder on superconducting properties: © 2017 American Chemical Society
inhomogeneous superconducting order parameter, phase fluctuation, pseudogap like features, enhancement of pairbreaking parameters, among others, have been observed in these systems.15−17 However, the vast and diverse applications demand further improvements of the electrical and mechanical properties of the films. The fundamental open questions, on the other hand, demand well-controlled deposited films, where the disorder can be tuned over a large parameter space without compromising crystalline quality. It is, therefore, of paramount importance, both from application and fundamental points of view, to develop a controlled growth technique of these films and link their various propertieselectrical, superconducting, and mechanical, for instanceswith structural characteristics. Typical sputtering techniques are used for the deposition of (Nb,Ti) N films,5,18−20 but the high deposition rate makes it difficult to control film thicknesses below 10 nm. Besides sputtering, growth of (Nb,Ti)N thin films by atomic layer deposition (ALD) is reported, while thin films of only the binary NbN compound are deposited by chemical vapor deposition (CVD) Received: February 6, 2017 Revised: June 23, 2017 Published: June 23, 2017 5824
DOI: 10.1021/acs.chemmater.7b00490 Chem. Mater. 2017, 29, 5824−5830
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Chemistry of Materials technique.21−25 Despite the significant amount of reports, it is still an open question whether the cubic structure is stabilized over the hexagonal phases. The other challenge is to achieve reproducibility and control of thickness and composition over epitaxial films. In this work, we demonstrate the deposition of superconducting epitaxial cubic (Nb,Ti)N thin films by the CVD process combined with novel characterization techniques to assess the structural and electrical properties of this material. In our earlier communication, we have demonstrated that the high-temperature CVD process can produce high-quality epitaxial superconducting NbN films, with the highest reported Tc for films with thickness 50 nm or less.26 Thermodynamic calculations provide the necessary information to identify the reactants of the gas phase. Through controlling the chemistry of the gas phase, we cover the full range in Nb and Ti composition inside the films. We find the cubic structure to be stable over a wide deposition temperature range (800−1300 °C), while the low deposition rate (70 nm/h) gives us the possibility to control the thickness of the films, down to 5 nm. Deposited films have high crystalline quality and an epitaxial relationship with used substrates. By using electron backscatter diffraction (EBSD), high-resolution TEM, and precession electron diffraction ASTAR/ACOM techniques (automated crystal orientation mapping),27,28 we performed phase and orientation mapping from macro to nanometer scale. Electrical transport measurements down to 4 K, indicate that the critical temperature of superconductivity (Tc) can be controlled by the Ti composition in the films. We therefore expect (Nb,Ti)N thin films grown by CVD to be useful in superconducting applications like single-photon detectors (SSPDs).
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scans were made with a Bruker D8 Advance diffractometer equipped with a copper source, a Ge (111) curved monochromator in accordance with the Johansson geometry and a Lynx-Eye 1D detector. The in-plane X-ray diffraction, pole figures and rocking curve measurements were performed with a Rigaku Smartlab five-circle diffractometer associated with a cupper rotating anode source. Specifically, for the rocking-curve measurements, a four-reflection Ge (220) monochromator was used to obtain a monochromatic and parallel (divergence about 18 arcsec) beam. Thickness of the samples was measured by X-ray reflectivity technique, using a PANalytical X’Pert Pro MPD diffractometer equipped with a copper source and a Göbel mirror. TEM samples were characterized by ASTAR/ACOM technique using a 2100F TEM-FEG JEOL. The structural and interface quality of the (Nb,Ti)N layers was investigated by TEM imaging with a JEOLJEM 2010 microscope operating at 200 kV with a 0.19 nm point-topoint resolution. EBSD analysis were carried out using an EDAX Hikari camera on a Zeiss Ultra 55 FEG-SEM, at 20 keV with an approximate probe current of 4 nA. Electrical Measurements. Electrical transport and Hall measurements were performed in a Quantum Design PPMS. For Hall measurements, the magnetic field was swept from 0 to 8 T. The kFS values were extracted from electrical resistivity and Hall coefficient at room temperature, as described in ref 26.
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RESULTS AND DISCUSSION Thermodynamic Analysis. In our CVD apparatus, TiClx(g) and NbClx(g) chlorides are produced in situ, through the chlorination of Nb(s) and Ti(s) pellets (M(s)+(x/2)Cl2(g) → MClx(g), where M is the metal). Thermodynamic calculations of this chlorination process using FactSage software show that TiCl3(g) and NbCl4(g) are the main chloride species, while TiCl4(g) and NbCl5(g) are also produced (see Figure 1).
EXPERIMENTAL SECTION
Thermodynamic Analysis. Thermodynamic analysis of the gaseous phase chemistry is made using Factsage thermodynamic software. For the chlorination process of Nb and Ti, the thermodynamic database SGPS is used. A new thermodynamic database is created from the thermodynamic data of B. Lee et al, where (Nb,T)iN is described as a solid solution.29 High-Temperature Halide CVD. Thin-film deposition of (Nb,Ti) N is implemented into a homemade quartz, cold wall, vertical, twochamber CVD reactor.30 The growth apparatus is already described in Mercier et al.24 Our reactor is composed of a chlorination chamber and a growth chamber. In the chlorination chamber, the chloride species (TiClx(g) and NbClx(g)) are produced in situ by chlorination of the corresponding metals with Cl2(g). Growth on (0001) oriented Al2O3 and 1 μm thick (0001) epitaxial AlN grown on (0001) Al2O3 substrates, is made in the same run in order to ensure similar deposition conditions. We have performed two sets of experiments, targeting first in the control of crystal structure for the deposition of cubic or hexagonal (Nb,Ti)N films and, second, the control of stoichiometry and thus of superconducting properties of cubic (Nb,Ti) N films with a thickness of 10 nm or less. In the first set of experiments, a predeposition step is used. After heating up the substrates and once the desired deposition temperature is reached, Cl2(g) is injected in the Nb(s) (99.99%) and Ti(s) (99.99%) source metals. One minute later, NH3(g) is injected in the reactor to initiate the deposition of (Nb,Ti)N, while H2(g) is used as a carrier gas. In the second set of experiments, no predeposition step is used, and the Cl2(g) and NH3(g) gases are injected simultaneously when the deposition temperature is reached. To control continuously the ratio of Nb/Ti in the gas phase and the stoichiometry of the films, we regulate the flow of Cl2(g) in the separated Nb and Ti chlorination tubes. Characterization Tools. Several X-ray diffractometers were used to perform the different kind of measurements in this study: θ/2θ
Figure 1. Calculated partial pressure pi of NbClx (dashed line) and TiClx(solid line) gaseous species as a function of the chlorination temperature of Nb(s) and Ti(s).
When similar chlorination conditions are applied in both metallic sources, the equilibrium partial pressure of Ti species is 3 to 4 orders of magnitude higher than Nb ones up to 650 °C. Above this temperature, the partial pressure of the main Ti chloride species decreases due to the formation of the TiCl2(s) solid phase. In a second thermodynamic calculation, the mentioned chloride species react with NH3(g) and H2(g) while both solid and gaseous phases are allowed to form. The cubic Nb, the hexagonal Nb2N and a cubic solid solution of (Nb,Ti)N are the only solid phases considered, based on the thermodynamic databases used. According to the calculations, the full amount of Nb and Ti from the gas phase is 5825
DOI: 10.1021/acs.chemmater.7b00490 Chem. Mater. 2017, 29, 5824−5830
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Chemistry of Materials
Figure 2. (a) HR-TEM image of a (Nb,Ti)N film deposited on a (0001) oriented sapphire substrate. The scale bar corresponds to 2 nm. Phase mapping (b,e) and orientation mapping (c,f) of a NbN and (Nb,Ti)N cross-sectional samples, using ASTAR technique. In-plane misorientation in the cubic NbN and (Nb,Ti)N area is plotted along a selected line (d,g).
carbide close to the film−substrate interface. HRTEM provides only local structural information, and statistics on a broader scale are needed. For that we use the ASTAR technique, which allows phase and orientation mapping down to the nanometer scale (scanning step is less than 1.5 nm) for an area of a few hundreds of nanometers.27 Phase mapping reveals that in both samples h-(Nb,Ti)N grows on the c-axis sapphire substrate, and during deposition a transition to δ-(Nb,Ti)N occurs (see Figure 2b,e). The cubic δ-(Nb,Ti)N consists of 50−150 nm grains with 60° degrees in-plane misorientation (see Figure 2c,d,f,g). A similar transition from hexagonal to the cubic phase is observed for films grown by rf magnetron sputtering.5 However, in this case, a mixture of hexagonal and cubic phase is also observed. We believe that during the predeposition or chlorination step, (Nb,Ti) is deposited on the substrate. When NH3(g) is injected in the reactor two phenomena occur in parallel. First, a “nitridation like” process of the deposited (Nb,Ti) occurs under reactive CVD leading to the formation of h-(Nb,Ti)N. Second, a layer of (Nb,Ti)N nucleates and grows under classic CVD conditions on the initially deposited (Nb,Ti) layer (see schematic illustration in Figure 3). There, under classic CVD
incorporated into the cubic (Nb,Ti)N solid solution that is formed. Variations of the deposition temperature (in the range of 800−1500 °C) and the NH3/H2 ratio in the gas phase does not affect significantly the incorporation of Nb and Ti in the film. Thus, the composition of the film is controlled mainly by the Nb/Ti ratio in the gas phase. Thin-Film Deposition. In the first set of experiments, we examine the stabilization of various (Nb,Ti)N crystal structures in the temperature range of 800−1300 °C. The Nb−N system forms more than five phases of both cubic and hexagonal structure that have similar X-ray diffraction patterns;23,24,31,32 thus, θ/2θ XRD scans are not sufficient for phase identification. In this respect, we perform additional in-plane XRD scans, along the (11−20) and (10−10) planes of sapphire and AlN substrates. We found that deposited films are a mixture of cubic δ-(Nb,Ti)N Fm3̅m phase (ICDD: 01-088-2404) and hexagonal (Nb,Ti)N for deposition temperatures between 800 and 1200 °C, while above 1200 °C hexagonal phases are absent. There are two hexagonal NbN phases that best match the XRD scans, the (ε and δ′ phases) with the space groups P6̅m2 (ICDD: 04004-3002) and P63/mmc (ICDD: 04-004-3004). Due to similar XRD patterns, it is not possible to distinguish which of the two phases appear in the films. In order to simplify the notation later in the paper, we will simply use h-(Nb,Ti)N, which stands for hexagonal (Nb,Ti)N, instead of ε and δ′. On (0001) sapphire substrate (0001), (10−10) and (10−11) h(Nb,Ti)N oriented grains appear, while when an (0001) AlN buffer layer is used, the (10−10) orientation is missing. The inplane orientation of the cubic and hexagonal grains with the sapphire and AlN substrates is the same as that found by Mercier et al.24 In order to understand the origin of the phase stabilization, we prepare two cross-sectional TEM samples from films with and without Ti, grown on sapphire substrate using the focused ion beam (FIB) technique. We perform phase and orientation mapping with the ASTAR technique and high-resolution TEM (HRTEM), Figure 2. HRTEM images show that the films consist of two layers parallel to the substrate with thickness ratio of 1/4 and 3/4 of total film thickness (only one example is shown in Figure 2a). The diffraction pattern generated by fast Fourier Transform (FFT) of the high-resolution image of the first layer close to the substrate corresponds to h-(Nb,Ti)N while the second domain is of the cubic δ-(Nb,Ti)N structure. No additional diffraction spots are found in the FFT generated patterns that could correspond to the presence of a chloride or
Figure 3. Schematic illustration for the formation of hexagonal (h(Nb,Ti)N) and cubic (δ-(Nb,Ti)N) thin films by reactive and classic CVD processes.
conditions, cubic δ-(Nb,Ti)N is always stabilized. When the deposition temperature is higher than 1200 °C, no h-(Nb,Ti)N phase appears in the films but only the cubic δ-(Nb,Ti)N stabilizes on both sapphire and AlN substrates. In the second set of experiments no predeposition step is used and we operate under classic CVD conditions, in order to 5826
DOI: 10.1021/acs.chemmater.7b00490 Chem. Mater. 2017, 29, 5824−5830
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Chemistry of Materials avoid the formation of hexagonal phases. Deposited films in the temperature range of 800−1300 °C, are of the cubic δ(Nb,Ti)N structure, free of hexagonal grains for the full range of Ti (or Nb) composition. The deposition rate is in the range from 50 to 150 nm/h, depending mainly on deposition temperature. The films are (111) oriented along the deposition direction and epitaxially deposited on sapphire and AlN substrates as found by pole figure X-ray scans of the (111) and (220) poles. There are six spots at around 71° on the (111) pole figure, which reveal that there are two domains in the cubic film, with a 60° in-plane twist. Twinned grains inside an epitaxial film are a well-known phenomenon which exists due to the stacking of atoms of a cubic structure on a hexagonal substrate. Out of-plane rocking curve measurements of the (111) plane, on the same samples, indicate a high crystalline quality of the deposited films, with a mosaicity less than 0.09° (324 arcsec). When Ti or a different substrate is used, no significant change in the mosaicity is observed, for films in the thickness range of 10−30 nm. However, the surface morphology is different. (Nb,Ti)N films show improved surface characteristics compared to NbN films on sapphire substrate, while the use of AlN substrate improves further the surface morphology (see Figure 4). The density of those grains
Figure 5. (a) Shift of (111) θ/2θ X-ray diffraction peak of δ-(Nb,Ti)N films with different Ti composition (x) grown on sapphire substrate. (b) Estimation of Ti composition (x) derived from the lattice parameter a of films grown at 1020 and 940 °C on sapphire substrate, using Vegard’s law.
By applying Vegard’s law on the lattice parameter (a) of the δ-(Nb,Ti)N films it is possible to estimate the Ti composition (x) of the deposited films (see Figure 5). The lattice parameter of the δ-NbN varies with deposition temperature and is smaller than the reference value of 4.44 Å. Such lattice parameter differences could originate from residual stress in the films, due to different thermal expansion coefficients and to the lattice mismatch between the films and the substrate. Our detailed inplane and out-of-plane analysis, combined with stress calculations, indicate that our films are stressed. However, one of the assumptions we make when using Vegard’s law is that the main effect on the lattice parameter differences we measure is due to Ti/Nb ratio variations and that stress effects are the same on the different samples. Deposition temperature appears to have a minor effect on the Ti or Nb composition, which is in agreement with the thermodynamic calculations.
Figure 4. Surface morphology of (Nb,Ti)N thin films grown on (0001) sapphire substrate with and without an (0001) AlN buffer layer, observed by scanning electron microscope (SEM).
varies from 1011/cm2 for δ-NbN/sapphire to 1010/cm2 for δ(Nb,Ti)N/sapphire. From AFM (atomic force microscopy) measurements, these grains appear to be triangular pillars with lateral dimensions of 25−30 nm and with an average height of 5 nm for the three cases investigated. It leads to a AFM rootmean-square roughness of 1.9 and 1.7 nm as measured on a 5 × 5 μm surface of, respectively, δ-NbN/sapphire and δ-(Nb,Ti)N/sapphire. No additional XRD diffraction peaks expected for the cubic (Nb,Ti)N are found. The origin of the faceted, triangular grains is not clear at present. In films with different Ti composition, the (111) δ-(Nb,Ti)N diffraction peak is shifted between the (111) 2θ position of cubic δ-NbN and c-TiN (see Figure 5). Such a shift is wellknown to appear due to the formation of a solid solution of two phases, in our case cubic δ-NbN and c-TiN. The evolution of the lattice parameter of the solid solution is usually fitted by the Vegard’s law (see eq 1). First-principle calculations show that the lattice parameter of (Nb,Ti)N depends almost linearly on Ti composition.33
a Nb1 − x TixN = (1 − x)a NbN + xaTiN
(1)
The solid-state transitions in the Nb−N system, while still under controversy, are already described in the literature.34,35 There, additional Nb-rich phases, like β-Nb2N and γ-Nb4N3, appear during the transition from Nb to NbN. However, in our case, no such additional phases appear according to XRD and TEM results. This might be due to the different growth conditions and rather complex chemistry of the CVD process compared to the one used in those studies. It is still an open question in the literature, whether the hexagonal NbN phases are stable or if they appear transiently during phase transitions in the Nb−N system. According to our results, the cubic δNbN is the only thermodynamically stable phase, and the hNbN is stabilized only through solid-state transitions. We find that the upper limit of stability of the h-NbN is 1200 °C, in 5827
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Chemistry of Materials good agreement with previous investigations.23,24,33,34 However, additional studies (e.g., annealing experiments) are required in order to draw a safer conclusion on the stability of h-NbN phases in the temperature range of 800−1300 °C. When Ti is added in the deposition, we obtain the same results. Ti is incorporated both into the hexagonal and the cubic layer, according to scanning transmission electron microscopy (STEM) scans (result not shown here). Thus, Ti does not influence the stabilization of the cubic or hexagonal phase but rather contributes to the formation of a (Nb,Ti)N solid solution. Given that the cubic δ-(Nb,Ti)N structure shows the most prominent superconducting properties, the size of cubic domains is important information to consider when dealing with the electrical properties of films. Structural defects can be generated at the grain boundaries of those domains and affect electron transport and thus the electrical properties of the films. With the length of superconducting nanowires in SSPDs being a few hundreds of nanometers, a more macroscopic observation of those twinned domains compared to the one given by ASTAR technique is required. EBSD is a technique capable of identifying twin variants of a cubic structure as for the case of 3C-SiC grown on hexagonal SiC substrates.36 We perform EBSD analysis on the surface of a 95 nm thick δ-NbN layer grown on (0001) sapphire substrate. The EBSD scan of a 4.4 μm × 2.9 μm area, at a step size of 15 nm, shows that there are two cubic domains with an in-plane twist relationship of 60° (in blue and red color), with a rather random distribution (see Figure 6). The size of those domains varies from 50 to 100 nm up to 1 μm in length.
variation of Ioffe−Regal parameter (kFS ) (a measure of disorder in electron transport37) and Tc as a function of Ti composition (x). In Figure 7a, we plot the variation of resistivity, normalized to the maximum resistivity of each film, as a function of
Figure 7. (a) Variation of resistance with temperature, close to the critical temperature of superconductivity Tc, of 10 nm thin δ-(Nb,Ti) N films grown on (0001) sapphire substrate, with different Ti fraction (x). The resistances have been normalized with respect to the maximum values. (b) The critical temperature of superconductivity (Tc) and Ioffe−Regal parameter (kFS ) of the same films, as a function of Ti composition (x).
temperature, close to superconducting transition. The figure clearly shows that the Tc of the films systematically increases with the increasing x for 0 ≤ x ≤ 0.46. This observation is consistent with (Nb,Ti)N films deposited by sputtering.38−40 Figure 7b shows the variation in Tc and kFS as a function of x. We see that both Tc and kFS increase monotonously with x. kFS is indeed expected to influence Tc: As shown by Anderson et al.41 the Coulomb pseudo potential increases with decreasing kFS . This, according to McMillan’s equation,42 results in an increase in Tc. Similar dependencies were observed in disordered NbN thin films,43 suggesting that kFS is the dominant parameter influencing Tc in the studied parameter regime. The question is now how kFS increases with x are linked: The observed increase of kFS with x, might seem surprising at first, but it is in agreement with the cleaner surface morphology observed in Figure 4 for larger x, indicating that this is the dominating effect on electrical disorder here.
Figure 6. EBSD mapping of the surface of a 95 nm thick δ-NbN film, showing two twinned cubic variants in red (domain A) and in blue colors (domain B). The red and blue boxes show the (001) pole figures corresponding to the two variants.
We demonstrate that this deposition technique can be used to explore the effect of disorder on superconducting and normal-state properties in a systematic way. For that, five δ(Nb,Ti)N films of thickness 10 ± 1 nm, with different Ti compositions (0 ≤ x ≤ 0.46), are grown in the same deposition conditions. Electrical transport and Hall measurements are performed down to 4 K, using a Quantum Design PPMS. While the detail of the superconducting and normal-state properties of the films will be published separately, here, we report the
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CONCLUSIONS We have presented for the first time the epitaxial deposition of (Nb,Ti)N thin films by chemical vapor deposition. We have performed a thorough structural study of the films. Our process allows the control of structure, stoichiometry, thickness and electrical properties of the deposited films. We found that the 5828
DOI: 10.1021/acs.chemmater.7b00490 Chem. Mater. 2017, 29, 5824−5830
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Chemistry of Materials cubic δ-(Nb,Ti)N is the thermodynamically stable phase during classic CVD process in the temperature range of 800−1300 °C. Hexagonal (Nb,Ti)N is formed as a solid-state transition of Nb, due to a “nitridation-like” process under reactive CVD, for deposition temperatures lower than 1200 °C. This opens the way to grow h-(Nb,Ti)N as a buffer layer or hard coating exploiting its interesting mechanical properties. By properly adjusting the Nb/Ti ratio in the gaseous phase, we control the Ti fraction in the full range of composition. The thicknesses of the films can be controlled down to approximately 5 nm, thanks to the low growth rate. By combining various characterization techniques, we were able to locate and measure the size of single domains of the cubic phase. The dependence of the critical temperature of superconductivity on Ti composition confirms the ability of the CVD process to control the superconducting properties of the films through the control of disorder.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Frederic Mercier: 0000-0002-5210-857X Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding
This work was financially supported by the French National Research Agency/grant ANR-14-CE26-0007-WASI. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Thierry Encinas from CMTC, Grenoble-INP, CNRS and Herve Roussel from LMGP, Univ. Grenoble Alpes, CNRS for the X-ray reflectivity and Bragg− Brentano measurements and fruitful discussions.
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