Epitaxial growth and physical properties of ternary nitride thin films by polymer-assisted deposition

The layered structure is a material architecture that includes the combination of discrete compositional or structural layers. This structure encompasses many material systems such as graphene-based materials,^1,2 oxide-based transition metal compounds,^3,4 sulfide materials and nitride compounds.⁵ These materials have proven to be so interesting due to the potential to have their physical properties greatly modulated by changing the components or the configuration of layers. Metal-nitride (ABN₂) materials have been intensely studied as the self-assembled layer-structure materials due to their various interesting and versatile physical properties such as high mechanical strength and hardness, as well as electronic, magnetic, and optical properties that carry the potential for technological applications.^6–8 For example, binary nitrides such as TiN^9–12 and MoN^13,14 have been shown to be the valuable materials in superconducting and catalytic applications. Ternary and complex nitrides have recently attracted much attention due to their significant potential or otherwise extend the physical phenomena found in binary nitrides to generate further knowledge and possibilities of these materials.^15–21 However, the research discussed above is mostly in bulk format which was synthesized by the high temperature ceramic method. Other form factors such as epitaxial thin films are valuable in research due to their malleability via engineering of external strain from substrate choice and orientation, and allow the researchers to investigate the interface, grain boundary, and bulk contributions of materials to observe the electrical and magnetic properties. In order to better understand the complex physical phenomena exhibited by ternary nitrides, high purity and structural quality such as epitaxial films are highly required yet challenging.

Ternary nitride thin films are commonly produced by metalorganic chemical vapor deposition^22–24 or molecular beam epitaxy^25,26 techniques. Chemical solution deposition techniques, such as polymer-assisted deposition (PAD), offer advantages in terms of relatively simple setup and low cost. The PAD is a versatile method, which has been used to synthesize different kinds of thin films including simple and complex metal oxides, semiconductors, metal-nitrides, and metal carbides.^27–29 Our previous work has demonstrated the growth of metal nitride thin film materials with high purity of phase and epitaxial structure.^20,21 However, the continued growth of high quality ternary nitride thin films with a wide range of compositions is necessary to further establish the structure-property relationships in order for these thin film materials to be utilized in technology applications. Here, we report the growth of epitaxial complex ternary nitrides FeMoN₂, (Fe_0.33Mo_0.67)MoN₂, CoMoN₂, and FeWN₂ by PAD technique and study the epitaxial growth and electrical properties. FeMoN₂ has been previously studied for its interesting magnetic and electrical transport properties, with the results that suggest a sensitivity to compositional and structural modulations of the ABN₂ lattice.^21,30 We choose different modes of variation of the FeMoN₂ structure to broadly investigate the effects of partial and whole substitution of the A and B-sites of this structure with a selection of (Fe_0.33Mo_0.67)MoN₂, CoMoN₂, and FeWN₂ materials to help provide deeper understanding of the complex composition-structure-property relationships of ternary nitride thin films.

All epitaxial ternary nitride films presented in the study were grown on the c-plane sapphire (Al₂O₃) substrates by PAD. The details on the precursor preparation are provided in the supplementary material. The precursor mixes were spin coated on c–cut sapphire substrate at 3000 rpm for 30 s. Then the film was annealed at 650 °C for 2 h in flowing NH₃ gas at an ambient pressure. Both the FeMoN₂ and (Fe_0.33Mo_0.67)MoN₂ thin film samples were determined to have thicknesses of approximately 50 nm, while the thickness of the FeWN₂ and CoMoN₂ films is approximately 10 nm.

The nitride films were characterized by X-Ray diffraction (XRD), using θ-2θ, rocking curve, and ϕ–scans to obtain information on the orientation, lattice parameters, and epitaxial quality of the thin films. High-resolution transmission electron microscopy (HRTEM) was employed to investigate the film microstructure (FEI Tecnai G2 F,²⁰ 200 kV, point resolution: 0.27 nm). Selected area electron diffraction (SAED) patterns were recorded to identify epitaxial relationship (JEOL 2010, 200 kV). The temperature dependence of film resistivity was observed using a physical property measurement system (Quantum Design PPMS Model 6000) from 2 to 400 K.

The CoMoN₂, FeMoN₂, (Fe_0.33Mo_0.67)MoN₂, and FeWN₂ share a similar structure of ABN₂ (A = Fe, Co, Fe/Mo; B = Mo, W) as illustrated in Figure 1.^5,6,16,31 It is composed of alternating layers of trigonal B–N prisms and triangular lattice layers of A-site atoms. In Figure 1(b), the structure is modified when the ratio of A:B in materials such as (Co_xMo_1-x)MoN₂ and (Fe_xMo_1-x)MoN₂ is less than 1. In this instance, B-site atoms begin to populate the triangular lattice layers.

FeWN₂ is a layered nitride, shown in Figure 1(a), consisting of Fe triangular lattice planes alternating with trigonal W–N prism slabs, as reported by Miura et al.^5,31 Very less information exists concerning the structure and properties of FeWN₂ outside of those reported by Bem and Loye.³² and the nonstoichiometric Fe_xWN₂ studies carried out by Miura et al.⁵ Most commonly, FeWN₂ materials have been reported as fabricated by ammonolysis of FeWO₄ precursor, producing nanocrystalline powder structures. Similar to FeWN₂, CoMoN₂ is proposed to exhibit a layered structure based on the previous reports. Its structure was reported by Bhattacharyya et al.¹⁸ in CoMoN₂ nanoparticles, and by Cao et al.⁶ in (Co_xMo_1-x)MoN₂ bulk nanocrystalline materials. Most of the literature that exists for the cobalt molybdenum nitride system focuses on the synthesis of Co₃Mo₃N₂. The structure and properties of FeMoN₂ and Fe_xMo_1-xMoN₂ have been scarcely investigated due to the difficulty in their fabrication, though most reports indicate ammonolysis as the preferred method for a successful growth.^16,30 (Fe_0.8Mo_0.2)MoN₂ nanocrystalline powders have been reported for Fe_xMo_1-xMoN₂ by Bem et al.¹⁶ 

Figure 2(a) shows the X-Ray diffraction (XRD) θ-2θ scan of the (Fe_xMo_1-x)MoN₂ (x = 0.33, 1) films on the c-plane sapphire (Al₂O₃) substrates. It indicates that the films are preferentially oriented along the c-axis and with no detectable impurity phases. All XRD results for the CoMoN₂ and FeWN₂ thin films are very similar (Figures S1 and S2), with the exception of slight modifications to the lattice parameters, to those shown in the (Fe_xMo_1-x)MoN₂ films in Figure 2. See supplementary material for additional characterization data and analysis on CoMoN₂ and FeWN₂ thin films. Figure 2(b) shows the rocking curve of the (004) film peak, with the full width at half maximum (FWHM) to be about 0.12°–0.13°, indicating that these films have good crystallinity. The FWHM values of all CoMoN₂ and FeWN₂ materials also fall within the range of 0.10°–0.15°. The in-plane orientation between the film and substrate was determined by the ϕ–scans of {$101¯4$} FeWN₂ family planes and {$112¯6$} Al₂O₃ family planes, as shown in Figure 2(c). Both film and substrate have six peaks that correspond to the six-fold symmetry of FeWN₂ and Al₂O₃. There is a 30° in-plane rotation between the two lattices when we consider the hexagonal unit cells of both materials. The [$11¯00$] orientation of FeWN₂ aligns with the [$21¯1¯0$] direction of Al₂O₃ and the [$112¯0$] orientation of FeWN₂ aligns with the [$011¯0$] direction of Al₂O₃.

The diffraction results of the FeWN₂ thin film agree with the hexagonal structures observed in the previously grown FeWN₂ powder materials by Miura and Bem.^5,17 Furthermore, a comparison of the θ-2θ scans for (Fe_0.33Mo_0.67)MoN₂ and FeMoN₂ (Figure 2(a)) reveals that the increase of Fe content in the (Fe_xMo_1-x)MoN₂ structure leads to a decrease of out-of-plane lattice parameter, as indicated by the slight increase in the 2θ angle of the film peaks. The obtained c-parameter values and corresponding data collected from the selected previous growth efforts are presented in Table I. Using FeMoN₂ as a reference, we can see that both whole (as in CoMoN₂) and fractional substitution (as in (Fe_0.33Mo_0.67)MoN₂) of the A-site in the (A_xB_1-x)BN₂ structures resulted in an increase in the out-of-plane lattice parameter, while the substitution of the B-site from FeMoN₂ to FeWN₂ shows a decrease in the out-of-plane lattice parameter. This change may be due to the variance in the ionic radii of the substitution elements, but since the data are not well-supported by the limited data available in bulk form of these materials, more investigation is required to establish an effective modulation relationship between the structure and composition of these materials.

Figure 3 shows the cross-sectional high-resolution transmission electron microscopy (HRTEM) image of the (Fe_xMo_1-x)MoN₂ films on the c-plane sapphire. The HRTEM image suggests clear and sharp interface between film and substrate. All films presented in this report show a similar quality of structure and clean delineation between film and substrate, with HRTEM and SAED images for CoMoN₂ and FeWN₂ thin films presented in Figure S3. A SAED pattern from a region including both film and substrate reveals the epitaxial growth of (Fe_xMo_1-x)MoN₂ films on Al₂O₃, with the orientation relationships of the films and substrates determined to be (0001)_film // (0001)_Sapp and [$11¯00$]_film // [$21¯1¯0$]_Sapp, which is consistent with the XRD results. Figure 3(e) shows an illustration of the orientation relationship between the film and the substrate, with a 30° in-plane rotation and possible lattice matching between the A-site layer of ABN₂ and the oxygen sublattice in the Al₂O₃ substrate. There are three identical possible lattice matching configurations, as shown in Figure 3(e). The in-plane shift of the matching between the film and the substrate has also been reported in the VO₂/SrTiO₃ and LaSrMnO₃/C-sapphire systems.^33,34 This 30° rotation between the FeMoN₂ epilayer and the Al₂O₃ substrate allows the FeMoN₂ lattice aligning itself with the oxygen sublattice in Al₂O₃, which is similar with the report in the ZnO/Al₂O₃ system.³⁵ Although the in-plane lattice parameter of Al₂O₃ is 4.785 Å, the periodicity of oxygen sublattice is 2.88 Å, which is very close to the lattice parameter of the ternary nitride thin film compositions investigated in this report, based on the previous bulk investigations (a = ∼2.84–2.87 Å).^6,16,30,32 This configuration reduces the lattice mismatch from around 40% to ∼1%. Similar orientation relationships were observed in the CoMoN₂ and FeWN₂ samples (results shown in Figure S3).

The normalized temperature-dependent resistivity of these thin films is shown in Figure 4. These films have a resistance of few hundred Ω and the resistivity is in the range of 0.1–1 mΩ cm. The FeWN₂ thin film shows a metallic behavior below 300 K. There is a slight upturn in the resistivity at temperatures below ∼20 K, which may be due to the defects and contribution from grain boundaries.^21,34 The FeWN₂ thin film grown by PAD shows a behavior that does not align with the results obtained by the previous reports in powder materials, and may indicate a change in the electronic properties of FeWN₂ in an epitaxial thin film.¹⁷ In particular, it seems that the grain boundary contribution at very low temperatures is significant. It is notable that the resistivity of the CoMoN₂ thin film is relatively insensitive to the changes in temperature within the measured temperature range, only showing a clearly increasing trend with the reduction in temperature below ∼40 K. In the Fe_xMo_1-xMoN₂ system, it should be noted that a large change in the temperature-dependent resistivity between the two samples was observed. The FeMoN₂ thin film exhibits the metallic behavior at the range of 50 K to 175 K. This is in contrast to (Fe_0.33Mo_0.67)MoN₂, which exhibits a semiconductor behavior all the way up to ∼340 K, with a small “bump” occurring in the region between 340 and 400 K. Previous research efforts have reported a relative temperature insensitivity in bulk Fe_xMo_1-xMoN₂, but the previous data is not available for thin films except in the case of FeMoN₂.¹⁶ The complexity of the electrical properties may be, as suggested by Luo et al., due to scattering from grain boundaries or a small amount of carbon or oxygen contamination within the grain boundaries which alters the electrical conductivity behavior of the film.²¹ 

In order to understand the temperature dependence of the resistivity of the thin films, a modified parallel connection channel model was employed^34,36 (see Sec. 5 of the supplementary material)

where $Sm$ and $Si$ are the contribution factors to modulate the ratio of contributions from the metallic and insulating components, respectively. $ρm$ is used here to express the resistivity of the metallic component, given by $ρm=ρ0+aTn$⁠, where $ρ0$ corresponds to the residual resistivity arising from defect scattering and the coefficient $a$ is used to represent the temperature dependent contribution of the metallic resistivity.^34,$ρcb$ and $ρi$ represent the resistivity arising from the Coulomb blockade effect and from the insulating contributions, and follow a thermal activation law given by $ρcb=exp (ΔEcb/kBT)$ and $ρi=exp (ΔEi/kBT)$⁠, respectively. Here, $ΔEcb$ and $ΔEi$ represent the effective activation energies for the Coulomb blockade effect and insulating phase, respectively. $kB$ is the Boltzmann constant and $T$ is the temperature, expressed in Kelvin. Although this model has been employed most often in magnetic manganite materials,^36–38 the behavior aligns well with the profiles that this model has described, with the contributions from metallic and semiconductor transport mechanisms. The modification to the parallel connection channel model introduces a contribution factor from defects or impurities and grain boundary Coulomb blockade effects, which significantly contribute at very low temperatures.

Although the FeWN₂, CoMoN₂, and FeMoN₂ thin films follow similar trends that are suitable to this model, the (Fe_0.33Mo_0.67)MoN₂ thin film does not show a clear metallic behavior region. The resistivity vs. temperature profile fits most closely to two regions of thermally activated semiconductor-like behavior with two distinct activation energies, which can possibly be correlated to a pure insulating phase coupled with the Coulomb blockade effect at a very low temperature. A calculation of the activation energies of two separate temperature regions was estimated by $ρ=exp (ΔE/kBT)$ where $ρ$ is the normalized resistivity of the sample and $ΔE$ is the effective energy at the low (below 5 K) and high (100–400 K) temperature ranges. The detailed fitting results are shown in Table II, with further information on (Fe_0.33Mo_0.67)MoN₂ presented in Figure S5. The diminished metallic contribution in the (Fe_0.33Mo_0.67)MoN₂ thin film is a potential indication of interstitial molybdenum A-site doping suppressing the conduction of the grains, which may suggest that the A-site triangular lattice in the FeMoN₂ layered nitride material is the main contributor of metallic conduction, as the metallic regions are also seen in the CoMoN₂ and FeWN₂ materials. Discontinuities of the A-site triangular lattice planes via defect formations are also expected to contribute to the Coulomb blockade effect in conduction between grain boundaries.³⁸ Although the mathematical fitting curves follow the trend seen in these samples, it is not able to differentiate between any intrinsic property of the materials and the strain contribution from the Al₂O₃ substrates. Possible vacancies at the A-site are also considered as opposed to A-site doping, since the effect of nitrogen nonstoichiometry is not yet well-defined. However, in the absence of structurally evident impurity phases such as MoN in FeMoN₂ thin films reported by Luo et al., we propose that the main contribution to the modulation of the electronic properties arises from the doping of the A-site. Considering the relative similarities in the structures and compositions of these materials, it can be seen that doping or substitution of the A- and B-sites can significantly alter the electrical transport properties of the ABN₂ epitaxial thin films.

In summary, the epitaxial ternary nitride films of FeMoN₂, (Fe_0.33Mo_0.67)MoN₂, CoMoN₂, and FeWN₂ were grown by the solution-based PAD technique. The structural analysis suggests good epitaxial quality and the suitability of the PAD technique for the production of these materials in potential future applications. The PAD technique shows distinct advantages of producing these high-quality materials as the epitaxial thin film coatings, which have not been fully explored by other techniques. Whole or fractional substitution of the metal sites, such as those occurring FeMoN₂ and (Fe_0.33Mo_0.67)MoN₂ or CoMoN₂ is suggested by temperature-dependent resistivity measurements to affect both transition temperatures and contributions between metallic and semiconductor behaviors, as well as the relative sensitivity of the resistivity of these materials to the changes in temperature.

See supplementary material for further structural information on CoMoN₂ and FeWN₂ thin films, mathematical fitting information and precursor preparation details.

The work at Los Alamos National Laboratory was supported by the NNSA's Laboratory Directed Research and Development Program and was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC52-06NA25396.