Enhancement of Mechanical Hardness in SnOxNy with a Dense High

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Enhancement of mechanical hardness in SnOxNy with a dense high-pressure cubic phase of SnO2 Hyo Jin Gwon, Na-Ri Kang, Yunju Lee, Sung Ok Won, Hye Jung Chang, Ji-Won Choi, Chong-Yun Kang, Seong Keun Kim, Beom Jin Kwon, Sahn Nahm, Ju-Young Kim, Jin-Sang Kim, and Seung-Hyub Baek Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02888 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 17, 2016

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Chemistry of Materials

Enhancement of Mechanical Hardness in SnOxNy with a Dense High-pressure Cubic Phase of SnO2 Hyo Jin Gwon †, ‡, Na-Ri Kang§, Yunju Lee∥, Sung Ok Won∥, Hye Jung Chang∥, ⊥, Ji-Won Choi†, Chong-Yun Kang†, Seong Keun Kim†, Beomjin Kwon†, Sahn Nahm‡, Ju-Young Kim§, Jin-Sang Kim †,

Seung-Hyub Baek†,⊥,*

†

Center for Electronic Materials, Korea Institute of Science and Technology, Seoul 02792, Korea

‡

Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea

§

School of Materials Science and Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Korea ∥

Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 02792, Korea

⊥

Department of Nanomaterials Science and Technology, Korea University of Science and Technology, Daejeon 34113, Korea ABSTRACT: Controlling crystalline phases in polymorphic materials is critical not only for the fundamental understanding of the physics of phase formation, but also for the technological application of forbidden, but potentially useful physical properties of the nominally unstable phases. Here, using tin oxide (SnO2) as a model system, we demonstrate a new way to enhance the mechanical hardness of an oxide by stabilizing a high-pressure dense phase through nitrogen integration in the oxide. Pristine SnO2 has a tetragonal structure at the ambient pressure, and undergoes phase transitions to orthorhombic and cubic phases with increasing pressure. Leveraging the enhanced reactivity of nitrogen in plasma, we are able to synthesize tin oxynitride (SnON) thin films with a cubic phase same as the high-pressure phase of SnO2. Such nitrogen-stabilized cubic SnON films exhibit a mechanical hardness of ~23±4 GPa significantly higher than even the nitride counterpart (Sn3N4) as the result of the shortened atomic distance of the denser, high-pressure cubic phase. Moreover, SnON has a heavily-doped, n-type semiconducting property with a controllable optical bandgap. Our work will provide new opportunities to search for and to utilize beneficial, but hidden physical properties which exist in a particular phase stable only at extreme conditions.

INTRODUCTION Understanding the relation between crystal structure and physical property of a solid has been a major subject in the 1-2 materials science and engineering . A polymorphic material, having an ability to form more than one crystalline phase, provides an exciting platform to explore this issue due to its versatile properties as the result of the diverse phase transitions. Polymorphism manifests in most binary metal oxides. For example, ZrO2 transforms from monoclinic phase at room temperature to tetragonal and cubic phases with in3-5 creasing temperature . Likewise, SnO2 transforms from tetragonal rutile phase (P42/mnm) at atmospheric pressure to orthorhombic CaCl2-type (Pnnm), orthorhombic α-PbO2type (Pbcn), and cubic fluorite-type phases (Pa3) with in6-8 creasing pressure . Each phase has its own stable regime as a function of temperature and pressure, as determined by both thermodynamics and kinetics.

The stable regime of a particular polymorphic phase can be changed by material engineering. Phase stabilization is an attempt to synthesize nominally unstable phases in a stable form by modifying the material to utilize their useful proper° ties at normal atmospheric conditions (1 atm, 25 C). Key approach of phase stabilization is to induce pseudo-pressure either internally from the material, or externally from the surrounding. The internal pressure can be controlled by doping. Substitution of cations with different-sized, or differentvalent ones can lead to the creation of the cation chemical pressure or oxygen vacancy-induced pressure. This method enables stabilizing high-temperature tetragonal and cubic 9-10 ZrO2 phases as well as high-pressure orthorhombic SnO2 11-12 phase at normal conditions. Epitaxial strain is a simple tool to impose an external pressure on the polymorphic material. Bi-axial strain arising from the lattice and thermal mismatch between films and substrates can stabilize the 13-14 nominally unstable phase . Phase stabilization technique has been an excellent tool for fundamental understanding of

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phase formation and transformation in polymorphic solids as well as for technological utilization of potentially beneficial properties found only at extreme environments. Here, we explore a new route of the phase stabilization using aliovalent anion substitution. We choose SnO2 as a model system. Leveraging the enhanced reactivity of nitrogen in the RF plasma, we are able to incorporate both nitrogen and oxygen into thin films to create tin oxynitride (SnON). We demonstrate the nitrogen-stabilized cubic SnON thin films, as described in Fig. 1. Note that the high-pressure cubic phase has never been realized before at atmospheric condi11-12 tions . Moreover, we report a high mechanical hardness of SnON thin films with almost two-fold enhancement over the normal tetragonal SnO2, which is attributed to the compact and dense crystal structure of SnON as a high-pressure phase. The key aspect of SnON is the distinctive characteristics of the bonding coordination and the ionicity/covalency between the two anions: oxygen ions are prone to have an ionic bonding nature with the low number of coordination while nitrogen ions have a covalent counterpart with the high 15-17 number of coordination . The coexistence of such distinct features plays a critical role to determine the crystal structure and the physical properties of SnON different from that of SnO2 and Sn3N4. Anion control has rarely been studied for the purpose of 18-19 phase modification in polymorphic materials . This might be hampered by the difficulty of integrating anions into the sample. Most anions are either a gaseous or a liquid state, thus it is difficult to precisely control the nominal composition of anions in a solid. Moreover, there exists an enormous difference of reactivity between oxygen and other anions. Oxygen is the most stable and predominant anion that is easily combined with most metal cations. However, other anions such as nitrogen and sulfur are less reactive than oxygen. The feature makes it difficult to synthesize the anionsubstituted samples.

EXPERIMENTAL SECTION Film Growth. We grew SnON thin films using RF magnetron reactive sputtering technique. The vacuum chamber was −6 pumped out by a cryopump down to ~1×10 Torr. The distance between the substrate heater and the sputtering target ° was 7 cm, and the substrate heater was set at 400 C during growth. We used Si/SiO2 substrate and (001) MgO single crystal substrate for polycrystalline and epitaxial SnON films. Prior to SnON growth, the target was sputtered to clean the surface for 15 min. Both Sn metal and rutile-SnO2 ceramic targets can be used. When pure Sn target (99.99%) is used, pure N2 (99.9999%) was flowing under 5 mTorr with various sputtering powers ranging from 25 W to 75 W. When SnO2 ceramic target is used, a mixture of NH3 and N2 with a ratio of 1:4 was flowing under 5 mTorr. The sputtering power was set at 15 W. Characterization. The crystal structure of SnON thin films were analyzed by High-Resolution X-Ray Diffraction (HRXRD, Rigaku, CuKα=1.5406 Å) equipped with a (220) Ge crystal 4-bounce hybrid monochromator for θ-2θ scan in the ° ° ° range of 20 ~85 with 0.02 step. φ-scans of the SnON (202) ° ° ° peaks were performed in the range of 0 ~360 with 0.02 step. In addition, the examinations of surface features were carried

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out by using atomic force microscope (AFM, Digital Instruments Dimension 3100). The chemical composition and bonding states in films are investigated by X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe) with an Al Ka radiation (1486.6 eV). The core level XPS spectra for N 1s, O 1s and Sn 3d were measured, and energy calibration was achieved by setting the hydrocarbon C 1s line at 284.6 eV. The composition of the film with high capacities was determined by Rutherford Backscattering Spectroscopy (RBS, 2+ ° 6SDH2) using a 2.0 MeV He ion beam impinging on the 5 ° tilted surface at a backscattering angle of 170 . The microstructure analysis was carried out using a Cscorrected transmission electron microscopy (FEI; Titan S 80300) operated at 300 kV. The cross-section TEM samples were prepared utilizing focused ion beam (FIB) (FEI; Helios Nano-Lab 600). Nanoindentation (DCM II module in Agilent G200) tests were carried out using a three-sided pyramidal diamond Berkovich indenter tip. All films were thicker than 200 nm. Maximum indentation depth was set as 30 nm. Nanoindentations were carried out at constant indentation strain rate of -1 0.05 s . Hardness and elastic modulus as a function of indentation depth were obtained by the continuous stiffness measurement (CSM). Hardness and Young’s modulus of SnON films were analyzed for indentation depth from 17 nm to 22 nm. This range of indentation depth was chosen to minimize possible nanoindentation experimental issues such as area function of nanoindenter at very early stage of contact and substrate effect at relatively deep indentation depth.

RESULTS AND DISCUSSION We synthesize SnON thin films using a reactive, magnetron sputtering technique where the reactivity of nitrogen can be substantially enhanced by plasma. The details on the growth are descried in the Methods section. Figure 2a shows the x-ray diffraction pattern of polycrystalline SnON thin films grown on amorphous glass substrates with a sputtering ° power of 50 W at 400 C. This is measured in a fixed glancing ° incident angle mode with 0.5 . The most stable SnO2 at normal conditions has the tetragonal, rutile phase (P42/mnm) with the lattice parameters of a = 4.738 Å and c = 3.187 Å. The measured diffraction patterns do not match with this phase (JCPDS Card No. 41-1445). However, the diffraction patterns are well-matched only with the high-pressure, cubic phase of SnO2 (JCPDS Card No. 50-1429). Figure 1a shows the phase transitions of SnO2 as a function of pressure. During hydrostatic compression, the tetragonal phase (P42/mnm) transforms into two different orthorhombic phases (Pnnm and Pbcn space groups), and continually transform to cubic phase (Pa3) in the range from 21 GPa to 40 GPa. On the other hand, during decompression, cubic SnO2 phase can be stable down to 10 GPa, exhibiting a hysteresis behavior. The lattice parameter (5.09 ± 0.01 Å) of our SnON thin films is wellmatched with the extrapolated one of cubic SnO2 phase down to atmospheric pressure, as shown in Fig. S1. This indicates that our cubic SnON thin films are the stabilized phase of cubic SnO2 phase. The indexing of diffraction peaks in Fig. 2a is based on this cubic phase.


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In order to investigate the intrinsic properties of our nitrogen-stabilized cubic SnON, we grow an epitaxial SnON thin film on (001) MgO substrate. Despite the large lattice mismatch between SnON (5.09 Å) and MgO (4.212 Å), an epitaxial (001) SnON film is successfully grown. Figure 2b shows an out-of-plane θ-2θ scan of the SnON/MgO sample. The XRD pattern indicates that the SnON film grown on (001) MgO substrate is purely (001)-oriented. The out-of-plane lattice parameter (5.09 Å) is the same as the (001) interplanar distance in polycrystalline SnON. This implies that the SnON film is fully relaxed on MgO substrate due to the large lattice mismatch. Azimuthal φ-scans of the SnON film show that both (202) SnON and (202) MgO peaks are positioned at the same φ angles with four-fold symmetry. This indicates that our SnON thin film has a cube-on-cube epitaxy on MgO substrate. The full width at half maximum (FWHM) of rocking ° curve of the (002) SnON peak is 0.10 (Fig. S2), showing a high crystalline quality. Figure 2d is a high-resolution transmission electron microscopy (HRTEM) image. The inset in Fig. 2d shows the fast Fourier transform (FFT) pattern taken from SnON film and MgO substrate. The HRTEM analysis also confirms an epitaxial relation of the SnON film to the MgO substrate. The high density of misfit dislocations at the SnON/MgO interface (Fig. S3) indicates that the SnON film is fully relaxed, as consistent with the XRD results. SnON becomes fully-relaxed even at a-few-unitcell thickness by accommodating misfit dislocations periodically. This initial layer serves as a well-matched buffer layer for the further growth of high-quality SnON thin film. Figure 2e shows atomic force microscope (AFM) images of the epitaxial SnON film surface. A 120 nm-thick SnON film on MgO exhibits a smooth surface with a two unit-cell height roughness at maximum. In order to confirm the incorporation of nitrogen into the thin films, we perform x-ray photoelectron spectroscopy (XPS) analysis. Figure 3 shows the XPS narrow-scan spectra for the epitaxial SnON film. For comparison, XPS results on SnO2 film are also plotted. Figure 3a clearly shows that the intensity of N 1s peak around 396.25 eV is strong in our SnON films while there is no peak in SnO2 films. This data indicates that substantial nitrogen atoms are incorporated in our SnON films over XPS measurement errors. Figure 3b and c show that how the binding energy of Sn-O is modified by N incorporation. When N is integrated into SnO2, the binding energy of Sn 3d is shifted to a lower energy level (Fig. 3b). This behavior is consistent with the previous reports on ox20-21 ynitrides . As the electronegativity of O (3.44) is larger than that of N (3.04), the binding energy between cation and anion becomes lowered as the N content increases. Moreover, the intensity of O 1s peak in SnON is reduced compared to that in SnO2 as a result of nitrogen incorporation (Fig. 3c). In oxynitrides, the ratio between nitrogen and oxygen is one of the important parameters to determine their physical properties. In order to control the nitrogen content in the film, we vary RF sputtering powers to control the density of reactive nitrogen species in the plasma. As the sputtering power increase, more nitrogen will be incorporated into the films. This is confirmed by the XPS results as shown in Fig. S4. The sputtering power ranging from 25 W to 75 W allows the formation of the purely-cubic SnON phase. XRD analysis reveals that secondary phases are formed below 25 W with the SnO2 phase and over 75 W with the Sn3N4 phase. The

composition of the SnON thin films are characterized by Rutherford Backscattering Spectroscopy (RBS). The composition of the SnON thin film grown at 25W, 50W and 75W sputtering powers were SnO0.8N1.15, SnO0.4N1.25 and SnO0.39N1.26, respectively. We characterize the optical properties of our SnON thin films grown on glass substrates with various sputtering powers. The optical transmittance is measured in the wavelength range of 250 nm - 800 nm (Fig. 4a). The optical transmittance decreases as the nitrogen content in the film increases. Based on these results, the optical band gap of the film is determined by applying the Tauc model in the high absorb2 ance region. Figure 4b shows the plot of (αhv) vs the photon energy (hv) of various SnON thin films with different nitrogen content. The optical band gap continuously decreases from 3.6 eV to 3.3 eV as the nitrogen content increases. The optical images in the inset of Fig. 4b clearly show that the SnON thin films are getting darker with increasing nitrogen content. This is consistent with the previous results that the addition of nitrogen increases the valence band maximum, 17 reducing the optical band gap . These results suggest that one can control the optical properties by the nitrogen content under the same cubic SnON crystal structure (Fig. S5). The electrical resistivity, mobility and carrier concentration are determined from the Hall measurement using van der Pauw method. Regardless of the nitrogen contents, all epitaxial (001) SnON films show an n-type, metal-like 3 -1 transport property with ~1.3×10 Scm . The carrier concentra20 -3 2 -1 -1 tion and the electron mobility are ~10 cm and ~30 cm V s , respectively, at room temperature. Such a highly conducting property is attributed to the dominant oxygen vacancy formation as a donor in our SnON films, which is a common feature of transparent conducting oxides (Fig. S5). In order to evaluate the mechanical property of our SnON thin films, we carry out the nanoindentation test on (001) epitaxial SnON thin films on (001) MgO substrate. The hardness and Young’s modulus are measured from the indentation force-depth curves. The details of experimental conditions of nanoindentation are described in Methods section. Figure 5 shows that hardness and Young’s modulus of our SnON thin films. For comparison, previous results on SnO2 and Sn3N4 are also plotted. Our SnON films show a significant enhancement of hardness (~23±4 GPa) and Young’s modulus (~230±20 GPa) over both SnO2 and Sn3N4. In order to validate our measurement method, we grow (100) epitaxial SnO2 thin films on (001) Al2O3 single crystal substrates (Fig. S6), and measure the mechanical properties using the same nanoindentation method. This result is consistent with the previous reports as shown in Fig. 5, which indicates that our results measured by the nanoindentation are valid. 22

Usually, a nitride is harder than an oxide counterpart . N has a higher number of coordination with metal cations than O. Such a higher coordination number of N is favorable to form a robust, 3-dimensional network, and this can lead to the tight binding of atoms in a unitcell. Moreover, N forms a covalent-like bonding while O does an ionic-like one. Covalent bonding is stronger than ionic bonding. Based on this framework, the previous reports that the mechanical hardness of binary oxides usually increases as the amount of N increases can be explained. For example, the mechanical hardness of a silicon oxynitride is larger than oxide, but smaller than nitride, SiO2 (8.32 GPa) < SiON (13.83 GPa)
Sn3N4 (7.047 g/cm ) > rutile SnO2 (6.994 -3 g/cm ). Also, the volume per metal atom, another common26-28 ly-used factor to relate with hardness , is listed in ascend3 3 ing order as: SnON (~32.97 Å ) < Sn3N4 (33.75 Å ) < rutile 3 SnO2 (35.77 Å ). This trend implies that the stabilized highpressure cubic phase of our SnON films leads to the improvement of the mechanical hardness of SnON over Sn3N4.

ACKNOWLEDGMENT

CONCLUSION We have demonstrated a way to stabilize a high-pressure cubic phase by incorporating N into nominally tetragonal SnO2. The nitrogen-stabilized cubic SnON exhibits highlyimproved mechanical hardness even over Sn3N4. This is attributed to the compact and dense crystal structure of cubic SnON as a high-pressure phase. We observed that the optical band gap of SnON can be controlled by the amount of nitrogen in SnON. Moreover, SnON films behave like a heavilydoped, n-type semiconductor due to the abundant oxygen vacancies. Recently, oxynitrides are gaining more attention for its potential benefits of merging physical properties of oxides and nitrides. For example, the improvement of light29-31 to-electricity conversion and the enhancement of me32-34 are reported. Our work will provide chanical strength new degrees of freedom to discover unexpected phenomena as well as to design functional properties through the anionassisted phase transformation.

ASSOCIATED CONTENT Supporting Information. Lattice parameters and phases of SnO2 as a function of pressure, rocking curve of (002) peak for SnON films deposited on (001) MgO substrates, inverse fast Fourier transform image of epitaxial SnON films on MgO substrates, structural characterization of the (100)SnO2/(001)Al2O3 structure, X-ray photoelectron spectroscopy analysis of O 1s peak of SnON films and band gap modification and electron carrier generation in SnON. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Author Contributions The manuscript was written through contributions of all authors.

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The authors declare no competing financial interest.

The authors gratefully acknowledge the financial support of the Korea Institute of Science and Technology (KIST) through 2E26370.

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Figure 1. Phase transition of SnO2 and the stabilization of cubic phase by nitrogen doping. a, Schematic illustration of variation of SnO2 phase as a function of pressure. The unitcell schematics of four polymorphic SnO2 phases are drawn based on Ref. 2. b, Schematic illustration of two routes to stabilize cubic phase in SnO2: by applying high-pressure (left) and by nitrogen doping (right).


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Figure 2. Structural characterization of polycrystalline and epitaxial SnON thin films. a, XRD pattern in glancing angle mode of polycrystalline SnON films on amorphous glass substrate. The XRD pattern of the high-pressure, cubic SnO2 (JCPDS #50-1429) is also presented for comparison. As the peak position changes with the magnitude of the external pressure, the reference peak position is shifted by extrapolating the lattice parameter versus pressure curve. (See Fig. S1) b, Out-of-plane θ-2θ scan of the 120 nm-thick, (001) epitaxial SnON films on (001) MgO substrates. c, φ-scan of the in-plane (202) SnON and (202) MgO diffraction peaks. d, A cross-sectional high-resolution TEM image at the SnON/MgO interface with a [100] zone axis. The insets are the fast Fourier transform (FFT) pattern of SnON film and MgO substrate. For better visibility, the TEM image is artificially colored. e, Surface morphology of the epitaxial (001) SnON film (120 nm) on (001) MgO structure.


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Figure 3. Chemical characterization of SnON films. X-ray photoelectron spectroscopy (XPS) analysis at a, N 1s b, Sn 3d and c, O 1s peak for SnON film on (001) MgO substrates.

Figure 4. Characterization of optical properties of polycrystalline SnON films grown by various sputtering powers on 2 transparent glass substrates. a, Optical transmittance spectra and b, the plot of (αhν) vs hν of SnON films grown with different RF powers. The inset shows the optical image of SnON films.


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Figure 5. Mechanical properties of epitaxial (001) SnON films on MgO measured by nanoindentation. a, hardness and b, modulus of the SnO2, Sn3N4 and SnON.

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Enhancement of Mechanical Hardness in SnOxNy with a Dense High

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