Design of Metastable Tin Titanium Nitride Semiconductor Alloys

Article pubs.acs.org/cm

Design of Metastable Tin Titanium Nitride Semiconductor Alloys Andre Bikowski,† Sebastian Siol,†,§ Jing Gu,†,∥ Aaron Holder,† John S. Mangum,‡ Brian Gorman,‡ William Tumas,† Stephan Lany,† and Andriy Zakutayev*,† †

National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Colorado School of Mines, 1500 Illinois Street, Golden, Colorado 80401, United States

‡

S Supporting Information *

ABSTRACT: We report on design of optoelectronic properties in previously unreported metastable tin titanium nitride alloys with spinel crystal structure. Theoretical calculations predict that Ti alloying in metastable Sn3N4 compound should improve hole effective mass by up to 1 order of magnitude, while other optical bandgaps remains in the 1−2 eV range up to x ∼ 0.35 Ti composition. Experimental synthesis of these metastable alloys is predicted to be challenging due to high required nitrogen chemical potential (ΔμN ≥ +1.0 eV) but proven to be possible using combinatorial cosputtering from metal targets in the presence of nitrogen plasma. Characterization experiments confirm that thin films of such (Sn1−xTix)3N4 alloys can be synthesized up to x = 0.45 composition, with suitable optical band gaps (1.5−2.0 eV), moderate electron densities (1017 to 1018 cm−3), and improved photogenerated hole transport (by 5×). Overall, this study shows that it is possible to design the metastable nitride materials with properties suitable for potential use in solar energy conversion applications. Ge3N4.23 Among group IVB nitrides, TiN along with Zr3N4 and Hf3N424,25 is used in hard coatings26,27 and in biomedical implants.28,29 Several mixed group IVA/IVB nitrides have been predicted to have interesting mechanical and electronic properties,30,31 but none of them have been synthesized so far. Much less is known about group IV nitride semiconductor applications, where most research to date has been directed to group IIIA compounds (i.e., AlN, InN, GaN) for solid-state lighting and high-power electronics.32 Among the few rare examples, (Sn,Ge,Si)3N4 alloys with experimentally determined spinel crystal structures33,34 have been theoretically proposed for the use in light-emitting diodes based on the tunable band gap values.35 Indeed, for example, the spinel γ-Sn3N436 has a band gap in the visible range, high optical absorption, and low electron effective mass (0.18m0, where m0 is the rest mass of the free electron).35,37 However, a major drawback for semiconductor applications of spinel Sn3N4 is the heavy hole effective mass (m*h = 12.9m0). In this paper, we present the design of metastable (Sn1−xTix)3N4 alloys, including theoretical calculations as well as experimental synthesis and characterization. According to the theoretical predictions, the Ti substitution on the octahedral lattice site of the Sn3N4 spinel structure causes a strong reduction of the hole effective mass, while other properties remain favorable for solar energy conversion. However, the (Sn1−xTix)3N4 alloys would require a nonequilibrium nitrogen chemical potential to be synthesized, based on the current structural models for Ti3N4.38,39 To successfully synthesize

1. INTRODUCTION Discovery of novel materials with desired properties among many possible candidates is one of the most important goals of modern material science. The basic science efforts in this area include the discovery of chemically plausible yet previously unreported “missing materials”.1,2 Applied material discovery efforts screen experimental crystallographic databases for thermoelectric, photovoltaic, battery, phosphor, and other functionalities.3,4 Complementary to such discovery is design of new materials, where the desired properties are achieved by manipulating chemical composition or crystal structures. Recent examples include design of entropy-stabilized oxides,5 ferroelectric materials,6 and semiconductor alloys.7 An interesting task for both design and discovery is the realization of metastable materials. This topic lies at the intercept of the two “grand challenges” for basic science:8 to design materials with tailored properties and to control materials away from equilibrium. For example, metastable polymorphs9 can be predicted by structure search methods10 and synthesized using high-pressure approaches or using highenergy precursors.11 Compared to the polymorphs, metastable semiconductors alloys7 (e.g., (Al,Ga)As or (In,Ga)N solid solutions) are a less scientifically famous but equally technologically important category of metastable materials that will be discussed in this manuscript. Nitrides are one of the most structurally diverse but chemically unexplored classes of materials in inorganic solidstate chemistry.12,13 There are many remaining open questions about nitride crystal structure,14,15 thermodynamic stability,16,17 and physical properties.18,19 Among group IVA nitrides, functional nitride examples include photocatalysis in C3N4,20,21 thermal stability in Si3N4,22 and catalytic activity in © 2017 American Chemical Society

Received: May 25, 2017 Revised: July 7, 2017 Published: July 7, 2017 6511

DOI: 10.1021/acs.chemmater.7b02122 Chem. Mater. 2017, 29, 6511−6517

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Chemistry of Materials

Figure 1. Theoretical predictions. (A) The (Sn1−xTix)3N4 spinel crystal structure (top), featuring tetrahedral (Td) and octahedral (Oh) sites, and the (Sn1−xTix)3N4 region of its stability (bottom), as a function of N, Sn, and Ti chemical potentials. The high nitrogen chemical potential of ΔμN > 1 eV (point “A”) is necessary to stabilize the Sn3N4−Ti2SnN4 alloys. (B) The mixing enthalpy (top), band gap energy (middle), and DOS effective masses (bottom) as a function of x in (Sn1−xTix)3N4, showing that the x = 0.17−0.33 compositions (yellow region) are most promising for solar energy conversion. (C) The band structure and projected density of states for the Sn3N4 and the (Sn1−xTix)3N4 alloy at x = 0.33. As envisioned, Ti substitution modifies the band dispersion through its low-energy 3d states.

these highly metastable (Sn1−xTix)3N4 spinel alloy thin films, we used combinatorial reactive sputtering in the presence of nitrogen plasma. Experimental measurements of the optical, electrical, and photoelectrochemical properties are consistent with the theoretical predictions. On the basis of both theoretical and experimental results, the alloys with x = 0.25−0.35 Ti composition may be promising for renewable hydrogen production using photoelectrochemical water splitting. Overall, this work illustrates the promise of the design of metastable nitride alloys (e.g., (Sn1−xTix)3N4) for solar energy conversion and other applications.

staying below m*e = 0.5m0 and the band gap (Eg) changes nonmonotonously, spanning the range of interest for solar energy conversion between 1 and 2 eV over the interval 0.17 < x < 0.33. In particular, the (Sn1−xTix)3N4 alloy at x = 0.25 composition has the predicted band gap close to 1.5 eV, which along with favorable electron and hole effective masses makes it especially interesting for photovoltaic (PV) or photoelectrochemical (PEC) applications (Figure 1b). To get deeper insight into the effect of Ti alloying on the electronic structure, we show in Figure 1c the calculated band structures and projected density of states (DOS) for (Sn1−xTix)3N4 with x = 0.33, in comparison with Sn3N4 (x = 0). The unoccupied Ti-d atomic orbitals hybridize with the occupied N-p states, leading to a corresponding Ti-d character in the valence band (VB). For reasons related to the symmetry of the octahedrally coordinated Ti sites,43 this interaction vanishes at the Brillouin zone center (Γ) for the highest VB and increases toward the zone edge. Correspondingly, the m*h is lowered from 2.0m0 to 0.70m0, and from 9.8m0 to 0.55m0 along the Γ-L and Γ-X directions, respectively. Note that these individual band effective masses (Figure 1c) are smaller than the DOS effective mass m*h = 1.4m0 (Figure 1b), which average over all k-space directions and integrate band degeneracies. The conduction band (CB) also acquires some Ti-d character, which leads to a reduced CB curvature and to an increase of the approximately isotropic m*e (Figure 1b) from extremely low value (0.16m0) in Sn3N4 (x = 0) to reasonably low value (0.40m0) in Sn2TiN4 (x = 0.33). Together, these changes in the conduction and valence band edges result in the nonmonotonic variation of the band gap shown in Figure 1b. Despite the encouraging electronic structure properties (Figure 1c), theoretical calculations predict that the (Sn1−xTix)3N4 alloys may be difficult to synthesize. To address their synthesizability, we calculated the formation enthalpies of the Sn3N4, Ti2SnN4, TiN, Ti3N4,38 and related compounds (Table S2). Their mixing enthalpies in an alloy are shown in Figure 1b, and the range of chemical potentials under which these compounds are stable against decomposition into competing phases are presented in Figure 1a. We find that

2. RESULTS AND DISCUSSION 2.1. Theoretical Prediction of (Sn1−xTix)3N4 Alloys. Spinels are a large and diverse class of materials40,41 with a wide range of functionalities.42 There are two cation coordinations (Figure 1a top), tetrahedral (Td) and octahedral (Oh), with different possible cation valences (I, II, III, IV, VI) in spinel oxides. However, the few known nitride spinels are all group IV compounds. This is because for every [A2+X]Oh[B+Y]TdN4−III combination with X, Y ≠ 4 (such as in Sn3N4), the higher valence of the nitrogen (III) requires at least one of A or B metals to have >4 oxidation state, which is energetically unfavorable. Regardless of the anion, the semiconductor properties of spinels can be tuned by isovalent substitutions on the Oh sites, since they are spatially interconnected in 3D, in contrast to isolated Td sites (Figure 1a top). Among the different 4-valent elements, group IVA transition metals that prefer octahedral coordination hold the most promise for band structure modification of spinel Sn3N4. For example, Ti has strong Oh site preference (Table S1 in Supporting Information) and also features low-lying unoccupied 3d states that can couple with the Sn3N4 valence band, modifying its hole effective mass. The calculated hole and electron effective masses, as well as the band gap, as a function of (Sn1−xTix)3N4 alloy composition are shown in Figure 1b. The most striking effect is the dramatic decrease of the density of states (DOS) effective mass for holes (m*h) from 12.9m0 for Sn3N4 to 1.4m0 for Ti2SnN4. At the same time, the electron effective mass remains relatively small, 6512


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Figure 2. Experimental realization: (A) (Sn1−xTix)3N4 experimental XRD pattern color map for films prepared at 280 °C, and the lattice constant calculated based on the (222) spinel peak, in comparison with Sn3N4 deposited at 150 °C, and with theoretical Sn3N4 and Ti2SnN4 references. The phase-pure spinel (SP) alloys form up to x = 0.45 composition, and above x = 0.75 rocksalt (RS) TiN is observed. The spinel and rocksalt crystal structures have similar underlying anion lattices, explaining the continuity of the (222)/(111) diffraction peaks. (B) The corresponding optical absorption spectra and resistivity measurements (0.1 to 10 Ω cm) for x = 0.2−0.5 composition indicate a moderate concentration of free carriers, and band gap values in the 1−2 eV range as predicted by theory. Structural phases formed during reactive sputtering, with the phase boundaries determined from XRD, are shown as dashed lines and shaded regions in the background.

peaks starts decreasing but no other XRD peaks appear. Thus, it can be concluded that phase-pure (Sn1−xTix)3N4 spinel alloy were synthesized up to x = 0.45 composition. The detailed XRD pattern at various compositions (Figure S4) and the summary table of the peak positions (Table S3) are provided in Supporting Information. The results of the (Sn1−xTix)3N4 optical absorption measurements are plotted on color scale in Figure 2b (top panel), with black-to-red transition indicating the optical absorption onset (energy at which absorption coefficient reaches 104 cm−1). These results show that for low Ti concentrations of x < 0.35 the absorption onset is close to 2.0 eV photon energy, higher than 1.7 eV for the Ti-free Sn3N4 (x = 0) grown at 150 °C. At higher Ti content (x > 0.35), there is a steep shift of the absorption onset to lower photon energy (Figure 2b) for the (Sn1−xTix)3N4 alloys grown at 280 °C. These optical results are consistent with the theoretical predictions (Figure 1b), supporting the accuracy of our calculations of other properties (e.g., effective masses). For the samples grown at higher temperature, an overall increase in absorption was observed (see Figures S5 and S6 in Supporting Information). This is likely due to precipitation of metallic tin (Sn) and titanium nitride (TiN) in this metastable material at these elevated temperatures. Figure 2b (bottom panel) shows the resistivity of the Sn− Ti−N films as a function of chemical composition and substrate temperature. At the intermediate Ti concentrations of interest in the range from x = 0.2 to x = 0.5, the resistivity is moderate, with maximum values in the 1.0−10 Ω cm range, similar to phase-pure Sn 3 N 4 values observed here and reported previously.37 Using the mobility determined by fitting the optical data for x = 0.66 Ti composition (1.4 cm2/(V s), Figure S6), the corresponding estimated free carrier concentration is 5 × 1017 to 5 × 1018 cm−3. These carrier concentrations are somewhat lower compared to those determined from Hall effect measurements (5 × 1019 cm−3 for x = 0.34), which may be attributed to lower mobility of these x = 0.34 samples compared to the optically measured x = 0.66 samples. Indeed,

the calculated mixing enthalpies are quite high (close to 0.4 eV), so nonequilibrium synthesis techniques are required to realize these metastable alloys. We also find that a very high nitrogen chemical potential of at least ΔμN = +1 eV is necessary to stabilize the spinel Ti2SnN4 phase. However, we note that achieving such high positive nitrogen chemical potential is not uncommon for other previously synthesized metastable nitrides,44,45 for example ΔμN (Sn3N4) = +0.4 eV and ΔμN (Cu3N) = +0.8 eV. 2.2. Experimental Realization of (Sn1−xTix)3N4 Alloys. To synthesize metastable (Sn1−xTix)3N4 alloys, combinatorial reactive sputtering with a nitrogen plasma source was used. Recently, we reported reactive magnetron sputtering with a nitrogen plasma source to produce ΔμN ≥ +1.0 eV in the case of Cu3N synthesis44 and used it to discover a hitherto unreported SnN binary compound.46 Thin films of metastable oxide alloy with high mixing enthalpies have been also successfully synthesized by our group,47,48 using highthroughput experimental (HTE) sputtering.49 The HTE approach relies on preparation of combinatorial sample libraries with mutually orthogonal gradients of chemical composition and substrate temperature, followed by spatially resolved characterization of the resulting structure and properties, as well as automated analysis of the large amounts of experimental data (see Methods for more details). Following this experimental approach, the (Sn1−xTix)3N4 alloy with x = 0 (i.e., Sn3N4) was synthesized in a phase-pure spinel structure in a broad range of studied substrate temperatures and target-substrate distances (Figure S1), indicating that the nitrogen chemical potential of at least ΔμN = 0.4 eV was achieved (Figure S2). With increasing Ti composition above x = 0.23, the diffraction peaks of the single spinel phase shift to higher angles (Figure 2a, top panel), for the samples grown at 280 °C (for other temperatures, see Figure S3). This corresponds to the decrease in the lattice constant from 9.0 Å at x = 0 to 8.6 Å at x = 0.67, as calculated from the position of the (222) spinel peak (Figure 2a, bottom panel). Above the x = 0.45 composition, the intensity of XRD 6513


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The single spinel phase is further supported by homogeneous composition of the x = 0.23 (Sn1−xTix)3N4 samples obtained by energy-dispersive X-ray (EDX) mapping. It is possible that the spinel phase is stabilized with assistance of oxygen observed by both EDX (Figure 3) and RBS (Figure S7) measurements, as known in the case of nitride ceramics.51 Nevertheless, XRD and microscopy results indicate that Ti is incorporated into the spinel lattice, at least in the x = 0.23−0.45 composition range. This is significant because it represents the first experimental case of the ternary nitride spinel alloys between the group IVA and group IVB elements. 2.3. (Sn1−xTix)3N4 Alloys with x = 0.67 Composition. As shown in Figure 1b, the full occupation of the Oh sites by Ti at x = 0.67 in (Sn1−xTix)3N4 leads to the stoichiometric spinel Ti2SnN4 compound as a new breaking point in the convex hull between Sn3N4 and Ti3N4. The theory also predicts that the synthesis of Ti2SnN4 would be challenging due to high positive nitrogen chemical potential that is required (>+1 eV). This may be the reason that, despite numerous prior theoretical studies,30 neither Ti2SnN4 nor any other predicted double-metal spinel nitrides have been experimentally reported yet. Hence, the synthesis of (Sn1−xTix)3N4 alloys with x = 0.67 composition is of basic scientific interest in this work. The experimental XRD observation (Figure 2a) indicates that, close to the x = 0.67 composition, the (Sn1−xTix)3N4 films become either amorphous or nanocrystalline. While most of the spinel peaks disappear at this composition, the (222) spinel peak merges into the rocksalt (111) peak of TiN (Figure S8), due to the similarity of the underlying octahedral planes (Figure 2a). With further increase in Ti content, the Ti3+ oxidation state becomes preferable, eventually leading to growth of rocksalt TiN (x = 1). So in this x > 0.67 Ti-rich region the alloy composition should be written as (Sn1−xTix)3+yN4, where y = 0...1. Given the XRD observations above, it is not clear if the (Sn1−xTix)3N4 alloy close to x = 0.67 composition is a nanocrystalline phase-pure spinel, a nanocrystalline phase mixture of spinel and rocksalt, or amorphous (question mark in Figure 2b). To understand the structure and phase of the (Sn1−xTix)3N4 alloys at x = 0.67 composition, we performed STEM HAADF imaging supplemented by SAED and EDX measurements (Figure 3). For the sample with this stoichiometry prepared at 280 °C, HAADF and SAED results indicate crystalline nanoscale grains, rather than mostly amorphous phase. Quantitative analysis of SAED patterns (Table S5) is consistent with the spinel structure, but the presence of (111) rocksalt peak cannot be ruled out due to its similar position with the spinel (222) peak. The lattice constant calculated from SAED data and XRD data are different by ∼10%, which may be related to some inaccuracy of both measurements and/or small deviations in preparation conditions between the samples. Interestingly, Figure 3b also shows spatial fluctuations of the Sn and Ti concentrations on the length scale of 20−30 nm, which point to a beginning spinodal decomposition52 of this metastable material. It is likely that the decomposition occurs between Ti-rich and Sn-rich alloy compositions, while maintaining the underlying fcc anion lattice that is common (Figure 2a inset) to both the spinel (Sn3N4 derived) and rocksalt (TiN derived). This interpretation would be consistent with the merging of the spinel (222) into the rocksalt (111) peak observed in XRD (Figure 2a) The experimental observations of spinodally decomposed (Sn1−xTix)3N4 alloys at x = 0.67 composition are inconsistent

the Hall effect measurements also indicate lower electron mobilities (0.3 cm2/(V s)) for the x = 0.34 samples, which might be related to stronger grain boundary scattering. The free electrons in this low-Ti composition range may be generated by unintentional oxygen doping in the (Sn1−xTix)3N4 samples, detected in this study using RBS (see Figure S7). Further reduction in base pressure of the synthesis chamber would be necessary to reduce the level of oxygen contamination. Because experimental measurements of the effective masses in semiconductors are difficult, we performed photoelectrochemical (PEC) experiments with our (Sn1−xTix)3N4 films. To test the theoretical prediction that Ti substitution should decrease its hole effective mass, we used the relative current of the photogenerated holes as a proxy for their effective masses. Consistent with the theoretical prediction (Figure 1b), we find that increasing the concentration x of Ti in the (Sn1−xTix)3N4 alloys leads to an increased photocurrent by as much as 5× for the films with x = 0.35, compared to the Sn3N4 baseline sample (Table S4 of Supporting Information). Another possible explanation is the change in the positions of the band edges with respect to water redox potentials. However, the absolute photocurrents observed for these samples were generally lower than previously reported for the Sn3N4 material,37 likely due to the smaller thickness of the absorber layers. The deposition of a NiOx layer on top of the Ti-rich (Sn1−xTix)3N4 alloy samples with x = 0.35 further enhanced the photocurrent by 50%. The possible reasons for the improved photocurrent might be the stabilizing effect of the deposited NiOx on the (Sn1−xTix)3N4 alloy surface, or the catalytic activity of NiOx toward water oxidation.50 Finally, to verify the absence of phase separation in the (Sn1−xTix)3N4 spinel alloys at the nanoscale that is imperceptible for XRD, Figure 3a shows results of detailed scanning transmission electron microscopy (STEM) studies of the (Sn1−xTix)3N4 alloys with x = 0.23. The high-angle annular dark-field (HAADF) images and selected area electron diffraction (SAED) patterns indicate nanocrysalline character of the samples and are consistent with spinel structure of the (Sn1−xTix)3N4 alloys (see Table S5 for a list of lattice spacings).

Figure 3. As shown in the middle and bottom panels, the x = 0.23 sample (A) is a homogeneous chemical composition of an alloy. For the x = 0.67 sample (B), elemental Sn/Ti segregation at 20−30 nm scale is observed by EDX analysis, indicative of spinodal decomposition on the underlying crystal lattice. The diffraction rings marked by blue circles correspond to the spinel lattice spacings, but the presence of some rocksalt peaks cannot be ruled out. 6514


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The results of this study point in several other promising future research directions. First, the chemical domain of the studied nitrides should be expanded from the group III and IV compounds to the rest of the periodic table. A crystallographic database exploration indicates that there is a number of previously unreported nitride compounds, both stable and metastable, that likely await their discovery. Second, there is no reason to limit the materials design of semiconductor alloys to isostructural end-point compounds such as Sn3N4 and Ti2SnN4 spinels. On the contrary, the exploration of the alloys between materials with different crystal structures may lead to emergence of new metastable phases with useful properties that the end-point compounds do not have.53 Overall, this study highlights the promise of “materials design” approach to metastable nitride semiconductor alloys for solar energy conversion and other technological applications.

with the calculated lower energy of Ti2SnN4 relative to the Sn 3N 4 and Ti 3 N4 binaries (Figure 1b). One possible explanation is that the very high nitrogen chemical potential of ΔμN > 1.0 eV required to grow Ti2SnN4 (Figure 1a) was not achieved during the synthesis experiments. However, this is unlikely because ΔμN > 1.0 eV was achieved in prior similar synthesis experiments.44 Another possibility is that the lower energy structure of Ti3N4 could decrease the phase stability of Ti2SnN4. Specifically, the calculated crystal structure model for Ti3N4 (orthorhombic CaTi2O4 type) resulted from a survey of a few likely prototype structures,38 so a more exhaustive structure search could be useful. It is unlikely that the observed inconsistency results from different vibrational entropy contributions to Ti2SnN4, Ti3N4, and Sn3N4, because this term is usually quite similar between such similar solids. According to theoretical calculations, the alloys with x = 0.67 composition have a small band gap of 0.2 eV. This narrow gap and the tendency to form Ti3+ at x > 0.67 can explain the high optical absorption throughout the experimentally measured spectral range (Figures S5 and S6). For these higher Ti concentrations, the measured resistivity also becomes low (ρ ∼ 10−3−10−2 Ω cm and x = 0.67, and ρ ∼ 10−4 Ω cm at x = 1.0), consistent with the band gap narrowing and Ti3+ formation. Thus, these x = 0.67 alloys are not suitable for solar energy conversion, in contrast to the (Sn1−xTix)3N4 alloys with x < 0.45 composition and promising semiconductor properties discussed above.

4. METHODS Theoretical results presented in this work were obtained by thermodynamic stability and electronic structure calculations from first principles. The density functional theory (DFT) calculations were performed using the VASP54 code in generalized gradient approximations (GGA55), with an on-site Coulomb interaction of U = 3 eV for Ti-d.56 The enthalpies of formation were determined using the fitted elemental reference energies (FERE) scheme.57 The electronic structure was calculated employing the PAW implementation of the GW method,58 starting from an initial GGA+U calculation. To approximately correct for the slower convergence behavior of the 3d orbitals, we employed an additional onsite potential Vd for transition metal cations (Vd = −1.3 eV for Ti-d). A larger number of such GW band-structure calculations are available at http://materials.nrel.gov.59 For the (Sn1−xTix)3N4 alloy calculations (x = 0, 0.5, 1.0, 1.5, 2.0), we used the 14 atom primitive cells of the spinel structure with the lowest energy among different possible nonequivalent cation arrangements. The density of states (DOS) effective mass was determined directly from the GW eigenvalue spectrum60 using T = 1000 K for integration. For more detailed description of the theoretical methods, the reader is referred to Supporting Information. To obtain experimental results presented in this paper, a highthroughput experimental (HTE) approach was used, briefly described in Results and Discussion. The compositional gradient across the 50 × 50 mm glass substrate was achieved by a tilted arrangement of the Sn and Ti sputtering sources at an angle of 30° with respect to the substrate normal. The temperature gradient was formed by partially connecting the substrate to the heated sample holder.61,62 The 0.5−2 μm thick Sn−Ti−N films were prepared by combinatorial radiofrequency (RF) magnetron cosputtering in a vacuum chamber with a base pressure below 5 × 10−5 Pa at a process pressure of 0.3 Pa using a volumetric flow ratio of Ar/N2 = 66/33% with an atomic nitrogen source to increase its effective chemical potential and a liquid nitrogencooled cryoshroud to decrease the base pressure. The resulting combinatorial sample libraries (4 × 11 grid) were investigated using spatially resolved measurements of composition and thickness (X-ray fluorescence), structure (X-ray diffraction), optical (UV−vis−NIR spectroscopy), and electrical properties (four-point probe). For selected samples cut out from the libraries, we measured the electron concentration (Hall effect), oxygen content (Rutherford backscattering), photoresponce (photoelectrochemical measurements), and microstructure (scanning transmission electron microscopy). The resulting large amounts of data were analyzed using custom-written routines in Wavemetrics Igor Pro. More information on our combinatorial high-throughput approach can be found in the literature,63,64 and more measurement details specific to this paper are provided in Supporting Information

3. CONCLUSIONS In this work, we demonstrated design of the (Sn1−xTix)3N4 metastable spinel alloys. The calculated optical band gap of the (Sn1−xTix)3N4 alloys has the 1−2 eV value in the x = 0.17−0.33 range. This is consistent with the measured strong optical absorption onset close to 2 eV for the (Sn1−xTix)3N4 thin film samples with low Ti concentrations (x ∼ 0.23−0.35). Further, theoretical calculations of hole effective masses are supported by experimental measurements of increased photoexcited hole current in (Sn1−xTix)3N4 alloys compared to pure Sn3N4 thin films. It is remarkable that the useful (Sn 1−x Ti x ) 3 N 4 optoelectronic properties emerge at the intermediate alloy compositions, first improving and then deteriorating with increasing Ti composition. This observation clearly illustrates the advantages of the materials design as a function of continuously changing chemical composition. An important result of this work is the experimental synthesis of the theoretically predicted, yet previously unreported, phasepure ternary group IV nitride spinel material (Sn1−xTix)3N4 with a Ti composition up to x = 0.45. This is one of the first experimental examples of mixed-cation group A/group B spinel nitride alloys reported in the literature. These (Sn1−xTix)3N4 low-Ti (x = 0.25) spinel alloys with moderate electron doping, favorable hole transport, and solar-matched bandgaps may be of technological relevance as photoanodes for the oxygen evolution reaction part of the photoelectrochemical water splitting. However, more work is necessary to improve the semiconductor properties of the low-Ti compositions for PEC applications and to understand the tendency toward nanoscale spinodal decomposition of the high-Ti alloy compositions. A possible route to address both of these objectives is the deposition of the (Sn1−xTix)3N4 films at low temperatures with a high nitrogen chemical potential and subsequent rapid thermal annealing for crystallization. 6515


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02122. Computational and experimental methods; calculated and experimental phase diagram for Sn−N; measured XRD patterns on color scale and regular scales; optical absorption of (Sn,Ti)3N4 films on color scale and regular scale; Rutherford backscattering results; (Sn,Ti)3N4 XRD peak position and intensities; calculated group IV cation Oh/Td site preferences; calculated formation enthalpies; measured spinel XRD peak positions; measured photocurrent results; lattice plane spacings from SAED (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sebastian Siol: 0000-0002-0907-6525 Andriy Zakutayev: 0000-0002-3054-5525 Present Addresses §

Department of Chemistry & Biochemistry, San Diego State University, San Diego, CA. ∥ Empa, Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, as part of the Energy Frontier Research Center “Center for Next Generation of Materials by Design: Incorporating Metastability” under contract no. DE-AC36-08GO28308 to NREL. The authors thank John Perkins for carrying out the RBS measurements, Marc Landry for his support in building up the sputtering system, and Gerbrand Ceder and David Ginley for useful discussions.

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Design of Metastable Tin Titanium Nitride Semiconductor Alloys

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