Synthesis of ZnSnN2crystals via a high-pressure metathesis reaction

Synthesis and Comprehensive Studies of Be-IV-N2 (IV=Si, Ge): Solving the Mystery of Wurtzite-Type Pmc21 Structures

The research for wurtzite-type ternary nitride semiconductors containing earth abundant elements with a stoichiometry of 1 : 1 : 2 was focused on metals like Mg or Zn, so far. The vast majority of these Grimm-Sommerfeld analogue compounds crystallize in the β-NaFeO2 structure, although a second arrangement in space group Pmc21 is predicted to be a viable alternative. Despite extensive theoretical and experimental studies, this structure has so far remained undiscovered. Herein, we report on BeGeN2 in a Pmc21 structure, synthesized from Be3N2 and Ge3N4 using a high-pressure high-temperature approach at 6 GPa and 800 °C. The compound was characterized by powder X-ray diffraction (PXRD), solid state nuclear magnetic resonance (NMR), Raman and energy dispersive X-ray (EDX) spectroscopy, temperature-dependent PXRD, second harmonic generation (SHG) and UV/Vis measurements and in addition also compared to its lighter homologue BeSiN2 in all mentioned analytic techniques. The synthesis and investigation of both the first beryllium germanium nitride and the first ternary wurtzite-type nitride crystallizing in space group Pmc21 open the door to a new field of research on wurtzite-type related structures.

Low-temperature synthesis of cation-ordered bulk Zn3WN4 semiconductor via heterovalent solid-state metathesis

Metathesis reactions are widely used in synthetic chemistry. While state-of-the-art organic metathesis involves highly controlled processes where specific bonds are broken and formed, inorganic metathesis reactions are often extremely exothermic and, consequently, poorly controlled. Ternary nitrides offer a technologically relevant platform for expanding synthetic control of inorganic metathesis reactions. Here, we show that energy-controlled metathesis reactions involving a heterovalent exchange are possible in inorganic nitrides. We synthesized Zn3WN4 by swapping Zn2+ and Li+ between Li6WN4 and ZnX2 (X = Br, Cl, F) precursors. The in situ synchrotron powder X-ray diffraction and differential scanning calorimetry show that the reaction onset is correlated with the ZnX2 melting point and that product purity is inversely correlated with the reaction's exothermicity. Therefore, careful choice of the halide counterion (i.e., ZnBr2) allows the synthesis to proceed in a swift but controlled manner at a surprisingly low temperature for an inorganic nitride (300 °C). High resolution synchrotron powder X-ray diffraction and diffuse reflectance spectroscopy confirm the synthesis of a cation-ordered Zn3WN4 semiconducting material. We hypothesize that this synthesis strategy is generalizable because many Li-M-N phases are known (where M is a metal) and could therefore serve as precursors for metathesis reactions targeting new ternary nitrides. This work expands the synthetic control of inorganic metathesis reactions in a way that will accelerate the discovery of novel functional ternary nitrides and other currently inaccessible materials.

Ammonothermal Crystal Growth of Functional Nitrides for Semiconductor Devices: Status and Potential

The state-of-the-art ammonothermal method for the growth of nitrides is reviewed here, with an emphasis on binary and ternary nitrides beyond GaN. A wide range of relevant aspects are covered, from fundamental autoclave technology, to reactivity and solubility of elements, to synthesized crystalline nitride materials and their properties. Initially, the potential of emerging and novel nitrides is discussed, motivating their synthesis in single crystal form. This is followed by a summary of our current understanding of the reactivity/solubility of species and the state-of-the-art single crystal synthesis for GaN, AlN, AlGaN, BN, InN, and, more generally, ternary and higher order nitrides. Investigation of the synthesized materials is presented, with a focus on point defects (impurities, native defects including hydrogenated vacancies) based on GaN and potential pathways for their mitigation or circumvention for achieving a wide range of controllable functional and structural material properties. Lastly, recent developments in autoclave technology are reviewed, based on GaN, with a focus on advances in development of in situ technologies, including in situ temperature measurements, optical absorption via UV/Vis spectroscopy, imaging of the solution and crystals via optical (visible, X-ray), along with use of X-ray computed tomography and diffraction. While time intensive to develop, these technologies are now capable of offering unprecedented insight into the autoclave and, hence, facilitating the rapid exploration of novel nitride synthesis using the ammonothermal method.

Tolerance Factor and Phase Stability of the KCoO2-Type AMN2 Nitrides

In order to help understand the structural stability of KCoO2-type ternary nitrides AMN2, referring to perovskite structure, a tolerance factor t is proposed to describe the size effect on the phase/symmetry options of the experimentally accessible AMN2 nitrides. This leads to a range of t values above 0.946 for structurally stable KCoO2-type AMN2 nitrides with t values around 0.970 for the orthorhombic and tetragonal phase boundary. In contrast, most of AMN2 nitrides exhibit α-NaFeO2-type structure with t ∼ 0.898-0.946 and cations ordered or disordered rocksalt structure while t below 0.898. Employing the proposed criterion, the structure formation for other ternary AMN2 compositions with lanthanum and alkaline earth cations for the A sites were predicted, which was testified through the synthesis attempts and complemented by formation energy evaluations. The efforts to synthesize the ternary Lanthanide and alkaline earth-based AMN2 nitrides were unsuccessful, which could associate the structural instability with the large formation energies of lanthanide nitrides LaMN2 and the greater tolerance factor of 1.048 for BaTiN2. The experimentally already synthesized AMN2 nitrides could be categorized into three types with different tolerance factors, and scarce AMN2 nitrides with lower formation energies would be accessible using different synthetic routes beyond the traditional solid-state synthesis method.

Nanocrystalline ZnSnN2 Prepared by Reactive Sputtering, Its Schottky Diodes and Heterojunction Solar Cells

ZnSnN₂ has potential applications in photocatalysis and photovoltaics. However, the difficulty in preparing nondegenerate ZnSnN₂ hinders its device application. Here, the preparation of low-electron-density nanocrystalline ZnSnN₂ and its device application are demonstrated. Nanocrystalline ZnSnN₂ was prepared with reactive sputtering. Nanocrystalline ZnSnN₂ with an electron density of approximately 10¹⁷ cm^-3 can be obtained after annealing at 300 °C. Nanocrystalline ZnSnN₂ is found to form Schottky contact with Ag. Both the current I vs. voltage V curves and the capacitance C vs. voltage V curves of these samples follow the related theories of crystalline semiconductors due to the limited long-range order provided by the crystallites with sizes of 2-10 nm. The I-V curves together with the nonlinear C^-2-V curves imply that there are interface states at the Ag-nanocrystalline ZnSnN₂ interface. The application of nanocrystalline ZnSnN₂ to heterojunction solar cells is also demonstrated.

Synthesis of Zn2NbN3ternary nitride semiconductor with wurtzite-derived crystal structure

Binary III-N nitride semiconductors with wurtzite crystal structure such as GaN and AlN have been long used in many practical applications ranging from optoelectronics to telecommunication. The structurally related ZnGeN₂or ZnSnN₂derived from the parent binary compounds by cation mutation (elemental substitution) have recently attracted attention, but such ternary nitride materials are mostly limited to II-IV-N₂compositions. This paper demonstrates synthesis and characterization of zinc niobium nitride (Zn₂NbN₃)-a previously unreported II₂-V-N₃ternary nitride semiconductor. The Zn₂NbN₃thin films are synthesized using a one-step adsorption-controlled growth that locks in the targeted stoichiometry, and a two-step deposition/annealing method that suppresses the loss of Zn and N. Measurements indicate that this sputtered Zn₂NbN₃crystalizes in cation-disordered wurtzite-derived structure, in contrast to chemically related rocksalt-derived Mg₂NbN₃compound, also synthesized here for comparison using the two-step method. The estimated wurtzite lattice parameter ratio of Zn₂NbN₃is 1.55, and the optical absorption onset is at 2.1 eV. Both of these values are lower compared to published Zn₂NbN₃computational values ofc/a= 1.62 andE_g= 3.5-3.6 eV. Additional theoretical calculations indicate that this difference is due to cation disorder in experimental samples, suggesting a way to tune the structural parameters and the resulting properties of heterovalent ternary nitride materials. Overall, this work expands the wurtzite family of nitride semiconductors to include Zn₂NbN₃, and suggests that related II₂-V-N₃and other ternary nitrides should be possible to synthesize.

A discussion on `Domain formation and phase transitions in the wurtzite-based heterovalent ternaries: a Landau theory analysis'

A scientific exchange on an earlier paper [Quayle (2020). Acta Cryst. A76, 410–420] has led to the clarification of some of the points.

Heterovalent ternary nitrides are considered one of the promising classes of materials for photovoltaics, combining attractive physical properties with low toxicity and element abundance. One of the front-runner systems under consideration is ZnSnN₂. Although it is nominally a ternary compound, no clear crystallographic evidence for cation ordering has been observed so far. An attempt to elucidate this discrepancy [Quayle (2020). Acta Cryst. A76, 410–420] was the trigger for an intensive discussion between the authors, and an agreement was reached to elaborate on some points in order to set things in perspective. Rather than using a conventional comment–answer scheme, this is published in the form of a joint discussion to celebrate constructive criticism and collegiality.

Synthesis of CaSnN2 via a High-Pressure Metathesis Reaction and the Properties of II-Sn-N2 (II = Ca, Mg, Zn) Semiconductors

A novel ternary nitride semiconductor, CaSnN₂, with a layered rock-salt-type structure (R3̅m) was synthesized via a high-pressure metathesis reaction. The properties and structures of II-Sn-N₂ (II = Ca, Mg, Zn) semiconductors were also systematically studied, and the differences among them were revealed by comparison. These semiconductor materials showed a rock-salt- or wurtzite-type structure depending on the combined effect of the synthetic conditions and the characteristics of the group II elements. Additionally, the rock-salt-type structures of CaSnN₂ and MgSnN₂ (i.e., the ambient-pressure phase) were different from those predicted using first-principles calculations. Further, on the basis of first-principles calculations and consideration of the pressure effect, the recovered CaSnN₂ sample showed an R3̅m structure. CaSnN₂ and MgSnN₂ showed a band gap of 2.3-2.4 eV, which is suitable for overcoming the green-light-gap problem. These semiconductors also showed a strong cathode luminescence peak at room temperature, and generalized gradient approximation (GGA) calculations revealed that CaSnN₂ has a direct band gap. These inexpensive and nontoxic semiconductors (II-Sn-N₂ semiconductors (II = Ca, Mg, Zn)), with mid band gaps are required as pigments to replace cadmium-based materials. They can also be used in emitting devices and as photovoltaic absorbers, replacing In_xGa_1-xN semiconductors.

Combinatorial Synthesis of Magnesium Tin Nitride Semiconductors

Nitride materials feature strong chemical bonding character that leads to unique crystal structures, but many ternary nitride chemical spaces remain experimentally unexplored. The search for previously undiscovered ternary nitrides is also an opportunity to explore unique materials properties, such as transitions between cation-ordered and -disordered structures, as well as to identify candidate materials for optoelectronic applications. Here, we present a comprehensive experimental study of MgSnN2, an emerging II-IV-N2 compound, for the first time mapping phase composition and crystal structure, and examining its optoelectronic properties computationally and experimentally. We demonstrate combinatorial cosputtering of cation-disordered, wurtzite-type MgSnN2 across a range of cation compositions and temperatures, as well as the unexpected formation of a secondary, rocksalt-type phase of MgSnN2 at Mg-rich compositions and low temperatures. A computational structure search shows that the rocksalt-type phase is substantially metastable (>70 meV/atom) compared to the wurtzite-type ground state. Spectroscopic ellipsometry reveals optical absorption onsets around 2 eV, consistent with band gap tuning via cation disorder. Finally, we demonstrate epitaxial growth of a mixed wurtzite-rocksalt MgSnN2 on GaN, highlighting an opportunity for polymorphic control via epitaxy. Collectively, these findings lay the groundwork for further exploration of MgSnN2 as a model ternary nitride, with controlled polymorphism, and for device applications, enabled by control of optoelectronic properties via cation ordering.

High electron mobility single-crystalline ZnSnN2 on ZnO (0001) substrates

Making a systematic effort, we have developed single-crystalline ZnSnN₂ on ZnO (0001) by reactive magnetron co-sputtering.

Alloy-Free Band Gap Tuning across the Visible Spectrum

We present evidence, from theory and experiment, that ZnSnN_{2} and MgSnN_{2} can be used to match the band gap of InGaN without alloying-by exploiting cation disorder in a controlled fashion. We base this on the determination of S, the long-range order parameter of the cation sublattice, for a series of epitaxial thin films of ZnSnN_{2} and MgSnN_{2} using three different techniques: x-ray diffraction, Raman spectroscopy, and in situ electron diffraction. We observe a linear relationship between S^{2} and the optical band gap of both ZnSnN_{2} (1.12-1.98 eV) and MgSnN_{2} (1.87-3.43 eV). The results clearly demonstrate the correlation between controlled heterovalent cation ordering and the optical band gap, which applies to a broad group of emerging ternary heterovalent compounds and has implications for similar trends in other material properties besides the band gap.

Nitridophosphates: A Success Story of Nitride Synthesis

Nitridophosphates and phosphorus nitrides are thoroughly investigated classes of nitrides. During thirty years of research, the methods for their synthesis evolved from the condensation of molecular precursors at moderate temperatures and ambient pressures to state-of-the-art high-pressure and high-temperature processes. Landmark breakthroughs made in recent years led to a comprehension-based proficiency in nitridophosphate synthesis that is illustrated by the large compositional and structural diversity of the nitridophosphates known today. Herein, we review the advances made in synthesis with regard to the prevalent problem of nitride synthesis: the susceptibility of nitride ions to oxidation.

Interplay between Composition, Electronic Structure, Disorder, and Doping due to Dual Sublattice Mixing in Nonequilibrium Synthesis of ZnSnN2 :O

The opportunity for enhanced functional properties in semiconductor solid solutions has attracted vast scientific interest for a variety of novel applications. However, the functional versatility originating from the additional degrees of freedom due to atomic composition and ordering comes along with new challenges in characterization and modeling. Developing predictive synthesis-structure-property relationships is prerequisite for effective materials design strategies. Here, a first-principles based model for property prediction in such complex semiconductor materials is presented. This framework incorporates nonequilibrium synthesis, dopants and defects, and the change of the electronic structure with composition and short range order. This approach is applied to ZnSnN₂ (ZTN) which has attracted recent interest for photovoltaics. The unintentional oxygen incorporation and its correlation with the cation stoichiometry leads to the formation of a solid solution with dual sublattice mixing. A nonmonotonic doping behavior as a function of the composition is uncovered. The degenerate doping of near-stoichiometric ZTN, which is detrimental for potential applications, can be lowered into the 10¹⁷ cm^-3 range in highly off-stoichiometric material, in quantitative agreement with experiments.

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

Nitrides represent an intriguing class of functional materials with a broad range of application fields. Within the past decade, the ammonothermal method became increasingly attractive for the synthesis and crystal growth of nitride materials. The ammonothermal approach proved to be eminently suitable for the growth of bulk III-nitride semiconductors like GaN, and furthermore provided access to numerous ternary and multinary nitrides and oxonitrides with promising optical and electronic properties. In this minireview, we will shed light on the latest research findings covering the synthesis of nitrides by this method. An overview of synthesis strategies for binary, ternary, and multinary nitrides and oxonitrides, as well as their properties and potential applications will be given. The recent development of autoclave technologies for syntheses at high temperatures and pressures, in situ methods for investigations of crystallization processes, and solubility measurements by ultrasonic velocity experiments is briefly reviewed as well. In conclusion, challenges and future perspectives regarding the synthesis and crystal growth of novel nitrides, as well as the advancement of autoclave techniques are discussed.

Single-layer ZnMN2 (M = Si, Ge, Sn) zinc nitrides as promising photocatalysts

The catalytic activity of single-layer ZnGeN₂ is better and the band edge positions can be effectively tuned to match better with the redox potentials of water by biaxial strain.

Conduction-band effective mass and bandgap of ZnSnN2 earth-abundant solar absorber

Pseudo III-V nitride ZnSnN₂ is an earth-abundant semiconductor with a high optical absorption coefficient in the solar spectrum. Its bandgap can be tuned by controlling the cation sublattice disorder. Thus, it is a potential candidate for photovoltaic absorber materials. However, its important basic properties such as the intrinsic bandgap and effective mass have not yet been quantitatively determined. This paper presents a detailed optical absorption analysis of disordered ZnSnN₂ degenerately doped with oxygen (ZnSnN_2-x O _x ) in the ultraviolet to infrared region to determine the conduction-band effective mass (m _c^*) and intrinsic bandgap (E _g). ZnSnN_2-x O _x epilayers are n-type degenerate semiconductors, which exhibit clear free-electron absorption in the infrared region. By analysing the free-electron absorption using the Drude model, m _c^* was determined to be (0.37 ± 0.05)m ₀ (m ₀ denotes the free electron mass). The fundamental absorption edge in the visible to ultraviolet region shows a blue shift with increasing electron density. The analysis of the blue shift in the framework of the Burstein-Moss effect gives the E _g value of 0.94 ± 0.02 eV. We believe that the findings of this study will provide important information to establish this material as a photovoltaic absorber.