Ammonothermal Synthesis and Optical Properties of Ternary Nitride Semiconductors Mg‐IV‐N₂, Mn‐IV‐N₂ and Li‐IV₂‐N₃ (IV=Si, Ge)

Abstract

Grimm-Sommerfeld analogous nitrides MgSiN₂ , MgGeN₂ , MnSiN₂ , MnGeN₂ , LiSi₂ N₃ and LiGe₂ N₃ (generally classified as II-IV-N₂ and I-IV₂ -N₃ ) are promising semiconductor materials with great potential for application in (opto)electronics or photovoltaics. A new synthetic approach for these nitride materials was developed using supercritical ammonia as both solvent and nitride-forming agent. Syntheses were conducted in custom-built high-pressure autoclaves with alkali metal amides LiNH₂ , NaNH₂ or KNH₂ as ammonobasic mineralizers, which accomplish an adequate solubility of the starting materials and promote the formation of reactive intermediate species. The reactions were performed at temperatures between 870 and 1070 K and pressures up to 230 MPa. All studied compounds crystallize in wurtzite-derived superstructures with orthorhombic space groups Pna2₁ (II-IV-N₂ ) and Cmc2₁ (I-IV₂ -N₃ ), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN₂ : 4.8 eV, MgGeN₂ : 3.2 eV, MnSiN₂ : 3.5 eV, MnGeN₂ : 2.5 eV, LiSi₂ N₃ : 4.4 eV, LiGe₂ N₃ : 3.9 eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.