Ammonothermal Synthesis and Optical Properties of Ternary Nitride Semiconductors Mg-IV-N-2, Mn-IV-N-2 and Li-IV2-N-3 (IV=Si, Ge)

Grimm-Sommerfeld analogous nitrides MgSiN2, MgGeN2, MnSiN2, MnGeN2, LiSi2N3 and LiGe2N3 (generally classified as II-IV-N-2 and I-IV2-N-3) 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 Pna2(1) (II-IV-N-2) and Cmc2(1) (I-IV2-N-3), respectively, which was confirmed by powder X-ray diffraction. Optical bandgaps were estimated from diffuse reflectance spectra using the Kubelka-Munk function (MgSiN2: 4.8eV, MgGeN2: 3.2eV, MnSiN2: 3.5eV, MnGeN2: 2.5eV, LiSi2N3: 4.4eV, LiGe2N3: 3.9eV). Complementary DFT calculations were carried out to gain insight into the electronic band structures of these materials and to corroborate the optical measurements.