Cation-Mutation Design of Quaternary Nitride Semiconductors Lattice-Matched to GaN

The search for new direct bandgap, earth-abundant semiconductors for efficient, high-quality optoelectronic devices, as well as photovoltaic and photocatalytic energy conversion has attracted considerable interest. One methodology for the search is to study ternary and multiternary semiconductors with more elements and more flexible properties. Cation mutation such as binary -> ternary -> quaternary for ZnS -> CuGaS2 -> Cu2ZnSnS4 and ZnO -> LiGaO2 -> Li2ZnGeO4 led to a series of new quaternary chalcogenide and oxide semiconductors with wide applications. Similarly, starting with GaN, ternary nitrides such as ZnSnN2 and ZnGeN2 have been designed and synthesized recently. However, quaternary nitride semiconductors have never been reported either theoretically or experimentally. Through a combination of the Materials Genome database with the first-principles calculations, we designed a series of quaternary nitride compounds I-III-Ge2N4 (I = Cu, Ag, Li, Na, K; III = Al, Ga, In) following the GaN -> ZnGeN2 -> I-III Ge2N4 mutation. Akin to Li2ZnGeO4, these quaternary nitrides crystallize in a wurtzite-derived structure as their ground state. The thermodynamic stability analysis shows that while most of them are not stable with respect to phase separation, there are two key exceptions (i.e., LiAlGe2N4 and LiGaGe2N4), which are stable and can be synthesized without any secondary phases. Interestingly, they are both lattice-matched to GaN and ZnO, and their band gaps are direct and larger than that of GaN, 4.36 and 3.74 eV, respectively. They have valence band edges as low as ZnO and conduction band edges as high as GaN, thereby combining the best of GaN and ZnO in a single material. We predict that flexible and efficient band structure engineering can be achieved through forming GaN/LiAlGe2N4/LiGaGe2N4 heterostructures, which have tremendous potential for ultraviolet optoelectronics.