Design, Synthesis, and Optoelectronic Properties of the High-Purity Phase in Layered AETM N 2 ( AE = Sr, Ba; TM = Ti, Zr, Hf) Semiconductors

We report the synthesis and optoelectronic properties of high phase-purity (>94 mol %) bulk polycrystals of KCoO -type layered nitrides N ( = Sr, Ba; and = Ti, Zr, Hf), which are expected to exhibit unique electron transport properties originating from their natural two-dimensional (2D) electronic structure, but high-purity intrinsic samples have yet been reported. The bulks were synthesized using a solid-state reaction between NH and N precursors with NaN to achieve high N chemical potential during the reaction. The N bulks are n-type semiconductors with optical band gaps of 1.63 eV for SrTiN , 1.97 eV for BaZrN , and 2.17 eV for BaHfN . SrTiN and BaZrN bulks show degenerated electron conduction due to the natural high-density electron doping and paramagnetic behavior in all of the temperature ranges examined, while such unintentional carrier generation is largely suppressed in BaHfN , which exhibits nondegenerated electron conduction. The BaHfN sample also exhibits weak ferromagnetic behavior at temperatures lower than 35 K. Density functional theory calculations suggest that the high-density electron carriers in SrTiN come from oxygen impurity substitution at the N site (O ) acting as a shallow donor even if the high-N chemical potential synthesis conditions are employed. On the other hand, the formation energy of O becomes larger in BaHfN because of the stronger -N chemical bonds. Present results demonstrate that the easiness of impurity incorporation is designed by density functional calculations to produce a more intrinsic semiconductor in wider chemical conditions, opening a way to cultivating novel functional materials that are sensitive to atmospheric impurities and defects.