Design, Synthesis, and Optoelectronic Properties of the High-Purity Phase in Layered N ( = Sr, Ba; = 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.