Direct Band Gap Semiconductors with Two- and Three-Dimensional Triel-Phosphide Frameworks (Triel=Al, Ga, In).

Abstract

Recently, several ternary phosphidotrielates and -tetrelates have been investigated with respect to their very good ionic conductivity, while less focus was pointed towards their electronic structures. Here, we report on a novel series of compounds, in which several members possess direct band gaps. We investigated the known compounds Li3AlP2, Li3GaP2, Li3InP2, and Na3InP2 and describe the synthesis and the crystal structure of novel Na3In2P3. For all mentioned phosphidotrielates reflectance UV-Vis measurements reveal direct band gaps in the visible light region with decreasing band gaps in the series: Li3AlP2 (2.45 eV), Li3GaP2 (2.18 eV), Li3InP2 (1.99 eV), Na3InP2 (1.37 eV), and Na3In2P3 (1.27 eV). All direct band gaps are confirmed by quantum chemical calculations. The unexpected property occurs despite different structure types. As a common feature all compounds contain EP4 tetrahedra, which share exclusively vertices for E=In and vertices as well as edges for E=Al and Ga. The structure of the novel Na3In2P3 is built up by a polyanionic framework of six-membered rings of corner-sharing InP4 tetrahedra. As a result, the newly designed semiconductors with direct band gaps are suitable for optoelectronic applications, and they can provide significant guidance for the design of new functional semiconductors.