Experimental investigation of phase equilibria around a ternary compound semiconductor Mg(MgxZn1-x)2P2 in the Mg-P-Zn system at 300 °C using Sn flux

Mg(MgxZn1-x)(2)P-2 was found to be a key material in Zn3P2-based solar cells, which have attracted attention as an earth-abundant photovoltaic technology. However, Mg(MgxZn1-x)(2)P-2 has not been investigated as a functional material thus far, and even the single-phase range in the Mg-P-Zn equilibrium phase diagram is currently unknown. Thus, in this work, we investigate the phase equilibria around Mg(MgxZn1-x)(2)P-2 in the Mg-P-Zn system using Sn as a flux for equilibrium experiments. We find that Mg(MgxZn1-x)(2)P(2)exists as a single phase with a trigonal P (3) over bar m1 structure from x = 0 to x = 0.55 based on our XRD and the SEM-EDS analyses. The lattice parameters, a and c, monotonically increase with increase in the Mg content. In addition, the Mg(MgxZn1-x)(2)P-2 phase can equilibrate with Zn3P2 only in the case of x = 0 (MgZn2P2) despite the wide composition range available. The lattice mismatch between MgZn2P2 and Zn3P2 is about 0.5%, which is consistent with our previous results of an electron diffraction study on Mg(MgxZn1-x)(2)P-2/Zn3P2 interfaces. We also find that the interface between Mg and Mg(MgxZn1-x)(2)P-2 is unstable and that the interface reaction is expected to result in the formation of the Mg-rich phosphide phase, which is air- and water-sensitive. The instability of the reaction product in the atmosphere can degrade the device via the formation of voids around the interface, which was observed in our previous work. Therefore, we deduce that Mg is not suitable as the surface electrode of Mg(MgxZn1-x)(2)P-2/Zn3P2 solar cells. The phase equilibria established in this study can aid in redesigning the device structure of Zn3P2-based solar cells and in further investigation of the physical properties of Mg(MgxZn1-x)(2)P-2.