Reactive Epitaxial Formation of a Mg-P-Zn Ternary Semiconductor in Mg/Zn₃P₂ Solar Cells

Zinc phosphide (Zn₃P₂) has attracted considerable attention as an environmentally benign and earth-abundant photoabsorber for thin-film photovoltaics. It is known that interdiffusion occurs at the Mg/Zn₃P₂ interface, which is a component of the record device, but the micro- and nanoscopic structures of the interface after interdiffusion have been controversial for over three decades. Here, we report on the formation of a Mg-P-Zn ternary semiconductor, Mg(Mg _xZn_{1- x})₂P₂, at the Mg/Zn₃P₂ interface. Interestingly, Mg(Mg _xZn_{1- x})₂P₂ is epitaxially grown on Zn₃P₂ with the orientation relationship of [21̅1̅0](0001)_Mg(Mg ||[100](011)_Zn due to interdiffusion. The lattice mismatch of the Mg(Mg _xZn_{1- x})₂P₂ layer on the Zn₃P₂ substrate is less than 0.5%, and this is favorable for carrier transport across the interface. Mg(Mg _xZn_{1- x})₂P₂ is the material suggested as "n-type Mg-doped Zn₃P₂" or "a Mg-P-Zn alloy" in the previous studies. Thus, only the optimization of Mg treatment as conducted in the previous studies is insufficient for the improvement of the cell performance. This work clarified that a suitable microstructure and band structure around Mg(Mg _xZn_{1- x})₂P₂/Zn₃P₂ heterointerface should be established.