Developing a method for scalable growth of defect-free Zn₃P₂ thin films for photovoltaic applications is a crucial step for moving away from scarce materials in thin film solar cells. Zn₃P₂ is an earth abundant semiconductor which has shown properties that would make it a suitable material for thin film photovoltaics applications. However, Zn₃P₂ thin films tend to grow with many defects which results in poor crystal quality. These defects stem from the fact that there are not many substrates with matching lattice and thermal expansion coefficients, thus many defects are formed during the growth and the cooling down process. Poor crystal quality is detrimental for photovoltaic applications which calls for further research in this topic.^[1][2] Research for producing high quality Zn₃P₂ films has resurfaced, mainly by using Selected Area Epitaxy (SAE). A mask can be used to restrict the epitaxial growth of Zn₃P₂ to nano sized holes which reduces the interface area and thus limiting defect formation. As the crystal grows out of the hole, it can fully relax and start growing on the mask laterally, which leads to the structures coalescing and forming a thin film. This method has been shown to work by using molecular beam epitaxy, with silicon oxide as a mask and indium phosphide substrates for Zn₃P₂.^[3] Alternative microfabrication methods and substrates are needed, as electron beam lithography (EBL) has a low-throughput and indium is a rare material which would deviate from earth-abundant perspective of Zn₃P₂ solar cells.

We successfully have shown the ability to grow Zn₃P₂ using SAE combined with metalorganic vapor phase epitaxy. To achieve high quality thin films, it is important to study the effects of different growth parameters on parasitic nucleation on the mask and selectivity before the pyramids coalesce. Temperature and V/II series experiments were performed to better understand the growth parameter effects. Another aspect in scaling Zn₃P₂ thin film growth is to try and move away from EBL and use Talbot Displacement lithography instead, which relies on optical interference to achieve sub-wavelength and quicker exposure of large area. We have explored two different pitch lengths for producing Zn₃P₂ pyramids on indium phosphide and studied the effects on epitaxial growth and coalescence. Alternative substrate to indium phosphate is silicon, which we are currently investigating.