Rekha Schnepf1 2 Aaron Martinez1 2 John Mangum1 3 Noemi Leick1 Paul Ndione1 Elisa Miller-Link1 Pauls Stradins1 2 Eric Toberer1 2 Adele Tamboli1 2

1, National Renewable Energy Laboratory, Golden, Colorado, United States
2, Colorado School of Mines, Golden, Colorado, United States
3, Colorado School of Mines, Golden, Colorado, United States

In this work, we present the effects of annealing on the structural and optical properties of polycrystalline ZnGeP2 films on Si substrates. As structural analogs to III-V materials, II-IV-V2 materials have the potential for optoelectronic applications beyond the present III-V materials. Currently, III-V optoelectronic devices enable fiber communications, solid-state lasers, light emitting diodes and high efficiency photovoltaics, but they rely on epitaxial heterostructures limited by lattice matching considerations. In contrast, II-IV-V2 materials can be lattice matched to silicon and have the potential for tunable electronic properties for fixed composition through control of cation ordering. Therefore, implementation of a material with similar properties to the III-Vs and lattice matched with silicon could be transformative for tandem photovoltaics. ZnGeP2 is one such material with a lattice matching within 1% of silicon and a band gap of 2.1 eV.
In order to control crystallinity and ordering independently of composition, stoichiometric amorphous ZnGeP2 films were grown and then annealed ex-situ. The crystallinity and ordering of the annealed films was studied with x-ray diffraction and transmission electron microscopy. To gain an understanding of how optical properties change with structure, spectroscopic ellipsometery was used to determine the optical constants and absorption coefficient of the films. Using these characterization techniques, we have confirmed the ability to control crystallinity and ordering of the films as a function of anneal temperature and time. Subsequently, with changes in crystallinity and ordering we observed variations in the optical properties of the films. From our results we can conclude that ZnGeP2 shows large potential for integration in Si-based devices.