Quantifying Cation Disorder in ZnGeP₂Thin Films Using Resonant Energy X-ray Diffraction

II-IV-V₂ materials, ternary analogs to III-V’s, are emerging for their potential applications in devices such as LEDs and solar cells. Controlling cation ordering in II-IV-V₂’s offers the potential to tune properties at nearly fixed compositions and lattice parameters. Cation disorder is prevalent in many II-IV-V₂’s and has a profound effect on properties – however, quantification of disorder remains difficult. In this work, we investigate two different methods to quantify cation ordering in ZnGeP₂thin films: a stretching parameter calculated from lattice constants (c/a), and an order parameter determined from the cation site occupancies (S). We use high resolution X-ray diffraction (HRXRD) to determine c/a and resonant energy X-ray diffraction (REXD) to extract S. REXD is critical to distinguish elements with similar Z-number (e.g. Zn and Ge) through X-ray diffraction. We found that samples with a c/a corresponding to the ordered chalcopyrite structure had only partially ordered S values. The optical absorption onset for these films occurred at lower energy than expected for fully ordered ZnGeP₂, indicating that S better captures disorder in these samples. This work highlights the importance of nuanced techniques (e.g. S) when analyzing more complex ternary systems.