Investigating Cation Disorder in ZnGeP₂ Thin Films Using Resonant Energy X-ray Diffraction

Ternary materials are being increasingly studied for their potential integration in solar cell and LED devices. Controlling cation disorder in these materials offers the potential to tune properties at nearly fixed compositions and lattice parameters. However, quantification of cation disorder is challenging. In this work, we present two methods used to quantify cation disorder in ZnGeP₂ thin films: a stretching parameter calculated from lattice constants, c/a, and an order parameter determined from site occupancies, S. We use high resolution X-ray diffraction to determine c/a and resonant energy X-ray diffraction (REXD) to extract S. REXD is necessary for distinguishing between elements with similar Z-number (Zn and Ge). We found that films with a c/a corresponding to the ordered structure had partially disordered S values. The optical absorption onset showed a trend that is consistent with the expected band gap narrowing as disorder is increased, indicating that S better represents the disorder in these films. This work highlights the complexities in characterizing cation disorder and the importance in choosing a metric that best reflects the physical properties of interest.