Vacancy defect influences on optoelectronic properties for In0.875Ga0.125As0.25P0.75: a first-principles study

InGaAsP photocathodes show great potential for near-infrared applications, particularly at 1.06 mu m. However, vacancy defects are unavoidable in InGaAsP crystals during their growth. Various defective models were constructed for In0.84375Ga0.125As0.25P0.75, In0.875Ga0.09375As0.25P0.75, In0.875Ga0.125As0.21875P0.75 and In0.875Ga0.125As0.25P0.71875 to study In, Ga, As and P vacancy defect influences respectively on In0.875Ga0.125As0.25P0.75 bulk properties. The electronic structure, formation energy, Mulliken population, electron density difference, and optical properties of defective crystals were calculated from first principles. Results show that In0.875Ga0.125As0.25P0.71875 has the lowest formation energy, implying that P vacancy defect is most easy formed. In and Ga vacancies are negatively charged and act as acceptors, whereas As and P vacancies are positively charged and act as donors. Increased populations of Ga-P and In-P bonds around the vacancies strengthen the covalency. In and Ga vacancies in the low-energy region significantly improve optical conductivity, reflectivity, and introduce new abnormal dispersion. Although As and P vacancies do not change the dispersion properties in the low energy region, they increase the refractive index.