Origin of the Indirect–Direct Band Gap Transition in GaP and Its Alloys

Indirect GaP-based photoelectric devices often exhibit low photoelectric conversion efficiencies. Numerous research efforts have been undertaken to facilitate indirect–direct transitions; however, a comprehensive understanding of the nature of these transitions remains elusive. In this study, we demonstrate that the occupied d orbitals, strain, and electronegativity are the three critical factors influencing the indirect–direct transition in GaP. Elevating the occupied d orbitals can raise the conduction band X and L valleys through s–d and p–d coupling while leaving the Γ valley unchanged, thereby facilitating the transition. The X valley possesses positive deformation potentials, whereas the Γ and L valleys exhibit negative ones. Consequently, a mere 0.6% tensile strain can convert the GaP band gap from indirect to direct. Additionally, larger electronegativity anions can lower the conduction band Γ valley via s–s coupling, further triggering the transition. Further investigation into the mechanisms at play in GaP alloys reveals that strain is the most sensitive factor and is crucial under most conditions. Based on these underlying mechanisms, we propose new alloys such as (GaP)_1–x(ZnO)_x and (GaP)_1–x(ZnS)_x. This work provides a framework for analyzing semiconductor indirect–direct transitions and offers insights for designing photoelectric devices based on indirect band gap semiconductors.