Due to the large number of possible defect sites characterized by different local chemical environments, investigating point defects in semiconductor alloys has been a long-standing challenge for first-principles defect calculations. In practice, it is often assumed that focusing on the energetically most favorable defect site is sufficient to provide a reliable estimation of the defect properties. Here, using a prototypical defect $\left({\mathrm{C}}_{\mathrm{N}}\right)$ in the ${\mathrm{Al}}_{0.5}{\mathrm{Ga}}_{0.5}\mathrm{N}$ alloy as an example, we show that the site occupation is highly important and critically impacts the defect-assisted nonradiative capture coefficients. We find a positive correlation between the hole capture coefficient $\left(C_p\right)$ and defect formation energy, which brings in a strong temperature dependence: the statistically averaged effective $C_p$ increases substantially with increasing the material growth temperature. Quantitative analysis demonstrates that the effective $C_p$ with equilibrium site occupations at the typical growth temperature is one order of magnitude higher than that with only the lowest-energy site considered. Further, we show that even though the lowest-energy configuration of ${\mathrm{C}}_{\mathrm{N}}$ in the alloy has the same first-nearest-neighbor environment as the one in GaN, $C_p$ in the two cases differs by more than an order of magnitude. The large difference of $C_p$ is primarily attributed to reduced electron-phonon coupling caused by lattice distortions in the alloy, which highlights the importance of lattice distortion in semiconductor alloys in addition to local chemistry. Our study provides essential insights for defect engineering in semiconductor alloys toward enhanced electronic and optoelectronic performance.

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• Carrier generation & recombination
• Defects
• Electron-phonon coupling
• Alloys
• Semiconductors
• Density functional theory