Bowing of the defect formation energy in semiconductor alloys

Using the first-principles method and special quasirandom structure approach, we have studied the formation energies of two prototype defects in alloys, Ge${}_{\mathrm{As}}$ in Al${}_{x}$Ga${}_{1\ensuremath{-}x}$As and Cu${}_{\mathrm{Cd}}$ in CdS${}_{x}$Te${}_{1\ensuremath{-}x}$. We find that giant bowing effects for the defect formation energy can exist in semiconductor alloys. The bowing effect originates from the concentrated distribution of defects at low energy sites caused by the defect wave-function localization and the size-mismatch-induced strain effect. Because the bowing effect can drastically reduce the defect formation energy---even in dilute semiconductor alloys---it can have wide applications, such as alloy-enhanced defect solubility in semiconductors.