Large binding-energy variation and alloy disorder in ${\mathrm{In}}_{0.79}$${\mathrm{Ga}}_{0.21}$${\mathrm{As}}_{0.44}$${\mathrm{P}}_{0.56}$

We have observed in the photoluminescence spectrum of ${\mathrm{In}}_{0.79}$ ${\mathrm{Ga}}_{0.21}$ ${\mathrm{As}}_{0.44}$ ${\mathrm{P}}_{0.56}$ a line presenting a striking novel feature: Its peak energy shifts continuously by ∼20 meV along a definite sample direction whereas the lower-energy lines do not move. We prove that this shift originates from a binding-energy variation (from 28 to 8 meV or less) in an alloy of constant composition. We interpret these results in terms of the effects of short-range alloy disorder around impurities.