We demonstrate that excitons in semiconductor alloys are subject
to competing localization effects due to disorder (random potential fluctuations)
and shallow point defects (impurities). The relative importance of these effects
varies with alloy chemical composition, impurity activation energy as well as
temperature. We evaluate this effect quantitatively for MgxZn1−xO : Al (0 6
x 6 0.058) and find that exciton localization at low (2 K) and high (300 K)
temperatures is dominated by shallow donor impurities and alloy disorder,
respectively
