We report here the characteristic features of a newly developed dielectric encapsulant cap layer, which after adequate thermal treatments, leads to a reproducible local alloy disorder in several III–V quantum well (QW) structures. Data are presented to demonstrate its universality, namely, its ability to promote alloy‐disorder free of charge carriers either on group III or group V sublattice depending on the type of the QW structure. ©1995 American Institute of Physics.