What is the origin of very strong photoluminescence in ZnSe/BeTe superlattices at liquid helium temperature?

The spatially indirect photoluminescence (PL) has been investigated in ZnSe/BeTe superlattices at various temperatures and exciting light intensities. At the temperature region higher than View the MathML source the PL spectrum showed two peak structures; one is attributed to localized states and another has free excitonic features. At liquid helium temperature the latter component became dominant and very strong PL which is irrelevant to a built-in electric field normal to the hetero-interfaces is observed. As possible origins of the anomalous PL, exciton complex or excitonic condensation states are proposed.