Exciton Spectra and Energy Transfer in CdTe/ZnTe Double Quantum Wells Grown by Atomic-Layer Epitaxy

The photoluminescence and excitation of luminescence spectra of a series of samples containing two CdTe layers D1 and D2 with a nominal thickness of 1.5 and 4 monolayers in a ZnTe matrix were studied. The samples differ in the width of the ZnTe spacer separating the D1 and D2 inserts and constituting 15, 25, 35, 45 and 55 monolayers (samples nos. 1–5, respectively). ZnTe layers are grown on a GaAs substrate by the standard molecular beam epitaxy mode, while CdTe inserts are grown in atomic layer epitaxy mode. It is shown that when the barrier thickness is less than 25 monolayers electronic states in the quantum wells formed by D1 and D2 layers are tunnel-coupled, and only one band is recorded in the emission spectrum, caused by the recombination of excitons in the deep quantum well. At larger thicknesses of the spacer, two bands I₁ and I₂ are observed in the luminescence spectrum, which are associated with the recombination of excitons in shallow (D1) and deep (D2) wells, respectively. It was found that the intensity ratio I₁/I₂ significantly depends on the energy and intensity of the excitation. Possible origin of these dependences are discussed.