Spectroscopic investigation of the electronic states in narrow coupled GaAs/AlAs quantum wells with indirect band structure

The optical properties of a specially designed GaAs/AlAs superlattice consisting of a periodic sequence of coupled quantum wells (CQWs) have been investigated by low- and high-excitation-intensity photoluminescence and photoluminescence excitation spectroscopy. The artificially layered semiconductor structure was grown by molecular-beam epitaxy and its structural parameters have been determined by x-ray diffraction. It exhibits the unique feature of symmetrically coupled quantum wells via the states and asymmetrically coupled quantum wells via the X states. The observed luminescence shows mainly type-II emission and a weak direct recombination characterized by a splitting of the emission band due to the symmetric-antisymmetric states in the coupled wells. At high photogenerated carrier density, the type-I recombination becomes dominant. The fine structure of the observed spectral features have been interpreted on the basis of a simple quantum-mechanical calculation for the band structure of the coupled-quantum-wells system, assuming two independent sets of coupled wells associated with the AlAs X-point and GaAs point states, and also by means of a tight-binding model.