Two different types of ‘natural’ quantum dots in narrow GaAs/AlGaAs QWs

We present the results of magneto-photoluminescence studies of ‘natural’ quantum dots (QDs) formed at the interfaces of narrow GaAs/AlGaAs quantum wells. The full spin–split quartet of excitonic states is observed over a wide range of magnetic field magnitude, B, and orientation θ. Using a novel microphotoluminescence technique based on the excitation and collection through a standard single-mode fibre, the excitonic fine structure was investigated with unprecedented accuracy. Two distinctly different types of QDs were found. The most common type shows a diamagnetic shift View the MathML source) accompanied by a small or vanishing zero-field splitting View the MathML source and an angle dependence of the diamagnetic shift, resulting from a larger lateral extent in the plane of the quantum well (QW) than perpendicular to it, as expected for ‘natural’ QDs. Additional to this type, a second less common type of QD was identified in the View the MathML source QW. This novel type exhibits a new form of excitonic confinement, resulting in a very small diamagnetic shift (View the MathML source) accompanied by a large zero-field splitting View the MathML source and an angle dependence of the diamagnetic shift typical for QDs with a smaller wavefunction extent in the plane of the QW than perpendicular to it. These highly-localised states with their enhanced exchange interaction and narrow linewidth allow the direct observation of the anticrossing between the lower J=±1 and upper J=±2 state. We argue that these states may be localised in strips of GaAs wider than the nearby well, which arise from a step in the growing interface being replicated at the second interface but with a small lateral displacement.