7 M ay 1 99 7 Lateral superlattices as voltage - controlled traps for excitons

We demonstrate the localisation of quantum well excitons in a periodic array of linear traps using photoluminescence experiments. The excitonic traps are induced by applying spatially alternating external voltages via interdigitated metal gates. The localisation originates from the periodical modulation of the strength of the quantum-confined Stark effect in the plane of the quantum well. In our experiments , the trap depth is easily tuned by the voltages applied to the interdigitated gates. Furthermore, we find that a perpendicular magnetic field reduces the exciton diffusion length. In short-period lateral superlattices, we observe a magnetic-field-induced stabilisation of excitons in the presence of strong in-plane electric fields. 1 Introduction Electro-optic interactions involving excitons in semiconductor microstructures and microdevices are currently attracting much interest [1]. Such devices are usually laterally microstructured in the plane of quantum wells. Consequently, it is interesting to study quantum well excitons in laterally varying potentials and, in particular, the possibility of localising them in distinct quasi-one-or quasi-zero-dimensional regions of a sample. Whereas the localisation of charged particles, like electrons or holes, is easily possible in electrostatic lateral potential superlattices [2], the localisation of neutral, but polarisable, excitons needs to make use of a more special mechanism. The spatial localisation of excitons in one-and zero-dimensional structures has been realised in semiconductors by various approaches [3, 4, 5]. One mechanism to induce exciton localisation is based on strain [3]. Other possibilities include inter-diffusion of a barrier material [4] and the preparation of self-organised lateral structures [5]. In all these cases, the exciton localisation cannot be changed by externally tuneable parameters. In the present study, we realise a voltage-tuneable localisation for excitons within the plane of a quantum well. Exciton transport in semiconductors can be induced by spatially non-uniform electric fields via the Stark effect. The change of the exciton energy by the electric field is −dE, where d is the exciton dipole moment induced by the electric field E. The energy −dE plays the role of the effective exciton potential, which will be denoted in the following as U ef f. If the electric field is dependent on the spatial coordinate , the potential U ef f can form excitonic traps. In principle, the trap potential U ef f can be controlled by external voltages. At low temperatures this mechanism of exciton localisation is effective if the localisation energy is significantly larger than the spatially random fluctuations in the exciton …