Résumé
Resonant tunnelling via single impurities placed in a single barrier was experimentally studied. The typically observed structures in the current-voltage characteristics seem to be paired. Such a pair can be interpreted as ground and excited states of an impurity, related to X-xy and X-z valleys, which is in agreement with the results obtained for big mesas. However, it is not clear why X-xy states can be seen without any phonon participation.