Ferromagnetic ordering in doped by Mn GaAs quantum wells supported by the upper Hubbard band of Mn centers

We have studied experimentally ferromagnetic properties of GaAs-AlGaAs quantum wells where the center of GaAs well was doped by Mn (x<0.1%) while the neighboring barrier was doped by shallow acceptor Be with an aim to occupy (at least, partially) the states of the upper Hubbard band of Mn acceptors in the well (AMn+). It was found that while the measurements of the anomalous Hall effect unambiguously demonstrated a presence of ferromagnetic ordering up to rather high temperatures (of 250 K), the conductivity demonstrated activated behavior with high activation temperatures (of the order of 100 meV). In contrast to our previous studies of the structures with undoped barriers, the Curie temperatures were significantly higher (150 K instead of 100 K), but conductivities of barrier-doped structures were order of magnitudes lower. We suggest a theoretical model explaining this behavior. It takes into account, first, the electrostatic potential of charged acceptors in the barrier (Be-) forming a potential bend in the near-barrier region which accepts some of the holes supplied by the barrier acceptors. Second, we take into account the fact that at least part of the supplied holes occupies the upper Hubbard band mentioned above. The latter fact can explain observed increase in the Curie temperature with respect to the structures with undoped barriers. The suppression of conductivity is explained by a presence of long-range potential produced by the randomly distributed charged centers within the barrier. Thus we deals with droplets of delocalized holes within the potential bend and partially occupied upper Hubbard band which, with a help of the indirect exchange, form the islands of ferromagnetically ordered Mn ions. The detailed discussion of further experimental studies necessary to confirm our model is also presented.