Fermi level effects on Mn incorporation in modulation-doped ferromagnetic III_1−xMn_xV heterostructures

The effect of increasing the Fermi level by modulation doping on the incorporation of Mn into the III-V host lattice-and hence on the ferromagnetic properties of III1-xMnx V alloys-is investigated in Ga1-xMnxAs/Ga1-yAlyAs heterojunctions and quantum wells. Introducing Be acceptors into the Ga1-yAlyAs barriers leads to an increase of the Curie temperature T-C of Ga1-xMnxAs, from 70 K in undoped structures to over 100 K in modulation-doped structures. This increase is qualitatively consistent with multi-band mean field theory simulation of carrier-mediated ferromagnetism. Here the crucial feature is that the increase of T-C occurs only in those structures where the modulation doping is introduced after the deposition of the magnetic layer, and that T-C actually drops when the Be-doped layer is grown first. Using ion channelling techniques we provide direct evidence that this latter reduction in T-C is directly correlated with an increased formation of magnetically inactive Mn interstitials. This formation of interstitials is induced by the shift of the Fermi energy as the holes are transferred from the barrier to the quantum well during the growth.