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We have carried out a comparative study between the magnetic and non-magnetic resonant tunnelling diodes (RTD’s) made of p-type GaAs and insulating AlAs thin films. The devices were fabricated by using a metal-organic chemical vapour deposition (MOCVD) technique for the growth of the AlAs/GaAs/AlAs quantum wells and a molecular-beam epitaxy (MBE) for the growth of the p-type emitter layer. In the case of the magnetic RTD's the emitter was heavily doped by the magnetic Mn atoms during the MBE growth, whereas beryllium was used as a dopant in the non-magnetic RTD's. The measured I-V characteristics show the first three resonant peaks due the tunnelling of the holes from the p-type emitter to the three lowest quantized energy levels HH1, LH1, and HH2 in the AlAs/GaAs/AlAs quantum well both in magnetic and non-magnetic RTD's. However, a large magnetic field dependence of the tunnelling current in rather low fields (B < 1T) appears only in the magnetic RTD’s. The observed decrease of current in the resonant peaks due to applied magnetic field in the case of the heavily Mn doped metallic emitters is explained as a consequence of the exchange interaction between the hole spins and the localized magnetic moments of the Mn atoms. This interaction changes those electronic states of the valence band and the density of states, which contribute to the spin-dependent tunnelling current. This may be related to the tunnelling anisotropic magnetoresistance (MR) observed in the spin dependent tunnelling processes in magnetic semiconductors. The origin of the observed double peak structure in the resonant peaks at low temperatures is discussed in the case of the lightly doped semiconducting magnetic emitter. The previously proposed explanation related to the band splitting due to a spontaneous magnetization below the Curie temperature is challenged, since the same effect has been observed in the non-magnetic RTD’s. Also the contribution of the large negative MR of the Mn-doped emitter to the observed large shifts of the resonant peaks with decreasing temperature and increasing magnetic field is analysed in the case of the magnetic RTD’S with a lightly doped emitter, where the MR of the emitter becomes the dominant effect instead of the spin dependent tunnelling.
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