Resonant magneto-tunnelling in GaAs/AlGaAs double-barrier heterostructures

Resonant magneto-tunnelling in GaAs/AlGaAs double-barrier heterostructures is studied at low temperatures. Magnetic fields aligned parallel to the barriers are used to map out the in-plane dispersion of the Γ-conduction band in GaAs/AlGaAs quantum wells. By rotating the magnetic field direction in the plane of the quantum well, it is demonstrated that the effective mass of confined Γ-conduction band electrons becomes anisotropic along the two orthogonal in-plane <110> directions when an electric field is applied perpendicular to the interfaces. On a qualitative level, the anisotropy can be understood by the orthogonal <110> orientations of bond-planes at opposite interfaces of the quantum well, whereby the symmetry is broken either by an electric field along [001] or by differences in interface roughness. On the quantitative level, a quantum mechanical model involving interface band mixing is presented to explain the experimental results. Tunnelling between a three-dimensional emitter contact and a two-dimensional electron system is studied in magnetic fields aligned perpendicular to the barriers of a double-barrier heterostructure. The differential conductance around the Fermi energy exhibits a pseudogap due to magnetic-field-induced correlations within the two-dimensional electron system. The pseudogap is shown to be thermally-activated and to depend on the two-dimensional electron density. Further, it is discussed how slow in-plane spin relaxation can create a bottleneck for tunnelling of electrons and how this could account for the observed enhancement of the pseudogap around filling factor v = 1. (author)