The large tunneling anisotropic magneto-resistance of a single p ++ -(Ga,Mn)As/n + -GaAs Zener-Esaki diode is evidenced in a perpendicular magnetic field over a large temperature and voltage range. Under an applied bias, the tunnel junction transparency is modified, allowing to continuously tune anisotropic transport properties between the tunneling and the ohmic regimes. Furthermore, an asymmetric bias-dependence of the anisotropic tunneling magneto-resistance is also observed: a reverse bias highlights the full (Ga,Mn)As valence band states contribution, whereas a forward bias only probes part of the density of states and reveals opposite contributions from two subbands.

We perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the I(V) characteristics measured at 30 mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS ͑LDOS͒ and Fermi edge singularities are observed.

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The Zeeman splittings of a Si shallow donor in AlAs and of a two-dimensional electron gas ͑2DEG͒ in GaAs are evidenced by resonant tunneling spectroscopy in submicrometer GaAs/AlAs/GaAs junctions. In magnetic field, the donor acts as a spin-sensitive probe of the spin-polarized density of states in the emitter. In the current-voltage characteristic the two splittings are resolved, which allows us to estimate the Landé g factors for the impurity g I = + 1.96Ϯ 0.16 and for the 2DEG. Because of spin conservation in the tunneling between the 2DEG and the donor, the relative sign of the two g factors can be determined.

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