Abstract
The Zeeman splittings of a Si shallow donor in AlAs and of a two-dimensional electron gas (2DEG) 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 gI=+1.96±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.