Resonant tunneling via donor X states in the AlAs barrier and binding energies of donors bound to X-XY and X-Z valleys

Magnetotransport in GaAs/AlAs/GaAs single-barrier heterostructures, incorporating unintentional donors in the barrier, is studied. Resonant tunneling is observed through the quasiconfined states in the AlAs layer which originate from the X-XY and X-Z conduction-band minima and through two distinct states of the donors bound to the X-XY and X-Z valleys. This allowes us to determine directly the binding energies of X-XY- and X-Z-related donors at the center of a 5-nm AlAs barrier as E-B(X-XY)approximate to70 meV and E-B(X-Z)approximate to50 meV, respectively. Furthermore, we observe an additional oscillatory fine structure of the donor resonances which we attribute to a difference in the binding energies of donors located at different position in the AlAs layer.