Resonant tunnelling via states of the X-related donors located at different atomic layer in AlAs barrier

Magnetotransport in GaAs/AlAs/GaAs single-barrier heterostructures incorporating unintentional donors in the barrier is studied. Resonant tunnelling was observed both through the quasiconfined states in the AlAs layer which originated from the XXY and XZ conduction band minima and through two distinct states of the donors bound to the XXY and XZ valleys. Furthermore, we observed an additional oscillatory fine structure of the donor resonances which we attribute to the difference in binding energies of donors located at different position of the AlAs layer. Magnetic field behaviour of the fine structure shows that the binding energy of X-related donors depends on both magnetic field and donor position in the barrier.