Tunneling via impurity states related to the X valley in a thin AlAs barrier

Abstract

Special features corresponding to resonance tunneling of electrons from the Γ valley of GaAs to the X valley of AlAs were observed in the current-voltage characteristics of single-barrier GaAs/AlAs/GaAs heterostructures. Tunneling both via the states related to the two-dimensional X xy and X z subbands and via the related impurity states was detected. It is shown that the energy position of such impurity states is largely controlled by two factors: (i) spatial confinement of the AlAs layer, which influences both the size-quantization energy levels of the X xy and X z subbands and the corresponding binding energies of impurity states, and (ii) the biaxial compression of the AlAs layer due to a mismatch of the AlAs and GaAs lattice parameters, which results in the splitting of the X xy and X z valleys. This made it possible to determine directly the binding energy of the impurity states; this energy was found to be ∼50 meV for the X z valley and ∼70 meV for the X xy valley.