RESONANT TUNNELING THROUGH THE BOUND-STATES OF A SINGLE DONOR ATOM IN A QUANTUM-WELL

We have observed a series of sharp peaks in the low-temperature I(V) characteristics of a gated 1-mu-m x 1-mu-m GaAs/(AlGa)As resonant tunneling diode, in which the gate is used to reduce the effective cross-sectional area from 0.7 to < 0.1-mu-m2. These peaks, which occur at voltages well below the calculated resonant threshold, show a weak dependence on temperature, magnetic field, and cross-sectional area. We argue that this subthreshold structure is due to an inhomogeneity which gives rise to a localized preferential current path, and we deduce that the spatial extent of the inhomogeneity is approximately 25 nm. The likely origin of the inhomogeneity is a donor impurity in the quantum well.