Low-dimensional resonant tunnelling and Coulomb blockade: a comparison of fabricated versus impurity confinement

Nanometre-scale fabrication techniques, combined with epitaxial resonant tunnelling structures, now routinely allow the study of quasi-0D confined electron systems. In addition to energy level separations that are tunable by the confining potentials, these systems can also exhibit Coulomb blockade. Surprisingly similar effects are also observed for conventional, unconfined resonant tunnelling devices. We have recently discovered that the turn-on characteristics of nearly all resonant tunnelling devices exhibit sharp peaks in conductance, attributable to tunnelling through single quantum well donor states. These unintentional donor states are distributed in energy, depending on position in the quantum well. We have performed electronic spectroscopy of these states, and consistently find binding energies approximately 10 meV greater than expected for a single quantum well donor due to quantum well fluctuations. We present detailed measurements of single-electron tunnelling through a single donor bound state utilizing simple non-confined heterostructures.