LOW-DIMENSIONAL RESONANT-TUNNELING AND COULOMB-BLOCKADE - A COMPARISON OF FABRICATED VERSUS IMPURITY CONFINEMENT

Nanometre-scale fabrication techniques, combined with epitaxial resona nt tunnelling structures, now routinely allow the study of quasi-0D co nfined electron systems. In addition to energy level separations that are tunable by the confining potentials, these systems can also exhibi t Coulomb blockade. Surprisingly similar effects are also observed for conventional, unconfined resonant tunnelling devices. We have recentl y discovered that the turn-on characteristics of nearly all resonant t unnelling devices exhibit sharp peaks in conductance, attributable to tunnelling through single quantum well donor states. These unintention al donor states are distributed in energy, depending on position in th e quantum well. We have performed electronic spectroscopy of these sta tes, and consistently find binding energies approximately 10 meV great er than expected for a single quantum well donor due to quantum well f luctuations. We present detailed measurements of single-electron tunne lling through a single donor bound state utilizing simple non-confined heterostructures.