Resonant tunnelling across double barrier structures

Three aspects of the resonant transmission of electrons across double barrier structures are described in this thesis. Chapter 3 describes a series of measurements of the d2I(V)/dV2 of a wide well double barrier structure taken with a magnetic field orientated at a tilt angle with respect to the direction of current flow through the sample. The measurements are explained using a semiclassical model, making use of the relationships that exist between the properties of the quantum well of the sample and the properties of its predominantly chaotic classical counterpart. The success of the explanation therefore provides experimental validation of the relationships. Chapter 4 describes the observation of a new conductance fluctuation phenomenon. Henini et al [4.01] have shown that the incorporation of a layer of Si donor impurities into the quantum well of a double barrier structure leads to a feature in a sample's G(V) at low bias, below the first continuum resonance. They have labelled this feature the single donor resonance (SDR). I observe conductance fluctuations within the SDR at low temperatures when a large magnetic field is applied to the sample perpendicular to the direction of current flow, BperpendicularJ, and show that these conductance fluctuations can be attributed to statistical variations of the rate at which isolated donor ground states are brought onto resonance with increasing bias. Their properties are partially consistent with the predictions of a theory developed by Larkin and Matveev to describe the fluctuations of the conductance of an idealised single barrier device in the voltage regime of impurity assisted tunnelling. Chapter 5 describes the observation of resonant structure within the subthreshold I(V) of a double barrier structure. I start the chapter with an explanation of the resonant structure's origin. Each resonant feature is due to the transmission of electrons through a localised state embedded in the well of the double barrier structure. Each localised state is associated with a cluster of donors. Fromhold et al have developed a theory to describe the tunnelling of electrons through a localised state embedded in the well of a double barrier structure. I continue the chapter with comparisons between the observed and predicted shape of a resonant feature at zero magnetic field and the observed and predicted dependence of a resonant feature upon magnetic fields BparallelJ and BperpendicularJ, and attempt to explain the origin of the inconsistencies encountered. (author)