Resonant tunnelling through zero dimensional quantum dots

When InAs is grown above GaAs or AlAs, a lattice mismatch between the two layers causes strain fields to develop. Above a critical thickness, these strain fields cause the InAs to form islands or dots. The dots are typically 10nm in diameter, and 4nm high. As the dimensions of the dots are below the typical electron wavelength, zero dimensional states are formed. In this thesis quantum dots are grown within an AlAs tunnel barrier. When a bias is applied between the top and bottom contacts of the device, resonant tunnelling is observed of electrons from the two dimensional electron gas formed in front of the AlAs tunnel barrier, through individual InAs quantum dots, despite the presence of several million dots within a typical sample. By measuring the tunnel current through a dot as a function of the applied bias, information can be gained as to the properties of the dot itself, and the electron gas from which tunnelling occurs. Conventional magneto-transport measurements are sensitive only to effects at the Fermi energy of the electron emitter gas. Due to the zero-dimensionality of the quantum dots, a single dot can be used as a spectroscopic probe of the occupied density of states of an electron gas at all energies between the Fermi energy and the subband edge, in a manner not available to any other measurement technique. Further, many body effects cause an enhancement of the tunnel current when the dot ground state is resonant with the Fermi energy of the two dimensional electron gas. The ground state of the quantum dot can also therefore be used to study many body processes at the Fermi level. (author)