TUNNELING SPECTROSCOPY OF LOW-DIMENSIONAL STATES

In this review a survey of tunnelling processes between barrier-separa ted two-dimensional (2D) systems and systems of different dimensionali ty is given. Tunnelling between barrier-separated 2D systems can be st udied on very different samples such as triple-barrier structures, dou ble-barrier structures with a two-dimensional emitter, double-barrier structures under hydrostatic pressure, double heterostructures, couple d quantum wells and also coupled 2D electron-hole systems. Pure 2D-2D tunnelling processes with individual contacts on both 2D systems, howe ver, are only reported on double heterostructures and coupled quantum wells. Using a transfer Hamiltonian formalism, it is shown that all re sonances in the tunnelling current have their origin in density of sta tes effects, transmission coefficients or the overlap integrals betwee n the initial and final states. 2D subband energies, background impuri ty concentrations, the effective mass and also non-parabolicity effect s can be determined quantitatively in terms of the transfer Hamiltonia n formalism. By nanofabrication, tunnelling processes between 2D syste ms and states of lower dimensionality (1D, 0D) can also be investigate d. Here, the tunnelling processes are mainly influenced by the overlap integral between the initial and final states. The corresponding reso nance positions in the tunnelling current strongly depend on the shape of the confining potential and, moreover, the current-voltage charact eristics turn out to be the Fourier transform of the 1D (0D) wavefunct ion of the final state. A brief survey of 1D-1D and 1D-0D tunnelling e xperiments is also given.