Tunneling spectroscopy of quantum dots using submicrometer-diameter ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ As-GaAs triple-barrier diodes

We have studied resonant tunneling (RT) through ${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$ As-GaAs triple-barrier diodes with diameters between 0.2 and 10 μm. We have observed fine structures just above and below the RT threshold voltage ${\mathrm{V}}_{\mathrm{th}}$ in the current-voltage curve of a submicrometer diode at T=4 K. With a diameter of 0.4–0.8 μm, the magnetic-field dependence and the sample dependence of the structure unambiguously show that (1) fine structures above ${\mathrm{V}}_{\mathrm{th}}$ are caused by resonant tunneling (RT) through zero-dimensional (0D) states and coupled-0D states confined by the diode side-wall depletion region with a parabolic potential, and (2) fine structures below ${\mathrm{V}}_{\mathrm{th}}$ are caused by RT through shallow-donor-bound states. Energy spacings of side-wall confined states and shallow-donor-bound energy are estimated to be 6.5 and 15–20 meV, respectively, which are consistent with a simple model. The tunneling current through the side-wall confined state is found to be proportional to the degeneracy of the 0D level confined by the parabolic potential. With a diameter of 0.2–0.3 μm, fine structures superimposed on the steplike structure were observed. We consider that those interplay between the 0D tunneling and a Coulomb blockade. The radius of a quantum dot estimated from a Coulomb staircase of 14 nm is in reasonable agreement with the extent of electron wavefunction in a quantum dot of 12 nm.