Single-electron tunneling and Coulomb charging effects in aysmmetric double-barrier resonant-tunneling diodes

Resonant tunneling is studied in an ultrasmall asymmetric GaAs-${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As double-barrier diode at low temperatures. In reverse bias, spikelike current-voltage characteristics are observed and assigned to electrons tunneling from zero-dimensional (0D) states in the accumulation layer to 0D states in the well. The 0D-0D tunneling reflects the single-electron spectrum without Coulomb charging effects. In forward bias, steplike characteristics are observed and ascribed to tunneling from one-dimensional subbands in the emitter contacts through 0D states in the well, accompanied by Coulomb charging effects. Moderate magnetic fields (B≊4 T) parallel to the current improve the flatness of the steps.