Observation of tunneling phenomena and the charging effect through small constricted regions in semiconductors fabricated with a focused ion beam at 4.2 K

Small tunnel barriers were formed on ${\mathrm{Al}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs by focused-ion-beam implantation. The samples were then measured with dc current at 4.2 K, and the I-V curves revealed two distinct regions depending on applied bias: a tunneling region at small bias voltage and a region where thermal current over the barrier is dominant at large bias voltage. A dip in the dI/dV curve was observed in the tunneling region in the source-drain voltage (${\mathit{V}}_{\mathrm{SD}}$) range from -5 to 5 mV. In a different sample, periodic and reproducible staircaselike steps with a periodicity Δ${\mathit{V}}_{\mathrm{SD}}$ of 35 mV were observed. This phenomenon is related to the Coulomb staircase, which occurs when the capacitance of a quantum dot is very small.