Bimodal counting statistics in single-electron tunneling through a quantum dot

We explore the full counting statistics of single-electron tunneling through a quantum dot using a quantum point contact as noninvasive high bandwidth charge detector. The distribution of counted tunneling events is measured as a function of gate and source-drain voltages for several consecutive electron numbers on the quantum dot. For certain configurations, we observe super-Poissonian statistics for bias voltages at which excited states become accessible. The associated counting distributions interestingly show a bimodal characteristic. Analyzing the time dependence of the number of electron counts, we relate this to a slow switching between different electron configurations on the quantum dot.