Observation of anti-bonding excited state in charging diagram of a few-electron double dot

We fabricated a unique hybrid vertical–lateral double dot device with two dots coupled in parallel to study the excitation energy spectra with the number of electrons in each dot and inter-dot level detuning as parameters. We measured the charging diagram for a finite bias voltage and observed the tunnel-coupled bonding state and anti-bonding state. The separation between the two states is a direct measure of the inter-dot tunnel coupling energy. We use this energy and other characteristic energies derived from the charging diagram to characterize the exchange interaction energy.