Spectroscopy of the local density-of-states in nanowires using integrated quantum dot

In a quantum dot (QD) an energy eigenstate can be shifted in energy with the aid of a gate voltage. It, hence, can serve as an ideal tunable spectrometer that measure the local density-of-states (DOS) as a function of energy of the electron systems to which it is attached. Semiconducting nanowires (NWs) have become popular quasi-one-dimensional (1d) systems in, for example, studies of the emergence of Majorana-like features in NWs proximitized by a superconductor (SC). Here we report on spectroscopic measurements using a QD integrated directly into the NW during the epitaxial growth as an energetically and spatially well-defined tunnel probe to perform dI/dV spectroscopy of discrete bound states in the NW lead segments. We will focus here on InAs NWs not covered by a SC with InP tunnel barriers and pure InAs lead segments that connect the QD to metallic source and drain contacts. Due to the 1d nature of the lead segments and the non-ideal coupling of the NW to source and drain, strong DOS features appear in the lead segments themselves. These are usually difficult to distinguish from the DOS features of the QD itself. By tuning a side-gate in close proximity of one of the lead segments, we can distinguish transport features related to the modulation in the lead DOS and to excited states in the QD. We implement a non-interacting capacitance model and derive expressions for the slopes of QD and lead resonances that appear in two-dimensional spectroscopy plots of dI/dV as a function of source-drain bias and gate voltage.