  Terahertz (THz) waves are a highly sensitive probe of free carrier
concentrations in semiconducting materials. However, most experiments operate
in the far-field, which precludes the observation of nanoscale features that
affect the material response. Here, we demonstrate the use of nanoscale THz
plasmon polaritons as an indicator of surface quality in prototypical quantum
devices properties. Using THz near-field hyperspectral measurements, we observe
polaritonic features in doped silicon near a metal-semiconductor interface. The
presence of the THz surface plasmon polariton indicates the existence of a thin
film doped layer on the device. Using a multilayer extraction procedure
utilising vector calibration, we quantitatively probe the doped surface layer
and determine its thickness and complex permittivity. The recovered multilayer
characteristics match the dielectric conditions necessary to support the THz
surface plasmon polariton. Applying these findings to superconducting
resonators, we show that etching of this doped layer leads to an increase of
the quality factor as determined by cryogenic measurements. This study
demonstrates that THz scattering-type scanning near-field optical microscopy
(s-SNOM) is a promising diagnostic tool for characterization of surface
dielectric properties of quantum devices.
