Terahertz nanospectroscopy of plasmon polaritons for the evaluation of doping in quantum devices

TL;DR

This paper demonstrates the use of nanoscale terahertz plasmon polaritons to evaluate doping profiles and surface quality in quantum devices, providing a new diagnostic tool for nanoscale material characterization.

Contribution

It introduces a method using THz near-field hyperspectral measurements to quantitatively analyze doped surface layers in quantum devices.

Findings

Detection of polaritonic features indicating doped layers

Quantitative measurement of layer thickness and permittivity

Etching of doped layers improves device quality factor

Abstract

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.