Near-Field Scanning Microwave Microscopy of Low-Dimensional Materials for Nanoelectronics
The near-field scanning microwave microscope (NSMM) combines broadband compatibility with the nanometer-scale spatial resolution of scanning probe microscopy. The NSMM is particularly well-suited to non-invasive characterization of electromagnetic properties, including dielectric permittivity and conductivity, which manifest as changes in the tip-sample admittance. Here, we apply NSMM to characterize low-dimensional material systems, both in isolation and integrated into devices. In semiconducting nanowires, we demonstrate sensitivity to dopant polarity, concentration, and spatial distribution. We further extend this technique to unipolar, two-dimensional transition metal dichalcogenides, such as MoS2 and WSe2. Finally, we examine spatial variations in carrier type in nanocrystalline tellurene, a one-dimensional van der Waals material. In ambipolar tellurene FETs, we demonstrate that the global device carrier equivalence does not arise from uniform carrier neutrality, but rather from coexisting n- and p-type regions. NSMM enables us to visualize this coexistence by mapping the carrier equivalent backgate voltage throughout the sample.