Abstract:

Optical microscopy has played an instrumental role in 2-dimensional (2D) materials research. In particular, the phenomenon of thin-film interference of light has been leveraged to improve contrast and vertical resolution of 2D materials down to the sub-nanometer scale, often via Fabry-Pérot (FP) thin-film resonators.

In this thesis, interference reflection microscopy (IRM) and backside absorbing layer microscopy (BALM), both of which harbor FP effects, are developed and utilized to study visibility and topographic inhomogeneities of the 2D semiconductor  MoS₂. Experimental contrast data are compared against Fresnel-based simulations of contrast. For IRM, an optimal configuration was found by tuning of incident wavelength and top medium refractive index, yielding ≈ 80% contrast. For BALM, the optical properties were measured for both the anti-reflective absorbing layer of nanometric Cr/Au, and an additional insulating AlO_x layer, where for the first time the contrast spectrum for this system was acquired and simulated, yielding a maximum experimental contrast of ≈ 79% for 2D  MoS₂ .

Simulations of the optical stack across a variable range of aperture stop diameters and FP layer thicknesses predict further improvement of BALM conditions for high-contrast  MoS₂ visibility. Additional aspects including z-focus, optical noise, image post-processing, and others were also considered. Building on the visibility aspects, a charge density imaging capability for 2D  MoS2 and other transition metal dichalcogenide crystals was developed by leveraging the charge-dependent complex refractive index near the wavelengths of the excitons.

Capacitors and field-effect transistors (FET) of  MoS₂ were realized, with multiple in operando experiments performed in widefield at throughputs up to 4 fps. In IRM mode, a liquid electrolyte gate was used, where charging delays and inhomogeneities due to intra- and inter-flakes resistances in polycrystalline  MoS2 are presented. For Schottky barrier  MoS₂ FETs, the drain versus gate voltage competition for control of the local charge density in the channel was studied for the first time by optical microscopy. Solid-state  MoS₂ capacitor devices integrated in a BALM optical stack are also presented for the first time, both by experiments and simulations. A preliminary solid-state FET device was realized, exemplifying the powerful idea of combining optical charge imaging with electrical characterization in tandem.

This work on visibility and charge imaging aspects aims to widen the role and impact of optical microscopy techniques in the space of 2D materials research.