Trion and exciton dynamics in two dimensional semiconductors

Abstract

Two-dimensional semiconducting systems have become increasingly important for a variety of applications including photo-detectors, high-power transistors and optoelectronics. With the discovery of the indirect-to-direct bandgap transition in atomically thin transition metal dichalcogenide (TMDs’) materials, a plethora of further applications and advances await. Optical properties in these materials are especially interesting to measure, due to presence of spin-valley coupling giving rise to valleytronic applications, and enhanced light emission (and absorption) with applications in optoelectronics. Optical studies in semiconductors near the bandgap primarily relate to the fundamental optical excitation of semiconductors, an exciton (a Coulomb-bound electron-hole pair). If the Coulomb interaction is strong enough, excitons may capture an extra electron or hole, forming charged excitons known as trions. Trions have shown to carry longer-lived spin information, and can drift under an electric field. The interaction between excitons and trions, is thus a technologically important issue in optoelectronics. The purpose of this dissertation is to measure the interactions between excitons and trions in a variety of two-dimensional systems, primarily in the new class of semiconducting two-dimensional materials, TMDs’. The interactions are measured for their character (coherent or incoherent) and dynamics. Utilizing a two-color pump-probe setup we uncover coherent coupling, between excitons and trions in monolayer molybdenum diselenide, an order of magnitude larger than traditional semiconductors (like gallium arsenide). Incoherent relaxation pathways towards trions, are measured via resonant excitation of excitons. A mobility edge within the exciton resonance is uncovered, with applications in quantifying transport properties of materials under study. Further, valley sensitive measurements are carried out on monolayer tungsten diselenide, revealing the long-lived trion spin polarization and ultrafast exciton valley relaxation. The possible spectroscopy feature of biexcitons is discussed in monolayer tungsten diselenide. Finally, measurements are extended to high mobility gallium arsenide quantum well systems, and electron-density dependent spin scattering mechanisms are uncovered. We further discuss the possibility to suppress spin relaxation, via gate voltage, in these gallium arsenide quantum wells.