Theory and simulation of electronic and optical properties of charged defects in two-dimensional semiconductors

Charged defects are frequently observed in quasi two-dimensional (2D) semiconductors such as monolayer transition-metal dichalcogenides (TMDCs) and few-layer black phosphorus (BP), and can have a significant effect on the performance of such materials in transport and optical applications. To aid the development of nanoscale devices fabricated using 2D materials, a detailed understanding of charged defects in these systems is needed. In this thesis, isolated charged defects are modelled in 2D semiconductors using the tight-binding approach which allows the use of large supercells which are required to capture the bound states that arise. The screened defect potential is described through an external potential determined via first-principles random-phase approximation. The binding energies and wavefunctions of bound states induced by charged defects in monolayer forms of MoS$_2$, WS$_2$, and BP as well as bilayer BP, were studied and the effect of varying the defect charge, defect height and the dielectric constant of possible substrates was investigated. It was found that for shallow defects in TMDCs, the binding energies of bound states can be mostly understood using effective mass theory. However, the presence of multiple low-energy valleys in the conduction and valence bands with different effective masses results in an interplay and competition between defect levels originating from different valleys. This results in resonant states that hybridise with the continuum bands, which has implications for transport, and results in the optical conductivity exhibiting excitonic resonances at energies below the optical band gap. Applying a similar analysis to monolayer BP, it was found that the substrate dielectric constant presents a promising means of control over both the donor and acceptor binding energies. In both monolayer and bilayer BP, it was found that the large difference between the armchair and zigzag effective masses of holes and electrons results in highly anisotropic wavefunctions of the defect states.Open Acces