Optoelectronics of two dimensional transition metal dichalcogenides

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Abstract

Two dimensional transition metal dichalcogenides provide a host of unique optoelectronic properties, attributed to their two dimensional nature and unique band structure, making them promising for future optoelectronics device applications. In the work presented in this thesis, we focus on the theoretical understanding and modelling of the optoelectronic properties of monolayer transition metal dichalcogenides, their heterostructures and multilayers. We studied the relaxation rates of photo-excited carriers leading to the formation of electron-hole pairs and their subsequent radiative recombination, resulting in emission of light. We find sub-ps relaxation times, attributed to the strong coupling of carriers with optical phonons, allowing the efficient formation of strongly bound multi-particle complexes such as excitons, trions and biexcitons, which can recombine radiatively if allowed by selection rules. We classify the various complexes according to their optical activity, and predict using diffusion quantum Monte Carlo calculations the resulting photoluminescence spectra in these materials. We proposed a novel, material specific, Auger process in WS2 and WSe2 involving dark excitons, which dominates over radiative processes for relatively low carrier densities, providing an explanation to the observed low quantum efficiencies in these materials. In the same pair of materials, we have shown how the ground state dark trions and biexcitons can become bright and recombine radiatively through an electron-electron intervalley scattering process, resulting in new observable lines in the photoluminescence spectra of these materials. The ability to form van der Waals heterostructures of two or more layers of these materials, allows for new degrees of freedom to be explored and utilised. The heterobilayer system made of MoSe2/WSe2 has a type-II band alignment, allowing for the formation of interlayer bound complexes with carriers localized on opposite layers. We studied the bound complexes formed in this bilayer system, localized on donor impurities. We used quantum Monte Carlo methods to obtain binding energies and wave functions, and calculated the radiative rates and doping dependent photoluminescence spectra of these complexes for closely aligned layers, and asymptotic behaviour for strongly misaligned layers. Finally, we studied few-layers of 2H-stacked transition metal dichalcogenides. The van der Waals quantum well structure results in the splitting of the conduction and valence bands into multiple subbands with energy spacings covering densely the infrared to far-infrared spectral range. We developed a hybrid k.p-tight binding model parameterised by DFT calculations of monolayer and bulk crystals of the studied materials. We used the model to describe the subband dispersions, transition energies, phonon induced broadening and resulting absorption lineshapes for both p-doped and n-doped few-layer films.

Keyword(s)

2D materials; Optoelectronics; Transition metal dichalcogenides

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