Van der Waals heterostructures with photo-oxidised high- κ dielectrics

Abstract

The emergence of atomically thin systems has underpinned significant discoveries in fundamental science and game changing innovation in novel technologies such as energy storage and data communication. In this thesis, different types of optoelectronic devices based on van der Waals (vdW) heterostructures are investigated. A high-dielectric (κ) material hafnium disulphide (HfS2) is embedded into these heterostructures and photo-oxidised into hafnium oxide (HfOx) by laser writing selectively and underneath the contacts. Moreover, HfOx as a gate dielectric for field-effect transistors (FET) instead of hexagonal boron nitride (h-BN) is also shown. A dielectric constant for hafnium oxide of ~15 is reported, which shows a novel way to introduce dielectrics in such complicated structures being compatible with two-dimensional 2D materials. Finally, the impact of the dielectric environment on monolayer tungsten diselenide (1L-WSe2) while been surrounded by different dielectric materials such as quartz, hexagonal boron nitride, indium selenide (In2Se3) and hafnium oxide, is demonstrated. The effect of the dielectric environment on the exciton binding energies and quasiparticle bandgap has been investigated by measuring the energy separation between the 1s and 2s states using transmission measurements. The exciton binding energy, as well as the electronic band gap, were found to decrease as the average dielectric constant increases. The largest reduction of band gap by ~300 meV is observed when WSe2 is encapsulated between HfOx compared with that of exposed WSe2 on quartz.