Optoelectronic devices based on atomically thin semiconductors and photo-oxidised HfOx

Abstract

The direction of research in solid state physics and technology has changed since the discovery of graphene. Now, a plethora of two-dimensional materials are being thoroughly investigated for their unique properties as well as for their implementation in next-generation optoelectronic devices. Of course, much effort is needed in order to reach the current level of modern electronics which is based on decades of research and development. For example, the level of miniaturisation modern technology requires can be achieved with atomically thin materials, driving Moore's Law forward. Conventional dielectrics exhibit high leakage currents when their dimensions are reduced to the nano-scale and the need for alternative materials compatible with two-dimensional electronics arises. However, the techniques that are being used for the growth and processing of conventional semiconducting materials are not always suitable with two-dimensional materials, which need special handling. These are some of the points that will be addressed in this PhD dissertation.

Here, a new method for generating a fundamentally two-dimensional high-k dielectric which can be automatically incorporated in atomically thin optoelectronics devices is presented. The photo-oxidation of hafnium disulfide, HfS2, is a straight-forward, non-invasive process that can be used to oxidise pristine few-layered HfS₂, opening new paths for applications ranging from optoelectronics to photonics. The resulting dielectric, Hafnium dioxide, HfO₂, exhibits outstanding properties that exceed those of silicon dioxide, SiO₂ and its atomically thin nature makes it an ideal insulating layer for next-generation nano-electronics.

Finally, the last part of this thesis is dedicated to a novel, CVD-grown, n-type monolayer of tungsten diselenide, WSe2. This is the first time negatively doped CVD-grown WSe₂ is reported, which opens the possibility of choosing the doping of the two-dimensional semiconductor before fabrication. For investigating and characterising this novel material, field-effect transistors are fabricated and characterised optoelectronically, shining light on the carriers' behaviour and the ability of the material in light-detection applications. Vacuum and ambient annealing of the WSe2 based devices highlights a possible way to control the doping level of the material, and thus the electrical behaviour of the devices.