Two dimensional materials synthesis for electronic and optoelectronic applications

Aspiotis, Nikolaos (2019) Two dimensional materials synthesis for electronic and optoelectronic applications. University of Southampton, Doctoral Thesis, 194pp.

Record type: Thesis (Doctoral)

Abstract

Atomically thin materials offer unique optical, electronic and physical properties due to quantum connement effects. Graphene has been the material that has primed the extensive research interest in the field. The lack of an energy bandgap in graphene helped to expand the research of 2D materials beyond graphene, in search for application tailored properties. The strongest overall candidate for electronic applications has since been Transition Metal Dichalcogenides (TMDCs). The metal-chalcogen bonds are strong covalent bonds that form stacked layers together by weak Van der Waals forces and can hence be easily separated to form individual layers. The significance of this ability lies in the fact that although TMDCs have an indirect bandgap in their bulk form, they transition to a direct bandgap in single layer form. This property is important for optoelectronic applications as it results in an enhanced photoluminescence quantum yield. A monolayer of such a material offers very high effective mobility that would otherwise require three times thicker single crystal silicon layer to reach. Transistors made of TMDCs have also been shown to reach the thermal transport limit achieving a subthreshold swing of as low as 60 mV/dec and on/off ratios of 10⁸. Those attributes make TMDCs an ideal candidate for next generation electronic and optoelectronic applications potentially replacing current material technologies. Due to the weak Van der Waals forces between layers one of the first methods explored to obtain single layers of graphene and TMDCs has been exfoliation and transfer techniques involving tape, chemical or mechanical methods. Those techniques viii have been providing very high quality single crystal layers with excellent electronic and optoelectronic properties. A direct drawback of these methods is the lack of scalability. For this reason, there has been a collective research effort in the community towards the development of direct growth methods for TMDCs that are scalable and can be used in traditional top-down fabrication processes. Scalable techniques have recently included RF sputtering, CVD and ALD techniques that use solid, metal halide or organic precursors. Most of those studies rely on the transfer of the TMDC after it has been grown in order to form electronic devices such as field effect transistors. The main reason for this is that during the growth process the dielectric integrity of the underlying SiO₂, on which the films are commonly grown, is compromised. This work aims to tackle the scalability of 2D materials by devising methods directly applicable to wafer scale production. In particular, for TMDCs a combination of Atomic Layer deposition and Thermal reaction is used to form a few layer MoS₂ on a SiO₂ substrate without the need for transfer to perform as an FET device. Using ALD, a thin layer of MoO₃ is first formed on the SiO₂ and then annealed in a CVD reactor in presence of H₂S. As the wafers are already coated with MoO₃ during the high temperature anneal in H₂S the SiO₂ quality is preserved removing the need to transfer to a fresh substrate and therefore enabling the practical upscale of the technology. This thesis discusses the methods developed by the author for growing 2D films of graphene, MoS₂ and HfS₂. The results from the characterization of the films at a variety of growing conditions provide a comprehensive guide to optimizing the film growth for optoelectronic and electronic applications. Moreover new fabrication protocols have been designed in order to accommodate the fragile nature of 2D materials while making high performance devices. This work provides an array of devices as performance demonstrators such as FET, fiber modulator, mechanochromic metamaterial and graphene photodetector. The most significant achievement of this work is the design of the full fabrication protocol for high performance FET devices and the resulting performance of these devices. It was demonstrated that a subthreshold slope of under 180 mV/dec and an on/off ratio of more than 10⁴ can be achieved with directly grown transistors in a readily scalable process.