Synthesis, Transfer, and Constructure of Two-Dimensional Materials for Semiconductor Devices

Abstract

Since 2004, two-dimensional (2D) materials have emerged as promising candidates for next-generation semiconductor devices. The unique properties of 2D materials make them highly attractive for applications in the semiconductor industry. However, several critical challenges still need to be addressed for the successful industrialization of 2D materials in the semiconductor market. The first major obstacle pertains to the large-scale, controllable synthesis of 2D materials with uniform quality. The synthesis of large-size 2D materials that can meet industrial standards is still challenging in the community. The second challenge involves technical issues associated with integrating 2D materials into conventional silicon-based semiconductor devices, such as the transfer technique of 2D materials and the contact issue between 2D materials and metal electrodes. In addition, despite the immense potential of 2D materials in flexible electronic devices owing to their exceptional surface-to-volume ratio and flexibility, their application potential is severely hindered by their inherent brittleness as atomic-thin materials. Therefore, in this thesis, our main focus lies on addressing the aforementioned challenges in order to facilitate the industrialization of 2D materials in the field of semiconductor technology. Specifically, we concentrate on investigating the chemical vapor deposition (CVD) synthesis techniques, exploring transfer and cleaning techniques for 2D materials, as well as constructing 3D structure of 2D materials, thereby expanding their potential applications in the semiconductor industry. Note that although chapters 2 and 5 are mainly focused on graphene, various 2D materials are discussed in chapter 3 (ice-aided transfer) and chapter 4 (ice-aided cleaning). Especially in chapter 3, although we choose transition metal dichalcogenides (TMDs) to demonstrate the transfer results, the techniques can be applied to many other materials, but that is not the aims of this thesis at this moment.

(1) CVD synthesis of 2D materials. The synthesis of 2D materials can be achieved using various methods, such as mechanical exfoliation, liquid-phase exfoliation, solvothermal method, and epitaxial growth. Among these methods, CVD which involves the deposition of precursor gases onto the substrate to grow atomically thin layers stands out as the most promising synthesis methods for large-scale synthesis of high-quality 2D materials. In this work, we studied the CVD synthesis of graphene, the most famous 2D material. By adjusting the growth parameter, including the growth temperature, growth time, and gas ratio, monolayer graphene with various flake sizes, nucleation density, and covered area can be obtained. This level of control on growth conditions enables the production of graphene samples customized to meet the specific requirements of various applications. In addition, seamless stitching of graphene domains can be achieved on the large single-domain copper, which paves the way to the synthesis of large-scale single crystal graphene.

(2) Ice-aided transfer and cleaning of 2D materials. After synthesis, 2D materials need to be transferred out from the original growth substrate to desired target substrate for further characterization and applications. In the conventional transfer method, which employs polymers as supporting layer, the polymer residues are difficult, if not impossible, to be totally removed, which can lead to the degradation of 2D materials quality and the performance of 2D material-based semiconductor devices. To address this issue, the ice-aided transfer was proposed, in which ice is the only material used in the transfer process. The atomic force microscopy studies showed the hexagonal 2D ice layer can be epitaxially formed between the substrates and 2D layers, enabling the firm attachment/facile detachment. In practice, the adhesion between various 2D materials and ice can be well controlled by temperature. Through such controlled adhesion of ice, this ice-aided transfer method can yield ultra-high quality and exceptional cleanliness in transferred 2D flakes and continuous 2D films, and applicable for a wide range of substrates. Furthermore, beyond transfer, ice can also be used for cleaning the surfaces of 2D materials at higher temperatures. These novel techniques could enable the unprecedented ultra-clean 2D materials surfaces and performance, and will contribute to the upcoming technological revolutions associated with the 2D materials.

(3) The large surface-to-volume ratio endows 2D materials with ultra-high flexibility and foldability, offering new opportunities for 3D structural design with 2D materials. In this work, a novel approach is developed for preparing the periodically ordered 3D graphene origami, by spontaneous buckling of originally flat graphene grown on the high index copper surfaces via chemical vapor deposition. The Cu surface steps and the thermal expansion misfit strain at the Cu/graphene interfaces upon cooling are critical in the origami generation and patterning. The origami shapes are controllable by the Cu growth surfaces and CVD conditions. More importantly, such periodic and stereotype graphene origami structures can be transferred from the Cu surface to other functional surfaces including soft and hard substrate, showing great promise in flexible and stretchable device applications.

Date of Award	1 Feb 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Thuc Hue LY (Supervisor)