Probing transient and steady-state electronic response in topological and 2D materials

• Main Author: Liu, Jiayun
• Other Authors: Chia Ee Min, Elbert
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Physics
• Online Access: https://hdl.handle.net/10356/157876
• Institution: Nanyang Technological University

Over the past half a century, technological advancement in electronics has progressed at breakneck speed. It has brought about powerful microprocessors which are ubiquitous in modern electronics. This advancement fueled by our ever-increasing demand for faster electronics gave rise to the doubling of computation power in these microprocessors every few years. This progress however cannot carry on forever as the transistors are approaching the quantum limit where problems like leakage current and heat dissipation are limiting the performance of microprocessors. Therefore, there is a search for alternative technologies for the next generation of electronics. Spin-based devices or spintronics have been proposed and are thought to be a good alternative to conventional Si-based electronics. Spintronic devices use the spin property of electrons to transmit and process data. The main advantages of spintronic devices are the fast spin state switching and lossless transfer of spin information which could potentially increase the processing speed as well as reducing power consumption. However, the development of spintronic devices is lackluster due to the lack of materials suitable for practical implementation. With the advent of novel materials like topological and 2D materials with unique spin properties, there is increasing interest in using these materials for spintronics applications. In my work, I studied the different spin related properties in these novel materials to search for suitable materials for spintronics applications. One of the most important metrics to evaluate spintronics devices is the spin Hall angle. It has recently been demonstrated that Tantalum Arsenide (TaAs) possesses giant spin Hall angle coupled with the large photocurrent response that was previously reported, this makes TaAs a suitable material for spintronics application. However, the transient and the large photocurrent response of TaAs is not well understood. In this thesis, the transient response of TaAs is studied with transient absorption spectroscopy and a microscopic origin for the large photocurrent observed in TaAs is proposed. Transition from conventional electronics to next generation spintronics is non-trivial considering the fundamental differences between the technologies. A reasonable intermediate step would be to integrate spintronic devices with conventional silicon-based electronics. The spin injection efficiency into silicon is explored with THz emission measurement. The measured spin injection efficiency is comparable to that of platinum, which is widely considered to have high efficiency. Transition metal dichalcogenide (TMD) has band structure that give rise to ‘valleys’ in the k-space. These valleys are coupled to the spins of the carriers through spin-valley coupling which allows for easy manipulation of spins in TMDs. The photocurrent properties of multi-layer MoS2 is studied with scanning photocurrent microscopy. The observed photocurrent has a geometrical dependence which is due to stress-induced shift of the valleys and the built-in field.