Light-tunable two-dimensional electronic devices

• Main Author: Zhu, Chao
• Other Authors: Liu Zheng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136721
• Institution: Nanyang Technological University

Two-dimensional materials have drawn great attentions due to their atomic thin nature and enriched excellent physical and chemical properties. With their high mobility, suppressed short channel effect, enhanced gate control, additional valley degree of freedom and the ability to form various van der Waals heterostructures, 2D materials are believed to be promising candidates for the next generation electronics and optoelectronics. A great variety of 2D material based electronic and optoelectronic devices have been fabricated, including the field effect transistors, photo detectors, solar cells, sensors, nonvolatile memories, and other novel functional devices. By now, a lot of efforts have been put on the control of electronic properties of 2D materials and 2D electronic devices. However, most studies have focused on the electric field control by the gate. Other control methods are insufficiently studied. Achieving various control of 2D electronics and optoelectronics is important, especially for cases where a gate control is inaccessible. Light as a powerful tool of localized excitation and heating, can achieve the tailoring and modifications of 2D materials with arbitrary patterns. On the other hand, due to the increasing demand of huge computation, the optical computing and optical-electronic hybrid computing become potential alternatives to the miniaturization of electronic transistors. Therefore, studying the light control of 2D electronic devices is fundamentally important and necessary for the wide applications of 2D materials in the future electronics, optoelectronics, and novel functional devices. To achieve the light control of electronic properties of 2D materials and 2D electronic devices, the thesis demonstrates the wide availability of light control on 2D materials and 2D electronics, where three systems are considered, including the metallic 1T-TaS2, the semiconducting 2H-WSe2, and the semiconductor/insulator heterostructure of narrow-gap BP and wide-gap SrTiO3. Specifically, the light tunability is demonstrated by: the light-tunable CDW phase transition in 1T-TaS2 and CDW oscillators, the light-ablation-induced controlled hole doping in 2H-WSe2 and the direct-light-patterned WSe2 logic circuits, and the wavelength-dependent persistent conductivity switching in BP/SrTiO3 heterostructure and the light-tunable multistage nonvolatile memory. Firstly, the light control of bias-induced CDW phase transition in 1T-TaS2 is demonstrated. The direct evidence from in-situ characterization of the bias-induced NCCDW-ICCDW phase transition was provided by the in-situ Raman measurement. The role of Joule heating in the bias-induced CDW transition was evaluated. Inspired by the Joule heating effect, the light heating was used to tune the bias-induced CDW phase transition. And based on this light-tunable phase transition, a CDW oscillator was demonstrated, where the oscillation frequency could be continuously tuned by the light intensity. Secondly, the laser-induced controlled hole doping in 2H-WSe2 is presented. Controllable light-ablation-induced hole doping of 2H-WSe2 was achieved. Various types of characterization were performed to understand the underlying mechanism. A further photocurrent mapping was performed on the laser patterned WSe2 P-N junction, indicating a complex junction profile could be achieved. A direct-light-patterned NOR gate circuit was demonstrated, showing comparable performances with other logic circuit fabrication methods. Thirdly, the light-induced wavelength-dependent persistent switching of conductivity in the BP/SrTiO3 heterostructure is demonstrated. Inspired by previous work of photodoping, BP devices on SrTiO3 were studied, showing an enhanced photoresponsivity as well as a persistent switching on the conductivity. Specifically, a visible light illumination could turn on the BP channel while a UV light illumination could turn off the BP channel. Wavelength-dependent photo switching behavior was studied under different temperatures to understand its mechanism. The light-tunable multistage nonvolatile memory was demonstrated in the BP/SrTiO3 device, which showed a very long retention time at low temperature.