Advanced Optoelectronic Memory Devices Based on Two-Dimensional Nanomaterials

基於二維納米材料的先進光電存儲器件

Student thesis: Doctoral Thesis

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Award date23 Aug 2024

Abstract

In the rapidly evolving modern technological environment characterized by the information age, electronic devices such as smartphones, computers, smart watches, and smart speakers have become an indispensable part of our daily lives.

In this context, optoelectronic devices based on two-dimensional (2D) materials have emerged as a promising alternative. 2D materials possess exceptional physical properties for electronic devices, including ultra-thinness, high electron mobility, and strong stability. Additionally, their unique optoelectronic characteristics enable functions that are unattainable by traditional silicon-electronic devices, such as optical computing, optical communication, and optical memory. This thesis study aims to delve into the potential of 2D material-based optoelectronic memories and comprehensively explore their implications for future optoelectronic applications. It consists of four main studies which will be highlighted as follows.

Firstly, starting from the perspective of novel materials, we investigated the feasibility of using 2D materials as substitutes for traditional silicon-based materials in the fabrication of Flash memory transistors. Specifically, we employed tellurium (Te) nanoflakes, synthesized via the hydrothermal method, as the charge-trapping layer for the Flash memory. The MoS2 and h-BN nanosheets were used as the channel and insulating layer materials. This approach led to the successful realization of a high-performance memory device based on the MoS2/h-BN/Te heterostructure. This device exhibited exceptional performance metrics, including an ultra-high on/off ratio of approximately 108, an ultra-fast switching speed of around 100 ns, excellent endurance exceeding 4000 cycles, and robust retention stability surpassing 4000 s. This outstanding performance demonstrates the great potential of 2D heterostructures for memory technologies.

Secondly, from the perspective of device mechanisms, we explored the effects of the crystal phase structure of the 2D materials on the performance of memory devices when used as the charge-trapping layer for a non-volatile memory device. During the research process, we explored various performance indicators of the memory device when the metallic phase 1T′-MoTe2 and the semiconductor phase 2H-MoTe2 nanosheets were used as floating gate layers, respectively. Comprehensive measurements revealed that memory devices constructed from MoS2/h-BN/1T′-MoTe2 demonstrated superior performance compared to those fabricated from the MoS2/h-BN/2H-MoTe2 heterostructure. The advantages of the former included an expanded memory window, accelerated switching speeds of about 100 ns, and an elevated extinction ratio of over 107. Furthermore, memory devices based on metallic phase heterostructures demonstrated high durability and cyclic stability. These findings underscore the critical role of crystal phases in determining the performance of memory devices.

Thirdly, from the standpoint of fabrication technologies, we further explored the impact of defect engineering on the properties of 2D memory devices. We aimed to enhance the performance of the device by performing plasma treatment on the charge-trapping layer of Flash devices. We chose PdSe2, which has great potential for large-scale preparation, as the floating gate layer and prepared a MoS2/h-BN/PdSe2 heterostructure. Comparative experimental analysis showed that the device after plasma treatment has a higher storage capacity and a smaller subthreshold swing. Significantly, the enhanced device demonstrates non-volatile optical response characteristics. This inventive approach offers a crucial understanding for future investigations into the performance of non-volatile optoelectronic memory devices.

Finally, considering the simplicity and multifunctionality of memory devices, we sought to simplify the 2D material memory structure while enhancing its versatility. We fabricated a reconfigurable memory device (RMD) based on MoS2/CuInP2S6 heterostructure that integrates the defect engineering-enabled interlayer defects and the ferroelectric polarization in CuInP2S6 to realize a simplified structure device for all-in-one sensing, memory and computing. The plasma treatment of the CuInP2S6 nanosheet effectively increases the interlayer defect density which significantly enhances the charge-trapping ability in synergy with ferroelectric properties. The device not only serves as a non-volatile electronic memory device but also can be reconfigured into optoelectronic memory mode or synaptic mode after controlling the ferroelectric polarization states in CuInP2S6. In memory mode, light control enabled non-volatile memory behavior, while in synapse mode, light control functioned as volatile memory behavior. In the end, we explored different application scenarios based on these distinct device behaviors, paving a new path for the development of multifunctional and simplified memory devices.

In summary, I introduced innovative concepts and designed and fabricated memory devices using advanced 2D materials. I conducted detailed electrical and optical tests on these devices and performed in-depth data analysis in conjunction with semiconductor physics to better understand the device mechanisms. Additionally, I studied the impact of phase structures and defect engineering techniques on the performance of memory devices based on these novel 2D materials. I also endeavored to streamline the structure of these memory devices while exploring their potential for multifunctionality and reconfigurability. From design, fabrication, and testing to analysis, my work has laid a certain foundation for the development of memory device mechanisms, design, and fabrication, thereby providing insights for future advancements in the field of 2D memory devices.