UC Berkeley

Two dimensional (2D) materials have gained much attention these years for their intrinsic physical properties and promising applications in the field of electronics, optics and mechanics. Those materials, especially the 2D transition metal dichalcogenides (TMD), are regarded as a potential replacement plan for silicon in the next generation semiconductor technology. The combination of 2D semiconductors and epitaxial grown single crystalline ferroelectrics enables the development of next generation emerging ferroelectric memory devices, and supplies a polarization controlled system to explore the fancy physical phenomena in the 2D crystals under high carrier density doping at the same time.

This thesis talks about the researches on two dimensional transistors with non-volatile functionalities originating from different mechanisms. Chapter 1 briefly introduced the research topics as well as motivations. Chapter 2 and 3 discussed details of two main components in the research: lead zirconate titanate (PZT) and molybdenum disulfide (MoS2) respectively. The MoS2 flakes were integrated with single crystalline PZT films directly and trap related hysteresis loops were observed in chapter 4. In chapter 5, methods to minimize the effects of interface states in the 2D ferroelectric structure were carried out. Correct handedness hysteresis loops were detected, with large loop size (>20V) and high ON/OFF ratio (>106), exhibiting the promising future of 2D ferroelectric devices. The switchable metal-insulator phase transitions of MoS2 controlled by ferroelectric polarization were also detected. In chapter 6, the clockwise hysteresis loops induced by interface states in the MoS2 2D dielectric transistor were investigated. Then a pulse gating technique with self-designed pulse sequences was developed, which effectively minimized the hysteresis loops induced by adsorbates and the intrinsic electrical properties of tested devices were eventually extracted.