Investigation of Transport Behavior in Two-Dimensional Ferroelectric Heterostructures

This dissertation summarizes an investigation of the polarization-related electronic transport behavior in the ferroelectric thin films and two-dimensional (2D) materials heterostructures using Scanning Probe Microscopy (SPM) techniques. The polarization-related resistive switching in hafnium oxide thin films-based ferroelectric tunnel junction has been demonstrated by employing semiconducting MoS2 as a top electrode. We explored a coupling between the semiconducting properties of MoS2 and the polarization of Hf0.5Zr0.5O2 resulted in an enhanced tunneling electroresistance effect of up to 3 orders of magnitude. These results provide a possible pathway for the fabrication of high-density non-volatile memory devices. These results are presented in Chapter 3. Resistive switching control using conducting domain walls as functional elements has been investigated using graphene/LiNbO3 heterostructures. One approach involves the modulation of resistance through the manipulation of domain wall density using super-coercive voltage. This approach requires higher energy to switch the polarization and can induce high leakage current that makes it deleterious. To overcome this drawback, we have developed a new approach that involves tuning of domain wall conductivity by a sub-coercive voltage without altering the domain configuration. These results are presented in Chapter 4. Chapter 5 describes modulation of the transport behavior of 2D MoS2 junctions by mechanical stress induced by the sharp probe of atomic force microscope (AFM). We show that the junction resistance can be reversibly tuned by up to 4 orders of magnitude by altering the mechanical force applied via AFM tip. Additionally, we show that AFM tip generates strain gradient inducing flexoelectric effect that leads to an enhancement of photovoltaic effect. Finally, we have discovered stable room temperature ferroelectricity with out-of-plane polarization in trigonally distorted 1T”-MoS2. Here, the polarization switching has been realized by the mechanical load applied via AFM probe. The piezoelectric and the electrical properties of MoS2 flakes are probed. Moreover, we show that flipped flakes of 1T”-MoS2 samples consist of monolayers of randomly oriented polarization, showing the possibility of head-head or tail-tail configuration. These results are presented in Chapter 6. Advisor: Alexei Gruverma