Piezo-generated charge mapping revealed through Direct Piezoelectric Force Microscopy

While piezoelectrics and ferroelectrics are playing a key role in many everyday applications, there are still a number of open questions related to the physics of those materials. In order to foster the understanding of piezoelectrics and ferroelectric and pave the way to future applications, the nanoscale characterization of these materials is essential. In this light, we have developed a novel AFM based mode that obtains a direct quantitative analysis of the piezoelectric coefficient d33. This nanoscale tool is capable of detecting and reveal piezo-charge generation through the direct piezoelectric effect at the surface of the piezoelectric and ferroelectric materials. We report the first nanoscale images of the charge generated in a thick single crystal of Periodically Poled Lithium Niobate (PPLN) and a Bismuth Ferrite (BiFO3) thin film by applying a force and recording the current produced by the materials. The quantification of both d33 coefficients for PPLN and BFO are 13 +- 2 pC/N and 46 +- 7 pC/N respectively, in agreement with the values reported in the literature. This new mode can operate simultaneously with PFM mode providing a powerful tool for the electromechanical and piezo-charge generation characterization of ferroelectric and piezoelectric materials

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

Fabrication and nano-scale characterisation of ferroelectric thin films

PhD ThesisThis thesis focuses on the fabrication and characterisation of BaTiO3 thin films. One of the aims is to deposit amorphous BaTiO3 films on conductive thin films through sputtering at temperatures compatible with semiconductor manufacturing, followed by post deposition annealing to crystallise these films. However, rapid thermal processing (RTP) is known to create pinholes and cracks due to thermal mismatches between the electrode and insulator, causing degradation of the film quality. Initial focus was to develop thin film electrodes which can withstand process temperatures above 800 C. Deposition conditions, including the nitrogen flow rate relative to that of argon during deposition were optimised to obtain TiNx with least resistivity and excellent material properties through reactive sputtering. TiNx films deposited at various nitrogen flow rates were then annealed in a non-oxidising condition and their properties were thoroughly studied. Films deposited at the highest nitrogen flow rate (95%) showed least variation in resistivity and showed excellent material properties even after a high temperature anneal. BaTiO3 films of varying thicknesses were deposited on TiNx using RF-sputtering and subjected to RTP at various temperatures. It was found that there exists a critical thickness for each RTP temperature below which BaTiO3 films are pinhole free. A process was then developed by depositing and annealing multiple layers of BaTiO3 films, with the thickness of each deposition less than the critical thickness. It was observed that the multi-layered films are stable and pinhole free with a smooth surface while the single layers of equivalent thicknesses showed cracked surfaces. Current-atomic force microscopy studies showed leakage current through large pinholes in single-layered films, whereas the pinholes were not the leakage path for multi-layered films. Metal-insulator-metal capacitor structures were also fabricated using BaTiO3 with TiNx top and bottom electrodes and the fringing effects in leakage characteristics were studied. Finally, the polarisation reversal mechanism in BaTiO3 was investigated using piezoresponse force spectroscopy (PFS). It was experimentally demonstrated that the polarisation reversal in these materials is a two-step process, which involves polarisation rotation and switching when the applied electric field is not parallel to the crystallographic orientation of the grain. However, it is a single step switching when the polarisation and the electric field are parallel, as widely perceived. The two step polarisation reversal was found to help [101] and [111] oriented grains to switch at a lower electric field compared to [001] grains.Engineering and Physical Sciences Research Council (EPSRC), UK: Intel Ireland

National Center for Advanced Information Components Manufacturing. Program summary report, Volume II

The National Center for Advanced Information Components Manufacturing focused on manufacturing research and development for flat panel displays, advanced lithography, microelectronics, and optoelectronics. This report provides an overview of the program, program history, summaries of the technical projects, and key program accomplishments

Ferroelectric Switching and d33 Mapping of Micro-Patterned Piezoelectrics by Piezo Force Microscopy

The domain configurations and piezoelectric coefficient of ferroelectric materials such as PZT and PMN-PT thin films can have a significant effect on the optimization of future electronic devices. Piezo Force Microscopy (PFM) is an ideal tool based on Atomic Force Microscopy that allows unique investigations of such nanoscale effects, and can further be implemented to monitor domain switching dynamics. Utilizing PFM, the domain orientations as well as switching dynamics can be tracked in-situ. As the lateral dimension plays an important role in ferroelectric properties since it influences in-plane strain, both normal and lateral domain orientations are uniquely mapped simultaneously. Leveraging a new method for fabricating ferroelectric mesas down to the nanoscale, coercive fields and switching kinetics have thereby been investigated for continuous strained films as well as geometrically strain-relieved samples. A piezoelectric enhancement at edges of engineered nano islands is directly observed, while switching activation energies have been calculated and related to the strain-relief behavior. Piezoelectric coefficients in the surface normal direction were precisely measured on 2 μm, 1 μm and 0.75 μm wide PMN-PT microfabricated structures. These revealed piezoelectric enhancements up to 500 nm from feature edges for 2 μm or wider structures, and an increasing overall enhancement throughout smaller 1 μm and 0.75 μm structures as the strain relief becomes more complete. These results are corroborated by X-RAY diffraction and dielectric measurements from collaborators. The enhancement of dielectric and piezoelectric properties for geometrically strain relieved structures should furthermore be applicable to in-plane piezoactuation. Accordingly, lateral piezoactuation for PMN-PT microstructures is recorded for 4 μm and 2 μm features revealing xiv direction-dependent edge enhancement as hypothesized. Such insights are crucial for ferroelectric strain engineering efforts, the development of new device mechanisms, and their ultimate performance

Ferroelectric-Semiconductor Systems for New Generation of Solar Cells

This dissertation includes two parts. In the first part the study is focused on the fabrication of multifunctional thin films for photovoltaic applications. There is no doubt about the importance of transforming world reliance from traditional energy resources, mainly fossil fuel, into renewable energies. Photovoltaic section still owns very small portion of the production, despite its fast growth and vast research investments. New methods and concepts are proposed in order to improve the efficiency of traditional solar cells or introduce new platforms. Recently, ferroelectric photovoltaics have gained interest among researchers. First objective in application of ferroelectric material is to utilize its large electric field as a replacement for or improvement of built-in electric field in semiconductor p-n junctions which is responsible for the separation of generated electron-hole pairs. Increase in built in electric field will increase open-circuit voltage of the solar cell. In this regard, thin films of ferroelectric hafnium dioxide doped with silicon have been fabricated using physical vapor deposition techniques. Scanning probe microscopy techniques (PFM and KPFM) have been employed to analyze ferroelectric response and surface potential of the sample. The effects of poling direction of the ferroelectric film on the surface potential and current-voltage characteristics of the cell have been investigated. The results showed that the direction of poling affects photoresponse of the cell and based on the direction it can either improved or diminished. In the second part of this work, epitaxial thin films have been synthesized with physical vapor deposition techniques such as sputtering and electron beam evaporation for the ultimate goal of producing multifunctional three-dimensional structures. Three-dimensional structures have been used for applications such as magnetic sensors, filters, micro-robots and can be used for modification of the surface of solar cells in order to improve light absorption and efficiency. One of the important techniques for producing 3-D structures is using origami techniques. The effectiveness of this technique depends on the control of parameters which define direction of bending and rolling of the film or curvature of the structure based on the residual stress in the structure after film’s release and on the quality and uniformity of the film. In epitaxially grown films, the magnitude and direction of the stress are optimized, so the control over direction of rolling or bending of the film can be controlled more accurately. For this purpose, deposition conditions for epitaxy of Zn, Fe, Ru, Ti, NaCl and Cr on Si, Al2O3 or MgO substrates have been investigated and optimized. Crystallinity, composition and morphology of the films were characterized using reflective high energy diffraction (RHEED), Auger electron spectroscopy (AES), energy dispersive X-ray (EDX), and scanning electron microscopy (SEM)

Active layers of high-performance lead zirconate titanate at temperatures compatible with silicon nano- and microelecronic devices

Applications of ferroelectric materials in modern microelectronics will be greatly encouraged if the thermal incompatibility between inorganic ferroelectrics and semiconductor devices is overcome. Here, solution-processable layers of the most commercial ferroelectric compound ─ morphotrophic phase boundary lead zirconate titanate, namely Pb(Zr0.52Ti0.48)O3 (PZT) ─ are grown on silicon substrates at temperatures well below the standard CMOS process of semiconductor technology. The method, potentially transferable to a broader range of Zr:Ti ratios, is based on the addition of crystalline nanoseeds to photosensitive solutions of PZT resulting in perovskite crystallization from only 350 °C after the enhanced decomposition of metal precursors in the films by UV irradiation. A remanent polarization of 10.0 μC cm−2 is obtained for these films that is in the order of the switching charge densities demanded for FeRAM devices. Also, a dielectric constant of ~90 is measured at zero voltage which exceeds that of current single-oxide candidates for capacitance applications. The multifunctionality of the films is additionally demonstrated by their pyroelectric and piezoelectric performance. The potential integration of PZT layers at such low fabrication temperatures may redefine the concept design of classical microelectronic devices, besides allowing inorganic ferroelectrics to enter the scene of the emerging large-area, flexible electronics