Ferroelectric field effect transistor (FeFET) memories based on a new type of ferroelectric material (silicon doped hafnium oxide) were studied within the scope of the present work. Utilisation of silicon doped hafnium oxide (Si:HfO2 thin films instead of conventional perovskite ferroelectrics as a functional layer in FeFETs provides compatibility to the CMOS process as well as improved device scalability. The influence of different process parameters on the properties of Si:HfO2 thin films was analysed in order to gain better insight into the occurrence of ferroelectricity in this system. A subsequent examination of the potential of this material as well as its possible limitations with the respect to the application in non-volatile memories followed. The Si:HfO2-based ferroelectric transistors that were fully integrated into the state-of-the-art high-k metal gate CMOS technology were studied in this work for the first time. The memory performance of these devices scaled down to 28 nm gate length was investigated. Special attention was paid to the charge trapping phenomenon shown to significantly affect the device behaviour.

This book provides comprehensive coverage of the materials characteristics, process technologies, and device operations for memory field-effect transistors employing inorganic or organic ferroelectric thin films. This transistor-type ferroelectric memory has interesting fundamental device physics and potentially large industrial impact. Among various applications of ferroelectric thin films, the development of nonvolatile ferroelectric random access memory (FeRAM) has been most actively progressed since the late 1980s and reached modest mass production for specific application since 1995. There are two types of memory cells in ferroelectric nonvolatile memories. One is the capacitor-type FeRAM and the other is the field-effect transistor (FET)-type FeRAM. Although the FET-type FeRAM claims the ultimate scalability and nondestructive readout characteristics, the capacitor-type FeRAMs have been the main interest for the major semiconductor memory companies, because the ferroelectric FET has fatal handicaps of cross-talk for random accessibility and short retention time. This book aims to provide the readers with development history, technical issues, fabrication methodologies, and promising applications of FET-type ferroelectric memory devices, presenting a comprehensive review of past, present, and future technologies. The topics discussed will lead to further advances in large-area electronics implemented on glass, plastic or paper substrates as well as in conventional Si electronics. The book is composed of chapters written by leading researchers in ferroelectric materials and related device technologies, including oxide and organic ferroelectric thin films.

"Non-volatile memories using ferroelectric capacitors, known as Ferroelectric Random Access Memory (FRAM) have been studied for many years, but they suffer from loss of data during read out process. Ferroelectric Field Effect Transistors (FeFETs), which are based on ferroelectric gate oxide, have been of recent interest for non-volatile memory applications. The FeFETs utilize the polarization of the ferroelectric layer incorporated into the transistor gate stack to control the channel conductivity. Therefore, in FeFET devices, the read out process is non-destructive because it is only processed by measuring the resistivity in the channel region. The drain current-gate voltage (ID-VG) characteristics of FeFETs exhibit a voltage shift due to polarization hysteresis known as the 'memory window', an important figure of merit of a FeFET that provides a window for the read voltage. A dielectric layer between semiconductor layer and the ferroelectric is required to reduce charge injection effect, and to compensate lattice mismatch between the ferroelectric and the semiconductor. In addition, a non-ferroelectric interfacial layer may form between the semiconductor and the ferroelectric layer. However, this dielectric layer causes a voltage drop since the system becomes equivalent to two serial capacitors. It also causes an electric field that opposes the polarization. Using a high permittivity material such as HfO2 reduces the voltage drop and the effect of depolarization. To date, the majority of the work involving FeFETs has been based on conventional ferroelectric materials such as Lead Zirconate Titanate (PZT) and Strontium Bismuth Tantalate (SBT). These materials are not compatible with standard IC processing and furthermore scaling thicknesses in PZT and SBT result in loss of polarization characteristics. Recently, ferroelectricity has been reported in doped hafnium oxide thin films with dopants such as Si, Al, and Gd. Particularly, silicon doped hafnium oxide (Si:HfO2) has shown promise. In this material, the remnant polarization considerably increases by decreasing the layer thickness. The lower permittivity of Si:HfO2 compared to that of PZT and SBT, allows to employ thinner films that reduce fringing effects. This study focuses on employing Si:HfO2 in short channel FeFETs. The study has two major objectives. First, to show that short channel FeFETs can be accomplished with large memory window. Second, to demonstrate the role of bulk layer thickness and permittivity on FeFET performance. N-channel metal oxide semiconductor FET (N-MOSFET) with printed channel length of 26 nm has been designed with Si:HfO2 as the ferroelectric layer, and TiN as the gate electrode. The effects of buffer layer thickness and permittivity and ferroelectric layer thickness on the memory window have been explored using Silvaco Atlas software that employs ferroelectric FET device physics developed by Miller et al. Polarization characteristics reported for Si:HfO2 have been incorporated in this model. The simulations performed in this study have shown that using Si:HfO2 as a ferroelectric material makes it possible to accomplish short channel FeFETs with good performance even without using buffer layers. This means it is possible to minimize depolarization effects. Using Si:HfO2 as a ferroelectric layer makes it possible to accomplish highly scaled and ultra-low-power FeFETs."--Abstract.

Ferroelectricity in Doped Hafnium Oxide: Materials, Properties and Devices covers all aspects relating to the structural and electrical properties of HfO2 and its implementation into semiconductor devices, including a comparison to standard ferroelectric materials. The ferroelectric and field-induced ferroelectric properties of HfO2-based films are considered promising for various applications, including non-volatile memories, negative capacitance field-effect-transistors, energy storage, harvesting, and solid-state cooling. Fundamentals of ferroelectric and piezoelectric properties, HfO2 processes, and the impact of dopants on ferroelectric properties are also extensively discussed in the book, along with phase transition, switching kinetics, epitaxial growth, thickness scaling, and more. Additional chapters consider the modeling of ferroelectric phase transformation, structural characterization, and the differences and similarities between HFO2 and standard ferroelectric materials. Finally, HfO2 based devices are summarized. Explores all aspects of the structural and electrical properties of HfO2, including processes, modelling and implementation into semiconductor devices Considers potential applications including FeCaps, FeFETs, NCFETs, FTJs and more Provides comparison of an emerging ferroelectric material to conventional ferroelectric materials with insights to the problems of downscaling that conventional ferroelectrics face

In 2011, Böscke et al. reported the unexpected discovery of ferroelectric properties in hafnia based thin films, which has since initiated many further studies and revitalized research on the topic of ferroelectric memories. In spite of many efforts, the unveiling of the fundamentals behind this surprising discovery has proven rather challenging. In this work, the originally claimed Pca21 phase is experimentally proven to be the root of the ferroelectric properties and the nature of this ferroelectricity is classified in the frame of existing concepts of ferroelectric materials. Parameters to stabilize this polar phase are examined from a theoretical and fabrication point of view. With these very basic questions addressed, the application relevant electric field cycling behavior is studied. The results of first-order reversal curves, impedance spectroscopy, scanning transmission electron microscopy and piezoresponse force microscopy significantly advance the understanding of structural mechanisms underlying wake-up, fatigue and the novel phenomenon of split-up/merging of transient current peaks. The impact of field cycling behavior on applications like ferroelectric memories is highlighted and routes to optimize it are derived. These findings help to pave the road for a successful commercialization of hafnia based ferroelectrics.

The hafnium based ferroelectric memories offer a low power consumption, ultra-fast operation, non-volatile retention as well as the small relative cell size as the main requirements for future memories. These remarkable properties of ferroelectric memories make them promising candidates for non-volatile memories that would bridge the speed gap between fast logic and slow off-chip, long term storage. Even though the retention of hafnia based ferroelectric memories can be extrapolated to a ten-year specification target, they suffer from a rather limited endurance. Therefore, this work targets relating the field cycling behavior of hafnia based ferroelectric memories to the physical mechanisms taking place within the film stack. Establishing a correlation between the performance of the device and underlying physical mechanisms is the first step toward understanding the device and engineering guidelines for novel, superior devices. In the frame of this work, an in-depth ferroelectric and dielectric characterization, analysis and TEM study was combined with comprehensive modeling approach. Drift and diffusion based vacancy redistribution was found as the main cause for the phase transformation and consequent increase of the remnant polarization, while domain pinning and defect generation is identified to be responsible for the device fatigue. Finally, based on Landau theory, a simple way to utilize the high endurance strength of anti-ferroelectric (AFE) materials and achieve non-volatility in state-of-the-art DRAM stacks was proposed and the fabrication of the world's first non-volatile AFE-RAM is reported. These findings represent an important milestone and pave the way toward a commercialization of (anti)ferroelectric non-volatile memories based on simple binary-oxides.

This thesis evaluates the viability of ferroelectric Si:HfO2 and its derived FeFET application for non-volatile data storage. At the beginning, the ferroelectric effect is explained briefly such that the applications that make use of it can be understood. Afterwards, the latest findings on ferroelectric HfO2 are reviewed and their potential impact on future applications is discussed. Experimental data is presented afterwards focusing on the ferroelectric material characteristics of Si:HfO2 that are most relevant for memory applications. Besides others, the stability of the ferroelectric switching effect could be demonstrated in a temperature range of almost 400 K. Moreover, nanosecond switching speed and endurance in the range of 1 million to 10 billion cycles could be proven. Retention and imprint characteristics have furthermore been analyzed and are shown to be stable for 1000 hours bake time at 125 oC. Derived from the ferroelectric effect in HfO2, a 28 nm FeFET memory cell is introduced as the central application of this thesis. Based on numerical simulations, the memory concept is explained and possible routes towards an optimized FeFET cell are discussed. Subsequently, the results from electrical characterization of FeFET multi-structures are presented and discussed. By using Si:HfO2 it was possible to realize the world's first 28 nm FeFET devices possessing i.a. 10k cycling endurance and an extrapolated 10 year data retention at room temperature. The next step towards a FeFET memory is represented by connecting several memory cells into matrix-type configurations. A cell concept study illustrates the different ways in which FeFET cells can be combined together to give high density memory arrays. For the proposed architectures, operational schemes are theoretically discussed and analyzed by both electrical characterization of FeFET multi-structures and numerical simulations. The thesis concludes with the electrical characterization of small FeFET memory arrays. First results show that a separation between memory states can be achieved by applying poling and incremental step pulse programming (ISPP) sequences. These results represent an important cornerstone for future studies on Si:HfO2 and its related applications.