Ekaterina Yurchuk

Impact of different dopants on the switching properties of ferroelectric hafniumoxide

The wake-up behavior of ferroelectric thin film capacitors based on doped hafnium oxide dielectrics in TiN-based metal–insulator–metal structures is reported. After field cycling a remanent polarization up to 40 µC/cm2 and a high coercive field of about 1 MV/cm was observed. Doping of HfO2 by different dopants with a crystal radius ranging from 54 pm (Si) to 132 pm (Sr) was evaluated. In all cases, an improved polarization–voltage hysteresis after wake-up cycling is visible. For smaller dopant atoms like Si and Al stronger pinching of the polarization hysteresis appeared with increasing dopant concentration and proved to be stable during cycling.

Ferroelectricity in Si‐Doped HfO2 Revealed: A Binary Lead‐Free Ferroelectric

Static domain structures and polarization dynamics of silicon doped HfO2 are explored. The evolution of ferroelectricity as a function of Si-doping level driving the transition from paraelectricity via ferroelectricity to antiferroelectricity is investigated. Ferroelectric and antiferroelectric properties can be observed locally on the pristine, poled and electroded surfaces, providing conclusive evidence to intrinsic ferroic behavior.

Ferroelectric hafnium oxide: A CMOS-compatible and highly scalable approach to future ferroelectric memories

With the ability to engineer ferroelectricity in HfO2 thin films, manufacturable and highly scaled MFM capacitors and MFIS-FETs can be implemented into a CMOS-environment. NVM properties of the resulting devices are discussed and contrasted to existing perovskite based FRAM.

Impact of Scaling on the Performance of HfO 2 -Based Ferroelectric Field Effect Transistors

The recently discovered ferroelectric behavior of HfO2-based dielectrics yields the potential to overcome the main challenges of the ferroelectric field-effect transistors (FeFETs) - CMOS compatibility as well as scalability to the state-of-the-art technology nodes of logic transistors. In this paper, we study the impact of scaling on the memory performance of FeFET devices employing Si:HfO2 ferroelectric films. The operation capability was proven down to a gate length of 28 nm. Program/erase characteristics, endurance behavior, and retention properties were analyzed for FeFETs with gate lengths scaled down to 32 nm. The detected difference in the performance between the long and short channel devices could be for the most part attributed to transistor short channel effects. In addition, the effect of temperature on the device properties of Si:HfO2-based FeFETs was investigated in detail. The program/erase speed was ascertained to be independent of temperature. On the other hand, increase in temperature resulted in reduced initial memory window accompanied by its slightly accelerated decay with time.

Doped Hafnium Oxide – An Enabler for Ferroelectric Field Effect Transistors

Ferroelectrics are very interesting materials for nonvolatile data storage due to the fact that they deliver very low power programming operation combined with nonvolatile retention. For 60 years researchers have been inspired by these fascinating possibilities and have tried to build ferroelectric memory devices that can compete with mainstream technologies in their respective time. The progress of the current concepts is limited by the low compatibility of ferroelectrics like PZT with CMOS processing. Therefore, PZT or SBT based 1T1C ferroelectric memories are not scaling below 130 nm and 1T ferroelectric FETs based on the same materials are still struggling with low retention and very thick memory stacks. Hafnium oxide, a standard material in sub 45 nm CMOS, can show ferroelectric hysteresis with promising characteristics. By adding a few percent of silicon and annealing the films in a mechanically confined manner. Boescke et al. demonstrated ferroelectric hysteresis in hafnium oxide for the first time. Recently, a large number of dopants including Y, Al, Gd and Sr have been used to induce ferroelectricity in HfO2. This paper reviews the current status of hafnium oxide based ferroelectrics, its application to field effect transistors and puts this approach into a wider context of earlier developments in the field.

From MFM Capacitors Toward Ferroelectric Transistors: Endurance and Disturb Characteristics of

Ferroelectric Si:HfO2 has been investigated starting from metal-ferroelectric-metal (MFM) capacitors over metal-ferroelectric-insulator-semiconductor (MFIS) and finally ferroelectric field-effect-transistor (FeFET) devices. Endurance characteristics and field cycling effects recognized for the material itself are shown to also translate to highly scaled 30-nm FeFET devices. Positive-up negative-down as well as pulsed Id-Vg measurements illustrate how ferroelectric material characteristics of MFM capacitors can also be identified in more complex MFIS and FeFET structures. Antiferroelectric-like characteristics observed for relatively high Si dopant concentration reveal significant trapping superimposed onto the ferroelectric memory window limiting the general program/erase endurance of the devices to 104 cycles. In addition, worst case disturb scenarios for a VDD/2 and VDD/3 scheme are evaluated to prove the viability of one-transistor memory cell concepts. The ability to tailor the ferroelectric properties by appropriate dopant concentration reveals disturb resilience up to 106 disturb cycles while maintaining an ION to IOFF ratio of more than four orders of magnitude.

{\rm HfO}_{2}

Ferroelectric field effect transistors (FeFETs) based on ferroelectric hafnium oxide (HfO2) thin films show high potential for future embedded nonvolatile memory applications. However, HfO2 films besides their recently discovered ferroelectric behavior are also prone to undesired charge trapping effects. Therefore, the scope of this paper is to verify the possibility of the charge trapping during standard operation of the HfO2-based FeFET memories. The kinetics of the charge trapping and its interplay with the ferroelectric polarization switching are analyzed in detail using the single-pulse ID-VG technique. Furthermore, the impact of the charge trapping on the important memory characteristics such as retention and endurance is investigated.

-Based FeFET Devices

Novel HfO2-based non-volatile ferroelectric field effect transistors (FeFETs) reveal integration and scaling properties superior to the devices utilizing perovskite-type ferroelectrics. However, until now the switching endurance of only 104 program/erase cycles could be proven. The mechanisms responsible for the cycling degradation have been scarcely studied so far. Therefore, the scope of this paper is to clarify the origin of the cycling degradation in HfO2-based FeFETs. Several possible degradation mechanisms - fatigue of the ferroelectric layer and degradation of the transistor gate stack - are proposed and investigated. The limited endurance properties were found to be linked to the transistor gate stack reliability rather than to the ferroelectric material itself. The gate leakage current measurements and the trapping analyses presented in this paper identified a degradation of the interfacial layer in the gate stack, which in turn is strongly linked to a reduction of ferroelectric memory window.

Charge-Trapping Phenomena in HfO 2 -Based FeFET-Type Nonvolatile Memories

We report the fabrication of highly scaled sub-0.3 μm ferroelectric field-effect transistors on the basis of ferroelectric HfO2. The electrical properties of 9 nm thick Si-doped HfO2 films depending on the silicon content and the annealing temperature were investigated. The most suitable fabrication conditions for the emergence of ferroelectricity were identified. The ferroelectric properties were verified up to temperatures of 170°C. N-channel MFIS-FETs (Metal-Ferroelectric-Insulator-Semiconductor Field-Effect Transistors) with poly-Si/TiN/Si:HfO2/SiO2/Si gate stack and channel lengths down to 260 nm were successfully fabricated. The switching characteristics, endurance and retention properties were analysed. Switching times of 10 ns were demonstrated. A memory window of 1.2 V was obtained with program/erase voltages of -6.5 V and +4 V and pulses as short as 50 ns. Endurance performance of up to 104 cycles was verified. Retention characteristics were measured at 25°C and 150°C. 10 years data retention was indicated for both temperatures by the extrapolation of the experimental data.

Origin of the endurance degradation in the novel HfO 2 -based 1T ferroelectric non-volatile memories

The film thickness dependence of ferroelectric Si:HfO2 (10 nm and 30 nm) was studied with a focus on ferroelectric field effect transistor (FeFET) memory applications based on a 28 nm bulk technology. Experimental P-E hysteresis of metal-ferroelectric-metal capacitor structures could be reproduced by a Preisach-based ferroelectric simulation model implemented in a commercially available TCAD environment. The experimentally observed thickness dependence of material characteristics was then used for demonstrating memory window widening, reduced interfacial field stress and decreased depolarization fields by FeFET TCAD modeling. Based on these findings, improved memory characteristics (memory window size, endurance, retention) can be anticipated for FeFET devices possessing the appropriate Si:HfO2 thickness.