Milan Pešić

Electric field cycling behavior of ferroelectric hafnium oxide.

HfO2 based ferroelectrics are lead-free, simple binary oxides with nonperovskite structure and low permittivity. They just recently started attracting attention of theoretical groups in the fields of ferroelectric memories and electrostatic supercapacitors. A modified approach of harmonic analysis is introduced for temperature-dependent studies of the field cycling behavior and the underlying defect mechanisms. Activation energies for wake-up and fatigue are extracted. Notably, all values are about 100 meV, which is 1 order of magnitude lower than for conventional ferroelectrics like lead zirconate titanate (PZT). This difference is mainly atttributed to the one to two orders of magnitude higher electric fields used for cycling and to the different surface to volume ratios between the 10 nm thin films in this study and the bulk samples of former measurements or simulations. Moreover, a new, analog-like split-up effect of switching peaks by field cycling is discovered and is explained by a network model based on memcapacitive behavior as a result of defect redistribution.

Complex Internal Bias Fields in Ferroelectric Hafnium Oxide.

For the rather new hafnia- and zirconia-based ferroelectrics, a lot of questions are still unsettled. Among them is the electric field cycling behavior consisting of (1) wake-up, (2) fatigue, and (3) the recently discovered subcycling-induced split-up/merging effect of transient current peaks in a hysteresis measurement. In the present work, first-order reversal curves (FORCs) are applied to study the evolution of the switching and backswitching field distribution within the frame of the Preisach model for three different phenomena: (1) The pristine film contains two oppositely biased regions. These internal bias fields vanish during the wake-up cycling. (2) Fatigue as a decrease in the number of switchable domains is accompanied by a slight increase in the mean absolute value of the switching field. (3) The split-up effect is shown to also be related to local bias fields in a complex situation resulting from both the field cycling treatment and the measurement procedure. Moreover, the role of the wake-up phenomenon is discussed with respect to optimizing low-voltage operation conditions of ferroelectric memories toward reasonably high and stable remanent polarization and highest possible endurance.

Evidence of single domain switching in hafnium oxide based FeFETs: Enabler for multi-level FeFET memory cells

Recent discovery of ferroelectricity in HfO2 thin films paved the way for demonstration of ultra-scaled 28 nm Ferroelectric FETs (FeFET) as non-volatile memory (NVM) cells [1]. However, such small devices are inevitably sensible to the granularity of the polycrystalline gate oxide film. Here we report for the first time the evidence of single ferroelectric (FE) domain switching in such scaled devices. These properties are sensed in terms of abrupt threshold voltage (VT) shifts leading to stable intermediate VT levels. We emphasize that this feature enables multi-level cell (MLC) FeFETs and gives a new perspective on steep subthreshold devices based on ferroelectric HfO2.

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

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.

Low leakage ZrO2 based capacitors for sub 20 nm dynamic random access memory technology nodes

During dynamic random access memory (DRAM) capacitor scaling, a lot of effort was put searching for new material stacks to overcome the scaling limitations of the current material stack, such as leakage and sufficient capacitance. In this study, very promising results for a SrTiO3 based capacitor with a record low capacitance equivalent thickness value of 0.2 nm at target leakage current are presented. Due to the material properties of SrTiO3 films (high vacancy concentration and low band gap), which are leading to an increased leakage current, a physical thickness of at least 8 nm is required at target leakage specifications. However, this physical thickness would not fit into an 18 nm DRAM structure. Therefore, two different new approaches to develop a new ZrO2 based DRAM capacitor stack by changing the inter-layer material from Al2O3 to SrO and the exchange of the top electrode material from TiN to Pt are presented. A combination of these two approaches leads to a capacitance equivalent thickness value...

Ultra-thin ZrO2/SrO/ZrO2 insulating stacks for future dynamic random access memory capacitor applications

Aiming for improvement of the ZrO2-based insulator properties as compared to the state-of-the-art ZrO2/Al2O3/ZrO2 stacks beyond 20 nm dynamic random access memory (DRAM) technology applications, ultra-thin (5 nm) ZrO2/SrO/ZrO2 stacks with TiN electrodes deposited by physical vapor deposition are addressed. By replacing the Al2O3 interlayer with SrO, the effective dielectric permittivity of the stack can be increased as indicated by electrical analysis. At the same time, no degradation of the insulating properties of the SrO-containing stacks and minor changes in the reliability, compared to an Al2O3 interlayer, are found. These results are indicating the possibility of further reducing the effective oxide thickness of the ZrO2-based stacks to come close to 0.5 nm for future DRAM capacitors.

Lanthanum-Doped Hafnium Oxide: A Robust Ferroelectric Material

Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO2 appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.

How to make DRAM non-volatile? Anti-ferroelectrics: A new paradigm for universal memories

The major disadvantages of DRAM memory cells are the very short retention time and high power consumption needed to refresh the stored information. Here, we present a new concept using a modified DRAM capacitor stack to enable non-volatile data storage. Recent reports verified anti-ferroelectric properties for pure ZrO2 dielectrics used in DRAM stacks. Anti-ferroelectric materials are well known for high endurance strength but at the same time volatile memory behavior. Based on Landau theory, we propose a simple way how non-volatility can be achieved in state-of-the-art ZrO2 based DRAM stacks. By employing electrodes with different workfunction values, a built-in bias is introduced within the AFE stack, thus creating two stable non-volatile states. Moreover, we report the fabrication of the worlds first non-volatile AFE-RAM. Detailed characterization proved high endurance and reliable operation of this non-volatile DRAM stack equivalent. In addition to the 1T-1C cell, we show a proof of concept for a MIS capacitor device which can be integrated in future AFE-FET based 1T memory architectures.

Insights into antiferroelectrics from first-order reversal curves

Antiferroelectric (AFE) HfO2 and ZrO2 based thin films are promising for energy and low power computing related applications. Here, we investigate 10 nm thin AFE Si:HfO2 films by means of first-order reversal curves (FORCs). Polarization-voltage, capacitance-voltage, and X-ray diffraction measurements confirm typical AFE behavior originating from the tetragonal phase. FORC analysis reveals two oppositely biased switching density peaks with a narrow distribution of coercive fields around 0.23 MV/cm, which is at least 4 times lower than that in typical ferroelectric HfO2 and ZrO2 films. The distributions along the internal bias field axis are much broader compared to the distribution of coercive fields. The exceptional stability of the switching density magnitude and coercive fields for up to 108 electric field cycles is demonstrated. Only small reductions of the internal bias fields are observed with cycling. These results highlight pathways towards improved cycling stability and variability of ferroelectr...

Root cause of degradation in novel HfO 2 -based ferroelectric memories

HfO2-based ferroelectrics reveal full scalability and CMOS integratability compared to perovskite-based ferroelectrics that are currently used in non-volatile ferroelectric random access memories (FeRAMs). Up to now, the mechanisms responsible for the decrease of the memory window have not been revealed. Thus, the main scope of this study is an identification of the root causes for the endurance degradation. Utilizing trap density spectroscopy for examining defect evolution with cycling of the device studied together with modeling of the degradation resulted in an understanding of the main mechanisms responsible for degradation of the ferroelectric behavior.