S. Starschich

About the deformation of ferroelectric hystereses

Studying ferroelectric hafnium oxide with focus on memory applications for the past years, discussions frequently involved the shape of measured polarization hystereses, its relation to the device performance, and how to optimize it. A perfect model-like hysteresis is of nearly rectangular shape and all deviations from this situation have to have a certain physical origin. Different phenomena and their impact on the shape of the polarization hystereses were reported in literature: Aging, imprint, fatigue, or dielectric interface layers to name a few examples. A collection of these phenomena is not easily found up to now. Thus, filling or at least reducing this gap is one of the goals of this work. Moreover, observing a pinched, slanted, or displaced hysteresis, it is quite tempting to try the reverse approach: a derivation of potential structural origins for this curve shape. First, the basics of the dynamic hysteresis measurement and the ferroelectric memories are briefly reviewed. The figures of interes...

Chemical solution deposition of ferroelectric yttrium-doped hafnium oxide films on platinum electrodes

Ferroelectric hafnium oxide films were fabricated by chemical solution deposition with a remnant polarization of >13 μC/cm2. The samples were prepared with 5.2 mol. % yttrium-doping and the thickness varied from 18 nm to 70 nm. The hafnium oxide layer was integrated into a metal-insulator-metal capacitor using platinum electrodes. Due to the processing procedure, no thickness dependence of the ferroelectric properties was observed. To confirm the ferroelectric nature of the deposited samples, polarization, capacitance, and piezoelectric displacement measurements were performed. However, no evidence of the orthorhombic phase was found which has been proposed to be the non-centrosymmetric, ferroelectric phase in HfO2.

Evidence for oxygen vacancies movement during wake-up in ferroelectric hafnium oxide

The wake-up effect which is observed in ferroelectric hafnium oxide is investigated in yttrium doped hafnium oxide prepared by chemical solution deposition. It can be shown that not the amount of cycles but the duration of the applied electrical field is essential for the wake-up. Temperature dependent wake-up cycling in a range of −160 °C to 100 °C reveals a strong temperature activation of the wake-up, which can be attributed to ion rearrangement during cycling. By using asymmetrical electrodes, resistive valence change mechanism switching can be observed coincident with ferroelectric switching. From the given results, it can be concluded that redistribution of oxygen vacancies is the origin of the wake-up effect.

An extensive study of the influence of dopants on the ferroelectric properties of HfO2

The ferroelectric properties of hafnium oxide based thin films prepared by chemical solution deposition (CSD) are investigated. In this extensive study, a wealth of strongly different dopants (size and valence) and dopant concentrations is used to induce ferroelectricity in 42 nm thin films. Using the same precursors and preparation conditions for all dopants a good comparability is given. In particular, the dopant size appears to have a crucial impact on the resulting ferroelectric properties. For smaller dopants only a small ferroelectric response is observed whereas for larger dopants the remanent polarization is increased significantly. The crystal phase for varying dopant concentrations and dopant sizes is investigated by grazing incidence X-ray diffractions (GI-XRD). A dominating cubic phase is found for doping concentrations showing the highest remanent polarization. Similar to first CSD studies on Y:HfO2, this is reflected in a prominent wake-up behavior, which is attributed to a phase transition from cubic to orthorhombic during field cycling.

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 and pure ZrO2 films

Ferroelectric and piezoelectric properties of Hf1-xZrxO2 (HZO) and pure ZrO2 films with a layer thickness of up to 390 nm prepared by chemical solution deposition (CSD) are investigated. The piezoelectric properties are measured using a double-beam laser interferometer (DBLI) and piezoresponse force microscopy. It is shown that for 100 nm thick films, the maximum remanent polarization is found for pure ZrO2 and reduces for the increasing hafnium content. A stable remanent polarization of 8 μC/cm2 is observed for ZrO2 film thicknesses between 195 and 390 nm. A piezoelectric coefficient of 10 pm/V is extracted from unipolar DBLI measurements. The observed thickness limitation for atomic layer deposition deposited HZO based ferroelectrics can be overcome by the CSD deposition technique presented in this work. Thick ZrO2 films are promising candidates for energy related applications such as pyroelectric and piezoelectric energy harvesting and electrocaloric cooling as well as for microelectromechanical systems.

Pulse wake-up and breakdown investigation of ferroelectric yttrium doped HfO2

The wake-up effect in yttrium doped hafnium oxide is investigated by pulse measurements, revealing the initial distribution of oxygen vacancies within symmetrical and asymmetrical layer stacks. It is shown that single pulses are sufficient to achieve a significant wake-up, whereby additional pulses lead to a stronger wake-up. Further cycling induces a degradation of the devices, which is explained by generation of oxygen vacancies and results in an electroforming step where subsequently resistive valence change mechanism switching is observed. The degradation and therefore the generation of oxygen vacancies show a strong frequency dependence, whereby the device lifetime is strongly increased for increasing frequencies.

Comparison of hafnia and PZT based ferroelectrics for future non-volatile FRAM applications

Ferroelectric random access memories (FRAM) are nonvolatile memories which allow a fast access time and a low power consumption. State-of-the-art devices are based on the perovskite lead zirconate titanate (PZT), which suffers from CMOS incompatibility resulting in scaling issues. The discovery of the ferroelectricity in doped hafnium oxide enabled scaled 3D memory devices. A variety of dopants has already been found to stabilize the orthorhombic phase, responsible for ferroelectric switching. Interestingly, among the variety of dopants only a mixed hafnium zirconium oxide (HZO) enabled a wide process window together with a large memory window. Here, we compare PZT with HZO and show that despite different dielectric properties similar memory relevant performance can be achieved. Additionally, analogous wake-up and degradation behavior can be observed in both materials. Moreover, we show experimental proof that a thermally driven wake-up in both material systems is caused by diffusion of ionic charges. Extracted activation energy hints to oxygen vacancy movement as the main cause for the wake-up effect in HZO.

Doped ZrO2 for future lead free piezoelectric devices

The ferroelectric and piezoelectric properties of doped ZrO2 prepared by chemical solution deposition (CSD) are investigated. Doping with different elements such as Mg, In, La, and Y leads to a stabilization of the constricted hysteresis. As shown in a previous work, for the constricted hysteresis of ZrO2, the piezoelectric response is significantly larger compared to ZrO2 with a normal hysteresis. The Mg doped ZrO2 shows a strong temperature and cycle stability. For the piezoelectric properties, a magnesium concentration of 7% shows the largest piezoelectric response with a piezoelectric coefficient of >10 pm/V, as well as the best cycle stability. Due to thicker films, which can be realized by the CSD technique, the shown doped ZrO2 films are a promising candidate for energy related applications such as piezoelectric energy harvesting as well as for microelectromechanical systems.