The transition metal oxides ZrO2 and HfO2 as well as their solid solution are widely researched and, like most binary oxides, are expected to exhibit centrosymmetric crystal structure and therewith linear dielectric characteristics. For this reason, those oxides, even though successfully introduced into microelectronics, were never considered to be more than simple dielectrics possessing limited functionality. Here we report the discovery of a field-driven ferroelectric phase transition in pure, sub 10 nm ZrO2 thin films and a composition- and temperature-dependent transition to a stable ferroelectric phase in the HfO2–ZrO2 mixed oxide. These unusual findings are attributed to a size-driven tetragonal to orthorhombic phase transition that in thin films, similar to the anticipated tetragonal to monoclinic transition, is lowered to room temperature. A structural investigation revealed the orthorhombic phase to be of space group Pbc21, whose noncentrosymmetric nature is deemed responsible for the spontaneous...

Ferroelectricity in Simple Binary ZrO2 and HfO2

The recent progress in ferroelectricity and antiferroelectricity in HfO2-based thin films is reported. Most ferroelectric thin film research focuses on perovskite structure materials, such as Pb(Zr,Ti)O3, BaTiO3, and SrBi2Ta2O9, which are considered to be feasible candidate materials for non-volatile semiconductor memory devices. However, these conventional ferroelectrics suffer from various problems including poor Si-compatibility, environmental issues related to Pb, large physical thickness, low resistance to hydrogen, and small bandgap. In 2011, ferroelectricity in Si-doped HfO2 thin films was first reported. Various dopants, such as Si, Zr, Al, Y, Gd, Sr, and La can induce ferro-electricity or antiferroelectricity in thin HfO2 films. They have large remanent polarization of up to 45 μC cm(-2), and their coercive field (≈1-2 MV cm(-1)) is larger than conventional ferroelectric films by approximately one order of magnitude. Furthermore, they can be extremely thin ( 5 eV). These differences are believed to overcome the barriers of conventional ferroelectrics in memory applications, including ferroelectric field-effect-transistors and three-dimensional capacitors. Moreover, the coupling of electric and thermal properties of the antiferroelectric thin films is expected to be useful for various applications, including energy harvesting/storage, solid-state-cooling, and infrared sensors.

Ferroelectricity and Antiferroelectricity of Doped Thin HfO2‐Based Films

The rapid increase in information in the big-data era calls for changes to information-processing paradigms, which, in turn, demand new circuit-building blocks to overcome the decreasing cost-effectiveness of transistor scaling and the intrinsic inefficiency of using transistors in non-von Neumann computing architectures. Accordingly, resistive switching materials (RSMs) based on different physical principles have emerged for memories that could enable energy-efficient and area-efficient in-memory computing. In this Review, we survey the four physical mechanisms that lead to such resistive switching: redox reactions, phase transitions, spin-polarized tunnelling and ferroelectric polarization. We discuss how these mechanisms equip RSMs with desirable properties for representation capability, switching speed and energy, reliability and device density. These properties are the key enablers of processing-in-memory platforms, with applications ranging from neuromorphic computing and general-purpose memcomputing to cybersecurity. Finally, we examine the device requirements for such systems based on RSMs and provide suggestions to address challenges in materials engineering, device optimization, system integration and algorithm design. Resistive switching materials enable novel, in-memory information processing, which may resolve the von Neumann bottleneck. This Review focuses on how the switching mechanisms and the resultant electrical properties lead to various computing applications.

Resistive switching materials for information processing

The structural, thermal, and dielectric properties of the ferroelectric phase of HfO2, ZrO2, and Hf0.5Zr0.5O2 (HZO) are investigated with carefully validated density functional computations. We find that the free bulk energy of the ferroelectric orthorhombic Pca21 phase is unfavorable compared to the monoclinic P21/c and the orthorhombic Pbca phase for all investigated stoichiometries in the Hf1−xZrxO2 system. To explain the existence of the ferroelectric phase in nanoscale thin films, we explore the Gibbs/Helmholtz free energies as a function of stress and film strain and find them unlikely to become minimal in HZO films for technological relevant conditions. To assess the contribution of surface energy to the phase stability, we parameterize a model, interpolating between existing data, and find the Helmholtz free energy of ferroelectric grains minimal for a range of size and stoichiometry. From the model, we predict undoped HfO2 to be ferroelectric for a grain size of about 4 nm and epitaxial HZO below 5 nm. Furthermore, we calculate the strength of an applied electric field necessary to cause the antiferroelectric phase transformation in ZrO2 from the P42/nmc phase as 1 MV/cm in agreement with experimental data, explaining the mechanism of field induced phase transformation.

The origin of ferroelectricity in Hf1−xZrxO2: A computational investigation and a surface energy model

The effects of annealing temperature (Tanneal) and film thickness (tf) on the crystal structure and ferroelectric properties of Hf0.5Zr0.5O2 films were examined. The Hf0.5Zr0.5O2 films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases.

Evolution of phases and ferroelectric properties of thin Hf0.5Zr0.5O2 films according to the thickness and annealing temperature

Incipient ferroelectricity is known to occur in perovskites such as SrTiO3, KTaO3, and CaTiO3. For the first time it is shown that the intensively researched HfO2 thin films (16 nm) also possess ferroelectric properties when aluminium is incorporated into the host lattice. Polarization measurements on Al:HfO2 based metal–insulator–metal capacitors show an antiferroelectric-to-ferroelectric phase transition depending on annealing conditions and aluminium content. Structural investigation of the electrically characterized capacitors by grazing incidence X-ray diffraction is presented in order to gain further insight on the potential origin of ferroelectricity. The non-centrosymmetry of the elementary cell, which is essential for ferroelectricity, is assumed to originate from an orthorhombic phase of space group Pbc21 stabilized for low Al doping in HfO2. The ferroelectric properties of the modified HfO2 thin films yield high potential for various ferroelectric, piezoelectric, and pyroelectric applications. Furthermore, due to the extensive knowledge accumulated by various research groups regarding the HfO2 dielectric, an immediate relevance of ferroelectric hafnium oxide thin films is anticipated by the authors.

Incipient Ferroelectricity in Al-Doped HfO2 Thin Films

Ferroelectric (FE) materials have a high potential for non-volatile data storage. The range of applications is determined by the properties of the used materials. Conventionally, materials like lead zirconate titanate (PZT), which possess only limited compatibility with standard semiconductor processing, are implemented. Integration into transistor devices is only possible with interfacial barriers and FE layers thickerthan100nmneedtobeusedinordertoachievenonvolatility. 1,2 Hence, no easy scaling of the devices according to current technology node requirements can be realized. Recently, doped hafnium oxide was shown to be FE. 3 The FE phase is formed when a composition at the boundary between the monoclinic and the tetragonal/cubic phase is crystallized and is enhanced when using mechanical confinement. In this paper, we present an overview of HfO2 based metal-insulator-metal (MIM) capacitors exhibiting FE polarization hysteresis for various dopants and their impact on the FE properties. The properties can also be controlled in HfxZr1−xO2 4,5 which will not be discussed at this point due to the different composition range of this system. For several reasons the attribution of a ferroelectric-like behavior toasimplyleakydielectricorelectrontrappingwasexcluded:Allhysteresis curves posses concave regions corresponding to a current maximum before the voltage maximum and show signs of saturation, 3‐9 in the capacitance-voltage measurements peaks correlated to the maximum slope in the hysteresis measurements, 3‐9 sub-loop, 8 fatigue behavior, 13 and a piezoelectric response 3 was confirmed, and structural measurements support the existence of an orthorhombic phase at the transition from the monoclinic to the tetragonal/cubic phase. 3‐9 In addition, ferroelectric field effect transistors successfully realized with Si:HfO2 show a threshold voltage shift in opposite direction to shifts attributed to charge trapping with a retention of the shift extrapolated up to ten years. 14‐16 The discovery of FE HfO2 opens up an interesting pathway to build highly scalable FE devices and the FE transistor as one potential application will be discussed.

Hafnium Oxide Based CMOS Compatible Ferroelectric Materials

https://researchportal.port.ac.uk/portal/files/2835363/Paper_Acta_Materialia_2015.pdf

Electric field and temperature scaling of polarization reversal in silicon doped hafnium oxide ferroelectric thin films

Clear distinctions in the breakdown behavior of crystalline and amorphous dielectrics in MIM capacitors are correlated to different polarity asymmetries in the C-V and I-V characteristics of these symmetrical devices. Physical analysis of the complete capacitor stack reveals an oxidation of the TiN bottom electrode interface, leading to TiON formation and nitrogen diffusion into the dielectric. This can be traced back to the growth behavior of the dielectric on the TiN substrate. The atomic layer deposition caused interface formation is enhanced for crystalline films compared to amorphous, resulting in the asymmetry differences seen in electrical and breakdown characterization. In addition, asymmetrical charge trapping and a higher trap density are assumed for crystalline dielectric. These effects are detected both for doped HfO2 and ZrO2 layers.

Detailed Correlation of Electrical and Breakdown Characteristics to the Structural Properties of ALD Grown HfO2- and ZrO2-based Capacitor Dielectrics

The ferroelectric behavior of capacitors based on hafnium oxide dielectrics is reported. Thin films of 7-30 nm thickness were found to exhibit ferroelectric polarization-voltage hysteresis loops when integrated into TiN-based metal-insulator-metal capacitors. A remanent polarization up to 25 μC/cm2 and a coercive field of about 1 MV/cm was observed when Si:HfO2 was used as a ferroelectric material. Switching times of 10 ns were demonstrated and the ferroelectric properties were verified in the temperature range from 80 K to 470 K. N-channel MFIS-FETs (Metal-Ferroelectric-Insulator-Semiconductor Field-Effect Transistors) with poly-Si/TiN/Si:HfO2/SiO2/Si gate stack with a gate length equal or below 260 nm were successfully fabricated. The switching characteristics, endurance and retention properties were analyzed. A memory window of 1.2 V was obtained. Endurance performance of up to 104 cycles was verified. Retention characteristics were measured and 10 years data retention was extrapolated at 25 °C and 150 °C.

Johannes Mueller

Papers

Incipient Ferroelectricity in Al-Doped HfO2 Thin Films

Hafnium Oxide Based CMOS Compatible Ferroelectric Materials

Electric field and temperature scaling of polarization reversal in silicon doped hafnium oxide ferroelectric thin films

Detailed Correlation of Electrical and Breakdown Characteristics to the Structural Properties of ALD Grown HfO2- and ZrO2-based Capacitor Dielectrics

Non-volatile data storage in HfO 2 -based ferroelectric FETs