Hafnium Oxide Based Ferroelectric Materials

In 2006, Tim Böscke was investigating doped HfO₂ and ZrO₂ dielectrics for DRAM capacitor applications at Qimonda (formerly Infineon), a manufacturer of dynamic random-access memories (DRAMs). Specifically, he tried to replicate results just published by Akira Toriumi’s group, which reported a maximum dielectric constant for 10 nm thick HfO₂ films containing 4-5 atomic percent Si. Toriumi had suggested that the higher dielectric constant could be caused by a tetragonal phase (t phase, space group: P4₂/nmc), which was difficult to distinguish from a non-centrosymmetric orthorhombic phase (oIII phase, space group: Pca2₁). At that time, in the phase diagrams of HfO₂ and ZrO₂ at room temperature and atmospheric pressure, only centrosymmetric nonpolar phases were reported, such as the t-phase or the monoclinic phase (m-phase, space group: P2₁/c). Böscke found a voltage-dependent capacitance enhancement for a particular concentration when measuring the capacitance as a function of voltage for HfO2-based capacitors with different Si concentrations, which could not be explained by normal paraelectric behavior. Through a detailed electrical and structural characterization of the capacitor, the enhancement was found to be due to ferroelectric switching. A previously unreported non-centrosymmetric orthorhombic Pca2₁ phase was suggested to cause this behavior. Subcycle and saturation polarization, field cycling endurance, fatigue, imprint, and piezoresponse experiments confirmed the ferroelectric properties.

For the first three years, the focus was on integrating ferroelectric HfO₂ into non-volatile memory devices, such as a ferroelectric field-effect transistor (FeFET) or a one-transistor/one-capacitor ferroelectric random access memory (FeRAM), because the initial results were obtained at a memory device manufacturer. Consequently, the first FeFET transistor was produced in 2007 using the 65 nm technology node, which was significantly smaller than any reported FeFET at the time. Unfortunately, Qimonda’s bankruptcy delayed further research; however, NaMLab and Fraunhofer IPMS-CNT continued the work, and GlobalFoundries in Dresden, Germany began fabricating FeFET devices at an even smaller 28 nm technology node.

In 2011, five years after the initial experiments, the first publication reporting these exciting findings appeared, followed by several others. The observation of similar ferroelectric performance in Hf_xZr_1-xO accelerated progress in the field, as this material offers a wide range of properties from mostly dielectric (x ≈ 1) to ferroelectric (x ≈ 0.5) to antiferroelectric (x ≈ 0). The research community had to be convinced that a simple oxide with a binary or ternary fluoride structure could have ferroelectric properties. Many groups joined the research effort to identify the proposed Pca2₁ o_III phase by diffraction in transmission electron microscopy, optimize the ferroelectric properties, and understand ferroelectricity’s origin in HfO2. Over the past decade, the applications of ferroelectric materials have expanded from ferroelectric capacitors, transistors, and tunneling interconnects for non-volatile memory applications to negative capacitors, logic-in-memory, neuromorphic computing, supercapacitors, and applications based on pyroelectrics or piezoelectrics. A rapidly growing number of publications is evidence of the strong research activity in this area.

During the last two years the main focus of development of ferroelectric HfO₂ based materials is the detailed understanding of the ferroelectric properties in thin doped HfO₂ layers. A variety of dopant materials (Si, Al, Ge, Y, Gd, La and Sr) were studied in addition to a mixed Hf_1-xZr_xO₂. A different ionic radius, smaller or larger than Hf and different valency can impact the phase formation (Figure 1). Deposition techniques included atomic layer deposition and physical vapor deposition. The ferroelectric orthorhombic Pca2₁ phase of HfO₂ is formed when the material is crystallized with a certain dopant or oxygen concentration at the phase boundary between the monoclinic and the tetragonal/cubic phase and is enhanced through mechanical confinement. Scanning transmission electron microscopy and electron diffraction methods confirmed the structure. Continuous research is ongoing aimed to understand the root cause of this previously unknown phase. In particular, the effect of interplay between the influence of different dopants and the amount of oxygen vacancies or interstitials present in the layer on phase formation is under investigation. A detailed analysis of Lanthanum doping confirmed the interaction between both. In fact, it was shown that two three-valent Lanthanum dopant atoms within a HfO₂ based layer introduced one oxygen vacancy.  Surface and interface energy of these grains together with residual stress generated during growth and crystallization anneal can be additional critical parameters. Ab initio simulations by partners at the Munich UAS and University of Connecticut confirmed the influence of the above mentioned factors on the phase stability of ferroelectric HfO₂. A qualitative model describing the influence of these basic parameters on the crystal structure of HfO₂ was proposed.

The polarization hysteresis for all dopants showed a maximum remanent polarization value between 15-40 μC/cm², depending on the dopant material. The highest values were obtained for Lanthanum doped HfO₂ with TiN electrodes. Piezo-response force microscopy (Oak Ridge Nat. Laboratory/Univ. Nebraska) in conjunction with transmission electron microscopy (North Carolina State University) measurements revealed domains within single grains with a diameter of ~20-30 nm for 10 nm thick films. The polycrystalline structure of the films caused a varying polarization orientation within the layer. The size distribution of the grains follows a Poisson distribution resulting in a grain size dependent coercive field and Curie temperature.

Future studies will focus on the structural basis of the ferroelectric properties and their impact on the ferroelectric switching behavior and how device cycling performance can be improved.