Recent progress for obtaining the ferroelectric phase in hafnium oxide based films: impact of oxygen and zirconium

Different causes for ferroelectric properties in hafnium oxide were discussed during the last decade including various dopants, stress, electrode materials, and surface energy from different grain sizes. Recently, the focus shifted to the impact of oxygen vacancies on the phase formation process. In this progress report, the recent understanding of the influence of oxygen supplied during deposition on the structural phase formation process is reviewed and supplemented with new data for mixed HfₓZr₁₋ₓOᵧ films. Even though polar and non-polar HfₓZr₁₋ₓOᵧ thin films are well characterized, little is known about the impact of oxygen exposure during the deposition process. Here, a combination of structural and electrical characterization is applied to investigate the influence of the oxygen and zirconium content on the crystallization process during ALD deposition in comparison to other deposition techniques. Different polarization properties are assessed which correlate to the determined phase of the film. Optimized oxygen pulse times can enable the crystallization of HfₓZr₁₋ₓOᵧ in a polar orthorhombic phase rather than a non-polar monoclinic and tetragonal phase

Genuinely Ferroelectric Sub-1-Volt-Switchable Nanodomains in HfₓZr₍₁₋ₓ₎ O₂ Ultrathin Capacitors

The new class of fully silicon-compatible hafnia-based ferroelectrics with high switchable polarization and good endurance and thickness scalability shows a strong promise for new generations of logic and memory devices. Among other factors, their competitiveness depends on the power efficiency that requires reliable low-voltage operation. Here, we show genuine ferroelectric switching in HfₓZr₍₁₋ₓ₎ O₂ (HZO) layers in the application-relevant capacitor geometry, for driving signals as low as 800 mV and coercive voltage below 500 mV. Enhanced piezoresponse force microscopy with sub-picometer sensitivity allowed for probing individual polarization domains under the top electrode and performing a detailed analysis of hysteretic switching. The authentic local piezoelectric loops and domain wall movement under bias attest to the true ferroelectric nature of the detected nanodomains. The systematic analysis of local piezoresponse loop arrays reveals a totally unexpected thickness dependence of the coercive fields in HZO capacitors. The thickness decrease from 10 to 7 nm is associated with a remarkably strong decrease of the coercive field, with about 50% of the capacitor area switched at coercive voltages ≤0.5 V. Our explanation consistent with the experimental data involves a change of mechanism of nuclei-assisted switching when the thickness decreases below 10 nm. The practical implication of this effect is a robust ferroelectric switching under the millivolt-range driving signal, which is not expected for the standard coercive voltage scaling law. These results demonstrate a strong potential for further aggressive thickness reduction of HZO layers for low-power electronics

Unveiling the double-well energy landscape in a ferroelectric layer

The properties of ferroelectric materials, which were discovered almost a century ago¹ , have led to a huge range of applications, such as digital information storage² , pyroelectric energy conversion³ and neuromorphic computing⁴⁻⁵ . Recently, it was shown that ferroelectrics can have negative capacitance⁶⁻¹¹, which could improve the energy efficiency of conventional electronics beyond fundamental limits¹²⁻¹⁴. In Landau–Ginzburg–Devonshire theory¹⁵⁻¹⁷, this negative capacitance is directly related to the doublewell shape of the ferroelectric polarization–energy landscape, which was thought for more than 70 years to be inaccessible to experiments¹⁸. Here we report electrical measurements of the intrinsic double-well energy landscape in a thin layer of ferroelectric Hf₀.₅Zr₀.₅O₂. To achieve this, we integrated the ferroelectric into a heterostructure capacitor with a second dielectric layer to prevent immediate screening of polarization charges during switching. These results show that negative capacitance has its origin in the energy barrier in a double-well landscape. Furthermore, we demonstrate that ferroelectric negative capacitance can be fast and hysteresis-free, which is important for prospective applications¹⁹. In addition, the Hf₀.₅Zr₀.₅O₂ used in this work is currently the most industry-relevant ferroelectric material, because both HfO₂ and ZrO₂ thin films are already used in everyday electronics²⁰. This could lead to fast adoption of negative capacitance effects in future products with markedly improved energy efficiency

Chemical Stability of IrO2_{2} Top Electrodes in Ferroelectric Hf0.5_{0.5}Zr0.5_{0.5}O2_{2} ‐Based Metal–Insulator–Metal Structures: The Impact of Annealing Gas

The IrO$_{2}$ top electrode chemistry of ferroelectric IrO$_{2}$/Hf$_{0.5}$Zr$_{0.5}$O$_{2}$ (HZO)/IrO$_{2}$ metal–insulator–metal (MIM) structures dependent on rapid thermal annealing in different gas atmospheres (nitrogen, oxygen, and forming gas [FG]) is investigated. Using hard X-ray photoelectron spectroscopy (HAXPES), the strongly modified chemical states of the IrO$_{2}$ layer dependent on the choice of annealing gas atmosphere are observed. For O$_{2}$ and N$_{2}$ anneals, the IrO$_2$ electrode remains either unaffected or is just slightly chemically attacked at the top. In contrast, FG annealing causes a complete reduction of the IrO$_{2}$ top electrode into metallic Ir. Surprisingly, oxygen is detected—unbound to Ir—which is incorporated in the metallic Ir layer. This mobile oxygen is thought to affect the electrical behavior of the IrO$_{2}$/HZO/IrO$_{2}$ device. In addition, it may serve as a test sample for future studies of the root causes of the role of oxygen-vacancy interactions at the interface, which can influence the performance instabilities in HZO-based MIM structures, such as wake-up, imprint, and fatigue

Enhanced Ferroelectric Polarization in TiN/HfO2_{2}/TiN Capacitors by Interface Design

The interface formation between ferroelectric HfO$_{2}$ layers and TiN bottom electrodes was studied by hard X-ray photoelectron spectroscopy and directly correlated to the electric polarization characteristics of the TiN/HfO$_{2}$/TiN capacitors. We consistently deduced the interface chemistry from HfO$_{2}$- and TiN-related core levels, dependent on the oxygen flow ṁ supplied before and during physical vapor deposition (PVD) growth of HfO$_{2}$. The results underline the critical, twofold impact of oxygen supply on HfO$_{2}$ and interface properties. When supplied before growth, the supplied oxygen stabilizes the TiN/HfO$_{2}$ interface by oxidation and formation of a self-limiting (noninsulating) TiO$_{2}$ intralayer. When supplied during growth, on the other hand, oxygen flows above a critical threshold reduce the oxygen vacancy concentration within the HfO$_{2}$ film. We reveal a direct relation between the maximum ferroelectric remanent polarization and a critical threshold PVD oxygen exposure flow rate. The results allow for advancement of the PVD growth process in terms of a more flexible design of the ferroelectric HfO$_{2}$ films with chemically stable TiN interfaces

Chemical Stability of IrO₂ Top Electrodes in Ferroelectric Hf_0.5Zr_0.5O₂‐based MIM Structures : The Impact of Annealing Gas

publishe

Chemical Stability of IrO 2

publishe

Genuinely Ferroelectric Sub-1-Volt-Switchable Nanodomains in HfxZr(1-x)O2 Ultrathin Capacitors

The new class of fully silicon-compatible hafnia-based ferroelectrics with high switchable polarization and good endurance and thickness scalability shows a strong promise for new generations of logic and memory devices. Among other factors, their competitiveness depends on the power efficiency that requires reliable low-voltage operation. Here, we show genuine ferroelectric switching in HfxZr(1-x)O2 (HZO) layers in the application-relevant capacitor geometry, for driving signals as low as 800 mV and coercive voltage below 500 mV. Enhanced piezoresponse force microscopy with sub-picometer sensitivity allowed for probing individual polarization domains under the top electrode and performing polarization domains under the top electrode and performing a detailed analysis of hysteretic switching. The authentic local piezoelectric loops and domain wall movement under bias attest to the true ferroelectric nature of the detected nanodomains. The systematic analysis of local piezoresponse loop arrays reveals a totally unexpected thickness dependence of the coercive fields in HZO capacitors. The thickness decrease from 10 to 7 nm is associated with a remarkably strong decrease of the coercive field, with about 50% of the capacitor area switched at coercive voltages <= 0.5 V. Our explanation consistent with the experimental data involves a change of mechanism of nuclei-assisted switching when the thickness decreases below 10 nm. The practical implication of this effect is a robust ferroelectric switching under the millivolt-range driving signal, which is not expected for the standard coercive voltage scaling law. These results demonstrate a strong potential for further aggressive thickness reduction of HZO layers for low-power electronics

Impact of vacancies and impurities on ferroelectricity in PVD- and ALD-grown HfO2_2 films

We investigate the emerging chemical states of TiN/HfO$_2$/TiN capacitors and focus especially on the identification of vacancies and impurities in the ferroelectric HfO$_2$ layers, which are produced either by physical vapor deposition (PVD) or atomic layer deposition (ALD). Depending on the specific growth conditions, we identify different mechanisms of oxygen vacancy formation. Corresponding spectral features are consistently observed for all HfO$_2$- and TiN-related core levels by hard x-ray photoelectron spectroscopy (HAXPES). In ALD-grown samples, we find spectral signatures for the electronic interaction between oxygen vacancies and nitrogen impurities. By linking the HAXPES results to electric field cycling experiments on the TiN/HfO$_2$/TiN capacitors, we discuss possible formation mechanisms and stabilization of the ferroelectric HfO$_2$ phase directly related to specific PVD or ALD conditions