Doped hafnium oxide was shown to exhibit a strong ferroelectric behavior. It was implied that doping allows the stabilization of a HfO2 non-centrosymmetric orthorhombic phase, a prerequisite for ferroelectric behavior. However, a number of reports showed comparable ferroelectric properties in undoped HfO2, although with different behaviors of the remanent polarization upon layer thickness. In this work, the ferroelectric properties and crystalline structure of doped and undoped HfO2 layers as a function of the layer thickness were studied. The results show similar properties for ultrathin layers. Indeed, both show a linear increase in the orthorhombic/tetragonal crystalline phase amount and of remanent polarization as a function of thickness up to 7 nm. For higher thicknesses, a strong increase in the monoclinic phase is observed in undoped layers alongside a strong decrease in the remanent polarization whereas, for doped layers, the remanent polarization continues to increase with thickness. The experimental results suggest that the stabilization of the undoped HfO2 orthorhombic ferroelectric phase is due to mechanical stress, which is particularly high in ultrathin layers but relaxes for thicker layers.

Sub-10 nm thick gadolinium-doped hafnia (Gd:HfO2) layers were grown in metal–insulator–metal (TiN/Gd:HfO2/TiN) stacks using a plasma-enhanced atomic layer deposition process. Thermally annealed Gd:HfO2 layers with a thickness of 8.8, 6.6, and 4.4 nm exhibited orthorhombic crystalline structure and showed ferroelectric properties. Indeed, polarization vs electric field hysteresis loops were recorded with 2Pr polarization ranging from 2 to 20 μC/cm2. The studied layers showed the same coercive electric field (∼2 MV/cm). Consequently, polarization switching voltage between +Pr and −Pr decreased down to 0.9 V for the thinnest layer. Remanent polarization cycling showed a strong wake-up effect, with no fatigue, up to 109, followed by a stabilization up to 1010 cycles, where 2Pr reached 33 μC/cm2 for 8.8 nm Gd:HfO2. This endurance result and the absence of noticeable remanent polarization fatigue can be attributed to the optimal chemical composition of the TiN/Gd:HfO2 interface, which is supposed to be at the origin of defect generation, mostly oxygen vacancies, that leads to ferroelectric polarization fatigue.

Resistance switching is studied in conductive bridge memory structures made from atomic layer deposited HfO2 and Ag active electrode. Inert electrode is varied by using different substrates (TiN, W, Pt). HfO2 crystallinity is modified by varying the deposition temperature (300/350 °C) and the film thickness (10/20 nm). Current–voltage characteristics, as well as current–time characteristics (to access to the switching kinetics), are studied according to the inert electrode nature and HfO2 structural properties. Results are discussed along resistance transition mechanisms which imply (i) the generation of oxygen vacancies by electronic injection at the inert electrode, (ii) Ag diffusion along oxygen vacancy paths, and (iii) the reduction of silver ions controlled by the inert electrode/HfO2 interface. Best characteristics, in terms of stability, are observed with Pt inert electrode and 10 nm films. Crystalline and amorphous films (10 nm) provide similar characteristics. In 10 nm films, TiN and W inert electrodes lead to variability in electrical properties (parasitic sets during reset, switching time dispersion). Such a variability is related to high electronic injection at the TiN/HfO2 and W/HfO2 interfaces which creates a high density of oxygen vacancy paths (Ag diffusion paths). In thicker and well-crystallized films (20 nm), progressive set is observed. This is ascribed to conduction along oxygen vacancy paths, which dominates over conduction along Ag conductive bridges.

The electronic structure and optical properties of monoclinic HfO2 (m-HfO2) and HfO2:Al2O3 alloys, from the density functional theory (DFT), are investigated. The calculated lattice parameters and optical properties of m-HfO2 are consistent with the experimental data. Upon alloying with Al2O3 (more than 25%), we observe that the bandgap of Hf-aluminate (HfAlO) increases. Moreover, some doping states in the top and bottom valence bands are induced, which enhance the visible absorption of HfO2. From the impedance spectroscopy analysis, it is observed that 90% of the Al2O3 content in HfO2 induces a reduction of oxygen vacancies (and ac conductivity) as well as an increase in the dielectric constant as compared to pure HfO2. In addition, from the J–V and C–V variations, both current density and capacitance voltage nonlinearities are reduced. The conduction mechanisms of HfO2 and HfAlO dielectrics are systematically investigated. According to the J–E plots, parameters like the optical dielectric constant and the effective barrier height are extracted. Results are consistent with the DFT calculations and show that the Hf0.1Al0.9O device may constitute a potential candidate for metal–insulator–metal capacitor applications.

In this work, we propose a novel integration in order to significantly reduce the High Resistance State vari-ability and to improve thermal stability in Oxide-based Resistive Random Access Memory (OxRRAM) devices. A novel device featuring a metallic liner, acting as a parallel resistance, is presented. To assess the effect of this solution, we compare the results with a standard OxRRAM cell structure. A very good stability of the resistive states, both in endurance and temperature, is highlighted and explained thanks to a conductive fila-ment based model.

This work presents an in-depth study of the thermal stability of low-k material in view of intermediate Back-End-Of-Line (iBEOL) for 3D sequential Integration. SiOCH ULK were analyzed after thermal annealing up to 600°C. Moreover, the stability and reliability of this ULK material coupled with W metal in line 1 integration is characterized up to 550°C, 5 h. We have demonstrated that low-k material can support a thermal budget of 500°C, 2h with limited outgassing. This result is fully compatible with 3D sequential integration.

In this paper, three-dimensional (3D) metal-insulator-metal (MIM) structures were obtained by atomic layer deposition of an aluminum doped TiO2 layer on a dense array of truncated conical holes etched in a silicon substrate. Different features of conical holes were fabricated in order to increase the developed area of the MIM structure. A capacitance density of nearly 185 nF/mm2 was obtained on an array of 19.2 μm deep holes with 5 and 10 μm bottom and top conical hole diameters, respectively. Interestingly, the increase in capacitance density scales with the developed area and no degradation of the electrical properties of the MIM structure has been observed. Indeed, the leakage current across the 3D MIM structures remains as small as in planar (2D) MIM structures.

The behavior of the capacitance switching of HfO2 Resistive non-volatile Memories is investigated in view of realizing a MEMImpedance (MEM-Z) device. In such a Metal Insulator Metal structure, the impedance value can be tuned by the adjustment of both resistance and capacitance values. We observe a strong variation of capacitance from positive to negative values in a single layer Metal Insulator Metal device made of HfO2 deposited by Atomic Layer Deposition, but unfortunately no memory effect is observed. However, in the case of a two layer structure, a device has been obtained with a memory effect where both resistance and capacitance values can be tuned simultaneously, with a variation of capacitance down to negative values to get an inductive behavior. Negative capacitance values are observed for voltage values near SET voltage. A schematic model based on shaped oxygen vacancy density is proposed to account for this capacitance variation. The oxygen vacancies can be either isolated or connected in the bulk of the oxide. (C) 2016 AIP Publishing LLC.

The electrical reliability of HfO2 based metal-insulator-metal capacitors is investigated under AC stress voltage. The capacitance-time (C-t) and conductance-time (G-t) responses are studied for different stress amplitudes and frequencies. Time-to-breakdown is observed to strongly depend on the electrode nature. Electrical degradation is discussed via a model based on oxygen vacancy/oxygen ions generation. Defect generation is controlled by the injecting nature of electrodes. Partial recovery, and so time-to-breakdown, are controlled by the ability of electrodes to store oxygen.

Dielectric spectroscopy is carried out for intrinsic and aluminum-doped TiO2 rutile films which are deposited on RuO2 by the atomic layer deposition technique. Capacitance and conductance are measured in the 0.1 Hz–100 kHz range, for ac electric fields up to 1 MVrms/cm. Intrinsic films have a much lower dielectric constant than rutile crystals. This is ascribed to the presence of oxygen vacancies which depress polarizability. When Al is substituted for Ti, the dielectric constant further decreases. By considering Al-induced modification of polarizability, a theoretical relationship between the dielectric constant and the Al concentration is proposed. Al doping drastically decreases the loss in the very low frequency part of the spectrum. However, Al doping has almost no effect on the loss at high frequencies. The effect of Al doping on loss is discussed through models of hopping transport implying intrinsic oxygen vacancies and Al related centers. When increasing the ac electric field in the MVrms/cm range, strong voltage non-linearities are evidenced in undoped films. The conductance increases exponentially with the ac field and the capacitance displays negative values (inductive behavior). Hopping barrier lowering is proposed to explain high-field effects. Finally, it is shown that Al doping strongly improves the high-field dielectric behavior.

In this paper, TiO2 layers grown on RuO2 by atomic layer deposition (ALD) using tetrakis (dimethyla-mino) titanium (TDMAT) and either oxygen plasma or H2O as oxygen source were analyzed using X-ray diffraction (XRD), Raman spectroscopy, and depth-resolved X-ray Photoelectron spectroscopy (XPS). The main objective is to investigate the surface chemical reactions mechanisms and their influence on the TiO2 film properties. The experimental results using XRD show that ALD deposition using H2O leads to anatase TiO2 whereas a rutile TiO2 is obtained when oxygen-plasma is used as oxygen source. Depth-resolved XPS analysis allows to determine the reaction mechanisms at the RuO2 substrate surface after growth of thin TiO2 layers. Indeed, the XPS analysis shows that when H2O assisted ALD process is used, intermediate Ti2O3 layer is obtained and RuO2 is reduced into Ru as evidenced by high resolution transmission electron microscopy. In this case, there is no possibility to re-oxidize the Ru surface into RuO2 due to the weak oxidation character of H2O and an anatase TiO2 layer is therefore grown on Ti2O3. In contrast, when oxygen plasma is used in the ALD process, its strong oxidation character leads to the re-oxidation of the partially reduced RuO2 following the first Ti deposition step. Consequently, the RuO2 surface is regenerated, allowing the growth of rutile TiO2. A surface chemical reaction scheme is proposed that well accounts for the observed experimental results.

Resistance switching in HfO2 metal-insulatormetal devices is investigated through current-voltage (I-V) and current-time (I-t) measurements. Depending on electrodes, a high-to-low resistance transition is observed in I-V characteristics at high or low voltages (OXRAM and CBRAM behaviors). This is correlated in the I-t characteristics to a progressive current increase, or to an abrupt current jump. Following the high-to-low resistance transition, several behaviors are observed: Non-switchable resistance (irreversible breakdown), evanescent low resistance state (spontaneous breakdown recovery), or fully switchable resistance (voltage-controlled reversible breakdown). The primary mechanism, common to both CBRAM and OXRAM devices, is hot electron injection at the cathode which leads to oxygen vacancies (defects) in the oxide bulk. Anode metal may diffuse along defect paths. In that case, the high-to-low resistance transition is due to the formation of metallic filaments across the oxide thickness (CBRAM case). When the anode metal diffusion is more difficult, the high-to-low resistance transition is ascribed to oxygen vacancy percolation paths (OXRAM case).

Voltage nonlinearity is studied in microelectronic metal / insulator (oxide) / metal capacitors. The influence of electrodes (work function, oxygen affinity) and test parameters (dc or ac bias, frequency) are investigated. A physical model is proposed to explain the capacitance-voltage relation. The model is based on electronic hopping trough oxygen vacancy defects.

This paper is devoted to the study of the electrical properties of Au/HfO2/TiN metal–insulator–metal (MIM) capacitors in three distinctive modes: (1) alternative mode (C–f), (2) dynamic regime [thermally stimulated currents, TSCs I(T)] and (3) static mode [I(V)]. The electrical parameters are investigated for different temperatures. It is found that capacitance frequency C–f characteristic possesses a low-frequency dispersion that arises for high temperature (T > 300 °C). Accordingly, the loss factor exhibits a dielectric relaxation (with an activation energy E a ~ 1.13 eV) which is intrinsically related to the diffusion of oxygen vacancies. The relaxation mechanisms of electrical defects in a dynamic regime (TSCs) analysis show that defect related to the TSC peak observed at 148.5 °C (E a ~ 1 eV) is in agreement with impedance spectroscopy (C–f). On the other hand, when the MIM structures are analyzed in static mode, the I–V plots are governed by Schottky emission. The extrapolation of the curve at zero field gives a barrier height of 1.7 eV.