Al-doped HfO2 Research Articles

Sub-5 nm Al-doped HfO2 ferroelectric thin films compatible with 3D NAND process

The HfO2-based ferroelectric thin films have provided new opportunity for ferroelectric field-effect transistors (FeFETs) to be the candidate for the next-generation non-volatile memory, because of their compatibility with complementary metal-oxide-semiconductor technology. Benefiting from their similar but simpler structure compared to commercial flash memory, FeFETs can be integrated into three dimensional (3D) NAND architectures, which is named as 3D Fe-NAND, promising for super large storage applications. However, the ultra-thin HfO2-based ferroelectric thin film with tolerance for 850 ℃ thermal treatment has not been realized, which is the prerequisite for the high-density and low-power-consumption 3D Fe-NAND. In this work, we systematically investigated the impact both of thermodynamic and kinetical process on the ferroelectricity and reliability of the Al-doped HfO2 thin films. By carefully designing the dopant concentration and annealing technique, a 4.5-nm-thick HfAlOx film annealed at 850 °C with good ferroelectricity with 2Pr of ∼20 μC/cm² and breakdown field of ∼8.6 MV/cm is achieved. Furthermore, device simulation demonstrates the enhanced memory window at low operation voltage when integrating thinner HfAlOx ferroelectric films into FeFETs, suggesting the competitiveness of ultra-thin HfAlOx ferroelectric films in low-power-consumption operation. This work provides the strategy to design high-performance, ultra-thin HfO2-based ferroelectric thin films compatible with 3D Fe-NAND architecture.

Low-thermal-budget crystallization of ferroelectric Al:HfO2 films by millisecond flash lamp annealing

We report the use of a low-thermal-budget annealing technique; flash lamp annealing (FLA), which provides an extremely short annealing time in the millisecond range, on the ferroelectric properties of Al-doped HfO2 (HAO) films. HAO annealed at 1000 °C with 5 ms shows a higher remanent polarization value of 24.9 μC cm−2 compared to rapid thermal annealing (RTA), without degradation of endurance. GIXRD shows a stronger peak intensity originating from the orthorhombic (o-) phase and is observed when using FLA, indicating the formation of a larger amount of the o-phase. We believe that this is a consequence of the low thermal budget of FLA, and that specifically FLA can minimize the relaxation of the compressive stress in the TiN electrodes, inducing a high tensile stress to the HAO films and therefore an enhancement of o-phase formation. These results indicate that FLA is a promising annealing method for HAO crystallization due to the enhancement of o-phase formation.

Effect of the number and distribution of Al2O3 atomic layer deposition cycles within HfO2 layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al2O3 atomic layer deposition (ALD) cycles into a 10-nm-thick HfO2 matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (Pr) is enabled through the formation of crystalline Al-doped HfO2 (Al:HfO2) by incorporating at least two Al2O3 ALD cycles evenly into the HfO2 film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al2O3 cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 105 cycles. Notably, an improved 2 Pr of ∼9 μC/cm2 is achieved when the Al2O3 layers are concentrated near the lower region of the HfO2. On the other hand, as the Al2O3 layers are intensively located in the upper region of the HfO2, a dielectric response is observed in the polarization–voltage and current–voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO2 effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO2 layer to form the orthorhombic phase.

Voltage ramp stress based lifetime-prediction model of advanced Al-doped HfO[formula omitted] dielectric for 2.5D MIMCAPs

The reliability of an Al-doped HfO2 dielectric used in a high density 2.5D MIMCAP is investigated by constant voltage stress (CVS) and voltage ramp stress (VRS) measurements. The good agreement of the results from the two techniques allows to propose a model for lifetime prediction based on the breakdown characteristics. The extracted activation energy shows a voltage dependence associated with a change in the degradation characteristics of the high-κ material at high fields.

Effects of Al doping concentration and top electrode on the ferroelectricity of Al-doped HfO2 thin films

Al-doped HfO2 (HAO) is regarded as one of the potential HfO2 ferroelectric materials owing to its compatibility with the front end of the line process in integration circuits. In this work, atomic layer deposited (ALD) HAO thin films with different Al-doping concentrations and the corresponding devices with Ti or W top electrodes have been explored. It is found that the HAO film properties and corresponding ferroelectric device performances depend greatly on the doping concentration. In the concentration range of 3.0%–5.0%, 4.5% is the most proper doping concentration to induce the ferroelectricity of the thin film. It is revealed by x-ray diffraction and photoelectron spectroscopy that the oxygen vacancies can be modulated by doping, thereby contributing to the formation of the ferroelectric orthorhombic phase. A high remnant polarization (2Pr) of 36.8 ± 0.7 μC/cm2 under 5 mV/cm sweeping is observed through electrical measurement in the device with a 4.5% HAO and W top electrode. In addition, it was found that the choice of W or Ti top electrodes would affect the leakage and breakdown characteristics of the device. This work reveals the mechanism of Al doping and electrode modulation of HfO2 device performance and promotes the development of HAO ferroelectric thin film devices.

Kinetically Stabilized Hafnia Ferroelectric of Al-Doped HfO$_{\text{2}}$ Film by Fast Ramping and Fast Cooling Process

Kinetically Stabilized Hafnia Ferroelectric of Al-Doped HfO$_{\text{2}}$ Film by Fast Ramping and Fast Cooling Process

Synthesis of Dielectric Al-Doped HfO2 Thin Film by Polymer-Assisted Deposition

As transistors scale down, demands for high-k dielectric materials increase. Among various dielectric materials, HfO2 have drawn attention because of its large band gap, high dielectric constant, and excellent physical/chemical stability. Although several solution-based synthesis have been developed, their dielectric properties are inferior to those obtained by conventional vacuum processes (e.g., atomic layer deposition, sputtering). This study demonstrates a solution-based polymer-assisted deposition (PAD) of high-k Al-doped HfO2 thin films exhibiting excellent dielectric properties comparable to the conventional deposition methods. The PAD process enables the control of film thickness (≥3 nm) and the homogenous Al doping in the entire thin film. The key to the success is HCl-mediated coordination of metal ions and polymer. Systematic analysis on the structure and chemical composition of the dielectric layer is carried out. It is found that the phase transition from monoclinic phase to a mixture of tetragonal and amorphous phases results in excellent dielectric properties. The 7.8 nm thick thin film doped with 4.1 at% of Al exhibits a high dielectric constant of 30.2 with a high areal capacitance (674 nF cm−2 at 1.0 kHz), a low leakage current (3.3 × 10−9 A cm−2 at 2.0 MV cm−1), and a high breakdown voltage (7.7 V).

Experimental study of endurance characteristics of Al-doped HfO2 ferroelectric capacitor

In this work, the endurance characteristics of Al-doped HfO2 (HAO)-based metal-ferroelectric-metal (MFM) capacitors (which were annealed at 1000 °C) with various doping concentrations were investigated. The doping concentration was optimized for the high annealing temperature (1000 °C) process. To investigate the impact of cycling pulses on the endurance characteristics of HAO-based MFM capacitor, the rise/fall time (t r/f ) and hold time (th ) for the cycling pulses were varied. Moreover, by adopting the recoverable fatigue process, the endurance characteristics under repetitive wake-up/fatigue processes were studied. The HAO capacitors achieved the remnant polarization (2Pr) of 23.767 μC cm−2 at pristine state under the high annealing temperature. Furthermore, it was demonstrated that the endurance characteristics (∼108 cycles) of the HAO capacitors were comparable to them of other HfO2-based ferroelectric capacitors. Lastly but not least, it turned out that the amount of oxygen and oxygen vacancies in the HAO thin film was dependent of doping concentrations for the film. The impact of oxygen and oxygen vacancies was quantitatively analyzed, in detail, with TEM, XPS and GIXRD analysis.

Understanding the Effect of Oxygen Content on Ferroelectric Properties of Al-Doped HfO Thin Films

To satisfy the demand of high-speed and low-power memory in the field of integrated circuits, different elements doped HfO2 ferroelectric tunnel junctions have been developed. In this work, we investigate the annealing atmosphere effects on the ferroelectric property of the Al-doped HfO2 (HfAlO) ferroelectric tunnel junctions based on first-principles calculation and experiments. The ferroelectric layer/electrode interface has the obvious effect on ferroelectric property of the devices under different oxygen content annealing atmosphere, including 0%, 50% and 100% O2. The first-principle analysis verified that stable o-phase proportion in HfAlO film owing to the co-existence of the Al and oxygen vacancy is the key element to stable ferroelectric performance of device. This work could promote the development of HfAlO ferroelectric tunnel junctions for next-generation in-memory computing applications.

Impact of annealing temperature on the remanent polarization and tunneling electro-resistance of ferroelectric Al-doped HfOx tunnel junction memory.

The ferroelectric characteristics of a metal-ferroelectric-metal (MFM) ferroelectric tunneling junction (FTJ) capacitor device are investigated herein. The device consists of an aluminum-doped hafnium oxide (HAO) insulator sandwiched between tungsten (W) and titanium nitride (TiN) metal electrodes. Rapid thermal annealing (RTA) is performed for 20 s under a nitrogen atmosphere at temperatures of 750 °C, 800 °C, and 850 °C to find that ferroelectricity with a large remanent polarization (Pr) of 41.28 μC cm-2 can be obtained at the optimum annealing temperature of 800 °C. The presence of ferroelectricity is confirmed by polarization-switching positive-up-negative-down (PUND) measurements and by the hysteric polarization-voltage (P-V) loop. All devices exhibit excellent reliability, with an endurance of up to ∼106 cycles and long retention characteristics. In addition, the interfacial paraelectric capacitance (Ci) values of the three HAO FTJs are investigated via pulse-switching measurements. The results indicate that the HAO film annealed at 800 °C for 20 s exhibits an excellent tunneling electro-resistance (TER) ratio of 186% and this is attributed to the extra paraelectric layer formed between the ferroelectric layer and the bottom electrode. The detailed findings of this study are expected to assist in the development of hafnium oxide-based ferroelectric non-volatile memory applications.

Oxygen‐Scavenging Effects of Added Ti Layer in the TiN Gate of Metal‐Ferroelectric‐Insulator‐Semiconductor Capacitor with Al‐Doped HfO2 Ferroelectric Film (Adv. Electron. Mater. 11/2022)

Metal–Ferroelectric–Insulator–Semiconductors The Ti-inserted top electrode structure is efficiently utilized to induce the oxygen scavenging effect at the interfacial layer of a semiconductor-based ferroelectric device. The Al-doped HfO2 ferroelectric film with the high crystallization annealing temperature triggers a more active decomposition process at the interfacial layer to enhance the ferroelectric switching and also reduce the interface trap density, leading to a more reliable device operation. More details can be found in article number 2200310 by Cheol Seong Hwang and co-workers.

Charge Storage and Reliability Characteristics of Nonvolatile Memory Capacitors with HfO2/Al2O3-Based Charge Trapping Layers.

Flash memories are the preferred choice for data storage in portable gadgets. The charge trapping nonvolatile flash memories are the main contender to replace standard floating gate technology. In this work, we investigate metal/blocking oxide/high-k charge trapping layer/tunnel oxide/Si (MOHOS) structures from the viewpoint of their application as memory cells in charge trapping flash memories. Two different stacks, HfO2/Al2O3 nanolaminates and Al-doped HfO2, are used as the charge trapping layer, and SiO2 (of different thickness) or Al2O3 is used as the tunneling oxide. The charge trapping and memory windows, and retention and endurance characteristics are studied to assess the charge storage ability of memory cells. The influence of post-deposition oxygen annealing on the memory characteristics is also studied. The results reveal that these characteristics are most strongly affected by post-deposition oxygen annealing and the type and thickness of tunneling oxide. The stacks before annealing and the 3.5 nm SiO2 tunneling oxide have favorable charge trapping and retention properties, but their endurance is compromised because of the high electric field vulnerability. Rapid thermal annealing (RTA) in O2 significantly increases the electron trapping (hence, the memory window) in the stacks; however, it deteriorates their retention properties, most likely due to the interfacial reaction between the tunneling oxide and the charge trapping layer. The O2 annealing also enhances the high electric field susceptibility of the stacks, which results in better endurance. The results strongly imply that the origin of electron and hole traps is different—the hole traps are most likely related to HfO2, while electron traps are related to Al2O3. These findings could serve as a useful guide for further optimization of MOHOS structures as memory cells in NVM.

Effects of etching process and annealing temperature on the ferroelectric properties of atomic layer deposited Al-doped HfO2 thin film

An investigation was conducted with regard to the effect of etching process on the ferroelectric (FE) characteristics of different device structures with Al-doped HfO2 thin films; further, the effect of the rapid thermal annealing temperature on the FE properties was elucidated using metal-ferroelectric-metal (MFM) capacitors using TiN electrodes with varying thickness and 4 at.% Al-doped HfO2 FE layer. The capacitors were annealed at different temperatures after lithography and etching process; this was aimed at incorporating the FE-orthorhombic phase. The samples annealed after patterning were able to obtain improved FE characteristics due to the amount of tensile stress. The MFM devices that were initially patterned were also studied as a reference. We found that even though it required higher temperature and shorter time to introduce the FE phase, it exhibited more stable as well as promising FE properties and electrical performances with a relatively large remnant polarization (2P r ∼ 60 μC cm−2), a coercive electric field of approximately 2 MV cm−1 and high switching current density with less leakage. Our results indicate how the FE properties of the HfO2-based thin films can be engineered through suitable process sequence and post-annealing conditions, thereby verifying the applicable flexibility of FE-HfO2 for semiconductor device integration.

The electrons' journey in thick metal oxides

Originally introduced in electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used in a multitude of electronic devices. Understanding charge transport mechanisms in metal oxides is of paramount importance for device optimization; however, a detailed and self-consistent discussion of electron conduction at all applied electric fields is lacking in the literature. In this work, we investigated the conduction mechanisms in three model systems, Al2O3, HfO2, and Al-doped HfO2 metal–insulator–metal capacitors, determining the path that the electrons travel within the metal oxide. Traps properties are extracted from experimental current–voltage characteristics using the Ginestra® simulation software. Furthermore, the analysis allowed to visualize the location of traps most involved in the conduction and the dominant transport mechanisms at each applied electric field. Despite the different oxide properties, a similar trend was recognized at low electric fields, the electron transport through the oxide is negligible, and the dominant contribution to the measured current is ascribed to the charge/discharge of traps located near the metal/oxide interfaces, leading to displacement currents. At high electric fields, the transport of electrons occurs through the defect rich oxides in the two following ways: if a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode; otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode.

Effects of post cooling on the remnant polarization and coercive field characteristics of atomic layer deposited Al-doped HfO2 thin films

A study was conducted on how the cooling rate after the phase change heat treatment required to secure the ferroelectric (FE) properties affects the ferroelectricity of Al-doped HfO2 (Al:HfO2) thin films. When cooling the thin film to room temperature after heat treatment, the cooling rate was controlled through slow chamber cooling, which maintains and cools the sample inside the heat treatment equipment, slightly faster air cooling, and rapid cooling using deionized water. We achieved the dramatic increase of remnant polarization (Pr) and coercive electric field (Ec) using rapid quenching after phase change heat treatment. The 2Pr and 2Ec values of rapid quenched FE Al:HfO2 thin film are approximately ∼100 μC/cm2 and ∼9.5 MV/cm, respectively, which are the highest records among HfO2-based FE thin films reported so far. These significant improvements are attributed to inducing homogeneously distributed defects complexes and higher stress/confinement strain within Al:HfO2 thin film, leading to preventing pinched hysteresis loop and stabilize orthorhombic crystal structure. Furthermore, endurance up to 2 × 106 cycles and reasonable projected 10 years retention properties were achieved, stably without wake-up and fatigue in quenched FE Al:HfO2 capacitor. These results propose a beneficial method to improve the ferroelectricity of HfO2-based thin films.

Thermal stability of antiferroelectric-like Al:HfO2 thin films with TiN or Pt electrodes

HfO2-based antiferroelectric-like thin films are increasingly being considered for commercial devices. However, even with initial promise, the temperature sensitivity of electrical properties such as loss tangent and leakage current remains unreported. 50 nm thick, 4 at. % Al-doped HfO2 thin films were synthesized via atomic layer deposition with both top and bottom electrodes being TiN or Pt. A study of their capacitance vs temperature showed that the Pt/Al:HfO2/Pt had a relative dielectric permittivity of 23.30 ± 0.06 at room temperature with a temperature coefficient of capacitance (TCC) of 78 ± 86 ppm/°C, while the TiN/Al:HfO2/TiN had a relative dielectric permittivity of 32.28 ± 0.14 at room temperature with a TCC of 322 ± 41 ppm/°C. The capacitance of both devices varied less than 6% over 1 to 1000 kHz from −125 to 125 °C. Both capacitors maintained loss tangents under 0.03 and leakage current densities of 10−9–10−7 A/cm2 between −125 and 125 °C. The TiN/Al:HfO2/TiN capacitor maintained an energy storage density (ESD) of 18.17 ± 0.79 J/cm3 at an efficiency of 51.79% ± 2.75% over the −125 to 125 °C range. The Pt/Al:HfO2/Pt capacitor also maintained a stable ESD of 9.83 ± 0.26 J/cm3 with an efficiency of 62.87% ± 3.00% over the same temperature range. Such low losses in both capacitors along with their thermal stability make antiferroelectric-like, Al-doped HfO2 thin films a promising material for temperature-stable microelectronics.

Ferroelectric enhancement of Al-doped HfO2 thin films by rapid electron beam annealing in a low thermal budget

In past few years, there was a great amount of research on ferroelectric Al-doped HfO2 (HAO) thin films which suffer from the need of high annealing temperatures to achieve significant ferroelectricity. In this work, we realize pronounced remnant polarization 2Pr~29μC/cm2 of HAO using rapid electron beam annealing (EBA) with a large area. The simulation of electron beam trajectories reveals that the effect of EBA concentrates on the region ~20 nm below the sample surface, which highly benefits the process integration where a low thermal budget is required. The energy-dispersive X-ray and high-angle annular dark-field analyses reveal the interdiffusion between Al and Hf in the HAO layer treated by EBA. The pronounced ferroelectricity of HAO can be accounted for by the lattice strain, which facilities the formation of the orthorhombic phase, due to the substitution of Al for Hf as supported by the fast Fourier transformation diffraction pattern.

Impact of Stack Engineering on HfOₓ/Al:HfOₓ-Based Flexible Resistive Memory Devices and Its Synaptic Characteristics

The effect of stack engineering of HfO_x/ Al-doped HfO_x(Al:HfO_x)-based flexible resistive random access memory (ReRAM) devices including its synaptic characteristic has been investigated. The flexible Al/HfO_x/Al:HfO_x/HfO_x/ITO/PET resistive memory device shows outstanding switching performance in terms of repeatability, statistical variation of parameters even in bending condition up to 5 mm. The switching mechanism has been explained by ohmic conduction in low resistance state (LRS) and the trap-controlled space charge limited conduction (SCLC) in high resistance state (HRS). The improved synaptic characteristics, such as long-term potentiation (LTP), long-term depression (LTD), spike-rate-dependent plasticity (SRDP), paired-pulse facilitation (PPF), and post-tetanic potentiation (PTP), are effectively mimicked with Al/HfO_x/Al:HfO_x/ITO/PET ReRAM, which is suitable for neuromorphic computing application.

Electrical characterization of memory capacitors for nonvolatile memory applications based on nanolaminated HfO2/Al2O3 and Al-doped HfO2 stacks

Memory capacitors with atomic-layer-deposited HfO2/Al2O3 nanolaminated layers and Al-doped HfO2 charge trapping layers were investigated through capacitance-voltage (C-V) and current-voltage (I-V) measurements. The dielectric constant of the multi-dielectric stack comprising 20-nm Al2O3 blocking oxide, a HfO2-based layer and 2.4-nm tunnel SiO2 does not depend on the manner of Al-introduction in HfO2.The stacks exhibit a negative oxide charge of about -5.1×1011 cm−2 and -2.5×1011 cm−2 for the structures with nanolaminated and doped layers, respectively. The Al-doping of HfO2 is found to produce lower leakage currents. A sublinear behavior of the current-voltage curves is observed in the range of -20 ÷ +10 V for both HfO2-based stacks. Memory windows of ∼ 1 V when charging with ±27-V voltage pulses are obtained; the data suggests that electron trapping is better pronounced in the HfO2/Al2O3 nanolaminate, while positive charge accumulation prevails in the Al-doped HfO2 layers.

Effects of Post Cooling on the Remnant Polarization and Coercive Field Characteristics of Atomic Layer Deposited Al-Doped Hfo2 Thin Filim

Effects of Post Cooling on the Remnant Polarization and Coercive Field Characteristics of Atomic Layer Deposited Al-Doped Hfo2 Thin Filim