Al-doped HfO2 Research Articles

Optimization of MIS type Non-Volatile Memory Device with Al-Doped HfO2 as Charge Trapping Layer

The reduction in physical size of the Non-volatile memory (NVM) demands the use of high-k dielectrics due to loss in charge trapping behavior. The large bandgap of Al2O3 (∼7.0 eV) proves it suitable for blocking and tunneling layer, while the high dielectric constant of HfO2 proves it suitable for charge trapping layer (CTL). In this paper, we propose the application of Al doped HfO2 used as CTL. The doping concentration has been varied by varying the number of sub-cycles (sequential cycles of HfO2, Al2O3) by ALD system. The optical energy bandgap of Al doped HfO2 observed to be increases from 4.91−5.20 eV as the Al incorporation content increases with 1:9−1:4 ALD sub-cycles. Oxygen vacancy defect in the thin film decreased to 17.87% for 1:4-Al doped HfO2 sample as verified from the XPS. The increase in Al incorporation leads to charge loss towards silicon substrate due to high bandgap energy. In addition, charge traps are not well performed due to reduction of internal defects in the film. After 10 years, the NVM device with Al doped HfO2 (1:9) CTL has improved charge retention characteristics by 14% as compared to a normal HfO2 charge trap layer NVM device.

Demonstration of Ferroelectricity in Al-doped HfO2 with a Low Thermal Budget of 500 °C

We report the experimental realization of ferroelectricity in Al-doped HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (HAO) with the lowest reported thermal budget of 500 °C. A HAO film annealed at 500 °C for 30 s exhibits ferroelectricity with a remnant polarization (Pr) of 2.95 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The Pr increases to 10.40 μC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> when annealing duration is increased to 5 min, and approaches the values annealed at much higher temperatures of 700 to 1000 °C in other previous works. The presence of ferroelectricity has been confirmed by a series of typical phenomena, including the polarization-switched Positive-Up-Negative-Down measurement, the hysteretic polarization-voltage loop, and the butterfly-shaped C-V curves. In addition, comparable endurance characteristics are also obtained with other HfO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based ferroelectric films.

Impact of the radiation effect on the energy storage density and wake-up behaviors of antiferroelectric-like Al-doped HfO2 thin films.

The effect of the γ-ray total dose radiation on the energy storage density (ESD) and the phase transition of antiferroelectric-like (AFE-like) Al-doped HfO2 (HfAlO) thin films was investigated. The ESD property and wake-up behavior of the phase transition during the field cycling of the AFE-like HfAlO thin films were quantified before and after the radiation. The efficiency of the AFE-like thin films for energy storage slightly decreases as the total dose increases from 200 krad (Si) to 5 Mrad (Si), which is attributed to the radiation-induced trapped defects at the interfaces of HfAlO/TiN. Both the J-E, C-V, and εr-f characteristics of the AFE-like HfAlO thin films were also measured before and after the radiation at the same electrodes. These results further confirm that the ferroelectricity of the thin films can be reduced due to the radiation oxide trapped defects. It is worth noting that an enhanced wake-up behavior of the AFE-like HfAlO thin films can be observed after the radiation, which indicates that the transition from the antiferroelectric phase to the ferroelectric phase could be accelerated by the increased radiation-induced defects.

Implementation of an electrically modifiable artificial synapse based on ferroelectric field-effect transistors using Al-doped HfO2 thin films.

Human brain-like synaptic behaviors of the ferroelectric field-effect transistors (FeFETs) were emulated by introducing the metal-ferroelectric-metal-insulator-semiconductor (MFMIS) gate stacks employing Al-doped HfO2 (Al:HfO2) ferroelectric thin films even at a low operation voltage. The synaptic plasticity of the MFMIS-FETs could be gradually modulated by the partial polarization characteristics of the Al:HfO2 thin films, which were examined to be dependent on the applied pulse conditions. Based on the ferroelectric polarization switching dynamics of the Al:HfO2 thin films, the proposed devices successfully emulate biological synaptic functions, including excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), and spike timing-dependent plasticity (STDP). The channel conductance of the FeFETs could be controlled by partially switching the ferroelectric polarization of the Al:HfO2 gate insulators by means of pulse-number and pulse-amplitude modulations. Furthermore, the 3 × 3 array integrated with the Al:HfO2 MFMIS-FETs was also fabricated, in which electrically modifiable weighted-sum operation could be well verified in the 3 × 3 synapse array configuration.

Impact of Radiation Effect on Ferroelectric Al-Doped HfO2 Metal-Ferroelectric- Insulator-Semiconductor Structure

The $\gamma $ -ray total dose radiation effects on ferroelectric Al-doped HfO2 (HfAlO) thin films with an n-type Si substrate were studied. The $I$ - $V$ , $P$ - $V$ , $C$ - $V$ and fatigue characteristics of the HfAlO-based Metal-Ferroelectric-Insulator-Semiconductor (MFIS) structure were analyzed with the increasing total dose from 0 to 1 Mrad (Si). The remnant polarization ( $P_{\mathrm {r}}$ ) values, the capacitance ( $C$ ) values and the switching voltage ( $V_{\mathrm {c}}$ ) of this MFIS gate structure decreased with the increasing total dose. A TCAD model of Metal/HfAlO/SiO2/Si (MFIS) was proposed to explain the degradation mechanism of ferroelectric properties of the MFIS structure. We find that the new interface defects caused by radiation can reduce the electric field strength with the increasing total dose, which can significantly influence the irradiation properties of the HfAlO/SiO2/n-Si gate structure. These results provide the basis to applications for the HfO2-based devices in radiation working environment.

Influence of specific forming algorithms on the device-to-device variability of memristive Al-doped HfO2 arrays

In this work, the influence of specific switching algorithms on device-to-device (D2D) variability of the forming process, in an integrated Al-doped HfO2 1T-1R 4 kbit resistive random access memory array, is investigated. The resistive devices are programed by using two different algorithms: the incremental step pulse and verify algorithm (ISPVA) at different temperatures and the constant amplitude pulse and verify algorithm (CAPVA) at different voltage amplitudes. The stabilized forming currents of both algorithms are compared in terms of their distributions, yields, and dispersions. The D2D distributions of the forming voltages of ISPVA and the forming times of CAPVA are fitted by Weibull distributions. The obtained Weibull parameters provide a link with the statistics governing the process. Finally, the authors discuss the importance of the ISPVA, CAPVA, temperature, and voltage amplitudes to improve the reliability of the forming process.

Crystallization annealing effects on ferroelectric properties of Al-Doped HfO2 thin film capacitors using indium–tin–oxide electrodes

To investigate the effect of indium-tin-oxide (ITO) electrode on the Al-doped HfO2 (Al:HfO2) ferroelectric thin films, we fabricated and characterized the ITO/Al:HfO2/ITO and ITO/Al:HfO2/TiN capacitors by changing the annealing conditions. The ferroelectric remnant polarization (2Pr) was obtained to be 13.25 μC/cm2 for the ITO/Al:HfO2/TiN capacitors with the post-deposition annealing, which was termed T1. The 2Pr decreased after the post-metallization annealing due to the interface degradation between the Al:HfO2 and ITO electrode. Alternatively, the switching time and activation field of the T1 for the ferroelectric polarization switching were 1.25 μs and 1.15 MV/cm. These parameters were sensitively influenced by the interfacial dead layer formation and the amounts of ferroelectric orthorhombic phase. Furthermore, the fatigue endurance of the T1 were improved by preventing the crowding of oxygen vacancies at interfaces between the Al:HfO2 and top electrodes, in which the polarization values did not experience marked variations even after the fatigue cycles of 108.

Polarization switching kinetics of the ferroelectric Al-doped HfO2 thin films prepared by atomic layer deposition with different ozone doses

Ferroelectric switching kinetics of the Al-doped HfO2 (Al:HfO2) thin films prepared by the atomic layer deposition process were investigated by varying the dose time of oxygen precursor (O3). When the O3 dose time was reduced to 3 s, the Al:HfO2 films exhibited an enhanced remnant polarization (2Pr) of 10.2 μC/cm2 due to the suppression of the monoclinic phase and the increase in the ratio of oxygen vacancy. Double-pulse switching and the Kolmogorov–Avrami–Ishibashi model were used to obtain detailed quantitative information on the switching kinetics of the Al:HfO2 films. The estimated values of switching time and activation energy showed the strong dependence of O3 dose. This suggests that the O3 dose condition can be a key control parameter to modulate the ferroelectric polarization switching dynamics of the Al:HfO2 thin films.

Characterization of metal–ferroelectric–metal–insulator–semiconductor structures using ferroelectric Al-doped HfO2 thin films prepared by atomic-layer deposition with different O3 doses

Metal–ferroelectric–metal–insulator–semiconductor (MFMIS) capacitors using Al-doped HfO2 (Al:HfO2) ferroelectrics were proposed and the effects of O3 doses were investigated. The memory window (MW) increased with increasing the areal ratios of the MIS to the MFM (SI/SF) and the largest MW was obtained at the largest SI/SF for both devices prepared with O3 doses of 3 and 5 s. Contrarily, the retention of the device prepared with O3 dose of 3 s could be improved compared with the device prepared with longer O3 doses. Thus, the SI/SF and O3 doses could be suggested as critical control parameters for the Al:HfO2 MFMIS capacitors.

Exploring New Metal Electrodes for Ferroelectric Aluminum-Doped Hafnium Oxide

In this paper, we explore new metal electrodes for ferroelectric capacitors based on Al-doped HfO2. We find that Ti/Pd, Ti/Au, and W top electrodes can induce much higher remanent polarization as compared to the traditional TiN top electrode. The endurance of the capacitors with Ti/Pd electrodes is also much better than that with TiN and W electrodes. These results indicate that Ti/Pd is a very promising candidate for ferroelectric Al-doped HfO2. In addition, we find that the remanent polarization reaches maximum when the annealing temperature is around 900 °C–950 °C. At a given annealing temperature, the optimal Hf-to-Al cycle ratio corresponding to the highest remanent polarization is around 23:1. With optimized process conditions, we demonstrate high-performance Ti/Pd gated ferroelectric Al-doped HfO2 capacitors with remanent polarization up to $20~\mu \text{C}$ /cm2, endurance higher than 108 cycles, and retention over ten years at room temperature. Another interesting feature of the ferroelectric capacitors with Ti/Pd electrodes is high tunability of polarization by external pulses, which will be important for neurosynaptic computing applications.

Physical properties of the ferroelectric capacitors based on Al-doped HfO2 grown via Atomic Layer Deposition on Si

Ferroelectric capacitors based on aluminium (Al) doped hafnium oxide (HfO2) thin films grown on silicon substrates were fabricated by Atomic Layer Deposition (ALD), taking into account two methods. The first one involved the growth of a binary oxide, in a laminar way, by alternating the ALD cycles of HfO2 and Al2O3, and the second, the two precursors were sequentially mixed on the surface. The composition and structure of deposited aluminium doped hafnium oxide (Al:HfO2) thin films have been studied using X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GIXRD). XPS measurements show the formation of opposite ferroelectric polarization areas. Via GIXRD, it was found that the Al:HfO2 films deposited on Si have a structure with polycrystalline domains. Recording and investigation of ferroelectric domains were performed by Piezoresponse Force Microscopy (PFM), while the electrical performances of obtained devices were analysed by capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The PFM measurements show there is a mechanical non-zero response even outside the written area and for an appropriate value of the electrical stress the difference in phase between successive areas is saturated to a value close to 180°. The atomic force microscopy (AFM) analyses indicate a very low value of roughness average, for all grown thin films, ~0.2 nm, for a thickness of ~7 nm. From C-V characteristics, the memory window was extracted and the calculated values were found to be 0.8 V for the device obtained by the first ALD method, and 0.44 V for the second one, respectively. Moreover, in the case of the device based on the ferroelectric layer grown by the second ALD method, the memory window extends over a much wider applied voltage domain, in the range (±4 V; ±8 V), at a signal of 100 kHz.

First principles study of effect of vaiable component Al on HfO&lt;sub&gt;2&lt;/sub&gt; resistance

&lt;sec&gt;In order to improve the resistance properties of HfO&lt;sub&gt;2&lt;/sub&gt; and increase the consistency and uniformity of conductive filaments formed by oxygen vacancies (VO), the first-principles calculation method based on density functional theory is used to study the micro-properties of Al-doped HfO&lt;sub&gt;2&lt;/sub&gt; resistive materials. The results show that the interval Al (Int-Al) is more suitable for being incorporated into HfO&lt;sub&gt;2&lt;/sub&gt;, and the closer to the relative position of VO the Int-Al, the faster the convergence rate of the resistive material tends to be stable, and the smaller the formation energy. The effects of different Int-Al concentrations on the formation of HfO&lt;sub&gt;2&lt;/sub&gt; supercells with VO defects show that when the concentration of doped Int-Al is 4.04%, the fractional charge state density map can form relatively good charge channels. The maximum and critical equipotential surface values are highest, which is conducive to improving the consistency and uniformity of the formation of conductive filaments in HfO&lt;sub&gt;2&lt;/sub&gt; resistive materials. The calculation of energy formation shows that the change is slow when the concentration of Int-Al is lower than 4.04%. When the concentration of Int-Al is higher than 4.04%, the abnormal increase occurs, which indicates that the defect system becomes more and more difficult to form with the increase of the concentration of Int-Al. &lt;/sec&gt;&lt;sec&gt;The introduction of the impurity and the VO defect destroy the original complete crystal structure, which causes the position of the atoms around the impurity to shift, and the valence electron orbit and the energy level of the crystal are changed, and the distribution of the internal charges of the HfO&lt;sub&gt;2&lt;/sub&gt; defect system is affected. In order to study the effect of the change of the lattice structure on the formation of the VO conductive filament, the VASP software package is used to calculate the relative ratio of the atoms in the lattice structure of the HfO&lt;sub&gt;2&lt;/sub&gt; defect system as the reference and the relative ratio of the HfO&lt;sub&gt;2&lt;/sub&gt; defect system after the optimizing the lattice structure. Further study of the change of lattice structure, when the concentration of doped Int-Al is 4.04%, shows that the defect formation energy decreases significantly, which is conducive to the formation of perfect conductive channel. The conductive channel has a certain reference significance for improving the performance of HfO&lt;sub&gt;2&lt;/sub&gt; based resistive variable memory materials.&lt;/sec&gt;

Sub-1 nm Equivalent Oxide Thickness Al-HfO2 Trapping Layer with Excellent Thermal Stability and Retention for Nonvolatile Memory

Memory stacks for charge trapping cells have been produced exploiting Al-doped HfO2, Al2O3, and SiO2 made by atomic layer deposition. The fabricated stacks show superior stability and electrical ch...

The Correlation Between the Preferred Orientation and Al Distribution of Al-Doped HfO2 Films by Plasma-Enhanced Atomic Layer Deposition

This research was mainly supported by the Global Frontier R&D Program (2013M3A6B1078874) on Center for Hybrid Interface Materials (HIM) funded by the Ministry of Science, ICT & Future Planning, Republic of Korea, and partially supported by the Future Semiconductor Device Technology Development Program (10067739) funded by Ministry of Trade, Industry & Energy (MOTIE) and Korea Semiconductor Research Consortium (KSRC), and this study has been conducted with the support of the Korea Institute of Industrial Technology development project (KITECH EO-18-0022). The first two authors (Da-Young Kim and Jaeho Jang) contributed equally to this paper.

Al-, Y-, and La-doping effects favoring intrinsic and field induced ferroelectricity in HfO2: A first principles study

III-valent dopants have shown to be most effective in stabilizing the ferroelectric, crystalline phase in atomic layer deposited, polycrystalline HfO2 thin films. On the other hand, such dopants are commonly used for tetragonal and cubic phase stabilization in ceramic HfO2. This difference in the impact has not been elucidated so far. The prospect is a suitable doping to produce ferroelectric HfO2 ceramics with a technological impact. In this paper, we investigate the impact of Al, Y, and La doping, which have experimentally proven to stabilize the ferroelectric Pca21 phase in HfO2, in a comprehensive first-principles study. Density functional theory calculations reveal the structure, formation energy, and total energy of various defects in HfO2. Most relevant are substitutional electronically compensated defects without oxygen vacancy, substitutional mixed compensated defects paired with a vacancy, and ionically compensated defect complexes containing two substitutional dopants paired with a vacancy. The ferroelectric phase is strongly favored with La and Y in the substitutional defect. The mixed compensated defect favors the ferroelectric phase as well, but the strongly favored cubic phase limits the concentration range for ferroelectricity. We conclude that a reduction of oxygen vacancies should significantly enhance this range in Y doped HfO2 thin films. With Al, the substitutional defect hardly favors the ferroelectric phase before the tetragonal phase becomes strongly favored with the increasing concentration. This could explain the observed field induced ferroelectricity in Al-doped HfO2. Further Al defects are investigated, but do not favor the f-phase such that the current explanation remains incomplete for Al doping. According to the simulation, doping alone shows clear trends, but is insufficient to replace the monoclinic phase as the ground state. To explain this fact, some other mechanism is needed.

Ferroelectric transistors with monolayer molybdenum disulfide and ultra-thin aluminum-doped hafnium oxide

In this letter, we demonstrate ferroelectric memory devices with monolayer molybdenum disulfide (MoS2) as the channel material and aluminum (Al)-doped hafnium oxide (HfO2) as the ferroelectric gate dielectric. Metal-ferroelectric-metal capacitors with 16 nm thick Al-doped HfO2 are fabricated, and a remnant polarization of 3 μC/cm2 under a program/erase voltage of 5 V is observed. The capability of potential 10 years data retention was estimated using extrapolation of the experimental data. Ferroelectric transistors based on embedded ferroelectric HfO2 and MoS2 grown by chemical vapor deposition are fabricated. Clockwise hysteresis is observed at low program/erase voltages due to slow bulk traps located near the 2D/dielectric interface, while counterclockwise hysteresis is observed at high program/erase voltages due to ferroelectric polarization. In addition, the endurances of the devices are tested, and the effects associated with ferroelectric materials, such as the wake-up effect and polarization fatigue, are observed. Reliable writing/reading in MoS2/Al-doped HfO2 ferroelectric transistors over 2 × 104 cycles is achieved. This research can potentially lead to advances of two-dimensional (2D) materials in low-power logic and memory applications.

Effect of acceptor doping on phase transitions of HfO2 thin films for energy-related applications

Fluorite structured HfO2 or ZrO2 thin films have been intensively studied for memory- and energy-related applications since their ferroelectricity was first reported in 2011. The phase transition between the nonpolar tetragonal and the polar orthorhombic phase in these new ferroelectric materials is believed to be promising for energy harvesting, energy storage and solid state cooling. The temperature dependent phase transition and resulting strong pyroelectric and electrocaloric effect have been reported for Si-doped HfO2 and (Hf,Zr)O2 thin films. In this study, the effect of acceptor (Al and Gd) doping into HfO2 thin films on their temperature dependent phase transition was systematically examined. The phase transitions in Al- and Gd-doped HfO2 thin films were much broader compared to Si-doped HfO2 and (Hf,Zr)O2 films. The maximum adiabatic temperature change (ΔT) values of Al- and Gd-doped HfO2 film were 5.7 and 3.1K, respectively. A giant negative electrocaloric effect with ΔT of −7.4K could be observed for Al-doped HfO2. The various factors which can potentially affect the phase transitions of HfO2 films, such as dopant size, grain size distribution, spatial dopant distribution, and oxygen vacancy distribution were carefully examined to understand the different phase transition behavior. From the various factors, the distribution of oxygen vacancies is suggested as the origin of the different phase transitions of HfO2 films doped with trivalent and tetravalent dopants.

Effects of Al dopants and interfacial layer on resistive switching behaviors of HfOx film

Abstract In this work, Al dopants were introduced into HfO x film by different methods to modulate the oxygen vacancies in the film or near the interface, and the resistive switching characteristics were investigated. By transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis, an interfacial layer was formed at HfO x /Al interface. The occurrence of interfacial layer resulted in the larger switching voltages and resistances in LRS for HfO x /Al/HfO x and HfO x /Al samples compared with HfO x :Al sample with uniform Al dopants and no interface. Besides, the different reset processes for Al-doped HfO x samples were demonstrated. Much uniform distribution of resistances can be observed for all Al-doped HfO x samples due to the control of oxygen vacancies by Al doping and the good retention properties were achieved for all samples. The models for underlying physical mechanisms were also proposed to illustrate the switching behaviors of the prepared samples.

Crystallization behaviors of ultrathin Al-doped HfO2 amorphous films grown by atomic layer deposition**Project supported by the National High Technology Research and Development Program of China (Grant No. 2015AA016501) and the National Natural Science Foundation of China (Grant Nos. 61574168 and 61504163).

In this work, ultrathin pure HfO2 and Al-doped HfO2 films (about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550 °C to 750 °C are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO2 and Al-doped HfO2 films are both amorphous. After 550-°C annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO2 film while the Al-doped HfO2 film remains amorphous. After annealing at 650 °C and above, a great number of HfO2 tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO2 film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.

Subthreshold swing improvement in MoS2 transistors by the negative-capacitance effect in a ferroelectric Al-doped-HfO2/HfO2 gate dielectric stack

Obtaining a subthreshold swing (SS) below the thermionic limit of 60 mV dec-1 by exploiting the negative-capacitance (NC) effect in ferroelectric (FE) materials is a novel effective technique to allow the reduction of the supply voltage and power consumption in field effect transistors (FETs). At the same time, two-dimensional layered semiconductors, such as molybdenum disulfide (MoS2), have been shown to be promising candidates to replace silicon MOSFETs in sub-5 nm-channel technology nodes. In this paper, we demonstrate NC MoS2 FETs by incorporating a ferroelectric Al-doped HfO2 (Al : HfO2), a technologically compatible material, in the FET gate stack. Al : HfO2 thin films were deposited on Si wafers by atomic layer deposition. Voltage amplification up to 1.25 times was observed in a FE bilayer stack of Al : HfO2/HfO2 with a Ni metallic intermediate layer. The minimum SS (SSmin) of the NC-MoS2 FET built on the FE bilayer improved to 57 mV dec-1 at room temperature, compared with SSmin = 67 mV dec-1 for the MoS2 FET with only HfO2 as a gate dielectric.