HfOx Film Research Articles

Resistive Switching Effect in TaN/HfOx/Ni Memristors with a Filament Formed under Local Electron-Beam Crystallization

The influence of an intense electron beam on a nonstoichiometric oxide HfOx (х@) layer of a TaN/HfOx/Ni memristor on its electrophysical properties is studied. It is found that the crystalline h-Hf, m‑HfO2, o-HfO2, and t-HfO2 phases are formed in the HfOx film under this impact. It is established that memristors demonstrate resistive switching at certain electron fluence values. At the same time, such memristors have resistive switching voltages several times lower than those of unirradiated memristors. In addition, they exhibit a multiple decrease in the spread of resistive switching voltages, as well as resistances in low- and high-resistance states. The current–voltage curves of the obtained memristors indicate that the charge transport in them is described by the space-charge-limited current mechanism.

Enhanced resistive switching performance of hafnium oxide-based devices: Effects of growth and annealing temperatures

Although hafnium oxide-based resistive switching (RS) devices have been explored extensively, extremely high operating voltages and currents hinder their application in low power operations. In this study, the growth temperature and annealing conditions have been varied to significantly improve the characteristics of hafnium oxide-based RS devices. Deposited using a rf magnetron sputtering at different temperatures, HfOx films are subsequently subjected to vacuum annealing to create a high density of oxygen vacancies. It is observed that the crystallinity, defect density, and RS device performance of these films are influenced by these thermal treatments. The HfOx film annealed at 450 ºC exhibited bipolar RS with the substantial improvement in the device performance, such as electroforming, set/reset voltages, and operating currents as low as ~1 V, 0.5/− 0.5 V, and ~1 µA, respectively, in addition to large ON/OFF ratio> 103, good endurance (>103 cycles), and retention capability (>104 s). The localized conductive filaments composed of oxygen vacancies are found to be responsible for the RS behavior in Al/HfOx/FTO devices, and the simple vacuum annealing approach considerably increases the proportion of oxygen vacancies. With the insights into the influence of annealing temperatures on the switching parameters, this work paves the way for the design of low-power devices for data storage applications.

Modulation of resistive switching and magnetism of HfOx film by Co doping

The coexistence of resistive switching and magnetism modulation of Co doped HfOx samples with different doping concentrations was reported in this work. The chemical composition, especially oxygen vacancies was analyzed by XPS technology and the influence of Co dopants on resistive switching and magnetism were demonstrated both experimentally and theoretically by first principles calculation. Improved switching performance such as on/off ratio, uniformity and reliability can be obtained by Co doping. In addition, 5.3 % Co doped HfOx sample behaved different magnetism properties in high resistance state and low resistance state (magnetization change of 61.4 %) due to the modulation of defects concentration. The switching behavior and magnetism were both related to the defects in HfOx film. Therefore, the filamentary model consisting of oxygen vacancies and Co ions were proposed to illustrate switching behavior and its relationship with magnetism. Results provide a new idea to clarify the switching mechanism of RRAM, and indicate the promising applications of Co doped HfOx film in the multifunctional electromagnetic devices.

Conduction mechanism and impedance analysis of HfO[formula omitted]-based RRAM at different resistive states

Conduction mechanism and impedance analysis of both the low resistance state (LRS) and high resistance state (HRS) are performed on TiN/HfOx/Pt resistive-switching memory devices at various temperatures. The dc conduction mechanism in LRS is dictated by Ohmic conduction, whereas by trap controlled space-charge-limited current conduction in HRS. The XPS analysis results on the surface of the HfOx film indicated the presence of oxygen deficiencies. The equivalent physical model under various resistance states is established by using ac impedance spectroscopy method, that is related to oxygen vacancies in the HfOx film. We further discussed the activation energy of dc and ac in the HRS and found the conduction occurs at the grain boundary. Through analysis of different resistance states of HfOx-based resistive switching memories, the conduction mechanism correlation between time and frequency domain is established.

Effect of the Oxygen Composition Control of HfOx Films on Threshold and Memory Switching Characteristics for Hybrid Memory Applications

AbstractConsidering a high‐density vertical X‐point memory array, the film thickness of an active device for the selector and memory needs to be scaled down. Here, the authors propose an AgTe/HfOx‐based hybrid memory device with a film thickness of less than 10 nm by adopting a bilayer (HfO1.8/HfO2) film using precise composition control. Non‐volatile switching characteristics can be obtained by considering the formation of stable nanoscale Ag filaments in the nanoporous region in non‐stoichiometric HfO1.8 deposited via sputtering. By contrast, volatile switching characteristics are achieved by an unstable atomic‐scale Ag path in stoichiometric HfO2 deposited via plasma‐enhanced atomic layer deposition. Based on density functional theory calculations, it is confirmed that the stoichiometry of the HfOx film is the key parameter that controls the switching characteristics. The bilayer (HfO1.8/HfO2) device exhibits excellent hybrid memory characteristics, such as distinct read margin (>1.5 V), high ON/OFF ratio (>106), and extremely low OFF current (≈pA), for vertical X‐point memory applications.

Improvement of Resistive Switching Performance in Sulfur-Doped HfOx-Based RRAM.

In order to improve the electrical performance of resistive random access memory (RRAM), sulfur (S)-doping technology for HfOx-based RRAM is systematically investigated in this paper. HfOx films with different S-doping contents are achieved by atmospheric pressure chemical vapor deposition (APCVD) under a series of preparation temperatures. The effect of S on crystallinity, surface topography, element composition of HfOx thin films and resistive switching (RS) performance of HfOx-based devices are discussed. Compared with an undoped device, the VSET/VRESET of the S-doped device with optimal S content (~1.66 At.%) is reduced, and the compliance current (Icc) is limited from 1 mA to 100 μA. Moreover, it also has high uniformity of resistance and voltage, stable endurance, good retention characteristics, fast response speed (SET 6.25 μs/RESET 7.50 μs) and low energy consumption (SET 9.08 nJ/RESET 6.72 nJ). Based on X-ray photoelectron spectroscopy (XPS) data and fitting of the high/low resistance state (HRS/LRS) conduction behavior, a switching mechanism is considered to explain the formation and rupture of conductive filaments (CFs) composed of oxygen vacancies in undoped and S-doped HfOx-based devices. Doping by sulfur is proposed to introduce the appropriate concentration oxygen vacancies into HfOx film and suppress the random formation of CFs in HfOx-based device, and thus improve the performance of the TiN/HfOx/ITO device.

Engineering Method for Tailoring Electrical Characteristics in TiN/TiOx/HfOx/Au Bi-Layer Oxide Memristive Devices

Memristive devices have led to an increased interest in neuromorphic systems. However, different device requirements are needed for the multitude of computation schemes used there. While linear and time-independent conductance modulation is required for machine learning, non-linear and time-dependent properties are necessary for neurobiologically realistic learning schemes. In this context, an adaptation of the resistance switching characteristic is necessary with regard to the desired application. Recently, bi-layer oxide memristive systems have proven to be a suitable device structure for this purpose, as they combine the possibility of a tailored memristive characteristic with low power consumption and uniformity of the device performance. However, this requires technological solutions that allow for precise adjustment of layer thicknesses, defect densities in the oxide layers, and suitable area sizes of the active part of the devices. For this purpose, we have investigated the bi-layer oxide system TiN/TiOx/HfOx/Au with respect to tailored I-V non-linearity, the number of resistance states, electroforming, and operating voltages. Therefore, a 4-inch full device wafer process was used. This process allows a systematic investigation, i.e., the variation of physical device parameters across the wafer as well as a statistical evaluation of the electrical properties with regard to the variability from device to device and from cycle to cycle. For the investigation, the thickness of the HfOx layer was varied between 2 and 8 nm, and the size of the active area of devices was changed between 100 and 2,500 µm2. Furthermore, the influence of the HfOx deposition condition was investigated, which influences the conduction mechanisms from a volume-based, filamentary to an interface-based resistive switching mechanism. Our experimental results are supported by numerical simulations that show the contribution of the HfOx film in the bi-layer memristive system and guide the development of a targeting device.

Uniform switching behavior of HfOx-based memory with gradual-grown filaments by interface modulation

HfOx/Cu/HfOx structures with Ti electrodes were fabricated to investigate the resistive switching characteristics. Stable and uniform switching behaviors can be observed for HfOx/Cu/HfOx sample except the few cycles at the beginning. XPS depth profile of the structure was carried out to explore the role of interface layer on chemical composition of HfOx film and the defects in the films were analyzed. In addition, the resistance in LRS was insensitive to device area. The processes of growth and dissolution of conductive filaments in HfOx film during switching cycles were investigated. The undissolved filaments in the bottom part of HfOx film promoted the growth of filaments in the subsequent set process. As the fixed Vos filaments and stable residual Cu filaments which can act as a precursor in upper and bottom part of the sample were formed, excellently uniform distribution of switching parameters can be obtained. Meanwhile, good endurance and retention properties can be observed for HfOx/Cu/HfOx sample. The migration of Vos and diffusion of Cu atoms under electric field were both responsible for the resistive switching behavior for HfOx/Cu/HfOx sample, and the underlying physical model of conductive filaments was proposed to illustrate the switching mechanism of HfOx/Cu/HfOx sample.

Improved resistive switching performance of graphene oxide-based flexible ReRAM with HfOx buffer layer

Flexible electronic devices have generated large research interest in recent years. Herein, we demonstrate a resistive random access memory (ReRAM) device fabricated on a flexible polyethylene terepthalate (PET) substrate using a bilayer of graphene oxide (GO) and HfOx films. The physical properties of GO and HfOx were characterized by a number of techniques such as X-ray diffraction (XRD), UV–Visible absorption spectroscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The GO/HfOx-bilayered ReRAM devices exhibited good switching behavior at low SET/RESET voltages and showed improved cell-to-cell uniformity of switching parameters compared to the devices without HfOx layer, due to the oxygen vacancies within the HfOx film as confirmed by XPS. The switching mechanism was explained by the formation/rupture of the conductive filament through the migration-induced oxidation/reduction reaction of the oxygen vacancies at the interface of GO/HfOx. From the I–V curve fitting, the conduction in low resistance state (LRS) was found to be governed by the Ohmic mechanism and the trap-controlled space charge limited current (SCLC) in high resistance state (HRS) was observed for both the cycles.

Resistance Change Behavior Under Voltage Pulses at Reset Process of Ti/HfO2/Au-Reram with Various HfO2 Layer Conditions

1. Introduction Recently the application of ReRAM to artificial synapse has been paid much attention in the field of neuromorphic computing([1],[2]). For the implementation of neuromorphic synaptic devices, gradual resistance change response by applying voltage pulse trains is required. We investigated the condition of Ti/HfOx/Au-ReRAM to possess gradual resistance change by changing sputtering conditions of HfOx layer. 2. Sample preparation and experiments Figure 1 shows the structure of Ti/HfOx/Au device. For deposition of HfOx film, we used reactive sputtering with Ar and O2 mixture gases. At this time, we adopted two types of Ar/O2 flow ratios, which is (1)7.8 : 15.6 sccm and (2)7.8 : 2.0 sccm. These devices have a bipolar memory characteristic. SET occurs when a positive voltage is applied, and RESET occurs when a negative voltage is applied. We measured resistance change of RESET process of the ReRAM device with applying DC voltage pulses. 3. Results and discussion Figure 2 shows resistance change behavior with applying voltage pulses at RESET process. Pulse width is 1.0 uA and pulse period is 2.0 uA. Figure 2(a) is the resistance change behavior of device (1) with constant amplitude voltage pulses. Resistance change was rarely observed when pulse amplitude is -1.5 V, however, gradual resistance change more than 1 digit was observed when pulse amplitude is -1.6 V. Moreover, when we applied -1.7 V, gradual resistance change more than 2 digits within 40 pulses was observed. In the case of this device, resistance change was ideal for the use of neuromorphic synaptic device. Figure 2(b) is the results of device (2). Gradual resistance change was observed after 60 pulses when pulse width is -1.5 V. However resistance increased more than 3 digits after a few pulses were applied when pulse amplitudes were -1.6 V and -1.7 V. In the case of device (2), resistance change is almost binary and it is not suitable for use of artificial synapse device. The difference of resistance change behavior may come from spatial variation of oxygen vacancies. For further understanding of the mechanisms, we will evaluate the difference of atomic composition of HfOx layer by XPS and XRD.

Silicon Surface Passivation by Atomic Layer Deposited Hafnium Oxide Films: Trap States Investigation Using Constant Voltage Stress Studies

Excellent silicon surface passivation is demonstrated by the HfOx film deposited using optimized atomic layer deposition (ALD) process. For pristine films, surface recombination velocity (SRV) as low as ~29 cm/s is achieved. The highest effective lifetime of 3 ms, corresponding to SRV of ~5 cm/s, is achieved for hydrogen ambient annealed films. Capacitance-voltage and conductance-voltage measurements confirm that the pristine film consists of positive fixed charges, which remain of the same type or change polarity depending on the postdeposition annealing ambient. Constant voltage stress (CVS) measurements reveal that acceptor-like trap states are present in the film. These trap states are intrinsic in nature and are incorporated in the film structure during ALD growth. CVS study gives an insight to the change in fixed charge polarity in nitrogen ambient annealing, which is not the case in hydrogen ambient. The reduction in surface recombination losses is explained as a combined effect of field and chemical passivation where former dominates in nitrogen and later is dominant in hydrogen ambient annealed films.

Enhanced resistive switching characteristics of HfOx insulator fabricated by atomic layer deposition and La(NO3)3·6H2O solution as catalytic oxidant

Nanocrystalline HfOx films were synthesized by an atomic layer deposition method using Hf[N(CH3)C2H5]4 as the metal precursor and La(NO3)3·6H2O solution as the oxidant. La(NO3)3·6H2O solution played the role of both oxidant and catalyst, catalytic oxidant, where the La element in the deposited HfOx films was under the detection limit. The introduction of La(NO3)3·6H2O solution instead of H2O effectively altered the surface roughness, crystalline status, and resistive switching properties of HfOx films. Although the crystalline structures of both HfOx films made with La(NO3)3·6H2O solution and H2O were monoclinic, the surface roughness of the HfOx film grown by using the La(NO3)3·6H2O solution oxidant is smoother than that using H2O. Moreover, resistive switching characteristics of the HfOx insulator deposited with the La(NO3)3·6H2O solution oxidant enhanced not only uniformity of switching parameters but also endurance.

Selective pulsed chemical vapor deposition of water-free HfOx on Si in preference to SiCOH and passivated SiO2

HfOx was selectively deposited on Si in preference to SiCOH using an oxygen-containing precursor, Hf(OtBu)4, via a pulsed chemical vapor deposition. The water-free process avoids nucleation by water on passivated oxide surfaces. The uniform HfOx film was deposited on Si by the thermal decomposition of the precursor, and the inherent selectivity of HfOx was retained over SiCOH until ~ 2 nm of HfOx was deposited on Si at 250 °C. Additional selectivity enhancement was demonstrated by employing molecular surface passivation and reversible adsorption/desorption of the precursor via control of the purge time. 1,1,3,3-tetramethyldisilazane (TMDS) and Bis(dimethylamino)dimethylsilane (DMADMS) were employed to passivate reactive –OH sites on SiO2 and SiCOH to increase the selectivity to ~ 7 nm HfOx deposition on Si before any significant nucleation occurred on TMDS passivated SiCOH. It is expected that the selective water-free HfOx deposition can be used for patterning nanoscale structures in MOSFET devices.

Modulation of resistive switching behavior of HfOx film by embedding a thin Al buffer layer

In this work, a thin Al buffer layer was embedded in HfOx film to improve its switching behavior. To investigate the role of the interfacial layer on the switching performance of the film, the micro-structure and chemical composition in and near the interface of the HfOx film was analyzed by XPS and TEM. Larger oxygen-deficiency of the HfOx film near the interface can be obtained due to the chemical reaction. Uniform distribution of switching parameters were observed for the HfOx/Al sample due to the creation of oxygen vacancies around Al ions which favor the formation of a fixed conductive path in the HfOx film, and the diffused Al ions in the film can enhance the local electric field and act as a precursor to induce the growth of filaments. In addition, better dissolution of filaments during the reset process led to an enhanced memory window. Results show that oxygen vacancies and Al ions were both responsible for the improved switching behaviors for the HfOx/Al sample. Finally, the switching mechanism for the HfOx/Al sample was illustrated based on the filamentary model.

Controlled surface oxidation of HfSe2 via oxygen-plasma treatment

Growing uniform surface oxides is vital for fabricating 2D materials based electronic and optoelectronic devices. However, the surface transition metal oxides (TMOs) of air-exposure transitional metal dichalcogenides (TMDs), such as HfOx on HfSe2, are island-type, with poor TMOs/TMDs interface quality. Here, we report a controlled surface oxidation of atomically thin HfSe2 flakes via oxygen-plasma treatment, where a dense atomically flat HfOx film is produced. This HfOx film is found to be a good surface passivation layer to prevent the inner part from further oxidation in air, which is also promising as a high-K dielectric in HfSe2-based devices.

Effect of rare element Ce doping concentration on resistive switching of HfOx film

The switching characteristics of rare element Ce-doped HfOx films were investigated. The effect of Ce doping on oxygen defects was analyzed by X-ray photoelectron spectroscopy (XPS) and first-principle calculations. The variations of valence state of Ce and oxygen vacancies with the increase of Ce doping concentration were demonstrated. Although Ce doping increased dopants-induced oxygen vacancies in HfOx film, the density of oxygen vacancies decreased as the doping concentration increased. Besides, the introduction of Ce dopants enlarged the difference in electronegativity of Hf and O which made it harder for O to escape from HfOx film under voltage. Hence the switching behaviors of Ce-doped HfOx film were affected by multiple factors and can be effectively improved with an appropriate doping concentration (4.3%). A physical model based on the formation and rupture of conductive filaments was proposed to clarify the switching behavior of Ce-doped HfOx samples.

Effects of hydrogen annealing temperature on the resistive switching characteristics of HfOx thin films

We investigated the effects of hydrogen annealing temperature on Pt/HfOx/Pt resistive switching memory devices. Memory devices annealed in a hydrogen environment exhibited unipolar resistive switching behaviors with low set, reset, and forming voltages owing to reduction of the HfOx film by the injected hydrogen. The devices annealed at 400 °C exhibited good endurance, lasting 7 times as long as non-annealed device. This improvement could be attributed to the generation of more oxygen vacancies in HfOx, which resulted in more conducting pathways forming during the resistive switching process. The devices annealed in the hydrogen environment also showed longer retention times than those of non-annealed devices. Therefore, hydrogen annealing improves the performance of this nonvolatile switching memory by enabling low power operation at lower operation voltages with greater reliability.

The resistive switching characteristics of Ni-doped HfOx film and its application as a synapse

The effect of Ni doping concentration on resistive switching characteristics of Au/HfOx:Ni/Pt structure was investigated, and the 3.1% Ni-doped HfOx film was used to imitate the function of biological synapse. From the X-ray absorption near edge structure analysis, the forms and the local electronic structure of Ni atoms in the HfOx films with different Ni doping concentrations were investigated. According to the X-ray photoelectron spectroscopy analysis, the Ni doping introduced more oxygen vacancies in HfOx films. Gradual Reset process and concentrated distribution of resistive switching parameters were achieved for 3.1% Ni-doped HfOx films, which could be used to emulate the “learning” and “forgetting” function of the biological synapse. Multiple resistance levels can be observed in the continuously pulse number under identical pulse width of positive or negative voltage pulses. Moreover, the application of pulse-train operation scheme is an effective method to control analog synaptic devices during the Reset process, which can contribute to understand the nature of the conductive nano-filament evolution.

Influneces of different oxygen partial pressures on switching properties of Ni/HfOx/TiN resistive switching devices

The HfOx-based resistive random access memory (RRAM) has been extensively investigated as one of the emerging nonvolatile memory (NVM) candidates due to its excellent memory performance and compatibility with CMOS process. In this study, the influences of deposition ambient, especially the oxygen partial pressure during thin film sputtering, on the resistive switching characteristics are discussed in detail for possible nonvolatile memory applications. The Ni/HfOx/TiN RRAMs are fabricated, and the HfOx films with different oxygen content are deposited by a radio frequency magnetron sputtering at room temperature under different oxygen partial pressures. The oxygen partial pressures in the sputter deposition process are 2%, 4% and 6% relative to engineer oxygen content in the HfOx film. Current-voltage (I-V) measurements, X-ray photoelectron spectroscopy, and atomic force microscopy are performed to explain the possible nature of the stable resistive switching phenomenon. Through the current-voltage measurement, typical resistive switching behavior is observed in Ni/HfOx/TiN device cells. It is found that with the increase of the oxygen partial pressure during the preparation of HfOx films, the stoichiometric ratio of O in the film is improved, the root mean square (RMS) of the surface roughness of the film slightly decreases due to the slower deposition rate under a higher oxygen partial pressure, and the high resistance state (HRS) current decreases. In addition, by controlling the oxygen content of the device, the endurance performance of the device is improved, which reaches up to 103 under a 6% oxygen partial pressure. The HfOx films prepared at a higher oxygen partial pressure supply enough oxygen ions to preserve the switching effect. As the oxygen partial pressure increases, the uniformity of the switching voltage is improved, which can be attributed to the fact that better oxidation prevents the point defects (oxygen vacancies) from aggregating into extended defects. Through the linear fitting and temperature test, it is found that the conduction mechanism of Ni/HfOx/TiN RRAM device cells in low resistance state is an ohmic conduction mechanism, while in high resistance state it is a Schottky emission mechanism. The interface between TE and the oxide layer (HfOx) is expected to influence the resistive switching phenomenon. The activation energy of the device is investigated based on the Arrhenius plots in HRS. A switching model is proposed according to the theory of oxygen vacancy conductive filament. Furthermore, the self-compliance behavior is found and explained.

Effects of Gibbs free energy of interfacial metal oxide on resistive switching characteristics of solution-processed HfOx films

Effects of bottom electrodes (BEs) of Al, Mo, and Pt on resistive switching characteristics of sol-gel HfOx films were investigated in this work. To avoid influences of plasma or thermal energy on HfOx RS characteristic, the top electrodes were formed by pressing indium balls onto the HfOx surface rather than by using a sputter or an evaporator. When using Mo as the BE, the as-deposited HfOx film can give a forming-free resistive switching behavior with low set/reset voltages of 0.28V / − 0.54V. In contrast, non-switching characteristics of the HfOx films were observed when using Al and Pt as the BEs. The HfOx conduction current was found to be highly dependent on the BE. However, when an annealing process at 350°C in an oxygen ambient was performed to the HfOx films on different BEs, the resistive switching behavior of the HfOx/Mo was absent while it can be found in the HfOx/Al sample. Differences in I-V characteristics of the HfOx films on different BEs were explained by considering Gibbs free energies of interfacial oxide layers. X-ray photoelectron spectroscopy (XPS) depth profile was used to examine the interfacial oxide layer. The resistive switching mechanism was also studied.