HfO2 Layer Research Articles

Breakdown transients in high-k multilayered MOS stacks: Role of the oxide–oxide thermal boundary resistance

In this work, breakdown transients of multilayered gate oxide stacks were analyzed to study the impact of the interfaces between oxides on the heat dissipation considering an electromigration-based progressive breakdown model. Using two distinct measurement setups on four different sets of samples, featuring two layers and three layers of Al2O3 and HfO2 interspersed, the breakdown transients were captured and characterized in terms of the degradation rate. Experimental results show that the number of oxide–oxide interfaces present in the multilayered stack has no visible impact on the breakdown growth rate among our samples. This strongly supports the interpretation of the bulk materials dominating the heat transfer to the surroundings of a fully formed conductive filament that shows no electrical differences between our various multilayered stack configurations.

Application of the eutectic high entropy alloy Nb0.73CoCrFeNi2.1 for high temperature joints

The eutectic high entropy alloy Nb0.73CoCrFeNi2.1 was manufactured by means of arc smelting and the obtained ingots were cut into 300-μm-thick foils, which were used as filler alloys to braze Crofer 22 APU to Hf-metallized yttria-stabilized zirconia (3YSZ). The brazing process was conducted in a vacuum furnace at 1200 °C for 5 min at a vacuum of 4.3·10–4 mbar. In order to minimize the intense diffusion and erosion of the steel substrate, a heating and cooling rate of 50 K/min was applied. Sound joints without any pores or flaws were obtained. The microstructure of the joints consisted of an HfO2 reaction layer at the ceramic interface and the same eutectic structure consisting of a Laves phase and a solid solution that was already detected in the smelted foil. The average hardness of the microstructure in the joint seam amounted to 352 ± 17 HV0.01 and the joints reached strength values up to 97 ± 7 MPa while the fracture area was always located at the ceramic interface in the HfO2 layer. Comparable joints, with AgCuTi3 as filler metal, brazed at 920 °C, only reached a shear strength of ~ 52 ± 2 MPa.

Fully Transparent Friction‐Modulation Haptic Device Based on Piezoelectric Thin Film

AbstractStanding Lamb waves in vibrating plates enable haptic interfaces. If the out‐of‐plane displacement of these waves exceeds 1 µm at frequencies above 25 kHz, a silent friction modulation can be created between a human finger and a vibrating plate. A fully transparent friction‐modulation haptic device based on a piezoelectric thin film is demonstrated. The antisymmetric Lamb mode induced at 73 kHz allows for a functional performance that fulfills all conditions for practical use. Out‐of‐plane displacement reaches 2.9 µm when 150 V unipolar voltage is applied. The average transmittance of the whole transducer reaches 75%. The key points of this technology are: 1) a thin HfO2 layer between lead zirconate titanate film and substrate that prevents chemical reaction between them; 2) the efficient integration of transparent indium tin oxide electrodes and solution‐derived piezoelectric lead zirconate titanate thin film onto optical‐grade fused silica; and 3) the use of a transparent insulating layer made of SU‐8 photoresist.

Implementation of Synaptic Device Using Various High-k Gate Dielectric Stacks.

We investigate the characteristics of short-term and long-term synaptic plasticity in a Si-based fieldeffect transistor (FET)-type memory device. An Al₂O₃/HfO₂/Si₃N₄/SiO₂ gate dielectric stack is used to realize short-term and long-term plasticity (STP/LTP). Si₃N₄ and HfO₂ layers are designed to charge trap layer for synaptic device. The mechanism of STP and LTP operation is analyzed by considering the device response to the potentiation and depression pulses and retention measurement of the memory functionality. To investigate the STP operation, paired pulse facilitation (PPF) measurement is performed. The retention characteristic is also studied to validate the LTP property of the device. By investigating a device with an Al₂O₃/HfO₂/Si₃N₄ stack as a control device, it is shown that the Al₂O₃/HfO₂/Si₃N₄/SiO₂ stack device is suitable for a synaptic device in neuromorphic systems.

Core-shell TiO2@HfO2 nanowire arrays with designable shell thicknesses for improved permittivity and energy density in polymer nanocomposites

In this paper, core-shell structured TiO2@HfO2 nanowire arrays with designable HfO2 layer thicknesses were constructed by atomic layer deposition (ALD) method. The effects of TiO2@HfO2 nanowire arrays filler and HfO2 thickness on dielectric and energy storage properties of polyvinylidene fluoride (PVDF) based nanocomposites were investigated. The HfO2 shell thicknesses were controlled via regulating the cycle times of ALD and increased from 2.4 nm to 5.4 nm with the cycle times increment from 13 times to 50 times. Owing to the ferroelectricity of the HfO2 and the hierarchical interfacial polarization originated from the multilevel interface in the composites, the permittivity of the TiO2@HfO2 nanowire array/PVDF nanocomposite with 50 deposition cycles reached 27.4 at 103 Hz, which was 105% larger than that of the TiO2 nanowire array/PVDF nanocomposite. Moreover, the maximal discharged energy density is 2.92 J/cm3 at a low electric field of 138 kV/mm, exceeding more than 97%.

Atomically flat HfO2 layer fabricated by mild oxidation HfS2 with controlled number of layers

In this work, homogeneous surface oxidation of hafnium disulfide (HfS2) is achieved by an extremely simple thermal treatment, i.e., mild oxidization in air. Due to the high thermal stability of hafnium dioxide (HfO2), atomically flat HfO2 is formed on top of unoxidized HfS2 and unlimited layer-by-layer oxidation is observed. The thickness of HfO2 can be controlled by oxidation temperature and time. The HfO2 layer fabricated by the oxidation of HfS2 is smooth and atomically flat with roughness comparable to that of pristine HfS2. Growth of a high-quality, uniform, atomically flat oxide film on top of semiconductor is the first step for the fabrication of field effect transistors and metal-insulator-semiconductor devices. Thus, our results will facilitate the future fabrication of HfO2 films with atomic-scale thickness.

Experimental Investigation of Pulsed Laser Deposition of Ferroelectric Gd:HfO2 in a CMOS BEOL Compatible Process

In this work, we report an experimental investigation of pulsed laser deposition (PLD) of thin Gd:HfO2 layers at 330 °C, which show ferroelectric behavior after annealing at 450 °C, compatible with...

Investigation on Threshold Voltage Adjustment of Threshold Switching Devices with HfO2/Al2O3 Superlattice on Transparent ITO/Glass Substrate.

Threshold voltage adjustment in threshold switching (TS) devices with HfO2/Al2O3 superlattice (by means of changing the cycle ratio of HfO2 to Al2O3 in atomic layer deposition) is investigated to implement a transparent cross-point array. TS devices with different cycle ratios (i.e., 3:1, 3:2, and 3:3) were fabricated and studied. The threshold voltage of the devices was increased from 0.9 V to 3.2 V, as the relative contents of Al2O3 layer in the superlattice were increased. At the same time, it is demonstrated that the off-resistance values of the devices were enhanced from 2.6 × 109 to 6 × 1010 Ω as the atomic layer deposition (ALD) cycle ratio of HfO2 to Al2O3 layer was adjusted from 3:1 to 3:3. However, the hold voltage and the on-current values were almost identical for the three devices. These results can be understood using the larger barrier height of Al2O3 layer than that of HfO2 layer.

Microstructure and properties of a novel designed interpenetrating network structure silicide/HfxZr1−xC multi-phase outer layer

In this paper, a novel interpenetrating network structure was designed for the outer layer of Zr-Hf-Si-C multi-layer and multi-phase coating for carbon matrix to improve the ablation resistance and anti-particle scour. The proposed interpenetrating network structure is prepared via a one-step method of embedding infiltration/thermal evaporation. The microstructural, micro-mechanical, and ablation resistance properties of the coating were studied. Results show that the silicide and HfxZr1−xC(0.50 < x < 0.71) penetrate each other in the outer layer. The mechanical properties have increased by 27.7% in the outer layer owing to the interpenetrating network structure. The multi-phase outer layer with silicide showed a significant improvement in the ablation resistance as compared to that of the multi-phase outer layer without silicide and could protect carbon matrix for >60 s at 2800 ± 70 °C. Under the action of the interpenetrating network structure silicide, the ablation centre and the corresponding transition region form a dense Zr(Hf)O2 layer and Zr(Hf)CxOy region, respectively.

Process Dependent Optimization of Dielectric and Metal Stacks for Multilevel Resistive Random-Access Memory

Recently, multilevel Resistive RAM (ReRAM) stacks have been demonstrated to have promising applications related to inference and learning in artificial intelligence. However, extensive studies on energy efficiency, repeatability, and retention during multilevel operations have not been undertaken previously. It is, therefore, required to investigate the device characteristics such as compliance current (CC) dependence, forming voltage, memory window, symmetry, and switching energy. Furthermore, understanding the impact of dielectric composition and/or electrode materials is necessary to optimize the device performance, such as switching characteristics, endurance, and energy efficiency. In this work, we have investigated a two-terminal ReRAM device, 10nm Ti/50nm TiN/7nm HfO2/1nm Al2O3/PVD Ti/TiN (Fig. 1), with bilayer switching dielectrics of HfO2 and Al2O3. In this case, the embedded Al2O3 layer was placed between the HfO2 layer and the top electrode (TE). We have evaluated the gradual resistance change capability with varying CC.The I-V characteristics of the ReRAM device, measured by a dc double sweep, is shown in Fig. 2. The device switched at a compliance current of 50nA with a forming voltage of 1.85 volts to set to low resistance state (LRS) from the high resistance state (HRS). The subsequent reset from LRS to HRS was obtained during the negative voltage ramp showing a bipolar switching operation (Fig. 2). With the variation of compliance current (50 nA to 70 nA), the LRS was gradually changed indicating multilevel LRS. It is known that by increasing the compliance current with a gradual set process, multiple conducting filaments (CFs) or thicker CFs can form setting the different LRS values and the annihilation of the CFs during the reset process can also moderately vary the HRS values. The resistance ratio between the HRS and LRS was observed to be more than 1000. The devices were subjected to 100 cycles of SET and RESET operations at different CCs without any failures. When the embedded Al2O3 layer was placed between the bottom electrode (BE) and the HfO2 layer, the switching compliance current was increased to 10 mA with a reduced forming voltage of 1.6V. Multilevel LRS states were observed for this device when the CC was varied from 10 mA to 12 mA for 100 cycles at each CCs.To further understand the behavior, TEM depth profile measurements were carried out, and it was observed that Al2O3 diffused from the peak at the as-deposited location into the HfO2 layer in both cases. While this improved the uniformity in switching behavior, the low switching energy consumption in the case of an embedded Al2O3 layer in between the HfO2 layer and the TE layer remains to be investigated. Replacing the PVD Ti layer (Fig. 1) with ALD Ti in the TE when an Al2O3 layer was between the bottom electrode (BE) and the HfO2 layer reduced the switching compliance current from 10 mA to 100 nA. This suggests that the metal quality of the TE can impact the switching behavior of the multilevel ReRAMs. Figure 1

Switching Dynamics Measurement of Ferroelectric Y-Doped HfO2 M-I-M Capacitor

Domain-switching kinetics of 5 mol% Y-doped HfO2 metal-insulator-metal (MIM) capacitor was characterized. The changes in polarization indicate the polarization reversal in HfO2. In this research, we show the polarization reversal of an 20-nm-thick Y-doped HfO2 deposited by atomic layer deposition (ALD) by switching the applied voltage (2.6 to 4.6V) and the pulse duration (1μs to 500μs). We use the measurements of Kolmogorov-Avrami-Ishibashi (KAI) model to extract the minimum switching time of the domains. Introduction Ferroelectric HfO2 layer has attracted attraction in CMOS and memory applications owing to its scaling ability and process compatibility. It has been demonstrated that doping of foreign elements, including Zr, Si, Al, Y, atoms under post-deposition annealing (PDA) or post-metallization annealing (PMA) induces the ferroelectricity in HfO2 [1]. The switching dynamics of ferroelectric Al- and Zr-doped HfO2 metal-insulator-metal (MIM) capacitors has been well-modeled by nucleation limited switching (NLS) model, where multigrain polarization takes place assuming a distribution in the switching time [2, 3]. In this study, we apply the KAI model to fit the switching dynamics of Y-doped HfO2 MIM capacitor using the following equation,, △P=Ps*(1-exp{-(t/τ)β })where △P, P s, τ and β are the partial, saturation polarization, switching time constant and a fitting parameter, respectively [4]. Experiments An MIM capacitor with a 20-nm-thick 5 mol% Y-doped HfO2 film was fabricated by atomic layer deposition (ALD). The top and bottom electrode were both W deposited by sputtering. PMA was then carried out at 500 oC for 1 minute. Typical polarization-voltage (P-V) hysteresis is shown in fig. 1, where a remanent polarization (P r) of 12 mC/cm2 was obtained. A voltage waveform shown in fig. 2 was applied to the electrode to measure the polarization reversal of the devices after certain polling process. Polarization reversal was extracted by applying pulses with various height ranging from 2.6 to 4.6 V and width from 1 ms to 500 ms followed by half P-V measurement. Results and discussions Fig 3 shows measured partial polarization with different pulse width and voltage. As the pulse width increases, △P increases, indicating that a certain time is required to fully switch the domains. Data fit using the KAI model has well reproduced the measured data, indicating the presence of multigrain switching. However, a β of 0.6 to 0.8, which is smaller than those of Zr-doped HfO2 (typically b=2.1), needs to be used to fit the data. This may imply the difference in the domain size among dopants incorporated in the HfO2 layer. Conclusion Switching dynamics of ferroelectric Y-doped HfO2 MIM capacitors has been measured. The device has shown to follow the multidomain switching based on KAI model as is explained for Zr-doped HfO2 ones, except for the difference in the fitting parameter.

Chemical and Biological Sensor Capsules for Real-Time Measurement of Cell Properties in Bioreactors

Introduction Real-time bioprocess monitoring and control is needed for the scalable production and deployment of cancer cell therapies at reasonable cost. For example, CAR-T cell therapy shows promise as an effective treatment for cancer with high (70-90%) remission rates.1 However, the cost of these treatments is too expensive for their mass adoption.2 One reason for the steep price is the difficulty scaling production while ensuring delivery of an efficacious therapy to the patient. One factor preventing scalability is the lack of effective process analytical technologies. As a potential solution to this issue, we have developed a fully integrated, wireless, 3D-printed sensor capsule to be used for multiplexed sensing of critical quality attributes (CQAs), namely pH, glucose, lactate, and IL-2 concentrations within cell bioreactors. CQAs are used not only as metrics for cell viability, but also as determinants of a cell’s ability to deliver efficacious treatment. Unlike current process monitoring technology, capsule sensors are buoyant and can be propelled using impellers within the bioreactor. This allows for measurements uniformly inside the bioreactor. To our knowledge, this is the first time that 3D-printed, wireless sensor capsules have been developed for use in cell bioreactors for cancer cell therapy. Methods The capsule consists of electrochemical sensors as well as data acquisition and wireless transmission electronics integrated into a capsule made of a biocompatible polymer (Figure 1). The sensors for glucose, lactate, pH, and IL-2 along with Ag on-chip pseudo-reference electrode (pseudo-RE) were fabricated using standard lithographic techniques on silicon substrates. The reference electrode consisted of a thin-film of Ag that was later coated with polyvinyl butyral (PVB) to improve stability.3 IL-2 and pH sensing was achieved using gold electrodes. A pH-sensitive Al2O3 or HfO2 layer is deposited through atomic layer deposition on the pH sensor to achieve pH-sensitivity. Glucose and lactate sensing was achieved amperometrically using enzymes, glucose or lactase oxidase respectively, immobilized on a Pt electrode biased to 0.7 V. Detection could also be achieved potentiometrically by using the pH sensor to detect changes in pH caused by conversion of the enzyme product, H2O2, to H+.Figure 1 shows the capsule and sensor design. The capsule was 3D printed using a Stratasys Connex-350 and was designed to be fully insulated from solution except for an opening on the top revealing the sensors. The sensor chip is installed within the cap of the capsule which can be screwed off, but with wired connection to the electronics housed in the body of the capsule. This allows for simple replacement of the sensor chip while retaining the capsule components for reuse. Results and Conclusions Before testing within the capsule, the sensors were tested using microfluidics. Figure 2 shows the performance of the Ag/PVB pseudo-RE. The pseudo-RE shows a stable potential in different pH buffer solutions and similar sensitivity results when compared to a commercial reference electrode for pH sensing. By having a stable microfabricated pseudoRE, the sensing components will not limit miniaturization of the capsule. Figure 3 shows the response of the pH, lactate, and glucose sensors at different concentrations. The pH sensor shows high sensitivity (~53 mV/pH) over the range of pH 3 to 7. Glucose and lactate sensing was tested in 1X PBS. The glucose sensor shows a linear response to glucose concentrations from 1-6 mM with saturation at ~8 mM validating the use of the sensor within cell culture media.4 The lactate sensor shows sensitivity in the concentration range of 1-5 mM with saturation occurring around ~6 mM. The division of T cells requires aerobic glycolysis where glucose is converted to lactate. However, not all glucose is expected to be converted. The lactate concentration expected to be produced is ~1 mM.5 Selectivity towards IL-2 is achieved using anti-IL2 antibodies. Figure 4a shows the functionalization scheme for detection of IL-2. A thiol self-assembled monolayer (SAM) is attached to the gold electrode and then modified for amine conjugation using NHS/EDC. Anti-IL2 can then bind to the SAM and unbound sites are blocked with ethanolamine. Figure 4b reveals antibody attachment occurs optimally at pH 5. Surface plasmon resonance shows successful attachment of 10 ug/mL of IL-2 to the functionalized gold sensor. The next step is to demonstrate IL-2 sensing potentiometrically. The capsule’s wireless sensing capability was validated using pH sensing in a beaker. The pH sensing results in Figure 5b were obtained wirelessly from the capsule and were nearly identical to the results measured using our microfluidic system and Keithley-4200 lab instrument (Fig. 5a). This work has successfully demonstrated wireless sensing using a 3D-printed sensor capsule and functionality of electrochemical sensors for multiple CQAs for cell growth in bioreactors. These sensor capsules can enable scalability of the cell manufacturing process while ensuring delivery of an efficacious treatment. Future work will focus on capsule sensor validation in cell growth media and monitoring of cell growth in bioreactors.

WITHDRAWN: Multi-stack insulator to minimise threshold voltage drift in ZnO FET sensors operating in ionic solutions

Abstract FET biosensors operating in an electrolyte experience a monotonic, temporal and relatively slow change in threshold voltage caused by the hydration of the insulator layer between the electrolyte and the FET's channel. Minimising this temporal change in threshold voltage is critical as, over time, the drain current of n-channel FETs decreases, making it difficult to distinguish between the signal generated in response to analyte - receptor binding events and the background noise generated by the electrolyte and the FET biosensor. While Rapid Thermal Annealing of the insulator layer is known to diminish threshold voltage drift and its negative effects, it is not compatible with a low temperature fabrication process of 200 °C. Our low temperature approach to minimising threshold voltage drift involves depositing a tri-layer insulator stack, consisting of a layer of HfO2 between two Al2O3 layers. Wetting ZnO NWFETs with PBS (10 mM phosphate, 150 mM KCl, pH 7.4) for an hour, showed that ZnO NWFETs with a stack insulator layer experienced a much smaller threshold voltage and drain current drift (100 mV, 0.064 nA) than ZnO NWFETs with a single material insulator layer (≥4300mV, 2.72 nA), Aluminium oxide in this case. Having established the resilience enhancing properties of the stack insulator layer on FETs operating in electrolytes of physiological relevant ionic concentrations; ZnO NWFETs with a stack insulator layer were shown to be capable of detecting the presence of the miDNA-21 strands. This, in effect, paves the way for miRNA sensing experiments in the near future and for exploring the potential of ZnO NWFETs as a diagnostic tool.

Four-terminal polycrystalline Ge1−xSnx thin-film transistors using copper-induced crystallization on glass substrates and their application to enhancement/depletion inverters

Polycrystalline germanium–tin (Ge1−xSnx) thin films with a thickness of 15 nm were grown at 500 °C using Cu-induced crystallization. The films were applied to fabricate four-terminal (4T) thin-film transistors (TFTs) on a glass substrate. The top gate (TG) was fabricated from 30 nm thick SiO2, while the bottom gate (BG) consisted of a double layer of HfO2 and SiO2 with a capacitance equivalent thickness of 16 nm. Aluminum-induced lateral metallization of the source and drain (SD) was implemented to decrease the parasitic resistance of the SD. The Ion/Ioff of the double gate approximately corresponded to 1 × 104. Furthermore, 4T poly-Ge1−xSnx TFTs operated with a field-effect mobility of 20 cm2 V−1 s−1 for the TG drive and 12 cm2 V−1 s−1 for the BG drive at a control gate voltage of 0 V in both cases. A p-channel enhancement/depletion inverter was validated based on the precise control of threshold voltage using a control gate.

(Invited) Reliability and Scaling Perspectives of HfO2-Based OxRAM

Hafnium oxide OxRAM (Oxide Random Access Memory) is a non-volatile memory technology that has been widely investigated over the last 15 years. Its working mechanism relies on the formation and rupture of a tiny conductive filament through a HfO2 layer sandwiched between an active (Ti) and inert (TiN) electrodes. This causes the electrical resistance to very over three orders of magnitudes between a low resistance state (LRS) and a high resistance state (HRS).OxRAM is a strong candidate to replace embedded Flash technology at scaled nodes. This is justified by several considerations but mainly driven by the low cost of integration and very high compatibility with standard CMOS technology. Yet OxRAM faces tremendous challenges in terms of reliability, variability and scaling.In this paper we first show our latest findings in terms of reliability on a Ti/HfO2(5nm)/TiN cell. Controlling LRS and HRS resistance distributions can be achieved with a proper write/erase algorithm design (see Fig. 1) and it is a key point to enable large array integration. A proper write/erase sequence ensures long bit endurance up to 107 cycles. Data retention is then evaluated at four different temperatures (125, 165, 200 and 260 °C) to extract a 10 years retention temperature of 85°C.Next, we discuss OxRAM scaling perspectives. In this context, it is essential to take into account both the resistive cell and its selector (most typically a MOS transistor). As recently shown [2], so far the OxRAM scaling has been mainly limited by the programming current, which determines the MOS selector size. Even if the OxRAM cell is integrated in the BEOL above its selector, the bitcell size is mostly determined by the underlying transistor. In addition, the OxRAM programming voltage dictates the transistor EOT and challenges the transistor reliability for scaled nodes. We demonstrated our OxRAM cell integration with a thin gate oxide FDSOI transistor (GO1/SG) and we show how this paves the OxRAM integration in a 28nm node.Eventually we discuss the ultimate scaling perspective of an OxRAM array in a Cross-Point architecture. To enable such architecture the development of a BEOL selector is mandatory and we focused on Ovonic Threshold Switching device (OTS) for this scope. The OTS works switching between a low-conductive state and a metastable conductive one, which is voltage-triggered. We demonstrated (see Fig. 3) a 1S1R (one selector one resistor) OxRAM cell where the HfO2 memory is co-integrated with a series Ge-Sb-Se-N (GSSN) OTS. We eventually discuss the limits of this architecture showing that a trade-off exists between the half pitch of the Cross-Point array metal lines and the size of the memory bank.REFERENCE[1] G. Sassine et al., 2018 IEEE International Reliability Physics Symposium (IRPS). [2] J. Sandrini et al., 2019 IEEE Electronic Device Meeting (IEDM). [3] D. Alfaro Robayo et al., 2019 IEEE 11th International Memory Workshop (IMW). Figure 1

Process Dependent Optimization of Dielectric and Metal Stacks for Multilevel Resistive Random-Access Memory

A HfO2/Al2O3 bilayer structure for a two-terminal ReRAM device with an intention of having multiple resistance states as a function of compliance current (CC) after forming was evaluated. A reduced power consumption was observed when the Al2O3 buffer layer was placed between the top electrode and the HfO2 layer as compared to when it is embedded between the HfO2 layer and the bottom electrode. Gradual resistance change capability was observed with varying CC. The switching power requirement increases even if the Al2O3 buffer layer thickness was decreased when the buffer layer was near the bottom electrode. It was demonstrated that by modifying the deposition process of the top metal layer the switching energy requirement can be altered.

Performance of H-diamond MOSFETs with high temperature ALD grown HfO2 dielectric

Hydrogen-terminated diamond (H-diamond) metal-oxide-semiconductor field effect transistors (MOSFETs) with different structures were fabricated on the same polycrystalline diamond plate and their performances were compared. A 28-nm-thick HfO2 layer grown at 300 °C by atomic layer deposition (ALD) system was used as both gate dielectric and passivation layer. Devices A and B have the same source-to-drain distance of 6 μm but different gate lengths of 2 μm (device A) and 6 μm (device B), and in device B the gate extends over the source/drain electrodes with the HfO2 film separating them. Device C has a similar structure with device A but a larger gate length (6 μm). Among these devices, device A shows the highest saturation drain current of 190.6 mA/mm at VGS = −8 V, and device B shows the lowest on-resistance of 61.61 Ω·mm. All the devices show a high on/off ratio of about 109 and a gate leakage current lower than 10−10 A. It is also extracted a constant carrier mobility of 37.1 cm2/Vs in the gated channel in a large VGS range of −2 V ≤ VGS ≤ −8 V, which indicates the good interface characteristics between H-diamond and HfO2 and contributes to the high device performances. These results indicate that high conductivity 2DHG and high performance MOSFET devices can also be achieved by depositing high temperature grown HfO2 on the H-diamond surface.

Nanolaminated Al2O3/HfO2 dielectrics for silicon carbide based devices

Nanolaminated aluminum oxide (Al2O3)/hafnium oxide (HfO2) thin films as well as single Al2O3 and HfO2 layers have been grown as gate dielectrics by the plasma enhanced atomic layer deposition technique on silicon carbide (4H-SiC) substrates. All the three dielectric films have been deposited at a temperature as low as 250 °C, with a total thickness of about 30 nm, and, in particular, the nanolaminated Al2O3/HfO2 films have been fabricated by alternating nanometric Al2O3 and HfO2 layers. The structural characteristics and dielectric properties of the nanolaminated Al2O3/HfO2 films have been evaluated and compared to those of the parent Al2O3 and HfO2 single layers. Moreover, the structural properties and their evolution upon annealing treatment at 800 °C have been investigated as a preliminar test for their possible implementation in the device fabrication flow chart. On the basis of the collected data, the nanolaminated films demonstrated to possess promising dielectric behavior with respect to the simple oxide layers.

Radiation sustenance of HfO2/β-Ga2O3 metal-oxide-semiconductor capacitors: gamma irradiation study

β-Ga2O3 is an interesting new generation wide bandgap semiconductor for power device applications. The gamma irradiation was performed on the HfO2/β-Ga2O3-based metal-oxide-semiconductor capacitors at 1 kGy and 200 kGy doses. The leakage current density variation at −1 V is 3.47 × 10−7 A cm−2, 5.90 × 10−7 A cm−2 and 4.91 × 10−6 A cm−2 for the pristine, 1 kGy and 200 kGy devices, respectively. In the case of substrate injection, the Schottky emission mechanism appears to be dominant in the 0.65 to 2.0 MV cm−1 electric field range. The barrier heights (Ni/HfO2/β-Ga2O3) 0.95 eV, 0.86 eV and 0.83 eV were extracted from Schottky emission for the pristine, 1 kGy and 200 kGy doses, respectively. However, as the dose increases to 200 kGy, the charge-trapping favors the trap-assisted Poole–Frenkel (PF) tunneling mechanism. In this case, the PF emission mechanism seems to be dominant for the pristine and 1 kGy in the range of 2.0–4.0 MV cm−1, whereas for 200 kGy it is 1.45 to 4.0 MV cm−1 which indicates that the defect assisted PF tunneling is predominant at 200 kGy irradiation dose. There is an increase in the density of oxide traps (Dot) changes from 1.14 × 1012 cm−2 eV−1 to 1.47 × 1012 cm−2 eV−1, and the density of interface traps has increased from 1.95 × 1011 cm−2 eV−1 to 3.80 × 1011 cm−2 eV−1 for the pristine and 200 kGy samples, respectively. The peaks of Photoluminescence show that the three bands of defects centered at 3.0, 2.7 and 2.2 eV. These peaks have possibly arisen from the vacancies of oxygen. At 200 kGy high dose, the defect band emissions were found at 2.9, 2.6 eV and a broad emission at 2.1 eV indicates the increase in the denisty of oxide-trapped charges within the HfO2 layer.

Memristor with a ferroelectric HfO2 layer: in which case it is a ferroelectric tunnel junction

New interest in the implementation of ferroelectric tunnel junctions has emerged following the discovery of ferroelectric properties in HfO2 films, which are fully compatible with silicon microelectronics technology. The coercive electric field to switch polarization direction in ferroelectric HfO2 is relatively high compared to classical perovskite materials, and thus it can cause the migration of non-ferroelectric charges in HfO2, namely charged oxygen vacancies. The charge redistribution would cause the change of the tunnel barrier shape and following change of the electroresistance effect. In the case of ambiguous ferroelectric properties of HfO2 ultrathin films, this oxygen-driven resistive switching effect can mimic the tunnel electroresistance effect. Here, we demonstrate two separate resistive switching regimes, depending on the applied voltage, in the same memristor device employing a ferroelectric Hf0.5Zr0.5O2 (4.5 nm) layer. The first regime originates from the polarization reversal, whereas the second one is attributed to the accumulation/depletion of the oxygen vacancies at the electrode interface. The modulation of the tunnel barrier causes the enhancement of ROFF/RON ratio in ∼20 times compared to the tunnel electroresistance effect. The developed device was used to formulate the criteria for unambiguous discrimination between the ferroelectric-and non-ferroelectric resistive switching effects in HfO2-based ferroelectric tunnel junctions.