HfO2 Buffer Layer Research Articles

Improved Ga2O3 films on variously oriented Si substrates with Al2O3 or HfO2 buffer layer via atomic layer deposition

Ga2O3 is an ultrawide-band-gap semiconductor with excellent properties and promising applications in the electronic and optoelectronics. For acting as the substrate for heteroepitaxial Ga2O3 films, Si has the advantages of wafer-level size, cheap price and nice compatibility whereas also perform the disadvantage of large thermal mismatch. In this paper, the apparent flower-like defects are exhibited on the macroscopic surfaces of annealed Ga2O3 films deposited on (100), (110) and (111) oriented Si substrates by atomic layer deposition (ALD) method. The reason is ascribed to the different thermal expansion coefficients between Ga2O3 and Si induced compressive and tensile stresses. The Al2O3 or HfO2 buffer layer intercalated between Ga2O3 films and Si substrates via successive ALD process suppress the flower-like defects effectively while the surface roughnesses are low to be 1.290 nm and 2.393 nm, respectively. XRD spectra demonstrate that buffer layer indeed relieves the stress of Ga2O3 films on Si substrates while the amorphous Al2O3 has better influence than the polycrystalline HfO2. The results are helpful to the improvement of growth quality and device property of Ga2O3.

Wedge-shaped HfO2 buffer layer-induced field-free spin-orbit torque switching of HfO2/Pt/Co structure

Field-free spin–orbit torque (SOT) switching of perpendicular magnetization is essential for future spintronic devices. This study demonstrates the field-free switching of perpendicular magnetization in an HfO2/Pt/Co/TaO x structure, which is facilitated by a wedge-shaped HfO2 buffer layer. The field-free switching ratio varies with HfO2 thickness, reaching optimal performance at 25 nm. This phenomenon is attributed to the lateral anisotropy gradient of the Co layer, which is induced by the wedge-shaped HfO2 buffer layer. The thickness gradient of HfO2 along the wedge creates a corresponding lateral anisotropy gradient in the Co layer, correlating with the switching ratio. These findings indicate that field-free SOT switching can be achieved through designing buffer layer, offering a novel approach to innovating spin–orbit device.

Enhanced Switching Reliability of Hf0.5Zr0.5O2 Ferroelectric Films Induced by Interface Engineering.

Ferroelectric materials have been widely researched for applications in memory and energy storage. Among these materials and benefiting from their excellent chemical compatibility with complementary metal-oxide-semiconductor (CMOS) devices, hafnia-based ferroelectric thin films hold great promise for highly scaled semiconductor memories, including nonvolatile ferroelectric capacitors and transistors. However, variation in the switched polarization of this material during field cycling and a limited understanding of the responsible mechanisms have impeded their implementation in technology. Here, we show that ferroelectric Hf0.5Zr0.5O2 (HZO) capacitors that are nearly free of polarization "wake-up"─a gradual increase in switched polarization as a function of the number of switching cycles─can be achieved by introducing ultrathin HfO2 buffer layers at the HZO/electrodes interface. High-resolution transmission electron microscopy (HRTEM) reveals crystallite sizes substantially greater than the film thickness for the buffer layer capacitors, indicating that the presence of the buffer layers influences the crystallization of the film (e.g., a lower ratio of nucleation rate to growth rate) during postdeposition annealing. This evidently promotes the formation of a polar orthorhombic (O) phase in the as-fabricated buffer layer samples. Synchrotron X-ray diffraction (XRD) reveals the conversion of the nonpolar tetragonal (T) phase to the polar orthorhombic (O) phase during electric field cycling in the control (no buffer) devices, consistent with the polarization wake-up observed for these capacitors. The extent of T-O transformation in the nonbuffer samples is directly dependent on the duration over which the field is applied. These results provide insight into the role of the HZO/electrodes interface in the performance of hafnia-based ferroelectrics and the mechanisms driving the polarization wake-up effect.

Pseudo-flexible resistive switching characteristics of nano-bowl-like NiO arrays on mica substrates

Nano-bowl-like NiO (nb-NiO) arrays on flexible Pt-coated mica substrates have been fabricated by electro-deposition and magnetron sputtering with the assistance of the monolayer colloidal sphere template. The nano-bowl array makes the polycrystalline NiO film thickness change periodically in the range of 150–450 nm, and the HfO2 buffer layer effectively depresses the local leakage current. A voltage which is lower than ±0.4 V can make the Ag/HfO2/nb-NiO/Pt cell reversibly switch between the high and the low resistance states within 11 μs. This cell exhibits a ratio of the high to the low resistance (RH/RL) of 103 ∼ 104, a data retention time over 104 s and an endurance of >1200 cycles. Driven by the non-uniform electric field, oxygen ions accumulated at the HfO2/NiO interface prefer to occur the Fowler-Nordheim tunneling at the thinnest bowl bottom, which decreases switching voltages. The bent cell almost keeps initial resistive switching properties before the bending radius (r) is below 2.5 cm. Though its properties greatly degrade when the r further reduces below 1.5 cm, it can recover most of initial memory window after returning to the flat state again, showing potential applications in new types of pseudo-flexible memory devices.

Temperature-dependent effect of modulation in graphene-supported metamaterials

We report on a novel effect of temperature-dependent modulation in graphene-supported metamaterials. The effect was observed during the theoretical analysis of a model graphene-supported electro-optical modulator having silicon dioxide (SiO2) or hafnium dioxide (HfO2) as a buffer dielectric layer. Comparative analysis of the two materials showed that they provide approximately the same maximum values for transmission and reflection modulation depths. However, in the case of a HfO2 buffer layer, a lower chemical potential of the graphene is required to achieve the maximum value. Moreover, theoretical calculations revealed that a lower gate voltage (up to 6.4 times) is required to be applied in the case of a HfO2 layer to achieve the same graphene chemical potential. The graphene layer was found to possesses high absorption (due to the additional resonance excitation) for some values of chemical potential and this effect is extremely temperature dependent. The discovered modulation effect was demonstrated to further increase the transmission modulation depth for the simple model structure up to 2.7 times (from 18.4% to 50.1%), while for the reflection modulation depth, this enhancement was equal to 2.2 times (from 24.4% to 52.8%). The novel modulation effect could easily be adopted and applied over a wide range of metadevices which would serve as a quick booster for the development of related research areas.

Preparation and characterization of HfO2/VO2/HfO2 sandwich structures with low phase transition temperature, excellent thermochromic properties, and superior durability

In the last few decades, smart windows made from VO2-based thermochromic films have attracted extensive attention, but their actual commercial applications are limited by low luminous transmittance (Tlum), low solar modulation ability (ΔTsol), high phase transition temperature (Tc), and poor durability. In this study, glass/HfO2/VO2/HfO2 tri-layer films were designed and deposited on glass substrates by pulse laser deposition. Crystal structures, surface morphology, surface roughness, electrical properties, and optical properties of as-prepared sandwich structure films were analyzed. Results showed that both HfO2 buffer layer and antireflection layer (ARL) were monoclinic phase and grew along the (020) and (−111) crystal planes, respectively. HfO2 buffer layer not only reduced Tc of VO2 film by about 20 °C, but also played an important role in regulating crystal quality and surface morphology of VO2 films. More importantly, by covering films with HfO2 ARL, Tlum and ΔTsol of VO2 film were greatly improved. In particular, when the thicknesses of HfO2 buffer layer and ARL were 80 nm and 120 nm, the obtained HfO2/VO2/HfO2 tri-layer film reached a balance between high Tlum (∼47.2%), high ΔTsol (∼9.1%) and low Tc (∼49.1 °C). In addition, after 216 h of boiling water treatment, Tlum and ΔTsol of HfO2/VO2/HfO2 film covered with 120 nm thick ARL still remained at 49.3% and 7.0%, showing excellent durability. This research provides a new strategy for designing VO2-based smart windows with high performance and good durability.

Investigation of the superconducting properties of NbN films deposited by DC magnetron sputtering on a high-k dielectric HfO2 buffer layer

The influence of the buffer layer of hafnia dielectric (HfO2) on the superconducting properties of niobium nitride films (NbN), produced by the technique of reactive magnetron depositing has been investigated for the first time. This study presents a comprehensive analysis of morphological, microstructural, and electrophysical parameters of thin NbN films. The dependence of transition critical temperature on the thickness of the HfO2 buffer layer has been obtained. For the first time, it has been demonstrated that the superconducting properties of niobium nitride films at the HfO2 buffer layer have high superconductive parameters, namely, T c ∼ 13 K (with a the film thickness of approximately 30 nm) and high critical current density of approximately 107 A cm−2. The relation between the thickness of the HfO2 buffer layer with the critical current density of niobium nitride films has been determined. It appeared that the values of critical current density increase significantly up to 13 MA cm− 2 when the thickness of the hafnia buffer layer is more than 2 nm.

Epitaxial growth technique for single-crystalline PbTiO3 thin film on Si substrate using an HfO2 buffer layer

We have succeeded in obtaining thin-film single crystals of PbTiO3 (PT) piezoelectrics on a Si single crystal with a few defects in the film at the extremely low temperature of 450 °C, which was considered difficult in the past. We confirmed that the obtained thin film demonstrates dielectric characteristics (lattice constant: a = 0.3904 nm, c = 0.4159 nm; relative dielectric constant: 120), along with an extremely high residual polarization value (Pr = 85 μC cm−2). A martensitic transformation in the HfO2 buffer layer plays an important role in the c-domain formation of the epitaxial PT film by an elastic lattice matching. This discovery will demonstrate a new function of HfO2 film and indicates that the system can be applied in a broad range of scenarios seeking to obtain single crystals for various functional oxide layer thin films on Si wafer substrates.

IMPROVEMENT IN STRUCTURAL, OPTICAL AND ELECTRICAL PROPERTIES OF ITO FILM THROUGH AlN AND HfO2BUFFER LAYERS

Indium Tin Oxide (ITO) films were deposited on glass substrate using radiofrequency (RF) magnetron sputtering technique. To improve the physical characteristics of the ITO film, AlN and HfO2buffer layers were deposited on glass prior to the film deposition. The ITO/glass, ITO/AlN/glass and ITO/HfO2/glass films were annealed using CO2laser and electrical oven heating methods. The crystallinity of the ITO film was improved due to the incorporation of AlN and HfO2buffer layers and also by the post-deposition annealing process. The optical transmittance of the ITO was also increased due to the presence of the buffer layers. Similarly, the annealed ITO films grown on buffer layers exhibited lower values of the sheet resistance as compared to the film deposited without buffer layers. The laser annealing technique was more found to be more effective in reducing the ITO sheet resistance.

Silicon-integrated lead-free BaTiO3-based film capacitors with excellent energy storage performance and highly stable irradiation resistance

The energy storage density and thermal stability of silicon-based lead-free BZTS film capacitors are greatly improved by inserting a high resistance dielectric HfO2 buffer layer between the BZTS thin film and the Si substrate.

Integration of perovskite Pb[Zr0.35Ti0.65]O3/HfO2 ferroelectric-dielectric composite film on Si substrate

PurposeThe purpose of this paper is to investigate the effect of high-k material HfO2 as a buffer layer for the fabrication of metal-ferroelectric-insulator-silicon (MFeIS) structures on Si (100) substrate.Design/methodology/approachRF-sputtered Pb[Zr0.35Ti0.65]O3 or (PZT) and plasma-enhanced atomic layer deposited HfO2 films were selected as the ferroelectric and high-k buffer layer, respectively, for the fabrication of metal-ferroelectric-insulator-silicon (MFeIS) structures on Si (100) substrate. Multiple angle ellipsometry and X-ray diffraction analysis was carried out to obtain the crystal orientation, refractive index and absorption coefficient parameters of the deposited/annealed films. In the different range of annealing temperature, the refractive index was observed in the range of 2.9 to 2 and 1.86 to 2.64 for the PZT and HfO2 films, respectivelyFindingsElectrical and ferroelectric properties of the dielectric and ferroelectric films and their stacks were obtained by fabricating the metal/ferroelectric/silicon (MFeS), metal/ferroelectric/metal, metal/insulator/silicon and MFeIS capacitor structures. A closed hysteresis loop with remnant polarization of 4.6 µC/cm2 and coercive voltage of 2.1 V was observed in the PZT film annealed at 5000 C. Introduction of HfO2 buffer layer (10 nm) improves the memory window from 5.12 V in MFeS to 6.4 V in MFeIS structure with one order reduction in the leakage current density. The same MFeS device was found having excellent fatigue resistance property for greater than 1010 read/write cycles and data retention time more than 3 h.Originality/valueThe MFeIS structure has been fabricated with constant PZT thickness and varied buffer layer (HfO2) thickness. Electrical characteristics shows the improved leakage current and memory window in the MFeIS structures as compared to the MFeS structures. Optimized MFeIS structure with 10-nm buffer layer shows the excellent ferroelectric properties with endurance greater than E10 read/write cycles and data retention time higher than 3 h. The above properties indicate the MFe(100 nm)I(10 nm)S gate stack as a potential candidate for the FeFET-based nonvolatile memory applications.

Impact of HfO2 buffer layer on the electrical characteristics of ferroelectric/high-k gate stack for nonvolatile memory applications

For the proposed work, the electrical properties of metal–ferroelectric–insulator–silicon (MFeIS) capacitors with Sr0.8Bi2.2Ta2O9 (SBT) ferroelectric film deposited on HfO2/Si substrate have been investigated. The SBT film was deposited by RF sputtering and HfO2 film by plasma-enhanced atomic layer deposition (PEALD). The structural characteristic of the deposited ferroelectric and dielectric films was obtained using X-ray diffraction and multiple angle ellipsometric analysis. XRD results indicate the polycrystalline and perovskite structure of the SBT film and amorphous structure of the HfO2 film annealed at different temperatures. SBT film deposited on the silicon substrate and annealed at 500 °C shows the maximum refractive index of 3.46 with the maximum grain size of 32 nm. Metal/ferroelectric/silicon (MFeS), metal/ferroelectric/metal (MFeM), metal/insulator/silicon (MIS) and metal/ferroelectric/insulator/silicon (MFeIS) structures were fabricated to obtain the electrical and ferroelectric properties. MFeIS structure with 10 nm buffer layer shows the improved memory window of 5 V as compared to the 3.07 V in the MFeS structures. MFeI(10 nm)S structure even shows endurance higher than 1012 read/write cycles and data retention for more than 8 h.

Integration of ferroelectric BIT and dielectric HfO2 on silicon substrate with high data retention and endurance for ferroelectric FET applications

For this proposed work, the electrical and ferroelectric properties of metal–ferroelectric–insulator–silicon (MFeIS) and metal–ferroelectric–insulator–metal (MFeIM) capacitors with Bi4Ti3O12 (BIT) ferroelectric film deposited on HfO2/Si substrate were investigated. Physical vapor deposition technique (RF sputtering) was carried out for the deposition of 100 nm ferroelectric and high-k dielectric film of 5, 10 and 15 nm thickness. The structural properties such as crystallographic phase, grain size with composition and refractive index of the deposited films were measured by X-ray diffraction, field emission scanning electron microscopy with energy dispersive spectroscopy (FESEM-EDS) and multiple angle ellipsometry. Metal/ferroelectric/silicon (MFeS), metal/ferroelectric/metal (MFeM), metal/insulator/silicon (MIS), MFeIS and MFeIM structures were fabricated to obtain the electrical and ferroelectric properties. Investigation shows that the MFeIS structure with 10 nm buffer layer demonstrates improved memory window of 8.81 V as compared to the 3.3 V in the MFeS structure. MFeIM with 10 nm HfO2 buffer layer shows maximum remnant polarization of 4.05 μC/cm2. MFeI (10 nm) S structure even shows endurance higher than 1013 read/write cycles and data retention for more than 10 years. The reliability of the ferroelectric and ferroelectric/dielectric stack was obtained by measuring the breakdown voltage characteristics.

Morphological and electrical properties of RF sputtered calcium copper titanate thin films with the incorporation of intermediate layer

In this research work, an attempt has been undertaken to improve the structural, morphological and electrical properties of RF sputtered CCTO thin films with the incorporation of HfO2 as a buffer layer and Zn as an intermediate layer. RF sputtering technique was used to deposit all the films with the optimized sputtering parameters. XRD pattern of HfO2 buffered CCTO thin films have shown an increase in (220) peak intensity with the enhancement in buffer layer thickness from 2.5 nm to 20 nm. Field emission scanning electron microscope images depicted the formation of smooth surface morphology. In addition, improved electrical and interface properties are observed with the increase in buffer layer thickness. CCTO films, deposited on 20 nm of HfO2 buffer layer, have shown lower leakage current of 3.15 × 10−7 Amp. In addition, reduced Dit value of 5.3 × 1010 eV−1cm−2 has been observed for film with 20 nm buffer layers. Improved morphological and electrical properties in CCTO films are attributed to suppression of defect states due to the presence of HfO2 buffer layer. Furthermore, the effect of different position of Zn intermediate layer, sandwiched between two CCTO layers, was also investigated. The appearance of splitted (220) peak has confirmed the incorporation of Zn in CCTO lattice. The surface morphology was found to be strongly dependent upon the multilayer geometry. Improved dielectric constant of 1078 at 100 kHz frequency is observed for the film configuration of 40-10-60, in which the Zn layer is situated towards the middle position.

Impact of plasma enhanced atomic layer deposited HfO2 buffer layer on the structural, electrical and ferroelectric properties of metal/ferroelectric/insulator/semiconductor gate stack for non-volatile memory applications

For the proposed work, electrical and ferroelectric characteristics of metal–ferroelectric–insulator–silicon (MFeIS) and metal–ferroelectric–insulator–metal (MFeIM) capacitor structures with Bi4Ti3O12 (BIT) as the ferroelectric film deposited on HfO2/Si substrate has been investigated. RF sputtering has been used for the deposition of BIT layer on the plasma enhanced atomic layer deposited (PEALD) HfO2/Si substrate. The structural properties like crystallographic phase, grain size with composition and refractive index of the deposited films has been measured using X-ray diffraction, Field emission scanning electron microscopy with energy dispersive spectroscopy (FESEM-EDS) and multiple angle ellipsometry. The electrical and ferroelectric properties has been obtained by fabricating metal/ferroelectric/silicon (MFeS), metal/ferroelectric/metal (MFeM), metal/insulator/silicon (MIS), metal/ferroelectric/insulator/silicon (MFeIS) and metal–ferroelectric–insulator–metal (MFeIM) structures and characterizing using keithley 4200 SCS and Precision Premier II ferroelectric tester. Results indicate the improved memory window from 3.3 V in MFeS structure to 6.3 V in MFeIS structure with 10 nm buffer. The MFeIM structure with 10 nm buffer layer shows the maximum remnant polarization of 4.11 μC/cm2 with endurance higher than 1012 read/write cycles and data retention time greater than 10 years.

The permittivity and refractive index measurements of doped barium titanate (BT-BCN)

While piezoelectric- ferroelectric materials offer great potential for nonvolatile random access memory, most commonly implemented ferroelectrics contain lead which imposes a challenge in meeting environmental regulations. One promising candidate for lead-free, ferroelectric material based memory is (1−x)BaTiO3−xBa(Cu1/3Nb2/3)O3 (BT-BCN), x=0.025. The samples studied here were grown on a Si substrate with an HfO2 buffer layer, thereby preventing the interdiffusion of BT-BTCN into Si. This study provides further insight into the physical behavior of BT-BCN that will strengthen the foundation for developing switching devices. The sample thicknesses ranged from 1.5 to 120 nm, and piezoelectric force microscopy was employed in order to understand the local ferroelectric behaviors. Dielectric constant as a function of frequency demonstrated enhanced frequency dispersion indicating the polar nature of the composition. The relative permittivity was found to change significantly with varying bias voltage and exhibited a tunability of 82%. The difference in the peak position during up and down sweeps is due to the presence of the spontaneous polarization. Furthermore, reflectometry was performed to determine the refractive index of samples with differing thicknesses. Our results demonstrate that refractive indices are similar to that of barium titanate. This is a promising result indicating that improved ferroelectric properties are obtained without compromising the optical properties.

Ultra-low voltage resistive switching of HfO2 buffered (001) epitaxial NiO films deposited on metal seed layers

A set of (001) epitaxial NiO films were prepared on highly textured (001) Pt seed layers using magnetron sputtering, and their resistive switching performance was measured. Cube-to-cube epitaxial relationships of NiO(001)//Pt(001) and NiO[001]//Pt[001] were demonstrated. Current-voltage measurements revealed that the Ag/(001)NiO/(001)Pt capacitor structures exhibited stable bipolar switching behavior with an ON/OFF ratio of 20 and an endurance of over 5 × 103 cycles. Furthermore, inserting a HfO2 buffer layer between the NiO film and the Ag top electrode increased the ON/OFF ratio to more than 103 and reduced the SET/RESET voltage to below ±0.2 V. These enhancements are attributed to the differing filament growth mechanisms that occur in the NiO and HfO2 layers. The present work suggests that Ag/HfO2/(001)NiO/(001)Pt capacitor structures are a promising technology for next-generation, ultra-low voltage resistive switching memory.

Efficient Piezoelectric Energy Harvesters Utilizing (001) Textured Bimorph PZT Films on Flexible Metal Foils

Extracting energy from low vibration frequencies (<10 Hz) using piezoelectric energy harvester promises continuous self‐powering for sensors and wearables. The piezoelectric compliant mechanism (PCM) design provides a significantly higher efficiency by fostering a uniform strain for its 1st mode shape, and so is interesting for this application. In this paper, a PCM energy harvester with bimorph Pb(Zr,Ti)O3 (PZT) films on Ni foil deposited by rf magnetron sputtering is shown to have high efficiency and large power for low frequency mechanical vibration. In particular, {001} textured PZT films are deposited on both sides of polished Ni foils with (100) oriented LaNiO3 seed layers on HfO2 buffer layers. The performance of PCM with an active area of 5.2 cm2 is explored for various excitation accelerations (0.02–0.16 g [g = 9.8 m s−2]) around 6 Hz. The PCM device provides a power level of 3.9 mW cm−2 g2 and 65% mode shape efficiencies.

The effects of a HfO2 buffer layer on Al2O3‐passivated indium‐gallium‐zinc‐oxide thin film transistors

AbstractIn this study, the effects of a HfO2 buffer layer between an Al2O3 passivation layer and an indium‐gallium‐zinc‐oxide (IGZO) thin film transistor (TFT) were investigated to enhance the environmental stability and electrical properties of the IGZO TFT. When applying the HfO2 buffer layer, the formation of HfAlx Oy at the Al2O3/HfO2 interface reduces the water vapor transmission rate (WVTR) due to densification of the passivation layer. Consequently, by adding the HfO2 buffer layer, WVTR of the TFT was highly improved; moreover, changes in the electrical characteristics can be prevented compared to both the non‐passivated and Al2O3‐passivated TFTs. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-κ Insulator Thin Films and Retention Properties of MFIS Diodes

ABSTRACTFerroelectric field effect transistor (FFET) is a promising candidate for non-volatile random access memory because of its high speed, single device structure, low power consumption, and nondestructive read-out operation. Currently, however, such ideal devices are commercially not available due to poor interface properties between ferroelectric film and Si substrate, such as leakage current and interdiffusion etc. So we choose YSZ and HfO2 insulating thin films as buffer layer due to they possess relatively high dielectric constant, high thermal stability, low leakage current, and good interface property with Si substrates. Two structural diodes of Pt/BNT/YSZ/Si and Pt/SBT/HfO2/Si were fabricated, and the microstructures, interface properties, C-V, I-V, and retention properties were investigated in detail. Experimental results show that the fabricated diodes exhibit excellent long-term retention properties, which is due to the good interface and the low leakage density, demonstrating that the YSZ and HfO2 buffer layers are playing a critical modulation role between the ferroelectric thin film and Si substrate.