Hf Layer Research Articles

The mechanical properties and wear resistance of Hf-Ta-N coatings prepared by double glow plasma alloying technology

Traditional metal nitrides often perform brittle fracture under heavy loads, limiting its use as wear resistant coatings. Doping a transition metal into binary nitrides can form ternary nitrides, which would have higher toughness and greater wear resistance. In this study, Hf-Ta-N coatings were prepared by double glow plasma alloying technology at different Ar/N2 ratios. The nitride content on the surface is not positively correlated with the nitrogen ratio. Under the 1:1 Ar/N2 ratio, the coating surface has the highest content of nitride. The Hf-Ta-N coatings can be divided into two parts, the outer Hf-Ta-N deposited layer and the inner Hf, Ta and Ti diffusion layer. XRD results reveal that the peaks shift due to the solid solution doping. With the rise of nitrogen flow rate, the hardness and elastic modulus increase first and then decrease, which is consistent with the nitride content on the surface. Under the 1:1 Ar/N2 ratio, the Hf-Ta-N coating prepared has the best wear resistance benefitting from its excellent mechanical property. Under the 3:1 Ar/N2 ratio, the Hf-Ta-N coating prepared was worn out during the friction test resulting from its poor mechanical property.

Novel behaviors of coercivity in GdFeCo/Hf/MgO heterostructure

It is commonly known that the coercivity (Hc) of the rare-earth/transition-metal (RE–TM) compound is tuned based on the varying RE content as compared to the TM component. The drawback of this approach is that the Hc changes are permanent. In this work, we investigate the coercivity behaviors of the GdFeCo/Hf/MgO heterostructure where the heavy metal Hf layer is inserted in the middle not only to convert charge current into spin current but also to prevent the oxygen diffusion effect. A strong geometry dependence of coercivity detected on Hall bar devices is attributed to the intrinsic properties of GdFeCo and elucidates that the oxidation issue from MgO on GdFeCo is prevented. By selecting a proper Gd content, we demonstrate that the coercivity can be altered flexibly in a wide range Hcmax/Hcmin≈200% via electric current. We develop a simple model to simulate the roles of Joule heating and spin–orbit torques in the novel behaviors of coercivity. It is verified that the excellent magnetic characteristics of GdFeCo are still preserved while manipulating coercivity by the electric current. Our findings broaden the new magnetic behaviors of RE–TM alloys, making them attractive for tunable magnetic anisotropy.

Orbit-Engineered Anisotropic Magnetoresistive Effect for Constructing a Magnetic Sensor with Ultrahigh Sensitivity.

A strong anisotropic magnetoresistance (AMR) effect induced by spin-orbit coupling is the basis for constructing a highly sensitive and reliable magnetic sensor. Presently, effective AMR enhancement in traditional films focuses on the modulation of the lattice or charge degree of freedom, leading to a general AMR ratio below 4%. Here, we demonstrate a different strategy to strengthen the AMR effect by tuning the orbital degree of freedom. By inserting an oxygen-affinitive Hf layer into a Ta/MgO/NiFe/MgO/Ta multilayer film, Fe-O orbital hybridization at the MgO/NiFe interface was modulated to trigger an effective orbital reconfiguration of Fe. In turn, the number of holes in the in-plane symmetric d orbits of Fe increased substantially, facilitating the s-d electron scattering to enhance the AMR ratio to 4.8%. By further micromachining the film into a Wheatstone bridge, we constructed a sensing element that displayed an ultrahigh sensitivity of 2.7 mV/V/Oe and a low noise detectability of 0.8 nT/√Hz. These findings help to advance the development of orbit-governed AMR sensors and provide an alternative method for tuning other orbit-related physical effects.

Radiation-induced phase separation in nanostructured Hf-In-C ternary thin films under irradiation with 200 keV Ar+ ion beam

Thin films consisting of 17 groups of Hf/In/C multilayers cyclically alternating layers of Hf, In and C each with a thickness of 4–5 nm were synthesized by ion sputtering using a 25 keV Ar+ ion beam with 400 µA current and targets made of pure hafnium, indium and carbon. The films were subsequently annealed in vacuum at 120°C for 24 hours to induce intermixing of elements phases, their interaction, and formation of the Hf-In-C nanostructures (including the Hf2InC MAX phase). After fabrication, a part of the pristine (as deposited) samples was irradiated by 200 keV Ar+ ions at high fluences 1015 and 1017 cm−2. Both samples (as prepared and irradiated) were analyzed by IBA nuclear analytical methods, as well as by AFM and TEM microscopic techniques, and by XPS and profilometry to understand the microstructural evolution. Moreover, nanoindentation analysis was performed to assess the effects of ion irradiation on the microstructure and mechanical properties of the films. The experimental results showed that thin Hf-In-C nanostructured films can be formed by ion sputtering with promising mechanical parameters. The irradiated Hf-In-C films were found to be resistant only up to a fluence of about 1015 Ar cm−2. At higher fluences it degrades, and Hf2InC transforms to the binary HfC0.95 phase due to sublimation of In. At 1017 cm−2 the original matrix (including M2AX) is destroyed, and instead, a mixture of MX binary phases (e.g. HfC0.95) and crystalline oxides (e.g. HfO2 and In2O3) are formed.

Nanoscale deposition of Group IVB elements on anodized surfaces to reduce friction

Reduction of dry friction losses is among top technical priorities. Recently, nanoscale deposition of Ti layers on anodic alumina demonstrated unusually low dry friction, but no clear comparison to other elements had been provided. In this study, magnetron sputtering and Atomic Layer Deposition (ALD) were used to form nanometric layers of Cu, Cr, Hf, Nb, Ti, Zr and their oxides on several types of anodic alumina. Elements of IVB group were more effective than layers of Cu, Cr or Hf, which are widely used in anti-wear coatings. Only layers of 15–75 nm thickness remained effective, while 180 nm or thicker layers did not reduce friction significantly. Properties of materials within nanometric layers appeared to contribute to friction reduction much more compared to insignificant effects of hardness, roughness or substrate interactions. Also, inert substrates might represent a favorable matrix for studying the fundamentals of dry friction on deposited nanoscale structures.

Performance enhancement of p-type organic thin-film transistors by surface modification of hybrid dielectrics

The quality of the dielectric/organic semiconductor interface is a critical issue, because it determines the charge transport properties in organic thin-film transistors (OTFTs). High- k organic-inorganic hybrid films have received considerable attention for their outstanding dielectric properties, including low leakage currents, high breakdown fields, and suitable band offsets against the organic semiconductor. However, Hf and Zr hybrid gate dielectrics on p-type OTFTs show poor charge transport properties in the organic semiconductor channel, due to the polaron disorder elicited by the high- k properties and the presence of the –N(CH 3 ) 2 polarity (hole trapper) on the dielectric/semiconductor interface. In this report, the surface of the Hf and Zr hybrid dielectrics was capped by an ultra-thin poly-1,3,5-trivinyl-1,3,5,-trimethyl-cyclosiloxane (pV3D3) layer formed via an initiated chemical vapor deposition (iCVD) process, to modify the hybrid dielectrics/semiconductor interface. The pV3D3-capped Hf and Zr hybrid OTFTs show an enhanced V T stability while a large amount of V T shift was observed from the Hf and Zr hybrid OTFTs. This large amount of V T shift is attributed to the hole trap sites originated by –N(CH 3 ) 2 on the uncapped hybrid dielectrics. Furthermore, the p-type OTFTs with the pV3D3-capped hybrid dielectrics show a higher mobility than those with the uncapped hybrid dielectrics. The presence of the non-polar/low- k pV3D3 on the hybrids contribute to narrow the density of state (DOS) in the organic channel, improving the charge transport properties. This combined approach using the bulk layer of Hf and Zr hybrid films and the pV3D3 capping layer can overcome the limitations of single-layer hybrid dielectrics and improve the overall device performance of the OTFTs. • Performance Enhancement of p-type OTFTs through simple surface modification. • Ultra-thin (~1 nm) non-polar low-k dielectric on the hybrid. • Simple process to change the surface properties. • Systematic investigation of interface quality which affect charge transport in organic semiconductor.

Enhanced spin–orbit torque switching in perpendicular multilayers via interfacial oxygen tunability

Spin–orbit torque (SOT) offers a promising pathway to electrically manipulate magnetization in perpendicular multilayers, but the ultrahigh current density required for SOT switching limits its applications. Here, we report that field-free SOT switching is achieved in perpendicular Ta/CoFeB/MgO multilayers by inserting ultrathin Mg or Hf layers. A critical current density of 1.18 × 107 A/cm2 is obtained in Ta/CoFeB/Mg(0.1 nm)/MgO multilayers for field-free SOT switching, which is 42% lower than that in the Ta/CoFeB/MgO sample. The results demonstrate that the enhanced SOT switching efficiency is determined by a modified Rashba interface induced by interfacial orbital hybridization due to the presence of an ultrathin inserted layer. Furthermore, SOT exhibits a significant dependence on the interfacial structure, especially the interfacial oxygen content. Our findings provide an effective insight into the interfacial manipulation of SOT-based spintronic devices.

Precise chemical state analyses of ultrathin hafnium films deposited on clean Si(111)-7 × 7 surface using high-resolution core-level photoelectron spectroscopy

Herein, well-defined ultrathin hafnium films on Si(111) -7 × 7 [Hf/Si(111)] were prepared by using electron beam heating in an ultrahigh vacuum (UHV) chamber at a pressure below 1.1 × 10−8 Pa. High-resolution core-level photoelectron spectroscopy of the Si 2p1/2, 3/2 and Hf 4f5/2, 7/2 levels combined with a synchrotron radiation X-ray light source was employed to reveal the chemical states at the interface and surface of ultrathin Hf/Si(111) films. Ultrathin Hf layers grow on clean Si(111)-7 × 7 surfaces by a lever rule that the first phase is not consumed when subsequent phases form. Therefore, it was found that the surface and interface of Hf/Si(111) certainly contains three components, which can be ascribed as metallic Hf layers, Hf monosilicide (HfSi), and Si-rich Hf silicide just above the Si substrate (HfSi4). The metallic Hf layers were observed to comprise some diffused Si atoms. According to the results obtained from photoelectron spectroscopy and scanning electron microscopy, ultrathin Hf layers change into HfSi2 islands on a bare Si(111)-7 × 7 surface after annealing at 1073 K. Small dot-shaped islands of HfSi2 appeared after annealing a two-monolayer thick Hf sample; whereas, rectangular HfSi2 islands including metallic Hf species randomly appeared after annealing a six-monolayer thick Hf sample. The long axes of the rectangle islands expanded in a direction that connected the corner holes in a model with dimers, adatoms, and stacking-faults (DAS model) of a clean Si(111)-7 × 7 surface. Our precise chemical analyses will be very useful for preparing well-defined interfaces and surface structures for obtaining next generation 3D metal-oxide-semiconductor field effect transistors and ferroelectric devices.

Initial oxidation processes of ultrathin hafnium film and hafnium disilicide islands on Si(100)-2 × 1 surfaces studied using core-level X-ray photoelectron spectroscopy

We investigated the initial oxidation of ultrathin hafnium (Hf) film on Si(100)-2 × 1 [Hf/Si(100)] using high resolution Hf 4f5/2, 7/2, Si 2p1/2, 3/2, and O 1s core-level photoelectron spectroscopy. Ultrathin Hf/Si(100) film was prepared using electron-beam evaporation in an ultrahigh vacuum chamber below 1.5 × 10−8 Pa. Our results revealed the formation of metallic Hf layers containing a few Si atoms on the Si(100)-2 × 1 substrate. A Hf monosilicide (HfSi) component was also formed in the vicinity of the interface region. Metallic Hf rapidly oxidized, transforming into hafnium dioxide (HfO2) and its suboxides, after exposure to O2 molecules at <4.1 Langmuir. In accordance with the oxidation of metallic Hf, Si atoms in metallic Hf layers were also oxidized into typical Si (sub)oxides and Hf silicate. The other HfSi component at the interface was nearly unreactive with O2 molecules. These facts suggest that the metallic Hf component plays a vital role in the initial oxidation of the ultrathin Hf/Si(100) film. After annealing from 873 K to 973 K, the Hf suboxides in low ionic valences progressed into fully oxidized HfO2. Once the annealing temperature reached ~1073 K, oxygen atoms were entirely removed from the ultrathin HfO2/Si(100) film containing SiO2 at the interface. Simultaneously, ultrathin HfO2 layers changed into islands of Hf disilicide (i-HfSi2) on a bare Si(100)-2 × 1 surface. The i-HfSi2 component showed slight reactivity with O2 molecules at 298 K. In contrast to the initial oxidation of clean Si(100)-2 × 1 surface, the dangling bonds on bare Si(100)-2 × 1 surface among i-HfSi2 oxidized preferentially. This was due to some back-bonds of the Si dimers on bare Si(100)-2 × 1 that were occupied by Hf atoms in the form of HfSi2.

The growth and unique electronic properties of the porous-alumina-assisted hafnium-oxide nanostructured films

Hafnium-oxide nanostructures of controlled composition and properties are in demand for modern electronic and optical engineering. Here hafnium-oxide nanostructured films are synthesized on substrates via the anodizing/re-anodizing of a thin Hf layer through a porous anodic alumina (PAA) film at 150/400 V in 0.2 m H3PO4 electrolyte and examined by SEM, EIS, and Mott-Schottky analysis. The films are composed of HfOx nanorods, which grow in the pores, anchored to a continuous HfO2 bottom layer by tiny HfOx nanoroots penetrating the alumina barrier layer. The understanding of film nucleation and growth is advanced through disclosing the hidden features and field-assisted modifications of the metal-oxide interfaces, such as O2-filled nanosized voids and individual hafnium-oxide nanoroots dominating within the alumina barrier layer and securing electron transport to each nanorod. The films reveal a unique combination of electrical properties, such that the bottom oxide behaves like an ideal dielectric whereas the roots and rods show semiconducting behavior. A 600 °C annealing at atmospheric pressure surprisingly leaves the rods and roots semiconducting and still amorphous while the annealing at 10−5 Pa transforms the entire film to a nanocrystallite-containing n-type semiconductor, of a high doping density of up to 2 1021 cm−3. Further anodic polarization of the PAA-free vacuum-annealed film generates dielectric anodic oxide over the film, of thickness proportional to applied potential until the thinner roots and finally the dominating root become fully blocked for electron conduction. Potential applications include high-k dielectrics for high-voltage electrolytic capacitors, semiconducting active layers for gas sensing, or photoanodes for photoelectrochemical water splitting.

Fabrication of NbTiN Josephson Junctions With Thermally Oxidized Hf Tunnel Barriers

We fabricated NbTiN Josephson junctions (JJs) with HfO x barriers on thermally oxidized Si substrates. Hf overlayers with different resistivities were used for the HfO x barrier formation. The Hf overlayer with low resistivity produced junctions with high quality parameters. The critical current density (J c ) was obtained in the range of 0.01-0.4 kA/cm 2 by changing the thermal oxidation conditions of the Hf layer. The critical current uniformity was estimated at a maximum-to-minimum spread of ±10.1% for 85 of the 100 JJs. We evaluated the specific capacitance, which was approximately 110 fF/μm 2 for the J c ranging from 0.1 to 0.4 kA/cm 2 . The quasiparticle tunnel characteristics were improved by using the NbN electrodes instead of the NbTiN electrodes.

Spin-orbit torques acting upon a perpendicularly magnetized Py layer

We show that Py, a commonly used soft ferromagnetic material with weak anisotropy, can become perpendicularly magnetized while depositing on a Ta buffer layer with Hf or Zr insertion layers (ILs) and an MgO capping layer. By using two different approaches, namely, harmonic voltage measurement and hysteresis loop shift measurement, the dampinglike spin-orbit torque (DL-SOT) efficiencies from Ta/IL/Py/IL/MgO magnetic heterostructures with perpendicular magnetic anisotropy are characterized. We find that though Ta has a significant spin Hall effect, the DL-SOT efficiencies are small in systems with the Ta/Py interface compared to that obtained from the control sample with the traditional Ta/CoFeB interface. Our results indicate that the spin transparency for the Ta/Py interface is much less than that for the Ta/CoFeB interface, which might be related to the variation of spin mixing conductance for different interfaces.

The effect of inserted layers on the anomalous Hall effect and perpendicular magnetic anisotropy in Ta/CoFeB/MgO heterostructures

In this work, Ta/M/CoFeB/MgO/Ta multilayers were designed to investigate the effect of M insertion on the anomalous Hall effect (AHE) and perpendicular magnetic anisotropy (PMA). Once inserted, Bi did not remain at the interface of Ta and CoFeB; it migrated to the surface of the films, which was not beneficial to PMA. X-ray photoelectron spectroscopic analyses revealed that the segregation of Ta was partly suppressed by the insertion of an Mo, Hf or Pt layer. However, the formation of a CoPt (or FePt) alloy decreased the PMA in the case of a Pt layer. The sample with Hf and Mo still exhibited a clear PMA after annealing at a 350 and 500 °C, respectively.

Hf layer thickness dependence of resistive switching characteristics of Ti/Hf/HfO2/Au resistive random access memory device

Resistive random access memory (ReRAM) devices with a HfO2 dielectric layer have been studied extensively owing to the good reproducibility of their SET/RESET switching properties. Furthermore, it was reported that a thin Hf layer next to a HfO2 layer stabilized switching properties because of the oxygen scavenging effect. In this work, we studied the Hf thickness dependence of the resistance switching characteristics of a Ti/Hf/HfO2/Au ReRAM device. It is found that the optimum Hf thickness is approximately 10 nm to obtain good reproducibility of SET/RESET voltages with a small RESET current. However, when the Hf thickness was very small (∼2 nm), the device failed after the first RESET process owing to the very large RESET current. In the case of a very thick Hf layer (∼20 nm), RESET did not occur owing to the formation of a leaky dielectric layer. We observed the occurrence of multiple resistance states in the RESET process of the device with a Hf thickness of 10 nm by increasing the RESET voltage stepwise.

Effects of Hf buffer layer at the Y-doped HfO2/Si interface on ferroelectric characteristics of Y-doped HfO2 films formed by reactive sputtering

10 nm-thick ferroelectric (FE) HfO2 films with 1.5 mol% yttrium-doping were fabricated by mid-frequency reactive magnetron co-sputtering deposition on bare Si and underlying metal Hf. Yttrium was incorporated into the HfO2 layers by simultaneously sputtering from Y and Hf metal targets under oxygen atmosphere. The effects of interfacial layer on structural and electrical properties of Y:HfO2 films have been systemically studied using high-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD) and electrical measurements. Based on TEM and XRD measurements, the presence of a non-centrosymmetric orthorhombic phase which is responsible for ferroelectricity in yttrium-doped HfO2 (Y:HfO2) films could be detected. As an initial protective layer, the ultra-thin metal Hf (~1 nm) layer covers the silicon surface and prevents the native oxide growth during the deposition and post-annealing process. Results reveal that the thickness of interfacial layer is significantly reduced by deposit a Hf buffer layer and a remanent polarization Pr of up to 14 μC/cm2 can be detected in P-V measurement. The strategy of using Hf buffer layer to improve electrical properties of HfO2 film is a feasible method for the fabrication of metal-insulator-semiconductor (MIS) capacitor based on FE-HfO2/Si gate stack.

Fast Low-Current Spin-Orbit-Torque Switching of Magnetic Tunnel Junctions through Atomic Modifications of the Free-Layer Interfaces

Future applications of spin-orbit torque will require new mechanisms to improve the efficiency for switching nanoscale magnetic tunnel junctions (MTJs), while also controlling the magnetic dynamics to achieve fast, nanosecond scale performance with low write error rates. Here we demonstrate a strategy to simultaneously enhance the interfacial magnetic anisotropy energy and suppress interfacial spin memory loss by introducing sub-atomic and monatomic layers of Hf at the top and bottom interfaces of the ferromagnetic free layer of an in-plane magnetized three-terminal MTJ device. When combined with a beta-W spin Hall channel that generates spin-orbit torque, the cumulative effect is a switching current density of 5.4 x 106 A/cm2, more than a factor of 3 lower than demonstrated in any other spin-orbit-torque magnetic memory device at room temperature, and highly reliable switching with current pulses only 2 ns long.

Characterization of the interfacial structure and perpendicular magnetic anisotropy in CoFeB‐MgO structures with different buffer layers

In this study, we describe the deposition of Hf and Mo metal layers individually on Ta to compose new buffer layers, ie, Ta/Hf and Ta/Mo, where CoFeB/MgO stacks are deposited using magnetron sputtering. The synthesised Ta/Hf buffer has higher surface roughness, while the Ta/Mo buffer has lower surface roughness as compared with the Ta buffer. The surface roughness of the buffer appears to influence the interface of the subsequently deposited layers, resulting in rougher or smoother CoFeB/MgO interfaces. Additionally, we present a report on the magnetic properties of Ta, Ta/Hf, and Ta/Mo buffer samples. As the annealing temperature is below 200 °C, the saturation magnetisation (Ms) values for all buffer layers increase at similar rates, whereas the effective magnetic anisotropy energy (Keff) values increase at varying rates. After annealing at 350 °C, Keff reaches its maximum value for Ta/Hf and Ta/Mo buffer layers, whereas the CoFeB/MgO interface width decreases to a minimum value. The width increases as the annealing temperature is increased over 350 °C, and Keff gradually decreases with increase in the annealing temperature. The CoFeB/MgO interface width is primarily dependent on the buffer/CoFeB interface width, which is a critical parameter to obtain high perpendicular magnetic anisotropy (PMA) and high‐quality films. This work provides perspectives for understanding and controlling PMA from the viewpoint of interfacial structure.

Interfacial chemical structure-modulated anomalous Hall effect in perpendicular Co/Pt multilayers

Abstract The enhancement of anomalous Hall effect (AHE) has been observed by inserting a Hf metallic layer at the Co/HfO2 interface in perpendicular HfO2/[Co/Pt]2/Co/HfO2 multilayers. It is displayed that the saturation anomalous Hall resistivity is 46% larger than that in Co/Pt multilayers without Hf insertion. Meanwhile, thermally stable AHE property is obtained in perpendicular HfO2/[Co/Pt]2/Co/Hf/HfO2 multilayers. The X-ray photoelectron spectroscopy analysis reveals that the improved AHE originates from the modulation of chemical states at the Co/HfO2 interface, owing to the insertion of the Hf layer.

Strong perpendicular magnetic anisotropy energy density at Fe alloy/HfO2 interfaces

We report on the perpendicular magnetic anisotropy (PMA) behavior of heavy metal (HM)/Fe alloy/MgO thin film heterostructures when an ultrathin HfO2 passivation layer is inserted between the Fe alloy and MgO. This is accomplished by depositing one to two atomic layers of Hf onto the Fe alloy before the subsequent rf sputter deposition of the MgO layer. This Hf layer is fully oxidized during the subsequent deposition of the MgO layer, as confirmed by X-ray photoelectron spectroscopy measurements. The HfO2 insertion generates a strong interfacial perpendicular anisotropy energy density without any post-fabrication annealing treatment, for example, 1.7 erg/cm2 for the Ta/Fe60Co20B20/HfO2/MgO heterostructure. We also demonstrate PMA even in Ni80Fe20/HfO2/MgO structures for low-damping, low-magnetostriction Ni80Fe20 thin films. Depending on the choice of the HM, further enhancements of the PMA can be realized by thermal annealing to at least 400 oC. We show that ultra-thin HfO2 layers offer a range of options for enhancing the PMA properties of magnetic heterostructures for spintronics applications.

The impact of Hf layer thickness on the perpendicular magnetic anisotropy in Hf/CoFeB/MgO/Ta films

The impact of Hf layer thickness on magnetic anisotropy is evaluated for Hf/CoFeB/MgO/Ta multilayer films commonly used in magnetic tunnel junctions (MTJs). An easy-axis magnetization transition from the in-plane to the out-of-plane direction is observed when the thickness of Hf is greater than ∼1.5 nm in our structure. Moreover, a critical Hf layer thickness exists for strong perpendicular magnetic anisotropy on the CoFeB/MgO interface that maintains the properties required for use in MTJs. We also perform X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD) to study the effects of film composition, chemical states, and crystallization on the magnetic anisotropy in Hf/FeCoB/MgO/Ta multilayers with Hf layers of various thicknesses.