Underlayer Material Research Articles

Effect of the underlayer on the elastic parameters of the CoFeB/MgO heterostructures

We investigated the thermally induced surface acoustic waves in CoFeB/MgO heterostructures with different underlayer materials. Our results show a direct correlation between the density and elastic parameters of the underlayer materials and the surface phonon dispersion. Using finite element method-based simulations, we calculate the effective elastic parameters (such as elastic tensor, Young’s modulus, and Poisson’s ratio) for multilayers with different underlayer materials. The simulation results, either considering the elastic parameters of individual layers or considering the effective elastic parameters of whole stacks, exhibit good agreement with the experimental data. This study will help us deepen our understanding of phonon properties and their interactions with other quasiparticles or magnetic textures with the help of these estimated elastic properties.

Strong Impact of Underlayers on the Voltage‐Controlled Magnetic Anisotropy in Interface Engineered Co/MgO Junctions with Heavy Metals

AbstractThe voltage‐controlled magnetic anisotropy (VCMA) effect in ferromagnets is of crucial interest for next‐generation non‐volatile magnetic memory technologies since it enables spin manipulation with low power consumption in addition to high‐speed operation and high writing endurance. Although intense efforts are made in the past decade to increase the efficiency of the purely electronic VCMA effect, the physical origin of this phenomenon is still elusive, particularly for interface‐engineered ferromagnetic materials. Here, a pathway is proposed to tune the VCMA effect by exploiting a combination of the insertion of heavy metals and underlayers. Focusing on Co/MgO junction deposited on Os underlayer, a large VCMA effect of −100 fJ V−1 m−1 is demonstrated with Pt insertion, which is ≈50% greater than the case deposited on Pt underlayer. On the other hand, smaller VCMA effect is observed with Ir insertion, and even a positive VCMA effect is observed with Os insertion, which are vastly different from the case deposited on Pt underlayer. The systematic variation of VCMA coefficient depending on the underlayer materials implies the effect of electron depletion from underlayers. The concepts demonstrated here can be applicable for other spintronic materials and may open a way to interfacial spin‐orbit engineering for VCMA effect.

Half‐Metallic Full‐Heusler and Half‐Heusler Compounds with Perpendicular Magnetic Anisotropy

Herein, a mechanism is proposed as to how thin films formed from a Heusler compound can simultaneously have both perpendicular magnetic anisotropy (PMA) and be half‐metallic. It is proposed that a thin film formed from a half‐metallic full‐Heusler or half‐Heusler compound that is cubic in the bulk can undergo a tetragonal distortion by adopting the lattice constant of the underlayer material during the thin film deposition process. The value of this distortion can be tuned by using underlayer materials with different in‐plane lattice constants. The distortion can thereby be optimized so that it is large enough to give rise to sufficient PMA, while, simultaneously, small enough to retain the half‐metallic properties (and, therefore, high tunneling magnetoresistance properties) of the Heusler compound. Density functional theory (DFT) calculations that are carried out for 90 full‐Heuslers and 147 half‐Heuslers that have been identified in the literature as half‐metals show that of these, 14 full‐Heusler and 59 half‐Heusler compounds display both half‐metallicity and PMA for optimal tetragonal distortions.

Computational design and optimization of nanostructured AlN deep-UV grating reflectors.

Deep-ultraviolet (DUV) optoelectronics require innovative light collimation and extraction schemes for wall-plug efficiency improvements. In this work, we computationally survey material limitations and opportunities for intense, wavelength-tunable DUV reflection using AlN-based periodic hole and pillar arrays. Refractive-index limitations for underlayer materials supporting reflection were identified, and MgF2 was chosen as a suitable low-index underlayer for further study. Optical resonances giving rise to intense reflection were then analyzed in AlN/MgF2 nanostructures by varying film thickness, duty cycle, and illumination incidence angle, and were categorized by the emergence of Fano modes sustained by guided mode resonances (holes) or Mie-like dipole resonances (pillars). The phase-offset conditions between complementary modes that sustain high reflectance (%R) were related to a thickness-to-pitch ratio (TPR) parameter, which depended on the geometry-specific resonant mechanism involved (e.g., guided mode vs. Mie dipole resonances) and yielded nearly wavelength-invariant behavior. A rational design space was constructed by pointwise TPR optimization for the entire DUV range (200-320 nm). As a proof of concept, this optimized phase space was used to design reflectors for key DUV wavelengths and achieved corresponding maximum %R of 85% at λ = 211 nm to >97% at λ = 320 nm.

Effects of underlayer on structures and magnetic properties of Fe<sub>65</sub>Co<sub>35</sub> alloy films

Fe<sub>100-<i>x</i></sub>Co<sub><i>x</i></sub> (<i>x</i> = 30–40) alloys have the highest saturation magnetizations, 4π<i>M</i><sub>s</sub> ≥ 24 kG (1 G = 10<sup>–4</sup> T). Therefore, FeCo thin flms have been widely used in microwave magnetic devices. However, the as-deposited FeCo film has a large coercivity, which is attributed to the large saturation magnetostriction and high magneto-crystalline anisotropy<i>.</i> On the basis of maintaining high saturation magnetization, adding an appropriate underlayer is a simple and effective method to reduce the coercivity of the film and facilitate the magnetic field-induced in-plane uniaxial magnetic anisotropy. Since these kinds of films are used in a high-frequency environment, the eddy current loss in GHz band must be considered. For a certain film material, the thinner the film, the lower the eddy current loss is. However, at present, the thickness of ferromagnetic layer is generally tens of nanometers or even hundreds of nanometers, which will not help to suppress the eddy current loss at high frequency. In the present study, to obtain FeCo films with good soft magnetic properties and excellent high-frequency characteristics, Fe<sub>65</sub>Co<sub>35</sub> alloy films with a thickness of 13 nm and different underlayers (Cu, Co and Ni<sub>80</sub>Fe<sub>20</sub>) are prepared by magnetron sputtering. The effects of different underlayer materials and different NiFe underlayer thickness values on the structures and magnetic properties of FeCo films are studied. The results show that the introduction of underlayers can increase the in-plane uniaxial magnetic anisotropies of films, and the soft magnetic properties of films are significantly improved. The reason why the good soft magnetic properties can be achieved is attributed to the grain refinement, the dipolar interaction between layers, and the reduction of surface roughness. For different underlayer materials with the same thickness, NiFe underlayer can obviously improve the soft magnetic properties of FeCo films: the covercivity of easy axis is 23 Oe. By changing the thickness of NiFe underlayer, the dynamic magnetic properties of films can be adjusted. The resonance frequency changes from 3.13 GHz for NiFe(1 nm)/FeCo(13 nm) film to 2.78 GHz for NiFe(9.3 nm)/ FeCo(13 nm) film. For all NiFe/FeCo bilayer films, the real part of the permeability <i>μ′</i> at low frequency has a large value of 350–450, and the damping coefficient <i>α</i> shows a small value of 0.01–0.02. In addition, the smaller film thickness can reduce eddy current loss, which contributes to its application in high-frequency microwave magnetic devices.

Crystallographically Oriented Hybrid Perovskites via Thermal Vacuum Codeposition

Hybrid lead halide perovskites typically form polycrystalline films that have multiple grain sizes and surface defects. A key engineering challenge toward commercialization is therefore the production of homogeneous, defect‐free large‐area devices achieving high efficiency. New market opportunities may arise from vacuum‐deposited perovskites if detailed understanding and control of crystal formation are available. Of the many factors that make reproducibility of device performance difficult, two variables are identified that have not yet been considered in detail: deposition speed and underlayer material selection. Herein, it is demonstrated that small changes in the perovskite growth rate (0.18–0.72 Å·s−1) substantially affect the preferred crystal orientation. Further, varying underlayer interfaces greatly influence the composition of the final perovskite and thus its energetic profile. The research aids control in fine‐tuning the perovskite film at the nanometer scale, which enables the reproducible fabrication of vertically aligned and micrometer‐sized grain features, highly demanded for in high‐quality semiconductors.

Vacuum processed large area doped thin-film crystals: A new approach for high-performance organic electronics

Rubrene single crystal domains with hundreds of micrometers size fully covering different substrates are achieved by thermal annealing of evaporated amorphous thin films with the help of a thin glassy underlayer. The sufficiently large energy level offset of the underlayer material and rubrene enables high performance staggered bottom gate rubrene crystalline transistors with maximum field-effect linear mobility over 5 cm2V−1s−1 (μAvg = 4.35 ± 0.76 cm2V−1s−1) for short channel devices of 20 μm, comparable to high quality rubrene bulk single crystals. Moreover, since molecular dopants up to several mole percent can be incorporated into the single crystals with a minimal disturbance of the lattice, the contact resistance of the transistors is significantly reduced to around 1 kOhm.cm by contact doping via adlayer epitaxy of p-type doped rubrene. Our results pave the way for novel high-performance organic electronics using crystalline active materials with mass-production compatible deposition techniques.

Effects of Crystal Orientation of Al Thin Film and Underlayer Materials on Stress-induced Nanowire Growth

Effects of Crystal Orientation of Al Thin Film and Underlayer Materials on Stress-induced Nanowire Growth

Preparation and Properties of Intrinsically Atomic-Oxygen Resistant Polyimide Films Containing Polyhedral Oligomeric Silsesquioxane (POSS) in the Side Chains.

The relatively poor atomic-oxygen (AO) resistance of the standard polyimide (PI) films greatly limits the wide applications in low earth orbit (LEO) environments. The introduction of polyhedral oligomeric silsesquioxane (POSS) units into the molecular structures of the PI films has been proven to be an effective procedure for enhancing the AO resistance of the PI films. In the current work, a series of POSS-substituted poly (pyromellitic anhydride-4,4′-oxydianiline) (PMDA-ODA) films (POSS-PI) with different POSS contents were synthesized via a POSS-containing diamine, N-[(heptaisobutyl-POSS)propyl]-3,5-diaminobenzamide (DABA-POSS). Subsequently, the effects of the molecular structures on the thermal, tensile, optical, and especially the AO-erosion behaviors of the POSS-PI films were investigated. The incorporation of the latent POSS substituents decreased the thermal stability and the high-temperature dimensional stability of the pristine PI-0 (PMDA-ODA) film. For instance, the PI-30 film with the DABA-POSS content of 30 wt% in the film exhibited a 5% weight loss temperature (T5%) of 512 °C and a coefficient of linear thermal expansion (CTE) of 54.6 × 10−6/K in the temperature range of 50–250 °C, respectively, which were all inferior to those of the PI-0 film (T5% = 574 °C; CTE = 28.9 × 10−6/K). In addition, the tensile properties of the POSS-containing PI films were also deteriorated, to some extent, due to the incorporation of the DABA-POSS components. The tensile strength (TS) of the POSS-PI films decreased with the order of PI-0 > PI-10 > PI-15 > PI-20 > PI-25 > PI-30, and so did the tensile modulus (TM) and the elongations at break (Eb). PI-30 showed the TS, TM, and Eb values of 75.0 MPa, 1.55 GPa, and 16.1%, respectively, which were all lower than those of the PI-0 film (TS = 131.0 MPa, TM = 1.88 GPa, Eb = 73.2%). Nevertheless, the incorporation of POSS components obviously increased the AO resistance of the PI films. All of the POSS-PI films survived from the AO exposure with the total fluence of 2.16 × 1021 atoms/cm2, while PI-0 was totally eroded under the same circumstance. The PI-30 film showed an AO erosion yield (Es) of 1.1 × 10−25 cm3/atom, which was approximately 3.67% of the PI-0 film (Es = 3.0 × 10−24 cm3/atom). Inert silica or silicate passivation layers were detected on the surface of the POSS-PI films after AO exposure, which efficiently prevented the further erosion of the under-layer materials.

Measurement of Peptide Coating Thickness and Chemical Composition Using XPS.

X-ray photoelectron spectroscopy is a highly surface-sensitive analytical technique, capable of providing quantitative information on the chemical composition of materials within the top ∼10nm of their surface. For samples consisting of distinct underlayer and overlayer materials, the thickness of the coating can also be determined if it falls within this ∼10nm information depth, which is often the case for peptide layers. Such measurements are simple to perform for flat samples and can also be performed on nanoparticulate samples provided that either the core radius or total particle radius are known. Here, we describe a straightforward protocol for obtaining such measurements from peptide coatings on both flat surfaces and nanoparticles, including preparation of nanoparticle samples from suspension, data acquisition, and analysis.

Underlayer material dependent symmetric and asymmetric behavior of voltage-controlled magnetic anisotropy in CoFeB films

Voltage-controlled magnetic anisotropy (VCMA), observed at the interfaces of ultrathin ferromagnetic metallic films and oxide layer, has proven to be a useful tool for the development of all-electric field controlled spintronics devices. Here, we have studied the symmetric and asymmetric behavior of VCMA in CoFeB/MgO heterostructures, grown on different underlayer materials, by measuring ferromagnetic resonance using spin pumping and inverse spin Hall effect technique. We observe symmetric behavior of VCMA in CoFeB films with Ta underlayer, whereas a systematic transformation from symmetric to asymmetric behavior of VCMA with decreasing CoFeB thickness is observed for Pt underlayer. We speculate that the increased interfacial roughness, defects and strain of ultrathin CoFeB films with Pt buffer layer probably leads to the complicated band structure at CoFeB/MgO interface resulting in asymmetric behavior of VCMA. The observed symmetric behavior of VCMA in control samples justifies the role of interfacial roughness, defects and discards the role of oxide overlayer on the observed asymmetric behavior of VCMA in ultrathin CoFeB films.

Perpendicular magnetic anisotropy in TbFeCo/MgO structure with Ta- and Hf-underlayer

In this study, the interfacial PMA in (Ta or Hf)/TbFeCo/MgO/Ta structures are investigated and the effectiveness of two kinds of heavy-metal Hf and Ta underlayers are explored. The PMA of TbFeCo is found to critically rely on several aspects like Tb-concentration (Tb sputtering power), underlayer material (Ta/Hf), annealing temperature and heating rate of annealing. The crystalline phase of the underlayer is investigated by XRD and resistivity measurements and optimized by changing the film thickness. It was observed that for the Hf underlayer the PMA could be obtained in as-deposited samples irrespective of the Tb-concentration with squareness close to unity. The post-annealing results show improved PMA in FeCo-dominant films; however, the PMA reduces when Tb-content is increased.

Perpendicular magnetic anisotropy in Pt/Co-based full Heusler alloy/MgO thin-film structures

Perpendicular magnetic anisotropy (PMA) in ultrathin magnetic structures is a key ingredient for the development of electrically controlled spintronic devices. Due to their relatively large spin-polarization, high Curie temperature and low Gilbert damping the Co-based full Heusler alloys are of special importance from a scientific and applications point of view. Here, we study the mechanisms responsible for the PMA in Pt/Co-based full Heusler alloy/MgO thin films structures. We show that the ultrathin Heusler films exhibit strong PMA even in the absence of magnetic annealing. By means of ferromagnetic resonance experiments, we demonstrate that the effective magnetization shows a two-regime behavior depending on the thickness of the Heusler layers. Using Auger spectroscopy measurements, we evidence interdiffusion at the underlayer/Heusler interface and the formation of an interfacial CoFe-rich layer which causes the two-regime behavior. In the case of the ultrathin films, the interfacial CoFe-rich layer promotes the strong PMA through the electronic hybridization of the metal alloy and oxygen orbitals across the ferromagnet/MgO interface. In addition, the interfacial CoFe-rich layer it is also generating an increase of the Gilbert damping for the ultrathin films beyond the spin-pumping effect. Our results illustrate that the strong PMA is not an intrinsic property of the Heusler/MgO interface but it is actively influenced by the interdiffusion, which can be tuned by a proper choice of the underlayer material, as we show for the case of the Pt, Ta and Cr underlayers.

Cu(111) preferential orientation on thin HfN film as a diffusion barrier

We examined the effect of barrier thickness on Cu grain orientation control. We demonstrate that 5 nm thick HfN films which showed excellent barrier properties in our previous study are applicable to the underlayer for Cu grain orientation control. The strong (111)-oriented Cu layer is enhanced by using the HfN barrier with a 5 nm thickness. The obtained results bring about the compatibility of the high performance barrier with a suitable underlayer to obtain the highly-oriented Cu layer. These results are also valid for the development of barrier or underlayer materials for use in Si-LSI and three-dimensional LSI technology.

The Role of Underlayers and Overlayers in Thin Film BiVO4 Photoanodes for Solar Water Splitting

AbstractLight‐driven water splitting with metal oxide semiconductor materials to produce H2 constitutes one of the most promising energy conversion technologies built on solar power. BiVO4 stands out as one of the most attractive metal oxides with reported photocurrents close to its theoretical maximum of 7.5 mA cm−2 at 1 sun illumination. The present work addresses the state‐of‐the‐art strategies to enhance the performance of this material for water oxidation by heterostructuring with different underlayer (SnO2 and WO3) and overlayer (NiOOH/FeOOH, Co–Pi, Co–Fe Prussian Blue derivative) materials, with particular emphasis on the physico‐chemical mechanisms responsible for the reported enhancements.

Magneto-plasmonics on perpendicular magnetic nanostructures consisting of a CoPt layer and noble-metal grains

The magneto-optical (MO) properties of perpendicular magnetic nanostructures consisting of a hexagonal close-packed Co80Pt20 nanolayer and noble-metal (Ag or Au) fine grains were investigated under polar Kerr measurement conditions. The samples exhibited an unusual MO hysteresis loop in which the Kerr rotation angle increased at a low magnetic field; this effect was observed in a different wavelength region for the CoPt–Ag and CoPt–Au samples. The nanostructures consisted of two magnetic regions of CoPt layers formed on the grains and on the underlayer. The increase in the Kerr angle was induced by the antiparallel magnetic alignment of these CoPt layers. The opposite MO polarity on the CoPt nanostructures was suggested in a micro-MO observation using scanning near-field polarized optical microscopy. The Ag and Au fine grains induced the MO phase reversal of CoPt in a different wavelength region for each sample. These MO behaviors were attributed to the influence of localized surface plasmons excited on the noble-metal grains. The magneto-plasmonic activities on the CoPt nanostructures were also affected by the underlayer material and the environmental conditions. The perpendicular magnetic nanostructures are expected to provide a new type of probe for chemical and biological sensing applications.

Modulating Electrocatalysis on Graphene Heterostructures: Physically Impermeable Yet Electronically Transparent Electrodes.

The electronic properties and extreme thinness of graphene make it an attractive platform for exploring electrochemical interactions across dissimilar environments. Here, we report on the systematic tuning of the electrocatalytic activity toward the oxygen reduction reaction (ORR) via heterostructures formed by graphene modified with a metal underlayer and an adlayer consisting of a molecular catalyst. Systematic voltammetric testing and electrochemical imaging of patterned electrodes allowed us to confidently probe modifications on the ORR mechanisms and overpotential. We found that the surface configuration largely determined the ORR mechanism, with adlayers of porphyrin molecular catalysts displaying a higher activity for the 2e- pathway than the bare basal plane of graphene. Surprisingly, however, the underlayer material contributed substantially to lower the activation potential for the ORR in the order Pt > Au > SiO x, strongly suggesting the involvement of the solution-excluded metal on the reaction. Computational investigations suggest that ORR enhancements originate from permeation of metal d-subshell electrons through the graphene layer. In addition, these physically impermeable but electronically transparent electrodes displayed tolerance to cyanide poisoning and stability toward long-term cycling, highlighting graphene as an effective protection layer of noble metal while enabling electrochemical interactions. This work has implications in the mechanistic understanding of 2D materials and core-shell-type heterostructures for electrocatalytic reactions.

Preparation of c-axis perpendicularly oriented ultra-thin L1-FePt films on MgO and VN underlayers

Ultra-thin L10-FePt films of 2 nm average thickness are prepared on (001) oriented MgO and VN underlayers epitaxially grown on base substrate of SrTiO3(001) single crystal. Detailed cross-sectional structures are observed by high-resolution transmission electron microscopy. Continuous L10-FePt(001) thin films with very flat surface are prepared on VN(001) underlayer whereas the films prepared on MgO(001) underlayer consist of isolated L10-FePt(001) crystal islands. Presence of misfit dislocation and lattice bending in L10-FePt material is reducing the effective lattice mismatch with respect to the underlayer to be less than 0.5 %. Formation of very flat and continuous FePt layer on VN underlayer is due to the large surface energy of VN material where de-wetting of FePt material at high temperature annealing process is suppressed under a force balance between the surface and interface energies of FePt and VN materials. An employment of underlayer or substrate material with the lattice constant and the surface energy larger than those of L10-FePt is important for the preparation of very thin FePt epitaxial thin continuous film with the c-axis controlled to be perpendicular to the substrate surface.

Formation of $L1_{0}$ -FePt(001) Ultra-Thin Films With Flat Surfaces Using VC and VN Underlayers

FePt alloy films of 2 to 10 nm thicknesses are prepared on (001) single-crystal underlayers of MgO, VN, and VC by employing a two-step method consisting of low-temperature deposition at 200 °C followed by high-temperature annealing at 600 °C. MgO, VN, and VC crystals are selected as underlayer materials, since the crystals have the same NaCl-type crystal structure and similar lattice constants of about 0.42 nm but different surface energies. The single-crystal underlayers are prepared by RF sputtering on the single-crystal substrates of MgO and SrTiO3 at 600 °C. The influences of underlayer material and FePt film thickness on the $L1_{0}$ ordering degree, the magnetic properties, and the surface morphology are systematically investigated. Epitaxial films involving $L1_{0}$ (001) crystal structure with the $c$ -axis normal to the substrate surface are formed on all the underlayers. As the film thickness decreases, the ratio of out-of-plane to in-plane lattice constant $c/a$ decreases and the ordering degree $S$ increases. Stronger perpendicular magnetic anisotropy is observed for an FePt film with a higher $S$ value. The film thickness trends of $c/a$ , $S$ , and magnetic properties are similar to those for FePt films thicker than 5 nm formed on MgO underlayers. However, when the thickness is decreased to 2 nm, de-wetting of deposited atoms occurs on MgO underlayers during annealing at 600 °C. The surfaces of 2-nm-thick films formed on MgO underlayers are composed of isolated islands. On the contrary, continuous surfaces are realized for the investigated film thickness range of 2–10 nm in the cases of VC and VN underlayers. The surface roughness of FePt film formed on VN underlayer ( $R_{a}$ : 0.1–0.2 nm) is lower than that of film formed on VC underlayer ( $R_{a}$ : 0.1–0.4 nm). This paper shows that the employment of a VN underlayer is effective in the preparation of $L1_{0}$ ordered FePt ultra-thin film with flat surface.

Research of influence of the underlayer material on the growth rate of carbon nanotube arrays for manufacturing non-volatile memory elements with high speed

This experimental work is devoted to the regimes of obtaining arrays of carbon nanotubes. Arrays of perpendicular nanotubes perpendicular to the surface were obtained by the method of Plasma-enhanced chemical vapor deposition. In this paper, geometric and electronic parameters of carbon nanotubes were investigated depending on the material of the sublayer. The rates of growth of carbon nanotubes on various structures were also determined. In the experiments for growth, structures such as Ni-Al-Si, Ni-V-Si, Ni-Ti-Si, Ni-Cr-Si were used. The growth rates for the intensive section were for the Ni-Cr-Si structure, the growth rate is about 1 μm / min, for the Ni-V-Si structure it is 0.55 μm / min. The growth rates for the saturation region for the Ni-Cr-Si structure, the growth rate is about 0.2 μm / min, for the Ni-V-Si structure 0.16 μm / min. The results obtained in this paper can be used to optimize the growth regimes perpendicularly oriented to the substrate carbon nanotubes, which are used as various elements in modern nanoelectronics.