Insertion Of Layers Research Articles

Fast, Highly Flexible, and Transparent TaOx-Based Environmentally Robust Memristors for Wearable and Aerospace Applications

Memristor devices that can operate at high speed with high density and nonvolatile capabilities have great potential for the development of high data storage and robust wearable devices. However, in real-time, the performance of memristors is challenged by their instability toward harsh working conditions such as high temperature, extreme humidity, photo irradiation, and mechanical bending. Herein, we introduce a TaOx/AlN-based flexible and transparent memristor device having stable endurance under extreme 2 mm bending (for more than 107 cycles) with an ON/OFF ratio of more than 2 orders of magnitude at 25 ns rapid switching. This device exhibits excellent flexibility under extreme bending conditions (bending radius of 2 mm) even with intense ultraviolet (UV) radiation. A thin AlN insertion layer having low dielectric and high thermal conductivity plays a crucial role in improving the switching stability and device flexibility. In particular, the devices exhibit excellent minimum switching fluctuations under UV irradiation, >106 s nonvolatility retention at high temperature (135 °C), various gas ambient, and damp heat test (humidity 95.5%, 83 °C) because of the indium metal drift during the switching process and high bonding energy of Ta–O. Most importantly, direct observation of indium metal strongly anchored in the TaOx switching layer during the switching process is reported for the first time via transmission electron microscopy, which provides clear insights into the switching phenomenon. Furthermore, the results of electrical and material analyses explain that our facile device design has excellent potential for wearable and aerospace applications.

Improved power conversion efficiency in n-MoS2/AlN/p-Si (SIS) heterojunction based solar cells

In the present work, MoS2/Si (n-p) and MoS2/AlN/Si (Semiconductor-Insulator-Semiconductor (SIS)) heterojunctions based solar cells have been fabricated by depositing ultrathin layers of MoS2 and AlN on p-type Si substrate by DC magnetron sputtering technique. The SIS heterojunction exhibits higher current and large threshold voltage as compared to conventional p-n junction in Current-Voltage (I-V) characteristics. It suggests that the tunnelling of charge carriers can be a better transport mechanism for solar cell applications. With the AlN layer insertion, the power conversion efficiency of the device increases from 2.92% to 3.53%. The insertion of ultrathin insulating AlN layer in between the n-MoS2/p-Si heterojunction lowers the static charge transfer and makes more effective tuning of the Fermi level. The obtained results demonstrate a novel route for the fabrication of efficient photovoltaic devices for next-generation energy harvesting applications.

Antiferromagnetic domain switching modulated by an ultrathin Co interlayer in the Fe/Co/CoO/MgO(001) system

Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic circular dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process modulated by the sub-nm thick Co inserting layer in a single crystalline Fe/Co/CoO/MgO(001). The CoO AFM domain switching occurs at lower temperature for the thicker Co interlayer, and the activation energy barrier of CoO AFM domain switching decreases as the Co interlayer thickness increases. The exchange coupling strength between the AFM spins in CoO layer and the ferromagnetic spins in the Fe/Co bilayer is found to be independent of the Co layer thickness. Our results suggest an approach to modulate the dynamic properties of AFM domains with an interfacial modification.

Origin of GaN-InGaN-GaN barriers in enhancing the hole injection for InGaN/GaN green light-emitting diodes

Abstract In this work, we propose to insert an In0.07Ga0.93N layer into the GaN quantum barriers for InGaN/GaN green light-emitting diodes (LEDs) to achieve the enhanced hole injection and improve the internal quantum efficiency (IQE). Nevertheless, we also find that the insertion layer cannot be too thin, and a properly thick insertion layer is recommended, because the polarization induced electric field in the GaN/In0.07Ga0.93N/GaN quantum barriers can accelerate holes, and thereby the hole blocking effect at the GaN/In0.07Ga0.93N interfaces and the increase for the valence band barrier height of p-EBL will be less significant. Other advantage is that the insertion In0.07Ga0.93N layer can also reduce the polarization induced electric field in the quantum wells, which is helpful to increase the radiative recombination rate.

Spin Dynamic Damping of Py Induced by Gd Capping

Magnetic dynamic damping is significantly important in determining the relaxation time of magnetic moments and the speed of magnetic memory devices. Here, magnetic dynamic behaviors of permalloy (Py) films capped with gadolinium (Gd) are studied, and the effects of the interface are investigated by the Cu insert layer. For Py/Gd bilayer films, both the saturation magnetization (M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) and the surface anisotropy constant (K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> ) are decreased with increasing Gd thickness, while the Gilbert damping (α) is enhanced. With the insert of Cu at Py/Gd interface, the above tendencies of M <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> , K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⊥</sub> , and α are suppressed. We attribute these phenomena to the antiferromagnetic coupling between Gd and Py near the interface, which are caused by the magnetic proximity effect (MPE) instead of the spin relaxation in the rare earth Gd themselves.

Gate-tunable linear magnetoresistance in molybdenum disulfide field-effect transistors with graphene insertion layer

Molybdenum disulfide (MoS2) holds great promise as atomically thin two-dimensional (2D) semiconductor for future electronics and opto-electronics. In this report, we study the magnetoresistance (MR) of MoS2 field-effect transistors (FETs) with graphene insertion layer at the contact interface. Owing to the unique device structure and high-quality contact interface, a gate-tunable linear MR up to 67% is observed at 2 K. By comparing with the MRs of graphene FETs and MoS2 FETs with conventional metal contact, it is found that this unusual MR is most likely to be originated from the contact interfaces between graphene and MoS2, and can be explained by the classical linear MR model caused by spatial fluctuation of carrier mobility. Our study demonstrates large MR responses in MoS2-based systems through heterojunction design, shedding lights for the future magneto-electronics and van der Waals heterostructures.

Effect of La0.67Sr0.33 MnO3 Insertion Layer on the Ferroelectric and Resistive Switching Behaviors of PbZr0.52Ti0.48O3/Nb:SrTiO3 Heterostructures

Recently, ferroelectric resistive switching (RS) effect in the ferroelectric/semiconductor heterostructures has been widely studied and the RS performance has been greatly improved. However, the relationships between ferroelectric and RS behaviors as well as interface structure of ferroelectric/semiconductor heterostructures need to be further studied. Herein, a [Formula: see text][Formula: see text]MnO3 (LSMO) layer with the thickness of 7 nm is inserted into [Formula: see text][Formula: see text]O3/Nb:SrTiO3 (PZT/NSTO) heterostructures, and its effects on the ferroelectric and RS behaviors are investigated. The PZT/NSTO heterostructures show significantly asymmetric ferroelectric loops, and the RS ratio in which can reach to three orders of magnitude. However, by inserting the LSMO layer, the ferroelectric loops became relatively symmetric, but the RS effect almost disappeared. It can be considered that the LSMO layer affects the interfacial energy band structure of the PZT/NSTO heterostructures, which makes ferroelectric polarization lose its effect on the modulation of the depletion layer width. Therefore, the existence of the adjustable depletion layer is very important for the RS effect of ferroelectric/semiconductor heterostructures.

Graftless Maxillary Sinus Floor Augmentation with Simultaneous Porcine Bone Layer Insertion: A 1- to 5-Year Follow-up Study.

Evidence suggests that maxillary sinus floor augmentation via a lateral approach can be performed without positioning a bone graft inside, when one or more implants can be placed simultaneously. The aim of this study was to test if the placement of a porcine cortical bone layer underneath the sinus membrane can increase bone formation and implant stability. One hundred seventy-two patients with posterior maxilla atrophy needing implant rehabilitation were selected. Two hundred six sinus augmentation procedures were performed via a lateral approach, and 295 implants were placed in the same session of the sinus elevation surgery. In all the surgeries, a porcine cortical bone layer was placed underneath the sinus membrane, without using any graft material. After 6 to 7 months of healing, the implants were uncovered, then restored with porcelain-fused-to-metal crowns and monitored with a followup of 1 to 5 years. The implant cumulative success rate was 95.2%, while the residual bone crest height changed from 2.67 ± 1.11 mm to 12.54 ± 1.42 mm, with an increase of 9.87 mm on average. Marginal bone resorption was 0.83 mm on average after 1 year of loading, while the mean implant stability measured at the moment of implant placement and 6 to 7 months later increased from an implant stability quotient (ISQ) of 62.61 ± 5.7 to an ISQ of 70.07 ± 8.2. This study confirms the validity of the graftless sinus elevation surgery when simultaneous implant placement is performed. The use of a porcine cortical bone layer seems to increase, from a radiologic point of view, the amount of bone around the implants, reducing healing time, cost, and biologic complications for the patient.

Engineering anisotropic magnetoresistance of Hall bars with interfacial organic layers

Tuning the magnetoresistance behavior of heterostructures composed of nonmagnetic and ferromagnetic (FM) materials is crucial for improving their applicability in electronic and spintronic devices. In this study, we investigate whether the integration of organic layers to NiFe/Pt junctions can result in the modification of the magnetic moment of the FM layer using iron phthalocyanines (FePc) and copper phthalocyanines (CuPc) as the interfacial layers for controlling the spin-charge conversion. Relaxation of the out-of-plane magnetic hard axis of the NiFe/Pt junctions is observed, as a result of the modification of the interfacial magnetic structure. The transport measurements of the fabricated hybrid Hall bar junctions with NiFe/FePc/Pt and NiFe/CuPc/Pt reveal that although the intrinsic anisotropic magnetoresistance of the present Hall bar is maintained with the integration of interfacial metal phthalocyanine (MPc) layers, a change in the magnetic response along the axis perpendicular to the in-plane of Hall bars is observed, owing to the insertion of the interfacial MPc layers. The present method of interface engineering via integration of organic interfacial layers can act as a model system for controlling the spin-charge conversion behavior of magnetic heterojunction toward the development of multifunctional molecular-engineered spintronic devices.

Cathodoluminescence Study of Dislocations in Step-Graded InGaP Buffer Layers of Metamorphic Single-Junction InGaAs Solar Cells

Epitaxial growth of lattice-mismatched materials is useful for solar cells, but lattice dislocations must be controlled for best device performance. It has been shown that metamorphic growth enables fabrication of InGaAs p–n junctions with good performances on GaAs substrates due to the insertion of buffer layers. Here, we investigate misfit and threading dislocations inside the step-graded InGaP buffer layers of a single-junction InGaAs solar cell by cathodoluminescence microscopy. Prior to measurement, the device edges were polished at various angles (less than 10° with respect to the substrate surface). By using this technique, cross sections of very thin layers can be directly imaged with a resolution that allows us to observe misfit and threading dislocations. In the present device, the densities of the two types of dark lines depend on the position in the buffer structure. In particular, near the InGaAs base layer, the density of the dark lines extending in the [110] direction is higher than that of the dark lines extending in the [1-10] direction. We believe that this difference in the dark line density is related to the surface morphology.

Magnetic properties of Co/Ni-based multilayers with Pd and Pt insertion layers

In this study, the influence of Pd and Pt insertion layers in Co/Ni multilayers (MLs) on their magnetic properties, e.g., magnetic anisotropies, saturation magnetization, coercivity, magnetic domain size, and Curie temperature, is investigated. We compare three series of [Co/Ni/X]N ML systems (X=Pd, Pt, no insertion layer), varying the individual Co layer thickness as well as the repetition number N. All three systems behave very similarly for the different Co layer thicknesses. For all systems, a maximum effective magnetic anisotropy was achieved for MLs with a Co layer thickness between 0.15 and 0.25 nm. The transition from an out-of-plane to an in-plane system occurs at about 0.4 nm of Co. While [Co(0.2 nm)/Ni(0.4 nm)]N MLs change their preferred easy magnetization axis from out-of-plane to in-plane after six bilayer repetitions, insertion of Pd and Pt results in an extension of this transition beyond 15 repetitions. The maximum effective magnetic anisotropy was more than doubled from 105 kJ/m3 for [Co/Ni]3 to 275 and 186 kJ/m3 for Pt and Pd, respectively. Furthermore, the insertion layers strongly reduce the initial saturation magnetization of 1100 kA/m of Co/Ni MLs and lower the Curie temperature from 720 to around 500 K.

Study of the perpendicular magnetic anisotropy, spin–orbit torque, and Dzyaloshinskii–Moriya interaction in the heavy metal/CoFeB bilayers with Ir22Mn78 insertion

The perpendicular magnetic anisotropy (PMA), current-induced spin–orbit torques (SOTs), and Dzyaloshinskii–Moriya interaction (DMI) in the as-grown W or Ta/Ir22Mn78(IrMn)/CoFeB/MgO stacks with varying IrMn layer thicknesses were investigated. The in-plane magnetized W/CoFeB/MgO sample becomes perpendicularly magnetized after inserting the IrMn layer without the requirement of the annealing process. The effective magnetization fields 4πMeff show a nonmonotonic dependence on the IrMn layer thickness, which reaches the maximum in magnitude at a thickness of tIrMn = 0.75 nm. The SOT effective fields corresponding to damping-like and field-like torques decrease with the insertion layer thickness. Moreover, the variation of the IrMn layer thickness leads to the change of the DMI in magnitude and sign change from positive (favoring right-handed chirality) to negative (favoring left-handed chirality). The realization of changing the PMA, SOTs, and DMI by inserting the IrMn layer provides more flexibility in the design of spintronic devices.

Perpendicular magnetic anisotropy in SrTiO3/Co/Pt films induced by oxygen diffusion from CaTiO3 spacer layer

The control of perpendicular magnetic anisotropy (PMA) of magnetic metals on a platform of perovskite oxides, especially on widely used SrTiO3 substrates, is of technological importance in integrating conventional metal spintronic devices with multifunctional perovskite oxides. In this work, we tuned the magnetic anisotropy of Co/Pt on SrTiO3 (001) substrates by inserting an interfacial calcium titanate layer with an amorphous state. The crystallinity and device microstructure were characterized by transmission electron microscopy. The Hall effect and magnetic measurement show that the PMA was achieved with the inserting layer. The electronic structure analysis by x-ray photoelectron spectroscopy suggests that out-of-plane magnetic anisotropy corresponds to the appearance of a significant density of Co–O bonding and a reduction of the Ti valence state in the calcium titanate layer. It is indicated that the amorphous calcium titanate layer could serve as an effective oxygen source to modify the chemical environment of the interfacial Co and Ti, resulting in the tuning of hybridization between Co and oxygen as well as the magnetic anisotropy. This work paves the way to engineer the magnetic anisotropy of metallic spintronic devices on the platform of perovskite oxides via oxygen diffusion for possible application in multifunctional spin orbital torque memory devices.

Influence of Cu Insertion Layer on Magnetic Properties of Co-Tb/Cu/Co-Tb Thin Films

This work focuses on the study of the influence of the insertion layer (IL) on the magnetic properties of thin films of Co-Tb. Thin films of Ta (5 nm)/Co1−xTbx (20 nm)/Cu (3 nm)/Co1−xTbx (20 nm)/Ta (5 nm) as well as Ta (5 nm)/Co1−xTbx (40 nm)/Ta (5 nm) with x = 0.10 and 0.15 were grown on a silicon substrate using DC magnetron sputtering. The X-ray diffraction patterns indicate the amorphous nature of all prepared films. The measurement of surface roughness of Ta (5 nm)/Co1−xTbx (20 nm) and Co0.85Tb0.15 (20 nm)/Cu (3 nm)/Co0.85Tb0.15 (20 nm) films using 3D optical profilometer indicates a high surface roughness. The measured out-plane and in-plane magnetic hysteresis (M-H) curves indicate that Ta/Co1−xTbx (40 nm)/Ta films (i.e., without any IL) exhibit nearly magnetic isotropic behavior. However, Ta/Co1−xTbx (20 nm)/Cu (3 nm)/Co1−xTbx (20 nm)/Ta films exhibit in-plane magnetic anisotropy. The in-plane anisotropy was found to be significant for the case of x = 0.10 film with larger effective anisotropy constant (Keff). The values of saturation magnetization and squareness (Sq) of films are found to decrease with Cu insertion layer and it is likely due to the worsening of the Co-Tb/Cu interface. The post-annealing of the Co-Tb films with Cu IL helps to improve the value of MS and Keff but it reduces the HC and the squareness of the hysteresis curve.

Schottky barrier height and modulation due to interface structure and defects in Pt|MgO|Pt heterojunctions with implications for resistive switching

The modulation of Schottky barrier height (SBH) due to defect migration has been suggested to be an important driving mechanism for resistive switching in metal–oxide–metal structures. Here, we explore the SBH and its modulation due to different interface structures and defects in the Pt|MgO|Pt(001) system using hybrid Heyd–Scuseria–Ernzerhof density functional theory. The computed magnitudes of SBH at Pt|MgO interfaces obtained using the generalized gradient approximation (local density approximation) functional are found to be significantly underestimated as compared to those obtained using hybrid functional. Furthermore, the magnitudes of SBH are found to depend critically on interface structures. In the case of defect-free Pt|MgO interfaces, the p-type SBH is found to be 4.13 eV and 3.04 eV for interfaces having adjacent Pt–O and Pt–Mg bonds, respectively. In addition, the SBH magnitudes are found to exhibit significant variation primarily due to nominal effective charges on interface layers which, in turn, are induced by interface defects such as O and Mg vacancies. The magnitudes of p-type SBH are found to increase (decrease) by ∼1.0–1.5 eV as the ionic layers with charge +1e (−1e) are introduced at the interface. The modulation in SBH due to interface ionic/polar layer is explained using a micro-capacitor model. Furthermore, the SBH is found to shift by ∼0.2 eV with the varying distance of O and/or Mg vacancies from the interface. Our results suggest that fluctuations in experimental resistive switching data in Pt|MgO structures may originate due to fluctuations in SBH induced by changes in interface atomic structure. The study also shows that SBH in Pt|MgO and related structures may be modulated in a controlled way by the insertion of interface polar layers. The lower and upper bounds of the SBH magnitudes are also estimated using a semi-empirical model expressed in terms of parameters such as charge neutrality level, ionization potential, pinning parameter, and metal work function. The quantitative results on the SBH modulation presented in the study may be expected to have important implications for resistive switching phenomenon in Pt|MgO and similar other structures.

Freestanding GaN substrate enabled by dual-stack multilayer graphene via hydride vapor phase epitaxy

The freestanding GaN substrate has a great potential to homo-epitaxy of optoelectronic and electronic devices with high reliability and performances. Here, we realized the growth of freestanding bulk GaN on dual-stack multilayer graphene as an insertion layer by the hydride vapor phase epitaxy (HVPE) method. Stacked multilayer graphene was fabricated on copper foil using chemical vapor deposition (CVD), following the overlap of top and bottom layers of graphene after removing the copper. Based on Raman spectra and X-ray photoelectron spectroscopy (XPS) experiments, it is found that the dual-stack structure of multilayer graphene is crucial for the self-separation of GaN substrate from sapphire. The exfoliation was attributed to the weak interaction force between the dual-stack multilayer graphene. This study can provide an efficient, practical, and mass-producible technique to fabricate freestanding GaN substrates, which exhibit immense application potential for the nitride-based optoelectronic and electronic devices.

Mechanical properties, thermal stability and oxidation resistance of TiN/CrN multilayer coatings

Chromium and titanium nitride coatings are well known for their high hardness and good wear resistance. Here, we research the mechanical and thermal properties of TiN/CrN_M1 and _M2 multilayers with bilayer period of ~17.8 and 6.7 nm by means of X-ray diffraction, Transmission electron microscopy, Scanning electron microscopy and Nanoindentation, which combines the advantages of TiN and CrN coatings. The interfacial strengthening arising from coherent interface leads to an increased hardness of ~27.5 GPa for TiN/CrN_M1 and ~29.4 GPa for TiN/CrN_M2, corresponding to ~26.4 Gpa for TiN and ~17.3 GPa for CrN. The TiN insertion layer retards the thermal decomposition process of CrN due to the higher binding energy of Ti–N bond than Cr–N, and thus evinces higher thermal stability of TiN/CrN multilayers. Furthermore, the TiN/CrN multilayers exhibit better oxidation resistance than TiN coatings.

Unraveling optimal interfacial conditions for highly efficient and reproducible organic photovoltaics under low light levels

The insertion of interfacial layers between the photoactive layer and electrodes is effective for improving the power conversion efficiency (PCE) of organic photovoltaics (OPVs). Further understanding on effects of the interlayers is needed to optimize OPV performance in low light applications, as the operating mechanism of OPVs varies depending on the light intensity. Here, the critical roles of the electron transport layer (ETL) on the performance of OPVs are demonstrated in different irradiation environments involving low-intensity lighting. ZnO ETL was found to deliver better performance and reproducibility compared with a conjugated polyelectrolyte ETL regardless of the irradiation conditions, showing high PCEs exceeding 18% under low light conditions such as a 500 lx light-emitting diode. In addition to the great charge selectivity and electronic properties, the uniform surface coverage of the bottom indium tin oxide electrode was a key factor in preventing shunt path formation and consequently reducing recombination and leakage current, which mainly determine the low light performance of OPVs. This study provides important design considerations for interfacial contact materials to achieve high-performance and reproducible OPVs that operate unconditionally over a wide range of light intensities for various applications.

Effect of Ply-level Hybridization and Insertion of Metal Layers on Impact Damage Modes and Compression Strength after Impact of Thin-ply Composite Laminates

Effect of Ply-level Hybridization and Insertion of Metal Layers on Impact Damage Modes and Compression Strength after Impact of Thin-ply Composite Laminates

Improved crystalline quality of ZnO with inserting multi-buffer layers

To improve the crystalline quality of zinc oxide (ZnO) for further application in solar cells, multi-buffer layers (mBL) were inserted into ZnO thin films and fluorine-doped tin oxide (FTO) glass, by radio frequency (RF) magnetron sputtering. The characterization of ZnO thin films was carried out by X-ray diffraction (XRD), photoluminescence (PL), Hall-effect measurements, atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), and DRUV-vis spectra (DRUV-vis). XRD and PL results show that the crystalline quality of ZnO has been successfully improved with inserted mBL, due to the increased crystallinity index of ZnO (002) and the increased near-band-edge (NBE) emission. The transmittance and bandgap of ZnO thin films has been slightly affected with inserted mBL.