Ultrathin MgO Barrier Research Articles

Plasmonic gold nanoparticle incorporated MgO-coated SnO2 photoanode for efficiency enhancement in dye-sensitized solar cells

SnO2 is an attractive semiconducting material suitable for application as the photoanode in dye sensitized solar cells (DSSCs) due to its wide energy band gap and notable photo stability. However, improved solar cell performance can be achieved only by using composites of SnO2 with other materials like MgO, ZnO, Al2O3 and CaCO3. In this study, plasmonic DSSCs were fabricated using MgO coated SnO2 (SnO2:MgO) based photoanodes incorporating gold nanoparticles (Au NP) having the size in the 30 – 35 nm range and sensitized with ruthernium N719 dye. Photoanodes were characterized by UV–VIS spectroscopy and the DSSCs were characterized by current–voltage (J–V) measurements, incident photon-to-electron conversion efficiency (IPCE) measurements and electrochemical impedance spectroscopy (EIS). Under the illumination of 100 mW cm−2 (AM 1.5), the efficiency (η) of the reference DSSC with pristine SnO2 photoanode was 1.52%, where as the efficiency of the optimized plasmonic DSSC with Au NP incorporated SnO2:MgO photoanode (Au: SnO2:MgO) was an impressive 4.69%. This efficiency enhancement of about 208% compared to the reference DSSC appears to be due to the increased open-circuit voltage (VOC) of 725.6 and increased short-circuit photocurrent density (JSC) of 9.06 mA cm−2 respectively evidently caused by the reduced electron recombination by ultra-thin MgO barrier layer and the enhanced light harvesting caused by the local surface plasmon resonance (LSPR) effect due to Au nanopraticles. EIS analysis showed that the incorporation of plasmonic Au metal nanoparticles leads to a decrease in the series resistance (RS) and the interfacial charge-transfer resistance (RCT) at the SnO2/electrolyte interface.

Study on ion transport mechanism in ultrathin electrolyte membrane for low temperature operation of solid oxide fuel cell

At the Centre for Innovative Integrated Electronic Systems (CIES) at Tohoku University, Japan, Professor Masaaki Niwa is exploring the science and technology of CoFeB/MgO/CoFeB based p-MTJ stack for STT-MRAM. 'My research is focusing on elucidation of interface structure of ultra-thin MgO barrier including failure analysis by means of advanced physical analyses such as transmission electron microscope, electron tomography as well as Synchrotron radiation experiments,' outlines Niwa. He explains that the MgO barrier used for p-MTJ is a tunnel barrier, and also points out that the gate dielectric film of a MOSFET is also a well-known typical tunnel barrier. 'I have been engaged in R&D of the gate stack of advanced CMOSFETs for many years while working at Panasonic,' he highlights. This involved the development of surface/interface control and reliability physics of the ultra-thin gate dielectric. 'This is unique because a person with the background of the MRAM and CMOS tries to elucidate the charge transfer mechanism in the ionic bonding ultrathin electrolyte film used for SOFC, which is the typical electrochemical phenomenon,' he explains. 'An important element of our work involves exploring the potential of SOFCs, which are energy conversion devices that produce electricity by electrochemically combining a fuel and an oxidant across an ionic conducting oxide electrolyte.' Niwa believes that SOFCs could hold the key to helping overcome serious energy and environmental problems as they boast high efficiency and are environmentally friendly.

Atomic layer deposition of a MgO barrier for a passivated black phosphorus spintronics platform

We demonstrate a stabilized black phosphorus (BP) 2D platform thanks to an ultrathin MgO barrier, as required for spintronic device integration. The in-situ MgO layer deposition is achieved by using a large-scale atomic layer deposition process with high nucleation density. Raman spectroscopy studies show that this layer protects the BP from degradation in ambient conditions, unlocking in particular the possibility to carry out usual lithographic fabrication steps. The resulting MgO/BP stack is then integrated in a device and probed electrically, confirming the tunnel properties of the ultrathin MgO contacts. We believe that this demonstration of a BP material platform passivated with a functional MgO tunnel barrier provides a promising perspective for BP spin transport devices.

The annealing effect on memory state stability and interlayer coupling in perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Abstract Perpendicular magnetic tunnel junctions (p-MTJs) attract great interest because of their excellent performance in spin-transfer-torque magnetic random access memories (STT-MRAMs). The annealing process is critical to achieve the required structural and magnetic properties, therefore obtaining high perpendicular tunneling magnetoresistance (p-TMR) and lower switching current. In this work, the CoFeB/MgO/CoFeB-based p-TMR stack was prepared and a high p-TMR value of 155.9% was obtained from CIPT. The p-MTJs were patterned with pillar sizes ranging from 50 nm to 1000 nm and were annealed upon different annealing temperatures (TA) of 300 and 340 °C. The resistance at parallel (RP) and antiparallel (RAP) state in p-MTJs was obtained upon the application of a ±30 mT perpendicular magnetic field, where the angle of magnetization between storage and reference layer at memory state, θP and θAP, were derived from the P and AP resistance ratios with and without magnetic field. The condition θP = θAP of about 63.5° was found in as-deposited samples, indicating a ferromagnetic interlayer coupling between storage and reference layers, where θP = 0° and θAP = 180° at TA = 300 °C and 340 °C demonstrate a stable memory state. The stability of the resistance state was modeled by considering the evolution of effective perpendicular magnetic anisotropy (Keff) and interlayer coupling constant (J), showing a method to evaluate the ratio of Keff/J via the memory state resistance characterization, where the Keff/J of about −1.26 was obtained in our as-deposited samples. Our study assesses the effect of TA on the θP and θAP and supplies the method to enhance the stability through the optimization of perpendicular magnetic anisotropy and interlayer coupling, so to better control the performance of p-MTJ-based STT-MRAMs.

Barrier breakdown mechanism in nano-scale perpendicular magnetic tunnel junctions with ultrathin MgO barrier

Recently, the perpendicular magnetic tunnel junctions (p-MTJs) arouse great interest because of its unique features in the application of spin-transfer-torque magnetoresistive random access memory (STT-MRAM), such as low switching current density, good thermal stability and high access speed. In this paper, we investigated current induced switching (CIS) in ultrathin MgO barrier p-MTJs with dimension down to 50 nm. We obtained a CIS perpendicular tunnel magnetoresistance (p-TMR) of 123.9% and 7.0 Ω·μm2 resistance area product (RA) with a critical switching density of 1.4×1010 A/m2 in a 300 nm diameter junction. We observe that the extrinsic breakdown mechanism dominates, since the resistance of our p-MTJs decreases gradually with the increasing current. From the statistical analysis of differently sized p-MTJs, we observe that the breakdown voltage (Vb) of 1.4 V is 2 times the switching voltage (Vs) of 0.7 V and the breakdown process exhibits two different breakdown states, unsteady and steady state. Using Simmons’ model, we find that the steady state is related with the barrier height of the MgO layer. Furthermore, our study suggests a more efficient method to evaluate the MTJ stability under high bias rather than measuring Vb. In conclusion, we developed well performant p-MTJs for the use in STT-MRAM and demonstrate the mechanism and control of breakdown in nano-scale ultrathin MgO barrier p-MTJs.

Asymmetric dielectric breakdown behavior in MgO based magnetic tunnel junctions

The time-dependent dielectric breakdown phenomenon (TDDB) has been investigated in a series of nominally identical MgO based magnetic tunnel junctions (MTJs) by pulsed voltage endurance test. Results from the pulsed endurance test reveal that the breakdown voltage is dependent on the polarity of the applied voltage. MTJs with “UP” current stress (Iup) (flowing from reference layer (RL) to free layer (FL)) show higher endurance than that of MTJs with “DOWN” current stress (Idown). We also found that bipolar stressing could result in reduced cumulative stress time before failure as compared to unipolar stressing. This could be explained by increased charge trapping/detrapping effects during bipolar stressing. The asymmetric breakdown behavior for different polarity was further supported by the different field acceleration slopes observed in the mean time to failure (MTTF) – voltage bias trend line. Symmetric pulse scheme breakdown measurements were also carried out at different temperatures ranging from 25–85°C. The time-dependent clustering model is applied here to best describe the breakdown statistics in view of the non-uniformity in percolation breakdown due to thickness variations (interface roughness) and other process-induced damages to the ultra-thin MgO barrier layer.

Magnetization reversal in sub-100 nm magnetic tunnel junctions with ultrathin MgO barrier biased along the hard axis

We report on room temperature magnetoresistance and low frequency noise in sub-100 nm elliptic CoFeB/MgO/CoFeB magnetic tunnel junctions with ultrathin (0.9 nm) barriers. For magnetic fields applied along the hard axis, we observe current induced magnetization switching between the antiparallel and parallel alignments at dc current densities as low as 4 × 106 A/cm2. We attribute the low value of the critical current to the influence of localized reductions in the tunnel barrier, which affects the current distribution. The analysis of random telegraph noise, which appears in the field interval near a magnetization switch, provides an estimate to the dimension of the pseudo pinholes that trigger the magnetization switching via local spin torque. Micromagnetic simulations qualitatively and quantitatively reproduce the main experimental observations.

PARAMETER SPACE FOR THERMAL SPIN-TRANSFER TORQUE

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with three monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magnetoresistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm2. The thermally generated torque is calculated ab initio using the Korringa–Kohn–Rostoker and nonequilibrium Green's function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

Incidence of in situ annealing on the nanoscale topographical/electrical properties of the tunnel barrier in sputtered epitaxial Fe/MgO/Fe multilayers

Technological improvements in the magnetotransport performance of Fe/MgO/Fe stacks require nanoscale control over the topographical and electrical properties of the ultrathin MgO barrier. We have statistically investigated the incidence of in situ annealing of the lower Fe layer on the nanoscale topographical/electrical properties of Fe/MgO bilayers and the structural and magnetic properties of Fe/MgO/Fe/Co multilayers prepared by sputtering. This annealing step improves the crystal quality of both the lower Fe and the upper Fe/Co layers, leading to an enhanced saturated magnetic moment. Finally, this annealing step substantially mitigates the presence of nanohills on the lower Fe layer and improves the uniformity of the height and/or the thickness of the MgO tunnel barrier. Our results pave the way for studies of nanoscale transport on micrometre-sized devices through a better understanding of, and control over, nanoscale hotspots in the tunnel barrier.

Study of dielectric breakdown distributions in magnetic tunneling junction with MgO barrier

The breakdown distribution of a magnetic tunnel junction (MTJ) with an ultrathin (∼1.2 nm) MgO barrier was studied, and two distinct distributions were identified. The breakdown distribution with high value demonstrates a wide peak-to-peak separation (∼13.4σ) to the critical spin torque induced switching voltage. However, the peak-to-peak separation is only ∼8.4σ for the devices showing low breakdown value. Both abrupt and gradual breakdown events were observed in two distributions. The dependence of the percentage of low breakdown devices as a function of bias polarity, test and stress conditions, MTJ film properties, and process conditions was investigated. The low breakdown percentage can be significantly reduced by increasing the RA value and MTJ process optimization.

In situ heat treatment of ultrathin MgO layer for giant magnetoresistance ratio with low resistance area product in CoFeB/MgO/CoFeB magnetic tunnel junctions

In order to promote the grain growth of ultrathin MgO barrier deposited on a CoFeB layer, in situ infrared (IR) heat treatment just after the deposition of MgO barrier was examined. In case that IR heat treatment was not applied, tunneling magnetoresistance (TMR) ratio of CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) was significantly decreased with decreasing resistance area (RA) product to less than 10 Ω μm2. On the other hand, TMR ratio of 206% was achieved at the RA product of 2.1 Ω μm2 when the IR heat treatment was applied. According to the cross sectional transmission electron microscope images for the samples with 0.76-nm-thick (∼4 ML) MgO barrier, the (001) oriented well crystallized structure with smooth interface was observed for the IR heated sample. Moreover, it revealed that the lateral grain size of MgO was significantly enlarged compared to that for the sample without IR heating. The improvement of TMR properties at low RA product region by the heat treatment might be due to the decrease in grain boundaries of MgO layer where the coherent tunneling of Δ1 electrons is not restricted.