Ion-assisted Oxidation Research Articles

The dependence of nano-contact magnetoresistance on the bulk scattering spin asymmetry in CoFe alloys with oxidation impurities.

Nano-contact magnetoresistance (NCMR) spin-valves (SVs) using an AlO x nano-oxide-layer (NOL) have numerous nanocontacts in the thin AlOx oxide layer. The NCMR theoretically depends on the bulk scattering spin asymmetry ([Formula: see text]) of the ferromagnetic material in the nanocontacts. To determine the relationship between NCMR and [Formula: see text], we investigated the dependence of NCMR on the composition of the ferromagnetic material Co1-xFex. The samples were annealed at 270 °C and 380 °C to enhance the MR ratio. For both annealing temperatures, the magnetorsistance ratio in the low-resistance area product region at less than 1 Ω μm2 was maximized for Co0.5Fe0.5. To evaluate [Formula: see text] exactly, we fabricated current-perpendicular-to-plane giant magnetoresistance SVs with Co1-xFex/Cu/Co1-xFex layers and used Valet and Fert's theory to solve the diffusion equation of the spin accumulation for a ferromagnetic layer/non-ferromagnetic layer of five layers with a finite diffusion length. The evaluated [Formula: see text] for Co1-xFex was also maximized for Co0.5Fe0.5. Additionally, to determine the difference between the experimental MR ratio of NCMR SVs and the theoretical MR ratio, we fabricated Co0.5Fe0.5 with oxygen impurities and estimated the decrease in [Formula: see text] with increasing oxygen impurity concentration. Our Co0.5Fe0.5 nano-contacts fabricated using ion-assisted oxidation may contain oxygen impurities, and the oxygen impurities might cause a decrease in [Formula: see text] and the MR ratio.

Formation and anisotropic magnetoresistance of Co/Pt nano-contacts through aluminum oxide barrier

We report on the observation of anisotropic magnetoresistance (AMR) in vertical asymmetric nano-contacts (NCs) made through AlOx nano-oxide layer (NOL) formed by ion-assisted oxidation method in the film stack of Co/AlOx-NOL/Pt. Analysis of NC formation was based on in situ conductive atomic force microscopy and transmission electron microscopy. Depending on the purity of NCs from Al contamination, we observed up to 29% AMR ratio at room temperature.

Oxidation and Annealing Process: Morphological Change and Nanocontact MR in Spin Valves With FeCo–AlO$_{x}$ NOL Spacer

We studied the formation mechanism of ferromagnetic nanocontacts (NCs) around 1-2 nm in diameter in thin AlOx nano-oxide layer (NOL) through in situ conductive atomic force microscopy (c-AFM), which is the only method suitable for measuring morphological changes in features this small. Spin valves (SVs) with Fe0.5Co0.5NCs in AlOx NOL fabricated by ion-assisted-oxidation (IAO) irradiated onto 1.3-nm-thick Al had nanocontact magnetoresistance (NCMR) characteristics with the MR ratio of 5% for 1 Ω μm2. According to measurements with c-AFM, these NCs were mostly located at the valleys between the AlOx grains or the edge of the grain. In other words, NCs were formed in the oxidation and cohesion process of Al by the IAO. Moreover, we found that the high-temperature annealing process improved the MR ratio to around 10% for 0.5 Ω μm2. The high-temperature annealing process may involve the AlOx morphological change and have led the Al atoms in NCs to migrate to AlOx by combining with oxygen as impurities. Then, the morphological changes and migration of Al caused NCs to be the low resistivity and the short length.

ＣＣＰ‐ＮＯＬを有するＣＰＰ‐ＧＭＲ膜のＭＲ変化率とナノ微細構造の関係

Current-Perpendicular-to-Plane Giant MagnetoResistance (CPP-GMR) spin valves with a Current-Confined-Path (CCP) Nano-Oxide Layer (NOL) were investigated from the viewpoint of the relation between MR ratio and nanostructure. We have reported that the CPP-GMR spin valves with the CCP-NOL fabricated by Ion-Assisted Oxidation (IAO) show higher MR ratio than those fabricated by Natural Oxidation (NO). In this report, the nanostructures of CPP-GMR spin valves are compared by means of the high-resolution Transmission Electron Microscope (TEM) and the Fast Fourier Transform (FFT) map. It was confirmed that IAO is effective for improving the crystalline structure not only for the CCP, but also for the free layer on the CCP. The well crystalline structure can decrease the resistivity of the metal path and enhance the spin-dependent scattering inside the free layer, which are considered to be the reasons for the enhancement of MR ratio for the IAO.

CPP-GMR films with a current-confined-path nano-oxide layer (CCP-NOL)

We investigated the film performance and nanostructure of current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin-valve film with a current-confined-path nano-oxide layer (CCP-NOL). By applying ion-assisted oxidation (IAO) for the CCP-NOL formation, we enhanced the MR ratio to 5.4% at a small RA value of 500 mΩ µm2 for conventional Co90Fe10 layers. Furthermore, the use of bcc-Fe50Co50 also increased the MR ratio to 8.2% at a small RA value of 580 mΩ µm2. A modified Valet–Fert model for the CCP-NOL showed that the MR enhancement by the IAO is due to the improvement in resistivity of the CCP, and that by Fe50Co50 is due to a larger spin-dependent interface scattering effect. Analysis by cross-sectional TEM and three-dimensional atom probe confirmed the formation of the CCP-NOL structure. A reliability test for test element devices showed almost no change even under acceleration stress. The CPP-GMR spin-valve film with the CCP-NOL is extendable to future high-density recording heads due to its potential for a higher MR ratio at a small value of RA.

Nanoconstricted structure for current-confined path in current-perpendicular-to-plane spin valves with high magnetoresistance

We have successfully observed a nanoconstricted structure for current-confined-path (CCP) effect in current-perpendicular-to-plane-giant-magnetoresistance (CPP-GMR) spin valves. By inserting an AlCu nano-oxide layer (NOL) formed by ion-assisted oxidation (IAO) between a pinned layer and a free layer, the MR ratio was increased while maintaining a small area resistance product (RA). The cross-sectional high-resolution transmission electron microscopy image of the sample with RA=380mΩμm2, ΔRA=16mΩμm2, and MR ratio=4.3% showed that an amorphous oxide layer is a main part of the NOL that blocks the electron conduction perpendicular to plane. Some parts of the NOL are punched through crystalline, metallic channels having a diameter of a few nanometers, which are thought to work as nanoconstricted electron conduction paths between the pinned layer and the free layer. Nano-energy-dispersive-x-ray-spectrum analysis also showed that Cu is enriched in the metallic channels, whereas Al is enriched in the amorphous oxide region, indicating that the metallic channel is made of Cu and the oxide is made of Al2O3. The nanoconstricted structure with good segregation between the metallic channel and the oxide layer enables us to realize a large MR ratio in CCP-CPP spin valves.

Current modulation of a hygroscopic insulator organic field-effect transistor

We have fabricated solution processable polymer transistors with high conductivity, requiring only a few volts for obtaining good current modulation. The devices can be fabricated and operated in air and the operation is greatly enhanced in humid atmosphere. Devices reach an On∕Off ratio of about 600 and a subthreshold swing of 500mV per decade operating on voltages less than 2V. In this letter the mechanism behind the current modulation is investigated, and we show that the current is modulated through ion-assisted oxidation and reduction of the semiconductor by ions moving vertically in the insulator material to the transistor channel.

MR Ratio Enhancement by NOL Current-Confined-Path Structures in CPP Spin Valves

We have compared the magnetoresistance (MR) performance of current-confined-path (CCP) current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) spin valve films with a nano-oxide-layer (NOL), made between natural oxidation (NO) and ion-assisted oxidation (IAO). For the NO, an MR ratio was only 1.5% at an RA of 370 m/spl Omega//spl mu/m/sup 2/, whereas for the IAO, an MR ratio was greatly increased to 5.4% at an RA of 500 m/spl Omega//spl mu/m/sup 2/. Fitted data by the Valet-Fert model showing larger MR enhancement effect by the IAO is explained by the improvement of the metal-purity of the Cu inside the CCP structure. By further improvement of metal-purity of the Cu, a large MR ratio of more than 30% can be expected at a small RA of 300 m/spl Omega//spl mu/m/sup 2/. The CCP-CPP spin valve film is a promising candidate for realizing high-density recording heads for 200 to 400-Gbpsi recording.

Ion-assisted oxidationによる極薄FeCo系酸化物を有するスペキュラースピンバルブ膜の高性能化

Ion-assisted oxidationによる極薄FeCo系酸化物を有するスペキュラースピンバルブ膜の高性能化

Specular spin-valve films with an FeCo nano-oxide layer by ion-assisted oxidation

We compared the specular spin-valve films with an Fe50Co50 nano-oxide layer (NOL) and a Co90Fe10 NOL in a pinned layer, prepared by natural oxidation (NO) and ion-assisted oxidation (IAO). For the IAO, an Ar-ion beam was used for the energy-assist effect during the oxidation, resulting in thermally stable NOL formation. With small oxygen exposures during the oxidation for the Fe50Co50 NOL by IAO, good ferromagnetic coupling through the NOL and high specularity at the NOL interface were concurrently obtained. Moreover, twisted coupling through the NOL was observed for the Fe50Co50 NOL by IAO for higher oxygen exposures. On the other hand, the NO did not cause large magnetoresistance (MR) enhancement for either the Co90Fe10 or Fe50Co50 NOLs, and the Co90Fe10 NOL by IAO caused weak magnetic coupling through the NOL, resulting in a small MR ratio. The Fe50Co50 NOL for small oxygen exposures is a good candidate for a final specular spin-valve film head for 100-Giga-bit per square inch recording.

Ion-assisted low-temperature oxidation for fabrication of strained Si1-xGex and Si1-x-yGexCy MOS capacitors

Ion-assisted oxidation has been carried out at a substrate temperature of 150 °C for fabrication of metal-oxide-semiconductor capacitors using fully strained Si1-xGex (x = 0.2 and 0.26) and partially strained Si1-x-yGexCy (Ge:C = 20:1 and 40:1) films. The oxide thickness is designed to consume a part of the Si-cap layer, leading to the formation of a valence band quantum well for confining the carriers away from the Si-SiO2 interface. The fixed oxide charge density and mid-gap interface trap density are found to be higher for the sample containing higher Ge and C concentration. The presence of hole-trapping centres is observed for all the samples. However, a decrease in the hole trap density is observed either with the reduction of Ge content or with the increase of carbon fraction in the film.

Ultra Low-Temperature Growth of High-Integrity Thin Gate Oxide Films by Low-Energy Ion-Assisted Oxidation

High integrity thin gate oxide films have been grown at a temperature as low as 450° C by direct oxidation of silicon. The bombardment of the silicon surface by low energy ions of argon and oxygen mixed plasma is utilized to activate the oxidation process. Dielectric breakdown field intensity of the oxide film of 12 MV/cm is obtained by the MOS capacitor evaluation. The precise control of the bombarding energy is essential in achieving the high-integrity thin gate oxide films.

Ultra-low-temperature growth of high-integrity gate oxide films by low-energy ion-assisted oxidation

The gate oxide films have been grown at a temperature as low as 450 °C by direct oxidation of silicon. Such a low-temperature oxidation has been realized by employing a precision controlled ion bombardment in an Ar/O2 mixed plasma for the surface activation. Perfectly controlled Ar ions give the bombardment energy for the oxide film growth. Dielectric breakdown fields of 10 MV/cm are achieved. Integration in a total low-temperature device process has been demonstrated by fabricating self-aligned Al-gate metal-oxide-silicon field effect transistor (MOSFET) formed without any heat processing over 450 °C. The precise control of the ion bombardment is quite essential for the low-temperature process.