Pressure PECVD Research Articles

3D Stackable Vertical‐Sensing Electrochemical Random‐Access Memory Using Ion‐Permeable WS2 Electrode for High‐Density Neuromorphic Systems

AbstractIon‐based electrochemical random‐access memory (ECRAM) is proposed for synaptic applications owing to its promising characteristics that have the potential to accelerate data processing through neuromorphic systems. However, attaining ideal synaptic functionalities and constructing high‐density vertical synapse arrays are challenging due to issues related to uncontrolled ion migration and constraints in 3D multi‐stacking. Here, a breakthrough using 3D stackable Li ion‐based vertical‐sensing ECRAM (VS‐ECRAM) is presented with an ion‐permeable ultrathin WS2 electrode synthesized through low‐temperature (200 °C) atmospheric‐pressure plasma‐enhanced chemical vapor deposition (AP‐PECVD). The direct AP‐PECVD of the WOx channel layer induces WS2 formation in the surface region, which exhibits sufficient electrical conductivity to function as an electrode. By utilizing the WS2 electrode as an ion‐barrier layer in the VS‐ECRAM synapse, excellent weight update linearity and cycling variability are achieved due to the finely controlled ion migration. Furthermore, a two‐layer stacked 3D VS‐ECRAM is successfully fabricated through the vertical WS2 formation, and independent weight updates without any disturbance are confirmed. Finally, a high pattern recognition accuracy of 95.22% is obtained using a multi‐layer perceptron‐based neural network. Therefore, the proposed 3D stackable WS2‐based VS‐ECRAM exhibits a strong potential for application in high‐density neuromorphic devices with excellent synaptic performances.

A substrate constituent Na-catalyzed growth of carbon nanotubes on glass substrate by atmospheric pressure PECVD

A growth mechanism of carbon nanotubes (CNTs) without any externally added catalyst is reported in this study. Instead of any externally added catalyst, the constituents of the used soda lime glass substrates seem to play the pivotal role in the growth of CNTs. XPS analysis of the synthesized CNTs reveal that among the various constituents of soda lime glass, sodium (Na) emerges as the primary catalyst for initiating CNT growth. The adopted PECVD technique involving an especially designed spiral-shaped fused hollow cathode source for generating the required atmospheric pressure plasma makes such synthesis easily feasible. The important key factors influencing CNT growth include substrate temperature and plasma pre-treatment of the substrate. Plasma pre-treatment of the substrate at an elevated temperature leads to the formation of catalyst nanoparticles on the surface of the substrate. This report features a detailed CNT growth mechanism proposed on the basis of optical emission spectroscopy and XPS analyses. To be precise, this study demonstrates a novel Na-catalysed synthetic approach for achieving uniform CNT growth on glass substrates without using any externally added catalyst, facilitating the preparation of clean CNTs.

Low-temperature homoepitaxial growth of β-Ga2O3 thin films by atmospheric pressure plasma-enhanced chemical vapor deposition technique

Low-temperature homoepitaxial growth of β-Ga2O3(-201) has been successfully demonstrated by the atmospheric pressure plasma-enhanced chemical vapor deposition technique. To search for low-temperature growth, temperature-dependent studies were carried out between 350 and 600 °C. A high N2 gas flow rate, low gallium source concentration, and high oxygen flow rate ratio played key roles in growing independent and homogeneous multiple nuclei of Ga2O3, leading to three-dimensional grain growth mode, single crystallinity, and the highest growth rate of ⁓0.17 µm/h at 350 °C. The highly reactive atmospheric pressure oxygen plasma actively led to epitaxial growth. The low thermal budget homoepitaxial growth is a record reduction reported thus far.

High-mobility and low subthreshold swing amorphous InGaZnO thin-film transistors by in situ H2 plasma and neutral oxygen beam irradiation treatment

In this work, staggered bottom-gate structure amorphous In–Ga–Zn–O (a-IGZO) thin film transistors (TFTs) with high-k ZrO2 gate dielectric were fabricated using low-cost atmospheric pressure-plasma enhanced chemical vapor deposition (AP-PECVD) with in situ hydrogenation to modulate the carrier concentration and improve interface quality. Subsequently, a neutral oxygen beam irradiation (NOBI) technique is applied, demonstrating that a suitable NOBI treatment could successfully enhance electrical characteristics by reducing native defect states and minimize the trap density in the back channel. A reverse retrograde channel (RRGC) with ultra-high/low carrier concentration is also formed to prevent undesired off-state leakage current and achieve a very low subthreshold swing. The resulting a-IGZO TFTs exhibit excellent electrical characteristics, including a low subthreshold swing of 72 mV dec−1 and high field-effect mobility of 35 cm2 V−1 s−1, due to conduction path passivation and stronger carrier confinement in the RRGC. The UV–vis spectroscopy shows optical transmittance above 90% in the visible range of the electromagnetic spectrum. The study confirms the H2 plasma with NOBI-treated a-IGZO/ZrO2 TFT is a promising candidate for transparent electronic device applications.

Electrical Characteristics of Atmospheric-Pressure Plasma Enhanced Chemical Vapor Deposition Fabricated Indium-Gallium-Zinc-Oxide Thin-Film Transistors with In-Situ Hydrogenation and Neutral Beam Oxidation

Low temperature poly-crystalline silicon thin-film transistors (LTPS-TFTs) and indium-gallium-zinc-oxide TFTs (IGZO-TFTs) are potential candidates for future technology of various displays. Due to its simple manufacturing process, low cost and good uniformity, IGZO-TFTs have been developed as main stream display technology. From the viewpoint of device electrical characteristics, a-IGZO TFTs have better field-effect mobility (>10 cm2/V * s), larger Ion/Ioff ratio (>106), smaller subthreshold swing (S.S) and good stability. In this study, atmospheric-pressure PECVD (AP-PECVD) is used to deposit a-IGZO active layer, during the deposition process In-Situ hydrogenation is applied to enhance the layer characteristics. Also, the layer is then surface oxidation treated by neutral beam system (NBS). The optimal TFTs device characteristics is reached with In-Situ hydrogenation of H2 90 sccm, and surface oxidation of 400 W NBS treatment. The field-effect mobility is 34.05 cm2/V * s, threshold voltage is 1.74 V, subthreshold swing is 62 mV/decade, and current ratio Ion/Ioff is 2.04×107. Compared with conventional plasma, NBS used in this study provides charge-free plasma which could enhance device electric characteristics.

Area selective atmospheric pressure PECVD of organosilicon precursors: Role of vinyl and ethoxy groups on silicon oxycarbide deposition patterns - A case study

An atmospheric pressure Dielectric Barrier Discharge plasma torch was used to perform selective area plasma enhanced chemical vapour deposition of four organosilicon precurors: Triethylvinylsilane (VTES), Vinyltriethoxysilane (VTEOS), Tetraethoxysilane (TEOS) and Tetraethylsilane (TES). The precursors were chosen in such a way that they differ only by the presence or absence of vinyl and/or ethoxy groups. Experimentally, major differences have been observed on the deposition speed, coating patterns and thin film composition. Precursor with vinyl group has confined deposition at the central region of plasma torch, in contrast precursor with only ethoxy groups has circular deposition in the periphery region of plasma torch, and precursors with both the vinyl and ethoxy groups has deposition at both locations. A detailed chemical analysis revealed the film composition to be silicon oxycarbide (SiOxCyH) with high content of carbon for the central region's coating whereas the peripherical coating is close to silica like (SiOx) with a minuscule carbon content. Coatings patterns and composition have been investigated in the light of gaseous flows mixing obtained from computational Fluid Dynamic (CFD) simulation. A strong correlation is evidenced for central SiOxCyH deposition of vinyl containing precursors, to the high concentration of precursor. Whereas the peripheral SiOx deposition of ethoxy containing precursors corresponds to high concentration of plasma species. Furthermore, the role of reactive plasma species on the underlying deposition mechanisms has been identified as crucial and has been purposed as the driving factor.

The Effect of the Process Parameters on the Composition and Properties of Silica-Like Films Deposited by Atmospheric Pressure PECVD in the System TEOS-He-O2

The Effect of the Process Parameters on the Composition and Properties of Silica-Like Films Deposited by Atmospheric Pressure PECVD in the System TEOS-He-O2

Preparation and properties of SnSb/C/DLC nanofibers with different precursors based on AP-PECVD

In order to explore the morphology and improved electrochemical performance of diamond-like carbon (DLC) deposited on SnSb/C nanofiber derived from different precursors by the atmospheric pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technology. SnCl4•5H2O/SbCl3 (I), Sn(CH3COO)2/Sb(CH3COO)3 (II) were used as SnSb precursors, respectively, the SnSb/C nanofiber membranes were prepared by electrospinning and carbonization, and then DLC was deposited on different SnSb/C nanofibers to obtain SnSb/C/DLC nanofibers. It is shown that the densified and irregular cubic-like DLC nanoparticles were deposited on SnSb/C (I) nanofibers. While on the SnSb/C(II) nanofibers, DLC nanoparticles exhibited as sparse and spherical. Meanwhile, both SnSb/C/DLC nanofibers have good structural stability, especially higher sp3/sp2 ratios DLC was shown in the SnSb/C/DLC (I) nanofiber than that of SnSb/C/DLC (II) one. By comparison, SnSb/C/DLC(I) nanofibers have excellent electrochemical performance, the capacity retention is 69.33% after 100 cycles, and the capacity retention rate from 20 to 100 cycles is 109.24%. This reveals that DLC materials with higher C–C content and uniform distribution have a positive effect on the aspect of inhibiting SnSb alloys volume expansion.

Open-air plasma deposition of polymer-supported silica-based membranes for gas separation

In this study, we developed an open-air atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) as a scalable technique to fabricate polymer-supported silica-based membranes for gas separation. Using the open-air AP-PECVD, we achieved a low-temperature deposition of silica-based molecular sieve membranes onto porous polymeric support in a system that can operate under open-air conditions. We investigated the effect of deposition conditions, such as precursor concentration and deposition temperature, on the membrane structure and permeation property. It was revealed that precursor concentration plays a vital role in determining membrane properties. The membranes prepared with low precursor concentration had an inorganic structure with granular morphology, resulting in poor permselective performance. In contrast, deposition at high precursor concentration led to the formation of a continuous layer with a silicone-like structure. By optimizing the precursor concentration, a membrane having a He permeance of 7.25 × 10−8 mol/(m2 s Pa) with a He/SF6 permeance ratio of 171 can be fabricated at room temperature. The deposition at elevated temperature was also effective in improving the permselective performance. By increasing the deposition temperature to 150 °C, the permselective performance of the membrane was improved to a He permeance of 7.7 × 10−8 mol/(m2 s Pa) and He/SF6 of 434. The resultant membranes also exhibited high selectivity for the separation of industrial relevant gas mixtures such as H2/N2 and H2/CH4 with the permeance ratios of 40 and 38, respectively. The present study demonstrates that open-air AP-PECVD is promising as a scalable technique to fabricate polymer-supported silica-based membranes with high productivity.

Low Temperature Open-Air Plasma Deposition of SrTiO3 Films for Solar Energy Harvesting: Impact of Precursors on the Properties and Performances.

Strontium titanate (STO) is a well-known oxide used in a wide variety of applications due to its excellent stability and optoelectronic properties. However, its integration in photoelectrocatalytic devices is limited by the lack of fast and scalable methods to produce robust films at a low temperature and atmospheric pressure. Herein, we report an atmospheric pressure plasma-enhanced chemical vapor deposition (AP-PECVD) approach for the synthesis of STO crystalline films and their applications for photoelectrochemical solar energy conversion. The film crystallinity, which plays a determinant role in the photoelectrochemical performance, was linked to the selected strontium precursor and injection method. Through thermal stability studies of the precursors [Sr(dpm), Sr(ipo), Sr(acac), and Ti(ipo)] and analysis of the solution droplet size, it was demonstrated that the closer thermal decomposition behavior and superior miscibility of the Sr(dpm) and Ti(ipo) precursors led to more homogeneous and crystalline films with the highest photoelectrochemical performance (16.5 μA cm-2 at 1.23 V vs RHE under 100 mW cm-2), which can be further improved by a factor of 3.4 using thermal annealing at 500 °C. Evidence of the impact of a strontium precursor on the properties of STO films is provided through thermogravimetric analysis, X-ray diffraction, energy-dispersive system, UV-vis, X-ray photoelectron spectroscopy, HIM-SIMS, and photoelectrochemical analysis.

Atmospheric-pressure PECVD synthesis of polymer-supported molecular sieving silica membranes for gas separation: Effect of pore size of polymeric support

Molecular sieving silica-based membranes were prepared on porous polymeric supports by atmospheric-pressure plasma-enhanced chemical vapor deposition. The effect of polymeric support pore size on the gas permeation properties of polymer-supported silica-based membranes was investigated using nanofiltration (NF) and ultrafiltration membranes as polymeric supports. The silica-based layer was uniformly formed on the surfaces of the polymeric supports, irrespective of their different pore sizes. The selectivity of the resultant polymer-supported membranes was found to increased with decreasing support pore size. The NF-supported membrane exhibited an improved H2/N2 permeance ratio of > 40, which was comparable to those of ceramic-supported membranes, demonstrating the successful synthesis of molecular sieving silica-based membranes onto a porous polymeric support.

Facile low-temperature route toward the development of polymer-supported silica-based membranes for gas separation via atmospheric-pressure plasma-enhanced chemical vapor deposition

The use of inexpensive porous polymers instead of conventional ceramics to support silica-based membranes can potentially minimize the cost of membrane synthesis. To develop polymer-supported silica-based membranes, lowering the high synthesis temperature of the silica-based top layer is essential. In this study, we explore the application of atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) for synthesizing polymer-supported silica-based membranes. The deposition of a continuous silica-based layer on an asymmetric polysulfone ultrafiltration membrane using hexamethyldisiloxane as an organosilicon precursor was achieved at ambient temperature and pressure via AP-PECVD. The gas permeation properties of the AP-PECVD-derived polymer-supported membranes strongly depended on the deposition duration, as prolonged deposition could reduce the remaining defects by covering the entire surface of the support with the plasma-deposited layer. The membranes showed increased selectivities for H2/N2 and H2/SF6 from 2.9 to 9.5, and from 4.5 to 184, respectively, exhibiting gas permeation dominated by molecular sieving. The present study demonstrates that AP-PECVD is a promising strategy for the fabrication of polymer-supported silica-based membranes for gas separation.

Gas-phase kinetics in atmospheric-pressure plasma-enhanced chemical vapor deposition of silicon films

Atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition of silicon (Si) films was numerically simulated. The AP plasma used for Si depositions was excited by a 150-MHz very high-frequency (VHF) electric power, which was capable of generating continuous glow discharges covering the entire electrode surface. The experimental film thickness profiles could be well fitted by the simulations by adjusting the electron density in the plasma. The results showed that, although neutral–neutral reactions proceed very rapidly due to the frequent collisions between the gas species, the dissociation of the source SiH4 molecules by electron impact is the key factor that governs the chemistry occurring in the AP-VHF plasma and promotes the film growth on the substrate. The input power dependences of electrical property of the Si films could be explained by the contribution of SiH3 radical to the deposition. It was also shown that, even though the plasma was continuous glow, the electron density changed in the direction of gas flow, suggesting that the very rapid nucleation of clusters and their growth into nanoparticles were occurring in the AP-VHF plasma.

Role of in-situ hydrogen plasma treatment on gate bias stability and performance of a-IGZO thin-film transistors

This work investigates the effect of an in situ hydrogen plasma treatment on gate bias stability and performance of amorphous InGaZnO thin-film transistors (TFTs) deposited by using atmospheric-pressure PECVD. The H2 plasma-treated a-IGZO channel has shown significant improvement in bias stress induced instability with a minuscule threshold voltage shift (ΔV th) of 0.31 and −0.17 V under positive gate bias stress (PBS) and negative gate bias stress (NBS), respectively. With the aid of the energy band diagram, the proposed work demonstrates the formation of negative species O2 − and positive species H2O+ in the backchannel under PBS and NBS in addition to ionized oxygen vacancy (Vo) defects at a-IGZO/ZrO2 interfaces are the reason for gate bias instability which could be effectively suppressed with in situ H2 plasma treatment. From the experimental result, it is observed that the electrical performance such as field-effect mobility (μ FE), on-off current ratio (I on/I off), and subthreshold swing improved significantly by in situ H2 plasma treatment with passivation of interface trap density and bulk trap defects.

TiO2 Coatings Via Atmospheric-Pressure Plasma-Enhanced Chemical Vapor Deposition for Enhancing the UV-Resistant Properties of Transparent Plastics.

Herein, TiO2 coatings were deposited on photodegradable polymers for protection from UV irradiation using the atmospheric-pressure plasma-enhanced chemical vapor deposition (AP-PECVD) technique. Polymethylmethacrylate (PMMA) and polycarbonate (PC) substrates were coated with titanium tetraisopropoxide as the precursor in an open-air atmospheric-pressure nonequilibrium argon plasma jet. The AP-PECVD-derived TiO2 coatings exhibited good adhesion to PMMA and PC. The TiO2 coatings could shield more than 99% of UV light in the wavelength range of 200–300 nm, without affecting the transmittance of visible light. UV irradiation tests on polymer films demonstrated that the degradation rates of PMMA and PC were significantly reduced by one-tenth after they were coated with TiO2 films.

How to Confer a Permanent Bio-Repelling and Bio-Adhesive Character to Biomedical Materials through Cold Plasmas

Plasma Enhanced–Chemical Vapor Deposition (PE-CVD) of polyethylene oxide-like (PEO)-like coatings represent a successful strategy to address cell-behavior on biomaterials. Indeed, one of the main drawbacks of organic and hydrophilic films, like PEO-like ones, often consists in their poor adhesion to the substrate, especially in biological fluids where the biomaterial is required to operate. In this paper, low pressure (LP) and aerosol-assisted atmospheric pressure (aerosol-assisted AP) PE-CVD of PEO-like coatings is compared. The stability of the two different classes of coatings was investigated, both in water and in the cell culture media, during cell culture experiments. The obtained results show that, when deposited at atmospheric pressure (AP), the adhesion of the PEO-like coatings to the substrate has to be granted by an intermediate gradient layer. This interlayer can match the properties of the substrate with that of the topmost coatings, and, in turn, can dramatically improve the coating’s stability in complex biological fluids, like the cell culture medium. An accurate modulation of the experimental conditions, both at LP and AP, allowed control of the film chemical structure and surface properties, to permanently promote or discourage the cellular adhesion on the surfaces of biomaterials.

Depositing chromium oxide film on alumina ceramics enhances the surface flashover performance in vacuum via PECVD

Surface flashover seriously restricts the electrical performance of insulating system in vacuum. Surface properties of the insulating materials are the key factors affecting the electrical property along the surface. In this paper, Atmospheric-Pressure Plasma Enhanced Chemical Vapor Deposition (AP-PECVD) is used to deposit chromium oxide film on alumina ceramics to improve its flashover voltage in vacuum. Chromium acetylacetonate and argon are used as the precursor and working gas, respectively. A homemade ultrasonic atomizer and gas distribution system are used to ensure the uniformity during the AP-PECVD. For comparison, deposition experiments are both carried out at atmospheric and sub-atmospheric pressures. The experimental results show that the surface flashover voltages in 10−4 Pa are increased by 20% and 26% after the PECVD at atmospheric and sub-atmospheric pressures, respectively. The physicochemical and electrical property indicates that more shallow traps are introduced into the surface of alumina ceramics. The initial potential of the deposited sample decreases by 36% after the AP-PEVCD. Therefore, the deposition of chromium oxide films on alumina ceramics by AP-PECVD can effectively inhibit the surface charge accumulation and improve the surface flashover voltage in vacuum by reducing the trap energy level and its density. Based on the SEEA (secondary electron avalanche) theory and the distribution of the surface traps, the effects of chromium oxide films deposited on alumina ceramics are summarized for the application of vacuum insulation. The feasibility of depositing chromium oxide films to improve the vacuum surface flashover voltage of alumina ceramics by PECVD is proved. The process of the PECVD at atmospheric pressure is flexible, which provides a promising technical support for enhancing surface flashover for other insulating materials.

Plasma Surface Engineering: An Enabling Technology Designed to Clean and Protect Printed Circuit Boards

Abstract Long term reliability and performance of printed circuit boards (PCBs) are strongly affected by the presence of surface contaminants from the manufacturing and assembly processes. Flux and solder residue, dust particles, oils and greases are often found on the assembled boards and can inhibit the successful application of conformal coatings that are used to protect the electronic components. Surface contaminants can cause coating delamination, dendritic growth, electromigration, corrosion and result in compromised coatings. In the first part of this paper, the fundamental mechanism of plasma-induced removal of organic contaminants from PCBs will be presented. While vacuum based plasmas are considered the traditional solvent-free technology for surface cleaning, a new approach involving air plasma operating under atmospheric pressure conditions is gaining interest due to its adaptability for industrial inline processing. The low concentration of oxygen that is available in the plasma gas is effective in vaporizing organic contaminants leaving behind a clean surface. Additionally, atmospheric plasma processes focusing on the development of functional nanocoatings on PCBs have been investigated. These plasma-enhanced chemical vapor deposition (PECVD) processes involve the delivery and vaporization of small volumes of solvent-free precursors that react with the plasma to form thin coatings on polymer substrates. Depending on the chemical structure of the precursor used, adhesion promoting, water repellant or electrical barrier coatings of 30–100nm thickness can be deposited. These protective functional coatings do not require any curing or special handling and no chemical waste is generated. The latest developments in atmospheric pressure PECVD for electronics protection will be presented in the second part of the paper. Besides the improvement of device performance and reliability, the application of PECVD has the potential to replace chemical substances such as primers known to have harmful impact on human health and the environment.

Pulsed very high-frequency plasma-enhanced chemical vapor deposition of silicon films for low-temperature (120 °C) thin film transistors

With the aim of improving silicon (Si) film quality deposited by atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition at a low substrate temperature of 120 °C, we examine the difference between continuous wave and pulsed plasmas for deposition characteristics depending on H2 and SiH4 flow rates and inpur VHF power. A specifically designed parallel-plate-type electrode system is used to generate the AP plasma, which is excited by 150 MHz very high-frequency (VHF) electric power. Hydrogen and monosilane diluted with helium are used as the process gases. The electrode system enables us to form turbulence-free one-dimensional gas flow in the plasma zone, thereby ensuring a deposition process without contamination of the substrate surface with dust particles. The electrical property of the deposited Si layers was evaluated by fabricating bottom-gate thin film transistors (TFTs). The performance of the bottom-gate TFTs revealed that the a-Si TFTs, i.e. those channel layers prepared with a pulse duty cycle of 10%, exhibit reasonably high field effect mobilities of around 1 cm2 · V–1 · s–1, suggesting that the pulse modulation of input VHF power is highly effective in achieving a plasma chemistry suitable for high-quality Si growths at low temperatures.

Study of Atmospheric-Pressure Plasma Enhanced Chemical Vapor Deposition Fabricated Indium Gallium Zinc Oxide Thin Film Transistors with In-Situ Hydrogen Plasma Treatment.

Amorphous InGaZnO (a-IGZO) Thin Film Transistors (TFTs) has been studied extensively for their perspective applications in next generation active-matrix displays such as liquid crystal displays and flat-panel displays, due to its better field-effect mobility (>10 cm²/V · S), larger Ion/Ioff ratio (>106), and better stability electrical. Hydrogen is known as shallow donors for n-type (channel) oxide semiconductors (Dong, J.J., et al. 2010. Effects of hydrogen plasma treatment on the electrical and optical properties of Zno films: Identification of hydrogen donors in ZnO. ACS Appl. Mater. Interfaces, 2, pp.1780-1784), and it is also effective passivator for traps (Tsao, S.W., et al., 2010. Hydrogen-induced improvements in electrical characteristics of a-IGZO thin-film transistors. Solid-State Electron, 54, pp.1497-1499). In this study, In-Situ hydrogen plasma is applied to deposit IGZO channel. With atmospheric-pressure PECVD (AP-PECVD), IGZO thin film can be deposited without vacuum system, large area manufacturing, and cost reducing (Chang, K.M., et al., 2011. Transparent conductive indium-doped zinc oxide films prepared by atmospheric pressure plasma jet. Thin Solid Films, 519, pp.5114-5117). The results show that with appropriate flow ratio of Ar/H₂ plasma treatment, the a-IGZO TFT device exhibits better performance with mobility (μFE) 19.7 cm²/V · S, threshold voltage (VT) 1.18 V, subthreshold swing (SS) 81 mV/decade, and Ion/Ioff ratio 5.35×107.