Showing papers in "Applied Physics Express in 2009"

531 nm Green Lasing of InGaN Based Laser Diodes on Semi-Polar {202̄1} Free-Standing GaN Substrates

Abstract: Lasing in pure green region around 520 nm of InGaN based laser diodes (LDs) on semi-polar {2021} free-standing GaN substrates was demonstrated under pulsed operation at room temperature. The longest lasing wavelength reached to 531 nm and typical threshold current density was 8.2 kA/cm2 for 520 nm LDs. Utilization of a novel {2021} plane enabled a fabrication of homogeneous InGaN quantum wells (QWs) even at high In composition, which is exhibited with narrower spectral widths of spontaneous emission from LDs than those on other planes. The high quality InGaN QWs on the {2021} plane advanced the realization of the green LDs.

Continuous-Wave Operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {2021} GaN Substrates

Abstract: Room-temperature continuous-wave operation of 520 nm InGaN-based green laser diodes on semi-polar {2021} GaN substrates was demonstrated. A threshold current of 95 mA corresponding to a threshold current density of 7.9 kA/cm2 and a threshold voltage of 9.4 V were achieved by improving the quality of epitaxial layers on {2021} GaN substrates using lattice-matched quaternary InAlGaN cladding layers and also by adopting a ridge-waveguide laser structure.

510–515 nm InGaN-Based Green Laser Diodes on c-Plane GaN Substrate

Abstract: We succeeded in developing InGaN-based green laser diodes (LDs) with a wavelength of 515 nm under continuous-wave (cw) operation by improving the growth condition of epitaxial layers and structures of LDs. The LD structures were grown on conventional c-plane free-standing GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage at 515 nm were 53 mA and 5.2 V, respectively. The lifetime of 510–513 nm LDs was estimated to be over 5000 h under cw operation with an optical output power of 5 mW at 25 °C.

Very High Mobility in Solution-Processed Organic Thin-Film Transistors of Highly Ordered [1]Benzothieno[3,2-b]benzothiophene Derivatives

Abstract: Field-effect mobility as high as 5 cm2/(V s) is achieved in solution-processed organic thin-film transistors with the development of a method for growing highly-oriented crystalline films of [1]benzothieno[3,2-b]benzothiophene derivatives. A droplet of the solution is sustained at an edge of a structure on an inclined substrate, so that the crystalline domain grows in the direction of inclination. The oriented growth realizes excellent molecular ordering that manifests itself in micrometer-scale molecular terraces on the surface as a result of the self-organizing function of the material. The unprecedented performance achieved using an easy fabrication process has increased attractiveness of organic thin-film transistors for industrial applications.

Growth of Core–Shell InP Nanowires for Photovoltaic Application by Selective-Area Metal Organic Vapor Phase Epitaxy

Abstract: We report on the formation of core–shell pn junction InP nanowires using a catalyst-free selective-area metalorganic vapor-phase epitaxy (SA-MOVPE) method. A periodically aligned dense core–shell InP nanowire array was fabricated and used in photovoltaic device applications. The device exhibited open-circuit voltage (VOC), short-circuit current (ISC) and fill factor (FF) levels of 0.43 V, 13.72 mA/cm2 and 0.57, respectively, which indicated a solar power conversion efficiency of 3.37% under AM1.5G illumination. This study demonstrates that high quality core–shell structure nanowire fabrication is possible by SA-MOVPE and that the nanowire arrays can be used in integrated nanowire photovoltaic devices.

Nanoparticle-Stabilized Cholesteric Blue Phases

Abstract: The authors report the expansion of the temperature range of cholesteric blue phases by doping nanoparticles. When spherical gold nanoparticles with a mean diameter of 3.7 nm were doped in a blue phase-exhibiting multi-component liquid crystal mixture, the temperature range of the cholesteric blue phase increased from 0.5 to 5 °C, while the clearing temperature decreased by approximately 13 °C. We believe that the mechanism stabilizing the cholesteric blue phase is similar to that of polymer-stabilized cholesteric blue phases: the nanoparticles accumulate in the lattice disclinations, stabilizing the overall cholesteric blue structure.

Room-Temperature Red Emission from a p-Type/Europium-Doped/n-Type Gallium Nitride Light-Emitting Diode under Current Injection

Abstract: We have succeeded in the growth of europium (Eu)-doped GaN layer grown by organometallic vapor-phase epitaxy (OMVPE) and demonstrated the first low-voltage operation of current-injected red emission from a p-type/Eu-doped/n-type GaN light-emitting diode (LED) at room temperature. The bright red emission was obtained with an applied voltage as low as 3 V under normal lighting conditions. At a dc current of 20 mA, the output power, integrated over the 5D0–7F2 transition in Eu3+ ions (around 621 nm), was 1.3 µW. This result suggests a novel way to realize GaN-based red LEDs and monolithic devices comprising red, green and blue GaN-based LEDs.

Voltage-Assisted Magnetization Switching in Ultrathin Fe80Co20 Alloy Layers

Abstract: Growing demands for the voltage-driven spintronic applications with ultralow-power consumption have led to new interest in exploring the voltage-induced magnetization switching in ferromagnetic metals. In this study, we observed a large perpendicular magnetic anisotropy change in Au(001)/ultrathin Fe80Co20(001)/MgO(001)/polyimide/indium tin oxide (ITO) junctions, and succeeded in realizing a clear switching of magnetic easy axis between in-plane and perpendicular directions. Furthermore, employing a perpendicularly magnetized film, voltage-induced magnetization switching in the perpendicular direction under the assistance of magnetic fields was demonstrated. These pioneering results may open a new window of electric-field controlled spintronics devices.

Reduction of Deep Levels and Improvement of Carrier Lifetime in n-Type 4H-SiC by Thermal Oxidation

Abstract: Significant reduction of major deep levels in n-type 4H-SiC(0001) epilayers by means of thermal oxidation is demonstrated. By thermal oxidation of epilayers at 1150–1300 °C, the concentration of the Z1/2 and EH6/7 centers has been reduced from (0.3–2)×1013 cm-3 to below the detection limit (1×1011 cm-3). The depth-profile analysis of the Z1/2 center has revealed that the Z1/2 center is eliminated to a depth of about 50 µm from the surface after thermal oxidation at 1300 °C for 5 h. The carrier lifetime in an n-type 4H-SiC epilayer measured by differential microwave photoconductance decay has been significantly improved from 0.73 µs (as-grown) to 1.62 µs (after oxidation: 1300 °C, 5 h×2). The reduction mechanism of the Z1/2 and EH6/7 centers is discussed.

Wide Color Gamut Backlight for Liquid Crystal Displays Using Three-Band Phosphor-Converted White Light-Emitting Diodes

Abstract: This work reports a wide color gamut backlight for liquid crystal display (LCD) utilizing a three-band white light-emitting diodes (LED). The LED backlight was fabricated by combining a blue LED chip with β-sialon:Eu (green) and CaAlSiN3:Eu (red) phosphors. This 2-phosphor-converted white LED shows a discrete spectrum with distinct separation of red, green, and blue primary colors due to a narrow and asymmetric emission band of the green phosphor. By applying typical color filters of LCDs and color-matching functions, a wide color gamut of 91.9% of the National Television Standard Committee standard is attained, compared to 71.6% for the conventional two-band white LED.

Large Interface Spin-Asymmetry and Magnetoresistance in Fully Epitaxial Co2MnSi/Ag/Co2MnSi Current-Perpendicular-to-Plane Magnetoresistive Devices

Abstract: Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) characteristics were investigated in a Co2MnSi (CMS)/Ag/CMS fully epitaxial device and compared to those in a CMS/Cr/CMS device systematically. Reflection high-energy electron diffraction and transmission electron microscopy images showed the two samples had no remarkable differences and little interdiffusion. The large spin-asymmetry of electron scattering was found at the CMS/Ag inteface compared with that at the CMS/Cr interface. Finally, the largest magneto-resistance (MR) ratio of 28.8% was observed at room temperature in the CMS/Ag/CMS CPP-GMR device.

Mobility Variations in Mono- and Multi-Layer Graphene Films

Abstract: The electric properties of mono- and multi-layer graphene films were systematically studied. The current modulation increased monotonically with a decrease in the layer number due to the reduction of the interlayer scattering. The carrier mobility in the monolayer was greater than that in the multilayer due to the linear dispersion relation. On the other hand, in the monolayer, the carrier transport was significantly sensitive to the charged impurity density due to the reduction in the screening effect, which caused larger mobility variation. The reduction of the charged impurity density is thus key for high mobility.

Fabrication of the Iron-Based Superconducting Wire Using Fe(Se,Te)

Abstract: We have fabricated the Fe(Se,Te) superconducting wire by a special process based on a powder-in-tube method. The pure Fe tube plays the role of not only the sheath but also the raw material for synthesizing the superconducting phases. We succeeded in observing zero resistivity current on the current?voltage measurements for the Fe(Se,Te) wire. Introduction of the pinning centers and fabricating a multi-core wire will enhance the critical current density for the next step.

Continuous Separation of Metallic and Semiconducting Carbon Nanotubes Using Agarose Gel

Abstract: We have developed a novel method to separate metallic and semiconducting single-wall carbon nanotubes (SWCNTs) with high purities using agarose gel. When an SWCNTs/sodium dodecyl sulfate (SDS) dispersion was applied to a column containing agarose gel beads, semiconducting SWCNTs were trapped by the beads, while metallic SWCNTs passed through the column. After the semiconducting SWCNTs adsorbed to the beads were eluted with sodium deoxycholate solution, the column could be used for repeated separation. Because this continuous, repeatable separation method is applicable to a low-cost, large-scale process, it should enable the industrial production of metallic and semiconducting SWCNTs.

(K,Na)NbO3-Based Multilayer Piezoelectric Ceramics with Nickel Inner Electrodes

Abstract: A lead-free (K,Na)NbO3-based multilayer piezoelectric ceramic with nickel inner electrodes was obtained. Its normalized electric-field-induced thickness strain (Smax/Emax) was 360 pm/V, where Smax denotes the maximum strain and Emax denotes the maximum electric field. This value of 360 pm/V is about half that for the Pb(Zr,Ti)O3-based ceramics that are widely used. However, the (K,Na)NbO3-based multilayer ceramic can achieve comparable displacement to that attainable with Pb(Zr,Ti)O3-based multilayer ceramics by stacking more ceramic layers with smaller thicknesses. Additionally, it is easier to reduce the ceramic-layer thickness by using nickel electrodes. The developed (K,Na)NbO3-based multilayer ceramic is thus considered a good candidate for lead-free piezoelectric actuators.

500-nm Optical Gain Anisotropy of Semipolar (1122) InGaN Quantum Wells

Abstract: We studied the effect of carrier population on light emission polarization of green InGaN quantum wells (QWs) on the semipolar (1122) plane. The 3 nm thick QWs emitting light at about 540 nm at low pumping power have electrical field (E) component E∥[1123] stronger than that E∥[1100]. However, we found that increasing the pumping power changed the sign of the polarization ratio. Using the varied stripe length (VSL) method, we measured the optical gain for light propagating ∥[1123] direction to be ~2 times that of light propagating ∥[1100] direction. We explain this behavior by inhomogeneous QW state filling.

Elimination of the Major Deep Levels in n- and p-Type 4H-SiC by Two-Step Thermal Treatment

Abstract: By thermal oxidation of 4H-SiC at 1150–1300 °C, the Z1/2 and EH6/7 concentrations can be reduced to below 1×1011 cm-3. By the oxidation, however, a high concentration of HK0 center (EV + 0.78 eV) is generated. Additional annealing in Ar at 1550 °C results in elimination of the HK0 center. Thus, all the major deep levels can be eliminated by the two-step thermal treatment. Based on the depth profiles of deep levels, a model for the defect generation and elimination is proposed. The carrier lifetime in 4H-SiC epilayers has been improved from 0.64 (as-grown) to 4.52 µs by this method.

One-Dimensional Optical Lattice Clock with a Fermionic 171Yb Isotope

Abstract: We demonstrate a one-dimensional optical lattice clock with ultracold 171Yb atoms, which is free from the linear Zeeman effect. The absolute frequency of the 1S0(F = 1/2)–3P0(F = 1/2) clock transition in 171Yb is determined to be 518 295 836 590 864(28) Hz with respect to the SI second.

Domain Wall Motion Induced by Electric Current in a Perpendicularly Magnetized Co/Ni Nano-Wire

Abstract: The authors show experimental results on domain wall motion induced by electric current in a Co/Ni nano-wire with perpendicular magnetic anisotropy. The motion was detected electrically by using the anomalous Hall effect. Threshold current density for the domain wall motion was found to decrease with decreasing the wire width, where the minimum threshold current density of approximately 5×1011 A/m2 was observed for the wire width of 70 nm.

Bistable Polymer Stabilized Cholesteric Texture Light Shutter

Abstract: We developed a bistable polymer stabilized cholesteric texture (PSCT) light shutter which can be switched between a transparent state and an opaque state by voltage pulses. The PSCT light shutter is switched to a transparent state by a low frequency voltage pulse and remains transparent after the pulse. It is switched to an opaque state by a high frequency voltage pulse and remains opaque after the pulse. It can be used for architectural and greenhouse windows and is very energy-efficient.

Impact of Cu Electrode on Switching Behavior in a Cu/HfO2/Pt Structure and Resultant Cu Ion Diffusion

Abstract: In a newly proposed switching device using polycrystalline HfO2 thin film with ion diffusion path, we have found that a Cu electrode could contribute to improved switching performance. Current–voltage measurements at room temperature revealed clear resistive switching, not accompanied by a forming process, in our Cu/HfO2/Pt structure. The current step difference from one state to the other one was in the order of 103–104, giving a sufficient on/off ratio. Voltage sweep polarity suggested that filamentary Cu paths were formed due to Cu ion diffusion and annihilated at the HfO2/Pt interface at reversed bias. This filament path formation and annihilation was the origin of the switching device performance.

Effect of Pressurization on the Dynamics of a Cavitation Bubble Induced by Liquid-Phase Laser Ablation

Abstract: We applied external high pressure to ambient water in liquid-phase laser ablation. As a result, it was found that the maximum volume Vmax of a cavitation bubble induced by laser ablation satisfied a scaling law of Vmax∝Pext-1 with Pext being the pressure applied to water. The effect of the pressurization was also observed in the shape of the second bubble induced by the collapse of the first cavitation bubble. These experimental results indicate that the dynamics of a cavitation bubble induced by liquid-phase laser ablation is controlled by the external pressure.

Anti-Reflection Structures on Lenses by Nanoimprinting Using Ordered Anodic Porous Alumina

Abstract: Anti-reflection (AR) structures composed of an ordered array of tapered pillars were formed on the surfaces of two types of lens, microlens and Fresnel lens, through the nanoimprinting of polymer using molds prepared from anodic porous alumina. The use of anodic porous alumina for the preparation of mold for nanoimprinting allows the formation of AR structures on the curved surface of the lens, and could effectively suppress the reflectance of incident light. The process enables the high-throughput production of polymer lenses with AR structures and will be useful for the preparation of various types of functional optical devices.

Ge/GeO2 Interface Control with High-Pressure Oxidation for Improving Electrical Characteristics

Abstract: High-pressure oxidation (HPO) of germanium (Ge) for improving electrical properties of Ge/GeO2 stacks was investigated. The capacitance–voltage (C–V) characteristics of metal/GeO2/Ge capacitors fabricated with HPO revealed improved electrical properties without any post-deposition annealing, and the interface states density (Dit) was reduced to 2×1011 eV-1 cm-2 near the midgap. Moreover, the refractive index of thermally oxidized GeO2 was increased by HPO. It is also discussed from a thermodynamic viewpoint of the Ge/GeO2 system that the GeO desorption from Ge/GeO2 stacks could be efficiently suppressed by HPO.

Terahertz Laser with Optically Pumped Graphene Layers and Fabri?Perot Resonator

Abstract: We propose and substantiate the concept of terahertz (THz) laser based on the optically pumped graphene layers and the resonant cavity of the Fabri–Perot type The pumping scheme which corresponds to the optical interband excitation of graphene followed by the emission of an optical phonons cascade provides the population inversion for the interband transitions in a relatively wide range of THz frequencies We demonstrate that the THz lasing in the device under consideration at room temperatures is feasible if its structure is optimized The frequency and output power of the generated THz radiation can be tuned by varying the distance between the mirrors

Growth of Semipolar (1122) GaN Layer by Controlling Anisotropic Growth Rates in r-Plane Patterned Sapphire Substrate

Abstract: Semipolar (1122) GaN was achieved by controlling anisotropic growth rates in a maskless r-plane patterned sapphire substrate. Upon optimizing the growth conditions, the growth rate of the GaN layer on etched c-plane-like sapphire was much higher than that on other planes such as the original r-plane sapphire. Singularly (1122)-oriented GaN was confirmed when GaN was grown on only the c-plane-like sapphire sidewall. The control of the anisotropic growth rate is useful for growing nonpolar and semipolar layers using maskless patterned substrates.

Improvement in Lasing Characteristics of GaN-based Vertical-Cavity Surface-Emitting Lasers Fabricated Using a GaN Substrate

Abstract: We compared the lasing characteristics of GaN-based vertical-cavity surface-emitting lasers (VCSELs) fabricated using a GaN substrate with those fabricated using a sapphire substrate. The original substrates are removed from the devices after the devices have been bonded to Si substrates. Consequently, with the exception of the cavity length, the two kinds of fabricated VCSELs have almost the same structures. The VCSELs fabricated using a GaN substrate have a higher maximum output power (0.62 mW) and longer lifetimes than those fabricated using a sapphire substrate. Even for the VCSELs fabricated with a GaN substrate, 10-min operation causes their threshold current to increase.

Standby-Power-Free Compact Ternary Content-Addressable Memory Cell Chip Using Magnetic Tunnel Junction Devices

Abstract: A compact ternary content-addressable memory (TCAM) cell of 3.15 µm2 with a 0.14 µm complementary metal oxide semiconductor process is realized by the use of nonvolatile magnetic tunnel junction (MTJ) devices with spin-injection write. This TCAM cell based on logic-in-memory architecture with nonvolatile MTJs needs no standby power, yet allows instant shut-down of the supply voltage without data backup to an external nonvolatile device.

Giant Ge-on-Insulator Formation by Si–Ge Mixing-Triggered Liquid-Phase Epitaxy

Abstract: Formation of giant single-crystalline Ge-on-insulator (GOI) with 400 µm length is demonstrated by using seeding lateral liquid-phase epitaxy (L-LPE). High quality (100), (110), and (111) oriented single-crystals are obtained on Si substrates covered with SiO2. A mechanism based on the solidification temperature gradient due to Si–Ge mixing and the thermal gradient due to latent heat at the growth front is proposed. An additional experiment on quartz substrates well supports the importance of Si–Ge mixing as a trigger for giant L-LPE of Ge. This process opens up the possibility of Ge-based fully-depleted field-effect transistors with high electron and hole mobilities.

A Kinetic Model of Silicon Carbide Oxidation Based on the Interfacial Silicon and Carbon Emission Phenomenon

Abstract: We proposed a kinetic model for thermal oxidation of silicon carbide, termed "silicon and carbon emission model", taking into account the Si and C emissions from the oxidation interface, which lead to a reduction of interfacial reaction rate. We used this model to calculate oxide growth rates and found that the derived growth rates showed a good fit with the measured rates over the entire oxide thickness for both the C and Si faces. We discussed the difference in oxidation mechanism between these polar faces in terms of the difference in parameter values deduced from the curve fits.