Showing papers in "Japanese Journal of Applied Physics in 2006"

Dye-Sensitized Solar Cells with Conversion Efficiency of 11.1%

Abstract: Dye-sensitized solar cells (DSCs) using titanium dioxide (TiO2) electrodes with different haze were investigated. It was found that the incident photon to current efficiency (IPCE) of DSCs increases with increase in the haze of the TiO2 electrodes, especially in the near infrared wavelength region. Conversion efficiency of 11.1%, measured by a public test center, was achieved using high haze TiO2 electrodes. This indicates that raising the haze of TiO2 electrodes is an effective technique for improvement of conversion efficiency.

Amorphous Oxide Semiconductors for High-Performance Flexible Thin-Film Transistors

Abstract: Recently, we have demonstrated the potential of amorphous oxide semiconductors (AOSs) for developing flexible thin-film transistors (TFTs). A material exploration of AOSs desired as the channel layer in TFTs is most important for developing high-performance devices. Here, we report our concept of material exploration for AOSs in high-performance flexible and transparent TFTs from the viewpoints of chemical bonding and electronic structure in oxide semiconductors. We find that amorphous In–Ga–Zn–O (a-IGZO) exhibits good carrier transport properties such as reasonably high Hall mobilities (>10 cm2V-1s-1) and a good controllability of carrier concentration from <1015 to 1020 cm-3. In addition, a-IGZO films have better chemical stabilities in ambient atmosphere and at temperatures up to 500 °C. The flexible and transparent TFT fabricated using a-IGZO channel layer at room temperature operated with excellent performances, such as normally-off characteristics, on/off current ratios (~106) and field-effect mobilities (~10 cm2V-1s-1), which are higher by an order of magnitude than those of amorphous Si:H and organics TFTs.

Grating Couplers for Coupling between Optical Fibers and Nanophotonic Waveguides

Abstract: Nanophotonic waveguides and components are promising for use in the large-scale integration of photonic circuits. Coupling light between nanophotonic waveguides and a single-mode fiber is an important problem and many different solutions have been proposed and demonstrated in recent years. In this paper, we discuss a grating coupler approach. Grating couplers can be placed anywhere on a circuit and can easily be integrated. We have experimentally demonstrated >30% coupling efficiency with a 1 dB bandwidth of 40 nm on standard wafers. Theoretically, the coupling efficiency can be improved to >90% using an optimized grating design and layer stack. The fabrication of the couplers in silicon-on-insulator and in indium phosphide membranes is also discussed.

Bent-Core Liquid Crystals: Their Mysterious and Attractive World

Abstract: Structures and properties of liquid crystalline phases formed by bent-core molecules are reviewed. At least eight phases designated as B1–B8 have been found, being unambiguously distinguished from phases formed by usual calamitic molecules due to a number of remarkable peculiarities. In addition to B1–B8 phases, smectic A-like phases and biaxial nematic phases formed by bent-core molecules are also reviewed. The most attractive aspects of this new class of liquid crystals are in polarity and chirality, despite being formed from achiral molecules. The bent-core mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without introducing chirality. Spontaneous chiral deracemization at microscopic and macroscopic levels occurs and is controllable. Moreover, achiral bent-core molecules enhance system chirality. The interplay between polarity and chirality provides chiral nonlinear optic effects. Further interesting phenomena related to polarity and chirality are also reviewed.

Blue, Green, and Amber InGaN/GaN Light-Emitting Diodes on Semipolar {11-22} GaN Bulk Substrates

Abstract: We demonstrate the fabrication of blue, green, and amber InGaN/GaN light-emitting diodes (LEDs) on semipolar {11-22} bulk GaN substrates. The {11-22}GaN substrates used in this study are produced by cutting out from a c-oriented GaN bulk crystal grown by hydride vapor epitaxy. The LEDs have a dimension of 320 ×320 µm2 and are packed in an epoxide resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA are 1.76 mW and 3.0%, respectively, for the blue LED, 1.91 mW and 4.1% for the green LED, and 0.54 mW and 1.3% for the amber LED. The maximum output powers obtained with a maximum current of 200 mA are 19.0 mW (blue), 13.4 mW (green), and 1.9 mW (amber), while the maximum EQEs are 4.0% at 140 mA (blue), 4.9% at 0.2 mA (green), and 1.6% at 1 mA (amber). It is confirmed that the emission light is polarized along the [1-100] direction, reflecting the low crystal symmetry of the {11-22} plane.

Electrical Properties and Depolarization Temperature of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 Lead-free Piezoelectric Ceramics

Abstract: The piezoelectric properties of a solid solution of the binary system, x(Bi1/2Na1/2)TiO3–(1-x)(Bi1/2K1/2)TiO3 [BNKT100x; x=0.50–0.98] were investigated, focusing on depolarization temperature, Td. Fine piezoelectric properties in lead-free piezoelectric ceramics were obtained near the morphotropic phase boundary (MPB) composition between the rhombohedral and tetragonal structures, and the highest electromechanical coupling factor, k33, and piezoelectric constant, d33, were 0.56 for BNKT84 and 157 pC/N for BNKT80, respectively. However, the Td of BNKT80 was low (174 °C). The Td of the MPB composition was low, and the Td near the MPB composition was sharply decreased. It is thought that BNKT70 is a candidate composition for lead-free actuator applications owing to its relatively large piezoelectric constant, d33 (126 pC/N), dynamic d33 (214 pm/V), and high depolarization temperature, Td (206 °C). In this study, we determined depolarization temperature, Td, from the temperature dependence of dielectric and piezoelectric properties.

Phase Tomography by X-ray Talbot Interferometry for Biological Imaging

Abstract: The X-ray phase tomography of biological samples is reported, which is based on X-ray Talbot interferometry. Its imaging principle is described in detail, and imaging results obtained for a cancerous rabbit liver and a mouse tail with synchrotron radiation are presented. Because an amplitude grating is needed to construct an X-ray Talbot interferometer, a high-aspect-ratio grating pattern was fabricated by X-ray lithography and gold electroplating. X-ray Talbot interferometry has an advantage that it functions with polychromatic cone-beam X-rays. Finally, the compatibility with a compact X-ray source is discussed.

Present Status and Future Prospect of Widegap Semiconductor High-Power Devices

Abstract: High-power device technology is a key technological factor for wireless communication, which is one of the information network infrastructures in the 21st century, as well as power electronics innovation, which contributes considerably to solving the energy saving problem in the future energy network. Widegap semiconductors, such as SiC and GaN, are strongly expected as high-power high-frequency devices and high-power switching devices owing to their material properties. In this paper, the present status and future prospect of these widegap semiconductor high-power devices are reviewed, in the context of applications in wireless communication and power electronics.

Piezoelectric Properties of BaTiO3 Ceramics with High Performance Fabricated by Microwave Sintering

Abstract: Hydrothermally synthesized BaTiO3 powders with nanoscale-sized particles were densified by microwave sintering. A sintered sample of the nanopowder fabricated by hydrothermal synthesis has a high piezoelectric constant d33 due to fabrication by microwave sintering. The maximum value of the piezoelectric constant d33 of a specimen fabricated by microwave sintering was approximately 350 pC/N for a small grain size of 2.1 µm. Detailed microstructures of the samples were observed by transmission electron microscopy (TEM) and scanning electron microscopy/electron backscattered diffraction analysis/orientation imaging microscopy (SEM/EBSD/OIM). The size of ferroelectric domains in the samples showing superior piezoelectric properties was less than 50 nm. SEM/EBSD/OIM observations revealed that the fraction of random boundaries was higher by approximately 10% in microwave sintered samples than in conventionally sintered ones. It is suggested that the small size of domain and the higher fraction of random boundaries might be responsible for the excellent piezoelectric properties of small grains, which can partially be attributed to domain size.

Ultra-High Efficiency White Light Emitting Diodes

Abstract: We fabricated the high luminous efficiency white light emitting diode (LED) and the high power white LED by using the patterned sapphire substrates and an indium–tin oxide (ITO) contact as a p-type electrode The high luminous efficiency white LED was the yellow YAG-phosphors-coated small-size (240 ×420 µm2) high efficiency blue LED with the quantum efficiency of 633% at a forward-bias current of 20 mA The luminous flux (Φ), the forward-bias voltage (Vf), the correlated color temperature (Tcp), the luminous efficiency (ηL), and the wall-plug efficiency (WPE) of the high luminous efficiency white LED are 86 lm, 311 V, 5450 K, 138 lm/W, and 417%, respectively The luminous efficiency is 15 times greater than that of a tri-phosphor fluorescent lamp (90 lm/W) The high power white LED was fabricated from the larger-size (1 ×1 mm2) blue LED with the output power of 458 mW at 350 mA Φ, Vf, Tcp, ηL, and WPE of the high power white LED are 106 lm, 329 V, 5200 K, 917 lm/W, and 277%, respectively, at 350 mA The WPE is greater than that of a fluorescent lamp (25%) in the visible region Moreover, the luminous flux of the high power white LED reaches to 402 lm at 2 A, which is equivalent to the total flux of a 30 W incandescent lamp

An Empirical Potential Approach to Wurtzite–Zinc-Blende Polytypism in Group III–V Semiconductor Nanowires

Abstract: The relative stability between wurtzite and zinc blende structures in group III–V semiconductor nanowires is systematically investigated based on an empirical potential, which incorporates electrostatic energy due to valence-bond and ionic charges. The energy differences between wurtzite and zinc blende structures of 12 compound nanowires with diameter of 1–22 nm show that the wurtzite nanowires are stabilized for small diameter. This structural trend is found to be due to the contribution of two- and three-coordinated atoms on the nanowire facets to the system energy. We also find that the critical diameters, where the nanowires turn out be bistable forming both wurtzite and zinc blende structures, exist at the diameter of 12–32 nm depending on the ionicity of semiconductors. The bistability implies the synthesis of nanowires exhibiting polytypes, and supports the experimental results in GaP, GaAs, InP, and InAs nanowires.

Phase Transition Temperatures and Piezoelectric Properties of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–BaTiO3 Lead-Free Piezoelectric Ceramics

Abstract: The phase transition temperatures of x(Bi1/2Na1/2)TiO3–y(Bi1/2K1/2)TiO3–zBaTiO3) [x+y+z=1, y:z=2:1] [abbreviate to BNBK2:1(x)] ceramics were investigated using electrical measurements. We discussed the determination of the depolarization temperature, Td, and defined the Td for (Bi1/2Na1/2)TiO3 (BNT)-based solid solutions. We also determined the rhombohedral–tetragonal phase transition temperatures, TR–T, for BNBK2:1(x), and verified them using dielectric and piezoelectric measurements. It was demonstrated that TR–T corresponded with Td at x=0.94. The existence of an intermediate phase with ferroelectric and antiferroelectric properties at temperatures higher than the Td around the morphotropic phase boundary (MPB) was also revealed.

Breakthroughs in Improving Crystal Quality of GaN and Invention of the p–n Junction Blue-Light-Emitting Diode

Abstract: Marked improvements in the crystalline quality of GaN enabled the production of GaN-based p–n junction blue-light-emitting and violet-laser diodes. These robust, energetically efficient devices have opened up a new frontier in optoelectronics. A new arena of wide-bandgap semiconductors has been developed due to marked improvements in the crystalline quality of nitrides. In this article, we review breakthroughs in the crystal growth and conductivity control of nitride semiconductors during the development of p–n junction blue-light-emitting devices. Recent progress mainly based on the present authors' work and future prospects of nitride semiconductors are also discussed.

Magnetic Damping in Ferromagnetic Thin Films

Abstract: We determined the Gilbert damping constants of Fe–Co–Ni and Co–Fe–B alloys with various compositions and half-metallic Co2MnAl Heusler alloy films prepared by magnetron sputtering. The ferromagnetic resonance (FMR) technique was used to determine the damping constants of the prepared films. The out-of-plane angular dependences of the resonance field (HR) and line width (ΔHpp) of FMR spectra were measured and fitted using the Landau–Lifshitz–Gilbert (LLG) equation. The experimental results fitted well, considering the inhomogeneities of the films in the fitting. The damping constants of the metallic films were much larger than those of bulk ferrimagnetic insulators and were roughly proportional to (g-2)2, where g is the Lande g factor. We discuss the origin of magnetic damping, considering spin–orbit and s–d interactions.

Lead-Free Barium Titanate Ceramics with Large Piezoelectric Constant Fabricated by Microwave Sintering

Abstract: A lead-free barium titanate (BaTiO3) ceramics with a high density and a large piezoelectric constant, d33, as manufactured at 1320°C by microwave sintering, using a pure fine powder with a particle size of 100 nm produced by hydrothermal synthesis. The density of the ceramic with a 3.4 µm grain size was more than 98.3% of the theoretical value. The ceramic after poling had a dielectric constant of e33T/e0 =4200, an electromechanical coupling factor planar mode of kp =36% and d33 =350 pC/N. The value of d33 is the largest one ever reported for lead-free BaTiO3 ceramics.

Current-Induced Magnetization Switching in MgO Barrier Based Magnetic Tunnel Junctions with CoFeB/Ru/CoFeB Synthetic Ferrimagnetic Free Layer

Abstract: We report the intrinsic critical current density (Jc0) in current-induced magnetization switching and the thermal stability factor (E/kBT, where E, kB, and T are the energy potential, the Boltzmann constant, and temperature, respectively) in MgO based magnetic tunnel junctions with a Co40Fe40B20(2 nm)/Ru(0.7–2.4 nm)/Co40Fe40B20(2 nm) synthetic ferrimagnetic (SyF) free layer. We show that Jc0 and E/kBT can be determined by analyzing the average critical current density as a function of coercivity using the Slonczewski's model taking into account thermal fluctuation. We find that high antiferromagnetic coupling between the two CoFeB layers in a SyF free layer results in reduced Jc0 without reducing high E/kBT.

Spinodal Decomposition under Layer by Layer Growth Condition and High Curie Temperature Quasi-One-Dimensional Nano-Structure in Dilute Magnetic Semiconductors

Abstract: We show that spinodal decomposition under layer by layer crystal growth condition leads to characteristic quasi-one-dimensional nano-structures in dilute magnetic semiconductors (DMS). It is found that the DMS systems can form rather large clusters with highly anisotropic shape even for low concentrations. It is suggested that the blocking phenomena in the super-paramagnetism, the magnetic dipole–dipole interaction and the network of the one-dimensional structures should be considered to understand the magnetism in DMS. Based on the present simulations, we propose that the delta-doping method can be effective approach to realize high Curie temperature.

Dislocation-Free m-Plane InGaN/GaN Light-Emitting Diodes on m-Plane GaN Single Crystals

Abstract: m-Plane (10-10) nonpolar InGaN-based light emitting diodes (LEDs) with no threading dislocations or stacking faults have been realized on m-plane GaN single crystals by conventional metal organic vapor phase epitaxy. The crystalline properties of the material, together with the structures of the LED devices, have been observed by scanning transmission electron microscopy. It is shown that dislocation-free nonpolar nitride layers with smooth surfaces can be obtained under growth conditions involving high V/III ratios, which are the optimized growth conditions for c-plane GaN. The peak wavelength of the electroluminescence emission obtained from the finished devices is 435 nm, which is in the blue region. The output power and the calculated external quantum efficiency are 1.79 mW and 3.1%, respectively, at a driving current of 20 mA.

Ultimate Top-down Etching Processes for Future Nanoscale Devices: Advanced Neutral-Beam Etching

Abstract: For the past 30 years, plasma etching technology has led in the efforts to shrink the pattern size of ultralarge-scale integrated (ULSI) devices. However, inherent problems in the plasma processes, such as charge buildup and UV photon radiation, limit the etching performance for nanoscale devices. To overcome these problems and fabricate sub-10-nm devices in practice, neutral-beam etching has been proposed. In this paper, I introduce the ultimate etching processes using neutral-beam sources and discuss the fusion of top-down and bottom-up processing for future nanoscale devices. Neutral beams can perform atomically damage-free etching and surface modification of inorganic and organic materials. This technique is a promising candidate for the practical fabrication technology for future nano-devices.

Terahertz Plasma Waves in Gated Graphene Heterostructures

Abstract: Plasma waves in graphene-based heterostructures with massless (neutrino-like) two-dimensional electron gas and with a highly conducting substrate (n+-Si) serving as a gate and an isolating gate layer (SiO2) are studied. Using the developed model, we show that the sufficiently long plasma waves exhibit a linear (sound-like) dispersion with the wave velocity determined by the gate layer thickness and the gate voltage. The plasma wave velocity in graphene heterostructures can significantly exceed the plasma wave velocity in the commonly employed semiconductor gated heterostructures. The gated graphene heterostructures can be used in different novel voltage tunable THz devices which utilize the plasma waves.

P-Channel Tunnel Field-Effect Transistors down to Sub-50 nm Channel Lengths

Abstract: Experimental results of p-channel silicon vertical tunnel field-effect transistors down to sub-50 nm channel length are shown. As predicted by two-dimensional simulations, we show that the device on-current is nearly independent of channel length scaling. As the drain current is determined by electrons tunneling from the valence band to the conduction band, we show that mobility does not play any role in determining the device characteristics. Low temperature measurements reveal weak positive temperature coefficient in the transfer characteristics due to the dependence of bandgap on temperature. However, as expected for the silicon devices, low on-current is observed. Thus, we propose a lateral tunnel FET on SiGe-on-insulator with high on-currents and symmetric performance in n-channel as well as p-channel operating modes.

Development and Application of Carbon Nanotubes

Abstract: In this review, we introduce the production methods and applications of carbon nanotubes. Carbon nanotubes are now attracting a broad range of scientists and industries due to their fascinating physical and chemical properties. Focusing on the chemical vapor deposition (CVD) method, we will briefly review the history and recent progress of the synthesis of carbon nanotubes for the large-scale production and double-walled carbon nanotube production. We will also describe effective purification methods that avoid structural damage, and discuss the electrochemical, composite, and medical applications of carbon nanotubes.

Impact of drain induced barrier lowering on read scheme in silicon nanocrystal memory with two-bit-per-cell operation

Abstract: The threshold voltages (Vth's) and read schemes of silicon nanocrystal memories with two bits per cell are examined by experiments and simulations. It is found that the drain induced barrier lowering (DIBL) has a marked effect on Vth's in the four states and thus on read schemes for detecting the four Vth's. It is also shown that the read scheme can be selected by controlling DIBL using device parameters including gate length, injected charge fraction, and injected charge density. Suitable read schemes for low-voltage and low-power applications are discussed.

Enhancement-Mode AlGaN/AlN/GaN High Electron Mobility Transistor with Low On-State Resistance and High Breakdown Voltage

Abstract: A low on-resistance and high-breakdown-voltage enhancement-mode (E-mode) AlGaN/AlN/GaN high electron mobility transistor (HEMT) was fabricated without the recessed-gate process. A thin AlGaN barrier layer (4.5 nm) was used for the normally-off operation. In order to decrease the on-state resistance, two different techniques are used. One is the side-ohmic contact, which has a low contact resistance, due to the direct contact with the two-dimensional electron gas (2DEG). The other is SiO2 passivation, which induces the sheet carriers and decreases the sheet resistance. As a result, an on-state resistance of 1.9 mΩcm2 and an off-state breakdown voltage of 610 V were achieved for the E-mode HEMT within a threshold voltage of approximately -0.1 V.

Protonation Sites in Chemically Amplified Resists for Electron-Beam Lithography

Abstract: In chemically amplified resists that utilize acid-catalytic reactions for pattern formation, proton dynamics is important from the viewpoints of the insoluble layer formation due to acid loss, the resolution decrease due to acid diffusion, and the image quality improvement due to base-quencher effects. For electron-beam lithography, the protons and anions of the acid are initially generated at different places. Protons migrate in the resist matrix toward counter anions, attracted by the opposite electric charges. However, the details of proton migration are still unclear. In this study, we investigated proton quenching in poly(4-hydroxystyrene) films using base quenchers with different proton affinities. When the proton affinity of the base quencher was increased, the equimolecular proton adduct of the acid-sensitive dye was quenched without postexposure bake. Although the proton affinity is a gas-phase value, the quenching effect correlated well with the proton affinity.

High-Temperature Metal-Organic Vapor Phase Epitaxial Growth of AlN on Sapphire by Multi Transition Growth Mode Method Varying V/III Ratio

Abstract: High-quality AlN layers were grown on c-plane sapphire substrates by high-temperature metal-organic vapor phase epitaxy AlN layers of about 9 µm in thickness with an atomically flat surface were obtained without cracks Multiple modulation of the V/III ratio during growth led to a reduction in the number of dislocations during the growth transition period The dislocation density of the AlN layers was found to be less than 3×108 cm-2

Growth of Super Long Aligned Brush-Like Carbon Nanotubes

Abstract: Efficient chemical vapor deposition (CVD) synthesis of super long (7 mm) aligned carbon nanotubes (CNTs) with highdensity is reported here. Activity of catalyst nanoparticles has been achieved for very long time periods (ca. 12 h) by optimization of experimental parameters. The relative levels of ethylene and water, as well as those of ethylene and H2, were found to be most important for achieving extended-time activity of the catalyst. Transmission electron microscope (TEM) images revealed that the nanotubes were mainly double-walled, but very few single-walled and multi-walled nanotubes were also present in the sample. [DOI: 10.1143/JJAP.45.L720]

Modified Oxygen Vacancy Induced Fermi Level Pinning Model Extendable to P-Metal Pinning

Abstract: Typical p-metals show similar effective work functions close to p+ polycrystalline silicon (poly-Si) pinning position irrespective of materials after high-temperature process. We found that this phenomenon can be explained by the modified Vo model taking into account the effect of Si substrate. Oxygen absorption by Si substrate and subsequent electron transfer to metal electrode clearly explain the p-metal Fermi level pinning as well as p+ poly-Si pinning. In addition, unsuppressed Fermi level pinning by insertion of barrier layer at p+ poly-Si/barrier layer/high-k gate stack, which is one of the open issues concerning p+ poly-Si pinning, has the same overall reaction scheme. The modified model also consistently explains this phenomenon.

Growth and Characterization of AlInN Ternary Alloys in Whole Composition Range and Fabrication of InN/AlInN Multiple Quantum Wells by RF Molecular Beam Epitaxy

Abstract: We studied on rf molecular beam epitaxy (RF-MBE) growth of AlInN ternary alloys on N-polarity GaN templates. The growable highest temperature for the AlInN ternary alloy with mid-composition range was about 600 °C, which was very similar to that of N-polarity InN epitaxy. The compositional and structural qualities of AlInN ternary alloys were quite poor, however, for growth temperatures above 580 °C. AlInN ternary alloys without apparent phase separation in the whole composition range could be grown at 550 °C, and their crystalline, electrical, and optical properties were characterized. The bowing parameter for the optical bandgap of AlInN ternary alloys was found to be 4.96±0.28 eV. Further we for the first time fabricated InN/AlInN multiple quantum wells (MQWs).

Floating Nanodot Gate Memory Devices Based on Biomineralized Inorganic Nanodot Array as a Storage Node

Abstract: The memory effect in floating nanodot gate field-effect-transistor (FET) was investigated by fabricating biomineralized inorganic nanodot embedded metal–oxide–semiconductor (MOS) devices. Artificially biomineralized cobalt (Co) oxide cores accommodated in ferritins were utilized as a charge storage node of floating gate memory. Two dimensional array of Co oxide core accommodated ferritin were, after selective protein elimination, buried into the stacked dielectric layers of MOS capacitors and MOSFETs. Fabricated MOS capacitors and MOSFETs presented a clear hysteresis in capacitance–voltage (C–V) characteristics and drain current–gate voltage (ID–VG) characteristics, respectively. The observed hysteresis in C–V and ID–VG are attributed to the electron and hole confinement within the embedded ferritin cores. These results clearly support the biologically synthesized cores work as charge storage nodes. This work proved the feasibility of the biological path for fabrication of electronic device components.