Showing papers in "Japanese Journal of Applied Physics in 1998"

Self-Ordering of Cell Configuration of Anodic Porous Alumina with Large-Size Pores in Phosphoric Acid Solution

Abstract: Large cell-sized anodic porous alumina with long-range ordering was fabricated using phosphoric acid solution. Self-ordering of the anodic alumina took place under long-period constant voltage anodization at 195 V, which generated the self-ordered anodic porous alumina with a cell size of 500 nm.

Direct Evidence that Dislocations are Non-Radiative Recombination Centers in GaN

Abstract: Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Cathode Ray Tube Lighting Elements with Carbon Nanotube Field Emitters

Abstract: We have manufactured cathode ray tubes (CRTs) equipped with field emitters composed of multiwalled carbon nanotubes. The fabricated CRTs (20 mm in diameter by 74 mm in length) are of a triode type, consisting of a cathode (nanotube field emitter arrays), a grid and an anode (phosphor screen). The manufactured CRTs are lighting elements, which are assembled to form a giant outdoor display. Stable electron emission, adequate luminance and long life are demonstrated. The CRT lighting elements presented here are the first practical products utilizing carbon nanotubes on an industrial scale.

High-Power, Long-Lifetime InGaN/GaN/AlGaN-Based Laser Diodes Grown on Pure GaN Substrates

Abstract: Epitaxially laterally overgrown GaN on sapphire was used to reduce the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. The GaN layer above the SiO2 mask area surrounding the window, corresponding to the lateral overgrowth, was nearly free of the threading dislocations. A high density of threading dislocations was observed in the vicinity of GaN grown in the window regions. InGaN multi-quantum-well-structure laser diodes (LDs) grown on pure GaN substrates, which were fabricated by removing the sapphire substrate, were demonstrated. The LDs with an output power of 5 mW exhibited a lifetime of more than 290 h and an estimated lifetime of 10,000 h despite a relatively large threshold current density. The far-field pattern of the LDs with a cleaved mirror facet revealed single-mode emission without any interference effects.

The new neutron powder diffractometer with a multi-detector system for high-efficiency and high-resolution measurements

Abstract: We describe a multipurpose neutron powder diffractometer for high-efficiency and high-resolution measurements, HERMES, which has been installed at the JRR-3M reactor in Japan Atomic Energy Research Institute. To obtain high-quality data for a short counting time, or with a small sample amount, HERMES has a multi-detector system which consists of 150 3He neutron detectors and simple window type collimation. The best resolution is Δ2θ of 18' and Δd/d of 3 ×10-3, where 2θ and d are scattering-angle and d-spacing, respectively. The duration of an ordinary powder diffraction experiment is 1–5 h under the present conditions in JRR-3M. Some typical results of powder diffraction experiments and performance are given in this paper. We also show a result of 2D intensity mapping of magnetic diffuse scattering from a single crystal of a spin glass material.

Formation of Silicon-Based Thin Films Prepared by Catalytic Chemical Vapor Deposition (Cat-CVD) Method

Abstract: This paper is a review of the catalytic chemical vapor deposition (Cat-CVD) method and properties of silicon-based thin films, such as amorphous-silicon (a-Si), polycrystalline-silicon (p-Si) and silicon nitride (SiNx) films, prepared by the Cat-CVD method. In the Cat-CVD method, also known as the hot-wire CVD (HWCVD) method, deposition gases are decomposed by catalytic cracking reactions with a heated catalyzer placed near the substrates, so that films are deposited at low substrate temperatures around 300°C without any help from the plasma. After explaining the deposition system and deposition mechanism, the properties of Cat-CVD a-Si, p-Si and SiNx films are described and the results are compared with those obtained by the conventional plasma CVD (PCVD) method. The superiority of the Cat-CVD method over the PCVD method is demonstrated.

High rate deposition of microcrystalline silicon using conventional plasma-enhanced chemical vapor deposition

Abstract: The deposition of hydrogenated microcrystalline silicon (µc-Si:H) at a relatively high working pressure is performed using a conventional radio-frequency plasma-enhanced chemical vapor deposition method. Correlation of the deposition rate and crystallinity with deposition parameters, such as working pressure, flow rate, dilution ratio and input RF power, are studied. It was found that the deposition rate exhibits a maximum at around 4 Torr and that the crystallinity of films decreases monotonically with increasing pressure. The combination of SiH4 depletion and high working pressure in the plasma is necessary to improve the crystallinity of films deposited at a high rate. Consequently, a high deposition rate of 9.3 A/s is achieved with high crystallinity and low defect density.

Microwave dielectric properties and far infrared reflectivity spectra of the (zr0.8sn0.2)tio4 ceramics with additives

Abstract: Microwave dielectric properties and far infrared reflectivity spectra were investigated in the (Zr0.8Sn0.2)TiO4 ceramics with 1 mol% Sb2O5, Sb2O3 and Nd2O3, respectively. Dielectric constant was not significantly changed, while the unloaded Q value was affected with additives. The unloaded Q of the pure (Zr0.8Sn0.2)TiO4 specimens was higher than that of the specimens with Sb2O3 and/or Nd2O3, but lower than that of the specimens with Sb2O5. The high unloaded Q value of 6600 at 7 GHz was obtained for the specimens with 1 mol% Sb2O5 due to elimination of oxygen vacancies. The effects of additives on the intrinsic microwave losses of the specimens were investigated by the infrared reflectivity spectra from 50 to 4000 cm-1, which were evaluated using the Kramers-Kronig analysis and classical oscillator model. The relative tendency of microwave dielectric properties calculated from the reflectivity data was in good agreement with the results by the Hakki and Coleman's method.

New Semiconductor Alloy GaAs_ Bi_x Grown by Metal Organic Vapor Phase Epitaxy

Abstract: A new semiconductor alloy material, GaAs1-xBix has been created by Metal Organic Vapor Phase Epitaxial (MOVPE) growth. A low growth temperature, such as 365°C, is required to obtain the alloy. X-ray diffraction measurements of alloy layers reveal that the diffraction patterns are satisfactory. The maximum GaBi content in the GaAsBi alloy estimated from the lattice constant is around 2%, which is consistent with that estimated from secondary ion mass spectroscopy (SIMS) measurements. In a photoluminescence (PL) measurement, a single peak spectrum is observed from 10 to 300 K. The temperature variation of the PL peak energy is as small as 0.1 meV/K.

Current and Temperature Dependences of Electroluminescence of InGaN-Based UV/Blue/Green Light-Emitting Diodes

Abstract: Current and temperature dependences of the electroluminescence of InGaN UV/blue/green single-quantum-well (SQW)-structure light-emitting diodes (LEDs) were studied. The emission mechanism of InGaN SQW-structure LEDs with emission peak wavelengths longer than 375 nm is dominated by carrier recombination at large localized energy states caused by In composition fluctuation in the InGaN well layer. When the emission peak wavelength becomes shorter than 375 nm, the conventional band-to-band emission mechanism becomes dominant due to poor carrier localization resulting from small In composition fluctuations. In addition, the quantum-confined Stark effect due to the piezoelectric field becomes dominant, which causes a low output power of the UV LEDs.

New Semiconductor Alloy GaAs1-xBix Grown by Metal Organic Vapor Phase Epitaxy

Abstract: A new semiconductor alloy material, GaAs1-xBix has been created by Metal Organic Vapor Phase Epitaxial (MOVPE) growth. A low growth temperature, such as 365°C, is required to obtain the alloy. X-ray diffraction measurements of alloy layers reveal that the diffraction patterns are satisfactory. The maximum GaBi content in the GaAsBi alloy estimated from the lattice constant is around 2%, which is consistent with that estimated from secondary ion mass spectroscopy (SIMS) measurements. In a photoluminescence (PL) measurement, a single peak spectrum is observed from 10 to 300 K. The temperature variation of the PL peak energy is as small as 0.1 meV/K.

Reduction of Etch Pit Density in Organometallic Vapor Phase Epitaxy-Grown GaN on Sapphire by Insertion of a Low-Temperature-Deposited Buffer Layer between High-Temperature-Grown GaN

Abstract: The etch pit density of organometallic vapor phase epitaxy (OMVPE)-grown GaN on sapphire was discovered to reduce drastically by the insertion of either a low-temperature-deposited AlN buffer layer or GaN buffer layer between high-temperature-grown-GaN on sapphire.

InGaN-Based Blue Light-Emitting Diodes Grown on Epitaxially Laterally Overgrown GaN Substrates

Abstract: InGaN single-quantum-well-structure blue light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown GaN (ELOG) and sapphire substrates. The emission spectra showed the similar blue shift with increasing forward currents for both LEDs. The output power of both LEDs was almost the same, as high as 6 mW at a current of 20 mA. The LED on sapphire had a considerable amount of leakage current in comparison with that on ELOG. These results indicate that the In composition fluctuation is not caused by dislocations, the dislocations do not act as nonradiative recombination centers in the InGaN, and the dislocations forms the leakage current pathway in InGaN.

Stress and Defect Control in GaN Using Low Temperature Interlayers

Abstract: In organometallic vapor phase epitaxial growth of Gail on sapphire, the role of the low- temperature-deposited interlayers inserted between high-temperature-grown GaN layers was investigated by in situ stress measurement, X-ray diffraction, and transmission electron microscopy. Insertion of a series of low temperature GaN interlayers reduces the density of threading dislocations while simultaneously increasing the tensile stress during growth, ultimately resulting in cracking of the GaN film. Low temperature AIN interlayers were found to be effective in suppressing cracking by reducing tensile stress. The intedayer approach permits tailoring of the film stress to optimize film structure and properties.

Effect of Surface Roughness on Room-Temperature Wafer Bonding by Ar Beam Surface Activation

Abstract: Using Ar beam etching in vacuum, strong bonding of Si wafers is attained at room temperature. With appropriate etching time, the bonding occurs spontaneously without any load to force two wafers together. However, surface roughness of the wafers increases during Ar beam etching. Because surface roughness has a strong influence on wafer bonding, long etching time degrades the bonding strength. Using atomic force microscope, we measured surface roughness enhancement caused by Ar beam etching, and investigated the relationship between surface roughness and bonding properties such as strength and interfacial voids. The results agree well with theoretical predictions using elastic theory and energy gain by bond formation. A guideline for successful room-temperature bonding is proposed from these results.

Negative Thermal Quenching Curves in Photoluminescence of Solids

Abstract: We have derived an analytical formula for the temperature dependence of the photoluminescence (PL) intensity when the "negative" thermal quenching phenomenon is observed. The formula was examined using available experimental data for GaAs and ZnS, and the agreement of the results was quite good. It was confirmed quantitatively that the principal mechanism of negative thermal quenching is the thermal excitation of electrons to the initial state of the PL transition from the eigenstates with smaller energy eigenvalues. In addition, it was revealed that the real activation energy for the nonradiative recombination processes is given by e+E' when the negative thermal quenching phenomenon is observed, where e is the activation energy estimated simply from the apparent slope in the experimental data, and E' is the activation energy for the negative thermal quenching phenomenon.

A New Nonlinear Optical Borate Crystal K2Al2B2O7 (KAB)

Abstract: A new nonlinear optical (NLO) borate crystal K2Al2B2O7 (Potassium Aluminum Borate, KAB) has been discovered. The structure has been established by 4-axis X-ray diffraction methods. The material crystallizes in the trigonal space group P321 with a = 8.5657(9) A, C = 8.463(2) A and Z = 3. KAB possesses a space arrangement similar to Sr2Be2B2O7 (SBBO). A KAB crystal with a dimensions of 30×15×1 mm3 was grown using the Top-Seeded Solution Growth (TSSG) method. The optical properties of KAB were measured.

Amber ingan-based light-emitting diodes operable at high ambient temperatures

Abstract: High-efficiency amber InGaN single-quantum-well (SQW) structure light-emitting diodes (LEDs) with a luminous efficiency of 10 lm/W were developed. At a current of 20 mA, the external quantum efficiency, the output power and the emission wavelength of the amber InGaN SQW structure LEDs were 3.3%, 1.4 mW and 594 nm, respectively. The output power of InGaN LEDs was about twice as high as that of AlInGaP LEDs. There was a large difference in the temperature dependence of the output power between InGaN and AlInGaP LEDs. When the ambient temperature was increased from room temperature to 80°C, the output power of AlInGaP LEDs decreased dramatically. On the other hand, the output power of the InGaN LEDs remained almost constant.

Atomic Layer Deposition of TiN Films by Alternate Supply of Tetrakis(ethylmethylamino)-Titanium and Ammonia

Abstract: Atomic layer deposition (ALD) of amorphous TiN films on SiO2 between 170°C and 210°C has been investigated by alternate supply of reactant sources, Ti[N(C2H5CH3)2]4 [tetrakis(ethylmethylamino)titanium:TEMAT] and NH3. Reactant sources were injected into the reactor in the following order:TEMAT vapor pulse, Ar gas pulse, NH3 gas pulse and Ar gas pulse. Film thickness per cycle was saturated at around 1.6 monolayers (ML) per cycle with sufficient pulse times of reactant sources at 200°C. The results suggest that film thickness per cycle could exceed 1 ML/cycle in ALD, and are explained by the rechemisorption mechanism of the reactant sources. An ideal linear relationship between number of cycles and film thickness is confirmed. As a result of surface limited reactions of ALD, step coverage was excellent. Particles caused by the gas phase reactions between TEMAT and NH3 were almost absent because TEMAT was segregated from NH3 by the Ar pulse. In spite of relatively low substrate temperature, carbon impurity was incorporated below 4 at.%.

Morphotropic phase boundary in solid solution systems of perovskite-type oxide ferroelectrics

Abstract: The origin of the appearance of the morphotropic phase boundary in the perovskite-type oxide solid solution systems and the increase in the dielectric susceptibilities in the vicinity of the boundary is theoretically clarified on the basis of a Landau-type free energy function. The dielectric susceptibilities are concretely expressed in terms of the model parameters, and found to diverge at the morphotropic phase boundary within the present model.

Crystal Structures and Phase Transformation in In2Se3 Compound Semiconductor

Abstract: Crystal structures and phase transformation in In2Se3 compound semiconductor have been studied by electron diffraction, high resolution electron microscopy and X-ray diffraction (XRD) together with optical absorption measurements. The time-temperature-transformation (TTT) diagram reveals that there exist only two phases in In2Se3 and the transformation temperature is 853 K. The transformation from the high temperature phase to the low requires a long incubation time for crystal nucleation and a relatively high temperature for crystal growth. The low and high temperature phases are the vacancy ordered in screw form (VOSF) phase and the layer structure phase, respectively. Both phases possess semiconducting optical properties and are constructed on the basis of a tetrahedral bonding structure. The VOSF phase is of a defect wurtzite structure, in which vacancies on 1/3 of the cation sites are ordered in screw form along the c-axis. The space group is P61 or P65 with a=7.14 A, c=19.38 A, Z=6. The layer structure is constructed of five-layer Se–In–Se–In–Se sets and the sets are linked by weak van der Waals' force with stacking sequence of ABC. The space group is R3m with a = 4.00 A, c = 28.80 A, Z=3 (indexed in hexagonal system). In the layer structure, structure vacancies on 1/3 of the cation sites aggregate to form a vacancy plane for every three In-planes. The structural difference between the two phases is most clearly characterized by the difference in coordination numbers of the Se atoms.

Electrical instability of hydrogenated amorphous silicon thin-film transistors for active-matrix liquid-crystal displays

Abstract: We investigated the threshold voltage shifts (ΔVT) of inverted-staggered hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) induced by steady-state (dc) and pulsed (ac) gate bias-temperature-stress (BTS) conditions. Our study showed that, for an equivalent effective-stress-time, ΔVT has an apparent pulse-width dependence under negative BTS conditions–the narrower the pulse width, the smaller the ΔVT. This gate-bias pulse-width dependence is explained by an effective-carrier-concentration model, which relates ΔVT for negative pulsed gate-bias stress to the concentration of mobile carriers accumulated in the conduction channel along the a-Si:H/gate insulator interface. In addition, our investigation of the methodology of a-Si:H TFT electrical reliability evaluation indicates that, instead of steady-state BTS, pulsed BTS should be used to build the database needed to extrapolate ΔVT induced by a long-term display operation. Using these experimental results, we have shown that a-Si:H TFTs have a satisfactory electrical reliability for a long-term active-matrix liquid-crystal display (AMLCD) operation.

A Novel Phase-Modulated Excimer-Laser Crystallization Method of Silicon Thin Films

Abstract: We propose a novel excimer-laser crystallization method that uses a phase-shift mask for large-grain growth of Si thin films on the glassy substrate. Due to interference effects of the laser light, the phase difference of light at the mask results in spatial modulation of light intensity at the sample surface, which triggers the lateral grain growth. Grains as large as 7 µm could be grown by a single-shot irradiation.

Stabilized platinum electrodes for ferroelectric film deposition using Ti, Ta and Zr adhesion layers

Abstract: Pt-based metallizations using different adhesion layers (Ti. Zr and Ta) were studied for use as electrodes for Ferroelectric thin films on oxidized silicon substrates. Different ways of oxidizing the adhesion layers prior to ferroelectric film growth are compared. with regard to obtaining stable. adherent Pt films of well-defined (111) orientation, while avoiding lead diffusion through the electrode. Upon in-situ deposition of PbTiO3 nt high excess lead flux. lead diffusion through the Pt film was found to depend strongly on the adhesion layer and the stabilization treatment. Pre-oxidation reduces leed diffusion during the later processing Ti diffuses through the electrode upon oxidation. whereas Ta and Zr stay in piece, in analogy to the diffusivities in the corresponding oxides. A novel oxidation treatment was developed to produce stable. adherent metallizations with controlled orientation and good barrier properties against lead diffusion.

Three-Dimensional Optical Data Storage in Vitreous Silica

Abstract: To achieve high bit densities (>10 GB/cm3) in optical memory, we accomplished a three-dimensional optical data storage system using vitreous silica as the recording material. We succeeded in high-density optical recording by focusing pulsed laser beams of 532 nm (full width at half maximum [FWHM] 30 ps) and 400 nm (150 fs). A recording density of 72.9 GB/cm3 was achieved, which corresponded to that of 100 compact disks, on a glass plate of (2×2) cm2 and 2.2 mm thickness. We found that the optical damage of silica occurs within 400 ps after irradiation by a single pulse for 30 ps at 532 nm. Three photoluminescence bands were found in the photomodified silica at 283 nm, 468 nm and 558 nm. All the three bands showed similar photoluminescence excitation spectra, i.e. a peak related to oxygen vacancy absorption at 250 nm.

The Measurement of High-Power Characteristics for a Piezoelectric Transducer Based on the Electrical Transient Response

Abstract: In this paper, the theoretical and experimental considerations on the high-power characteristics of a piezoelectric transducer are presented. First, we propose a new measurement method for the large vibration amplitude region using an electrical transient response to eliminate the effect of temperature rise. Then, a new loss mechanism concerning the piezoelectric effect is explained by observing the relationship between the vibration velocity and the current and their harmonics. A `piezoelectric loss' is newly introduced in the equivalent circuit model to describe the real loss phenomena clearly.

Nitrogen Doping Effect on Phase Change Optical Disks

Abstract: The nitrogen doping effect on the Ge–Sb–Te recording layer was quantitatively examined. We succeeded in the quantitative analysis of the nitrogen concentration in the Ge–Sb–Te–(N) recording layer by secondary ion mass spectrometry (SIMS) observation. The nitrogen concentration could be finely controlled at a high deposition rate of 4.7 nm/s. The addition of a small amount of nitrogen remarkably improved the overwrite cycle numbers. We found that the most suitable nitrogen concentration was from 2 to 3 at%. We proposed a model to explain the nitrogen atom function in the recording layer. The nitrogen atoms produced nitrides, which are condensed near the grain boundaries of Ge–Sb–Te microcrystals. This resulted in the formation of very thin wrappings, which wrap the crystal grain in a manner similar to that of the peel of a peach and suppressed the micro-material flow. We achieved 8×105 overwrite cycles at λ=790 nm, NA=0.50 and using the pit position modulation (PPM) recording method where the minimum bit length is 0.87 µm.

Three-Dimensional Optical Memory Using Glasses as a Recording Medium through a Multi-Photon Absorption Process

Abstract: The feasibility of multilayered optical data storage was examined in SiO2 and Ge–SiO2 glasses by using a focused 800 nm, 120 fs, 200 kHz pulsed laser. The glasses are transparent in the wavelengths ranging from 300 to 1000 nm. After the irradiation with the 800 nm tightly focused femtosecond laser, a refractive-index bit was formed in the glasses. The relationships between the size of refractive-index bit versus average power and irradiation duration of the laser pulse have been examined. ESR spectra showed a significant increase in the concentration of SiE' and GeE' color centers in the laser irradiated glasses. Writing 3-dimensional refractive-index bit inside the transparent glasses based on a multi-photon absorption process is expected to become a useful method used to fabricate optical memory with both an ultra-high storage density and an ultra-high recording speed.

Role of Dislocation in InGaN Phase Separation.

Abstract: The role of dislocation for luminescence in InGaN grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) method was investigated by cathodoluminescence (CL) and atomic force microscopy (AFM). The CL emission area and dark spots between InGaN and GaN layers in InGaN/GaN single quantum well (SQW) and multiple quantum well (MQW) structures showed completely one to one correspondence. These results indicate that dislocations in InGaN work as non-radiative recombination centers. Furthermore it was confirmed that the phase separation in InGaN is caused by spiral growth due to mixed dislocations, and such a growth mechanism is discussed.

InGaN/GaN/AlGaN-Based Laser Diodes Grown on GaN Substrates with a Fundamental Transverse Mode

Abstract: An InGaN multiquantum-well (MQW)-structure laser diode (LD) was grown on an epitaxially laterally overgrown GaN on sapphire. The lowest threshold current densities between 1.2 and 2.8 kA/cm2 were obtained when the number of InGaN well layers was two. The InGaN MQW LD was grown on a free-standing GaN substrate that was obtained by removing the sapphire substrate. The LDs with cleaved mirror facets showed an output power as high as 30 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed by reducing the ridge width to a value as small as 2 µm. The lifetime of the LDs at a constant output power of 5 mW was about 160 h under CW operation at an ambient temperature of 50°C, due to a high threshold current density of 6 kA/cm2.