Table of contents

The dependence of the electrical efficiency on the injection current was studied in detail in InGaN/GaN multiple quantum wells (MQWs) blue light emitting diodes. When the InGaN quantum well thickness increased from 2 to 3.5 nm, or the Al component in p-AlGaN changed from 0.1 to 0.2, it was found that the electrical efficiency decreased dramatically, while a thin Mg-doped GaN layer inserted between p-AlGaN and MQWs with optimized Mg concentration can enhance the electrical efficiency effectively. Analysis shows that the injection efficiency was dramatically affected by the interface states due to the strong stress at the interface between p-AlGaN blocking layer and MQWs active region and the capability of electrons arriving at the interface.

Improvement of Al-polar AlN layer quality was accomplished by three-stage flow-modulation metalorganic chemical vapor deposition (FM-MOCVD). In this method, the unit of the FM-MOCVD sequence was composed of three stages; Stage I for simultaneous source supply, Stage II for trimethylaluminum supply, and Stage III for ammonia supply, which were cyclically repeated. The AlN quality revealed by X-ray diffraction strongly depended on the time of Stage I. A growth model was proposed considering the surface coverage of the islands nucleated during Stage I. Exciton fine structures were eventually observed by low-temperature cathodoluminescence reflecting the tremendously improved crystalline quality.

We report on state-of-the-art AlGaN/GaN heterostructure field-effect transistor (HFET) technology in the scope of millimeter-wave applications. 60-nm-long-gate HFETs having 4- and 6-nm-thick Al_0.4Ga_0.6N barrier layers and SiN passivation layers formed by catalytic chemical vapor deposition (Cat-CVD) were fabricated on 4H-SiC substrates. Both structures had low sheet resistances of 200–220 Ω/sq that were due to not only high mobilities of 1900–2000 cm²/(V·s) but also high electron densities of (1.4-1.7)×10¹³ cm^-2, which were provided by the high-Al-composition barrier layers and the Cat-CVD SiN. The devices with the 4- and 6-nm-thick barriers had maximum drain current densities of 1.4 and 1.6 A/mm and peak extrinsic transconductances of 448 and 424 mS/mm, respectively. Maximum f_T and f_max reached 190 and 251 GHz, respectively.

A novel method for fabricating trench structures on GaN was developed. A smooth non-polar (1100) plane was obtained by wet etching using tetramethylammonium hydroxide (TMAH) as the etchant. A U-shape trench with the (1100) plane side walls was formed with dry etching and the TMAH wet etching. A U-shape trench gate metal oxide semiconductor field-effect transistor (MOSFET) was also fabricated using the novel etching technology. This device has the excellent normally-off operation of drain current–gate voltage characteristics with the threshold voltage of 10 V. The drain breakdown voltage of 180 V was obtained. The results indicate that the trench gate structure can be applied to GaN-based transistors.

We have carried out epitaxial lift-off (ELO) of In_0.57Ga_0.43As/In_0.56Al_0.44As metamorphic high electron mobility heterostructures and their van der Waals bonding (VWB) on AlN ceramic substrates. Using a metamorphic heterostructure with an AlAs sacrificial layer and an InGaAs graded buffer grown on GaAs(001), thin film Hall-bar devices on AlN ceramic substrates were successfully fabricated by ELO and VWB. The Hall-bar devices exhibit very high electron mobilities, such as 11000 cm²/(V s) at room temperature (RT) and 84000 cm²/(V s) at 12 K. The RT mobility is the highest ever reported for ELO devices. This is the first report on ELO for metamorphic devices.

Magnetic tunnel junctions (MTJs) with half-metallic electrodes are expected to show a large tunnel magnetoresistance (TMR) ratio, according to Julliere's model. A Co₂MnSi Heusler alloy is theoretically expected to possess a half-metallic electronic state. Experimentally, at low temperature, Co₂MnSi(100)/Al-oxide/CoFe junctions exhibited a large TMR ratio. We fabricated MTJs with high-quality (110)-oriented Co₂MnSi electrodes and investigated the TMR effects. We obtained a TMR ratio of about 40% at room temperature and 120% at 2 K, respectively. However, we observed degradation of the energy gap of Co₂MnSi in the minority spin band from the conductance–voltage characteristics. We infer that the interface of Co₂MnSi(110) possesses no half-metallic property.

[Fe₃Si/FeSi₂]₂₀ superlattices were prepared on Si(111) at an elevated substrate temperature of 300 °C, and the magnetoresistance ratio and interlayer coupling strengths were enhanced by approximately 100 and 34%, respectively, as compared to those of superlattices deposited at room temperature. While the elevated substrate temperature degraded the interface sharpness, the crystalline orientation and the crystallinity of the Fe₃Si layers were apparently enhanced. The latters strongly influence on the interlayer coupling and the magnetoresistance ratio. This implies that quantum well states are tightly formed under the well-ordered crystalline planes, and the spin diffusion lengths are improved due to the enhanced crystallinity.

We established a new method for evaluating quantitatively the silicon atomic displacement as a function of the depth from the surface induced by arsenic implantation into a silicon wafer. A simulation based on a convolution integral was developed successfully to reproduce the experimental depth profiles of isotopes in the arsenic-implanted ²⁸Si/³⁰Si isotope superlattices, from which the average distance of the silicon displacements due to the collisions with implanted arsenic is obtained. We show that it takes the average displacement of ∼0.5 nm to make the structure appear amorphous by transmission electron microscopy.

The drift mobility, carrier density and conductivity of the two-dimensional electron gas (2DEG) confined in the tensilely strained 15 nm Si quantum well (QW) of SiGe heterostructures were obtained by mobility spectrum analysis at room-temperature. The highest 2DEG drift mobility of 2900 cm² V^-1 s^-1 with carrier density of 1×10¹¹ cm^-2 were observed in the Si QW with -0.9% tensile strain. However, the increase of strain up to -1.08% resulted in the decline of 2DEG drift mobility down to 2670 cm² V^-1 s^-1 and the pronounced increase of carrier density up to 4.4×10¹¹ cm^-2. Nevertheless, the pronounced enhancement of 2DEG conductivity was observed.

We have epitaxially grown p-Si/β-FeSi₂/n-Si double heterostructures light-emitting diodes (LEDs) on Si(111) substrates by molecular-beam epitaxy. The 1.6 µm electroluminescence intensity measured at room temperature (RT) was improved significantly for LEDs constructed using a thick β-FeSi₂ active layer (190 nm) embedded in heavily-doped Si p–n diodes formed on floating-zone Si(111) substrates. The external quantum efficiency was increased up to approximately 0.02% at RT.

Microscopic structures of amorphous Ge₂Sb₂Te₅ films, electrically phase-changed using co-planar electrodes, have been studied through electrical and structural investigations. Microscope images show a structured phase-change region consisting of a narrow channel, banks on both sides, and rough peripheral regions. The room-temperature resistivity is ∼10^-3 Ω·cm, which has a metallic temperature dependence. Micro-Raman scattering spectra at the channel and the bank exhibit peaks due to crystalline Te and tellurides. X-ray diffraction patterns from the films, which contain many channels, present crystalline peaks ascribable to cubic GeTe and other compounds. These observations suggest that the Ge₂Sb₂Te₅ melt is liable to phase-separate under electrical self-heating.

We have investigated superconductive properties of nano-scale Nb wires fabricated by a simple lift-off process with magnetron sputtering. The superconductive properties of the Nb wires were remarkably improved by employing highly plasma resistant electron-beam resist ZEP520A combined with a thin Ti passivation layer. This optimized fabrication process yielded a 300-nm-wide Nb wire with the same transition temperature as that of the reference Nb film. Thereby, a highly transparent Nb/Cu junction was successfully fabricated.

The critical current densities (J_c) of MgB₂ thin films deposited on α-Al₂O₃ fabricated by a precursor and post-annealing process have been investigated in high magnetic fields of up to 30 T. J_c values of 2.8×10⁵, 3.0×10⁴, and 3.7×10³ A/cm² were obtained at 4.2 K in external magnetic fields of 10, 20, and 26 T respectively, applied parallel to the film surface. These values were comparable to those seen for Nb₃Sn in fields up to 18 T, and surpassed them in fields over 18 T. The superconducting transition temperature was T_c^onset=29.6 K and T_c^zero=28.5 K. Grain connectivity was estimated to be 38.4%. The H_c2(0) estimated from the resistivity measurements was 45 T. Transmission electron microscope (TEM) observations of the microstructures suggested that grains 10–20 nm in size exist that show no epitaxial growth relationship to the sapphire substrate. J_c enhancements in fields up to ∼26 T are due to the strong grain boundary pinning associated with the small grains and the small size of MgO precipitates.

The depletion layer formed at the interface of aluminum (Al) with poly(3-hexylthiophene-2, 5-diyl) (P3HT) has been studied, using the bias dependent photoluminescence (PL) spectra in indium tin oxide (ITO)/P3HT/Al sandwiched cells. A quenching in the PL intensity has been observed under the reverse bias conditions, which has been attributed to the increase in the depletion layer width. A direct relationship between the depletion layer width and the PL quenching has been derived and explained.

Organic field-effect transistors based on octyl-subustituted oligo-p-phenylenevinylene have been studied. We have succeeded in improving field-effect hole mobilities by thermal treatment at liquid crystalline phase characterized as a highly ordered smectic phase. The field-effect mobility of the device as vacuum-evaporated was calculated to be 6.9×10^-3 cm² V^-1 s^-1 whereas the mobility was enhanced to 1.7×10^-1 cm² V^-1 s^-1 after annealing the device at 100 °C for 12 h. From the surface morphology of the films observed by using atomic force microscope, the enhancement is found to be attributed to reduction of defect density in the film because of the thermal movement of liquid-crystal molecules.

We fabricated two-input NAND gates composed of p-channel pentacene and n-channel C₆₀ transistors. The logic devices were prepared on flexible polymer substrates through a shadow mask process. Correct NAND logic functionality was demonstrated at a wide voltage range of 2–7 V. From voltage transfer characteristics of the NAND gates, we obtained impressive signal gains up to 120 and large noise margins in the given voltage range.

Whitening of polymer light-emitting diodes (PLEDs) based on the blue-emitting poly(9,9-dioctylfluorene) (PDOF) films was possible by dispersing vapor of an orange fluorescent dye 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM) into the film by means of the solution-free vapor transportation method (VTM). Devices prepared with this method showed good color stability with bias voltage increase, while those formed with conventional spin-coating, where dyes and polymers were mixed in a solution (solution-mixed), showed color change from yellow to white-yellow. The maximum luminance of the PLED formed by the VTM was higher than that formed by conventional spin-coating process.

We fabricated highly efficient organic light-emitting diodes (OLEDs) using the solution processible iridium complexes which have long alkyl or cycloalkyl substituents in an ancillary ligand as a host material and a red phosphorescent iridium complex as an emitting guest by spin coating method. The OLEDs emitted red phosphorescence from the guest and the luminance reached around 10,000 cd/m². The efficiency of the OLED improved after we added an electron transport material (ETM) to the emitting layer. The OLED using a mixture of the alkyl substituted iridium complex and ETM as the host showed an external quantum efficiency of 10%.

We developed a waveguide-integrated Si nano-photodiode (PD) with a surface plasmon (SP) antenna for on-chip optical clock distribution. The interfacial periodic nano-scale metal–semiconductor–metal Schottky electrodes were shown to function as an SP optical antenna and also as an optical coupler between a SiON waveguide and a very thin Si-absorption layer. Furthermore, a very high speed response of 17 ps as well as enhanced photoresponsivity was achieved for a 10-µm coupling length. By using this technology, we fabricated a prototype of a large-scale-integration (LSI) on-chip optical clock system and demonstrated 5 GHz of optical clock circuit operation connected with a 4-branching H-tree structure.

A 1.5-µm nonreciprocal-loss waveguide optical isolator having improved transverse-magnetic-mode (TM-mode) isolation ratio was developed. The device consisted of an InGaAlAs/InP semiconductor optical amplifier waveguide covered with a ferromagnetic epitaxial MnSb layer. Because of the high Curie temperature (T_c=314 °C) and strong magneto-optical effect of MnSb, the nonreciprocal propagation of 11–12 dB/mm has been obtained at least up to 70 °C.

We have developed a new precise fabrication technology of a lenticular lens sheet with black stripe patterns for a rear-projection television. The black stripe patterns have to be accurately aligned to cylindrical lenses on the lenticular lens sheet and be able to absorb ambient light. Our proposed technology utilized a photo-sensitive material with a surface modifier which controls surface free energy for surface modification. Using this material, we could obtain two patterned areas which had different surface free energies. Furthermore, fabrication of black stripe patterns on the lenticular lens sheet was carried out by a self-alignment method. This lenticular lens sheet with black stripe patterns had similar optical properties compared with a conventional one.

A single-mode oxide-confined vertical-cavity surface-emitting laser (VCSEL) with deeply-etched half-ring-shaped holey structure for fiber-optic applications is demonstrated. Single fundamental mode continuous-wave output power of 2.6 mW has been achieved in the 850 nm range, with a threshold current of approximately 1 mA. Side-mode suppression ratio (SMSR) larger than 26 dB has been measured. Contrary to the previously reported surface relief methods, the deep-etched (approximately 2.2 µm in depth) holey structure in this paper provides another approach to achieve single-mode operation of the VCSEL with a larger oxide aperture.

A new simple interference exposure method using a phase-shifting mask was discussed on the basis of Fourier synthesis for fabricating blazed gratings. Phase-shifting mask was designed with 244 nm exposure-light wavelength to launch multiple diffraction beams so that resultant interference pattern fit to required optical intensity profile. Fine surface-relief pattern on SiO₂ mask for 3-µm-period sawtooth optical-intensity profile was fabricated by electron-beam direct-writing lithography with 30 nm scanning step and relief height of 65 nm. Sawtooth-like intensity profile was demonstrated with theoretically predicted interference visibility.

We propose that four-wave mixing (FWM) microscopy can be applied to three-dimensional mapping of refractive index (RI) structure inside transparent samples. We derive an analytical relationship between the RI and the intensity of the FWM signal that is due to nonresonant optical nonlinearity. By using the relationship, the RI profile can be directly and quantitatively obtained from the intensity distribution of the FWM signal. We experimentally demonstrate the RI profiling of a phase grating fabricated in a non-alkali glass.

We report on an optical parametric oscillator (OPO) based on a large aperture periodically poled 5 mol % MgO-doped LiNbO₃ (PPMgLN). A high average power Q-switched Nd:YAG laser operating at 1.064 µm producing 10 ns pulses at a repetition rate of 10 Hz was used to pump the OPO. Total output pulse energy of 126 mJ at 2 µm with 62% slope efficiency was achieved at relatively low pump power intensity. Temperature tuning behavior of the quasi-phase matched OPO was also observed.

A concentrically patterned photonic crystal mirror with polarization selectivity fabricated by the autocloning technique was used for the generation of radially and azimuthally polarized beams from an Nd:YAG laser cavity. By adopting the photonic crystal mirror as the output coupler of the cavity, both radially and azimuthally polarized beams were obtained with similar output power as that of an unpolarized beam generated by a conventional, non-polarization-selective output coupler.

Octave spanning high quality super continuum is generated in all fibre system. A 10 nJ and 104 fs high energy ultrashort soliton pulse is generated from 1 ps fiber chirped pulse amplification laser using large mode area photonic crystal fibre. The conversion efficiency from 1 ps pulse into ultrashort soliton pulse is 33%. A 1.05–2.20 µm over one octave spanning high quality super continuum with ±5 dB uniformity is generated using the high energy soliton pulse and highly nonlinear fiber.

We fabricated a multi-mesa-channel (MMC) structure by forming a periodic trench just under a gate electrode to improve the uniformity of effective electric field in the channel in an AlGaN/GaN high electron mobility transistor (HEMT). A unique performance, i.e., a nearly temperature-independent saturation drain current, was observed in the MMC device in a wide temperature range. A two-dimensional (2D) potential calculation indicates that the mesa-side gate effectively modulates the potential, resulting in a field surrounding 2D electron gas. Such a surrounding-field effect and a relatively lower source access resistance may be related to a unique current behavior in the MMC HEMT.

We report Coulomb blockade oscillations in an atomically thin graphite ribbon fabricated by the micromechanical cleavage technique. Aperiodic current oscillations as a function of the gate voltage indicate the formation of multiple Coulomb islands inside the thin graphite ribbon. We conclude that the Coulomb islands originate from puddles of electrons and holes caused by the inhomogeneous interface between the ribbon and the substrate.

A semiclassical Monte Carlo simulation was run to estimate the performances of a monolayer and a bilayer (with vertical electric field of 1 V/nm applied) graphene-channel field-effect transistor (FET). The vertical field produces a band gap of 0.16 eV and gives semiconductive properties in the bilayer graphene. Electrons in monolayer graphene show a notable velocity overshoot of up to 7.6×10⁷ cm/s. A sub-0.1 ps transit time is also expected in a 65-nm channel device. The performance of a bilayer graphene-channel FET is inferior to a monolayer graphene one, but comparable with that of an InP high electron mobility transistor (HEMT). This lower performance may be attributed to the electron effective mass produced by the vertical field.

In usual thermal imprint process, mold patterns are transferred by applying mechanical pressures at elevated temperatures. We have developed a new imprint process for glass, which utilizes electrostatic force as a driving force for deformation. In the method, high dc voltage is applied between a mold and a glass specimen so that the mold becomes an anode. This method enables glass forming at lower temperatures with smaller mechanical force compared to usual thermal imprint process. In addition, this method is suitable for small patterns less than 1 µm. It is expected that this method realizes large-area and high-efficiency nano-forming process for glass.

A structural model is described for the shallow thermal donors (STDs), which typically consist of seven identical absorption peaks that are caused by electronic transition. In the STD family, attention was paid to the 247 cm^-1 peak that appears to strongly depend on the condition of both the crystal growth and the annealing. We examined NO configuration and the six kinds of NO+O_i configuration. Semi-empirical molecular orbital calculations for these complexes suggested that not only the complex with the C_2v symmetry but also the asymmetric NO+O_i complexes could simultaneously exist in the Si crystal. We concluded that the 247 cm^-1 peak, which was highly unstable and behaved oddly during annealing, might be the NO complex and that the other six STD peaks might correspond to the six kinds of NO+O_i complexes.

The process simulator is a key technology in extreme ultraviolet (EUV) lithography, which is regarded as the ultimate in projection lithography. The requirement for the accuracy of process simulators has become increasingly strict with the shrinkage of feature size. In chemically amplified EUV resists, acid generators decompose through a reaction with thermalized electrons (∼25 meV). This sensitization mechanism of EUV resists is analogous to that induced by an electron beam (EB). However, the acid distribution in EUV resists is different from that in EB resists because of the multispur effect, which is caused by the charged intermediates narrowly distributed around the absorption points of EUV photons. In this work, the authors formulate a proposed point spread function for the EUV lithography process based on chemically amplified resists.

We introduce a system to measure the mechanical properties of fluids under high shear deformation rates of up to 10⁶ s^-1. The newly developed micro-fluid emission nozzle remarkably extended the variety of liquids to, for example, organic solvents, strong alkalis and acids, and viscous fluids. The dynamic behavior of a fluid after the head-on collision of the two emitted particles was observed by the stroboscopic method. The surface tension and viscosity were determined from the resonant frequency and decay constant of the resonant oscillation by comparison with numerical simulation. An approximate theory to describe the large-amplitude oscillation is also presented.

We studied proximity effect in 30 keV electron beam (EB) drawing with calixarene resist for patterned media and quantum devices. Using about 15-nm-thick calixarene resist on Si substrate in conventional EB drawing system, the proximity effect has been studied by forming and observing 20-, 25-, 30-, and 40-nm-pitch resist dot arrays and measuring exposure dosage intensity distribution (EID) function. As a result, the proximity effect is negligible small due to comparing with some dot sizes in center, side and corner of 2 µm square with 25×25 nm² pitch dot arrays. In addition, the proximity effect parameter η in EID function is less than 0.3. It is clear that the EB drawing and calixarene resist system is very suitable for forming ultrahigh packed dot arrays pattern. We demonstrated 20×20 nm² pitch resist dot arrays (about 1.6 Tb/in.²) with a dot diameter of about 14 nm and the same size as everywhere in the pattern.

The trade-off between resolution, sensitivity, and line edge roughness (LER) is the most serious problem for the development of sub-30 nm resists based on chemical amplification. Because of this trade-off, the increase in acid generation efficiency is essentially required for high-resolution patterning with high sensitivity and low LER. In this study, we investigated the dependences of acid generation efficiency on the molecular structure and concentration of acid generators upon exposure to extreme ultraviolet (EUV) radiation. The acid generation efficiency (the number of acid molecules generated by a single EUV photon) was obtained within the acid generator concentration range of 0–30 wt % for five types of ionic and nonionic acid generators.

A laser-cooling experiment of a 40 keV ²⁴Mg⁺ beam was carried out in the small laser-equipped storage ring (S-LSR). A laser co-propagating with the beam and an induction accelerator were utilized in the experiment. The lowest longitudinal temperature achieved in the present experiment was 3.6 K for 3×10⁴ ions stored in the ring. It was found that the number of stored ions is related to the temperature at the final equilibrium state of the laser cooling. This relation shows that the longitudinal temperature of the laser-cooled beam linearly couples with the transverse one through intra-beam scattering.