Table of contents

We have succeeded in enhancing the photoalignment efficiency of polyimide containing azobenzene in the backbone structure by exposing the corresponding precursor (polyamic acid: Azo-PAA) film to alkyl-amine vapor prior to photoalignment. The Azo-PAA film absorbed alky-amines and swelled by 300%. The photoinduced rotation of the Azo-PAA backbone structure occurred more easily in the swollen film. Most of the alkyl-amines in the swollen film desorbed during thermal imidization. As a result of the photoalignment efficiency enhancement, we also succeeded in expanding the controllable pretilt angle range of liquid crystals up to 38° without the appearance of threadlike disclination loops.

Lasing oscillation at wavelengths up to 1045 nm at room temperature has been realized from GaAs_1−xBi_x Fabry–Perot laser diodes (FP-LDs) by electrical injection, and the temperature characteristics of GaAs_1−xBi_x FP-LDs are revealed for the first time. The characteristic temperature T₀ of the GaAs_0.97Bi_0.03 FP-LD in the temperature range between 15 and 40 °C (T₀ = 125 K) is similar to that reported for typical 0.98 µm InGaAs/GaAs LDs. The temperature coefficient of the lasing wavelength in GaAs_0.97Bi_0.03 FP-LDs is reduced to 0.17 nm/K, which is only 45% of that of GaAs FP-LDs.

In this paper, we describe the fabrication and characterization of a GaN-based light-emitting diode (LED) on a GaN-on-silicon platform. A freestanding membrane structure eliminates the absorption of the emitted light by a silicon substrate and reduces the number of confined optical modes, leading to higher photoluminescence intensity. Compared with an LED with a silicon substrate, the current–voltage characteristics of a freestanding membrane LED demonstrate a lower turn-on voltage and a steeper current–voltage profile. Both anomalous positive capacitance peak and negative capacitance are observed in the capacitance–voltage measurements, which correspond well to the current–voltage characteristics. The measured electroluminescence intensity is significantly increased for a freestanding membrane LED. These experimental results show that our proposed substrate removal technology is promising for the fabrication of a high-performance membrane LED for diverse applications.

We fabricated GaN light-emitting diodes with a layer of graphene as a transparent electrode. A 3-nm-thick Al layer was deposited on the graphene layer by electron-beam evaporation. This Al layer plays an important role in protecting the graphene layer during the device fabrication process. Moreover, this Al layer can also enhance the light emission of GaN light-emitting diodes through the investigation of electroluminescence spectra. The significantly improved light emission is attributed to the current expansion, the enhanced plasmonic density of states, and the decreased non-radiative recombination rate.

The feasibility of a highly efficient and air-stable organic light-emitting diode (OLED) was examined. A phosphorescent OLED not containing an air-sensitive material was fabricated by employing an inverted structure with an air-stable electron injection layer. Efficient electron injection from the bottom cathode to the emitting layer was demonstrated from the fact that the device characteristics of the inverted OLED were almost the same as those of a conventional OLED. No dark spot formation was observed after 250 days in the inverted OLED encapsulated by a barrier film with a water vapor transmission rate of 10⁻⁴ g m⁻² day⁻¹.

A magnetic toroidal dipole is constructed using a kind of planar metamaterial, on the basis of four asymmetry split ring resonators. Traditional electric and magnetic multipoles have been suppressed, and therefore, the toroidal moment can be enhanced by the resonant plasmonic response with rational arrangements of meta-atoms. In particular, the magnetic toroidal dipole is weakly coupled to external excitation, and unit transmittance has been achieved as a result of the enhanced toroidal response. This superior performance, with little effect of the dielectric loss, offers a new mechanism in the design of low-loss or electromagnetically induced transparent materials.

The optical properties of a graphene material are analyzed first and a novel graphene–metal hybrid plasmonic switching in the near-infrared band is demonstrated in this study. The proposed hybrid structure is composed of metal–insulator–metal (MIM) waveguides and a graphene material. The switching is achieved by controlling the gate voltage of the graphene sheet. The switch's functionality is verified by two-dimensional finite-difference time-domain (FDTD) simulation. As applications, two different plasmonic switch structures are presented. The graphene–metal hybrid plasmonic structure may have potential applications in ultracompact integrated circuits and nanophotonic networks.

The influence of film formation on light-trapping properties of silicon thin-film solar cells prepared on randomly textured substrates was studied. Realistic interface morphologies were calculated with a three-dimensional (3D) surface coverage algorithm using the measured substrate morphology and nominal film thicknesses of the individual layers as input parameters. Calculated interface morphologies were used in finite-difference time-domain simulations to determine the quantum efficiency and absorption in the individual layers of the thin-film solar cells. The investigation shows that a realistic description of interface morphologies is required to accurately predict the light-trapping properties of randomly textured silicon thin-film solar cells.

We present high-quality a-SiO_x:H solar cells with a very thin i-layer of 100 nm fabricated at a low temperature of 100 °C. To boost the photocurrent with such a thin absorber, we suggested the application of a low-index MgF₂ buffer at the n-type nanocrystalline silicon oxide (n-nc-SiO_x:H)/Ag nanotextured interface to suppress the absorption loss at the Ag back contact. The introduction of MgF₂ of only a few nanometers (∼4 nm) thickness enhanced the reflection at the n-nc-SiO_x:H/Ag interface, which resulted in the reinforcement of the short-circuit current by about 7.3% from 9.60 to 10.30 mA/cm² while almost maintaining V_oc and FF. We demonstrated the efficiency improvement of up to 7.66% by MgF₂ at the back contact.

Spectrum distortion induced by pump depletion of pump–probe-based stimulated Brillouin scattering along an optical fiber is investigated theoretically and experimentally. Spectral hole burning is observed in the middle of the fiber. The critical condition for spectrum distortion is defined, and normalized parameters are introduced to describe the critical condition with different injected powers in arbitrary-length fibers. The experimental observation is in reasonable agreement with the numerical analysis. The different spectrum distortion may introduce novel applications to programmable optical fiber filtering.

We present a simple method for significantly broadening the bandwidth of a metamaterial absorber formed using only a low-conductivity alloy ring and an alloy ground plane separated by a dielectric layer. The >80% relative absorption bandwidth of the device is improved to 101.2%, which is much higher than previously reported values. The absorption mechanism for the broadband alloy absorber involves the overlapping of two different resonance frequencies. In particular, the electromagnetic energy is entirely dissipated in the alloy layer, which makes it insensitive to the loss of the dielectric layer and widens the range of choices for potential dielectric materials.

Generation of practical random numbers (RNs) by a spintronics-based, scalable truly RN generator called "spin dice" was demonstrated. The generator utilizes the stochastic nature of spin-torque switching in a magnetic tunnel junction (MTJ) to generate RNs. We fabricated eight perpendicularly magnetized MTJs on a single-board circuit and generated eight sequences of RNs simultaneously. The sequences of RNs of different MTJs were not correlated with each other, and performing an exclusive OR (XOR) operation among them improved the quality of the RNs. The RNs obtained by performing a nested XOR operation passed the statistical test of NIST SP-800 with the appropriate pass rate.

We investigate the voltage breakdown of the spin transfer torque magnetic random access memory (STT-MRAM) with perpendicular magnetic tunnel junctions (pMTJs). Different breakdown behaviors are observed for RF-MgO pMTJs and naturally oxidized MgO pMTJs. While the time-to-failure body distribution of the naturally oxidized MgO follows the Weibull distribution, that of RF-MgO follows the lognormal distribution. This result suggests distinctly different dielectric breakdown mechanisms for naturally oxidized MgO and RF-MgO. For low failure probability, the progressive voltage breakdown of RF-MgO (associated with the lognormal distribution) results in an order-of-magnitude reliability improvement over the abrupt breakdown of the naturally oxidized MgO. We show that RF-MgO is suitable for perpendicular STT-MRAM applications.

InAs nanowire with 〈110〉 orientation is proposed for use as an electron spin transport channel for application to spintronics devices, particularly the Datta–Das spin transistor. Stable zinc blende crystal NWs were grown using a molecular beam epitaxy system. Subsequently, global back-gate NW field effect transistors were fabricated, and the superiority of the electrical transport properties within our resultant 〈110〉 NWs was demonstrated by comparing the field-effect mobility with a 〈111〉 NW control sample. Additionally, single NW Hall-bar devices were fabricated, which allowed us to obtain the transport properties accurately, and Hall effect measurements were successfully taken at different temperatures.

We demonstrate charge and spin current transport in a graphene-based lateral spin valve using yttrium oxide (Y-O) as a tunneling barrier between graphene and a ferromagnetic electrode. A Y-O layer grown on graphene is flat, with a root-mean-square roughness of 0.17 nm, which is much lower than that of conventional barrier materials. This flatness allows the utilization of a very thin but well-defined tunneling barrier, leading to a large spin signal of ∼20 Ω and a high spin injection efficiency of 15% with a low contact resistance of ∼1 kΩ. These findings represent important progress toward the realization of graphene-based spintronics applications.

We investigated the enhancement of carrier lifetime in lightly Al-doped p-type 4H-SiC epilayers (N_A ≃ 2 × 10¹⁴ cm⁻³) by postgrowth processing. A carrier lifetime of 2.8 µs in an as-grown epilayer is increased to 5.1 µs by carbon vacancy elimination, i.e., thermal oxidation at 1400 °C for 48 h. It reaches 10 µs by subsequent hydrogen annealing at 1000 °C for 10 min. The carrier lifetime in the as-grown epilayer is also increased to 4.0 µs by only hydrogen annealing. These results suggest that, in addition to carbon vacancy, there is another lifetime killer in p-type SiC, which cannot be eliminated by thermal oxidation but can be passivated by hydrogen annealing.

GaN films were grown on a multilayer graphene (MLG)/amorphous SiO₂ stack by pulsed sputtering deposition and their structural properties were investigated. The GaN films on MLG show high c-axis orientation. In addition, the GaN films exhibit coexisting zincblende and wurtzite phases, but the zincblende phase is suppressed by the insertion of AlN interlayers. The polarity control of the GaN films was demonstrated using AlN interlayers with and without surface oxidation. These results indicate that the proposed technique can yield high-quality Ga-polarity GaN films on MLG for potential use in large-area GaN-based optical and electronic devices.

The mechanism of solid-state dewetting is investigated using spatial correlation analysis of holes formed in thermally annealed silver films. This work demonstrates that silver thin films can evolve to form a hole pattern with substantial spatial correlation during solid-state dewetting. The spatial correlation is observed only in films annealed under oxygen for an amount of time necessary to generate a sufficiently high hole number density. This indicates that the holes form via heterogeneous nucleation, and that the nucleation process becomes increasingly spatially correlated due to the pre-existence of other holes.

Epitaxial deposition rate and production yield of Si epitaxy have been improved by reducing the pressure during mesoplasma chemical vapor deposition to attain a rate of 490 nm/s and a yield of 60% at 3 Torr while maintaining a Hall mobility as high as 210 cm²·V⁻¹·s⁻¹. Decreasing the pressure increased the local density of excited atomic H in the plasma near the deposition region. This increased density potentially increased the instability of the Si–Cl gases and also promoted the impingement dynamics of the growth-precursor clusters, which were likely the cause of the improved production yield and film quality.

We demonstrate high conversion efficiency for extreme ultraviolet (EUV) emission at 6.5–6.7 nm from multiple laser beam-produced one-dimensional spherical plasmas. Multiply charged-state ions produce strong resonance emission lines, which combine to yield intense unresolved transition arrays (UTAs) in Gd, Tb, and Mo. At an optimum laser intensity of 1 × 10¹² W/cm², which is estimated to yield an electron temperature of around 100 eV, the maximum in-band EUV conversion efficiency (CE) was observed to be 0.8%, which is one of the highest values ever reported due to the reduction of plasma expansion loss.

An ultrathin-body Ge-on-insulator (GeOI) wafer having a bonded thin Al₂O₃/SiO₂ hybrid buried oxide layer was fabricated using an epitaxially grown Ge film on Si as a Ge donor layer. The epitaxial Ge film was confirmed to have a negligibly low density of crystal-defect-induced p-type carriers and was successfully transferred to form the GeOI wafer. Strong Al₂O₃/SiO₂ bonding effectively suppressed Ge exfoliation during the wafering process. The obtained device-grade GeOI layer and strong bonding strength between Al₂O₃ and SiO₂ are potentially advantageous for future Si-based complementary metal–insulator–semiconductor (CMIS) fabrication processes utilizing large-diameter Si wafers.

In this paper, we successfully demonstrate multifunctional surfaces based on scaffolding biomimetic structures, namely, hybrid salvinia leaves with moth-eye structures (HSMSs). The novel fabrication process employs scalable polystyrene nanosphere lithography and a lift-off process. Systematic characterizations show the biomimetic HSMS exhibiting superhydrophobic, self-cleaning, antiadhesive, and antireflective properties. Furthermore, the resulting surface tension gradient (known as the Marangoni effect) leads to a superior air retention characteristic in the HSMS under water droplet impact, compared with the traditional hybrid lotus leaf with a moth-eye structure (HLMS). Such results and learnings pave the way towards the attainment and mass deployment of dielectric surfaces with multiple functionalities for versatile biological and optoelectronic applications.

A horizontal vibrational motion of biological tissue generated by a femtosecond laser-induced impulsive force was directly detected for the first time as angular shift of the cantilever of an atomic force microscope (AFM), which was directly in contact with the tissue. The motion of a small plant stem (diameter: 160 µm) on the force loading was detected by the torsional motion of the AFM cantilever. The sensitivity of the method was evaluated by a numerical simulation with the finite element method (FEM). The results conclusively demonstrated the efficacy of this method for nano-scale detection of the horizontal motion of biological micro-objects.

The requirement of producing concentration gradients rapidly and controllably is essential in many biochemical applications. The use of microdroplets is advantageous as the droplets act as isolated open reaction vessels that are proficient in containing a variety of reagents. We present a concept of producing concentration gradients over an array of open wells, using a concentrated primary drop which, whilst in motion, merges with successive buffer droplets and, as a result, leaves behind droplets of different concentrations. The primary drop is positioned to slide down a 90° incline over several secondary drops, whereby merging, mixing, and detaching occurs within each well.

We proposed and investigated a solution to the inverse acoustic cloak problem, an anti-stealth technology to make cloaks visible, using the time-lapse reversal (TLR) method. The TLR method reconstructs the image of an unknown acoustic cloak by utilizing scattered acoustic waves. Compared to previous anti-stealth methods, the TLR method can determine not only the existence of a cloak but also its exact geometric information like definite shape, size, and position. Here, we present the process for TLR reconstruction based on time reversal invariance. This technology may have potential applications in detecting various types of cloaks with different geometric parameters.