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Surface plasmons resonance is a promising way to improve the luminescence efficiency of light-emitting materials including InGaN/GaN-based quantum wells; however, it has rarely been used for GaN epilayer with metal nanoparticles (NPs). We demonstrated in enhancing the bandgap emission from GaN epilayer by localized surface plasmon resonance (LSPR) in a quadrupole oscillation mode using Ag NPs fabricated on the GaN. The shapes of the Ag NPs fabricated on the GaN substrates gradually changed over time and were eventually destroyed completely. This should be the reason why previous studies of enhancement of GaN-based materials by using Ag NPs have not performed as good results as expected. This problem was solved by employing oxide thin films to protect the Ag NPs from destruction or aggregation and obtain enhanced photoluminescence of the GaN epilayer. The localized and enhanced electric fields have been calculated by the finite difference time domain (FDTD) method to elucidate the enhancement mechanism. However, it has been still difficult to use this method to reproduce the enhanced emissions caused by the quantum interaction between the surface plasmons and excitons. We reproduced the highly efficient emissions attributed to the Purcell effect by evaluating the nanoantenna effect using the FDTD method. This technique confirmed that an oxide thin film between the GaN substrate and Ag NPs played an effective role in the local enhancement of the electromagnetic field and light extraction from the LSPR mode when several types of oxide thin films were considered.

We investigate the internal quantum efficiencies (IQEs) of AlGaN/AlN quantum wells (QWs) on (0001) c- and semipolar (1[Formula: see text]02) r-planes in the far-ultraviolet C (far-UVC) region using time-integrated photoluminescence and time-resolved PL spectroscopies. Stronger emissions from r-QWs are observed, especially at shorter wavelengths, indicating that the r-QWs exhibit higher IQEs than the c-QWs. Analyses of the experimental results suggest that the main reason for the higher IQEs of r-QWs in the far-UVC region is shorter radiative lifetimes and an increase in a slow decay component, which might be related to a reduction in the number of nonradiative recombination centers. These findings indicate the superiority of the semipolar r-plane for fabricating QWs that emit in the far-UVC region.

Transition metal dichalcogenides (TMDs) with only a few atoms thickness provide an excellent solution to scale down current semiconductor devices. Many studies have demonstrated that molybdenum disulfide (MoS2), a member of TMDs, is promising as a channel material to fabricate field-effect transistors (FETs). However, the carrier mobility in MoS2 FET is always far lower than the theoretical prediction. Although this poor performance can be attributed to the defects, it still lacks a quantitative analysis clarifying the correlation between carrier mobility and defect density. In this work, by using scanning tunneling microscopy, we directly counted the defects in MoS2 FETs with different carrier mobility. We found that vacancies and impurities equally contribute to carrier mobility and the total defect density induces a power-law decreasing tendency to the carrier mobility of MoS2 FET. Our current results directly prove that the reduction of point defects can exponentially improve the carrier mobility of FETs made by TMDs.

A carbon-containing amorphous film is deposited on metalorganic-vapor-phase-epitaxy-grown AlN or AlGaN templates by flowing propane, aluminum, and nitrogen gases at 1010 °C. The deposited ∼1-nm-thick layers show p-type conductivity with a sheet carrier density of ∼1 × 1013 cm−2. When the film is deposited on Mg-doped Al0.8Ga0.2N prior to metallization, it acts as a hole injection layer into p-type AlGaN and shows nearly a double increase in hole current. The transmittance of the deposited layer is ∼90% in the 200–400-nm wavelength range. Hence, the deposited layer is a promising hole injection layer into high-Al-content p-type AlGaN.

The time-domain waveform of the terahertz (THz) electric field emitted from the charge-ordered organic ferroelectrics α − (ET)2I3 [ET:bis(ethylenedithio)-tetrathiafulvalene)] can be significantly modulated depending on incident fluence of femtosecond pulses. Weak irradiation triggers the emission, showing ∼1 THz oscillation in the time-domain waveform with more than 5 oscillation cycles and a corresponding narrow spectral shape. In contrast, strong irradiation almost completely suppresses the trailing portion of the waveform after the first cycle, resulting in over 3 times broader bandwidth. As revealed by time-resolved experiments, this modulation arises from the cooperative nature of photoinduced melting of the charge order accompanying quenching of infrared-activity of the intermolecular vibrations which strongly couple with the emission.

We report on the synthesis and properties of wafer-scale two-dimensional/three-dimensional (2D/3D) n-SnSe2/n-GaN(0001) heterojunctions. The hexagonal crystal structure of crystalline SnSe2 grown by molecular beam epitaxy was confirmed via in-situ reflection high-energy electron diffraction and off-axis X-ray diffraction. Current-voltage (I-V) measurements of SnSe2/GaN diodes exhibited 9 orders of magnitude rectification, and the SnSe2/GaN heterojunction barrier height was estimated to be 1 eV using capacitance-voltage measurements and internal photoemission measurements. Vertical electronic transport analyzed using temperature-dependent I-V measurements indicates thermionic field emission transport across the junction. This work demonstrates the potential of epitaxial growth of large area high quality 2D crystals on 3D bulk semiconductors for device applications involving carrier injection across 2D/3D heterojunctions.

We conducted photoluminescence (PL) mapping to elucidate the detailed mechanism of PL enhancement based on the energy transfer from excitons to surface plasmon polaritons (SPPs) in silver-coated InGaN quantum wells (QWs). The PL mapping for bare InGaN QWs showed positive or negative correlations between the PL peak intensity and wavelength. These correlations are normally caused by exciton localization and the quantum confined Stark effect, respectively; however, they did not appear in the silver-coated region of the InGaN QWs, and the wavelength distribution shifted into shorter wavelengths due to the SP-induced PL enhancement. These results suggest that the energy transfer from the excitons to the SPPs should be much faster than that in the exciton localization and charge screening processes of the piezoelectric field in QWs.

Over the past decade, it has become apparent that the extreme sensitivity of 2D crystals to surface interactions presents a unique opportunity to tune material properties through surface functionalization and the mechanical assembly of 2D heterostructures. However, this opportunity carries with it a concurrent challenge: an enhanced sensitivity to surface contamination introduced by standard patterning techniques that is exacerbated by the difficulty in cleaning these atomically thin materials. Here, we report a templated MoS2 growth technique wherein Mo is deposited onto atomically stepped sapphire substrates through a SiN stencil with feature sizes down to 100 nm and subsequently sulfurized at high temperature. These films have a quality comparable to the best MoS2 prepared by other methodologies, and the thickness of the resulting MoS2 patterns can be tuned layer-by-layer by controlling the initial Mo deposition. The quality and thickness of the films are confirmed by scanning electron, scanning tunnel...

InxGa1−xN multiple quantum wells (QWs) with [0001], ⟨112¯2⟩, and ⟨112¯0⟩ orientations have been fabricated by means of the regrowth technique on patterned GaN template with striped geometry, normal planes of which are (0001) and {112¯0}, on sapphire substrates. It was found that photoluminescence intensity of the {112¯2} QW is the strongest among the three QWs, and the internal quantum efficiency of the {112¯2} QW was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {112¯2} QW was about 0.38ns at low temperature, which was 3.8 times shorter than that of conventional [0001]-oriented InxGa1−xN QWs emitting at a similar wavelength of about 400nm. These findings strongly suggest the achievement of stronger oscillator strength owing to the suppression of piezoelectric fields.

Splitting between the field-cooled dielectric constant and the zero-field-cooled dielectric constant was observed for a diluted system of LiTaO3 nanoparticles (diameter 30 Å) embedded in amorphous SiO2. At the applied field frequency of 100 kHz, the real part of the field-cooled dielectric constant diverged from that of the zero-field-cooled one at 380 °C. The bifurcation point of the history-dependent dielectric constant rose from 310 to 540 °C upon increasing the field frequency from 10 to 1000 kHz. Bulk LiTaO3 powders showed no splitting in the history-dependent dielectric constant and the maximum at 645 °C in the real part of the dielectric constant, despite the variation of frequency. Both the splitting of the history-dependent dielectric constant and the frequency dependence of the bifurcation point suggest that the LiTaO3 nanoparticles with a single-domain structure were in the superparaelectric state as a consequence of insignificant cooperative interactions among the nanoparticles in the diluted system. The energy barrier of 0.9 eV separating two (+p and –p) polarization states corroborated the potential of the LiTaO3 nanoparticle for ultrahigh-density recording media applications.

We have developed fabrication processes for epitaxial NbN/MgO/NbN trilayers and epitaxial NbN/MgO/NbN tunnel junctions. We estimated the junction specific capacitances (CS) for designing fully epitaxial NbN–superconductor-insulator-superconductor (SIS) mixers by using Fiske steps. Tunnel junctions measuring 40×3 and 20×3 μm2 were fabricated, and Fiske steps caused by external magnetic fields were clearly observed in the I–V characteristics. The CS was estimated as a function of the product of the normal-state resistance and the junction area (RNA), and they were 73–120 fF/μm2 at 8800–90 Ω μm2. We fabricated and tested a fully epitaxial NbN/MgO/NbN–SIS mixer. The mixer showed low-noise properties over the Nb gap frequency, and the lowest DSB receiver noise was 260 K at 795 GHz.

Precise identification of recombination dynamics based on local, radiative, and nonradiative recombination has been achieved at room temperature in a blue-light-emitting InxGa1−xN/GaN single-quantum-well structure by comparing the photoluminescence (PL) spectra taken by illumination-collection mode (I-C mode) and those by illumination mode (I-mode) in scanning near-field microscopy. The PL data mapped with PL lifetimes, as well as with PL spectra, revealed that the probed area could be classified into four different regions whose dominating processes are (1) radiative recombination within a probing aperture, (2) nonradiative recombination within an aperture, (3) diffusion of photogenerated excitons/carriers out of an aperture resulting in localized luminescence, and (4) the same diffusion process as (3), but resulting in nonradiative recombination.

A large frequency dependence of the maximum dielectric constant temperature was observed for LiTaO3 nanocrystals (the diameter ≈20 A) dispersed in mesoporous silicate. At the applied field frequency of 100 kHz, the maximum temperatures in the real and imaginary parts were 365 and 345 °C, respectively. The maximum temperature in the real part is apparently lower than the paraelectric–ferroelectric transition temperature (645 °C) of bulk LiTaO3. The maximum temperature in the imaginary part rose from 285 to 420 °C with increasing frequency from 10 to 1000 kHz. Since the bulk LiTaO3 shows no relaxor behavior, such superparaelectric behavior is obviously a consequence of nanominiaturization of LiTaO3 crystal and insignificant cooperative interactions between the nanoparticles.

Semipolar {n[n¯]01} InGaN/GaN quantum wells (QWs) (n = 1−3) are fabricated on top of GaN microstructures, which consist of semipolar {1Ī01} facets. Semipolar planes are obtained via regrowth of three-dimensional structures on (0001) GaN templates under controlled growth conditions. Compared to QWs on {1Ī01} facets, {n[n¯]01} ridge QWs show an intense emission at ∼ 440 nm. Time resolved photoluminescence reveals that the radiative lifetime of excitons in {n[n¯]01} InGaN ridge QWs at 13 K is 310 ps, which is comparable to that in {1Ī01} QWs. The estimated internal quantum efficiency at room temperature is as high as 57%.

Spatial distribution of photoluminescence (PL) with spectral, spatial, and/or time resolution has been assessed in an InxGa1−xN single-quantum-well (SQW) structure using scanning near-field optical microscope (SNOM) under illumination-collection mode at 18 K. The near-field PL images revealed the variation of both intensity and peak energy in PL spectra according to the probing location with the scale less than a few hundredths of a nanometer. PL linewidth, the value of which was about 60 meV in macroscopic PL, was as small as 11.6 meV if the aperture size was reduced to 30 nm. Clear spatial correlation was observed between PL intensity and peak wavelength, where the regions of strong PL intensity correspond to those of long wavelength. Time-resolved SNOM–PL study showed the critical evidence that supports the model of diffusion of carriers to potential minima.

Narrow luminescence lines from self-assembled CdSe quantum dots (QDs) are studied at room temperature by using time-resolved photoluminescence spectroscopy. The narrowest line of the luminescence has a full width at half maximum of 2.1 meV at room temperature. The narrow luminescence lines are mainly the contribution of CdSe QDs with a longer recombination lifetime to the luminescence.

We performed photoluminescence (PL) and degenerate four-wave-mixing (FWM) spectroscopy in epitaxially laterally overgrown GaN at 10 K. Optical transitions based on exciton complexes such as biexciton emission, exciton–exciton scattering, electron–hole plasma, and so on, were revealed by PL under a wide range of excitation densities. The FWM signals were observed from states below the fundamental excitonic absorption edge, showing that nonlinear photoswitching can be performed with a transmittance geometry. The origin of such nonlinearlity was discussed by correlating with feasible many-body effects between excitons, biexcitons, and free carriers.

We propose a three-dimensional photonic crystal structure having a wide full photonic band gap in the optical regime, which can be fabricated by an alternating-layer deposition and etching (drilling) process. This fabrication process is much simpler than that previously reported. The combination of current lithographic technology and autocloning bias-sputtering deposition is a promising way of realizing these photonic crystals.

We demonstrate a self-healing effect of unwanted defects in the autocloning process: autocloning is a previously proposed technique to fabricate multidimensional periodic nanostructures by stacking up corrugated multilayers. With the self-healing effect, aperiodic perturbations in the initial periodic shape of the structure immediately disappear and the surface shape becomes periodic in a few cycles of stacking. If the perturbations exist in photonic crystal, they cause light-scattering loss and also make the boundaries between photonic bands and band gaps unclear. In other words, they make the attenuation of light at the frequency in photonic band gaps weak. Consequently, to attain uniformity of the shape automatically with this effect is very important in the fabrication process for practical photonic crystal devices. In this letter, we verify this phenomenon experimentally, and discuss the mechanism by comparison with process simulation.

(Ga,Mn)As digital ferromagnetic heterostructures are grown by incorporating submonolayer planes of MnAs into GaAs using molecular beam epitaxy. Structural and magnetic measurements indicate single-crystalline superlattice structure and ferromagnetic order with Curie temperatures (TC) up to 50 K. By varying the spacing between neighboring Mn layers, we observe that TC initially decreases with increasing spacer thickness, followed by a regime with weak dependence on the spacer thickness. The persistence of ferromagnetism for interlayer spacings of at least 200 ML (∼560 A) suggests that the individual Mn layers are ferromagnetic.

Dynamical behavior of optical gain formation has been assessed at room temperature in the InxGa1−xN multi-quantum-well (MQW) based laser diodes (LDs) by employing pump and probe spectroscopy with a pulse width of 150 fs. The LDs are composed of (a) In0.1Ga0.9N–In0.02Ga0.98N MQW and (b) In0.3Ga0.7N–In0.05Ga0.95N MQW, whose stimulated emissions correspond to near ultraviolet (390 nm) and blue (440 nm), respectively. The optical gain was contributed from the nearly delocalized states [the lowest-quantized MQW levels (LQL)] in the sample (a), while it was from highly localized levels with respect to the LQL by 500 meV for the sample (b). It was found that the photogenerated carriers rapidly (less than 1 ps) transferred to the LQL, and then relaxed to the localized tail within the time scale of about 5 ps, giving rise to the optical gain. Such gain spectra were saturated and other bands appeared in the vicinity of the LQL under higher photoexcitation.

We study the optical polarization properties of confined structures in InGaN/GaN single quantum well devices. Using a near-field optical setup we investigated the photoluminescence maps with a polarization-modulation method. If the optical emissions have a preferred polarization orientation, our apparatus yields a signal that is proportional to the degree of polarization. We could demonstrate that within the quantum well there are localized submicrometer centers that emit strongly oriented light. This points toward the existence of quantum-dot like confined asymmetric domains hidden within the quantum well.

The tunneling barrier height of NbN/AlN/NbN tunnel junctions was measured by investigating the barrier thickness dependence of the current density, and the junction interface was studied by cross-sectional transmission electron microscopy (TEM). We found that the current density of the junctions has two distinct types of dependency on the AlN barrier thickness, corresponding to two average barrier heights in different regions for the current density. The TEM observations showed that the junctions had a very smooth and clear electrode–barrier interface, and the crystal structures of the counterelectrode NbN films were strongly dependent on the thickness of AlN barriers. The average barrier height was estimated to be 2.35 eV in the low-Jc region, Jc 5 kA/cm2.

We found a coherent array of tungsten ultrafine particles of about 700 nm around a laser-irradiated mark on a single-crystalline surface of tungsten. The tungsten specimens were irradiated by a short-pulse Nd:YAG laser under low pressure in an inert gas atmosphere. Two types of coherent arrays were observed that included the two-dimensional cubic and hexagonal systems. Such a coherent array of ultrafine particles may have potential applications for emission devices of plasma display and microelectronic devices.

Light-beam steering that is extremely wavelength dependent has been demonstrated by using photonic crystals fabricated on Si. The scanning span reached 50° with only a 1% shift of incident wavelength at around 1 μm. The resulting angular dispersion is two orders of magnitude larger than that achieved with conventional prisms or gratings. The application of such superprism phenomena promises to enable the fabrication of integrated micro lightwave circuits that will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.

We demonstrate that the apparent emission colors of InGaN-based light-emitting diodes using microstructured multifacet quantum wells as active layers can externally be controlled over a wide spectral range that encompasses green to blue or white at a color temperature of 4000K to blue along the Planckian locus. The controllability relies on facet-dependent polychromatic emissions. The pulsed current operation with the appropriate duties varied their relative intensities and the consequent apparent colors without seriously affecting the total number of emitted photons, particularly for the blue to green variation.

Radiative and nonradiative recombination processes have been studied in ultraviolet In0.02Ga0.98N light-emitting diodes (LEDs), by employing photoinduced voltage, electroreflectance, and time-resolved photoluminescence spectroscopy. It was found that excitons in the In0.02Ga0.98N active layer are weakly localized at low temperature, and delocalized above about 70 K, showing a three-dimensional feature in the temperature dependence of the radiative lifetime. The external quantum efficiency (ηext) of this LED is about ten times higher than the LED whose active layer is composed of GaN. Improvement of the ηext value by the addition of a small amount of In to the active layer is attributed to the suppression of the density of the nonradiative recombination center (NRC), as well as possibly to where the origin of the NRC is changed.

We previously demonstrated a process for fabricating three-dimensional (3D) periodic nanostructures composed of corrugated a-Si/SiO2 multilayers, which behave as 3D photonic crystals. In this process, bias sputtering is a key technique by which the pattern is self-forming. This letter clarifies the mechanism of the self-shaping effect of bias sputtering by comparing deposition simulation and experiments. The mechanism is decomposed into three main effects: diffuse incidence of neutral particles of film material, sputter etching by normally incident rare-gas ions, and subsequent redeposition of sputtered film material. Specifically, redeposition has a self-adjusting effect on the depth of holes or valleys, and is the key of formation of stable patterns.

Although extensive efforts have continued to develop Ohmic contacts for p-type GaN, which have specific contact resistance (ρc) lower than that (ρc∼10−2 Ω cm2) of conventional Ni/Au contacts, to the best of our knowledge no breakthrough has been reported in open literature. We demonstrated that bilayered Ta/Ti contacts have a ρc value of around 3×10−5 Ω cm2 for p-type GaN with a hole concentration of 7×1017 cm−3. This contact has resistance low enough to manufacture blue laser diodes, but deterioration of the ρc value during room-temperature storage is the key issue.

We have fabricated optical waveguides in three-dimensional (3D) photonic crystals and observed propagation of light beams. Light beams with wavelengths of 1.15 μm propagate along the line defects formed in the 3D photonic crystals. The 3D photonic crystals consist of a-Si/SiO2 multilayers laminated alternately by rf bias sputtering on a periodically hollowed silica substrate with a triangular lattice. The pit diameter is 0.2 μm and the pitch of the lattice is 0.5 μm. The thickness of each laminated layer is 0.2 μm. Line defects are formed normal to the surface by laminating a-Si/SiO2 multilayers with ten periods on the substrate in which the corrugation patterns have been omitted in a certain area corresponding to the core. The measurements of transmittance normal to the surface show that the wavelength of 1.15 μm used in observation of propagation is in the passband for the one-dimensional periodic region corresponding to the core and in the stop band for the 3D periodic region corresponding to the cladding, respectively. Measurements show good agreement with finite-difference time-domain calculations.

The stimulated emissions from semipolar InGaN laser diode (LD) structures grown on (11[overline 2]2) GaN substrates are observed at room temperature under photopumped conditions. The measured emission peaks are in the photon energy range from 2.62 eV (474 nm) to 3.05 eV (405 nm), and the emission intensity has a threshold behavior with respect to the pumping power. A strong in-plane optical anisotropy is observed between the two perpendicular directions, [[overline 1] [overline 1]23] and [1[overline 1]00], due to anisotropic matrix elements, which depend on the crystal orientation; the stimulated emission measured along the [[overline 1] [overline 1]23] direction occurs with a lower threshold pumping power at a lower energy compared to that obtained along the [1[overline 1]00] direction. The experimental results and the valence band calculations indicate that the transverse-electric mode with an electric vector along the [1[overline 1]00] direction is dominant for gain formation in semipolar and nonpolar InGaN LDs. Compared to c-plane InGaN LDs, semipolar InGaN LDs have comparable or less threshold pumping powers.