Showing papers in "Applied Physics Express in 2014"
TL;DR: In this paper, an ionizing-radiation-induced luminescence characterization system equipped with a pulse-width-tunable X-ray source was developed, where the excitation pulse width was tunable from nano-to millisecond ranges.
Abstract: To evaluate the X-ray-induced afterglow phenomenon, we developed an ionizing-radiation-induced luminescence characterization system equipped with a pulse-width-tunable X-ray source. The system consists of a pulse X-ray tube and a detector system based on photon counting. The excitation pulse width was tunable from nano- to millisecond ranges, and the dynamic range of the X-ray-induced afterglow was 106. Conventional scintillators for X-ray CT or security systems, namely, Bi4Ge3O12, CdWO4, Tl-doped CsI, and Tb and Pr-codoped Gd2O2S, were evaluated for the performance test. Results show that the afterglow time profiles of these scintillators are consistent with generally known results with high accuracy.
304 citations
TL;DR: In this article, the near-band-edge optical responses of solution-processed CH3NH3PbI3 on mesoporous TiO2 electrodes, which are utilized in mesoscopic heterojunction solar cells, were studied.
Abstract: We studied the near-band-edge optical responses of solution-processed CH3NH3PbI3 on mesoporous TiO2 electrodes, which is utilized in mesoscopic heterojunction solar cells. Photoluminescence (PL) and PL excitation spectra peaks appear at 1.60 and 1.64 eV, respectively. The transient absorption spectrum shows a negative peak at 1.61 eV owing to photobleaching at the band-gap energy, indicating a direct band-gap semiconductor. On the basis of the temperature-dependent PL and diffuse reflectance spectra, we clarified that the absorption tail at room temperature is explained in terms of an Urbach tail and consistently determined the band-gap energy to be ~1.61 eV at room temperature.
281 citations
TL;DR: In this paper, the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate were reported and the light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% respectively.
Abstract: We report on the optical properties of the InGaN-based red LED grown on a c-plane sapphire substrate. Blue emission due to phase separation was successfully reduced in the red LED with an active layer consisting of 4-period InGaN multiple quantum wells embedding an AlGaN interlayer with the Al content of 90% on each quantum well. The light output power and external quantum efficiency at a dc current of 20 mA were 1.1 mW and 2.9% with the wavelength of 629 nm, respectively. This is the first demonstration of a nitride-based red LED with the light output power exceeding 1 mW at 20 mA.
211 citations
TL;DR: In this paper, the effect of nanoscale pinholes in compact TiO2 layers on the device performance was investigated, and it was shown that the ALD-based layer acts as an efficient hole-blocking layer in perovskite solar cells; it offers a large shunt resistance and enables a high power conversion efficiency.
Abstract: A uniform and pinhole-free hole-blocking layer is necessary for high-performance perovskite-based thin-film solar cells. In this study, we investigated the effect of nanoscale pinholes in compact TiO2 layers on the device performance. Surface morphology and film resistance studies show that TiO2 compact layers fabricated using atomic layer deposition (ALD) contain a much lower density of nanoscale pinholes than layers obtained by spin coating and spray pyrolysis methods. The ALD-based TiO2 layer acts as an efficient hole-blocking layer in perovskite solar cells; it offers a large shunt resistance and enables a high power conversion efficiency of 12.56%.
202 citations
TL;DR: In this article, the authors found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys.
Abstract: We found a giant Seebeck effect in semiconducting single-wall carbon nanotube (SWCNT) films, which exhibited a performance comparable to that of commercial Bi2Te3 alloys. Carrier doping of semiconducting SWCNT films further improved the thermoelectric performance. These results were reproduced well by first-principles transport simulations based on a simple SWCNT junction model. These findings suggest strategies that pave the way for emerging printed, all-carbon, flexible thermoelectric devices.
183 citations
TL;DR: Generation of practical random numbers (RNs) by a spintronics-based, scalable truly RN generator called "spin dice" was demonstrated and the RNs obtained by performing a nested XOR operation passed the statistical test of NIST SP-800 with the appropriate pass rate.
Abstract: 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.
179 citations
TL;DR: In this paper, the authors reported the characteristics of vertical GaN-based trench metal-oxide-semiconductor field effect transistors on a free-standing GaN substrate with a blocking voltage of 16 kV.
Abstract: This paper reports the characteristics of vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate with a blocking voltage of 16 kV The high blocking voltage was obtained by using field plate edge termination around the isolation mesa of the transistor To our knowledge, the blocking voltage is the highest ever reported for vertical GaN-based transistors on free-standing GaN substrates Normally off operation with a threshold voltage of 7 V is also demonstrated
164 citations
TL;DR: In this paper, a high growth rate of 2.2 nm/min for β-Ga2O3(010) homo-epitaxial growth was achieved by optimizing the O flux between 650 and 750 °C.
Abstract: β-Ga2O3(010) homo-epitaxial growth was performed by plasma-assisted molecular beam epitaxy. Under Ga-rich conditions and for growth temperatures above 650 °C, the growth rate was independent of the Ga/O ratio (>1). A high growth rate of 2.2 nm/min for β-Ga2O3(010) was achieved by optimizing the O flux between 650 and 750 °C. Under Ga-rich conditions between growth temperatures of 500–900 °C, smooth surfaces with rms roughness below 1 nm were realized. We found that the slightly Ga-rich conditions between 650 and 750 °C were optimal for β-Ga2O3(010) growth with a smooth surface and a high growth rate.
116 citations
TL;DR: In this paper, the authors reveal the origin of the large absorption coefficient in BaSi2, which is roughly two to eighty times larger at ħω − Eg = 0.5 eV than other conventional absorbers such as Si, GaAs, CdTe, CuInSe2, and Cu2ZnSnS4.
Abstract: Employing a hybrid density functional theory, we reveal the origin of the large absorption coefficient in BaSi2, which is roughly two to eighty times larger at ħω − Eg = 0.5 eV than other conventional absorbers such as Si, GaAs, CdTe, CuInSe2, and Cu2ZnSnS4. This is explained on the basis of the electronic band structure of BaSi2, whereby the lowest conduction band (CB) has a small dispersion, owing to the localized Ba-d states, resulting in flat bands. Consequently, these flat bands contribute to a wide range of optical absorption in the low-energy region and lead to high optical activity in BaSi2.
98 citations
TL;DR: In this article, the microstructures of these devices were investigated by X-ray diffraction, transmission electron microscopy, and their calculations, and the result showed an increase in the efficiencies of the devices, due to microstructural change of the perovskite compound layers.
Abstract: TiO2/CH3NH3PbI3-based photovoltaic devices were fabricated. The microstructures of these devices were investigated by X-ray diffraction, transmission electron microscopy, and their calculations. Structure analysis indicated phase transformation of the perovskite structure from a tetragonal to a cubic system by annealing, which resulted in the improvement of photovoltaic properties of the devices. Effects of a multiple spin-coating method using a mixture solution were also investigated. The result showed an increase in the efficiencies of the devices, due to the microstructural change of the perovskite compound layers.
96 citations
TL;DR: In this article, the negatively charged silicon-vacancy (SiV−) has 70% of its emission to the zero-phonon line (ZPL), in contrast to the positively charged nitrogen vacancy (NV−), which has a broad spectrum.
Abstract: Among promising color centers for single-photon sources in diamond, the negatively charged silicon-vacancy (SiV−) has 70% of its emission to the zero-phonon line (ZPL), in contrast to the negatively charged nitrogen vacancy (NV−), which has a broad spectrum. Fabricating single centers of useful defect complexes with high yield and excellent grown-in defect properties by ion implantation has proven to be challenging. We have fabricated bright single SiV− centers by 60-keV focused ion beam implantation and subsequent annealing at 1000 °C with high positioning accuracy and a high yield of 15%.
TL;DR: In this paper, the orientation of more than 99% of the nitrogen-vacancy (NV) centers can be aligned along the [111] axis by chemical vapor deposition homoepitaxial growth on (111) substrates.
Abstract: Nitrogen-vacancy (NV) centers in diamond have attracted significant interest because of their excellent spin and optical characteristics for quantum information and metrology. To exploit these characteristics, precise control of the orientation of the NV axis in the lattice is essential. Here we show that the orientation of more than 99% of the NV centers can be aligned along the [111] axis by chemical vapor deposition homoepitaxial growth on (111) substrates. We also discuss the alignment mechanisms. Our result enables a fourfold improvement in the magnetic field sensitivity and opens new avenues to the optimum design of NV center devices.
TL;DR: In this article, the authors show that the giant spin Hall effect (GSHE) magnetoresistive random access memory (MRAM) can enable better energy delay and voltage performance than MTJ spin torque devices at 10?30 nm scaled nanomagnet dimensions.
Abstract: We show that the giant spin Hall effect (GSHE) magnetoresistive random access memory (MRAM) can enable better energy delay and voltage performance than MTJ spin torque devices at 10?30 nm scaled nanomagnet dimensions. We propose a dense bit cell composed of a folded electrode to enable scaling to sub-10 nm CMOS. We derive the energy-delay trajectory and energy-delay product of GSHE and MTJ devices with an energy minimum at the magnetic characteristic time. Optimized GSHE devices with PMA can enable low voltage (<0.1 V), scaled dimensions, and fast switching time (100 ps) at an average switching energy approaching 100 aJ/bit.
TL;DR: In this article, a 10-layer stack-type multilayer actuator with a normalized strain Smax/Emax of 720 pm/V at 5 kV/mm was constructed.
Abstract: Lead-free 0.96[Bi1/2(Na0.84K0.16)1/2(Ti(1−x)Nbx)O3]–0.04SrTiO3 (BNKTN–ST, with x = 0–0.030) ceramics were synthesized by a conventional solid-state reaction technique. Polarization and strain hysteresis loops indicated a significant disruption of ferroelectric order accompanied by an enhanced field-induced strain (S = 0.438%) with a high normalized strain Smax/Emax of 876 pm/V at 5 kV/mm. Their reproducibility was confirmed by the fabrication of a 10-layered stack-type multilayer actuator, which demonstrated a normalized strain Smax/Emax of 720 pm/V.
TL;DR: In this article, a TiN/HfO2/ITO memory device is fabricated, which shows stable bipolar resistive switching behavior, as well as excellent data retention and good endurance.
Abstract: In this work, a TiN/HfO2/ITO memory device is fabricated, which shows stable bipolar resistive switching behavior, as well as excellent data retention and good endurance. Moreover, a very low SET voltage of 0.2 V is achieved with a self-compliance current effect. The result brings about an obvious reduction in SET power to 160 µW, which is crucial for future high-density resistive switching memories. On the basis of the conducting filament theory, a possible resistive mechanism is discussed to explain the low SET voltage and self-compliance current phenomenon.
TL;DR: In this article, the authors investigated the microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer and obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW.
Abstract: Microwave oscillation properties of spin torque vortex oscillators (STVOs) consisting of an FeB vortex free layer were investigated. Because of a high MR ratio and large DC current, a high emission power up to 3.6 µW was attained in the STVO with a thin FeB free layer of 3 nm. In STOs with a thicker FeB layer, e.g., 10 nm thick, we obtained a large Q factor greater than 6400 while maintaining a large integrated emission power of 1.4 µW. Such excellent microwave performance is a breakthrough for the mutual phase locking of STVOs by electrical coupling.
TL;DR: In this article, the performance potentials of silicene nanoribbon (SiNR), germanene nanorsoribbons (GeNR), and graphene nanoribrbons (GNR) as field effect transistor (FET) channel materials were assessed.
Abstract: Silicene or germanene is a monolayer honeycomb lattice made of Si or Ge, similar to graphene made of C. In this work, we have assessed the performance potentials of silicene nanoribbon (SiNR), germanene nanoribbon (GeNR), and graphene nanoribbon (GNR), which all have a sufficient band gap to switch off, as field-effect transistor (FET) channel materials. We have demonstrated that, by comparing at the same band gap of ∼0.5 eV, the GNR FET maintains an advantage over SiNR or GeNR FETs under an ideal transport situation, but SiNR and GeNR are attractive channel materials for high-performance FETs as well.
TL;DR: In this article, an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 ± 5 to 225 ± 130 µW/(m·K2) was reported.
Abstract: We report an increase in the thermoelectric power factor of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) from 23 ± 5 to 225 ± 130 µW/(m·K2) in high-humidity conditions. This enhancement was caused mainly by an increase in the apparent Seebeck coefficient, which could be related to morphological change after water absorption or electrochemical reaction of PEDOT in air. Our results demonstrate a positive effect of water in the PEDOT:PSS system and indicate the need for well-controlled measurement conditions, particularly humidity, in evaluating the performance of conducting organic materials.
TL;DR: In this paper, recessed-gate Al2O3/AlGaN/GaN normally-off metaloxide-semiconductor high-electron-mobility transistors (MOS-HEMTs) on 8 in. Si.
Abstract: We report recessed-gate Al2O3/AlGaN/GaN normally-OFF metal–oxide–semiconductor high-electron-mobility transistors (MOS-HEMTs) on 8 in. Si. The MOS-HEMTs showed a maximum drain current of 300 mA/mm with a high threshold voltage of +2.4 V. The quite low subthreshold leakage current (~10−8 mA/mm) yielded an excellent ON/OFF current ratio (9 × 108) with a small, stable subthreshold slope of 74 mV/dec. An atomic-layer-deposited Al2O3 layer effectively passivates, as no significant drain current dispersions were observed. A high OFF-state breakdown voltage of 825 V was achieved for a device with a gate-to-drain distance of 20 µm at a gate bias of 0 V.
TL;DR: In this article, a yellow persistent phosphor of Ce3+−Cr3+codoped Gd3Al2Ga3O12 transparent ceramics was developed by a solid-state reaction.
Abstract: We have developed a yellow persistent phosphor of Ce3+–Cr3+-codoped Gd3Al2Ga3O12 transparent ceramics prepared by a solid-state reaction. The yellow persistent luminescence due to the Ce3+: 5d–4f transition was observed even after 460 nm blue-light excitation as well as after UV excitation. The chromaticity coordinate of the persistent luminescence in the ceramic phosphor is located at , which appears really yellow compared with the color coordinate of the well-known SrAl2O4:Eu2+–Dy3+ or other conventional blue or green persistent phosphors. The luminance values for the transparent ceramic sample 1, 5, and 30 min after the blue excitation was ceased are 731, 63, and 8 mcd/m2, respectively.
TL;DR: In this paper, a stackable complementary metaloxide-semiconductor field effect transistor (CMOSFET) on interlayer dielectrics for three-dimensional (3D) large-scale integrated devices was investigated.
Abstract: To realize a stackable complementary metal–oxide–semiconductor field-effect transistor (CMOSFET) on interlayer dielectrics for three-dimensional (3D) large-scale-integration devices, we investigated poly-Ge thin films formed by flash lamp annealing. The process resulted in crystalline grains of micrometer-order size, and the Hall-effect mobility of holes was as high as 200 cm2 V−1 s−1. A depletion-type trigate poly-Ge channel pMOSFET with a gate length of 80 nm formed on a poly-Ge film exhibited a drive current of 280 µA/µm at a drain voltage of −1 V and a gate overdrive of −1 V. The operation of inversion-type short-channel trigate poly-Ge nMOSFETs was also demonstrated.
TL;DR: In this paper, the authors presented experimental and modeling results for extending the 3-dB modulation bandwidth of a 980-nm quasi-singlemode (QSM) vertical-cavity surface-emitting laser (VCSEL) with a passive transverse-coupled cavity (TCC).
Abstract: This paper presents experimental and modeling results for extending the 3-dB modulation bandwidth of
a 980-nm quasi-single-mode (QSM) vertical-cavity surface-emitting laser (VCSEL) with a passive
transverse-coupled cavity (TCC). While the bandwidth of a conventional VCSEL is 9 GHz, the 3-dB
modulation bandwidth of a QSM TCC VCSEL with the same epi-wafer structure can reach 27 GHz,
which is three times larger than the conventional VCSEL without optical feedback. A clear eye opening
is obtained for large-signal modulations at 36 Gbps. A numerical simulation for further enhancement of
the bandwidth is also conducted
TL;DR: In this article, the authors performed spectroscopic observations of the 698-nm clock transition in 87Sr confined in an optical lattice using a laser linewidth transfer technique.
Abstract: We performed spectroscopic observations of the 698-nm clock transition in 87Sr confined in an optical lattice using a laser linewidth transfer technique. A narrow-linewidth laser interrogating the clock transition was prepared by transferring the linewidth of a master laser (1064 nm) to that of a slave laser (698 nm) with a high-speed controllable fiber-based frequency comb. The Fourier-limited spectrum was then observed for an 80-ms interrogating pulse. We determined that the absolute frequency of the 5s2 1S0–5s5p 3P0 clock transition in 87Sr is 429 228 004 229 872.0 (1.6) Hz referenced to the SI second.
TL;DR: In this paper, a high-energy noiselike rectangular pulse was generated in a mode-locked figure-eight fiber laser, which appeared to have a rectangular shape on the oscilloscope.
Abstract: We report on the generation of a high-energy noiselike rectangular pulse in a mode-locked figure-eight fiber laser. The noiselike pulse appeared to have a rectangular shape on the oscilloscope. The pulse duration increased with increasing pump power, while the peak amplitude remained constant, which is very similar to the pulse evolution of dissipative soliton resonance. However, the pulse type is confirmed as a noiselike pulse using an autocorrelator. With the maximum pump power of 350 mW, the 135 nJ noiselike rectangular pulse with 76 ns duration was achieved. The results provide a new guideline for clarifying an alternative formation mechanism of the high-energy rectangular pulses in fiber lasers.
TL;DR: In this article, the authors proposed the coalescence growth of GaN by the Na-flux method and demonstrated the possibility of enlarging the diameter of high-quality GaN crystals.
Abstract: Low-curvature and large-diameter GaN wafers are in high demand for the development of GaN-based electronic devices. Recently, we have proposed the coalescence growth of GaN by the Na-flux method and demonstrated the possibility of enlarging the diameter of high-quality GaN crystals. In the present study, 2 in. GaN wafers with a radius of curvature larger than 100 m were successfully produced by the Na-flux coalescence growth technique. FWHMs of the 002 and 102 GaN X-ray rocking curves were below 30.6 arcsec, and the dislocation density was less than the order of 102 cm−2 for the entire area of the wafer.
TL;DR: The feasibility of a highly efficient and air-stable organic light-emitting diode (OLED) was examined in this paper, where a phosphorescent OLED not containing an air-sensitive material was fabricated by employing an inverted structure with an airstable electron injection layer.
Abstract: 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.
TL;DR: In this paper, a planar plasmonic filter consisting of two graphene ribbons coupled by a graphene ring is constructed on monolayer graphene and investigated using the finite-difference time-domain method.
Abstract: A planar plasmonic filter consisting of two graphene ribbons coupled by a graphene ring is constructed on monolayer graphene and investigated using the finite-difference time-domain method. The simulation results reveal that the edge modes indeed enhance the electromagnetic coupling between objects. The structure exhibits perfect band-pass filtering effect tuning and is optimized by means of the gate voltage. The simulation results are confirmed by the resonance theory of the ring. This structure is a real electrically controlled filter operating in the mid-infrared region. Our studies support the fabrication of ultracompact planar devices for optical processing.
TL;DR: In this article, the authors investigated microwave oscillation in a sombrero-shaped nano-contact spin-torque oscillator under an external magnetic field with an out-of-plane component.
Abstract: We investigated microwave oscillation in a sombrero-shaped nano-contact spin-torque oscillator under an external magnetic field with an out-of-plane component. By increasing the out-of-plane field, we observed an abrupt change from in-plane precession to out-of-plane precession around the effective demagnetization field of the free layer. The out-of-plane precession under optimized conditions yielded not only a fundamental oscillation peak with a very large Q factor (3200) but also higher-order harmonic peaks with frequencies up to at least 45 GHz. A high Q factor and high-frequency oscillations are of great importance for device applications such as telecommunications, radar sensors, and high-speed magnetic field sensors.
TL;DR: In this article, the authors demonstrate a high-voltage low on-resistance AlN/GaN/AlGaN double heterostructure grown by metal-organic chemical vapor deposition on a silicon (111) substrate using a total buffer thickness of less than 2 µm.
Abstract: We demonstrate a high-voltage low on-resistance AlN/GaN/AlGaN double heterostructure grown by metal–organic chemical vapor deposition on a silicon (111) substrate using a total buffer thickness of less than 2 µm. A fully scalable local substrate removal technique was developed to dramatically enhance the off-state breakdown voltage of the transistors. The three-terminal breakdown voltage of these devices using a gate–drain distance of 15 µm increased significantly, from 750 V to 1.9 kV, after local substrate removal. The high two-dimensional electron gas carrier density (2.3 × 1013 cm−2) associated with the low sheet resistance enables a record combination of a specific on-resistance (1.6 mΩ cm2) and high breakdown voltage for GaN-on-Si transistors.
TL;DR: In this paper, the role of defects in reducing the internal quantum efficiency (IQE) of InxGa1−xN/GaN multiple quantum wells (MQWs) as the emission wavelength increased from approximately 450 to 530 nm was investigated.
Abstract: Deep-level optical spectroscopy (DLOS) and photoluminescence (PL) were used to understand the role of defects in reducing the internal quantum efficiency (IQE) of InxGa1−xN/GaN multiple quantum wells (MQWs) as the emission wavelength increased from approximately 450 to 530 nm, i.e., the "green gap". DLOS studies of light emitting diodes (LEDs) identified QW defects whose concentration increased significantly with increasing x. The effect of increased QW defect density on IQE was assessed by examining the PL of MQW samples. Green-emitting MQWs had lower IQE and required higher pump power to reach peak IQE, corroborating the important impact of enhanced non-radiative recombination at defects.