The 50 most-cited articles in Japanese Journal of Applied Physics, 2019

Abstract Gallium nitride (GaN) is an attractive material for high-frequency and high-power devices. Due to the availability of relatively high-quality free-standing bulk GaN substrates, the research and development of vertical GaN devices on GaN substrates has made significant progress in recent years, and various transistors and diodes based on vertical GaN with excellent characteristics have been reported. This paper reviews the current status and recent progress of vertical GaN power device development reported from companies and research institutions, which includes the technological development of our recent research results of Schottky barrier diodes and trench MOSFETs. Key remaining issues for practical applications are also described.

Abstract The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of “new” nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III–V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductor NbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by nitride semiconductors.

Abstract We present a vertical GaN planar metal-oxide-semiconductor field-effect transistor (MOSFET) fabricated by an all ion implantation process. The fabricated MOSFET shows an on-resistance of 2.78 mΩ cm 2 and a breakdown voltage of 1200 V, by applying the short cell pitch design to reduce the on-resistance and a Mg and N sequential implantation to improve the breakdown voltage of the pn-junction. By evaluating each on-resistance component in the fabricated vertical GaN planar MOSFET using the simultaneously formed test structures, an effective on-resistance of the active region excluding the source parasitic resistance is 1.4 mΩ cm 2 . Consequently, it was demonstrated that an all ion implantation process can fabricate a vertical GaN planar MOSFET with a high breakdown voltage and low on-resistance. This result will greatly contribute to the realization of GaN power devices.

Abstract The status and challenges in the development of GaN power devices are reviewed. At present, normally-off gate injection transistors (GITs) on Si are commercially available. The updated structure known as a hybrid-drain-embedded GIT provides superior reliability that contributes to the stable operation of compact power switching systems with high efficiency. The fabricated vertical GaN transistor on GaN as a future challenge demonstrates extremely low specific on-state resistance and high breakdown voltage. Metal-insulator-semiconductor-gate GaN transistor is also a technical challenge for faster switching, since it would give greater freedom of gate driving as a result of both high threshold voltage and widened gate voltage swing. Normally-off operation free from hysteresis in the current–voltage characteristics is confirmed in a recessed-gate AlGaN/GaN heterojunction field effect transistor using AlON as a gate insulator. Fast switching characteristics are experimentally confirmed for both of the newly developed GaN devices, indicating their great potential for practical…

Abstract Spontaneous orientation polarization (SOP) is inherent in the evaporated films of many organic semiconducting molecules with a permanent dipole moment. A significant electric field is formed in the film due to SOP. Consequently, the properties of organic light-emitting diodes (OLEDs) incorporating such films are influenced. The polarization charge appearing at heterointerfaces dominates the charge injection and accumulation properties. Moreover, SOP correlates with device degradation. In this article, we review the SOP of organic semiconductor films and its influences on the device properties of OLEDs.

Abstract The influence of aluminum mole fraction of Al x Ga 1- x N/Al y Ga 1- y N multiple quantum wells (MQWs) on the optical polarization, light extraction efficiency (LEE) and external quantum efficiency (EQE) of deep ultra violet light emitting diodes in the wavelength range between 264 and 220 nm is investigated. The on-wafer EQE decreases from 0.6% to 0.00013% in this wavelength range. Polarization resolved photoluminescence and electroluminescence measurements show a change from dominant transverse-electric to dominant transverse-magnetic polarized light emission with increasing aluminum mole fraction in the MQW. The quantitative agreement with k · p calculations allow to ascribe this shift to a change of the characteristic of the Γ 7+ valance band. Ray tracing simulations predict a reduction of the on-wafer LEE from 4% to 1.5%. Therefore the dramatic drop of the EQE in this wavelength range cannot be attributed to a drop in LEE and is most likely dominated by charge carrier injection and radiative recombination efficiency.

Abstract In this review, the history of research and development of the Na-flux method for growing single GaN crystals is summarized from its discovery in 1994 until the present. Underlying the development of the Na-flux method, which has become one of the more important technologies for growing high quality GaN crystals, there have been several important innovations without which it would have been impossible to achieve current technical levels. Here, we describe the development of the Na-flux method through these innovations, including a method for controlling nucleation by adding carbon, single- and multipoint seed techniques, and a hybrid of the flux-film coated and multipoint seed approaches.

Abstract The fascinating point of 2D and layered materials is that they can be assembled into van der Waals (vdW) heterostructures, in which atomic layers are integrated by vdW force. There are almost infinite potential combinations in vdW heterostructures owing to the multiple degrees of freedom, i.e., the choice of materials, stacking order, and lateral orientation angle at the interfaces. In this article, we review the fabrication technique of vdW heterostructures, which has played an essential role in the development of the 2D materials research field. First, we describe the primary technique of mechanical exfoliation to fabricate and identify high-quality atomic layers. We then discuss the assembly of atomic layers into vdW heterostructures. Finally, we introduce the recent advancement of fabrication techniques using autonomous robotic assembly. We hope this article would help the readers to acquire basic knowledge of vdW assembly and motivate them to fabricate vdW heterostructures.

Abstract Different causes for ferroelectric properties in hafnium oxide were discussed during the last decade including various dopants, stress, electrode materials, and surface energy from different grain sizes. Recently, the focus shifted to the impact of oxygen vacancies on the phase formation process. In this progress report, the recent understanding of the influence of oxygen supplied during deposition on the structural phase formation process is reviewed and supplemented with new data for mixed Hf x Zr 1- x O y films. Even though polar and non-polar Hf x Zr 1- x O y thin films are well characterized, little is known about the impact of oxygen exposure during the deposition process. Here, a combination of structural and electrical characterization is applied to investigate the influence of the oxygen and zirconium content on the crystallization process during ALD deposition in comparison to other deposition techniques. Different polarization properties are assessed which correlate to the determined phase of the film. Optimized oxygen pulse times can enable the crystallization…

Abstract Due to its outstanding polarization properties and the possibility of lattice-matched growth on GaN, Sc x Al 1-x N is a promising material among group III nitrides providing a wide field of potential applications in modern semiconductor technology. However, epitaxial growth of Sc x Al 1-x N by MBE is still in an early stage of research. In this work, Sc x Al 1-x N samples were grown by plasma-assisted MBE on GaN-on-sapphire templates under a variety of growth conditions and pulsed supply of Sc and Al, resulting in compositions ranging from Sc 0.02 Al 0.98 N to Sc 0.69 Al 0.31 N. Samples grown in the highly metal-rich regime showed phase degradation and high surface roughness, whereas growth in the N-rich and intermediate regime led to phase purity and surface roughness as low as 0.7 nm. Electrical characterization revealed a 2DEG for Sc 0.2 Al 0.8 N with a sheet resistance of 215 Ω/□, a Hall mobility of 553 cm 2 V −1 ·s −1 , and a sheet carrier density of 5.26 × 10 13 cm −2 at 77 K.

Abstract A new principle for the inductor is proposed theoretically in terms of spiral magnets. In sharp contrast to the conventional inductor made by the solenoid with ferromagnetic core, the bulk sample itself acts as an inductor without any composite structure. Furthermore, the inductance L in the present inductor is inversely proportional to the cross section A of the system, which enables to reduce the size of the device.

Abstract Organic electronics such as organic field-effect transistors (OFET), organic light-emitting diodes (OLED), and organic photovoltaics (OPV) have flourished over the last three decades, largely due to the development of new conjugated materials. Their designs have evolved through incremental modification and stepwise inspiration by researchers; however, a complete survey of the large molecular space is experimentally intractable. Machine learning (ML), based on the rapidly growing field of artificial intelligence technology, offers high throughput material exploration that is more efficient than high-cost quantum chemical calculations. This review describes the present status and perspective of ML-based development (materials informatics) of organic electronics. Although the complexity of OFET, OLED, and OPV makes revealing their structure-property relationships difficult, a cooperative approach incorporating virtual ML, human consideration, and fast experimental screening may help to navigate growth and development in the organic electronics field.

Abstract We have investigated the effects of alternate current poling (ACP) for four different plates of [100]-oriented (100 – x )Pb(Mg 1/3 Nb 2/3 )O 3 –x PbTiO 3 (PMN– x PT, x = 28, 29, 30, 31) single crystals (SCs) obtained from one ingot. A record high piezoelectric coefficient d 33 of 3000 pC N –1 for PMN–30PT SCs, and a high free dielectric permittivity of 8900 for PMN–28PT SCs by ACP, 27% higher than those of conventional direct current poling (DCP), were confirmed. X-ray analysis revealed a higher (004) peak intensity and angle of ACP SCs compared to those of DCP. We assume that ACP creates a monoclinic multiphase (M B and M C ) and eliminates some portion of the tetragonal phase in SCs. In addition, (004) peak shifts were different between the different PT composition SC samples. This work demonstrates an ACP process with excellent cost performance can offer promising prospects and practical value for PMN– x PT SC piezoelectric transducers.

Abstract GaN power ICs provide an elegant solution for high-frequency power switching applications. This paper will first discuss the GaN power integration platform, which requires not only a high-voltage power switch, but also peripheral low-voltage transistors, diodes, resistors, capacitors, etc. The low-voltage components are preferably fabricated using the same process steps of the power switch to be cost-effective. As an example of a GaN power IC, an integrated gate driving circuit is demonstrated. By adopting a charge pump unit, the novel gate driver in this work enables rail-to-rail output voltage, fast switching speed and enhanced reliability. For the design of GaN power ICs, the unique device physics of GaN power devices should be carefully considered. The V th of the GaN power transistor is found to be bias-dependent and to exhibit dynamic behavior during power switching operation. The mechanism of this dynamic V th will be explained and its impact on circuit operation will be illustrated.

Abstract The dielectric, piezoelectric and physical properties of the [001]-oriented rectangular plate and beam mode of Pb[(Mg 1/3 Nb 2/3 ) 0.52 (Yb 1/2 Nb 1/2 ) 0.15 Ti 0.33 ]O 3 (PMN-PYbN-PT) ternary single crystals (SC) poled by alternate current poling (ACP) were investigated. The dielectric permittivity ε 33 T / ε 0 and average piezoelectric coefficient d 33 were 6800 and 2490 pC/N, respectively, for the rectangular plate sample after ACP, which were 31% and 41% larger than those of the same sample after conventional direct current poling. The highest value of d 33 reached 2750 pC N −1 . The dielectric loss was reduced by 31%; nevertheless, the d 33 improved after ACP. The beam mode electromechanical coupling factor ( k 33 ′ ) was 91% by using ACP, showing the highest value among all relaxor-PT SC systems reported so far. The regular periodic stripe domains were observed in ACP samples, showing high domain wall density and uniform domain size. This work demonstrates that the high performance after ACP is an effective method of improving the relaxor-PT SC transducers.

Abstract Here, we investigate the dislocations in β -Ga 2 O 3 single crystals grown by edge-defined film-fed growth (EFG) and halide vapor-phase epitaxy (HVPE) using synchrotron X-ray topography. The (001)- and ( 2 ¯ 01 )-oriented crystals grown in the [010] direction by EFG exhibited dislocations along the 〈010〉 direction with some dislocations oriented in a line; in addition, wandering dislocations were observed on the (001) surface. Based on the invisibility criterion, the Burgers vector of some dislocations was determined to be 〈010〉. On the other hand, in the (001) film grown by HVPE over the EFG substrate, threading dislocations propagating in the [001] direction were observed. Furthermore, it was found that the dislocations on the substrate grown by EFG were inherited by the film formed by HVPE: a dislocation was generated in the film grown by HVPE at both ends of the void defects in the substrate grown by EFG.

Abstract A vertical p–n diode with a simple edge termination structure on a GaN free-standing substrate is demonstrated. The edge of this device is terminated simply by etching a drift layer deeply and vertically. A device simulation revealed that the electric field at the device edge was more relaxed and uniformly applied by etching the mesa deeper than the depletion region. The fabricated device showed low leakage current and avalanche capability, and its breakdown characteristics could be reproduced many times. By emission microscopy observation, we found that there was no leakage current at the side wall of the device and that avalanche breakdown occurred throughout the inside of the device. This indicates that the electric field crowding at the side wall of the device was completely suppressed and a uniform electric field distribution was obtained by this structure.

Abstract GaN-based high-electron-mobility transistors (HEMTs) fabricated on SiC substrates were successfully bonded to single-crystal diamond heat spreader by a surface activated bonding method with a thin titanium (Ti) cover layer. A thin Ti layer on the diamond improved the bonding strength of the SiC/diamond interface since the formation of an amorphous layer on the diamond surface was suppressed. This SiC/diamond bonding process was applied to high-output power InAlGaN/GaN HEMTs on diamond heat spreaders. The structure with the diamond successfully reduced the thermal resistance of the devices and enabled their high-power operation.

Abstract The feasibility of reservoir computing based on dipole-coupled nanomagnets is demonstrated using micro-magnetic simulations at 0 K. The static-magnetization directions of the nanomagnets are used as reservoir states. To update these states, we change the magnetization of one nanomagnet according to a single-bit-sequential input signal. We also change the uniaxial-anisotropy of the other nanomagnets using a voltage-induced magnetic anisotropy change to facilitate the flow of information, storage, and linear/nonlinear calculations. As a result, the reservoir computing output matrix was found to be trainable to perform AND/OR/XOR operations with an input delay of up to three bits.

Abstract We investigated dry and wet etchings of β -Ga 2 O 3 and fabricated vertical Schottky barrier diodes (SBDs) with mesa termination using the optimal etching condition. Using an inductively-coupled plasma reactive-ion etching with a nickel-hard mask, a β -Ga 2 O 3 (010) mesa structure with a smooth sidewall is obtained at an etching rate of 77 nm min −1 in BCl 3 /Cl 2 mixture gas. By immersing β -Ga 2 O 3 (001) vertical SBDs with mesa termination in hot phosphoric-acid solution, the specific on resistance and ideality factor of the SBDs are reduced to 0.91 mΩcm 2 and 1.03, respectively. Current density at reverse bias is in good agreement with thermionic field emission model.

Abstract In order to avoid sudden catastrophic hard breakdown in high breakdown voltage vertical GaN p–n junction diodes, punch-through induced breakdown structures have been newly considered. Mg acceptor concentration in the p-GaN layer was reduced so that the punch-through breakdown occurred before the hard breakdown. By using a wafer with triple drift layers and the p-GaN layer with lowered Mg concentration of 3 × 10 17 cm −3 grown on a freestanding n-GaN substrate, the diode showed a high breakdown voltage of 4.9 kV with high reverse avalanche capabilities against sudden increase of reverse current over 5 orders of magnitudes. No degradation was observed after fifteen repetitive measurements.

Abstract The process of forming the ferroelectric orthorhombic phase was investigated for epitaxial 7% Y-doped (YHO7) films using in situ high-temperature X-ray diffraction. Epitaxial YHO7 films were grown on (111) ITO-coated (111)YSZ substrates by pulsed laser deposition at room temperature and a subsequent heat treatment process. Films deposited at room temperature were crystallized as paraelectric monoclinic phase. The monoclinic phase partially changes to tetragonal phase above 600 °C and perfectly transformed around 950 °C during heating. The change from tetragonal phase to orthorhombic phase was detected at 300 °C, corresponding to the Curie temperature under the cooling process. These results clearly suggest that the tetragonal phase was more stable at 1000 °C for YHO7 films on heating than the other phases, and the formation of this tetragonal phase—the high-temperature paraelectric phase of the ferroelectric orthorhombic phase—is key to the formation of the ferroelectric orthorhombic phase.

Abstract This review introduces the latest progress on gallium nitride (GaN)-based visible vertical-cavity surface-emitting lasers (VCSELs) with features such as plane and curved distributed Bragg reflectors (DBRs) made of dielectric materials and boron ion implantation to form current apertures. This novel class of GaN-based VCSELs allow small apertures down to 3 μm and long cavities of more than 20 μm without the occurrence of diffraction loss. These structures have enabled low threshold currents (e.g., I th = 0.25 mA), high efficiency operation (e.g., WPE = 9.5%), and robust fabrication processes with high lasing yield. All those characteristics are the best among those previously reported GaN-based VCSELs. The proposed structure should facilitate the production of VCSELs formed on semi-polar plane GaN substrates and arrayed VCSELs, which are expected to realize novel light emitters that have been previously difficult to fabricate, such as green VCSELs and watt-class blue VCSEL arrays.

Abstract Metal-oxide semiconductors are widely used for thin-film transistors for flat-panel displays, but they do not seem to be limited to the conventional applications. In this review, emerging applications using metal-oxide semiconductor thin-film devices are reviewed, which may be novel killer applications in the future. First, the essential advantages are enumerated, and, next, based on the essential advantages, several proposals of emerging applications from many organizations are introduced. First, power devices using metal-oxide semiconductor thin-film devices are proposed, because they can be tolerant against high voltage, and the carrier mobility is high. Second, sensing devices are proposed, because the electrical characteristic can be sensitive to peripheral environment, and the thin-film devices can be fabricated on large areas with low cost. Third, thermoelectric devices are proposed, because the Seebeck coefficients can be improved, and the cost performance is the most important. Fourth, computing applications, including many kinds of related applications, are…

Abstract We investigate the current-dependent and temperature-dependent efficiency droops (“J-droop” and “T-droop”, respectively) in InGaN-based blue and AlGaInP-based red light-emitting diodes (LEDs). It is found that the blue and red LEDs show different droop behaviors with increasing current density and temperature. The J-droop is significant in the blue LED while the T-droop is severe in the red LED. In case of the blue LED, the carrier accumulation caused by the saturation of the radiative recombination rate is thought to increase the quasi-Fermi level rapidly, thus causing the J-droop. On the other hand, the T-droop of the red LED is influenced by redistribution of carriers due to the increased thermal energy with a small barrier height in the AlGaInP material system. The comparison of different droop characteristics of blue and red LEDs helps understand the recombination mechanisms of both LEDs and provides useful insight for improving the device performance further.

Abstract Multilayer graphene was synthesized by overlayer growth of graphene on a monolayer graphene template using a chemical vapor deposition method under a high process temperature of 1300 °C. Structural analysis using Raman spectra revealed that the synthesized multilayer graphene forms highly crystalline graphene layers with a turbostratic stacking structure. Atomic force microscope images indicated that the step edges of the grown graphene layer proceed via lateral growth mode. The electrical transport properties of the synthesized multilayer graphene showed higher conductivity and carrier mobility than those of the monolayer graphene template. The improvement of the electrical transport properties is caused by the turbostratic stacking structure that has the electronic band dispersion similar to that of monolayer graphene. This result means that the synthesis of graphene layers grown on the graphene template is effective to improve the carrier transport properties in multilayer graphene sheets.

Abstract Hf 0.5 Zr 0.5 O 2 thin films are not always ferroelectric. This work investigates the impact of annealing temperature and time on the crystalline structures and dielectric properties of 10 nm thick Hf 0.5 Zr 0.5 O 2 thin films. It is found that the tetragonal phase crystal is formed from the amorphous film firstly, then transforms to the orthorhombic and monoclinic phases, in accordance with the annealing temperature and time. The volume fraction of the orthorhombic phase in the film, which is known as the origin of ferroelectricity, becomes dominant in a certain range of the annealing condition. Thus, the annealing temperature and time are responsible for the anti-ferroelectric, ferroelectric, and paraelectric characteristics of Hf 0.5 Zr 0.5 O 2 thin films. This phase transformation behavior is discussed from the viewpoint of formation energies of the respective crystal phases, which accompanies the changes of unit cell volumes. The competition of transformation rates between the tetragonal to orthorhombic and the orthorhombic to monoclinic is key for the formation of…

Abstract This study aimed to estimate the dependence of the effects of radish ( Raphanus sativus ) seed irradiation with atmospheric cold plasma (CP) on physiological seed condition or dormancy status. Seeds were treated for different durations with a scalable dielectric barrier discharge (DBD) device. The relationship between the effect of CP on germination kinetics with changes in the content of phytohormones in seeds stored for different periods of time after harvesting (1, 5, and 7 months) was estimated. CP treatment efficiently stimulated germination only in freshly harvested (not after-ripened seeds), and these changes correlated with a decrease in abscisic acid (ABA) and an increase in gibberellin (GA) content. CP treatment induced a fast decrease in ABA content and ABA/GA ratio in after-ripened seeds (5 months after harvesting) but did not stimulate germination. Primary evidence of direct ABA modification by the reactive compounds from the gaseous phase of CP was obtained.

Abstract In this paper, we investigated the dependence of threshold power density on the Al 0.55 Ga 0.45 N underlying layer film thickness in ultraviolet-B band (UV-B) lasers on various AlN wafers (four types). We also prepared and compared AlN templates for AlN freestanding substrates, AlN films fabricated by metalorganic vapor phase epitaxy, and annealed sputtered AlN templates at high temperature. The initial growth of AlGaN became three-dimensional by inserting a homoepitaxial Ga-doped AlN layer between the AlN template and Al 0.55 Ga 0.45 N, before it shifted to two-dimensional growth. It is possible to reduce the dislocation in Al 0.55 Ga 0.45 N using this mode. The dependence of AlGaN film thickness and that of the AlN template on samples with an inserted homoepitaxial Ga-doped AlN layer were studied. Compared with Al 0.55 Ga 0.45 N having a thickness of 5 μ m, there was almost no noticeable difference between the dark spot density characterized by cathodoluminescence and the threshold power density in UV-B lasers for the AlN template. Besides, the characteristics were…

Abstract Sources of carrier compensation in n-type and p-type GaN layers grown by metalorganic vapor phase epitaxy were quantitatively identified by a combination of Hall-effect analysis and deep level transient spectroscopy. For n-type GaN, we identified three electron compensation sources: residual carbon atoms likely sitting on nitrogen sites (C N ), an electron trap at the energy level of E C –0.6 eV (the E3 trap), and self-compensation appearing with increasing donor concentration. We showed that the C N also play a key role in hole compensation in p-type GaN by forming donor-like charged states. We also investigated the reduction of acceptor concentrations ( N a ) in highly Mg-doped GaN. Atomic-resolution scanning transmission electron microscopy revealed that electrically inactive Mg atoms of 3/2 atomic layers are segregated at the boundary of pyramidal inversion domains. The N a reduction can be explained by this Mg segregation.

Abstract Power integration is essential for the fully utilization of advanced GaN devices in power conversion applications due to the reduced parasitic inductance, low on-state resistance, and high-temperature operation. This paper presents a GaN-based monolithic integration design with optimized gate drivers for high-temperature DC–DC converters. Four different gate drivers are experimentally evaluated for integration with boost converters based on enhancement (E)-mode AlGaN/GaN metal–insulator–semiconductor heterojunction-field-effect-transistors (MIS-HFETs). The optimized gate driver, consisting of DCFL (Direct-Coupled FET Logic) inverters and a buffer amplifier, can operate over a wide temperature range (from 25 °C to 250 °C). Furthermore, a 100 kHz, 5 V/11 V ( V IN / V OUT ) boost converter prototype with the proposed monolithic integration design was built and found to operate successfully under high temperatures (HTs) up to 250 °C. These results validate the advantages of GaN-based monolithic integration techniques in achieving HT, high power density, and high efficiency…

Abstract We propose a method to evaluate the carrier transport properties in the inversion layer of 4H–SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) experimentally. Our approach differs from conventional methods, which have adjusted the parameters in conventional mobility models. Intrinsic phonon-limited mobility ( μ phonon ) in the SiC MOSFET was observed by suppressing the severe impact of Coulomb scattering on the SiC MOS inversion layer by lowering the acceptor concentration ( N A ) of the p-type well region to the order of 10 14 cm −3 . In this study, we investigated the carrier transport properties in the inversion layer of Si-face 4H–SiC MOSFETs with nitrided oxide. It is revealed that the μ phonon of the SiC MOSFET is a quarter or less than the conventionally presumed values. Additionally, surface roughness scattering is found not to be the most dominant mobility-limiting factor even at high effective normal field ( E eff ) for the SiC MOSFET. These results demonstrate that conventional understanding of carrier scattering in the SiC MOS inversion layer…

Abstract We synthesized Cr 3+ , Cr 4+ -codoped Mg 2 SiO 4 phosphors and studied their optical properties for application as a novel wideband near-infrared (NIR) light-source. The phosphors have an ultra-wideband NIR luminescence spectrum of 650–1400 nm. The origin should be the 4 T 2 → 4 A 2 transition of Cr 3+ and the 3 T 2 → 3 A 2 transition of Cr 4+ . The luminescence spectrum spans over one octave range and covers the “optical window in biological tissues”. We successfully achieved an ultra-wideband luminescence of 650–1400 nm with a maximum output power of 367.5 μ W by combining a blue LED with Cr 3+ and Cr 4+ -codoped Mg 2 SiO 4 phosphors in a single package.

Abstract Mechanisms, materials, and processes of high-resolution printing techniques dedicated to printed electronics are reviewed. Advanced printing methods, including reverse offset printing and adhesion contrast planography, use absorption of ink solvents by polydimethylsiloxane (PDMS) to semi-solidify inks before ink-transfer. The patterning principle transforms from wetting to adhesion and fracture; resolution problems encountered during the patterning of liquid inks (e.g. spreading, splitting, coalescence, bulges, and coffee ring shapes) can be avoided, and single- μ m features can be achieved. After summarizing the fundamentals of reverse offset printing, details on the patterning mechanism and contact mechanics encountered during the process are shown together with the design concept of the ink formulation. Complementary processes for multilayered electronic devices (e.g. wet-on-wet for high-throughput production, buried electrode formation, taper formation, and vertical interconnections) and the prediction of pattern size integrity are presented.

Abstract We fabricated p−n diodes under different growth pressures on free-standing GaN substrates of the same quality and observed a noteworthy difference in the reverse leakage current. A large reverse leakage current was generated by nanopipes, which were formed from screw dislocations in the homoepitaxial layer. There were two types of screw dislocation observed in this study. The first type already existed in the substrate and the other was newly generated in the epilayer by the coalescence of edge and mixed dislocations. An increase in the growth pressure suppressed the transformation of screw dislocations into nanopipes, which led to a reduction in the reverse leakage current. To reduce the leakage current further, it is necessary to apply growth conditions that do not transform screw dislocation into nanopipes and to use a free-standing substrate without threading dislocations, that become nanopipes.

Abstract This study reports the method of exploring new dielectric materials by combining a large set of first principles calculations and machine learning. A database of dielectric constants was constructed using the first principles calculations. Crystal structures of 3382 candidate compounds were obtained from the Materials Project database. Harmonic phonon calculations were made to select the compounds showing no imaginary phonon modes. The dielectric constants were then calculated using the density function perturbation theory resulting in 2504 compounds to be constructed in the database. Machine learning methods were adopted to correct the calculated dielectric constants for the systematic errors found between the calculated and the experimental dielectric constants. A random forest model with 68 feature variables successfully predicted dielectric constants within the 50% error range of the logarithmic of the dielectric constant. The predicted dielectric constants for most of the compounds were in the range 3–100.

Abstract In order to detect internal defects of concrete, we have studied noncontact non-destructive acoustic inspection and have detected and visualized internal defects using acoustic vibration excitation on the target surface by aerial sound waves and two-dimensional vibration velocity distribution by a laser Doppler vibrometer. Because a high sensitive scanning laser Doppler vibrometer (SLDV) is used, as the sound pressure increases, the reverberation of sound waves from the surroundings may cause resonance in the galvano mirror system of the laser head of the SLDV. For real concrete structures, for example a tunnel inner wall (distance about 5–10 m), there was no problem because the sound pressure was not so large. In the case of a viaduct, it was measured remotely (about 30 m), but because it is surrounded released space, the influence was not so large. However, in a closed space surrounded by concrete, the sound pressure needs to be increased as the distance increases, and the influence of the reverberation from the surroundings on a laser head cannot be ignored. Therefore,…

Abstract We investigated the formation mechanism of sidewall striations in etched holes with high aspect ratios, with focus on the roles of energetic ions and fluorocarbon radicals. Striations were observed on the sidewalls of holes with high aspect ratios in stacked dielectric films, despite the smooth sidewall of the mask. Argon plasma treatment increased the degree of striation on the carbon mask, while striation was prevented with a cyclic process consisting of a fluorocarbon deposition step and an argon plasma treatment step. The blanket films were irradiated with oblique ion beams in order to simulate the surface reactions on the sidewalls of high aspect ratio holes. Argon ion beams oriented 85° from the surface normal formed severe striations on the blanket fluorocarbon film surface, despite the low roughness of the SiO 2 film surface. A thicker fluorocarbon film was observed in regions with striations than deeper regions with smooth sidewalls. The striations formed on the fluorocarbon films at the sidewalls of high aspect ratio holes and transferred to the dielectric films…

Abstarct The impact of controlling Ga-oxide (GaO x ) interlayers in SiO2/GaO x /GaN gate stacks is investigated by means of physical and electrical characterizations. Direct deposition of SiO2 insulators produces thin GaO x interlayers, and subsequent oxidation treatment attains high-quality insulator/GaN interface. However, the Ga diffusion into the SiO2 layers severely degrades the breakdown characteristics of GaN-MOS devices. To improve reliability of such devices, we proposed a two-step procedure with the initial SiO2 deposition conducted under nitrogen-rich ambient, followed by thick SiO2 capping. We found that this two-step procedure enables nitrogen incorporation in the insulator/GaN interface to stabilize GaN surface. Consequently, the Ga diffusion into the SiO2 overlayer during the oxidation annealing is effectively suppressed. The proposed method allows us to achieve a SiO2/GaO x /GaN stacked structure of superior electrical property with improved Weibull distribution of an oxide breakdown field and with interface state density below 1010 cm−2 eV−1.

Abstract Selective area growths of α -Ga 2 O 3 were demonstrated on c-, m-, and a-plane sapphire substrates by the mist-CVD method using SiO 2 as a mask material. We successfully achieved the coalescence of the α -Ga 2 O 3 on the mask with the 10 1 ¯ 0 stripe on an a-plane sapphire substrate by epitaxial lateral overgrowths, owing to the enhanced lateral overgrowth, that is, the largest ratio of the lateral to vertical growth rates of 0.87 among the substrate orientation and the stripe direction investigated. In cross-sectional transmission electron microscopy images, dislocations were not observed in the α -Ga 2 O 3 layer on the mask, while the dislocations on the window area were propagated without bending.

Abstract Cu-to-Cu direct bonding was successfully achieved in 10 s with 〈111〉-oriented nanotwinned Cu (nt-Cu) bumps in ambient N 2 . The bonding temperature and pressure were 300 °C and 90 MPa, respectively. A nearly void-free interface and a low bump resistance of 4.9 mΩ can be observed after a short-time bonding process. Besides, longer bonding times of 60 s and 30 s were employed, but the resistances of the Cu joints did not decrease significantly when the bonding time increased to 60 s. However, the nt-Cu columnar grains started to recrystallize during the 60 s bonding and started detwinning in 10 s bonding. Yet, the bonding interface remained under such a short bonding time.

Abstract This paper demonstrates a modulation-doped fisseld-effect transistor (MODFET) and a metal-semiconductor field-effect transistor (MESFET) using β -(AlGa) 2 O 3 (010). Ohmic contacts on Sn-doped (Al 0.15 Ga 0.85 ) 2 O 3 exhibit a fairly linear behavior, which has a specific contact resistivity and sheet resistance of 9 × 10 −5 Ω cm 2 and 75 kΩ/□, respectively. The MODFET with Sn-doped (Al 0.08 Ga 0.92 ) 2 O 3 barrier layer showed a breakdown voltage of 610 V for gate-drain spacing ( L gd ) of 8 μ m, while the (Al 0.16 Ga 0.85 ) 2 O 3 -channel MESFET exhibited a breakdown voltage of 940 V for L gd of 20 μ m. These results show the great potential of (AlGa) 2 O 3 transistors for high-power applications.

Abstract Wafer bonding technology is one of the key technologies for high-density three-dimensional integration. We demonstrated 300 mm wafer-level hybrid bonding with Cu and an interlayer dielectric (ILD) layer with the design of 1 × 1 μ m 2 pads with 2 μ m pitch. In this sequence of the bonding process, the structure of Cu bonding pads was controlled to be precisely convex to the ILD layer by chemical-mechanical polishing. Then the bonding process was performed in vacuum. Lastly, the bonded wafers were annealed at 200 °C for the postannealing process. As a result, hybrid wafer bonding with few voids was successfully accomplished.

Dramatic advances are being made in dry processing technologies.Atomic scale precision below 10 nm is now possible with fine patterning technologies for high-volume manufacturing of semiconductor devices.The isotropic and anisotropic nature of both film deposition and etching is versatile for nanoscale fabrication of three-dimensional features, such as high-aspect-ratio (HAR) features.Here we conduct a systematic review of the literature over the last 40 years to evaluate the history and progress of dry processes with regard to fine pattern transfer, HAR feature formation, and multiple patterning as lithographic techniques.Finally, we address the major challenges shared across the plasma science and technology community.

Abstract Heterojunction channel engineering is discussed as a means of enhancing the performance and bias stress stability of In–Ga–Zn–O thin-film transistors (IGZO TFTs). A heterojunction channel was formed by depositing In-rich IGZO on IGZO111 with an atomic ratio of In:Ga:Zn=1:1:1, whose energy band lineup was type-II with a potential well of 0.4 eV for electrons. The thickness of the bottom IGZO111 layer strongly affected the field-effect mobility ( μ FE ) of the TFT owing to electron confinement effect at the heterojunction interface. μ FE was enhanced from 12.4 cm 2 V −1 s −1 for homogeneous IGZO111 TFT to 24.7 cm 2 V −1 s −1 for the heterojunction TFT. Device simulation results revealed that electron confinement in the heterojunction channel played an important role in determining μ FE in the TFT. A channel engineering approach will provide a method of overcoming the trade-off between μ FE and the positive-bias stress stability of oxide TFTs.

Abstract From the previous studies, one of the challenges in the oxide vapor phase epitaxy (OVPE) method was suppressing poly-crystal generation for thick GaN growth. In this study, thick GaN growth was realized by controlling the supersaturation ratio using thermodynamic analysis, and an OVPE-GaN wafer of 300 μ m-thick was obtained. As a result of evaluating the quality of the OVPE-GaN wafer, it was confirmed that both the high oxygen concentration and the high crystallinity were achieved. The resistivity was 7.75 × 10 −4 Ω cm and the threading dislocation density (TDD) was 8.8 × 10 4 cm −2 . It was newly found that the OVPE method can reduce TDD from the order of 10 6 cm −2 to the order of 10 4 cm −2 with a grown thickness of less than 500 μ m. From these results, it was proved that the OVPE method has a great potential of manufacturing low resistivity and low TDD GaN wafers.

Abstract For accelerating the development of GaN power-switching devices, current knowledge on the origins and dynamic properties of the major intrinsic nonradiative recombination centers (NRCs) in Mg-doped GaN (GaN:Mg) are reviewed, as lightly to heavily doped p-type planar GaN segments are required but certain compensating defects including NRCs hinder their formation. The results of complementary time-resolved photoluminescence and positron annihilation spectroscopy measurements on the epitaxial and ion-implanted GaN:Mg formed on low dislocation density GaN substrates indicate the following: major intrinsic NRCs are the clusters of Ga vacancies (VGas) and N vacancies (VNs), namely VGa(VN)2 in the epitaxial GaN:Mg and (VGa)3(VN)3 in the ion-implanted GaN:Mg after appropriate thermal annealings. The minimum electron capture-cross-sections of VGa(VN)2 and (VGa)3(VN)3 are commonly the middle of 10−13 cm2 at 300 K, which is approximately four times larger than the hole capture-cross-section of the major intrinsic NRCs in n-type GaN, namely VGaVN divacancies, being 7 × 10−14 cm2.

In this work, we investigate AlN/sapphire templates grown by metalorganic vapor phase epitaxy (MOVPE) subjected to an intermediate high temperature annealing (HTA) step at different AlN film thicknesses. To avoid cracking of the MOVPE layers during HTA at 1700 °C these layers have to be grown under low incorporation of tensile strain. For each sample the growth is stopped at a certain thickness between 230 nm and 1.2 μ m followed by HTA. X-ray rocking curve FWHM of 0002 and 10–12 reflections lead to an estimation of threading dislocation densities (TDD) as low as cm −2 for the thickest AlN layers of 880 nm and 1.2 μ m after annealing. For all layer thicknesses MOVPE growth is subsequently continued to a total AlN layer thickness of 1.5 μ m to reach for a smooth surface. The change of the AlN strain state after HTA leads to an increased wafer curvature at room temperature and compressive strain in the subsequently grown AlN layers increasing the wafer bow at room temperature. Hence, to serve as a base layer, e.g. for ultraviolet light emitting diodes, a trade-off between low TDD,…

Abstract The current – voltage characteristics of Al/Ti/4H-SiC Schottky barrier diodes have been investigated in the 85–445 K temperature range by means of a combined numerical and analytical simulation study. Simulation results showed a good agreement with measurements in the whole explored current range from 10 μ A to 10 mA. The main device electrical parameters, namely the barrier height (BH) and ideality factor, were found to be strongly temperature - dependent. In particular, the ideality factor decreases while the BH increases with increasing temperature. The observed behaviours have been successfully interpreted by using the thermionic emission theory with a triple Gaussian distribution of the BH in three different temperature ranges, i.e. 85 ≤ Δ T 1 ≤ 135 K, 180 ≤ Δ T 2 ≤ 270 K, and 315 ≤ Δ T 3 ≤ 445 K. The corresponding Richardson constants are A 1 * = 149.26 A cm −2 K −2 , A 2 * = 138.19 A cm −2 K −2 , and A 3 * = 173.21 A cm −2 K −2 , respectively. These values are close to the theoretical result of 146 A cm −2 K −2 expected for n - type 4H-SiC. Finally, it has been…

Abstract This paper investigates the porosity-dependent tensile mechanical properties of porous silver films that are 8–10 μ m thick. The silver films are fabricated with a pressure press; the use of different pressures changes the porosity ( p ) in the range of 5% to 25%. p is determined using scanning electron microscopy cross-sectional images of the films. Stress–strain (S–S) curves are obtained by tensile tests performed on the porous and bulk silver films. The conventional ductile plateau disappears from the porous films. Augmented ultimate tensile strength and a smaller breaking strain occur when p is in the range 6%–14%.