Assessing material qualities and efficiency limits of III–V on silicon solar cells using external radiative efficiency

The paper presents a quantitative approach to the investigation and comparison of the material qualities of III–V on silicon (III–V/Si) solar cells by using external radiative efficiencies. We use this analysis to predict the limiting efficiencies and evaluate the criteria of material quality in order to achieve high‐efficiency III–V/Si solar cells. This result yields several implications for the design of high‐efficiency III–V/Si solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Conceptual design of rain radar for the tropical rainfall measuring mission

Specifications for a spaceborne rain radar for tropical rainfall measurement are described. A spaceborne rain radar has problems peculiar to rain observation from space. The radar must have a fast scanning mechanism to cover a large swath. Very weak rain echoes compared to the sea or land surface signal must be detected. These capabilities must be attained under the severe power consumption and mass limitations of the satellite bus. The fast scanning requirement forces application of an electrically scanning mechanism. This requirement also causes a severe limitation of the available number of independent samples. The requirement for weak rain echoes excludes application of the pulse compression technique, which is a very conventional technique for other active microwave sensors on board satellites. Under these constraints, a rain radar with an electrically scanning planar antenna at 13-8 GHz is proposed.     

Detection of characteristic signals from as-doped (<1 at.%) regions of silicon by transmission electron microscopy and convergent-beam electron diffraction

Characteristic signals were detected from As-doped (< 1 at.%) regions of silicon by dark-field transmission electron microscopy and convergent-beam electron diffraction. A slight intensity increase was observed in 220 dark-field images, which may be explained by an increase of scattering amplitude due to the As doping. The doped region showed a much higher intensity in 004 dark-field images. The characteristic high intensity was observed for specimens with As concentrations of about 0.09-0.8 at.%. Convergent-beam electron diffraction patterns obtained from the As-doped region showed a characteristic rocking curve for 004 reflection. These characteristics should originate from incoherent elastically scattered electrons due to a static lattice distortion around the doped As atoms. The observed characteristics in dark-field images and rocking curves of the 004 reflection should be a good probe not only for investigating the concentration of doped atoms in Si lattice, but also for the amount of impurity and/or point defects in other crystalline materials.     

Metal Single-Atom Cocatalyst on Carbon Nitride for the Photocatalytic Hydrogen Evolution Reaction: Effects of Metal Species

Single-atom (SA) catalysts exhibit high activity in various reactions because there are no inactive internal atoms. Accordingly, SA cocatalysts are also an active research fields regarding photocatalytic hydrogen (H₂) evolution which can be generated by abundant water and sunlight. Herein, it is investigated whether 10 transition metal elements can work as an SA on graphitic carbon nitride (g-C₃N₄; i.e., gCN), a promising visible-light-driven photocatalyst. A method is established to prepare SA-loaded gCN at high loadings (weight of ≈3 wt.% for Cu, Ni, Pd, Pt, Rh, and Ru) by modulating the photoreduction power. Regarding Au and Ag, SAs are formed with difficulty without aggregation because of the low binding energy between gCN and the SA. An evaluation of the photocatalytic H₂-evolution activity of the prepared metal SA-loaded gCN reveals that Pd, Pt, and Rh SA-loaded gCN exhibits relatively high H₂-evolution efficiency per SA. Transient absorption spectroscopy and electrochemical measurements reveal the following: i) Pd SA-loaded gCN exhibits a particularly suitable electronic structure for proton adsorption and ii) therefore they exhibit the highest H₂-evolution efficiency per SA than other metal SA-loaded gCN. Finally, the 8.6 times higher H₂-evolution rate per active site of Pd SA is achieved than that of Pd-nanoparticles cocatalyst.     

Photoluminescence characteristics and pit formation of InGaN/GaN quantum-well structures grown on sapphire substrates by low-pressure metalorganic vapor phase epitaxy

We have examined how a growth interruption, caused by closing group-III sources, affects the crystalline quality of InGaN/GaN quantum-well (QW) structures grown by metalorganic vapor phase epitaxy. The QW samples were characterized by their photoluminescence (PL), and by atomic force microscopy (AFM), transmission electron microscopy (TEM), and energy dispersive x-ray (EDX) microanalysis. The PL peak wavelength was strongly dependent on the duration of the growth interruption and on the number of QW layers. AFM measurements revealed that the size of the open hexagonally shaped pits in the QW structures increased dramatically as the interruption duration was lengthened. Through TEM and EDX microanalysis, we found that the formation of these hexahedronal pits, formed due to the growth interruption, causes a large fluctuation in the In composition, especially around the pits, and the presence of such pits in an underlying QW layer strongly affects the In incorporation into the upper QW layers, leading to significant growth-rate variation in an InGaN QW layer and red-shifting of the PL spectra when a multiple-QW structure is grown.     

Analog Circuit Design Methodology in a Low Power RISC Microprocessor

There are various kinds of analog CMOS circuits in microprocessors. IOs, clock distribution circuits including PLL, memories are the main analog circuits. The circuit techniques to achieve low power dissipation combined with high performance in newest prototype chip in the Super H RISC engines are described. A TLB delay can be decreased by using a CAM with a differential amplifier to generate the match signal. The accelerator circuit also helps to speed up the TLB circuit, enabling single-cycle operation. A fabricated 96-mm² test chip with the super H architecture using 0.35-m four metal CMOS technology is capable of 167-MHz operation at 300 Dhrystone MIPS with 2.0-W power dissipation.     

Photorefractive device using self-assembled monolayer coated indium-tin-oxide electrodes

Photorefractive (PR) performances of methyl-substituted poly(triarylamine) (PTAA)-based PR device were demonstrated using chemically modified electrodes (CMEs) of self-assembled monolayer (SAM) coated indium-tin-oxide (ITO) electrodes. The SAM-ITO electrodes successfully suppressed dark current which blocks the formation of space-charge field and also causes the dielectric breakdown. The PR device consisted of composite of PTAA, 4-azacycloheptylbenzylidenemalononitrile (7-DCST), N-ethylcarbazole (ECz), and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) sandwiched between the SAM-ITO electrodes. The PR devices showed the PR performances: optical gain Γ, refractive index Δn, diffraction efficiency η, response time τ, sensitivity S, phase shift Φ, trap-limited field E_q, number density of traps N_T, and space-charge field E_SC. The remarkable response time of 11.3 ms was achieved at the low electric field of 20 V μm⁻¹, which was comparable to the response time of high-definition television (HDTV) quality of 16 ms. Our approach will widen the usage of higher mobility materials to photorefractive field and give us more favorable materials to achieve the best performance of photorefractivity in the future.     

Observation of iron silicide formation by plan-view transmission electron microscopy

The formation and the phase transitions of iron silicide by solid-phase epitaxy have been investigated by means of plan-view transmission electron microscopy, which enables us to observe a clean interface between Fe and Si. Layers of Fe were deposited on Si (100) at room temperature in an ultrahigh vacuum chamber. The sample was annealed in the electron microscope at a temperature between 673 and 1073 K. After annealing at 673 K, FeSi crystallites were formed with various orientations. When the annealing temperature was increased to 973 K, we found that the crystallites suddenly started to coalesce into grains of several hundreds of nanometers in size and polycrystalline beta-FeSi2 was formed. These phase transitions were also confirmed with electron energy-loss spectroscopy.     

Solution‐Processed Silicane Field‐Effect Transistor: Operation Due to Stacking Defects on the Channel

2D silicon nanomaterials have unique potential for use in applications owing to their many different exotic electronic properties. Field‐effect transistors are fabricated based on free‐standing silicanes through a solution process. Owing to the sensitive surface and the nanometer thickness, the devices require the use of fabrication conditions similar to those of lithium‐ion batteries to prevent oxidation of the sheets. Reliable transistor performance is observed at room temperature in a channel thinner than 3 nm, as drain voltage dependent transfer curves current modulation, depending on the edge effect of the silicane, although the transistor property is modest (hole mobility of 1.8 cm² V^?1 s^?1). The results suggest the feasibility of other air‐sensitive 2D nanomaterials for applications in nanoelectronic devices.