Yoshiaki Yazawa

GaInNAs: A Novel Material for Long-Wavelength-Range Laser Diodes with Excellent High-Temperature Performance.

We propose a novel material, GaInNAs, that can be formed on GaAs to drastically improve the temperature characteristics (T0) in long-wavelength-range laser diodes. The feasibility of our proposal is demonstrated experimentally.

Visible photoluminescence of Ge microcrystals embedded in SiO2 glassy matrices

Ge microcrystals embedded in SiO2 glassy matrices were formed by a radio‐frequency magnetron cosputtering technique and then annealed at 800 °C for 30 min. The average radius of the Ge microcrystals in SiO2 was determined to be about 3 nm by means of Raman spectroscopy and high resolution electron microscope. The annealed sample showed a strong room temperature luminescence with a peak at 2.18 eV. This is consistent with quantum confinement of electrons and holes.

Room-Temperature Pulsed Operation of GaInNAs Laser Diodes with Excellent High-Temperature Performance

We have successfully operated GaInNAs laser diodes with a pulsed current at room temperature. The lowest threshold current density was about 0.8 kA/cm2, and the lasing wavelength was about 1.2 µ m. Characteristic parameters such as internal quantum efficiency and the gain constant were measured, and excellent high-temperature performance was observed. The characteristic temperature was 127 K in the temperature range from 25 to 85° C.

Extremely large N content (up to 10%) in GaNAs grown by gas-source molecular beam epitaxy

Abstract GaNAs GaP strained single quantum wells are fabricated on GaP wafers by gas-source molecular beam epitaxy in which a nitrogen radical is used as the nitrogen source. The structure and luminescence properties of the quantum wells are investigated by transmission electron microscopy and photoluminescence measurements. The N content in the GaNAs quantum wells was estimated to be about 10%, which is about one order of magnitude larger than previously reported and more than sufficient for fabricating laser diodes on a Si wafer.

Analysis of Band Offset in GaNAs/GaAs by X-Ray Photoelectron Spectroscopy

We used X-ray photoelectron spectroscopy (XPS) to measure the energy discontinuity in the valence band (ΔEv) of Ga1-xNxAs/AlAs (x=0, 0.014, 0.034) and estimated ΔEv of GaNAs/GaAs by using the Al2p energy level as a reference. The change in ΔEv for GaNAs/GaAs with an increasing nitrogen content was -(0.019±0.053) eV/%N. This suggests that the valence-band edge (Ev) in GaNAs decreases in proportion to the nitrogen content. Based on the decrease in the bandgap energy of GaNAs, we found that the energy discontinuity in the conduction band (ΔEc) of GaNAs/GaAs is -(0.175±0.053) eV/%N. This large effect of bandgap bowing on the conduction band indicates that an ideal carrier confinement in the well can be obtained by using GaInNAs as an active layer in long-wavelength laser diodes.

Design and characterization of flat-plate static-concentrator photovoltaic modules

Using a ray-tracing program to analyze the characteristics of monofacial-cell and bifacial-cell-types of flat-plate static-concentrator photovoltaic modules revealed that about 90% of the annual irradiation can be collected by a monofacial-cell module with a concentration ratio of 1.5 and by a bifacial-cell module with concentration ratio of 2.0. Preliminarily, small modules were fabricated and dependence of optical performance on light incident angles were measured. The optical collection efficiency of a monofacial-cell module with a concentration ratio of 1.5 was 87.6% and that of bifacial-cell module with a concentration ratio of 2.0 was 85.6%.

Static concentrator photovoltaic module with prism array

To obtain cost-effective photovoltaic modules, we have developed static prism-array concentrator modules consisting of prism concentrators about 4 mm thick assembled unidirectionally under a 3.2-mm-thick cover glass. Calculating the optical collection efficiency for the annual solar irradiation in Tokyo, we found that the theoretical efficiency of the modules is 94.4% when the geometrical concentration ratio is 1.88 and that it is 89.1% when that ratio is 2.66, respectively. Fabricating prism-array-concentrator modules with a geometrical concentration ratio of 2.66, we obtained a maximum optical collection efficiency of 82% with a flat reflector and 81.7% with a V-grooved reflector.

CHARACTERIZATION OF THE REFRACTIVE INDEX OF STRAINED GAINNAS LAYERS BY SPECTROSCOPIC ELLIPSOMETRY

We have characterized the refractive index of strained GaInNAs layers. Using spectroscopic ellipsometry (SE), the variation in optical constants of GaInNAs layers, about 6 nm thick with a nitrogen content lower than 1%, can be clearly observed. Analysis of the SE data, including the strain effect in the layer, clarified that the refractive index of GaInNAs increases in proportion to the nitrogen content. While the trend for increase in refractive index with a decrease in the bandgap energy is the same as that observed in conventional III–V alloy semiconductors, the rate of increase is found to be much larger than that in GaInAs. This result suggests a large density of states in the conduction band characteristics of this type of material system that includes nitrogen atoms.

GaNAs grown by gas source molecular beam epitaxy

Abstract The two main keys to growing GaNAs are reducing layer thickness and using a highly efficient N source. Through careful and precise growth control, GaNAs is grown with a N content of up to about 10% by gas source molecular beam epitaxy in which a N radical is used as the N source.

Thin-film c-Si solar cells prepared by metal- induced crystallization

We prepared a thin-film polycrystalline silicon solar cell using metal-induced crystallization (MIC) of an amorphous silicon film and a thin Ni layer. The MIC using a 0.6-nm-thick Ni layer produced a highly activated n-type crystalline layer at a 5501C annealing temperature. The Ni concentration in the i-layer of a solar cell prepared by successively depositing i- and p-layers on an MIC n-layer using plasma-enhanced CVD was lower than 1 10 16 /cm 3 . This solar cell was highly responsive in the long-wavelength region of its quantum efficiency, indicating that the n/i interface and i-layer region near the n-layer were of high quality. r 2002 Elsevier Science B.V. All rights reserved.