Yoichi Nabetani

Effect of oxygen on the electronic band structure in ZnOxSe1−x alloys

The effect of alloying small amounts of ZnO with ZnSe on the electronic band structure has been studied. Optical transitions in molecular-beam-epitaxy-grown ZnOxSe1−x epitaxial films (0⩽x⩽1.35%) were investigated using photoreflectance and photoluminescence spectroscopies. The fundamental band-gap energy of the alloys was found to decrease at a rate of about 0.1 eV per atomic percent of oxygen. The pressure dependence of the band gap was also found to be strongly affected by O incorporation. Both the effects can be quantitatively explained by an anticrossing interaction between the extended states of the conduction band of ZnSe and the highly localized oxygen states located at approximately 0.22 eV above the conduction-band edge.

Epitaxial growth and large band-gap bowing of ZnSeO alloy

ZnSeO alloy was grown by molecular-beam epitaxy up to 1.3% O composition on GaAs substrate using rf plasma. The crystal structure of epitaxial ZnSeO alloy was zinc-blende. O composition was estimated by a strain-free lattice constant. No phase separation was observed by in situ reflection high-energy electron diffraction and x-ray diffraction. Photoluminescence intensity was larger than that of ZnSe. The peak energy shifted toward lower energies with increasing O composition. The band-gap energy determined by photoluminescence excitation spectra decreased with increasing O composition. A bowing parameter as high as 8 eV was obtained. This large band-gap bowing widens the controllable energy-gap range of II-VI semiconductor.

Temperature dependence of the band gap of ZnSe1−xOx

We have studied the temperature dependence of the band gap of molecular-beam-epitaxy-grown ZnSe1−xOx films (x=0–0.021) using photoluminescence spectroscopy from 15 to 280 K. The temperature dependence of the band gap decreases with increasing oxygen concentration, which can be quantitatively explained by an anticrossing interaction between the highly localized oxygen defect states and the extended states of the conduction band.

Temperature dependence and bowing of the bandgap in ZnSe1−xOx

We investigated the temperature dependence of the bandgap of untreated and hydrogen-irradiated ZnSe1−xOx (x=0.23%–0.90%) alloys by photoluminescence from T=10 K to room temperature. The variation of the bandgap energy with T is similar to that of ZnSe, and does not depend on the oxygen concentration. This indicates that oxygen incorporation in ZnSe does not lead to the carrier localization observed for nitrogen incorporation in GaAs and GaP. Correspondingly, no interaction between hydrogen and oxygen is observed in hydrogenated ZnSe1−xOx.

Improvement of InAs quantum-dot optical properties by strain compensation with GaNAs capping layers

Two kinds of self-assembled InAs quantum dots (QDs) grown on GaAs (001) substrates were studied. One is capped with GaAs layers and the other with GaNAs strain-compensating layers. Photoluminescence (PL) measurements on the two kinds of InAs QDs showed distinct dependence on the selection of the capping layers. The homogeneity and luminescence efficiency of the InAs QDs were much improved when the net strain was reduced with GaNAs layers. These results demonstrate the importance of net strain compensation for the improved optical quality of InAs QDs.

Passivation of an isoelectronic impurity by atomic hydrogen: The case of ZnTe:O

We investigated the optical properties of ZnTe:O∕GaAs before and after atomic hydrogen irradiation. Oxygen incorporation gives rise to energy levels associated with single O atoms, O–O pairs, and O clusters, and to a blueshift of the energy gap of the material with respect to that of pure ZnTe∕GaAs. All of these effects disappear progressively after irradiation with H, which also leads to an increase in the tensile strain of the epilayer. These observations provide experimental evidence of H-induced passivation of an isoelectronic impurity in II–VI alloys.

Direct liquid cooling module with high reliability solder joining technology for automotive applications

We developed the direct-liquid-cooling IGBT module which enabled downsizing of a power control unit for HEV system and high reliability simultaneously. This module eliminates thermal grease by unifying a ceramic substrate and a heat sink. It contributes this module realized the reduction of thermal resistance 30 % compared to the conventional indirect liquid cooling type. High thermal conductive Si3N4 ceramics for the substrate and lightweight aluminum heat sink that are suitable for automotive use demand are applied. The technological challenge of this module is to overcome the decrease of the reliability of the joint by large CTE mismatch between substrate and heat sink. We developed the Sn-Sb based solder material which can attain high reliability for automotive use with large CTE mismatch components. And IGBT module with this new solder is applied to HEV.

Theory of strain states in InAs quantum dots and dependence on their capping layers

The dependence of strain states in InAs self-assembled quantum dots (QDs) on their capping layers was investigated by valence-force field model calculations. An InAs QD on (001) GaAs and embedded in a GaNAs capping layer and the one with its dot surface terminated with nitrogen (N) and embedded in a GaAs capping layer show reduced compressive strain within the QDs in the (001) growth plane due to the lateral expansion of the QDs, while the one embedded in an InGaAs capping layer shows enhanced tensile strain along the [001] growth direction. The strain energies around the center of the InAs QDs with the GaNAs capping layer and with the N-surface termination are lowered compared with those for conventional GaAs capping layers. The burying conditions of InAs QDs also modify the sizes of QDs. The stress distributions obtained by strain energy mapping showed that In atoms around the top of QDs undergo inward stress. This inward stress prevents In segregation and explains the experimentally observed improved o...

Deposition of Zinc Oxide Thin Films in Supercritical Carbon Dioxide Solutions

This article reports the deposition of ZnO thin films in supercritical CO2 solutions likely for the first time. Zincacetylacetonate was dissolved in supercritical CO2, together with oxygen, and was processed at 8–13 MPa and 230–380 °C. Continuous crystalline films were obtained on Si and sapphire at temperatures higher than 280 °C. The-band-edge emission at 370 nm was confirmed in photoluminescence spectra.

Room-Temperature Formation of a ZnO-Based Adhesion Layer for Nanoprecision Cu/Glass Metallization

This study demonstrated that a thin ZnO layer functions as a strong adhesion layer between Cu and glass, even when Cu films are formed near room temperature. The adhesion strength was studied with a scratch tester and the films and interfaces were characterized by scanning transmission electron microscopy. The origin of the high adhesion strength was discussed in terms of the presence of an extremely thin (approximately 10 nm) intermixing layer consisting of Cu, Pd, Zn, O, and Si.