Keiichiro Sakurai

Spatial composition fluctuations in blue-luminescent ZnCdO semiconductor films grown by molecular beam epitaxy

Abstract The optical and structural properties of textured ZnCdO films that exibit a newly found distinct blue luminescence have been investigated. Microscopic compositional fluctuations of Cd, associated with hexagonal grain structures, were observed. Models for the blue-luminescence process are proposed.

Terawatt-scale photovoltaics: Trajectories and challenges

Coordinating technology, policy, and business innovations The annual potential of solar energy far exceeds the worlds total energy consumption. However, the vision of photovoltaics (PVs) providing a substantial fraction of global electricity generation and total energy demand is far from being realized. What technical, infrastructure, economic, and policy barriers need to be overcome for PVs to grow to the multiple terawatt (TW) scale? We assess realistic future scenarios and make suggestions for a global agenda to move toward PVs at a multi-TW scale.

Improvements of ZnO qualities grown by metal-organic vapor phase epitaxy using a molecular beam epitaxy grown ZnO layer as a substrate

Zinc oxide (ZnO) of high quality was homoepitaxially grown by metal-organic vapor phase epitaxy (MOVPE) on molecular beam epitaxy (MBE)-grown ZnO layers after the pretreatment of the underlying MBE-ZnO at 1000°C in N2 which resulted in an atomically flat surface. In photoluminescence at 15 K, the 3 meV line width of the emission from donor-bound-excitons (D0X) and the observation of the fourth phonon replica of the emission from free-excitons (EX) have demonstrated the high potential of MOVPE growth of ZnO toward optical applications.

Alkali incorporation control in Cu(In,Ga)Se2 thin films using silicate thin layers and applications in enhancing flexible solar cell efficiency

Control of the alkali doping level in Cu(In,Ga)Se2 (CIGS) films was demonstrated using alkali-silicate thin layers of various thickness deposited on substrates prior to the sputtering of the Mo back contact layer. Not only the alkali density in the CIGS film, but also the Ga composition distribution in CIGS films, CIGS grain size, and consequent photovoltaic performance showed variations with the silicate layer thickness. Using alkali-silicate thin layers as an alkali source material, 17.4% and 17.7% efficiency flexible CIGS solar cells have been demonstrated on Ti and zirconia substrates, respectively.

Effects of oxygen plasma condition on MBE growth of ZnO

We report growth condition optimizations of ZnO films by plasma-assisted molecular beam epitaxy. Effects of oxygen plasma conditions on film quality were evaluated by photoluminescence, X-ray characterization and scanning electron microscopy. From the growth experiments on c-plane sapphire and bulk ZnO (+ c, - c) substrates, it was suggested that the surface morphology is strongly related with the growth direction of ZnO parallel to its c-axis, and was found to be controllable by optimization of oxygen plasma conditions.

Photoluminescence characterization of Zn1−xMgxO epitaxial thin films grown on ZnO by radical source molecular beam epitaxy

The authors report that high-quality Zn1−xMgxO alloys are very brilliant light emitters, even more brilliant than ZnO, particularly in the high-temperature region; both the emission bandwidth and the oscillator strength of the photoluminescence from Zn1−xMgxO alloys increase remarkably with increasing Mg composition ratio x. The authors have revealed that the increase in the oscillator strength is mainly due to the increase in the activation energy required for the nonradiative recombination processes. Therefore, it is suggested that the localization of excitons, because of the compositional fluctuation, takes place in Zn1−xMgxO alloys and that the degree of the localization increases with increasing x.

Growth of ZnO by Molecular Beam Epitaxy Using NO2 as Oxygen Source

Nitrogen dioxide was applied for molecular beam epitaxy of ZnO on (0001) sapphire substrates. Equipped with liquid nitrogen cryoshrouds, the pressure inside the chamber was kept below 1×10-7 Torr during growth. The samples were characterized by reflection high energy electron diffraction, X-ray diffraction, photoluminesence and scanning electron microscopy. The ZnO crystals clearly showed excitonic optical properties up to room temperature. Effects of low-temperature buffer layers for better surface morphology and reproducibility were also investigated.

Cu(In1-xGax)Se2 growth studies by in situ spectroscopic light scattering

The growth of polycrystalline Cu(In1−xGax)Se2 thin films using a three-stage deposition process has been investigated by in situ diffuse spectroscopic light scattering in the wavelength range of 400–800 nm. Differences in the time dynamics of the intensity at different wavelengths can be explained by the development of surface roughness at different length scales. Of specific interest is the development of roughness around the stoichiometric points of film formation. It is shown that on-line monitoring of the spectroscopic light scattering can be used for process control and for adjustment of the final film roughness.

Blue Photoluminescence from ZnCdO Films Grown by Molecular Beam Epitaxy

Blue luminescence from textured ZnCdO films grown by molecular beam epitaxy was observed. The properties of ZnCdO films grown at various Zn/Cd partial pressures were studied by several techniques including X-ray diffraction, fluorescence (FL) microscopy and Auger electron spectroscopy. With increasing CdO content, the properties of ZnCdO films changed sharply at specific Zn/Cd partial pressures. Some of the films grown at approximately the threshold pressures showed blue luminescence at room temperature, with large Stokes shift leaving the absorption edge in the UV region. By FL microscopy, it was observed that the blue luminescent regions were nonuniformly distributed, suggesting composition fluctuation.

Growth of polycrystalline Cu(In,Ga)Se2 thin films using a radio frequency-cracked Se-radical beam source and application for photovoltaic devices

Cu(In,Ga)Se2 (CIGS) thin films were grown using a rf-cracked Se-radical beam source. A unique combination of film properties, a highly dense and smooth surface with large grain size, is shown. These features seem to have no significant influence on the photovoltaic performance. Defect control in bulk CIGS leading to corresponding variations in the electrical and photoluminescence properties was found to be possible by regulating the Se-radical source parameters. A competitive energy conversion efficiency of 17.5%, comparable to that of a Se-evaporative source grown CIGS device, has been demonstrated from a solar cell fabricated using a Se-radical source grown CIGS absorber.