Keita Kataoka

Direct measurement of band offset at the interface between CdS and Cu2ZnSnS4 using hard X-ray photoelectron spectroscopy

We directly and non-destructively measured the valence band offset at the interface between CdS and Cu2ZnSnS4 (CZTS) using hard X-ray photoelectron spectroscopy (HAXPES), which can measure the electron state of the buried interface because of its large analysis depth. These measurements were made using the following real devices; CZTS(t = 700 nm), CdS(t = 100 nm)/CZTS(t = 700 nm), and CdS(t = 5 nm)/CZTS(t = 700 nm) films formed on Mo coated glass. The valence band spectra were measured by HAXPES using an X-ray photon energy of 8 keV. The value of the valence band offset at the interface between CdS and CZTS was estimated as 1.0 eV by fitting the spectra. The conduction band offset could be deduced as 0.0 eV from the obtained valence band offset and the band gap energies of CdS and CZTS.

P-type doping of GaN by magnesium ion implantation

Magnesium ion implantation has been performed on a GaN substrate, whose surface has a high thermal stability, thus allowing postimplantation annealing without the use of a protective layer. The current–voltage characteristics of p–n diodes fabricated on GaN showed distinct rectification at a turn-on voltage of about 3 V, although the leakage current varied widely among the diodes. Coimplantation with magnesium and hydrogen ions effectively suppressed the leakage currents and device-to-device variations. In addition, an electroluminescence band was observed at wavelengths shorter than 450 nm for these diodes. These results provide strong evidence that implanted magnesium ions create acceptors in GaN.

Stoichiometric water splitting using a p-type Fe2O3 based photocathode with the aid of a multi-heterojunction

Fe2O3-based photocathodes are one of the least expensive options for hydrogen generation by water splitting. Although p-type N,Zn-doped Fe2O3 (N,Zn–Fe2O3) has been reported to possess a negative conduction band minimum position sufficient for photocathodic hydrogen generation, the efficiency and stability of the resulting H2 production is low and the reaction is sacrificial. In the present work, analysis by hard X-ray photoelectron spectroscopy (HAXPES) showed that these negative characteristics result from the self-redox reaction of p-type Fe2O3. Based on this result, a TiO2 layer was introduced onto the surface of p-type N,Zn–Fe2O3 to passivate surface defects. In addition, to ensure efficient electron transfer, a thin Cr2O3 layer was also inserted between N,Zn–Fe2O3 and a bottom side conductive oxide layer to generate a favorable band alignment for hole transfer. The resulting Pt/TiO2/N,Zn–Fe2O3/Cr2O3 electrode exhibits a highly stable, significantly enhanced cathodic photocurrent during H2 production under AM 1.5 irradiation. The mechanism providing this improvement was investigated by combining electrochemical impedance spectroscopy, open-circuit voltage decay analysis and scanning tunneling electron microscopy-energy dispersive X-ray spectroscopy. Stoichiometric water splitting without an external electrical bias was also demonstrated by connecting the Fe2O3-based photocathode to an n-type SrTiO3−x photoanode, representing the first-ever example of stoichiometric overall water splitting using an Fe-based photocathode.

Band slope in CdS layer of ZnO:Ga/CdS/Cu2ZnSnS4 photovoltaic cells revealed by hard X-ray photoelectron spectroscopy

For compound semiconductor photovoltaic cells with a common structure of the window-layer (WL)/buffer-layer (BL)/absorbing-layer (AL), the band slope in BLs, affecting the conversion efficiency, was directly and non-destructively measured by hard X-ray photoelectron spectroscopy. We demonstrated that the band slope in CdS-BLs sandwiched between WLs and Cu2ZnSnS4 (CZTS)-ALs reflected the trend of the work functions of WLs ( ϕ WL). This result implies that the larger downward band slope to the WL can be achieved using a smaller ϕ WL. The relatively large downward band slope of ∼0.5 eV to the WL was estimated in our ZnO:Ga/CdS/CZTS sample with a higher conversion efficiency of 9.4%, which indicates that the conversion efficiency of CZTS cells can be improved by a larger downward band slope to the WL.

ADSORPTION AND REACTION OF NITRIC OXIDE ON Si(111)-Au SURFACES

We studied adsorption and reaction of nitric oxide on Si(111)7 × 7, 5 × 2-Au,

Band offset of Al1− x Si x O y mixed oxide on GaN evaluated by hard X-ray photoelectron spectroscopy

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ATOMIC STRUCTURE ANALYSIS OF ULTRA THIN IRON SILICIDE FILMS BY STEREO ATOMSCOPE

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Ion implantation technique for conductivity control of GaN

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Enhancements of photoluminescence intensity in high-quality floating-zone Si by thermal annealing in vacuum

, and 6 × 6-Au surfaces with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy (STM). We found that NO gas reacts most strongly with the 7 × 7 surface, strongly with the 5 × 2-Au surface, and little with the other Si(111)-Au surfaces. STM results indicated that the NO exposure removes adatoms and erodes the row structure on the 5 × 2-Au surface at room temperature.

Iron silicides grown by solid phase epitaxy on a Si(111) surface : Schematic phase diagram

An Al1− x Si x O y mixed oxide has been deposited on GaN by plasma-enhanced atomic layer deposition. The band diagrams between the mixed oxide and GaN for various Si atom fraction x values are determined by hard X-ray photoelectron spectroscopy for the first time. The band gap of the mixed oxide increases with increasing x. This dependence has a large bowing parameter of 1.5 eV. We have successfully obtained conduction band offset (ΔE C) and valence band offset (ΔE V) as a function of x: ΔE C (eV) = 1.6 + 0.4x + 1.2x 2 and ΔE V (eV) = 1.7 + 0.34x + 0.36x 2. These relationships enable us to design GaN metal–oxide–semiconductor devices using the Al1− x Si x O y mixed oxide.