Yoshitaka Tomomura

Luminescence properties of blue La1−xCexAl(Si6−zAlz)(N10−zOz) (z∼1) oxynitride phosphors and their application in white light-emitting diode

This letter reports blue oxynitride phosphors of La1−xCexAl(Si6−zAlz)(N10−zOz) (z∼1) (termed JEM crystal phase) and their application for the white light-emitting diodes (LEDs). The JEM phosphor can be excited by 405nm light efficiently, and its spectrum can be tuned widely by changing the Ce concentration. The emission spectrum of this phosphor is as wide as 110nm in full width at half maximum, which is convenient to solid state lighting. The preparation of white LED was attempted by using a 405nm InGaN chip and oxynitride phosphors in this work. High color rendering index >95 was achieved in white LED with various correlated color temperatures, indicating the suitability of the JEM phosphor in solid-state lightings.

Growth of ZnS by Metalorganic Chemical Vapor Deposition

ZnS epilayers are grown on GaP and GaAs substrates by a low pressure metalorganic chemical vapor deposition technique using dietylzinc (DEZ) or dimethylzinc (DMZ) and H2S as source materials. Good epitaxial layers of ZnS are grown at a substrate temperature TG of 370 to 400°C at a flow rate ratio, [H2S]/[DEZ] or [DMZ]3.0. A typical growth rate of ZnS layers at TG=375°C is ~4 µm/hr for the DEZ/H2S system and ~3 µm/hr for the DMZ/H2S system.

Growth of ZnSe/MgS strained‐layer superlattices by molecular beam epitaxy

Growth of ZnSe/MgS strained‐layer superlattices (SLSs) has been carried out by molecular beam epitaxy. The crystal quality of SLSs degraded with MgS thickness of more than about 3 monolayers. Photoluminescence due to the transition between quantized levels was observed. Photoluminescence peak energies higher than 3.0 eV at 77 K were observed, which are comparable to that of ZnMgSSe quaternary material.

Homoepitaxial growth of ZnS single crystal thin films by molecular beam epitaxy

Abstract Homoepitaxial ZnS single crystal films have been grown on ZnS(100) substrates at a low temperature of 260°C by molecular beam epitaxy using a high-temperature S beam heated up at 800°C. Crystallinity, surface morphology and growth rate of the films grown under various beam conditions (Zn beam pressure P Zn =(0.1−1.8) x 10 -6 Torr, S beam pressure P S =(1.6−3.7) x 10 -6 Torr and beam pressure ratio P S / P Zn =1–32) were investigated. Under the above beam conditions, the growth rate was independent of P Zn , proportional to the square of P S , and was higher on (100)4°–5° off substrates than on (100) substrates. From these results, the growth kinetics were presumed to be governed by a pairwise interaction of S molecules with a Zn atom at a step site on the growth surface. ZnS single crystal films with good crystallinity and flat surfaces were successfully grown at high beam pressure ratios ( P S / P Zn > 4). On the other hand, crystallinity and surface morphology of the grown films became worse on decreasing the beam pressure ratio ( P S / P Zn

Photo-assisted homoepitaxial growth of ZnS by molecular beam epitaxy

Abstract A novel thin film growth technique, photo-assisted homoepitaxy of ZnS usingmolecular beam epitaxy (MBE), has been carried out with elemental zinc (Zn) and elemental sulphur (S) as source materials. The crystallinity of ZnS was greatly improved by irradiation with 350 nm monochromated ultraviolet light from a xenon lamp. The growth rate was about 0.6 μm/h at 260°C. The crystal quality was characterized by RHEED patterns, surface photographs and photoluminescence spectra. Finally, the irradiation intensity dependence and the growth mechanism were discussed.

On the origin of fine structure in the photoluminescence spectra of the β-sialon:Eu2+ green phosphor

Abstract The photoluminescence (PL) and PL excitation (PLE) spectra of Si6−zAlzOzN8−z (β-sialon):Eu2+ phosphors with small z values (z=0.025–0.24) were studied at room temperature and 6 K. The PL and PLE spectra exhibit fine structure with the PL lines being as sharp as 45–55 nm even at room temperature; this fine structure was enhanced by decreasing the z value. These results can be used for expanding the color gamut of liquid crystal displays, particularly in the blue–green region. From low-temperature measurements, the fine PLE structure was ascribed to discrete energy levels of 7FJ states. The 4f65d excited states of Eu2+ are considered to be localized near the 4f orbital. This is because the bonding of Eu2+ with surrounding atoms is ionic rather than covalent. Lattice phonon absorptions were also observed in the PLE spectrum, revealing that the optically active Eu2+ ions are located in the β-sialon crystal. The PL spectrum of the sample with the smallest z value (0.025) consists of a sharp zero-phonon line and lattice phonon replicas, which results in a sharp and asymmetric spectral shape.

Homoepitaxial growth of low-resistivity-Al-doped ZnS single crystal films by molecular beam epitaxy

Abstract Low-resistivity-Al-doped ZnS homoepitaxial films have been grown by molecular beam epitaxy. The lowest resistivity of 2.2×10 -3 Ω cm was obtained at an Al concentration of 4.6×10 19 cm -3 in the ZnS(100) single crystal films grown at 260°C. Th e electron concentration was 3.9×10 19 cm -3 . Films with Al concentrations between 10 18 and 10 19 cm -3 showed excellent photolumi nescence spectra which exhibit strong and dominant near-band-edge emission of 3.785 eV at 77 K.

Dependence of Electrical Properties on Work Functions of Metals Contacting to p-type ZnSe

In order to search a possibility to prepare Ohmic contacts to p-ZnSe by depositing a single metal element, the electrical properties and thermal stability were studied by a current–voltage (I–V) method for a variety of metals (In, Cd, Nb, W, Cu, Ag, Au, Ni, Pt, and Se) contacting to nitrogen-doped ZnSe layers grown by the molecular beam epitaxy technique. All the metals were deposited on the ZnSe substrates which were chemically cleaned prior to metal deposition in order to remove the native oxide layers. The turn-on voltages (V T) and differential resistances were determined by the I–V curves for these contacts before and after annealing. The contacts were divided into three groups by the V T values: (1) In, Cd, Nb, and W contacts with V T~16 V, (2) Au and Pt contacts with V T larger than 9 V, and (3) Cu, Ag, Ni, and Se contacts with V T~3 V. The lowest V T value of 2 V was achieved by the Cu contact. However, this Cu contact deteriorated during storage at ambient temperature. The V T value of ~3 V was routinely obtained for the Ag and Ni contacts. The Ni contact is the most promising materials for the device application among the contact materials studied in the present experiment, although the linear relation in the I–V behavior was not obtained. The electrical properties of these contacts during annealing at temperatures below 300° C were studied, and no significant reduction of the V T value was obtained in the present contacts. The present study suggests that the deposition of a single metal element to p-ZnSe with doping level of 1×1018 cm-3 would not provide low resistance Ohmic contact.

SCHOTTKY BARRIER HEIGHTS OF METALS CONTACTING TO P-ZNSE

Schottky barrier heights (SBH’s) of a variety of metals (In, Cd, Nb, Ti, W, Cu, Ag, Au, Ni, Pt, and Se) contacting to p-ZnSe grown by a molecular beam epitaxy method were measured by an internal photoemission, capacitance–voltage (C–V), and/or current density–voltage (J–V) method. The internal photoemission method could not measure accurately the SBH’s of these metals due to a strong interference of the monochromatic incident light in the p-ZnSe epilayer. The C–V method measured a SBH value of 1.23 eV for the Au contact and 1.13 eV for the Ni contact, but did not measure the SBH’s of other metals due to strong hysteresis of the C–V curves. The SBH’s of these metals were successfully measured by the J–V method to be 1.2±0.1 eV. The present experiment showed that the SBH values were independent of the work functions of the contact metals, indicating that the Fermi-level could be pinned at the p-ZnSe/metal interface. In addition, turn-on voltages conventionally used to evaluate the electrical properties of t...

Effects of surface cleaning on electrical properties for Ni contacts to p‐type ZnSe

As a first step to achieve our goal to prepare Ohmic contacts to p‐type ZnSe using a conventional deposition and annealing method, the microstructure and chemical composition of the ZnSe surface were analyzed by x‐ray photoelectron spectroscopy, reflection high‐energy electron diffraction, and reflection electron microscopy. A thin native oxide layer consisting of ZnO and SeOx and adsorbed carbons were observed on the ZnSe surface which was grown by the molecular beam epitaxy technique. Most of the oxide layer was removed by saturated bromine water solution. A strong correlation was observed between the surface composition and the electrical properties measured by a current‐voltage method for Ni contacting to the N‐doped p‐type ZnSe substrates. The turn‐on voltage (which corresponds to a breakdown voltage for back‐to‐back Schottky contacts) was significantly reduced by removing the oxide layer using saturated bromine water solution. The present experiment suggested that the interfacial oxide layer is an e...