Hideyuki Toyota

Effect of Ce Co-Doping on CaGa2S4:Eu Phosphor: II. Thermoluminescence

Thermoluminescence of Eu and Ce co-doped CaGa 2 S 4 has been investigated for the first time. All co-doped samples were found to exhibit a long afterglow of the Eu emission having a peak energy of ∼2.20 eV. Analyses of measured transient thermoluminescence curves showed two activation energies, ∼0.2 eV and ∼0.94 eV, for most co-doped samples. These two activation energies are shown to be explainable by the model which includes one trap level with two activation processes, and hole release from the terminal state of the Eu ion. Experimentally observed thermoluminescence curves are shown to be well reproduced by the theoretically formulated model which takes into account the above-mentioned hole release process.

Effect of Ce Co-Doping on CaGa2S4:Eu Phosphor: I. Energy Transfer from Ce to Eu Ions

The resonance-type energy transfer process has been investigated for the first time in Ce and Eu co-doped CaGa2S4 polycrystalline samples with different dopant concentrations in the range from 0.2 at.% to 4 at.%. The energy transfer from Ce to Eu ions increases with increasing Ce and Eu concentrations. The values of the rate determined from measured Ce emission decay time constants with different dopant concentrations showed a reasonable agreement with those obtained from intensity analyses of the Ce emission which takes into account the re-absorption effect by Eu ions. In addition, the observed change of the transfer rate with dopant concentration was discussed in terms of the distance between Ce and Eu ions.

Light scattering and ellipsometric studies at and below energy gap: polycrystalline calcium and strontium thiogallates

An approach based on peculiar angles is considered in relation with the problem of restoration of the dielectric function from ellipsometric measurements at and below energy gap on polycrystalline materials with macroscopic roughness. It is shown that at least for macroscopically and mildly rough surfaces, scattering changes only the amplitude diminution while the phase remains unchanged. This offers a possibility to restore the ellipsometric angles unperturbed by scattering provided that the parameters of surface distribution are known. The ellipsometric data on polycrystalline calcium and strontium thiogallates are briefly overviewed in connection with the performed analysis. The results of comparison show that both multiple scattering and overlayer-type effects can be involved in the transient region between diffraction and geometric optics.

Inhomogeneous distribution of manganese atoms in ferromagnetic ZnSnAs2:Mn thin films on InP revealed by three-dimensional atom probe investigation

Atomic-scale Mn distributions in ferromagnetic ZnSnAs2:Mn thin films grown on InP substrates have been studied by applying three-dimensional atom probe (3DAP) microscopy. It is found that Mn atoms in cross-sectional 3DAP maps show the presence of inhomogeneities in Mn distribution, which is characteristic patterns of a spinoidal decomposition phase with slightly high and low concentration regions. The high Mn concentration regions are expected to be coherently clustered MnAs in the zinc-blende structure, resulting in the formation of Mn-As random connecting patterns. The origin of room-temperature ferromagnetism in ZnSnAs2:Mn on InP can be well explained by the formation of atomic-scale magnetic clustering by spinoidal decomposition without breaking the continuity of the zinc-blende structure, which has been suggested by previous theoretical works. The lattice-matching between magnetic epi-layers and substrates should be one of the most important factors to avoid the formation of secondary hexagonal MnAs ...

Characterization of GaSb/AlGaSb Multi-Quantum-Well Structures Grown on Si(001) Substrates

We prepared GaSb/AlGaSb multi-quantum-well (MQW) structures on n-type silicon (001) substrates by molecular beam epitaxy (MBE). To minimize dislocations in MQW layers and decrease the total thickness of the epitaxial layer, we employed not only an AlSb initiation layer but also a superlattice buffer layer (SL-BL) in a moderately thick GaSb buffer layer. For comparison we also fabricated other MQW structures on a considerably thick GaSb buffer layer without SL-BL. The obtained atomic force microscopy (AFM), transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated that the definite MQW structures for both the samples and the quality of MQW layers prepared using SL-BL were generally better than those of the reference sample. The PL emission of these samples at about 1.30–1.55 µm was observed at room and low temperatures. The dependence of PL emission energy on GaSb well width was well explained by the finite square well potential model.

Dielectric Function Spectra and Intraband Density-of-States in TlSe

Room temperature spectroellipsometric study of TISe was performed in the photon energy region 1.5-4.6eV. The tensor components of the dielectric function spectrum of TlSe were obtained. Interband density-of-states (DOS) derived from XX (YY) and ZZ components of the obtained dielectric function spectrum was then compared with the calculated intraband DOS, taking into account for selection rules and transition probabilities. It is found that contribution of the interband optical transitions in Γ, T and N points quite plausibly explains main structures in the obtained dielectric function spectra. However, some structures cannot be explained due to optical transitions in these symmetry points. A corresponding assignment is given and discussed.

Magnetic phase change in Mn-doped ZnSnAs2 thin films depending on Mn concentration

The relationship between Mn concentration and Curie temperature (TC) is studied for Mn-doped ZnSnAs2 ferromagnetic semiconductors, epitaxially grown on InP substrates by molecular beam epitaxy. In the ferromagnetic phase, Mn distributions in a (Zn,Mn,Sn)As2 thin film with 7.2 cation percent (cat. %) Mn are investigated using three-dimensional atom probe tomography. The results indicate an inhomogeneous distribution which spreads to a relatively high Mn concentration of 9.0 at. % (at. %). In the paramagnetic phase, it is found that the paramagnetic to ferromagnetic transition takes place sharply with a TC of 334 K when the Mn doping concentration increases to about 4 cat. % Mn, which corresponds to a magnetic percolation threshold for ferromagnetism in (Zn,Mn,Sn)As2. An effective Curie temperature ⟨TC⟩ is considered to bridge the Curie temperatures obtained experimentally to those calculated theoretically in inhomogeneous magnetic semiconductors. The behavior of magnetism in Mn-doped ZnSnAs2 can be explain...

Effects of cationic growth conditions of molecular beam epitaxy on ferromagnetic properties of Mn-doped ZnSnAs2 thin films

The effects of cationic growth parameters of molecular beam epitaxy on the crystalline and magnetic properties of Mn-doped ZnSnAs2, (Zn,Sn,Mn)As2, thin films were investigated in order to control ferromagnetism at room temperature. The combination of cationic beam equivalent pressures of Zn, Sn, and Mn atoms was varied at an optimal substrate temperature of 320 °C. Clear hysteresis loops were observed for the samples at 300 K, indicating ferromagnetism at room temperature. We found that the change in Zn flux does not significantly affect the composition of (Zn,Sn,Mn)As2 thin films and ferromagnetic properties and that ferromagnetic properties definitely depend on the amounts of Mn and Sn fluxes. X-ray photoelectron spectroscopy analysis revealed that the sum of the amounts of Mn2+ and Mn4+, which may contribute to the ferromagnetism in (Zn,Sn,Mn)As2 thin films, was approximately 60–70% of the total Mn amount with respect to ferromagnetic (Zn,Sn,Mn)As2 samples. This result indicates that the ferromagnetic properties could be controlled in multilayered (Zn,Sn,Mn)As2 by changing the combination of Zn, Sn, and Mn fluxes.

Crystalline Quality and Structure of MBE-Grown Ferromagnetic Semiconductor ZnSnAs2:Mn Thin Films Revealed by High-Resolution X-ray Diffraction Measurements

Abstract The crystalline structure and quality of ZnSnAs2:Mn films grown by molecular beam epitaxy (MBE) on InP substrates are two of the most important issues in preparing spintronic structures such as magnetic quantum wells and spin-based transistors. We conducted high-resolution X-ray diffraction (XRD) measurements to clarify the crystalline structure of ZnSnAs2:Mn thin films. The XRD measurements reveal with very high accuracy that the samples have a sphalerite structure with high epitaxial quality. These results, within the limits of measurement accuracy support the previous assumption that ZnSnAs2:Mn thin films are coherently clustered ferromagnetic semiconductors without breaking the continuity of the sphalerite structure, leading to ferromagnetism with a high Curie temperature close to that of zincblende MnAs.

Antiferromagnetic-ferromagnetic phase transition in (Zn,Sn,Mn)As2 epitaxial thin films

The magnetization of (Zn,Sn,Mn)As2 thin films epitaxially grown on InP(001) substrates exhibited an anomalous temperature dependence, increasing slightly with temperature, compared with estimates based on the mean-field theory (MFT). Assuming that ferromagnetic (FM) and antiferromagnetic (AF) phases coexist at low temperature, these anomalous magnetic properties can be well explained by an AF-FM transition through a paramagnetic phase. The Neel temperature TN of the AF phase is estimated from the threshold of the difference curve between the experimental and theoretical data, assuming the magnetization of the FM phase, which follows the MFT. The estimated TN rapidly increases by ∼50 K in the range of Mn concentrations where the hexagonal MnAs (h-MnAs) phase precipitates. This can be explained by the superparamagnetic behavior of the precipitated h-MnAs nanoclusters, which possess a high blocking temperature.