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We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of 1 / 3 [ 1 1 ¯ 01 ] and 1 / 3 [ 11 2 ¯ 0 ] , while the m-plane α-Ga2O3 thin film displays Burgers vectors of 1 / 3 [ 2 1 ¯ 1 ¯ 0 ] and 1 / 3 [ 1 1 ¯ 01 ]. This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3. [ABSTRACT FROM AUTHOR]

We performed Ge doping of α-Ga2O3 thin films grown on m-plane sapphire substrates using mist chemical vapor deposition. Although the typical growth rate was high at 4 μm/h, the resultant α-Ga2O3 thin films exhibited high crystallinity. We controlled the carrier density in the range of 8.2 × 1016–1.6 × 1019 cm−3 using bis[2-carboxyethylgermanium(IV)]sesquioxide as the Ge source. The highest mobility achieved was 66 cm2 V−1 s−1 at a carrier concentration of 6.3 × 1017 cm−3. Through secondary ion mass spectrometry analysis, a linear relationship between the Ge concentration in the α-Ga2O3 thin films and the molar ratio of Ge to Ga in the source solution was established. The quasi-vertical Schottky barrier diode fabricated using the Ge-doped α-Ga2O3 thin films exhibited an on-resistance of 7.6 mΩ cm2 and a rectification ratio of 1010. These results highlight the good performance of the fabricated device and the significant potential of Ge-doped α-Ga2O3 for power-device applications. [ABSTRACT FROM AUTHOR]

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K. [ABSTRACT FROM AUTHOR]

In this study, we developed a metal–semiconductor field-effect transistor (MESFET) using a Ge-doped α-Ga2O3 channel layer grown via mist chemical vapor deposition (CVD). As a buffer layer, Fe-doped α-Ga2O3 was deposited between the Ge-doped α-Ga2O3 channel layer and the m-plane sapphire substrate to reduce the influence of threading dislocations and suppress current leakage. Furthermore, an n+ contact layer heavily doped with Ge was deposited on the channel layer to reduce the contact resistance. The carrier concentration and Hall mobility of the channel layer were 2.1 × 1017 cm−3 and 44 cm2 V−1 s−1, respectively. The transfer length method indicates that the contact between the metal and the n+ layer exhibits Ohmic behavior with a resistance as low as 16 Ω mm. The MESFET exhibited a maximum current of 24 mA/mm and an on-resistance of 587 Ω mm at VGS = 2 V. The Ion/Ioff ratio exceeded 109. The breakdown voltage was 364 V, the leakage current between the drain and the source was below 10−5 mA/mm, and the power figure of merit was 1.2 MW/cm2. These results demonstrate that the mist CVD-derived Ge-doped α-Ga2O3 can give rise to a MESFET with good performance. [ABSTRACT FROM AUTHOR]

Rutile-type GeO2 (r-GeO2) with an ultrawide bandgap of ∼4.7 eV has emerged as a promising material for next-generation power-electronic and optoelectronic devices. We performed transmission electron microscopy (TEM) observation to analyze the structural properties of r-GeO2 film on r-TiO2 (001) substrate at an atomic level. The r-GeO2 film exhibits a threading dislocation density of 3.6 × 109 cm−2 and there exist edge-, screw-, and mixed-type dislocations in the film as demonstrated by two-beam TEM. The edge-type dislocations have Burgers vectors of [100] and/or [110]. The bandgap of the r-GeO2 film is 4.74 ± 0.01 eV as determined by electron energy loss spectroscopy. [ABSTRACT FROM AUTHOR]

The combination of metasurface with optical emitters provides a unique opportunity to control the emission. The metasurface effect strongly depends on the spectral overlap between the internal electronic transitions of the emitter and the optical resonances of the metasurface. Elaborate design of the metasurface could realize the resonances at both absorption and emission wavelengths of the emitter, but it usually leads to complexity in fabrication. In this work, we propose a very simple strategy to obtain the resonances at both wavelengths by sandwiching the emitter layer with a pair of metasurfaces designed for absorption and emission, respectively. For this purpose, we use a sticker of Al metasurface, which is the array of Al nanoparticles embedded in a flexible polymer film that can be stuck on any clean surfaces. The metasurface stack is prepared by simply placing the sticker, resonating at the emission wavelength, on the emitter layer deposited on the TiO 2 metasurface resonating at the absorption wavelength. The overall enhancement achieved for the stack of the metasurfaces can be understood roughly as the multiplication of contributions from the respective metasurfaces. Employing a sticker provides an easy-to-make way of constructing a stack of metasurfaces, which increases the degree of freedom in designing the metasurface-coupled emitters. [ABSTRACT FROM AUTHOR]

Although titanium dioxide ( TiO 2) is a promising constituent of the metasurface operative in the visible, the experimental demonstration is limited so far because TiO 2 is intrinsically chemically/physically stable and is hard to be processed into nanostructures with high precision. In this paper, we develop a facile pathway to fabricate the TiO 2 metasurface via oxidation of Ti nanoparticle array that can be made by the conventional lift-off process. Under an optimized heat-treatment procedure in air, Ti nanoparticles are converted to TiO 2 nanoparticles with a size expansion predictable by the molar volume mismatch between Ti and TiO 2 , while the global periodic arrangement is retained. We apply this technique to a Ti nanoparticle array fabricated on the phosphor plate of yttrium aluminum garnet doped with Ce 3 + (YAG:Ce) and demonstrate the directional outcoupling of emission through the metasurface. The photoluminescence from the YAG:Ce plate is directionally enhanced in the forward direction, as large as three times as much compared to that from the flat YAG:Ce plate without the metasurface. Because of the high transparency and lossless feature of TiO 2 in the visible, the present metasurface does not lower the total quantum yield of the system consisting of the YAG:Ce plate and the TiO 2 metasurface, which is beneficial for the solid-state-lighting application. [ABSTRACT FROM AUTHOR]

An array of nanocylinders can modulate photoluminescence (PL) of emitters via simultaneous excitation of local resonances and light diffractions. Regarding the local resonance, localized surface plasmon polaritons (LSPP) of metals has been commonly applied, while the Mie resonance of dielectrics can also be a candidate. Since these two resonances are excited in different classes of materials, experimental comparisons between their enhancement efficiencies are rarely made. In this study, we fabricated periodic arrays of Si and Al nanocylinders with the identical design and compared their PL outcoupling abilities. We deposited ultraviolet (UV)-absorbing and highly luminous dielectric films on the arrays and observed enhanced PLs from the films on the nanocylinder arrays upon a UV laser excitation. The optical transmission with varied incident angles revealed that the dispersion of the hybrid mode of diffraction with the Mie resonance in the Si nanocylinder array was sharper than that of the diffraction-LSPP hybrid in the Al nanocylinder array. The PL enhancement by the Si nanocylinder array was more selective both spectrally and spatially than that by the Al nanocylinders, which could be attributed to the difference between the mode profiles. Our findings experimentally verified the advantage of choosing Si as a constituent of the cylinder array to provide highly directional light sources. [ABSTRACT FROM AUTHOR]

White light-emitting diodes (LEDs), light sources that combine blue LEDs and yellow phosphors, are equipped with bulky optics such as lenses, mirrors, and/or reflectors to shape the light into the required directions. The presence of bulky optics causes optical loss and limits the design. Here, a periodic array of metallic nanocylinders, which exhibits a high scattering efficiency owing to the excitation of localized surface plasmon resonance, is proposed as an alternative means of achieving a directional output without the limitations of bulky optics. A prototype of a directional light emitter is fabricated consisting of an Al nanocylinder array on a yellow phosphor plate and a blue laser. The array shapes the yellow luminescence into the forward direction and generates directional quasi-white light (correlated color temperature of 4900 K). The intensity enhancement reaches a factor of five in the forward direction and is further improved up to a factor of seven by the deposition of a multilayer dichroic mirror on the back side of the phosphor plate, resulting in conversion efficiencies as high as 90 lm/W. Our results pave the way toward the development of efficient and compact directional white-light-source devices without any bulky optics. [ABSTRACT FROM AUTHOR]

Periodic arrays of metallic nanoparticles act as metasurfaces to control light at the interface. Although the combination of a variety of functional materials with metasurfaces should open a rich scientific research field, the application degree of freedom is limited severely by the fabrication process. We developed a metasurface sticker that can be stuck on any surface and gives control over in- and outcoupling of light. We demonstrate a photoluminescence outcoupling by placing a sticker on the luminescent layers: the emission pattern is modulated both spatially and spectrally in a way defined by the metasurface sticker. The sticker is useful for enhancing optical response from many different surfaces including those that are incompatible with the nanofabrication processes and should help the integration of metasurfaces into functional devices. [ABSTRACT FROM AUTHOR]

In this study, photoluminescence (PL) under high excitation intensity as a function of crystalline size was systematically investigated through ZnO nanocrystalline films prepared by spin-coating a colloidal solution of ZnO nanoparticles obtained using the microemulsion method. Annealing of the films at 723, 633, and 593 K allowed us to tune the crystalline radius R. PL studies distinguished different regimes of crystalline size according to the ratio of R to the effective Bohr radius aB(R/aB). For the sample annealed at 723 K (R/aB=7.2), the peak of stimulated emission due to the exciton-exciton collisions appeared on the low-energy side of the exciton emission with an increase in excitation intensity. A further increase in excitation intensity eventually resulted in the occurrence of an electron-hole plasma (EHP) accompanied by consequent band gap renormalization, which indicates that high excitation intensity provokes the dissociation of excitons. For the sample annealed at 633 K (R/aB=4.7), the stimulated emission was observed while the transition to EHP was obscure. For the sample annealed at 593 K (R/aB=2.1), only emissions due to the recombination of the electron-hole pair were observed, and stimulated emission did not appear even when the excitation intensity was increased. The transition from free-exciton emission to donor-bound exciton emission was observed in temperature dependence of PL only for the sample with R/aB=7.2. The origin of annihilation of the stimulated emission with a size reduction is discussed based on nonradiative Auger recombination. [ABSTRACT FROM AUTHOR]

We have developed a technique by which poling and ion implantation in solid state are simultaneously accomplished. The technique has been applied to 2Ag2O·3Na2O·25ZnO·70TeO2 (in mol %) glass. The glass is sandwiched by two cover glasses containing Na+ and then the thermal poling is carried out. We have accidentally found that Ag nanoparticles are selectively precipitated in the vicinity of the anode-side glass surface. X-ray photoelectron spectroscopy reveals that the glass composition of the anode-side glass surface becomes rich in Na+ after the thermal poling, which is caused by the Na+ implantation from the cover glass at the anode side. [ABSTRACT FROM AUTHOR]

EuS microcrystal-embedded SiO[sub 2], Al[sub 2]O[sub 3], and TiO[sub 2] thin films have been prepared by using rf sputtering method. X-ray diffraction analysis indicates that EuS microcrystal is precipitated as a single phase in Al[sub 2]O[sub 3] and TiO[sub 2] films while precipitation of EuS is not detected in SiO[sub 2] film. Faraday effect attributable to the 4f[sup 7]-4f[sup 6]5d transition of Eu[sup 2+] in EuS microcrystalline phase is observed in Al[sub 2]O[sub 3] and TiO[sub 2] films. In particular, Faraday rotation angle observed for EuS-embedded TiO[sub 2] film is large; for instance, the magnitude of Verdet constant for as-deposited TiO[sub 2] film prepared without heating of substrate during the sputtering is 0.15 deg/cm Oe at wavelength of 700 nm. This value is larger by two orders of magnitude than those of Eu[sup 2+]- or Tb[sup 3+]-containing oxide glasses which show the largest Faraday rotation angle among rare-earth-containing glasses, and is comparable to the value for EuSe single crystal. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the effect of surface etching on second-harmonic (SH) intensity and Maker fringe pattern for poled borosilicate glasses containing CuCl. Observation of a drastic decrease in the SH intensity with anode-side surface etching; Oscillation in the fringe which originates from the interference of SH waves generated from both surfaces.

Presents information on a study which examined the second harmonic generation for tellurite glasses. Poling temperature dependence of second harmonic intensity; Experimental procedure; Results; Discussion.

Deals with a study which applied two-band theory in place of the Debye theory to analyze the temperature dependence of recoil-free fraction and the characteristic temperatures of the iron-boron and iron-phosphorus amorphous alloys. Details of recoil-free fraction and characteristic temperatures; Methodology of the study; Results and discussion.

Provides information on an experimental study examining poling temperature dependence of optical second harmonic intensity for Na2O-ZnO-TeO2 glasses. Methodology used to conduct the experiment; Detailed information on the poling of a glass sample; Results of the study.

Reports that Faraday effect measurements were carried out at room temperature for sodium borate glasses containing Eu2+ ions prepared under a reducing atmosphere. Analysis of the wavelength dependence of the Verder constant.

Demonstrates the fluorescence line narrowing spectroscopy of sodium borate glasses containing a samarium ion and its isoelectonic europium ion under a pulsed tunable dye laser excitation. Temporal changes of the emission line; Dynamics of excited states for both ions; Site-to-site variations of the inhomogeneous broadening of 5D0-7F0 transitions.

Presents a study that prepared transparent glass ceramics containing β-lead-fluorine:thulium crystallites by heat treatment of glasses in a germanium dioxide-lead-oxygen-lead-fluorine-thulium-fluorine system. Average crystallite size of β-lead-fluorine; Temperature range by which the glass ceramics were prepared; Incorporation of thulium into β-lead-fluorine crystalline phase.

Presents a study which found amorphous oxides with ferromagnetic character in the strontium-iron-oxygen and bismuth-iron-oxygen iron-based systems. X-ray diffraction patterns of the systems; Magnetic curves observed in the specimens; Magnetic hysteresis of the systems.

Studies persistent spectral hole burning of Eu3+ ions in sodium aluminosilicate with several aluminum oxide contents. Application of the hole burning spectroscopy to silicate and aluminosilicate glasses doped with Eu3+; Linear dependence of homogenous linewidth with T between 4.2 and 8k.

Two dimensional pulse-based electron spin transient nutation (2D-ESTN) spectroscopy is a powerful tool for determining the spin quantum number and has been applied to BaTiO3 fine powder in order to identify the origin of the continuous wave electron spin resonance (CW-ESR) signal around g = 2.00. The signal is frequently observed in BaTiO3 ceramics, and the correlation between the signal intensity and positive temperature coefficient of resistivity (PTCR) properties has been reported to date. The CW-ESR spectrum of BaTiO3 fine particles synthesized by the sol-gel method shows a typical asymmetric signal at g = 2.004. The 2D-ESTN measurements of the sample clearly reveal that the signal belongs to the S = 5/2 high spin state, indicating that the signal is not due to a point defect as suggested by a number of researchers but rather to a transition metal ion. Our elemental analysis, as well as previous studies, indicates that the origin of the g = 2.004 signal is due to the presence of an Fe3+ impurity. The D value (second-order fine structure parameter) reveals that the origin of the signal is an Fe3+ center with distant charge compensation. In addition, we show a peculiar temperature dependence of the CW-ESR spectrum, suggesting that the phase transition behavior of a BaTiO3 fine particle is quite different from that of a bulk single crystal. Our identification does not contradict a vacancy-mediated mechanism for PTCR. However, it is incorrect to use the signal at g = 2.00 as evidence to support the vacancy-mediated mechanism. [ABSTRACT FROM AUTHOR]

We have prepared a zinc ferrite (ZnF2O4) thin film 85 nm thick deposited on a silica glass substrate by using a radio frequency sputtering method. Faraday effect measurements have been carried out not only for as-deposited but also for annealed thin films. The thin film annealed at 300 °C as well as the as-deposited thin film exhibit a large Faraday rotation angle at a wavelength of around 390 nm. In particular, the thin film annealed at 300 °C manifests the largest Faraday effect among the present thin films; the rotation angle of 1.65°/μm is attained at a wavelength of 386 nm. The thin films 1.08 μm thick exhibit a large magnetization at room temperature, and the dependence of the magnetization on the external magnetic field is suggestive of a ferrimagnetic behavior. [ABSTRACT FROM AUTHOR]

Presents a study which examined the poling temperature variation of optical second-harmonic intensity induced by electrical poling in 30ZnO-70TeO[sub2], 5MgO-25ZnO-70TeO[sub2] and 10MgO-20ZnO-70TeO[sub2] glasses. Theoretical background; Computational approach; Results.

Presents a study that investigated the second-order doppler shift in Mössbauer spectra of iron-boron and iron-phosphorus amorphous alloys. Estimation of changes in second-order doppler shift with temperature in Mössbauer effect measurements; Details on how to obtain the characteristic temperature representing the bond strength of iron atoms in crystalline solid.

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K. [ABSTRACT FROM AUTHOR]

Photoreactive random media containing silver nanoparticles (Ag NPs) have been fabricated, and hole burning effects based on the interference of multiply scattered light have been investigated. Through the analysis of hole profiles, transport mean free path ℓ is estimated for the light with the wavelength of 585 nm. We find that ℓ in the samples embedded with Ag NPs is much shorter compared with that in a medium containing the same amount of titania (TiO2) NPs, indicating that Ag NPs scatter 585 nm lightwaves much stronger than TiO2 NPs because of their larger scattering cross sections due to the localized surface plasmon resonance. [ABSTRACT FROM AUTHOR]

We have prepared random lasers that consist of macroporous titania monoliths infiltrated with dye solution and that operate close to light localization regime. When the excitation pulse energy exceeds a threshold, discrete spectral lines ascribed to laser oscillation appear on a featureless emission peak. No pulse-to-pulse variation was observed in the spectral positions of lasing lines, indicating that the lasing modes are localized in the medium. We demonstrated selective excitation of lasing modes by pumping at different positions on the sample. [ABSTRACT FROM AUTHOR]

We report on epitaxial growth and magnetic properties of EuTiO3 thin films with a perovskite structure. Single crystalline EuTiO3 films with atomically flat surface are grown on (001) surface of SrTiO3 by pulsed laser deposition (PLD) and subsequent annealing in reducing atmosphere. The as-deposited films possess the crystal structure with an elongated c-axis and tend to stabilize ferromagnetically ordered Eu2+ spins at low temperatures. Postannealing at 1000 °C relaxes the out-of-plane lattice strain, and brings about a drastic change in magnetic structure; the annealed film becomes an antiferromagnet below Néel temperature of 5.1 K. The change in magnetic properties accompanied by the modification in lattice constant is discussed. [ABSTRACT FROM AUTHOR]

Ti4+-doped oxide glass is revealed to yield intense greenish emission by excitations with ultraviolet (UV) light and near-infrared femtosecond pulsed laser (NIFPL). The emission profile obtained by UV excitation can be well traced by NIFPL. Compared with Ta5+-doped oxide glasses reported previously, Ti4+-doped glasses exhibit distinctive characteristics in optical absorption, excitation, and emission spectra, indicating that intense tunable emissions can be readily achieved by selecting nd0 ions as dopants in glasses. The glass materials containing nd0 ions are expected to find applications in high density optical storage and three-dimensional color displays. [ABSTRACT FROM AUTHOR]

The authors report on the fabrication of room-temperature ferrimagnetic semiconductor thin films composed of 0.6FeTiO3∙0.4Fe2O3 solid solution on α-Al2O3 (0001) substrates using a pulsed laser deposition method. A single ordered phase (R3ˉ symmetry) is obtained under very limited deposition conditions including oxygen partial pressure of 2.0 x10-3 Pa and substrate temperature of 700 °C. The thin film with the ordered phase is an n-type semiconductor and ferrimagnetic with the Curie temperature >400 K. The Hall effect measurements at room temperature suggest that the carrier spins are polarized. [ABSTRACT FROM AUTHOR]

Epitaxial thin films composed of 0.7FeTiO3·0.3Fe2O3 solid solution have been prepared on α-Al2O3 (0001) substrates by a pulsed laser deposition method, and their electrical and magnetic properties have been examined. A single phase of the ordered phase can be obtained under limited deposition conditions: oxygen partial pressure of 1.0×10-3 Pa and substrate temperature of 600–700 °C. The as-deposited film is semiconducting and ferrimagnetic below room temperature, while subsequent annealing in vacuum leads to the Curie temperature above room temperature. On the other hand, the thin films with the disordered phase appear to be antiferromagnetic and also insulating. [ABSTRACT FROM AUTHOR]

The authors report on an intense blue emission centered at 420 nm under ultraviolet excitation from pentavalent tantalum ion-doped silicate glasses. The blue emission is distinctly different from emission due to defects in silica glass, which is also reflected in both absorption spectrum and remarkably enhanced emission. The authors suggest that localized Ta 5d0 energy level is responsible for the blue emission. [ABSTRACT FROM AUTHOR]

Mechanical milling of a mixture of powders composed of 2 mol % MnO2 and 98 mol % ZnO leads to a ferromagnetic phase with Curie temperature above 300 K. The magnetic field dependence of magnetization manifests a residual magnetization of 2.84×10-4 emu/g and a coercive force of 76 Oe at 300 K; the residual magnetization of the as-milled sample is larger than those of heat-treated samples. It is speculated that the ferromagnetic behavior is imposed to Zn-doped manganese oxide phases formed on the surface of MnO2 particles via the mechanical milling. [ABSTRACT FROM AUTHOR]

Studies the effect of poling on surface crystallization behavior of BaTiO[sub 3] in 15BaO-15TiO[sub 2]-70TeO[sub 2] glass and on second-harmonic generation of the resultant glass ceramics. Increase in the fraction of crystalline phases precipitated at the glass surface; Results of x-ray diffraction studies.

We have prepared a three-dimensional magnetophotonic crystal and observed modulated Faraday rotation around its photonic band gap. The magnetophotonic crystal was prepared by the self-assembly of highly monodispersed silica spheres, which were heavily impregnated with maghemite (γ-Fe2O3) nanoparticles. The slab sample with a thickness of 10 μm shows a clear photonic band gap centered at λ=620 nm in optical transmittance. Faraday rotation is notably modified inside the photonic band gap. [ABSTRACT FROM AUTHOR]