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The performance of ultraviolet (UV) laser diodes (LDs) with a low Al mole fraction AlGaN cladding layer was investigated by varying the thicknesses of the waveguide layer. It is found that (1) the loss of carriers in the waveguide layer is much larger than that in blue or green LDs due to the shallower quantum well and consequently a weaker carrier confinement of UV lasers. (2) Carrier loss in the waveguide layer can be suppressed by using a thinner waveguide layer. Therefore, the threshold current of LDs can be reduced. (3) The ultraviolet GaN/AlGaN single quantum well laser diodes lasing at 366 nm are fabricated by using relatively thin AlGaN waveguide layers and low Al mole fraction AlGaN cladding layers. [ABSTRACT FROM AUTHOR]

Coherently strained BiFeO3 epitaxial films deposited on (001)-oriented (LaAlO3)0.3(Sr2AlTaO6)0.7 have a tetragonal crystal form, a stress-distorted version of the rhombohedral phase. A conversion from coherent BiFeO3 to a new, tilted pseudotetragonal phase with the c/a ratio exceeding 1.2 is observed beyond the critical thickness of 60 nm. X-ray reciprocal space maps display that this highly elongated metastable structure is monoclinically distorted by ~0.2° and exhibits an out-of-plane tilt of ~3°. These observations are at odds with traditional understandings that a coherent epilayer should turn into its parent structure upon increasing the thickness, providing a new insight into the strain relaxation mechanism of epitaxial films. We show that in the heating and cooling cycles, the transition between these two phases is completely reversible and is associated with the alleviation of thermal stress. Our results reveal that the coherent BiFeO3 epilayer with tetragonal symmetry stabilized by moderate compressive strain behaves as a structural bridge that links the thermally stable rhombohedral phase and the metastable tetragonal-like phase with a giant axial ratio. Moreover, the finding of a BiFeO3 phase mixture in our study extends the threshold in-plane strain of the stress-driven morphotropic phase boundary to a value as low as -2.3%. [ABSTRACT FROM AUTHOR]

Hysteresis behavior for ultrathin magnetic films was investigated. Control of interface morphology becomes very important in designing ultrathin magnetic layers.

NaZn13-type La(Fe0.94Co0.06)11.8Si1.2 alloys were manufactured to investigate the influence of their microstructural change on magnetic refrigeration performance during magnetic field cycling. The magnetic refrigeration performance measurements indicate that both the large magnetic entropy change value (ΔS=14.1 J kg-1 K-1) and maximum adiabatic temperature change (Δ=2.2K) are favorable for the alloys to be superior candidate of magnetic refrigerants. However, the alloys exhibit nearly 10% decrease of ΔS and ΔT when they performed cycling ten-thousand times. More than thousand times of cycles induce local stress and grain cleavages presented by the accumulation of irreversible microstructure changes such as micro-cracks and sub-boundaries. According to the domain observation for the alloys with different field cycles, these microstructure characteristics accompany with the reconfiguration of the local magnetic domains and increase of domain wall energy, which are considered to be the reason of the decrease of ΔS. [ABSTRACT FROM AUTHOR]

The optical and structural properties of InGaN/GaN multi-quantum wells (MQWs) with different thicknesses of low temperature grown GaN cap layers are investigated. It is found that the MQW emission energy red-shifts and the peak intensity decreases with increasing GaN cap layer thickness, which may be partly caused by increased floating indium atoms accumulated at quantum well (QW) surface. They will result in the increased interface roughness, higher defect density, and even lead to a thermal degradation of QW layers. An extra growth interruption introduced before the growth of GaN cap layer can help with evaporating the floating indium atoms, and therefore is an effective method to improve the optical properties of high indium content InGaN/GaN MQWs. [ABSTRACT FROM AUTHOR]

The significant effect of the thickness of Ni film on the performance of the Ohmic contact of Ni/Au to p-GaN is studied. The Ni/Au metal films with thickness of 15/50 nm on p-GaN led to better electrical characteristics, showing a lower specific contact resistivity after annealing in the presence of oxygen. Both the formation of a NiO layer and the evolution of metal structure on the sample surface and at the interface with p-GaN were checked by transmission electron microscopy and energy-dispersive x-ray spectroscopy. The experimental results indicate that a too thin Ni film cannot form enough NiO to decrease the barrier height and get Ohmic contact to p-GaN, while a too thick Ni film will transform into too thick NiO cover on the sample surface and thus will also deteriorate the electrical conductivity of sample. [ABSTRACT FROM AUTHOR]

In Mn50Ni41-xSn9Cox ribbons, the exchange bias field is very sensitive to the Co content. Based on both theoretical and experimental studies, it has been found that with increasing Co content, the pinned phase (ferromagnetic phase) remains almost unchanged while the pinning phase is changed from a canonical spin glass to a cluster spin glass and finally to a ferromagnetic phase. Changing the Co content in Mn50Ni41-xSn9Cox alloys has been proven to be an effective way of tuning the magnetic anisotropy and the phase structure of the pinning phase. With different Co contents, a continuous tuning of the exchange bias field from 345 Oe to 3154 Oe is realized. [ABSTRACT FROM AUTHOR]

The influence of unintentionally doped carbon impurities on electrical resistivity and yellow luminescence (YL) of low-temperature (LT) grown Mg doped GaN films is investigated. It is found that the resistivity of Mg doped GaN films are closely related to the residual carbon impurity concentration, which may be attributed to the compensation effect of carbon impurities. The carbon impurity may preferentially form deep donor complex CN-ON resulting from its relatively low formation energy. This complex is an effective compensate center for MgGa acceptors as well as inducing YL in photoluminescence spectra. Thus, the low resistivity LT grown p-type GaN films can be obtained only when the residual carbon impurity concentration is sufficiently low, which can explain why LT P-GaN films with lower resistivity were obtained more easily when relatively higher pressure, temperature, or NH3/TMGa flow rate ratio were used in the LT grown Mg doped GaN films reported in earlier reports. [ABSTRACT FROM AUTHOR]

In metallic glasses (MGs), a clear correlation has been established between plasticity and the Poisson's ratio. Such a correlation between the two distinctive macroscopic mechanical properties is challenging to explain from a microstructure perspective. We studied the microstructural origin of the Poisson's ratio and plasticity criterion in various MGs and find a correlation between the relative concentration of flow units and Poisson's ratio: the MGs with higher concentration of flow units show a larger Poisson's ratio and better plasticity. We have explained the empirical correlation between ductility and the Poisson's ratio based on microscopic flow units in MGs. [ABSTRACT FROM AUTHOR]

The effect of quantum barrier (QB) thickness on performances of InGaN/GaN multiple-quantum-well light-emitting diodes (MQW LEDs) with relatively large barrier layer thicknesses has been investigated. It is found that the density and averaged size of V-defects increases with QB thickness, resulting in larger reverse- and forward-bias current in LEDs. Electroluminescence measurement shows that LED with thinner QB has higher internal quantum efficiency but lower efficiency droop-onset current density, which should be ascribed to the faster saturation of carrier leakage into V-defects. Correspondingly, above the droop-onset current density, severer Auger recombination and carrier overflow are induced by higher carrier density due to the less V-defect related carrier leakage, leading to the more serious droop phenomenon in LEDs with thinner QB. [ABSTRACT FROM AUTHOR]

It has been postulated that metallic glasses, in contrast to their crystalline counterparts, exhibit nano-scale structural heterogeneity which is crucial for understanding the long-standing issues of relaxations and deformation of glasses. We fabricate micrometer scale metallic glassy fibers (MGFs) with different diameters and structural configurations, and find that the thinner MGFs cooled down with faster cooling rates have smaller superconducting transition temperatures and wider transition widths. We show that the superconducting properties correlate with the heterogeneous microstructure of metallic glasses and can be used as a novel way to experimentally characterize the structural heterogeneity of metallic glasses. [ABSTRACT FROM AUTHOR]

We characterize the evolution of flow units associated with the flow 'defects' in metallic glass by monitoring the fictive temperature change of a typical metallic glass upon isothermal annealing below its glass transition temperature. The correlations between the fictive temperature Tf, enthalpy change, and the concentration of flow units have been obtained. Such correlations help in understanding the evolution process of flow units, structural feature, and structural relaxation behaviors in metallic glasses, and can rationalize effects of the cooling rate, aging, and annealing on properties and structure of metallic glasses. [ABSTRACT FROM AUTHOR]

The unique thermoplastic forming ability of metallic glasses in their supercooled liquid state makes them the ideal embossing materials for miniature fabrication. However, the understanding and controlling of micro filling process that is crucial for miniature fabrication and their applications remain fundamental, yet presently unresolved issues. Here, the mold filling kinetics of a model Pd-based metallic glass in supercooled liquid state is studied using different Si micro molds with different channels. A universal kinetic equation, which can describe the filling kinetics of viscous metallic supercooled liquid in micro molds with irregular shapes, is obtained. [ABSTRACT FROM AUTHOR]

In the structure of conventional InGaN-based laser diodes, considerable optical absorption loss exists in the p-doped region, especially in the p-doped upper waveguide layer. A structure of InGaN-based laser diodes with an undoped InGaN upper waveguide layer instead of p-doped GaN was designed and optimized. Instead of being located between AlGaN EBL and AlGaN cladding layer, the undoped InGaN layer was positioned between the AlGaN electron blocking layer and the active region of multi-quantum well. The optical and electrical characteristics of this type of laser diodes were simulated by the commercial software lastip, and it was found that the optical absorption loss was obviously lowered. Further theoretical analysis showed that the undoped InGaN upper waveguide layer eliminated the influence of electron leakage current, leading to a reduction of threshold current density and an increase of output light power of the InGaN-based laser diodes. [ABSTRACT FROM AUTHOR]

Bulk metallic glasses have excellent thermoforming ability in their wide supercooled liquid region. We show that large-size metallic glass grating (∼8 × 8 mm2) with fine periodicity and ultra smooth surface feature can be readily fabricated by hot embossing. The method for fabrication of gratings is proved to be much cheaper, and requires low pressure and short time (less than 30 s). The metallic glass gratings exhibit comparable optical properties such as rainbow-like spectrum when shone by fluorescent lamp light. [ABSTRACT FROM AUTHOR]

We have investigated the effect of light Si-doping on the optical properties of high quality GaN films with the method of low temperature photoluminescence. It is found that the peak (Ix) at 3.473 eV always appears in the photoluminescence spectra of lightly Si-doped GaN. The relative intensity of peak Ix to heavy-hole free exciton peak increases linearly with the increasing concentration of Si doping, providing a strong support to the assignment that Ix originates from inelastic scattering of free excitons by Si donors. In addition, a rarely reported peak (Px) at 3.365 eV can only be clearly observed in the PL spectrum of unintentionally doped GaN sample. Its intensity is found to reduce dramatically with the decrease of residual carbon concentration based on the secondary ion mass spectrometry analysis. Px is attributed to the excitons bound to carbon-related complex defects. [ABSTRACT FROM AUTHOR]

The spectral responses of back-illuminated p-i-n AlGaN ultraviolet photodetector under different bias voltage are investigated. The device structure is composed of p-GaN/p-Al0.3Ga0.7N/i-Al0.3Ga0.7N/n-Al0.6Ga0.4N and Ohmic contacts, where the p-GaN layer is very thin. There are two peaks in the spectral response of photocurrent located at 350 and 290 nm, respectively. It is found that in some devices the relative intensity and phase of these two peaks may change strongly with applied forward bias. A detailed analysis suggests that the possible Schottky-type-like behavior of metal and p-GaN contact and an unsatisfactory doping of p-GaN and p-AlGaN are responsible for the abnormal phenomenon. [ABSTRACT FROM AUTHOR]

The carrier distribution and recombination dynamics of InGaN/GaN multiple quantum well (MQW) light-emitting diode structure are investigated. Two emission peaks were observed in the low temperature photoluminescence spectra of an InGaN/GaN MQW structure, but only one peak was observed in the electroluminescence (EL) spectra. Combined with the spatially resolved cathodoluminescence (CL) measurements, it is found that the electrically injected carrier distribution is governed by hole transport and diffusion in InGaN/GaN MQW structure due to the much lower mobility of hole. And the electron and hole recombination of EL occurs predominantly in the QWs that are located closer to the p-GaN layer. [ABSTRACT FROM AUTHOR]

Confinement factor and absorption loss of AlInGaN based multiquantum well laser diodes (LDs) were investigated by numerical simulation based on a two-dimensional waveguide model. The simulation results indicate that an increased ridge height of the waveguide structure can enhance the lateral optical confinement and reduce the threshold current. For 405 nm violet LDs, the effects of p-AlGaN cladding layer composition and thickness on confinement factor and absorption loss were analyzed. The experimental results are in good agreement with the simulation analysis. Compared to violet LD, the confinement factors of 450 nm blue LD and 530 nm green LD were much lower. Using InGaN as waveguide layers that has higher refractive index than GaN will effectively enhance the optical confinement for blue and green LDs. The LDs based on nonpolar substrate allow for thick well layers and will increase the confinement factor several times. Furthermore, the confinement factor is less sensitive to alloys composition of waveguide and cladding layers, being an advantage especially important for ultraviolet and green LDs. [ABSTRACT FROM AUTHOR]

The relationship between band gap and biaxial stress in wurtzite ZnO thin films has been investigated by side-inclination x-ray diffraction technique and optical absorbance spectrum as well as ab initio calculation. The experimental result shows that differing from other semiconductor thin films with hexagonal structure, such as GaN, the band gap of ZnO thin films increases with the increase in biaxial tensile stress. For explaining the difference, ab initio calculation is performed to simulate the relationship between band gap and biaxial stress of wurtzite ZnO and GaN. The calculated result indicates that the band gap of ZnO increases under biaxial tensile stress but GaN is opposite, supporting our experimental result. The band offset calculation shows that the conduction-band minimum (CBM) and the valence-band maximum (VBM) of ZnO and GaN offset to low energy under biaxial tensile stress. The VBM offset of ZnO is larger than the CBM, responsible for the increase in band gap. The VBM offset of GaN is smaller than the CBM, responsible for the decrease in band gap. The difference between ZnO and GaN is attributed to the strong p-d coupling in valence band of ZnO, but strong p-p coupling in valence band of GaN. [ABSTRACT FROM AUTHOR]

It is studied whether there is any regular relationship between the yellow luminescence band and electron mobility of n-type GaN. For a series of GaN samples grown with the same Si doping, it is found that the electron mobility decreases with an increase of relative intensity of yellow luminescence, accompanied by an increase of edge dislocation density. Further research indicates that it is acceptors introduced by edge dislocations which lead to the concomitant changes of yellow luminescence and electron mobility. Similar changes are induced by Si doping in the n-type GaN samples with relatively low edge dislocation density. However, the relationship between the yellow luminescence and electron mobility of n-type GaN is not a simple one. A light Si doping may simultaneously increase yellow luminescence and electron mobility when Si doping plays a dominant role in reducing the carrier scattering. This means that even the intensity of yellow luminescence is often used as an indicator of material quality for GaN, it does not have any monotonous correlation with the electron mobility of GaN. [ABSTRACT FROM AUTHOR]

The recombination mechanisms of nitrogen-related emissions in p-type ZnO films were investigated by photoluminescence (PL) measurements. The enhanced peak at 3.260 eV was confirmed as a donor-acceptor pair (DAP) transition and the emission around 3.310 eV was assigned to the free electron to acceptor (FA) recombination by studying the temperature evolution of DAP and FA luminescence, calculating the energy level of the corresponding nitrogen acceptor, and measuring the decay time of DAP transition. By comparing the PL spectra of the samples with various hole concentrations, it can be found that as the hole concentration increases, the DAP peak significantly dominates the spectra. In addition, the acceptor binding energy is estimated to be about 120 meV from the FA transition, which is in good agreement with the value calculated by a hydrogenic acceptor model. [ABSTRACT FROM AUTHOR]

Field-induced unidirectional anisotropy of Co-based amorphous ribbons was discussed by magnetization measurement. The shifted hysteresis loops of Co58Fe5Ni10Si11B16 amorphous ribbons were obtained by annealing the samples in longitudinal magnetic field. Here, the feasibility of employing the longitudinal pulse field to tailor the anisotropy characteristic is demonstrated. It is found that the shifted loops can be technically controlled by enhancing the pulse field to modulate the magnetic anisotropy from unidirectional to uniaxial, and even back to unidirectional. The surface domains scan gives strong evidence that the pulse field can be one of the skillful methods to navigate the unidirectional anisotropy in the amorphous ribbons for potential applications. [ABSTRACT FROM AUTHOR]

Optical properties of light-hole free exciton (FXB) in GaN epilayers were investigated by using near-resonance photoluminescence (PL) and time-resolved PL techniques. In contrast to the case of off-resonance PL where only heavy-hole free excitons (FXA) have strong response, FXB band can be well resolved in the near-resonance PL spectra. The variable-temperature near-resonance PL spectra show that the linewidth of FXB broadens faster than the FXA with increasing temperature. Moreover, the luminescence lifetime of FXB is found to be shorter than that of FXA. [ABSTRACT FROM AUTHOR]

Exchange bias phenomena of nanocrystalline FeCuNbSiB ribbons have been investigated at room temperature. It was found that the rougher surface of ribbons annealed in longitudinal field played a main role in bringing the bias behavior in our ribbons. The topography of ribbon surfaces and the configuration of magnetic domains were specially observed in order to make clear the dependence of bias behavior on the induced magnetic anisotropy. A simple phenomenological explanation was also put forward to promote the comprehension of loop shift in ribbons and its possibility of potential applications. [ABSTRACT FROM AUTHOR]

Time-resolved light-current curves, spectra, and far-field distributions of ridge structure InGaN multiple quantum well laser diodes grown on sapphire substrate are measured with a temporal resolution of 0.1 ns under a pulsed current condition. Results show that the thermal lensing effect clearly improves the confinement of the higher order modes. The thermal lens leads to a lower threshold current for the higher order modes, a higher slope efficiency, and a change in the lasing mode of the device. The threshold current for the higher modes decreases by about 5 mA in every 10 ns in a pulse, and the slope efficiency increases by 7.5 times on the average when higher modes lase. [ABSTRACT FROM AUTHOR]

The glass transition and the microstructural characteristics of (Zr[sub 0.59]Ti[sub 0.06]Cu[sub 0.22]Ni[sub 0.13])[sub 100-x]Al[sub x] ( 10≤x≤16) bulk metallic glasses (BMG) are investigated as function of Al content. It is found that the Al content has strong effects on the glass transition, microstructure, and properties of the alloy. The marked increase of the Debye temperature and shear modulus with increasing Al content indicates that the different Al incorporations result in microstructural change in atomic short range of the BMG. The increase of the glass transition temperature with the increase Al content is consistent with the large variation of properties with increase of Al content. The change of the microstructure of the glass-forming alloy is responsible for the change of the properties. [ABSTRACT FROM AUTHOR]

Reports that strains in cubic GaN films grown on GaAs were measured by a triple-axis x-ray diffraction method. Residual strains in the as-grown epitaxial films; Dependence of the relief of strains in the GaN films on the growth conditions.

Studies the pressure dependence of ultrasonic attenuation in Zr[sub 41]Ti[sub 14]Cu[sub 12.5]Ni[sub 10-x]Be[sub 22.5]C[sub x] bulk metallic glasses by using a pulse echo overlap method. Glass forming ability; Effects of carbon addition on changes of the alpha[1] and alpha[sub s].