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The radiation effects of metal-oxide-semiconductor (MOS) capacitors based on an Al2O3 gate dielectric were studied using 30 MeV Si heavy ions. To give insights into the types of defects induced by Si ion irradiation in the gate oxide, synergistic experiments involving γ-ray irradiation and Si ion irradiation were carried out. The results revealed that the defects in the as-grown Al2O3 layer were found to be hole trapping centers, whereas Si ion induced new defects were electron trapping centers. The experimental and simulation data of current density-voltage (J-V) curves confirmed that the transmission mechanism of leakage current in the Al2O3 layer was mainly dominated by the Frenkel-Poole mechanism. In detail, the variations of leakage current were mainly due to the attributions of the local built-in electric field-assisted current leakage and the conductive path-assisted current leakage and were found to be dependent on the irradiation condition and Al2O3 thickness. Lastly, the decrease of the gate oxide capacitance of MOS capacitors was attributed to the increase of the series resistance and leakage current in MOS capacitors. [ABSTRACT FROM AUTHOR]

Ferroelectric switching is unambiguously demonstrated for the first time in a III-V semiconductor based material: Al1-xScxN—A discovery which could help to satisfy the urgent demand for thin film ferroelectrics with high performance and good technological compatibility with generic semiconductor technology which arises from a multitude of memory, micro/nano-actuator, and emerging applications based on controlling electrical polarization. The appearance of ferroelectricity in Al1-xScxN can be related to the continuous distortion of the original wurtzite-type crystal structure towards a layered-hexagonal structure with increasing Sc content and tensile strain, which is expected to be extendable to other III-nitride based solid solutions. Coercive fields which are systematically adjustable by more than 3 MV/cm, high remnant polarizations in excess of 100 μC/cm2—which constitute the first experimental estimate of the previously inaccessible spontaneous polarization in a III-nitride based material, an almost ideally square-like hysteresis resulting in excellent piezoelectric linearity over a wide strain interval from −0.3% to + 0.4% and a paraelectric transition temperature in excess of 600 °C are confirmed. This intriguing combination of properties is to our knowledge as of now unprecedented in the field of polycrystalline ferroelectric thin films and promises to significantly advance the commencing integration of ferroelectric functionality to micro- and nanotechnology, while at the same time providing substantial insight to one of the central open questions of the III-nitride semiconductors—that of their spontaneous polarization. [ABSTRACT FROM AUTHOR]

The energy barrier heights between two recently reported Ta-based amorphous metals (TaWSi and TaNiSi), TaN, and atomic layer deposited Al2O3 and HfO2 insulators are measured in metal/insulator/metal (MIM) structures with Au top electrodes using internal photoemission (IPE) spectroscopy. For Al2O3, the Ta-based metal barrier heights, φBn, increase with increasing metal work function, ΦM, for TaN, TaWSi, and TaNiSi, respectively. For HfO2, however, the barrier heights are relatively constant for all three metals φBn,TaNiSi ≈ φBn.TaWSi ≈ φBn.TaN. The difference between HfO2 and Al2O3 is attributed to enhanced Fermi-level pinning due to a larger dielectric constant. The slope parameter, S, was found to be roughly 0.89 and 0.44–0.69 for Al2O3 and HfO2, respectively. For devices with a TaWSi bottom electrode, a comparison was also made between Al and Au top electrodes. Significantly, smaller barrier heights were obtained with an Au top electrode than with an Al top electrode, 0.6 eV and 0.8 eV lower for HfO2 and Al2O3, respectively. IPE energy barriers are found consistent with current-voltage asymmetry of MIM diodes, whereas Schottky model predictions of barrier heights were inconsistent. [ABSTRACT FROM AUTHOR]

A reactor based on the association of a pulsed laser nanoparticle source and a pulsed laser deposition process is presented. This process uses two independent nano-second pulsed lasers. The nanoparticle source, based on a quenching of a plasma plume, is accurately described. Nanoparticles produced by this dedicated laser-based source present intrinsically quasi-monodisperse size in the range of 1–10 nm. This monodispersity is essential to correlate nanoparticle size and related properties. For silver nanoparticles, a change in a characteristic parameter, the opening t-time of the quenching valve, (conditioning the species residence time) from 280 μs to 580 μs, leads to a nanoparticle size increase by a factor of about two (from 2.5 nm to 4.3 nm). Consequently, the size modulation allows the synthesis of nanoparticles and resulting nanocomposites which present drastically different properties. A kinetic model in good agreement with the experiment shows two nanoparticle growth modes, i.e., a monomer accretion and a cluster coagulation. Revealing the potential of the pulsed laser reactor, different nanocomposite materials constituted by Ag nanoparticles associated with oxide thin films (Al2O3, VO2) matrix are optically characterized. A surface plasmon resonance (λSPR) in the visible–near IR regime is evidenced, and huge modulation and tunability are obtained linked to the host matrix nature and nanocomposite architecture. Moreover, the metal insulator transition capacity of a vanadium dioxide matrix makes λSPR tunable as a function of temperature. This type of nanocomposite appears pertinent for their great potential in both nano-photonics and nano-sensors. [ABSTRACT FROM AUTHOR]

Complex oxide heterointerfaces with the two-dimensional electron gas have drawn a lot of attention due to their emerging properties. However, most of them are prepared using the method with relatively high energy particles, which would inevitably lead to some defects. Here, a key challenge in the quasi-two-dimensional electron gas at spinel-type Al2O3/SrTiO3 heterointerfaces using a spin coating method is addressed. The grown Al2O3 films have the atomic-scale smooth surface and the thickness is about 70 nm. The γ-Al2O3 layer with a thickness of about 3 nm in proximity to SrTiO3 and the amorphous Al2O3 layer on the top of γ-Al2O3 are observed for the heterointerface annealed at 800 °C. The heterointerfaces at annealed temperatures above 750 °C exhibit a metallic behavior, which is attributed to the dominant layer of γ-Al2O3. The sheet carrier density is about 3.1 × 1015 cm−2 and the Hall mobility is 4924.4 cm2 V−1 s−1 at 15 K at the heterointerface annealed at 800 °C. Our work provides a low-cost way for the large-scale and large-area production of two-dimensional electron gas at high-quality oxide interfaces. [ABSTRACT FROM AUTHOR]

Scalable architectures characterized by quantum bits (qubits) with low error rates are essential to the development of a practical quantum computer. In the superconducting quantum computing implementation, understanding and minimizing material losses are crucial to the improvement of qubit performance. A new material that has recently received particular attention is indium, a low-temperature superconductor that can be used to bond pairs of chips containing standard aluminum-based qubit circuitry. In this work, we characterize microwave loss in indium and aluminum/indium thin films on silicon substrates by measuring superconducting coplanar waveguide resonators and estimating the main loss parameters at powers down to the sub-photon regime and at temperatures between 10 and 450 mK. We compare films deposited by thermal evaporation, sputtering, and molecular beam epitaxy. We study the effects of heating in a vacuum and ambient atmospheric pressure as well as the effects of pre-deposition wafer cleaning using hydrofluoric acid. The microwave measurements are supported by thin film metrology including secondary-ion mass spectrometry. For thermally evaporated and sputtered films, we find that two-level state are the dominant loss mechanism at low photon number and temperature, with a loss tangent due to native indium oxide of ∼ 5 × 10 − 5 . The molecular beam epitaxial films show evidence of the formation of a substantial indium-silicon eutectic layer, which leads to a drastic degradation in resonator performance. [ABSTRACT FROM AUTHOR]

ScAlMgO4—easily cleaved in c-plane—forms a natural superlattice structure of a ScO2 layer and two Al0.5Mg0.5O layers stacking along c-axis. ScAlMgO4 is one of the RAMO4-type layered multicomponent oxides and a promising lattice-matching substrate material for InGaN and ZnO. Identification of the topmost layer and the surface atomic structure of the cleaved ScAlMgO4 (0001) are investigated by the X-ray crystal truncation rod scattering method. It is confirmed that ScAlMgO4 is cleaved between the two Al0.5Mg0.5O layers. The two parts separated at this interlayer are inversion symmetric to each other and without surface charge. This prevents parallel-plate-capacitor-like electrostatic force during the cleavage. Two different mechanisms are proposed for the two types of cleavage caused by the impact of a wedge and by the in-plane stress due to an overgrown thick GaN film. It is also revealed that about 10%–20% of the topmost O atoms are desorbed during a surface cleaning at 600 °C in ultra-high vacuum. Surface observations using reflection high-energy electron diffraction are possible only after the high-temperature cleaning because the electrical conduction caused by the oxygen deficiency prevents the charge-up of the insulating sample. [ABSTRACT FROM AUTHOR]

The fragment impact response of two plastic-bonded explosive (PBX) formulations was studied using explosively driven aluminum fragments. A generic aluminum-capped detonator generated sub-mm aluminum particles moving at hypersonic velocities. The ability of these fragments to initiate reaction or otherwise damage two PBX materials was assessed using go/no-go experiments at standoff distances of up to 160 mm. Lower density PBX 9407 (RDX-based) was initiable at up to 115 mm, while higher density PBX 9501 (HMX-based) was only initiable at up to 6 mm. Several techniques were used to characterize the size, distribution, and velocity of the particles. Witness plate materials, including copper and polycarbonate, and backlit high speed video were used to characterize the distribution of particles, finding that the aluminum cap did not fragment homogeneously but rather with larger particles in a ring surrounding finer particles. Finally, precise digital holography experiments were conducted to measure the three-dimensional shape and size of the fastest-moving fragments, which ranged between 100 and 700 μm and traveled between 2.2 and 3.2 km/s. Crucially, these experiments showed variability in the fragmentation in terms of the number of fragments at the leading edge of the fragment field, indicating that both single and multiple shock impacts could be imparted to the target material. These types of data are critical for safety experiments and hydrocode simulations to quantify shock-to-detonation transition mechanisms and the associated risk-margins for these materials. [ABSTRACT FROM AUTHOR]

Three types of energetic semiconductor bridges (ESCBs) through integrating different Al/MoOx energetic multilayer nanofilms on a semiconductor bridge have been investigated in this study. The relationships among the critical firing energy, critical firing time, total firing time, and ignition energy as well as the input energy utilization efficiency of these initiators were analyzed. The principal component analysis (PCA) was utilized in the experiments to evaluate the output energy magnitude based on the ignition duration, the maximum flame area, reaction ratio, and other parameters. The results obtained were as follows: (1) The critical firing energy is positively proportional to the modulation periods of nanofilms for the initiators discharged with identical voltage, while the total firing energy and the input energy utilization efficiency do not change significantly; (2) by using PCA, the composite score of the ESCB/50 nm, ESCB/150 nm, ESCB/1500 nm, and SCB is measured at 3.025, 0.250, -1.433, and -1.842, respectively, discharge with 30 V/47 µF, which indicated that the output energy of ESCBs can be increased significantly by decreasing the modulation periods of nanofilms. [ABSTRACT FROM AUTHOR]

Vacancy-type defects in Al0.1Ga0.9N were probed using a monoenergetic positron beam. Al0.1Ga0.9N layers with different carbon doping concentrations ([C]=5x1017-8x1019cm-3) were grown on Si substrates by metalorganic vapor phase epitaxy. The major defect species in Al0.1Ga0.9N was determined to be a cation vacancy (or cation vacancies) coupled with nitrogen vacancies and/or with carbon atoms at nitrogen sites (CNs). The charge state of the vacancies was positive because of the electron transfer from the defects to CN-related acceptors. The defect charge state was changed from positive to neutral when the sample was illuminated with photon energy above 1.8 eV, and this energy range agreed with the yellow and blue luminescence. For the sample with high [C], the charge transition of the vacancies under illumination was found to be suppressed, which was attributed to the trapping of emitted electrons by CN-related acceptors. With increasing [C], the breakdown voltage under the reverse bias condition increased. This was explained by the trapping of the injected electrons by the positively charged vacancies and CN-related acceptors. [ABSTRACT FROM AUTHOR]

A prototype 200 µm diameter Al0.52In0.48P p+-i-n+ mesa photodiode (2 µm i-layer) was characterised at temperatures from 100 °C to -20 °C for the development of a temperature tolerant photon counting X-ray spectrometer. At each temperature, X-ray spectra were accumulated with the AlInP detector reverse biased at 0V, 5 V, 10 V, and 15V and using different shaping times. The detector was illuminated by an 55Fe radioisotope X-ray source. The best energy resolution, as quantified by the full width at half maximum (FWHM) at 5.9 keV, was observed at 15V for all the temperatures studied; at 100 °C, a FWHM of 1.57 keV was achieved, and this value improved to 770 eV FWHM at -20 °C. System noise analysis was also carried out, and the different noise contributions were computed as functions of temperature. The results are the first demonstration of AlInP's suitability for photon counting X-ray spectroscopy at temperatures other than ≈20 °C. [ABSTRACT FROM AUTHOR]

In this study, numerical simulations of suspensions of Al2O3 particles in corn oil at various volume fractions were performed to explore the mechanisms involved in the electro-mechanical behavior of positive polarized particles suspended in an insulated suspending medium traveling through a rotating flow channel while exposed to a nonuniform AC electric field. A theoretical model was developed to simulate the trajectories of individual particles under the action of dielectrophoretic (DEP), dipole-dipole interparticle, viscous, and gravitational forces in a laminar flow. The results demonstrated that particles began to aggregate along the edges of electrodes with electric field application. New particles coming from more distant regions then gradually joined the vertices of particle aggregates, leading to the growth of chain-like clusters along the electric field gradient. Predictions showed good agreement with the results observed in a previous experiment. The new findings of this study are that the DEP force always tends to confine particles near the electrode edges, whereas the clustering of particles on the electrodes arises from the competition between particle interaction due to the DEP force and the long-range dipole-dipole interparticle interaction with neighboring particles. This process became more apparent with higher volume fractions of particle. [ABSTRACT FROM AUTHOR]

In this paper, the performance of an Al0.52In0.48P 63Ni radioisotope cell is reported over the temperature range of -20 °C to 140°C. A 400 µm diameter p+-i-n+ (2 µm i-layer)Al0.52In0.48P mesa photodiode was used as a conversion device in a novel betavoltaic cell. Dark current measurements on the Al0.52In0.48P detector showed that the saturation current increased increasing the temperature, while the ideality factor decreased. The effects of the temperature on the key cell parameters were studied in detail showing that the open circuit voltage, the maximum output power, and the internal conversion efficiency decreased when the temperature was increased. At -20 °C, an open circuit voltage and a maximum output power of 0.52V and 0.28 pW, respectively, were measured. [ABSTRACT FROM AUTHOR]

Gated parallel wire structures obtained from inverted-modulation-doped heterojunctions made of high-In-content metamorphic InGaAs/InAlAs were investigated. The narrowest wire width was found to be ~190nm made using electron beam lithography and reactive ion etching. Magnetotransport was measured at low temperatures. Weak anti-localization and suppression with applied negative gate voltages were observed in low-mobility wide wires (1360 nm), which were considered for a two-dimensional system. The dependence on the gate voltage of spin-orbit coupling parameters was also obtained by fitting. The parameters decreased as the negative gate voltages increased. The trend might originate not from the electron system at the InGaAs/InAlAs interface but from the other electron system accumulated at the Al2O3/InGaAs interface, which can also contribute to conductivity. In high-mobility narrow wires (190 nm), which are close to a one-dimensional system, weak anti-localization peaks were still observed, indicating strong spin-orbit coupling. In addition, the critical widths of wires corresponding to zero conductance were estimated to be <100 nm. Therefore, our metamorphic modulation doped heterojunctions seem suitable for smaller spin-FETs. [ABSTRACT FROM AUTHOR]

Very thin aluminum-doped zinc oxide (AZO) films with a well-defined (0001) orientation and a surface roughness of 0.357 nm were deposited on amorphous glass substrates at a temperature of 200 °C by radio frequency magnetron sputtering, which are promising, particularly in terms of orientation evolution, surface roughness, and carrier transport, as buffer layers for the subsequent deposition of highly (0001)-oriented AZO polycrystalline films of 490 nm thickness by direct current (DC) magnetron sputtering. Sintered AZO targets with an Al2O3 content of 2.0 wt. % were used. DC magnetron sputtered AZO films on bare glass substrates showed a mixed (0001) and the others crystallographic orientation, and exhibited a high contribution of grain boundary scattering to carrier transport, resulting in reduced Hall mobility. Optimizing the thickness of the AZO buffer layers to 10 nm led to highly (0001)-oriented bulk AZO films with a marked reduction in the above contribution, resulting in AZO films with improved Hall mobility together with enhanced carrier concentration. The surface morphology and point defect density were also improved by applying the buffer layers, as shown by atomic force microscopy and Raman spectroscopy, respectively. [ABSTRACT FROM AUTHOR]

The fragmentation and constitutive response of aluminum-based compacts were examined under dynamic conditions using mesostructured powder compacts in which the interfaces between the powders (sizes of 40, 100, and 400 μm) were tailored during the swaging fabrication process. Fragmentation was induced in ring samples of this material through explosive loading and was examined through high speed photography, laser interferometry, and soft capture of fragments. Fragment velocities of around 100 m/s were recorded. The fragment mass distributions obtained correlated in general with the interfacial strength of the compacts as well as with the powder size. Experimental results are compared with fragmentation theories to characterize the behavior of reactive powders based on the material's mesostructure by introducing the fracture toughness of the compacts. The mean fragment size is calculated using a modified form of Mott's theory and successfully compared with experimental results. [ABSTRACT FROM AUTHOR]

The effect of carbon doping on the structural and electrical properties of GaN buffer layer of AlGaN/GaN high electron mobility transistor (HEMT) structures has been studied. In the undoped HEMT structures, oxygen was identified as the dominant impurity using secondary ion mass spectroscopy and photoluminescence (PL) measurements. In addition, a notable parallel conduction channel was identified in the GaN buffer at the interface. The AlGaN/GaN HEMT structures with carbon doped GaN buffer using a CBr4 beam equivalent pressure of 1.86×10-7 mTorr showed a reduction in the buffer leakage current by two orders of magnitude. Carbon doped GaN buffers also exhibited a slight increase in the crystalline tilt with some pits on the growth surface. PL and Raman measurements indicated only a partial compensation of donor states with carbon acceptors. However, AlGaN/GaN HEMT structures with carbon doped GaN buffer with 200 nm thick undoped GaN near the channel exhibited good 2DEG characteristics. [ABSTRACT FROM AUTHOR]

Electronic and optical properties of non-trivial semipolar AlInGaN/InGaN quantum well (QW) structures are investigated by using the multiband effective-mass theory and non-Markovian optical model. On vicinal c-plane GaN substrate miscut by a small angle (θ<40°) from c-plane, the AlInGaN/InGaN system is shown to have ~3 times larger spontaneous emission peak intensity than the conventional InGaN/GaN system at green wavelength. It is attributed to much larger optical matrix element of the quaternary AlInGaN/InGaN system, derived from the reduction of internal electric field induced by polarizations. This effect exceeds the performance-degrading factor of smaller quasi-Fermi-level separation for the quaternary AlInGaN/InGaN system than that for the conventional InGaN/GaN system. Results indicate that the use of quaternary III-nitride QWs on vicinal substrates may be beneficial in improving the performance of optical devices emitting green light. [ABSTRACT FROM AUTHOR]

The temperature dependence of avalanche multiplication and breakdown voltage in Al0.52In0.48P, lattice-matched to GaAs, has been measured on a series of p+-i-n+ and n+-i-p+ diodes with nominal avalanche region thicknesses ranging from 0.068 to 1.0 µm from 77.8 to 298 K. From this, impact ionization coefficients as a function of temperature have been determined. For a given avalanche region thickness, Al0.52In0.48P exhibits temperature coefficient of breakdown voltage smaller than those of Ga0.52In0.48P and Al0.6Ga0.4As by approximately 1.6 x and 2.0 x, respectively. Our analysis shows that the alloy disorder potential and alloy composition ratio may be responsible for the large variation in temperature coefficient of breakdown voltages observed in a range of III-V ternary semiconductors. [ABSTRACT FROM AUTHOR]

Optical phonons are measured to probe the origins of the reported anomalously high piezoelectric response in aluminum scandium nitride (Al1-xScxN). Epitaxial layers with 0≤x≤0.16 deposited on sapphire(0001) exhibit a refractive index below the band gap, which increases from 2.03 for x = 0 to 2.16 for x = 0.16, corresponding to a dielectric constant ε∞= 4.15 + 3.2x. Raman scattering shows that zone-center E2(H) and A1(TO) phonon modes shift to lower frequencies with increasing x, following linear relationships: ω(E2(H)) = 658-233x (cm-1) and ω(A1(TO))=612-159x (cm-1). Similarly, zone-center E1(TO) and A1(LO) phonon mode frequencies obtained from specular polarized infrared reflectance measurements red-shift to ω(E1(TO)) = 681-209x (cm-1) and ω(A1(LO)) = 868-306x (cm-1). The measured bond angle decreases linearly from 108.2° to 106.0°, while the length of the two metal-nitrogen bonds increase by 3.2% and 2.6%, as x increases from 0 to 0.16. This is associated with a 3%-8% increase in the Born effective charge and a simultaneous 6% decrease in the covalent metal-N bond strength, as determined from the measured vibrational frequencies described with a Valence-Coulomb-Force-Field model. The overall results indicate that bonding in Al-rich Al1-xScxN qualitatively follows the trends expected from mixing wurtzite AlN with metastable hexagonal ScN. However, extrapolation suggests non-linear composition dependencies in bond angle, length, and character for x ≥ 0.2, leading to a structural instability that may be responsible for the reported steep increase in the piezoelectric response. [ABSTRACT FROM AUTHOR]

This paper reports an efficient method to improve the crystal quality of GaN Gunn diode with AlGaN hot electron injecting layer (HEI). An evident reduction of screw dislocation and edge dislocation densities is achieved by the strain management and the enhanced lateral growth in high temperature grown AlGaN HEI layer. Compared with the top hot electron injecting layer (THEI) structure, the bottom hot electron injecting layer (BHEI) structure enhances the crystal quality of transit region due to the growth sequence modulation of HEI layer. A high Hall mobility of 2934 cm2/Vs at 77 K, a nearly flat downtrend of Hall mobility at the temperature ranging from 300 to 573 K, a low intensity of ratio of yellow luminescence band to band edge emission, a narrow band edge emission line-width, and a smooth surface morphology are observed for the BHEI structural epitaxy of Gunn diode, which indicates that AlGaN BHEI structure is a promising candidate for fabrication of GaN Gunn diodes in terahertz regime. [ABSTRACT FROM AUTHOR]

An extraordinary enhancement of magneto-optical Kerr effect was demonstrated in HfO2/Co/HfO2/Al/substrate composite structure by experimental measurements as well as theoretical calculations. Giant polar Kerr rotation was observed within a broad spectral region, and especially a peak of -7.92° was found at wavelength of 496 nm, which is nearly forty times as large as that of a single Co film. In addition, a Kerr rotation reversal was also observed in short wavelength region in the longitudinal geometry. We proposed that the multiple reflections and optical interference in the optical cavities lead to the enormous enhancement and modulation of the Kerr rotation. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) of high-density GaAs nanowires (NWs) encapsulated by a double AlGaAs/GaAs shell is studied. Two lines are found and assigned to the radiative recombinations of photoexcited electrons confined in the center of the GaAs core and at the heteroboundary between the outer GaAs shell and the inner AlGaAs one with the holes in the core and the holes confined at the heteroboundary between the core and the inner AlGaAs shell. The simple model, based on representation of the valence band structure using two levels, well accounts for the observed temperature dependence of the integrated photoluminescence intensities. The proposed double shell structure with tunneling transparent inner shell sets conditions for easy control of the emission energy of the heterostructured nanowires. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of nearly lattice matched n-type InxAl1-xN (x ≈ 0.18) grown by metal organic chemical vapor deposition (MOCVD) are investigated with particular attention to the potentially conductive GaN template and InAlN/GaN interfacial polarization charges. The thermoelectric properties of InAlN are measured over a range of carrier densities and through temperatures as high as 815 K. The maximum room temperature ZT was found to be 0.007 at a carrier density of 6.4 × 1019 cm-3. The ZT of InAlN at this carrier density increases to 0.05 at 815 K. It is also shown that the interfacial charge in InAlN/GaN structures and the resulting two dimensional electron gas (2DEG) lead to greatly improved electron mobility and power factor when 2DEG conduction is dominant. Using this strategy, a 250% improvement in power factor is realized as the thickness of InAlN is decreased from 290 nm to 34 nm. Methods for extending these power factor enhancements to thicker materials are discussed. [ABSTRACT FROM AUTHOR]

Combustion temperatures from impact initiated nickel-aluminum (NiAl) and aluminum-polytetrafluoroethylene (Al-PTFE) materials have been measured using a high-speed two-camera imaging pyrometer. The materials were launched with a nominal velocity of 1700 m/s into a sealed chamber. Upon impact into a steel anvil chemical reactions were initiated and a flame propagated through the chamber. The measured temperature after impact was 3600 K (NiAl) and 3300 K (Al-PTFE). [ABSTRACT FROM AUTHOR]

The effect of microstructure on electromagnetic properties was investigated in Al18B4O33w/Co and Al18B4O33w/FeCo composite particles. Resistivity (ρ) and saturation magnetization (Ms) were introduced to determine the relationship between microstructure and electromagnetic properties indirectly. The ρ of Co coatings decreased by nine orders of magnitude when annealed in H2, which substantially affected permittivity and permeability. Intense dielectric relaxation occurred in 2 GHz to 18 GHz band, particularly when the ρ was lower than 1.16 × 10-3 Ω·cm, and a strong eddy current effect began to emerge when the ρ decreased to 5 × 10-4 Ω·cm. An unusual increase in the permeability of the Co coatings was identified, and enhanced exchange-coupling interaction was correspondingly revealed, which was assumed to be related to microstructure evolution. The influence of microstructure observed in Al18B4O33w/Co was further found in the Al18B4O33w/CoFe composite particles. This study suggested that the introduction of ρ and Ms was helpful in demonstrating the relationships between microstructure and electromagnetic properties. However, the quantitative characterization of microstructures and the identification of intrinsic electromagnetic parameters should be studied further. [ABSTRACT FROM AUTHOR]

We investigate the influence of the AlGaN barrier thickness in Al0.14Ga0.86N/GaN heterostructures on both the electrical properties of the heterostructure itself as well as on high electron mobility transistors fabricated on these structures. With increasing barrier thickness, we observe decreasing sheet resistances, transconductances, and threshold voltages. The observed changes are well-described by modelling. We demonstrate that an increase in the barrier thickness in AlGaN/GaN heterostructures results in an increase in available high-frequency input power swing before turn-on of the Schottky gate. The device long-term stability under direct current stress is not affected by the increase in barrier thickness as shown by on-wafer reliability tests at 150 °C base plate temperature. These results pave the way towards AlGaN/GaN transistors offering high robustness under extreme mismatch conditions as well as excellent high-frequency power performance. [ABSTRACT FROM AUTHOR]

Very low density growth of GaAs quantum dots in self-assembled nanoholes created by gallium droplet etching is demonstrated. The emission energy of the quantum dots can be accurately controlled by the GaAs deposition amount, from 1.8 to 1.6 eV, independently of the dot density which can be reproducibly controlled over the range 0.2-2×10-8 cm-2 by the gallium deposition rate. The ensemble full-width-half-maximum is <10 meV and single-dot linewidths of 40 μeV (limited by our spectral resolution) have been measured. Additionally, shallow mounds on the sample surface allow the buried GaAs/AlGaAs dots to be located. A simple method to reliably predict the emission energy based on the shape of the nanohole is presented. [ABSTRACT FROM AUTHOR]

We report the growth and characterization of Co2FeAl nanowires. Nanowires are grown using electrospinning method and the diameters range from 50 to 500 nm. These nanowires exhibit cubic crystal structure with a lattice constant of a=5.639 Å. The nanowires exhibit ferromagnetic behavior with a very high Curie temperature. The temperature dependent magnetization behavior displays an anomaly in the temperature range 600-850 K, which disappears at higher external magnetic fields. [ABSTRACT FROM AUTHOR]

The formation of two-dimensional carrier gases in gated GaN/AlGaN/GaN heterostructures is investigated theoretically. It is shown that under certain conditions a two-dimensional hole gas at the upper GaN/AlGaN interface can be formed in addition to the two-dimensional electron gas at the lower AlGaN/GaN interface. For the calculations, a Schrödinger-Poisson solver and a simple analytical model developed in the present work are used. Conditions for the formation of a two-dimensional hole gas are elaborated. It is shown that once a two-dimensional hole gas is created, it shields the coexisting two-dimensional electron gas which will result in a diminishing effect of the gate voltage on the two-dimensional electron gas. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the dependence of elastic constants cij on composition for random wurtzite quaternary AlxGayIn1-x-yN alloy in the whole concentration range. The study takes as its starting point the cij parameters for zinc blende phase calculated earlier by the authors on the basis of valence force field model. To obtain the wurtzite constants from cubic material parameters the Martin transformation is used. The deviations from linear Vegard-like dependence of cij on composition are analyzed and accurate quadratic fits to calculated moduli are presented. The influence of nonlinear internal strain term in the Martin transformation is also investigated. Our general results for quaternary AlxGayIn1-x-yN alloys are compared with the recent ab initio calculations for ternaries GaxIn1-xN and AlxIn1-xN (Gorczyca and Łepkowski, Phys. Rev. B 83 203 201, 2011) and good qualitative agreement is found. [ABSTRACT FROM AUTHOR]

An investigation on the influence of Al/CuO reactive multilayer films (RMFs) additives on exploding foil initiator was performed in this paper. Cu film and Cu/Al/CuO RMFs were produced by using standard microsystem technology and RF magnetron sputtering technology, respectively. Scanning electron microscopy characterization revealed the distinct layer structure of the as-deposited Al/CuO RMFs. Differential scanning calorimetry was employed to ascertain the amount of heat released in the thermite reaction between Al films and CuO films, which was found to be 2024 J/g. Electrical explosion tests showed that 600 V was the most matching voltage for our set of apparatus. The explosion process of two types of films was observed by high speed camera and revealed that compared with Cu film, an extra distinct combustion phenomenon was detected with large numbers of product particles fiercely ejected to a distance of about six millimeters for Cu/Al/CuO RMFs. By using the atomic emission spectroscopy double line technique, the reaction temperature was determined to be about 6000-7000 K and 8000-9000 K for Cu film and Cu/Al/CuO RMFs, respectively. The piezoelectricity of polyvinylidene fluoride film was employed to measure the average velocity of the slapper accelerated by the explosion of the films. The average velocities of the slappers were calculated to be 381 m/s and 326 m/s for Cu film and Cu/Al/CuO RMFs, respectively, and some probable reasons were discussed with a few suggestions put forward for further work. [ABSTRACT FROM AUTHOR]

The electrical properties, presence of deep electron and hole traps and photoluminescence spectra were measured for undoped a-GaN films grown by metal-organic chemical vapor deposition (MOCVD) in a two-stage process using a high V/III ratio at the first stage and low V/III ratio at the second stage. Growth was performed on r-sapphire substrates with a high temperature GaN nucleation layer. The films showed a full width at half maximum of 450-470 arcseconds for the (11-20) x-ray rocking curve with little anisotropy with respect to the sample rotation around the growth direction. The stacking fault (SF) density determined by selective etching was ∼5 × 104 cm-1. The residual donor concentration was 1014-1015 cm-3, with a very low density (2.5 × 1013 cm-3) of electron traps located at Ec - 0.6 eV, which are believed to be one of the major non-radiative recombination centers in nonpolar GaN. Consequently, the films showed a high intensity of bandedge luminescence with negligible contribution from defect bands associated with SFs. In contrast to previously studied nonpolar GaN films, the a-GaN layers showed a high concentration of gallium-vacancy-related acceptors near Ev + 1 eV and a strong yellow luminescence band, both indicating that growth conditions were effectively N-rich. a-AlGaN/GaN heterojunctions with thin heavily Si doped AlGaN barriers made on a-GaN substrates showed two-dimensional electron gas (2DEG) concentrations of 1.2 × 1013 cm-3, with 2DEG mobility of 80 cm2/Vs. Capacitance-voltage profiling of Schottky diodes on these HJs suggest that the 2DEG is fully depleted by the built-in voltage of the Schottky diode. [ABSTRACT FROM AUTHOR]

We report a simulation of quantum cascade lasers based on the integration of a number of optoelectronic models on both microscopic and macroscopic scales. On the microscopic scale, quantum mechanical computation was performed to find the quantization states and a rate equation approach was used to compute the optical gain. On the macroscopic scale, we solved the drift-diffusion equations with modification of current density to account for long-range carrier transport, including quantum tunneling, mini-band tunneling, and hot carrier transport. Multiple lateral optical modes were computed by solving a scalar wave equation as an eigenvalue problem. Finally, multiple lateral mode laser cavity photon rate equations were solved with the drift-diffusion equations in a self-consistent manner to predict the lasing characteristics of a quantum cascade laser. The simulation compared the integrated models with experimental data from a number of AlInGaAs/InP systems with variable quantum wells and at different temperatures. Reasonable agreements with experiments have been obtained for both electrical and lasing characteristics. [ABSTRACT FROM AUTHOR]

The accurate determination of the chemical composition of multicomponent antimonide layers still remains difficult. The problem becomes even more complicated when group III antimonides are grown in a MBE chamber that is also used for the growth of group III arsenides. In this paper, the composition of MBE grown AlGaAsSb layers is investigated in the context of unintentional arsenic incorporation. Control of the As concentration and a determination of the AlGaSb composition are crucial for an accurate calculation of the AlGaAsSb stoichiometry. It has been found that when using diffraction measurements with an As detection limit of 0.087%, the Al content in Al0.5Ga0.5Sb is determined with an accuracy of ±1%. Taking into account the GaSb reference from secondary ion mass spectrometry, the accuracy can be increased to ±0.52% of Al. The Al/Ga ratio determined for AlGaSb layers is further used for the calculation of the As content in AlGaAsSb alloys grown under the same technological conditions as the ternary layer. As a result, the procedure for the accurate determination of stoichiometry coefficients in this material is recommended. [ABSTRACT FROM AUTHOR]

The steady state and time-resolved photoluminescence (PL) spectra of cubic AlxGa1-xN have been measured for 0 < x < 1. The intensity of the room temperature PL increases by an order of magnitude when the AlN content increases from x = 0 to x = 0.95. Additionally, the PL decay slows down with the decrease of temperature and increase of x. These results show that strong localization of carriers on alloy composition fluctuations plays a large role in determining the intensity and temporal evolution of the PL. The activation energy for the localized carriers increases with the increase of x and reaches the value of 55 meV at x = 0.95. [ABSTRACT FROM AUTHOR]

Four-wave mixing (FWM) in an InxGa1-xAs/AlAsySb1-y intersubband transition (ISBT) optical waveguide operating in the 1.55-μm wavelength region is discussed. The third-order optical nonlinear susceptibilities of InxGa1-xAs/AlAsySb1-y multiple quantum wells (MQWs) are estimated theoretically. ISBT excitation causes large third-order optical nonlinearity. χ(3) values of more than 1 × 1016 m2/V2 are obtained. The propagation of interacting waves for FWM in InxGa1-xAs/AlAsySb1-y ISBT optical waveguides is discussed. The interaction length is limited not only by phase matching but also by ISBT absorption. Sufficiently high FWM conversion efficiencies were obtained theoretically and experimentally for realizing coherent wavelength convertors. FWM in an InxGa1-xAs/AlAsySb1-y ISBT waveguide is a promising method for realizing compact and high-efficiency coherent wavelength conversion. [ABSTRACT FROM AUTHOR]

In this work we report on the characteristics of a (Ni/Au)/AlGaN/GaN/SiC Schottky barrier diode (SBD). A variety of electrical techniques, such as gate current-voltage (I-V), capacitance-voltage (C-V), and deep level transient spectroscopy (DLTS) measurements have been used to characterize the diode. The behavior study of the series resistance, RS, the ideality factor, n, the effective barrier height, [uppercase_phi_synonym]b, and the leakage current with the temperature have emphasized an inhomogeneity of the barrier height and a tunneling mechanism assisted by traps in the SBD. Hence, C-V measurements successively sweeping up and down the voltage have demonstrate a hysteresis phenomenon which is more pronounced in the temperature range of 240 to 320 K, with a maximum at ∼300 K. This parasitic effect can be attributed to the presence of traps activated at the same range of temperature in the SBD. Using the DLTS technique, we have detected one hole trap having an activation energy and a capture cross-section of 0.75 eV and 1.09 × 10-13cm2, respectively, seems to be responsible for the appearance of the hysteresis phenomenon. [ABSTRACT FROM AUTHOR]

Secondary electron imaging combined with immersion lens and through-the-lens detection has been used to analyze various semiconductor junctions. Dopant contrast imaging was applied for multi-doped 4H-SiC, growth-interrupted n+/p and n/n+ homoepitaxial interfaces, and an AlGaAs/GaAs p-n junction light-emitting diode structure. Dopant contrast was explained by the local variation in secondary electron escape energies resulting from the built-in potential difference. The effect of varying electron affinity on contrast for the heterostructures is also discussed. The contrast profile of the n-doped AlGaAs compared reasonably well to the simulated valence bandedge energy using a previously determined efficiency of dopant ionization. [ABSTRACT FROM AUTHOR]

Degradation under high current stress of AlGaN quantum well based light emitting diodes emitting at 285 and 310 nm has been studied using electroluminescence, time-resolved photoluminescence and current-voltage experimental techniques. The measurements have revealed that during aging decrease of the emission intensity is accompanied by increase of the tunneling current, increase of the nitrogen vacancy concentration and partial compensation of the p-doping. The main role in the device degradation has been ascribed to formation of tunneling conductivity channels, probably, via activation of the closed core screw dislocations with the help of nitrogen vacancies. Carrier lifetimes in the quantum wells and the p-cladding were found to be unaffected by the aging process, suggesting that the nonradiative recombination has a lesser influence on the device degradation. [ABSTRACT FROM AUTHOR]

The objective of this paper is to investigate the influence of shallow traps on the signals from Al2O3:C,Mg obtained using time-resolved optically stimulated luminescence (TR-OSL) measurements through experiments and numerical simulations. TR-OSL measurements of Al2O3:C,Mg were carried out and the resulting optically stimulated luminescence (OSL) curves were investigated as a function of the temperature. The numerical simulations were carried out using the rate-equations for a simplified model of Al2O3:C,Mg containing two types of luminescence centers with different luminescence lifetimes and three types of electron traps (a shallow trap, a main dosimetric trap, and a thermally disconnected deep trap). Both experimental results and simulations show that the OSL signals during and between the stimulation pulses are affected by the presence of shallow traps. However, with an appropriate choice of timing parameters, the influence of shallow traps can be reduced by calculating the difference between the signals during and between stimulation pulses. Therefore, TR-OSL can be useful in dosimetry using materials having a large concentration of shallow traps and OSL components with short luminescence lifetimes, for example Al2O3:C,Mg and BeO. Our results also show that the presence of shallow traps has to be taken into account when using the TR-OSL for discrimination between luminescence centers with different luminescence lifetimes, or separation between the OSL from different materials based on their characteristic luminescence lifetimes. The experimental results also show evidence of thermal assistance in the OSL process of Al2O3:C,Mg. [ABSTRACT FROM AUTHOR]

Detailed spectral response (SR) modeling for heterojunction bipolar phototransistors (HPTs) is presented in this work. All the related physical parameters are taken into account for the resolution of photogenerated excess minority carrier continuity equations in the active layers of the HPT. The layer dependence of the optical flux absorption profile at near-bandgap wavelengths is also investigated and its generalization as a single-exponential has been refuted for HPTs based on GaAs material systems (InGaP-GaAs/AlGaAs-GaAs). The variation in the responsivity of the device with changing base width is analyzed at various wavelengths and a detailed experimental setup for optical characterization of HPTs is also provided. The measured results at 635, 780, 808, and 850 nm show good agreement to the modeled data, validating the newly developed theoretical model. [ABSTRACT FROM AUTHOR]

We present theoretical and experimental results on the bonding and structural characteristics of AlN:Ag thin film nanocomposites obtained by means of density functional theory (DFT) computations, high resolution transmission electron microscopy (HRTEM) observations, Auger electron spectroscopy (AES), and x-ray diffraction (XRD) measurements. From the theoretical calculations it was determined that the presence of the Ag substitutional of N or Al atoms affects the electronic density of states (EDOS) of the resulting systems. In particular, occupied energy states are introduced (between others) that lie within the energy gap of the AlN matrix due to Ag-d, Al-p (accompanied with a charge transfer from Al to Ag), Ag-p, and N-p hybridizations, respectively. The effect is predicted to be even more pronounced in the case of Ag nanoparticle inclusions affecting the EDOS of the composite system. These predictions were verified by the HRTEM images that gave unequivocal evidence for the presence and stability of Ag nanoparticles in the AlN matrix. In addition, the AES data suggested a metal-metal (Ag-Al) bonding preference, while the XRD patterns revealed that the atomic Ag dispersions in the AlN thin films results in a small elongation of the Wurtzite lattice, which is in agreement with the DFT predictions. These results may useful in tailoring the electronic response of AlN-based systems and the design of devices for various opto-electronic applications. [ABSTRACT FROM AUTHOR]

Spectra of the interband spin photocurrent due to Rashba and Dresselhaus spin splittings have been experimentally investigated in InGaAs/AlGaAs quantum wells at room temperature. The Rashba- and Dresselhaus-induced circular photogalvanic effect (CPGE) spectra are found to be quite similar in the spectral regions corresponding to the transitions 1e1hh (the first conduction to the first valence sub-band of heavy hole) and 1e2hh. The ratio of Rashba- and Dresselhaus-induced CPGE currents for the transition 1e1hh is estimated to be 4.95. The magnitude of the Rashba-induced CPGE current is up to several tens of nA/W for the transition 1e1hh, which is 1 order of magnitude larger than that obtained in GaN/AlGaN superlattices. Comparing the CPGE spectrum with reflectance-difference and photoreflectance spectra, we find that the large Rashba spin splitting is mainly induced by a large indium atom segregation effect and by the internal field in the quantum wells. [ABSTRACT FROM AUTHOR]

Quasistatic measurements of room-temperature anelastic relaxation were used to characterize the properties of shear transformation zones (STZs) in amorphous Al86.8Ni3.7Y9.5 in the dilute limit. Using a combination of nanoindenter cantilever bending and mandrel bend relaxation techniques, anelastic relaxation was measured over times ranging from 1 s to 3 × 107 s. Direct spectrum analysis yields relaxation-time spectra, which display seven distinct peaks. The results were analyzed using a linear dashpot-and-spring model, combined with transition-state theory, to yield several STZ properties. These reveal a quantized hierarchy of STZs that differ from each other by one atomic volume. Potential STZs occupy a large volume fraction of the solid. They access their ergodic space, with the ratio of forward-to backward jump rates ranging from 1.03 to 4.3 for the range of stress values used. [ABSTRACT FROM AUTHOR]

A theoretical study of Al1-xGaxN, based on the full-potential linearized augmented plane wave method, is used to investigate the variations in the bandgap, optical properties, and nonlinear behavior of the compound with the change in the Ga concentration. It is found that the bandgap decreases with the increase in Ga. A maximum value of 5.50 eV is determined for the bandgap of pure AlN, which reaches a minimum value of 3.0 eV when Al is completely replaced by Ga. The static index of refraction and dielectric constant decreases with the increase in the bandgap of the material, assigning a high index of refraction to pure GaN when compared to pure AlN. The refractive index drops below 1 for higher energy photons, larger than 14 eV. The group velocity of these photons is larger than the vacuum velocity of light. This astonishing result shows that at higher energies the optical properties of the material shifts from linear to nonlinear. Furthermore, frequency dependent reflectivity and absorption coefficients show that peak values of the absorption coefficient and reflectivity shift toward lower energy in the ultraviolet (UV) spectrum with the increase in Ga concentration. This comprehensive theoretical study of the optoelectronic properties predicts that the material can be effectively used in the optical devices working in the visible and UV spectrum. [ABSTRACT FROM AUTHOR]

Using transmission electron microscopy we investigate the influence of AlSb monolayers and substrate surface preparation on the microstructure of GaSb grown on GaAs (001) by molecular beam epitaxy. The geometric phase analysis method is used to analyze the interface dislocation type and the residual strain, as well as the dislocation core behavior versus the thickness of the AlSb interface layer. A quantitative measurement of the local Burgers vectors shows that the misfit dislocations at the GaSb/GaAs interface are always 60° dislocations. They are arranged in pairs which are more or less distant. For the samples with the lower threading dislocation density, the average distance between the 60° pairs is smaller, the interface is flatter and the local strain is more relieved. These results show that understanding the atomic structure of interfaces may be of great help in improving the quality of GaSb grown on GaAs substrates. [ABSTRACT FROM AUTHOR]

The wurtzite AlGaN/GaN Gunn diode with tristep-graded Al composition AlGaN as hot electron injector is simulated by using an improved negative differential mobility model of GaN. The results show that the oscillation mode of Gunn diode gradually shifts from dipole domain mode toward accumulation mode with increase in temperature, and the mode shift closely depends on the injector length. At the temperatures of 300-400 K, 0.6 and 0.4 μm Gunn diodes normally generate the oscillation of dipole domain mode, yielding the fundamental oscillation frequencies of 332-352 GHz and 488-508 GHz, respectively, with the dc/rf conversion efficiencies of 2%-3% and the output power densities of 109-1010 W cm-3. At higher temperatures, the diodes generate the accumulation mode oscillation, and the highest frequency approaches 680 GHz and 977 GHz, respectively, with the dc/rf conversion efficiencies of 0.5%-1%. [ABSTRACT FROM AUTHOR]

In this work we theoretically study the vertical transport in GaN/AlGaN resonant tunneling diodes in the ballistic regime. Heterostructures based on III-nitride compounds are characterized by a large conduction band discontinuity and a presence of an internal electric field, both of which have important effects on the electronic transport. Using the transfer matrix formalism, we investigate the effect of the energetic barrier height on the resonant current. Our calculations show an increase in the peak to valley ratio (PVR) with increasing Al content of the barriers which arises from the large decrease in the valley current. Furthermore, we show that the current resonances are different for positive and negative applied voltages. We also demonstrate that, due to the asymmetry of the conduction band profile, only one current direction leads to a significant PVR. Finally, we present an approach to achieve large PVR in both current directions by reducing the asymmetry induced by the internal field. [ABSTRACT FROM AUTHOR]

An analytical model, with incorporation of transferred-electron effect, for drain-current characteristics of AlGaN/GaN heterostructure field-effect transistors (HFETs) is presented. The transferred-electron effect is often neglected in modeling the drain-current of III-V HFETs. The broad steady-state electron drift-velocity overshoot of GaN in comparison to other direct semiconductors such as GaAs and InP, in addition to the larger difference between the peak and saturation drift-velocity, and the wider band gap of this semiconductor suggest the importance of the incorporation of transferred-electron effect (i.e., steady-state drift-velocity overshoot) in modeling the drain-current of these devices. Simulation results are compared with the results of the adoption of a saturating drift transport model, which has been recently used in modeling the drain-current of these devices. Comparisons between the two models demonstrate the importance of the consideration of transferred-electron effect, especially as the Ohmic contact quality is improved. [ABSTRACT FROM AUTHOR]