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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Epitaxial growth of In-rich InGaN on yttria-stabilized zirconia and its application to metal–insulator–semiconductor field-effect transistors

We grew In-rich In{sub x}Ga{sub 1-x}N films on yttria-stabilized zirconia (YSZ) substrates at low temperatures by pulsed sputtering deposition. It was found that single-crystal In{sub x}Ga{sub 1-x}N (0.63 ≤ x ≤ 0.82) films can be prepared without significant compositional fluctuations at growth temperatures below 500 °C. It was also found that the electrical properties of InGaN are strongly dependent on In composition, growth temperature, and film polarity. N-channel operation of the metal–insulator–semiconductor field-effect transistor (MISFET) with an ultrathin InGaN channel on the YSZ substrates was successfully demonstrated. These results indicate that an InGaN-based MISFET is a promising device for next-generation high-speed electronics.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Fabrications and application of single crystalline GaN for high-performance deep UV photodetectors

High-quality single crystalline Gallium Nitride (GaN) semiconductor has been synthesized using molecule beam epitaxy (MBE) technique for development of high-performance deep ultraviolet (UV) photodetectors. Thickness of the films was estimated by using surface profile meter and scanning electron microscope. Electronic states and elemental composition of the films were obtained using Raman scattering spectroscopy. The orientation, crystal structure and phase purity of the films were examined using a Siemens x-ray diffractometer radiation. The surface microstructure was studied using high resolution scanning electron microscopy (SEM). Two types of metal pairs: Al-Al, Al-Cu or Cu-Cu were used for interdigital electrodes on GaN film in order to examine the Schottky properties of the GaN based photodetector. The characterizations of the fabricated prototype include the stability, responsivity, response and recovery times. Typical time dependent photoresponsivity by switching different UV light source on and off five times for each 240 seconds at a bias of 2V, respectively, have been obtained. The detector appears to be highly sensitive to various UV wavelengths of light with very stable baseline and repeatability. The obtained photoresponsivity was up to 354 mA/W at the bias 2V. Higher photoresponsivity could be obtained if higher bias was applied but it would unavoidably result in a higher dark current. Thermal effect on the fabricated GaN based prototype was discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Discovery of ferromagnetism with large magnetic anisotropy in ZrMnP and HfMnP

ZrMnP and HfMnP single crystals are grown by a self-flux growth technique, and structural as well as temperature dependent magnetic and transport properties are studied. Both compounds have an orthorhombic crystal structure. ZrMnP and HfMnP are ferromagnetic with Curie temperatures around 370 K and 320 K, respectively. The spontaneous magnetizations of ZrMnP and HfMnP are determined to be 1.9 μ{sub B}/f.u. and 2.1 μ{sub B}/f.u., respectively, at 50 K. The magnetocaloric effect of ZrMnP in terms of entropy change (ΔS) is estimated to be −6.7 kJ m{sup −3} K{sup −1} around 369 K. The easy axis of magnetization is [100] for both compounds, with a small anisotropy relative to the [010] axis. At 50 K, the anisotropy field along the [001] axis is ∼4.6 T for ZrMnP and ∼10 T for HfMnP. Such large magnetic anisotropy is remarkable considering the absence of rare-earth elements in these compounds. The first principle calculation correctly predicts the magnetization and hard axis orientation for both compounds, and predicts the experimental HfMnP anisotropy field within 25%. More importantly, our calculations suggest that the large magnetic anisotropy comes primarily from the Mn atoms, suggesting that similarly large anisotropies may be found in other 3d transition metal compounds.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Self-cavity lasing in optically pumped single crystals of p-sexiphenyl

Organic single-crystal self-cavities are prepared by solution growth of p-sexiphenyl (p-6P). Based on Fabry-Pérot feedback inside a quasi-lozenge-shaped platelet crystal, edge-emitting laser is obtained under optical pumping. The multimode lasing band appears at the 0-1 or 0-2 vibronic progressions depending on the excitation conditions which affect the self-absorption effect. Cavity-size dependence of amplified spontaneous emission (ASE) is investigated with laser-etched single crystals of p-6P. As the cavity length of square-shaped crystal is reduced from 100 to 10 μm, ASE threshold fluence is decreased probably due to size-dependent light confinement in the crystal cavity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Short-pulse excitation of microwave plasma for efficient diamond growth

To realize a variety of potential applications of diamonds, particularly in the area of power electronics, it is indispensable to improve their growth efficiency. Most conventional approaches have tried to achieve this simply by increasing the gas temperature; however, this makes it difficult to grow large diamond crystals. To improve the growth efficiency while lowering the gas temperature, we propose that using a pulse-modulated microwave plasma with a sub-millisecond pulse width can enhance the power efficiency of the growth rate of single-crystal diamonds. We found that using a sub-millisecond pulse-mode discharge could almost double the growth rate obtained using continuous mode discharge for a fixed average microwave power and gas pressure. A comparison between experimental observations of the optical emission spectra of the discharge and a numerical simulation of the gas temperature suggests that a decrease in the gas temperature was achieved, and highlights the importance of electron-dominated reactions for obtaining the enhancement of the growth rate. This result will have a large impact in the area of diamond growth because it enables diamond growth to be more power efficient at reduced temperatures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High-resolution 2-D Bragg diffraction reveal heterogeneous domain transformation behavior in a bulk relaxor ferroelectric

In-situ measurement of fine-structure of neutron Bragg diffraction peaks from a relaxor single-crystal using a time-of-flight instrument reveals highly heterogeneous mesoscale domain transformation behavior under applied electric fields. We observed that only 25% of domains undergo reorienta- tion or phase transition contributing to large average strains, while at least 40% remain invariant and exhibit microstrains. Such insights could be central for designing new relaxor materials with better performance and longevity. The current experimental technique can also be applied to resolve com- plex mesoscale phenomena in other functional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2016

V{sub 3}O{sub 5}: Insulator-metal transition and electric-field-induced resistive-switching

V{sub 3}O{sub 5} is one of the two vanadium oxides that exhibit insulator to metal transitions above room temperature. We report on the DC I–V characteristics of polycrystalline samples and single-crystals of V{sub 3}O{sub 5} over a wide range of currents. For all samples self-heating induced hysteretic nonlinear conductivity followed at higher currents by onset of negative differential resistivity regime, and finally, at highest currents, by switching to the metallic state. Slow switching towards a partially transformed state with a prolonged memory is typical of polycrystalline samples. High currents applied in the metallic state of one of the single crystals affected the oxygen content of the material and even caused appearance and disappearance of a VO{sub 2} inclusion. Simple and reproducible I–V plots were obtained for a single crystal with the currents that barely induced the metallic state.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Mechanism of polarization switching in wurtzite-structured zinc oxide thin films

The properties of a potentially new class of ferroelectric materials based on wurtzite-structured ZnO thin films are examined using the first-principles calculations. Theoretical P-E hysteresis loops were calculated using the fixed-D method for both unstrained and (biaxially) strained single crystals. Ferroelectric polarization switching in ZnO (S.G. P6{sub 3}mc) is shown to occur via an intermediate non-polar structure with centrosymmetric P6{sub 3}/mmc symmetry by displacement of cations relative to anions in the long-axis direction. The calculated coercive electric field (E{sub c}) for polarization switching was estimated to be 7.2 MV/cm for defect-free monocrystalline ZnO. During switching, the short- and long-axis lattice parameters expand and contract, respectively. The large structural distortion required for switching may explain why ferroelectricity in this compound has not been reported experimentally for pure ZnO. Applying an epitaxial tensile strain parallel to the basal plane is shown to be effective in lowering E{sub c} during polarization, with a 5% biaxial expansion resulting in a decrease of E{sub c} to 3.5 MV/cm. Comparison with calculated values for conventional ferroelectric materials suggests that the ferroelectric polarization switching of wurtzite-structured ZnO may be achievable by preparing high-quality ZnO thin films with suitable strain levels and low defect concentrations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Thermal modelling of high-power laser diodes mounted using various types of submounts

Using three-dimensional thermal modelling of a highpower 980-nm laser diode with a stripe contact width of 100 μm as an example, we analyse the thermal parameters of high-power laser diodes mounted using submounts. We consider a range of thermal conductivities of submounts that includes parameters of widely used thermal compensators based on AlN, BeO and SiC, as well as on CuW and CuMo composites and polycrystalline and single-crystal synthetic diamond with high thermal conductivity. Taking into account experimental overall efficiency vs. pump current data, we calculate the temperature of the active layer as a function of the width, thickness and thermal conductivity of the submount at thermal loads corresponding to cw output powers of 10, 15 and 20 W. (lasers)

36 MATERIALS SCIENCE↗
DOE OSTI2014

Investigation of free charge carrier dynamics in single-crystalline CVD diamond by two-photon absorption

By using the methods of transient gratings (TGs) and induced absorption, we have studied the kinetics of plasma of free charge carriers (FCCs) created by the action of a picosecond laser pulse in two high-purity diamond single crystals synthesised from the gas phase. The gratings with different spatial periods have been excited at the wavelengths of 266 or 213 nm (above and below the fundamental absorption edge in diamond) and probed with continuous-wave radiation in the visible region. At the moderate FCC concentrations (∼7 × 10{sup 17} cm{sup -3}), the coefficient of ambipolar diffusion and the carrier recombination time of two crystals are 20.3 and 18.9 cm{sup 2} s{sup -1} and 30 and 190 ns, respectively. The increase in the carrier concentration up to 5 × 10{sup 19} cm{sup -3} reduces the TG lifetime. We have determined the conditions under which the relaxation of the grating of carriers leads to the formation of a thermal grating, with the amplitude sufficient for its experimental observation. (interaction of laser radiation with matter. laser plasma)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Laser radiation frequency doubling in a single-crystal fibre based on a stoichiometric LiNbO{sub 3} crystal

We demonstrate the employment of single-crystal optical fibres based on lithium niobate for doubling the laser radiation frequency. The measured characteristics of the fibre confirm its high quality and spatial homogeneity. Parameters of the frequency doublers for neodymium laser radiation (λ = 1 mm) based on fibre and bulk single crystals are compared. Single crystals are grown by the method of laser-heated pedestal growing with heating by radiation of a CO{sub 2} laser (LHPG-method). (nonlinear optical phenomena)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spectroscopy and tunable continuous-wave operation of Cr{sup 2+}:Zn{sub 1-x}Mg{sub x}Se single crystal around 2.5 μm

Hexagonal single crystals of the Cr{sup 2+}:Zn{sub 1-x}Mg{sub x}Se solid solution are grown by the Bridgman – Stockbarger technique. We study their optical and photoluminescent characteristics and determine the temperature dependences of the photoluminescence kinetics within the range 5 – 440 K. The crystals possess a broad gain band in the range from 2 to 3.5 μm and exhibit a weak quenching of luminescence with a quantum yield exceeding 80% at room temperature. Tunable continuous-wave oscillation is demonstrated in the region of 2.5 μm. (lasers)

36 MATERIALS SCIENCE↗
DOE OSTI2015

Absorption spectra and nonlinear transmission (at λ = 2940 nm) of a diffusion-doped Fe{sup 2+}:ZnSe single crystal

Transmission spectra of a ZnSe sample diffusion-doped with Fe{sup 2+} ions have been measured in the wavelength range 500 – 7000 nm. A broad absorption band in the range 500 – 1500 nm has been observed in both doped and undoped regions of the sample. This band is possibly due to deviations from stoichiometry in the course of diffusion doping. The transmission of the Fe{sup 2+}:ZnSe sample at a wavelength of 2940 nm has been measured at various dopant concentrations and high peak pulse intensities (up to 8 MW cm{sup -2}). The samples have been shown to be incompletely bleached: during a laser pulse, the transmission first increases, reaches a maximum, and then falls off. Our results suggest that the incomplete bleaching cannot be accounted for by excited-state absorption. The incomplete bleaching (as well as the transmission maximum) is due to the heating of the sample, which leads to a reduction in upper level lifetime and, accordingly, to an increase in absorption saturation intensity. (nonlinear optical phenomena)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Carbon photonics

The properties of new carbon materials (single-crystal and polycrystalline CVD diamond films and wafers, single-wall carbon nanotubes and graphene) and the prospects of their use as optical elements and devices are discussed. (optical elements of laser devices)

36 MATERIALS SCIENCE↗
DOE OSTI2016

Room-temperature 1.2-J Fe{sup 2+}:ZnSe laser

The characteristics of a laser based on a Fe{sup 2+}:ZnSe single crystal pumped by an electric-discharge HF laser at room temperature are studied. The HF laser beam diameter on the crystal surface was 17 mm. The achieved laser energy was 1.2 J with an efficiency of ∼ 25% with respect to the pump energy. (letters)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Yb : YAG thin-rod laser amplifier with a high pulse energy for a fibre oscillator

High (more than ten times) small-signal gain is demonstrated in an Yb : YAG single crystal thin rods with pumping by a fibre-coupled diode laser. A four-pass amplifier for a fibre master oscillator with an average output power exceeding 15 W at a pulse repetition rate of 3 MHz is fabricated based on this active element. The small-signal gain in the developed amplifier is 26 dB, which is comparable with the gain in regenerative amplifiers. The possibility of obtaining sub-millijoule pulse energy at a pulse repetition rate of tens of kilohertz is shown. (extreme laser radiation: physics and fundamental applications)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Room-temperature Fe{sup 2+}:ZnS single crystal laser pumped by an electric-discharge HF laser

A room-temperature laser based on a Fe{sup 2+}:ZnS single crystal has been studied. The crystal was pumped by a nonchain electric-discharge HF laser with a light pulse FWHM of ∼140 ns. The pump spot diameter on the crystal surface (d) varied from 3.9 to 11.6 mm. The slope efficiency at these values of d was 37% – 39 %, respectively. The maximum laser energy was 380 mJ (at d=11.6 mm) with the total efficiency with respect to the absorbed pump energy of ∼19%. A further increase in the laser energy was not achieved because the crystal was destroyed at an incident pump energy and power of 3.4 J and ∼24 MW, respectively. The damaged region has a form of a long narrow channel shifted from the entrance surface inside the active element. (lasers)

42 ENGINEERING↗
DOE OSTI2016

IR luminescence of Fe{sup 2+} : ZnSe single crystals excited by an electron beam

Spectral-kinetic characteristics of the cathodoluminescence of Fe{sup 2+} : ZnSe single crystals are studied at T='300' and 80 K. These characteristics correspond to the IR luminescence spectrum and kinetics of Fe{sup 2+} ions in ZnSe crystals optically excited into the impurity absorption band. The obtained results open a real possibility of pumping Fe{sup 2+} : ZnSe lasers by hot electrons. (letters)

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗