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This article describes efforts to achieve fast deposition of thick Quasi-Phase-Matched (QPM) GaP structures with high surface and structural quality on oriented patterned (OP) templates in a Hydride Vapor Phase Epitaxial (HVPE) process. These QPM structures will be incorporated in devices for conversion of frequencies from the near infrared to the mid infrared and THz regions, where powerful and tunable sources are in great demand for both military and civilian applications. In contrast with GaAs - the most studied OP QPM material - the two-photon absorption of GaP is predicted to be extremely low, which allows pumping with a number of convenient sources between 1 – 1.7 μm. Unpatterned GaP layers up to 370 μm thick were grown with growth rates up to 93 μm/hr with high reproducibility on bare substrates. The layers demonstrated smooth surface morphology with RMS < 1 nm and high structural quality with FWHM equal to 39 arcsec for layers grown on GaP and 112 arcsec for those grown on GaAs. Growth on OP-GaP templates resulted in 142 μm thick QPM structures deposited at a growth rate of 71 μm/h with good vertical (normal to the layer surface) propagation of the initial pattern. When the growth was performed on OP-GaAs one of the domains showed a trend toward a faceting growth. Further investigations are in progress to equalize the vertical and lateral growth of the two domains, and determine the best orientation of the substrate and pattern in order to achieve structures thick enough for high power nonlinear applications.

We report synthesis of phase pure multiferroic YMnO3 thin films on sapphire (0001) with conducting ZnGaO buffer contact layer. Films were prepared by using pulsed laser deposition technique and characterized using x-ray diffraction, scanning electron microscopy, energy dispersive absorption spectroscopy, and magnetic field dependent dielectric measurement techniques. Structural characterizations indicated phase purity and epitaxial nature of the films. The dielectric response indicated an anomaly in dielectric constant [variant_greek_epsilon] and tan δ in the vicinity of 30 K, well below the bulk Néel temperature ∼70 K. This anomaly in [variant_greek_epsilon] and tan δ and its magnetic field dependence is explained as an influence of strain due to lattice mismatch between the substrate and YMnO3 film. A substantial enhancement in magnetocapacitance was also observed for magnetic field applied parallel to ab plane of the film. Our results show that it is possible to tune the multiferroic property of YMnO3 via changes in ferroelastic route. [ABSTRACT FROM AUTHOR]

Radiation effects on graphene field effect transistors (GFETs) with hexagonal boron nitride (h-BN) thin film substrates are investigated using 60Co gamma-ray radiation. This study examines the radiation response using many samples with varying h-BN film thicknesses (1.6 and 20 nm thickness) and graphene channel lengths (5 and 10 μm). These samples were exposed to a total ionizing dose of approximately 1 Mrad(Si). I-V measurements were taken at fixed time intervals between irradiations and postirradiation. Dirac point voltage and current are extracted from the I-V measurements, as well as mobility, Dirac voltage hysteresis, and the total number of GFETs that remain properly operational. The results show a decrease in Dirac voltage during irradiation, with a rise of this voltage and permanent drop in Dirac current postirradiation. 1.6 nm h-BN substrate GFETs show an increase in mobility during irradiation, which drops back to preirradiation conditions in postirradiation measurements. Hysteretic changes to the Dirac voltage are the strongest during irradiation for the 20 nm thick h-BN substrate GFETs and after irradiation for the 1.6 nm thick h-BN GFETs. Failure rates were similar for most GFET types during irradiation; however, after irradiation, GFETs with 20 nm h-BN substrates experienced substantially more failures compared to 1.6 nm h-BN substrate GFETs. [ABSTRACT FROM AUTHOR]

We use transmission electron microscopy to study orientation-patterned GaAs layers very attractive for applications in terahertz and infrared frequency conversion devices. We observe regularly distributed inversion domains separated by inversion boundaries, together with undesirable microtwin defects originating at these boundaries. Atomic resolution aberration-corrected scanning transmission electron microscopy allowed us to resolve the GaAs dumbbells leading to a direct determination of the growth polarity of particular domains and determination of the alternating Ga-Ga and As-As bonds at the {110}-type antiphase boundary planes. We also determined observed microtwins as rotation twins called orthotwins, the defect that can cause optical losses. [ABSTRACT FROM AUTHOR]

Orientation patterned (OP)-GaAs crystals are very promising as nonlinear optical materials. They are suitable for mid-infrared and terahertz laser sources, by frequency conversion of shorter wavelength pump sources. OP-GaAs crystals must contain low concentrations of defects and must be homogeneous to reduce fluctuations, in the refractive index and the concomitant optical propagation losses. Understanding of the defects with electrooptic signature is crucial to improve the growth conditions for reducing their presence. Spectrally resolved cathodoluminescence imaging is used to study the main defects and how they are distributed throughout the OP-GaAs crystal (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]