No abstract

We report the use of two Raman signatures, the Bi-induced longitudinal-optical-plasmon-coupled (LOPC) mode and the GaAs Fröhlich scattering intensity, present in nominally undoped (100) GaAs1−yBiy to predict the 300K photoluminescence intensity and Bi composition (y) in GaAs1−yBiy. The LOPC mode is used to calculate the hole concentration in GaAs1−yBiy epitaxial layers. A linear relationship between hole concentration and photoluminescence intensity is found for a range of samples grown at various temperatures and growth rates. In addition, the composition (y) of Bi in GaAs1−yBiy is also found to be linearly related to the GaAs Fröhlich scattering intensity.

We have examined the morphology and composition of embedded nanowires that can be formed during molecular beam epitaxy of GaAs(1-x)Bi(x) using high angle annular dark field ('Z-contrast') imaging in an aberration-corrected scanning transmission electron microscope. Samples were grown in Ga-rich growth conditions on a stationary GaAs substrate. Ga-rich droplets are observed on the surface with lateral trails extending from the droplet in the [110] direction. Cross-sectional scanning transmission electron microscopy of the film reveals epitaxial nanowire structures of composition ∼GaAs embedded in the GaAs(1-x)Bi(x) epitaxial layers. These nanowires extend from a surface droplet to the substrate at a shallow angle of inclination (∼4°). They typically are 4 μm long and have a lens-shaped cross section with major and minor axes dimensions of 800 and 120 nm. The top surface of the nanowires exhibits a linear trace in longitudinal cross-section, across which the composition change from ∼GaAs to GaAs(1-x)Bi(x) appears abrupt. The bottom surfaces of the nanowires appear wavy and the composition change appears to be graded over ∼25 nm. The droplets have phase separated into Ga- and Bi-rich components. A qualitative model is proposed in which Bi is gettered into Ga droplets, leaving Bi depleted nanowires in the wakes of the droplets as they migrate in one direction across the surface during GaAs(1-x)Bi(x) film growth.

We have performed de Haas-van Alphen measurements of the Fermi surface of α-uranium single crystals at ambient pressure within the α 3 charge density wave (CDW) state from 0.020 K -10 K and magnetic fields to 35 T using torque magnetometry. The angular dependence of the resulting frequencies is described. Effective masses were measured and the Dingle temperature was determined to be 0.74 K ± 0.04 K. The observation of quantum oscillations within the α 3 CDW state gives new insight into the effect of the charge density waves on the Fermi surface. In addition we observed no signature of superconductivity in either transport or magnetization down to 0.020 K indicating the possibility of a pressure-induced quantum critical point that separates the superconducting dome from the normal CDW phase.71.18.+y 71.45.Lr 71.25.Pi Uranium first isolated in the 1920's [1] has remained a challenge for condensed matter physics since the first scientific work on it began almost 90 years ago. The orthorhombic alpha phase of uranium (α-U) provides a unique setting to understand the role of felectrons in the complex behavior of the actinides. The 20-atom unit cell of uranium and the three low temperature charge density waves are unique among the elements. These transitions, which are located at 43 K (α 1 ), 37 K (α 2 ) and 23 K (α 3 ), result in the volume of the new unit cell below 23 K growing by a factor of 72 to ~6000 A 3 [1]. Uranium also undergoes high temperature phase transitions to the beta and gamma structures that render it impossible to grow high-quality single crystals of the alpha phase by the usual methods. In addition the crystal structure of α-U resembles corrugated cardboard along the [010] direction [2] makes the crystal particularly susceptible to twinning. Single crystals were serendipitously grown at Argonne National Laboratory while separating uranium from fission products in simulated spent uranium fuel [3]. These crystals proved to be of much higher quality than any previously available. Measurements on these crystals provided new insights into the physics of α-U [4,5]. Although many outstanding

The strain dynamic of thin film AlN is investigated before and after the deposition of a GaN epitaxial layer using plasma assisted molecular beam epitaxy. X-ray diffraction ω/2θ-scan and asymmetric reciprocal space mapping analysis show that the deposition of GaN alters the strain state of the underlying AlN template. The in-plane lattice constant of the AlN is found to increase upon growth of GaN, giving rise to a more relaxed GaN epitaxial layer. Hence, the subsequent GaN epitaxial thin film possesses better structural quality especially with lower screw dislocation density and flat surface morphology which is evidenced by the X-ray diffraction ω-scan, room temperature photoluminescence, and atomic force microscopy analysis. Such relaxation of AlN upon GaN deposition is only observed for relatively thin AlN templates with thicknesses of 20 nm–30 nm; this effect is negligible for AlN with thickness of 50 nm and above. As the thicker AlN templates already themselves relax before the GaN deposition, the localized strain fields around the misfit dislocations prohibit further change of lattice parameters.