Inversion domains in AlN grown on (0001) sapphire

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2 {+-} 0.5{sup o} from the c-axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH

Optical parametric generation of a midinfrared continuum in orientation-patterned GaAs

We have generated an ultrabroad mid-infrared continuum by using single-pass optical parametric generation (OPG) in orientation-patterned GaAs (OP-GaAs). The spectrum spans more than an octave, from 4.5 to 10.7 m, measured 20 dB down from the peak. The 17.5 mm long, 0.5 mm thick, all-epitaxially-grown OPGaAs sample with a 166.6-m quasi-phase-matching period was pumped with 3.1-3.3 m wavelength, 1 ps pulses up to 2 J in energy. The OPG threshold was observed at 55 nJ pump energy with the pump polarized along the [111] crystal direction. The slope efficiency near threshold was 51%, and the external conversion efficiency was as high as 15%. © 2006 Optical Society of America OCIS codes: 190.2620 Gallium arsenide is a promising material for parametric frequency conversion because of its large nonlinear susceptibility (d 14 = 94 pm/ V for frequency doubling of f =4 m), 1 broad infrared transparency range ͑ = 0.9-17 m͒, and high thermal conductivity. Also of interest are its highly symmetric nonlinear susceptibility tensor and linear optical isotropy, which facilitate nonlinear optical interactions that use a rich variety of polarization configurations. However, the optical isotropy of GaAs prevents birefringent phase matching; efficient interactions can instead be obtained by quasi-phase-matching. Orientation-patterned GaAs (OP-GaAs) is a type of quasi-phase-matched (QPM) GaAs in which periodic inversions of the crystallographic orientation are grown into the material. The advantage of OP-GaAs compared with other QPM GaAs methods [such as diffusion-bonded GaAs (Ref. 2) and Fresnel QPM GaAs (Ref. 3)] is that OP-GaAs has lithographically defined periods, which permit excellent periodicity control and access to small QPM periods. To fabricate OP-GaAs, a process based on photolithography and molecular beam epitaxy is used to grow a thin-film template with periodic crystal inversions. 4,5 A thick film (0.5-1 mm) is then grown upon this template by hydride vapor phase epitaxy to produce bulk OP-GaAs. 6,7 With OP-GaAs, efficient second-harmonic generation, 1 difference-frequency generation, Anomalously wide tuning bandwidths for parametric processes are associated with wavelength 0 , where the group-velocity dispersion, d 2 k /d 2 , of a material goes to zero. 11 If one pumps a nonlinear crystal at 0 / 2, broadband gain around the degenerate signal-idler wavelength can be obtained. Use of OPG is a convenient way to investigate the broadband gain that occurs for GaAs when pumping at ϳ 0 / 2 = 3.31 m. Slightly detuning the pump from 0 / 2 and choosing an appropriate QPM period to compensate for the phase-mismatch results in even broader gain bandwidths (at the expense of some gain variation), as plotted i

Argyrin B a non-competitive inhibitor of the human immunoproteasome exhibiting preference for β1i

Inhibitors of the proteasome have found broad therapeutic applications however, they show severe toxicity due to the abundance of proteasomes in healthy cells. In contrast, inhibitors of the immunoproteasome, which is upregulated during disease states, are less toxic and have increased therapeutic potential including against autoimmune disorders. In this project, we report argyrin B, a natural product cyclic peptide to be a reversible, non-competitive inhibitor of the immunoproteasome. Argyrin B showed selective inhibition of the β5i and β1i sites of the immunoproteasome over the β5c and β1c sites of the constitutive proteasome with nearly 20-fold selective inhibition of β1i over the homologous β1c. Molecular modelling attributes the β1i over β1c selectivity to the small hydrophobic S1 pocket of β1i and β5i over β5c to site-specific amino acid variations that enable additional bonding interactions and stabilization of the binding conformation. These findings facilitate the design of immunoproteasome selective and reversible inhibitors that may have a greater therapeutic potential and lower toxicity

Aluminum Nitride Crystal Growth by Halide Vapor Transport Epitaxy

AbstractHalide vapor transport epitaxy (HVTE) is demonstrated for growth of AlN layers with thickness up to 50 μm at deposition rates up to 60 μm/h. The HVTE process uses an aluminum chloride amine adduct as the aluminum source of both aluminum and nitrogen. This new technique eliminates the main difficulties of the conventional hydride VPE growth, where aluminum oxidation and the strong reactivity of aluminum chloride with fused silica create the potential for oxygen contamination. This study shows the effect of temperature, gas flow velocities, and reactor pressure on the growth rate and layer quality. It is found that the growth rate and the layer quality strongly depend on the gas ratio. The species of carrier gas, the flow rates and partial pressures can be used as tools to optimize growth rate and to avoid any etching effects resulting from reverse chemical reactions. The crystalline layer quality as determined by x-ray rocking curve measurement shows FWHM of 300–900 and 500–1300 arc-sec for (002) and (102) planes, respectively.PACS: 81.05.Ea; 81.15.Kk; 68.55.Jk.</jats:p

Inversion domains in AlN grown on (0001) sapphire

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2 {+-} 0.5{sup o} from the c-axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH

Structural Study of V-like Columnar Inversion Domains in AlN Grown on Sapphire

ABSTRACTV-like columnar inversion domains with a divergence angle of about 4.5° ± 1° grown in AlN films with N-polarity were studied using transmission electron microscopy (TEM) and atomic force microscopy. Such domains emerge at the surface forming a small islands in form of hexagonal, truncated pyramids. A model of such pyramid was proposed. TEM studies indicate a displacement of c/2 along the [0001] direction at the inversion domain boundary. A boundary itself is composed of long segments located on the {1100} planes, which are alternated by short segments on some inclined planes.</jats:p