The structural, morphological, and defect properties of mixed anion, InAs y P 1Ϫy and mixed cation, In x Al 1Ϫx As metamorphic step-graded buffers grown on InP substrates are investigated and compared. Two types of buffers were grown to span the identical range of lattice constants and lattice mismatch ͑ϳ1.1-1.2%͒ on ͑100͒ InP substrates by solid source molecular beam epitaxy. Symmetric relaxation of ϳ90% in the two orthogonal ͗110͘ directions with minimal lattice tilt was observed for the terminal InAs 0.4 P 0.6 and In 0.7 Al 0.3 As overlayers of each graded buffer type, indicating nearly equal numbers of ␣ and  dislocations were formed during the relaxation process and that the relaxation is near equilibrium and hence insensitive to asymmetric dislocation kinetics. Atomic force microscopy reveals extremely ordered crosshatch morphology and very low root mean square ͑rms͒ roughness of ϳ2.2 nm for the InAsP relaxed buffers compared to the InAlAs relaxed buffers ͑ϳ7.3 nm͒ at the same degree of lattice mismatch with respect to the InP substrates. Moreover, phase decomposition is observed for the InAlAs buffers, whereas InAsP buffers displayed ideal, step-graded buffer characteristics. The impact of the structural differences between the two buffer types on metamorphic devices was demonstrated by comparing identical 0.6 eV band gap lattice-mismatched In 0.69 Ga 0.31 As thermophotovoltaic ͑TPV͒ devices that were grown on these buffers. Clearly superior device performance was achieved on InAs y P 1Ϫy buffers, which is attributed primarily to the impact of layer roughness on the carrier recombination rates near the front window/emitter interface of the TPV devices.
The strain relaxation mechanism and defect properties of compositionally step-graded InAs y P 1−y buffers grown by molecular beam epitaxy on InP have been investigated. InAsP layers having lattice misfits ranging from 1% to 1.4% with respect to InP, as well as subsequently grown lattice matched In 0.69 Ga 0.31 As overlayers on the metamorphic buffers were explored on both ͑100͒ and 2°offcut ͑100͒ InP substrates. The metamorphic graded buffers revealed very efficient relaxation coupled with low threading dislocation densities on the order of ͑1-2͒ ϫ 10 6 cm −2 for the range of misfit values explored here. A detailed analysis via high resolution x-ray diffraction revealed that the strain relaxed symmetrically, with equivalent numbers of ␣ and  dislocations, and to greater than 90% for all cases, regardless of substrate offcut. Further analysis showed the relaxation to always be glide limited in these materials when grown on a graded buffer compared to a single step layer. The threading dislocation density was observed by plan-view transmission electron microscopy to be constant for the range of misfit values studied here in the top layer of the graded structures, which is attributed to the very efficient use of residual dislocations and the dominance of dislocation glide over nucleation in these graded anion metamorphic buffers, suggesting great promise for metamorphic devices with lattice constants greater than that of InP to be enabled by InAsP metamorphic structures on InP.
Relaxed, high-quality, compositionally step-graded InAsyP1−y layers with an As composition of y=0.4, corresponding to a lattice mismatch of ∼1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4×106 cm−2 within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1−xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1−y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III–V devices.
Abnormalities in zinc metabolism have been linked to many diseases, including different kinds of cancers and neurological diseases. The present study investigates the fragmentation pathways of a zinc chaperon using a model peptide with the sequence acetyl-His1-Cys2-Gly3-Pro4-Tyr5-His6-Cys7 (analog methanobactin peptide-5, amb5). DFT/M05-2X and B3LYP geometry optimizations of [amb5–3H+Zn(II)]− predicted three lowest energy conformers with different chelating motifs. Direct dynamics simulations, using the PM7 semiempirical electronic structure method, were performed for these conformers, labeled a, b, and c, to obtain their fragmentation pathways at different temperatures in the range 1600–2250 K. The simulation results were compared with negative ion mode mass spectrometry experiments. For conformer a, the number of primary dissociation pathways are 11, 14, 24, 70, and 71 at 1600, 1750, 1875, 2000, and 2250 K, respectively. However, there are only 6, 10, 13, 14, and 19 pathways corresponding to these temperatures that have a probability of 2% or more. For conformer b, there are 67 pathways at 2000 K and 71 pathways at 2250 K. For conformer c, 17 pathways were observed at 2000 K. For conformer a, for two of the most common pathways involving C–C bond dissociation, Arrhenius parameters were calculated. The frequency factors and activation energies are smaller than those for C–C homolytic dissociation in alkanes due to increased stability of the product ions as a result of hydrogen bonding. The activation energies agree with the PM7 barriers for the C–C dissociations. Comparison of the simulation and experimental fragmentation ion yields shows the simulations predict double or triple cleavages of the backbone with Zn(II) retaining its binding sites, whereas the experiment exhibits single cleavages of the backbone accompanied by cleavage of two of the Zn(II) binding sites, resulting in b- and y-type ions.
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