Thermal activation and deactivation of grown‐in defects limiting the lifetime of float‐zone silicon

By studying the minority carrier lifetime in recently manufactured commercially available n‐ and p‐type float‐zone (FZ) silicon from five leading suppliers, we observe a very large reduction in the bulk lifetime when FZ silicon is heat‐treated in the range 450–700 °C. Photoluminescence imaging of these samples at the wafer scale revealed concentric circular patterns, with higher recombination occurring in the centre, and far less around the periphery. Deep level transient spectroscopy measurements indicate the presence of recombination active defects, including a dominant center with an energy level at ～E_v + 0.5 eV. Upon annealing FZ silicon at temperatures >1000 °C in oxygen, the lifetime is completely recovered, whereby the defects vanish and do not reappear upon subsequent annealing at 500 °C. We conclude that the heat‐treatments at >1000 °C result in total annihilation of the recombination active defects. Without such high temperature treatments, the minority carrier lifetime in FZ silicon is unstable and will affect the development of high efficiency (>24%) solar cells and surface passivation studies.     

A new and useful method for the macrocyclization of linear peptides

A new and useful procedure for the macrocyclization of linear peptides is described. The natural amino acid side chains of tyrosine (phenol), lysine (alkylamine), and histidine (imidazole) react in an intramolecular fashion with a pendent pyridine-N-oxide-carboxamide, which is selectively activated by the phosphonium salt, PyBroP. The reaction is mild, rapid, and efficient with a potentially large substrate scope. Multiple examples are provided with full characterization and analyses, including a novel aza-variant of the C-O-D ring system of vancomycin.     

Characteristics of spontaneous fission of <Superscript>250</Superscript>No

This study describes an experiment on investigating the properties of spontaneous fission of shortlived neutron-deficient nuclei synthesized in the reaction of complete fusion ⁴⁸Ca + ²⁰⁴Pb = ²⁵²No*. The experiment is performed using the SHELS separator and the beam of multicharged ions at U-400 accelerator (LNR, JINR). Two activities undergoing spontaneous fission, which can be related to the ground and isomeric states of ²⁵⁰No nucleus, are registered. The half-lives, total kinetic energies of fission fragments, and neutron multiplicities are measured for the short-lived nuclei. The average number of neutrons per fission for the activity with t _1/2 = 5.1 ± 0.3 μs is = 4.38 ± 0.13 μs, and for nuclei with the half-life t _1/2 = 36 ± 3 μs it is xxxxx.     

Anisotropic profile decay on perturbed Au(111) vicinal surfaces

The anisotropy of the surface free energy has a large influence on the decay rate of periodic surface profiles at temperatures below the roughening temperature. This has been demonstrated by a study of the periodic surface profiles that were etched into vicinal Au(111) crystals in two orientations: with their wavevector parallel (ψ = 0) and perpendicular (ψ = π/2) to the intrinsic step direction of the vicinal surface. Due to this modulation, steps become either oscillatory in shape or remain straight, with their average separation modulated. Accordingly, profile decay is driven primarily by step self-energy (ψ = 0) or step interaction energy (ψ = π/2). The rate of decay was measured under ultra-high vacuum conditions at 750°C using a scanning tunneling microscope for imaging. A large anisotropy of decay was observed on two vicinal crystals of 1.5 and 5° miscut, with the ψ = 0 decay being much faster than with ψ = π/2). The results are evaluated and discussed in the framework of recent theory.