Evolution of hydrogen and helium co-implanted single-crystal silicon during annealing

H+H+ was implanted into single-crystal silicon with a dose of 1×1016/cm21×1016/cm2 and an energy of 30 KeV, and then He+He+ was implanted into the same sample with the same dose and an energy of 33 KeV. Both of the implantations were performed at room temperature. Subsequently, the samples were annealed in a temperature range from 200 to 450 °C450 °C for 1 h. Cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry/channeling, elastic recoil detection, and high resolution x-ray diffraction were employed to characterize the strain, defects, and the distribution of H and He in the samples. The results showed that co-implantation of H and He decreases the total implantation dose, with which the surface could exfoliate during annealing. During annealing, the distribution of hydrogen did not change, but helium moved deeper and its distribution became sharper. At the same time, the maximum of the strain in the samples decreased a lot and also moved deeper. Furthermore, the defects introduced by ion implantation and annealing were characterized by slow positron annihilation spectroscopy, and two positron trap peaks were found. After annealing, the maximum of these two peaks decreased at the same time and their positions moved towards the surface. No bubbles or voids but cracks and platelets were observed by cross-sectional transmission electron microscopy. Finally, the relationship between the total implantation dose and the fraction of hydrogen in total implantation dose was calculated. © 2001 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70387/2/JAPIAU-90-8-3780-1.pd

Defect and strain in hydrogen and helium coimplanted single-crystal silicon

In this paper we studied the processes of blistering and exfoliation on the surface of crystal silicon, the evolution of defects/strains in the crystal silicon caused by hydrogen and helium coimplantation during annealing, and the formation of platelets and bubbles in the crystal. It is shown that H + and He + coimplantation produces a synergistic effect, which greatly decreases the total implantation dose, compared with either just H + or He + implantation. We also present the effect of coimplantation and analyse the different roles of H and He in the process of exfoliation during annealing. It seems that the essential role of hydrogen is to interact chemically with the defects in the silicon and create H-stabilized platelets, while the role of helium is to effuse into these platelets and exert a pressure on the inner surface of these platelets. The damage caused by coimplantation is lower than by hydrogen implantation (at the dose that exfoliation requires).Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48907/2/d10102.pd

Comparison between the different implantation orders in H + and He + coimplantation

H + and He + were implanted into single crystals in different orders (H + first or He + first). Subsequently, the samples were annealed at different temperatures from 200 °C to 450 °C for 1 h. Cross sectional transmission electron microscopy, Rutherford backscattering spectrometry and channelling, elastic recoil detection were employed to characterize the defects and the distribution of H and He in the samples. Furthermore, the positron traps introduced by ion implantation and annealing were characterized by slow positron annihilation spectroscopy. Both orders in the coimplantation of H and He have the ability to decreases the total implantation dose after annealing. No bubbles or voids but cracks and platelets, were observed by cross sectional transmission electron microscopy. The different implantation orders affect the density of interstitial atoms and positron traps.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48910/2/d10406.pd

A novel method to identify pre-microRNA in various species knowledge base on various species

Abstract Background More than 1/3 of human genes are regulated by microRNAs. The identification of microRNA (miRNA) is the precondition of discovering the regulatory mechanism of miRNA and developing the cure for genetic diseases. The traditional identification method is biological experiment, but it has the defects of long period, high cost, and missing the miRNAs that but also many other algorithms only exist in a specific period or low expression level. Therefore, to overcome these defects, machine learning method is applied to identify miRNAs. Results In this study, for identifying real and pseudo miRNAs and classifying different species, we extracted 98 dimensional features based on the primary and secondary structure, then we proposed the BP-Adaboost method to figure out the overfitting phenomenon of BP neural network by constructing multiple BP neural network classifiers and distributed weights to these classifiers. The novel method we proposed, from the 4 evaluation terms, have achieved greatly improvement on the effect of identifying true pre-RNA compared to other methods. And from the respect of identifying species of pre-RNA, the novel method achieved more accuracy than other algorithms. Conclusions The BP-Adaboost method has achieved more than 98% accuracy in identifying real and pseudo miRNAs. It is much higher than not only BP but also many other algorithms. In the second experiment, restricted by the data, the algorithm could not get high accuracy in identifying 7 species, but also better than other algorithms

Construction of the Tetrahedral Trifluorophosphine Platinum Cluster Pt₄(PF₃)₈ from Smaller Building Blocks

The experimentally known but structurally uncharacterized Pt₄(PF₃)₈ is predicted to have an <i>S</i>₄ structure with a central distorted Pt₄ tetrahedron having four short PtPt distances, two long Pt–Pt distances, and all terminal PF₃ groups. The structures of the lower nuclearity species Pt­(PF₃)_<i>n</i> (<i>n</i> = 4, 3, 2), Pt₂(PF₃)_<i>n</i> (<i>n</i> = 7, 6, 5, 4), and Pt₃(PF₃)₆ were investigated by density functional theory to assess their possible roles as intermediates in the formation of Pt₄(PF₃)₈ by the pyrolysis of Pt­(PF₃)₄. The expected tetrahedral, trigonal planar, and linear structures are found for Pt­(PF₃)₄, Pt­(PF₃)₃, and Pt­(PF₃)₂, respectively. However, the dicoordinate Pt­(PF₃)₂ structure is bent from the ideal 180° linear structure to approximately 160°. Most of the low-energy binuclear Pt₂(PF₃)_<i>n</i> (<i>n</i> = 7, 6, 5) structures can be derived from the mononuclear Pt­(PF₃)_<i>n</i> (<i>n</i> = 4, 3, 2) structures by replacing one of the PF₃ groups by a Pt­(PF₃)₄ or Pt­(PF₃)₃ ligand. In some of these binuclear structures one of the PF₃ groups on the Pt­(PF₃)_<i>n</i> ligand becomes a bridging group. The low-energy binuclear structures also include symmetrical [Pt­(PF₃)_<i>n</i>]₂ dimers (<i>n</i> = 2, 3) of the coordinately unsaturated Pt­(PF₃)_<i>n</i> (<i>n</i> = 3, 2). The four low-energy structures for the trinuclear Pt₃(PF₃)₆ include two structures with central equilateral Pt₃ triangles and two structures with isosceles Pt₃ triangles and various arrangements of terminal and bridging PF₃ groups. Among these four structures the lowest-energy Pt₃(PF₃)₆ structure has an unprecedented four-electron donor η²-μ₃-PF₃ group bridging the central Pt₃ triangle through three Pt–P bonds and one Pt–F bond. Thermochemical studies on the aggregation of these Pt-PF₃ complexes suggest the tetramerization of Pt­(PF₃)₂ to Pt₄(PF₃)₈ to be highly exothermic regardless of the mechanistic details

A contribution to the study of negative polarity in GMA welding

GMAW using the electrode with negative polarity (DCEN) has been frequently suggested as a potential means of increasing production capacity. The objective of this work was to further study the performance of negative polarity in GMAW of carbon steels. In this project phase, bead-on-plate welds were carried out in flat position to assess the effect of different potential shielding gas compositions on bead geometry, finishing and spattering. The characteristics were compared with DCEP at the same current, but depositing the same volume of material per unit of length (more industrial related comparison). The arc length was kept the same by adjusting voltage to reach shortest arcs, yet with suitable non short-circuiting metal transfer mode. An approach to measure bead convexity was also proposed and assessed. The results showed that DCEN is feasible as a means of increasing GMAW production capacity. However, to become DCEN applicable with GMAW, the results suggest an Ar based blend with around 6.5 % of O2 is the most appropriate shielding gas, as much as that there is a demand for a standard electronic controlled power source able to work in constant current mode. Funders: Swedish Agency for Economic and Regional Growth, 20200328.Mapla