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    A new generation of 99.999% enriched 28Si single crystals for the determination of Avogadro's constant
    (Sèvres : Bureau, 2017) Abrosimov, N.V.; Aref’ev, D.G.; Becker, P.; Bettin, H.; Bulanov, A.D.; Churbanov, M.F.; Filimonov, S.V.; Gavva, V.A.; Godisov, O.N.; Gusev, A.V.; Kotereva, T.V.; Nietzold, D.; Peters, M.; Potapov, A.M.; Pohl, H.-J.; Pramann, A.; Riemann, H.; Scheel, P.-T.; Stosch, R.; Wundrack, S.; Zakel, S.
    A metrological challenge is currently underway to replace the present definition of the kilogram. One prerequisite for this is that the Avogadro constant, NA, which defines the number of atoms in a mole, needs to be determined with a relative uncertainty of better than 2  ×  10−8. The method applied in this case is based on the x-ray crystal density experiment using silicon crystals. The first attempt, in which silicon of natural isotopic composition was used, failed. The solution chosen subsequently was the usage of silicon highly enriched in 28Si from Russia. First, this paper reviews previous efforts from the very first beginnings to an international collaboration with the goal of producing a 28Si single crystal with a mass of 5 kg, an enrichment greater than 0.9999 and of sufficient chemical purity. Then the paper describes the activities of a follow-up project, conducted by PTB, to produce a new generation of highly enriched silicon in order to demonstrate the quasi-industrial and reliable production of more than 12 kg of the 28Si material with enrichments of five nines. The intention of this project is also to show the availability of 28Si single crystals as a guarantee for the future realisation of the redefined kilogram.
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    Electronic Properties and Structure of Boron–Hydrogen Complexes in Crystalline Silicon
    (Weinheim : Wiley-VCH, 2021-9-17) De Guzman, Joyce Ann T.; Markevich, Vladimir P.; Coutinho, José; Abrosimov, Nikolay V.; Halsall, Matthew P.; Peaker, Anthony R.
    The subject of hydrogen–boron interactions in crystalline silicon is revisited with reference to light and elevated temperature-induced degradation (LeTID) in boron-doped solar silicon. Ab initio modeling of structure, binding energy, and electronic properties of complexes incorporating a substitutional boron and one or two hydrogen atoms is performed. From the calculations, it is confirmed that a BH pair is electrically inert. It is found that boron can bind two H atoms. The resulting BH2 complex is a donor with a transition level estimated at E c–0.24 eV. Experimentally, the electrically active defects in n-type Czochralski-grown Si crystals co-doped with phosphorus and boron, into which hydrogen is introduced by different methods, are investigated using junction capacitance techniques. In the deep-level transient spectroscopy (DLTS) spectra of hydrogenated Si:P + B crystals subjected to heat-treatments at 100 °C under reverse bias, an electron emission signal with an activation energy of ≈0.175 eV is detected. The trap is a donor with electronic properties close to those predicted for boron–dihydrogen. The donor character of BH2 suggests that it can be a very efficient recombination center of minority carriers in B-doped p-type Si crystals. A sequence of boron–hydrogen reactions, which can be related to the LeTID effect in Si:B is proposed.