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High-purity isotopically enriched germanium has been prepared by a hydride method, and the main factors capable of reducing the germanium yield in the monogermane pyrolysis process have been identified. To raise the germanium yield, we have optimized pyrolysis conditions in a laboratory-scale flow apparatus. At a monogermane flow rate at the reactor inlet of 30 mL/min, the optimal monogermane decomposition temperature is 420–450°C. Such conditions ensure a decomposition rate of ~6 g/h and a high product yield and make it possible to obtain most of the germanium in polycrystalline form. Using isotopically enriched monogermane pyrolysis, we have obtained high-purity isotopically enriched germanium samples in the form of polycrystalline ingots with a yield above 95%. The ingots are n-type and range in resistivity from 25 to 50 Ω cm. The content of regulated impurities in the 72Ge-,73Ge-, 74Ge-, and 76Ge-enriched germanium ingots thus prepared does not exceed 0.01 to 1 ppm by weight.

A bulk nanostructured material based on oxidized silicon nanopowder was fabricated using a spark plasma sintering technique. Structural investigations revealed that this material has the composition of ∼14 nm core Si granules inside an SiO2 shell. Photoluminescence measurements have shown that the emission spectra lie in the energy range of 0.6–1.1 eV, which is not typical of the emissions of the Si/SiO2 nanostructures reported in numerous papers. This result can be explained by the formation of energy states in the bandgap and the participation of these states in both electronic transport and photoluminescence emission. Annealing of the sample leads to a decrease in defect density, which in turn leads to quenching of the 0.6–1.1 eV photoluminescence. In this case ∼1.13 eV inter-band transitions in the Si core start to play a dominant role in radiative recombination. Thus, the possibility of controlling the photoluminescence emission over a broad wavelength range was demonstrated.

This paper examines sources of carbon impurities in polycrystalline monoisotopic 28Si prepared by a hydride method. Analytical data on the concentrations of carbon-containing impurities in volatile silicon compounds (28SiH4 and 28SiH4), process gases (Ar and H2), and polycrystalline 28Si are used to identify the major sources of carbon in the polycrystalline 28Si prepared by the hydride method. These are the starting 28SiH4 and calcium hydride used in 28SiH4 conversion into 28SiH4. The rate of carbon intake into polycrystalline silicon from the apparatus material during the monosilane pyrolysis process does not exceed 9 × 1011 cm–2 h–1. Polycrystalline silicon has been precipitated from monosilane with different concentrations of hydrocarbon impurities. At hydrocarbon concentrations in the range 10–4 to 10–3 mol %, the carbon concentration in the monosilane correlates with that in the silicon obtained from it. High-purity monosilane has been used to prepare polycrystalline 28Si samples with concentrations of carbon impurities in the range (0.8–2.3) × 1015 cm–3. Based on calculations of the carbon impurity distribution along the length of a zone-refined ingot, we examine the effect of the initial carbon concentration in the starting polycrystal on the yield of single-crystal monoisotopic 28Si. Requirements are formulated for the carbon concentration in polycrystalline 28Si which ensure a high yield of single crystals with parameters suitable for metrological applications.

A metrological challenge is currently underway to replace the present definition of the kilogram. One prerequisite for this is that the Avogadro constant, N A, 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.

Elements the 3 and 5 of groups of the Periodic System and carbon are the most important impurities in silicon. The estimation technique of carbon, boron and phosphorus impurity content in high-purity monosilane has been proposed. The technique involves the preparation of polycrystalline silicon by silane decomposition, growing a test single crystal by the floating zone melting method and analysis of single crystal samples by IR spectroscopy. Calculation of impurity concentration in polycrystalline silicon has been performed using results on their content in the test single crystal samples, data on impurity distribution in the liquid-solid system and sample coordinates along the ingot length. Effective impurity distribution coefficients in the solid-liquid system for specific growing conditions have been calculated using the Burton-Prim-Slichter equation. Results for the test silicon samples with natural isotopic composition and 28Si isotope enriched ones obtained from monosilane samples with different impurity contents have been reported. Results on IR spectroscopic studies of impurity composition for the test silicon single crystal are in agreement with the concentration data obtained by chromatography. The concentration of 3 and 5 group impurities in monosilane were in the range 4x10−9−2x10−6 at. %, and for carbon concentration was 2x10−6–6x10−4 at.%. The measurement uncertainty by IR spectroscopy does not exceed 15% for carbon impurity and 20% for boron and phosphorus. We show that the upper limit of carbon content in monosilane detected using this method is determined by its solubility in silicon, while the bottom limit depends on the detection accuracy of the IR spectroscopy technique and possible background contamination.

The technology of the growth of Si, Ge, and Si1–xGex layers by molecular-beam epitaxy with the use of a sublimation source of monoisotopic 30Si or 28Si and/or gas sources of monogermane 74GeH4 is demonstrated. All of the epitaxial layers are of high crystal quality. The secondary-ion mass spectroscopy data and Raman data suggest the high isotopic purity and structural perfection of the 30Si, 28Si, 74Ge, and 30Si1–x74Gex layers. The 30Si layers doped with Er exhibit an efficient photoluminescence signal.

We report on the high-pressure synthesis of novel nano- and microcrystalline diamonds with germanium–vacancy (Ge–V) color centers emitting at 602 nm. The synthesis was carried out in non-metallic growth systems C–H–Ge and C–H–O–Ge enriched with germanium and carbon isotopes. We demonstrate germanium and carbon isotope shifts in the fine structure of the luminescence, which allows us to unambiguously associate the center with the germanium impurity entering into the diamond lattice. We show that there are two ground-state energy levels with the separation of 0.7 meV and two excited-state levels separated by 4.6 meV in the electronic structure of the center and suggest a split-vacancy structure of this center. High-intensity and narrow-line emission of high-pressure synthesized small diamonds with Ge–V centers makes them promising candidates for single-photon emitters.

Two known representatives of the split-vacancy complexes in diamond, the negatively charged silicon-vacancy ${\mathrm{SiV}}^{\ensuremath{-}}$ and recently discovered germanium-vacancy ${\mathrm{GeV}}^{\ensuremath{-}}$ defects, were comparatively studied for their photoluminescence (PL) and complementary optical absorption spectra. The observed strong difference between luminescence and absorption spectra indicates a strong frequency defect, that is the difference of binding energies of impurity atom in the ground and excited electronic states, in these color centers. The presence of frequency defect is well supported by first-principle calculations. The obtained results accompanied with isotopic effects shed light on the structure of these centers in the ground and excited electronic states that would open the doorway to their theoretical description.

The low dielectric losses of an isotropically pure single crystal $^{28}$Si sample were determined at a temperature of 20 mK and at powers equivalent to that of a single photon. Whispering Gallery Mode (WGM) analysis revealed large Quality Factors of order $2\times10^6$ (dielectric loss $\sim 5\times10^{-7}$) at high powers, degrading to $7\times10^5$ (dielectric loss $\sim 1.4\times10^{-6}$) at single photon energy. A very low-loss narrow line width paramagnetic spin flip transition was detected with extreme sensitivity in $^{28}$Si, with very small concentration below $10^{11}$~cm$^{-3}$ (less than 10 parts per trillion) and g-factor of $1.995\pm0.008$. Such determination was only possible due to the low dielectric photonic losses combined with the long lifetime of the spin transition (low magnetic loss), which enhances the magnetic AC susceptibility. Such low photonic loss at single photon energy combined with the narrow line width of the spin ensemble, indicate that single crystal $^{28}$Si could be an important crystal for future cavity QED experiments., Comment: 5 pages, 5 figures

The results of the precise measurement of the refractive index of stable germanium isotopes Ge72, Ge73, Ge74, and Ge76 single crystals with high enrichment and a germanium single-crystal Genat of natural isotopic composition with the Fourier-transform interference refractometry method from 1.94 to 20 μm with 0.1 cm−1 resolution and accuracy of 2×10−5 to 1×10−4 are shown. The content of 72 impurities measured by the mass spectrometric method was below 10−5–10−6 wt. %. Oxygen and carbon concentrations measured by IR spectroscopy in all crystals are within 5×1015 cm−3. All germanium crystals have a hole conductivity with specific electric resistivity of 45–50 Ohm×cm. The coefficients of the generalized Cauchy dispersion function approximating the experimental refractive index values all over the measuring range are given. The transmission and Raman spectra are also given.

The results of studying the IR absorption spectra of shallow donors and acceptors in high-purity stable 28Si(99.99%), 29Si(99.92%), and 30Si(99.97%) isotope single crystals grown by the method of float zone melting were reported. The content of residual boron, phosphorus, and arsenic impurities in the studied single crystals was determined with a detection limit of 1 × 1012, 4 × 1011, and 1 × 1012 at./cm3, respectively. The results of the IR spectroscopic estimation of the content of shallow donors and acceptors were in good agreement with the data obtained from the Hall effect measurements for the concentration of free charge carriers. The parameters of the absorption lines of boron and phosphorus impurities in the single crystal of silicon isotopes were studied. The change in the isotope composition of silicon was shown to lead to the shift of the energy spectrum of shallow impurity sites towards higher energies with an increase in the atomic weight of an isotope.

The heat capacity of high-purity isotopically-enriched germanium Ge-76 has been measured in the range of 2.5–15 K. In this range, the heat capacity of Ge-76 is 6–15% higher than the heat capacity of germanium of the natural isotopic composition, which is determined by a change in the average mass.

The fine structure of the spectra of a shallow-level Li donor center and a Fe0 deep donor (S = 1), which occupy tetrahedral interstices in a silicon lattice, is studied in monoisotopic silicon 28Si due to considerable narrowing of the lines of ESR spectra. In the case of the Li donor center, experimental data are found to confirm the role of internal strains in the crystal when observing the ESR spectra of the ground state 1s T2 and state E at T = 3.8–10 K with g < 2.000. The anisotropy in the distribution of strains, which turned out to have a tetragonal type, is investigated using the angular dependences of the line width of the spin resonance corresponding to the triplet state T2z of Li. Similar anisotropy is found in the case of the introduction of Fe0 ions into the initial crystals based on the theory of angular dependences of the width of ESR lines caused by the transitions −1 → 0 and 0 → +1 (ΔMs = 1) in comparison with the transition −1 → +1 (ΔMs = 2).

The temperature dependence of the thermal conductivity κ(T) of single-crystal silicon highly enriched in 29Si (99.919%) isotope has been measured in the temperature range 2.4–410 K. At low temperatures (T < 6 K) in the boundary phonon scattering regime, the thermal conductivity of the 29Si crystal is higher than that of the 28Si (99.983%) crystal. At high temperatures where the thermal conductivity is determined by anharmonic processes of phonon scattering, the thermal conductivity of 29Si is lower than that of 28Si. The conclusions of the theory of phonon thermal conductivity on the mass dependence of κ(T) agree with the experimental results.

Precision studies of Raman scattering in isotopically pure 28Si, 29Si, and 30Si single crystals have been performed in the temperature range from 8 to 300 K. It was found that the quantum effects manifest themselves noticeably at low temperatures (T ≤ 100 K). This can be seen from the difference ~(0.4 ± 0.1) cm−1 between the normalized frequencies of two extreme 28Si and 30Si isotopes, while at room temperature, quantum effects are not visible and are comparable with the measurement error of frequencies (0.1 cm−1). Raman scattering in silicon isotopes in the region of noticeable quantum effects has been studied under hydrostatic conditions at pressures to 12 GPa and at a temperature of Т = 80 K. Quantum effects are not changed under compression within the accuracy of measurements.

The possibility of using infrared spectrometry for the determination of the total oxygen and carbon impurity in polycrystalline silicon of the natural isotope composition and that enriched with the 28Si isotope was studied for samples synthesized by different methods. The results of determining these impurities by the optical method are compared to those obtained by independent methods of analysis. The conditions of IR spectrometric analysis of the silicon synthesized by deposition from the gas phase are determined. It is shown that, for IR spectrometry, the upper boundaries of the analytical range of oxygen and carbon in polycrystalline silicon are 1 × 1018 and 2 × 1018 cm−3; and the limits of their detection are 8 × 1015 and 5 × 1015 cm−3 at a sample thickness of 0.5 and 0.2 cm, respectively.

An attempt is described to replace the present definition of the kilogram with the mass of a certain number of silicon atoms. A prerequisite for this is that the Avogadro constant, NA, is determined with a relative uncertainty of better than 2 × 10 −8 . For the determination, silicon crystals are used. However, the difficulty arising thereby is the measurement of the average molar mass of natural Si. Consequently, a worldwide collaboration has been launched to produce approximately a 5 kg 28 Si single crystal with an enrichment factor greater than 99.985% and of sufficient chemical purity so that it can be used to determine NA with the targeted relative measurement uncertainty mentioned above. In the following, the first successful tests of all technological steps will be reported (enrichment of SiF4, distillation into silane and chemical purification, chemical vapour deposition of polycrystalline 28 Si, floating zone growth of a dislocation-free single crystal) and new equipment for the production of high-purity 28 Si with an enrichment of not less than 99.99% will be described. All steps are well defined by a Technical Road Map (TRM28) and all key results are measured by new mass spectrometric, IR spectroscopic and other chemical and physical methods, such as Hall effect, photoluminescence, laser scattering and x-ray topographic methods (TRM for Analytical Monitoring and Certification, TRM28-AMC). The initial enrichment of the gas is >0.999 95 and the depletion during the entire process is

We have assessed isotope dilution during Czochralski growth of single crystals of stable silicon isotopes from crucibles of quartz glass of natural isotopic composition. A procedure has been developed for applying protective coatings from isotopically enriched silicon dioxide to crucibles. Comparison of the isotopic compositions of isotopically enriched polycrystalline silicon and 28Si, 29Si, and 30Si single crystals grown from it demonstrates that the use of protective coatings makes it possible to preclude isotope dilution in the crystal growth step.

A process has been developed for the preparation of single-crystal 29Si from 29Si-enriched silane. A silicon single crystal has been grown with a 29Si content over 99.9 at %. The oxygen and carbon concentrations in the crystal are under 1 × 1016 cm−3, and its resistivity exceeds 1 kΩ cm.

We have prepared 28Si, 29Si and 30Si single crystals with enrichments above 99.9 at % and a silicon single crystal of natural isotopic composition. The oxygen and carbon concentrations in all the crystals are within 5×1015 cm-3, and the content of metal impurities is 0.01 — 0.1 ppma. The refractive index of the crystals has been determined in the range 1.05–25.5 μm using interference refractometry, and its dispersion and material dispersion have been determined.

The effect of evaporation of phosphorus impurities from the melt is investigated as well as the contaminating effect of quartz glass crucibles on residual content and distribution of this impurities by length of high-purity germanium single crystals. The residual content of phosphorus impurities is mainly influenced by the contaminating effect of crucible material and its distribution by length of crystals is described by the model accounting for the impurities income from crucible material. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The thermal conductivity of isotopically enriched 28Si (99.896%) was measured from 80 to 300 K. The data are shown to be well fitted by the Holland model.

This paper investigates certain characteristics of p-n structures, prepared on a base of germanium obtained by the hydride method. The purpose of the study is to estimate the degree of purity of the germanium and its applicability for the preparation of the detectors with a high energy resolution. Monocrysstals of the p type were used. On completion of diffusion of the lithium in the samples by means of the procedure described, a protective groove was cut out. The apparatus spectra of Fe-55 and Am-241 are shown. The characteristics of the detectors obtained confirm the excellent quality and high purity of the germanium hydride.