O. N. Godisov

High Purity Isotopically Enriched 29Si and 30Si Single Crystals: Isotope Separation, Purification, and Growth

We report the successful isotope separation and bulk single crystal growth of 29Si and 30Si stable isotopes. The isotopic enrichments of the 29Si and 30Si single crystals determined by mass spectrometry are 99.23% and 99.74%, respectively. Both crystals have the electrically active net-impurity concentration less than 1015 cm-3. Thanks to the result of this work and the 28Si crystals we grew previously, high quality single crystals of every stable Si isotope (28Si, 29Si, and 30Si) have been made available for a wide variety of basic research and industrial applications.

Probing of the shallow donor and acceptor wave functions in silicon carbide and silicon through an EPR study of crystals with a modified isotopic composition

The spatial distributions of the unpaired-electron wave functions of shallow N donors in SiC crystals and of shallow P and As donors in silicon crystals were determined by studying crystals with a modified content of the 29Si and 13C isotopes having a nonzero nuclear magnetic moment. As follows from the present EPR and available ENDOR data, the distribution of donor electrons in SiC depends substantially on the polytype and position in the lattice; indeed, in 4H-SiC, the unpaired electrons occupy primarily the Si s and p orbitals, whereas in 6H-SiC these electrons reside primarily in the s orbitals of C. The electron distributions for the N donor in the hexagonal position, which has a shallow level close to that obtained for this material in the effective-mass approximation, and for the donor occupying the quasi-cubic position differ substantially. The EPR spectrum of N in quasi-cubic positions was observed to have a hyperfine structure originating from a comparatively strong coupling with the first two coordination shells of Si and C, which were unambiguously identified. The effective-mass approximation breaks down close to the N donor occupying the quasi-cubic position, and the donor structure and the donor electron distribution become less symmetric. In silicon, reduction of the 29Si content brought about a substantial narrowing of the EPR line of the shallow P and As donors and an increase in the EPR signal intensity, as well as a noticeable increase in the spin-lattice relaxation time T1. This offers the possibility of selectively studying these spectra by optically exciting a region of the crystal in order to shorten T1 and thereby precluding EPR signal saturation only in the illuminated part of the material. This method may be used to advantage in developing materials for quantum computers based on donors in silicon and SiC.

Peculiarities of neutron-transmutation phosphorous doping of Si30 enriched SiC crystals: Electron paramagnetic resonance study

We have obtained a high concentration of P donor dopants in 6H‐SiC enriched with Si30 and irradiated with thermal neutrons. It was established that annealing at a relatively low temperature of 1300°C, i.e., 500–600°C lower than that used for annealing SiC with the natural isotope composition after neutron-transmutation doping, gives rise to an electron paramagnetic resonance (EPR) signal corresponding to three different shallow P (sP) donors with large hyperfine interactions. The correlated changes of these sP centers in all the annealing experiments and the similarities to the spectra of shallow N donors demonstrate that these sites have shallow donor levels and a similar electronic structure and that they belong to different lattice sites: two quasicubic and hexagonal. The phosphorus at these three sites is suggested to occupy the C position. Simultaneously the low-temperature EPR signal from another set of P-related donor centers having a small, strongly anisotropic hyperfine interaction is observed. I...

Preparation and properties of isotopically pure polycrystalline silicon

A method for extracting silicon isotopes was developed. Polycrystalline 28Si, 29Si, and 30Si samples isotopically enriched to 99.95, 99.5, and 99.9%, respectively, were prepared. Energies of optical phonons in the isotopically pure samples show the expected dependence on the average atomic mass.

Neutron transmutation doping of silicon 30Si monoisotope with phosphorus

Phosphorus-doped silicon 30Si monoisotope samples with a highly homogeneous impurity distribution at a concentration of 5 × 1016 cm−3 were obtained for the first time by means of neutron transmutation doping.

The Way of Making Monoisotopic FZ-Si Crystals for the New kg Mass Unit Standard and for Basic Research

LeibnitzInstitute for Crystal Growth, Max-Born-Str. 2, D-12489 Berlin, Germany Science and Technical Center “Centrotech” 198096 St.Petersburg. Russia 3 Institute of the Chemistry of High-Pure Substances (ICHPS) of the RAS, Tropinin-Str. 49, 603600 Nizhny Novgorod, Russian Federation VITCON Projectconsult GmbH, Dornbluthweg 5, D07743 Jena, Germany Physikalisch Technische Bundesanstalt, Bundesallee 100 38116 Braunschweig, Germany

Separation of Silicon Isotopes by Silicon Tetrafluoride–Silane Technology

Silane enriched in silicon isotopes was obtained in high yield by reacting SiF4 with a solution of NaAlH4 in diethylene glycol dimethyl ether in a purpose-designed apparatus. Chemical analyses are presented for isotopically enriched silicon obtained by the thermal decomposition of silane.

Specific features of transmutational doping of 30Si-enriched silicon crystals with phosphorus: Studies by the method of electron spin resonance

Electron spin resonance (ESR) is used to study the neutron transmutation doping of silicon crystals enriched with 30Si isotope: phosphorus donors and radiation defects produced in the course of transmutational doping are observed. The ESR signals related to the phosphorus uncontrolled impurity in 30Si before transmutational doping (the P concentration is ∼1015 cm−3) and phosphorus introduced by neutron irradiation with doses ∼1 × 1019 cm−2 and ∼1 × 1020 cm−2 (the P concentrations are ∼5 × 1016 and ∼7 × 1017 cm−3, respectively) are studied. As a result of drastic narrowing of the phosphorus ESR lines in 30Si, the intensity of lines increased appreciably, which made it possible to measure the phosphorus concentration in the samples with a small volume (down to 10−6 mm−3). The methods for determining the concentration of P donors from hyperfine structure in the ESR spectra of isolated P atoms, exchange-related pairs, and clusters that consist of three, four, and more P donors are developed. In the region of high concentrations of P donors, in which case the hyperfine structure disappears, the concentration of P donors was estimated from the exchange-narrowed ESR line.

Probing of the Wave Function of Shallow Donors and Acceptors by EPR in SiC Crystals with Changed Isotopic Composition

The conclusion which is drawn from the EPR line broadening and narrowing of the N shallow donor in an isotope enriched and non-enriched 4H-SiC and 6H-SiC crystals along with previous ENDOR results shows that the spin-density distribution over the C and Si nuclei differs between the 4H-SiC and 6H-SiC polytypes. The main part of the spin density in 4H-SiC is located on the Si sublattice. In contrast, in 6H-SiC the main part of the spin density is located on the C sublattice. An explanation for the difference in the electronic wave function of the N donor in 4HSiC and 6H-SiC can be found in the large difference in the band structure of two polytypes and in the position of the minima in the Brillouin zone.

Isotope-pure 28Si layers grown by VPE

Vapor-phase epitaxy was used to grow Si layers enriched to 99.96% with 28Si isotope. Secondary-ion mass spectrometry and Raman spectroscopy were used to demonstrate the high quality of the epitaxial material obtained.