N. A. Sobolev

Rhombohedral-to-orthorhombic transition and multiferroic properties of Dy-substituted BiFeO3

Investigation of crystal structure, ferroelectric, and magnetic properties of polycrystalline Bi1−xDyxFeO3 (0.1≤x≤0.2) samples was carried out. X-ray diffraction study revealed composition-driven rhombohedral-to-orthorhombic R3c→Pnma phase transition at x∼0.15. Both structural phases were found to coexist in a broad concentration range. Piezoresponse force microscopy found suppression of the parent ferroelectric phase upon dysprosium substitution. Magnetometric study confirmed that the A-site doping induces appearance of a weak ferromagnetic behavior. Both the ferroelectric and magnetic properties were shown to correlate with a structural evolution.

OH- in synthetic and natural coesite

Abstract The incorporation of hydrogen into the coesite structure was investigated at pressures ranging from 4.0-9.0 GPa and temperatures from 750-1300 °C using Al and B doped SiO2 starting materials. The spectra show four sharp bands (ν1, ν2a, ν2b, and ν3) in the energy range of 3450-3580 cm-1, consistent with the hydrogarnet substitution [Si4+(T2) + 4O2- = vaT2 + 4OH-], two weak sharp bands at 3537 and 3500 cm-1 (v6a and ν6b) attributed to B-based point defects, and two weaker and broad bands at 3300 and 3210 cm-1 (ν4 and ν5) attributed to substitution of Si4+ by Al3+ + H. More than 80% of the dissolved water is incorporated via the hydrogarnet substitution mechanism. The hydrogen solubility in coesite increases with pressure and temperature. At 7.5 GPa and 1100 °C, 1335 H/106 Si is incorporated into the coesite structure. At 8.5 GPa and 1200 °C, the incorporation mechanism changes: in the IR spectra four new sharp bands appear in the energy range of 3380-3460 cm-1 (ν7-ν10) and the ν1-ν3 bands disappear. Single crystal X-ray diffraction, Raman spectroscopy, polarized single-crystal and in situ high-pressure FTIR spectroscopy confirm that the new bands are due to OH- in coesite. The polarization and high-pressure behavior of the ν7-ν10 OH bands is quite different from that of the ν1-ν3 bands, indicating that the H incorporation in coesite changes dramatically at these P and T conditions. Quantitative determination of hydrogen solubility in synthetic coesite as a function of pressure, temperature, and chemical impurity allow us to interpret observations in natural coesite. Hydrogen has not previously been detected in natural coesite samples from ultra high-pressure metamorphic rocks. In this study, we report the first FTIR spectrum of a natural OH-bearing coesite. The dominant substitution mechanism in this sample is the hydrogarnet substitution and the calculated hydrogen content is about 900 ζ ± 300 H/106 Si. The coesite occurs as an inclusion in diamond together with an OH-bearing omphacite. The shift of the OH-bands of coesite and omphacite to lower energies indicates that the minerals are still under confining pressure.

Radiation hardness of InGaAs/GaAs quantum dots

The interaction between point defects in the matrix and excitons localized in self-organized InGaAs/GaAs quantum dots is investigated for structures irradiated by protons. The exciton ground state is demonstrated to be unaffected by radiation doses up to 1014 p/cm2. The close proximity of radiation-induced defects leads to a strong nonmonotonous temperature dependence of the luminescence yield: Carriers are lost via tunneling from excited quantum dot states to irradiation-induced defects below ∼100 K, whereas at higher temperatures, carriers escape to the barrier and are captured by defects.

The electronic structure and magnetic properties of transition metal-doped silicon carbide

The band structure and magnetic properties of cubic (3C) and hexagonal (6H) polytypes of silicon carbide doped with 3d transition metals have been studied by ab initio calculations. We demonstrate that for 3C-SiC Cr and Mn produce half-metallic ferromagnetic solutions at both (Si and C) substitution sites, but with different magnetic moments, while SiC:Fe remains paramagnetic. A similar situation has been observed for 6H-SiC; however, Fe on the Si site at low concentrations leads to a ferromagnetic ordering.

Avalanche breakdown-related electroluminescence in single crystal Si:Er:O

Er3+-related electroluminescence (EL) at ∼1.54 μm from single-crystal silicon light-emitting diodes fabricated by erbium and oxygen co-implantation and subsequent annealing has been observed in the avalanche breakdown regime in the 80–300 K temperature range. The EL intensity decreased by a factor of 2 with a temperature increase from 80 to 300 K. The room-temperature yield under the reverse bias was over one order of magnitude higher than that under the forward bias.

Photoluminescence and structural defects in erbium-implanted silicon annealed at high temperature

The behavior of luminescence spectra and structural defects in single crystal Czochralski silicon after erbium implantation at 1 MeV energy and 1×1013 cm−2 dose with subsequent annealing at 1100 °C for 0.25–3 h in an argon or chlorine-containing ambience was studied by photoluminescence (PL), transmission electron microscopy, and chemical etching/Nomarski microscopy. We have found that annealing in the chlorine-containing ambience gives rise to dislocation loops and pure edge dislocations with dominant dislocation-related lines in the PL spectrum. Pure edge dislocations are responsible for the appearance of the lines. The Er-related lines due to the intra-4f shell transitions in the rare-earth ions dominate in the PL spectra and no structural defects are observed after annealing in argon. The observed differences in the optical and structural properties of Si:Er are associated with intrinsic point defects generated during the implantation and annealing.

Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the be...

Defect engineering in implantation technology of silicon light-emitting structures with dislocation-related luminescence

Results obtained in development of physical foundations of ion implantation technology for fabrication of silicon light-emitting structures (LESs) based on dislocation-related luminescence and intended for operation at wavelengths close to ∼1.6 μm are summarized. The development of the concept of defect engineering in the technology of semiconductor devices makes it possible to determine the fundamental aspects of the process of defect formation; reveal specific features of the emission spectra related to changes in the implantation conditions of Er, Dy, Ho, O, and Si ions and the subsequent annealing; and design light-emitting structures with a desirable spectrum of luminescent centers and extended structural defects. The technological conditions in which only a single type of extended structural defect (Frank loops, perfect prismatic loops, or pure edge dislocations) is introduced into the light-emitting layer are found, which enables analysis of the correlation between the concentration of extended defects of a certain type and the intensity of lines of the dislocation-related luminescence. The key role of intrinsic point lattice defects in the origination and transformation of extended structural defects and luminescent centers responsible for the dislocation-related luminescence is revealed. It is found that the efficiency of luminescence excitation from the so-called D1 centers, which are of particular interest for practical applications, varies by more than two orders of magnitude between structures fabricated using different technological procedures. High-efficiency silicon light-emitting diodes with room-temperature dislocation-related luminescence have been fabricated.

Anomalous temperature dependence of erbium-related electroluminescence in reverse biased silicon p–n junction

Electroluminescence (EL) and electrophysical characteristics of erbium and oxygen coimplanted and annealed p–n junctions, characterized by higher values of the Er3+-related EL intensity at ∼1.54 μm in the breakdown regime at 300 K as compared with that at 85 K, have been studied in the temperature range from 85 to 300 K. Hole traps in the Er–O codoped n layer were found to be responsible for the anomalous EL behavior. Er-related EL was observed in the same samples in avalanche breakdown at high temperatures and in tunnel breakdown at low temperatures.

Silicon LEDs emitting in the band-to-band transition region: Effect of temperature and current strength

The parameters of silicon light-emitting diodes (LEDs) prepared through boron implantation into n-Si, followed by annealing at 700–1200°C, were studied. The maximum room-temperature internal quantum efficiency of electroluminescence (EL) in the region of band-to-band transitions was estimated as 0.4% and reached at an annealing temperature of 1100°C. This value did not vary more than twofold within the operating temperature range 80–500 K. The EL growth and decay kinetics was studied at various currents. Following an initial current range of nonlinear dependence, the EL intensity scaled linearly with the current. It is shown that interpretation of this result will apparently require a revision of some present-day physical concepts concerning carrier recombination in silicon diodes.