Bibliothek

Notizen: Abstract The luminescence of single-crystalline 6H-SiC plates after electrochemical etching has been investigated. The photoluminescence spectrum was found to change strongly after etching; the decay times of separate bands were determined. Just as in the case of silicon, the change in the photoluminescence could be due to the formation of nanostructures.

Notizen: Abstract Heterojunctions were obained by mechanical clamping of anodized 6H-SiC wafers to wafers of layered III–VI semiconductors (InSe and GaSe) at 300 K. On account of the high degree of perfection of the cleavage surfaces, a strong and quite perfect electrical contact is formed. The photo-emf spectrum of the heterojunctions has the form of a wide band. The longwavelength edge of this band is due to the narrow-gap component and the short-wavelength edge is due to the narrow-gap component and the short-wavelength edge is due to absorption in SiC.

Notizen: Abstract Epitaxial-diffused 6H-SiC diodes incorporating a high-resistivity interlayer in the base were studied; the resistance of this interlayer varied when the forward-bias voltage was applied. It is shown that, in spite of the absence of direct indications of the effects of the series resistance (the capacitance is independent of frequency and the value of capacitive cutoff voltage is small), the capacitance measurements for such structures may be incorrect.

Notizen: The weak-field dielectric dispersion (100 Hz–1.8 GHz) studies both of pure and Fe2+/Fe3 modified Cd2Nb2O7 ceramics over the temperature range of 90–380 K are presented and discussed from the viewpoint of relaxor and glassy properties of the system. It is revealed that Cd2Nb2O7 pyrochlore is intolerant of the addition of 25 mol % Fe2+ or Fe3+ for Cd2+. From the x-ray diffraction analysis, pure Cd2Nb2O7 forms a single-phase pyrochlore, while the compositions Cd1.5Fe0.52+Nb2O7 and Cd1.5Fe0.53+Nb2O7 give CdNb2O6 columbite doped with Fe2+ or Fe3+ on the Cd sites (〈8 and 〈2 mol %, respectively), except for minor amount of parasitic hematite. The novel CdNb2O6 type compounds are not ferroelectrics, unlike Cd2Nb2O7. In the latter, at TC=196 K the dielectric relaxation due to the motion of ferroelectric domain walls driven by an external ac electric field is observed. A polydispersive dielectric response of Cd2Nb2O7 around 188 K has characteristics of the relaxor ferroelectrics with glassy behavior (like PMN). Near the characteristic freezing temperature of the zero-field-cooled state (Tf=183 K) the dielectric absorption spectra and the relaxation-time distribution strongly broaden and tend to flatten out, while below Tf the imaginary part of the dielectric permittivity becomes nearly frequency independent. The dielectric response of Cd2Nb2O7 dominating far below TC (around 150 K) and that of Fe2+/Fe3+ doped CdNb2O6 between 90 and 380 K are typical of glass-forming systems at temperature far above Tglass. The relaxational process is characterized by (i) a significant frequency dependence of the peak permittivity position, (ii) non-Arrhenius behavior, and (iii) increasing asymmetry of the dielectric absorption spectrum at the low-frequency side with decreasing temperature, without broadening the relaxation-time distribution and freezing the peak-absorption frequency. It is proposed that although the nature of structural disorder in Cd2Nb2O7 pyrochlore and Fe2+/Fe3+ doped CdNb2O6 columbite is different, in both systems the off-center displacements of the A-site ions act as a random field and are responsible for the relaxor and dipolar glass-like behavior upon cooling. The Debye-like HF dielectric relaxation (1 MHz–1.8 GHz) observed both in Cd2Nb2O7 and its isostructural analog Cd2Ta2O7 at RT and higher (a centrosymmetric phase) is attributed to fluctuations in polarization of the dynamically reoriented O(7th)–Cd–O(7th) dipoles due to dynamical off-center location of Cd ions. © 2001 American Institute of Physics.

Notizen: Deep centers in n-type 4H–SiC and 6H–SiC irradiated with 8 MeV protons have been investigated by capacitance spectroscopy and electron paramagnetic resonance (EPR). Samples were fabricated by sublimation epitaxy or commercially produced by CREE Inc. Research Triangle Park, NC. It is showed that irradiation of wide-band gap semiconductors may lead to an increase in the concentration of uncompensated donors in an n-type material. The spectrum of deep centers in both SiC polytypes is independent of the technology of material growth or type of charged particles. However, the parameters and behavior of the radiation defects in 6H– and 4H–SiC are different. A conclusion about the possible nature of the irradiation induced centers is made on the basis of annealing behavior and EPR data. The obtained results show that proton irradiation can be used in SiC device fabrication technology for producing local high-resistance regions in the semiconductor. © 2000 American Institute of Physics.

Notizen: The Dubna electron cyclotron resonance ion source DECRIS-14-2 was put into regular operation at the U-400M cyclotron in the beginning of 1995. During the past two years the source has produced a wide range of ion beams for physics experiments as well as for the testing of the beam transport lines and the cyclotron itself. Ions of gases, such as He, N, O, Ne, and Ar were successfully delivered and accelerated in the cyclotron. The ion source has shown good performance especially in the case of middle charge state ions (e.g., 600 eμA of Ar8+) as well as high operational reliability. Significant progress in metal ion production has been achieved by introducing a new microoven for the evaporation of metal samples. This microoven in combination with an additional tantalum sheet installed inside the discharge chamber has made it possible to obtain Li2+ ion beams in excess of 200 eμA. © 1998 American Institute of Physics.

Notizen: The new electron cyclotron resonance (ECR) ion source, the Dubna electron cyclotron resonance ion source (DECRIS)-14-2, was designed and built for use with the isochronous cyclotron U-400M. The source assembly was completely finished by the end of 1994 and in January of 1995 the source was tested on the ECR-test bench. In February the source was installed in the cyclotron and in April the first physical experiment on the accelerated beam was carried out. Here we present some design aspects of this source and the results of the first operation on the cyclotron. © 1996 American Institute of Physics.

Notizen: Production of the intense accelerated 48Ca ion beam is the key problem in the experiments on synthesizing of new heavy nuclei. For this purpose an axial injection system with the electron cyclotron resonance (ECR)-4M ion source was created for the U400 cyclotron. The task was to achieve an accelerated beam with an intensity of 0.5 pμA of 48Ca5+ at the 48Ca consumption of ∼0.5 mg/h. To solve this problem, a new method for the solid material feed into the ECR source was developed. The combination of a micro oven with a hot tantalum sheet inside the discharge chamber allowed the production of intense beams of ions of metals with relatively low melting point. The present article describes the method, technique, and experimental results on the production of 48Ca ion beam at the U-400 cyclotron from the ECR-4M ion source. The analysis of the working substance balance in the ion source including the ion beam extraction and material regeneration is performed. The analysis based on the experimental data has shown that the efficiency of Ca atom transformation into ion beam is close to that obtained for the gases such as Ar, and the intensity of 48Ca5+ constitutes about 20% of the extracted ion beam. © 2000 American Institute of Physics.

Notizen: Two electron cyclotron resonance (ECR) sources, DECRIS-14-2 and ECR4M, are presently in permanent operation at the Flerov Laboratory of Nuclear Reactions cyclotrons, U400M and U400, respectively. A wide range of ions of gases from He up to Xe was delivered by the sources and accelerated in the cyclotrons. Major efforts were made in the production of high current stable ion beams of solids with relatively low melting points in mass ranges from Li up to Bi. Both the evaporator and the MIVOC technique were used. Among the solids a beam as exotic as 48Ca was produced at the U400 cyclotron with high efficiency. The main results on production of ion beams of gases and solids are reported. For further development of ECR ion sources a test bench was designed and built. The test bench is equipped with the new DECRIS-3 ion source. The parameters of the test bench and ECR source are described. © 2000 American Institute of Physics.