The upconversion mechanism of Er3+ ions has been studied for lead-germanate glasses containing Er2O3 concentrations from 0.1 mol % to 2 mol %. Intense green emission was observed at room temperature due to 4S3/2→4I15/2 transition excited by a cw near-infrared laser beam at 797 nm. This green emission shows a similar intensity for samples with different Er3+ ion concentrations. A weak blue emission of 410 nm originating from the 2H9/2→4I15/2 transition was also observed. This blue emission and a red emission from the 4F9/2 level increase with the increase in Er3+ ion concentration. The bright green emission is attributed to the excited level absorption while the blue emission is due to a third step excitation where energy transfer between excited ions owing to their Coulomb interaction plays a key role.

The level structure of 7 Se was investigated via the decay of Br produced in the Ni ("O,pn) reaction. 32 transitions were assigned to Se on the basis of half-life, energy, and/ or relative y-ray yields for different beam energies with 18 transitions confirmed by coincidence data. 32 transitions are placed into the decay scheme which consists of the following levels: 862.0, 3225.9; and 3239.5 keV. The half-life of~~B r was measured to be 1.31+ 0.04 min. RADIOACTIVITY +Br; measured Tii2, E&, I&, pp coin. Se deduced levels, Jp.

The photoresponse of ZnO nanowires irradiated with photons having energies below the band gap of ZnO ͑3.4 eV͒ was studied before and after deposition of CdTe quantum dots via a pulsed electron-beam technique. The small amount of deposited CdTe did not increase the dark current of the samples. However, a substantial increase in the steady state photocurrent was observed after CdTe deposition suggesting a clear photosensitization effect. Results revealed that CdTe influences the photoconductivity transients of ZnO by minimizing its interaction with oxygen in air as well as providing additional traps that serve to increase the photocurrent time constant.

Several new high-spin states are observed in the prompt ␥-rays emitted from the neutron-rich, odd-Z 145,147 La fragments produced in the spontaneous fission of 252 Cf. Alternating parity bands are extended up to spins 41/2 and 43/2 in 145,147 La, respectively. A new band completes the evidence for two sets of parity doublets expected for octupole correlations. Eight B(E1)/B(E2) ratios ͑four new͒ between two bands in 145 La are all essentially constant and somewhat larger than similar ratios in 143,144 Ba, where stable octupole deformation and/or correlations are reported. The new ratios out of the 31/2 Ϫ levels in both nuclei show a sharp spike compared to other states, presumably from a strong reduction in E2 strengths in this backbending region. In 145 La, collective bands show competition and coexistence between symmetric and asymmetric shapes. Band crossings occur in both nuclei around បϷ0.26-0.30 MeV. Their backbends are associated with the alignment of two i 13/2 neutrons according to cranked shell model calculations. ͓S0556-2813͑99͒08402-2͔ PACS number͑s͒: 21.10. Re, 23.20.Lv, 27.60.ϩj, 25.85.Ca

Electrical contact properties and graphitic structures of metal/carbon/4H–SiC structures are investigated. Metals studied include Ni, Co, Cr, NiCr, Ti, W, Mo, Al, and Au. Ohmic contacts are formed on Ni/C, Co/C, Cr/C, and NiCr/C films on 4H–SiC with n-type, C-face, and a doping concentration of 1.8×1019 cm−3 . Only Ni/C and Co/C films exhibit Ohmic contact behavior on SiC with n-type, Si-face, and a doping concentration of 1.6×1018 cm−3. Ni and Co are well known as excellent graphitization catalysts. Raman spectra show that the formation of graphitic carbon is related to the formation of Ohmic contacts in the annealed metal/carbon/SiC structures. Generally accepted catalytic graphitization mechanisms are applied to explain the scanning electron microscopy images, which demonstrate a relationship between the catalytically reacted morphology and Ohmic contact behavior. This study provides evidence that the metals with better catalytic graphitization activities form better Ohmic contacts on metal/carbon/SiC structures.

Cadmium zinc telluride has been developed as a room temperature radiation detector in recent years. The issue of decreasing surface leakage current, which affects the energy resolution of radiation response, was investigated extensively by various surface oxidation treatments. In this study, an aqueous solution of hydrogen peroxide (H2O2) was used in the oxidation of CdZnTe wafers in order to decrease surface leakage current. The surface chemical composition and its morphology modification due to this oxidation process were investigated. X-ray photoelectron spectroscopy analysis revealed that the main chemical species left after H2O2 oxidation are TeO2 along with a small amount of a Cd-related oxide. We found no obvious evidence of the presence of Zn-related oxides. The oxide thickness reaches a saturation after 5 min of H2O2 oxidation. The atomic force microscopy images revealed that, at this stage of saturation, the surface roughness and the mean diameter of the main features were about 1.3 μm and 97 nm, respectively. The decrease of 37% in the surface leakage current was related to the oxide layers formed by H2O2 oxidation.

Graphitic features are detected on 4H-SiC surface following oxidation and etching using surface enhanced Raman spectroscopy (SERS). The electronic state of the carbon is sp2 on both the Si- and C-faces of 4H-SiC. The structures of the “carbon clusters” consist of two-dimensional graphitic flakes less than 2 nm and one-dimensional polyenes. The degree of graphitization on the C-face SiC is higher than those on the Si-face SiC. This study provides experimental evidence for “carbon clusters” existing on SiC surfaces following oxidation at atmospheric pressure and demonstrates that SERS is an effective technique to probe low concentration species on the SiC surface.

We demonstrate ohmic contacts to SiC using carbon films after thermal annealing. Carbon films are deposited on 4H-SiC and 6H-SiC substrates using a radio frequency sputtering method. The carbon/SiC samples convert from Schottky behavior to ohmic behavior after annealing in the temperature range from 1150 to 1350°C. Nanosize graphitic flakes are identified after annealing by Raman spectroscopy and are associated with the ohmic behavior of the contacts. The contact behavior of carbon films on 4H-SiC and 6H-SiC after annealing are compared and the polytype of the SiC has no effects on the structural evolution of the carbon films during annealing. This study reveals that the structural evolution of carbon is associated with formation of ohmic contacts on SiC and that nanosize graphitic flakes play a determinative role in the formation of ohmic contacts.