Carbon structural transitions and ohmic contacts on 4H-SiC

The structural properties of sputtered carbon films on SiC are investigated using X-ray photoelectron spectroscopy (XPS) and Raman scattering. The as-deposited films are amorphous with an sp²/sp³ ratio of 1. The sp² carbon structures gradually increase with increasing temperatures and consist of amorphous aromatic-like carbon, polyene-like carbon, and nano-size graphite flakes. Schottky contacts on carbon/SiC are converted to ohmic contacts after annealing. The concentration of nano-graphitic flakes relative to the aromatic-like and polyene-like carbon increases nearly linearly with annealing temperature. Stacked graphitic structures are not observed. The specific contact resistivities are at 10⁻³–10⁻⁴Ωcm² on the carbon/SiC after annealing from 1050°C to 1350°C.     

Pain-reducing properties of rewarding electrical brain stimulation in the rat.

In 4 experiments with 13 male Charles River rats, electrodes implanted along the medial forebrain bundle were screened for self-stimulation and stimulation-induced analgesia. Analgesia was defined by changes in unconditioned or escape responses to footshock. Almost all electrodes produced both self-stimulation and analgesia or neither. Thresholds for the 2 effects were highly correlated. Brain stimulation produced an analgesic aftereffect comparable in duration with the poststimulation enhancement of performance in self-stimulation (the priming effect). The refractory period of neurons underlying analgesia, assessed by behavioral means, was similar to that previously found for the priming effect in self-stimulation (.8-1.2 msec). Results suggest a common neural system mediating electrical analgesia and the priming effect of self-stimulation. (25 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Observation of differences in the quenching of the Photocurrent in GaAs containing EL2 and EL0

The quenching of the photocurrent and photo-Hall effect of several undoped semi-insulating gallium arsenide samples has been measured and compared with the deep-level photoluminescence spectra from neighboring samples. Samples that show either EL2 (0.68 eV) or ELO (0.63 eV) photoluminescence have distinctly different photocurrent quenching behaviors. EL2 samples show a photocurrent decrease of several orders of magnitude, and a change fromn-type to p-type conduction during quenching at 80 K with 1.1 eV light. ELO samples show a reduction in photocurrent of less than an order of magnitude with no change in the carrier type at this temperature. Photo-Hall effect experiments at 80 K indicate that the conduction isn-type for the ELO samples, but changes fromn- to p-type during the quench for the EL2 samples. The temperature dependence of the quenching has also been studied. EL2 samples show little variation in the range 10-80 K, while ELO samples show significant quenching similar to EL2 after the temperature is reduced below 70 K. These results indicate that defects other than EL2 can significantly affect photocurrent quenching experiments.     

Shallow acceptor levels in 4H- and 6H-SiC

Shallow impurities are the principal means of affecting the electrical properties of semiconductors in order to induce desired characteristics. They can be used to isolate a region by introducing carriers of opposite charge, or they can be used to enhance the number of carriers of a particular type. Thermal admittance spectroscopy has been used to determine the activation energies of the principal p-type dopants, Al and B, in 4H and 6H-SiC, and temperature dependent Hall effect measurements were used to study the shallow B acceptors in 6H-SiC. The accept or species B and Al occupy inequivalent lattice sites in the Si sublattice, and would be expected to exhibit distinct energy levels for each site in analogy to the well known donor energy levels of N. Activation energies for B in 6H-SiC were found to be E_h=E_v+0.27 eV, E_k1=E_v+0.31 eV, and E_k2=E_v+0.38 eV. Al acceptors in 4H-SiC were found to exhibit two energy levels at E_h=E_v+0.212 eV and E_k=E_v+0.266 eV.