Effects of hydrogen on the formation of SiGe/Si heterostructures
In this report, we review and discuss our results obtained on the surface segregation of Ge atoms and surfactant effects of hydrogen atoms in Si overlayer growth on Ge/Si(100) by gas-source molecular beam epitaxy (GSMBE) using GeH/sub 4/ and Si/sub 2/H/sub 6/. The layer-by-layer growth rate of the Si overlayers was observed in-situ by reflection high-energy electron diffraction (RHEED) intensity oscillation periods.
Surfactant effect of H atoms on the suppression of Ge segregation in Si overgrowth on Ge(n ML)/Si(100) substrates by gas source molecular beam epitaxy
Hydrogen surfactant effect on the surface segregation of Ge atoms has been studied from layer-by-layer changes in the growth rate of Si overlayers on Ge/Si(100) substrates with Ge thicknesses of 0-6 ML by gas source molecular beam epitaxy using Si 2 H 6 . The layer-by-layer growth rates were in-situ measured by reflection high-energy electron diffraction (RHEED) intensity oscillation periods. The growth rate of Si overlayers decreases exponentially with increasing Si thickness and eventually approaches that of Si/Si(100) homoepitaxial growth at substrate temperatures of 440-550°C, which corresponds to the change in surface coverages of Ge atoms. The decay length of the surface concentration of Ge atoms is suppressed by an increase in Si 2 H 6 gas pressure. The temperature dependence of the decay length has an activation energy of 1.3 eV for Si 2 H 6 gas pressures of 5×10 -6 to 5×10 -5 Torr. These facts can be explained as an effect of H atoms existing on the surface
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%