The incorporation of carbon in Si1−yCy alloys grown using silane and methylsilane by low-pressure rapid thermal chemical vapor deposition is investigated. Substitutional carbon content determined by x-ray diffraction analysis is compared to total carbon concentration measured by secondary ion mass spectrometry. Lower growth temperatures (<600 °C) and higher silane partial pressures are observed to significantly improve substitutional carbon incorporation. At 550 °C, to within experimental error, fully substitutional carbon incorporation is observed over the range of compositions studied (0–1.8 at. % carbon). Fourier transform infrared spectroscopy is also used to verify the presence of substitutional carbon.
Schottky diodes fabricated on in situ doped n-type Si/Si1−x−yGexCy/Si heterostructures grown by chemical vapor deposition were used for admittance spectroscopy in order to study the impact of carbon on the conduction band offsets. Samples with a nominal Ge concentration of 20 at. % and carbon fractions up to 1.3 at. % were studied. In these experiments, the measurement frequency was swept continuously from 1 kHz to 5 MHz, and the temperature was scanned in small increments from 20 to 300 K. Admittance signals in these samples were found to originate from three sources, namely doping freeze-out, band offsets, and traps. Signals arising from the band offsets indicate a conduction band edge lowering for Si/Si1−x−yGexCy of ∼33±22 meV/at. % C. A trap-related admittance signal at an energy of 228±25 meV below the Si conduction band was observed in the Si1−x−yGexCy sample with the highest C fraction (1.3 at. %). The trap energy measured by admittance spectroscopy is in close agreement with the activation energy of 230 meV, which has been reported in the literature for a complex involving interstitial carbon. The conduction band offset in a Si/Si1−yCy sample with 0.95 at. % C was also measured by both admittance spectroscopy and Schottky capacitance–voltage profiling. The two techniques yield excellent agreement, with Si/Si0.9905C0.0095 conduction band offsets of 48±10 and 55±25 meV, respectively.
Metal-oxide-semiconductor (MOS) capacitors fabricated on in situ doped n-type Si/Si1−x−yGexCy and Si/Si1−yCy epitaxial layers were used to study the conduction band offsets in these heterojunctions. The heterostructures were grown epitaxially in a rapid thermal chemical vapor deposition reactor. Si/Si1−x−yGexCy samples with a nominal Ge concentration of 20 at. % and carbon fractions up to 1.3 at. % were studied. Carbon fractions up to 1.6 at. % were studied for the Si/Si1−yCy samples. Gate oxides were formed by thermal oxidation of the Si cap at 750 °C. X-ray diffraction measurements confirm that the processing did not affect the strain in the layers. Devices exhibit well-behaved high frequency and quasistatic capacitance–voltage (C–V) characteristics indicating the high electronic quality of the material. Capacitance–voltage measurements performed over a range of temperatures were used to extract the band offsets. Confinement of electrons at the heterointerface is apparent in the C–V curves of the Si/Si1−yCy MOS capacitors. Comparison of the measured C–V data to one-dimensional device simulations yields a conduction band edge lowering of ∼65 meV per at. % C in the Si1−yCy samples. The Si1−x−yGexCy samples, on the other hand show no evidence of electron confinement. Based on a sensitivity analysis of this technique, it is estimated that the conduction band offset in these samples is less than 30 meV. The smaller offsets in Si/Si1−x−yGexCy compared to Si/Si1−yCy can be explained by the competition between strain compensation and the intrinsic chemical effect of carbon in Si1−x−yGexCy.
The first demonstration of n-MOSFETs fabricated using strained Si1-yCy surface channels is reported. Tensile-strained Si1-yCy layers with substitutional carbon contents up to 0.8 atomic percent were epitaxially grown on <100> Si substrates by rapid thermal chemical vapor deposition, using silane and methylsilane as the silicon and carbon precursors. n-MOSFETS were fabricated using standard MOS processing with reduced thermal exposure to minimize the possibility of strain relaxation. A remote plasma CVD oxide was employed to form the gate oxide. The Si1-yCy devices exhibit electrical characteristics that are typical for Si n-MOSFETs, with good turn-on and subthreshold characteristics. MOS capacitance-voltage analysis demonstrates comparable oxide interface qualities for the Si1-yCy and Si control devices. No carbon-related leakage current is observed in source and drain diode junctions. Characterization of the MOSFET electron inversion layer mobility at room temperature shows comparable mobilities, within the sensitivity of the measurement, for the Si1-yCy and Si control devices. This is in contrast to the mobility enhancement observed in n-MOSFETs fabricated using tensile- strained Si grown on relaxed Si1-xGex layers. At low temperatures, the inversion layer mobility of Si1-yCy devices is lower than that of the Si controls, and appears to be affected by Coulomb and possibly random alloy scattering.
that since the fraction of the bed exposed to the high temperature is small, the activity of the catalyst, as measured by the gas oil test, did not reflect this compositional change.The top part of unstabilized samples after 133 cycles showed a reduction of cell parameter from 24.67 to 24.44 Á, which is close to 24.35 Á found for the stabilized sample. Its crystallinity also compared favorably with that obtained by the standard stabilization procedure. Comparison with the crystallinity of the unstabilized sample indicated that the unit cell shrinkage was not accompanied by further loss of crystallinity. However, the stabilization reaction is apparently slower than the crystal destruction reaction at >1300 °F. As a result, the unstabilized sample at the bottom of the bed was substantially destroyed by the high temperatures.
ConclusionsThe most significant finding of this study is the failure to simulate the commercial deactivation experience in the laboratory. It is clear that at temperatures typical of normal commercial operation, little hydrothermal deactivation occurred; only at substantially higher temperature did deactivation become significant. It is possible that gas-phase temperatures in regenerator cyclones sometimes could exceed 1400 °F and, during upsets, the dilute phase regenerator temperature can also reach such levels. These results would suggest that the catalyst deactivation rate could be significantly improved if these high-temperature excursions could be avoided.In commercial practice, the make-up catalyst in its ammonium form is added directly to the dense phase of the fluidized regenerator. Although the x-ray data indicate that the zeolite in the equilibrium catalyst is in its stabilized form, it is believed that additional resistance to deactivation could be achieved if the make-up catalyst were prestabilized.Literature Cited
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