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.
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