Evidence of quantum size effect in nanocrystalline silicon by optical absorption
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Abstract:
The optical absorption spectrum in nanocrystalline silicon $(n\ensuremath{-}\mathrm{Si})$ was determined from both light transmittance and reflectance measurements. We observed that $n\ensuremath{-}\mathrm{Si}$ has a phonon structure in the optical absorption spectrum. This structure originates from momentum-conserving TO phonon absorption and emission, and provides direct evidence that $n\ensuremath{-}\mathrm{Si}$ is an indirect-band-gap semiconductor with quantum size effects. By using small-angle x-ray scattering to measure the nanocrystal size distribution, we found that the band-gap widening varies as ${(1/L)}^{1.6}$ with decreasing nanocrystal diameter L.Keywords:
Nanocrystalline material
Nanocrystalline silicon
We employed plasma enhanced chemical vapor deposition technique to fabricate nanocrystalline Si films at a low temperature of 250 degrees C by using SiCl4 and H2 as source gases. The evolution of microstructure of the films with deposition periods shows that nanocrystalline Si can be directly grown on amorphous substrate at the initial growth process, which is in contrast to the growth behavior observed in the SiH4/H2 system. Furthermore, it is interesting to find that the area density of nanocrystalline Si as well as grain size can be controlled by modulating the concentration of SiCl4. By decreasing the SiCl4 concentration, the area density of nanocrystalline Si can be enhanced up to 10(11) cm(-2), while the grain size is shown to decrease down to 10 nm. It is suggested that Cl plays an important role in the low-temperature growth of nanocrystalline Si.
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