Nanocrystalline silicon (nc-Si) is traditionally grown using a high dilution of SiH4 in H2 which leads to the formation of single crystal silicon crystallites within an amorphous silicon (a-Si) matrix. This approach typically leads to crystallites that are too large to be quantum confined. If nc-Si could be grown with quantum confined silicon nanoparticles (SiNPs) the band gap could be tuned, which might allow for the fabrication of all silicon multiple junction photovoltaic cells or the realization of hot carrier collection.
To produce nc-Si with quantum confined SiNPs we have developed a system to decouple the two growth processes. For the SiNP growth, a highly dilute SiH4 in Ar precursor is introduced to a quartz tube where a plasma decomposes the precursor to nucleate and grow SiNPs. These are injected into a standard parallel plate plasma enhanced chemical vapor deposition reactor where the a-Si is grown. For the growth of the nc-Si the SiNPs and a-Si are sequentially deposited to give a multiple layer structure.
To create highly compact SiNPs and produce smooth layers, a slit aperture is placed on the exit of the quartz tube. The slit width controls the pressure in the quartz tube, which controls both the size of the SiNPs and the velocity that the SiNPs have on exiting the SiNP reactor. Computational fluid dynamics modeling of the process shows that higher reactor pressures lead to longer residence times in the plasma (larger SiNPs) and high gas velocities exiting the SiNP reactor (high density films). This is confirmed experimentally showing there is a trade off between smaller SiNPs and the density of the SiNP layer.
To overcome this we have also mixed SF6 into the gas stream to reduce the SiNP diameter while maintaining a high SiNP reactor pressure.Films of nc-Si have been grown with both SF6 treated and untreated SiNPs. By controlling the a-Si thickness we are able to control the crystal fraction of the nc-Si films, as observed by a change in the ratio of the a-Si peak to the c-Si peak from Raman spectroscopy. From scanning electron microscopy images of focused ion beam prepared cross sections of the center of nc-Si films, a-Si and SiNP layers can be clearly observed.
However, since the thickness of SiNP layer has a Gaussian spatial profile, film morphology varies from alternating SiNP/a-Si layers at the middle to regions where the SiNP layer is thin enough that the nanoparticles are fully encapsulated in a-Si, similar to traditional nc-Si. This allows a continuous variation from one extreme to the other to be studied. Electron spin resonance measurements show a low density of defect states indicating very high quality nc-Si. Temperature dependent photoluminescence and photothermal deflection spectroscopy have also been used to characterize the optical properties of this unique material. In additional we are using THz spectroscopy to understand the carrier lifetime of the nc-Si to explore carrier transfer from a-Si to the SiNPs.
Colorado School of MInes, Renewable Energy Materials Research Science and Engineering Center
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