Nanocrystalline silicon/silicon oxide superlattices: Fabrication, characterization and applications in nano-flash memories

Fabrication and characterization of nanoscale Si structures are of great technological importance due to their potential applications in nanoelectronic devices. In this dissertation, I investigate structural, optical and electrical properties of Si nanocrystals that are formed by solid-phase crystallization of nanometer-thick (2.5–20 nm) amorphous Si films sandwiched between SiO₂ layers.; Si nanocrystals are fabricated as nanocrystalline (nc)-Si/amorphous (a)-SiO ₂ superlattices (SLs) using radio-frequency magnetron sputtering and controlled crystallization. High resolution X-ray diffraction shows that the average size of Si nanocrystals is controlled by the thickness of the initially deposited amorphous Si layer. Si nanocrystals are densely packed laterally as proved by atomic force microscopy. Auger electron spectroscopy, small-angle X-ray reflectivity and high-frequency capacitance measurements demonstrate that nc-Si/SiO₂ interfaces are planar, chemically abrupt and nearly defect free. The nc-Si SLs are thermally stable at temperatures up to 1100°C. As determined by Raman spectroscopy and complementary transmission electron microscopy, crystallization process induces self-organization among Si nanocrystals leading to their narrow size distribution, specific shapes and crystallographic orientation. These nanocrystals may have spherical or rectangular shapes. Laterally elongated nanocrystals possess preferred ⟨111⟩ crystallographic orientation along the SL axis. Energy quantization of holes in Si nanocrystals is inferred from photoconductance and photocurrent measurements. When placed as a floating gate, nanocrystals can be charged and can retain this charge for several hours at room temperature. This proves that Si nanocrystals can be used in nano-flash memory cells.; Presented results demonstrate that control over structural parameters of Si nanocrystals has been achieved and that solid-phase crystallization of nanometer-thick amorphous Si films is one of the most promising techniques for fabrication of Si-based nanoelectronic devices.