Large Ordered Arrays of Si Nanocrystals Achieved by Controlled Template Nucleation
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tion, ultrasoft pseudopotentials, supercells, and plane waves. In particular, supercells or “superstructures” of Si-SiO2-Si with 7–8 planes of Si layers separating the SiO2 layers were used. The researchers concluded that H+ is the only charged state in the supercell because all positively charged defects were more stable than their neutral states by ~3 eV when their Fermi energy approached the top of the Si valence band. The researchers found that as an H+ atom approaches the interface, it is repelled by other H+ atoms, and either is immediately trapped inside a suboxide bond or SiO2 protrusion, or migrates laterally throughout the supercell with energy barriers of 0.3–0.5 eV, with the barriers for leaving the cell being much larger. The research team also calculated how the reaction between H+ and Si-H defects occurs mechanistically. As H+ comes within 1.6 Å of the Si–H bond, a H+–H–Si bridge forms, followed by release of H2 and the formation of a positive defect (D+ ). This was shown with electron-density plots. These plots, according to the research team, show the depassivation process as H+ approaches the Si–H bond, bridge formation, and subsequent defect formation. Total energies released from this reaction were 1.3 eV,
but the cost for the reaction was 1.6 eV. According to the report, this reaction was also found to be reversible with annealing in H2 to show an energy barrier of 1.6 eV. MATHEW M. MAYE
Large Ordered Arrays of Si Nanocrystals Achieved by Controlled Template Nucleation The process of self-ordering nucleation of nanocrystals was previously demonstrated by using strained layers in non-latticematched systems and by chemically employing two-dimensional (2D) polymer matrices. A research group headed by Philippe Fauchet and Leonid Tsybeskov at the University of Rochester and Q. Xie from Semiconductor Product Sector in Mesa, Ariz. have demonstrated a way of managing silicon nucleation by facilitating the long-range ordering of uniformly sized and shaped nanocrystals through a controlled template. As reported in the November issue of Nano Letters, the research team used inverse-pyramidshaped holes with submicrometer 2D periodicity to control the in-plane properties of the crystals. The a-Si/SiO2 superlattices were deposited by sputtering and crystallized by high-temperature rapid
thermal annealing (RTA) on this template. Using maskless interferometric photolithography and reactive-ion etching, a periodic array of holes was transferred to an underlying oxide layer. Anisotropic KOH etching on exposed (100) Si was used to form an inverted pyramid due to a high etch ratio of (100) surfaces over (111) planes. Etching was carried out at 70°C for 3 min using a dilute (25%) KOH solution, yielding holes shaped like inverted pyramids with atomically smooth (111) walls. A 10-period a-Si/SiO2 superlattice was grown on the predefined template by sputtering in the presence of alternate Ar/Ar + O2 plasma. Using a Si target, the researchers made films with a final thickness of 50 Å for the a-Si layers and 30 Å for
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