G. F. Grom

Ordering and self-organization in nanocrystalline silicon

The spontaneous formation of organized nanocrystals in semiconductors has been observed during heteroepitaxial growth and chemical synthesis. The ability to fabricate size-controlled silicon nanocrystals encapsulated by insulating SiO2 would be of significant interest to the microelectronics industry. But reproducible manufacture of such crystals is hampered by the amorphous nature of SiO2 and the differing thermal expansion coefficients of the two materials. Previous attempts to fabricate Si nanocrystals failed to achieve control over their shape and crystallographic orientation, the latter property being important in systems such as Si quantum dots. Here we report the self-organization of Si nanocrystals larger than 80 Å into brick-shaped crystallites oriented along the 〈111〉 crystallographic direction. The nanocrystals are formed by the solid-phase crystallization of nanometre-thick layers of amorphous Si confined between SiO2 layers. The shape and orientation of the crystallites results in relatively narrow photoluminescence, whereas isotropic particles produce qualitatively different, broad light emission. Our results should aid the development of maskless, reproducible Si nanofabrication techniques.

PHONON-ASSISTED TUNNELING AND INTERFACE QUALITY IN NANOCRYSTALLINE SI/AMORPHOUS SIO2 SUPERLATTICES

We report on the interface quality and phonon-assisted tunneling in nanocrystalline Si (nc-Si)/amorphous SiO2 (a-SiO2) superlattices (SLs) prepared by magnetron sputtering and thermal crystallization of nanometer-thick a-Si layers. Phonon-assisted tunneling is observed in a bipolar nc-Si based structure, which confirms that the nc-Si/a-SiO2 junction is not only abrupt but also nearly defect free. The conclusion is supported by capacitance–voltage measurements from which the estimated interface defect density is found to be ∼1011 cm−2 for an eight-period SL. Such high quality interfaces hold considerable promise for the development of nc-Si SL quantum devices.

Self-organization and ordering in nanocrystalline Si/SiO2 superlattices

Abstract The solid phase crystallization of nanometer-thick layers of disordered Si confined between layers of amorphous SiO 2 has been achieved using high temperature annealing. For ultrathin Si layers (∼1– 3 nm thick) crystallization was not possible even after extensive annealing at temperatures up to 1100°C, because of the high strain fields introduced by the SiO 2 layers. However, for thicker layers (∼4– 20 nm thick) a variety of Si nanocrystals ranging in shape from spheres to bricks could be spontaneously formed and, in suitable cases, oriented along the 〈1 1 1〉 crystallographic direction. This formation of organized nanocrystals is an important step towards the construction of Si/SiO 2 quantum devices.

A Memory Device Utilizing Resonant Tunneling in Nanocrystalline Silicon Superlattices

A quantum structure based on Si/SiO 2 and fabricated using standard Si technology has strong potential for applications in non-volatile and scaled dynamic memories. Among standard requirements, such as long retention time and endurance, a structure utilizing resonant tunneling offers lower bias operation and faster write/read cycle. In addition, degradation effects associated with Fowlher-Nordheim (FN) hot electron tunneling can be avoided. Superlattices of nanometer size layers of silicon and silicon dioxide were obtained by sputtering. The size of the silicon nanocrystallites (nc-Si) is fixed by the thickness of the silicon layer which limits the size dispersion. A detailed analysis of the storage of charges in the dots, as a function of the nanocrystals size, is investigated using capacitance methods. Constant voltage and constant capacitance techniques are used to monitor the discharge of the structure. Room temperature non-volatile memory with retention times as long as months is evidenced.

Structural Characterization of Nc-Si/A-Sio 2 Superlattices Subjected To Thermal Treatment

The morphology of nanocrystalline (nc)-Si/amorphous (a)-SiO 2 superlattices (SLs) is studied using Raman spectroscopy in the acoustic and optical phonon ranges, transmission electron microscopy (TEM), and atomic force microscopy (AFM). It is demonstrated that high temperature annealing (up to 1100°C) and oxidation in O 2 /H 2 O ambient do not destroy the SL structure, which retains its original periodicity and nc-Si/a-SiO 2 interface abruptness. It is found that oxidation at high temperatures reduces the defect density in nc-Si/a-SiO 2 SLs and induces the lateral coalescence of Si nanocrystals (NCs). The size, shape, packing density, and crystallographic orientation of the Si nanocrystals are studied as a function of the oxidation time.

Optical and structural characterization of nc-Si/a-SiO2 superlattices

Nanocrystalline (nc)-Si/amorphous (a)-SiO2 superlattices (SLs) have been studied by transmission electron microscopy, Auger elemental microanalysis (AEM), Raman spectroscopy and optical reflection spectroscopy. Recrystallized Si/SiO2 SL is extremely stable under high temperature annealing (up to 1100 degree(s)C) and aggressive wet thermal oxidation: AEM and Raman spectroscopy of folded acoustic phonons show no changes in periodicity in the growth direction and the abruptness of the nc-Si/a-SiO2 interfaces. Furthermore, Raman spectroscopy in the optical phonon range indicates that the annealing decreases the defect density in the Si nanocrystals, possibly due to Si-Si bond rearrangement accompanied by surface reconstruction and surface defect passivation by oxygen.