Rishikesh Krishnan

Effect of oxidation on charge localization and transport in a single layer of silicon nanocrystals

The effect of oxidation on charge transport and retention within a sheet of silicon (Si) nanocrystals was investigated with an electrostatic force microscope. Single layers of nanocrystals with smooth and abrupt Si/SiO2 interfaces were prepared by thermal crystallization of thin amorphous Si layers, followed by an oxidation treatment for isolating the nanocrystals. Controlled amounts of charge were injected into the nanocrystals and their in-plane diffusion was monitored in real time. Rapid transport of the injected charge occurred for the nonoxidized nanocrystals. Oxidation of the nanocrystal layer resulted in suppression of lateral transport. The nanocrystals oxidized for 30 min retained the injected charge in a well-defined, localized region with retention times of the order of several days. These long-term charge retention characteristics indicate that nanocrystals prepared by this process could be attractive candidates for nonvolatile memory applications.

Periodic Two-dimensional Arrays of Silicon Quantum Dots for Nanoscale Device Applications

We report a successful unification of standard lithographic approaches (top down), anisotropic etching of atomically smooth surfaces, and controlled crystallization of silicon quantum dots (bottom up) to produce silicon nanoclusters at desired locations. These results complement our previous demonstration of silicon nanocrystal uniformity in size, shape, and crystalline orientation in nanocrystalline silicon (nc-Si)/SiO 2 superlattices, and could lead to practical applications of silicon nanocrystals in electronic devices. The goal of this study was to induce silicon nanocrystal nucleation at specific lateral sites in a continuous amorphous silicon (a-Si) film. Nearly all previous studies of silicon nanocrystals are based on films containing isolated nanocrystals with random lateral position and spacing. The ability to define precise two-dimensional arrays of quantum dots would allow each quantum dot to be contacted using standard photolithographic techniques, leading to practical device applications like high-density memories. In this work, a template substrate consisting of an array of pyramid-shaped holes in a (100) silicon wafer was formed using standard microfabrication techniques. The geometry of this substrate then influenced the crystallization of an a-Si/SiO 2 superlattice that was deposited on it, resulting in preferential nucleation of silicon nanoclusters near the bottom of the pyramid holes. These clusters are clearly visible in transmission electron microscopy (TEM) images, while no clusters have been observed on the planar surface areas of the template. Possible explanations for this selective nucleation and future device structures will be discussed.

Charge Retention in Single Silicon Nanocrystal Layers

Silicon (Si) nanocrystals formed by controlled thermal crystallization of amorphous silicon dioxide (a-SiO 2 )/amorphous silicon (a-Si)/amorphous silicon dioxide (a-SiO 2 ) layers hold considerable promise for application in non-volatile memory products and optoelectronic devices. The size of the nanocrystals is fixed by the thickness of the Si layer and strong quantum confinement is provided in the vertical (growth) direction by the insulating a-SiO 2 layers. However, the extent of quantum confinement in the lateral dimensions remains to be established. Electron energy loss spectroscopy (EELS) measurements performed within a scanning transmission electron microscope (STEM) indicate that the nanocrystals are laterally isolated by approximately 2nm of a-SiO 2 . The confinement potential provided by this barrier is insufficient to localize carriers within a nanocrystal for prolonged durations and can permit quantum mechanical tunneling via wave function overlap between adjacent nanocrystals. Charge leakage kinetics within a sheet of Si nanocrystals was studied using electric force microscopy. Approximately 750 electrons were injected within a 100nm radius circular patch with an atomic force microscope cantilever. The entire charge dissipated from this area in 70min via lateral conduction routes. With a goal of localizing the injected charge and enhancing its retention time, the samples were subjected to relatively low temperature dry oxidation at 750°C. After 20 min of oxidation, retention times above 400 minutes were observed.