Charge Retention in Single Silicon Nanocrystal Layers
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Charge Retention in Single Silicon Nanocrystal Layers Rishikesh Krishnan, Todd D. Krauss1, and Philippe M. Fauchet Dept. of Electrical & Computer Eng, University of Rochester, Rochester, NY14627, USA 1 Dept. of Chemistry, University of Rochester, Rochester, NY 14627, USA ABSTRACT Silicon (Si) nanocrystals formed by controlled thermal crystallization of amorphous silicon dioxide (a-SiO2)/amorphous silicon (a-Si)/amorphous silicon dioxide (a-SiO2) 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-SiO2 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-SiO2. 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. INTRODUCTION The unique size dependent optical and electronic properties of semiconductor nanocrystals have attracted much attention in recent years [1]. Novel devices with nanocrystal constituents can be easily tailored by varying the size of the crystallites. Over the past decades, Metal Oxide-Semiconductor (MOS) based microelectronics has evolved and matured with silicon being the material of choice. Hence an all-silicon based nanotechnology would be amenable for integration with conventional micro-fabrication technology. Si nanocrystals have now been proposed in many applications, including light emitting diodes [2], photonic crystals [3], non-volatile memory products [4], and single electron transistors [5]. Recent reports of stimulated light emission from nanocrystalline silicon (nc-Si) have raised the exciting prospect of a Si based laser [6]. For a practical implementation of these technologies, it is necessary to have a system with well-understood and preferably well-defined material properties. Si nanocrystals formed by controlled thermal crystallization of a-Si/a-SiO2 layers may be one such system. It has been shown that nanocrystals prepared by this procedure have a monodiperse size distribution (along the
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