Charge Retention in Single Silicon Nanocrystal Layers

Abstract

ABSTRACTSilicon (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.