Spatially Resolved Transport Studies And Microscopy Of Ultrathin Metal Oxide Semiconductor Structures

The objective of the contract was the investigation of hot electron transport on a microscopic scale in metal oxide semiconductor (MOS) structures in order to assess intrinsic transport properties in SiO2 and defect generation processes that lead to oxide failure. The technique used to achieve microscopic resolution was Ballistic Electron Emission Microscopy (BEEM), a variant of the Scanning Tunneling Microscope (STM). In BEEM, use is made of the nearly monochromatic and highly forward focused electron beam of the STM to inject hot electrons through the thin metal 'gate' and into the conduction band of the SiO2. The measured minimum electron energy required to achieve injection and subsequent detection in the Si defines the oxide barrier potential. A research highlight was the demonstration of image force effects in electron transport through MOS structures. The observation was made that the oxide barrier potential decreased with the application of an oxide field. This effect was attributed to screening by metal electrons and was described by classical image force powering, an often-ignored concept by the MOS science and engineering community. The results led to a new 'dynamic' value for the dielectric constant of SiO2, which was confirmed by theoretical modeling of the MOS transport process. Monte Carlo simulations were made of the spreading of the beam as it traverses the oxide. The inclusion of image force effects was necessary for agreement with experiment.